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Geochemistry of Carboniferous peralkaline felsic volcanic rocks, central New Brunswick, Canada: examination of uranium potential.

ABSTRACT

Felsic volcanic rocks from the ca. 335 Ma Cumberland Hill Formation of southern New Brunswick consist of peralkaline rhyolite and trachyte. These rocks are interpreted to have undergone extensive fractional crystallization from an alkali basaltic magma, although their different geochemical signatures indicate that the rhyolite was not derived from the trachyte via continuous fractional crystallization. The rhyolite is highly enriched in incompatible trace elements including uranium (up to ca. 20 ppm). Cumberland Hill peralkaline rhyolite suggests that a potentially economic source of uranium exists in the basin, based on the elevated uranium concentrations and high potential for remobilization into surrounding sedimentary units.

RESUME

Les roches volcaniques felsiques de la Formation de Cumberland Hill, au sud du Nouveau-Brunswick, dont l'origine remonte a quelque 335 Ma se composent de rhyolite et de trachyte hyperalcalines. On estime que ces roches ont subi une cristallisation fractionnee intense a partir d'un magma de basalte alcalin, meme si leurs diverses signatures geochimiques indiquent que la rhyolite n'est pas issue de la trachyte par un processus permanent de cristallisation fractionnee. La rhyolite est fortement enrichie d'elements traces incompatibles, au nombre duquel se trouve de l'uranium (concentration qui peut atteindre environ 20 ppm). Si l'on se fie aux fortes concentrations d'uranium et a la grande possibilite de remobilisation dans les unites sedimentaires environnantes, la presence de la rhyolite hyperalcaline dans la Formation de Cumberland Hill porte a croire que le bassin pourrait contenir une source d'uranium vraisemblablement rentable.

[Traduit par la redaction]

INTRODUCTION

Igneous rocks are sparsely distributed throughout the Middle Devonian-Permian Maritimes Basin (Fyffe and Barr 1986; Dunning et al. 2002). They appear to decrease in volume through time from: (i) Middle to Late Devonian tholeiire, minor alkali basalt, bimodal gabbro and A-type to evolved I-type granite; (ii) Lower Carboniferous bimodal igneous rocks, and most sparsely, (iii) Upper Carboniferous bimodal igneous rocks (New Brunswick Department of Natural Resources 2010a). The felsic igneous rocks, as well as the interbedded lithic sandstone, conglomerate, and feldspathic sandstone of the Cumberland Hill Formation, the topic of this paper, were considered part of the Late Wesphalian Pictou Group, and consequently associated with the latter episode of magmatism based on stratigraphic relationships (Fyffe and Barr 1986). However, recent mapping indicated that these volcanic rocks lie within the Visean to early Namurian Mabou Group, and therefore are associated with the middle episode of volcanic activity (St. Peter 1997; New Brunswick Department of Natural Resources 2010a). A mid-Visean U-Pb zircon date of 335 [+ or -] 2 Ma substantiates this interpretation (New Brunswick Department of Natural Resources 2010a). Hence the felsic volcanic rocks in the Cumberland Hill Formation appear to have approximately the same age as several alkali mafic units in New Brunswick (e.g., the Hardwood Ridge, Royal Road, and Queenstown basalts), and the tholeiitic-alkali mafic rocks of the Cap aux Diables Formation in the Magdalen Islands (Barr et al. 1985; Fyffe and Barr 1986; LaFleche et al. 1998; New Brunswick Department of Natural Resources 2010a).

The mafic rocks have continental within-plate characteristics (Barr et al. 1985), and HIMU-OIB Pb-isotopic signatures that have been related either to a mantle plume (Pe-Piper and Piper 1998) or to decompression melting below a pull-apart rift (LaFleche et al. 1998). However, the plume model (Murphy et al. 1999; Keppie and Krogh 1999) appears inconsistent with U-Pb chronology of Devonian to early Carboniferous volcanic rocks within the southern Magdalen Basin (Dunning et al. 2002).

In this paper, we selected key samples from the Cumberland Hill Formation in order to petrologically and geochemically characterize the rocks and compare the rhyolite and trachyte to similar rocks elsewhere in the Maritimes Basin. This work is critical when assessing uranium potential in the Cumberland Hill Formation, as similar rocks in the Maritimes Basin have been identified as hosts of economic uranium mineralization (e.g., Dahlkamp 1993; Plant et al. 1999; Cuney 2009; Nash 2010).

GEOLOGICAL SETTING

Late Devonian to Early Permian rocks of New Brunswick comprise the western part of the Maritimes Basin of Atlantic Canada. The Maritimes Basin is a successor basin within which there are a number of sub-basins separated by fault-bounded ridges of basement (Fig. 1a). In New Brunswick, these rocks occur within deep depositional centres or subbasins, or on shallowly buried or partially exposed basement uplifts and platforms (e.g., St. Peter and Johnson 2009).

The volcanic rocks of the Cumberland Hill Formation crop out as several inliers and are unconformably overlain by the Upper Carboniferous Pictou Group on the New Brunswick Platform (Fig.1b). The outcrops are likely related to a single volcanic centre partially hidden beneath Pictou Group rocks, because the area incorporating the inliers clearly defines a circular area of high magnetic response on aeromagnetic maps (Thomas and Kiss 2005).

Fyffe and Barr (1986) examined some of the felsic volcanic rocks in the vicinity of Cumberland Hill, including lava flows and tuffs in a rhyolitic unit and underlying trachytic unit. Subsequently, St. Peter (1997) delineated the distribution of these units in more detail, depicting the trachyte as mostly underlying, but in part interdigitating with, the rhyolite (Fig.2). There is also one massive intrusive porphyry plug with several smaller dykes/pipes, interpreted by Thomas and Kiss (2005) as volcanic feeder dykes, based on their bulls-eye positive magnetic anomalies (50-375 nT) within the magnetically anomalous area. The exact thickness of these units is unknown as they are poorly exposed; however, both trachyte and rhyolite units are believed to be tens of metres thick (St. Peter 1997).

PETROGRAPHY

The rhyolite occurs as aphanitic to porphyritic flows and ignimbritic tuffs with flow banding and microporphyritic to porphyritic textures (Fig. 3a). Flow-aligned phenocrysts of feldspar (mainly anorthoclase; Fyffe and Barr 1986; 0.1-0.5 mm in length) and quartz (0.3-1 mm) are enclosed in a quartzofeldspathic groundmass with rare green clinopyroxene (hedenbergite; Fyffe and Barr 1986). The groundmass is composed of quartz, feldspar, devitrified glass, minor magnetite, ilmenite, pyrite, rare galena, fluorite and accessory minerals similar to those in trachyte samples (i.e., microlites of zircon, apatite, monazite, and allanite). The rhyolite is also extensively altered, although to a lesser degree than the trachyte, with most primary magmatic phases replaced by a secondary assemblage (i.e., carbonate, sericite, albite, and chlorite).

The trachyte occurs in two varieties, porphyritic and aphanitic, the latter of which is dominant. The porphyritic trachyte contains phenocrysts of feldspar (sanidine or anorthoclase) up to 2 mm in size, rare clinopyroxene, and plagioclase (Fig. 3b). The groundmass is largely composed of devitrified glass with phenocrysts of feldspar, clinopyroxene, and quartz, and rare ilmenite, hematite, sphalerite, and galena. Accessory minerals (fine-grained microlitic zircon, apatite, monazite, and allanite) were identified and confirmed using a scanning electron microscope including backscattered electron imaging, energy-dispersive X-ray spectroscopy and X-ray element mapping. The aphanitic variety displays a typical trachytic texture with flow-aligned feldspars (0.1-0.5 mm in length) enclosed in a fine-grained matrix similar to that of the porphyritic type. Both varieties are pervasively altered, with only relicts of primary minerals present, and secondary minerals dominated by calcite, sericite, chlorite, albite, and hematite alteration.

GEOCHEMISTRY

Fifteen samples were collected from the Cumberland Hill Formation, crushed and pulped in a soft iron swing mill, then analyzed for major elements using glass discs by X-ray fluorescence (XRF) spectrometry in the Nova Scotia Regional Geochemical Centre at Saint Mary's University, Halifax. Samples were subsequently analyzed for trace element compositions using a Perkin Elmer Optima 3000 inductively coupled plasma-mass spectrometer in the Activation Laboratories in Ancaster, Ontario. Prior to analysis, samples were digested in nitric acid after lithium metaborate fusion to ensure the complete dissolution of refractory accessory minerals. Precision and accuracy of the XRF data are reported by Dostal et al. (1994). The analytical error of the trace element determinations is 2-10%.

[FIGURE 1 OMITTED]

According to the [Na.sub.2]O + [K.sub.2]O vs Si[O.sub.2]classification of LeMaitre et al. (1989; Fig. 4a) and the Zr/Ti[O.sub.2]vs Si[O.sub.2]diagrain (Fig. 4b) of Winchester and Floyd (1977), the rocks can be subdivided into two groups, trachyte and peralkaline rhyolite, analogous to comendite/pantellerite fields according to Winchester and Floyd (1977). Most rocks are peralkaline with a high agpaitic index (AI = mole (Na+K)/A1; Shand 1951) and acmite in their CIPW normative mineralogy. Considering loss on ignition-free analyses (Table 1), the trachyte on average (n = 7, 1[sigma]) has moderate Si[O.sub.2] (65 [+ or -] 2.48 wt. %) accompanied by high total alkalis (9.5 [+ or -]1.81 wt. %), low CaO (2.5 [+ or -]1.21 wt. %), high Fe[O.sup.t]/MgO (ca. 25) and elevated concentrations ofhigh field strength elements (HFSE) and other incompatible elements (e.g., Zr + Nb + Ce +Y = 1334 [+ or -] 409.70; Whalen et al. 1987). The average of the rhyolitic rocks (n = 8, 1[sigma]) has higher Si[O.sub.2] (75.0 [+ or -] 0.66 wt. %), equivalent total alkali concentrations (9.0 [+ or -] 0.29 wt. %), very low CaO (0.23 [+ or -] 0.08 wt. %), high Fe[O.sup.t]/MgO (ca. 140), and high HFSE and other incompatible elements (e.g., Zr + Nb + Ce + Y = 3126 [+ or -] 161.75; Whalen et al. 1987).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The major and trace element signatures of the volcanic rocks of the Cumberland Hill Formation are interpreted to vary predominantly due to the degree of fractionation. Conversely, a heterogeneous magma source could also contribute to the variations exhibited. In order to determine the fractionation trends, Nb was utilized as a differentiation index following White et al. (2006; Fig. 5). With increasing concentration of Nb, the trachyte samples display positive correlation with [Al.sub.2][O.sub.3], and negative correlation with Ti[O.sub.2]. The reduction in Ti[O.sub.2]concentration can most likely be attributed to crystallization of Fe-Ti oxides. Rhyolite samples, in contrast, have relatively constant [Al.sub.2][O.sub.3] (Fig. 5a) and Ti[O.sub.2](Fig. 5b). Trace elements also display different fractionation trends in trachyte and rhyolite samples. Trachyte samples show marked decreases in Ba with increasing Nb (Fig. 5c), reflecting the crystallization of feldspars, as well as an increase in incompatible trace elements including Zr, La, and Ga (Fig. 5d, e, f). However, the abundances of these trace elements in the rhyolite remain virtually unchanged with increasing Nb. The contrast in fractional crystallization trends between trachyte and rhyolite is highlighted by the CaO-[K.sub.2]O-[Na.sub.2]O ternary diagram (Fig. 6), which illustrates that trachyte samples plot in a trend away from the CaO apex, whereas rhyolite sample variation is parallel to the [K.sub.2]O-[Na.sub.2]O join. This difference is most readily explained by the crystallization of Ca-bearing plagioclase and clinopyroxene in the trachyte magma, and alkali feldspar crystallization in the rhyolite magma.

The chondrite-normalized rare earth element (REE) patterns for each suite (Fig. 7a) are strongly enriched in light REE (LREE). The rhyolite has higher LREE contents with Lan values of ca. 440-510 compared to the trachyte with Lan ca. 270-430. The trachyte shows steeper LREE to heavy rare earth element (HREE) profiles, with [La.sub.n]/[Yb.sub.n] ratios ca. 8-11 compared to ca. 4-6 in rhyolite. A similar relationship is observed for [La.sub.n]/[Sm.sub.n], which in the trachyte is ca. 2.6-3.4 and in the rhyolite is ca. 2.2-2.6. The shape of the REE patterns in the trachyte does not change significantly, although the absolute abundances are escalating with the increasing degree of differentiation. The REE patterns of the rhyolite are characterized by a pronounced negative Eu anomaly (average Eu/[Eu.sup.*] = 0.08), consistent with low-pressure fractional crystallization of alkali and/or plagioclase feldspar. Primitive mantle-normalized trace element patterns in both rock types (Fig. 7b) are fairly similar. Both are highly fractionated and peak at Th-Nb; however, the rhyolite displays strong, negative Ba, Sr, and moderately negative Eu anomalies, whereas the trachyte contains only slightly negative anomalies in these elements. The Nb/Ta ratios of the trachyte (14.27 -[+ or -] 0.47) and rhyolite (13.91 [+ or -] 2.48) are the same, but slightly lower than that of the mantle (typically ~ 17.5; Sun and McDonough 1989).

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The abundances of U and Th in the felsic rocks of the Cumberland Hill Formation are high and variable. Rhyolite display significantly higher contents of U (14.0 [+ or -] 4.0 ppm, n = 8), and Th (33.0 [+ or -] 3.1 ppm, n = 8) compared to trachyte (U = 2.4 [+ or -] 2.4 ppm, n = 7; Th = 12 [+ or -]1.4 ppm, n = 7). The large range in U may be related to devitrification and alteration of the rhyolite groundmass. The rhyolite samples also have a higher U/ Th ratio (0.44 [+ or -] 0.13, n = 8) than trachyte (0.32 [+ or -] 0.09, n = 7), which is comparable to the whole rock or melt inclusion data from non-mineralized peralkaline rhyolite from various U districts in the world (Cuney and Kyser 2009). The increase of U/Th ratio in the rhyolite compared to trachyte may reflect monazite fractionation. Based on the variability of the U and Th concentrations in the rhyolite and trachyte samples, we interpret that remobilization of these elements has occurred. This remobilization was likely caused by devitrification of the groundmass because zircon is modally insignificant, and not the principal host of these elements based on the lack of correlation of radioactive trace elements with Zr (see Dupuy and Dostal1983).

[FIGURE 6 OMITTED]

PETROGENESIS

Overall the geochemical and mineralogical similarities of the spatially and temporally associated trachyte and rhyolite imply that they are genetically related. However, discontinuous and/or contrasting variation trends and significant differences in incompatible trace element ratios indicate that the rhyolite was not derived from the trachyte via continuous fractional crystallization, although fractional crystallization may be responsible for the variations within the individual suites. The evolution of the trachyte in the melt phase involved crystallization of feldspars, clinopyroxene, and Fe-Ti oxides, whereas the evolution of the rhyolite was dominated by crystallization and fractionation of alkali feldspar.

[FIGURE 7 OMITTED]

An alternative model to continuous fractional crystallization for the origin of the rhyolite is assimilation-fractional crystallization. However, the low and overlapping Th/Ta ratio of the trachyte and rhyolite (Fig. 8a), a sensitive indicator of crustal contamination (Gorton and Schandl 2000), demonstrates that assimilation-fractional crystallization and crustal contamination processes cannot solely account for derivation of the rhyolite from the trachyte. In addition, Pe-Piper and Piper (1998) reported an [[epsilon].sub.Nd] value of +3, with an assumed age of ca. 310 Ma from the Cumberland Hill rhyolite. This relatively high value implies that significant assimilation of crustal material is not likely to have taken place.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

K/Rb ratios in trachyte samples are much higher than in rhyolite samples (Fig. 9a, b), which suggest that the evolution of the rhyolite may have been affected by fluids (Dostal and Chatterjee 1995). Again, this mechanism cannot explain the origin of the rhyolite, but rather indicates that fluids played an important role during the evolution of these rocks and perhaps even led to the mobilization of U (Cuney and Kyser 2009; Nash 2010) in the rhyolite (Fig. 9b).

A comparison with similar rocks elsewhere (e.g., Pecerrillo et al. 2003) suggests that the Cumberland Hill trachyte and rhyolite could represent various stages of fractional crystallization in an evolving alkali basaltic magma. The absence of a negative Nb anomaly suggests that the parent magmas were not derived from subcontinental lithospheric mantle, but more plausibly originated from asthenospheric mantle. This interpretation is supported by the rhyolite [[epsilon].sub.Nd] value of +3 (Pe-Piper and Piper 1998), which implies upward migration of enriched asthenospheric magma into a juvenile lithosphere.

LaFleche et al. (1998) and Pe-Piper and Piper (1998) described alkali basaltic rocks of comparable age from other parts of the Maritimes Basin. The presence of alkali gabbroic rocks of a similar age (Johnson 2008) in the area suggests that the felsic rocks of the Cumberland Hill Formation could be derived from such parent magma by fractional crystallization. In fact, peralkaline felsic volcanic complexes are typically associated with a shallow-seated alkali intrusions ranging in composition from gabbro to highly fractionated granitic rocks. Although geochemical data are insufficient from the mafic alkali rocks in the vicinity of the Cumberland Hill Formation, chemical analyses of alkali basaltic rocks reported by LaFleche et al. (1998) from the Magdalen Islands indicate that similar basalts could have been a parent to trachyte and rhyolite. Trachyte of similar compositions have been documented to evolve from alkali basalt by extensive fractional crystallization (Peccerillo et al. 2003) and peralkaline rhyolite could have evolved from such magma by further fractional crystallization in a subvolcanic magma chamber. A likely scenario for the Cumberland Hill felsic rocks is that they are the products of fractional crystallization of alkali basaltic magma, and that the trachyte and rhyolite represent separate magma pulses with the latter being younger having undergone more extensive fractionation prior to emplacement.

ECONOMIC POTENTIAL

Silica-oversaturated peralkaline felsic volcanic rocks, particularly rhyolite, are commonly enriched in U and represent a potential source of uranium for volcanic-related hydrothermal U deposits, such as the Streltsovka caldera (Transbaikalia, Russia: Ischukova 1997; Chabiron et al. 2001, 2003; Cuney and Kyser 2009; Nash 2010), sediment-hosted U deposits in siliciclastic sedimentary basins, such as the U deposits in the Tim Mersoi basin in Niger (Forbes et al. 1984; Forbes 1989; Plant et al. 1999; Pagel et al. 2005; International Atomic Energy Agency 2009), and the New Horton mineralization (U-Cu-Ag-Au) within the Carboniferous basin (Boyd 1978; McLeod and Smith 2010; New Brunswick Department of Natural Resources 2010b).

The crystallization of U-bearing accessory minerals in highly depolymerized peralkaline melts is typically suppressed (Cuney 2009; Cuney and Kyser 2009) leading to progressive enrichment of U in residual melts during fractional crystallization and entrapment of U in the glassy groundmass. Subsequently U can be leached during the alteration or devitrification of the glass (Nash 2010). Cuney (2009) and Cuney and Kyser (2009) inferred that ignimbritic tuffs, in which U is hosted in the glass, are the most favourable type of volcanic rock; such rhyolitic pyroclastic rocks are abundant in the Cumberland Hill Formation.

Rhyolite of the Cumberland Hill Formation has relatively high abundances of U compared to associated trachyte and could be a source of U mineralization. The U concentrations in these rhyolite samples are elevated compared to similar felsic and mafic Late Tournaisian-Early Visean deposits of the Maritimes Basin (Magdalen Islands, 0.4-1.9 ppm, Central New Brunswick, 0.8-1.9 ppm and the Cobequid Highland dykes, 0.2 ppm; Pe-Piper and Piper 1998). The abundances of U in the rhyolite are comparable to the non-mineralized U concentrations of the peralkaline rhyolite of the Streltsovka caldera (Fig. 7b), the largest, volcanic-related, U ore field in the world (Cuney 2009). Despite similar U concentration, it is important to consider where U is hosted in rhyolite. When U is hosted in zircon or other resistant accessory minerals it cannot be readily remobilized. Based on current geochemical work, it appears U is present within the glassy groundmass of the rhyolite in the Cumberland Hill Formation and can therefore be released and re-deposited and thus could be a source of U within U- (Cu-V-Ag) deposits within local parts of the Carboniferous basin.

CONCLUSIONS

Felsic volcanic rocks from the Cumberland Hill Formation consist of peralkaline rhyolite and trachyte. Both contain elevated concentrations of incompatible trace elements, with the rhyolite more enriched and fractionated than the trachyte. Although both rock types appear to be genetically related, differences in their geochemical signatures imply the rhyolite was not derived from the trachyte by continuous fractional crystallization. Examination of the Th/Ta ratio also indicated crustal contamination cannot account for the observed compositional contrast. A preferred model is that the trachyte and rhyolite originated from the same or highly similar parent magma, and represent separate magma pulses with the latter being younger, having undergone more extensive fractionation prior to emplacement. The absence of a negative Nb anomaly suggests the parent magma was not derived from subcontinental lithospheric mantle, but more plausibly originated from an asthenospheric mande source. Lastly, Cumberland Hill rhyolite contains elevated U concentrations, with U hosted within the glassy groundmass. Since U can be leached and re-deposited, the rhyolite could be a source of sediment-hosted U mineralization within the basin.

ACKNOWLEDGEMENTS

A grant from the Geological Survey Branch of the New Brunswick Department of Natural Resources and Energy (to JD) covered fieldwork expenses. Analytical work was predominantly funded by a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant (to JD), and in part by a Society of Economic Geologists Student Research Grant (to TRG). The manuscript benefited from constructive reviews by David Lentz, an anonymous reviewer, and journal editor Sandra Barr.

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TARYN R. GRAY (1) *, JAROSLAV DOSTAL (1), MALCOLM MCLEOD (2), DUNCAN KEPPIE (3), YUANYUAN ZHANG (4)

(1.) Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada

(2.) New Brunswick Department of Natural Resources and Energy, Geological Surveys Branch, P.O. Box 5040, Sussex, New Brunswick E4E 5L2, Canada

(3.) Instituto de Geologia, Universidad Nacional Autonoma de Mexico, 04510, Mexico

(4.) Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing, China 100871

* Corresponding author <Taryn.Gray@smu.ca>

Date received: 30 Apri12010 [paragraph] Date accepted: 16 September 2010

Doi: 10.4138/atlgeol.2010.010
Table 1. Major and trace element concentrations in trachyte
and rhyolite from the Cumberland Hill Formation.

                                          Trachyte

                              46[degrees]01'56"N, 65[degrees]51'57"W
                            to 46[degrees]01'50"N, 65[degrees]52'04"W

                               1-202   1-203   1-206   1-207

Si[O.sub.2] (wt. %)            61.71   59.93   66.95   63.63
Ti[O.sub.2]                    0.48    0.47    0.63     0.37
[Al.sub.2][O.sub.3]            14.29   14.14   13.87   14.94
[Fe.sub.2][O.sub.3.sup.T]      9.30    8.80    4.07     7.83
MnO                            0.18    0.25    0.11     0.22
MgO                            0.13    0.37    0.20     0.08
CaO                            1.52    2.36    2.30     1.69
[Na.sub.2]O                    4.78    4.43    3.73     5.77
[K.sub.2]O                     4.93    5.02    4.97     5.16
[P.sub.2][O.sub.5]             0.14    0.13    0.23     0.09
LOI                            1.84    2.48    2.84     0.66
Total                          99.30   98.37   99.89   100.44
Pb (ppm)                       26.0    13.0    18.0     21.0
Zn                              250     150     110     210
W                              1.60    1.40    1.80     1.40
Rb                             81.0    79.0    92.0     104
Cs                             0.60    0.20    1.30     0.70
Ba                              384     403     552     259
Sr                              107     281     100     42.0
Ga                             36.0    34.0    26.0     40.0
Ta                             6.72    6.68    3.67     8.09
Nb                             95.5    92.6    51.5     118
Hf                             22.3    21.9    11.7     24.9
Zr                              991     944     557     1150
Y                              86.90   83.3    51.7     101
Th                             11.5    11.1    11.3     12.2
U                              2.36    2.35    4.81     3.71
La                              114     116    93.0     139
Ce                              239     243     195     286
Pr                             25.9    26.3    20.7     30.5
Nd                             92.5    94.8    70.3     110
Sm                             20.8    20.7    14.9     23.7
Eu                             5.33    5.40    3.62     5.40
Gd                             19.3    19.6    13.3     22.2
Tb                             3.12    3.04    2.02     3.49
Dy                             16.9    16.6    10.8     19.7
Ho                             3.15    3.12    2.00     3.66
Er                             8.85    8.61    5.61     10.0
Tm                             1.27    1.24    0.81     1.42
Yb                             8.35    7.98    5.29     9.07
Lu                             1.30    1.30    0.80     1.42

                                          Trachyte

                            46[degrees]01'56"N, 65[degrees]51'57"W
                            to 46[degrees]01'50"N, 65[degrees]52'04"W

                                1-01    1-12    1-14

Si[O.sub.2] (wt. %)            64.15    62.39   62.30
Ti[O.sub.2]                     0.60    0.63    0.38
[Al.sub.2][O.sub.3]            13.75    13.51   14.43
[Fe.sub.2][O.sub.3.sup.T]       6.11    5.10    7.84
MnO                             0.19    0.17    0.22
MgO                             0.37    0.29    0.07
CaO                             3.13    4.50    1.38
[Na.sub.2]O                     3.03    2.21    6.17
[K.sub.2]O                      4.85    3.87    5.68
[P.sub.2][O.sub.5]              0.17    0.19    0.08
LOI                             4.02    6.31    0.62
Total                          100.37   99.17   99.17
Pb (ppm)                        9.00    16.0    90.0
Zn                              90.0    60.0     210
W                               1.10    3.50    0.70
Rb                              98.0    91.0     124
Cs                              1.70    1.70    0.70
Ba                              664      398     280
Sr                              93.0    96.0    39.0
Ga                              30.0    28.0    39.0
Ta                              3.70    4.33    8.29
Nb                              51.1    65.6     118
Hf                              14.4    16.0    31.0
Zr                              679      755    1550
Y                               54.4    62.3    88.8
Th                              12.6    13.6    14.8
U                               4.65    5.79    3.99
La                              86.0    88.3     120
Ce                              183      189     259
Pr                              21.7    22.1    30.6
Nd                              86.6    88.8     122
Sm                              17.1    17.5    24.8
Eu                              3.40    3.55    4.85
Gd                              14.0    14.7    21.5
Tb                              2.10    2.26    3.38
Dy                              11.3    12.6    18.4
Ho                              2.17    2.46    3.56
Er                              5.91    6.78    9.72
Tm                              0.88    1.02    1.45
Yb                              5.61    6.50    9.37
Lu                              0.89    1.03    1.48

                                         Rhyolite

                            46[degrees]01'11"N, 65[degrees]51'15"W
                            to 46[degrees]01'13"N, 65[degrees]51'14"W

                               2-01    2-8A    2-8B    2-09

Si[O.sub.2] (wt. %)            74.22   72.81   73.40   72.72
Ti[O.sub.2]                    0.17    0.17    0.17    0.17
[Al.sub.2][O.sub.3]            11.64   11.23   11.44   11.25
[Fe.sub.2][O.sub.3.sup.T]      3.39    4.16    4.13    4.13
MnO                            0.04    0.10    0.10    0.12
MgO                            0.02    0.02     --     0.02
CaO                            0.11    0.15    0.15    0.22
[Na.sub.2]O                    3.65    3.93    4.21    3.67
[K.sub.2]O                     4.59    4.96    4.84    5.56
[P.sub.2][O.sub.5]              --     0.01     --     0.01
LOI                            0.96    1.31    1.17    1.99
Total                          98.79   98.85   99.61   99.85
Pb (ppm)                       15.0    62.0    21.0    37.0
Zn                              390     280     300     360
W                              3.20    6.60    8.20    2.90
Rb                              308     290     299     347
Cs                             1.90    1.70    1.90    1.90
Ba                             15.00   4.00     --     12.0
Sr                             25.0    16.0    17.0    37.0
Ga                             56.0    54.0    58.0    57.0
Ta                             15.1    18.4    18.9    18.4
Nb                              281     235     287     222
Hf                             57.3    51.1    55.5    53.0
Zr                             2640    2190    2420    2290
Y                               177     179     175     179
Th                             29.6    31.2    30.8    38.7
U                              20.80   9.27    11.70   18.50
La                              164     150     151     144
Ce                              337     331     334     330
Pr                             39.3    39.1    39.5    37.7
Nd                              152     155     155     147
Sm                             35.6    36.6    37.3    35.3
Eu                             3.06    3.17    3.17    3.11
Gd                             33.2    34.9    35.9    34.6
Tb                             6.08    6.04    6.11    6.12
Dy                             33.3    34.5    34.3    35.1
Ho                             6.62    6.67    6.67    7.16
Er                             18.2    18.1    17.8    19.5
Tm                             2.85    2.64    2.63    2.95
Yb                             17.6    17.7    15.5    17.0
Lu                             2.54    2.68    2.23    2.36

                                         Rhyolite

                            46[degrees]01'11"N, 65[degrees]51'15"W
                            to 46[degrees]01'13"N, 65[degrees]51'14"W

                               2-10    2-11    2-12     2-13

Si[O.sub.2] (wt. %)            71.69   71.85   72.42   74.29
Ti[O.sub.2]                    0.17    0.17    0.18     0.18
[Al.sub.2][O.sub.3]            11.60   11.71   11.65   11.93
[Fe.sub.2][O.sub.3.sup.T]      4.37    4.22    4.36     3.38
MnO                            0.10    0.10    0.09     0.03
MgO                            0.02    0.05    0.02     0.03
CaO                            0.31    0.26    0.23     0.34
[Na.sub.2]O                    3.81    4.26    4.50     5.33
[K.sub.2]O                     4.97    4.53    4.18     3.46
[P.sub.2][O.sub.5]              --      --      --       --
LOI                            1.80    2.06    1.03     1.40
Total                          98.84   99.22   98.66   100.37
Pb (ppm)                       28.0    82.0    24.0     17.0
Zn                              300     300     310     290
W                              3.50    3.40    22.9     9.40
Rb                              306     262     244     192
Cs                             2.10    1.50    1.70     1.10
Ba                             6.00    9.00     --      9.00
Sr                             39.0    38.0    21.0     28.0
Ga                             57.0    56.0    58.0     55.0
Ta                             18.2    18.4    19.4     18.7
Nb                              249     223     306     208
Hf                             50.4    51.7    54.5     55.0
Zr                             2300    2280    2380     2450
Y                               184     162     187     191
Th                             32.9    29.2    33.1     35.0
U                              13.80   16.10   10.20   13.80
La                              142     146     148     146
Ce                              327     315     319     322
Pr                             37.5    38.3    38.7     38.7
Nd                              147     151     152     152
Sm                             35.5    36.1    36.3     36.4
Eu                             3.08    3.18    3.12     3.20
Gd                             34.7    34.8    35.5     36.0
Tb                             6.12    6.00    6.27     6.35
Dy                             35.1    33.0    35.9     36.1
Ho                             6.97    6.24    7.13     7.20
Er                             19.0    16.6    19.4     19.6
Tm                             2.93    2.55    3.01     2.97
Yb                             17.9    15.7    18.1     17.7
Lu                             2.63    2.30    2.60     2.56

Note:-symbol indicates no data available, or value below
detection limits
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Author:Gray, Taryn R.; Dostal, Jaroslav; McLeod, Malcolm; Keppie, Duncan; Zhang, Yuanyuan
Publication:Atlantic Geology
Article Type:Report
Geographic Code:1CANA
Date:Jan 1, 2010
Words:6636
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