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).

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 of nonmarine clastic sedimentary rocks deposited mainly in alluvial–lacustrine environments. relationships including rare contacts between the var- arc extending from southern Canada to northwestern ious units indicate that the sequence of emplacement Mexico. They record Tertiary extension following the is gneisses, gabbros and granites (Kontak and Rey- Laramide Orogeny (Coney, 1980; Dickinson, 2002). nolds, 1994), and that the emplacement of all the units Typically, they consist of an older metamorphic–plu- postdates Acadian deformation of the Meguma meta- tonic basement overprinted upwards by a metamor- sedimentary rocks. 40Ar/ 39Ar dates are interpreted to phic carapace of gently dipping, domal, greenschist– record cooling ages from the Liscomb Complex at amphibolite facies with lineated and foliated myloni- 375F3 Ma in amphibole (gneisses), 373F4to tic–gneissic fabrics. This carapace is overlain by a 367F3 Ma in muscovite (gneisses and granites), decollement zone with sliding and detachment kine- and 385 to 367 Ma in biotite (gneisses, granites and matic indicators. This zone is, in turn, overlain by an gabbros) (Kontak and Reynolds, 1994), providing a unmetamorphosed cover attenuated by subhorizontal younger limit for the emplacement of the Liscomb faults. The amplitude of most of these core complexes Complex. is V4km(Coney, 1980), and estimates of the pres- Clarke et al. (1993) inferred that the gneisses were sures of formation for the mylonitic fabrics are 3–3.5 emplaced as a domal uplift that may have intruded the kb (=10–13 km) (Davis et al., 1980). On the other Meguma Group through diapirism. However, the hand, core complexes in the arc–backarc Cyclades presence of paragneisses is more typical of core com- region (Aegean Sea, Greece) developed at 5–7 kb plexes. The classic Cordilleran metamorphic core (=20–25 km depth) and 700–380 8C, were exhumed complexes occur in a narrow belt within the magmatic at a rate between 1–2 km/my, and cooled at a rate of J. Dostal et al. / Lithos 86 (2006) 77–90 81

29 8C/my (Lister et al., 1984). Even deeper exhuma- xenoliths (Clarke et al., 1993). They contain xenoliths tion has been reported in the active core complexes of of both Liscomb gneisses and Meguma Group rocks. the D’Entrecasteaux Islands, a rifted arc complex in The granitic rocks of the complex are mainly grano- Papua New Guinea, in which gneissic domes include diorites and monzogranites (sensu Streckeisen and Le eclogites formed at depths of 45–75 km (=13–21 kb) Maitre, 1979) similar to those of the SMB. Both and temperatures of 730–900 8C(Hill et al., 1992). the granodiorites and monzogranites contain biotite Isothermal exhumation at a rate of 15 km/my was (+/Àmuscovite). In addition, many granites contain followed by cooling at N100 8C/my. These core com- garnet and other noteworthy accessory minerals, in- plexes rise through a region of density inversion cluding zircon, monazite and apatite. where ophiolites overlie less-dense continental crust The only other evidence for the nature of the base- and are located in a continental rift that passes later- ment beneath the Meguma Group comes from xeno- ally into the active Woodlark Basin sea-floor spread- liths in ~368 Ma mafic lamprophyre dykes (the Tangier ing system. Unroofing took place by faulting and Dykes on Fig. 1), which intrude the Meguma Group to shearing at the boundary the gneiss domes. Present the south of the Liscomb Complex (Fig. 1: Kempster et day surface uplift has led to topographic elevations of al., 1989). The xenoliths include three rock types: up to 2.5 km. Clockwise P–T–t paths and short-lived sapphirine granulites, mafic gneisses and garnetiferous thermal pulses associated with extensional deforma- quartzo–feldspathic gneisses (Owen et al., 1988; Owen tion and the intrusion of sills occur in all of these core and Greenough, 1991). Mineral core compositions for complexes (Lister and Baldwin, 1993). Poor exposure the early (pre-dyke) metamorphic event in the sapphi- of the Liscomb Complex means that most of these rine granulites and quartzo–feldspathic gneisses indi- characteristics cannot be observed. However, the gen- cate minimum temperatures of z600 8C at pressures of tle dips in the foliation of the Liscomb gneisses, the ~450–600 MPa (Owen et al., 1988), whereas rim com- domal shape of the high-grade gneisses and their positions and M2 (syn-dyke) assemblages in the meta- discordance with the surrounding Meguma Group pelitic rocks imply conditions of 725–795 8C and 700– are consistent with a core complex interpretation. 900 MPa (Owen and Greenough, 1991). These xeno- The gneisses of the Liscomb Complex include a liths indicate that a mafic unit was emplaced at or variety of migmatitic and non-migmatitic rocks ranging before 629F4 Ma into pelitic metasedimentary rocks from augen gneiss (quartz–K-feldspar–plagioclase– containing ~880–1050 Ma detrital zircons (Greenough biotite–muscovite–sillimanite) through hornblende– et al., 1999). The xenoliths underwent a high-grade biotite gneiss (quartz–K-feldspar–plagioclase–horn- metamorphic event at 378F1 Ma (U–Pb concordant blende–biotite) to quartzo–feldspathic gneiss (quartz– zircon age; Greenough et al., 1999). K-feldspar–plagioclase–biotite–muscovite–sillimanite– garnet) and sillimanite schist (quartz–plagioclase–cor- dierite–biotite–sillimanite) (Clarke et al., 1993). Al- 3. Geochronology though these mineral assemblages are typical of the upper amphibolite facies, that these are mostly ret- 3.1. Analytical methods rograde assemblages is indicated by the presence of coronas around pyroxene cores and zoned minerals, U–Pb isotopic analyses of zircon and monazite such as Mn-rich garnet cores and ternary feldspar from the Liscomb Complex were done at the Univer- cores with compositions indicative of temperatures sity of North Carolina using the procedure of Ratajeski N900 8C(Clarke et al., 1993; Kontak and Reynolds, et al. (2001). All zircon fractions were highly abraded, 1994). Clarke et al. (1993) and Kontak and Rey- but monazites were not. All reported errors are of two nolds (1994) estimated pressures of 640 to 820 MPa sigma. The zircon and monazite were obtained from and temperatures of 760 to 980 8C. three samples: granite LG-1 (location: N 45814.777V The mafic rocks form two separate intrusions and W 62846.732V) and garnet–biotite–sillimanite (Chatterjee et al., 1989) composed of amphibole/clin- gneisses LG-120 (Si-poor; probably a residual rock; opyroxene-bearing gabbros and diorites that contain location: N 45816.482V W62839.923V) and LG-122 significant proportions of both cognate and exotic (location: N 45816.402V W62840.003V; Fig. 2). 82 J. Dostal et al. / Lithos 86 (2006) 77–90

3.2. Results

Analyses from eight single-grain and one four-grain fractions of zircon from granite LG-1 form a recent Pb- loss line anchored by two concordant and two nearly- concordant points (Fig. 3A, Table 1). Regression of the zircon data yields an upper intercept at 373.8F2.7 and a lower intercept suggestive of recent Pb loss. The upper intercept age is interpreted as representing the time of crystallization of the Liscomb granite. Four single monazite grains from the same sample fall above the concordia, likely due to excess 206Pb caused by the incorporation of 230Th (e.g., Scha¨rer, 1984). In these cases, the 207Pb/ 235U ages (373.4 to 374.0 Ma; Table 1) yield the most reliable estimates. Four multi-grain and four single-grain zircon analyses from (probably restitic) gneiss sample LG-120 form a recent Pb-loss discordant trend with an upper intercept at 376.9F2.3 Ma. This age is supported by two concordant monazite analyses (Fig. 3B). We inter- pret the upper intercept age as representing the time of granulite facies metamorphism in the Liscomb metamorphic suite. The lack of any inheritance in these data suggests that either the pelitic protolith lacked detrital zircon, or it was all consumed to crystallize new zircon. Seven single-grain and one two-grain zircon fractions from gneiss sample LG-122 also form a discordant trend anchored by one concordant point (Fig. 3C), but with an upper intercept at 373.9F7.2 Ma. As these zircons are extremely rich in U and radiogenic Pb, whereas their common Pb content is small (Table 1), the error ellipses are much smaller than those of the other two samples, and thus the upper intercept age represents a good estimate of the time of granulite facies metamorphism.

4. Geochemistry

4.1. Analytical methods

The major and trace element analyses of samples used for geochronology are given in Table 2. Major and some trace (Rb, Sr, Ba, Zr, Nb, Y, Ga, Co, Cr, Ni, V and Zn) elements in these samples were analyzed Fig. 3. U–Pb concordia diagrams for (A) the Liscomb granodiorite (sample LG-1), (B) a garnet–biotite–sillimanite mafic gneiss (sam- with an X-ray fluorescence spectrometer at the Geo- ple LG-120) and (C) a garnet–biotite–sillimanite felsic gneiss (sam- chemical Centre of the Department of Geology, Saint ple LG-122). Mary’s University, Halifax. Additional trace elements Table 1 U–Pb isotopic data for Liscomb Complex Analysis #, fraction (number of grains) Weight Totala Totalb Totalb U Pb Atomic ratios Ages (Ma) (mg)a (ppm) (ppm) U Pb Com.Pb 206 Pbb / 206 Pbc / 206 Pbc / % Errord 207 Pbc / % Errord 207 Pbc / % Errord 206 Pb/ 207 Pb/ 207 Pb/ re (ng) (pg) (pg) 204 Pb 208 Pb 238 U 235 U 206 Pb 238 U 235 U 206 Pb

Liscomb granite (LG-1; N 45814.777V W62846.732V) 1) Fragment of acicular prism #1 (1) 0.60 0.82 44.0 4.71 1366 73 614 11.504 0.05379 1.346 0.40119 1.359 0.05410 0.192 337.7 342.5 375.0 0.990 2) Fragment of acicular prism #1 (1) 0.86 0.64 37.0 6.07 749 43 386 9.718 0.05458 1.671 0.40688 1.694 0.05407 0.269 342.6 346.6 374.0 0.987 3) Acicular prism #2 (1) 2.38 2.78 165.7 22.1 1168 70 460 18.326 0.05573 0.399 0.41553 0.432 0.05408 0.161 349.6 352.8 374.4 0.928 4) Acicular prism #3 (1) 2.55 1.94 109.3 10.9 761 43 659 38.741 0.05691 0.550 0.42419 0.569 0.05406 0.146 356.8 359.0 373.7 0.966 5) Fragment of acicular prism #4 (1) 1.36 0.82 47.7 5.88 607 35 531 14.739 0.05774 1.263 0.43065 1.281 0.05409 0.213 361.9 363.6 374.8 0.986 6) Stubby prism (1) 0.74 1.12 62.6 5.97 1504 84 710 32.585 0.05807 0.907 0.43279 0.938 0.05405 0.230 363.9 365.2 373.2 0.969 7) Fragment of acicular prism #1 (1) 0.71 1.19 68.5 6.16 1682 97 736 18.941 0.05865 0.850 0.43729 0.861 0.05407 0.131 367.4 368.3 374.1 0.988

8) Fragment of acicular prism #4 (1) 1.52 0.99 59.1 5.86 652 39 652 11.998 0.05906 1.010 0.44042 1.101 0.05408 0.415 369.9 370.5 374.4 0.926 77–90 (2006) 86 Lithos / al. et Dostal J. 9) Four fragments of acicular prism #4 (4) 0.71 1.09 64.2 6.70 1537 91 632 20.092 0.05947 0.946 0.44362 1.225 0.05410 0.750 372.4 372.8 375.2 0.791 10) Monazite 1 (1) 0.35 2.70 655.3 11.0 7701 1872 942 0.279 0.05998 0.388 0.44451 0.402 0.05375 0.099 375.5 373.4 360.4 0.969 11) Monazite 2 (1) 0.26 1.65 475.8 9.17 6318 1825 701 0.226 0.06017 0.604 0.44529 0.619 0.05367 0.127 376.7 374.0 357.4 0.979 12) Monazite 3 (1) 7.99 16.93 5165.9 24.4 2119 647 2647 0.210 0.06028 0.089 0.44523 0.157 0.05357 0.124 377.3 373.9 353.0 0.612 13) Monazite 4 (1) 2.42 9.08 2101.0 8.02 3756 869 4334 0.297 0.06033 0.121 0.44748 0.136 0.05380 0.062 377.6 375.5 362.6 0.891

Liscomb gneiss (LG-120; N 45816.482V W62839.923V) 1) Large fragment (1) 3.28 3.96 236.3 9.10 1206 72 1500 4.861 0.05388 0.282 0.40045 0.293 0.05390 0.077 338.3 342.0 366.9 0.964 2) Large acicular prism fragment (1) 2.24 1.07 57.2 2.29 478 26 1663 14.131 0.05514 0.139 0.41099 0.203 0.05405 0.142 346.0 349.6 373.3 0.713 3) Small stubby prisms (2) 3.05 0.38 22.3 1.11 125 7 1255 6.593 0.05661 0.244 0.42188 0.437 0.05405 0.346 355.0 357.4 373.0 0.613 4) Small flat prisms (2) 3.41 0.97 65.4 13.2 284 19 284 8.205 0.05754 1.042 0.42697 1.249 0.05381 0.678 360.7 361.0 363.3 0.840 5) Thin acicular prism fragments (6) 5.08 0.90 56.3 1.59 177 11 2143 5.490 0.05889 0.125 0.43930 0.161 0.05410 0.099 368.9 369.8 375.2 0.788 6) Medium stubby prisms (3) 5.29 0.51 32.6 1.31 96 6 1466 4.663 0.05890 0.192 0.43958 0.273 0.05413 0.186 368.9 370.0 376.6 0.732 7) Spheriodal with tips (1) 1.56 0.42 25.7 1.44 269 16 1115 6.719 0.05906 0.194 0.44043 0.259 0.05409 0.164 369.9 370.6 374.7 0.774 8) Fat medium prism (1) 3.53 0.67 39.1 1.55 189 11 1652 10.952 0.05950 0.152 0.44373 0.219 0.05409 0.151 372.6 372.9 374.7 0.722 9) Monazite (1) 1.67 1.28 713.5 12.5 765 427 404 0.104 0.05984 0.889 0.44624 0.919 0.05409 0.222 374.6 374.6 374.6 0.970 10) Monazite (2) 2.83 2.04 1160.8 36.5 721 410 229 0.104 0.05984 0.577 0.44780 0.809 0.05427 0.558 374.7 375.7 382.3 0.724

Liscomb gneiss (LG-122; N 45816.402V W62840.003V) 1) Clear square fragment (1) 4.68 2.97 163.3 1.15 635 35 8456 5.181 0.05123 0.100 0.37936 0.114 0.05370 0.054 322.1 326.6 358.7 0.879 2) Clear flat prism (1) 3.53 2.02 112.1 1.33 571 32 5076 5.676 0.05248 0.069 0.39138 0.128 0.05409 0.108 329.7 335.4 374.7 0.544 3) Medium stubby prism #1 (1) 3.97 2.76 164.5 1.20 696 41 8241 5.680 0.05629 0.067 0.41799 0.091 0.05386 0.062 353.0 354.6 365.2 0.736 4) Large pink fragment #1 (1) 27.77 10.21 600.3 1.13 368 22 32945 7.318 0.05741 0.150 0.42807 0.246 0.05408 0.195 359.9 361.8 374.2 0.610 5) Large pink fragment #2 (1) 21.36 9.74 573.0 1.10 456 27 32440 7.463 0.05757 0.049 0.42922 0.071 0.05408 0.052 360.8 362.6 374.2 0.685 6) Medium stubby prism #2 (1) 10.92 2.97 181.5 1.12 272 17 9845 5.846 0.05794 0.060 0.43089 0.096 0.05394 0.074 363.1 363.8 368.6 0.637 7) Medium thin prisms (2) 2.03 1.34 81.3 1.23 660 40 4100 6.976 0.05893 0.076 0.43925 0.112 0.05406 0.082 369.1 369.7 373.6 0.688 8) Medium stubby pink prism (1) 7.53 2.29 140.5 1.02 304 19 8572 7.342 0.05988 0.068 0.44649 0.090 0.05408 0.058 374.9 374.8 374.4 0.760 a Weight estimated from measured grain dimensions and assuming zircon=4.67 g/cm3, monazite=5.0 g/cm3, ~20% uncertainty affects only U and Pb concentrations. b Corrected for fractionation (0.18F0.09%/amu —Daly) and spike. c Corrected for fractionation, blank, and initial common Pb. d Errors quoted at 2s. e 207 Pb/235 U–206 Pb/238 U correlation coefficient of Ludwig (2001). 83 84 J. Dostal et al. / Lithos 86 (2006) 77–90

Table 2 land. The precision for the trace elements is between Chemical compositions of the dated rocks from the Liscomb 2% and 8% of the values cited (Dostal et al., 1986; Complex Longerich et al., 1990). The Sr and Nd isotopic ratios Sample no. Granite LG-1 Gneiss LG-120 Gneiss LG-122 of rocks from the same outcrops reported in Table 2 SiO2 (wt. %) 66.30 44.66 53.45 are from Clarke et al. (1993). TiO2 0.61 0.94 0.87 Al O 15.67 32.01 17.87 2 3 4.2. Liscomb granite and gneiss Fe2O3 4.22 13.43 8.17 MnO 0.09 1.38 0.18 MgO 1.49 4.93 6.88 Granitic rocks of the Liscomb Complex (Fig. 4) CaO 2.24 0.51 7.63 typically have SiO2 contents ranging from 66 to 73 Na2O 3.30 1.13 2.68 wt.%. They are peraluminous (mol. Al2O3 NCaO+ K2O 3.57 0.81 1.07 Na2O+K2O), with K2ONNa2O and K2ON3.5 wt.%. P2O5 0.26 0.05 0.13 LOI 1.00 0.90 1.06 Their K/Rb ratios (210–170) are slightly lower than Total 98.75 100.75 99.99 those of average crustal compositions (~230; Shaw, Cr (ppm) 26 243 123 1968). The chondrite-normalized patterns are enriched Ni 14 193 30 in light REE, display slightly fractionated heavy REE, Co 10 89 38 V 77 211 163 and are accompanied by a negative Eu anomaly. Their Zn 80 242 77 (La/Yb)n ratios range from ~10 to 17, whereas Rb 172 34 37 (Gd/Yb)n ratios range from 1.5 to 4 (Fig. 5). The Ba 979 300 268 eNd values (À2.7 to À5.9) and initial Sr isotopic Sr 194 98 320 ratios (0.70793 to 0.70875) of the Liscomb granites Ga 22 54 17 Ta 1.34 1.03 0.44 (Clarke et al., 1993) are typical of crustally-derived Nb 18.0 18.3 8.1 granitic rocks (Faure, 2001), and, more specifically, S- Hf 5.69 3.80 2.98 type granites (Clarke, 1992). The major (Fig. 4) and Zr 247 157 122 trace element abundances (Fig. 5) as well as the Nd Y141219 and Sr isotopic characteristics (Fig. 6) of the Liscomb Th 14.2 9.69 La 36.4 28.0 23.4 granites are within the variation range of the SMB, Ce 77.7 57.7 46.0 suggesting that the Liscomb granite probably repre- Pr 9.48 6.42 5.70 sents a satellite body of the SMB derived from a Nd 37.8 24.3 21.5 common or similar source. Sm 7.72 4.55 4.41 The Liscomb gneisses constitute a heterogeneous Eu 1.33 0.65 0.98 Gd 5.32 3.24 3.68 group of non-migmatitic and migmatitic rocks with Tb 0.74 0.51 0.59 SiO2 contents ranging from b46 to N72 wt.%, and Dy 3.59 2.79 3.55 Al2O3 contents ranging from 12 to N32 wt.%. Al2O3 Ho 0.55 0.47 0.74 shows a negative correlation with SiO2 and is high in Er 1.45 1.34 2.19 the sillimanite-bearing gneisses (~25 wt.%) but low in Tm 0.21 0.19 0.33 Yb 1.32 1.27 2.17 the quartz–feldspathic gneisses (~13 wt.%). The gar- Lu 0.20 0.19 0.33 net–hornblende gneisses are high in CaO (~5 wt.%), 87 86 Sr/ Sri 0.707926 0.714273 MgO (~4 wt.%) and Al2O3 (~18 wt.%). The Nd and eNd À5.12 À10.43 Sr isotopic ratios of the Liscomb gneisses (Fig. 6) are 143 144 Nd/ Ndi 0.511893 0.511621 highly variable, ranging from relatively low values of Isotopic ratios corrected to 375 Ma; isotopic data from Clarke et al. eNd (~+1) and initial Sr isotopic ratios (~0.706– 0.708) in augen gneisses and garnet–hornblende (the rare-earth elements [REE], Hf, Ta, Nb and Th) gneisses, to high radiogenic values in metapelites were analyzed in all these samples by inductively (NÀ10 and 0.714–0.716, respectively). Augen coupled plasma-mass spectrometry using a Na2O2- gneisses are compositionally similar to the peralumi- sintering technique at the Department of Earth nous granites. Most rocks are likely metamorphosed Sciences of the Memorial University of Newfound- felsic igneous rocks and clastic sedimentary rocks, J. Dostal et al. / Lithos 86 (2006) 77–90 85

Fig. 4. Albite–orthoclase–anorthite plot for the normative compositions of granitic rocks from the South Mountain Batholith and Liscomb Complex, showing fields of some granitoid rocks after O’Connor (1965). The field delineated by the dotted line includes the average compositions of various granitoid rock types from the South Mountain Batholith (MacDonald et al., 1992; Clarke et al., 1997). The average chemical compositions of four granitoids of the Liscomb Complex (granodiorite L9, monzogranite L10, leucomonzogranite L11 and leucogranite L12; Clarke et al., 1993) as well as the dated granite sample (LG-1) all plot into the SMB field.

particularly pelites. Some SiO2-poor rocks are proba- similar to that which formed the Liscomb granites. bly residual rocks, related to melt extraction, whereas The absence of leucosomes in the Liscomb gneisses other gneisses may represent an untapped but fertile indicates that they were not partially molten. Howev- source rock. These gneisses which resemble the er, the presence of a megacrystic gneissic granite source rocks have REE patterns very close to those intruding them suggests that partial melting took of the North American shale composite and average place at a greater depth than is presently exposed. upper continental crust (Fig. 5). Compositions of the metapelitic xenoliths of the Tangier Dykes are comparable to those of the Lis- 4.3. Source of magma comb gneisses (Eberz et al., 1991; Clarke et al., 1993), although the relation between the Liscomb gneisses Peraluminous granites of the Liscomb Complex, and Tangier xenoliths is uncertain. Isotopic data from like those of the SMB, were formed predominantly by the Tangier pelitic xenoliths show relatively uniform the partial melting of metasedimentary rocks. Sr and eNd values, but variable Sr values (Eberz et al., 1991). Nd isotopic data show that the source of the SMB Nevertheless, the Nd–Sr isotopic ratios for the xeno- cannot be in the Meguma Group (Fig. 6; Clarke and liths (Fig. 6) overlap those of the Liscomb gneisses Halliday, 1985; Clarke et al., 1988). The Liscomb and of the SMB granites. Dostal et al. (2004) also granitic rocks, like those of the SMB, are probably show that the Pb isotopic composition of K-feldspar products of the partial melting of a deeper-seated from the SMB granites overlaps that of xenoliths in crustal source, the basement of the Meguma Group. the Tangier Dykes. Thus, the Tangier pelitic xenoliths Nd and Sr isotopic analyses show that the Liscomb could also represent a source material for some of the granites fall in an intermediate position between Lis- SMB granitoids. comb augen gneisses and Liscomb metapelites, im- plying that the granites could be genetically related to 4.4. Meguma basement the gneisses (Clarke et al., 1993). Some Liscomb gneisses probably represent a fertile source similar The Meguma Terrane granitoids appear to have to that from which the Devonian peraluminous gran- been derived from a source comparable to the Lis- ites of the Meguma Terrane were derived. This is comb gneisses and Tangier xenoliths, which are com- consistent with the major and trace element composi- posed of pelitic metasedimentary rocks interpreted as tions, which indicate that partial melting of such deep-seated (mid-crustal) basement rocks underlying gneisses could generate a peraluminous granitic melt the Meguma Group. The isotopic data (Fig. 6) imply 86 J. Dostal et al. / Lithos 86 (2006) 77–90

that the Tangier metasedimentary xenoliths, like the Liscomb gneisses, are not high-grade equivalents of the Meguma Group, but belong to a distinct basement unit (Clarke et al., 1997). The Pb isotopic compositions and detrital zircon ages of the crustal xenoliths from the lamprophyre dykes of Tangier suggest that they are a part of the Avalon basement, which is thought to underlie/underthrust the Meguma Group rocks (Greenough et al., 1999; Dostal et al., 2004). Although no detrital zircons were identified in the Liscomb gneisses, enough parallels are drawn with the lithology, geochemistry and geochronology of these xenoliths to postulate that the gneisses are Ava- lonian as well. Likewise, the source of the SMB magma has Pb isotope characteristics comparable to the Avalon basement in coastal Maine and southern New Brunswick (Dostal et al., 2004), which suggests that the Meguma Terrane is, at least in part, underlain by Avalonian basement.

5. Discussion and conclusions

The U–Pb data from the Liscomb Complex support an upper amphibolite–granulite facies metamorphism at 377F2 Ma, which overlaps the 378F1 Ma metamorphism recorded in a granulite xenolith from a lamprophyre dyke located 30 km to the south of the Liscomb Complex (Greenough et al., 1999). This metamorphism took place at pressures of 640–820 MPa, which indicate depths of 24–29 and 26–33 km, respectively (Fig. 7; Owen and Greenough, 1991; Owen et al., 1988). On the other hand, crystallization of the Liscomb granite is dated at 374F3 Ma, and occurred at pressures of ~300 MPa (=depth of ~10–12 km; Fig. 7; Clarke et al., 1993; Kontak and Fig. 5. Chondrite-normalized rare-earth element compositions: (A) Reynolds, 1994). Although these ages overlap within the average of twelve two-mica granodiorites (L9), twelve two-mica error, the mean ages are consistent with local temporal monzogranites (L10) from the Liscomb Complex (Clarke et al., observations that the granite cuts the gneiss. This near 1993), and the dated granodiorite LG-1; (B) the average of SMB synchronicity suggests that middle crustal granulite/ granodiorite (GD), monzogranite (MG), as well as the average leucomonzogranite of the Davis Lake Pluton (LMG), which is upper amphibolite facies metamorphism and partial one of the intrusions of the SMB composite (Dostal and Chatterjee, melting were complementary processes. 1995, 2000); (C) the average of 17 garnet–hornblende gneisses Granitic magmatism in the Liscomb Complex is (L2), 10 quartzo–feldspathic gneisses (L3) and 14 sillimanite schists synchronous (within error) with all but one of the (L4) of the Liscomb Complex (after Clarke et al., 1993) compared granite plutons in the Meguma Terrane, which range to the North America shale composite (NASC; Gromet et al., 1984) F F and the average for the upper continental crust (UC; Taylor and in age from 378.5 2 to 370 3Ma(Keppie and McLennan, 1985). Normalizing values are after Sun and McDo- Krogh, 1999). Such voluminous granitic magmatism nough (1989). suggests a fertile source (Vielzeuf et al., 1990) that J. Dostal et al. / Lithos 86 (2006) 77–90 87

87 86 Fig. 6. Initial Sr/ Sr ratio versus qNd (375 Ma) for granites (Â) and gneisses (crosses) of the Liscomb Complex (Clarke et al., 1993), as well as those of the metapelitic xenoliths (open circles; Eberz et al., 1991) of the Tangier Dykes. The field of the South Mountain Batholith (SMB) is after Clarke and Halliday (1980) and Clarke et al. (1988), whereas that of Meguma Group sedimentary rocks is after Eberz et al. (1991).

was probably dominated by juvenile sedimentary rock protholiths rather than a reworked older basement (Sawyer, 1998; Brown, 2001). The peraluminous nature of the Meguma granitoids also suggests a metasedimentary source. The aluminosilicate-bearing Liscomb and Tangier gneisses, some of which are compositionally similar to sedimentary rocks, can be a fertile source of granite magma, and their Nd, Sr and Pb isotopic signatures suggest that they are the source of the Meguma granites. The similarity between the isotopic signatures of the granites and their source rocks is consistent with the popular assumption that magmas image their source region (Brown, 2001). Cooling ages of 369F3 and 368F3Ma(Kontak and Reynolds, 1994) on muscovite and biotite, respectively, were obtained from the granite of locality LG-1. Furthermore, because the northern margin of the Meguma Terrane is unconformably overlain by the Horton Group, the oldest part of which is late Fammenian (~365–360 Ma) (Martel et al., 1993), this age is consistent with a projection of the cooling curve through amphibole, muscovite and biotite to Fig. 7. The Liscomb Complex and Tangier xenolith data plotted on the surface, which suggests exhumation by ~364 (A) a Pressure–Temperature diagram, and (B) an Age–Temperature Ma (Fig. 7B). This suggests that, whereas develop- diagram. Note that only the overlapping age range is shown. Data are from Owen et al. (1988), Owen and Greenough (1991), Clarke ment of the core complex raised the granulite facies et al. (1993), Kontak and Reynolds (1994), Greenough et al. (1999) rocks from a depth of ~30 km to a depth of ~10–11 and this study. km in ~3 million years, exhumation to the surface 88 J. Dostal et al. / Lithos 86 (2006) 77–90 required an additional ~10 million years (i.e., at a rate Bouyx, E., Blaise, J., Brice, D., Degardin, J.M., Goujet, D., Gour- of ~54 8C/my; Fig. 7). These data are consistent with vennec, R., Le Menn, J., Lardeux, H., Morzadec, P., Paris, F., 1997. Biostratigraphie et paleobiogeographie du Siluro–Devo- the rates of melt production, segregation, ascent and nien de la zone de Meguma (Nouvelle-Ecosse, Canada). Cana- crystallization predicted by experimental studies (Har- dian Journal of Earth Sciences 34, 1295–1309. ris et al., 2000). Solar et al. (1998) inferred that the Braun, I., Kriegsman, L.M., 2001. Partial melting in crustal xeno- process of granite generation from the peak of meta- liths and anatectic migmatites: a comparison. Physics and morphism to the intrusion of granitic plutons takes Chemistry of the Earth. Part A: Solid Earth and Geodesy 26, 261–266. less than 1 Ma. Brown, M., 2001. Orogeny, migmatites and leucogranites: a review. It is noteworthy that these ~375 Ma magmatic and Proceedings of Indian Academy of Sciences, Earth Planetary metamorphic events in the Meguma Terrane, and Sciences, vol. 110, pp. 313–336. exhumation by ~364 Ma, are contemporaneous with Chatterjee, A.K., Richard, L.R., Clarke, D.B., 1989. Magmatic and the onset of extensional tectonics in south-eastern subsolidus amphiboles from the Ten Mile Lake intrusion, Lis- comb Complex, Nova Sotia. Geological Association of Canada- Canada. The extension was initiated in the Middle– Mineralogical Association of Canada, Program with Abstracts Late Devonian with the rift-related basalts and alluvial 14, A98. fan deposits of the McAras Brook Formation near the Clarke, D.B., 1992. Granitoid Rocks. Chapman & Hall, London. southern margin of the Maritimes Basin (Dostal et al., 283 pp. 1983; Keppie, 1993; Keppie et al., 1997). This is Clarke, D.B., Halliday, A.N., 1980. Strontium isotope geology of the South Mountain Batholith, Nova Scotia. 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