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U-Pb geochronology and geochemistry from the northeastern New River belt, southern New Brunswick, Canada: significance of the Almond Road Group to the Ganderian platformal margin.

INTRODUCTION AND REGIONAL GEOLOGICAL SETTING

The New River belt (NRB) is the westernmost of three Proterozoic lithotectonic belts within the broader northeastern Appalachian orogen in southern New Brunswick (Fig. 1). These Proterozoic belts represent the peri-Gondwanan microcontinents of Ganderia and Avalonia (Barr and White 1996b; Fyffe et al. 2009, 2011; Johnson et al 2012; van Staal and Barr 2012; Barr et al. 2014a, b). The easternmost Proterozoic rocks are in the Caledonia belt, which is the only belt in southern New Brunswick assigned to Avalonia (Barr and White 1996a). The Brookville and New River belts are interpreted to be part of Ganderia; these rocks form the basement to younger, Early Paleozoic arcs (Penobscot and Popelogan-Victoria) and back-arc complexes (Tetagouche-Exploits) exposed in south-central, central and northern New Brunswick, and central Newfoundland (van Staal et al. 2003; Wilson 2003; Hibbard et al. 2006; Zagorevski et al. 2010; Fyffe et al. 2011). The succession of Proterozoic rocks in the NRB and Caledonia belt are similar in that both contain Ediacaran volcanic and comagmatic rocks that formed in two separate pulses, one around ~ 625 Ma and a later one at ~ 555 Ma, and both lacking rocks representative of the ~70 million year hiatus. The Brookville belt that now separates the NRB and Caledonia belt shares only the later Ediacaran magmatic episode (~ 555 Ma) and includes an older ~ 650 Ma carbonate platform signifying a Late Cryogenian passive margin believed to have developed separately from Avalonia (Barr et al. 2014c).

The NRB belt extends from islands in Head Harbour Passage in the Bay of Fundy and trends northeast to the headland west of Maces Bay on mainland southern New Brunswick where it continues inland for a distance of over 160 kms (Figs. 1, 2). The NRB is truncated to the east by the Belleisle Fault, which separates it from the Early Silurian volcanic rocks of the Kingston Group (Figs. 1). The Kingston Group, and partially time-equivalent Mascarene Group that in New Brunswick is preserved within the NRB, represent an Early Silurian arc and back-arc complex, respectively, which were built on NRB Proterozoic basement and the overlying Early Paleozoic arc/back-arc complexes during an episode of northwest-directed (NW-dipping) subduction (Fyffe et al. 1999; Barr et al. 2002). To the northwest, the NRB is juxtaposed against Cambrian to Ordovician rocks of the St. Croix and Annidale belts along the Sawyer Brook and Taylor Brook faults, respectively. To the southwest, the NRB trends toward coastal Maine where the Proterozoic rocks are likely to underlie the Cambrian Ellsworth Schist and Castine Volcanics of the Ellsworth belt (Fyffe and Fricker 1987; Schulz et al. 2008; Fyffe et al. 2009).

The NRB is typified by (1) Neoproterozoic arc-related, comagmatic volcanic and plutonic rocks of early Ediacaran (ca. 629-622 Ma) and late Ediacaran (ca. 555-551 Ma) age (Johnson and McLeod 1996; Johnson 2001; Johnson and Barr 2004), (2) latest Ediacaran-earliest Cambrian (ca. 540 Ma) relatively shallow marine to subaerial pyroclastic volcanic and sedimentary rocks of the Belleisle Bay Group and (3) post-540 Ma cover of Cambrian marine sedimentary and minor volcanic rocks. The Cambrian cover rocks occur in three separate structural panels that subparallel the northeasterly strike of the belt, with the most inboard panel being closest to the Kingston belt, and the most outboard panel closest to the Annidale belt (Johnson 2001; Johnson et al. 2009; Fyffe et al. 2011). From farthest inboard to farthest outboard these younger Cambrian rocks are divided into the Saint John, Almond Road, and Ellsworth groups, respectively. All three sequences are either directly underlain by the Belleisle Bay Group, as is the case with the Almond Road Group and Saint John Group in the Long Reach area, or linked to the Belleisle Bay Group by means of detrital zircon as is the case for the Ellsworth Group (Fyffe et al. 2009). Although the quartz-rich Almond Road Group was not previously dated, it was considered to be approximately the same age as the Matthews Lake Formation of the Ellsworth Group and Glen Falls Formation of the Saint John Group (e.g., Fyffe et al. 2011; fig. 4). The age of orthoquartzite and quartzose sandstone in the Matthews Lake Formation, the lower unit of the Ellsworth Group in the NRB, is constrained by earliest Cambrian detrital zircon in the basal beds (Fyffe et al. 2009) and a ca. 514 Ma U--Pb age for the overlying volcanic rocks (McLeod et al. 2003). The Early Cambrian age for orthoquartzite in the Glen Falls Formation, in the lower part of the Saint John Group, is also fairly well-constrained by dated ash beds, macrofossils, and acritarch assemblages in overlying and underlying formations (e.g., Tanoli and Pickerill 1988; Landing et al. 2008; Palacios et al 2011, 2017), but the age of the Almond Road Group was poorly constrained until now.

We report new U--Pb (zircon) data that provide upper and lower age limits on the Almond Road Group. The maximum age of the basal quartzite in the group is Early Cambrian (late Fortunian), based on the youngest concordant detrital zircon population at ca. 530 Ma. The minimum age is Early Ordovician (early Floian) based on a U--Pb age of ca. 475 Ma for the cross-cutting West Scotch Settlement porphyry. Together these data support previous interpretations that Early Cambrian quartz-rich strata in the Ellsworth, Almond Road, and Saint John groups once formed a continuous, distal to proximal, Ganderian sandstone facies spanning all three structural panels of the NRB (Johnson et al. 2009; Fyffe et al. 2011) and may have extended across the Avalonian Caledonia belt as well.

STRATIGRAPHY OF THE NORTHEASTERN NEW RIVER BELT

The northeastern NRB is defined here as the structural panel between the Belleisle and Taylor Brook faults, west of Belleisle Bay and northeast of the Long Reach (Fig. 2). The Taylor Brook Fault shows evidence that it was initially a thrust fault that has been cut by a late steep structure that presently defines a large part of the boundary between the NRB and Annidale belt. With the exception of small, local fault-bounded units, the Silurian Mascarene Group is restricted to the area south of the Fanning Brook Fault, along the complexly faulted southern margin of the NRB in this area (Fig. 2). This lack of Silurian cover may be due to the oblique east-northeast-trend of the Fanning Brook Fault, which cuts across the regional northeast structural trend exposing older rocks on its north side (Fig. 1). Consequently this structural panel is the only one in which the basement rocks on the north side of the Mascarene basin are exposed.

The stratigraphic succession in the northeastern NRB is composed of the Almond Road Group and underlying Belleisle Bay Group. The Belleisle Bay Group in this area comprises two formations; the Browns Flat and Grant Brook formations. The Browns Flat Formation is not dated, but the overlying and in part laterally equivalent Grant Brook Formation yielded a U--Pb zircon age of 541 [+ or -] 3 Ma age for a slightly reworked intermediate to felsic tuff. Previous attempts to date the Grant Brook Formation yielded a maximum age of 542 [+ or -] 9 Ma (unpublished data from T. Krogh in Johnson et al. 2009), which is considered to be close to the depositional age based on the more recent age data. Historically the vast majority of rocks now included in the Belleisle Bay and Almond Road groups were assigned to the Silurian Mascarene Group (e.g., McCutcheon and Ruitenberg 1987). However, a pre-Silurian age for the Grant Brook Formation is demonstrated by the intense hornfelsing of the unit near the contact with the Early Ordovician (479 [+ or -] 2 Ma) Stewarton Gabbro (Johnson et al. 2009, 2012) and the Middle Ordovician (467 [+ or -] 3 Ma) age obtained for the cross-cutting Bull Moose Hill porphyry (Johnson et al. 2012).

Almond Road Group

The Almond Road Group is divided into the Snider Mountain Formation and overlying Ketchum Brook Formation (Johnson et al. 2009). The group is exposed north of Evandale on the east side of the Saint John River and on Spragg, Taylors and Ketchum brooks near The Grant (Fig. 2). It is also exposed farther northeast on Snider Mountain where it is overstepped by Carboniferous rocks of the Maritimes Basin.

Snider Mountain Formation

The Snider Mountain Formation is named after a section of rocks on Snider Mountain north of Sussex (Figs. 1, 2). On the Snider Mountain Road the formation is composed of white to grey orthoquartzite and intraformational quartzite-pebble-rich conglomerate that appears to be concordantly underlain by pink feldspathic sandstone and pink to reddish arkosic pebble conglomerate of the Grant Brook Formation. There is a gap of a few metres in the exposure, but the quartzite unit contains rare red siltstone clasts presumed to be from the Grant Brook Formation. Along strike to the northeast on Beatty Brook, similar pink feldspathic sandstone and pebbly arkosic conglomerate are succeeded down-section by thick-bedded, volcaniclastic rocks, reworked tuffs, and various pyroclastic volcanic rocks that are also part of the Grant Brook Formation.

The Snider Mountain Formation is also well-exposed on a tributary of Jones Brook where is crosses the Almond Road, east of the Saint John River (Fig. 3). Here it is composed of thick-bedded (65-70 cm), white, granular orthoquartzite (Fig. 4), intercalated with thin-bedded, fine-grained, light grey, feldspathic quartzite. The lack of pebble conglomerate at this location may indicate that these strata were deposited farther offshore than those exposed at the type section on Snider Mountain. The quartzite unit is about 20 metres thick and is separated from underlying purple and greyish green ash tuffs of the Grant Brook Formation by a small (< 5 m) concealed zone. The orthoquartzite is succeeded upsection to the south by dark grey siltstone interbedded with fine- to medium-grained, quartz-rich, feldspathic sandstone of the overlying Ketchum Brook Formation on the south side of Almond Road (it should be noted that much of the outcrop on the south side of the road is now covered by rip-rap during 2017 road improvements). Orthoquartzite from this locality on the north side of Almond Road was sampled for detrital zircon. Results are described below in the Geochronology section.

The Ketchum Brook Formation is characterized by the exposures on Spragg, Taylors and Ketchum brooks (Fig.5), and in the Beulah area and south of Almond Road (Fig. 3). The formation is divided into a lower sedimentary member and an upper mafic volcanic member. The sedimentary rocks comprise light grey thin- to thick-bedded (5-50 cms), fine- to medium-grained, slightly micaceous, quartz-rich, feldspathic sandstone intercalated with dark grey to black laminated siltstone and shale and minor thinly-laminated, light green siltstone, and shale. Near Beulah on a west-flowing tributary of Jones Brook the dark shale dominates and it is interbedded with rare limestone and calcareous siltstone with well-developed cone-in-cone structures. Cone-in-cone structures are diagenetic concretionary features resembling horn corals. Younging direction determined from the cone-in-cone structures show that the clastic rocks are overlain by the mafic volcanic rocks. Intercalations of dark shale and mafic tuff can be viewed on Highway 705 southeast of Beulah (Fig. 3). The mafic volcanic member comprises light grey, commonly bleached, highly amygdaloidal basalt flows and grey green, basaltic lithic tuff dominated by coarse mafic volcanic lapilli.

On Ketchum Brook south of the second bridge on Elm Flat Road only the lower sedimentary member is exposed and here is composed of thick-bedded (40-50 cm), light grey, medium-grained, micaceous, quartz-rich feldspathic sandstone, with thinner (2 cm) interbeds of finer grained, light grey quartzose sandstone and thinly laminated, light green siltstone and shale. To the north on Taylors Brook, it grades upwards into interbedded dark grey to black laminated siltstone and light grey, very fine-to medium-grained, micaceous, feldspathic sandstone. Sandstone beds range in thickness from about 2-20 cm. The Ketchum Brook Formation is intruded by a small number of mostly unnamed, felsic and mafic intrusive plugs and rhyolite domes.

West Scotch Settlement porphyry

The West Scotch Settlement porphyry is a small, hypabyssal felsic intrusion that is less than 1 kilometre in diameter. The intrusion is exposed on Elm Flat Road approximately 2.4 km north of its junction with the West Scotch Settlement Road and on Taylor Brook where it intrudes the Ketchum Brook Formation (Fig.5). The West Scotch Settlement porphyry has abundant euhedral to anhedral feldspar and quartz phenocrysts in an aphantic matrix.

The intrusion exhibits a rusty weathered surface due to the presence of iron-oxide from abundant disseminated pyrite. The feldspar phenocrysts and feldspar in the groundmass are pervasively altered to buff white clay. The intrusion is exposed sporadically along Taylors Brook for about 500 metres north of the junction with the Elm Flat Road, at which point it intrudes dark siltstone and white sandstone of the Ketchum Brook Formation. The intrusion-sediment contact is very irregular and dark sedimentary fragments are incorporated into the intrusion at the contact, which is brecciated and contains quartz veins with pyrite.

GEOCHRONOLOGY

West Scotch Settlement porphyry

Sample NB09-03 (UTM 20T -5063608 N, 277333 E) was collected from outcrop on the west side of Elm Flat Road where it turns and crosses over Taylors Brook (Fig.5).

Analytical methods and results

U--Pb analyses were conducted by Greg Dunning at the Geochronology Laboratory in the Department of Earth Sciences at Memorial University of Newfoundland using chemical abrasion - thermal ionization mass spectrometry (CA-TIMS) methods. The analyses were carried out according to the technique oudined in Sanchez-Garcia et al. (2008). The zircons were annealed, and then etched using the Mattinson (2005) technique, which is very effective at eliminating altered portions of zircon that have undergone lead-loss. The ages were calculated using the decay constants recommended by Jaffey et al. (1971). The U--Pb isotopic composition is shown in Table 1.

The zircon population in plane light under the picking microscope and the cathodoluminescence images of the representative grains are shown on Figures 6a and 6b. The sample has three concordant points of 3-5 zircon prisms each (Fig. 7). The three points yield [.sup.206]Pb/[.sup.238]U ages as follows: Z1 - 476 [+ or -] 3; Z2 - 475.5 [+ or -] 2.3; Z3 - 474.6 [+ or -] 3 Ma. The weighted average of these at the 95% confidence interval is 475.4 [+ or -] 1.6 Ma (MSWD = 0.24) indicating an Early Ordovician (Floian) age for the West Scotch Settlement porphyry.

Snider Mountain Formation

Sample NB12-313 (UTM 19T - 5054899 N, 731021 E) was collected from orthoquartzite outcrop on a tributary of Jones Brook less than 20 metres north of where it crosses the Almond Road (Figs. 3, 4). Detrital zircon from the sample is shown in Figure 8.

Analytical methods

Detrital zircon U-Th-Pb laser-ablation, inductively coupled plasma mass spectrometry (LA-ICPMS) analyses were conducted at the Texas A & M University R. Ken Williams Radiogenic Isotope Geosciences Laboratory. Rock samples were crushed and zircon grains concentrated using standard crushing and density separation (jaw and disc crusher, Wilfley table, heavy liquids) methods. Zircon grains were separated from other dense minerals by hand-picking in a petrie dish under a binocular microscope, but no further separation was performed on the bulk zircon aliquot recovered from heavy liquids. The bulk zircon aliquot was piled and quartered repeatedly in the petrie dish to obtain an unbiased (or minimally biased) sub-sample of about 1000 grains. This zircon sub-sample was mounted on double-stick tape and encased in a 2.5cm diameter epoxy disc, along with fragments of NIST 610, NIST 612, one primary reference material (zircon 91500; Wiedenbeck et al. (1995) and two secondary reference materials (zircons R33 and FC-1; Black et al. 2004; Paces and Miller 1993, respectively). The disk was abraded with 2000 grit sandpaper to expose the interior of zircon grains and polished to 0.25[micro]m on a diamond-suspension lap wheel.

LA-ICPMS analyses were conducted on a Thermo Scientific iCAP RQ quadrupole mass spectrometer running in standard high-sensitivity (STDS) mode connected to an esi/NWR 193nm 4ns pulsed excimer laser system equipped with a two-volume sample cell (TV-2). Instrument settings and run parameters are given in Table 2 and analytical data in Appendix A. Data are reduced using Iolite v. 3.5 (Paton et al. 2011) under the U--Pb Geochron4 data reduction scheme (Paton et al. 2010). Analysis of the primary reference material, each treated separately as an unknown, indicates an internal analytical reproducibility of U--Pb ages to better than 0.7%. The average accuracy of secondary reference materials (Appendix A) is better than 2.25% (FC-1) and better than 1.5% (R33).

Results

A total of 177 zircon grains were analyzed from sample NB12-313. A gap in dates at around 700 Ma provides a convenient cut-off below which the [.sup.206]Pb/[.sup.238]U dates are preferred and above which the [.sup.207]Pb/[.sup.206]Pb dates are preferred. Of the analyzed grains, 51 were either >10% discordant or were of <10% precision on their preferred U--Pb date. The age spectrum is dominated by a peak centered at about 575 Ma (Fig. 9a). It is important to note that in a detrital suite it is unlikely that these represent a single population of zircon; therefore at least some of the individual analyses that cluster around 575 Ma may actually be younger, within error of the 560-550 Ma volcanic and co-magmatic plutonic rocks now exposed in the NRB. The apparent youngest grain has a [.sup.206]Pb/[.sup.238]U age of 515 [+ or -] 5.5 Ma, (Fig. 9b, Appendix A) but it lies within error of a fairly continuous distribution of dates with no clear "cluster" of youngest dates and thus the data do not provide clear evidence for providing a quantifiable maximum age of deposition. The youngest population of zircons yielded a Concordia age of 529.2 [+ or -] 3.6 Ma indicating a depositional age of ca. 530 Ma or younger (Fig. 9c).

CHEMICAL DATA

A total of 22 samples of mafic to intermediate volcanic rocks were chosen for analysis. The sampled units include the Ketchum Brook Formation in the Almond Road Group (4 samples), and the Grant Brook (12 samples) and Browns Flat (6 samples) formations in the Belleisle Bay Group. The majority of volcanic rocks in the Browns Flat and Ketchum Brook formations are basalt, whereas the composition of the Grant Brook Formation volcanic rocks is mainly andesitic or basaltic andesite (Fig. 10a). Three of the analysed samples from the latter formation contained > 63% Si[O.sub.2] (Appendix B), although only one of these plots in the rhyolite- dacite field based on immobile trace element ratios (Fig.lOa). Immobile trace element ratios were also utilized to determine the magmatic affinity of the volcanic rocks in the northeastern NRB (Fig. 10b). The Th/Yb against Zr/Y bivariate diagram of Ross and Bedard (2009) was chosen as these authors have demonstrated that it is an improvement on Barrett and McLean's (1994, 1999) single ratio diagrams using Zr versus Y, La versus Yb and Th versus Yb, although the data are still somewhat ambiguous. The basalt in the Browns Flat Formation plot almost exclusively in the transitional field, although one sample was clearly calc-alkaline (Fig. 10b). Basalt samples from the Ketchum Brook Formation are similar in that the four samples span the tholeiitic, transitional, and calc-alkaline fields. The one tholeiitic sample in the Ketchum Brook Formation was collected from the same general area as another that plots well into the transitional field but the reason why the trace element ratios are so different is not clear. In contrast, the Grant Brook Formation is clearly calc-alkaline based on Th/Yb and Zr/Y ratios (Fig. 10b).

On N-MORB-normalized trace element spider diagrams all of the samples exhibit depletion in Nb relative to Th or La (Fig. 11) that is typical of arc magmas. The Belleisle Bay Group exhibits three slightly different patterns that correlate directly with the composition of the volcanic rocks. As would be expected the most felsic samples in the Grant Brook Formation show the most pronounced depletion in Ti (Figs, 11b, 11c, and 11d). Basalts from the Browns Flat and Grant Brook formations are similar and lend support to the field evidence that suggests these formations are in part laterally equivalent. The younger Ketchum Brook Formation basalt samples exhibit patterns similar to those in the Belleisle Bay Group and are indistinguishable from them based on trace element patterns alone.

On a tectonic discrimination diagram using high field strength elements such as the Th-Hf-Ta of Wood (1980), all of the volcanic rocks in the Belleisle Bay and Almond Road groups plot in the volcanic arc basalt field (Fig. 12a) and are displaced above the diagonal line representing mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) on the Th/Yb versus Nb/Yb diagram (Fig. 12b) of Pearce (2008), indicating that they interacted with continental crust or were erupted in a supra-subduction zone environment (Pearce and Peate 1995).

Previously published chemical data for the Belleisle Bay Group to the southwest in the Long Reach area (Johnson and Barr 2004) are compared to the results of this study. The latest Neoproterozoic to earliest Cambrian volcanic succession in the Long Reach area were divided into the Browns Flat and Beulah Camp formations, which were later assigned to the Belleisle Bay Group by Johnson et al. (2009). The Beulah Camp Formation is lithologically similar to the Grant Brook Formation and this correlation is corroborated by the chemical data. Basalts from the Beulah Camp Formation, like those in the Grant Brook were also chemically similar to those in the Browns Flat Formation (Johnson and Barr 2004). Major and trace element signatures in the former indicate that it also was likely erupted in a continental margin subduction zone (Johnson and Barr 2004). The Browns Flat Formation basalts in that area showed two different geochemical trends, one indicative of a within-plate setting and the other exhibiting characteristics of highly unevolved basalts suggesting a complex tectonic history.

DISCUSSION AND CORRELATION

Early Cambrian quartz-rich clastic sedimentary successions that overlie latest Ediacaran to earliest Cambrian volcanic and volcaniclastic to arkosic sedimentary rocks of the Belleisle Bay Group occur in all three structural panels within the NRB (Fyffe et al. 2011; fig. 4). These quartz-rich successions include the Matthews Lake Formation of the Ellsworth Group, the Snider Mountain Formation of the Almond Road Group, and the Glen Falls Formation of the Saint John Group. The age of the Glen Falls Formation is bracketed by a ca. 530 Ma ash bed from near the base of the underlying Ratcliffe Brook Formation and a ca. 510 Ma ash bed in the overlying Hanford Brook Formation (Landing et al. 1998; Landing and Westrop 1998; Isachasen et al. 1994; Palacios et al. 2011, 2017). A similar age is indicated for the Matthews Lake Formation as the overlying Mosquito Lake Road Formation is ca. 514 Ma (McLeod et al. 2003) and the youngest detrital zircon in the Matthews Lake Formation is ca. 540 Ma, (Fyffe et al. 2009). Although beyond the scope of this paper it is interesting to note the Glen Falls Formation quartzite in the Caledonia Terrane near Saint John shows a nearly unimodal detrital zircon population at ca. 540 Ma (Barr et al. 2012), essentially identical to the population distribution of zircon in the Matthews Lake Formation in the NRB, suggesting a similar source in the Early Cambrian. The age of the Almond Road Group is now bracketed between the Early Cambrian (late Terreneuvian) and Early Ordovician (Tremadocian) indicating that it is possible that all three quartzite sequences in the NRB are Stages 2-3 of the Cambrian (using the time scale of Cohen et al. 2013).

The textural maturity and bedforms that characterize the Glen Falls Formation indicate that it was deposited in a near-shore, high energy marine environment (Tanoli and Pickerill 1987; Landing and Westrop 1998). Although detailed sedimentological analysis was not undertaken on the Snider Mountain or Matthews Lake formations preliminary observations suggest a somewhat more distal depositional environment for these rocks. The most obvious is the lack of large-scale trough cross-bedding or other indications of wave action or fluvial processes that would be present in a shallow marine setting and the Snider Mountain Formation on Jones Brook contains no conglomerate at all. Although primary sedimentary features are rarely preserved in the more strongly deformed Matthews Lake Formation, some of the conglomerates have a mud-rich matrix and lithologically varied clasts that may have been deposited as debris flows. The Matthews Lake Formation as a whole tends to be a thicker, somewhat finer grained, quartzose sandstone sequence with local conglomerate debris flows that pass upwards into Mosquito Lake Formation containing sandstone with iron-rich siltstone lenses suggesting deposition in relatively deeper water. This led Fyffe et al. (2009, 2011) to suggest that the Almond Road and Matthews Lake may be more outboard facies of the tidally-influenced quartzite of the Glen Falls Formation, therefore would represent outer and inner platform sequences, respectively, deposited on NRB basement during the latter part of the Early Cambrian.

The arc-like chemistry of the Belleisle Bay Group is similar to the chemistry of mafic volcanic rocks in the upper Almond Road Group and the younger Late Cambrian to Early Ordovician Annidale Group, which occurs in the adjacent lithotectonic belt. The Annidale Group preserves the remains of the Penobscot back arc basin and a record of the Early Ordovician Penobscot orogeny (Johnson et al. 2009; 2012), one of the oldest orogenic events within the Appalachian orogen (Zagorevski et al 2007; van Staal and Barr 2012). In New Brunswick, the Penobscot back arc basin is interpreted to have developed along a continental margin represented by the Proterozoic rocks of the NRB and Brookville belt (Johnson et al. 2009, 2012; Fyffe et al. 2011). The Ediacaran rocks in these belts are interpreted to represent ensialic arc magmatism along the margin of Amazonia before it separated from Gondwana and became the Ganderian microcontinent (e.g., van Staal et al. 2012 and references therein). The arc signature of the ca. 540 Ma volcanic rocks in the Belleisle Bay Group supports the idea that the NRB was part of this continental margin in the Late Neoproterozoic and earliest Cambrian. The upwards transition from volcaniclastic sandstone and redbeds in the upper part of the Belleisle Bay Group to the widespread occurrence of quartz-rich sedimentary rocks by late Terreneuvian (530-520 Ma) suggest a rift-to-drift transition, possibly signalling the breakup of Gondwana. In other areas of Ganderia arc magmatism was shut-off at around 540 Ma due to ridge-trench collision that transformed the Gander margin into a strike-slip regime (e.g., Rogers et al. 2006). Exhumation of older rocks within the arc during such a collision could explain the prominence of ca. 575 Ma detrital zircon in the Almond Road Group. Around 515 Ma the NRB underwent renewed arc volcanism as represented by rocks in the upper part of the Almond Road Group and the Mosquito Lake Road Formation (Johnson and McLeod 1996). This mag-matism may be related to the development of the Penobscot arc and the formation of the Annidale back-arc basin by the Late Cambrian (ca. 495 Ma) (Johnson et al. 2012).

Rocks spanning the time period between the ca. 515 Ma Mosquito Lake Road Formation and ca. 495 Ma Annidale Group are not present in the NRB but they do occur in Maine. The chemical and isotopic data for the ca. 509-502 Ma Ellsworth Schist and overlying Castine Volcanics (Stewart 1998) in Maine show they are not arc-related, but that they likely developed within a proto- oceanic rift, such as in the Gulf of California (Schulz et al. 2008). Fyffe et al. (2009) considered the lithological characteristics of the Ellsworth volcanic sequence in coastal Maine to be similar to the Matthews Lake and Mosquito Lake Road formations in the NRB but the sequences are not temporally equivalent, therefore it remains unclear clear if this accounts for the contrast in tectonic environment or if it is due to along strike differences in an extremely complex long-lived convergent margin such as the Early Paleozoic Gander margin.

ACKNOWLEDGMENTS

The authors would like to extend their appreciation to Sherri Furey and Amanda Langille at the U--Pb laboratory at Memorial University Newfoundland and laboratory staff at Texas A & M University. We thank Sandra Barr and Chris White for initiating the detrital zircon project and facilitat-ing the U--Pb dating at Texas A & M and Chris White for providing the Concordia diagram for the Snider Mountain Formation. Journal reviewers Les Fyffe and Doug Reusch made many thoughtful comments that significantly im-proved the manuscript. We also wish to acknowledge Mal-colm McLeod who conducted field mapping in this area and made many contributions to our understanding of the geology. Geochemical data diagrams were produced using Geochemical Data Toolkit (GCDKit).

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Editorial responsibility: David P. West, Jr.
Appendix A. U-Th-Pb detrital zircon isotope analyses for NB12-313.

Analysis    Pb (1)   U (1)    Th (1)   [.sup.206]Pb/   [.sup.208]Pb/
Identifier           (ppm)             [.sup.204]Pb    [.sup.206]Pb
(NB313)

a_01         224.5    247.7     253.0   19000           3.366
a_02          74.9    331.0     258.1   11900           3.63
a_03          27.3    199.0     104.8    9400           5.62
a_07          64.0    185.8      85.2   17000           6.91
a_09          94.2    484.0     351.0   14000           4.239
a_10          89.5    445.0     347.0   19000           3.75
a_11         192.0    560.0     189.0  160000           9.58
a_12         171.2    399.0     196.3  101000           6.42
a_13          57.7    424.0     172.4   29000           7.48
a_14          20.6    141.0      74.8    7900           6.09
a_15         115.0    155.0     106.0   17600           5.3
a_16         247.0    757.0     665.0   77000           3.125
a_17          79.7    414.0     114.6   78000          11.64
a_18          58.1    309.0     203.9   24000           4.62
a_19          37.6    272.0     130.3   23000           5.66
a_21         113.7    412.0     393.0   45000           2.962
a_23          68.1    318.0     272.0   43000           3.581
a_24         111.2    527.0     130.0  160000          12.54
a_25         157.0    450.0     596.0   58000           2.58
a_26          61.9    259.0     198.1   38000           4.093
a_28          55.3    236.2     197.2   22000           3.72
a_29          29.6    139.5     103.4   22300           4.04
a_30          70.3    262.2     250.8   18000           3.138
b_31         218.0    960.0     297.0  142000          10.08
b_32         169.0    732.0     654.0   33000           3.36
b_33          58.5    285.0     214.0   11700           4.14
b_34          29.5    140.0     117.4    6600           3.79
b_36         127.8    325.0     150.8   27100           6.58
b_38          30.3    117.4     114.7    6400           3.24
b_39          46.7    274.0     175.9    3900           4.62
b_40         298.3    833.0     465.0   38000           6.28
b_41          45.6    417.0     174.2   12000           7.62
b_42          91.6    350.0     357.0   16000           3.35
b_44         116.7    190.3     145.1   35000           4.281
b_45          28.6    123.0     105.6    7300           3.62
b_46          38.0    191.0     131.0   14800           4.6
b_47          98.6    403.2     365.6   11000           3.432
b_48         308.0    890.0     362.0  170000           7.91
b_49          83.3    759.0     145.1   26000          15.66
b_50          97.3    388.3     351.0   20000           3.62
b_52          41.9    168.1     169.9   13900           3.422
b_53         158.9   1257.0     537.0   59000           6.46
b_55          61.7    295.0     197.0   25000           4.587
b_56          63.1    260.3     227.0   18000           3.56
b_57          23.9     87.0      83.9    6200           3.39
b_58         131.8    486.5     450.0   34000           3.142
b_59          48.0    194.0     172.0   27000           3.209
b_60         113.2    395.0     465.0   26000           2.83
c_61          58.9    271.5     217.0   17000           4.06
c_62          82.9    578.0     306.0   58000           5.816
c_63          74.9    434.6     281.0   14000           4.69
c_64          71.9    324.0     247.0   39000           3.88
c_65         201.0     77.6     268.0   17000           1.045
c_66          95.7    295.0     339.0   20000           2.729
c_67         156.0    492.0     568.0   32000           2.886
c_68          93.7    321.0     346.0   40000           3.011
c_71          31.5   1016.0      22.8   93000          44.7
c_72          63.3    181.0     247.0   80000           2.608
c_73          75.1    313.0     262.0   46000           3.47
c_75         138.0    401.0     511.0   42000           2.71
c_80         261.9    755.0     783.0  500000           2.82
c_81          57.5    204.7     213.4   68000           3.015
c_82          56.7    287.4     220.8   45000           4.129
c_84          62.6    148.0      82.7  210000           5.99
c_86          98.7    380.0      92.7  250000          12.6
c_87          27.8    132.0      91.0   28000           4.93
c_89        1063.0   2374.0    3600.0  298000           1.969
d_100         79.8    264.1     315.5   48000           2.906
d_101         21.3     83.4      77.6    5300           3.49
d_102         65.8    233.0     266.0   90000           2.99
d_104         66.3    207.6     252.4   18000           2.666
d_106        303.0    643.0     352.0  102000           6.66
d_107        271.0    602.0     860.0   20000           2.355
d_108         93.8    402.0     398.0   80000           3.482
d_109         82.2    474.0     287.6   16000           5.16
d_110         88.2    218.6     148.5   10000           4.799
d_111         36.1    115.3     140.6    5500           2.736
d_112        130.6    262.4     191.4   29000           4.86
d_113        482.0    232.0     510.0   50000           1.755
d_116         38.8    147.2      60.3   11000           8.7
d_117        185.8    736.0     220.0   34400          11.14
d_118        185.0    748.0     768.0   16000           3.22
d_119         56.0     57.3     109.2    7200           1.757
d_91          70.2    283.2     269.0   50000           3.2
d_94         174.0    676.0     588.0  117000           3.38
d_95         118.0    664.0     413.0   89000           5.1
d_96          77.0    257.0     268.0  100000           3.26
d_97          44.7    271.0     159.9   37000           5.34
d_98          60.5    255.6      89.0    1000          10.76
e_121         93.0    276.0     351.0   19000           2.59
e_123        386.0    920.0     512.0  170000           6.41
e_124        416.0   1538.0     712.0   26500           8.3
e_125         46.9     82.1      64.9   21500           4.624
e_127        126.1    622.0     459.0   18000           4.04
e_130         32.8    173.0     123.5   12500           4.53
e_133        405.0    954.0     536.0  153000           6.31
e_136         33.0    228.0     130.8   16800           5.97
e_138        170.0    472.0     233.0   24100           6.69
e_139         65.2    256.0     242.0   15900           3.472
e_140        194.0    766.0     708.0   54000           3.65
e_142         73.7    234.2     280.0   12100           2.8
e_144         53.0    169.0     223.0    9000           2.87
e_147         25.9     96.5     101.3    5600           3.34
e_148        120.1    388.6     477.0     900           2.93
e_149        181.0    501.0     685.0   10200           2.595
f_151         28.7    205.4     110.2     400           5.85
f_152        100.0    504.0     167.7  157000          10.64
f_153        248.0    690.0     344.0   24700           6.95
f_154        135.0    443.0     524.0   63000           3.3
f_155         67.2    240.1     263.0   42000           3.25
f_156         65.5    277.4     244.2   37000           3.91
f_157         65.1    188.0      84.5   27000           7.97
f_158         98.2    282.0     387.0   88000           2.598
f_160         60.6    246.0     241.0   44000           3.471
f_161         48.8    199.0     184.0   1E+06           3.63
f_163        144.6    392.0     573.0  710000           2.289
f_164         36.9     52.3      23.9  170000           7.97
f_166         87.6    357.0     313.0  590000           3.99
f_168        133.7    475.0     218.0  870000           7.8
f_169         93.6   1092.0      96.3   46500          30.8
f_173         16.2     52.4      21.7   52000           8.25
f_174        134.0    387.0     148.4    5100           8.39
f_175        101.7    362.0     391.0   51000           3.205
f_176         70.8    273.9     276.9   48000           3.465
f_178         43.0    225.0     156.7   25000           4.63
f_180        211.0    620.0     799.0   34000           2.82
Excluded analyses. <10% precision and/or >10% discordant
a_04         141.4    333.0     278.0     426           2.173
a_05         367.0   1069.0    1114.0     892           2.298
a_06         239.0    543.0     376.0     606           2.625
a_08          31.4    115.7      95.4     770           3.21
a_20         106.2    385.0     379.0    3710           2.894
a_22         155.0    629.0     492.0    8600           3.463
a_27         123.3    192.8     101.4    3420           3.73
b_35         115.2    248.0     117.0     842           1.993
b_37          39.7    211.0     124.0   23000           5.18
b_43          65.7    359.0     206.9    4300           4.71
b_54          93.8    375.0     323.0   25000           3.33
c_69         125.6    663.0     390.2   24100           4.485
c_70         142.6    895.0     485.0   43700           5.13
c_74          58.6    209.2     139.4   21300           3.17
c_76          85.0    128.0     103.5   25700           3.82
c_77          63.5    135.3     123.1    5210           1.81
c_78          79.9    318.0     272.0   21200           3.33
c_79          77.2    236.7     251.0   41000           2.47
c_83          92.6    485.0     250.0   93000           4.76
c_85          91.7    371.0     305.0  134000           3.54
c_88         723.0   1149.0    2280.0  143000           1.81
c_90         133.0    286.0     546.0  140000           1.807
d_103         75.3    195.0     179.5    5710           2.234
d_105        399.0    726.0    1482.0   22500           1.542
d_114         87.3    350.1     315.9   30000           3.26
d_115         60.1    256.0     176.0   11900           3.57
d_120         36.6    116.6     129.3    2100           2.722
d_92          22.2     68.3      75.0   40000           2.81
d_93         138.9    342.8     410.0   10300           2.093
d_99         307.0    678.0    1100.0   53000           2.15
e_122        451.0   2141.0    1640.0    7200           3.55
e_126        140.0    321.0     588.0    5800           1.761
e_128        104.4    344.1     291.0    5600           3.03
e_129        130.0    105.5     142.0     492           1.011
e_131         60.8    168.5     228.0    2000           2.331
e_132        110.7    200.0     207.0     928           1.775
e_134         74.3    301.0     196.5    6100           4.08
e_135         52.0    157.0     168.0    1000           2.879
e_137        111.6    394.0     282.0    2120           3.123
e_141         74.4    275.4     214.1    2620           3.31
e_143        167.0    203.4     750.0    1800           1.46
e_145         29.8    118.6     106.2     300           3.69
e_146         70.0    174.1     149.9    1050           2.266
e_150        139.1    416.0     368.0    1490           2.62
f_159        371.0    779.0    1230.0   11400           2.091
f_167         95.0    267.0     330.0   58000           2.78
f_170        122.0    298.1     430.0    5100           2.226
f_171         45.2    117.0      48.4    1030           3.94
f_172        224.0   1328.0     882.0    5600           5.59
f_177        122.0   1100.0     207.0    3900          11.7
f_179         73.5    206.0     206.0     920           2.623
f_151         28.7    205.4     110.2     400           5.85
d_103         75.3    195.0     179.5    5710           2.234
d_114         87.3    350.1     315.9   30000           3.26
d_120         36.6    116.6     129.3    2100           2.722
e_122        451.0   2141.0    1640.0    7200           3.55
e_141         74.4    275.4     214.1    2620           3.31

Analysis     [+ or -] 2[sigma] (2)  [.sup.208]Pb/  [+ or -]
Identifier                          [.sup.232]Th   2[sigma] (2)
(NB313)

a_01         0.065                  0.0935         0.004
a_02         0.078                  0.0294         0.001
a_03         0.12                   0.0267         0.001
a_07         0.14                   0.0769         0.004
a_09         0.088                  0.0271         0.001
a_10         0.085                  0.026          0.001
a_11         0.99                   0.1017         0.005
a_12         0.16                   0.09           0.004
a_13         0.26                   0.0353         0.002
a_14         0.29                   0.0292         0.002
a_15         0.24                   0.1087         0.005
a_16         0.098                  0.0368         0.002
a_17         0.16                   0.0716         0.003
a_18         0.075                  0.0293         0.001
a_19         0.19                   0.029          0.002
a_21         0.097                  0.0296         0.001
a_23         0.064                  0.0262         0.001
a_24         0.43                   0.0875         0.004
a_25         0.1                    0.0284         0.001
a_26         0.081                  0.0328         0.002
a_28         0.15                   0.029          0.001
a_29         0.1                    0.0293         0.001
a_30         0.087                  0.0291         0.001
b_31         0.15                   0.0757         0.003
b_32         0.21                   0.026          0.001
b_33         0.18                   0.0278         0.001
b_34         0.14                   0.0263         0.001
b_36         0.13                   0.0874         0.004
b_38         0.067                  0.0275         0.001
b_39         0.14                   0.0269         0.001
b_40         0.22                   0.0645         0.003
b_41         0.44                   0.0281         0.002
b_42         0.14                   0.0276         0.001
b_44         0.073                  0.0834         0.004
b_45         0.16                   0.0284         0.002
b_46         0.19                   0.0302         0.002
b_47         0.055                  0.0281         0.001
b_48         0.12                   0.0875         0.004
b_49         0.64                   0.06           0.003
b_50         0.14                   0.0293         0.001
b_52         0.091                  0.0264         0.001
b_53         0.2                    0.0296         0.002
b_55         0.082                  0.0326         0.002
b_56         0.096                  0.0296         0.002
b_57         0.15                   0.0304         0.002
b_58         0.063                  0.0299         0.001
b_59         0.098                  0.029          0.001
b_60         0.14                   0.0257         0.001
c_61         0.14                   0.0286         0.001
c_62         0.095                  0.0285         0.001
c_63         0.16                   0.0277         0.001
c_64         0.13                   0.03           0.001
c_65         0.098                  0.0791         0.004
c_66         0.069                  0.0297         0.001
c_67         0.084                  0.0288         0.001
c_68         0.09                   0.0284         0.001
c_71         9.3                    0.159          0.049
c_72         0.085                  0.0275         0.001
c_73         0.1                    0.0297         0.002
c_75         0.2                    0.0286         0.002
c_80         0.059                  0.0363         0.002
c_81         0.082                  0.029          0.002
c_82         0.094                  0.0266         0.001
c_84         0.19                   0.0807         0.004
c_86         0.28                   0.1118         0.005
c_87         0.15                   0.032          0.002
c_89         0.068                  0.0301         0.001
d_100        0.042                  0.0274         0.001
d_101        0.11                   0.029          0.002
d_102        0.11                   0.0261         0.001
d_104        0.051                  0.0278         0.001
d_106        0.28                   0.0918         0.004
d_107        0.075                  0.034          0.002
d_108        0.099                  0.0254         0.001
d_109        0.11                   0.0299         0.002
d_110        0.086                  0.0626         0.003
d_111        0.068                  0.027          0.001
d_112        0.34                   0.0713         0.004
d_113        0.051                  0.1008         0.005
d_116        0.23                   0.068          0.003
d_117        0.32                   0.0873         0.005
d_118        0.11                   0.0253         0.001
d_119        0.051                  0.054          0.003
d_91         0.13                   0.027          0.001
d_94         0.1                    0.03           0.002
d_95         0.24                   0.0314         0.002
d_96         0.12                   0.0304         0.002
d_97         0.15                   0.0293         0.001
d_98         0.96                   0.0725         0.004
e_121        0.11                   0.0286         0.002
e_123        0.13                   0.0806         0.004
e_124        0.71                   0.0579         0.004
e_125        0.099                  0.0781         0.004
e_127        0.16                   0.0282         0.002
e_130        0.12                   0.0286         0.001
e_133        0.26                   0.0801         0.004
e_136        0.13                   0.0272         0.001
e_138        0.27                   0.0789         0.005
e_139        0.089                  0.0289         0.001
e_140        0.14                   0.0292         0.001
e_142        0.12                   0.0284         0.001
e_144        0.15                   0.0267         0.001
e_147        0.1                    0.0276         0.001
e_148        0.14                   0.0276         0.001
e_149        0.092                  0.0285         0.001
f_151        0.3                    0.0273         0.002
f_152        0.15                   0.0648         0.003
f_153        0.14                   0.0782         0.004
f_154        0.21                   0.0286         0.001
f_155        0.12                   0.0275         0.001
f_156        0.1                    0.0296         0.001
f_157        0.81                   0.084          0.004
f_158        0.067                  0.0277         0.001
f_160        0.093                  0.0268         0.001
f_161        0.088                  0.0289         0.002
f_163        0.065                  0.026          0.001
f_164        0.23                   0.1686         0.009
f_166        0.24                   0.0311         0.002
f_168        0.28                   0.0663         0.003
f_169        1.2                    0.1021         0.006
f_173        0.28                   0.0808         0.004
f_174        0.67                   0.099          0.005
f_175        0.096                  0.0282         0.001
f_176        0.081                  0.0277         0.001
f_178        0.1                    0.0296         0.001
f_180        0.22                   0.0296         0.002
Excluded ana
a_04         0.04                   0.0529         0.003
a_05         0.058                  0.033          0.002
a_06         0.097                  0.0668         0.004
a_08         0.1                    0.0336         0.002
a_20         0.075                  0.0296         0.001
a_22         0.097                  0.0323         0.002
a_27         0.15                   0.1231         0.007
b_35         0.059                  0.118          0.012
b_37         0.23                   0.0333         0.002
b_43         0.14                   0.0327         0.002
b_54         0.12                   0.0299         0.002
c_69         0.059                  0.0343         0.002
c_70         0.14                   0.0307         0.001
c_74         0.13                   0.0429         0.003
c_76         0.12                   0.0864         0.004
c_77         0.048                  0.0526         0.003
c_78         0.13                   0.0315         0.002
c_79         0.12                   0.0304         0.002
c_83         0.11                   0.0382         0.002
c_85         0.11                   0.0323         0.002
c_88         0.18                   0.0367         0.002
c_90         0.089                  0.0263         0.001
d_103        0.092                  0.0438         0.003
d_105        0.021                  0.0286         0.001
d_114        0.12                   0.0279         0.002
d_115        0.11                   0.0352         0.002
d_120        0.082                  0.0303         0.002
d_92         0.092                  0.0323         0.002
d_93         0.053                  0.0359         0.002
d_99         0.17                   0.0299         0.001
e_122        0.21                   0.0308         0.003
e_126        0.024                  0.0255         0.001
e_128        0.11                   0.0377         0.002
e_129        0.037                  0.1008         0.007
e_131        0.095                  0.0293         0.002
e_132        0.044                  0.0565         0.003
e_134        0.11                   0.0409         0.002
e_135        0.077                  0.0347         0.002
e_137        0.07                   0.0427         0.003
e_141        0.14                   0.0383         0.002
e_143        0.13                   0.0271         0.002
e_145        0.17                   0.0302         0.002
e_146        0.065                  0.0497         0.003
e_150        0.25                   0.0388         0.003
f_159        0.066                  0.0332         0.002
f_167        0.2                    0.0327         0.002
f_170        0.057                  0.0308         0.002
f_171        0.23                   0.1004         0.006
f_172        0.16                   0.0284         0.002
f_177        1.2                    0.071          0.012
f_179        0.087                  0.0392         0.002
f_151        0.3                    0.0273         0.002
d_103        0.092                  0.0438         0.003
d_114        0.12                   0.0279         0.002
d_120        0.082                  0.0303         0.002
e_122        0.21                   0.0308         0.003
e_141        0.14                   0.0383         0.002

Analysis      [.sup.207]Pb/   [+ or -]      [.sup.207]Pb/  [+ or -]
Identifier    [.sup.206]Pb    2[sigma] (2)  [.sup.235]U    2[sigma] (2)
(NB313)

a_01           0.1096          0.0012       4.887          0.084
a_02           0.0597          0.0011       0.768          0.017
a_03           0.0582          0.0011       0.698          0.015
a_07           0.0976          0.0014       3.723          0.078
a_09           0.0588          0.001        0.719          0.015
a_10           0.0603          0.0011       0.715          0.018
a_11           0.1198          0.0021       5.92           0.16
a_12           0.1064          0.0014       4.601          0.11
a_13           0.0651          0.00083      1.017          0.019
a_14           0.0625          0.0015       0.791          0.02
a_15           0.1236          0.0017       6.201          0.11
a_16           0.0659          0.0013       1.084          0.036
a_17           0.0853          0.00092      2.857          0.049
a_18           0.0592          0.00087      0.7825         0.015
a_19           0.0611          0.0014       0.767          0.023
a_21           0.0622          0.0011       0.812          0.021
a_23           0.0581          0.0011       0.682          0.016
a_24           0.1026          0.0014       4.161          0.099
a_25           0.0601          0.00085      0.761          0.015
a_26           0.0635          0.0012       0.929          0.022
a_28           0.0591          0.001        0.779          0.018
a_29           0.061           0.0012       0.797          0.019
a_30           0.0636          0.0016       0.817          0.024
b_31           0.0916          0.00092      3.335          0.054
b_32           0.0667          0.0017       0.817          0.028
b_33           0.0607          0.0012       0.738          0.018
b_34           0.0588          0.0012       0.694          0.016
b_36           0.1085          0.0015       4.475          0.093
b_38           0.0579          0.0011       0.73           0.017
b_39           0.0594          0.0012       0.745          0.021
b_40           0.0998          0.0014       3.613          0.09
b_41           0.0598          0.0011       0.73           0.022
b_42           0.0592          0.001        0.736          0.016
b_44           0.0984          0.0013       3.965          0.076
b_45           0.0606          0.0015       0.763          0.022
b_46           0.0594          0.0009       0.768          0.017
b_47           0.0599          0.00081      0.755          0.015
b_48           0.1031          0.0012       4.409          0.077
b_49           0.0762          0.0011       1.988          0.048
b_50           0.0607          0.0012       0.822          0.019
b_52           0.0636          0.0016       0.77           0.02
b_53           0.0617          0.0011       0.803          0.022
b_55           0.0612          0.00098      0.905          0.017
b_56           0.0603          0.0013       0.755          0.02
b_57           0.0627          0.0023       0.824          0.03
b_58           0.0596          0.0013       0.791          0.024
b_59           0.0608          0.0016       0.745          0.023
b_60           0.06            0.0011       0.6857         0.013
c_61           0.0593          0.0009       0.766          0.015
c_62           0.0594          0.00068      0.7479         0.013
c_63           0.059           0.0011       0.731          0.019
c_64           0.0611          0.0012       0.794          0.018
c_65           0.0958          0.0015       3.481          0.072
c_66           0.0595          0.00091      0.78           0.018
c_67           0.0605          0.00072      0.7935         0.014
c_68           0.0599          0.001        0.774          0.019
c_71           0.0711          0.0024       1.103          0.038
c_72           0.0627          0.00098      0.806          0.016
c_73           0.0668          0.0017       0.856          0.027
c_75           0.0635          0.0014       0.827          0.021
c_80           0.0594          0.0013       0.849          0.024
c_81           0.0641          0.0015       0.796          0.026
c_82           0.0611          0.0013       0.761          0.021
c_84           0.1033          0.0018       3.787          0.075
c_86           0.13            0.0013       6.48           0.11
c_87           0.0643          0.0014       0.937          0.02
c_89           0.0675          0.0015       0.922          0.024
d_100          0.0585          0.0012       0.722          0.017
d_101          0.06            0.0019       0.775          0.025
d_102          0.0597          0.0011       0.695          0.015
d_104          0.0598          0.0011       0.759          0.018
d_106          0.1096          0.0011       5.006          0.087
d_107          0.0689          0.0014       1.093          0.034
d_108          0.059           0.00079      0.693          0.014
d_109          0.0626          0.0015       0.86           0.028
d_110          0.0825          0.0015       2.449          0.062
d_111          0.0611          0.0016       0.792          0.022
d_112          0.0935          0.0013       3.313          0.087
d_113          0.1249          0.0012       6.448          0.11
d_116          0.0888          0.0012       2.874          0.066
d_117          0.1183          0.0023       5.13           0.12
d_118          0.0621          0.0012       0.733          0.02
d_119          0.0782          0.002        2.011          0.056
d_91           0.0623          0.0019       0.752          0.022
d_94           0.0671          0.0014       0.917          0.025
d_95           0.0631          0.0013       0.804          0.02
d_96           0.0607          0.00098      0.807          0.02
d_97           0.0596          0.0012       0.793          0.022
d_98           0.0905          0.0013       3.01           0.066
e_121          0.063           0.0014       0.764          0.018
e_123          0.1017          0.0011       3.999          0.075
e_124          0.1067          0.0019       3.604          0.097
e_125          0.0945          0.0014       3.644          0.074
e_127          0.0629          0.0025       0.805          0.044
e_130          0.058           0.0011       0.712          0.016
e_133          0.0955          0.001        3.676          0.074
e_136          0.0615          0.00094      0.771          0.015
e_138          0.0974          0.0016       3.536          0.081
e_139          0.059           0.0012       0.761          0.019
e_140          0.0592          0.00065      0.7639         0.014
e_142          0.0601          0.0013       0.791          0.018
e_144          0.0651          0.0021       0.773          0.026
e_147          0.0612          0.0016       0.77           0.021
e_148          0.0619          0.0015       0.757          0.021
e_149          0.0664          0.0012       0.857          0.019
f_151          0.0667          0.0025       0.816          0.038
f_152          0.0815          0.00076      2.483          0.04
f_153          0.0986          0.0011       3.553          0.062
f_154          0.0621          0.0009       0.812          0.016
f_155          0.0601          0.0011       0.771          0.017
f_156          0.0608          0.0013       0.785          0.02
f_157          0.0988          0.0014       3.8            0.077
f_158          0.0581          0.001        0.749          0.016
f_160          0.0623          0.0019       0.785          0.027
f_161          0.0608          0.0016       0.786          0.022
f_163          0.0602          0.0017       0.772          0.026
f_164          0.2078          0.003        17.1           0.37
f_166          0.0646          0.0019       0.821          0.023
f_168          0.0857          0.0012       2.696          0.055
f_169          0.0972          0.0013       3.497          0.059
f_173          0.0942          0.0019       3.329          0.078
f_174          0.1201          0.0019       5.332          0.12
f_175          0.0622          0.0012       0.791          0.018
f_176          0.0603          0.0012       0.772          0.017
f_178          0.0634          0.0014       0.81           0.018
f_180          0.0634          0.0014       0.805          0.021
Excluded ana
a_04           0.1351          0.0018       1.935          0.042
a_05           0.0956          0.0022       1.206          0.034
a_06           0.1189          0.0041       2.005          0.074
a_08           0.0734          0.0021       0.962          0.031
a_20           0.0786          0.0027       0.967          0.038
a_22           0.0702          0.0011       0.941          0.025
a_27           0.1254          0.0028       4.67           0.13
b_35           0.2041          0.0058       2.75           0.11
b_37           0.0695          0.0019       0.895          0.027
b_43           0.0714          0.0014       0.955          0.026
b_54           0.0708          0.0019       0.91           0.027
c_69           0.0798          0.0011       1.003          0.021
c_70           0.0704          0.001        0.856          0.017
c_74           0.1011          0.004        1.375          0.067
c_76           0.1282          0.0031       4.896          0.093
c_77           0.1819          0.004        2.368          0.066
c_78           0.0972          0.0033       1.227          0.042
c_79           0.071           0.0027       0.906          0.041
c_83           0.0745          0.0021       1.063          0.03
c_85           0.073           0.0016       0.936          0.027
c_88           0.0763          0.0013       1.055          0.025
c_90           0.0678          0.0015       0.827          0.02
d_103          0.1356          0.0055       1.763          0.079
d_105          0.0778          0.0012       0.995          0.018
d_114          0.0728          0.0029       0.917          0.036
d_115          0.0867          0.0024       1.163          0.052
d_120          0.0809          0.003        0.997          0.041
d_92           0.075           0.0037       0.955          0.046
d_93           0.1227          0.0032       1.582          0.05
d_99           0.0706          0.0012       0.989          0.025
e_122          0.0844          0.0047       0.933          0.069
e_126          0.0794          0.0013       0.885          0.021
e_128          0.0913          0.0052       1.263          0.081
e_129          0.348           0.012        5.94           0.32
e_131          0.0871          0.0039       1.021          0.055
e_132          0.1837          0.0051       2.684          0.091
e_134          0.085           0.0018       1.247          0.03
e_135          0.0793          0.0015       1.056          0.023
e_137          0.1048          0.0024       1.403          0.037
e_141          0.0987          0.0016       1.282          0.027
e_143          0.0709          0.0018       0.994          0.028
e_145          0.0719          0.0019       0.977          0.029
e_146          0.1405          0.0036       1.923          0.052
e_150          0.1099          0.0043       1.504          0.067
f_159          0.0716          0.0014       1.008          0.028
f_167          0.0783          0.0017       1.068          0.036
f_170          0.0839          0.0019       1.114          0.032
f_171          0.1242          0.0045       2.843          0.1
f_172          0.0707          0.0015       0.92           0.027
f_177          0.0828          0.0035       1.668          0.061
f_179          0.1001          0.003        1.329          0.045
f_151          0.0667          0.0025       0.816          0.038
d_103          0.1356          0.0055       1.763          0.079
d_114          0.0728          0.0029       0.917          0.036
d_120          0.0809          0.003        0.997          0.041
e_122          0.0844          0.0047       0.933          0.069
e_141          0.0987          0.0016       1.282          0.027

Analysis     [.sup.206]Pb/   [+ or -]      Rho (3)   [.sup.208]Pb/
Identifier   [.sup.238]U     2[sigma] (2)            [.sup.232]Th
(NB313)

a_01         0.3206          0.0029        0.82772   1807
a_02         0.0927          0.0013        0.55915    586
a_03         0.0865          0.00088       0.47591    531.8
a_07         0.2742          0.0032        0.75536   1497
a_09         0.08836         0.00086       0.59279    540
a_10         0.0851          0.0013        0.69431    519.2
a_11         0.3567          0.009         0.77707   1957
a_12         0.3119          0.0056        0.83962   1741
a_13         0.1131          0.001         0.80048    701
a_14         0.09132         0.00099       0.32985    581
a_15         0.3624          0.0042        0.63189   2091
a_16         0.1191          0.004         0.82562    731
a_17         0.2425          0.0023        0.82343   1397
a_18         0.09546         0.00093       0.66029    584.4
a_19         0.0902          0.002         0.65111    577
a_21         0.0945          0.0017        0.73049    590
a_23         0.0851          0.001         0.59856    522.3
a_24         0.294           0.0057        0.81921   1696
a_25         0.0919          0.0011        0.70253    564.9
a_26         0.1062          0.0011        0.63389    653
a_28         0.0957          0.0014        0.68238    578
a_29         0.0949          0.0011        0.57121    583
a_30         0.0932          0.0013        0.51814    580
b_31         0.2653          0.0025        0.81964   1475
b_32         0.0894          0.0026        0.68764    518
b_33         0.089           0.0011        0.59183    553
b_34         0.0861          0.001         0.46668    525
b_36         0.2996          0.0038        0.76186   1694
b_38         0.09162         0.00097       0.59515    548
b_39         0.0909          0.002         0.70182    537
b_40         0.2636          0.0051        0.82794   1262
b_41         0.0887          0.0016        0.82342    560
b_42         0.09043         0.00079       0.69291    550
b_44         0.2938          0.0033        0.7409    1618
b_45         0.0909          0.0015        0.51596    566
b_46         0.0944          0.0015        0.72878    602
b_47         0.0915          0.0011        0.74644    559
b_48         0.3107          0.0033        0.76106   1695
b_49         0.1889          0.003         0.81696   1177
b_50         0.0984          0.0016        0.54257    583
b_52         0.0879          0.0012        0.32169    527
b_53         0.0946          0.0021        0.76091    590
b_55         0.1072          0.0016        0.56867    648
b_56         0.0909          0.0011        0.59798    390
b_57         0.0956          0.0014        0.18228    605
b_58         0.0966          0.0022        0.69717    596
b_59         0.0893          0.0012        0.5317     578
b_60         0.0832          0.00092       0.34725    511.9
c_61         0.09429         0.00098       0.64136    569
c_62         0.09132         0.00097       0.76873    567.6
c_63         0.09            0.0014        0.70483    551.2
c_64         0.0943          0.0011        0.49968    598
c_65         0.2658          0.0033        0.65577   1538
c_66         0.0953          0.0014        0.75851    592
c_67         0.09504         0.00088       0.78109    573.6
c_68         0.0936          0.0014        0.74547    566
c_71         0.1128          0.0014        0.23566    3080
c_72         0.09379         0.00097       0.62544    548
c_73         0.0939          0.0017        0.59103    591
c_75         0.095           0.0022        0.59095    570
c_80         0.1039          0.0023        0.64733    721
c_81         0.0902          0.0018        0.70381    577
c_82         0.0905          0.0011        0.68057    530
c_84         0.267           0.0029        0.48014   1568
c_86         0.363           0.0029        0.92882   2142
c_87         0.1065          0.0013        0.25051    637
c_89         0.1002          0.002         0.56023    599
d_100        0.08922         0.00099       0.49135    545.3
d_101        0.094           0.0011        0.23146    578
d_102        0.0849          0.001         0.52132    520
d_104        0.0918          0.0012        0.63698    554.7
d_106        0.3327          0.0031        0.88958   1775
d_107        0.1153          0.0019        0.806      676
d_108        0.0853          0.0011        0.75871    505.9
d_109        0.1008          0.0022        0.67705    595
d_110        0.2158          0.0032        0.70621   1226
d_111        0.0944          0.0014        0.37117    538
d_112        0.2583          0.0056        0.84865   1391
d_113        0.3766          0.0036        0.88942   1942
d_116        0.2335          0.003         0.86397   1330
d_117        0.3162          0.0062        0.60355   1691
d_118        0.0857          0.0012        0.74222    505
d_119        0.1878          0.0034        0.44541   1063
d_91         0.0876          0.0018        0.28944    539
d_94         0.0989          0.0021        0.6554     598
d_95         0.0922          0.0012        0.56174    625
d_96         0.0964          0.0017        0.76015    606
d_97         0.0964          0.0017        0.69011    583
d_98         0.2391          0.0038        0.75618   1415
e_121        0.0883          0.0015        0.43704    569
e_123        0.2839          0.0042        0.81749   1567
e_124        0.2441          0.0071        0.80049   1136
e_125        0.2786          0.0039        0.68397   1520
e_127        0.0923          0.003         0.69355    562
e_130        0.0889          0.0012        0.53984    569
e_133        0.2778          0.0039        0.87171   1557
e_136        0.09078         0.00097       0.62255    542
e_138        0.263           0.0051        0.71017   1547
e_139        0.0934          0.0014        0.58033    576
e_140        0.0933          0.0011        0.81983    581.7
e_142        0.0951          0.0014        0.39802    566
e_144        0.08619         0.00099       0.28819    533
e_147        0.0913          0.0011        0.31265    551
e_148        0.08842         0.00086       0.51321    550
e_149        0.0933          0.0011        0.58136    568.2
f_151        0.0884          0.0019        0.61232    545
f_152        0.2206          0.002         0.87192   1268
f_153        0.2603          0.0026        0.80297   1521
f_154        0.0947          0.001         0.69574    569
f_155        0.0927          0.0011        0.55799    548.4
f_156        0.0931          0.0012        0.5452     590
f_157        0.2776          0.0041        0.71496   1630
f_158        0.0932          0.0011        0.59312    552
f_160        0.0911          0.0015        0.46265    534
f_161        0.0936          0.0013        0.36503    576
f_163        0.0928          0.0022        0.56288    520
f_164        0.5944          0.0082        0.75389   3146
f_166        0.0926          0.0014        0.17512    618
f_168        0.2275          0.0031        0.72989   1297
f_169        0.2594          0.0028        0.6103    1964
f_173        0.2569          0.0036        0.51553   1569
f_174        0.3219          0.0039        0.73902   1908
f_175        0.0921          0.0012        0.53187    561
f_176        0.0925          0.0012        0.45013    551.9
f_178        0.0927          0.0013        0.3255     589
f_180        0.0926          0.0018        0.56279    588
Excluded ana
a_04         0.103           0.0013        0.82662   1042
a_05         0.0908          0.0014        0.57854    657
a_06         0.1213          0.0016        0.3565    1307
a_08         0.0943          0.0014        0.46015    668
a_20         0.089           0.0017        0.48566    589
a_22         0.0964          0.0015        0.84923    643
a_27         0.2691          0.0045        0.59719   2344
b_35         0.0971          0.0014        0.86725   2240
b_37         0.0937          0.0012        0.42283    662
b_43         0.0973          0.0017        0.69586    650
b_54         0.0938          0.0013        0.42828    595
c_69         0.0914          0.0011        0.78012    682.4
c_70         0.0884          0.0012        0.69905    610
c_74         0.0989          0.0021        0.60882    848
c_76         0.2788          0.0047        0.09413   1674
c_77         0.0948          0.0016        0.61449   1035
c_78         0.0919          0.0011        0.19807    626
c_79         0.0929          0.0023        0.5421     605
c_83         0.1048          0.0017        0.28949    758
c_85         0.0931          0.0012        0.69639    641
c_88         0.1006          0.0013        0.71555    729
c_90         0.0887          0.0015        0.46625    524
d_103        0.095           0.0016        0.42828    865
d_105        0.09309         0.00089       0.52258    569
d_114        0.0921          0.0023        0.29479    556
d_115        0.0977          0.0029        0.79641    698
d_120        0.0899          0.0016        0.43227    603
d_92         0.092           0.0016        0.11268    643
d_93         0.0933          0.0013        0.58234    712
d_99         0.1014          0.0016        0.75095    595
e_122        0.079           0.0021        0.78206    612
e_126        0.07987         0.00081       0.82661    508.6
e_128        0.0982          0.0018        0.51259    748
e_129        0.1229          0.0034        0.8315    1939
e_131        0.0845          0.0011        0.76036    584
e_132        0.1056          0.0016        0.5921    1111
e_134        0.1062          0.0018        0.51208    809
e_135        0.0957          0.0012        0.5013     688
e_137        0.09691         0.00099       0.51112    844
e_141        0.094           0.0011        0.64456    759
e_143        0.1015          0.0017        0.45514    541
e_145        0.0981          0.0013        0.45553    601
e_146        0.0992          0.0015        0.37093    981
e_150        0.0992          0.0031        0.51367    768
f_159        0.1013          0.0016        0.72794    660
f_167        0.0978          0.0019        0.79579    650
f_170        0.0964          0.0015        0.6202     612
f_171        0.1666          0.0051        0.40008   1933
f_172        0.0952          0.0018        0.69261    566
f_177        0.1457          0.0049        0.27712   1390
f_179        0.0968          0.0014        0.46708    778
f_151        0.0884          0.0019        0.61232    545
d_103        0.095           0.0016        0.42828    865
d_114        0.0921          0.0023        0.29479    556
d_120        0.0899          0.0016        0.43227    603
e_122        0.079           0.0021        0.78206    612
e_141        0.094           0.0011        0.64456    759

Analysis    [+ or -]      [.sup.207]Pb/  [+ or -]      [.sup.207]Pb/
Identifier  2[sigma] (2)  [.sup.206]Pb   2[sigma] (2)  [.sup.235]U
(NB313)

a_01        78             1790            20          1799.4
a_02        28              585            39           578.3
a_03        25              526            43           537
a_07        72             1574            28          1575
a_09        25              551            33           549.5
a_10        24              614            39           547.3
a_11        90             1951            31          1963
a_12        79             1735            25          1748
a_13        33              771            26           713.2
a_14        29              680            51           591.1
a_15        97             2004            25          2005
a_16        36              797            42           745
a_17        62             1320            21          1369.8
a_18        27              568            31           586.5
a_19        32              634            51           577
a_21        28              683            35           602.7
a_23        24              524            42           527.3
a_24        77             1669            25          1664
a_25        26              601            31           574.4
a_26        30              719            40           666.4
a_28        27              562            38           584.1
a_29        28              628            43           594.6
a_30        28              718            54           605
b_31        64             1459            18          1488.6
b_32        26              823            54           606
b_33        26              618            42           560.7
b_34        25              544            45           534.4
b_36        75             1772            25          1725
b_38        26              517            43           555.5
b_39        28              587            19           564
b_40        64             1618            26          1551
b_41        29              591            40           556
b_42        26              566            38           559.7
b_44        72             1590            25          1626
b_45        29              617            55           574
b_46        31              577            34           577.8
b_47        25              599            30           570.7
b_48        74             1678            22          1713.2
b_49        53             1096            28          1110
b_50        28              624            41           609
b_52        26              719            53           579
b_53        29              659            37           598
b_55        30              637            34           654.2
b_56        29              609            45           570.8
b_57        34              675            77           609
b_58        28              580            46           591
b_59        28              617            56           564
b_60        23              592            37           529.8
c_61        26              574            32           577.9
c_62        26              577            25           566.7
c_63        26              560            41           556.3
c_64        28              634            42           593.1
c_65        68             1537            30          1521
c_66        28              581            32           584.8
c_67        26              623            26           592.8
c_68        27              590            37           581.2
c_71        850             948            65           754
c_72        26              690            33           599.6
c_73        29              820            53           627
c_75        29              718            45           611.1
c_80        41              578            47           624
c_81        30              736            49           593
c_82        26              634            45           574
c_84        73             1680            31          1589
c_86        93             2095            17          2042.4
c_87        30              737            43           670.6
c_89        28              847            45           663
d_100       25              541            47           551.7
d_101       29              569            68           581
d_102       25              580            40           535.2
d_104       25              598            43           572.8
d_106       79             1790            18          1819.3
d_107       36              888            43           749
d_108       24              567            28           534.4
d_109       34              684            49           629
d_110       61             1251            35          1256
d_111       28              631            54           591
d_112       67             1495            26          1486
d_113       84             2025            18          2037.8
d_116       65             1394            27          1373
d_117       98             1927            33          1840
d_118       24              671            41           557.8
d_119       52             1137            52          1117
d_91        28              670            65           569
d_94        34              837            45           660
d_95        30              705            43           598.5
d_96        31              619            35           600
d_97        28              580            42           592
d_98        68             1431            28          1408
e_121       29              696            48           576
e_123       69             1653            20          1633
e_124       69             1741            33          1549
e_125       69             1512            28          1558
e_127       42              691            81           598
e_130       28              522            44           545.4
e_133       68             1538            21          1565
e_136       26              652            32           580
e_138       81             1577            28          1534
e_139       27              563            42           573.8
e_140       26              574            25           576
e_142       27              598            45           591
e_144       28              761            70           580
e_147       27              622            57           579
e_148       26              664            53           572
e_149       26              809            39           627.8
f_151       31              808            79           604
f_152       57             1230            18          1267.6
f_153       69             1596            21          1538.3
f_154       28              669            31           603.3
f_155       26              598            40           579.5
f_156       28              623            48           587.8
f_157       77             1597            25          1591
f_158       26              523            38           566.8
f_160       25              668            65           587
f_161       29              612            56           588
f_163       26              601            60           580
f_164       150            2885            23          2939
f_166       30              749            60           608
f_168       60             1327            27          1326
f_169       99             1566            24          1527.1
f_173       81             1500            38          1487
f_174       90             1953            27          1872
f_175       26              679            40           591.1
f_176       25              603            43           580
f_178       28              709            46           602.1
f_180       29              712            46           599
Excluded ana
a_04        49             2162            23          1092
a_05        33             1534            45           802
a_06        66             1925            61          1114
a_08        34             1013            60           685
a_20        28             1163            67           685
a_22        30              942            36           673
a_27        130            2027            40          1759
b_35        210            2846            46          1336
b_37        47              904            53           648
b_43        30              959            40           679
b_54        30              940            54           656
c_69        31             1188            27           704.8
c_70        28              935            30           627.2
c_74        48             1628            73           875
c_76        77             2067            42          1801
c_77        51             2664            37          1231
c_78        41             1577            68           817
c_79        30              946            82           654
c_83        40             1041            52           737
c_85        31             1006            43           669
c_88        47             1096            33           730
c_90        25              868            40           611.6
d_103       49             2147            68          1027
d_105       26             1135            30           700.7
d_114       30              993            82           660
d_115       40             1345            54           781
d_120       31             1198            75           700
d_92        35             1070           110           685
d_93        38             1989            46           962
d_99        28              939            36           697
e_122       48             1251            96           661
e_126       23             1183            33           642.5
e_128       43             1440           110           821
e_129       120            3702            57          1959
e_131       32             1307            89           711
e_132       58             2676            45          1321
e_134       39             1309            41           821
e_135       34             1178            41           731.2
e_137       50             1712            45           889
e_141       43             1594            31           836.9
e_143       36              946            51           700
e_145       31              970            54           691
e_146       51             2238            49          1093
e_150       51             1788            71           930
f_159       32              974            41           707
f_167       42             1159            48           736
f_170       29             1291            46           762
f_171       100            1998            62          1363
f_172       29              941            42           665
f_177       220            1254            77           995
f_179       42             1611            53           856
f_151       31              808            79           604
d_103       49             2147            68          1027
d_114       30              993            82           660
d_120       31             1198            75           700
e_122       48             1251            96           661
e_141       43             1594            31           836.9

Analysis     [+ or -]      [.sup.206]Pb/  [+ or -]      %
Identifier   2[sigma] (2)  [.sup.238]U    2[sigma] (2)  disc. (4)
(NB313)

a_01         14            1793           14              0.36
a_02          9.8           571.2          7.4            1.23
a_03          9.1           534.8          5.2            0.41
a_07         17            1562           16              0.83
a_09          8.4           545.8          5.1            0.67
a_10         11             526.6          7.5            3.78
a_11         23            1965           43             -0.10
a_12         20            1750           27             -0.11
a_13          9.8           690.9          5.9            3.13
a_14         11             563.3          5.9            4.70
a_15         15            1996           19              0.45
a_16         17             725           23              2.68
a_17         13            1399           12             -2.13
a_18          8.3           587.7          5.5           -0.20
a_19         13             557           12              3.47
a_21         12             582            9.9            3.43
a_23          9.4           526.2          6.1            0.21
a_24         20            1660           29              0.24
a_25          8.7           566.4          6.5            1.39
a_26         11             650.8          6.4            2.34
a_28         10             589            8.2           -0.84
a_29         11             584.6          6.5            1.68
a_30         13             574.2          7.5            5.09
b_31         13            1517           12             -1.91
b_32         16             552           15              8.91
b_33         11             549.3          6.8            2.03
b_34          9.6           532.6          6              0.34
b_36         17            1689           19              2.09
b_38         10             565            5.7           -1.71
b_39         12             561           12              0.53
b_40         20            1508           26              2.77
b_41         13             548            9.4            1.44
b_42          9.1           558            4.7            0.30
b_44         16            1660            16            -2.09
b_45         13             561            8.6            2.26
b_46          9.6           581.5          8.6           -0.64
b_47          8.9           564            6.7            1.17
b_48         15            1744           16             -1.80
b_49         16            1119           18             -0.81
b_50         11             605.1          9.5            0.64
b_52         12             543.2          6.8            6.18
b_53         13             582           12              2.68
b_55          9.2           656.1          9.3           -0.29
b_56         12             560.6          6.5            1.79
b_57         16             588.6          8.2            3.35
b_58         13             594           13             -0.51
b_59         13             551.6          7.3            2.20
b_60          8.1           515.1          5.5            2.77
c_61          8.7           580.8          5.8           -0.50
c_62          7.8           563.3          5.7            0.60
c_63         11             555.3          8.2            0.18
c_64         10             581            6.6            2.04
c_65         16            1519           17              0.13
c_66         10             586.8          8             -0.34
c_67          8.1           585.2          5.2            1.28
c_68         11             576.5          8              0.81
c_71         18             689.2          7.9            8.59
c_72          9.2           577.8          5.7            3.64
c_73         15             579           10              7.66
c_75         12             585           13              4.27
c_80         13             637           14             -2.08
c_81         14             557           11              6.07
c_82         12             558.6          6.3            2.68
c_84         16            1525           15              4.03
c_86         14            1998           15              2.17
c_87         10             652            7.3            2.77
c_89         13             615           12              7.24
d_100         9.8           550.9          5.8            0.15
d_101        11             578.9          6.4            0.36
d_102         9.1           525.1          6              1.89
d_104        11             565.9          7.1            1.20
d_106        15            1851           15             -1.74
d_107        16             703           11              6.14
d_108         8.6           527.5          6.2            1.29
d_109        15             619           13              1.59
d_110        18            1260           17             -0.32
d_111        13             581.5          8.1            1.61
d_112        19            1480           29              0.40
d_113        15            2060           17             -1.09
d_116        17            1352           16              1.53
d_117        20            1770           30              3.80
d_118        11             530            6.9            4.98
d_119        19            1109           18              0.72
d_91         13             541           10              4.92
d_94         13             608           13              7.88
d_95         11             568.6          7.3            5.00
d_96         11             593           10              1.17
d_97         12             593           10             -0.17
d_98         17            1381           20              1.92
e_121        10             545.1          8.7            5.36
e_123        15            1610           21              1.41
e_124        21            1407           37              9.17
e_125        16            1583           19             -1.60
e_127        24             569           18              4.85
e_130         9.2           548.9          7.4           -0.64
e_133        16            1580           20             -0.96
e_136         8.8           560.1          5.7            3.43
e_138        18            1504           26              1.96
e_139        11             575.5          8.2           -0.30
e_140         8             574.9          6.3            0.19
e_142        10             587            9.1            0.68
e_144        15             532.9          5.8            8.12
e_147        12             563.2          6.5            2.73
e_148        12             546.2          5.1            4.51
e_149        10             574.8          6.4            8.44
f_151        21             546           11              9.60
f_152        12            1286.3         10             -1.48
f_153        14            1491           13              3.07
f_154         8.9           583.2          6              3.33
f_155        10             571.5          6.7            1.38
f_156        11             573.6          6.8            2.42
f_157        16             1578          21              0.82
f_158         9.4           574.3          6.2           -1.32
f_160        15             561.8          9              4.29
f_161        12             576.5          7.9            1.96
f_163        15             572           13              1.38
f_164        21            3006           33             -2.28
f_166        13             570.7          8.5            6.13
f_168        15            1321           16              0.38
f_169        14            1486           14              2.69
f_173        19            1473           18              0.94
f_174        19            1798           19              3.95
f_175        10             567.6          7.2            3.98
f_176        10             570.3          7              1.67
f_178        10             571.5          7.5            5.08
f_180        12             571           10              4.67
Excluded ana
a_04         14             631.7          7.7           42.15
a_05         16             560.2          8.5           30.15
a_06         25             738            9.4           33.75
a_08         17             581            8.1           15.18
a_20         19             549           10             19.85
a_22         13             593.4          8.8           11.83
a_27         23            1536           23             12.68
b_35         28             597.5          8             55.28
b_37         14             577.3          7.3           10.91
b_43         13             598           10             11.93
b_54         14             578.2          7.4           11.86
c_69         11             563.5          6.7           20.05
c_70          9.5           546.1          7             12.93
c_74         28             608           13             30.51
c_76         16            1585           23             11.99
c_77         20             583.9          9.3           52.57
c_78         21             566.7          6.3           30.64
c_79         22             573           14             12.39
c_83         16             642           10             12.89
c_85         14             573.8          6.9           14.23
c_88         12             617.8          7.5           15.37
c_90         11             547.6          9             10.46
d_103        28             584.8          9.3           43.06
d_105         9.4           573.7          5.2           18.12
d_114        19             568           14             13.94
d_115        25             600           17             23.18
d_120        21             554.6          9.6           20.77
d_92         27             567.6          9.7           17.14
d_93         20             575.2          7.9           40.21
d_99         13             622.5          9.2           10.69
e_122        33             490           12             25.87
e_126        11             495.3          4.9           22.91
e_128        34             604           11             26.43
e_129        44             747           20             61.87
e_131        28             523            6.7           26.44
e_132        25             647            9.6           51.02
e_134        14             651           11             20.71
e_135        11             588.9          6.9           19.46
e_137        16             596.2          5.8           32.94
e_141        12             579.1          6.2           30.80
e_143        14             622.8          9.7           11.03
e_145        15             603.1          7.8           12.72
e_146        20             609.7          8.9           44.22
e_150        27             610           18             34.41
f_159        14             622            9.4           12.02
f_167        17             602           11             18.21
f_170        17             593.4          8.9           22.13
f_171        27             993           28             27.15
f_172        15             586           10             11.88
f_177        22             877           28             11.86
f_179        19             595.5          8.1           30.43
f_151        21             546           11              9.60
d_103        28             584.8          9.3           43.06
d_114        19             568           14             13.94
d_120        21             554.6          9.6           20.77
e_122        33             490           12             25.87
e_141        12             579.1          6.2           30.80

Primary reference material (5)
91500 (n=87)  15.2  84.9  29.7  46793  9.358  0.336  0.0542  0.003

91500 (n=87)  0.0749  0.00174  1.851  0.05  0.17932  0.00297  0.52548

91500 (n=87)  1062.6  12.4  1053.6  9.0  1063.5  3.7  1064.7  3.2

Secondary reference material(s) (5)

FC1 (n=49)  240.6  696.5  446.5  4E+06  5.573  0.107  0.057   0.003
R33 (n=28)   33.7  243.2  175.8  49260  4.96   0.14   0.0206  0.001

FC1 (n=49)  0.0766  0.00114  2.0129  0.042  0.1906   0.00266  0.69378
R33 (n=28)  0.0561  0.00117  0.5236  0.013  0.06768  0.00073  0.49626

FC1 (n=49)  1119.6  7.5  1100.0   5.3   1116.7  1.9  1119.5  1.8
R33 (n=28)   410.7  3.9   443.0  16.1    426.0  1.3   421.6  0.8

(1) U-Th-Pb concentrations referenced to either NIST 612 glass or 91500
zircon; concentration uncertainty approximately [+ or -] 20%. (2)
Isotope ratios not corrected for common Pb.
(3) Dates calculated with decay constants of Jaffey et al. (1971) and
238U/235U = 137.818 (Hiess, 2012) using Iolite v. 3.5 (Paton et al.
2011) and U-Pb Geochron4 DRS (Paton et al. 2010). Preferred dates used
in plots are [.sup.206]/[.sup.238] dales less than 700 Ma and 207/206
dates greater than 700 Ma. (4) Discordance calculated as
(1-([.sup.206]Pb/[.sup.238]U age/ [.sup.207]Pb/[.sup.235]U age))* 100.
(5) Concentration data are means of all anlayses; dales and isotope
ratios are weighted means of all analyses <2% discordant. Average
reproducibility of individual U--Pb dates from primary reference
material is better than 0.7% (reduced separately as unknowns) and
average accuracy of secondary reference material is better than

2.25% (FC-1) and better than 1.5% (R33).

Appendix B. Major and trace element chemical data for mafic and
intermediate volcanic rocks in the Almond Road and Belleisle Bay
groups.

Sample                        10SJ-10  10SJ-15           1630225
Unit                             1     Almond Rd. Group
Zone                            20          20           19
Easting                     266888      275060           732153
Northing                   5055869     5061551           5054959

Si[O.sub.2]                     50.29       53.97         49.11
Ti[O.sub.2]                      1.15        1.07          1.66
[Al.sub.2][O.sub.3]             16.61       17.93         18.69
[Fe.sub.2][O.sub.2.sup.1]        9.47        6.88         11.80
MnO                              0.14        0.10          0.18
MgO                              6.18        3.48          5.26
CaO                              4.71        2.43          3.54
[Na.sub.2]O                      5.40        7.36          5.02
[K.sub.2]O                       0.27        0.06          1.09
[P.sub.2][O.sub.5]               0.16        0.23          0.22
Cr                              80          10            10
Ni                              50          10            10
Co                              31          19            34
Sc                              30          16            29
V                              231         130           262
Cu                              50          40            40
Pb                               2.5         7             2.5
Zn                              50         100            80
Ga                              15          16            19
Cs                               0.5         0.8           0.25
Rb                               3           0.5           9
Ba                              95         144           572
Sr                             149         281           341
Ta                               0.2         0.24          0.3
Nb                               2.3         4             3
Hf                               2.1         4.2           3.6
Zr                              92         196           131
Y                               21.4        33.2          31
Th                               1.34        2.76          1.2
U                                0.39        0.59          0.8
La                               6.14       12.00          9.90
Ce                              14.70       30.60         25.40
Pr                               2.05        4.17          3.80
Nd                               9.89       19.00         18.40
Sm                               3.10        4.95          4.90
Eu                               0.99        1.40          1.79
Gd                               3.51        5.21          5.30
Tb                               0.65        0.91          0.90
Dy                               3.84        5.87          5.70
Ho                               0.81        1.20          1.10
Er                               2.32        3.55          3.40
Tm                               0.34        0.56          0.50
Yb                               2.17        3.80          3.20
Lu                               0.36        0.62          0.47
Zr/Y                             4.30        5.90          4.23
Th/Yb                            0.62        0.73          0.38

Sample                     1630322    06SJ-057    10SJ-11    10SJ-34
Unit                                                         Browns
Zone                                                         Flat Fm.
Easting                     19         20          20         20
Northing                   733116     280351      269501     294433
                           5055170    5062935     5055154    5076280

Si[O.sub.2]                 47.79     48.05        50.40      52.52
Ti[O.sub.2]                  0.82       2.31        2.76       1.04
[Al.sub.2][O.sub.3]         18.01      13.97       13.06      15.36
[Fe.sub.2][O.sub.2.sup.1]    9.57      12.19       13.30       8.51
MnO                          0.13       0.23        0.19       0.14
MgO                          5.25       4.91        3.65       6.73
CaO                          8.01       8.21        4.09       6.57
[Na.sub.2]O                  4.97       3.61        4.64       2.95
[K.sub.2]O                   0.05       0.74        0.10       0.17
[P.sub.2][O.sub.5]           0.21       0.35        1.42       0.21
Cr                         180         50          10        280
Ni                          80         30          10        150
Co                          36         31          14         36
Sc                          38         38          34         26
V                          239        357         116        183
Cu                          70         40          10         40
Pb                           2.5        2.5         8          7
Zn                          60        100          90         50
Ga                          15         20          20         15
Cs                           0.25       0.25        0.3        0.4
Rb                           1         14           3          4
Ba                          41        265        1105        107
Sr                         111        486         282        976
Ta                           0.1        0.4         0.52       0.61
Nb                           1          6           6.3        8.4
Hf                           1.6        4.1         3.1        2.8
Zr                          50        151         136        131
Y                           21         42          48.6       16.8
Th                           0.7        1.7         2.93       3.64
U                            0.3        0.5         0.74       0.92
La                           4.90      13.30       21.40      19.80
Ce                          11.80      31.90       50.90      42.20
Pr                           1.69       4.62        7.25       4.90
Nd                           8.60      22.20       36.20      20.30
Sm                           2.50       6.10        9.76       4.29
Eu                           0.95       2.21        3.75       1.27
Gd                           3.00       7.00       10.70       3.69
Tb                           0.60       1.20        1.72       0.56
Dy                           3.70       7.90        9.90       3.46
Ho                           0.80       1.60        1.93       0.68
Er                           2.30       4.60        5.36       1.97
Tm                           0.34       0.67        0.76       0.29
Yb                           2.10       4.30        4.54       1.76
Lu                           0.32       0.64        0.72       0.30
Zr/Y                         2.38       3.60        2.80       7.80
Th/Yb                        0.33       0.40        0.65       2.07

Sample                      10SJ-37    10SJ-38    10SJ-41    10SJ-16
Unit
Zone                        20         20         20         20
Easting                     288534     288574     270126     297403
Northing                    5071884    5071840    5053942    5080347

Si[O.sub.2]                 48.62        52.20     49.24       55.43
Ti[O.sub.2]                  0.65         1.29      0.65        1.85
[Al.sub.2][O.sub.3]         15.30        18.64     17.67       19.92
[Fe.sub.2][O.sub.2.sup.1]    8.38        11.18      9.45        5.68
MnO                          0.14         0.15      0.18        0.12
MgO                         11.01         4.80      8.46        1.43
CaO                          8.14         1.45      4.15        2.48
[Na.sub.2]O                  2.39         4.72      4.49        6.56
[K.sub.2]O                   0.14         0.55      0.07        2.09
[P.sub.2][O.sub.5]           0.07         0.16      0.08        0.72
Cr                         790           70        60          10
Ni                         260           40        50          10
Co                          46           35        37          10
Sc                          37           36        32          25
V                          221          305       219          99
Cu                          50          130       110           5
Pb                           2.5          2.5       2.5        16
Zn                          50           70        70         100
Ga                          13           19        15          26
Cs                           3            2.5       2.8         2.2
Rb                           3           13         1          76
Ba                         147          194        51         461
Sr                         178          330       280         538
Ta                           0.07         0.27      0.06        1.2
Nb                           1.3          3.8       0.8        17.5
Hf                           1.3          2.8       1.1         8.9
Zr                          49          118        39         428
Y                           15.7         24.3      10.7        59.5
Th                           0.77         2.07      0.37       12.8
U                            0.17         0.39      0.09        2.33
La                           3.75         9.00      3.04       72.20
Ce                           7.79        20.30      7.66      162.00
Pr                           1.10         2.58      1.08       19.80
Nd                           5.68        12.10      5.76       81.10
Sm                           1.70         3.45      1.61       17.00
Eu                           0.65         1.01      0.58        4.46
Gd                           2.19         3.56      1.78       13.90
Tb                           0.41         0.70      0.33        2.07
Dy                           2.86         4.38      2.09       11.20
Ho                           0.61         0.92      0.42        2.20
Er                           1.79         2.91      1.22        6.15
Tm                           0.28         0.46      0.18        0.87
Yb                           1.87         3.00      1.25        5.71
Lu                           0.29         0.49      0.21        0.89
Zr/Y                         3.12         4.86      3.64        7.19
Th/Yb                        0.41         0.69      0.30        2.24

Sample                      10SJ-18          10SJ-19     10SJ-21
Unit                        Grant Brook Fm
Zone                        20               20          20
Easting                     297325           297275      297181
Northing                    5080680          5080855     5081009

Si[O.sub.2]                   50.18           48.37       61.23
Ti[O.sub.2]                    1.64            0.95        1.30
[Al.sub.2][O.sub.3]           20.59           17.71       14.98
[Fe.sub.2][O.sub.2.sup.1]     10.30            9.89        6.71
MnO                            0.16            0.28        0.30
MgO                            2.45            6.68        3.16
CaO                            6.09            4.65        3.50
[Na.sub.2]O                    3.48            4.31        1.46
[K.sub.2]O                     1.31            1.62        2.97
[P.sub.2][O.sub.5]             0.35            0.16        0.38
Cr                            70             220          10
Ni                            50              70          10
Co                            36              46          12
Sc                            27              36          17
V                            142             219         102
Cu                            10              90           5
Pb                            12               5          14
Zn                           100              60          70
Ga                            19              21          20
Cs                             1               2.7         6.2
Rb                            37              58          96
Ba                           286             454         653
Sr                           270             319         274
Ta                             0.58            0.21        0.81
Nb                             7.9             3.1        11.7
Hf                             4.1             1.8         5.7
Zr                           198              80         276
Y                             29.4            16.7        38.3
Th                             5.69            2.17        8.7
U                              1.47            0.47        2.18
La                            28.40           12.70       41.90
Ce                            67.00           24.90       92.90
Pr                             8.74            3.08       11.40
Nd                            38.00           14.20       46.30
Sm                             8.67            3.50       10.10
Eu                             2.23            1.08        2.66
Gd                             7.02            3.67        8.46
Tb                             1.09            0.57        1.27
Dy                             6.05            3.31        7.21
Ho                             1.12            0.65        1.39
Er                             3.19            1.96        3.94
Tm                             0.45            0.29        0.57
Yb                             2.77            1.92        3.75
Lu                             0.45            0.30        0.59
Zr/Y                           6.73            4.79        7.21
Th/Yb                          2.05            1.13        2.32

Sample                      10SJ-23    10SJ-33    10SJ-39    NB07-02
Unit                                                         Grant Brook
                                                               Fm
Zone                        20         20         20         20
Easting                     297048     288829     288779     275706
Northing                    5081255    5071219    5071391    5061120

Si[O.sub.2]                  49.66      47.08       50.96     63.21
Ti[O.sub.2]                   1.07       2.62        0.82      0.79
[Al.sub.2][O.sub.3]          18.27      15.49       17.24     16.03
[Fe.sub.2][O.sub.2.sup.1]    10.39      13.62        8.25      6.80
MnO                           0.22       0.20        0.13      0.20
MgO                           6.84       4.19        5.39      1.70
CaO                           5.70       5.30        3.84      1.35
[Na.sub.2]O                   3.17       4.51        6.15      4.28
[K.sub.2]O                    0.22       0.16        0.26      1.19
[P.sub.2][O.sub.5]            0.15       0.36        0.23      0.17
Cr                          100         50          90        30
Ni                           60         30          40        20
Co                           46         37          30        17
Sc                           35         39          23        20
V                           286        361         178       141
Cu                           60         50          40        40
Pb                           13          7          10        14
Zn                           60        130          50        80
Ga                           19         22          18        18
Cs                            0.5        1           2.4       4.2
Rb                            5          4          10        46
Ba                          151         89         214       221
Sr                          602        169         734       219
Ta                            0.16       0.54        0.36      0.55
Nb                            2.9        7.3         4.7       6.5
Hf                            2.4        4.5         3.5       3.7
Zr                          105        192         147       164
Y                            26         35.2        22.7      25
Th                            3.43       2.03        5.64      6.58
U                             0.98       0.54        1.25      1.44
La                           16.80      13.00       22.80     24.50
Ce                           38.10      32.10       51.00     51.70
Pr                            5.12       4.27        6.24      6.20
Nd                           22.80      20.60       26.50     25.70
Sm                            5.21       5.78        5.50      5.74
Eu                            1.44       1.63        1.38      1.34
Gd                            4.88       6.14        4.61      5.01
Tb                            0.82       1.10        0.72      0.79
Dy                            4.74       6.82        4.22      4.65
Ho                            0.94       1.39        0.87      0.91
Er                            2.78       4.19        2.51      2.78
Tm                            0.40       0.65        0.38      0.42
Yb                            2.59       4.40        2.54      2.85
Lu                            0.41       0.71        0.42      0.49
Zr/Y                          4.04       5.45        6.48      6.56
Th/Yb                         1.32       0.46        2.22      2.31

Sample                      06SJ-044  06SJ-063    10SJ-33A    10SJ-40
Unit
Zone                        20        20          20          20
Easting                     297196    289023      288829      290201
Northing                    5079767   5073595     5071219     5074627

Si[O.sub.2]                   65.11     56.95      68.61        61.24
Ti[O.sub.2]                    1.34      0.96       0.59         0.40
[Al.sub.2][O.sub.3]           14.78     16.39      14.62        17.54
[Fe.sub.2][O.sub.2.sup.1]      6.81      7.91       3.72         5.13
MnO                            0.08      0.13       0.06         0.15
MgO                            1.94      4.76       0.77         1.32
CaO                            1.25      2.86       2.25         1.34
[Na.sub.2]O                    6.27      5.45       4.46         8.18
[K.sub.2]O                     0.56      0.16       1.69         0.07
[P.sub.2][O.sub.5]             0.60      0.19       0.17         0.19
Cr                            10        50         10           10
Ni                            10        40         10           10
Co                             8        26          5            4
Sc                            17        28          9            6
V                             S3       182         31           18
Cu                            10        90          5            5
Pb                            19        12          5            2.5
Zn                            80        60         15           15
Ga                            15        15         18           13
Cs                             0.25      3.5        3.1          0.4
Rb                            19         6         55            0.5
Ba                           142       141        222           29
Sr                           217       445        102          108
Ta                             0.8       0.4        0.79         0.16
Nb                            11         5          8.6          2.6
Hf                             6.8       3.9        5.3          2.3
Zr                           252       136        253          112
Y                             40        25         31.5         21.1
Th                             8.8       7.1        7.34         1.36
U                              1.8       1.6        1.35         0.43
La                            36.70     30.40      26.40         7.62
Ce                            70.60     68.30      55.90        18.30
Pr                            10.20      7.92       6.33         2.52
Nd                            44.40     33.80      25.50        11.40
Sm                             9.50      7.00       5.57         3.09
Eu                             2.42      1.97       1.26         0.88
Gd                             8.50      5.90       5.14         3.25
Tb                             1.40      1.00       0.91         0.56
Dy                             8.20      5.40       5.66         3.43
Ho                             1.60      1.00       1.16         0.74
Er                             4.60      2.90       3.56         2.24
Tm                             0.66      0.42       0.55         0.36
Yb                             4.20      2.70       3.79         2.58
Lu                             0.62      0.39       0.63         0.45
Zr/Y                           6.30      5.44       8.03         5.31
Th/Yb                          2.10      2.63       1.94         0.53


SUSAN C. JOHNSON (1*), GREGORY R. DUNNING (2), AND BRENT V. MILLER (3)

(1.) New Brunswick Department of Energy and Resource Development, Geological Survey Branch, Sussex, New Brunswick E4E 7H7, Canada

(2.) Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland and Labrador A1B 3X5, Canada

(3.) Department of Geology and Geophysics, Texas A & M University, College Station, Texas 77843-3115, USA

Corresponding author <susan.johnson@gnb.ca>

doi: 10.4138/atlgeol.2018.005

Date received: 02 January 2018 f Date accepted: 17 February 2018
Table 1. U-Pb isotope composition and age for West Scotch Settlement
intrusion (NB09-03).

                                     Concentration        Measured
                           Wt.       U      [Pb.sub.rad]  Total
Fraction                   (mg) (a)  (ppm)  (ppm) (b)     common
                                                          Pb (pg)

Z1 5 sml clr euh prm Etch  0.005     604    50.3          6
Z2 3 sml clr euh prm Etch  0.003     460    38.6          4
Z3 3 sml clr euh prm Etch  0.003     785    66.0          6

                           Measured        Corrected atomic ratios (c)
                           [.sup.208] Pb/  [.sup.206] Pb/ [.sup.238] U
Fraction                   [.sup.206] Pb

Z1 5 sml clr euh prm Etch  0.2056          0.07663 [+ or -] 48
Z2 3 sml clr euh prm Etch  0.2155          0.07655 [+ or -] 40
Z3 3 sml clr euh prm Etch  0.2223          0.07640 [+ or -] 48

                           Corrected atomic ratios (c)
                           [.sup.207] Pb/ [.sup.235] U
Fraction

Z1 5 sml clr euh prm Etch  0.5995 [+ or -] 52
Z2 3 sml clr euh prm Etch  0.5971 [+ or -] 32
Z3 3 sml clr euh prm Etch  0.5976 [+ or -] 38

                           Corrected atomic ratios (c)
                           [.sup.207] Pb/ [.sup.206] Pb
Fraction

Z1 5 sml clr euh prm Etch  0.05675 [+ or -] 42
Z2 3 sml clr euh prm Etch  0.05658 [+ or -] 18
Z3 3 sml clr euh prm Etch  0.05672 [+ or -] 16

                                             Age (Ma)
                           [.sup.206] Pb/  [.sup.207] Pb/
Fraction                   [.sup.238] U    [.sup.235] U

Z1 5 sml clr euh prm Etch  476             477
Z2 3 sml clr euh prm Etch  475             475
Z3 3 sml clr euh prm Etch  475             476

                            Age (Ma)
                           [.sup.207] Pb/
Fraction                   [.sup.206] Pb

Z1 5 sml clr euh prm Etch  482
Z2 3 sml clr euh prm Etch  475
Z3 3 sml clr euh prm Etch  481

Z, zircon; 3 and 5 No. of grains in the analysis; clr, clear; euh,
euhedral; prm, prism; sml, small; Etch, zircons annealed, then etched
with HF; (chemical abrasion of Mattinson 2005); (a) Weights of grains
were estimated, with potential uncertainties of 25%-50% for small
samples; (b) Radiogenic lead; (c) Atomic ratios corrected for
fractionation, spike, laboratory blank of 0.6-2.0 picograms (pg) common
lead, and initial common lead at the age of the sample calculated from
the model of Stacey and Kramers (1975), and 0.3 pg U blank. 2 [sigma]
uncertainties are reported after the ratios and refer to the final
digits.

Table 2. Analytical Parameters.

laser                  esi/NWR 193 nm 4 ns excimer
background/washout     14 s/8 s
laser repetition rate  15 Hz
spot size/shape        30 [micro]m / circle
fluence                3.25 J [cm.sup.-2]
carrier/makeup gas     0.6 l/min He, 0.8 l/min Ar
mass spectrometer      Thermo Scientific iCAP RQ
plasma RF power        1550 W
total duty cycle       195 ms
                       [.sup.48]Ti (10), [.sup.88]Sr (10), [.sup.96]Zr
                       (2.5), [.sup.179]Hf (2.5),
isotopes measured      [.sup.202]Hg (10), [.sup.204]Pb (20),
                       [.sup.206]Pb (20), [.sup.207]Pb
(dwell times in ms)    (50), [.sup.208]Pb (10), [.sup.232]Th (10),
                       [.sup.235]U (20), [.sup.238]U
                       (10), [.sup.232]Th [.sup.16]O (10),
                       [.sup.238]U[.sup.16]O (10)
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Author:Johnson, Susan C.; Dunning, Gregory R.; Miller, Brent V.
Publication:Atlantic Geology
Article Type:Report
Geographic Code:1CNBR
Date:Jan 1, 2018
Words:20365
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