The White Rock Formation in the Yarmouth area of the Meguma terrane of southern Nova Scotia consists mainly of mafic tuffaceous rocks with less abundant mafic flows, epiclastic and clastic sedimentary rocks, and minor intermediate and felsic crystal tuff. It is divided into seven map units that appear to young from west to east, inconsistent with a previously assumed synclinal structure. The White Rock Formation is flanked on both northwest and southeast by mainly the Cambrian to Lower Ordovician Halifax Formation; the western contact is interpreted to be a sheared disconformity, whereas the eastern contact appears to be a major brittle fault and shear zone that juxtaposes different crustal levels. The granitic Brenton Pluton forms a faulted lens within the eastern shear zone. A felsic tuff from the upper part of the White Rock Formation yielded a U–Pb zircon age of Ma, identical within error to published ages for the Brenton Pluton and felsic volcanic rocks near the base of the White Rock Formation in the Torbrook area of western Nova Scotia. The chemical characteristics of the mafic volcanic rocks and associated mafic intrusions consistently indicate alkalic affinity and a continental within-plate setting. The felsic volcanic rocks and Brenton Pluton have chemical characteristics of within-plate anorogenic granitic rocks, and the pluton is interpreted to be comagmatic with the felsic volcanic rocks. The igneous activity may have occurred in response to extension as the Meguma terrane rifted away from Gondwana in the latest Ordovician to Early Silurian. Epsilon Nd values are similar to those in voluminous Devonian plutonic rocks of the Meguma terrane, and the magmas appear to have been derived from similar sources.
Igneous rocks of Silurian age are a significant component of the northern Appalachian orogen (e.g., Chandler et al. 1987; Dunning et al. 1990; Bevier and Whalen 1990; Doig et al. 1990; Barr and Jamieson 1991). The widespread distribution of these rocks of similar age has led to the suggestion that they are an overstep sequence, with the implication that the terranes of the orogen were assembled along the Laurentian margin by the Silurian (e.g., Chandler et al. 1987; Keppie and Krogh 2000; Lynch 2001). However, enhanced knowledge of the petrological characteristics, tectonic settings, and precise ages of the rocks is required to confirm or disprove this model. This paper contributes to the understanding of Silurian tectonic evolution of the orogen by providing such information about Silurian igneous rocks in the most outboard terrane in the orogen, the Meguma terrane of southern Nova Scotia (Fig. 1).
In the Meguma terrane, Silurian igneous rocks include volcanic units in the White Rock Formation in the Yarmouth, Cape St. Mary, and Torbrook areas, and granitic rocks in the Brenton Pluton in the Yarmouth area (Fig. 1). Recently published U–Pb (zircon) ages of 442 ± 4 Ma for a rhyolitic tuff in the White Rock Formation in the Torbrook area and Ma for the Brenton Pluton (Keppie and Krogh 2000) confirm identical Silurian ages for these units. However, it was still not known if the volcanic rocks in the Yarmouth area are of the same age as those in the Torbrook area because the relationship of the Brenton Pluton to the volcanic rocks was not clear from previous studies (e.g., Hwang 1985). Furthermore, previous petrochemical studies of the volcanic rocks and the Brenton Pluton had yielded inconclusive results about their chemical affinity and tectonic setting (Sarkar 1978; O’Reilly 1976).
The purpose of this paper is to report the results of a field and petrological study of the Brenton Pluton and the White Rock Formation in the Yarmouth area, and to present a U–Pb age from a felsic tuff in the White Rock Formation. The results show that the volcanic rocks and the pluton are cogenetic and have important implications for tectonic evolution of the Meguma terrane.
The Meguma terrane is dominated by the Meguma Group and the South Mountain Batholith (Fig. 1). Traditionally, the Meguma Group is divided into two formations (Keppie 2000). The Goldenville Formation typically consists of massive to locally laminated or cross-laminated, fine-grained metasandstone, interbedded with minor metasiltstone and slate. The conformably overlying Halifax Formation consists mainly of thinly laminated slate and metasiltstone. The maximum age of the Meguma Group is constrained by detrital 40Ar/39Ar muscovite ages of 600–550 Ma (Hicks et al. 1999; Muir 2000) and comparable U–Pb dates from detrital zircon and titanite in the Goldenville Formation (Krogh and Keppie 1990). Middle Cambrian fossils occur in the upper part of the Goldenville Formation (Pratt and Waldron 1991), and Early Tremodocian fossils in the upper part of the Halifax Formation (Doyle 1979; White et al. 1999). Hence, it is likely that both formations are dominantly Cambrian, although the Halifax Formation extends into at least the Early Ordovician (e.g., Keppie 2000).
Slate, quartzite, and (or) metavolcanic rocks of the White Rock Formation overlie the Halifax Formation along the western margin of the Meguma terrane (Fig. 1). The original nature of the contact between the two formations is debated, because of the effects of later deformation, and has been interpreted as both conformable and unconformable (e.g., Schenk 1995; White et al. 1999; Keppie 2000). Metavolcanic rocks are present in the White Rock Formation in the Yarmouth, Cape St. Mary, and Torbrook areas, but are absent in the Digby and Wolfville areas (White et al. 1999; Ferguson 1990). Rhyolitic tuff at the base of the formation in the Torbrook area yielded a U–Pb (zircon) age of 442 ± 4 Ma (Keppie and Krogh 2000). Metasedimentary rocks in the upper part of the formation in the Digby area yielded Upper Silurian vertebrate and crinoid remains (Bouyx et al. 1997), indicating that the upper part of the formation in that area is laterally equivalent to the fossiliferous upper Silurian Kentville and New Canaan formations in the Wolfville area. The U–Pb age and fossil control indicate that the White Rock Formation, in western Nova Scotia at least, is mainly Silurian in age. Single and multigrain detrital 40Ar/39Ar muscovite ages from the White Rock Formation in the Digby area indicate ages, probably metamorphic, of ca. 500 Ma in the source area, somewhat younger than equivalent data from the Meguma Group (Muir 2000).
The Torbrook Formation conformably overlies the White Rock Formation in the Digby and Torbrook areas (Fig. 1) and consists of poorly cleaved, grey metasiltstone and calcareous metasiltstone with minor slate, metasandstone, marble, and rare ironstone. It contains Early Devonian (Lochkovian to lower Emsian) shelly fossils (Bouyx et al. 1997).
The Meguma Group and overlying White Rock and Torbrook formations were deformed into northeast-trending, generally upright, folds with near-horizontal axes during the Acadian Orogeny (Keppie and Dallmeyer 1987). Detailed 40Ar/39Ar studies on single and multigrain muscovite and whole-rock samples suggest that deformation and greenschist-facies metamorphism occurred at ca. 400–390 Ma, soon after deposition of the Early Devonian Torbrook Formation (Muecke et al. 1988; Kontak et al. 1998; Hicks et al. 1999; Muir 2000). However, amphibolite-facies metamorphism in the Yarmouth area may have occurred at ca. 380 Ma, based on the U–Pb age of monazite from the Brenton Pluton interpreted to be of metamorphic origin (Keppie and Krogh 2000).
The ca. 370 Ma South Mountain Batholith and satellite plutons (Fig. 1) consist mainly of medium- to coarse-grained monzogranite and granodiorite with megacrysts of K-feldspar. These intrusions produced well-developed contact metamorphic aureoles with abundant andalusite and cordierite that are superimposed on regional fabrics. In the northern part of the Meguma terrane, adjacent to the Cobequid–Chedabucto Fault, similar granitic rocks are associated with gabbro and high-grade gneiss in the Liscomb Complex (Fig. 1). The Liscomb Complex was emplaced after deformation of the surrounding metasedimentary rocks of the Meguma Group and experienced rapid uplift and exhumation in the Late Devonian (Clarke et al. 1993; Kontak and Reynolds 1994). Petrological data indicate that the gneissic rocks in the complex are unrelated to metasedimentary rocks of the Meguma terrane, and they have been interpreted to represent underlying Avalon terrane rocks (Clarke et al. 1993).
Along the southwestern margin of the Meguma terrane, several steep, northeast-trending shear zones overprint earlier folds (Keppie and Dallmeyer 1995; Culshaw and Liesa 1997; White et al. 1999, 2001; Horne et al. 2000). This Middle Carboniferous (Culshaw and Reynolds 1997) deformation is postulated to be related to strike-slip movement along the terrane-bounding Cobequid–Chedabucto fault system and may record juxtaposition of the Meguma terrane against North America.
Stratigraphy of the White Rock Formation in the Yarmouth area
The White Rock Formation in the Yarmouth area forms a northeast-trending belt with a maximum width of about 15 km in the well-exposed coastal section (Figs. 1, 2). The southeastern margin is in faulted contact with slate of the Halifax Formation in the south and metasandstone of the Goldenville Formation farther to the north (Fig. 2). This fault is inferred to be a brittle structure that juxtaposes chlorite- and biotite-grade rocks of the Halifax and Goldenville formations against staurolite-grade, typically schistose, rocks of the White Rock Formation. The brittle fault is located within the broader Chebogue Point shear zone of Culshaw and Liesa (1997), previously referred to as the Deerfield shear zone by Keppie and Dallmeyer (1995), that affects both the White Rock Formation and the Meguma Group. However, in the coastal section north of Chebogue Point (Fig. 2), the metamorphic grade in the White Rock Formation appears lower than in the same unit to the north, and the rocks seem less sheared than those of the adjacent Halifax Formation.
On the northwest, the position of the contact between the White Rock and Halifax formations is poorly defined because of lack of exposure both inland and on the coast. The most southerly outcrops of slate of the Halifax Formation at Cranberry Point are separated by nearly 3 km of no outcrop from the most northerly outcrop of metasedimentary rocks assigned to the White Rock Formation (Fig. 2). The contact position shown on Fig. 2 is based on a strong aeromagnetic signature that appears to be associated with the Halifax Formation (Fig. 2). The Halifax Formation and possibly the adjacent White Rock Formation in that area are deformed in the Cranberry Point shear zone (Culshaw and Leisa 1997).
The rocks of the White Rock Formation have been metamorphosed to amphibolite facies, and the combined effects of metamorphism and deformation have obscured primary sedimentary and volcanic features in many outcrops. However, based on macroscopic relict igneous features such as phenocrysts, amygdales, lithic and crystal clasts, and bombs, metavolcanic units are interpreted to include both flows and varied tuffaceous rocks. Possible pillow structures are preserved locally and, overall, the sequence appears to have been deposited in a mainly subaqueous setting, with minor subaerial eruptions suggested by coarsely vesicular mafic flows and rare ignimbritic felsic units that contain vestiges of flow-banding and flattened pumice fragments. Some basaltic flows preserve amygdaloidal tops, and sedimentary and reworked tuffaceous units locally preserve cross-bedding and graded bedding that enable the determination of younging direction.
Basaltic to andesitic rocks now consist mainly of amphibole, plagioclase, quartz, and epidote, with less abundant biotite, chlorite, opaque minerals, carbonate minerals, and apatite. Andesitic rocks have a lower mafic index than basaltic rocks. Texture at the thin-section scale is metamorphic, but varies widely, probably in response to variations in the original igneous grain size and texture (MacDonald 2000). Intermediate to felsic rocks include crystal tuff dominated by feldspar with less abundant quartz grains in a recrystallized quartzofeldspathic groundmass with abundant muscovite and epidote. More detailed petrographic descriptions are given in MacDonald (2000).
The White Rock Formation in the Yarmouth area previously has been interpreted to occur in a synclinal structure (Taylor 1967; Lane 1979; Hwang 1985; Culshaw and Liesa 1997). MacDonald (2000) also inferred the presence of a syncline in her division of the formation into lithologic units. Her units were based on detailed observations in well-exposed coastal sections, and projected inland using sparse outcrops and aeromagnetic data (King 1997). However, follow-up work (White et al. 2001) suggested that the aeromagnetic signatures may not be a reliable way to trace specific units inland because the signatures appear to vary along strike. More importantly, the rocks appear to be consistently right-way-up and to young to the southeast, without repetition of distinctive lithologic units. Hence, although stratigraphic relations are complicated by numerous shear zones and faults (Fig. 2), the observations call into question the presence of a syncline. Instead, we have re-interpreted the units of MacDonald (2000) in terms of a southeast-dipping succession of seven units, as shown on Fig. 2 and described below.
(1) The stratigraphically lowermost unit is dominantly metasedimentary. The lower part consists of tuffaceous metasandstone and metasiltstone beds with calc-silicate beds and lenses. Upward, these rocks become interlayered with laminated phyllite and slate, the latter containing trace fossils (as yet unidentified). The phyllite and slate grade upward into metasiltstone with distinctive garnetiferous schist layers up to 2 m thick that contain locally abundant chloritoid and are associated with sulphide-rich metasandstone layers. Thickbedded, massive to well-laminated quartzite occurs near the top of the unit.
(2) Above the quartzite is a mainly mafic metavolcanic unit. The lower part consists of mafic crystal and lithic-crystal tuff interlayered with minor intermediate to felsic crystal to crystal-lithic tuff. It also includes a boudinaged quartzite layer, like those in the underlying unit. The metavolcanic rocks are overlain by tuffaceous metasandstone interlayered with thinly bedded metasiltstone and metasandstone. These rocks grade upward into andesitic lithic to lithic-crystal tuff.
(3) In faulted contact above the andesitic rocks is a unit of predominantly mafic volcanic and metasedimentary rocks. The lower part consists of mafic lithic tuff and flows. The middle part consists of well-laminated metasiltstone and metasandstone, locally interlayered with amphibole-rich tuffaceous metasandstone and mafic crystal tuff and flows, and includes a massive quartzite layer 2 m wide. The metasandstone sequence contains calc-silicate lenses, abundant trace fossils of chondrites, and rare brachiopod shells (Lane 1979). Up-section, the metasedimentary package is in sharp contact with a thick package of mafic lithic to lithic-crystal tuff and associated flows interlayered with minor tuffaceous metasandstone. The uppermost part of the unit contains intermediate crystal tuff and conglomeratic layers.
(4) The tuffaceous rocks and conglomerate are in faulted contact with a dominantly mafic volcanic unit. The lower part consists of massive flows and mafic lithic tuff, locally interlayered with tuffaceous metasandstone. The middle part of the unit is well exposed at Cape Forchu and consists of thinly bedded mafic lithic tuff and tuffaceous sandstone with well preserved original volcanic and sedimentary features (Figs. 3a, 3b) and minor calc-silicate lenses. The upper part of the unit is separated from the middle part by a shear zone and consists of massive mafic flows and lithic to crystal-lithic tuff. Locally the flows contain pillow-like structures (Fig. 3c) and brecciated flow tops.
(5) Overlying the mafic unit is another mainly metasedimentary unit. The lower part consists of thickly bedded metasandstone with abundant calc-silicate nodules and interbedded chloritoid-spotted slate. The middle part consists of chloritoid-spotted slate interbedded with metasiltstone, and near the top of the unit, the metasedimentary rocks are interlayered with mafic lithic tuff and flows and tuffaceous metasandstone.
(6) Above the metasedimentary rocks, in gradational contact, is a sequence of mafic lithic tuff and flows interlayered with minor tuffaceous metasandstone. The mafic flows locally contain peperite-like structures, and many have amygdaloidal upper parts, 1–2 m wide, that indicate younging to the east. Cross-bedding and graded bedding in metasandstone layers also indicates younging to the east. Unit 6 also contains two distinctive felsic crystal-tuff layers, each about 10 m thick. The lower felsic layer can be traced inland for several kilometres in sporadic outcrops. The upper felsic layer was sampled for U–Pb dating (see in the following text).
(7) The uppermost unit contains a mixture of metasedimentary and metavolcanic rocks. Through most of the unit, the metasedimentary rocks are staurolite-bearing micaceous phyllite and schist locally interlayered with thin horizons (< 30 cm thick) of metasandstone and amphibolite. The metavolcanic rocks are mafic to rarely intermediate lithic tuff interlayered with tuffaceous metasandstone, and occur in a central belt flanked by metasedimentary rocks. Exposures along the coastal section north of Chebogue Point are not like inland exposures of the unit in that they lack phyllite and schist and consist of metasiltstone and metasandstone with interlayered mafic rocks that could be either flows or sills. No evidence for younging direction was observed, and these rocks appear to be in faulted contact with more deformed slate of the Halifax Formation. Large areas of medium- to coarse-grained amphibolite of uncertain protolith, as well as scattered outcrops of interlayered quartzite and amphibolite, occur in the vicinity of Brazil Lake (Fig. 2) and appear also to be part of unit 7.
This new stratigraphic interpretation suggests that the White Rock Formation in the Yarmouth area, may be at least 10 km thick, with neither the top nor bottom definitively exposed. The contrast in metamorphic grade at both the northwestern and southeastern contacts between the White Rock Formation and the adjacent Halifax and Goldenville formations suggests that neither of these contacts is stratigraphic, in contrast to the assumption in the syncline interpretation (e.g., Taylor 1965; Hwang 1985). Lack of a preserved stratigraphic relationship is further supported by the presence of major shear zones in both areas and the pinching out of the Halifax Formation along the southeastern contact. However, the interpretation is considered preliminary because of the scarcity of inland exposures and the presence of major shear zones and faults within the succession, in addition to those at the margins (Fig. 2; Culshaw and Leisa 1997; White et al. 2001).
The Brenton Pluton (Taylor 1967) is elongate and fault bounded, with the western contact faulted against the uppermost unit of the White Rock Formation and the eastern contact faulted against the Halifax Formation (Fig. 2). The pluton consists dominantly of grey, medium-grained, syenogranite to monzogranite (O’Reilly 1976; MacDonald 2000) that is strongly foliated and lineated, except for local areas in the central part of the pluton, where it is equigranular and lacks foliation. The foliation strikes north-northeast and dips steeply to the northwest (White et al. 2001). Stretching lineations have moderate plunge to the southwest, west, and northwest along the western margin and to the east and northeast near the eastern margin. This dome-like pattern is consistent with upward motion of the pluton within the Chebogue Point shear zone. Although previous workers reported a contact metamorphic aureole associated with the pluton (e.g., Taylor 1967; O’Reilly 1976; Hwang 1985), that interpretation appears to have been based mainly on the presence of staurolite- and garnet-bearing rocks in the vicinity of the pluton. As described above, these rocks are typical of unit 7 of the White Rock Formation, and their distribution is not spatially related to the pluton.
The granite contains approximately equal amounts (ca. 30–35%) of quartz and K-feldspar, lesser amounts of plagioclase (20–25%), minor biotite and muscovite, and accessory apatite, garnet, monazite, zircon, and opaque minerals. The foliation is defined by parallel elongate grains of feldspar and mica, and clusters (augen) of quartz and feldspar. Myrmekitic intergrowths of quartz and plagioclase (< An10) are common. The K-feldspar is perthitic microcline, for which O’Reilly (1976) reported compositions of Or90–94. Accessory garnet is spessartine-rich almandine (O’Reilly 1976). O’Reilly (1976) interpreted the garnet to be of igneous rather than xenocrystic origin because he inferred the pre-intrusion metamorphic grade of the host rocks to be too low for garnet to be present. He also interpreted the pluton to have been recrystallized under biotite-grade conditions due to the absence of deformation features in mica around garnet augen. However, if the monazite age of 380 ± 3 Ma represents the time of metamorphism of the pluton, as suggested by Keppie and Krogh (2000), then the grade during metamorphism and deformation was even higher, and the garnet could be of metamorphic origin.
Abundant mafic sills occur in the White Rock Formation and the adjacent Halifax Formation. They are typically foliated, medium to coarse grained and boudinaged parallel to foliation. Some sills contain plagioclase phenocrysts, and some appear to be somewhat amygdaloidal near their eastern (top) margins, although some such units could be flows rather than sills. Crosscutting intrusions (dykes) are rare. Also present locally are areas of amphibolite that appear to represent small plutons. The amphibolitic mineralogy in all of these mafic bodies is similar to that in the mafic flows of the White Rock Formation, to which they appear to be related. No mafic intrusions were observed in the Brenton Pluton.
Samples were selected for chemical analysis on the basis of relatively fine grain size and mineralogical homogeneity. Coarse crystal or lithic tuff and porphyritic or amygdaloidal units were avoided. Major and trace element data (Table 1) were obtained for 16 samples from mafic flows and fine-grained tuff, three tuffaceous samples of intermediate composition, five felsic tuffaceous samples, 16 samples from the Brenton Pluton, and 12 samples from mafic sills, dykes, and small plutons in both the White Rock Formation and the adjacent Halifax Formation. Seven samples were also analyzed for rare-earth elements (Table 2). Analytical methods are given in the table footnotes, and sample descriptions are presented in MacDonald (2000). Relatively few intermediate samples are represented in the data set because they represent a minor part of the sequence, and most are crystal and lithic tuffs that are unlikely to represent magmatic compositions. Sarkar (1978) previously reported chemical data from the White Rock Formation in the study area; however, rock type and sample locations were not well documented, and the data set is of variable quality. MacDonald (2000) made a detailed comparison with this earlier data set and showed that it is generally consistent with the data from the present study, although the older data are more scattered. The only consistent chemical difference appears to be somewhat higher Nb values in the data set of Sarkar (1978).
Mafic samples from the present study range in SiO2 content from about 43 to 52 wt.% (Fig. 4). Samples interpreted to be from mafic flows generally display less chemical variation than those from tuffaceous rocks. The tuffaceous samples show a particularly wide spread in Al2O3 and MgO, correlating with variation in relative abundance of plagioclase and amphibole (Figs. 4b, 4d), and are unlikely to represent unmodified magmatic compositions. Samples from mafic sills, dykes, and plutons are chemically similar to the mafic volcanic rocks, supporting a genetic relationship as feeder dykes and hypabyssal intrusions. Although six of the seven samples from mafic flows plot in the “igneous spectrum” in terms of Na and K compositions, in contrast to the more scattered compositions of tuffaceous samples (Fig. 5a), such mobile elements are not considered to be reliable indicators of magmatic characteristics in metamorphosed rocks, and hence the following interpretations rely on the typically more immobile elements (e.g., Floyd and Winchester 1978). TiO2 and Zr and Nb and Zr show co-variation in the mafic samples (Figs. 5b, 5c), suggesting that at least their ratios are likely to reflect the original igneous values. The spread of Nb/Y ratios suggests that the suite is transitional between subalkalic and alkalic (Fig. 5d), but there is no evidence for separate “slightly alkaline basalt-trachyte” and “olivine tholeiite – rhyolite” series, as suggested by Sarkar (1978).
On the commonly used Ti–Zr–Y tectonic setting discrimination diagram, the mafic samples generally plot in the within-plate basalt field (Fig. 6a). On diagrams that discriminate alkalic and tholeiitic within-plate basalts, they display alkalic characteristics (Figs. 6b, 6c), and a multi-element variation diagram for selected elements (after Pearce 1996) shows most similarity to the average within-plate alkalic basalt (Fig. 6d). Chondrite-normalized rare-earth element (REE) patterns for the mafic volcanic and sill samples show near-parallel smooth patterns with no significant Eu anomalies (Fig. 7a). Light REE (LREE) enrichment (La/Sm = 3–4) is typical of mafic alkalic rocks.
The intermediate tuff samples are characterized by abundant sodic plagioclase, consistent with their high Al2O3 and Na2O contents (Figs. 4b, 4f). They contain little or no quartz and variable amounts of muscovite, reflected in the low SiO2 (ca. 60%) and varied K2O contents (Fig. 4g). They appear anomalous compared to both more mafic and more felsic samples in many of their chemical features, especially the high Al2O3, Na2O, Zr and Nb contents (e.g., Figs. 4b, 4f, 5b, 5c) and seem unlikely to represent magmatic compositions.
The felsic tuff samples have a range in SiO2 (ca. 71–80%), high K2O, and moderate Na2O contents, consistent with the presence of abundant quartz and alkali feldspar, and minor amounts of chlorite, epidote, and opaque minerals. Their immobile element values show that they are unrelated to the mafic samples (Figs. 5b, 5c), and Nb/Y ratios suggest that they are mildly alkalic (Fig. 5d). The samples plot in the within-plate field on the Nb–Y tectonic setting diagram for granite (Fig. 8a) and have Zr and Ga/Al values like those of “A-type” granite (Fig. 8b), but distinction between “mantlederived” or “crust-derived” melt is unclear (Fig. 8c). REE analyses were obtained for one sample, and show LREE enrichment and a strong negative Eu anomaly (Fig. 7b).
The analyzed samples from the Brenton Pluton show little chemical variation, with SiO2 content about 75–77% (Fig. 4). They are generally similar in composition to the felsic tuff samples, but show less variation (Figs. 4, 5). Like the felsic tuff samples, they plot in the within-plate granite field (Fig. 8a), have compositions consistent with those of “A-type” granite (Fig. 8b), and are ambiguous in terms of mantle or crustal origin (Fig. 8c). The chondrite-normalized REE pattern is also similar to that of the felsic tuff sample, showing LREE enrichment (La/Sm = 4.9) and relatively flat heavy REE, with a strong negative Eu anomaly (Fig. 7b). The chemical data are consistent with a comagmatic relationship between the Brenton Pluton and felsic tuffaceous rocks of the White Rock Formation.
Six zircon fractions were analysed from felsic tuff sample LAM004-1 (Table 3). Four fractions of prismatic zircon grains are strongly discordant, whereas two fractions of small, colourless needles plot on or near concordia (Fig. 9). Three of the prismatic fractions and the concordant needle fraction (Z6) are colinear with an upper intercept age of 2061 ± 24 Ma and a lower intercept age of 438 ± 10 Ma (Fig. 9). However, fraction Z6 is concordant at Ma, and that age is interpreted to represent the crystallization age of the tuff. The upper intercept age for sample LAM004-1 is likely to represent inherited zircon and is similar to some of the detrital zircon ages in the Meguma Group (Krogh and Keppie 1990).
The crystallization age is identical within error to the age of Ma reported by Keppie and Krogh (2000) for granite from the Brenton Pluton. The similarity in age is consistent with the comagmatic relationship between the felsic volcanic rocks and the Brenton Pluton suggested by the chemical data. The ages are also identical within error to the crystallization age of 442 ± 4 Ma reported by Keppie and Krogh (2000) for crystal tuff at the base of the White Rock Formation in the Torbrook area, 100 km to the northeast (Fig. 1).
Sm-Nd isotopic data
Dated rhyolitic tuff sample LAM004-1 and a sample from the Brenton Pluton were analysed for Sm and Nd isotopes (Table 4). The εNd values are 1.37 and –0.42, with depleted mantle model ages of 1209 Ma and 1321 Ma, respectively. The difference suggests more crustal input in the Brenton Pluton compared to the White Rock Formation. However, the range of values is similar to that reported by Keppie et al. (1997) for three samples from the dated rhyolite unit at the base of the White Rock Formation in the Torbrook area (Fig. 10). No mafic samples were analysed in the present study, but data for mafic samples from Cape St. Mary and the Torbrook area show a spread of values consistent with varying amounts of crustal contamination of magmas derived from depleted mantle (Fig. 10).
The values from the felsic volcanic rocks and the Brenton pluton are within the mid- to high part of the range reported for widespread Devonian granitic plutons in the Meguma terrane, including South Mountain Batholith. They are consistent with derivation from sources similar to those represented by gneissic rocks in the Liscomb complex (Clarke et al. 1997), but show little influence from the metasedimentary rocks of the Meguma Group, which have highly negative εNd values (Fig. 10). The mafic volcanic rocks of the White Rock Formation show generally less evidence of crustal involvement than Devonian mafic intrusions (Fig. 10).
Previous interpretations of the Yarmouth area (cf. Taylor 1967; Lane 1979; Hwang 1985; Schenk 1995; Culshaw and Liesha 1997; MacDonald 2000) used variations on a synclinal model. These interpretations were based in part on extrapolation from better documented synclinal structures farther to the northeast (Fig. 1), as well as the presence of the Halifax Formation on both the northwestern and southeastern margins of the White Rock Formation in the Yarmouth area, and inferred repetition of units in the White Rock Formation. However, the evidence presented here, including the presence of a major shear zone and abrupt change in metamorphic grade on the southeast between the White Rock Formation and adjacent Halifax and locally Goldenville formations, consistent younging directions toward the southeast in the White Rock Formation, and no obvious repetition of units, at least in the well-exposed coastal section, does not support a synclinal model.
The White Rock Formation in the Yarmouth area differs from the formation in other areas in its high proportion of volcanic rocks (Fig. 11). At Cape St. Mary, where the section is only about 160 m in thickness, metavolcanic rocks constitute less than 40% (Kendall 1981). In the Torbrook area, volcanic rocks occur only in the lower, poorly exposed 20% of the ca. 600 m – thick section (Lane 1975, 1979). In the Digby and Wolfville areas, volcanic rocks do not appear to be present at all. In contrast, metavolcanic rocks constitute more than 80% of the formation in the Yarmouth area (Fig. 11), where the thickness of the formation appears to be at least 10 000 m. In both the Cape St. Mary and Torbrook sections, a felsic volcanic unit occurs at the base of the exposed section, whereas the rare felsic volcanic rocks in the Yarmouth area occur scattered through the middle units in the succession. However, felsic units in the Yarmouth and Torbrook areas have yielded the same U–Pb zircon age, within error. Although published chemical data are limited, analyses from Keppie et al. (1997) plotted with the data from this study (Figs. 4⇑⇑⇑–8) suggest that the volcanic rocks in the Cape St. Mary and Torbrook areas are similar to those of the Yarmouth area. The lack of obvious chemical evolution within the huge volcanic pile in the Yarmouth area suggests that the voluminous magmas may have formed over a relatively short time interval, and hence that the whole sequence may have formed at ca. 440 Ma. Although the original relationship between the Brenton Pluton and the White Rock Formation cannot be demonstrated due to high-grade regional metamorphism and sheared contacts, its similar U–Pb (zircon) age and chemical characteristics indicate that the two units formed at the same time (Fig. 11). The higher metamorphic grade in rocks of the White Rock Formation in the vicinity of the pluton is consistent with exposure of deeper crustal levels in that area, and the Brenton Pluton may represent a sheared and metamorphosed intrusion emplaced deep in the volcanic pile.
Schenk (1995) correlated the upper part of the volcanic sequence in the Yarmouth area with the New Canaan Formation of the Wolfville area. However, this interpretation is not supported by the U–Pb age presented here, which shows that the volcanic rocks in the Yarmouth area are latest Ordovician or Early Silurian, whereas the age of the New Canaan Formation is well constrained to the Late Silurian by fossils, both in the formation itself and in the underlying Kentville and laterally equivalent upper part of the White Rock Formation (Fig. 11; Bouyx et al. 1997). Like the White Rock Formation, the volcanic rocks in the New Canaan Formation have within-plate alkalic characteristics (James 1998), and their presence indicates that an extensional tectonic regime may have lasted throughout the Silurian in the Meguma terrane.
Sedimentary units of the White Rock Formation are interpreted generally to record relatively shallow-water deposition in a continental shelf setting, with times of subaerial exposure indicated by the felsic tuffs of the Cape St. Mary and Torbrook areas (Lane 1979; Schenk 1995). A similar shallow-marine depositional environment is indicated by the sedimentary and volcanic units that constitute the formation in the Yarmouth area. Continental provenance is indicated by the quartz-rich siliciclastic rocks in the lower units of the formation. The within-plate alkalic chemical signature of the volcanic rocks is consistent with their formation in an extensional setting. The extension may reflect separation of the Meguma terrane from Gondwana (van Staal et al. 1998), and hence the U-Pb ages provide important constraints on the timing of this event.
The Early Silurian age obtained from the rhyolite at the base of the White Rock Formation in the Torbrook area suggests that a major disconformity occurs between the formation and the underlying Halifax Formation that contains early Tremodoc fossils (Schenk 1970; Doyle 1979; Waldron 1992; White et al. 1999). The rhyolitic tuff dated in the Yarmouth area is the same age as that in the Torbrook area (Fig. 11), but is underlain by a thick volcanic succession. Although the unit could conceivably represent a long period of time extending back in time through the Ordovician to the Tremodoc, it is unlikely that such a lithologically and chemically coherent pile of volcanic rocks would have formed over such a long period of time, as noted above. Hence the western contact of the White Rock and Halifax formations in the Yarmouth area is probably also a major disconformity, somewhat modified by later shearing, representing an age gap of as much as 60 Ma. During this time, the environment changed from the deep-water continental slope – abyssal plain setting represented by the Meguma Group to the shallow-marine setting represented by the White Rock Formation.
The White Rock Formation has been interpreted to be part of an overstep sequence that includes rocks of similar age in the Avalon terrane and extends across much of the northern Appalachian orogen (Keppie and Krogh 2000). However, volcanic rocks of similar age in the Kingston terrane of southern New Brunswick (Barr et al. 1999), the Sarach Brook Metamorphic Suite of Cape Breton Island (Barr and Jamieson 1991; Barr and Raeside 1998), and the LaPoile Group of southern Newfoundland (Chorleton 1980) have volcanic-arc signatures and are interpreted to represent a convergent plate margin between the Avalon terrane and more inboard peri-Gondwanan terranes in the Silurian (White and Barr 2001).
The relationship between the Meguma and Avalon terranes in the Paleozoic remains controversial. Some workers have suggested that the two terranes joined in the Devonian through subduction (e.g., Clarke et al. 1997), whereas others have suggested that the two terranes travelled together after Avalon rifted away from Gondwana in the Early Ordovician (Keppie and Krogh 2000). Although rocks interpreted to have formed in a similar setting as the White Rock Formation also occur in the Antigonish Highlands of northern mainland Nova Scotia (Murphy et al. 1991, 1996), the associated sedimentary units there are mainly redbeds and do not resemble those of the White Rock Formation. If the withinplate alkalic volcanic rocks of the White Rock Formation represent rifting of the Meguma terrane from Gondwana at ca. 440 Ma, then the Avalon and Meguma terranes are unlikely to have been connected in the Silurian. Given the extent and impact of the ca. 400–390 Ma Acadian orogeny in the Meguma terrane and its relative absence in the Avalon terrane, it is unlikely that the two terranes were fellow travellers in the Devonian.
The White Rock Formation in the Yarmouth area of Nova Scotia is dominated by within-plate alkalic mafic metavolcanic rocks, with minor intermediate and felsic rocks, and interlayered siliciclastic and tuffaceous metasedimentary rocks. The rocks formed in a subaqueous marine to subaerial environment, perhaps related to rifting of the Meguma terrane from Gondwana. Although contacts are now tectonic, the Brenton Pluton is the same age as and likely was comagmatic with the felsic volcanic units of the White Rock Formation. The ca. 440 Ma age suggests that a major unconformity separates the White Rock Formation from the underlying Halifax Formation, from which the youngest fossils recovered are Early Ordovician.
This paper is based mainly on the M.Sc. thesis project of the senior author at Acadia University, Wolfville, Nova Scotia. The work was supported by the Natural Sciences and Engineering Research Council through a research grant to SMB, an Acadia University graduate assistantship, and the Nova Scotia Department of Natural Resources. The U–Pb dating was done by JFK while a research associate in the Department of Earth Sciences at Memorial University, St. John’s, Newfoundland. We thank journal reviewers Dan Kontak and Brendan Murphy for their helpful and perceptive comments that led to substantial improvement in the paper.
- Received April 28, 2001.
- Accepted September 18, 2001.
- Published on the NRC Research Press Web site at http://cjes.nrc.ca on February 22, 2002.
- © 2002 NRC Canada