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Journal of Petrology Volume 41 Number 12 Pages 1777-1804 2000
© Oxford University Press 2000
Petrology and Isotopic Composition of a Grenvillian Basement Fragment in the Northern Appalachian Orogen: Blair River Inlier, Nova Scotia, Canada
1DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF NORTH CAROLINA AT CHAPEL HILL, CB 3315, MITCHELL HALL, CHAPEL HILL, NC 27599-3315, USA
2DEPARTMENT OF GEOLOGY, ACADIA UNIVERSITY, WOLFVILLE, N.S., B0P 1X0, CANADA
Received January 4, 1999; Revised typescript accepted May 3, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGROCK TYPES AND PETROGRAPHYGEOCHEMISTRYSm-Nd ISOTOPESNEW AGE CONSTRAINTSDISCUSSION AND COMPARISON WITH...CONCLUSIONSREFERENCES |
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Mesoproterozoic metaplutonic rocks in northern Cape Breton Island, Nova Scotia, occur in a tectonic inlier within the Appalachian orogen. Although they have been multiply metamorphosed and variably deformed, the petrology and geochemistry of these rocks provide insight into the tectonomagmatic evolution of easternmost Laurentia. Anorthosite, syenite, and granitoid plutons (1100–980 Ma) intruded the Sailor Brook and Polletts Cove River gneisses. New Nd isotopic data are presented from a biotite-rich part of the Sailor Brook gneiss (
Ndi = -0·7), two anorthosite samples (Ndi = +2·1 and +2·8), and a charnockite unit (Ndi = -0·4). New U–Pb zircon data from the anorthosite yield 207Pb/206Pb ages between 975 and 1095 Ma. Disrupted U–Pb systematics preclude a unique age interpretation, but the 
1095 Ma single-grain date is a minimum age for the anorthosite. Field relations, major and trace-element geochemistry, and isotopic characteristics indicate that the anorthosite and charnockite were probably coexisting melts, but not differentiates of a single parent magma. The lithological and chemical characteristics of the Lowland Brook Syenite are similar to those of a contemporaneous belt of ultrapotassic plutons in the southern Grenville Province, and both have the chemical characteristics of certain modern continental-margin arc magmas. These data indicate that the Mesoproterozoic units in the Blair River inlier were juvenile crustal additions to eastern Laurentia during Grenville-related orogenic events. KEY WORDS: anorthosite; charnockite; granulite; Mesoproterozoic; Nd isotopes; petrology; U–Pb zircon
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGROCK TYPES AND PETROGRAPHYGEOCHEMISTRYSm-Nd ISOTOPESNEW AGE CONSTRAINTSDISCUSSION AND COMPARISON WITH...CONCLUSIONSREFERENCES |
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In eastern North America, Mesoproterozoic Laurentian basement rocks and their Neoproterozoic to mid-Paleozoic volcanic and sedimentary cover preserve a record of the Proterozoic to mid-Paleozoic geologic history of eastern Laurentia. The record includes Mesoproterozoic orogeny on the ancient North American craton, break-up of a Neoproterozoic supercontinent, development of a passive continental margin, and mid-Paleozoic orogeny. Exposures of Mesoproterozoic Laurentian basement in easternmost Canada and the USA occur in a discontinuous band of inliers in the Appalachian orogen from Newfoundland to Alabama (Fig. 1). They consist mostly of rocks formed or deformed during
1190–980 Ma Grenvillian tectonic and magmatic events, and are distinct from Neoproterozoic and Paleozoic units of accreted Appalachian terranes in that they include granulite-facies metamorphic rocks and, in many cases, anorthosite–charnockite–mangerite suites (Bartholomew, 1984
, and papers therein; Pettingill et al., 1984; Rankin et al., 1989, 1993; Owen & Erdmer, 1990; Hughes et al., 1997).
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The sinuous trend of the band of basement inliers broadly reflects promontories and re-entrants that formed the eastern margin of Laurentia after Neoproterozoic to early Paleozoic rifting and is broadly mimicked in the trend of the Appalachian structural front (Fig. 1; Thomas, 1977; Rankin et al., 1993; Thomas & Whiting, 1995). The inliers expose samples of the easternmost parts of Laurentia that are otherwise buried beneath the Appalachian orogen (Bartholomew & Lewis, 1992). Hence, documenting the chemical and isotopic characteristics of the basement inliers will aid in understanding the Mesoproterozoic to Neoproterozoic crustal components that amalgamated to form the Grenville Province of Laurentia (e.g. Pettingill et al., 1984; Ashwal et al., 1986; Moore, 1986; DeWolf & Mezger, 1994; Sinha et al., 1996). Furthermore, because the basement inliers were deformed and metamorphosed during the Paleozoic, they can aid our understanding of the timing and style of Appalachian-related events in ways that far-traveled and more tectonically disrupted outboard terranes cannot (e.g. Rodgers, 1995; Goldberg & Dallmeyer, 1997). For example, the uncertain provenance of at least one Grenvillian basement-like terrane within the Appalachian orogen (Goochland terrane, Hibbard & Samson, 1995; Aleinikoff et al., 1996) and the recognition of broadly ‘Grenvillian’ terranes within Paleozoic orogenic belts of Mexico and South America (Patchett & Ruiz, 1989; Teixeira et al., 1989; Kay et al., 1996; Tosdal, 1996; Restrepo-Pace et al., 1997) have focused attention on long-recognized problems of global paleogeography (e.g. Hoffman 1991; Dalla Salda et al., 1992; Eide & Torsvik, 1996; Keppie et al., 1996; Dalziel, 1997). Isotopic studies show potential for distinguishing among Grenville-age orogenic belts outside of the Grenville Province of North America (e.g. Pettingill et al., 1984; Patchett & Ruiz, 1989; Sinha et al., 1996; Wareham et al., 1998).
In this paper, we describe the petrology and isotope geochemistry of Mesoproterozoic rocks in the Blair River inlier, a fragment of Grenvillian basement exposed in northwestern Cape Breton Island, Nova Scotia, Canada (Fig. 2). An earlier publication (Miller et al., 1996) documented the Grenvillian age of the inlier and an overprinting Paleozoic tectonomagmatic event, but did not include petrological data and interpretations. By examination of sparsely preserved relict igneous minerals and textures and high-grade metamorphic mineral assemblages, it is possible to see through the widespread Paleozoic amphibolite-facies metamorphism and gain insight into the Mesoproterozoic tectonomagmatic history of the inlier (see Fig. 3). In addition, we present new U–Pb and Nd isotopic data that help to clarify the age relations and isotopic characteristics of the major metaplutonic units in the inlier. The similarity of these data to equivalent data from the Grenville Province of Laurentia supports previous interpretations that the Blair River inlier is a fragment of the Grenvillian basement of Laurentia.
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGROCK TYPES AND PETROGRAPHYGEOCHEMISTRYSm-Nd ISOTOPESNEW AGE CONSTRAINTSDISCUSSION AND COMPARISON WITH...CONCLUSIONSREFERENCES |
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Cape Breton Island is located at a promontory in the Laurentian continental margin and exposes a condensed section of the northern Appalachian orogen (Fig. 1 and Fig. 2 inset; Barr et al., 1987
, 1995, 1998; Lin et al., 1994). Four tectonostratigraphic zones of Cape Breton Island (Fig. 2 inset) correspond to large-scale tectonic subdivisions of the Appalachian orogen (Barr & Raeside, 1986, 1989). The Mira terrane in the southeast contains temporally distinct (680 Ma, 620 Ma, and 570 Ma) belts of continental-margin, arc-related and post-orogenic volcanic and plutonic rocks (Barr, 1993; Bevier et al., 1993; Barr & White, 1996; Barr et al., 1998), overlain by Cambrian to Ordovician sedimentary rocks bearing Acado-Baltic fauna (Landing, 1991). Similarities of these units to those in eastern Newfoundland indicate that the Mira terrane is part of the Avalon terrane (Barr et al., 1998). The Bras d’Or terrane, located northwest of the Mira terrane and separated from it by a Carboniferous fault (Barr et al., 1995), is a complex assemblage of 565–555 Ma continental-margin arc plutons and 500 Ma post-orogenic plutons intruded into metasedimentary, metavolcanic, and low-pressure gneissic rocks (Raeside & Barr, 1990; Farrow & Barr, 1992). The presence of a fault-bounded belt of Cambrian to Ordovician volcanic and sedimentary units that contain an Acado-Baltic fauna has been interpreted to show that the Bras d’Or terrane, like the Mira terrane, is part of Avalon (Keppie, 1990; Landing, 1991). However, the Bras d’Or terrane also has been interpreted to be a peri-Gondwanan terrane distinct from Avalon that may represent the basement of the Gander terrane of central Newfoundland (Barr et al., 1995, 1998). The Aspy terrane, located north and west of the Bras d’Or terrane (Fig. 2, inset), consists mainly of Ordovician to Silurian sedimentary and volcanic units that were deformed and metamorphosed in the Silurian–Devonian (Barr & Jamieson, 1991; Raeside & Barr, 1992). It may originally have been part of the sedimentary–volcanic cover of the Bras d’Or terrane (Lin, 1993; Chen et al., 1995), although the two terranes are now juxtaposed by post-Devonian faults (Barr et al., 1995). The Aspy terrane appears to include elements of both the Gander and Exploits terranes of central Newfoundland (Barr et al., 1998).
The Blair River inlier forms the northernmost part of Cape Breton Island, separated from the adjacent Aspy terrane by Red River and Wilkie Brook fault zones (Fig. 2). The inlier onsists mainly of several composite orthogneissic units, intruded by less deformed plutons of varied compositions including anorthosite, gabbro, syenite, and granite. Miller et al. (1996) reported U–Pb dates confirming that major units in the inlier are of Proterozoic age, including the Sailor Brook gneiss (>1217 Ma), Lowland Brook Syenite (1080 +5/-3 Ma), Red River Anorthosite Suite (>996 Ma, now considered to be >1095 Ma), and Otter Brook gneiss (978 +6/-5 Ma). They also showed that high-grade metamorphism of the Sailor Brook gneiss occurred at 1035 +12/-10 Ma, and that the Red River Anorthosite Suite was metamorphosed at 996 +6/-5 Ma. Both the igneous and metamorphic ages coincide with major thermal episodes in the southeastern Grenville Province of North America (e.g. McLelland et al., 1997; Ketchum et al., 1998; Martignole & Friedman, 1998).
Paleozoic igneous activity in the Blair River inlier is demonstrated by the 435 +7/-3 Ma age (Miller et al., 1996) of the Sammys Barren granite (Fig. 2). Other probable Paleozoic igneous units include the Fox Back Ridge diorite–granodiorite and an unnamed syenite body (Fig. 2), as well as scattered small gabbroic plutons and mafic and felsic dikes that are too small to show on the scale of Fig. 2 (Miller, 1997). Paleozoic amphibolite-facies metamorphism is reflected in 425 Ma titanite ages from Proterozoic units, and subsequent cooling through hornblende, muscovite and phlogopite 40Ar/39Ar, and rutile U–Pb, closure temperatures lasted until 410 Ma (Miller et al., 1996). A later, probably Devonian, greenschist-facies overprint is most intense near chlorite-grade shear zones and brittle fault zones.
The Proterozoic units of the Blair River inlier are distinct in both age and composition from rocks in other parts of Cape Breton Island and in northern Appalachian outboard terranes in general. The Blair River inlier contains rock types similar to those characteristic of the Grenville Province of Laurentia and similar to those in other Grenvillian basement inliers in the Appalachian orogen, such as the Steel Mountain, Indian Head, and Long Range inliers in western Newfoundland (e.g. Owen & Erdmer, 1990; Rankin et al., 1993). As in the Blair River inlier, Silurian metamorphism and granite intrusions have also been documented in western Newfoundland (Cawood et al., 1994). Thus, the Blair River inlier is interpreted to be an exposure of Laurentian Grenvillian basement that was deformed, metamorphosed, and intruded by granite during Appalachian orogenic events (Barr et al., 1987, 1995, 1998; Miller et al., 1996).
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ROCK TYPES AND PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGROCK TYPES AND PETROGRAPHYGEOCHEMISTRYSm-Nd ISOTOPESNEW AGE CONSTRAINTSDISCUSSION AND COMPARISON WITH...CONCLUSIONSREFERENCES |
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The major Mesoproterozoic meta-igneous units in the Blair River inlier are the Sailor Brook, Polletts Cove River, and Otter Brook gneisses, the Red River Anorthosite Suite and an associated charnockite unit, other smaller anorthosite bodies, and the Lowland Brook Syenite (Fig. 2). Field relationships and other salient features of these units are described below; petrographic characteristics of the major units are summarized in Table 1.
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Sailor Brook gneiss
The Sailor Brook gneiss forms the northwestern part of the Blair River inlier (Fig. 2). The characteristic rock type is granular gneiss of mafic to intermediate composition, in places with weak banding defined by varying proportions of ferromagnesian minerals. Two-pyroxene, granulite-facies metamorphic mineral assemblages consisting of augite, hypersthene, plagioclase (An33–42), K-feldspar, hastingsitic hornblende, quartz, and Fe–Ti oxides are preserved in some of the mafic gneiss and in mafic layers in banded gneiss (e.g. Fig. 3a). However, the majority of samples contain amphibolite-facies assemblages, but preserve relict textures and minerals comparable with those in the granulite-facies rocks, and hence are interpreted also to have had granulite-facies protoliths. For example, in many samples, amphibole, quartz, and plagioclase occur in mosaic assemblages pseudomorphous after clinopyroxene, which is preserved rarely as fragments or cores in the mosaic. Amphibole grains commonly retain vestiges of pyroxene cleavage, or contain Fe–Ti oxide inclusions that mimic pyroxene cleavage (compare Fig. 3c and d). Feldspars are highly altered, but grain outlines retain a granular texture. Locally migmatitic areas contain felsic bands of recrystallized and xenoblastic sericitized plagioclase (An22–34), non-perthitic K-feldspar, and quartz that may represent relict leucosomes.
Felsic gneiss is a minor component of the Sailor Brook unit, but where present has granular texture and weak foliation defined by elongate clusters of Fe–Ti oxides, recrystallized feldspar, and recovered quartz ribbons. The feldspar occurs as large equant perthitic and antiperthitic grains in a finer-grained matrix of microcline and plagioclase, as well as xenoblastic, lensoid, or ribbon quartz, Fe–Ti oxides, and large apatite grains.
Polletts Cove River gneiss
The Polletts Cove River gneiss (Fig. 2) is an extensive and heterogeneous unit that includes a variety of rock types, although further subdivision is not possible because of limited exposure. The characteristic lithologies are quartzofeldspathic gneiss and amphibolite, intruded by abundant granitic pegmatite and aplite dikes (Table 1). Variably foliated fine-grained chlorite-rich gneiss and schist are also abundant, and contain thin, coarse-grained granitic sheets that may have originated as pegmatitic dikes, now boudinaged and aligned parallel to the foliation in the host rock. Rare calc-silicate lenses are present, and appear to be associated with shear zones and faults. Some outcrops consist of granoblastic tonalitic to dioritic gneiss, similar to the Sailor Brook gneiss, or coarse-grained quartzofeldspathic gneiss similar to the Otter Brook gneiss (see below) but such areas are too small to be mapped separately. Also present are small, highly sheared and altered outcrops of anorthosite and leucogabbro, and dikes and small bodies of metagabbro.
The quartzofeldspathic gneiss is fine to medium grained and varies from granitic to granodioritic in composition. It contains both perthitic and non-perthitic K-feldspar, plagioclase of highly variable composition (An40 to An5), quartz, biotite, chlorite, and hornblende. The gneissic foliation is commonly defined by recrystallized quartz ribbons, and the preferred orientation of biotite and hornblende.
Amphibolite in the Polletts Cove River gneiss varies from massive to foliated to gneissic, and contains varying proportions of magnesio-hornblende, plagioclase, and quartz, with accessory apatite and titanite. Strongly foliated varieties contain up to 20% biotite and chlorite that appear to have replaced hornblende. Networks of granitic dikelets in the amphibolite consist of 0·5–3 mm grains of coarse-patch antiperthite in a finer-grained recrystallized matrix of microcline, equigranular plagioclase (An22), and elongate recrystallized quartz. Fine-grained green to gray schist and gneiss is dominated by chlorite, epidote and sericite, and contains pegmatite boudins. These rocks appear to be the low-grade, highly sheared equivalent of the amphibolite. Calc-silicate rocks are rare, and located mainly within areas dominated by amphibolite and green–gray gneiss. They contain augen of tremolite and diopside, wrapped by a foliation-defining matrix of phlogopite and calcic plagioclase.
Metagabbroic rocks occur as dikes and small bodies. They have an altered subophitic texture in which randomly oriented plagioclase (An40) laths are inclusions in or intergrown with multigranular aggregates of hornblende pseudomorphous after clinopyroxene. Minor anorthosite occurs in small layers or lenses in the Polletts Cove River gneiss and is altered extensively to sericite and saussurite. Fe–Ti oxide minerals in these rocks commonly have titanite rims. Wispy, pale green mafic layers contain aligned fibrous chlorite and epidote. Other leucocratic rocks of uncertain origin are commonly sheared and highly altered.
Otter Brook gneiss
The Otter Brook gneiss (Fig. 2) is typically a biotite- and hornblende-rich, garnet-bearing, quartzofeldspathic augen to flaser gneiss, and is interpreted to be an orthogneiss because of its relative homogeneity. The 978 Ma zircon age (Miller et al., 1996) is interpreted to represent the igneous crystallization age of the granitic protolith. In addition to orthogneiss, the Otter Brook unit also contains massive and foliated amphibolite, metagabbro dikes, <2 m wide lenses of calc-silicate rocks within fault and shear zones, and rare small anorthosite bodies (Table 1). These minor units are petrographically identical to those in the Polletts Cove River gneiss, described above. Granitic pegmatite dikes cut the biotite–garnet quartzofeldspathic gneiss and foliated amphibolite, but not the metagabbro dikes and calc-silicate lenses, and hence appear to pre-date the metagabbro and calc-silicate rocks. This observation is significant because contacts between the metagabbro or calc-silicate lenses are commonly faulted or not exposed. The metagabbro is younger than the gneiss and later pegmatites, but pre-dated 425 Ma metamorphism (Miller et al., 1996). We speculate that the metagabbro could be related to the Late Proterozoic rifting of eastern Laurentia, and that the calc-silicate rocks may be remnants of Lower Paleozoic cover sequences. Isotopic analyses to test these hypotheses are under way.
Typical Otter Brook gneiss consists of coarse-grained K-feldspar with serrated grain boundaries, plagioclase (An23–30), recrystallized quartz lenses, hornblende, garnet, biotite, and relict clinopyroxene (Table 1). The garnet grains (57% almandine component and 29% grossular component, with minor pyrope and spessartine) are highly resorbed and separated from biotite and hornblende by a metamorphic reaction zone of mainly K-feldspar with small amounts of plagioclase and muscovite. Amphibole is mostly pargasitic to hastingsitic hornblende with ragged grain boundaries, erratic compositional zoning, and numerous opaque inclusions. Hornblende grains in low-strain zones preserve poikiloblastic or mosaic textures that are characteristic of altered pyroxene. Hastingsitic hornblende is partly altered to biotite, but pargasitic hornblende appears stable with biotite. Biotite is partly altered to chlorite and K-feldspar. Rare small fragments of relict clinopyroxene (ferrosilite) are incompletely replaced by mosaic-textured aggregates of hornblende or are preserved in strain shadows adjacent to garnet porphyroblasts. Large and abundant accessory minerals include zircon, titanite, apatite, and allanite.
Red River Anorthosite Suite
The Red River Anorthosite Suite consists of a central core of massive anorthosite that grades outward into gabbroic rocks. The western part of the suite consists of a layered anorthosite–gabbro unit, with rare pyroxenite layers. Minor Fe–Ti oxide and apatite rocks (jotunite and nelsonite) occur in the gradational zone between the layered unit and the adjacent charnockitic unit (Fig. 2). Many samples from the Red River Anorthosite Suite are deformed and metamorphosed, with sericitized feldspar and chloritized mafic minerals, but most preserve some relict igneous minerals and textures from which an estimate of the primary mineralogy can be inferred (e.g. Fig. 3c and d).
The least altered or recrystallized anorthosite contains unzoned, subequigranular and semi-tabular to granular plagioclase grains (Fig. 3b), locally with 120° grain boundary intersections. The lack of zoning and the subhedral plagioclase grain shapes are characteristic of primary textures in adcumulate anorthosite, but this texture is also very similar to published examples of partly recrystallized anorthosite (Kehlenbeck, 1972; Ashwal, 1993). Textures in partly recrystallized anorthosite include incipient granulation of randomly oriented massive anorthosite and well-developed mortar texture adjacent to rounded plagioclase porphyroclasts (An45–55) in a finer-grained matrix of polygonal plagioclase (An40–50) aggregates. Ashwal (1993) interpreted similar textures to suggest deformation by magmatic ascent as a crystal mush or recrystallization during later intrusion of surrounding plutons (e.g. charnockite). Recrystallization associated with regional metamorphism is also possible. Granular anorthosite is finer grained and equigranular, with straight grain boundaries intersecting at 120°, and may be either the end product of complete recrystallization or a near-perfect adcumulate. The two rocks are texturally indistinguishable except where adcumulate plagioclase preserves igneous zoning patterns. Mafic minerals in the anorthosite are restricted to sparsely distributed centimeter-scale polymineralic clusters, most of which are oblate to augen-shaped and define a weak gneissic foliation. They consist of relict cores of orthopyroxene and rarely clinopyroxene, surrounded by biotite, hornblende, chlorite, epidote, and opaque oxide minerals.
The anorthosite either is in faulted contact with, or grades over a distance of 3–10 m into, gabbroic rocks. Compositions in the gradational zone range from leucogabbronorite to gabbro to pyroxene–hornblende gabbronorite. The gabbro is coarse grained and preserves relict subophitic texture, but in most samples, hornblende and actinolite have either partly or completely replaced pyroxene (Fig. 3c). Gabbro typically contains granular plagioclase (An32–45), partly or completely altered pyroxene, brown ferroan- to magnesio-hornblende, biotite, Fe–Ti oxide minerals and accessory apatite. Altered gabbro contains more quartz (up to 5%) than does relatively fresh gabbro. Gabbronorite is commonly altered but is distinguishable from gabbro by the distinctive alteration textures of orthopyroxene (schillerized) and clinopyroxene (uralitized) and by polymineralic mosaics of amphibole, plagioclase, and quartz (e.g. Fig. 3d). Rare samples preserve a gabbroic texture, but pyroxene is either rimmed by or completely recrystallized to granular aggregates of magnesio-hornblende.
The gabbroic rocks either change abruptly across faults, or grade, through various scales of layering and various degrees of compositional segregation, into rocks with pronounced rhythmic mafic and felsic layers. Mafic layers are of gabbro, gabbronorite, and rarely pyroxenite composition, and contain relict pyroxene (mainly clinopyroxene), light green and olive green amphibole, biotite, plagioclase (An30–35), and minor quartz, epidote, and chlorite. Felsic layers are of anorthosite to leucogabbro composition, and are composed predominantly of plagioclase (An28–32), quartz, and minor relict clinopyroxene or amphibole aggregates after clinopyroxene. Rare pyroxenite layers are 0·5–1 m thick, and have mainly concordant but locally cross-cutting sharp contacts with interlayered anorthosite–leucogabbro. The rhythmic layers are commonly 2–6 cm thick but locally up to 1 m thick and are laterally extensive. Individual centimeter-scale layers can be traced across entire outcrops (up to 30 m) without pinching out. The layers are generally flat lying and are locally gently folded. The scale and general style of layering are similar to those in layered gabbro complexes associated with massif-type anorthosite or layered mafic intrusions (e.g. Morse, 1968, 1982; Woussen et al., 1988). However, the layering is different in detail. In layered gabbro complexes (e.g. Skaergaard; Conrad & Naslund, 1989) cumulate layers are marked by an orthopyroxene-rich basal lithology, grading upward into anorthosite. Common cumulate layering in massif-type anorthosite is also asymmetric (i.e. has a ‘way up’) in mode and grain-size gradation and is generally at a larger scale (>0·5 m). Rhythmic layering is common in massif-type anorthosite bodies, but centimeter-scale layers do not have great lateral extent (Wiebe, 1988, 1990; Ashwal, 1993). Lamination as a result of flow, cumulation in a convecting magma chamber, and crystal compaction can produce centimeter-scale sharply defined layers and a preferred orientation of crystals (e.g. Higgins, 1991). A combination involving at least one of these igneous processes, perhaps enhanced by deformation and metamorphism, is a possible explanation for the details of the layering observed in the Red River Anorthosite Suite.
Charnockitic rocks
Charnockitic rocks generally occur in a discontinuous band at the margins of the Red River Anorthosite Suite, in faulted contact with the Polletts Cove River gneiss (Fig. 2), and also occur as sheared lenses within the Wilkie Brook fault zone. They occur adjacent to and may be gradational with the layered gabbro unit of the Red River Anorthosite Suite, although rocks in the transition zone are altered and deformed, and difficult to distinguish from one another in the field. Rocks lacking pyroxene (monzonite and granodiorite) are considered part of the charnockite unit because they can be traced into, and are texturally similar to, hypersthene-bearing charnockite. Charnockitic rocks commonly display agneissic foliation that is defined by elongate hypersthene and ovoid salite grains, oriented biotite, and quartz ribbons.
Charnockitic rocks contain the best preserved high-grade metamorphic mineral assemblages of any unit in the Blair River inlier. They commonly have a gneissic foliation defined by highly elongate hypersthene, equidimensional or slightly ovoid clinopyroxene (salite), preferentially oriented biotite, annealed quartz ribbons, perthitic K-feldspar, and plagioclase (An35), all in a fine-grained recrystallized feldspathic matrix. Some samples contain granular magnesio- to ferroan pargasitic hornblende and others lack Fe–Mg silicate minerals entirely. Fe–Ti oxide minerals (rimmed by titanite in metamorphosed samples), zircon, and apatite are ubiquitous accessory minerals.
Delaneys Brook anorthosite and other small anorthosite bodies
The Delaneys Brook anorthosite is the best exposed of numerous scattered small bodies of anorthosite that occur in the Blair River inlier in addition to the Red River Anorthosite Suite (Fig. 2). Most of these small bodies occur in the Polletts Cove River gneiss and are too small to map as separate units. The small anorthosite bodies are generally highly altered, but parts of the Delaneys Brook anorthosite preserve fine- to medium-grained equigranular polygonal plagioclase (An50) with diffuse normal zoning patterns. Its fine grain size and sharply defined zoning suggest that it cooled rapidly, like anorthosite dikes described from other massif-type bodies (Wiebe, 1979). In the center of the Delaneys Brook body, medium-grained granular anorthosite contains blocky or tabular plagioclase grains with rational grain boundaries, which are separated by irregularly shaped, interstitial patches of poikilitic, reversely zoned plagioclase grains with inclusions of quartz and Fe–Ti oxide minerals. These poikilitic plagioclase patches may represent trapped intercumulus liquid in an otherwise nearly perfect adcumulate rock. Rare, 1·5 cm diameter, orthopyroxene grains display schillerized bronzite cores that are rimmed by clinopyroxene and hornblende.
Lowland Brook Syenite
The Lowland Brook Syenite forms a large pluton in the northern part of the inlier (Fig. 2), but smaller syenite bodies occur throughout the gneissic units. The syenite is typically medium to coarse grained, and commonly displays an anastomosing gneissic foliation defined by oriented hornblende and recrystallized zones of feldspar. Low-strain zones on the order of tens of meters wide contain massive syenite. Within the low-strain zones are mafic enclaves, xenoliths, and an intrusion breccia of granular and migmatitic gneiss correlated with the Sailor Brook gneiss. These field observations indicate that the Lowland Brook Syenite intruded and may have assimilated parts of the Sailor Brook gneiss. Gneissic syenite contains large perthitic and antiperthitic feldspar porphyroclasts surrounded by fabric-defining mafic layers. The latter contain abundant green magnesio- to actinolitic hornblende and blocky brown ferro-edenitic hornblende in flattened elongate mosaic-texture aggregates that contain biotite, Fe–Ti oxide minerals, and relict pyroxene. Titanite occurs both as rims around Fe–Ti oxide minerals and as separate spindle-shaped grains within the amphibolite-facies fabric. Perthitic and antiperthitic feldspar porphyroclasts contain coarse lamellae of orthoclase (rarely microcline) and oligoclase (An15–22) and are partly recrystallized to an equigranular matrix of separate feldspars. Recrystallized matrix grains are orthoclase and andesine (An38). Both feldspars are moderately to highly sericitized.
Massive syenite in low-strain zones is coarse grained and consists almost entirely of orthoclase microperthite. Plagioclase lamellae in the microperthite are albite (An8–10). The microperthite is recrystallized at grain edges into mortar-texture mantles or fine-grained granular aggregates of separate K-feldspar and plagioclase (An35). Quartz is a minor constituent (<2%) and is most common in mafic patches, where it is associated with clinopyroxene of salite to ferroaugite composition, Fe–Ti oxide minerals that lack titanite rims, biotite partly altered to chlorite, and abundant large zircon grains. Titanite is not present in these samples, but rutile is common as exsolution lamellae in pyroxene. Both ferro-edenitic hornblende and fibrous actinolitic hornblende have partially replaced clinopyroxene.
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GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGROCK TYPES AND PETROGRAPHYGEOCHEMISTRYSm-Nd ISOTOPESNEW AGE CONSTRAINTSDISCUSSION AND COMPARISON WITH...CONCLUSIONSREFERENCES |
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Gneissic units
Fifteen samples from the Sailor Brook and Otter Brook gneissic units were selected for whole-rock chemical analysis to investigate their chemical characteristics (Table 2). No samples from the Polletts Cove River gneiss were analyzed because of the wide lithological variation and lack of detailed mapping and sampling as a result of limited exposure.
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The gneissic samples range in SiO2 content from about 47% to 65%, consistent with their mafic to intermediate mineralogy. The range of compositions in samples from the Sailor Brook gneiss generally encompasses that of the four samples from the Otter Brook gneiss, although the latter are mainly higher in Al2O3 and lower in MgO (Table 2). The gneissic samples have compositional ranges and display trends typical of igneous suites, not unlike those displayed by younger plutonic units in the Blair River inlier (Figs 4 and 5), and by gneissic units with igneous protoliths from other Grenvillian suites (e.g. McLelland & Chiarenzelli, 1990a; Bartholomew et al., 1991; Hughes et al., 1997). Igneous protoliths are also suggested by other discrimination diagrams (Fig. 6a and b) and most samples follow a trend in TiO2 and FeOt/MgO typical of calc-alkalic suites (Fig. 6c). Overall, the chemical data are consistent with the interpretation from petrographic features that the rocks are orthogneisses.
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Red River Anorthosite Suite
Analyzed samples from the Red River Anorthosite Suite include anorthosite (and altered anorthosite), leucogabbro, layered rocks, and pyroxenite (Table 2). A few samples from the smaller anorthosite bodies in the Blair River inlier were also analyzed. The major-element compositions of the samples from the Red River Anorthosite Suite (Fig. 4) generally reflect its change in modal composition from anorthosite in the central core, grading outward to leucogabbro and then to the gabbro–leucogabbro layered unit (Fig. 2). As a whole, the anorthosite samples from both the Red River Anorthosite Suite and from the smaller anorthosite bodies tend to be higher in the feldspar components (Al2O3, CaO, Na2O, K2O, and compatible trace elements) and lower in Fe2O3t, MgO, MnO, and trace elements such as Zr, Y and V (Figs 4 and 5) compared with the more mafic members of the suite. The CaO, Na2O, and K2O contents are variable and reflect both original plagioclase composition and degree of alteration. Samples with sericitized plagioclase have higher SiO2, Na2O and K2O, and lower CaO compared with the less altered samples (Fig. 4). This difference is consistent with the observed variation in plagioclase compositions from An
50 in freshest anorthosite samples to An<29 in metamorphosed and altered samples, and with the presence of up to 5% modal quartz in altered samples.
As in other massif-type anorthosite suites, the field relations and geochemical trends are most easily explained by crystal fractionation and accumulation (e.g. Buddington, 1972; Haskin & Salpas, 1992; Ashwal, 1993; Markl & Frost, 1999). For example, Fe2O3t and MgO decrease and Al2O3 and Sr increase from layered rocks through leucogabbro to anorthosite (Fig. 7a–c). This trend reflects the gradational change in modal proportions of primary cumulate plagioclase and pyroxene (e.g. Simmons & Hanson, 1978). Strontium is concentrated in the anorthosite because of its compatibility in plagioclase, and decreases progressively from anorthosite to gabbro and pyroxenite (Fig. 7c). In contrast, incompatible elements such as Zr (Figs 5b and 7c) are depleted in all lithologies of the anorthosite suite. In granitoid suites, Zr may be used as a fractionation index, but in the anorthosite suite, Zr concentrations are generally low (<100 ppm). Plotted against Sr, the Zr-enrichment trend shows a deflection at the layered unit where the charnockite trend begins (Fig. 7c); this is taken as evidence for contamination of the layered unit with the more evolved charnockitic material. As in other Proterozoic anorthosite massifs, for example, the Morin and Marcy anorthosites (Anderson & Morin, 1968; Ashwal, 1978, 1993; Simmons & Hanson, 1978; McLelland et al., 1994), the geochemical trends (such as increasing Al2O3 with increasing SiO2; Fig. 4c) are generally opposite to those of normal granitoid fractionation trends.
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Mafic and felsic layers in a sample from the layered unit were analysed separately. The mafic layer is compositionally similar to bulk gabbroic samples from the layered unit, although higher in Fe2O3t and MgO and lower in Al2O3 (Fig. 4). However, the felsic layer is not like the leucogabbroic samples or most anorthosite samples in that it has high SiO2 content and low abundances of most other components, including Al2O3 (Figs 3, 4 and 7). It is chemically similar to the altered anorthosite sample with highest SiO2 content, and to one of the charnockite samples that also has high SiO2 content (Figs 3, 4 and 7). More sampling is required to interpret these apparently anomalous compositional variations.
Rare earth element (REE) profiles for anorthositic samples (Fig. 8; Table 3) show light REE (LREE) enrichment, a moderately negative slope, and a large positive Eu anomaly, all features that are typical indicators of high degrees of plagioclase concentration and common characteristics of massif-type anorthosite (Ashwal, 1993). Leucogabbro samples have higher REE concentrations, a flatter profile, and no Eu anomaly. The sample from the layered unit is further enriched in total REE relative to the other parts of the suite and has a small negative Eu anomaly. The REE enrichment through the series anorthosite–leucogabbro–layered rocks coincides with decreasing size of Eu anomalies. This pattern is consistent with the interpretation that these rocks formed by progressive accumulation of plagioclase from a parent magma of approximately basaltic composition, leaving a more mafic cumulate residue. The lack of large negative Eu anomalies to complement the large positive Eu anomalies in the anorthosite may be due to our limited sampling of the more mafic end-members of the suite. By comparison, the most mafic rocks in the Marcy anorthosite massif have REE concentrations of up to 100 times chondrite (10 times the concentration in Marcy anorthosite) and display large negative Eu anomalies (Ashwal & Seifert, 1980). We have not analyzed mafic rocks with similar relative REE enrichments.
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Charnockitic rocks
Analyzed charnockitic rocks include monzogabbro (mangerite and jotunite), tonalite (enderbite), and granodiorite (opdalite). The charnockitic samples span a wide range in SiO2 from 54 to 74% (Fig. 4; Table 2). On Harker variation diagrams, they display fairly tight trends like those typical of granitoid suites, such as decreasing Al2O3, Fe2O3t, MgO, TiO2, and CaO with increasing SiO2 (Fig. 4). K2O tends to increase with SiO2 (Fig. 4f), but both K2O and Na2O (Fig. 4d, f) show considerable scatter. The lowest-silica samples from the charnockitic unit are intermediate between or overlap slightly in composition samples from leucogabbro and layered units of the anorthosite suite. On most diagrams the charnockite fractionation trend begins at values between the leucogabbro and layered unit and is opposite to the anorthosite trend; this difference is most obvious in Al2O3 (Fig. 4c) and in Zr vs Sr (Fig. 7c). Similar major-element relationships between anorthosite and charnockite suites have been observed in the Morin anorthosite [Buddington, 1972; data compilation given by Ashwal (1993)]. The overlap in compositions near the layered unit may be the result of contamination and ‘smearing’ of the contact between the charnockite and layered unit [as recognized in the field and as occurs commonly in other massif-type anorthosites (Ashwal, 1993)], perhaps as a result of some combination of contemporaneous emplacement, autometamorphism, and high-grade regional metamorphism.
Charnockite samples have higher REE compared with anorthosite and leucogabbro units, convex-upward to highly enriched LREE, and a small or moderately negative Eu anomaly (Fig. 8). Charnockite samples with the lowest REE and small negative Eu anomalies may be contaminated slightly by the layered unit. Otherwise, the REE patterns are not consistent with the charnockite having formed by differentiation from a magma that fractionated significant amounts of plagioclase.
Lowland Brook Syenite
Samples from the Lowland Brook Syenite have a fairly narrow range in SiO2 contents (56–63%; Table 2), but comparatively wider variations in other components. The six samples with highest SiO2 contents represent relatively massive syenite with low mafic-mineral contents, no plagioclase phenocrysts, and little or no titanite. The other samples are gneissic syenite, and contain more abundant mafic minerals, as well as plagioclase phenocrysts and titanite. Their mineralogy is reflected in their higher MgO, Fe2O3t, TiO2 and CaO, and lower K2O contents.
The Lowland Brook body is shoshonitic with K2O above 3·6% (Fig. 4f) and K2O/Na2O between 0·7 and 1·4. Al2O3 contents are high (16–18%) but most samples are metaluminous. Trace element spider diagrams (Fig. 9) show LILE and LREE enrichment relative to chondrite, and pronounced negative Th, Nb, Sr and Ti, and positive Zr anomalies. These chemical characteristics are very similar to those of ultrapotassic and shoshonitic suites in the Elzevir terrane of the Central Metasedimentary Belt (Figs 1 and 9; Corriveau & Gorton, 1993) and are typical of Group III, ultrapotassic suites (Foley et al., 1987), which form in response to subduction at active continental margins.
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Sm–Nd ISOTOPES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGROCK TYPES AND PETROGRAPHYGEOCHEMISTRYSm-Nd ISOTOPESNEW AGE CONSTRAINTSDISCUSSION AND COMPARISON WITH...CONCLUSIONSREFERENCES |
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We present here new neodymium isotopic analyses for two samples from the Red River Anorthosite Suite, one from the charnockite unit and one from a biotite-rich component of the Sailor Brook gneiss, and include previously published Nd data (Fig. 10, Table 4; Raeside & Dickin, 1990
; Barr et al., 1998). Six gneiss samples have initial Nd values (at an assumed crystallization age of 1220 Ma) between +2·1 and -0·67, with TDM ages of 1·48–1·65 Ga.
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The initial Nd values of three anorthosite bodies in the Blair River inlier are +2·1 to +3·5 (Fig. 10) with TDM ages of 1·25–1·36 Ga. The charnockite sample has an initial Nd value of -0·41, with a TDM of 1·64 Ga. These data are consistent with the charnockite being derived from the crust, and hence having a more isotopically evolved Nd isotopic composition compared with the anorthosite. The Nd data are also consistent with the anorthosite being mainly mantle derived, with a small amount of crustal contamination. With its low Sm and Nd concentrations, the anorthosite (2–3 ppm Nd; Tables 3 and 4) would require only very minor contamination by more highly evolved and Nd-rich crustal rocks such as the charnockite (20 ppm Nd) or the gneissic units (40 ppm) to have a significant influence on the Nd isotopic composition of the anorthosite. This conclusion is supported by unradiogenic feldspar Pb isotopic compositions and by the 
18O values of +7·0
to +8·3 (Ayuso et al., 1996).
In accordance with the chemical characteristics of continental-margin arc magmas, initial Nd values for the Lowland Brook Syenite are significantly lower than the model depleted mantle at -0·53 and +0·72 and with TDM ages of 1660 and 1470 Ma (Fig. 10; Table 4). Ayuso et al. (1996) obtained, from the Lowland Brook Syenite, 18O values of +8·0 to +8·5 and feldspar Pb isotope compositions that are even more unradiogenic than the presumed mantle-derived (albeit contaminated) anorthosite suite. Calculated at 1080 Ma, Nd values for the Sailor Brook gneiss and Polletts Cove River gneiss range from +0·84 to -1·50. Thus, it is likely that the Nd isotopic composition of the subduction-related syenite has been significantly affected by assimilation of country rocks such as the Sailor Brook and Polletts Cove River gneisses, as was suggested above based on field relations.
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NEW AGE CONSTRAINTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGROCK TYPES AND PETROGRAPHYGEOCHEMISTRYSm-Nd ISOTOPESNEW AGE CONSTRAINTSDISCUSSION AND COMPARISON WITH...CONCLUSIONSREFERENCES |
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Miller et al. (1996)
obtained a minimum age of 996 +6/-5 Ma for the Red River Anorthosite Suite by dating what they interpreted to be metamorphic zircon grains. New U–Pb zircon data (Fig. 11; Table 4) help to constrain further the age of the anorthosite. Five new analyses, including two multiple-grain and three single-grain fractions, are variably discordant and yield 207Pb/206Pb ages from 976 to 1095 Ma. These discordant data do not form a simple Pb-loss or mixing line, but instead probably show the effect of multiple stages of lead loss. This interpretation is consistent with the previous observation of at least two episodes of metamorphism (Miller et al., 1996). In the case of multiple generations of Pb loss, the oldest 207Pb/206Pb age of 1095·3 ± 1·5 Ma would be a new minimum crystallization age for the Red River Anorthosite Suite. Other massif-type anorthosites have proven to be similarly difficult to date accurately because of zircon growth and Pb loss during high-grade metamorphism (Aleinikoff et al., 1996; Ashwal et al., 1999), crustal contamination (Ashwal et al., 1998), and other complications to geochronologic systematics (Frost et al., 1988; Aleinikoff et al., 1996).
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DISCUSSION AND COMPARISON WITH MESOPROTEROZOIC ROCKS OF EASTERN LAURENTIA |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGROCK TYPES AND PETROGRAPHYGEOCHEMISTRYSm-Nd ISOTOPESNEW AGE CONSTRAINTSDISCUSSION AND COMPARISON WITH...CONCLUSIONSREFERENCES |
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The earliest recognized event in the Proterozoic evolution of the Blair River inlier was the crystallization of protoliths of the Sailor Brook and Polletts Cove River gneiss units at some time before 1217 Ma (Miller et al., 1996
). These units had tonalitic to dioritic igneous protoliths derived from variably evolved source regions. The 1350–1185 Ma Elzevirian event added substantial amounts of arc-related crust, including tonalite and trondhjemite suites, to both the Central Metasedimentary Belt and the Adirondack Mountains (Lumbers et al., 1990; McLelland et al., 1997). Known or probable Elzevirian-age, arc-related tonalitic gneiss is also found in both the Green Mountain massif of Vermont (Fig. 1; Ratcliffe et al., 1991) and the New Jersey Highlands (Volkert, 1993; McLelland et al., 1997). The tonalitic country rock gneisses in the Blair River inlier record at least two major episodes of deformation and metamorphism and thus arc-type geochemical signatures cannot be discerned. However, the similarity of rock types, the juvenile isotopic characteristics, and ages are all broadly consistent with their genesis during the Elzevirian event. The 1035 Ma metamorphism of the Sailor Brook gneiss is within the Ottawan phase (1100–1000 Ma) of Grenvillian orogenesis. Metamorphism of this age is widespread throughout the Grenville Province (Easton, 1986; Schärer et al., 1986; Van Breemen et al., 1986), including in the Central Gneiss Belt in Ontario and Quebec (Corrigan, 1990; Jamieson et al., 1992; Culshaw et al., 1997) and along the Central Metasedimentary Belt Boundary Zone (1065–1029 Ma; Van Breemen & Hanmer, 1986). The peak of the Grenvillian Orogeny in the Adirondacks of New York also occurred during the Ottawan phase (Rawnsley et al., 1987; McLelland et al., 1988; Mezger et al., 1988; McLelland & Chiarenzelli, 1990a, 1990b).
Grenville-age magmatism in the Blair River inlier is represented by the Red River Anorthosite Suite, the Lowland Brook Syenite, and the granodioritic protolith of the Otter Brook gneiss. Ashwal (1993) classified the Red River body as a Proterozoic massif-type anorthosite, on the basis of the petrographic reports by Jenness (1966) and Dupuy et al. (1986). Massif-type anorthosite suites are a distinguishing feature of Mesoproterozoic magmatism in the Grenville Province (Fig. 1), and anorthosite–charnockite suites also occur in some basement inliers in the Appalachians [Bartholomew (1984) and papers therein].
The Red River Anorthosite Suite displays most of the petrographic and geochemical characteristics typical of Proterozoic massif-type anorthosite complexes. For example, in other massif-type anorthosite suites, the lithological and geochemical gradation from massive anorthosite to leucogabbro, layered rocks, pyroxenite, and massive gabbro is thought to represent fractional crystallization (Ashwal, 1993). Although present exposure is limited, the Red River Anorthosite Suite contains areas of nearly pure (>90%) plagioclase of intermediate composition (An50), genetically related leucogabbroic rocks, along with comagmatic but probably not cogenetic charnockitic rocks. Furthermore, the suite lacks large volumes of mafic or ultramaficcumulates. The scale and general style of compositional layering in the Red River Anorthosite Suite resemble those of well-studied layered gabbro complexes associated with massif-type anorthosite or layered mafic intrusions (e.g. Morse, 1968, 1982; Woussen et al., 1988). Apparent lithological gradation between charnockite and anorthosite suites has been attributed to the smearing of contacts during the high-grade metamorphism that attended emplacement of charnockite (Ashwal, 1993). Similar processes may account for chemical similarities between the layered unit and the lowest-SiO2 parts of the charnockite. Some Mesoproterozoic anorthosite suites are interpreted as mantle-derived gabbroic differentiates that have been variably contaminated by crustal rocks represented by cogenetic, but not comagmatic, charnockitic rocks (e.g. Emslie, 1985; Ashwal, 1993; McLelland, 1994; Valley et al., 1995). Jotunite and nelsonite bodies are thought to be the latest-crystallizing parts of many massif-type anorthosite complexes (e.g. Goldberg, 1984; Owens et al., 1993; McLelland et al., 1994; Darling & Florence, 1995). All of these features are seen, affected by various degrees of deformation and metamorphism, in the Red River Anorthosite Suite.
The 1080 Ma Lowland Brook Syenite is nearly identical in age and chemical characteristics to a belt of ultrapotassic rocks in the Elzevir terrane of the Central Metasedimentary Belt, emplaced between 1074 and 1089 Ma (Corriveau et al., 1990; Corriveau & Gorton, 1993). These Grenvillian potassic plutons consist of a felsic, quartz to slightly nepheline normative shoshonitic suite, and a felsic to ultramafic, silica-undersaturated, potassic to ultrapotassic suite. The shoshonitic suite is alkaline, with K2O/Na2O between 0·8 and 1·2, has 16–19 wt % Al2O3, and 13–51 ppm of Nb. The chemistry of the felsic suites led Corriveau (1990) to interpret these plutons as subduction related and emplaced in a Mesoproterozoic island arc. Pehrsson et al. (1996) grouped volumetrically minor mafic suites with the highly potassic units of the Central Metasedimentary Belt. These workers interpreted the plutons to reflect subduction at an active continental margin. Both the Lowland Brook Syenite and the ultrapotassic plutons in the Central Metasedimentary Belt are similar chemically to Group III potassic rocks, which Foley et al. (1987) interpreted to be related to either continental-margin or island-arc subduction-related orogenic zones. They are depleted in high-field strength elements, and the calc-alkaline to shoshonitic character of both suggests magmatism in similar arc settings. However, the field evidence for intrusion into older crust, isotopic evidence of a source or significant contamination by isotopically evolved host rocks, and the scarcity of low-K phases and mafic rocks, are more consistent with magmatism at an active continental margin. Corrigan & Van Breemen (1997) interpreted the period from 1089 to 1056 Ma as a time period of voluminous mantle- and lower-crustal-derived plutonism in the allochthonous terranes of the south–central and southeastern portions of the Grenville orogen (Fig. 1; Central Metasedimentary Belt and Shawinigan domain of eastern Quebec).
The youngest Mesoproterozoic magmatic event of the Blair River inlier is the crystallization of the protolith of the Otter Brook gneiss at 978 Ma. The age and general lithology of the Otter Brook gneiss resemble those of a band of 956–966 Ma granite, monzonite, and quartz syenite bodies in eastern Labrador that is part of a larger belt of late-Grenvillian plutons in the easternmost Grenville Province (Gower et al., 1991). Plutonic and gneissic units that range from 980 to 950 Ma are found as basement inliers in western Newfoundland, Vermont, and Virginia [Baadsgaard et al., reported by Owen & Erdmer (1990); see also Herz (1984), Herz & Force (1984), Karabinos (1988), and Karabinos & Aleinikoff (1988, 1990)].
The Nd isotopic data from units of the Blair River inlier and from Mesoproterozoic rocks within the Appalachian orogenic belt suggest that Mesoproterozoic juvenile magmatism, with some more evolved components or contaminants, took place in the parts of easternmost Laurentia that are now sparsely exposed. Anorthosite from the Blair River inlier displays higher initial Nd values than other anorthosite bodies from Laurentian basement exposures in the Blue Ridge Province (Fig. 10). Like the Marcy anorthosite (Ashwal & Wooden, 1983, 1985), the Blair River anorthosite shows some evidence for contamination by isotopically evolved rocks such as charnockite. Except for one sample from the Blue Ridge Province, gneissic host rocks of other basement inliers in the Appalachian orogen are isotopically evolved compared with Mesoproterozoic plutons. Felsic plutons in these basement inliers typically display more positive Nd values than do the syenite and charnockite of the Blair River inlier. The most highly evolved rocks of the basement inliers are gneisses of the Long Range Inlier, which display initial Nd of about -4 (Fig. 10). A biotite gneiss sample from the Polletts Cove River gneiss would be similarly evolved if its age were 1000 Ma instead of (our assumed) 1220 Ma. The range of initial Nd values (+2·1 to -0·67; Fig. 10) for the gneissic country rocks in the Blair River inlier indicates that these are a complex combination of variably evolved crust. Neodymium data from the charnockite and syenite are consistent with significant contamination by, or a source within, rocks with the Nd characteristics of the gneissic country rocks (note Nd evolution trajectories in Fig. 10). Neodymium data from the anorthosite indicate that, if these rocks are mantle derived, then they contain at least a small degree of crustal contamination. Despite the subduction-like signature implied by the chemical similarity to Group III ultrapotassic suites of Foley et al. (1987), initial Nd values (-0·53 and +0·72; Fig. 10) for the Lowland Brook Syenite indicate an isotopically evolved source. Both of these characteristics are typical of magmatism at an active continental margin.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGROCK TYPES AND PETROGRAPHYGEOCHEMISTRYSm-Nd ISOTOPESNEW AGE CONSTRAINTSDISCUSSION AND COMPARISON WITH...CONCLUSIONSREFERENCES |
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The Sailor Brook gneiss and Polletts Cove River gneiss consist of intermediate to mafic orthogneissic units, the protoliths of which probably crystallized contemporaneously with the Elzevirian-related events in the Grenville Province. The Red River Anorthosite Suite consists of an outward-grading sequence of massive anorthosite, leucogabbro, gabbro, and a layered unit of anorthosite and gabbro. Pyroxenite and massive gabbro dikes cut the layered unit. Zircon U–Pb data from the Red River Anorthosite Suite show complex systematics that may reflect multiple events of Pb loss, and constrain the crystallization age to >1095 Ma. Cogenetic, but not comagmatic, charnockite generally separates the Red River Anorthosite Suite from the gneissic country rocks. Neodymium isotopic data are consistent with contamination of anorthosite by charnockite. These data, combined with the trace element and REE characteristics of both rock types, indicate that they are not related by simple differentiation of a common parental melt. A large syenite body in the Blair River inlier lithologically and chemically resembles rocks from a belt of ultrapotassic rocks in the Central Metasedimentary Belt of the Grenville Province. Major- and trace-element characteristics of both are similar to those of rocks generated by subduction at active orogenic zones. However, Nd isotopic data show that the syenite was not derived from a depleted mantle source, but instead reflects either major contamination by, or derivation from, rocks with Nd signatures like those of the gneissic country rocks in the Blair River inlier. Late Grenvillian magmatism in the Blair River inlier is recorded by the granitic to granodiorite protolith of the Otter Brook gneiss. Mesoproterozoic plutonic and gneissic rocks in the Blair River inlier resemble those of other Laurentian basement rocks exposed in eastern North America. However, the Blair River inlier apparently lacks isotopically juvenile Grenville-age felsic plutonism. On the basis of these lines of evidence, the Blair River inlier is considered to be an exposed fragment of easternmost Laurentia, although it is probably detached from autochthonous Laurentian basement at depth.
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ACKNOWLEDGEMENTS |
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This paper is derived in part from the Ph.D. dissertation of B.V.M. at Dalhousie University, Halifax, Nova Scotia. We thank G. R. Dunning for the extensive body of geochronologic data obtained previously and for many geological discussions. R. P. Raeside and R. A. Jamieson made many helpful comments and clarifications. Some of the anorthosite and charnockite chemical data are from the M.Sc. thesis of Robert Bekkers, and most of the Lowland Brook Syenite chemical data are from the B.Sc. honors thesis of Stuart Deveau, both at Acadia University. B.V.M.’s dissertation project was funded by NSERC research grants to S.M.B., R. P. Raeside, and R. A. Jamieson. New isotopic data presented here were obtained by B.V.M. while a post-doctoral fellow at the Syracuse University Radiogenic Isotope Geochemistry Laboratory and supported by a National Science Foundation grant to S. D. Samson. We thank journal reviewers C. van Staal and D. Rankin, and editor S. Sorensen for their helpful comments, which led to substantial improvement in the manuscript.
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FOOTNOTES |
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*Corresponding author. Telephone: 919-966-4516. e-mail: bvmiller{at}email.unc.edu
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REFERENCES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGROCK TYPES AND PETROGRAPHYGEOCHEMISTRYSm-Nd ISOTOPESNEW AGE CONSTRAINTSDISCUSSION AND COMPARISON WITH...CONCLUSIONSREFERENCES |
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