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Evidence for the Granulite–Granite Connection

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Evidence for the Granulite–Granite Connection Lithos 86 (2006) 77–90 www.elsevier.com/locate/lithos Evidence for the granulite–granite connection: Penecontemporaneous high-grade metamorphism, granitic magmatism and core complex development in the Liscomb Complex, Nova Scotia, Canada Jaroslav Dostala,*, Duncan J. Keppieb, Pierre Jutrasa, Brent V. Millerc, Brendan J. Murphyd aDepartment of Geology, Saint Mary’s University, Halifax, Nova Scotia B3H 3C3, Canada bInstituto de Geologia, Universidad Nacional Autonoma de Mexico, Mexico DF 04510, Mexico cRadiogenic Isotope Geochemistry, Department of Geology & Geophysics, Texas A&M University, College Station, Texas 77843-3115, USA dDepartment of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada Received 11 June 2004; accepted 14 April 2005 Available online 9 June 2005 Abstract Upper amphibolite–granulite facies gneisses and granites of the Liscomb Complex (Nova Scotia, Canada), which are exposed in a core complex within the Cambro–Ordovician Meguma Group of southern Nova Scotia, yielded concordant U–Pb zircon/monazite ages of 377F2 and 374F3 Ma, respectively. Geochronological and geochemical data suggest a single Devonian high-grade metamorphic event, which generated the granitic magma by partial melting of the fertile Liscomb gneisses at a depth of ~30 km. The melting was also synchronous with an extensional event during which the gneisses were uplifted in a core complex associated with the intrusion of granitoids to a depth of ~10 km. Subsequently, the gneisses and granites underwent rapid exhumation before the deposition of unconformably overlying late Fammenian rocks at ~364 Ma. These events took place during terminal stages of the Acadian Orogeny and the onset of extensional tectonics in Atlantic Canada during the Middle–Late Devonian. The close temporal and spatial association of Liscomb gneisses/granulites and granites, their major and trace element compositions, and their overlapping isotopic characteristics confirm the hypothesis that high-grade metamorphism and generation of granitic melt are complementary processes. As the Liscomb granites are of similar age, mineralogy and chemistry to the voluminous granitoid plutons found throughout the Meguma Terrane, a similar process is indicated for the rest of the terrane. D 2005 Elsevier B.V. All rights reserved. Keywords: Granite; Granulite; Core complex; Zircon dating; Melting * Corresponding author. E-mail address: [email protected] (J. Dostal). 0024-4937/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2005.04.002 78 J. Dostal et al. / Lithos 86 (2006) 77–90 1. Introduction isons because upper and mid-lower crusts are rarely exposed together, making a direct connection One hypothesis for the origin of voluminous difficult. Furthermore, geochronological data for granitoid rocks that intrude into the upper crust minerals with high blocking temperatures such as links it to melt generated during granulite facies zircon are generally missing for genetically-related metamorphism in the mid-lower crust (e.g., Viel- granulites and granites. However, an unusual situa- zeuf et al., 1990). Tests for this hypothesis have tion in which middle crustal granulite-facies been sought in mid-lower crustal granulites (e.g., gneisses and upper crustal granitoid rocks crop LeFort, 1986; Solar and Brown, 2001) and in lower out together occurs in the Liscomb Complex of crustal xenoliths within volcanic suites (e.g., Braun southern Nova Scotia, Canada (Fig. 1), thereby and Kriegsman, 2001). These tests have been gen- providing a rare opportunity to test the granulite– erally limited to geochemical and isotopic compar- granite connection. Fig. 1. Geological map of the Meguma Terrane of southern Nova Scotia, showing the major intrusions of Late Devonian granitoid rocks, including the South Mountain Batholith (SMB) as well as the Liscomb Complex, the xenolith-bearing lamprophyre dykes of Tangier (Greenough et al., 1999) and the Cambro–Ordovician Meguma Group. The area of study, shown in Fig. 2, is indicated by a star. Note that the lamprophyres form a swarm of narrow dykes along the eastern shore of Nova Scotia. The insert displays eastern Canada, northeastern USA, and the location of the map. It also shows the lithotectonic terranes of the Canadian Appalachians (terranes: M=Meguma; A=Avalon; G=Gander; D=Dunnage; H=Humber) and the Minas Fault (MF) separating the Meguma and Avalon terranes. J. Dostal et al. / Lithos 86 (2006) 77–90 79 2. Geological setting shallow marine rocks (Martel et al., 1993). The oldest of these rocks is of late Fammenian age (~365–360 The Liscomb Complex is located within the Ma according to Okulitch, 2003). Meguma Terrane of the Canadian Appalachians Many authors have inferred that the Meguma (Fig. 1). The Meguma Terrane, most outboard terrane Group and overlying Siluro–Devonian units repre- of the northern Appalachians, is juxtaposed against sents a Cambrian to Early Devonian passive margin the Avalon Terrane along the Minas (Cobequid-Che- bordering northwest Africa that was subsequently dabucto) Fault Zone. Both these terranes were accret- transferred to Laurentia during the Acadian Orogeny ed to North America (Laurentia) during continental (e.g., Schenk, 1997). This was based primarily upon: collision in the early to middle Paleozoic (Williams (i) proposed stratigraphic correlations between the and Hatcher, 1983). In particular, the Meguma was Cambro–Silurian strata in the Meguma Terrane and accreted in the Devonian during the final closure of coeval seccessions in Morocco (Schenk, 1997), and the Rheic Ocean. The Meguma Terrane is composed (ii) the Middle Devonian age of the Acadian Orogeny, mainly of the ~10 km thick Cambro–Ordovician tur- the oldest accretionary event recognized in the bidite succession of the Meguma Group which con- Meguma Terrane. Alternatively, it has been proposed tains Gondwanan fauna (Pratt and Waldron, 1991). that the Cambrian to Early Devonian strata of the Detrital zircons from a lower unit of the Meguma Meguma Terrane was either thrust over the Avalon Group yielded ~3.0 Ga, 2.0 Ga and 600 Ma ages, Terrane (e.g., Greenough et al., 1999) or may repre- also indicating a Gondwanan (West African) source sent a passive margin bordering the Avalon micro- (Krogh and Keppie, 1990). The Meguma turbidites continent, which would imply that the Meguma Group (wackes and pelites) are disconformably to uncon- was deposited on Avalonian continental crust (e.g., formably overlain by Siluro–Devonian shallow-ma- Keppie and Dostal, 1991; Keppie et al., 2003). Detri- rine and continental rocks. The youngest of these tal zircon studies show contrasting provenance for rocks contains Early Devonian (Lochkovian to lower Avalonian and Meguma Cambro–Ordovician sedi- Emsian) fossils (Boucot, 1975; Bouyx et al., 1997). mentary rocks (Krogh and Keppie, 1990; Keppie et These Cambrian to Devonian rocks were deformed al., 1998), whereas Siluro–Devonian sedimentary and metamorphosed to a lower greenschist to amphib- rocks contain similar age suites (Murphy et al., 2004). olite facies under low pressure metamorphic condi- The basement of the Meguma Group is only tions during the Devonian Acadian Orogeny at about exposed in the Liscomb Complex (Fig. 2), an assem- 405–370 Ma (Keppie and Dallmeyer, 1995; Hicks et blage of high-grade gneisses and mafic plutonic al., 1999), shortly after the deposition of the Lower rocks that were intruded by granitoid rocks (Giles Devonian rocks. This was accompanied by the intru- and Chatterjee, 1986, 1987; Clarke et al., 1993; sion of voluminous peraluminous granitoids of the Kontak and Reynolds, 1994). The complex, which South Mountain Batholith (SMB), Liscomb Complex crops out over an area of ~240 km2, cuts across and satellite plutons that were emplaced at a depth of greenschist facies metasedimentary rocks of the ~10–12 km around 380–370 Ma (Clarke et al., 1997; Meguma Group near the northern margin of the Kontak and Reynolds, 1994). Meguma Terrane (Fig. 1). The SMB is the dominant granitoid body of the Exposure of the Liscomb Complex is very poor. Meguma Terrane. It spreads over an area of about Strong shearing and at least 2 m wide contact aureole 7300 km2 (Fig. 1) and contains rocks ranging from were observed in the only exposure of the contact megacrystic biotite granodiorite, with up to 20% bio- between a foliated granite/gneiss of the Liscomb tite, to equigranular leucogranite containing less than Complex and the Meguma Group. The gneisses 2% biotite. The granitic bodies produced a distinct have a distinct foliation that is oblique to that in the contact metamorphic aureole. This was followed by surrounding Meguma Group and the fold traces and rapid exhumation, as documented by 40Ar/ 39Ar mica lithologic boundaries of the Meguma Group appear to cooling ages of ~375–360 Ma (Keppie and Dall- be sharply truncated by the border of the Liscomb meyer, 1995), before being unconformably overlain Complex, indicating that its emplacement is post- by Upper Devonian to Carboniferous continental and folding (Fig. 2). Within the Liscomb Complex, field 80 J. Dostal et al. / Lithos 86 (2006) 77–90 Fig. 2. Geological map of the Liscomb Complex (modified from Kontak and Reynolds, 1994 and Clarke et al., 1993) showing the sample locations. Note that due to poor exposure, most contacts are inferred. Structural information is from Faribault (1891), Fletcher and Faribault (1891a,b) and Kontak and Reynolds (1994). The Late Devonian to Early Carboniferous (Tournaisian) Horton Group consists
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