Field relations, age, and tectonic setting of metamorphic and plutonic rocks in the Creignish Hills--North Mountain area, southwestern Cape Breton Island, Nova Scotia, Canada.
The Creignish Hills and North Mountain areas of western Cape Breton Island (Fig. 1) consist mostly of Neoproterozoic metasedimentary and plutonic rocks typical of the Bras d'Or terrane of Barr and Raeside (1989) and Raeside and Barr (1990). These areas have been mapped and studied by numerous workers over the years (e.g., Kelly 1967; Milligan 1970; White et al. 1990; Campbell 1990; Justino 1991; Lynch and Brisson 1996; Keppie et al. 1998a; White et al. 2003; Wessel et al. 2005), and the resulting interpretations are varied, hampered by complex field relations and limited age constraints. White and Boehner (2008) compiled and evaluated existing data to produce a revised 1:50 000-scale geological map of the Whycocomagh NTS sheet, and Swanton et al. (2010) extended the mapping to the northeast to include metamorphic and plutonic rocks in the Lewis Mountain-Aberdeen Ridge area (Fig. 2).
The purpose of this paper is to provide an overview of the pre-Devonian geology based on these maps, and to present a new detrital zircon age spectrum from a metasedimentary unit in the Creignish Hills which better constrains the age of the metamorphic units in the area. We also take the opportunity to publish the full database for U-Pb zircon ages from the area reported only in preliminary form by White et al. (2003), and to present previously unpublished total fusion 40Ar/39Ar ages for muscovite from a metasedimentary unit in the Creignish Hills. In addition, we re-examine chemical data from Neoproterozoic plutons in these areas in the light of the current age information, and propose revised names for some of the rock units in the area based on all of the currently available data.
The Bras d'Or terrane of central Cape Breton Island is characterized by low- to high-grade metamorphic rocks intruded by abundant plutons of mainly Late Neoproterozoic age (Fig. 1). Barr and Raeside (1989) and Raeside and Barr (1990) documented tectonostratigraphic differences between these characteristic components of the Bras d'Or terrane and rocks of similar age that characterize the Avalonian Mira terrane of southeastern Cape Breton Island. Those differences have been supported by a number of subsequent studies (e.g., Barr et al. 1995, 1998; Potter et al. 2008a, b), and led to the inclusion of the Bras d'Or terrane, along with similar areas in central Newfoundland and southern New Brunswick, in Ganderia by Hibbard et al. (2006). In contrast, a number of other studies have focused on age similarities between rock units in the Bras d'Or and Mira terranes, and have produced alternative models based on the interpretation that these areas were not separate in the Neoproterozoic (e.g., Keppie and Dostal 1998; Keppie et al. 1990, 1998a, 2000).
The boundary between the Bras d'Or and Mira terranes of Barr and Raeside (1989) is placed in Bras d'Or Lake between North Mountain and Sporting Mountain (Fig. 1). It extends to the northeast through the Boisdale Hills where it coincides with a Carboniferous fault system (Macintosh Brook Georges River). From there it has been postulated to extend across the Cabot Strait to the south coast of Newfoundland (Barr et al. 1998, 2014a; Rogers et al. 2006). To the southwest, the terrane boundary is suggested to be offset at the Strait of Canso by the Canso fault and inferred to follow the western margin of the Creignish Hills (King 2002; Barr et al. 2012).
The northern boundary of the Bras d'Or terrane is assumed to be located north of the Creignish Hills beneath Carboniferous rocks. This interpretation is based on the occurrence southwest of Lake Ainslie of rhyolite and granite similar to those in the Lake Ainslie-Gillanders Mountain area of the Aspy terrane (Barr and Jamieson 1991). To the northeast, the boundary is inferred to follow high-strain zones through the central Cape Breton Highlands (Fig. 1). However, the relationship between the Bras d'Or and Aspy terranes is uncertain; available evidence suggests that they are linked in a complex basement/cover relationship, and that Bras d'Or terrane "basement" is likely present under Aspy terrane (e.g., Lin 1993, 1995, 2001; Lin et al. 2007; Price et al. 1999).
Traditionally, most metamorphic rocks in the Bras d'Or terrane were termed the George River Series or Group (e.g., Milligan 1970; Keppie 1979). Raeside and Barr (1990) subdivided these rocks into two assemblages based on metamorphic grade: mainly lower grade units collectively termed the George River metamorphic suite and mainly higher grade and typically gneissic units termed the Bras d'Or metamorphic suite. They also proposed local names for components of both of these metamorphic suites in different areas of the terrane because correlations among them could not be verified. Keppie (2000) termed the higher grade units the Bras d'Or Gneiss, rather than metamorphic suite, but the latter term is retained here as not all components of the unit are gneissic. Rocks of the George River and Bras d'Or metamorphic suites are everywhere separated from one another by known or inferred faults, or by plutonic units, and hence their relationship is enigmatic. Similarities in rock types suggest that they are the same rocks with different metamorphic histories (e.g., Barr et al. 2013) but even if so, their distribution and geological relations are difficult to explain, as exemplified by the map patterns in Figures 1 and 2.
GEORGE RIVER METAMORPHIC SUITE IN THE CREIGNISH HILLS--NORTH MOUNTAIN AREAS
The lower grade (mainly greenschist facies) metamorphic rocks in the Creignish Hills have been named the Blues Brook Formation (Raeside and Barr 1990; Campbell 1990). A compilation of previous work combined with their own mapping led White and Boehner (2008) to divide the Blues Brook Formation into 5 unnamed members, based on dominant rock types (Fig. 2): (1) slate interbedded with minor metasandstone, metasiltstone, and metacarbonate rocks (member si); (2) metacarbonate rock interbedded with minor metasandstone, metasiltstone, slate, and quartzite (member ca); (3) mainly quartzite interbedded with minor metacarbonate rocks (member qz); (4) a volcanic unit consisting mainly of metamorphosed andesitic to basaltic lithic and lithic crystal tuff with minor basalt flows (member vt); and (5) metasandstone and metasiltstone interbedded with minor slate, metacarbonate rocks, quartzite, and rare basaltic lithic tuff (member ss). Similarities in rock types and their map distribution suggest that the members were originally a single stratigraphic succession, although younging directions are poorly preserved and lateral equivalency is possible.
In the extension of the Creignish Hills into the Lewis Mountain--Aberdeen Ridge area to the northeast, Swanton (2010) and Swanton et al. (2010) also recognized the Blues Brook Formation, including a small area of massive white quartzite (member qz; Fig. 2). However, the remaining rocks in the formation in that area could not be readily assigned to any of the other members of White and Boehner (2008) and hence two additional members were identified: member ps, mainly pelitic schist interbedded with subordinate metacarbonate rocks, amphibolite, and quartzite and member qf, quartzofeldspathic schist with quartzite, metaconglomerate, and minor amphibolite (Fig. 2). To the east along the shore of Bras d'Or Lake, Swanton et al. (2010) identified a separate formation, Aberdeen Ridge, which consists of muscovite-chlorite quartzite with minor amphibolite.
In the North Mountain area to the south (Figs. 1, 2), the lower grade metamorphic rocks have been named the Malagawatch Formation (Raeside and Barr 1990; Justino 1991). The Malagawatch Formation has not yet been subdivided into members but consists of an assemblage of rock types similar to those of the Blues Brook Formation, including metasiltstone, slate, calcitic and dolomitic marble, calc-silicate rocks, minor quartzite, and mafic metavolcanic rocks (Justino 1991).
Rocks of the Blues Brook Formation are regionally metamorphosed to greenschist facies and grade into lower amphibolite facies in the Whycocomagh Mountain area (Armitage 1989; Campbell 1990; Swanton 2010; Swanton et al. (2010). The metamorphic mineral assemblage in most of the pelitic rocks consists of chlorite, muscovite, biotite, and albitic plagioclase whereas in the Whycocomagh Mountain area the grade is higher and garnet and andalusite are present. Most of the metacarbonate rocks are monomineralic (calcite or dolomite) but locally contain talc, muscovite, phlogopite, garnet, and minor tremolite. In the higher metamorphic grade calc-silicate rocks diopside and forsterite are present (Armitage 1989). The mafic metavolcanic rocks have been metamorphosed to assemblages of actinolite, chlorite, and albitic plagioclase; stilpnomelane has also been reported (Campbell 1990). Contact metamorphic effects are locally well developed around some of the plutonic units and most evident in the pelitic rocks where a hornfelsic texture is preserved containing randomly oriented biotite, cordierite, and rare andalusite.
The protolith age of these metamorphic units is constrained to greater than ca. 586 Ma by dating of mainly undeformed cross-cutting plutons, as described below. However, they have generally been assumed to be much older than 586 Ma, perhaps Mesoproterozoic, based on the presence of stromatolite-like structures in the Malagawatch Formation, similar to those in the Green Head Group of southern New Brunswick which are of inferred Early Proterozoic age (Hofmann 1974; White and Barr 1996; White et al. 2007). This interpretation has been supported by detrital zircon populations that are mainly Archean and Mesoproterozoic in quartzite samples from both areas (Keppie et al. 1998a; Barr et al. 2003). However, a sample of schist from the Blues Brook Formation (Fig. 2) yielded 2 euhedral zircon grains with slightly discordant [sup.206]Pb/[sup.238]U ages of 637 [+ or -] 3 Ma and 638 [+ or -] 2 Ma (Keppie et al. 1998a). Keppie et al. (1998a) interpreted the schist to have formed from a felsic volcanogenic unit, and the ages to be the age of the igneous protolith. If so, these dates are a maximum age for the Blues Brook Formation and, by inference, other units in the George River metamorphic suite. A younger [sup.40]Ar/[sup.39]Ar cooling age of 455 [+ or -] 1 Ma was reported for muscovite from the same area (Keppie et al. 2000). Campbell (1990) obtained older muscovite ages of ca. 625 Ma from elsewhere in the same unit, but those data are unpublished and of uncertain reliability.
BRAS D'OR METAMORPHIC SUITE IN THE CREIGNISH HILLS--NORTH MOUNTAIN AREAS
Keppie (1993), Keppie et al. (1998a), and Wessel et al. (2005) interpreted the presence of a folded shear zone between the low- and high-grade rocks in the Skye Mountain area of the Creignish Hills. They postulated that this shear zone repeatedly intersects the present erosion surface, implying that it is a low-angle tectonic boundary in which the high-grade rocks occur in structural domes. That interpretation was not adopted by White and Boehner (2008), who divided the high-grade metamorphic rocks in the Creignish Hills into two units, one at the northeastern tip for which they used the name Skye Mountain metamorphic suite, and the other, which they termed the Melford Formation, in two areas to the southwest: a fault-bounded block across the trend of the Creignish Hills in the northeast, and a smaller area near Melford in the central part of the Creignish Hills (Fig. 2). This distinction was made because the rocks in the latter two areas are not generally gneissic and differ in rock types from those in the area at the northeastern tip. We here introduce the name Chuggin Road instead of Skye Mountain for the rocks at the northeastern tip of the Creignish Hills because that area is not geographically located on Skye Mountain, and furthermore, the name Skye Mountain has long been used for plutonic rocks in the Skye Mountain area (see below). The area of high-grade rocks shown by Armitage (1989), Raeside and Barr (1990), and White and Boehner (2008) on Whycocomagh Mountain was reassigned by Swanton (2010) and Swanton et al. (2010) to the Blues Brook Formation as noted above, based on rock types and metamorphic grade. In the North Mountain area, rocks of the Bras d'Or metamorphic suite are known as the Lime Hill gneissic complex and occur in a large area in the south-central part of North Mountain (Fig. 2).
As described by Raeside (1990), Raeside and Barr (1990), Campbell (1990), and Sangster et al. (1990) the high-grade metamorphic rocks in both the Chuggin Road and Lime Hill units include biotite, bioitite-cordierite, and sillimanite-bearing paragneiss, migmatitic paragneiss, marble, quartzite, amphibolite, and tonalitic orthogneiss. In contrast, the Melford Formation has somewhat different rock types, including biotite, biotite-cordierite, sillimanite, and garnet-bearing schist, marble, quartzite, and granitic orthogneiss (White and Boehner 2008).
Keppie et al. (1998a) provided age constraints on rocks in the Melford Formation of White and Boehner (2008). A muscovite-biotite 'paragneiss' from the southern block yielded mainly mid-Proterozoic ages, but the youngest grain is ca.977Ma (Keppie et al. 1998a). A pelitic paragneiss inferred to have a volcanogenic protolith in the fault-bounded block at Skye Mountain yielded zircon grains with 206Pb/238U ages between 688 Ma and 694 Ma (Keppie et al. 1998a). These ages were interpreted to be the best estimate of igneous crystallization ages for the inferred volcanogenic protolith. A monazite grain interpreted to be of metamorphic origin in the same sample yielded a concordant age of 552 [+ or -] 8 Ma. Farther north and also in the Melford Formation of White and Boehner (2008), a foliated granitic sheet, considered to be syntectonic, yielded two concordant monazite grains with ages of 551 [+ or -] 1 Ma, interpreted to be igneous crystallization ages (Keppie et al. 1998a). Two zircon grains yielded discordant but similar ages of ca. 553 Ma. These ages provide a minimum age for the Melford Formation. In contrast to these Precambrian ages, 40Ar/39Ar ages from muscovite in the Melford Formation are younger: 441, 449, 461, 473, and 485 Ma (Campbell 1990; Dallmeyer and Keppie 1993; Keppie et al. 1998a). Similar 40Ar/39Ar ages were reported for muscovite (484, 524 Ma) and biotite (485 Ma) from the area of Melford Formation to the south near Melford (Campbell 1990; Keppie et al. 2000) and muscovite from the Melford pluton (Keppie et al. 2000). An igneous crystallization age of ca. 561 Ma reported by White et al. (2003) for orthogneiss in the Chuggin Road metamorphic suite is described below.
In the case of the Lime Hill gneissic complex, ages are constrained by a concordant U-Pb monazite date from paragneiss at 549 [+ or -] 2 Ma, and syn- and post-tectonic plutons that yielded discordant U-Pb (zircon) data with lower intercepts of 539 [+ or -] 1 Ma and 545-540 Ma (Sangster et al. 1990). An additional constraint is provided by a concordant monazite age of 553 [+ or -] 1 Ma from an anatectic granite sheet in pelitic paragneiss, interpreted to represent the time of peak metamorphism (Keppie et al. 1998a). Keppie et al. (1998a) reported that another monazite grain and rounded zircon grains from the same sheet yielded older but discordant ages.
Dioritic to granitic plutons intruded both the low- and high-grade metamorphic units in both the Creignish Hills and North Mountain areas (Fig. 2). White et al. (1990) assigned most of the plutonic rocks in the Creignish Hills area to the Creignish Hills pluton, and subdivided the pluton into five units based on modal mineralogy; texture, and lithogeochemistry: tonalite-diorite, granodiorite-tonalite, granodiorite-monzogranite, coarse-grained monzogranite, and fine-grained monzogranite. This work was done in the early days of geochronological work in Cape Breton Island, and the authors were much influenced by an Rb-Sr isochron using a range of rock types which indicated an age of about 446 Ma. However, [sup.40]Ar/[sup.39]Ar cooling ages of ca. 540 Ma from hornblende in the tonalite--diorite unit indicated a much older minimum age for that body. Furthermore, the granitic components of the pluton had petrological similarities to the Kellys Mountain and Cape Smokey plutons which had yielded U-Pb (zircon) ages of 498 and 493 Ma, respectively. Overall, it was concluded based on the available data that the rocks are (questionably) of Ordovician age. Two small plutons in the northeastern part of the belt were together named the Skye Mountain pluton, described as separate bodies of gabbro and granodiorite, and assigned an age anywhere between Precambrian and Carboniferous (White et al. 1990). These plutons were reported to differ from any of the components of the Creignish Hills pluton based on descriptions provided by earlier workers but were not part of the study by White et al. (1990).
Knowledge of the ages of these plutons was much improved as a result of work by Keppie et al. (2000), who reported U-Pb (zircon) ages of 553 [+ or -] 2 for a sample from the largest tonalite-diorite body of the Creignish Hills pluton. This age is consistent with the previously reported ca. 544 Ma hornblende cooling age from that body (Keppie et al. 1990). However, a smaller body of tonalite (River Denys pluton of Keppie et al. 2000) yielded a younger U-Pb age of 540+3 Ma, although 40Ar/39Ar ages for amphibole from the same sample and another sample in the same unit are older (ca. 550 and 551 Ma; Keppie et al. 1990). Keppie et al. (2000) concluded that, overall, these 540-550 Ma ages "post-date closely the time of intrusion", and that the River Denys Pluton is part of the same magmatic event as the 553 Ma diorite of the Creignish Hills pluton. The age of the Creignish Hills pluton was further constrained by a ca. 553 Ma age for the main granitic unit reported by White et al. (2003) and presented below.
In contrast to these Neoproterozoic ages, a sample reported to be from the gabbro-diorite body of the Skye Mountain pluton yielded a Silurian age of 438 [+ or -] 2 Ma (Keppie et al. 1998b, 2000). A sample from the separate granitic body to the west yielded only old (ca. 737 Ma) discordant zircon grains, but Keppie et al. (2000) inferred a Silurian age, based on muscovite plateau ages in the surrounding host rocks, especially an age of 441 Ma from what they interpreted to be the contact metamorphic aureole adjacent to the granite. Keppie et al. (2000) and Wessel et al. (2005) showed the Skye Mountain granite and gabbro-diorite as contiguous bodies on their maps, whereas previous maps (White et al. 1990; Horton 1994) showed them as separate bodies. Horton (1994) interpreted them to be unrelated based on their very different petrographic and chemical characteristics. He considered the foliated granitic sheet in the Melford Formation north of the pluton to be related to the granite and hence its ca. 551 Ma age to be the age of the pluton.
After a re-evaluation of outcrop locations and additional mapping in the area, White and Boenher (2008) confirmed that the two plutons are separated by a belt of rocks of the Melford Formation. They included the western (granitic) pluton in the Creignish Hills pluton, to which it is petrographically similar, and inferred a similar ca. 550 Maage. Hence, to avoid further confusion, the name Skye Mountain is retained here for only the gabbroic-dioritic body and a new name, Skye Mountain Road, is introduced for the western body, named after the road on which it is best exposed.
The small granitic body termed the Melford pluton by Keppie et al. (2000) yielded an older U-Pb (zircon) age of 586 [+ or -] 2 Ma, although this two-point upper intercept age is of limited reliability. The pluton intruded gneissic rocks assigned by Keppie et al. (2000) to the Bras d'Or Gneiss (Melford Formation in this study), and was interpreted to be late syn-tectonic based on the presence of a weak foliation parallel to that in its host rocks. White and Boehner (2008) showed that the pluton is larger than suggested by Keppie et al. (2000), and that it is faulted on its southwestern and northwestern edges against rocks of the Blues Brook Formation. They included the pluton in the granitic unit of the Creignish Hills pluton, in spite of its apparently somewhat older age and the occurrence of muscovite for which Keppie et al. (2000) reported a cooling age of 472 Ma. Muscovite is not a typical component of the Creignish Hills granite but has also been reported in part of the Skye Mountain Road pluton (Horton 1994).
Plutons form most of the North Mountain area, and were studied in detail by Justino (1991) and Justino and Barr (1994). Those authors divided the plutonic rocks into the Big Brook and Marble Mountain plutons, of mainly granodiorite composition, and the West Bay Pluton, composed of coarse-grained monzogranite. The latter pluton is cut off by a fault according to the map by Giles et al. (2010) but likely extends south of the fault with minor offset, based on the presence of outcrops of similar granite in the Sugar Camp gypsum quarry (Fig. 2). Small tonalitic to dioritic bodies (Mill Brook quartz diorite and related plutons) occur along the margins of these large plutons. Age constraints are provided by [sup.40]Ar/[sup.39]Ar hornblende cooling ages of 555 [+ or -] 5 Ma apparently from the Marble Mountain biotite granodiorite unit (based on approximate location shown by Keppie et al. 1990) and 550 [+ or -] 5 Ma and 545 [+ or -] 6 Ma on two separate bodies of the Mill Brook quartz diorite (Keppie et al. 1990) and the ca. 553 Ma age for the West Bay Granite reported by White et al. (2003) and presented below.
In this paper we report new detrital zircon ages for a sample (SMB14-189A) collected from the same location as sample 11Z reported by Keppie et al. (1998a). We resampled this site in order to further investigate the hints of young zircon grains in sample 11Z. The methods and data table are presented in Appendix A. We also resampled at the same location in the Melford Formation as paragneiss sample 188Z of Keppie et al. (1998a) but the sample yielded no zircon grains suitable for analysis.
We also present the data for the ages published in preliminary form by White et al. (2003). The methods and data tables are presented in Appendix B.
Muscovite was separated from a sample of mica schist interlayered with marble in unit ss near the southern margin of the Creignish Hills (Fig. 2). The analytical method and data are presented in Appendix C (Table Cl).
Sample SMB 14-189A from the Blues Brook Formation (UTM coordinates 5090806N, 637216E) yielded a small amount of tiny rounded zircon grains. Clear areas on the zircon grains were targeted for analysis so as to avoid cracks. U-Pb analyses are all near- concordant (Appendix Table Al) and give a wide range of Neoproterozoic to Archean ages (Fig. 3a). A subset of brownish grains proved to be relatively old, and included the two Archean grains (Table Al). Over half of the grains are grouped into a relatively tight cluster with [sup.206]Pb/[sup.238]U ages that vary over the range of about 600-700 Ma (Fig. 3a). Deposition of the unit was therefore at or younger than 600 Ma and much of the detritus in the sample was derived from Neoproterozoic rocks.
Sample SMB02-81, collected from an active quarry near Chuggin Road northwest of Whycocomagh (UTM coordinates 5094125N, 642350E), is a fine- to medium-grained tonalitic orthogneiss that consists mostly of quartz, plagioclase, and biotite. It is interpreted as an orthogneiss based on its homogeneous appearance in outcrop and on the presence of relict weakly zoned plagioclase grains visible in thin section. A relatively high Th/U ratio (mostly >0.5) in the zircon is also consistent with an igneous origin. The zircon population obtained from the sample is characterized by colourless, good-quality euhedral and subhedral prisms ranging from equant to elongate with length to breadth ratio of 3:1 to 4:1. Many equant grains are distinctly zoned with an apparent euhedral overgrowth component, but this component is not present on most long prisms. Four fractions consisting of long prisms (length:breadth >3:1), both with and without tiny fluid inclusions, are concordant or near-concordant at 565-560 Ma (Fig. 4a; Table Bl). Their average [sup.207]Pb/[sup.206]Pb age of 561 [+ or -] 3 Ma is interpreted as the igneous crystallization age of the tonalite, although we acknowledge that a slightly older age of ca. 565 [+ or -] 3 Ma (based on the [sup.206]Pb/[sup.238]U age of analysis Z1) is possible. Two additional fractions (1 and 4 grains each) of equant, colourless grains give discordant results indicating mid-Proterozoic inheritance (Fig. 4a). The age constrains the age of metasedimentary components of the Chuggin Road and other parts of the Bras d'Or metamorphic suite to greater than ca. 565 Ma.
The Creignish Hills pluton was sampled for dating on a logging road in the south-central part of the Creignish Hills (UTM coordinates 5071750N, 626375E). Sample SMB02-79 is typical of the medium- to coarse-grained monzogranitic unit that forms most of the pluton (White et al. 1990). It consists of quartz, K-feldspar (perthitic orthoclase), and plagioclase in approximately equal proportions, with minor biotite and hornblende. It yielded abundant equant to 2:1, euhedral, four-sided zircon prisms of varying size. Fluid inclusions are relatively common and opaque mineral inclusions were also observed. Four concordant or near-concordant single grain and multigrain fractions (Table B1) have an average [sup.207]Pb/[sup.206]Pb age of 553 +/- 3 Ma (Fig. 4b) which we interpret as the crystallization age of the granite.
The West Bay Granite was sampled in a quarry at the southwestern margin of North Mountain (UTM coordinates 5066850N, 641000E). The dated sample is coarse-grained monzogranite, with K-feldspar megacrysts up to 5 cm in length. Mafic minerals (biotite and possibly hornblende) have been replaced by chlorite. The zircon population in this sample appears identical to that for sample SMB02-79, and three near-concordant single grain and 2-grain fractions (Table B1) yield an identical crystallization age of 553 [+ or -] 2 Ma (Fig. 4c).
Muscovite schist sample SMB06-111 was collected from a small quarry in the Blues Brook Formation near the faulted southeastern margin of the Creignish Hills (Fig. 2). To reduce the possibility of homogenization of Ar isotopes during traditional incremental heating of muscovite and to reduce any core-rim effect (i.e., intracrystalline age discordance), standard single-grain, laser fusion [sup.40]Ar/[sup.39]Ar ages were used (e.g., Hames et al. 2008). Hence, each analysis is a mean age of the muscovite grain (Appendix C). Total fusion analyses of 23 single muscovite grains yielded apparent ages ranging from 488 Ma to 430 Ma (Table Cl). Displayed on a histogram (Fig. 5), ages cover the range between those high- and low-age cutoffs. The simplest interpretation is that these ages represent partial resetting of older (>500 Ma) muscovite at about 430 Ma (see further discussion below). Because the dated sample is from near the major faulted margin of the Creignish Hills, it is likely that the resetting is a result of deformation (e.g., Mulch and Cosca 2004).
Age of the George River and Bras d'Or metamorphic suites
Keppie and Dostal (1998) interpreted that the George River metamorphic suite (Blues Brook and Malagawatch formations) in the Creignish Hills and North Mountain areas consists of two parts--an older non-volcanogenic sedimentary package (quartzite, marble, schist, phyllite) and a younger volcanogenic package in which similar rocks are interlayered with felsic volcanogenic rocks and mafic flows. Based on their U-Pb data (summarized above), they estimated depositional ages of -977-700 Ma and -694-637 Ma, respectively, for these two packages. They showed that the mafic volcanic rocks in the inferred younger package are within-plate tholeiite, and attributed them to a backarc basin associated with a volcanic arc of similar age in the Stirling belt of the now-adjacent Avalonian Mira terrane. Our sample from the same location as the sample that yielded the -694 and 637 Ma ages contains abundant grains as young as 600 Ma, showing that the maximum depositional age for the unit is younger than suggested by the data of Keppie et al. (1998a). These new data are inconsistent with the interpretation made by Keppie and Dostal (1998) that the rocks formed in a back-arc related to the birth of arc magmatism in Avalonia because the main pulse of arc magmatism in many parts of Avalonia, including the now-adjacent Mira terrane, is at about 620 Ma and arcrelated rocks as old as 680 Ma are documented in the Mira terrane (Barr et al. 1996, 1998). These rocks are clearly older than the depositional age of at least part of the Blues Brook and Malagawatch formations. It also makes the chemical characteristics documented by Keppie and Dostal (1998) difficult to interpret, as discussed further in the next section.
Based on field relations and similarities in rock types, it seems unlikely that the components of the Blues Brook and Malagawatch formations are of widely different ages. White and Boehner (2008) interpreted the units to form a single stratigraphic succession. The minimum age is constrained by the abundant plutons with ages of ca. 550 Ma that intruded these rocks. Hence it seems that these sediments were deposited, regionally metamorphosed and deformed, and intruded by abundant plutons in less than 50 million years.
Only sparse detrital zircon data are available from other metasedimentary rocks in the Creignish Hills and Bras d'Or terrane in general (Keppie et al. 1998a; Barr et al. 2003), but a larger database is available from the Brookville terrane of southern New Brunswick. The detrital zircon age signature displayed by sample SMB 14-189A is similar, especially in its Late Mesozoic to Neoproterozoic to part, to that of the Martinon Formation in New Brunswick, in which the most abundant zircon grains have ages between 650 and 600 Ma, and many fewer grains have ages between 1000 Ma and 2200 Ma (Fig. 3b). Like the Blues Brook Formation, the Martinon Formation is part of the low-grade metasedimentary assemblage in the Brookville terrane, and closely associated with quartzite and marble, which locally contains stromatolite-like structures. Similarity in detrital zircon signatures between the >1000 Ma grains in the associated quartzite as well as in the high-grade assemblage (Brookville Gneiss) in the terrane led Barr et al. (2014b) to conclude that the high- and low-grade rocks are related, although the quartzite and paragneiss age spectra lack the 650-600 Ma zircon grains. Based on the results of Keppie et al. (1998a), the same situation exists in the Creignish Hills, although one sample of paragneiss from the Melford Formation contains grains as young as 688 Ma and 694 Ma (Keppie et al. 1998a). Resampling at that locality for the present study (black star, Fig. 2) did not result in any zircon grains which could have better constrained those younger ages.
The age of ca. 561 Ma from orthogneiss in the Chuggin Road complex (Fig. 4a) provides a minimum age for the Bras d'Or metamorphic suite and a maximum age for regional metamorphism. It is supported by several monazite and zircon ages of ca. 550 Ma reported by Keppie et al. (1998a) and Sangster et al. (1990) from the Creignish Hills and North Mountain areas (Figs. 6a, b). All of these data indicate a major thermal event at that time, coincident with regional metamorphism and widespread magmatic activity as discussed further in the subsequent section.
The assemblage of pelitic, psammitic, and carbonate rocks in the Blues Brook and Malagawatch formations and the likely correlative high-grade units indicates that they formed on a passive margin in a tropical climate, as also inferred for the Brookville terrane in southern New Brunswick (Barr et al. 2014b). Barr et al. (2014b) suggested that this passive margin was located on the Proto-Andean--Caribbean edge of Amazonia after it had separated from Laurentiaby ca. 650 Ma.
Pluton ages and geochemistry
The U-Pb igneous crystallization ages from zircon and monazite in plutonic rocks ranging from dioritic to granitic in the Creignish Hills and North Mountain areas cluster at about 550 Ma (Fig. 6a). The fact that 40Ar/39Ar cooling ages from amphibole in these units are similar to the zircon ages (Figs. 6a, b) indicates that these plutons cooled quickly and hence were probably epizonal, as also noted by Keppie et al. (1990) and Dallmeyer and Keppie (1993). In contrast to previous suggestions based on less reliable age estimates (White et al. 1990; Raeside and Barr 1990), the granitic rocks are the same age as the dioritic and tonalitic rocks and did not form in a separate, much younger igneous event.
This interpretation is further supported by chemical characteristics which show coherence among all of the known and inferred Neoproterozoic plutons in the Creignish Hills and North Mountain areas, as illustrated in Figure 7. With the exception of 3 new analyses (Table Dl), the chemical data depicted on this figure have been previously published in theses and papers. Hence we are not presenting the data again but include instead a list of sample numbers and data sources (Table D2), as the analyses may not have been identified with the correct age or pluton in the original paper.
As illustrated in Figure 7, the analyses from the Skye Mountain Road pluton (data from Horton 1994) are similar to those from the other granitic and granodioritic plutons, consistent with the interpretation that it is the same ca. 550 Ma age and not Silurian like the Skye Mountain diorite-gabbro. Similarly, the River Denys tonalite is chemically similar to other dioritic and tonalitic components of the Creignish Hills pluton, consistent with the interpretation of Keppie et al. (1998a) that the plutonic rocks are part of a single magmatic episode, even though ages show some variation. No chemical data are available from the possibly older (ca. 586 Ma) Melford pluton to indicate whether or not it is chemically similar to the other plutons, but White and Boehner (2008) correlated it with the coarse-grained granitic unit of the Creignish Hills pluton based on petrographic features.
With an igneous crystallization age of ca. 561 Ma, the tonalitic orthogneiss in the Chuggin Road metamorphic suite is also slightly older than the dominant ca. 550 Ma plutons of the Creignish Hills--North Mountain area, but dioritic, tonalitic, and granodioritic plutons elsewhere in the Bras d'Or terrane have yielded U-Pb (zircon) crystallization ages as old as 565 Ma (Fig. 6a). The Chuggin Road orthogneiss sample has chemical characteristics that are generally similar to other plutonic samples with similar silica content, but it has higher CaO and hence plots somewhat separately in terms of normative mineralogy (Fig. 7e).
Overall, samples from all of the plutons show the range of compositions typical of I-type granitoid suites (Fig. 7). They lie on a calc-alkalic trend on an AFM diagram (Fig. 8a), and range from metaluminous in the more mafic samples to peraluminous in the felsic samples (Fig. 8b). They have trace element characteristics of volcanic-arc granites, evolving toward within-plate characteristics in the most felsic (evolved) samples (Fig. 8c). These characteristics are shared by Neoproterozoic plutons throughout the Bras d'Or terrane (e.g., Farrow and Barr 1992; Raeside and Barr 1990; Grecco and Barr 1999; Wasylik et al. 2005; Swanton 2010). It is clear that a huge volume of plutonic rocks was emplaced in the Bras d'Or terrane mainly between about 565-550 Ma and that they formed in an Andeantype continental margin subduction zone. A similar interpretation has been made for the Brookville terrane of southern New Brunswick, although the pluton ages there are overall younger at 550-525 Ma (White et al. 2002).
Significance of muscovite ages
As noted above, 40Ar/39Ar cooling ages from muscovite in both plutonic and metamorphic rocks of the Creignish Hills are younger than most of the pluton crystallization ages, whereas the hornblende ages reflect pluton cooling at ca. 550-540 Ma (Figs. 6a, b). Examining the geographic distribution of the muscovite ages reported by Dallmeyer and Keppie (1993) from the Creignish Hills, most if not all samples are from proximity to the major shear zones which cross the area. The sample dated in the present study is from near the faulted southeastern margin of the Blues Brook Formation (Fig. 2). The age data shown in Figure 5 are statistically too few to attach significance to the internal peaks, but if the upper and lower bounds of the data have significance, they suggest that muscovite with a minimum age of 500 Ma was variably reset at about 430 Ma. The other age data (U-Pb zircon and monazite and 40Ar/39Ar amphibole) suggest that the likely initial age of the muscovite is ca. 550 Ma, the approximate time of regional metamorphism and pluton emplacement in the Bras d'Or terrane. Scattered plutons elsewhere in the Bras d'Or terrane with ages of ca. 500 Ma are post-tectonic, and little other evidence exists for a ca. 500 Ma thermal event in the Bras d'Or terrane (Figs. 6a, b). The variable resetting of the muscovite ages could have been related to the emplacement of the Skye Mountain dioritegabbro at ca. 438 Ma, but given that the young muscovite ages occur widely through the Creignish Hills (Dallmeyer and Keppie 1993), it seems more likely that the resetting was related to deformation in the major shear zones in the area (Fig. 2). More detailed 40Ar/39Ar work throughout the Bras d'Or terrane is needed in order to interpret the Paleozoic thermal history as recorded in these muscovite ages.
The U-Pb ages presented here for detrital zircon in the Blues Brook Formation of the Creignish Hills confirm the hints of Late Neoproterozoic ages reported by Keppie et al. (1998a) and show without doubt that the depositional age of the unit is no greater than about 600 Ma. Although it is possible that some components of the formation are much older, such as the quartzite in which the youngest detrital zircon age obtained is ca. 1000 Ma (Keppie et al. 1998a), similarities in rock types and contiguous field relations suggest that this is not the case. Based on similarities in rock types and previously published detrital zircon ages of ca. 690 Ma (Keppie et al. 1998a), it is likely that the high-grade metasedimentary rocks of the Melford Formation and Chuggin Road complex are of the same age, and that they represent the same or stratigraphically equivalent units at higher metamorphic grade, rather than unrelated units. The minimum ages of both the low1 and high-grade units in the Creignish Hills and nearby North Mountain are provided by cross-cutting syn- and post-tectonic plutons with ages of mainly 565-550 Ma, indicating that sediments were deposited, regionally metamorphosed and deformed, and intruded by plutons in less than 40-50 million years. The assemblage of pelitic, psammitic, and carbonate rocks indicates that a passive margin in a tropical climate was quickly changed to an active Andean-type continental margin in which voluminous calc-alkaline dioritic to granitic plutons were emplaced. This sedimentary and tectonic history is characteristic of the Bras d'Or terrane and is shared by its likely correlative, the Brookville terrane in southern New Brunswick (Barr et al. 2014b).
This paper assembles information gathered over many years and we are grateful to those who contributed information as cited in the paper, but especially Janet Campbell whose MSc thesis laid the groundwork for much of the subsequent work, and also her supervisor Rob Raeside. Funding was provided by Geological Survey of Canadas Targeted Geoscience Initiative 1 (TGI-1) and the Nova Scotia Department of Natural Resources. Sandra Barr's work on this project was funded mainly by grants over many years from the Natural Sciences and Engineering Research Council of Canada. Special thanks to Tracy Lenfesty and Janelle Brenton for providing help in the departmental library and Trevor MacHattie is thanked for comments on an earlier draft of the manuscript. We thank the journal reviewers Rob Raeside and Deanne van Rooyen for their helpful comments and suggestions which improved the manuscript. This paper is published with permission of the Director, Nova Scotia Department of Natural Resources.
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Editorial responsibility: David P. West, Jr.
Schist sample SMB14-189A was processed and analyzed in the Jack Satterly Geochronology Laboratory at the University of Toronto. It was crushed using a jaw crusher followed by a disk mill. Initial separation of heavy minerals was carried out with a Wilfley table, followed by paramagnetic separations with the Frantz isodynamic separator and density separations using bromoform and methylene iodide. Final sample selection for geochronology was by hand picking under a microscope, choosing the freshest, least cracked zircon grains. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) was used to obtain age data using a VG Series 2 Plasmaquad ICPMS and 213 nm New Wave laser system. Sensitivity was enhanced by the use of a 75 1/sec rotary pump (S-option) connected to the expansion chamber. Whole sample grains were mounted on double-sided tape for LA-ICPMS analysis. This has the advantage that interesting samples (e.g., the youngest grains from a detrital population) can be easily removed and re-dated by ID-TIMS and the amount of material available for analysis is maximized for small grains. The disadvantage is that cores and overgrowths or cracks and altered domains cannot be easily distinguished.
Grains were partially ablated using a 213 nm laser beam with diameters of 18-25 microns at 5 Hz and 40% power. For zircon, data were collected on [sup.88]Sr (10 ms), [sup.206]Pb (30 ms), [sup.207]pb (90 ms), [sup.232]Th (10 ms) and [sup.238]U (20 ms). Sr is a proxy for alteration or the presence of inclusions. Data with excessive Sr signals were rejected. Immediately prior to each analysis, the spot was pre-ablated over a larger area than the beam diameter for about 10 sec to clean the surface and remove any surface alteration. Following a 10 sec period of baseline accumulation the laser sampling beam was turned on and data were collected for 25 seconds. Rasters and sampling were separated by a 50 sec washout period. About 150 measurement cycles per sample were produced and ablation pits are about 15 microns deep. In some cases data profiles show rapidly varying emission due to chemical zoning of zircon. Instability was dampened through the use of a 25 ml mixing chamber in-line with the He flow transporting the ablated sample to the plasma. Data were edited and reduced using custom VBA software (UTILLAZ program) written by the D. Davis. [sup.206]Pb/[sup.238]U show only slight fractionation caused by hole depth through the run and most of the [sup.207]Pb/[sup.206]Pb and [sup.206]Pb/[sup.238]U data can be averaged. No corrections were made for common Pb since the [sup.204]Pb peak is too small for useful measurement and common Pb should be negligible in fresh zircon and monazite. The Th/U ratio of zircon can be a useful petrogenetic indicator and was also measured, although it is only a rough estimate because the ratio is not constant in the standard. Low Th/U (<0.1) is characteristic of metamorphic zircon. Igneous zircon usually shows Th/U the range 0.1-1.0. Zircon from sample DD8-17, a quartz diorite from the northwest Ontario dated at 3002 [+ or -] 2 Ma (Tomlinson et al. 2003) was used as a standard. Sets of 3 sample measurements are bracketed by measurements on standards.
For Precambrian samples, [sup.207]Pb/[sup.206]Pb ages are more accurate and precise than [sup.207]Pb/[sup.235]U or [sup.206]Pb/[sup.238]U ages. Numerical results of U-Pb isotopic analyses by LA-ICPMS are given in Table Al. Errors are at one sigma. These reflect reproducibility of individual data profiles. They do not include error from sample-standard reproducibility, which may be several percent for Pb/U but is unlikely to exceed 1 percent for [sup.207]Pb/[sup.206]Pb. Concordia data are plotted using the Isoplot program of Ludwig (2003). Error ellipses and regression errors are given at 95% confidence levels. U decay constants are from Jaffey et al. (1971).
Table A1. LA-ICPMS analyses on zircon from Creignish Hills metasedimentary sample SMB14-189A. Isotopic ratios spot/sample U [sup.206]pb Th/U (ppm) (ppm) 1 SMB14-189-06 72 7 0.9 2 SMB14-189-42 156 15 0.99 3 SMB14-189-26 258 26 1.75 4 SMB14-189-21 109 11 1.09 5 SMB14-189-71 152 15 2.39 6 SMB14-189-02 201 20 0.61 7 SMB14-189-15 115 12 1.02 8 SMB14-189-48 1122 114 0.28 9 SMB14-189-69 366 37 1.07 10 SMB14-189-55 438 45 1.03 11 SMB14-189-34 136 14 1.39 12 SMB14-189-33 215 22 1.66 13 SMB14-189-09 * 536 55 1.09 14 SMB14-189-03 152 16 2.95 15 SMB14-189-19 323 34 1.35 16 SMB14-189-49 77 8 1.12 17 SMB14-189-38 128 13 1.13 18 SMB14-189-41 228 24 0.03 19 SMB14-189-36 184 19 2.07 20 SMB14-189-23 218 23 2.18 21 SMB14-189-24 123 13 0.9 22 SMB14-189-51 187 20 0.89 23 SMB14-189-13 190 20 1.59 24 SMB14-189-44 330 35 0.62 25 SMB14-189-39 160 17 0.98 26 SMB14-189-45 425 45 1.01 27 SMB14-189-57 155 17 1.13 28 SMB14-189-37 71 8 2.24 29 SMB14-189-40 133 14 1.68 30 SMB14-189-31 485 53 1.65 31 SMB14-189-17 134 15 1.43 32 SMB14-189-11 362 40 0.83 33 SMB14-189-22 327 36 0.61 34 SMB14-189-52 114 13 0.97 35 SMB14-189-08 447 49 1.01 36 SMB14-189-27 359 40 1.5 37 SMB14-189-32 306 34 2.09 38 SMB14-189-10 57 6 1.27 39 SMB14-189-16 589 66 1.22 40 SMB14-189-25 321 36 1.71 41 SMB14-189-30 296 33 0.83 42 SMB14-189-20 464 53 0.72 43 SMB14-189-60 203 23 0.46 44 SMB14-189-53 247 28 0.88 45 SMB14-189-35 528 61 1.57 46 SMB14-189-54 180 22 1.02 47 SMB14-189-12 159 20 1.08 48 SMB14-189-50 101 13 0.49 49 SMB14-189-01 631 79 0.54 50 SMB14-189-56 197 27 0.15 51 SMB14-189-18 1486 249 0.06 52 SMB14-189-46 12 2 0.5 53 SMB14-189-04 13 2 1.11 54 SMB14-189-05 196 35 0.95 55 SMB14-189-62br 857 165 0.29 56 SMB14-189-68br 4410 879 0.08 57 SMB14-189-43 209 43 0.35 58 SMB14-189-07 160 33 0.49 59 SMB14-189-47 152 32 0.45 60 SMB14-189-64br 311 72 0.77 61 SMB14-189-63br 201 48 0.78 62 SMB14-189-29 168 42 0.7 63 SMB14-189-59 266 67 0.32 64 SMB14-189-65br 295 76 0.45 65 SMB14-189-28 225 61 0.85 66 SMB14-189-70 255 71 1.28 67 SMB14-189-58 102 29 0.83 68 SMB14-189-61br 218 75 0.84 69 SMB14-189-14 20 7 0.85 70 SMB14-189-66br 91 48 0.62 71 SMB14-189-67br 118 64 1.82 Isotopic ratios spot/sample [sup.207]Pb/ [+ or -] l[sigma] [sup.235]U 1 SMB14-189-06 0.965 0.031 2 SMB14-189-42 0.834 0.015 3 SMB14-189-26 0.855 0.017 4 SMB14-189-21 0.845 0.025 5 SMB14-189-71 0.843 0.012 6 SMB14-189-02 0.867 0.018 7 SMB14-189-15 0.871 0.021 8 SMB14-189-48 0.852 0.015 9 SMB14-189-69 0.849 0.01 10 SMB14-189-55 0.875 0.018 11 SMB14-189-34 0.849 0.021 12 SMB14-189-33 0.844 0.018 13 SMB14-189-09 * 0.914 0.016 14 SMB14-189-03 0.863 0.02 15 SMB14-189-19 0.92 0.017 16 SMB14-189-49 0.881 0.022 17 SMB14-189-38 0.842 0.019 18 SMB14-189-41 0.87 0.015 19 SMB14-189-36 0.892 0.019 20 SMB14-189-23 1.026 0.02 21 SMB14-189-24 0.896 0.024 22 SMB14-189-51 0.875 0.016 23 SMB14-189-13 0.827 0.018 24 SMB14-189-44 0.922 0.014 25 SMB14-189-39 0.906 0.018 26 SMB14-189-45 0.91 0.014 27 SMB14-189-57 0.881 0.017 28 SMB14-189-37 0.889 0.024 29 SMB14-189-40 0.924 0.019 30 SMB14-189-31 0.928 0.016 31 SMB14-189-17 0.955 0.02 32 SMB14-189-11 0.941 0.017 33 SMB14-189-22 0.949 0.018 34 SMB14-189-52 0.915 0.019 35 SMB14-189-08 0.938 0.018 36 SMB14-189-27 0.939 0.017 37 SMB14-189-32 0.961 0.017 38 SMB14-189-10 0.988 0.033 39 SMB14-189-16 0.965 0.016 40 SMB14-189-25 1.012 0.017 41 SMB14-189-30 0.963 0.016 42 SMB14-189-20 0.959 0.016 43 SMB14-189-60 0.987 0.016 44 SMB14-189-53 1.098 0.021 45 SMB14-189-35 0.974 0.016 46 SMB14-189-54 1.08 0.02 47 SMB14-189-12 1.101 0.025 48 SMB14-189-50 1.136 0.024 49 SMB14-189-01 1.137 0.021 50 SMB14-189-56 1.33 0.033 51 SMB14-189-18 1.689 0.024 52 SMB14-189-46 1.74 0.073 53 SMB14-189-04 1.726 0.087 54 SMB14-189-05 1.855 0.032 55 SMB14-189-62br 2.049 0.017 56 SMB14-189-68br 2.094 0.028 57 SMB14-189-43 2.279 0.034 58 SMB14-189-07 2.287 0.041 59 SMB14-189-47 2.356 0.037 60 SMB14-189-64br 2.801 0.032 61 SMB14-189-63br 2.91 0.028 62 SMB14-189-29 2.982 0.051 63 SMB14-189-59 3.173 0.057 64 SMB14-189-65br 3.348 0.034 65 SMB14-189-28 3.61 0.058 66 SMB14-189-70 3.806 0.041 67 SMB14-189-58 3.917 0.064 68 SMB14-189-61br 5.654 0.055 69 SMB14-189-14 5.697 0.145 70 SMB14-189-66br 13.601 0.144 71 SMB14-189-67br 14.977 0.152 Isotopic ratios spot/sample [sup.206]Pb/ [+ or -] l[sigma] error [sup.238]U corr. 1 SMB14-189-06 0.0951 0.0014 0.4709 2 SMB14-189-42 0.0985 0.0011 0.6237 3 SMB14-189-26 0.0991 0.0012 0.5998 4 SMB14-189-21 0.0993 0.0014 0.4842 5 SMB14-189-71 0.1009 0.0009 0.6554 6 SMB14-189-02 0.1013 0.0013 0.6093 7 SMB14-189-15 0.1015 0.0014 0.5641 8 SMB14-189-48 0.1015 0.0014 0.7751 9 SMB14-189-69 0.1018 0.0009 0.7359 10 SMB14-189-55 0.1018 0.0014 0.6625 11 SMB14-189-34 0.1031 0.0015 0.565 12 SMB14-189-33 0.1033 0.0013 0.5974 13 SMB14-189-09 * 0.1034 0.0013 0.6925 14 SMB14-189-03 0.1036 0.0014 0.6042 15 SMB14-189-19 0.1042 0.0012 0.6248 16 SMB14-189-49 0.1042 0.0014 0.5274 17 SMB14-189-38 0.1046 0.0013 0.5469 18 SMB14-189-41 0.1048 0.0012 0.6694 19 SMB14-189-36 0.1051 0.0013 0.5536 20 SMB14-189-23 0.1052 0.0013 0.6322 21 SMB14-189-24 0.1052 0.0018 0.6516 22 SMB14-189-51 0.1053 0.0013 0.6699 23 SMB14-189-13 0.1056 0.0014 0.6145 24 SMB14-189-44 0.1064 0.0011 0.6918 25 SMB14-189-39 0.1066 0.0013 0.6154 26 SMB14-189-45 0.1069 0.0012 0.7346 27 SMB14-189-57 0.1071 0.0012 0.5727 28 SMB14-189-37 0.1077 0.0016 0.5307 29 SMB14-189-40 0.1079 0.0014 0.6386 30 SMB14-189-31 0.1092 0.0013 0.705 31 SMB14-189-17 0.1097 0.0014 0.5947 32 SMB14-189-11 0.1098 0.0013 0.649 33 SMB14-189-22 0.1098 0.0013 0.6539 34 SMB14-189-52 0.11 0.0014 0.6325 35 SMB14-189-08 0.1102 0.0014 0.6723 36 SMB14-189-27 0.1108 0.0014 0.6698 37 SMB14-189-32 0.1112 0.0013 0.6231 38 SMB14-189-10 0.1113 0.0018 0.4787 39 SMB14-189-16 0.1115 0.0013 0.7347 40 SMB14-189-25 0.112 0.0014 0.7368 41 SMB14-189-30 0.113 0.0013 0.6731 42 SMB14-189-20 0.1135 0.0013 0.6711 43 SMB14-189-60 0.1139 0.0012 0.6971 44 SMB14-189-53 0.1143 0.0013 0.5894 45 SMB14-189-35 0.1152 0.0013 0.702 46 SMB14-189-54 0.1225 0.0016 0.6878 47 SMB14-189-12 0.1237 0.0017 0.6091 48 SMB14-189-50 0.124 0.0016 0.5881 49 SMB14-189-01 0.1254 0.0016 0.7144 50 SMB14-189-56 0.1372 0.0024 0.7139 51 SMB14-189-18 0.1677 0.0018 0.7651 52 SMB14-189-46 0.1689 0.0033 0.4709 53 SMB14-189-04 0.1719 0.0039 0.4552 54 SMB14-189-05 0.1777 0.002 0.6514 55 SMB14-189-62br 0.1929 0.0014 0.8311 56 SMB14-189-68br 0.1994 0.0024 0.904 57 SMB14-189-43 0.2056 0.0024 0.7671 58 SMB14-189-07 0.2078 0.0023 0.6346 59 SMB14-189-47 0.2083 0.0025 0.7708 60 SMB14-189-64br 0.2317 0.0021 0.7916 61 SMB14-189-63br 0.2408 0.0019 0.8141 62 SMB14-189-29 0.2487 0.0029 0.6921 63 SMB14-189-59 0.252 0.0037 0.814 64 SMB14-189-65br 0.2568 0.0021 0.7979 65 SMB14-189-28 0.2727 0.0034 0.7655 66 SMB14-189-70 0.2794 0.0023 0.7701 67 SMB14-189-58 0.2844 0.0032 0.6988 68 SMB14-189-61br 0.3452 0.0028 0.8421 69 SMB14-189-14 0.3554 0.0058 0.6464 70 SMB14-189-66br 0.5255 0.0044 0.8003 71 SMB14-189-67br 0.5384 0.0043 0.793 Apparent age (Ma) summary spot/sample [sup.207]Pb/ [+ or -] l[sigma] [sup.207]Pb/ [sup.206]U [sup.235]U 1 SMB14-189-06 1030 56 686 2 SMB14-189-42 655 31 616 3 SMB14-189-26 693 33 627 4 SMB14-189-21 666 55 622 5 SMB14-189-71 623 23 621 6 SMB14-189-02 676 34 634 7 SMB14-189-15 683 42 636 8 SMB14-189-48 635 25 626 9 SMB14-189-69 621 17 624 10 SMB14-189-55 684 33 638 11 SMB14-189-34 594 44 624 12 SMB14-189-33 576 38 621 13 SMB14-189-09 * 746 27 659 14 SMB14-189-03 619 39 632 15 SMB14-189-19 743 31 662 16 SMB14-189-49 652 45 642 17 SMB14-189-38 543 40 620 18 SMB14-189-41 610 28 636 19 SMB14-189-36 659 38 648 20 SMB14-189-23 949 30 717 21 SMB14-189-24 666 43 650 22 SMB14-189-51 612 29 638 23 SMB14-189-13 483 38 612 24 SMB14-189-44 702 23 663 25 SMB14-189-39 660 34 655 26 SMB14-189-45 664 23 657 27 SMB14-189-57 590 33 641 28 SMB14-189-37 598 49 646 29 SMB14-189-40 678 34 664 30 SMB14-189-31 661 26 667 31 SMB14-189-17 713 36 681 32 SMB14-189-11 680 29 673 33 SMB14-189-22 697 30 678 34 SMB14-189-52 614 34 660 35 SMB14-189-08 664 30 672 36 SMB14-189-27 655 29 672 37 SMB14-189-32 696 30 684 38 SMB14-189-10 755 60 698 39 SMB14-189-16 700 23 686 40 SMB14-189-25 791 24 710 41 SMB14-189-30 668 27 685 42 SMB14-189-20 649 27 683 43 SMB14-189-60 704 24 697 44 SMB14-189-53 919 32 753 45 SMB14-189-35 651 25 690 46 SMB14-189-54 738 28 743 47 SMB14-189-12 760 38 754 48 SMB14-189-50 821 36 771 49 SMB14-189-01 798 27 771 50 SMB14-189-56 937 36 859 51 SMB14-189-18 1015 19 1004 52 SMB14-189-46 1060 73 1023 53 SMB14-189-04 1009 88 1018 54 SMB14-189-05 1088 26 1065 55 SMB14-189-62br 1122 9 1132 56 SMB14-189-68br 1099 11 1147 57 SMB14-189-43 1207 19 1206 58 SMB14-189-07 1193 27 1208 59 SMB14-189-47 1246 19 1229 60 SMB14-189-64br 1375 13 1356 61 SMB14-189-63br 1374 11 1384 62 SMB14-189-29 1359 24 1403 63 SMB14-189-59 1453 20 1451 64 SMB14-189-65br 1520 12 1492 65 SMB14-189-28 1548 19 1552 66 SMB14-189-70 1601 13 1594 67 SMB14-189-58 1622 22 1617 68 SMB14-189-61br 1938 9 1924 69 SMB14-189-14 1900 35 1931 70 SMB14-189-66br 2722 10 2722 71 SMB14-189-67br 2840 10 2814 Apparent age (Ma) summary spot/sample [+ or -] l[sigma] [sup.206]Pb/ la [sup.238]U 1 SMB14-189-06 16 586 2 SMB14-189-42 9 606 3 SMB14-189-26 9 609 4 SMB14-189-21 14 610 5 SMB14-189-71 7 620 6 SMB14-189-02 10 622 7 SMB14-189-15 11 623 8 SMB14-189-48 8 623 9 SMB14-189-69 5 625 10 SMB14-189-55 10 625 11 SMB14-189-34 12 633 12 SMB14-189-33 10 634 13 SMB14-189-09 * 9 634 14 SMB14-189-03 11 635 15 SMB14-189-19 9 639 16 SMB14-189-49 12 639 17 SMB14-189-38 10 642 18 SMB14-189-41 8 643 19 SMB14-189-36 10 644 20 SMB14-189-23 10 645 21 SMB14-189-24 13 645 22 SMB14-189-51 8 645 23 SMB14-189-13 10 647 24 SMB14-189-44 7 652 25 SMB14-189-39 10 653 26 SMB14-189-45 8 655 27 SMB14-189-57 9 656 28 SMB14-189-37 13 659 29 SMB14-189-40 10 660 30 SMB14-189-31 8 668 31 SMB14-189-17 11 671 32 SMB14-189-11 9 672 33 SMB14-189-22 9 672 34 SMB14-189-52 10 673 35 SMB14-189-08 9 674 36 SMB14-189-27 9 677 37 SMB14-189-32 9 680 38 SMB14-189-10 17 680 39 SMB14-189-16 8 681 40 SMB14-189-25 9 684 41 SMB14-189-30 8 690 42 SMB14-189-20 9 693 43 SMB14-189-60 8 695 44 SMB14-189-53 10 698 45 SMB14-189-35 8 703 46 SMB14-189-54 10 745 47 SMB14-189-12 12 752 48 SMB14-189-50 11 753 49 SMB14-189-01 10 762 50 SMB14-189-56 14 829 51 SMB14-189-18 9 1000 52 SMB14-189-46 27 1006 53 SMB14-189-04 32 1023 54 SMB14-189-05 11 1054 55 SMB14-189-62br 6 1137 56 SMB14-189-68br 9 1172 57 SMB14-189-43 11 1205 58 SMB14-189-07 13 1217 59 SMB14-189-47 11 1220 60 SMB14-189-64br 8 1343 61 SMB14-189-63br 7 1391 62 SMB14-189-29 13 1432 63 SMB14-189-59 14 1449 64 SMB14-189-65br 8 1473 65 SMB14-189-28 13 1555 66 SMB14-189-70 9 1588 67 SMB14-189-58 13 1613 68 SMB14-189-61br 8 1911 69 SMB14-189-14 22 1960 70 SMB14-189-66br 10 2722 71 SMB14-189-67br 10 2777 Apparent age (Ma) summary spot/sample [+ or -] l[sigma] disc (%) 1 SMB14-189-06 8 45 2 SMB14-189-42 7 8 3 SMB14-189-26 7 13 4 SMB14-189-21 8 9 5 SMB14-189-71 5 1 6 SMB14-189-02 7 8 7 SMB14-189-15 8 9 8 SMB14-189-48 8 2 9 SMB14-189-69 5 -1 10 SMB14-189-55 8 9 11 SMB14-189-34 9 -7 12 SMB14-189-33 8 -11 13 SMB14-189-09 * 8 16 14 SMB14-189-03 8 -3 15 SMB14-189-19 7 15 16 SMB14-189-49 8 2 17 SMB14-189-38 7 -19 18 SMB14-189-41 7 -6 19 SMB14-189-36 7 2 20 SMB14-189-23 7 34 21 SMB14-189-24 11 3 22 SMB14-189-51 7 -6 23 SMB14-189-13 8 -36 24 SMB14-189-44 7 8 25 SMB14-189-39 8 1 26 SMB14-189-45 7 1 27 SMB14-189-57 7 -12 28 SMB14-189-37 9 -11 29 SMB14-189-40 8 3 30 SMB14-189-31 8 -1 31 SMB14-189-17 8 6 32 SMB14-189-11 7 1 33 SMB14-189-22 8 4 34 SMB14-189-52 8 -10 35 SMB14-189-08 8 -2 36 SMB14-189-27 8 -4 37 SMB14-189-32 7 2 38 SMB14-189-10 10 10 39 SMB14-189-16 8 3 40 SMB14-189-25 8 14 41 SMB14-189-30 8 -4 42 SMB14-189-20 8 -7 43 SMB14-189-60 7 1 44 SMB14-189-53 8 25 45 SMB14-189-35 8 -8 46 SMB14-189-54 9 -1 47 SMB14-189-12 10 1 48 SMB14-189-50 9 9 49 SMB14-189-01 9 5 50 SMB14-189-56 14 12 51 SMB14-189-18 10 2 52 SMB14-189-46 18 6 53 SMB14-189-04 22 -1 54 SMB14-189-05 11 3 55 SMB14-189-62br 7 -1 56 SMB14-189-68br 13 -7 57 SMB14-189-43 13 0 58 SMB14-189-07 12 -2 59 SMB14-189-47 13 2 60 SMB14-189-64br 11 3 61 SMB14-189-63br 10 -1 62 SMB14-189-29 15 -6 63 SMB14-189-59 19 0 64 SMB14-189-65br 11 3 65 SMB14-189-28 17 -1 66 SMB14-189-70 12 1 67 SMB14-189-58 16 1 68 SMB14-189-61br 13 2 69 SMB14-189-14 28 -4 70 SMB14-189-66br 19 0 71 SMB14-189-67br 18 3 Field relations, age, and tectonic setting of metamorphic and Plutonic rocks in the Creignish Hills--North Mountain area, southwestern ... Notes: * High Sr; analyses are arranged from lowest to highest [sup.206]Pb/[sup.238]U age; br = brownish; error correl. = Error correlation coefficient for concordia coordinates; disc (%) = discordance; relation between ages and concordia coordinates: Y = [sup.206]Pb/[sup.238]U = EXP(L238*([sup.206/238]Age)) /1; X = [sup.207]Pb/[sup.235]U = EXP(L23 5*([sup.207/235]Age)) /1. [sup.207]Pb/[sup.206]Pb = 137.88*X/Y; U decay constants (L238 & L235) from Jaffey et al. (1971).
Samples SMB02-79, SMB02-81, and SMB02-83 were processed at the ROM using a jaw crusher for initial crushing, a disk mill for sample reduction to sand-sized particles, and a Wilfley table, heavy liquids (bromoform and methylene iodide) and a Frantz magnetic separator for isolation of heavy mineral fractions. Zircon was selected from the least paramagnetic fraction by hand picking under a binocular microscope. All picked zircons were given an air abrasion treatment to eliminate cracked grains and remove exterior surfaces (Krogh 1982). Final selection of grains for analysis was then made. The weight of each fraction (typically one or two grains) was either estimated or measured using a microbalance. As the weights are normally small (generally <3 micrograms), both the measured and estimated weights are accurate only to about [+ or -]50%. However, this only affects the calculation of Pb and U concentrations and has no influence on age data.
The selected grains were washed in 4N and then 7N HN03 and then loaded into Teflon bombs with HF and a measured amount of [sup.205]Pb-[sup.235]U isotopic tracer solution (Krogh 1973). Dissolution occurred over four to five days at 195[degrees]C. No chemical isolation of U and Pb was carried out on the dissolved grains. Fractions were dried down with phosphoric acid and then loaded with silica gel onto outgassed rhenium filaments. The isotopic compositions of Pb and U were measured using a single Daly collector with a pulse counting detector in a solid source VG354 mass spectrometer. A detector mass discrimination of 0.14% per atomic mass unit (AMU) and a deadtime of 22.5 nsec were employed for Daly detector measurements. A thermal source mass discrimination correction of 0.1% per atomic mass unit for both Pb and U was also used.
The assigned laboratory blank for U is 0.2 pg. Total measured common Pb in samples was below 1 pg in nearly all cases and was assigned the isotopic composition of the lab blank. Error estimates were calculated by propagating known sources of analytical uncertainty for each analysis including ratio variability (within run), uncertainty in the fractionation correction, and uncertainties in the isotopic composition of laboratory blank. Decay constants used are those of Jaffey et al. (1971). All uncertainties in the text below and on concordia diagrams are given at the 95% confidence level. Discordia lines and concordia intercept ages were calculated by the method of Davis (1982) using the in-house program ROMAGE. Average [sup.207]Pb/[sup.206]Pb ages or [sup.206]Pb/[sup.238]U ages were also calculated using ROMAGE. Probability of fit measures the scatter of analyses with respect to a Pb-loss line. A value of around 50% would be expected for unimodal data sets with correctly chosen analytical errors. Low probability of fit suggests real differences in Pb loss history and/or zircon crystallization age. Data are presented in Table B1.
Table B1. U-Pb isotopic data for meta-igneous and igneous samples. Isotopic ratios Weight U Sample Fraction (mg) (ppm) SMB02-81 Chuggin Road tonalitic orthogneiss jk9pl22 Z1 clr 3:1 subh pr incl (1) 0.0013 137 jk9pl23 Z2 clr 3:1 subh pr (1) 0.0017 228 jk9pl24 Z3 clr 3:1 subh pr (1) 0.0012 343 jk9pl25 Z4 clr 3:1 subh pr (3) 0.0020 121 jkl0pl4 Z5 clr eq subh pr (1) 0.0063 133 jklOpl5 Z6 clr eq mf euh pr (4) 0.0052 89 SMB02-79 biotite granite, Creignish Hills pluton jk9p73 Z1 2:1 euh clr pr incl (1) 0.0062 173 jk9p74 Z2 2:1 euh clr pr incl (1) 0.0048 83 jk9p75 Z3 2:1 euh clr pr (2) 0.0042 95 jk9plll Z4 2:1 euh clr pr (3) 0.0076 77 SMB02-83 biotite granite, West Bay pluton jk9p76 Z1 euh 2:1 clr pr incl (1) 0.0057 104 jk9p77 Z2 euh 2:1 clr pr (2) 0.0030 165 jk9p78 Z3 euh 2:1 clr pr (2) 0.0030 197 Isotopic ratios Th/ total Pb Sample Fraction U (pg) SMB02-81 Chuggin Road tonalitic orthogneiss jk9pl22 Z1 clr 3:1 subh pr incl (1) 0.51 17.1 jk9pl23 Z2 clr 3:1 subh pr (1) 0.60 37.7 jk9pl24 Z3 clr 3:1 subh pr (1) 0.49 37.9 jk9pl25 Z4 clr 3:1 subh pr (3) 0.72 24.4 jkl0pl4 Z5 clr eq subh pr (1) 1.10 153.1 jklOpl5 Z6 clr eq mf euh pr (4) 0.31 73.4 SMB02-79 biotite granite, Creignish Hills pluton jk9p73 Z1 2:1 euh clr pr incl (1) 0.52 99.3 jk9p74 Z2 2:1 euh clr pr incl (1) 0.58 37.8 jk9p75 Z3 2:1 euh clr pr (2) 0.53 37.5 jk9plll Z4 2:1 euh clr pr (3) 0.63 54.1 SMB02-83 biotite granite, West Bay pluton jk9p76 Z1 euh 2:1 clr pr incl (1) 0.51 55.0 jk9p77 Z2 euh 2:1 clr pr (2) 0.54 46.4 jk9p78 Z3 euh 2:1 clr pr (2) 0.51 54.9 Isotopic ratios common [sup.206]Pb/ Sample Fraction Pb (pg) [sup.204]Pb SMB02-81 Chuggin Road tonalitic orthogneiss jk9pl22 Z1 clr 3:1 subh pr incl (1) 0.2 5097 jk9pl23 Z2 clr 3:1 subh pr (1) 0.5 4605 jk9pl24 Z3 clr 3:1 subh pr (1) 0.1 20104 jk9pl25 Z4 clr 3:1 subh pr (3) 0.2 7519 jkl0pl4 Z5 clr eq subh pr (1) 0.3 31394 jklOpl5 Z6 clr eq mf euh pr (4) 0.2 29824 SMB02-79 biotite granite, Creignish Hills pluton jk9p73 Z1 2:1 euh clr pr incl (1) 0.5 11557 jk9p74 Z2 2:1 euh clr pr incl (1) 0.3 8404 jk9p75 Z3 2:1 euh clr pr (2) 7.3 332 jk9plll Z4 2:1 euh clr pr (3) 0.2 21183 SMB02-83 biotite granite, West Bay pluton jk9p76 Z1 euh 2:1 clr pr incl (1) 0.4 9003 jk9p77 Z2 euh 2:1 clr pr (2) 0.2 14458 jk9p78 Z3 euh 2:1 clr pr (2) 0.2 14490 Isotopic ratios [sup.206]Pb/ [+ or -] Sample Fraction [sup.238]U 2[sigma] SMB02-81 Chuggin Road tonalitic orthogneiss jk9pl22 Z1 clr 3:1 subh pr incl (1) 0.0916 0.0004 jk9pl23 Z2 clr 3:1 subh pr (1) 0.0906 0.0004 jk9pl24 Z3 clr 3:1 subh pr (1) 0.0884 0.0004 jk9pl25 Z4 clr 3:1 subh pr (3) 0.0911 0.0004 jkl0pl4 Z5 clr eq subh pr (1) 0.1506 0.0005 jklOpl5 Z6 clr eq mf euh pr (4) 0.1572 0.0006 SMB02-79 biotite granite, Creignish Hills pluton jk9p73 Z1 2:1 euh clr pr incl (1) 0.0883 0.0005 jk9p74 Z2 2:1 euh clr pr incl (1) 0.0889 0.0004 jk9p75 Z3 2:1 euh clr pr (2) 0.0897 0.0004 jk9plll Z4 2:1 euh clr pr (3) 0.0888 0.0005 SMB02-83 biotite granite, West Bay pluton jk9p76 Z1 euh 2:1 clr pr incl (1) 0.0889 0.0003 jk9p77 Z2 euh 2:1 clr pr (2) 0.0887 0.0003 jk9p78 Z3 euh 2:1 clr pr (2) 0.0887 0.0005 Isotopic ratios [sup.207]Pb/ [+ or -] Sample Fraction [sup.235]U 2[sigma] SMB02-81 Chuggin Road tonalitic orthogneiss jk9pl22 Z1 clr 3:1 subh pr incl (1) 0.742 0.004 jk9pl23 Z2 clr 3:1 subh pr (1) 0.734 0.004 jk9pl24 Z3 clr 3:1 subh pr (1) 0.718 0.003 jk9pl25 Z4 clr 3:1 subh pr (3) 0.739 0.003 jkl0pl4 Z5 clr eq subh pr (1) 1.461 0.005 jklOpl5 Z6 clr eq mf euh pr (4) 1.681 0.006 SMB02-79 biotite granite, Creignish Hills pluton jk9p73 Z1 2:1 euh clr pr incl (1) 0.714 0.004 jk9p74 Z2 2:1 euh clr pr incl (1) 0.720 0.003 jk9p75 Z3 2:1 euh clr pr (2) 0.725 0.025 jk9plll Z4 2:1 euh clr pr (3) 0.717 0.003 SMB02-83 biotite granite, West Bay pluton jk9p76 Z1 euh 2:1 clr pr incl (1) 0.718 0.003 jk9p77 Z2 euh 2:1 clr pr (2) 0.718 0.002 jk9p78 Z3 euh 2:1 clr pr (2) 0.716 0.004 Apparent age (Ma) summary [sup.206]Pb/ [+ or -] Sample Fraction [sup.238]U 2[sigma] SMB02-81 Chuggin Road tonalitic orthogneiss jk9pl22 Z1 clr 3:1 subh pr incl (1) 564.9 2.4 jk9pl23 Z2 clr 3:1 subh pr (1) 558.9 2.5 jk9pl24 Z3 clr 3:1 subh pr (1) 546.2 2.3 jk9pl25 Z4 clr 3:1 subh pr (3) 562.2 2.4 jkl0pl4 Z5 clr eq subh pr (1) 904.1 3.0 jklOpl5 Z6 clr eq mf euh pr (4) 941.0 3.2 SMB02-79 biotite granite, Creignish Hills pluton jk9p73 Z1 2:1 euh clr pr incl (1) 545.8 3.1 jk9p74 Z2 2:1 euh clr pr incl (1) 549.2 2.2 jk9p75 Z3 2:1 euh clr pr (2) 553.8 2.4 jk9plll Z4 2:1 euh clr pr (3) 548.1 2.9 SMB02-83 biotite granite, West Bay pluton jk9p76 Z1 euh 2:1 clr pr incl (1) 549.2 1.9 jk9p77 Z2 euh 2:1 clr pr (2) 548.5 1.7 jk9p78 Z3 euh 2:1 clr pr (2) 547.7 2.7 Apparent age (Ma) summary [sup.207]Pb/ [+ or -] Sample Fraction [sup.206]Pb 2[sigma] SMB02-81 Chuggin Road tonalitic orthogneiss jk9pl22 Z1 clr 3:1 subh pr incl (1) 559.3 9.2 jk9pl23 Z2 clr 3:1 subh pr (1) 558.9 7.2 jk9pl24 Z3 clr 3:1 subh pr (1) 562.9 4.7 jk9pl25 Z4 clr 3:1 subh pr (3) 561.1 7.7 jkl0pl4 Z5 clr eq subh pr (1) 939.8 3.6 jklOpl5 Z6 clr eq mf euh pr (4) 1136.3 4.2 SMB02-79 biotite granite, Creignish Hills pluton jk9p73 Z1 2:1 euh clr pr incl (1) 551.4 4.2 jk9p74 Z2 2:1 euh clr pr incl (1) 555.4 6.9 jk9p75 Z3 2:1 euh clr pr (2) 553.4 69.6 jk9plll Z4 2:1 euh clr pr (3) 553.1 7.8 SMB02-83 biotite granite, West Bay pluton jk9p76 Z1 euh 2:1 clr pr incl (1) 551.2 4.6 jk9p77 Z2 euh 2:1 clr pr (2) 554.2 3.9 jk9p78 Z3 euh 2:1 clr pr (2) 552.1 4.3 Apparent age (Ma) summary disc. Sample Fraction (%) SMB02-81 Chuggin Road tonalitic orthogneiss jk9pl22 Z1 clr 3:1 subh pr incl (1) -1.0 jk9pl23 Z2 clr 3:1 subh pr (1) 0.0 jk9pl24 Z3 clr 3:1 subh pr (1) 3.1 jk9pl25 Z4 clr 3:1 subh pr (3) -0.2 jkl0pl4 Z5 clr eq subh pr (1) 4.1 jklOpl5 Z6 clr eq mf euh pr (4) 18.5 SMB02-79 biotite granite, Creignish Hills pluton jk9p73 Z1 2:1 euh clr pr incl (1) 1.1 jk9p74 Z2 2:1 euh clr pr incl (1) 1.2 jk9p75 Z3 2:1 euh clr pr (2) -0.1 jk9plll Z4 2:1 euh clr pr (3) 0.9 SMB02-83 biotite granite, West Bay pluton jk9p76 Z1 euh 2:1 clr pr incl (1) 0.4 jk9p77 Z2 euh 2:1 clr pr (2) 1.2 jk9p78 Z3 euh 2:1 clr pr (2) 0.8 Notes: Data are from abraded zircons (Krogh 1982). Decay constants used are those of Jaffey et al. (1971). Abbreviations: Z = zircon; euh = euhedral; subh = subhedral; eq = equant; pr = prism; clr = colourless; lbr = light brown; mf = multifaceted; incl = inclusions; 2:1, 3:1, etc. = length:breadth ratio; Number in brackets indicates number of grains analysed. Th/U--based on radiogenic [sup.208]Pb/ [sup.206]Pb ratio and [sup.207]Pb/[sup.206]Pb age. Common Pb//total common Pb, blank isotopic composition: [sup.206]Pb/204Pb = 18.221, [sup.207]Pb/2MPb = 15.612,[sup.208]Pb/204Pb = 39.36. disc. (%) = percent discordance for the given [sup.207]Pb/[sup.206]Pb age.
Chips of sample SMB06-111 were crushed gently, then sieved and washed. Typically, about 20 muscovite grains or grain fragments (-0.3-1.0 mm in diameter) were handpicked from each sample. The selected grains were placed individually into holes machined in aluminum disks and then irradiated in the McMaster University nuclear reactor. The flux monitors were the hornblende standard, MMhb-1, and the sanidine standard, Fish Canyon tuff. An age of 525 Ma was used for the former and 28.2 Ma for the latter. The age of the Fish Canyon standard is the value proposed recently by Kuiper et al. (2008), and the age of MMhb-1 is the one obtained from intercalibration of the two standards in the Dalhousie laboratory. Our value for MMhb-1 agrees with the ca. 526 Ma age derived from the intercalibrations of Renne et al. (1998) and with the ca. 523 Ma age suggested by Schoene and Bowring (2006) based on their U-Pb data on co-existing minerals. For whole-grain laser analysis, our Nd-YAG system was operated in the IR (at 1064 nm). In continuous mode with the beam expanded to approximately cover the grain, power was increased in a series of steps until complete fusion was achieved. All isotopic analyses were made using a VG 3600 mass spectrometer. Data are presented in Table C1.
Table C1. Total fusion data for muscovite grains in sample SMB06-111. [sup.39]Ar % [sup.37]Ar/ [sup.36]Ar/ (mV) ATM [sup.39]Ar [sup.40]Ar J = 0.0024 [+ or -] 2.4E-05 1 G56-01 4 14.9 0 0.00051 2 G56-02 19.1 6.8 0 0.00023 3 G56-03 11 8.7 0 0.0003 4 G56-04 8.2 9 0 0.00031 5 G56-05 11.4 8.4 0 0.00029 6 G56-06 9.2 8.8 0 0.0003 7 G56-07 8.6 9.1 0 0.00031 8 G56-08 6.5 12 0 0.00041 9 G56-09 12.3 7.6 0 0.00026 10 G56-10 14.6 6.9 0 0.00023 11 G56-11 11.8 8 0 0.00027 12 G56-12 13.6 7.5 0 0.00026 13 G56-13 12.8 6.9 0 0.00023 14 G56-14 5.8 13.3 0 0.00045 15 G56-15 8.3 9.5 0 0.00032 16 G56-16 12.5 7.9 0 0.00027 17 G56-17 6.1 12.2 0 0.00041 18 G56-18 5.1 12.5 0 0.00043 19 G56-19 6.5 11.1 0 0.00038 20 G56-20 8.7 9.4 0 0.00032 21 G56-21 10.6 7.9 0 0.00027 22 G56-22 11.9 7.9 0 0.00027 23 G56-23 15.4 5.9 0 0.0002 [sup.39]Ar/ % ITC Age (Ma) [+ or -] [sup.40]Ar l[sigma] J = 0.0024 [+ or -] 2.4E-05 1 G56-01 0.007 0 476.1 [+ or -] 3.1 2 G56-02 0.008 0 458.7 [+ or -] 2.5 3 G56-03 0.008 0 437.6 [+ or -] 2.4 4 G56-04 0.007 0 462.4 [+ or -] 2.5 5 G56-05 0.008 0 459.2 [+ or -] 2.5 6 G56-06 0.008 0 445.0 [+ or -] 2.3 7 G56-07 0.007 0 460.7 [+ or -] 2.4 8 G56-08 0.007 0 456.4 [+ or -] 2.6 9 G56-09 0.007 0 472.4 [+ or -] 2.5 10 G56-10 0.008 0 457.6 [+ or -] 2.4 11 G56-11 0.008 0 444.2 [+ or -] 2.3 12 G56-12 0.008 0 429.6 [+ or -] 2.2 13 G56-13 0.008 0 441.8 [+ or -] 2.2 14 G56-14 0.008 0 438.1 [+ or -] 2.6 15 G56-15 0.008 0 432.8 [+ or -] 2.3 16 G56-16 0.008 0 449.6 [+ or -] 2.4 17 G56-17 0.007 0 473.9 [+ or -] 2.6 18 G56-18 0.007 0 487.8 [+ or -] 2.8 19 G56-19 0.007 0 468.6 [+ or -] 2.7 20 G56-20 0.007 0 476.1 [+ or -] 2.6 21 G56-21 0.007 0 478.0 [+ or -] 2.4 22 G56-22 0.008 0 449.4 [+ or -] 2.3 23 G56-23 0.007 0 475.7 [+ or -] 2.3 Notes: [sup.37]Ar/[sup.39]Ar, [sup.36]Ar/[sup.40]Ar, and [sup.39]Ar/ [sup.40]Ar are corrected for mass spectrometer discrimination, interfering isotopes, and system blanks; % IIC = interfering isotopes correction.
New analytical data are presented in Table Dl, which also includes the analytical methods as footnotes. Sources of other chemical data are summarized in Table D2.
Table D1. Previously unplublished chemical data. ** Sample # SMB[0.sub.2]-81a SCGQ1 SCGQ2 Unit Chuggin Road West Bay West Bay N 5094125 5063307 * 5063307 * E 642350 631658 * 631658 * Si[0.sub.2] (wt.%) 67.92 74.82 71.93 Ti[0.sub.2] 0.443 0.24 0.36 [Al.sub.2][O.sub.3] 15.08 12.7 13.76 [Fe.sub.2][O.sub.3] 4.68 1.3 2.75 MnO 0.066 0.[0.sub.2] 0.06 MgO 1.71 0.13 0.65 CaO 4.42 0.13 1.21 [Na.sub.2]O 2.89 2.33 3.12 [K.sub.2]O 1.57 5.36 4.64 [P.sub.2][O.sub.5] 0.143 0.06 0.1 LOI 0.95 1.5 1.1 Total 99.87 98.59 99.68 Ba (ppm) 497 222 453 Rb 59 218 206 Sr 357 32 130 Y 15 38 60 Zr 151 178 197 Nb 12 14 18 Th 3 29 22 Pb 9 18 31 Ga 15 14 15 Zn 49 17 60 Cu 63 5 5 Ni <3 5 7 V 91 7 26 Cr 8 6 7 Notes: Sample locations are UTMs (N, northing; E, easting) in Grid Zone 20T, NAD83. * Location estimated from Google Earth. ** Analyses by X-ray fluorescence at the Regional Geochemical Centre, Saint Mary's University, Halifax, Nova Scotia. Major elements and some trace elements were measured using fused glass disks and other trace elements were measured using pressed powder pellets. Analytical error is generally less than 5% for major elements and 2-10% for trace elements. Fe203l is total Fe as Fe203. LOI is loss on ignition at 1000[degrees]C. Table D2. Sources of chemical data for plutonic units. Data sources are in reference list. Sample # Unit JC148 Skye Mountain Road granite/granodiorite SKI 80 Skye Mountain Road granite/granodiorite SK181 Skye Mountain Road granite/granodiorite SK184 Skye Mountain Road granite/granodiorite SK270 Skye Mountain Road granite/granodiorite SK271A Skye Mountain Road granite/granodiorite SK271B Skye Mountain Road granite/granodiorite SK275 Skye Mountain Road granite/granodiorite SK282 Skye Mountain Road granite/granodiorite JC555 River Denys tonalite JC806 River Denys tonalite Kep-063 River Denys tonalite Kep-064 River Denys tonalite Kep-066 River Denys tonalite Kep-128 River Denys tonalite CH03 Creignish Hills pluton (tonalite) CH10 Creignish Hills pluton (tonalite) F14-1000 Creignish Hills pluton (tonalite) F14-1005 Creignish Hills pluton (tonalite) F14-1010 Creignish Hills pluton (tonalite) F14-1047 Creignish Hills pluton (tonalite) F14-1054 Creignish Hills pluton (tonalite) F14-1070 Creignish Hills pluton (tonalite) Kep-100 Creignish Hills pluton (tonalite) Kep-101 Creignish Hills pluton (tonalite) Kep-110 Creignish Hills pluton (tonalite) Kep-150 Creignish Hills pluton (tonalite) Kep-222 Creignish Hills pluton (tonalite) Kep-325 Creignish Hills pluton (tonalite) F14-1029 Creignish Hills pluton (fine-grained granite) F14-1103 Creignish Hills pluton (fine-grained granite) F14-1107 Creignish Hills pluton (fine-grained granite) F14-1117 Creignish Hills pluton (fine-grained granite) F14-1118 Creignish Hills pluton (fine-grained granite) F14-1134 Creignish Hills pluton (fine-grained granite) FI 1-1011 Creignish Hills pluton (granodiorite) FI1 1021 Creignish Hills pluton (granodiorite) Fll-1038 Creignish Hills pluton (granodiorite) Fll-1078 Creignish Hills pluton (granodiorite) F14-1210 Creignish Hills pluton (granodiorite) F14-1211 Creignish Hills pluton (granodiorite) CH16 Creignish Hills pluton (granite) CH24 Creignish Hills pluton (granite) CH27 Creignish Hills pluton (granite) Fll-1016 Creignish Hills pluton (granite) Fll-1041 Creignish Hills pluton (granite) FI1-1042 Creignish Hills pluton (granite) F11-1051 Creignish Hills pluton (granite) F14-1041 Creignish Hills pluton (granite) F14-1078 Creignish Hills pluton (granite) F14-1098 Creignish Hills pluton (granite) F14-1099 Creignish Hills pluton (granite) F14-1143 Creignish Hills pluton (granite) F14-1158 Creignish Hills pluton (granite) F14-1224 Creignish Hills pluton (granite) SMBO2-810 Chuggin Road orthogneiss DS09-02 Lewis Mountain tonalite DS09-09 Lewis Mountain tonalite DS09-12A Lewis Mountain tonalite DS09-54A Lewis Mountain diorite DS09-55 Lewis Mountain diorite DS09-59 Lewis Mountain diorite DS09-04 Lewis Mountain monzogranite DS09-18A Lewis Mountain monzogranite DS09-25 Lewis Mountain monzogranite DS09-28 Lewis Mountain monzogranite DS09-29 Lewis Mountain monzogranite DS09-30 Lewis Mountain monzogranite DS09-62 Lewis Mountain monzogranite DS09-63 Lewis Mountain monzogranite MJ-018 Marble Mountain pluton MJ-103 Marble Mountain pluton MJ-175 Marble Mountain pluton MJ-181 Marble Mountain pluton MJ-189 Marble Mountain pluton MJ-195 Marble Mountain pluton MJ-363 Marble Mountain pluton MJ-370 Marble Mountain pluton MJ-058 Marble Mountain pluton MJ-068 Marble Mountain pluton MJ-072 Marble Mountain pluton MJ-080 Marble Mountain pluton MJ-097 Marble Mountain pluton MJ-151 Marble Mountain pluton MJ-378 Marble Mountain pluton MJ-199 Big Brook granodiorite MJ-208 Big Brook granodiorite MJ-336 Big Brook granodiorite MJ-340 Big Brook granodiorite MJ-394 Big Brook granodiorite MJ-500 Big Brook granodiorite MJ-501 Big Brook granodiorite MJ-421 Mill Brook quartz diorite MJ-257 West Bay granite MJ-267 West Bay granite MJ-268 West Bay granite MJ-285 West Bay granite MJ-290 West Bay granite MJ-316 West Bay granite MJ-581 West Bay granite MJ-588 West Bay granite MJ-591 West Bay granite SCGQ1 West Bay granite SCGQ2 West Bay granite Sample # Data source JC148 Campbell 1990 SKI 80 Horton 1994 SK181 Horton 1994 SK184 Horton 1994 SK270 Horton 1994 SK271A Horton 1994 SK271B Horton 1994 SK275 Horton 1994 SK282 Horton 1994 JC555 Campbell 1990 JC806 Campbell 1990 Kep-063 Keppie et al. 2000 Kep-064 Keppie et al. 2000 Kep-066 Keppie et al. 2000 Kep-128 Keppie et al. 2000 CH03 White et al. 1990 CH10 White et al. 1990 F14-1000 White et al. 1990 F14-1005 White et al. 1990 F14-1010 White et al. 1990 F14-1047 White et al. 1990 F14-1054 White et al. 1990 F14-1070 White et al. 1990 Kep-100 Keppie et al. 2000 Kep-101 Keppie et al. 2000 Kep-110 Keppie et al. 2000 Kep-150 Keppie et al. 2000 Kep-222 Keppie et al. 2000 Kep-325 Keppie et al. 2000 F14-1029 White et al. 1990 F14-1103 White et al. 1990 F14-1107 White et al. 1990 F14-1117 White et al. 1990 F14-1118 White et al. 1990 F14-1134 White etal. 1990 FI 1-1011 White et al. 1990 FI1 1021 White etal. 1990 Fll-1038 White et al. 1990 Fll-1078 White et al. 1990 F14-1210 White et al. 1990 F14-1211 White et al. 1990 CH16 White et al. 1990 CH24 White et al. 1990 CH27 White et al. 1990 Fll-1016 White et al. 1990 Fll-1041 White et al. 1990 FI1-1042 White et al. 1990 F11-1051 White etal. 1990 F14-1041 White et al. 1990 F14-1078 White et al. 1990 F14-1098 White et al. 1990 F14-1099 White etal. 1990 F14-1143 White et al. 1990 F14-1158 White etal. 1990 F14-1224 White et al. 1990 SMBO2-810 This study Table D1 DS09-02 Swanton 2010 DS09-09 Swanton 2010 DS09-12A Swanton 2010 DS09-54A Swanton 2010 DS09-55 Swanton 2010 DS09-59 Swanton 2010 DS09-04 Swanton 2010 DS09-18A Swanton 2010 DS09-25 Swanton 2010 DS09-28 Swanton 2010 DS09-29 Swanton 2010 DS09-30 Swanton 2010 DS09-62 Swanton 2010 DS09-63 Swanton 2010 MJ-018 Justino 1991 MJ-103 Justino 1991 MJ-175 Justino 1991 MJ-181 Justino 1991 MJ-189 Justino 1991 MJ-195 Justino 1991 MJ-363 Justino 1991 MJ-370 Justino 1991 MJ-058 Justino 1991 MJ-068 Justino 1991 MJ-072 Justino 1991 MJ-080 Justino 1991 MJ-097 Justino 1991 MJ-151 Justino 1991 MJ-378 Justino 1991 MJ-199 Justino 1991 MJ-208 Justino 1991 MJ-336 Justino 1991 MJ-340 Justino 1991 MJ-394 Justino 1991 MJ-500 Justino 1991 MJ-501 Justino 1991 MJ-421 Justino 1991 MJ-257 Justino 1991 MJ-267 Justino 1991 MJ-268 Justino 1991 MJ-285 Justino 1991 MJ-290 Justino 1991 MJ-316 Justino 1991 MJ-581 Justino 1991 MJ-588 Justino 1991 MJ-591 Justino 1991 SCGQ1 This study Table Dl SCGQ2 This study Table Dl
Chris E. White (1) *, Sandra M. Barr (2), Donald W. Davis (3), David S. Swanton (2), John W.F. Ketchum (4), and Peter H. Reynolds (5)
(1.) Nova Scotia Department of Natural Resources, Halifax, Nova Scotia B3J 2T9, Canada
(2.) Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada
(3.) Department of Geology, University of Toronto, 22 Russell St., Toronto, Ontario M5S 3B1, Canada
(4.) Northwest Territories Geoscience Office, PO Box 1500, 4601-B 52 Avenue, Yellowknife, Northwest Territories X1A 2R3, Canada
(5.) Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada
* Corresponding author <email@example.com>
Date received: 11 August 2015 * Date accepted: 01 December 2015
Please note: Some tables or figures were omitted from this article.
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|Author:||White, Chris E.; Barr, Sandra M.; Davis, Donald W.; Swanton, David S.; Ketchum, John W.F.; Reynolds,|
|Date:||Jan 1, 2016|
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