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New U-Pb (zircon) age and geochemistry of the Wedgeport pluton, Meguma terrane, Nova Scotia.


A new U-Pb (zircon) age of 357 [+ or -] 1 Ma for the Wedgeport pluton in the Meguma terrane of southwestern Nova Scotia confirms that it is younger than other granitoid plutons of the Meguma terrane but not as young as the ca. 316 Ma age based on previous geochronological work. The pluton consists of mainly homogeneous biotite monzogranite characterized by abundant accessory phases including garnet, zircon, epidote, titanite, and ilmenite. Major minerals are quartz, plagioclase (albite-oligoclase), and perthitic microcline. The pluton has consistently high Si[O.sub.2] content (average ca. 75%) but shows some internal variation consistent with fractional crystallization of feldspar and biotite, together with accessory phases such as zircon. Tectonic setting during emplacement of the pluton was likely within plate, but classification is ambiguous because the pluton has characteristics of all three granite types. Peraluminous composition and presence ofilmenite and aluminous minerals such as Fe-rich biotite and garnet suggest S-type affinity, but other characteristics such as high-level emplacement, accessory fluorite, and some chemical characteristics (high Ga/Al ratio, elevated Nb and Y) indicate A-type affinity, whereas abundant titanite and overall chemical character is compatible with felsic I-type composition. The granite has specialized characteristics such as elevated Rb relative to Sr and Ba, and high U and Th compared to all of the average granite types. High [[epsilon].sub.Nd] values (+2 to +3) indicate a relatively primitive source compared to other plutons of the Meguma terrane. The Wedgeport pluton appears to be unique in the Meguma terrane in both age and composition, although similar plutons may occur in inboard terranes of the Appalachian orogen.


Une nouvelle datation au U-Pb (a partir de zircon) situant a environ 357 Ma le pluton de Wedgeport a l'interieur du terrane de Meguma dans le Sud-Ouest de la Nouvelle-Ecosse, confirme qu'il est plus recent que les autres plutons granitiques du terrane de Meguma, mais qu'il a plus des 316 Ma qu'on lui avait attribues dans le cadre de travaux geochronologiques anterieurs. Le pluton est constitue de monzogranite a biotite principalement homogene caracterise par des phases accessoires abondantes, notamment du grenat, du zircon, de l'epidote, de la titanite et de l'ilmenite. Les principaux mineraux presents sont le quartz, le feldspath plagioclase (albite-oligoclase) et la microcline perthitique. Le pluton a une teneur constamment elevee en Si[O.sub.2] (en moyenne environ 75 %), mais il presente une certaine variation interne correspondant a une cristallisation fractionnee des feldspaths et de la biotite, ainsi que des phases accessoires, dont du zircon. L'intrusion du pluton est probablement survenue dans un cadre tectonique intra-plaque, mais sa classification est ambigue parce que le pluton possede des caracteristiques des trois types de granites. Sa composition hyperalumineuse et la presence d'ilmenite et de mineraux alumineux, comme de la biotite riche en fer et du grenat, laissent supposer une affinite de type S, mais d'autres aracteristiques, comme l'intrusion de haut niveau, la fluorine accessoire et certaines caracteristiques chimiques (rapport Ga/A1 eleve, teneur elevee en Nb et Y), temoignent d'une affinite de type A, tandis que l'abondance de titanite et son caractere chimique general sont compatibles avec une composition de type I felsique. Le granite presente des caracteristiques specialisees, telles qu'une quantite elevee de Rb par rapport au Sr et au Ba, et des teneurs elevees en U et en Th comparativement a tous les types de granites moyens. Les valeurs [[epsilon].sub.Na] elevees (+2 a +3) signalent une source relativement primitive comparativement aux autres plutons du terrane de Meguma. Le pluton de Wedgeport semble unique a l'interieur du terrane de Meguma tant du point de vue age que composition, meme si des plutons analogues peuvent etre presents dans des terranes interieurs.


The Wedgeport pluton in southernmost Nova Scotia differs in both age and composition from the abundant Devonian plutons that characterize the Meguma terrane (Fig. 1). Its specialized characteristics were recognized in the Nova Scotia "granite boom" of the late 1970s and early 1980s, when the poorly exposed pluton was extensively explored and drilled for Sn, W, and U (Cant et al. 1978; Wolfson 1983). Subsequently, the pluton was reported to be significantly younger than the other plutons of the Meguma terranc, with a late Carboniferous UPb (zircon) and Rb-Sr whole rock age of 316 [+ or -] 5 and 323 [+ or -] 12 Ma, respectively (Cormier et al. 1988). This age is similar to some [sup.40]Ar/[sup.39]Ar ages from mica in plutonic and metasedimentary units and shear zones in the area (e.g., Reynolds et al. 1981, 1987; Dallmeyer and Keppie 1987, 1988; Muecke et al. 1988; Keppie and Dallmeyer 1995; Culshaw and Rewlolds 1997). The pluton also yielded younger [sup.40]Ar/[sup.30]Ar biotite and Rb-Sr mineral ages of ca. 257 Ma (Reynolds et al. 1981; Cormier et al. 1988), which, together with the inferred crystallization age, were used to suggest that Permian-Carboniferous plutonism was a significant factor for economic mineralization in the Meguma Terrane (Cormier et al. 1988). It was suggested more recently that emplacement of the Wedgeport pluton may have been linked to extension related to delamination (Keppie and Dallmeyer 1995) or transtension along shear zones as a result of relative motion of the Gondwanan and Laurentian plates (Pe-Piper and Jansa 1999).


The Southwest Nova Mapping Project (White 2003) provided an opportunity to revisit the Wedgeport pluton with detailed mapping and sampling, which was used as the basis for a BSc honours thesis project by the senior author. Furthermore, the previously reported U-Pb (zircon) age was based on a single bulk zircon fraction and thus is not reliable by current standards; hence, the opportunity was also taken to re-date the pluton. The purpose of this paper is to report the results of these studies, together with a compilation of some earlier work on the pluton. The results show that the pluton is older than was indicated by previous dating, but still younger than other granitoid rocks of the Meguma terrane.


The Wedgeport pluton (Taylor 1967) intrudes thickly bedded metasandstone, metasiltstone, and slate of the New Harbour Member, the lower part of the Goldenville Formation (White and King 2002; White 2003) (Figs. 1,2). The Goldenville Formation is overlain by the mainly pelitic Halifax Formation. In the Yarmouth area west of the Wedgeport pluton (Fig. 1), metavolcanic and metasedimentary rocks of the late Ordovician to Silurian White Rock Formation disconformably overlie or are in faulted contact with the Halifax Formation (MacDonald et al. 2002; White et al. 2001). These units were deformed and metamorphosed during the Acadian orogeny at ca. 395-380 Ma (Hicks et al. 1999), prior to emplacement of the South Mountain Batholith and related plutons at ca. 378-372 Ma (see Clarke et al. 1997 for a summary). The Meguma terrane was juxtaposed with the Avalon terrane to the north by Devonian-Carboniferous dextral transcurrent motion along the Cobequid-Chedabucto fault system (e.g., Keppie 2000). Mantle-derived mafic igneous activity in the Middle Devonian to Carboniferous may have provided the heat source to generate these granitoid magmas by crustal melting (Tate and Clarke 1995). Ongoing localized deformation occurred along shear zones during the Carboniferous (Culshaw and Liesa 1997; Culshaw and Reynolds 1997).



Although the Wedgeport pluton is poorly exposed and outcrop is sporadic, second derivative acromagnetic data (King 1997a, b, and unpublished data) combined with several drill holes (Cant et al. 1978) clarify the position of the contact in areas of limited or no outcrop. The pluton intruded along the western limb of a north-south-trending, north-plunging anticline (Fig. 2). It produced a narrow (approximately 400 m wide) contact metamorphic aureole of spotted hornfels containing andalusite and cordierite superimposed on biotite zone assemblages (Cullen 1983). Abundant garnet also was noted during the present study in the more pelitic beds in the metasandstone. The contact metamorphic mineral assemblage is indicative of the albite-epidote to hornblende-hornfels facies, and indicates that the pluton was emplaced at relatively shallow crustal depth, about 3-4 km below surface. The contact zone is well exposed on the western shore of Pinkneys Point (Fig. 2), where the metasedimentary rocks are intruded by granitic dykes from the pluton.

The mainly granitic pluton contains scattered fine- to medium-grained, biotite-rich, round granodioritic enclaves that range in size from a few 10s of cm to 1-2 m in longest dimension. None of the enclaves were included in the present study. Narrow (2-5 cm wide, rarely > 1m), pale grey veins and pods of aplite and pegmatite intrude the pluton. Near the margins of the pluton the granite is heterogeneous with textures that range from fine-grained to porphyritic. In addition, igneous layering is present, defined by alternating biotite-rich and feldspar-quartz-rich bands that are broadly parallel to the contact with the adjacent country rocks. Miarolitic cavities are present locally but are generally uncommon (Wolfson 1983). These textural variations suggest that a significant amount of fluid was present during crystallization of the pluton, especially near the margins.

The pluton and adjacent country rock are cut by several thin (1-2 mm to several cm) quartz, quartz-tourmaline, quartz-carbonate, and greisen veins. Typically, the quartz-carbonate and greisen veins contain various sulphide minerals, fluorite, scheelite, white mica, and cassiterite (Wolfson 1983). Northeast- to east-trending shear zones, reported to contain cassiterite, wolframite, molybdenite, and scheelite, cut the pluton, and appear rusty as the result of weathering of associated sulphide minerals, mainly pyrite and arsenopyrite (e.g., Wolfson, 1983). Mylonitic fabrics are common in the shear zones which display well-developed asymmetric quartz augen and C-S fabrics indicating dextral sense of motion (White 2003).

The pluton is cut by lamprophyre and alkaline olivine diabase dykes of early Jurassic age (Cant et al. 1978; Chatterjee and Keppie 1981; Wolfson 1983; Pe-Piper and Reynolds 2000; White and King 2002; White 2003).


The Wedgeport pluton is composed mainly of medium- to coarse-grained biotite monzogranite which grades in composition to syenogranite (Fig. 3). Texture is typically hypidiomorphic equigranular and, locally, inequigranular to porphyritic, especially near the margins as described above. Microcline is the dominant feldspar, and simple Carlsbad twins are visible in some grains suggesting that orthoclase was the original form. Perthitic texture is typically present, and albitic lamellae have compositions similar to those of separate plagioclase grains in the rock. Granophyric texture occurs only in samples from the northwestern part of the intrusion; that area may represent the shallowest part at the time of emplacement. Subhedral plagioclase is slightly to extensively altered to saussurite and sericite. Microprobe analyses of plagioclase in 6 samples showed a range in composition from [An.sub.1] to [An.sub.17]; zoned plagioclase grains do not greatly vary in composition from the core to the rim (MacLean 2003). Anhedral quartz grains commonly show undulatory extinction.


Biotite forms about 15% of the rock, and typically is partially or wholly altered to chlorite and muscovite. Microprobe analyses (MacLean 2003) revealed that the biotite has a high iron content (Fe/Fe+Mg = 0.73 to 0.83) and tetrahedral Al contents of 2 to 2.2, close to the annite end member. Common inclusions in biotite are titanite, zircon, apatite, ilmenite, epidote, and probably monazite (not easily identified because of small grain size). Titanite is the most abundant accessory mineral in the samples and mainly occurs in association with biotite and ilmenite; the latter mineral is partially or in some cases entirely altered to titanite. Many samples also contain minor amounts of garnet and fluorite. Electron microprobe analyses revealed that the garnet is of almandine-grossular composition with high manganese content (MacLean 2003).

Chatterjee and Keppie (1981) and Chatterjee et al. (1985) divided the Wedgeport pluton into the Goose Bay and Pinkneys Point units, forming the eastern and western parts of the pluton, respectively, with the mutual contact not exposed. They described the Goose Bay granite as garnetiferous, and similar to parts of the South Mountain Batholith. In contrast, the Pinkneys Point granite was described as a more variable unit, containing topaz granite, greisenized granite, albitite, and greisen, and associated with mineralization, including cassiterite, wolframite, scheelite, and a variety of similar lithophile-element minerals (Chatterjee et al. 1985). Chatterjee and Keppie (1981) suggested that the Pinkneys Point pluton may have been the source of fluids that resulted in alteration and mineralized veins in the Wedgeport pluton and adjacent rocks. Observations made during the present study do not provide any evidence to support this subdivision; garnet was observed in samples distributed throughout the pluton, and features ascribed to the Pinkneys Point unit are more likely a reflection of the excellent exposure in the Pinkneys Point section and its position near the margin of the pluton.


Previous Work

U-Pb and Rb-Sr data presented by Cormier et al. (1988) indicated that the Wedgeport pluton crystallized at 316 [+ or -] 5 Ma, but was reheated at 257 [+ or -] 8 Ma during a thermal event. However, the U-Pb data of Cormier et al. (1988) are difficult to re-assess as no U isotopic information was provided and some aspects of the data are inconsistent between data table and text. Additionally, only one zircon fraction was analyzed, and presumably that fraction was large and multigrain. The zircon was reported to be of poor quality, and it is unlikely that it was air-abraded prior to analysis because at the time of the work, the abrasion technique (Krogh 1982) was not widely used.

An [sup.40]Ar/[sup.39]Ar plateau age on biotite at 258 [+ or -] 8 Ma has also been reported from the Wedgeport pluton (Reynolds et al. 1981); however, this sample showed appreciable alteration of biotite to chlorite (Reynolds et al. 1981). The similarity between the Rb-Sr mineral isochron ages (Cormier et al. 1988) and the [sup.40]Ar/[sup.39]Ar age from chloritized biotite suggests that both the Rb-Sr and [sup.40]Ar/[sup.39]Ar systems may record the timing of pervasive alteration in the Wedgeport pluton as opposed to a re-heating event.

Even younger hornblende and biotite [sup.40]Ar/[sup.39]Ar plateau ages (ca. 231 to 222 Ma and ca. 209 to 203 Ma) were reported for the lamprophyre and alkaline olivine diabase dykes that occur in the area (Reynolds et al. 1987; Pe-Piper and Reynolds 2000).

Analytical Methods

A 20 kg monzogranite sample was processed at the Royal Ontario Museum in Toronto using standard techniques, including heavy liquids and a Frantz magnetic separator for isolation of heavy mineral fractions. Zircon was selected from the least paramagnetic fraction by hand picking grains 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 (consisting of one to four grains) was determined using an unpublished spreadsheet calculation that estimates fraction weights from photomicrograph measurements of individual zircons. Uncertainties on the estimated weights have not been calculated but are likely to be <30% in most cases. This uncertainty only affects the calculation of Pb and U concentrations and has no influence on age data.

The selected grains were washed in 4N and 7N HN[O.sub.3] 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 out-gassed 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 dead time 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.

Assigned laboratory blank for U is 0.2 pg. Total measured common Pb in each fraction was below 1 pg in all cases and was assigned the isotopic composition of lab blank (see footnotes to Table 1). 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). Uncertainties 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. The average [sup.206]Pb/[sup.238]U age reported below was also calculated using ROMAGE.

New Results

Zircon grains in the Wedgeport granite sample are dominated by euhedral, 4-sided, well-formed igneous prisms ranging from small to large (Fig. 4a, b). Most grains are cracked and/or contain inclusions but the selected grains are free of these features. Six fractions of small and large grains were abraded and analysed separately (Table 1). Fractions Z2, Z3, and Z5 are multigrain fractions of small prisms with inclusions. On the concordia diagram (Fig. 5), these analyses plot distinctly below and to the left of two concordant analyses (Z4, Z6) of larger single prisms. Fraction Z1, also a single large grain, is older than all other fractions. Although interpretation of these data is not straightforward, we consider the following scenario to be the most likely. Concordant fractions Z4 and Z6, with identical [sup.206]Pb/[sup.238]U ages of 357 Ma, are interpreted to give the igneous crystallization age of the granite. The weighted mean [sup.206]Pb/[sup.238]U age of these fractions is 357.3 [+ or -] 1.1 Ma. Fraction Z5 is interpreted to consist of ca. 357 Ma zircons that experienced some modern-day Pb loss, and fraction Z1 appears to contain an inherited component. The age of this component is modelled as 1340 + 830/-500 Ma, the upper intercept age of a discordia line through fractions Z1, Z4, and Z6. The remaining fractions (Z2, Z3) have [sup.207]Pb/[sup.206]Pb ages significantly older than 357 Ma and therefore modern-day Pb loss alone cannot account for their discordance. We suggest that Pb loss combined with the presence of a minor inherited component best explains the discordance of these analyses.


We consider the weighted mean [sup.206]Pb/[sup.238]U age of 357.3 [+ or -] 1.1 Ma to be the best estimate for the crystallization age of the Wedgeport pluton. The previously reported age of 316 [+ or -] 5 Ma (Cormier etal. 1988), which was based on a single analysis of highly fractured and metamict grains, may be erroneously young due to significant Pb loss from these zircons.



We acquired major and trace element data (Table 2) for 18 samples from surface outcrops of the Wedgeport pluton (Fig. 2). Rare-earth element data were obtained for 5 of these samples (Table 3), and Sm-Nd isotopic data for the dated sample (Table 4). Wolfson (1983) had reported whole-rock analyses of 8 samples from drill core in the Wedgeport pluton, but her data included only a limited suite of trace elements. Pe-Piper and Jansa (1999) also obtained chemical data from 7 samples from the pluton, in order to compare to offshore plutons sampled in drill core. Those data include a suite of trace elements and in two samples, also rare-earth elements, comparable to the present data set, although no petrographic descriptions were provided. Locations for these samples (G. Pe-Piper, written communication, 2004) are shown on Fig. 2. In addition, Chatterjee et al. (1985) presented a large amount of chemical data from the Wedgeport pluton in the form of variation diagrams; however, their focus was on mineralization processes, not petrogenesis and tectonic setting, and the actual chemical data for those samples were not published.

Major and trace elements

All analyzed samples from the Wedgeport pluton have high silica content (Table 2) that averages about 75%. Although scattered, most major element oxides show a negative correlation with Si[O.sub.2] (Fig. 6). Exceptions are [Na.sub.2]O and [K.sub.2]O (Fig. 6f, g) which show little or no correlation with Si[O.sub.2]. These variations mainly reflect variations in the amount of modal biotite and plagioclase in the analyzed samples. No consistent differences are apparent between samples that contain garnet compared to those that do not, or between the porphyritic sample and the other samples. The two aplite samples do not differ significantly in major element composition from their host rocks. In terms of major element composition, the Wedgeport pluton does not show a clear affinity with any of the average A-type and felsic I- and S-type granites of Whalen et al. (1987). For example, [Na.sub.2]O contents are similar to that in the average I-type but lower than in the average A-type and higher than in the average S-type (Fig. 6f), whereas MgO and CaO contents are most similar to the average A-type (Fig. 6d, e) and [P.sub.2][O.sub.5] content to the average S-type (Fig. 6h).


Trace elements show more diagnostic features (Fig. 7). Both Ba and Sr show negative correlation with Si[O.sub.2], and concentrations below those typical of average felsic I- and S-type granite and most similar to average A-type (Figs. 7a, b). On the other hand, Rb values show no correlation with Si[O.sub.2] and are elevated compared to all three granite types (Fig. 7c). This feature is also shown on the Ba-Rb-Sr ternary diagram (Fig. 8a), in which the samples plot in the area of enriched granite according to El Bouseily and El Sokkary (1975).


The trace elements Y, Nb, and Ga show scatter and no consistent correlation with Si[O.sub.2] (Figs. 7d, e, f), whereas Zr shows a weak negative correlation (Fig. 7g). The abundances of all of these elements are below those of the average A-type granite, although they tend to be higher than those in the average felsic I- and S-type granites. V show strong negative correlation with Si[O.sub.2], and higher values than I-, S-, and especially A-type granites (Fig. 7h). Values in the Pe-Piper and Jansa (1999) samples are uniformly lower than those obtained during the present study, most likely due to a systematic analytical problem in one of the data sets. Both Pb and Zn show negative correlation with Si[O.sub.2] (Fig. 7i, j). Pb tends to be higher in samples that contain garnet, and Zn values are low compared to the average A-type granite. Concentrations of Ni, Cu, and C are low and commonly below the detection limit of the analytical method (Table 2), and hence are not displayed graphically.

Th and U values are scattered, but overall higher than those in the average felsic I-, S-, or A-type granites (Fig. 9a, b). A plot of Th against U shows a positive correlation (Fig.9c), and two trends are suggested that correspond to the "unaltered" (lower U) and "altered" (higher U) sample trends identified by Chatterjee et al. (1985). The majority of samples analyzed during the present study, as well as those of Pe-Piper and Jansa (1999), follow the "unaltered" trend.


The Wedgeport pluton is not easily assigned to I-, S-, or A-type on the basis of major or trace element compositions compared to the averages from Whalen et al. (1987). Whalen et al. (1987) used the Ga/AL ratio as a distinguishing feature of these granite types, and plots of [Na.sub.2]O+[K.sub.2]O and Nb against Ga/Al suggest a composition intermediate between A-type and felsic I- and S-type granites (Fig. 8b, c). However, the presence of iron-rich biotite and abundant titanite and fluorite are more consistent with A-type character. A plot of [Al.sub.2][O.sub.3] / [Na.sub.2]O + [K.sub.2]O against [Al.sub.2][O.sub.3] / CaO + [Na.sub.2]O + [K.sub.2]O shows that the Wedgeport pluton is peraluminous (Fig. 8d). Other peraluminous characteristics of the Wedgeport pluton include the presence of garnet in many samples.

The samples plot at the junction of the within-plate, syncollisional, and volcanic-arc granitoid fields on the Rb against Y+Nb discrimination diagram (Fig. 8e) but more definitively in the A-type field on the Nb-Y diagram (Fig. 8f).

Rare-Earth Element Data

Rare-earth element (REE) data were obtained for 5 samples (four granite and one aplite) during the present study (Table 3), and an additional two samples are available from Pe-Piper and Jansa (1999). The patterns for all of the granite samples are similar, and show moderate enrichment in the light REE, a large negative Eu anomaly, and a flat heavy REE at 20 to 40 times chondritic values (Fig. 10). The aplite sample shows a strongly contrasting pattern, with no enrichment in the light REE and increasing enrichment in the heavy REE. No obvious mineralogical differences such as high garnet content are evident in the aplite to explain the REE pattern.


Isotopic Data

The Sm-Nd data for the dated sample 016-WO2-004A (Table 4) are similar to those reported for two samples by Pe-Piper and Jansa (1999). The [[epsilon].sub.Nd] value at 357 Ma is 2.1, whereas the two samples from Pe-Piper and Jansa (1999) have values (at 357 Ma) of 2.1 and 3.1. These relatively high positive values contrast markedly with reported [[epsilon].sub.Nd] values for other granitic plutons in the Meguma terrane, which have negative values ranging from -1.4 to -5.2 at 372 Ma (Clarke et al. 1988).

The positive values in the Wedgeport pluton samples suggest a relatively primitive source and allow for little contamination from the metasedimentary host rocks of the Meguma Group, which have very negative [[epsilon].sub.Nd] values of ca. -10 (Clarke et al. 1988). Instead, they are similar to values from minor mafic plutons of the Meguma terrane, for which mainly positive values have been reported (Tate and Clarke 1995). They are also similar to values reported by MacDonald et al. (2002) for ca. 440 Ma rhyolite in the White Rock Formation in the Yarmouth area ([[epsilon].sub.Nd] = 1.37), although the ca. 440 Ma Brenton pluton has a lower value ([[epsilon].sub.Nd] = -0.42).


The revised age reported here for the Wedgeport pluton shows that it is 40 million years older than previously assumed, that is earliest Carboniferous (Okulitch 2002) as opposed to late Carboniferous. Hence the pluton cannot be associated with the late thermal events at ca. 320-315 Ma in the Meguma terrane, nor with the shear zones of late Carboniferous age, as suggested by previous workers (Dallmeyer and Keppie 1987; Keppie and Dallmeyer 1995; Culshaw and Reynolds 1997; see compilation in White 2003).

The 357 [+ or -] 1 Ma age is significantly younger than the ages from the South Mountain Batholith and its satellite plutons, Shelburne and Barrington Passage (Fig. 1), all of which have yielded ca. 372 Ma ages. It is also much younger than the nearby Brenton Pluton (Fig. 1), which is Silurian (MacDonald et al. 2002). No other igneous activity of ca. 357 Ma is known in the Meguma terrane, although abundant plutons of similar age occur north of the Cobequid-Chedabucto fault system in northern mainland Nova Scotia (Dunning et al. 2002). However, the latter plutons differ from the Wedgeport pluton in that they are part of bimodal gabbro-granite suites with clear A-type features, and occur in association with voluminous bimodal volcanic rocks. Pe-Piper and Jansa (1999) compared offshore plutons with onshore plutons, including the Wedgeport pluton, and showed that none of the offshore plutons has petrochemical characteristics that match those of the Wedgeport pluton.

A possible link may exist between the Wedgeport pluton and mineralization in spodumene-bearing pegmatite bodies in the Brazil Lake area north of the Wedgeport pluton (Fig. 1). Pegmatite in this area occurs as elongate lenses in deformed metavolcanic and metasedimentary rocks of the Silurian White Rock Formation. Although U-Pb dating of tantalite indicated that the pegmatite crystallized at 378 [+ or -] 1 Ma, molybdenite from an adjacent quartz vein yielded a Re/Os age of 354 [+ or -] 3 Ma (Kontak et al. 2003). This result indicates that mineralization in the Brazil Lake area is of similar age to the crystallization age of the Wedgeport pluton. Although it does not prove a genetic link between the two areas, the similar ages, younger than ages of other plutons known in the Meguma terrane, may be significant.

In addition to its anomalous age, the Wedgeport pluton also has anomalous petrochemical characteristics. It cannot be classified as any of the commonly accepted granite end-members, and its tectonic setting is not evident from its petrochemical features using commonly applied discrimination diagrams. However, a within-plate tectonic setting is most likely, both from the petrochemical features and the inferred tectonic situation of the Meguma terrane in the earliest Carboniferous. At that time, the Meguma terrane was being uplifted and unroofed, and shedding sedimentary material into Carboniferous basins. The northwestern part of the South Mountain Batholith was exposed by 357 Ma when the Wedgeport pluton was being emplaced, as conglomerate in the overlying units contains debris derived from the batholith. However, it is clear that the area around the Wedgeport pluton was still buried. The depth of emplacement of the Wedgeport pluton was likely epizonal, based on the presence of a well-developed contact metamorphic aureole containing cordierite and andalusite, and localized development of porphyritic, granophyric, and inequigranular textures near the margins.

The Wedgeport pluton appears to be a unique intrusion in the Meguma terrane, both in terms of its age and its composition, and remains an enigma in terms of its origin.
Table 1. U-Pb isotopic data for Wedgeport pluton sample 016-W02-004A

Reference Fraction (a) Weight U
 (mg) (ppm)

jk10p16 Z1 euh clr 2:1 pr (1) 0.0028 265
jk10p17 Z2 euh clr 3:1 pr incl (3) 314
jk10p18 Z3 euh clr 3:1 pr incl (3) 415
jk10p22 Z4 eq clr ball (1) 308
jk10p23 Z5 euh 2:1 pr (4) 345
jk10p75 Z6 clr ball (1) 226

Reference Th (b) total Pb com.(c)
 U (pg) Pb (pg)

jk10p16 0.53 44.8 0.2
jk10p17 0.53 59.6 0.3
jk10p18 0.47 72.7 0.3
jk10p22 0.42 89.3 0.5
jk10p23 0.39 54.9 0.4
jk10p75 0.62 62.2 0.4

Reference [sup.206]Pb [sup.206]Pb 2 [sigma]
 [sup.204]Pb [sup.238]U

jk10p16 15796 0.0575 0.0002
jk10p17 13947 0.0566 0.0002
jk10p18 15268 0.0565 0.0002
jk10p22 10838 0.0570 0.0003
jk10p23 8044 0.0561 0.0002
jk10p75 8911 0.0570 0.0003

Reference [sup.207] Pb 2 [sigma]
 [sup.235] U

jk10p16 0.428 0.002
jk10p17 0.421 0.001
jk10p18 0.419 0.002
jk10p22 0.422 0.002
jk10p23 0.415 0.001
jk10p75 0.422 0.002

 Ages (Ma)
Reference [sup.206]Pb 2 [sigma] [sup.207]Pb 2 [sigma]
 [sup.238]U [sup.206]Pb

jk10p16 360.4 1.1 373.3 4.6
jk10p17 355.0 1.1 368.1 4.9
jk10p18 354.4 1.4 363.0 4.4
jk10p22 357.4 1.6 356.0 4.9
jk10p23 351.6 1.0 358.3 4.8
jk10p75 357.1 1.7 362.4 5.0

Reference disc. (d)

jk10p16 3.6
jk10p17 3.7
jk10p18 2.4
jk10p22 -0.4
jk10p23 1.9
jk10p75 1.5

Data are from abraded zircons (Krogh 1982). Decay constants used are
those of Jaffey et al. (1971).

Notes: (a) Abbreviations: Z - zircon; euh - euhedral; eq - equant;
pr - prism; clr - colourless; incl - inclusions; 2:1, 3:1,
etc. - length: breadth ratio; Number in brackets indicates number of
grains analysed. (b) Th/U - based on radiogenic [sup.208]Pb/[sup.206]Pb
ratio and [sup.207]Pb/[sup.206]Pb age. (c) Com. Pb = total common
Pb; blank isotopic composition: [sup.206]Pb/[sup.204]Pb =18.221,
[sup.207]Pb/[sup.204]Pb = 15.612, [sup.208]Pb/[sup.204]Pb = 39.36.
(d) Disc. = percent discordance for the given [sup.207]Pb/[sup.206]Pb

Table 2. Chemical data for samples from the Wedgeport pluton

Map # Sample Si[O.sub.2] Ti[O.sub.2]

Biotite monzogranite

 1 P13-W02-023 72.81 0.30
 2 09-W01-222 73.22 0.33
 3 016-W02-003 (g) 73.26 0.29
 4 09-W01-223(g) 74.45 0.26
 5 09-W02-019 74.56 0.35
 6 09-W02-017 74.57 0.23
 7 09-W02-010B(g) 74.58 0.17
 8 09-W02-033A(g) 74.76 0.13
 9 09-W02-009(g) 75.10 0.18
 10 09-W02-014(g) 75.79 0.15
 11 09-W01-221C 75.98 0.18
 12 09-W02-082 76.09 0.19
 13 016-W02-004a(g) 76.45 0.15
 14 09-W02-034A(p) 76.58 0.12
 15 09-W02-010A(g) 77.28 0.17
 16 09-W02-015(g) 77.51 0.13

Aplite dykes

 17 016-W02-004b 75.55 0.10
 18 09-W02-033C 76.04 0.08

Sample [Al.sub.2] [[Fe.sub.2] MnO
 [O.sub.3] [O.sub.3]

Biotite monzogranite

P13-W02-023 13.45 2.34 0.03
09-W01-222 13.99 2.52 0.05
016-W02-003 (g) 13.25 2.52 0.05
09-W01-223(g) 13.50 2.34 0.06
09-W02-019 13.74 2.77 0.04
09-W02-017 13.31 2.03 0.04
09-W02-010B(g) 12.62 1.71 0.04
09-W02-033A(g) 12.47 1.55 0.03
09-W02-009(g) 12.39 1.65 0.03
09-W02-014(g) 12.77 1.55 0.04
09-W01-221C 13.41 1.51 0.02
09-W02-082 13.02 1.77 0.05
016-W02-004a(g) 12.01 1.59 0.03
09-W02-034A(p) 12.46 1.07 0.02
09-W02-010A(g) 12.79 1.88 0.04
09-W02-015(g) 12.43 1.37 0.04

Aplite dykes

 17 12.56 1.18 0.02
 18 12.37 1.18 0.03

 Sample MgO CaO [Na.sub.2]O

Biotite monzogranite

P13-W02-023 0.29 0.70 3.80
09-W01-222 0.51 0.74 3.34
016-W02-003 (g) 0.36 0.85 3.52
09-W01-223(g) 0.40 0.87 3.63
09-W02-019 0.50 0.48 3.70
09-W02-017 0.31 0.92 3.74
09-W02-010B(g) 0.22 0.45 3.30
09-W02-033A(g) 0.16 0.48 3.64
09-W02-009(g) 0.22 0.51 3.24
09-W02-014(g) 0.17 0.46 3.51
09-W01-221C 0.25 0.68 3.47
09-W02-082 0.24 0.79 3.59
016-W02-004a(g) 0.15 0.34 3.27
09-W02-034A(p) 0.10 0.37 3.43
09-W02-010A(g) 0.20 0.44 3.44
09-W02-015(g) 0.17 0.44 3.51

Aplite dykes

 17 0.09 0.42 3.82
 18 0.10 0.47 3.61

 Sample [K.sub.2]O [P.sub.2]

Biotite monzogranite

P13-W02-023 4.43 0.08
09-W01-222 5.38 0.14
016-W02-003 (g) 4.72 0.09
09-W01-223(g) 4.77 0.09
09-W02-019 4.05 0.11
09-W02-017 4.47 0.08
09-W02-010B(g) 4.96 0.06
09-W02-033A(g) 4.94 0.05
09-W02-009(g) 4.79 0.06
09-W02-014(g) 4.72 0.05
09-W01-221C 5.25 0.06
09-W02-082 4.51 0.06
016-W02-004a(g) 4.67 0.04
09-W02-034A(p) 4.91 0.03
09-W02-010A(g) 4.16 0.06
09-W02-015(g) 4.76 0.04

Aplite dykes

 17 4.23 0.03
 18 4.50 0.03

 Sample LOI Total Ba

Biotite monzogranite

P13-W02-023 0.74 98.97 302
09-W01-222 0.70 100.92 314
016-W02-003 (g) 0.64 99.55 269
09-W01-223(g) 0.60 100.97 277
09-W02-019 1.12 101.42 167
09-W02-017 0.51 100.21 389
09-W02-010B(g) 0.61 98.72 261
09-W02-033A(g) 0.50 98.71 141
09-W02-009(g) 0.88 99.05 152
09-W02-014(g) 0.69 99.89 75
09-W01-221C 0.41 101.23 174
09-W02-082 0.20 100.51 279
016-W02-004a(g) 0.28 98.98 58
09-W02-034A(p) 0.40 99.49 82
09-W02-010A(g) 0.39 100.85 109
09-W02-015(g) 0.66 101.05 136

Aplite dykes

 17 0.61 98.61 29
 18 0.49 98.90 171

 Sample Rb Sr Y

Biotite monzogranite

P13-W02-023 220 71 37
09-W01-222 342 73 46
016-W02-003 (g) 348 54 56
09-W01-223(g) 292 61 40
09-W02-019 208 87 67
09-W02-017 261 64 41
09-W02-010B(g) 343 30 48
09-W02-033A(g) 335 27 46
09-W02-009(g) 315 29 53
09-W02-014(g) 343 25 48
09-W01-221C 352 35 49
09-W02-082 263 64 48
016-W02-004a(g) 344 18 52
09-W02-034A(p) 281 20 81
09-W02-010A(g) 348 17 72
09-W02-015(g) 301 25 26

Aplite dykes

 17 320 20 63
 18 291 15 32

 Sample Zr Nb Th

Biotite monzogranite

P13-W02-023 196 16 32
09-W01-222 203 28 20
016-W02-003 (g) 186 25 41
09-W01-223(g) 176 20 34
09-W02-019 228 25 46
09-W02-017 150 20 23
09-W02-010B(g) 121 25 30
09-W02-033A(g) 107 23 39
09-W02-009(g) 130 25 38
09-W02-014(g) 120 25 31
09-W01-221C 137 23 37
09-W02-082 139 28 19
016-W02-004a(g) 124 23 48
09-W02-034A(p) 125 23 51
09-W02-010A(g) 131 24 32
09-W02-015(g) 111 22 39

Aplite dykes

 17 110 20 52
 18 96 22 37

 Sample Pb Ga Zn

Biotite monzogranite

P13-W02-023 19 19 29
09-W01-222 29 20 40
016-W02-003 (g) 18 20 32
09-W01-223(g) 33 19 45
09-W02-019 13 21 38
09-W02-017 22 19 27
09-W02-010B(g) 45 18 28
09-W02-033A(g) 24 16 22
09-W02-009(g) 22 18 23
09-W02-014(g) 45 20 21
09-W01-221C 17 21 16
09-W02-082 25 20 27
016-W02-004a(g) 22 17 22
09-W02-034A(p) 16 19 14
09-W02-010A(g) 51 17 32
09-W02-015(g) 17 19 27

Aplite dykes

 17 20 21 13
 18 21 21 15

 Sample Cu Ni V

Biotite monzogranite

P13-W02-023 <4 6 36
09-W01-222 4 12 42
016-W02-003 (g) <4 5 37
09-W01-223(g) <4 5 34
09-W02-019 <4 9 42
09-W02-017 <4 4 29
09-W02-010B(g) 14 5 23
09-W02-033A(g) <4 6 18
09-W02-009(g) 5 5 24
09-W02-014(g) 13 5 21
09-W01-221C <4 6 27
09-W02-082 <4 6 26
016-W02-004a(g) 7 5 20
09-W02-034A(p) <4 7 16
09-W02-010A(g) 4 5 23
09-W02-015(g) <4 6 19

Aplite dykes

 17 6 12 17
 18 <4 6 13

 Sample Cr Co U

Biotite monzogranite

P13-W02-023 <4 71 7
09-W01-222 14 61 7
016-W02-003 (g) <4 85 8
09-W01-223(g) 7 71 7
09-W02-019 6 73 8
09-W02-017 5 72 7
09-W02-010B(g) <4 82 8
09-W02-033A(g) <4 73 8
09-W02-009(g) <4 73 10
09-W02-014(g) <4 83 8
09-W01-221C 21 74 7
09-W02-082 <4 83 9
016-W02-004a(g) 6 84 10
09-W02-034A(p) 11 89 15
09-W02-010A(g) 6 69 13
09-W02-015(g) <4 82 7

Aplite dykes

 17 0 81 12
 18 <4 82 12

Notes: (g) = has garnet; (p) = porphyritic. Analyses were done by X-ray
Fluorescence at the Regional Geochemical Centre, Saint Mary's
University, Halifax, Nova Scotia. Major elements and some trace
elements were determined on fused glass disks and other trace elements
were measured in pressed powder pellets (see for
description of the methodology, and accuracy/precision information).
Analytical error is generally less than 5% for major elements and 2-10%
for trace elements. [[Fe.sub.2][O.sub.3].sup.t] is total Fe as
[Fe.sub.2][O.sub.3]. LOI is loss on ignition at 1000[degrees]C.

Table 3. Rare-earth element, Hf, and Ta data * for samples from the
Wedgeport pluton

Map # Sample La Cc Pr

 3 016-W02-003 33.85 65.86 7.42
 4 009-W01-223 22.65 50.99 4.92
 14 009-W02-033C 3.14 8.94 1.00
 16 016-W02-004a 31.48 63.21 7.28
 17 009-W02-034A 2S.19 53.73 6.28

Sample Nd Sm Eu Gd

016-W02-003 26.81 5.92 0.47 6.06
009-W01-223 17.68 3.92 0.28 3.96
009-W02-033C 4.05 1.48 0.08 1.84
016-W02-004a 25.82 5.59 0.17 5.19
009-W02-034A 23.05 6.04 0.14 6.53

 Sample Tb Dy Ho Er

016-W02-003 1.05 6.63 1.30 3.98
009-W01-223 0.70 4.53 0.90 2.87
009-W02-033C 0.39 2.96 0.62 2.23
016-W02-004a 0.93 5.96 1.18 3.67
009-W02-034A 1.24 8.31 1.69 5.55

 Sample Tm Yb Lu Hf

016-W02-003 0.65 4.40 0.65 6.04
009-W01-223 0.45 3.18 0.49 4.01
009-W02-033C 0.42 3.38 0.53 3.21
016-W02-004a 0.60 4.26 0.62 4.33
009-W02-034A 0.94 6.80 1.04 3.98

 Sample Ta

016-W02-003 5.00
009-W01-223 4.53
009-W02-033C 5.25
016-W02-004a 5.52
009-W02-034A 7.72

* Analyses at Memorial University of Newfoundland by ICP-MS, using the
[Na.sub.2][O.sub.2] sinter method (Longerich et al. 1990).

Table 4. New Sm-Nd isotopic data for sample 016-WO2-04A from the
Wedgeport pluton

 Nd Sm [sup.147]Sm [sup.143]Nd
 (ppm) (ppm) [sup.144]Nd [sup.144]Nd 2[sigma]

 26.52 5.964 0.136 0.512602 0.000030

 (ppm) [[epsilon].sub.Nd(0)] [[epsilon].sub.Nd(t)]

 26.52 -0.7 2.1

 (ppm) T.sub.(DM)]

 26.52 873 Ma

Notes: Analyses by Alain Potrel, Memorial University of Newfoundland.
Sm and Nd contents and Nd isotopic composition were analyzed using a
multicollector Finnigan Mat 262 mass spectrometer in static mode. Nd
isotopic ratio are normalized to [sup.146]Nd/[sup.144]Nd = 0.7219. The
reported values were adjusted to La Jolla Nd standard
([sup.143]Nd/[sup.144]Nd = 0.511860). During the course of data
acquisition replicates of the standard gave a mean value of
[sup.143]Nd/[sup.144]Nd = 0.511886 [+ or -] 26 (2[sigma], n=18). The
in-run precisions on Nd isotopic ratio are given at 95% confidence
level. Error on Nd isotopic compositions are <0.002% and errors on the
[sup.147]Sm/[sup.144]Nd ratio are estimated to be less than 0.1%. The
[[epsilon].sub.Nd] values are calculated using a
[sup.147]Sm/[sup.144]Nd = 0.1967 and [sup.143]Nd/[sup.144]Nd = 0.512638
values for the present day chondrite uniform reservoir (CHUR).
[sup.147]Sm decay constant is 6.54 [10.sup.-12] [y.sup.-1] (Steiger and
Jager 1977). The depleted mantle model age, [T.sub.(DM)], was
calculated both with respect to depleted mantle with
[[epsilon].sub.Nd(0)] value of +10 isolated from the CHUR since 4.55 Ga
and following a linear evolution with respect to the De Paolo (1988)
mantle model.


This project was funded mainly by the Nova Scotia Department of Natural Resources through the Southwest Nova Mapping Project. Barr's contributions were funded by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada. We thank Georgia Pe-Piper for providing locations for samples reported by Pe-Piper and Jansa (1999). Ken Currie and Linda Ham provided helpful reviews which improved the manuscript.


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Editorial responsibility: Ron K. Pickerill

Natalie J. MacLean, (1) Sandra M. Barr, (1) * Chris E. White, (2) and John W.F. Ketchum (3) ([dagger])

(1.) Department of Geology, Acadia University, Wolfville, NS B4P 2R6, Canada

(2.) Nova Scotia Department of Natural Resources, P.O. Box 698, Halifax, NS B3N 2T9, Canada

(3.) Jack Satterly Geochronology Lab, Royal Ontario Museum, 100 Queen's Park, Toronto, ON M5S 2C6, Canada

* Corresponding author <>

([dagger]) Current address: GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia

Date received: March 22, 2004 * Date accepted: June 29, 2004
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Author:MacLean, Natalie J.; Barr, Sandra M.; White, Chris E.; Ketchum, John W.F.
Publication:Atlantic Geology
Geographic Code:1CNOV
Date:Nov 1, 2003
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