JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 8
PAGES 1477^1503
2009
doi:10.1093/petrology/egp038
Evaluating the Origin of Garnet, Cordierite, and
Biotite in Granitic Rocks: a Case Study from the
South Mountain Batholith, Nova Scotia
SASKIA ERDMANN1*, REBECCA A. JAMIESON1 AND
MICHAEL A. MACDONALD2
1
DALHOUSIE UNIVERSITY, DEPARTMENT OF EARTH SCIENCES, HALIFAX, NS B3H 3J5, CANADA
2
NOVA SCOTIA DEPARTMENT OF NATURAL RESOURCES, MINERAL RESOURCES BRANCH, HALIFAX, NS B3J 3M8,
CANADA
RECEIVED JUNE 16, 2008; ACCEPTED MAY 25, 2009
ADVANCE ACCESS PUBLICATION JULY 6, 2009
Evaluating the origin of garnet, cordierite, and biotite in granites
provides important insight into closed- and open-system magma evolution. We present field, textural, major- and trace-element mineral
chemical, and Sr^Nd whole-rock isotopic data on garnet-,
cordierite-, and biotite-rich zones from the peraluminous South
Mountain Batholith. We infer that: (1) garnet-rich zones of decimeter to meter size with 30 vol. % large, subhedral garnet with
abundant inclusions of detrital country-rock monazite represent partially assimilated metapelitic country rocks, where garnet is the
incongruent product of biotite-dehydration melting; (2) cordieriterich zones tens of meters to kilometers in dimension, with 5 vol. %
large, subhedral to euhedral, zoned cordierite, formed by crystallization from relatively evolved magmas and subsequent crystal accumulation; (3) biotite-rich zones with large, subhedral to euhedral
biotite with abundant euhedral apatite inclusions, making up
80 vol. % (centimeter-scale) or 25 vol. % (kilometer-scale)
of the rocks, formed dominantly by fractional crystallization throughout the chemical evolution of the batholith. Our results suggest that
for garnet and cordierite, a combination of textural and mineral
chemical characterization is probably sufficient to determine their
origin in granites. However, for biotite and other readily equilibrated
minerals, evaluating both mineral and rock textures and majorelement, trace-element, and isotopic compositions is essential.
I N T RO D U C T I O N
tion; garnet
The origin of garnet, cordierite, and biotite crystals in
granitoid magmasçwhether phenocrysts (Miller &
Stoddard, 1981; Clemens & Wall, 1984; Erdmann et al.,
2005), xenocrysts (Bouloton, 1992; Fourcade et al., 2001;
Gottesmann & Fo«rster, 2004), restite or resistate (Chappell
et al., 1987; Drummond et al., 1988; Dahlquist et al., 2005),
or secondary reaction products (Kontak & Corey, 1988;
Beard et al., 2005; Stevens et al., 2007)çremains a widely
debated topic in granite petrogenesis. Possible mechanisms
for the formation of zones enriched in these minerals
include magma fractionation, mingling, mixing, and flow
(Barrie're, 1981; Didier & Barbarin, 1991; Weinberg et al.,
2001; Milord & Sawyer, 2003; Wiebe et al., 2007), countryrock contamination and partial assimilation (Pitcher &
Berger, 1972; McBirney, 1979; Defant et al., 1988; Didier,
1991; Saito et al., 2007), concentration and partial assimilation of source-rock solids (White & Chappell, 1977; Le
Fort, 1991; Barbey et al., 1999; Chappell, 2004; Stevens
et al., 2007), and localized secondary magmatic or metasomatic replacement of primary solids (Didier & Dupraz,
1985; Kontak & Corey, 1988; Beard et al., 2005; Clarke,
2007). Characterizing the origin of garnet, cordierite, and
biotite, and zones enriched in these minerals, is one important step in constraining closed- and open-system magma
evolution. Once their origin is resolved, analysis of garnet
*Corresponding author. Telephone: 902 494 3362. Fax: 902 494 6889.
E-mail: [email protected]
ß The Author 2009. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oxfordjournals.org
KEY WORDS:
assimilation; biotite; cordierite; fractional crystalliza-
JOURNAL OF PETROLOGY
VOLUME 50
major-element and oxygen isotopic compositions may constrain pressure^temperature conditions during crystallization and mantle and supracrustal components that
contributed to its formation (Valley, 1986; Chamberlain &
Conrad, 1993; Anderson, 1996; Lackey et al., 2006a). The
analysis of cordierite major- and trace-element and volatile compositions has the potential to link pressure^
temperature conditions, magma volatile contents, and
components that contributed to the cordierite formation
(Harley, 1994; Anderson, 1996; Thompson et al., 2002b;
Rigby & Droop, 2008). Correctly interpreting the origin
of garnet, cordierite, and biotite crystals in igneous rocks,
however, is not necessarily straightforward. Mineral textures and compositions rarely uniquely identify their origins, and not all potential source rocks and country-rock
contaminants may be known. Moreover, evidence for
xenocrysts, restite or resistate may be obscured, or obliterated, by endothermal, decompression, melting, dissolution,
redox, and ion exchange reactions (Chappell et al., 1987;
Wall et al., 1987; Green, 1994; Beard et al., 2005; Clarke,
2007). Nevertheless, studies that integrate analysis of textural features and mineral compositions have been successful in identifying components and processes involved in
granite formation in great detail (e.g. Waight et al., 2000;
Gagnevin et al., 2005; Lackey et al., 2006a).
Here we use a combination of field, textural, major- and
trace-element mineral chemical, and whole-rock Sr^Nd
isotopic data to infer the origin of garnet, cordierite, and
biotite in the peraluminous, granitic South Mountain
Batholith (SMB) of southern Nova Scotia, Canada.
We first discuss the potential of various evaluation criteria
to discriminate between garnet, cordierite, and biotite
crystals of different origins. We then describe garnet-, cordierite-, and biotite-rich zones in the SMB, and interpret
their origin and the process of their formation. We
conclude by recommending the most useful criteria for
identifying the origin of garnet, cordierite, and biotite in
granites, and point out potential problems in data interpretation. We distinguish between the ‘source rocks’ that partially melted to produce the granitic magma of the SMB
and the ‘country rocks’ (host or wall-rocks) that were in
contact with the ascending and emplaced magmas. By
‘resistate’ we mean parts of the original source-rock assemblage that did not melt or dissolve (see Shelley, 1993;
Vernon, 2007), and by ‘restite’ we mean crystals that
formed through incongruent melting or dissolution of the
source rocks (see White & Chappell, 1977; Barbey, 1991).
We reserve the term ‘contamination’ to refer to the addition
of country-rock material to the magma, including ‘xenoliths’ (country-rock fragments), ‘xenocrysts’ (crystals of
the original country-rock assemblage), and ‘peritectic
xenocrysts’ (formed though incongruent melting or dissolution of country-rock assemblages, e.g. via biotite-dehydration melting) (see Clarke, 2007). Mineral abbreviations
NUMBER 8
AUGUST 2009
used in this paper follow those recommended by Kretz
(1983) and Spear (1993); all other abbreviations are defined
in the legend of Table 1.
Geological setting
The Middle to Late Devonian South Mountain Batholith
(SMB) of southern Nova Scotia consists of peraluminous
granodiorites to leucogranites intruded into metasedimentary rocks of the Meguma Supergroup (MSG) (Fig. 1;
McKenzie & Clarke, 1975; Schenk, 1995; MacDonald,
2001; White et al., 2008). Texturally variable garnet
(trace amounts), cordierite (5 vol. %), and biotite
(25 vol. %) crystals are present throughout the batholith
(MacDonald & Horne, 1988; MacDonald, 2001; Clarke &
Erdmann, 2005). The highest concentrations include:
(1) garnet-rich zones (c. 01^1m) with 30 vol. % large,
subhedral, and typically biotite-rimmed garnet (Figs 1a,
2a and 3a) (Jamieson, 1974); (2) cordierite-rich zones
(c. 10^1000 m) with 5 vol. % large, subhedral to euhedral
grain clusters or single grains of cordierite (Figs 1a, 2b
and 3b) (MacDonald, 2001; Erdmann et al., 2005);
(3) biotite-rich zones with large, subhedral to euhedral biotite, forming 80 vol. % (centimeter-scale) or 25 vol. %
(kilometer-scale) of the rocks (Figs 1a, 2c and 3c)
(MacDonald, 2001). Garnet-rich zones occur in granodiorites, monzogranites, and leucomonzogranites (Fig. 1a).
Cordierite-rich zones are restricted to coarse-grained leucomonzogranites and some muscovite^biotite monzogranites (Fig. 3b). Biotite-rich zones are present throughout
the batholith, but are most common in granodiorites and
monzogranites (Figs 1a and 3c).
The host rocks of the SMB comprise mainly metapelitic
rocks of the Halifax Group and metapsammitic rocks of
the underlying Goldenville Group, which form the dominant units within the late Neoproterozoic to early
Ordovician MSG (Fig. 1; McKenzie & Clarke, 1975;
Schenk, 1995; Hicks et al., 1999; MacDonald, 2001; White
et al., 2006; Waldron et al., 2008). These rocks were affected
by Acadian deformation and greenschist- to amphibolitefacies regional metamorphism (c. 410^390 Ma; Kontak
et al., 1998; Hicks et al., 1999; Morelli et al., 2005) prior to
intrusion of the SMB at c. 380 Ma (Kontak et al., 2003,
2004). Contact metamorphism overprinted earlier assemblages and fabrics within 500^2000 m of the SMB contact
(Mahoney, 1996; Jamieson et al., 2005; Hart, 2006).
Contacts are typically sharp, although locally diffuse on a
centimeter to decimeter scale. The SMB and its adjacent
MSG host rocks contain similar mineral assemblages,
including variable proportions of quartz, plagioclase,
K-feldspar, biotite, muscovite, cordierite, garnet, andalusite, ilmenite, rutile, pyrrhotite, chalcopyrite, apatite, monazite, and zircon (MacDonald, 2001; Erdmann et al., 2005;
Jamieson et al., 2005; Clarke & Carruzzo, 2007). However,
the Sr^Nd isotopic signatures of the MSG and SMB are
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ERDMANN et al.
GARNET, CORDIERITE, AND BIOTITE IN GRANITES
Table 1: Properties of garnet, cordierite, and biotite from the South Mountain Batholith and the Meguma Supergroup
Crystal type
Mineral texture
Characteristic inclusions
Chemical composition
Zoning pattern
Some Qtz, Bt, Ap, Fe–Cu-sulphides,
Alm-dominated, Rich in Y, low in
uz–nz
Fe–Ti-oxides, 4500 Ma Mnz
V relative to s, eh MSG Grt, TE
Garnet
SMB
l, sh; Fig. 5a, b, d
concentrations similar to l, sh MSG Grt,
but lower in V
MSG
s, eh; Fig. 11a
Rare to abundant Qtz, Bt, graphite,
Alm-dominated, Sps-rich, Rich in V,
Fe–Ti-oxides
low in Y relative to l, sh MSG and
uz–rz
SMB Grt
MSG
l, sh; Fig. 11b, c
Some Qtz, Bt, Ap, Fe–Cu-sulphides,
Alm-dominated, Rich in Y, low in
Fe–Ti-oxides
V relative to s, eh MSG Grt,
uz–nz
TE concentrations similar to SMB Grt,
but higher in V
Cordierite
SMBy
l, eh, sector twinned; Fig. 9a, b
Few Qtz, Bt
Low-Mg cordierite, Rich in Li, Be, Cs,
nz
and Ga relative to MSG Crd;
TE concentrations similar to oz
SMB Crd
SMBy
l, eh, sector twinned; Fig. 9a, d
Few Qtz, Bt
Low-Mg cordierite, Rich in Li, Be,
oz
Cs, and Ga relative to MSG Crd.
TE concentrations similar to nz
SMB Crd
MSG
l, sh; Fig. 11d–f
Abundant Qtz, Bt, Fe–Ti-oxides,
Low-Na cordierite, Poor in Li, Be, Cs,
graphite Sil
and Ga relative to nz and oz SMB Crd.
Ann-dominated, TE concentrations
uz–rz
Biotite
SMB, Gd
s–l, eh; Fig. 10a–c
eh Ap, few Fe–Ti-oxides,
uz
Fe–Cu-sulphides
similar to near-contact MSG Bt
SMB, Mng
s–l, eh
eh Ap, few Fe–Ti-oxides
Ann-dominated, TE composition n.a.
uz
MSG
s–l, eh; Figs 10d,f and 11a,d
Few Fe–Ti-oxides, Fe–Cu-sulphides,
Ann-dominated, TE composition similar
uz
sh apatite
to SMB Bt from Gd
Abbreviations throughout text, figures, and tables: SMB, South Mountain Batholith; MSG, Meguma Supergroup;
TB, Tangier Basement; HP, Halifax Pluton; CR, country rock; MPEL, metapelite; MPSA, metapsammite; Gd, granodiorite; Mng, monzogranite; Fg, fine-grained; Cg, coarse-grained; l, large; s, small; sh, subhedral; eh, euhedral; TE, trace
element; n.a., not analyzed; elemental ratios for garnet are: XFe ¼ Fe/(Fe þ Ca þ Mg þ Mn); XMg ¼ Mg/
(Fe þ Ca þ Mg þ Mn); XMn ¼ Mn/(Fe þ Ca þ Mg þ Mn); XCa ¼ Ca/(Ca þ Fe þ Mg þ Mn); nz, normally zoned; rz, reversely
zoned: oz, oscillatory zoned; uz, unzoned. Normal zoning indicates a core–rim decrease in Mg, and an increase in Fe and
Mn; reverse zoning indicates a core–rim decrease in Fe and Mn, and an increase in Mg. Common accessory phases in
garnet, cordierite, and biotite are zircon and monazite; xenotime may occur in addition.
Near the SMB contact and in xenoliths.
yTextural data and major-element compositions reported by Erdmann et al. (2005).
distinct, with the more evolved SMB rocks showing a trend
towards an MSG-like isotopic signature (Clarke et al.,
1988; Eberz et al., 1991; Dostal et al., 2004).
At the present exposure level, the SMB is in contact with
roughly equivalent volumes of Halifax and Goldenville
Groups, but regional structural and geophysical
reconstructions suggest that SMB magmas must have
intruded mainly Goldenville Group rocks at depth
(Jackson et al., 2000; Culshaw & Lee, 2006). The basement to the SMB and MSG is not exposed. The only
direct evidence for the nature of potential SMB source
rocks comes from lower crustal xenoliths in mafic
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Fig. 1. (a) The South Mountain Batholith (SMB) dominantly intruded rocks of the Meguma Supergroup (MSG). Goldenville Group, metapsammite-dominated; Halifax Group, metapelite-dominated. Map after Keppie (2000); abundance of biotite and cordierite from MacDonald
(2001). Garnet-rich zones marked are the most important occurrences. Frames outline the location of Fig. 3b and c, but it should be noted that
different geological features are shown in Fig. 1 and Fig. 3b and c. (b) Schematic stratigraphy and metamorphic evolution of the Meguma
Terrane and the Tangier Basement (TB; after Keppie 2000). Age data from Krogh & Keppie (1999) and Greenough et al. (1999). Dominantly
granitic and minor lamprophyre intrusions, including the ‘Tangier Dykes’. Not exposed in SW Nova Scotia. The relationship of the Meguma
Terrane and the Avalonian Tangier Basement is debated (see Culshaw & Lee, 2006). They may have collided in the Devonian, may have been
connected since the Late Ordovician^Early Silurian, or rocks of the Meguma Supergroup may have been deposited on Tangier Basement.
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ERDMANN et al.
GARNET, CORDIERITE, AND BIOTITE IN GRANITES
Fig. 2. Characteristic outcrop photographs of garnet-, cordierite-, and biotite-rich zones in the SMB. (a) Garnet-rich zones have 30 vol. %
large, subhedral garnet crystals. (b) Cordierite-rich zones have 5 vol. % large, subhedral to euhedral cordierite crystals. (c) Biotite-rich
zones have 80 vol. % (centimeter-scale) or 25 vol. % (kilometer-scale) of large, subhedral to euhedral biotite. Gd, granodiorite.
lamprophyre dykes coeval with the SMB, commonly
referred to as the ‘Tangier dykes’ from a welldocumented locality east of the SMB (Owen et al., 1988;
Tate & Clarke, 1993; Greenough et al., 1999). The xenoliths
include pelitic and garnet-, orthopyroxene-, hornblende-,
and (or) sapphirine-bearing quartzofeldspathic gneisses,
rare quartzite, amphibolite, and gabbroic to granitic
rocks. Concentrically zoned garnet is present
in gneisses, cordierite is absent, and unzoned biotite is
common (Owen et al., 1988). Whole-rock Sr^Nd isotopic and Pb isotopic compositions suggest that these xenoliths represent fragments of the lower crustal source rocks
of the SMB and other coeval granitic intrusions in southern Nova Scotia (Eberz et al., 1991; Tate & Clarke,
1993; Dostal et al., 2004). For convenience, we refer to
the xenoliths and their lower crustal equivalents as
‘Tangier Basement’.
Origin of garnet, cordierite, and biotite
in the SMB
Theoretically, garnet-, cordierite-, and biotite-rich zones in
the SMB may be (1) primary crystallization products of
magmas derived from the Tangier Basement; (2) restite or
resistate derived from the Tangier Basement source rocks;
(3) primary crystallization products of relatively small
volumes of magma derived from the MSG; (4) xenocrysts
from the original MSG country-rock assemblage; (5) peritectic xenocrysts formed as a result of the assimilation of
the MSG country rocks (garnet or cordierite, not biotite);
(6) crystals of secondary magmatic origin; or (7) crystals
of retrograde metasomatic origin, formed by fluids derived
from the SMB or the MSG. However, SMB garnet has
higher almandine (XFe ¼ 066^078) and Ge (13 ppm),
and lower grossular contents (XCa ¼ 003^005) than
garnet from the Tangier Basement (XFe ¼ 047^066,
XCa ¼ 005^027; Ge 17 ppm) (Allan & Clarke, 1981;
Owen et al., 1988; Erdmann, 2006), and cordierite is absent
from exposed Tangier Basement rocks (Owen et al., 1988;
Eberz et al., 1991). These observations, combined with
locally large volumes of cordierite and biotite (up to 25%
in kilometer-scale zones), make a Tangier Basement restite
or resistate origin extremely unlikely (see Sawyer, 2001;
Vernon, 2007). Moreover, the studied rocks show little or
no evidence of hydrothermal alteration (Jamieson, 1974;
MacDonald & Horne, 1988; Erdmann et al., 2005), precluding a retrograde metasomatic origin (see Kontak &
Corey, 1988). We therefore focus our attention on the possibilities that garnet, cordierite, and biotite of the SMB represent either magmatic minerals or xenocrysts.
Based on a combination of textures, major-element
chemistry, and d18O garnet compositions from a relatively
limited number of samples, garnet-rich zones in the SMB
have been interpreted to represent partially assimilated
metapelitic country rocks (Jamieson, 1974; Clarke &
Erdmann, 2005; Lackey et al., 2006b). However, the origin
of the cordierite- and biotite-rich zones in the SMB, which
are volumetrically much more significant than the garnetrich zones, remains controversial. They have been
variously interpreted to have formed by fractional crystallization (MacDonald & Horne, 1988; Horne et al., 1989,
1990) or high degrees of country-rock contamination
(Clarke & Erdmann, 2005; Erdmann et al., 2005). Testing
these hypotheses requires integration of a broad array of
field, petrographic, compositional, and isotopic data.
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NUMBER 8
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M E T H O D S A N D DATA
Fig. 3. Dimensions and distribution of mineral concentrations in the
SMB. Figure 1 shows the locations of (b) and (c); (c) shows the location of (a). Sample numbers (e.g. E465A, Stop1.8A) are shown on
white background. (a) Garnet-rich zones are decimeters to meters in
size. (b) Cordierite-rich zones in the Halifax Pluton of the SMB.
Map after MacDonald & Horne (1988). (c) Large-scale biotite-rich
and biotite-poor zones in granodiorites and biotite monzogranites of
the central part of the SMB [data for Stage II intrusions and the
Halifax Pluton (HP) of the SMB are not available]. Biotite-rich
zones occur near the country-rock contact and kilometers away. Map
after Horne et al. (1989).
Our field data are from the northeastern margin and the
Halifax Pluton of the SMB, where exposure is better than
elsewhere in the batholith. The distribution of cordieriteand biotite-rich zones (Fig. 3b and c) is adapted from
MacDonald & Horne (1988) and MacDonald (2001). We
characterize the minerals of interest in terms of grain size
and shape, inclusion types and relations, zoning, and twinning. Reference to ‘texturally similar’ crystals means that
all textural properties described are identical.
For garnet, cordierite, and biotite, we present new
major-element data for 900 crystals from 60 samples,
and trace-element data for 150 crystals from 30 samples.
The major-element mineral compositions were determined
using a JEOL 8200 electron microprobe at Dalhousie
University; trace-element concentrations were obtained by
inductively coupled plasma mass spectrometry at
Memorial University, Newfoundland. We refer to a core^
rim decrease in Mg, and an increase in Fe and Mn as
normal zoning (i.e. reflecting normal growth in a cooling
magmatic system), and a core^rim decrease in Fe and Mn
with increasing Mg as reverse zoning. Cations for garnet
are calculated on the basis of 12 oxygen atoms per formula
unit (a.p.f.u.), for cordierite on 18 oxygen atoms, and for
biotite on 22 oxygen atoms.
In addition, 75 monazite crystals from four samples were
analyzed for major- and trace-element compositions and
chemical dating by electron microprobe (see Gagne¤, 2004;
Gagne¤ et al., in press). Samples were selected from the
Goldenville Group in the contact aureole (E430W), a
metapsammitic MSG xenolith hosted by biotite granodiorite (E468), and garnet-rich (E465A) and biotite-rich
(E471D) granodiorite (locations in Fig. 3b and c). Despite
potential Pb contamination during polishing, which could
produce apparent ages that are up to 10% too old
(Gagne¤, 2004), the data are capable of distinguishing monazite formed during crystallization of the SMB
(c. 390^370 Ma; Clarke & Halliday, 1980; Kontak et al.,
2003) from detrital monazite inherited from the MSG
country rocks (4500 Ma; Krogh & Keppie, 1990)
(Fig. 1b). However, monazites from the Tangier Basement
and SMB granites (c. 390^370 Ma; Greenough et al., 1999;
Kontak et al., 2003), and Acadian (regional) metamorphic
monazite in the MSG (c. 410^390 Ma; Hicks et al., 1999;
Morelli et al., 2005; T. Barresi & R. A. Jamieson, unpublished data), are geochronologically indistinguishable
using this method.
Whole-rock Sr^Nd isotopic data are presented for one
garnet-rich granodiorite (E465A), one cordierite-rich
monzogranite (Stop1.8A), two biotite-rich granodiorites
(E471D, E430Bt3), and five granitic ‘common host’ samples
in which the minerals of interest are absent or sparse
(E465B, Stop1.8B, E471L, E430Bt1, E430Bt2; locations in
Fig. 3). All macroscopic xenoliths were removed from the
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ERDMANN et al.
GARNET, CORDIERITE, AND BIOTITE IN GRANITES
Fig. 4. Mineral concentrations in the SMB may be the result of (a, b) the partial assimilation of country rocks or (c, d) the accumulation of
solids from the magma, including country-rock contaminants. (b) Mineral concentrations resulting from partial assimilation of country rocks
should have textures and compositions different from their common host (sample 1 vs 2). (d) Crystals in mineral concentrations resulting from
the accumulation of solids from the magma may have textures and compositions similar to, or different from, their common host, depending
on the degree of equilibrium between both (sample 3 vs 4).
samples to minimize the potential contribution from unrelated country-rock material. The analyses were performed
by thermal ionization mass spectrometry (TIMS) at the
Pacific Centre for Isotopic and Geochemical Research
(PCIGR) at the University of British Columbia by
Dominique Weis, using a Thermo Finnigan Triton system.
E VA L UAT I O N C R I T E R I A
Previous studies have shown that garnet, cordierite, and
biotite in the SMB may have a magmatic or a countryrock origin (as summarized above), and we therefore
focus our description and discussion on these two possibilities (Fig. 4). We evaluate our samples against the following
criteria.
Mineral textures
Mineral textures ideally reflect the growth conditions of a
crystal, and may also record dissolution or deformation
events (e.g. Lofgren, 1974; Tsuchiyama, 1985; Davidson
et al., 2007). Properties such as grain size and shape, inclusion types and relations, zoning, twinning, deformation
microstructures, and ocelli or corona growth textures
have been used to interpret crystal origins in igneous
rocks (garnet: Zeck, 1970; Allan & Clarke, 1981; Clemens
& Wall, 1984; Munksgaard, 1985; cordierite: Zeck, 1972;
Clemens & Wall, 1984; Erdmann et al., 2005; Garc|¤ aMoreno et al., 2007; biotite: Milord & Sawyer, 2003;
Charlier et al., 2007). Some textural features, such as oscillatory zoning, are relatively robust indicators of a crystal’s
origin, in this case recording magmatic or hydrothermal
growth (see reviews by Pearce, 1994; Shore & Fowler,
1996). Others, such as grain size, may define one or more
crystal populations, but additional crystal-scale features
or chemical analyses may be necessary to determine their
origin (e.g. Marsh, 1988; Clarke et al., 2005). Inclusions are
among the best genetic indicators: their patterns are
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JOURNAL OF PETROLOGY
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commonly distinct for magmatic and metamorphic
growth; they commonly include datable and (or) characteristic accessory phases (e.g. zircon, monazite, Fe^Ti
oxides); they are commonly protected from reaction with
the evolving host magma (Guo et al., 1996; Vernon &
Paterson, 2002; Charoy & Barbey, 2008).
Mineral assemblage
The mineral assemblages in granites and their country
rocks may overlap, although some differences are likely.
However, mineral concentrations dominated by primary
magmatic material may also contain contaminants (e.g.
Clarke & Clarke, 1998; Prevec et al., 2005; Gagnevin et al.,
2008; Fig. 4c and d), and it is, therefore, essential to decipher the relationship between the various concentrated
components (e.g. Fig. 4b vs 4d). Moreover, the original
minerals of some contaminants may be obscured by assimilation reactions during textural and chemical re-equilibration with the magmatic assemblage (Beard et al., 2005;
Clarke, 2007).
Dimension
The dimension of primary magmatic mineral concentrations is primarily controlled by the mechanism of formation and physicochemical system conditions (Jellinek &
Kerr, 1999; Collins et al., 2006; Z›a¤k & Klominsky, 2007).
On the other hand, the mass and volume of mineral concentrations resulting from contamination are limited by
the thermal state of the magma system, the rate of heat
transfer between magma and country rocks, the composition and mineralogy of the contaminants, and the mechanism of assimilation (Barboza & Bergantz, 1998; Spera &
Bohrson, 2001). The larger a mineral concentration, the
more probable is a primary magmatic origin, but significant volumes of country rocks may locally be assimilated
and subsequently concentrated (e.g. Green, 1994; Beard,
2008).
Spatial distribution
Primary magmatic mineral concentrations may be distributed throughout an igneous body, or may be restricted to
specific units, depending on the thermal, physical, and
chemical evolution of the system (Sparks & Marshall,
1986; Barbey et al., 2008). Minerals derived from the country rocks may correlate spatially with the types of country
rocks in contact with the magma (e.g. Poulson et al., 1991;
Barnes et al., 2004). However, contaminants may also be
assimilated or concentrated away from exposed contacts
with the in situ country rocks (e.g. if a large xenolith sank
into a magma chamber) (Clarke et al., 1998). The spatial
distribution of a mineral concentration is, therefore, a
helpful but ambiguous guide to its origin.
NUMBER 8
AUGUST 2009
Magmatic host
Concentrations dominated by primary magmatic minerals
should reflect the mineralogy of their host, so that the
restriction of certain mineral concentrations to certain
magmatic units may point towards a primary magmatic
origin (e.g. Clarke & Clarke, 1998; Fourcade et al., 2001).
Minerals derived from contamination may be found
throughout different magmatic host rocks, but they may
also be restricted to specific magmatic hosts, if variations
in magma temperature or composition caused locally
selective dissolution (Watson & Harrison, 1983; Edwards
& Russell, 1998; Clarke & Carruzzo, 2007).
Mineral composition
Mineral compositions, including zoning, may distinguish
crystal populations of different origins (e.g. Geist et al.,
1988; Belousova et al., 2006; Davidson et al. 2007). If two or
more compositionally distinct mineral populations exist in
a given rock, the crystals may have distinct origins, such
as primary magmatic and xenocrystic (e.g. Philips et al.,
1981; Spell et al., 2001; Charoy & Barbey, 2008). However,
the differences could also reflect changes in pressure, temperature, fO2, the composition of the system, or local disequilibrium during prolonged crystallization (Pichavant
et al., 2007; Salisbury et al., 2008). In granitic magmas,
slow intracrystalline diffusion rates may partly preserve
the initial compositions of millimeter-sized grains, but
smaller crystals, and (or) those with high diffusion rates,
may equilibrate with the host magma (Brady, 1995;
Edwards & Russell, 1998).
Whole-rock composition
Differences in whole-rock elemental and isotopic composition between country-rock contaminants and their host
magma may help to characterize the origin of a mineral
concentration. However, identifying foreign material
based on whole-rock compositional data is complicated if
assimilation was accompanied by partial melting, because
this produces solids and liquids of different chemical and
probably different isotopic composition (Watson &
Harrison, 1984; Tommasini & Davies, 1997; Knesel et al.,
1999). For example, the solid products of partial melting
may retain most of the Nd of the bulk contaminant [e.g. if
rare earth element (REE)-bearing accessory phases are
preserved; Bea et al., 1994]. The liquid products of partial
melting, on the other hand, may inherit most of the Sr
from the bulk contaminant (e.g. if plagioclase melts;
Hammouda et al., 1996). The physical separation of solid
and liquid contaminants may therefore lead to decoupling
of elemental and isotopic contaminant signatures, and
selective contamination by solids or liquids may produce
whole-rock isotopic compositions distinct from both contaminants and the main magma. Concentrations of refractory solids derived from partial melting of country-rock
contaminants will probably differ from the bulk
1484
ERDMANN et al.
GARNET, CORDIERITE, AND BIOTITE IN GRANITES
Fig. 5. Garnet-rich zones in the SMB. (a) Outcrop photograph (E465). Outlines mark most garnet grains, which are rimmed by large biotite
crystals. (b) Photomicrograph (plane-polarized light) of garnet intergrown with small biotite (Bt1), and partly rimmed by large biotite (Bt2).
(c) Photomicrograph (plane-polarized light) of garnet within a xenolith. The garnet is texturally similar to the SMB garnet in (b). (d) X-ray
map showing Mn core^rim zoning in an SMB garnet. High-Mn zones occur along grain boundaries and micro-cracks, suggesting that the
garnet crystal may have been originally unzoned, or weakly zoned.
contaminant in some, but not necessarily in all, elemental
and isotopic compositions. However, primary magmatic
mineral concentrations may also be chemically and (or)
isotopically distinct from their common host, if the host
magma experienced contamination, magma mixing, or
isotopic fractionation in a long-lived magma chamber subsequent to their formation (Davies & Halliday, 1998;
Hildreth & Fierstein, 2000).
R E S U LT S
Garnet-rich zones
Garnet-rich zones, on the scale of decimeters to a few
meters, are present in nearly all rock types of the SMB,
but they are most common along and near contacts with
the metapelitic Halifax Group. There, the garnet-rich
zones make up 2 vol. % of the rocks (Fig. 3a). Garnet
crystals are subhedral, and up to c. 2 cm in size (Fig. 5;
Table 1). They may be intergrown with small biotite crystals (Bt1), and are typically rimmed by large biotite
crystals (Bt2) (Fig. 5a and b). Inclusions in garnet are relatively rare, the most common being euhedral apatite, monazite, zircon, and Fe^Ti-oxides. Garnet crystals form up
to 30 vol. % of the garnet-rich zones (Figs 2a and 5a).
Other characteristic phases include small to large, inclusion-rich cordierite, biotite, Fe^Cu-sulphides, and small
quartz, plagioclase and Fe^Ti-oxide grains. Xenoliths, particularly micro-xenoliths (51cm), are more common in
the garnet-rich zones than in the adjacent garnet-poor
granites, and some xenoliths contain garnet that is texturally similar to that in the garnet concentrations themselves (Fig. 5c).
Garnet is almandine-rich (XFe ¼ 066^075; FeO ¼ 306^
335 wt %) with significant pyrope and spessartine components (XMg ¼ 012^027, MgO ¼ 29^70 wt %; XMn ¼
003^013, MnO ¼15^57 wt %) (Fig. 6a; Table 2). The
crystals are zoned, showing 50 mm wide, low-pyrope,
high-spessartine and high-almandine rims (Fig. 5d).
Calculated temperatures for garnet cores paired with adjacent xenolithic biotite give 780^8208C at 400 MPa
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Fig. 6. Major- and trace-element concentrations for (a, b) garnet, (c, d) cordierite, and (e, f) biotite from SMB granites and country rocks of
the Meguma Supergroup, including xenoliths. Gray areas outline characteristic compositional fields for the SMB mineral concentrations.
Symbols are similar for (a) and (b), (c) and (d), and (e) and (f). Abbreviations are defined in the legend of Table 1.
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ERDMANN et al.
GARNET, CORDIERITE, AND BIOTITE IN GRANITES
Table 2: Major-element compositions for garnet, cordierite, and biotite from SMB and MSG
Garnet
SMB,l,sh,uz–nz (11)
MSG,s,eh,uz–rz (2)
MSG,yl,sh,uz–nz (5)
s
Ø 7
Max.
Min.
s
Ø 18
Max.
Min.
3630
061
3618
3647
000
004
006
011
3591
018
3666
3769
3592
047
003
002
008
016
001
004
2178
2064
032
2068
3350
3057
084
3045
2081
2032
015
2074
2118
2026
022
3270
2882
116
3209
3655
2874
366
569
148
143
277
808
1087
536
171
589
1024
346
MgO
429
699
291
210
135
241
304
150
062
291
426
187
CaO
127
152
065
076
023
126
146
106
012
108
129
095
010
Cr2O3
011
021
Total
9994
000
006
008
017
001
005
016
039
013
008
Ø 32
Max.
Min.
SiO2
3726
3840
TiO2
005
014
Al2O3
2105
FeO
3223
MnO
9920
s
9963
Cordierite
SMB,zl,eh,nz (5)
SMB,zl,eh,oz (6)
MSG,ys–l,sh,uz–rz (3)
s
Ø 13
Max.
Min.
s
Ø 30
Max.
Min.
4677
020
4721
4798
000
001
001
002
4660
039
4842
4956
4743
056
000
001
001
003
000
001
3198
3084
031
3188
1167
1034
039
1063
3272
3135
036
3276
3346
3202
038
1191
926
086
970
1014
906
055
077
043
008
030
050
058
040
006
056
066
042
MgO
502
541
446
006
028
543
623
482
039
725
760
672
CaO
001
003
022
000
001
001
003
000
001
002
006
000
001
Ø 14
Max.
Min.
SiO2
4704
4748
TiO2
001
003
Al2O3
3139
FeO
1103
MnO
s
Na2O
115
150
091
016
108
153
089
019
020
032
003
010
K2O
000
000
000
—
000
002
000
001
001
009
000
002
Total
9619
9675
9893
Biotite
SMB,Gd,s–l,eh,uz (5)
SMB,Mng,s–l,eh,uz (2)
MSG,ys–l,eh,uz (24)
Ø 44
Max.
Min.
s
Ø8
Max.
Min.
s
Ø 294
Max.
Min.
s
SiO2
3429
3485
3388
021
3384
3538
3285
08
3498
3871
3143
138
TiO2
343
410
250
033
346
393
249
041
321
430
139
043
Al2O3
1936
2003
1855
034
1964
2043
1830
08
1894
2093
1702
073
FeO
2109
2209
1996
064
2197
2244
2109
05
2106
2282
1852
104
MnO
036
045
024
006
038
049
031
01
038
059
002
008
MgO
775
841
733
026
659
696
601
03
737
897
584
062
CaO
000
010
000
002
000
002
000
001
002
031
000
003
Na2O
015
023
007
004
017
021
013
003
018
077
010
006
K2O
959
997
896
023
964
984
926
022
955
1025
820
031
Cr2O3
009
016
002
004
011
013
010
001
007
023
000
005
BaO
007
033
009
007
—
—
—
—
002
022
000
005
F
013
033
000
014
027
033
020
005
024
054
000
015
Total
9632
9607
9602
Data acquired using a JEOL 8200 electron microprobe, operating at 15 keV and 15 nA, with a 3–10 mm spot size, and 40 s
counting times. Data given are average concentrations (e.g. Ø of 32 analyses), maximum (Max.) and minimum (Min.)
concentrations, and standard deviation (s). Abbreviations follow those of Table 1.
Number of samples.
yAnalyses of crystals near the SMB contact and in xenoliths.
zData from Erdmann et al. (2005).
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Table 3: Trace-element compositions (ppm) for garnet, cordierite, and biotite from SMB and MSG
Garnet
SMB,l,sh,uz-nz (9)
Ø 21
Max.
MSG,s,eh,uz-rz (1)
Min.
s
Ø3
Max.
MSG,l,sh,uz-nz (2)
Min.
s
Ø3
Max.
Min.
s
V
137
270
33
75
444
467
404
29
350
410
318
42
Cr
182
458
13
133
551
716
345
154
183
215
144
29
Zn
69
91
49
11
53
62
40
9
68
75
61
6
Ga
11
14
10
1
13
13
12
1
14
16
10
3
Ge
Y
8
13
6
2
10
12
7
2
11
12
9
2
1399
2684
774
462
115
186
75
50
1284
1412
1157
128
Cordierite
SMB,l,eh,nz (4)
SMB,l,eh,oz (2)
MSG,s-l,sh,uz-rz (2)
Ø 10
Max.
Min.
s
Ø 7
Max.
Min.
s
Ø4
Max.
Min.
s
Li
2126
2485
1695
232
2078
2293
1805
135
347
460
393
25
Be
287
320
231
29
345
635
181
153
33
65
23
18
B
39
50
9
15
19
34
2
10
22
34
21
6
Ga
65
69
58
3
67
75
60
4
31
49
33
6
Cs
154
174
142
11
155
204
128
20
1
3
1
1
Biotite
SMB,Gd,s-l,eh,uz (23)
SMB, common Gd (19)
s
MSG,s-l,eh,uz (57)
Max.
Min.
s
355
750
144
307
378
210
26
174
238
371
49
225
445
27
426
611
864
1465
Ø 57
Max.
Min.
Li
233
332
182
40
V
366
401
293
23
Cr
190
239
124
Zn
432
594
Rb
522
564
Ba
1854
3632
Ø 8
Min.
s
Ø 23
Max.
193
304
618
95
51
295
678
124
92
98
31
253
467
71
63
337
137
61
282
533
124
80
545
308
79
427
618
188
78
2736
518
570
1157
2774
441
446
109
Data acquired by inductively coupled plasma mass spectrometry, using a NUWAVE 213 nm NdYAG laser system and a
Hewlett-Packard 4500plus quadrupole mass spectrometer. Minimum laser spot size was 40 mm; the laser was operated at
10 Hz. Total acquisition time per analysis was 90 s, with a 30 s measurement of the gas blank. Garnet and cordierite traceelement concentrations given for all elements with concentrations 42s; biotite trace-element concentrations given for
most abundant trace elements. Data given are average concentrations (e.g. Ø of 9 analyses), maximum (Max.) and
minimum (Min.) concentrations, and standard deviation (s). Abbreviations follow those of Table 1.
Number of samples.
(using the thermometers of Perchuk & Lavrent’eva, 1983;
Dasgupta et al., 1991; Bhattacharya et al., 1992). Traceelement concentrations are 770^2700 ppm for Y,
15^460 ppm for Cr, 35^270 ppm for V, 50^90 ppm
for Zn, 10^15 ppm for Ga, and 5^15 ppm for Ge
(Fig. 6b; Table 3). Monazite inclusions in garnet range
from homogeneous to complexly zoned, yielding core
apparent ages mainly 4500 Ma (Fig. 7). The whole-rock
isotopic composition of the analyzed garnet-rich zone has
lower 2Ndi, but similar 87Sr/86Sri relative to the host granodiorite from the same outcrop (Fig. 8; Table 4; E 465A vs
E 465B). Garnet-rich and garnet-poor granodiorites have
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ERDMANN et al.
GARNET, CORDIERITE, AND BIOTITE IN GRANITES
Table 4: Sr^Nd isotopic data for garnet, biotite, and cordierite concentrations and their common SMB host granites
Rock type:
Gd
Mng
Gd
Grt-rich
Grt-poor
Crd-rich
Crd-poor
Crd-poor
Bt-rich
Bt-poor
Bt-rich
Bt-poor
Bt-poor
Sample:
E465A
E465B
Stop1.8A
Stop1.8B
Stop1.8B
E471D
E471L
E430Bt3
E430Bt2
E430Bt1
t (Ma)y
372
372
364
364
364
372
372
372
372
372
Sr (ppm)
223
251
58
47
47
174
216
276
278
242
Rb (ppm)
127
125
284
333
333
253
129
140
96
88
87
0718153
0717336
0778154
0814792
0814507
0730993
0718344
0718342
0716180
0715638
87
Sr/86Sr
Sr/86Sri
0709434
0709711
0705356
0709553
0709268
0708732
0709201
0710576
0710893
0710071
Nd (ppm)
405
375
170
121
121
870
382
445
300
226
Sm (ppm)
88
75
43
30
30
175
78
90
64
46
143
0512284
0512305
0512336
0512349
0512321
0512310
0512310
0512286
0512280
0512281
143
0511966
0512012
0511973
0511993
0511965
0512015
0512011
0511989
0511967
0511981
–38
–29
–38
–34
–40
–28
–29
–33
–37
–35
Nd/144Nd
Nd/144Ndi
eNdi
Bulk-Earth parameters (CHUR) used are: 87Rb/86Sr ¼ 00827, 87Sr/86Sr ¼ 07045, 147Sm/144Nd ¼ 01967, and 143Nd/144Nd
¼ 0512638. Reference standards were NBS987, La Jolla, and GSP-2 (Pretorius et al., 2006; PCIGR reference values).
Sample analyzed twice.
yReference age from Clarke & Halliday (1980).
compositions intermediate to those of Tangier Basement
and MSG rocks, but the garnet-rich zone more closely
resembles the Nd isotopic composition of the MSG than
its host (Eberz et al., 1991; Fig. 8b).
Cordierite-rich zones
Cordierite-rich zones, on the scale of tens of meters to several kilometers, are present in muscovite^biotite monzogranites and coarse-grained leucomonzogranites of the
Halifax Pluton (Figs 2b and 3b). In addition, monzogranites and leucomonzogranites elsewhere in the SMB consistently contain cordierite (Fig. 1a), although large-scale
cordierite-rich zones are not exposed. Cordierite-rich
zones and other granites have up to 5 vol. % cordierite
and abundant K-feldspar phenocrysts, with a matrix mineralogy similar to the adjacent hosts. The cordierite forms
small clusters or single grains of dominantly large, subhedral to euhedral, sector-twinned, inclusion-poor crystals
(Fig. 9a, b and d; Table 1). Inclusions are typically quartz,
biotite, and various accessory minerals. Xenoliths are rare
in both cordierite-rich and common leucomonzogranites
and monzogranites. Texturally similar cordierite crystals
are not present in or adjacent to xenoliths, but large, subhedral to euhedral, inclusion-poor cordierite is locally
enriched in51dm wide zones along the contact with metapelitic country rocks (Fig. 9c). In the Halifax Pluton, cordierite-rich zones making up c. 20 vol. % of the exposure
are distributed subparallel to, but several kilometers away
from, the external contact (Fig. 3b). High concentrations
of cordierite in the Halifax Pluton and elsewhere in the
SMB are commonly associated with K-feldspar megacryst-rich monzogranites.
Cordierite cores have XMg [Mg/(Mg þ Fe)] between
041 and 054, MnO concentrations of 043^077 wt %
(Mn ¼ 004^007 a.p.f.u.), and Na2O concentrations of
09^15 wt % (Na ¼ 010^032 a.p.f.u.) (Fig. 6c; Table 2).
They show both oscillatory or normal zoning in Fe, Mg,
and Mn (Fig. 9d; Erdmann et al., 2005). Trace-element concentrations in normally and oscillatory zoned cordierite
are 1700^2500 ppm for Li, 180^640 ppm for Be,
130^200 ppm for Cs, 60^75 ppm for Ga, and 550 ppm
for B (Fig. 6d; Table 3). Trace-element concentrations are
similar in normally and oscillatory zoned crystals; variations between core, intermediate, and rim zones show no
obvious trends. Within our sample set, cordierite-rich monzogranite is characterized by one of the lowest eNdi
values. The low 87Sr/86Sri of the sample is interpreted to
reflect alteration. Cordierite-poor monzogranite has a similarly low eNdi and low to medium 87Sr/86Sri (Fig. 8;
Table 4). The eNdi signatures of both cordierite-bearing
and cordierite-poor monzogranites are equivalent to that
of the garnet-rich granodiorite (Fig. 8).
Biotite-rich zones
Biotite-rich zones are common throughout the SMB, particularly in granodiorites and monzogranites. They make
up 80 vol. % (centimeter-scale) or 25 vol. % (kilometer-scale) of the rocks (Figs 1 and 3c). These zones are
dominated by large, subhedral to euhedral single grains
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Fig. 7. Apparent ages for monazite of SMB and MSG rocks. Detrital monazites of the MSG occur in the in situ country-rock sample (E430W)
and in the garnet-rich zone (E465A), but are absent from the xenolith sample (E468) and the biotite-rich zone (E471D). Pb contamination probably produced apparent ages that are up to 10% too old (see Gagne¤, 2004); for reference, we show accurate monazite ages for the Tangier
Basement (TB), the SMB, and monazites from the MSG that grew during regional metamorphism. Major-element concentrations were characterized using a 20 kV, 20 nA focused electron beam, and counting times of 40 s. Trace-element compositions were subsequently determined applying similar conditions, but a beam current of 200 nA, and counting times of 600 s.
or centimeter-size clusters (Fig. 10a^d; Table 1). The most
common inclusions are zircon, monazite, and euhedral
apatite; less abundant are Fe^Ti-oxides, Fe-Cu-sulphides,
and xenotime (Fig. 10c and d). Biotite is texturally similar
to that in relatively biotite-poor host granites, and both
biotite-rich and biotite-poor granites have a similar phase
assemblage (Fig. 10c and d vs Fig. 10e). Xenoliths (millimeter- to meter-scale) are commonly, but not universally,
present; biotite in xenoliths is generally similar to, but
finer-grained than that in the granitic biotite concentrations (Figs 1 and 10d, f), and lacks the euhedral apatite
inclusions typical of biotite in the granites (Fig. 10f).
Within the Halifax Pluton and along the northern part of
the SMB, biotite-rich zones are most common near the
external contact, but a similar distribution is not evident
elsewhere (Figs 1a and 3c). Biotite-rich zones have been
observed against both metapelitic and metapsammitic
MSG country rocks and xenoliths.
Biotite-rich zones in SMB granodiorites contain biotite
with XMg [Mg/(Mg þ Fe)] values between 038 and 042,
TiO2 of 25^41wt % (Ti ¼ 024^046 a.p.f.u.), and MnO
of 024^045 wt % (Mn ¼ 004^006 a.p.f.u.) (Fig. 6e;
Table 2). Trace-element concentrations are 180^330 ppm
for Li, 290^400 ppm of V, 120^240 ppm for Cr,
370^590 ppm for Zn, 450^560 ppm for Rb, and
610^3600 ppm for Ba (Fig. 6f; Table 3). In SMB monzogranites, biotite XMg can be as low as 034, but Ti and
Mn concentrations are similar to those in granodiorites
(Fig. 6e; Table 2; trace-element data not available).
However, biotite major and trace-element compositions
from biotite-rich zones in granodiorites and monzogranites
overlap with those from host SMB granites and MSG
rocks (Fig. 6e and f; Table 3), and biotite in all samples
studied is unzoned. In biotite-rich zones, euhedral to subhedral monazite inclusions in biotite are compositionally
homogeneous, with core apparent ages of c. 500^400
Ma (Fig. 7).
The Sr^Nd isotopic signature of a large (45 m 10 m)
biotite-rich granodiorite near the country-rock contact
(sample E471D) has one of the highest eNdi and lowest
87
Sr/86Sri compositions in the SMB, closest to the composition of the Tangier Basement source rocks and furthest
from the composition of the MSG country rocks (Eberz
et al., 1991; Clarke et al., 1993; Fig. 8; Table 4). Biotite-poor
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ERDMANN et al.
GARNET, CORDIERITE, AND BIOTITE IN GRANITES
Fig. 8. Sr^Nd whole-rock isotopic compositions for SMB, MSG country rocks, and inferred Tangier Basement source rocks. Compositional
fields shown in (a) summarize previous isotopic data from Clarke et al. (1988, 1993), and Eberz et al. (1991). New data shown in (a) and (b) are
given inTable 4. All data are initialized to an age of 372 Ma for granodiorites, garnet-rich, and biotite-rich zones, and to 364 Ma for monzogranites and cordierite-rich zones (in accordance with previous data). (a) The SMB granites have isotopic compositions intermediate between the
Tangier Basement and the MSG. Evidence for alteration of the cordierite-rich sample exists in thin section and in its low Rb concentration.
(b) Shaded fields include the garnet-, cordierite-, and biotite-rich samples and their common host granites. (#) Sample was analyzed twice.
Inset map shows the sample locations for outcrop E430: xenoliths are shown in grey; black marks show the approximate distribution of biotite
in the SMB granodiorite (white).
granodiorite from the same outcrop (E471L) is
isotopically similar to the biotite-rich granodiorite, with
slightly higher 87Sr/86Sri and slightly lower eNdi values
(Fig. 8b). In contrast, granodiorite from a decimeter-scale
biotite-rich zone along a country-rock^SMB contact
(E430Bt3) has a more MSG-like isotopic signature, with
eNdi and 87Sr/86Sri values similar to those of nearby biotitepoor granodiorites (E430Bt3 vs E430Bt1 and Bt2) (Fig. 8).
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Fig. 9. Cordierite in megacrystic monzogranites. (a) Outcrop photograph (Stop1.8A). Arrows mark cordierite. (b) Photomicrograph (partly
crossed polarizer) showing a large, euhedral, sector-twinned cordierite crystal. Continuous-line arrows mark quartz inclusions; dashed arrows
mark laser pits. (c) Photomicrograph (plane-polarized light) of cordierite within metapelitic country rocks (MPEL CR) and in an adjacent
monzogranite (Fig. 11f shows the same contact). Cordierite of the country rocks is small, subhedral, and inclusion-rich. Cordierite of the monzogranite is large, subhedral to euhedral, showing few large inclusions. (d) X-ray map showing an oscillatory zoned cordierite (modified from
Erdmann et al., 2005). It should be noted that cordierite is partially pinitized and that the pattern of oscillatory zoning is overprinted by lowMg zones along grain boundaries and micro-cracks. The zoning profile shows relative Mg concentrations from core to rim; the dashed lines
mark concentric low- and high-Mg zones.
Meguma Supergroup country rocks
Near the contact with the SMB, Goldenville Group metapsammites contain the assemblage quartz^plagioclase^
biotite^muscovite^cordierite^pyrrhotite, with accessory
ilmenite and apatite (Fig. 11a and d). Halifax Group metapelites contain the assemblage andalusite^cordierite^
biotite^plagioclase^K-feldspar/muscovite^graphite^
pyrrhotite^ilmenite (Fig. 11b, c, e and f).
Garnet is rare in MSG rocks near the SMB contact,
except in the Mn-rich transition zone between the
Goldenville and Halifax Groups. In these rocks it forms
fine- to medium-grained, Mn-rich (XMn ¼ 01^05) euhedral crystals with abundant fine-grained inclusions (this
study; R. A. Jamieson, unpublished data; Fig. 11a; Table 1).
Elsewhere, Mn- and Ca-rich garnet is associated with calcareous concretions in both Goldenville and Halifax lithologies. Large, subhedral garnet porphyroblasts, texturally
and chemically similar to those in SMB garnet-rich zones,
occur locally in metapelitic rocks at the immediate contact with the SMB (Fig. 11b and c). They are partially to
completely surrounded by plagioclase^K-feldspar^
quartz-dominated leucosomes, in which feldspars show
euhedral^subhedral crystal faces against quartz. The metapelite adjacent to the leucosomes contains small, anhedral
plagioclase, K-feldspar, and quartz crystals along some
grain boundaries and triple junctions, resembling small
melt pools produced in experiments and identified in relatively undeformed migmatites (Rosenberg & Riller, 2000;
Marchildon & Brown, 2002). Within leucosome layers,
garnet porphyroblasts make up 30 vol. %. They have
almandine-rich cores (XFe ¼ 065^080, FeO 287^366 wt
%), with significant pyrope and spessartine components
(XMg ¼ 006^009, MgO 19^43 wt %; XMn ¼ 008^024,
MnO 35^102 wt %), and low-pyrope, high-spessartine
and high-almandine rims (Fig. 6a; Table 2).
Concentrations of Y, Cr, Zn, Ga, and Ge are similar to
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Fig. 10. Biotite-rich zones in the SMB. (a) Outcrop photograph showing part of a large-scale biotite-rich zone (E471D). (b) Outcrop photograph of a small-scale biotite-rich zone (Bt-rich Gd) within an apophysis intruding metapsammitic country rocks (MPSA CR) (E430).
(c) Photomicrograph of a sample from the large-scale biotite-rich zone shown in (a). (d) Photomicrograph of a xenolith rimmed by a biotiterich granodiorite (Bt-rich Gd; Fig. 2c shows the same contact). Biotite in the granodiorite forms larger crystals than in the country rock; the contact is sharp. (e) Photomicrograph of a common granodiorite. (f) Photomicrograph of a xenolith in contact with a fine-grained granodiorite
(Fg Gd). Biotite and other crystals in the rim zone of the xenolith are larger than in the core, and as large as those in the granite. All photomicrographs taken in plane-polarized light. Arrows in (c)^(e) mark some of the euhedral apatite inclusions in biotite; dashed lines mark some of
the contacts between country rocks and granites.
those of the SMB garnet, with V concentrations higher
in country-rock porphyroblasts than in SMB garnet
(320^410 ppm vs 35^270 ppm) (Table 3, Fig. 6b).
Cordierite is abundant (20 vol. %) in metapelites and
common in metapsammites close to the contact. Crystals
contain abundant small inclusions of quartz, biotite,
graphite (in metapelites), Fe^Ti-oxides, or (rarely) sillimanite (variety fibrolite), which commonly define a prehornfels fabric and which coarsen towards the SMB
contact (Fig. 11d^f; Table 1). The abundant inclusions and
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Fig. 11. Garnet, cordierite, and biotite in country rocks of the MSG. (a) Small, subhedral to euhedral, inclusion-rich garnet. (b) Large, subhedral garnet rimmed by leucosome, occurring in a metapelitic layer 1m away from the SMB contact. (c) Photomicrograph (plane-polarized
light) showing part of a garnet crystal from the metapelitic layer of (b). (d) Photomicrograph (partly crossed polarizer) of inclusion-rich cordierite in a metapsammitic country rock (marked by dashed lines). (e) Photomicrograph (plane-polarized light) of cordierite (partially pinitized) and andalusite porphyroblasts in a metapelitic country rock. (f) Cordierite in metapelitic rocks of the MSG is most abundant along the
contact with the monzogranite. Cordierite also occurs in the monzogranite, concentrated in a c. 10 cm wide contact zone against the metapelites
(photomicrograph of contact in Fig. 9c).
anhedral, ovoid form distinguish this cordierite from the
large, subhedral to euhedral, inclusion-poor crystals in
SMB cordierite-rich zones. Major-element compositions
partly overlap, but trace-element concentrations are
different (Fig. 6c and d). Country-rock cordierite has XMg
of c. 056^059, MnO concentrations of 042^066 wt %
(004^006 a.p.f.u.), and Na2O concentrations of 503 wt
% (506 a.p.f.u.) (Fig. 6c; Table 2). Crystals are unzoned
or reversely zoned (with high-Mg rims). Concentrations
of Li, Be, Ga, and Cs are significantly lower, whereas B
concentrations are higher than in SMB cordierite
(Fig. 6d; Table 3).
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Fig. 12. Sketch illustrating crystal types of various origins that may occur in the granites of the South Mountain Batholith: (1, 2) Primary magmatic crystals (1, components dominantly cognate magmatic; 2, components dominantly derived from the Meguma Supergroup); (3) xenocrysts;
(4) peritectic xenocrysts derived from country rocks; (5) secondary magmatic crystals. Xenocrysts and peritectic xenocrysts may develop primary magmatic overgrowths or dissolve. Secondary magmatic crystals may consume primary magmatic crystals or xenocrysts.
Biotite typically makes up c. 5^15 vol. % of the rocks. It
forms small to large, subhedral to euhedral crystals, texturally similar to, but on average finer-grained than biotite
in the SMB granites (Fig. 10 vs Fig. 11a and b; Table 1).
Common inclusions are zircon, monazite, anhedral apatite, Fe^Ti-oxides, Fe^Cu-sulphides, or graphite. In any
one sample, country-rock biotite compositions are similar,
but they vary significantly between outcrops as a function
of metamorphic grade (Hart, 2006). Near the SMB contact
and in xenoliths, biotite has XMg between 032 and 045,
TiO2 concentrations of 14^43 wt % (017^049 a.p.f.u.),
and MnO concentrations of 5059 wt % (002^005
a.p.f.u.) (Fig. 6e; Table 2). Trace-element concentrations
of xenolithic biotite are similar to those of the SMB
biotite, but show a larger range of compositions (Fig. 6f;
Table 3).
Monazite in the MSG contact aureole and in xenoliths
forms small, anhedral crystals, which coarsen towards the
contact (T. Barresi & R. A. Jamieson, unpublished data).
They are relatively homogeneous to complexly zoned.
Calculated core apparent ages from country rock sample
(E430W) are in the range 511^780 Ma, consistently older
than core apparent ages from our xenolith sample (E468),
which range from 387 to 429 Ma (Fig. 7).
DISCUSSION
In the following sections we consider evidence for, rather
than against, a particular origin of the garnet, cordierite,
and biotite concentrations in the SMB, including: (1) a
xenocrystic or peritectic xenocrystic country-rock origin;
(2) a primary magmatic origin; (3) a secondary magmatic
origin; (4) a cognate magmatic or a hybrid magmatic
origin (Fig. 12).
Origin of the garnet-rich zones
Xenocrystic or peritectic xenocrystic country-rock origin
Abundant old, 4500 Ma monazite inclusions in SMB
garnet are compelling evidence for an inherited, countryrock origin. However, the only country-rock garnets that
are texturally similar and chemically overlapping with the
SMB garnets are leucosome-rimmed, subhedral grains in
some metapelites near the contact (Table 1). Minor compositional differences (e.g. in V) between SMB and leucosome-rimmed country-rock garnet (Fig. 6; Table 1) may
reflect lower temperatures in the country rock, formation
of the SMB garnets at a deeper level in the system or,
given our small sample set, may have no genetic significance. Temperatures of 780^8208C calculated from SMB
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garnet cores and xenolithic biotite are compatible with formation of garnet as the incongruent product of incipient
biotite dehydration melting. However, these represent minimum temperatures if biotite compositions were affected
by partial retrograde re-equilibration, as suggested by
zoned garnet rims. Microstructures in the leucosomes and
adjacent metapelitic rocks are indicative of in situ anatectic,
and not solid-state, formation (see Vernon, 1999; Sawyer,
2008). Given the textural and compositional similarities
between leucosome-rimmed country-rock garnet and
SMB garnet, we infer that SMB garnet formed during partial melting of xenoliths, and that they are thus peritectic
xenocrystic crystals derived from metapelitic country
rocks.
Polygonal quartz, biotite, plagioclase, Fe^Ti-oxides, and
inclusion-rich cordierite crystals in the garnet-rich zones
resemble equivalent minerals in the MSG. Although association with country-rock xenocrysts does not require a
foreign origin for the garnet crystals, the similarities are
most logically explained if they represent xenocrysts of
partially assimilated MSG country rocks. The relative
scarcity of xenocrysts (55 vol. %) indicates that the original country-rock assemblage was largely consumed in a
melting
reaction
of
the
form
Bt þ Kfs þ Pl
þ Qtz þ And ¼ Grt þ L. Large crystals of quartz, plagioclase, K-feldspar and biotite in the garnet-rich zones are
inferred to be mostly primary magmatic (Fig. 5b).
However, coarse-grained biotite rims on garnet (some
Bt2), interpreted as the product of a garnet-consuming
hydration reaction during cooling, are probably of secondary magmatic origin.
Garnet-rich zones in the SMB are observed mainly
along and near contacts with the metapelite-dominated
Halifax Group (Fig. 1a), consistent with our interpretation
that they formed by partial assimilation of metapelitic
country rocks. Maximum amounts of c. 2 vol. % of assimilated metapelites and dimensions of the order of decimeters to a few meters are compatible with an inferred
country-rock origin, because assimilation through partial
melting on this degree and scale would have consumed
only a small amount of magmatic heat (see Thompson
et al., 2002a; Erdmann, 2006; Glazner, 2007).
The analyzed garnet-rich zone has low eNdi but similar
87
Sr/86Sri, relative to its host granodiorite (Fig. 8), indicating mixing between magma and contaminants. We suggest
that the isotopic composition reflects partial assimilation
of MSG metapelites through partial melting of xenoliths
and country rock in direct contact with the pluton, followed by mixing of country-rock-derived melt with the
main magma. During partial melting of metapelites, Nd
may have remained in refractory accessory phases, or
may have partitioned into garnet (Bea et al., 1994;
Villaseca et al., 2003). In contrast, Sr is likely to have been
partitioned into the melt (Kd Sr for garnet is 002;
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Sisson & Bacon, 1992), where it partially equilibrated with
the main magma by diffusion (see Pichavant et al., 2007).
Based on all datasets evaluated, we suggest that the
garnet-rich zones in the SMB represent partially assimilated metapelitic xenoliths, and that the garnet crystals
have a peritectic xenocrystic origin. The garnet in these
zones differs in texture and composition from primary
magmatic garnet (Allan & Clarke, 1981; MacDonald,
2001) and metasomatic garnet (Kontak & Corey, 1988)
reported elsewhere within the SMB.
Origin of cordierite-rich zones
Xenocrystic or peritectic xenocrystic country-rock origin
Cordierite is abundant in the MSG country rocks, and cordierite xenocrysts and peritectic xenocrystic crystals in the
SMB have been reported previously (Maillet & Clarke,
1985; Clarke & Erdmann, 2005; Erdmann et al., 2005).
The distribution of 51dm wide cordierite-rich zones in
contact with some metapelitic country rocks (Figs 9c and
11f), and whole-rock Nd isotopic signatures suggesting the
presence of contaminants, may also point towards a
country-rock origin. As textural contrasts rule out a xenocrystic origin, here we evaluate the possibility that some
SMB cordierite represents peritectic xenocrysts.
Melting experiments on MSG metapelites yielded up to
20 vol. % peritectic xenocrystic cordierite with 410 vol. %
of other xenocrysts and peritectic xenocrysts (Crd þ Fe^
Ti-oxides þ L; Erdmann et al., 2007). Partial melting of
similar xenolithic material in the SMB magma would
have yielded abundant cordierite, with the remaining
assimilated components distributed in various primary
magmatic crystals. This could explain the scarcity of obvious contaminants in the cordierite-rich zones and their
MSG-like isotopic signature. If SMB cordierite is of peritectic xenocrystic origin, derived from metapelitic MSG
country rocks, 1^5 vol. % cordierite implies the presence
of c. 5^25 vol. % of assimilated country rock material
(Erdmann et al., 2007). For granodiorite and more felsic
magmas of the SMB, heating and assimilation of metapelitic rocks initially at greenschist- to amphibolite-facies conditions is unlikely to exceed c. 25 wt % (see Thompson
et al., 2002a; Glazner, 2007). Calculations using the AFC
model of Spera & Bohrson (2001) suggest that assimilation
through bulk melting was limited to less than 4^8 wt %
( 4^9 vol. %) of the mass of the SMB, and partial assimilation with 50% partial melting therefore to less than
c. 8^16 wt % ( 8^18 vol. %) (Erdmann, 2006). At least
in the zones of the highest cordierite concentrations, a peritectic xenocrystic origin for cordierite is thus most likely if
they represent large-scale, post-assimilation accumulations
rather than in situ melt products.
Primary magmatic origin
Even though a peritectic xenocrystic origin is conceivable
for some SMB cordierite, spatial relations, whole-rock
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GARNET, CORDIERITE, AND BIOTITE IN GRANITES
isotopic data, and oscillatory zoning indicate that most is
of primary magmatic origin. Normally zoned, subhedral
to euhedral cordierite crystals may have a different origin
from spatially associated oscillatory zoned grains of similar
abundance. However, textural and compositional similarities between both types point towards a similar origin
(Table 1). Possible explanations for the apparent normal
zoning are a cutting effect in thin section, the partial loss
of oscillatory zoning patterns by diffusion, or a local cause
for the oscillatory zoning.
Although not in itself conclusive, the lack of evidence for
assimilation (e.g. partially digested xenoliths) within the
main cordierite-rich zones also supports a primary magmatic origin of most SMB cordierite. Moreover, the
restriction of cordierite-rich zones to muscovite^biotite
monzogranites and coarse-grained leucomonzogranites
(Figs 1a and 3c) is also consistent with magmatic crystallization. Cordierite is stable below c. 8008C in haplogranitic
melts with A/CNK between 119 and 132 (Acosta-Vigil
et al., 2003), consistent with inferred crystallization temperatures and A/CNK 4119 in the SMB magmas (Clarke
et al., 2004). However, the in situ crystallization of up to 5
vol. % of cordierite seems unlikely, and we favour the
interpretation that cordierite, commonly associated with
K-feldspar megacrystic SMB rocks, was physically
accumulated.
Cognate or hybrid magmatic origin
Whole-rock Nd isotopic data suggest that MSG-derived
material is present in the cordierite-rich monzogranites
(Fig. 8), but both cordierite-rich and cordierite-poor monzogranites have similar isotopic compositions. The cordierite-rich zones, therefore, do not appear to represent
preferentially contaminated rocks. We suggest that both
monzogranites include a significant MSG component,
derived from some combination of selective contamination
by accessory minerals and a melt derived from MSG country rocks at deeper crustal levels.
Although xenocrystic cordierite has been reported from
the SMB (Maillet & Clarke, 1985; Erdmann et al., 2005;
Erdmann, 2006), we conclude that most cordierite in the
SMB is of primary magmatic origin, and that the cordierite-rich zones formed as a result of fractional crystallization and crystal accumulation. We have not found any
evidence for a secondary magmatic or hydrothermal
origin (e.g. replacement of garnet).
zones elsewhere in the granites may thus also result from
country-rock assimilation, even in the absence of abundant
obvious xenocrysts. The apparent ages of monazite inclusions from the large-scale biotite-rich zone overlap with
those both from the MSG contact aureole and from the
SMB granites (Fig. 7), and thus permit, but do not require,
a xenocrystic origin for the host biotite.
Primary magmatic origin
Spatial distribution, and textural and chemical similarities,
may indicate a xenocrystic origin for biotite in the smallscale biotite-rich zones of the SMB. However, other data
point towards a primary magmatic origin for most SMB
biotite, such as the scarcity of other obvious xenocrysts in
the biotite-rich zones and the presence of abundant euhedral apatite inclusions in biotite. As argued above, heating
and assimilation of MSG country rocks was probably thermally limited to less than c. 25 wt % (see Thompson et al.,
2002a; Erdmann, 2006; Glazner, 2007). With a maximum
of c. 15 vol. % ( 17 wt %) of biotite in the MSG rocks,
even 25 wt % ( 22^24 vol. %) country-rock assimilation
could have contributed only up to c. 4 wt % ( 4 vol. %)
biotite to the SMB. For the majority of biotite crystals in
the SMB, we therefore suggest a magmatic origin.
The high eNdi and low 87Sr/86Sri isotopic signature of
sample E471D suggest that this large-scale biotite-rich
zone is one of the isotopically least contaminated rocks of
the SMB (Fig. 8), consistent with the formation of the biotite-rich zones by fractional crystallization (Clarke et al.,
1993). Although the biotite-rich granodiorite from the
SMB contact (E430Bt3) has a country-rock-like isotopic
composition, country-rock isotopic components are
equally abundant in the two biotite-poor host samples
(Fig. 8). Therefore, no obvious genetic relationship between
the occurrence of biotite and country-rock material exists,
suggesting that we probably overlooked the presence of
micro-scale contaminants in all three samples.
Biotite-rich zones are present in all SMB units (Fig. 1a),
and their decreasing biotite content with increasing
host-rock differentiation index suggests that biotite was
concentrated from an evolving magma. Either physical
accumulation against magma chamber walls or local
undercooling may account for the common occurrence of
biotite-rich zones along country-rock contacts (e.g.
Vernon, 1991).
Primary or secondary magmatic origin
Origin of biotite-rich zones
Xenocrystic country-rock origin
Biotite, like cordierite, is an abundant constituent of the
MSG country rocks, and therefore, a likely xenocryst in
the SMB. Locally, in centimeter- to decimeter-scale contact zones with country rocks, SMB granites contain xenocrystic biotite, together with other obvious xenocrysts and
micro-xenoliths (Clarke & Erdmann, 2005). Biotite-rich
Given the tendency of biotite to equilibrate with its host
magma, it is impossible to determine from textures or mineral chemistry alone which biotite in the SMB is primary
magmatic and which is secondary magmatic. However,
the scarcity of replacement textures (with the exception of
secondary biotite in garnet-rich zones) suggests that the
majority of magmatic biotite in the SMB is of primary
origin.
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Fig. 13. Our estimate of the potential of crystal and whole-rock features to determine the origin of garnet, cordierite, and biotite in granitic
rocks.
A resistate origin for biotite in the SMB, or a secondary magmatic origin after resistate or restite, is theoretically also possible. However, the absence of obvious
resistate or restite crystals or relics (e.g. garnet, orthopyroxene, sapphirine) in the SMB rocks makes a secondary magmatic origin after source-rock solids relatively
unlikely. In addition, general concerns about the inheritance of large amounts of source-rock solids in
granites (see Sawyer, 2001; Vernon, 2007), and euhedral
apatite inclusions in many biotite crystals, argue against a
resistate origin (as outlined above for country-rock xenocrysts). Nevertheless, the source-like signature of our
large-scale biotite-rich samples suggests that these possibilities need further evaluation (e.g. the study of zircon
inclusions).
On the basis of our datasets, the origin of single biotite
crystals in the SMB cannot be determined unambiguously,
given the lack of universally diagnostic features.
Xenocrystic and secondary magmatic biotite are locally
present in our samples, and may occur in minor amounts
throughout the batholith (Clarke & Erdmann, 2005).
However, based on whole-rock compositions and euhedral
apatite inclusions in many biotite crystals, we favor the
interpretation that biotite in the biotite-rich zones is dominantly primary magmatic.
Evaluating the origin of garnet, cordierite,
and biotite in granitic rocks
The potential of different textural, mineralogical, compositional, and spatial criteria in evaluating the origin of
garnet, cordierite, and biotite in granitic rocks is highly
variable, as summarized in the Introduction. We suggest
that for garnet and cordierite, inclusion types and patterns,
other mineral textures, and mineral zoning patterns are
the best indicators of their origin (Fig. 13). Major- and
trace-element compositions of both garnet and cordierite,
if preserved, may also point towards a specific origin,
although small crystals may compositionally re-equilibrate
at magmatic temperatures (e.g. Brady, 1995; Edwards &
Russell, 1998). Trace-element compositions of garnet are
largely controlled by partitioning between garnet and
REE-rich accessory phases (e.g. zircon, monazite), and
may therefore not be diagnostic (Bea et al., 1994).
For biotite, we anticipate that the types and textures of
inclusions may be by far the best guide to the origin of a
given crystal. Mineral textures are similar in both igneous
and metamorphic environments, and diffusivities in biotite
are too high to preserve initial distinguishing chemical
characteristics (e.g. Brady, 1995). Given the abundance of
biotite in granites, determining its origin is important, but
it may be necessary to resort to the characterization of a
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ERDMANN et al.
GARNET, CORDIERITE, AND BIOTITE IN GRANITES
statistically relevant number of inclusions (e.g. zircon) or
more sophisticated isotopic methods than those employed
in the present study.
For garnet and cordierite, an evolved peraluminous leucogranite host may point towards a primary magmatic
origin; alternatively, garnet- and cordierite-rich zones in
variably evolved rock types point towards a country-rock
origin. For biotite, the magmatic host rock is less likely to
imply a particular origin, given its occurrence in most
metaluminous to peraluminous, and primitive to evolved,
granitoids (Fig. 13). Whole-rock isotopic Nd signatures for
garnet concentrations may partly reflect the origin of the
garnet, given its high Nd partition coefficient (Bea et al.,
1994; Villaseca et al., 2003). However, unless garnet is particularly abundant, the whole-rock Nd is likely to reside
mainly in accessory phases. For cordierite and biotite,
Sr and Nd have low partition coefficients, or easily reequilibrate (Bea et al., 1994; Ewart & Griffin, 1994;
Hammouda & Cherniak, 2000). The whole-rock Sr^Nd
isotopic signature of cordierite- and biotite-rich zones
therefore reflects the host assemblage, and not cordierite
or biotite.
CONC LUSIONS
Acquiring field, textural, chemical, and isotopic data to
evaluate the origin of minerals in igneous rocks is ideal,
but may not always be practical. We suggest that for less
readily equilibrating minerals, such as garnet and cordierite, a combination of textural and major-element mineral
chemical data may suffice to determine their origin. In
the present case, these data lead us to conclude that
garnet in SMB garnet-rich zones is probably of peritectic
xenocrystic origin. Problems with relying on field and
whole-rock compositional data alone are illustrated by our
evaluation of the origin of cordierite in the SMB. In this
case, a xenocrystic origin might have been inferred from
field data (abundant in country rocks and along countryrock contacts) and whole-rock isotopic data (countryrock-like signature). In contrast, the combination of
mineral chemistry and texture and correlation with
evolved host-rock compositions leads us to conclude that
most SMB cordierite is of primary magmatic origin.
For readily equilibrating minerals such as biotite, a combined evaluation of crystal-scale and whole-rock features
is essential, including inclusion relations and whole-rock
isotopic compositions. In the present case, a xenocrystic
origin might have been inferred on the basis of field and
textural data (abundant and texturally similar in country
rocks and along country-rock contacts) and in accordance
with mineral chemical data (compositionally similar to
biotite in adjacent country rocks). In contrast, a primary
magmatic (or possibly secondary magmatic) origin is
inferred for most SMB biotite based on contrasting
inclusion types and patterns between biotite in the batholith compared with that in the country rocks.
AC K N O W L E D G E M E N T S
We thank N. Daczko, D. Kelsey, and G. Stevens for their
constructive reviews, and G. Clarke and A. Lumsden for
very considerate editing. We sincerely thank D. Barrie
Clarke for initiating this study, for stimulating discussions,
for helpful comments on earlier versions of this paper, and
for support from his NSERC Discovery Grant for the
acquisition of data. We thank A. Dunn for support in
the field, G. Brown for the polished thin sections,
P. Stoffyn-Egli for assistance with the electron microprobe,
W. Diegor and M. Tubrett for help with the ICP-MS analyses and data reduction, and D. Weis for carrying out the
TIMS analyses. S. Erdmann acknowledges support from a
Killam predoctoral scholarship while collecting the data,
and a postdoctoral fellowship from the DAAD while working on the manuscript.
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