www.elsevier.com/locate/lithos
Lithos 86 (200
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, CanadabInstituto 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, USAdDepartment 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
0024-4937/$ - s
doi:10.1016/j.lit
* Correspondi
E-mail addre
6) 77–90
ee front matter D 2005 Elsevier B.V. All rights reserved.
hos.2005.04.002
ng author.
ss: [email protected] (J. Dostal).
J. Dostal et al. / Lithos 86 (2006) 77–9078
1. Introduction
One hypothesis for the origin of voluminous
granitoid rocks that intrude into the upper crust
links it to melt generated during granulite facies
metamorphism in the mid-lower crust (e.g., Viel-
zeuf et al., 1990). Tests for this hypothesis have
been sought in mid-lower crustal granulites (e.g.,
LeFort, 1986; Solar and Brown, 2001) and in lower
crustal xenoliths within volcanic suites (e.g., Braun
and Kriegsman, 2001). These tests have been gen-
erally limited to geochemical and isotopic compar-
Fig. 1. Geological map of the Meguma Terrane of southern Nova Scoti
including the South Mountain Batholith (SMB) as well as the Liscom
(Greenough et al., 1999) and the Cambro–Ordovician Meguma Group. T
the lamprophyres form a swarm of narrow dykes along the eastern shore of
and the location of the map. It also shows the lithotectonic terranes o
G=Gander; D=Dunnage; H=Humber) and the Minas Fault (MF) separat
isons because upper and mid-lower crusts are
rarely exposed together, making a direct connection
difficult. Furthermore, geochronological data for
minerals with high blocking temperatures such as
zircon are generally missing for genetically-related
granulites and granites. However, an unusual situa-
tion in which middle crustal granulite-facies
gneisses and upper crustal granitoid rocks crop
out together occurs in the Liscomb Complex of
southern Nova Scotia, Canada (Fig. 1), thereby
providing a rare opportunity to test the granulite–
granite connection.
a, showing the major intrusions of Late Devonian granitoid rocks,
b Complex, the xenolith-bearing lamprophyre dykes of Tangier
he area of study, shown in Fig. 2, is indicated by a star. Note that
Nova Scotia. The insert displays eastern Canada, northeastern USA,
f the Canadian Appalachians (terranes: M=Meguma; A=Avalon;
ing the Meguma and Avalon terranes.
J. Dostal et al. / Lithos 86 (2006) 77–90 79
2. Geological setting
The Liscomb Complex is located within the
Meguma Terrane of the Canadian Appalachians
(Fig. 1). The Meguma Terrane, most outboard terrane
of the northern Appalachians, is juxtaposed against
the Avalon Terrane along the Minas (Cobequid-Che-
dabucto) Fault Zone. Both these terranes were accret-
ed to North America (Laurentia) during continental
collision in the early to middle Paleozoic (Williams
and Hatcher, 1983). In particular, the Meguma was
accreted in the Devonian during the final closure of
the Rheic Ocean. The Meguma Terrane is composed
mainly of the ~10 km thick Cambro–Ordovician tur-
bidite succession of the Meguma Group which con-
tains Gondwanan fauna (Pratt and Waldron, 1991).
Detrital zircons from a lower unit of the Meguma
Group yielded ~3.0 Ga, 2.0 Ga and 600 Ma ages,
also indicating a Gondwanan (West African) source
(Krogh and Keppie, 1990). The Meguma turbidites
(wackes and pelites) are disconformably to uncon-
formably overlain by Siluro–Devonian shallow-ma-
rine and continental rocks. The youngest of these
rocks contains Early Devonian (Lochkovian to lower
Emsian) fossils (Boucot, 1975; Bouyx et al., 1997).
These Cambrian to Devonian rocks were deformed
and metamorphosed to a lower greenschist to amphib-
olite facies under low pressure metamorphic condi-
tions during the Devonian Acadian Orogeny at about
405–370 Ma (Keppie and Dallmeyer, 1995; Hicks et
al., 1999), shortly after the deposition of the Lower
Devonian rocks. This was accompanied by the intru-
sion of voluminous peraluminous granitoids of the
South Mountain Batholith (SMB), Liscomb Complex
and satellite plutons that were emplaced at a depth of
~10–12 km around 380–370 Ma (Clarke et al., 1997;
Kontak and Reynolds, 1994).
The SMB is the dominant granitoid body of the
Meguma Terrane. It spreads over an area of about
7300 km2 (Fig. 1) and contains rocks ranging from
megacrystic biotite granodiorite, with up to 20% bio-
tite, to equigranular leucogranite containing less than
2% biotite. The granitic bodies produced a distinct
contact metamorphic aureole. This was followed by
rapid exhumation, as documented by 40Ar / 39Ar mica
cooling ages of ~375–360 Ma (Keppie and Dall-
meyer, 1995), before being unconformably overlain
by Upper Devonian to Carboniferous continental and
shallow marine rocks (Martel et al., 1993). The oldest
of these rocks is of late Fammenian age (~365–360
Ma according to Okulitch, 2003).
Many authors have inferred that the Meguma
Group and overlying Siluro–Devonian units repre-
sents a Cambrian to Early Devonian passive margin
bordering northwest Africa that was subsequently
transferred to Laurentia during the Acadian Orogeny
(e.g., Schenk, 1997). This was based primarily upon:
(i) proposed stratigraphic correlations between the
Cambro–Silurian strata in the Meguma Terrane and
coeval seccessions in Morocco (Schenk, 1997), and
(ii) the Middle Devonian age of the Acadian Orogeny,
the oldest accretionary event recognized in the
Meguma Terrane. Alternatively, it has been proposed
that the Cambrian to Early Devonian strata of the
Meguma Terrane was either thrust over the Avalon
Terrane (e.g., Greenough et al., 1999) or may repre-
sent a passive margin bordering the Avalon micro-
continent, which would imply that the Meguma Group
was deposited on Avalonian continental crust (e.g.,
Keppie and Dostal, 1991; Keppie et al., 2003). Detri-
tal zircon studies show contrasting provenance for
Avalonian and Meguma Cambro–Ordovician sedi-
mentary rocks (Krogh and Keppie, 1990; Keppie et
al., 1998), whereas Siluro–Devonian sedimentary
rocks contain similar age suites (Murphy et al., 2004).
The basement of the Meguma Group is only
exposed in the Liscomb Complex (Fig. 2), an assem-
blage of high-grade gneisses and mafic plutonic
rocks that were intruded by granitoid rocks (Giles
and Chatterjee, 1986, 1987; Clarke et al., 1993;
Kontak and Reynolds, 1994). The complex, which
crops out over an area of ~240 km2, cuts across
greenschist facies metasedimentary rocks of the
Meguma Group near the northern margin of the
Meguma Terrane (Fig. 1).
Exposure of the Liscomb Complex is very poor.
Strong shearing and at least 2 m wide contact aureole
were observed in the only exposure of the contact
between a foliated granite/gneiss of the Liscomb
Complex and the Meguma Group. The gneisses
have a distinct foliation that is oblique to that in the
surrounding Meguma Group and the fold traces and
lithologic boundaries of the Meguma Group appear to
be sharply truncated by the border of the Liscomb
Complex, indicating that its emplacement is post-
folding (Fig. 2). Within the Liscomb Complex, field
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.
J. Dostal et al. / Lithos 86 (2006) 77–9080
relationships including rare contacts between the var-
ious units indicate that the sequence of emplacement
is gneisses, gabbros and granites (Kontak and Rey-
nolds, 1994), and that the emplacement of all the units
postdates Acadian deformation of the Meguma meta-
sedimentary rocks. 40Ar / 39Ar dates are interpreted to
record cooling ages from the Liscomb Complex at
375F3 Ma in amphibole (gneisses), 373F4 to
367F3 Ma in muscovite (gneisses and granites),
and 385 to 367 Ma in biotite (gneisses, granites and
gabbros) (Kontak and Reynolds, 1994), providing a
younger limit for the emplacement of the Liscomb
Complex.
Clarke et al. (1993) inferred that the gneisses were
emplaced as a domal uplift that may have intruded the
Meguma Group through diapirism. However, the
presence of paragneisses is more typical of core com-
plexes. The classic Cordilleran metamorphic core
complexes occur in a narrow belt within the magmatic
arc extending from southern Canada to northwestern
Mexico. They record Tertiary extension following the
Laramide Orogeny (Coney, 1980; Dickinson, 2002).
Typically, they consist of an older metamorphic–plu-
tonic basement overprinted upwards by a metamor-
phic carapace of gently dipping, domal, greenschist–
amphibolite facies with lineated and foliated myloni-
tic–gneissic fabrics. This carapace is overlain by a
decollement zone with sliding and detachment kine-
matic indicators. This zone is, in turn, overlain by an
unmetamorphosed cover attenuated by subhorizontal
faults. The amplitude of most of these core complexes
is V4 km (Coney, 1980), and estimates of the pres-
sures of formation for the mylonitic fabrics are 3–3.5
kb (=10–13 km) (Davis et al., 1980). On the other
hand, core complexes in the arc–backarc Cyclades
region (Aegean Sea, Greece) developed at 5–7 kb
(=20–25 km depth) and 700–380 8C, were exhumed
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-
tion has been reported in the active core complexes of
the D’Entrecasteaux Islands, a rifted arc complex in
Papua New Guinea, in which gneissic domes include
eclogites formed at depths of 45–75 km (=13–21 kb)
and temperatures of 730–900 8C (Hill et al., 1992).
Isothermal exhumation at a rate of 15 km/my was
followed by cooling at N100 8C/my. These core com-
plexes rise through a region of density inversion
where ophiolites overlie less-dense continental crust
and are located in a continental rift that passes later-
ally into the active Woodlark Basin sea-floor spread-
ing system. Unroofing took place by faulting and
shearing at the boundary the gneiss domes. Present
day surface uplift has led to topographic elevations of
up to 2.5 km. Clockwise P–T–t paths and short-lived
thermal pulses associated with extensional deforma-
tion and the intrusion of sills occur in all of these core
complexes (Lister and Baldwin, 1993). Poor exposure
of the Liscomb Complex means that most of these
characteristics cannot be observed. However, the gen-
tle dips in the foliation of the Liscomb gneisses, the
domal shape of the high-grade gneisses and their
discordance with the surrounding Meguma Group
are consistent with a core complex interpretation.
The gneisses of the Liscomb Complex include a
variety of migmatitic and non-migmatitic rocks ranging
from augen gneiss (quartz–K-feldspar–plagioclase–
biotite–muscovite–sillimanite) through hornblende–
biotite gneiss (quartz–K-feldspar–plagioclase–horn-
blende–biotite) to quartzo–feldspathic gneiss (quartz–
K-feldspar–plagioclase–biotite–muscovite–sillimanite–
garnet) and sillimanite schist (quartz–plagioclase–cor-
dierite–biotite–sillimanite) (Clarke et al., 1993). Al-
though these mineral assemblages are typical of the
upper amphibolite facies, that these are mostly ret-
rograde assemblages is indicated by the presence of
coronas around pyroxene cores and zoned minerals,
such as Mn-rich garnet cores and ternary feldspar
cores with compositions indicative of temperatures
N900 8C (Clarke et al., 1993; Kontak and Reynolds,
1994). Clarke et al. (1993) and Kontak and Rey-
nolds (1994) estimated pressures of 640 to 820 MPa
and temperatures of 760 to 980 8C.The mafic rocks form two separate intrusions
(Chatterjee et al., 1989) composed of amphibole/clin-
opyroxene-bearing gabbros and diorites that contain
significant proportions of both cognate and exotic
xenoliths (Clarke et al., 1993). They contain xenoliths
of both Liscomb gneisses and Meguma Group rocks.
The granitic rocks of the complex are mainly grano-
diorites and monzogranites (sensu Streckeisen and Le
Maitre, 1979) similar to those of the SMB. Both
the granodiorites and monzogranites contain biotite
(+/�muscovite). In addition, many granites contain
garnet and other noteworthy accessory minerals, in-
cluding zircon, monazite and apatite.
The only other evidence for the nature of the base-
ment beneath the Meguma Group comes from xeno-
liths in ~368Mamafic lamprophyre dykes (the Tangier
Dykes on Fig. 1), which intrude the Meguma Group to
the south of the Liscomb Complex (Fig. 1: Kempster et
al., 1989). The xenoliths include three rock types:
sapphirine granulites, mafic gneisses and garnetiferous
quartzo–feldspathic gneisses (Owen et al., 1988; Owen
and Greenough, 1991). Mineral core compositions for
the early (pre-dyke) metamorphic event in the sapphi-
rine granulites and quartzo–feldspathic gneisses indi-
cate minimum temperatures ofz600 8C at pressures of
~450–600 MPa (Owen et al., 1988), whereas rim com-
positions and M2 (syn-dyke) assemblages in the meta-
pelitic rocks imply conditions of 725–795 8C and 700–
900 MPa (Owen and Greenough, 1991). These xeno-
liths indicate that a mafic unit was emplaced at or
before 629F4 Ma into pelitic metasedimentary rocks
containing ~880–1050 Ma detrital zircons (Greenough
et al., 1999). The xenoliths underwent a high-grade
metamorphic event at 378F1 Ma (U–Pb concordant
zircon age; Greenough et al., 1999).
3. Geochronology
3.1. Analytical methods
U–Pb isotopic analyses of zircon and monazite
from the Liscomb Complex were done at the Univer-
sity of North Carolina using the procedure of Ratajeski
et al. (2001). All zircon fractions were highly abraded,
but monazites were not. All reported errors are of two
sigma. The zircon and monazite were obtained from
three samples: granite LG-1 (location: N 45814.777Vand W 62846.732V) and garnet–biotite–sillimanite
gneisses LG-120 (Si-poor; probably a residual rock;
location: N 45816.482V W 62839.923V) and LG-122
(location: N 45816.402V W 62840.003V; Fig. 2).
Fig. 3. U–Pb concordia diagrams for (A) the Liscomb granodiorite
(sample LG-1), (B) a garnet–biotite–sillimanite mafic gneiss (sam-
ple LG-120) and (C) a garnet–biotite–sillimanite felsic gneiss (sam-
ple LG-122).
J. Dostal et al. / Lithos 86 (2006) 77–9082
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., Scharer, 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 anal-
yses 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 meta-
morphic 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 frac-
tions from gneiss sample LG-122 also form a discor-
dant 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 radio-
genic 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
with an X-ray fluorescence spectrometer at the Geo-
chemical Centre of the Department of Geology, Saint
Mary’s University, Halifax. Additional trace elements
Table 1
U–Pb isotopic data for Liscomb Complex
Analysis #, fraction (number of grains) Weight
(mg)aTotala Totalb Totalb U
(ppm)
Pb
(ppm)
Atomic ratios Ages (Ma)
U
(ng)
Pb
(pg)
Com.Pb
(pg)
206Pbb /204Pb
206Pbc /208Pb
206Pbc /238U
% Errord 207Pbc /235U
% Errord 207Pbc /206Pb
% Errord 206Pb /238U
207Pb /235U
207Pb /206Pb
re
Liscomb granite (LG-1; N 45814.777V W 62846.732V)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
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 W 62839.923V)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 W 62840.003V)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).
J.Dosta
let
al./Lith
os86(2006)77–90
83
Table 2
Chemical compositions of the dated rocks from the Liscomb
Complex
Sample no. Granite LG-1 Gneiss LG-120 Gneiss LG-122
SiO2 (wt. %) 66.30 44.66 53.45
TiO2 0.61 0.94 0.87
Al2O3 15.67 32.01 17.87
Fe2O3 4.22 13.43 8.17
MnO 0.09 1.38 0.18
MgO 1.49 4.93 6.88
CaO 2.24 0.51 7.63
Na2O 3.30 1.13 2.68
K2O 3.57 0.81 1.07
P2O5 0.26 0.05 0.13
LOI 1.00 0.90 1.06
Total 98.75 100.75 99.99
Cr (ppm) 26 243 123
Ni 14 193 30
Co 10 89 38
V 77 211 163
Zn 80 242 77
Rb 172 34 37
Ba 979 300 268
Sr 194 98 320
Ga 22 54 17
Ta 1.34 1.03 0.44
Nb 18.0 18.3 8.1
Hf 5.69 3.80 2.98
Zr 247 157 122
Y 14 12 19
Th 14.2 9.69
La 36.4 28.0 23.4
Ce 77.7 57.7 46.0
Pr 9.48 6.42 5.70
Nd 37.8 24.3 21.5
Sm 7.72 4.55 4.41
Eu 1.33 0.65 0.98
Gd 5.32 3.24 3.68
Tb 0.74 0.51 0.59
Dy 3.59 2.79 3.55
Ho 0.55 0.47 0.74
Er 1.45 1.34 2.19
Tm 0.21 0.19 0.33
Yb 1.32 1.27 2.17
Lu 0.20 0.19 0.3387Sr / 86Sri 0.707926 0.714273
eNd �5.12 �10.43143Nd/ 144Ndi 0.511893 0.511621
Isotopic ratios corrected to 375 Ma; isotopic data from Clarke et al.
J. Dostal et al. / Lithos 86 (2006) 77–9084
(the rare-earth elements [REE], Hf, Ta, Nb and Th)
were analyzed in all these samples by inductively
coupled plasma-mass spectrometry using a Na2O2-
sintering technique at the Department of Earth
Sciences of the Memorial University of Newfound-
land. The precision for the trace elements is between
2% and 8% of the values cited (Dostal et al., 1986;
Longerich et al., 1990). The Sr and Nd isotopic ratios
of rocks from the same outcrops reported in Table 2
are from Clarke et al. (1993).
4.2. Liscomb granite and gneiss
Granitic rocks of the Liscomb Complex (Fig. 4)
typically have SiO2 contents ranging from 66 to 73
wt.%. They are peraluminous (mol. Al2O3NCaO+
Na2O+K2O), with K2ONNa2O and K2ON3.5 wt.%.
Their K/Rb ratios (210–170) are slightly lower than
those of average crustal compositions (~230; Shaw,
1968). The chondrite-normalized patterns are enriched
in light REE, display slightly fractionated heavy REE,
and are accompanied by a negative Eu anomaly. Their
(La /Yb)n ratios range from ~10 to 17, whereas
(Gd /Yb)n ratios range from 1.5 to 4 (Fig. 5). The
eNd values (�2.7 to �5.9) and initial Sr isotopic
ratios (0.70793 to 0.70875) of the Liscomb granites
(Clarke et al., 1993) are typical of crustally-derived
granitic rocks (Faure, 2001), and, more specifically, S-
type granites (Clarke, 1992). The major (Fig. 4) and
trace element abundances (Fig. 5) as well as the Nd
and Sr isotopic characteristics (Fig. 6) of the Liscomb
granites are within the variation range of the SMB,
suggesting that the Liscomb granite probably repre-
sents a satellite body of the SMB derived from a
common or similar source.
The Liscomb gneisses constitute a heterogeneous
group of non-migmatitic and migmatitic rocks with
SiO2 contents ranging from b46 to N72 wt.%, and
Al2O3 contents ranging from 12 to N32 wt.%. Al2O3
shows a negative correlation with SiO2 and is high in
the sillimanite-bearing gneisses (~25 wt.%) but low in
the quartz–feldspathic gneisses (~13 wt.%). The gar-
net–hornblende gneisses are high in CaO (~5 wt.%),
MgO (~4 wt.%) and Al2O3 (~18 wt.%). The Nd and
Sr isotopic ratios of the Liscomb gneisses (Fig. 6) are
highly variable, ranging from relatively low values of
eNd (~+1) and initial Sr isotopic ratios (~0.706–
0.708) in augen gneisses and garnet–hornblende
gneisses, to high radiogenic values in metapelites
(N�10 and 0.714–0.716, respectively). Augen
gneisses are compositionally similar to the peralumi-
nous granites. Most rocks are likely metamorphosed
felsic igneous rocks and clastic sedimentary rocks,
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.
J. Dostal et al. / Lithos 86 (2006) 77–90 85
particularly pelites. Some SiO2-poor rocks are proba-
bly residual rocks, related to melt extraction, whereas
other gneisses may represent an untapped but fertile
source rock. These gneisses which resemble the
source rocks have REE patterns very close to those
of the North American shale composite and average
upper continental crust (Fig. 5).
4.3. Source of magma
Peraluminous granites of the Liscomb Complex,
like those of the SMB, were formed predominantly by
the partial melting of metasedimentary rocks. Sr and
Nd isotopic data show that the source of the SMB
cannot be in the Meguma Group (Fig. 6; Clarke and
Halliday, 1985; Clarke et al., 1988). The Liscomb
granitic rocks, like those of the SMB, are probably
products of the partial melting of a deeper-seated
crustal source, the basement of the Meguma Group.
Nd and Sr isotopic analyses show that the Liscomb
granites fall in an intermediate position between Lis-
comb augen gneisses and Liscomb metapelites, im-
plying that the granites could be genetically related to
the gneisses (Clarke et al., 1993). Some Liscomb
gneisses probably represent a fertile source similar
to that from which the Devonian peraluminous gran-
ites of the Meguma Terrane were derived. This is
consistent with the major and trace element composi-
tions, which indicate that partial melting of such
gneisses could generate a peraluminous granitic melt
similar to that which formed the Liscomb granites.
The absence of leucosomes in the Liscomb gneisses
indicates that they were not partially molten. Howev-
er, the presence of a megacrystic gneissic granite
intruding them suggests that partial melting took
place at a greater depth than is presently exposed.
Compositions of the metapelitic xenoliths of the
Tangier Dykes are comparable to those of the Lis-
comb gneisses (Eberz et al., 1991; Clarke et al., 1993),
although the relation between the Liscomb gneisses
and Tangier xenoliths is uncertain. Isotopic data from
the Tangier pelitic xenoliths show relatively uniform
eNd values, but variable Sr values (Eberz et al., 1991).Nevertheless, the Nd–Sr isotopic ratios for the xeno-
liths (Fig. 6) overlap those of the Liscomb gneisses
and of the SMB granites. Dostal et al. (2004) also
show that the Pb isotopic composition of K-feldspar
from the SMB granites overlaps that of xenoliths in
the Tangier Dykes. Thus, the Tangier pelitic xenoliths
could also represent a source material for some of the
SMB granitoids.
4.4. Meguma basement
The Meguma Terrane granitoids appear to have
been derived from a source comparable to the Lis-
comb gneisses and Tangier xenoliths, which are com-
posed of pelitic metasedimentary rocks interpreted as
deep-seated (mid-crustal) basement rocks underlying
the Meguma Group. The isotopic data (Fig. 6) imply
Fig. 5. Chondrite-normalized rare-earth element compositions: (A)
the average of twelve two-mica granodiorites (L9), twelve two-mica
monzogranites (L10) from the Liscomb Complex (Clarke et al.,
1993), and the dated granodiorite LG-1; (B) the average of SMB
granodiorite (GD), monzogranite (MG), as well as the average
leucomonzogranite of the Davis Lake Pluton (LMG), which is
one of the intrusions of the SMB composite (Dostal and Chatterjee,
1995, 2000); (C) the average of 17 garnet–hornblende gneisses
(L2), 10 quartzo–feldspathic gneisses (L3) and 14 sillimanite schists
(L4) of the Liscomb Complex (after Clarke et al., 1993) compared
to the North America shale composite (NASC; Gromet et al., 1984)
and the average for the upper continental crust (UC; Taylor and
McLennan, 1985). Normalizing values are after Sun and McDo-
nough (1989).
J. Dostal et al. / Lithos 86 (2006) 77–9086
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 composi-
tions 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 meta-
morphism 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, crystal-
lization 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
Reynolds, 1994). Although these ages overlap within
error, the mean ages are consistent with local temporal
observations that the granite cuts the gneiss. This near
synchronicity suggests that middle crustal granulite/
upper amphibolite facies metamorphism and partial
melting were complementary processes.
Granitic magmatism in the Liscomb Complex is
synchronous (within error) with all but one of the
granite plutons in the Meguma Terrane, which range
in age from 378.5F2 to 370F3 Ma (Keppie and
Krogh, 1999). Such voluminous granitic magmatism
suggests a fertile source (Vielzeuf et al., 1990) that
Fig. 6. Initial 87Sr / 86Sr ratio versus qNd (375 Ma) for granites (�) 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).
Fig. 7. The Liscomb Complex and Tangier xenolith data plotted on
(A) a Pressure–Temperature diagram, and (B) an Age–Temperature
diagram. Note that only the overlapping age range is shown. Data
are from Owen et al. (1988), Owen and Greenough (1991), Clarke
et al. (1993), Kontak and Reynolds (1994), Greenough et al. (1999)
and this study.
J. Dostal et al. / Lithos 86 (2006) 77–90 87
was probably dominated by juvenile sedimentary rock
protholiths rather than a reworked older basement
(Sawyer, 1998; Brown, 2001). The peraluminous na-
ture of the Meguma granitoids also suggests a meta-
sedimentary 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 368F3 Ma (Kontak
and Reynolds, 1994) on muscovite and biotite, re-
spectively, 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
the surface, which suggests exhumation by ~364
Ma (Fig. 7B). This suggests that, whereas develop-
ment of the core complex raised the granulite facies
rocks from a depth of ~30 km to a depth of ~10–11
km in ~3 million years, exhumation to the surface
J. Dostal et al. / Lithos 86 (2006) 77–9088
required an additional ~10 million years (i.e., at a rate
of ~54 8C/my; Fig. 7). These data are consistent with
the rates of melt production, segregation, ascent and
crystallization predicted by experimental studies (Har-
ris et al., 2000). Solar et al. (1998) inferred that the
process of granite generation from the peak of meta-
morphism to the intrusion of granitic plutons takes
less than 1 Ma.
It is noteworthy that these ~375 Ma magmatic and
metamorphic events in the Meguma Terrane, and
exhumation by ~364 Ma, are contemporaneous with
the onset of extensional tectonics in south-eastern
Canada. The extension was initiated in the Middle–
Late Devonian with the rift-related basalts and alluvial
fan deposits of the McAras Brook Formation near the
southern margin of the Maritimes Basin (Dostal et al.,
1983; Keppie, 1993; Keppie et al., 1997). This is
consistent with evidence which suggests that exten-
sion is an essential element in the development of core
complexes (Dickinson, 2002). It is inferred to have
facilitated uplift of the Liscomb gneisses and intrusion
of the granites, which ascended very rapidly. It is
suggested that the onset of pull-apart tectonics along
the Minas Fault system may have generated a sudden
relaxation at the root of the Acadian Orogen, which
led to the rapid ascent of the Liscomb gneiss diapir
and voluminous granitic melts.
Acknowledgements
This study was financially supported by the Natu-
ral Sciences and Engineering Research Council of
Canada. JDK would like to acknowledge support
from the PAPIIT Project IN103003-3. We would
like to thank Ian Buick and Victor Owen for construc-
tive reviews of the manuscript. We also thank Peter
Giles and A.K. Chatterjee for their help in the field
and in the selection of sampling sites, as well as
Randolph Corney and Aaron Vaughan for their help
in the preparation of the manuscript.
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