![Map showing the distribution of migmatite in the Tumbledown and Weld anatectic domains (TAD and WAD), western Maine, with structure section A-A′. Heterogeneous migmatite is undifferentiated on the structure section. Stereograms are lower-hemisphere, equal-area (Schmidt) projections [selected from Solar & Brown (2001)]. Orientation data for fabric elements in metasedimentary rocks (left of map) show the alternation of zones of contrasting strain as explained in text (AFZ, zone of apparent flattening strain; ACZ, zone of apparent constrictional strain). Orientation data for fabric elements in migmatites (below map) show the same alternation for stromatic and heterogeneous types, respectively. Structure section A-A′ illustrates the listric geometry of the CMB shear zone system as it cuts migmatite in the TAD and WAD (after Brown & Solar, 1999; Solar & Brown, 2001). Short line segments are the intersections of foliation in AFZs with the plane of section. Avalon-like rocks are so designated based upon Nd isotope signatures measured from granodiorite samples from the Phillips pluton (Figs 1 and 2; north end of the WAD; Pressley & Brown, 1999) that are consistent with a source similar to that of the Avalon Composite Terrane (see Fig. 1). The depth to the Avalon Composite Terrane was determined using interpretations of geophysical data, as summarized by Brown & Solar (1999). Stations 95-97 and 95-121 are located for references to specimens from these in text and figures.](https://www.researchgate.net/profile/Gary-Solar/publication/31486786/figure/fig2/AS:670711710490627@1536921541213/Map-showing-the-distribution-of-migmatite-in-the-Tumbledown-and-Weld-anatectic-domains_Q320.jpg)
Content uploaded by Gary S. Solar
Author content
All content in this area was uploaded by Gary S. Solar
Content may be subject to copyright.
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 PAGES 789–823 2001
Petrogenesis of Migmatites in Maine, USA:
Possible Source of Peraluminous
Leucogranite in Plutons?
GARY S. SOLAR∗AND MICHAEL BROWN
LABORATORY FOR CRUSTAL PETROLOGY, DEPARTMENT OF GEOLOGY, UNIVERSITY OF MARYLAND,
COLLEGE PARK, MD 20742, USA
RECEIVED JUNE 24, 1999; REVISED TYPESCRIPT ACCEPTED JULY 4, 2000
KEY WORDS: anatexis of pelite; Maine; migmatite; peraluminous granite;
In Maine, Siluro-Devonian turbidites were metamorphosed under
plutons
high-T–low-Pfacies series conditions during deformation within a
Devonian crustal-scale shear zone system, defined by kilometer-scale
straight belts of apparent flattening strain that anastomose around
lozenges of apparent constrictional strain. At upper amphibolite
facies grade, metapelites are partially melted, the onset of which is INTRODUCTION
recorded by a migmatite front. The resulting migmatites are stromatic The process of generation, segregation, ascent and em-
or heterogeneous, and smaller-volume granites form sheets or cylinders placement of granite magma during orogeny has im-
according to the structural zone in which they occur, suggesting that portant implications because melt transfer affects the
migmatites and granites record syntectonic melt flow through the thermal and rheological behavior of the crust during
deforming crust. Common leucogranite of the nearby coeval Phillips orogenesis (e.g. Collins & Vernon, 1991; Stu
¨we et al.,
pluton, which was emplaced syntectonically, was sourced from 1993; Brown & Solar, 1999). Our conception of how
crustal rocks with geochemical characteristics similar to those of the melt is generated and segregated is well developed (e.g.
host Siluro-Devonian succession. Migmatites have melt-depleted Wickham, 1987; Johannes, 1988; Allibone & Norris,
compositions relative to metapelites. Leucosomes are peraluminous 1992; Sawyer, 1994, 1998; Brown et al., 1995; Rushmer,
and represent the cumulate products of fractional crystallization and 1996; Milord et al., 2001). We also understand well how
variable loss of evolved fractionated liquid. Among the heterogeneous granite magma is emplaced in both extensional and
migmatites are schlieric granites, the geochemistry of which suggests contractional tectonic settings (e.g. Hutton & Reavy,
melt accumulation before fractional crystallization and loss of the 1992; Grocott et al., 1994; Paterson et al., 1996; Benn et
evolved liquid. Smaller-volume granites are peraluminous with a al., 1998; Brown & Solar, 1998b; Cruden, 1998; Paterson
range of chemistries that reflect variable entrainment of residual & Miller, 1998). However, the mechanism by which melt
plagioclase and biotite, accumulation of products of fractional is transferred from source to sink during orogeny remains
crystallization and loss of most of the evolved liquid. Common a matter of debate (e.g. Clemens & Mawer, 1992;
leucogranite of the Phillips pluton and larger granites in the D’Lemos et al., 1992; Brown, 1994; Rutter, 1997; Sawyer,
migmatites have compositions that suggest crystallization of evolved 1998; Brown & Solar, 1999; Miller & Paterson, 1999;
liquids derived by fractional crystallization of primary muscovite Weinberg, 1999).
dehydration melts. We infer that the leucogranite represents the Within migmatites, the geometry of leucosomes and
crystallized fugitive liquid from a migmatite source similar to that smaller-volume granites may record the melt flow net-
exposed nearby. Water transported through the shear zone system work through the crust (e.g. Brown & Rushmer, 1997;
dissolved in melt was exsolved at the wet solidus to cause retrogression Sawyer, 1998; Brown & Solar, 1999; Brown et al., 1999),
particularly so if the leucosomes do not record solid-statein sub-solidus rocks and retrograde muscovite growth in migmatites.
∗Corresponding author. Present address: Department of Earth
Sciences, SUNY College at Buffalo, 1300 Elmwood Avenue, Buffalo,
NY 14222, USA. Telephone: 716-878-6731. Fax: 716-878-4524.
E-mail: solargs@bscmail.buffalostate.edu Oxford University Press 2001
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
strain. At the outcrop scale, the presence of granite rocks were intruded syntectonically by Early to Middle
located in structurally controlled sites within migmatite, Devonian plutons (e.g. Bradley et al., 1998; Solar et al.,
such as interboudin partitions and strain shadows (e.g. 1998).
Stromgard, 1973), fractures and fold hinge zones (e.g. Strain was partitioned during Devonian dextral tran-
Collins & Sawyer, 1996), and dilatant shear surfaces (e.g. spression of the northern Appalachians ( Fig. 1) so that
Brown, 1994) suggests melt flow through the migmatite inboard oblique contraction was accommodated within
during deformation (e.g. Brown & Rushmer, 1997). In a the CMB shear zone system (Solar & Brown, 1999, 2001)
partially molten rock, melt is segregated during de- whereas concomitant dextral-transcurrent displacement
formation by moving down gradients in melt pressure to was accommodated within the Norumbega shear zone
create leucosomes (Brown, 1994; Brown et al., 1995; system (West & Hubbard, 1997; West, 1999). De-
Rutter, 1997; Marchildon & Brown, 2001). If deformation formation had become localized within the Norumbega
and melting are coeval, cyclic inflation and collapse of shear zone system by Carboniferous time (Hubbard et
melt flow conduits caused by build-up of melt pressure al., 1995; West & Hubbard, 1997; Ludman, 1998; West,
and periodic draining of the source moves melt through 1999), which juxtaposed the ACT against the CMB by
the crust (e.g. Brown & Solar, 1998a, 1999; Weinberg, orogen-parallel translation.
1999).
This paper reports variations in the structure, mineral
assemblage and geochemistry of migmatites, and the Regional structural geology and
form, petrography and geochemistry of granites from an metamorphism
area in western Maine, USA. The area includes the
The CMB shear zone system is the principal Devonian
northern limit of migmatite in the Central Maine belt,
(Acadian) structure of western Maine (Fg. 1). It consists
which marks the termination of a diachronous ‘meta-
of two types of kilometer-scale structural zones (Solar &
morphic high’ that extends from eastern Connecticut,
Brown, 2001). These are NE-striking straight belts of
through central Massachusetts and New Hampshire, into
steeply to vertically dipping planar and moderately to
Maine [formally called the ‘Acadian metamorphic high’
steeply plunging linear structures associated with tight
by, for example, Schumacher et al. (1990)]. In an earlier
folds that alternate with and anastomose around zones
study we have shown that regional metamorphism and
in which the orientations and development of planar
crystallization of migmatite leucosomes and granites were
structures are variable and folds are gentle to open. The
coeval (Solar et al., 1998). In this study we compare the
preferred orientation of bladed muscovite and ribbons
geochemistry of metapelite source rocks, migmatites and
or rods of polycrystalline quartz aggregates define the
leucogranites to evaluate the hypothesis that migmatites
and leucogranites in western Maine are cogenetic. Our penetrative metamorphic mineral fabrics that are distinct
conclusions refute the common belief that there is no in each structural zone (S > L fabrics in straight belts;
genetic relationship between migmatites and leuco- LqS fabrics in intervening zones). We interpret these
granites, and have implications for the petrogenesis of fabrics to record contrasting styles of finite strain, and
migmatites and leucogranites in other orogens. we refer to these as zones of apparent flattening-to-plane
strain (AFZs; Fig. 1) and zones of apparent constrictional
strain (ACZs; Fig. 1). Solar & Brown (2001) have pos-
tulated that this pattern indicates deformation local-
GEOLOGY OF WESTERN MAINE ization, with greater strain accommodation, and therefore
Tectonic setting larger tectonic displacements within the AFZs. The pelite
layers exhibit porphyroblast–matrix microstructures that
The Central Maine belt (CMB) of the northern Ap-
suggest syntectonic growth of porphyroblast minerals
palachians is the principal tectonostratigraphic unit of
during progressive tightening of regional-scale folds (Solar
the eastern part of New England and New Brunswick
& Brown, 1999, 2000, 2001). Because a well-developed
(Fig. 1). The CMB lies between Ordovician rocks of the
NE-plunging mineral elongation lineation, defined by
Bronson Hill belt (BHB) to the WNW (Ratcliffeet al.,
the same metamorphic minerals at each grade, penetrates
1998, and references therein), and Neoproterozoic-to-
all zones, Solar & Brown interpreted matrix mineral
Silurian rocks of the Avalon Composite Terrane (ACT)
growth as syntectonic. Thus, the fabrics of the prograde
to the SSE (e.g. West et al., 1995; Cocks et al., 1997).
metamorphic minerals recorded the regional tectonic
The CMB is composed of a succession of Siluro-Devonian
strain ellipsoid.
turbidites (interlayered pelite and psammite) that was
The high-T–low-Pmetamorphic field gradient reflects
deformed and metamorphosed at greenschist to upper
syntectonic polymetamorphism (e.g. Guidotti, 1989;
amphibolite facies conditions during the Early Devonian
Solar & Brown, 1999) related to pluton-driven thermal
Acadian orogeny (e.g. Bradley, 1983; Smith & Barreiro,
1990; Robinson et al., 1998). These metasedimentary pulses (De Yoreo et al., 1989) that overprint a regionally
790
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
Fig. 1. Maps showing the location of the area of study in New England (inset), and the principal geological features of this area and the location
of Fig. 2 (main map). The Central Maine belt (CMB) metasedimentary rocks are shown unornamented. The NW margin of the CMB is in
contact with the Bronson Hill belt (BHB) at a zone of fine-grained tectonite within a zone of apparent flattening strain (AFZ). ACZ, zone of
apparent constrictional strain; CT, Connecticut; MA, Massachusetts; ME, Maine; NY, New York; RI, Rhode Island; VT, Vermont.
elevated thermal gradient (Brown & Solar, 1999). Sep- syntectonically (Solar & Brown, 1999, 2000), it follows
that both metamorphism and plutonism were syn-
arate periods of metamorphism (e.g. Guidotti, 1963,
chronous with deformation.
1989, 1993; Holdaway et al., 1982; Chamberlain &
England, 1985; Eusden & Barreiro, 1988; Smith & Barre-
iro, 1990) are interspersed with plutonism (e.g. Tomascak
et al., 1996; Bradley et al., 1998; Solar et al., 1998). PETROLOGY AND FIELD RELATIONS
Metamorphic grade in the area of Fig. 1 varies from
OF MIGMATITE
garnet zone through staurolite zone and lower sillimanite
zone to upper sillimanite zone, achieving anatexis in The Tumbledown and Weld anatectic domains ( TAD
metapelites along the high-Tprograde path at depths and WAD, respectively; Fig. 2) are separated based upon
equivalent to >15 km (Holdaway et al., 1997; Brown & map pattern and regional structure (Solar & Brown,
Solar, 1998b). The age of regional metamorphism is 2001). Migmatite in these domains varies from strongly
405 to 399 ±2 Ma, based upon U–Pb data from foliated metasedimentary rock with a few millimeter-
metamorphic monazite grains from upper amphibolite- scale leucosomes per square meter ( Fig. 3), in which relict
facies mica schist (Smith & Barreiro, 1990). In com- primary structures are preserved, to rocks structurally
parison, concordant U–Pb zircon and monazite ages disrupted by the migmatization process (diatexis; see
from granite bodies in migmatite, as well as plutons, Brown, 1973; Fig. 4) and schlieric granite ( Fig. 5a).
range from c. 408 to c. 404 Ma (Solar et al., 1998); this Leucosome density and disruption of relict primary struc-
suggests that granite emplacement was contemporaneous tures both increase across strike from the migmatite front
with metamorphism (Brown & Solar, 1998a, 1998b,(Solar, 1999). Similarly, sheets and cylinders of granite
progressively dominate outcrops of migmatite both along1999). Because metamorphic mineral fabrics developed
791
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
and across strike of migmatite layers (Figs 3d and 5b). >1–2 m long; both have low width-to-length ratios of
>0·05 (Fig. 3a and b). Melanosomes range from 0·1 to
Many outcrops of migmatite in both the TAD and WAD
0·6 mm, rarely up to 1 mm, in thickness and are invariably
consist of domains and/or blocks in which some relict
in contact with leucosomes. They are composed of >80
primary pelite–psammite interlayers are preserved. Lay-
vol. % biotite, accompanied by fibrolite, minor quartz,
ers in such blocks resemble those common in the strati-
plagioclase and retrograde chlorite. Biotite grains, gen-
graphic succession outside the TAD and WAD, down-
erally >1 mm long, are clustered, and show a strongly
temperature from the migmatite front, which suggests
preferred orientation that defines a lepidoblastic texture
the protolith of the migmatites was the CMB meta-
and a foliation parallel to leucosome edges. The in-
sedimentary succession. This is supported by the con-
tervening host rock layers are 2–10 cm thick. They con-
tinuation of the regional structure across the migmatite
tain biotite, quartz, sillimanite (mostly fibrolite), garnet,
front (Fig. 1).
pyrrhotite and/or ilmenite, muscovite (skeletal), and loc-
The TAD and WAD consist of two subdomains where
ally plagioclase, tourmaline and clinozoisite. Typically,
the structure of the migmatite and the distribution of
fibrolite has grown at the expense of primary muscovite
leucosome reflect the pattern of strain. We map these
in the foliation; the fibrolite forms a fabric in addition
subdomains as stromatic and heterogeneous migmatite
to the penetrative biotite foliation, and these sillimanite–
that correspond typically, but not precisely, to AFZs and
biotite folia alternate with quartz–feldspar folia (Fig.
ACZs, respectively. This structural relation led Brown &
3c). Fabrics are oriented sub-parallel to fabrics in the
Solar (1999) to interpret heterogeneous migmatite to be
metasedimentary rocks outside the migmatite domains
within the cores of regional thermal antiforms (in ACZs),
down-temperature from the migmatite front (Fig. 2,
flanked by stromatic migmatites (in AFZs). Transition
see stereograms). Elongate fibrolite aggregates define a
zones are present between the migmatite subdomains,
steeply plunging lineation visible in the field, and elongate
and between vein migmatite and diatexite within the
quartz aggregates define a weak sub-horizontal lineation
heterogeneous migmatite (Fig. 2). Commonly, bodies of
seen only in cut hand specimens and suitably oriented
schlieric granite are found within transition zones (see
thin sections.
Fig. 5a) that occur at AFZ–ACZ boundaries. In the
In leucosomes, grains are equant and anhedral with
northern part of the TAD and WAD, layers are pro-
average sizes ranging from 0·1 mm in the thinner leuco-
gressively eliminated across strike from the migmatite
somes to 4 mm in the thicker leucosomes. Quartz is
front by increasing volume of leucosome and disruption
present in approximately equal proportion to plagioclase.
by apparent flow, as stromatic migmatite grades into
An exception is rare leucosomes in which quartz is >70
diatexite.
vol. %, where the quartz grains are <0·1 to 3 mm in
diameter and plagioclase is >1 mm in length. Quartz
shows undulatory extinction, except in grains <0·1 mm
Migmatite domain 1: stromatic migmatite in diameter. Quartz grains in the quartz-rich leucosomes
Approximately half of the exposed migmatite in western display serrated edges to suggest grain-size reduction. In
Maine is stromatic (Fig. 3; Menhert, 1968), characterized leucosomes <1 mm thick, a magmatic foliation is defined
by a planar structure in which each layer is min- by biotite and opaque mineral grains (>10 m). Leuco-
eralogically and texturally distinct. This type of migmatite somes >5 mm thick consist of plagioclase, with normal
is found mostly in AFZs (Fig. 2), and corresponds to zoning in several irregular or patchy concentric shells,
metatexite (Brown, 1973; Ashworth, 1985), being com- and interstitial quartz. Some leucosomes may contain up
posed of discrete millimeter- or centimeter-thick sheet-like to 5 vol. % anhedral K-feldspar. Coarser irregular biotite
but discontinuous bodies of granite (leucosome) separated is inferred to be residual, whereas finer-grained euhedral
from medium-colored high-grade metamorphic host rock biotite grains are inferred to be neocrystalline. Although
by dark-colored selvedges (melanosome). The ori- textures generally appear to be the result of crystallization
entations of migmatite layers and mineral fabrics are in the presence of a melt phase (see Vernon & Collins,
concordant (Fig. 3), as reflected by the consistent steeply 1988) and, therefore, undeformed, many leucosomes of
dipping orientation of these structures at all scales within <1 cm thickness are locally overprinted by solid-state
each structural zone (Fig. 2). At the regional scale, the strain that has produced common sub-grain boundaries
layers are parallel to those of metasedimentary rocks in in quartz or serrated edges of quartz accompanied by
the same structural zone (Fig. 2, see stereograms). coronas of grains (<0·1 mm in diameter) that lack sub-
Leucosomes are trondhjemitic, being composed of grain boundaries.
plagioclase, quartz, biotite and muscovite (Fig. 3); they Leucosomes show ‘pinch-and-swell’ structure in three
make up >3 vol. % of stromatic migmatite at outcrop. dimensions ( Fig. 3b). A longer wavelength in the sub-
Millimeter-scale leucosomes range from >1to25cmin horizontal dimension suggests that the maximum ap-
parent ‘pinch’ is sub-vertical and down-dip, consistentlength, whereas centimeter-scale leucosomes are typically
792
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
Fig. 2. Map showing the distribution of migmatite in the Tumbledown and Weld anatectic domains (TAD and WAD), western Maine, with
structure section A–A′. Heterogeneous migmatite is undifferentiated on the structure section. Stereograms are lower-hemisphere, equal-area
(Schmidt) projections [selected from Solar & Brown (2001)]. Orientation data for fabric elements in metasedimentary rocks (left of map) show
the alternation of zones of contrasting strain as explained in text (AFZ, zone of apparent flattening strain; ACZ, zone of apparent constrictional
strain). Orientation data for fabric elements in migmatites (below map) show the same alternation for stromatic and heterogeneous types,
respectively. Structure section A–A′illustrates the listric geometry of the CMB shear zone system as it cuts migmatite in the TAD and WAD
(after Brown & Solar, 1999; Solar & Brown, 2001). Short line segments are the intersections of foliation in AFZs with the plane of section.
Avalon-like rocks are so designated based upon Nd isotope signatures measured from granodiorite samples from the Phillips pluton ( Figs 1 and
2; north end of the WAD; Pressley & Brown, 1999) that are consistent with a source similar to that of the Avalon Composite Terrane (see Fig.
1). The depth to the Avalon Composite Terrane was determined using interpretations of geophysical data, as summarized by Brown & Solar
(1999). Stations 95-97 and 95-121 are located for references to specimens from these in text and figures.
with kinematic indicators in the metasedimentary rocks host rock layers. Centimeter- to meter-scale sub-vertical
tabular bodies of granite cut outcrops of stromatic mig-(Solar & Brown, 2001). This triaxial, oblate shape is
similar to that defined by the mineral grains in the matite at concordant to weakly discordant angles to
793
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
Fig. 3. Features of the stromatic migmatite. (a) Pavement outcrop in Swift River, Roxbury, Maine (Station 95-103), showing typical stromatic
migmatite millimeter-thick leucosomes of trondhjemitic composition (lighter-colored, low width-to-length ratio bodies). Leucosomes are concordant
with the sub-vertical foliation in the melt-depleted host rock, separated by the darker-colored millimeter-thick melanosomes. 012°is to the left,
parallel to the long dimension of the field of view. (b) Pavement outcrop in Swift River, north of Mexico, Maine (Station 96-132), illustrating
‘pinch-and-swell’ structure of concordant leucosomes. This outcrop is located within 1 km of the transition zone on the west side of TAD. Strike
of foliation and trend of leucosomes are subparallel to those in (a); 018°is to the right, parallel to the long dimension of the field of view. (c)
Bt, Sil and Qtz +Pl grain-shape foliation in melt-depleted host in thin section (plane-polarized light) cut along the weakly defined steeply east-
plunging lineation, across the steeply dipping foliation (Noisy Brook, Roxbury, Maine; Station 95-148). Top of the field of view is a centimeter-
thick trondhjemite leucosome. (d) ‘Pinched-and-swelled’ composite granite sheets in stromatic migmatite of (a) (Station 95-103; view toward
012°). The ‘pinch’ of the sheets is more extreme in vertical section, sub-parallel to the weakly defined steeply plunging mineral lineation in the
rock. Also, the centimeter-scale composite layers in the thickest sheet at right are typical, and are observed up to meter scale.
the planar structures. Many of these granite sheets are migmatite is found exclusively in ACZs. The orientation
composite (Fig. 3d; see also Brown & Solar, 1999), and of grain-shape fabrics and geometry of leucosomes vary
most have a ‘pinch-and-swell’ structure with a longer more in these rocks than in the stromatic migmatite ( Fig.
wavelength in the sub-horizontal direction. In one area, 2, see stereograms). Weak foliations and lineations in
within 1 km along strike a progressive increase occurs in heterogeneous migmatite are defined by sillimanite
the proportion of meter-scale composite granite sheet to (mostly fibrolite) and biotite ( Fig. 4b).
host stromatic migmatite such that the migmatite be- There are two types of heterogeneous migmatite, vein
comes disrupted ultimately to occur only as isolated migmatite in the south and SW and diatexite in the
schollen in granites (Fig. 5b) that make up a sheeted north and NE [Fig. 2; terminology after Menhert (1968)
granite complex. and Brown (1973), as modified by Ashworth (1985)].
Contacts between the two types are gradational over tens
of meters in transition zones. Vein migmatite shows
Migmatite domain 2: heterogeneous phlebitic structure (Menhert, 1968), and meter-scale com-
migmatite positional layers interpreted to be relict from the protolith
(Fig. 4e). Diatexite, in contrast, is a rock in which the
A regular planar structure is absent in the remainder of
protolith structures are not observed, suggesting de-
the exposed migmatite in western Maine; we refer to
this type of migmatite as heterogeneous. This type of struction by diatexis (e.g. Fig. 4a and b; see Brown, 1973;
794
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
Fig. 4. Features of heterogeneous migmatite. (a) Diatexite typical of the TAD (Rt. 17, north of Roxbury, Maine; Station 95-30), with a
weak mineral fabric and lack of layers relative to stromatic migmatite (see Fig. 3a). (b) Typical texture of diatexite in thin section (Station
95-100) cut across the foliation and along the longest dimension of the grain-shape fabric (generally moderately to steeply east-plunging).
(c) Pavement outcrop of diatexite in Swift River, south of Roxbury, Maine (Station 95-35), with prolate-shaped schollen of calc-silicate
rock; 300°is to the left along the long dimension of the field. Leucosome is concentrated at the schollen margins. (d) Concordant, broadly
cylindrical-shaped granite body in diatexite in Swift River, south of Roxbury, Maine (Station 95-49). At the right–center is a meter-scale
block of biotite–garnet schist inside the granite that is separate, and whose foliation is discordant with the diatexite that makes up the
rest of the outcrop (>80°). (e) Pavement outcrop of typical vein migmatite of the southern part of the TAD, north of Mexico, Maine
(Station 96-65); 333°is to the left along the long dimension of the field. (f ) Texture of vein leucosome (Qtz +Pl) and melt-depleted
host (Bt +Pl +Grt +Qtz) in thin section (plane-polarized light; Station 96-65). This section is from the outcrop shown in (e), cut
along the long dimension of the leucosome and across the host foliation.
Sawyer, 1998). Vein migmatite shows sharp leucosome that together reflect original metasedimentary layers.
contacts (Fig. 4e and f ). In contrast, contacts between Centimeter-scale pod- or lens-shaped trondhjemitic
leucocratic and melanocratic domains in diatexite are leucosomes are separated by centimeter-scale ana-
diffuse and gradational at the centimeter scale stomosing darker host layers similar to the melanosomes
(Fig. 4c). of stromatic migmatite (Fig. 4e). Leucosomes make up
>15 vol. % on outcrop surfaces, and display ‘pinch-
Vein migmatite and-swell’ structure (Fig. 4e and f ), in which thickness
varies from 3 to 10 cm. Individual pods are up to 20 cm
Vein migmatite occurs as meter-scale units alternating
with weakly migmatized meter-scale semi-pelitic units long, yielding width-to-length ratios up to 0·5.
795
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
Fig. 5. (a) Schlieren-rich granite in the transition zone between stromatic migmatite and diatexite, west side of the TAD (Walker Mountain,
north of Roxbury, Maine; Station 95-215). The fabric is concordant with foliation and layers in the adjacent stromatic migmatite, and parallel
to the length of the north–south trending transition zone (000°is left along the long dimension of the field). The schollen within the granite are
calc-silicate-rich psammite that have internal structure similar to metasedimentary rocks of the CMB. Leucosome is concentrated at the interface
with the granite. (b) Sub-planar granite sheets in stromatic migmatite that parallel the migmatite foliation and layers (Station 95-51; 012°is left).
These sheets ‘pinch-and-swell’ to ‘pinch’ out the stromatic migmatite host along the fabric and length of the sheets. (c) Cylindrical granite bodies
in diatexite that contains a block of biotite–garnet schist (Station 95-49), apparently residual after partial melting of diatexite. (d) Texture of the
block of schist in (c) (plane-polarized light). Biotite defines a nearly decussate texture. Finer-grained groundmass is approximately equigranular
biotite +plagioclase.
Leucosomes consist of subequant and anhedral quartz, (variable dip direction) and weak, moderately plunging,
down-dip lineation.
plagioclase and K-feldspar with no discernible grain-
shape fabric. Larger leucosomes may contain anhedral
garnet grains of 1–2 mm diameter. Quartz grains have Diatexite
subgrain boundaries and undulatory extinction, but to a Diatexite varies at outcrop from ‘patchy’ leucosome-
lesser degree than counterparts in stromatic migmatite. dominated to biotite–sillimanite-dominated rock, and
Quartz generally displays the largest grains (>5mm in outcrop to outcrop from schlieren-rich migmatite to
diameter), whereas plagioclase is usually >1mm in schlieric granite with schollen of vein migmatite and
length, and grains commonly show inhomogeneous nor- unmigmatized calc-silicate-rich psammite (Fig. 5a). Most
mally compositionally zoned concentric shells of >50 mtypes are characterized by a discontinuous, weakly de-
width. fined foliation of variable attitude (Fig. 4a). Leucosomes
The melanosomic host rock contains sillimanite (mostly and leucocratic domains make up >9–15 vol. % of
fibrolite), biotite, muscovite, plagioclase, quartz, pyrrho- diatexite outcrop surfaces, and are generally uniformly
tite and/or ilmenite, garnet and locally K-feldspar. Sil- distributed.
limanite is found as clots within both muscovite and Discrete leucosomes appear as centimeter-scale
plagioclase (Fig. 4f ). Some prismatic sillimanite grains quartzo-feldspathic mineral segregations that vary from
are up to 0·1 mm across, but sillimanite more commonly diffuse to sharp at their margins (Fig. 4c). Shapes of
occurs in clots up to 5 mm in diameter. Biotite (1–3 mm leucosomes are subequant in pavement outcrops, 1–2 cm
in length), muscovite, and elongate untwinned plagioclase in diameter, and elongate down-dip of the sillimanite
(up to 6 mm in length) all show a distinct grain-shape fabric in the host rock, with lengths up to 20 cm and
width-to-length ratios of up to 0·1. Some leucosomesfabric, defining a strong moderately dipping foliation
796
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
anastomose along their lengths; these show a fabric & Solar (1999) for a discussion]. Entrained blocks of
strongly foliated biotite–garnet schist are found in thedefined by biotite and opaque minerals that we infer to
be of magmatic origin, and formed where melt and interior of the granite cylinders. The granite cylinders
lack a fabric, except proximal to the margin of theseentrained crystals flowed around residue and/or un-
melted protolith. Thus leucosomes tend to be rod-shaped, blocks (Fig. 5c).
with a long dimension that plunges moderately to steeply
ENE. This linear structure is sub-parallel to both the
weakly defined mineral elongation lineation in the host Interpretation
rock and the strongly defined mineral elongation lineation In both subdomains, the common matrix hosting the
in the metasedimentary rocks outside the migmatite leucosome is sillimanite (mostly fibrolite), biotite, garnet,
domains down-temperature from the migmatite front quartz, plagioclase, opaque phases (usually ilmenite), and
(Fig. 2). These fabrics define a triaxial, strongly prolate coarse, skeletal muscovite books that cut the fabric and
shape similar to that of the metasedimentary rocks in the are interpreted to be a retrograde feature. Fibrolite and
same structural zones (ACZs; Fig. 2). biotite are the main fabric-forming phases (Fig. 3c), and
Leucosomes contain quartz, plagioclase, biotite, fibrolite has apparently grown at the expense of primary
pyrrhotite and/or ilmenite, and locally K-feldspar and fabric-forming muscovite to suggest it was produced by
muscovite. Grains are 0·5 mm in size with subequant the breakdown of muscovite. Migmatite leucosomes are
and anhedral shapes. Some quartz grains have moderate discrete to diffuse with a common mineralogy of pla-
undulatory extinction with a few sub-grain boundaries. gioclase, quartz, muscovite, and locally K-feldspar and
Plagioclase commonly shows several irregular or patchy biotite. The microstructure of leucosomes shows crystal
normally compositionally zoned concentric shells of faces and mineral films along grain boundaries that
>50 m width. Larger biotite with concentrations of suggest some crystallization from melt (Sawyer, 1999),
opaque mineral grains at the edges is inferred to be and melt-present formation. Locally, leucosomes exhibit
residual, whereas smaller euhedral grains without the biotite foliation that is concordant with fabrics in the
opaque grains are inferred to be neocrystalline. adjacent host rock.
Although the dark host rock is unlike the unmigmatized On the basis of the petrography, the melt-producing
protolith it is not truly melanosomic, making the usual reaction at the migmatite front probably was
migmatite nomenclature unsuitable in this case. Diffuse
domains of the host rock interfinger with the leucosome muscovite +plagioclase +quartz +water →melt +
at the millimeter-scale, and consist of biotite, sillimanite sillimanite +biotite
(mostly fibrolite) and garnet, with accessory muscovite,
closely followed by
plagioclase, quartz, pyrrhotite and/or ilmenite, and K-
feldspar. Fibrolite has grown at the expense of muscovite muscovite +plagioclase +quartz →melt +
and untwinned plagioclase to form a younger generation sillimanite +K-feldspar +biotite
of foliation-forming minerals. Different proportions of
these minerals account for the gradual variation of di- as these two reactions are closely spaced at low P
atexite from more quartzo-feldspathic (leucocratic) to (Thompson & Tracy, 1976). At a depth of >15 km these
more ferro-magnesian (melanocratic) types. In the ex- reactions indicate Tof >700°C. Given the limited amount
treme case, either schlieric granite is formed (leucocratic of water-rich metamorphic volatile phase that can be
diatexite; Fig. 5a), or the mineral assemblage is dominated stored in rocks at upper amphibolite facies conditions,
by biotite, sillimanite and garnet with <10 vol. % we expect that melting will be dominated by the muscovite
plagioclase +quartz, to give the rock a melanocratic dehydration reaction. Patin
˜o Douce & Harris (1998)
appearance (Fig. 5c and d). In all varieties of diatexite, investigated experimentally melting of metapelites similar
sillimanite is found as clots within both plagioclase and to those in the CMB, using two schists with different
retrograde muscovite. Although grain size varies, it is modes from the hanging wall of the Main Central Thrust
generally >3 mm, with sillimanite clots up to 5 mm. in the Himalayas.
Biotite (1–3 mm in length), muscovite and elongate un- In Maine, the absence of primary muscovite in mig-
twinned plagioclase (up to 6 mm in length) show a matites, in comparison with the metasedimentary rocks
preferred grain-shape fabric that defines a weak, mod- outside the migmatite front where muscovite averages
erately to steeply dipping foliation and strong, moderately >25 vol. % in the mode, and the universal occurrence
ENE-plunging, down-dip lineation. of sillimanite as a fabric-forming phase with biotite in
Most outcrops of diatexite are cut by meter-scale, the migmatites suggest that the material that hosts the
cylindrical granite bodies (Fig. 4d). These granite bodies leucosome is depleted of melt. Further, the generally K-
are elongate subparallel to the mineral lineation in the feldspar-poor nature of the leucosomes suggests melt
has been lost from the migmatites as a whole. Thesediatexite, and to the rod-shaped leucosomes [see Brown
797
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
observations are consistent with syntectonic mig- metapelite compositions of the Siluro-Devonian strati-
graphic succession. Portions of the same specimens used
matization, and consequent syntectonic melt extraction
for whole-rock analyses of two of the stromatic migmatites
from the migmatite domains. We will evaluate this pos-
(95-103, Fig. 3a; and 96-286), two of the heterogeneous
tulate in the light of the contemporaneous deformation
migmatites (vein migmatite: 96-65, Fig. 4e; diatexite: 95-
after describing the geochemistry of all rock types, but
35, Fig. 4c) and one of the schlieric granites (from the
for the remainder of this paper we will refer to darker
northern TAD transition zone: 95-215, Fig. 5a) were
host rock that does not form a distinct melanosome like
separated into the different structural components, and
those in the stromatic migmatite as melt-depleted host
analyzed separately. Major, minor and trace element
rock.
data for all rocks were analyzed by X-ray fluorescence
(XRF) (Tables 1–4), and trace elements for 32 of the
rocks, including the rare earth elements (REE), were
The Phillips pluton analyzed by inductively coupled plasma mass spec-
Immediately NE of the WAD (Fig. 2) is the coeval Phillips trometry (ICP-MS) (Tables 5 and 6). Methods and ana-
pluton, which is interpreted to be hemi-ellipsoidal with lytical uncertainties concerning these data are given in
long dimension parallel to the regional moderately NE- the Appendix.
plunging lineation (Brown & Solar, 1998b, 1999; Pressley
& Brown, 1999). It is located in an ACZ, similar to the
diatexites, and it has a similar geometry to the smaller- Metasedimentary rocks
volume cylinders of granite found in the heterogeneous Metasedimentary rocks range from >53 to >71 wt %
migmatite domains. These observations have been used SiO
2
(Table 1). Al
2
O
3
and (FeO∗+MgO +TiO
2
)
to suggest a relationship between structure, granite ascent decrease, (CaO +Na
2
O) shows no systematic change,
and emplacement (Brown & Solar, 1999). The geo- and K
2
O decreases slightly with increasing SiO
2
(Fig. 6).
chemistry of common leucogranite (>95%) from the V decreases systematically, Rb/Sr increases slightly, and
Phillips pluton has been interpreted to reflect an origin Rb, Sr, Zr and Ba show no systematic change with
by muscovite dehydration melting of a source with geo- increasing SiO
2
(Fig. 7). In appropriate plots of major
chemical characteristics similar to the metasedimentary oxides ( Fig. 6), the field defined by the sample of meta-
rocks of the CMB (Pressley & Brown, 1999). The re- sedimentary rocks lies between projected compositions
maining >5% of the Phillips pluton is granodiorite of biotite, garnet and muscovite, and albite-rich pla-
interpreted to reflect an origin by biotite dehydration gioclase and quartz.
melting of a source geochemically similar to ‘Avalon- Two metasedimentary rocks from this study (95-97
like’ rocks (see Fig. 1). The granites in the migmatites do and 95-173) and those reported by Cullers et al. (1974,
not possess this latter component. For these reasons, we 1997) have similar REE compositions and concentrations,
evaluate what relation exists between the migmatites, and have similarly shaped chondrite-normalized REE
the smaller-volume granites in the migmatites and the patterns ( Fig. 8). These patterns have slight negative Nd
common leucogranite of the Phillips pluton. anomalies, otherwise they are smooth and straight to
concave-upward in the heavy REE (HREE), with similar
overall steepness over a large range of La
N
/Lu
N
(5·5–38).
Although the two specimens of this study lie at the lower
GEOCHEMISTRY end of this range (Table 5), they have similar La
N
/Sm
N
Major and trace element compositions were determined and Gd
N
/Lu
N
ratios to those of the larger sample of
for representative whole-rock specimens of (1) pelite Cullers et al. (1974, 1997). All patterns have negative Eu
layers of the CMB metasedimentary rocks, (2) stromatic anomalies (Eu/Eu∗=0·65 and 0·88 for the two speci-
migmatite, (3) heterogeneous migmatite, and (4) granites mens of this study). Overall, the shape and steepness of
from within the migmatite domains. For the Phillips the patterns are similar to those of the North American
pluton we use the published analyses of 10 common shale composite (Taylor & McLennan, 1985).
leucogranite specimens from Pressley & Brown (1999).
All specimens were selected for analysis as described by
Solar (1999). The metasedimentary rocks were collected
Stromatic migmatite
from lower to upper amphibolite facies outcrops and
from both AFZs and ACZs. This was done to account for Relative to the metasedimentary rocks, the stromatic
any variation in composition resulting from progressive migmatite sample has less SiO
2
and CaO, more Al
2
O
3
,
metamorphism and/or contrasting strain ac- ( FeO∗+MgO +TiO
2
) and K
2
O, and similar Na
2
O
commodation. The suite of metasedimentary rocks is, ( Table 2; Fig. 6). With increasing SiO
2
, stromatic mig-
matite decreases in (FeO∗+MgO +TiO
2
), Al
2
O
3
andtherefore, taken to be representative of the range of
798
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
Table 1: Whole-rock compositions (XRF): metasedimentary rocks
Formation:1Quimby Rangeley (Silurian) Perry Mountain (Silurian) Sm Dc
Station: 96-77 96-80 95-173 95-232 96-3 96-31 96-84 96-187 95-97 96-16 96-70 96-86 96-88 96-120 96-261 96-264 96-190
(calc- (semi-
silicate pelite)
rock)
wt %
SiO261·4 65·0 64·8 62·4 70·8 58·6 62·6 56·0 67·7 57·5 60·1 60·9 64·4 70·3 52·7 55·2 54·7
TiO21·07 0·78 0·78 0·91 0·84 0·83 0·99 0·93 0·78 0·96 0·99 1·12 0·93 0·74 0·97 1·06 1·02
Al2O316·3 15·9 17·9 17·8 13·7 17·0 18·2 19·6 15·8 22·1 20·5 18·4 17·1 13·5 21·6 20·0 22·4
FeO∗7·23 6·41 6·51 7·56 3·93 6·48 5·83 7·52 5·83 6·65 7·58 8·07 6·61 5·62 8·45 6·83 8·05
MnO 0·08 0·06 0·06 0·12 0·05 0·22 0·08 0·10 0·04 0·12 0·18 0·17 0·06 0·18 0·15 0·33 0·16
MgO 2·83 2·55 2·22 2·50 1·87 3·19 2·44 3·14 1·66 2·25 2·15 2·39 1·78 1·85 4·15 4·49 2·66
CaO 1·34 0·20 0·29 0·34 0·78 5·57 0·23 0·91 0·24 0·12 0·15 0·59 0·16 0·26 0·20 1·92 1·02
Na2O 2·17 1·74 1·17 1·14 1·03 1·25 1·36 2·58 0·81 0·50 0·78 1·42 1·03 0·59 0·99 1·42 0·81
K2O 2·98 3·14 3·80 4·77 4·44 3·51 3·95 5·37 3·97 5·16 4·64 3·83 3·70 4·13 6·96 5·26 5·04
P2O50·11 0·14 0·08 0·11 0·12 0·23 0·10 0·09 0·21 0·08 0·06 0·15 0·08 0·13 0·07 0·10 0·15
LOI 3·71 4·10 2·30 2·80 2·55 2·04 3·91 3·50 2·39 4·43 2·84 2·81 3·52 2·07 3·65 2·92 3·37
Total 99·2 100·0 99·8 99·6 100·0 99·0 99·7 99·7 99·5 99·8 99·9 99·9 99·4 99·3 99·8 99·5 99·4
ppm
Sn 4 <627 5 3 <6 4 8 10 <5 5 4 4 18 11 <6 4
Nb 913 12 1414141416 14161619161314 18 15
Zr 200 198 149 183 302 155 216 151 157 165 166 267 198 163 151 243 155
Y 2924 15 2421311826 25242928252223 31 28
Sr 130 116 122 97 69 350 88 113 73 95 117 100 54 50 93 83 134
Rb 104 106 138 147 144 148 139 251 182 167 180 173 137 224 364 174 199
Pb 822 19 1114261736 22263028251316 20 14
Ga 19 20 21 26 18 23 23 25 20 29 28 23 20 19 33 27 22
Zn 115 89 97 87 69 94 79 113 24 94 154 105 85 78 156 113 93
Ni 25 7 23 25<4341141 36213243113447 66 18
V 166 128 129 148 94 118 132 147 113 167 140 130 135 99 198 148 149
Cr 69 51 67 82 60 71 89 83 86 105 105 92 76 63 120 120 83
Ba 444 560 663 1080 518 480 584 745 553 673 613 558 608 461 772 881 295
Co 15 8 14 17 6 17 6 4 16 13 17 21 <8 17 21 29 15
Data in less than or equal to three significant figures.
1Formations are designated by ages of sedimentary deposition; Ordovician Quimby Formation, Sm, Silurian Madrid Formation; Dc, Devonian Carrabassett
Formation.
2Where values are given as <, this means that the concentration is below the detection limit for that element as indicated.
∗Total Fe as FeO.
799
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
Table 2: Whole-rock and component compositions (XRF): stromatic migmatite
Station: 95-60 95-70 95-72 95-102 95-127 95-148 95-151 95-152 95-208 95-103 96-286
(calc- WR1host1melano1leuco1WR1host1leuco1
psammite)
wt %
SiO251·743·965·956·653·154·646·455·652·055·455·954·668·064·960·874·0
TiO21·33 1·32 1·03 0·99 1·29 1·03 1·28 1·01 1·12 1·02 1·14 1·24 0·51 0·87 0·86 0·17
Al2O318·826·115·722·423·620·423·822·121·817·816·715·017·115·719·315·3
FeO∗9·50 9·87 6·56 6·20 6·96 9·19 9·55 8·81 9·38 11·512·314·63·86 6·51 5·91 1·16
MnO 0·07 0·10 0·17 0·05 0·14 0·16 0·13 0·17 0·16 0·08 0·10 0·10 0·04 0·15 0·11 0·04
MgO 3·74 3·71 2·55 2·18 2·66 3·62 3·61 2·87 3·68 2·63 3·00 3·29 1·16 2·57 2·42 0·38
CaO 2·12 0·20 0·36 0·05 0·34 0·31 0·67 0·24 0·28 0·51 0·09 0·05 0·95 0·90 0·25 1·29
Na2O2·23 0·70 1·12 0·80 1·03 1·02 1·95 0·82 0·96 1·36 0·47 0·42 2·76 1·97 1·44 3·38
K2O5·35 8·09 3·62 5·46 5·23 5·18 7·40 5·22 4·91 4·59 4·29 4·45 3·33 3·93 5·84 2·45
P2O50·16 0·08 0·10 0·04 0·06 0·08 0·06 0·11 0·10 0·06 0·04 0·07 0·08 0·12 0·04 0·06
LOI 4·60 5·12 3·02 4·92 5·55 4·03 4·88 3·28 4·84 4·95 5·77 6·07 2·59 2·30 2·99 1·19
Total 99·699·2 100·099·7 100·099·699·7 100·099·299·999·899·8 100·099·9 100·099·5
ppm
Sn 9 <525 6 7<5 67<5 8 8<5 5 <6<6 6
Nb 18816162118 191719 21253010 1514 4
Zr 267 109 180 145 204 156 228 152 172 186 231 276 82 155 143 56
Y 34 20 16 9 24 25 29 31 31 39 34 87 23 28 24 8
Sr 232 169 116 77 148 51 153 124 49 59 18 14 128 134 58 161
Rb 160 94 173 190 189 225 307 225 189 188 147 197 105 199 277 82
Pb 61 19 15 23 28 15 23 18 19 24 12 9 31 14 18 23
Ga 22 24 19 29 34 31 36 28 32 28 29 26 19 22 24 13
Zn 154 122 101 98 156 153 174 89 144 135 145 176 54 93 96 23
Ni 54 119 6 17 24 46 27 29 55 32 46 79 15 18 23 4
V 142 181 124 178 219 185 204 171 192 156 161 151 74 149 163 6
Cr 93 285 70 97 113 101 102 103 105 105 108 93 32 83 61 <27
Ba 1280 651 399 622 484 420 1600 667 407 218 202 180 217 295 618 582
Co 34 65 10 10 21 28 15 18 31 15 28 32 10 15 18 <5
A/CNK31·77 1·47
A/NK 2·10 1·87
TZr (°C)4—729
1WR, whole rock; host, melt-depleted host (a.k.a. mesosome or paleosome), melano, melanosome; leuco, leucosome.
2Where values are given as <, this means that the concentration is below the detection limit for that element as indicated.
3Corrected for apatite.
4Model calculated zircon saturation temperature in a granitic melt (from Watson & Harrison, 1983); where no temperature is given for an appropriate specimen,
the specimen has Zr in excess of saturation, and no TZr can be calculated.
∗Total Fe as FeO.
800
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
Table 3: Whole-rock and component compositions (XRF): heterogeneous migmatite
Diatexite Vein migmatite
Station: 95-30 95-53 95-100 95-179 96-33 96-52 96-117 96-130 96-157 95-35 95-49 96-65 96-153
(calc- WR1leuco1diatexite biotite–garnet schist2host1leuco1
psammite) a b c
wt %
SiO259·656·062·651·858·663·363·255·154·262·775·761·742·639·345·241·392·452·5
TiO21·09 1·13 0·95 1·15 1·03 1·03 0·89 1·02 1·03 0·92 0·14 0·89 2·05 2·71 1·84 1·55 0·02 0·89
Al2O318·922·316·622·218·015·720·220·022·617·615·117·720·318·820·528·24·57 24·3
FeO∗8·15 5·68 7·48 9·26 8·47 7·80 6·31 8·28 8·12 7·04 0·75 6·79 15·118·313·112·10·30 6·36
MnO 0·20 0·08 0·31 0·26 0·26 0·26 0·07 0·15 0·23 0·24 0·02 0·16 0·55 0·62 0·46 0·31 0·03 0·09
MgO 3·30 2·63 2·76 3·58 3·03 2·67 2·47 3·20 2·92 2·74 0·41 2·71 5·85 7·01 5·30 4·95 0·05 2·91
CaO 0·44 0·01 0·65 0·13 0·68 0·41 0·45 1·03 0·18 0·33 0·86 0·82 1·45 0·70 2·11 0·35 0·45 0·92
Na2O1·41 0·48 1·29 0·77 1·27 1·27 1·16 4·59 0·81 1·08 2·39 1·76 2·24 1·26 3·04 1·22 0·67 2·43
K2O4·64 7·46 4·39 6·97 5·36 5·16 3·53 3·99 6·22 4·68 2·93 4·24 6·51 7·52 5·57 6·64 0·79 5·94
P2O50·11 0·04 0·17 0·09 0·34 0·11 0·08 0·10 0·09 0·10 0·26 0·18 0·04 0·01 0·04 0·04 0·18 0·14
LOI 2·52 3·92 2·45 3·67 2·74 2·49 1·85 2·50 3·48 2·58 1·56 2·72 2·82 3·33 2·41 3·30 0·47 3·07
Total 100·0 100·099·799·999·8 100·0 100·099·999·9 100·0 100·099·799·499·699·5 100·099·999·6
ppm
Sn 7864877410 7 6<6
36<6<6 64 7
Nb 18 24 17 23 20 18 16 16 17 18 3 18 25 32 22 31 <3 35
Zr 220 198 196 178 191 242 142 149 162 116 14 186 437 529 307 264 6 279
Y 272926353529262428 22 11 18518035 47740
Sr 75 30 85 79 79 56 83 139 66 86 190 98 127 58 166 125 90 97
Rb 204 274 188 257 282 319 145 203 256 206 69 194 343 406 303 286 14 233
Pb 17 18 19 25 21 19 13 48 27 26 37 26 24 17 26 27 8 38
Ga 27 26 22 34 27 23 26 23 32 26 9 26 37 35 40 45 4 35
Zn 138 104 115 125 147 134 94 124 122 119 8 100 245 307 208 201 <8 116
Ni 55 6 35 49 48 46 23 45 51 40 <3 35 107 145 88 80 <3 26
V 162 186 130 202 151 136 143 151 177 136 16 147 329 427 296 237 4 94
Cr 88 101 69 126 78 75 84 95 114 72 <15 90 169 214 159 135 <15 40
Ba 435 680 462 591 311 208 612 297 753 418 1010 437 580 611 630 1010 526 839
Co 30 7 22 26 27 28 14 23 30 20 4 17 51 64 45 45 <4 17
A/CNK41·84 1·92
A/NK 2·09 2·29
TZr (°C)5——
1WR, whole rock; leuco, leucosome; host, melt-depleted host.
2Block of biotite–garnet schist in granite cylinder (Fig. 5c).
3Where values are given as <, this means that the concentration is below the detection limit for that element as indicated.
4Corrected for apatite.
5Model calculated zircon saturation temperature in a granitic melt (from Watson & Harrison, 1983); where no temperature is given for an appropriate specimen,
the specimen has Zr in excess of saturation, and no TZr can be calculated.
∗Total Fe as FeO.
801
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
Table 4: Whole-rock compositions (XRF): granite
Schlieric granite1Sheets in stromatic migmatite Cylinders in diatexite Phillips pluton2
leucogranite
Station: 95-215 95-53 95-72 95-103 95-121 95-49 96-11 96-126 96-108
mean 1
WR3shlieren leuco3
wt %
SiO269·659·674·667·571·076·073·668·868·671·672·474·01·08
TiO20·52 1·15 0·14 0·57 0·31 0·11 0·12 0·43 0·46 0·29 0·31 0·09 0·03
Al2O316·517·615·116·215·214·814·716·116·015·215·214·70·63
FeO∗3·86 9·19 1·29 3·55 2·44 0·72 0·70 2·81 2·94 1·99 2·01 0·78 0·18
MnO 0·21 0·38 0·06 0·06 0·05 0·01 0·03 0·05 0·07 0·04 0·05 0·03 0·01
MgO 1·33 3·34 0·37 1·44 0·82 0·27 0·16 1·46 0·98 0·60 0·65 0·18 0·06
CaO 0·64 0·14 2·07 2·37 1·72 1·36 0·57 2·64 3·11 2·50 2·47 0·65 0·11
Na2O1·50 0·51 4·51 4·65 3·69 2·93 3·68 4·49 4·07 4·33 4·85 3·53 0·42
K2O3·52 5·01 1·04 1·90 3·03 2·46 4·99 2·05 2·21 2·55 1·58 4·83 0·46
P2O50·09 0·06 0·10 0·18 0·26 0·10 0·29 0·17 0·13 0·13 0·09 0·24 0·06
LOI 2·09 2·81 0·72 1·31 1·15 1·22 0·82 0·82 0·79 0·42 0·55 0·72 0·11
Total 99·999·8 100·099·799·799·999·699·899·499·7 100·099·80·19
ppm
Sn 453 48615 4<5
45 3 10 2
Nb 13 21 3 9 10 3 18 9 7 10 7 11 3
Zr 120 245 45 211 89 70 52 187 265 206 144 49 11
Y 24 42 9 11 14 10 15 8 12 10 11 8 2
Sr 92 20 314 211 139 207 55 211 223 515 129 66 9
Rb 128 244 33 122 137 62 244 101 104 86 85 241 28
Pb 17 744 23284232 2313 3121 416
Ga 22 29 12 19 17 11 20 17 16 16 17 17 1
Zn 52 152 12 65 49 13 31 51 55 38 48 33 11
Ni 16 53 4 10 7 <3 <6 13 <6 <3 <6 <6
V 82 202 16 54 21 8 <5 42 29 12 24 <6
Cr 48 112 24 31 33 <15 18 35 <20 <28 <20 <28
Ba 337 427 108 213 184 601 159 239 367 1030 158 157 33
Co 11 36 3 11 4 6 <4 11 9 <5 <6 <6
A/CNK52·28 1·25 1·19 1·26 1·52 1·24 1·15 1·11 1·08 1·09 1·26 0·07
A/NK 2·62 1·77 1·67 1·63 1·97 1·28 1·68 1·76 1·54 1·57 1·33 0·07
TZr (°C)6700 813 749 750 707 800 830 807 778 712 4
1See Fig. 5a.
2Ten data from Pressley & Brown (1999); TZr calculated using data from the seven garnet-free leucogranite specimens.
3WR, whole rock; leuco, leucosome.
4Where values are given as <, this means that the concentration is below the detection limit for that element as indicated.
5Corrected for apatite.
6Model calculated zircon saturation temperature in a granitic melt (from Watson & Harrison, 1983); where no temperature is given for an appropriate specimen,
the specimen has Zr in excess of saturation, and no TZr can be calculated.
∗Total Fe as FeO.
802
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
Table 5: Whole-rock and component compositions (ICP-MS): metasedimentary rocks, stromatic migmatite and diatexite
Metased. rocks Stromatic migmatite Diatexite
Station: 95-173 95-97 95-72 95-148 95-152 95-103 96-286 95-30 95-53 95-35 95-49
Sr Sp
WR host leuco WR WR1host leuco WR leuco diat. biotite–garnet schist
abc
ppm
Ba 612 514 425 470 665 254 250 258 295 296 649 648 491 791 449 1100 420 600 638 712
Th 10·411·015·817·815·813·918·86·60 13·313·315·29·84 17·318·315·91·66 13·626·53·70 40·6
Nb 14·516·318·019·619·422·527·510·515·916·516·44·58 20·626·119·62·99 20·326·634·024·8
Y16·531·421·833·141·737·941·426·035·837·528·410·039·543·631·210·925·371·1 100 52·1
Hf 4·42 4·42 5·18 4·66 4·50 5·34 6·55 2·44 4·41 4·38 4·17 1·61 6·49 5·62 5·01 0·46 5·73 14·216·39·51
Ta 1·03 2·51 1·19 1·29 1·39 1·68 2·09 0·74 1·00 0·98 1·15 0·45 1·36 1·55 1·32 0·22 1·55 1·70 2·21 1·36
U2·07 2·39 3·15 3·03 3·59 3·66 4·70 2·32 3·10 3·18 3·80 1·87 4·22 3·78 4·21 0·78 3·01 5·36 3·17 5·42
Pb 21·019·818·118·221·119·011·029·518·318·315·724·620·914·226·639·625·618·310·724·7
Rb 147 178 174 225 226 166 146 105 208 209 286 85·9 211 270 211 72·1 210 334 402 303
Cs 14·652·314·69·58 22·57·63 6·11 2·74 8·89 9·08 15·52·20 11·815·211·61·71 9·58 16·920·318·7
Sr 123 76·0 121 53·0 129 63·024·0 130 136 135 63·0 171 82·032·094·0 196 98·0 135 60·0 176
Sc 23·318·220·928·230·519·620·013·619·321·424·04·10 25·833·320·92·80 23·352·459·247·5
La 23·236·645·053·647·846·865·019·645·245·746·620·554·558·948·96·14 46·371·211·4 109
Ce 43·872·786·9 103 92·692·3 126 39·287·987·891·739·3 105 110 95·212·287·0 137 21·1 207
Pr 4·81 8·23 9·76 11·310·410·914·54·57 9·96 9·91 10·43·98 11·612·710·51·45 9·60 15·02·35 23·0
Nd 18·032·737·144·640·643·256·917·739·339·239·314·645·849·740·45·94 38·557·89·05 89·0
Sm 3·86 7·47 7·76 9·50 9·07 9·61 12·34·08 9·23 8·99 7·01 3·02 9·51 10·48·85 1·54 8·44 11·32·13 17·3
Eu 1·02 1·51 1·48 1·37 1·83 1·29 1·29 1·08 1·23 1·37 1·19 1·04 1·65 1·33 1·55 1·63 1·46 1·94 0·67 2·44
Gd 3·24 6·71 6·68 7·57 7·73 7·91 9·52 3·68 7·61 7·81 5·31 2·45 8·11 8·75 7·26 1·99 6·81 9·55 4·36 13·2
Tb 0·52 1·07 0·95 1·14 1·26 1·34 1·59 0·73 1·26 1·21 0·91 0·37 1·28 1·39 1·14 0·37 1·00 1·74 1·38 1·91
Dy 3·11 6·26 4·86 6·41 7·67 7·92 8·97 4·86 7·31 7·07 5·29 2·02 7·46 8·00 6·21 2·28 5·23 11·713·610·1
Ho 0·62 1·22 0·87 1·24 1·57 1·56 1·69 1·02 1·40 1·35 1·08 0·36 1·44 1·55 1·17 0·42 0·95 2·62 3·65 1·93
Er 1·75 3·21 2·27 3·24 4·22 4·09 4·35 2·92 3·55 3·49 3·05 0·87 3·78 3·98 3·05 1·02 2·32 7·90 11·84·93
Tm 0·29 0·47 0·37 0·49 0·62 0·59 0·62 0·42 0·50 0·49 0·47 0·13 0·56 0·56 0·47 0·14 0·35 1·22 1·70 0·76
Yb 1·97 2·89 2·50 3·00 3·90 3·51 3·72 2·61 2·89 2·81 3·03 0·77 3·45 3·31 2·91 0·73 2·16 8·00 10·64·77
Lu 0·34 0·44 0·43 0·47 0·60 0·53 0·56 0·40 0·43 0·43 0·48 0·12 0·54 0·51 0·45 0·10 0·34 1·27 1·58 0·75
LaN/LuN7·18·61112 8·29·112 5·111111018 1012 11 6·214 5·80·75 15
LaN/SmN3·13·83·63·63·33·13·33·03·13·24·24·33·63·63·52·53·54·03·44·0
GdN/LuN1·21·91·92·01·61·92·11·12·22·31·42·61·91·92·02·42·50·93 0·34 2·2
Eu/Eu∗0·88 0·65 0·63 0·49 0·67 0·45 0·36 0·85 0·45 0·50 0·60 1·20·57 0·42 0·59 2·80·59 0·57 0·67 0·49
TMz (°C)2988 981 868
Data are in less than or equal to three significant figures; Sr, Rangeley Formation; Sp, Perry Mountain Formation; WR, whole rock; host, melt-depleted host (a.k.a.
mesosome or paleosome); leuco, leucosome; diat., diatexite.
1Repeat fusion.
2Model calculated monazite saturation temperature in a peraluminous granitic melt (from Montel, 1993).
803
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
Table 6: Whole-rock and component compositions (ICP-MS): vein migmatite, schlieric granite and granite
Vein migmatite Schlieric granite Granite in stromatic migmatite Granite in diatexite Phillips pluton
leucogranite2
Station: 96-65 95-215 95-53 95-72 95-103 95-121 95-49 96-11 96-108 96-126
mean 1
host leuco WR WR1melano leuco leuco1
ppm
Ba 1040 560 356 344 401 106 111 233 175 613 160 256 386 159 921
Th 25·50·18 9·68 9·74 23·02·97 2·81 7·43 6·48 8·14 4·01 11·813·511·419·4
Nb 30·00·51 12·612·323·23·45 3·41 9·94 10·73·14 19·68·52 7·97 8·86 9·60
Y48·28·19 32·535·856·812·612·811·215·111·116·79·32 13·412·511·4
Hf 7·87 0·20 3·32 3·36 7·37 1·27 1·34 5·31 2·53 2·10 1·92 5·08 6·08 4·25 4·95
Ta 2·27 0·08 1·10 1·08 1·91 0·35 0·35 0·86 0·85 0·41 2·65 0·76 0·80 0·99 0·90
U4·89 0·27 3·03 3·21 6·17 1·03 1·05 1·78 2·43 2·80 10·51·96 4·12 3·04 2·39
Pb 26·79·34 18·818·711·439·439·924·828·140·532·624·614·621·829·4
Rb 252 16·1 133 131 248 36·435·7 127 139 65·8 247 115 107 91·588·2
Cs 16·20·27 5·05 4·96 12·41·58 1·64 9·95 11·92·10 12·66·29 4·95 3·54 2·44
Sr 126 93·097·095·022·0 306 306 214 145 216 59·0 216 229 134 522
Sc 31·22·30 20·720·227·86·00 5·40 10·28·60 3·10 3·10 5·40 5·90 6·00 4·40
La 52·91·46 26·427·266·011·611·819·518·122·18·33 27·334·323·358·4—
Ce 108 3·67 52·052·7 127 21·521·730·435·842·218·347·570·040·695·721·75·5
Pr 13·60·53 5·85 5·79 14·52·34 2·31 3·34 3·83 4·51 1·99 5·09 5·77 4·27 9·32 —
Nd 54·02·61 22·922·556·58·79 8·77 12·314·617·17·69 19·020·416·331·59·06 2·6
Sm 11·91·09 5·12 5·03 12·71·90 1·94 2·36 3·74 3·79 2·68 3·60 3·83 3·60 4·45 2·64 0·78
Eu 2·16 0·59 1·30 1·28 1·44 2·56 2·56 1·02 0·94 1·81 0·35 1·10 1·16 0·86 0·97 0·50 0·07
Gd 9·87 1·50 4·69 4·78 11·01·73 1·70 2·03 3·34 2·84 3·12 2·50 2·86 3·04 2·88 2·63 0·61
Tb 1·70 0·31 0·84 0·87 1·78 0·31 0·32 0·33 0·57 0·41 0·64 0·36 0·42 0·44 0·39 —
Dy 10·31·86 5·51 5·86 10·12·05 2·09 2·06 3·05 2·18 3·44 1·86 2·43 2·34 2·06 1·81 0·38
Ho 2·10 0·30 1·22 1·33 2·13 0·44 0·46 0·41 0·52 0·39 0·50 0·33 0·46 0·43 0·37 —
Er 5·85 0·70 3·68 4·04 6·08 1·31 1·32 1·10 1·19 0·94 1·10 0·83 1·22 1·15 0·89 0·76 0·10
Tm 0·87 0·09 0·57 0·62 0·91 0·19 0·20 0·16 0·16 0·14 0·14 0·12 0·19 0·19 0·14 —
Yb 5·34 0·63 3·64 3·94 5·73 1·22 1·22 0·97 0·92 0·82 0·78 0·78 1·23 1·35 0·90 0·66 0·10
Lu 0·83 0·11 0·58 0·62 0·87 0·18 0·19 0·16 0·13 0·12 0·10 0·13 0·21 0·24 0·15 —
LaN/LuN6·61·44·74·67·96·56·4 131418 8·421171041—
LaN/SmN2·80·83·23·43·33·83·85·23·03·72·04·85·64·18·3—
GdN/LuN1·51·71·00·96 1·61·21·11·63·22·83·82·31·71·62·4—
Eu/Eu∗0·61 1·40·81 0·80 0·37 4·34·31·40·81 1·70·37 1·11·10·80 0·83 —
CeN/YbN13 5·0
GdN/YbN3·61·1
TMz (°C)3777 1020 917 958 973 992 902 1010 1040 988 1090 889 8
WR, whole rock; host, melt-depleted host; melano, melanosome; leuco, leucosome; REE ratios are calculated upon chondrite-normalized data.
1Repeat fusion.
2Ten data from Pressley & Brown (1999; TIMS, isotope dilution); TMz calculated using data from the seven garnet-free leucogranite specimens.
3Model calculated monazite saturation temperature in a peraluminous granitic melt (from Montel, 1993).
804
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
Fig. 6. Major and minor oxide compositions of CMB metasedimentary rocks and migmatites. Also plotted are migmatite components and
granite (XRF; Tables 1–4). Compositions of major minerals found in all rock types in the study area are based on analyses reported by Deer et
al. (1997). Mineral symbols are from Kretz (1983). Total Fe is treated as FeO, and reported as FeO∗. Lines connect migmatite components
(leucosome, melanosome and melt-depleted host) and their whole-rock compositions. MBS is ‘muscovite–biotite schist’from Patin
˜o Douce &
Harris (1998). Specimens 95-35 and 95-103 are indicated for reference to the use of these in REE modeling (see text and Fig. 13).
K
2
O (Fig. 6). With increasing SiO
2
, (Na
2
O+CaO) similar to those of the metasedimentary rocks (see Figs
8 and 9; Table 5). All patterns have negative Eu anomalies
does not change systematically. The average Rb and V
(Table 5) similar to metasedimentary rocks; the overall
contents are greater than those of metasedimentary rocks,
shape and steepness of these patterns are similar to
and both decrease with increasing SiO
2
(Fig. 7). Sr, Zr
those of the North American shale composite (Taylor &
and Ba contents are lower, whereas Rb/Sr ratios are
McLennan, 1985).
higher than those of metasedimentary rocks; these data
do not vary systematically with increasing SiO
2
(Fig. 7).
Stromatic migmatite compositions lie between projected
compositions of biotite, garnet and sillimanite, and albite-
Diatexite
rich plagioclase and quartz.
Chondrite-normalized REE patterns of stromatic mig- Compared with metasedimentary rocks and stromatic
matite have slight negative Nd (Fig. 9), otherwise the migmatite, the diatexite sample has a smaller range of
patterns are smooth and straight to concave-upward, SiO
2
,Al
2
O
3
and (FeO∗+MgO +TiO
2
) (Table 3; Fig.
6), it is poorer in CaO, and it displays similar Na
2
O,with REE compositions and overall patterns that are
805
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
Fig. 7. Trace element and trace element ratio variations as a function of wt % SiO
2
for CMB metasedimentary rocks and migmatites. Also
plotted are migmatite components and granite (XRF; Tables 1–4). Symbols and fields as listed in Fig. 6.
which increases with increasing SiO
2
.K
2
O contents tend diatexite patterns have negative Eu anomalies (Table 5)
similar to the metasedimentary rocks; the overall shape
to be greater than in metasedimentary rocks, especially
and steepness of these patterns are similar to those of the
at low SiO
2
contents; K
2
O decreases systematically with
North American shale composite (Taylor & McLennan,
increasing SiO
2
, and more steeply than for the stromatic
1985).
migmatite sample (Fig. 6). K
2
O, Rb and Rb/Sr ratios
are higher than those of stromatic migmatite, and Sr
contents are lower than those of both metasedimentary
rocks and stromatic migmatite (Fig. 7).
Granite
Chondrite-normalized REE patterns of diatexite (Fig.
9) are smooth, straight to concave-upward with negative All leucogranites from within the migmatites are per-
Nd anomalies, and are similar in slope to those of aluminous (A/CNK =1·1–1·5; Table 4), but in detail
the metasedimentary rocks and fall within the range of granite in sheets in the stromatic migmatite (within AFZs)
is more peraluminous (A/CNK =1·2–1·5) than granitemetasedimentary rocks (see Figs 8 and 9; Table 5). All
806
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
in cylinders in diatexite (within ACZs; A/CNK =1·1–Although Eu anomalies are small and unpronounced,
1·2). All granites display a limited variation in SiO
2
and unlike the granite in sheets, two of the granite patterns
Al
2
O
3
, and low (FeO∗+MgO +TiO
2
) that decreases have the same positive Eu anomalies, whereas the other
with increasing SiO
2
(Fig. 6). With increasing SiO
2
,two have similar negative Eu anomalies [Eu/Eu∗=1·1
(Na
2
O+CaO) decreases whereas K
2
O increases (Fig. (two specimens), 0·80 and 0·83]. The overall shape and
6). There is a systematic difference between granites steepness of these patterns are similar to those of the
found in stromatic migmatite (in AFZs) and those in North American shale composite (Taylor & McLennan,
heterogeneous migmatite (in ACZs) with respect to their 1985).
trace element compositions (Tables 4 and 6; Figs 7 and
9). Relative to granite in the diatexite domain, granite
in the stromatic migmatite commonly has lower Sr, Zr, Other rocks and migmatite components
Ba, Th, Hf and light REE (LREE), and higher Rb. For
The schlieric granite specimen (95-215, Fig. 5a) con-
granite 95-121 (from the west margin of the TAD, Fig.
sistently plots between the migmatites and the granites
2), Zr, Sr, Sr/Ba, LREE, middle REE (MREE), Th and
(Figs 6 and 7). Three specimens from a block of biotite–
Hf are lower, and Rb, V, Ta, Y, Nb and Rb/Sr are
garnet schist from one cylindrical body of granite (outcrop
higher than the other three granite specimens from the
95-49; Figs 4d and 5c) are poor in SiO
2
, and rich in
stromatic migmatite, and are similar to the average of
Al
2
O
3
, (FeO∗+MgO +TiO
2
) and K
2
O (Table 3),
the common leucogranite from the Phillips pluton ( Tables
and plot between the field of CMB metasedimentary
4 and 6; Figs 7 and 9).
rocks and these two minerals (Figs 6 and 7). The two
The two types of granite, sheets in stromatic migmatite
specimens of melt-depleted host from stromatic migmatite
(in AFZs) and cylinders in heterogeneous migmatite (in
plot within the range of SiO
2
for the whole-rock stromatic
ACZs), show differences in LREE concentrations and
migmatite with similar K
2
O, Rb and V, but at higher
chondrite-normalized REE patterns (Fig. 9). Granite
(FeO∗+MgO +TiO
2
) and Zr, and lower Al
2
O
3
,
from sheets has lower REE concentrations relative to
(Na
2
O+CaO), Na
2
O, Sr and Ba (Figs 6 and 7). These
metasedimentary rocks, and has steep La
N
/Lu
N
ratios
rocks have the highest Rb/Sr ratios of the suite (Fig.
(Table 6), but differently shaped REE patterns. Like
7). The melt-depleted host from vein migmatite 96-65
metasedimentary rocks, REE patterns of granite in the
commonly plots near the specimens from the block of
sheets have negative Nd anomalies; however, the patterns
biotite–garnet schist, with the exception of Al
2
O
3
and Ba
are not as smooth, and have inflection in the HREE
(higher), and V (lower).
part. Otherwise, these patterns are straight to concave-
Leucosomes from both types of migmatite are more
upward in the HREE. The LREE patterns are similar,
peraluminous than the granites (A/CNK =1·5–1·9).
with the exception of 95-121, which is lower in con-
Leucosomes of the stromatic migmatite and diatexite 95-
centration and has shallower slope (La
N
/Sm
N
=3·0–5·2;
35 resemble the granite sheets of the stromatic migmatite
2·0 for 95-121). The HREE patterns of the granite in
in respect of Zr, Sr, Th, Hf and REE, but resemble the
the sheets are similar in concentration and slope (Table
granite cylinders of the diatexite with respect to Rb.
6; Fig. 9), and this slope is similar relative to HREE of
Most leucosomes do not plot near the field of common
the metasedimentary rocks. Two of these granite REE
leucogranite from the Phillips pluton, except in SiO
2
vs
patterns have negative Eu anomalies [Eu/Eu∗=0·37
Al
2
O
3
, (Na
2
O+CaO), Zr and Ba (Figs 6 and 7). The
(95-121) and 0·81] and two have positive Eu anomalies
leucosome of schlieric granite 95-215 is significantly
(Eu/Eu∗=1·4 and 1·7). The overall shape and steep-
different from the other leucosome separates, with the
ness of these patterns are similar to those of the North
highest Na
2
O/K
2
O ratio and Sr contents, and the lowest
American shale composite (Taylor & McLennan,
Ba contents of the suite (Table 4). This leucosome also
1985).
has a different composition from the average of the
Granite from cylinders has similar REE concentrations
common leucogranite of the Phillips pluton. These ob-
and steep chondrite-normalized REE patterns (Table 6;
servations suggest that the different leucosomes and gran-
Fig. 9). Like metasedimentary rocks, REE patterns of the
ites are not related by simple melt segregation and
granite in the cylinders have negative Nd anomalies;
accumulation. Assuming all of these rocks were derived
however, the patterns are not as smooth, but have no
from a similar source, other processes must account for
inflection in the HREE part in the manner of granite in
these differences.
the sheets. The patterns are straight to concave-upward
Relative to metasedimentary rocks, melt-depleted
in the HREE. In comparison with patterns of the CMB
host and two of the three specimens of the biotite–garnet
metasedimentary rocks and granite in sheets (in AFZs),
schist (a and c) have higher REE concentrations, but
the LREE patterns of the granite in the cylinders (in
similarly shaped chondrite-normalized REE patterns
ACZs) have steeper slopes, but the HREE patterns are
similar in concentration and slope (Table 6; Fig. 9). ( Figs 8 and 9). Specimen b is low in LREE and
807
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
Fig. 8. Chondrite-normalized REE patterns of metasedimentary rocks and migmatite components (ICP-MS; Tables 5 and 6 except for the
majority of metasedimentary rocks). Data from Cullers et al. (1974, 1997) were obtained by radiochemical neutron activation analysis (RNAA),
with the exception of Nd [isotope dilution, thermal ionization mass spectrometry (TIMS)], and are here renormalized to chondrites. Data for
the Carrabassett Formation were obtained by isotope dilution and TIMS (from Pressley & Brown, 1999), and are here renormalized to chondrites.
Normalization is to data of Evansen et al. (1978; as reported by Taylor & McLennan, 1985). RNAA analyses of metasedimentary rocks of the
Perry Mountain Formation from Cullers et al. (1974, 1997) that were determined to have undergone diagenetic removal of the LREE (Cullers
et al., 1997) are excluded.
highest in HREE concentrations consistent with the overall shape and steepness of these patterns are similar
to those of the North American shale composite (Taylorhigher garnet content in that part of the block (e.g.
Hanson, 1980; see Fig. 5c for the centimeter-scale & McLennan, 1985).
Relative to metasedimentary rocks, leucosomes arevariation in the block). Like metasedimentary rocks,
REE patterns for the biotite–garnet schist have negative lower in REE concentrations, and have a larger range
of slopes in their chondrite-normalized REE patternsNd anomalies; otherwise the patterns are smooth, and
are straight to concave-upward (Fig. 9). All LREE (Tables 5 and 6; Figs 8 and 9), owing to fanning of
the LREE patterns ( Fig. 9). The HREE patterns ofpatterns are similar (Figs 8 and 9), but although most
HREE patterns of melt-depleted host and the schist leucosomes are similar in slope, except leucosome 95-
103 (Gd
N
/Lu
N
=12–2·4; 1·1 for 95-103), but shallowerare similar in concentration and slope, schist specimen
bisdifferent (Gd
N
/Lu
N
=0·9–2·2; 0·3 for b), and is relative to HREE of the metasedimentary rocks. Also,
with the exception of 95-103, which has a negative Eushallower relative to HREE of the metasedimentary
rocks. All of these patterns have negative Eu anomalies, anomaly ( Table 5; Fig. 9), all of these leucosome REE
patterns have variously positive Eu anomalies).similar to metasedimentary rocks and migmatites. The
808
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
Fig. 9. Chondrite-normalized REE patterns of migmatites, granites, schlieric granite and leucogranite from the Phillips pluton (ICP-MS; Tables
5 and 6). Data plotted for the Phillips pluton rocks, and for schlieric granite P-28, were obtained by isotope dilution and TIMS (Pressley &
Brown, 1999). Patterns for 95-35, 95-49, 95-103 and 95-121 are indicated for reference to text and Figs 6, 7, 10 and 13.
University of Maryland. Although there is variation in
BIOTITE COMPOSITION X
Mg
[where X
Mg
=Mg/(Mg +Fe) on a molar basis]
Biotite in smaller-volume granites and migmatite and Ti in atoms per formula unit (a.p.f.u.; based on
leucosomes varies from aggregates of grains forming 22 oxygens) between specimens, within each specimen
schlieren and individual or small clusters of irregularly compositional variation is insignificant (at 2based on
shaped grains, interpreted possibly to be residual, to counting statistics). Granite in sheets has biotite with X
Mg
smaller more bladed euhedral grains, interpreted to be of 0·36–0·37 and 0·52, and Ti a.p.f.u. of 0·22–0·26
magmatic. The composition of each type of biotite has and 0·19–0·20, respectively, in two specimens, whereas
been determined at multiple sites in several grains in granite in cylinders has biotite with X
Mg
of 0·48–0·49 and
each of one stromatic migmatite leucosome (95-103), one 0·40–0·39, and Ti a.p.f.u. of 0·26–0·32 and 0·33–0·35,
diatexite leucosome (95-100), two specimens of granite respectively, in two specimens, regardless of size, shape
from cylinders in diatexite (95-49 and 96-11) and two or position in the rock texture. Leucosome in stromatic
specimens of granite from sheets in stromatic migmatite migmatite and diatexite has biotite with X
Mg
of 0·49 and
(95-72 and 95-103) using a JEOL 8900R electron micro- 0·44, and Ti a.p.f.u. of 0·23–0·22 and 0·15, respectively,
regardless of size, shape or position in the rock texture.probe analyzer in wavelength-dispersive mode at the
809
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
There are three options for the origin of the granites
DISCUSSION and leucosomes in the Maine migmatites:
The central question is whether leucosomes and granites (1) these rocks could represent equilibrium melts (res-
in migmatite are related to granite in the nearby coeval idue equilibrates with liquid before melt segregation), in
plutons. Melt loss from migmatites is implied by the K- which case the range in composition observed is con-
feldspar-poor nature of the leucosomes. This is supported trolled by P,T,aH
2
O, protolith composition and the
by the lack of mass balance of trace elements between degree of melting, the amount of entrained residual
migmatite components (Tables 2 and 4, stromatic mig- material and any fractionation during crystallization.
matite and diatexite, respectively), suggesting open-sys- (2) The rocks could represent disequilibrium melts
tem behavior at the scale of hand specimens. The ‘pinch- (residue does not equilibrate with liquid before melt
and-swell’structure of granite sheets as described above segregation) if melting is rapid and deformation allows
is consistent with melt flow during deformation and weak fast segregation to preserve disequilibrium compositions
strain during or after emplacement of these sheets. In (Ayers & Harris, 1997). Rapid melting and segregation
addition, the strong correlation of regional fabrics across have been argued for muscovite dehydration melting
the migmatite front in the same structural zone supports (Rubie & Brearley, 1987; Brearley & Rubie, 1990), and
the interpretation that migmatization occurred while the rapid melting may occur in response to fluid ingress.
rocks were accommodating strain. Thus, we postulate Although compositions may have been modified by frac-
that leucosomes and smaller-volume granites record evi- tionation during crystallization, fast segregation as ex-
dence of syntectonic melt flow within and through the pected during deformation may limit the potential for
migmatite, and that granite in plutons apparently outside entrainment of residual material.
the migmatites at the level exposed represents evolved (3) These rocks could be the result of crystallization
melt that escaped syntectonically from a similar source of mixtures of liquid and crystals, where the crystals may
to the migmatites exposed in the TAD and WAD. be residual or cumulate, or both, and where some or
most of the evolved melt may have escaped from the
crystallization site, aided by and/or enhancing the de-
Origin of leucosomes and granites formation.
We can constrain the processes involved using the ex- In reality, equilibrium is an unlikely expectation during
perimental results of Patin
˜o Douce & Harris (1998). The syntectonic melting in nature, and some degree of dis-
composition of the muscovite–biotite schist used in that equilibrium is to be expected. Thus, under conditions of
study (MBS), lies within the field of CMB meta- limited disequilibrium we may anticipate equilibrium
sedimentary rocks (Figs 6 and 10), and melt compositions between melt and newly crystallized peritectic minerals,
produced from MBS plot among the data for Maine but not necessarily between melt and residual minerals.
granites and leucosomes (Figs 6 and 10). Most of the In the next section we discuss the petrogenesis of the
experiments of Patin
˜o Douce & Harris (1998) involved migmatite leucosomes and smaller-volume granites, and
the metamorphic volatile phase absent reaction we investigate any possible relationship with the common
leucogranite of the Phillips pluton.
22Ms +7Pl +8Qtz →25 melt +5Kfs +5Sil +2Bt.
This reaction is widely considered appropriate for pro-
ducing leucogranitic melt from metasedimentary proto- Petrogenesis
liths of metapelitic composition in collisional origins (e.g. Protolith of the migmatites and chemical differentiation
Patin
˜o Douce, 1999). Some of the experiments by Patin
˜oduring migmatization
Douce & Harris (1998) were run with added H
2
O,
resulting in the reaction On the basis of the structural evolution of western Maine
(Solar & Brown, 2001), and the field relations and geo-
9Ms +15Pl +7Qtz +xH
2
O→31 melt. chemistry of the migmatites and granites described herein,
we propose a model of progressive separation of melt and
The amount of melt is limited by protolith muscovite
residue during deformation of the CMB metasedimentary
content in the first case, and plagioclase content and
succession. Below the solidus, these rocks were under-
the amount of added H
2
O in the second case. The
going syntectonic prograde metamorphism that is as-
compositions of these melts vary from granite (muscovite
sumed to have been effectively isochemical. Although
dehydration and lower-pressure water-fluxed melting)
centimeter-scale quartzo-feldspathic- and mica-rich do-
to trondhjemite (higher-pressure water-fluxed melting).
main segregations occur in the metapelite layers (Solar
Migmatite textures show that biotite was apparently
& Brown, 2001) to suggest redistribution of silica (Ague,
stable on a regional basis (as described above), indicating
1994), we have collected specimens of protolith rocks
that the biotite dehydration melting reaction was not
crossed during regional metamorphism. that have undergone this process, and a reduction in
810
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
silica by open-system behavior does not account for the the specimen is enriched in the feldspathic components
lower SiO
2
in migmatites relative to metasedimentary compared with the protolith composition. Granite speci-
rocks sample. Therefore, we interpret differences in geo- men 95-121 plots within the field of common leucogranite
chemistry as due to syntectonic melting and various from the Phillips pluton.
secondary processes during melt–residuum separation, In Fig. 10, the migmatite leucosomes are seen to define
such as melt loss or gain, residue entrainment, fractional an array of compositions from melt dominated, plotting
crystallization, and accumulation of cumulate phases and close to the MBS melt compositions in the experiments
loss of evolved melt. of Patin
˜o Douce & Harris (1998), to cumulate dominated,
The three samples of the populations of meta- plotting close to a cumulate composed of >80% pla-
sedimentary rocks, stromatic migmatite and diatexite gioclase and >20% biotite. Variable loss of a K-rich
analyzed in this study are not statistically different in liquid is implied. In contrast, the smaller-volume granites
respect of their geochemical compositions (Solar, 1999). define a triangular field between the MBS melts and the
Thus, on the basis of comparative geochemical ar- cumulate join between plagioclase and biotite, with the
guments we cannot reject the null hypothesis that leucosome array as the bottom edge and extending along
partial melting of the metasedimentary rocks was the the cumulate join from >20 to >35% biotite. The
process that formed the migmatites. On the basis of common leucogranite of the Phillips pluton and granite
the field relations and geochemistry discussed above, 95-121 crystallized from a K-enriched (evolved) liquid in
metasedimentary rocks similar in composition to those comparison with the MBS melts.
of the CMB are inferred to be the protolith for the From data in Fig. 10, we infer that none of the smaller-
TAD and WAD migmatites. volume granites has a likely melt composition, although
For (FeO∗+MgO +TiO
2
), Al
2
O
3
, (Na
2
O+CaO) 95-103 appears to have a melt-dominated composition.
and V it is apparent that the depleted nature of stromatic Most of the smaller-volume granites have a variable
migmatite and diatexite is not balanced by the smaller- cumulate composition that we presume incorporates
volume granites alone (Figs 6 and 7). As linear trends some residual biotite; a K-rich liquid has been partially
on Harker plots are a characteristic feature of restite lost from these rocks. Indeed, all analyzed granites from
unmixing (e.g. Chappell et al., 1987), these plots indicate within diatexite appear to have largely cumulate com-
that differential separation of melt from residual solid positions probably with variable amounts of included
material was not the sole petrogenetic process involved residual biotite and/or plagioclase, and only a minor
in producing the variation observed. amount of retained K-rich liquid. However, in contrast
If mass balance is preserved at all scales during melting, to some studies (Friend et al., 1985; Sawyer, 1998; Milord
melt segregation and transfer, and crystallization of the et al., 2001), texturally distinct biotite in the Maine
melt, then the processes involved may be tracked in the migmatites and smaller-volume granites we analyzed does
ternary plot K–(Fe∗+Mg +Ti)–(Na +Ca) (Fig. 10). not have significantly different composition, and we are
In such a plot, biotite lies along the edge (Fe∗+Mg unable to identify residual biotite on the basis of chemical
+Ti)–K; it represents the major residual phase. The composition.
feldspar join is represented by the edge (Na +Ca)–K, Most oxide and trace element concentrations of
close to which lie melts produced from crustal protoliths. migmatite components, except for the schlieric granite
Residual compositions will be displaced from the field of (95-215), have straight connecting lines between the
metasedimentary rocks toward (Fe∗+Mg +Ti)–K, components and their respective whole-rock com-
whereas leucosome and granite compositions will trend positions ( Figs 6 and 7). The leucosome of schlieric
toward the feldspar join. In Fig. 10, the field of CMB granite 95-215 is lower in K
2
O and Rb, and higher
metasedimentary rocks lies between muscovite and gar- in (Na
2
O+CaO), Na
2
O and Sr than other leucosomes.
net, and close to biotite. Migmatite compositions are The difference is particularly conspicuous in Fig. 10,
displaced toward the (Fe∗+Mg +Ti)–K edge in where all connecting lines between leucosomes and
comparison with the CMB metasedimentary rock field,
whole-rock compositions are subparallel except for the
whereas granites and leucosomes are weakly clustered
leucosome from the schlieric granite (95-215). We
toward the feldspar join, closer to the (Na +Ca) apex
interpret these relations to indicate that migmatites do
than the K apex. The three specimens from the block
indeed represent in situ segregation, generally with
of biotite–garnet schist plot between biotite and pla-
variable loss of a K-rich liquid from partly cumulate
gioclase, but much closer to biotite, and are displaced
leucosomes, except for the schlieric granite, which we
from the CMB metasedimentary rock field toward garnet,
suggest was formed by melt accumulation before
reflecting the dominance of these phases in the rock ( Fig.
fractional crystallization and loss of most of the K-
5d). The whole-rock chemistry of the schlieric granite
rich liquid imposed the final composition on the largely
specimen plots between the field of CMB meta-
sedimentary rocks and the feldspar join, suggesting that cumulate leucosome.
811
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
Fig. 10. Results of major element modeling of melt–residuum separation during migmatization of CMB metasedimentary rocks as explained
in the text. Mineral abbreviations are from Kretz (1983). MBS is ‘muscovite–biotite schist’starting material from Patin
˜o Douce & Harris (1998).
Dashed arrows a
1
–b
1
and a
2
–b
2
through the MBS melt compositions track the model cumulate and fractionated melt trend during 20% fractional
crystallization of plagioclase and biotite. Tick marks on the line connecting plagioclase and biotite are in 20% increments of biotite content.
Specimens 95-35, 95-97, 95-103 and 95-121 are indicated for reference in the text and figures. The model subtraction trend in the composition
of 95-97 shown as the thick line is representative of the model change in composition of the rock by removal of 20% MBS melt.
zircon in a granite melt (e.g. Rapp & Watson, 1986).
Temperature regime of melting
Monazite saturation temperatures (902–1090°C, with the
To investigate further the temperature regime during exception of the leucosome from the vein migmatite)
melting, we estimate the saturation temperature of these are much higher than zircon saturation temperatures
rocks from the solubility of zircon and monazite in melt, (700–830°C). The zircon saturation temperatures are
obtained from Zr and LREE concentrations relative to consistent with muscovite dehydration melting, whereas
the major element compositions [Tables 2–4 and 6; from the spurious high monazite saturation temperatures in-
expressions of Watson & Harrison (1983) and Montel dicate entrainment of residual monazite.
(1993), respectively]. Temperatures in the range
750–800°C would be consistent with muscovite de-
Rb/Sr ratios
hydration melting. Spurious high temperatures might
indicate entrainment of residual accessory phases, Rb/Sr ratios have been used as a discriminator of water-
whereas low calculated temperatures argue against sig- fluxed vs dehydration melting (Harris & Inger, 1992;
nificant entrainment of accessory phases. If the solubilities Harris et al., 1995), although care must be exercised as
of accessory phases are kinetically controlled, the tem- some degree of disequilibrium may be involved in the
peratures inferred from zircon solubilities should be sys- melting process, which may reduce the Rb/Sr ratio of
tematically higher than inferred from monazite the melt regardless of the aH
2
O during melting (Harris
et al., 1993). Rb/Sr ratios of granite (excepting 95-121)solubilities, as a result of the more rapid dissolution of
812
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
and leucosome specimens are in the range 0·2–1, which garnet schist in the cylindrical body of granite 95-49
(Fig. 5) suggests it is an extreme example of residue fromtaken at face value are consistent with a low Rb–Sr
fractionation during water-fluxed melting of the CMB partial melting of CMB metasedimentary rock.
The dashed lines in Fig. 10 show the trends producedmetasedimentary rocks (Rb/Sr =0·8–2·5; Harris &
Inger, 1992; Harris et al., 1995). However, the postulated by fractional crystallization of a cumulate composed of
20% biotite +80% plagioclase (a
1
) and 35% biotite +loss of a K-rich liquid from some granite and leucosomes
is likely to have lowered Rb/Sr ratios in those rocks by 65% plagioclase (a
2
), respectively, from a melt com-
position similar to the 6 kbar MBS melt of Patin
˜o Doucepreferential fractionation of Rb from Sr (e.g. Harris &
Inger, 1992). Further, the incorporation of residual biotite & Harris (1998). Three important conclusions follow
from this exercise:in most of these granites will have raised Rb relative to
Sr (e.g. Hanson, 1980). Thus, any conclusions about type (1) the migmatite leucosomes could be produced by
fractional crystallization of assemblage a
1
and variableof melting based on Rb and Sr distributions in these
smaller-volume granites are likely to be flawed. melt loss, except for leucosome 95-103, which must
include entrained residual biotite;In contrast, common leucogranite from the Phillips
pluton has Rb, Sr and Ba covariations and K
2
O contents (2) the smaller-volume granites could be produced by
fractional crystallization of an assemblage similar to a
2
,that are interpreted to reflect control by peritectic K-
feldspar in the source (Pressley & Brown, 1999). Rb/Sr with variable melt loss and entrained residual plagioclase
and/or biotite;ratios for the Phillips pluton of 1·5–8·5 (of which most
are 3–5), suggest a high Rb–Sr fractionation during (3) the common leucogranite of the Phillips pluton can
be modeled by subtraction of 0–20% cumulate biotitederivation by moderate aH
2
O muscovite dehydration
melting of the CMB metasedimentary rocks. Granite and plagioclase (with biotite pplagioclase) from a melt
similar to the MBS melt composition and crystallization ofspecimen 95-121, which plots commonly with the Phillips
pluton leucogranite, also has a large Rb/Sr ratio (4·5; the resultant evolved K-rich liquid. Because the migmatite
leucosomes and smaller-volume granites individually areFig. 7), which suggests that it too may have crystallized
from a liquid composition evolved from melt that was very much smaller in volume than the Phillips pluton,
this suggests accumulation at the site of the Phillips plutonproduced by muscovite dehydration. We suggest that
conclusions about type of melting based on Rb–Sr dis- of many small batches of evolved melt, consistent with
the interpretation of Rb–Sr–Ba and Nd isotope data bytributions in these relatively large volume granites may
be more robust than for the smaller-volume granites. Pressley & Brown (1999).
Modeling Rb–Sr–Ba relationsModeling major element compositions
Leucosomes and smaller-volume granites are enrichedFrom arguments presented above, the apparent residual
composition of the migmatites may be the result of open- in Sr and depleted in Rb relative to the metasedimentary
rocks, the migmatites and the common leucogranite ofsystem behavior whereby melt was removed. If the CMB
metasedimentary rocks are a reasonable analogue for the Phillips pluton, although Ba contents are similar,
except for the common leucogranite of the Phillips pluton,the protolith for the migmatites and granites, then the
petrogenetic relations can be investigated by modeling which has Ba contents at the low end of the range for
leucosomes and smaller-volume granites. In principle, itsubtraction of MBS melt from the representative meta-
sedimentary rock 95-97 (Fig. 10). The residuum trend should be possible to test our modeling of the major
element compositions by using the results to model theshown for 20% melt extraction from that specimen (thick
line beginning at the rock composition and projected Rb–Sr–Ba relations of each of the different groups of
rocks. There are, however, significant problems inherentaway from MBS melt) extends to the limit of data for
migmatites, closer to the (Fe∗+Mg +Ti)–K edge in modeling the large ion lithophile elements (LILE).
The first issue concerns the assumption of equilibriumthan the field of CMB metasedimentary rocks. This is
consistent with a calculated average stoichiometric melt melting, with no entrainment of residual minerals in the
melt, and with the LILE partitioned between crystalsproduction of >28 vol. %, based on the average modal
muscovite in CMB metasedimentary rocks, assuming and melt in accordance with experimentally determined
partition coefficients. Equilibrium melting may be un-some of the melt is retained in the migmatites as
leucosome, consistent with the observations described likely during crustal anatexis, particularly during de-
formation, and there are few LILE mineral–melt partitionabove. Therefore, migmatite specimens can be modeled
as residual after melt removal from rocks similar in data for conditions of crustal melting and crystallization
of granitic systems. Further, the existing data exhibitcomposition to the CMB metasedimentary rocks. In
this light, the SiO
2
-poor, [Al
2
O
3
–(FeO∗+MgO +sufficient variation that modeled trends at best will be
qualitative. This variation reflects a number of importantTiO
2
)–K
2
O]-rich composition of the block of biotite–
813
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
controls on the partition coefficients, such as the structure migmatite front. Further, the chondrite-normalized REE
pattern for this rock plots at about the middle of the
and composition of the melt (e.g. Lagache & Carron,
sample range (Fig. 8).
1982), mineral composition (e.g. Blundy & Wood, 1991;
Examination of the protolith normalized REE patterns
Icenhower & London, 1996), temperature (Icenhower
in Fig. 12 shows that whole-rock compositions of the
& London, 1995; Chappell, 1996) and water content
migmatites resemble the protolith closely, but are gen-
(Mahood & Hildreth, 1983). The second issue is the
erally enriched in the total REE, probably reflecting melt
tendency for leucosome to maintain mineral–mineral
loss at the scale of the hand specimen relative to the
equilibrium with melanosome or the melt-depleted host
protolith. All granites have REE patterns that lie well
during crystallization (Nabelek, 1999), which makes the
below the protolith composition, and all but granite 95-
use of mineral–melt partition coefficients inappropriate
121 have positive Eu anomalies. One granite specimen
for some of the processes involved.
from the diatexite shows LREE enrichment relative to
The mineral–melt partition coefficients we chose to
the protolith, and two have positive Ce anomalies that
use are given in Table 7, together with the modes for
may reflect incorporation of monazite from the source.
the initial mineral assemblage in the metasedimentary
The melt-depleted host material in the migmatites is
protolith and the residue from partial melting. Although
REE enriched, with the exception of one specimen that
our modeling produced realistic Rb, Sr and Ba con-
shows MREE depletion. Leucosomes show variable total
centrations in residue and melt, modeling fractional crys-
REE depletion, shallow LREE slopes, inflected and vari-
tallization of this melt composition according to the
able HREE patterns and distinct positive Eu anomalies.
results obtained from our modeling of the major elements
Schlieric granites are slightly depleted in the REE, al-
does not reproduce the observed data for leucosomes
though 95-215 shows slight HREE enrichment, and both
and smaller-volume granites very well (Fig. 11). We
specimens show positive Eu anomalies. The common
conclude that modeling the LILE distributions during
leucogranite from the Phillips pluton is REE depleted in
complex, multistage processes of crustal melting and
a similar fashion to the smaller-volume granites, and is
fractional crystallization in granitic systems is premature.
closely similar to the pattern of granite 95-121. Com-
plementing the suite of granites, the three analyses from
The REE the block of biotite–garnet schist (95-49) show strong
Chondrite-normalized REE plots of these rocks are con- total REE enrichment, with the exception of the LREE
sistent with a petrogenetic relation between the meta- of one specimen (b) that may reflect the higher proportion
sedimentary rocks, the migmatites and the granites. of garnet in that specimen (e.g. Hanson, 1980). Each,
Migmatite whole-rock REE patterns are similar to those however, has a negative Eu anomaly similar to the
of the CMB metasedimentary rocks. REE patterns in patterns from the migmatite whole rock and melt-de-
the granites are also similar in slope, but are systematically pleted host.
lower in abundance. Typical upper-crustal granites that We have not tried to model REE patterns for the
are derived from metasedimentary protoliths have en- multistage model we postulate based on the major ele-
riched LREE contents relative to chondritic values (e.g. ment compositions because assumptions necessary for
Taylor & McLennan, 1985). This is true for the suite of such modeling are not met by the Maine migmatites and
specimens from Maine, where granites from the diatexite granites. REE modeling requires that (1) leucosomes and
domains are the most enriched (La >100 times chon- granites do not contain significant amounts of xenocrystic
drites) and common leucogranite from the Phillips pluton material, (2) the protolith composition can be un-
is the least enriched (La >20 times chondrites). Chon- ambiguously identified, (3) leucosome and granite REE
drite-normalized REE patterns show that most of the concentrations have not been significantly disturbed
granites and all of the leucosomes are LREE depleted either by subsolidus processes or by fluid infiltration, and
relative to the metasedimentary rocks, consistent with (4) leucosomes and granites have melt compositions and
the postulate that these are the putative protolith for the that have undergone fractional crystallization. None of
melts (Miller & Mittlefehldt, 1982; Mittlefehldt & Miller, these criteria can be satisfied given the dynamic nature
1983; Le Fort et al., 1987; Sevigny et al., 1989; Breaks & of the syntectonic migmatization and melt flow in our
Moore, 1992; Wark & Miller, 1993; Nabelek & Glascock, Maine example. In addition, mineral–melt partition co-
1995; Tomascak et al., 1996; Pressley & Brown, 1999; efficients for trace elements in silicic melts are dependent
Milord et al., 2001). Taking the CMB metasedimentary on composition (Mahood & Hildreth, 1983; Blundy &
rocks as the protolith for the TAD and WAD migmatites, Wood, 1991), and insufficiently well known to justify the
and the source for the granites, we recast the REE data complexity of modeling required in this case.
by normalizing to one representative metasedimentary The role of biotite is likely to be the key to under-
rock (specimen 95-97). We choose this metasedimentary standing the REE variations. As widespread biotite break-
down did not occur, biotite in the protolith probably didrock because it was collected from close proximity to the
814
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
Table 7: Data used in modeling Rb–Sr–Ba relations and results
Mineral mode (%)
1
Average protolith Residuum after melt removal
Ms 25 0
Bt 20 31
Qtz 20 15
Pl 10 3
Kfs 0 8
Sil 10 22
Grt 5 7
Other 10 14
Model 1 Model 2
Rb Sr Ba Rb Sr Ba
K
d
Pl
2
0·125 9·91·08 0·09 12·11·2
K
d
Bt
2
3·20·4467 23 2 0·04 10
K
d
Kfs
2
1·55·97·30·713 20
D
protolith
0·653 1·08 4·71 0·409 1·22 2·12
D
residue
1·11 0·892 7·74 0·678 1·39 4·71
Batch melting (ppm)
Protolith
3
165 96 616 165 96 616
Melt 220 90·8 168 287 83·0 341
Residue 170 92·8 815 146 104 790
Fractional crystallization (ppm)
Cumulate (a)
1% a
1
163 678 877 136 737 990
20% a
1
173 152 339 153 116 652
1% a
2
264 566 1360 218 610 1410
20% a
2
253 172 261 230 141 702
Residual liquid (b)
1% b
1
221 84·6 160 289 76·0 334
20% b
1
233 19·062·0 323 11·9 220
1% b
2
220 85·9 155 288 77·4 330
20% b
2
210 26·129·8 303 17·9 164
1
Abbreviations after Kretz (1983).
2
Partition coefficients from: Model 1, Nash & Crecraft (1985); Model 2, Blundy & Wood (1991) and Icenhower & London
(1995).
3
Weighted average of 15 CMB metasedimentary rocks.
not contribute significantly to the REE budget of the biotite addition to or subtraction from the melt. Thus,
REE-bearing accessory minerals such as apatite, mon-melt except as an entrained phase. Because accessory
minerals in migmatites commonly are included in the azite and zircon may not contribute significantly to the
melt if sequestered in residual biotite, leaving the meltmajor phases (e.g. Bea, 1996), minerals such as biotite
control the behavior of these phases. As common ac- depleted in the REE (e.g. Rapp & Watson, 1986; Sevigny,
1993; Watt & Harley, 1993; Nabelek & Glascock, 1995).cessory minerals in granites have distinctively different
REE compositions (Fig. 13c), exactly which phases are Correspondingly, the REE enrichment of the migmatites
is probably due to concentration of biotite and its includedincluded in biotite will control in detail the effect of
815
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
Fig. 11. Result of Rb–Sr–Ba modeling of melt–residuum separation during migmatization of CMB metasedimentary rocks as explained in the
text, and 20% fractional crystallization of Bt +Pl cumulates. Bt and Pl compositions with subscripts NC and IL refer to data of Nash &
Crecraft (1985) and Icenhower & London (1995), respectively. R, model residuum; a and b, model compositions of these points in Fig. 10. Data
used are shown in Table 7.
accessory minerals in the residue (Bea, 1996; Watt et al., The LREE-enriched granite in cylinders in diatexite
(Fig. 12) may be a consequence of incorporation of1996).
The LREE concentrations in the granites and the monazite into the melt ( Fig. 13) by the physical
destruction of entrained biotite (e.g. Sawyer, 1998).leucosomes are inconsistent with the calculated tem-
peratures for monazite saturation (902–1090°C), be- H
2
O recycling
cause monazite would be totally dissolved under those
conditions. As a test of residual biotite incorporation Poikilitic muscovite crystals and minor chlorite occur
into the granites to elevate LREE contents, we model parallel to foliation and lineation throughout migmatites
two-component mixing of the REE composition of of the TAD and WAD. Although we postulate loss of
biotite (and its included accessory minerals) from a K-rich melt from the migmatites, K-feldspar does
the migmatites with the migmatite leucosomes by occur locally in some leucosomes and its absence from
incremental addition (e.g. Milord et al., 2001). Two of other leucosomes may be due to replacement by
the results are listed in Table 8, and plotted as muscovite. Given the steep orientation of these fabrics,
protolith-normalized patterns in Fig. 13a and b. Mixing we interpret the retrograde growth of muscovite and
residual biotite with leucosome from stromatic migmatite chlorite to record buoyancy-aided fluid flow parallel
95-103 does not reproduce the REE pattern of the to the fabrics in the rocks. This fluid is likely to
granite sheet that cuts it (Fig. 13a). Addition of >2·5have been derived from crystallizing melts within the
vol. % residual biotite to the leucosome composition migmatites ( Fig. 14). As melt crystallizes in the de-
results in higher LREE and HREE concentrations than forming rocks, liberated H
2
O may promote melting in
in the granite. In contrast, the leucosome from diatexite adjacent units at suprasolidus conditions or retrogression
95-35 is depleted in LREE relative to the granite at subsolidus conditions. Such a mechanism has been
cylinder that cuts it (Fig. 13b), and although addition postulated by Holk & Taylor (1997) to explain ho-
of 25 vol. % residual biotite causes the LREE pattern mogenization of oxygen isotopic compositions in mid-
of the mixture to resemble that of the granite, the crustal rocks of the Thor–Odin metamorphic core
HREE content is enriched relative to the granite complex in British Columbia. No influx of water-rich
composition. If biotite were removed from the magma volatile phase is necessarily implied or required, and
during ascent, the REE composition would decrease we postulate that this is the principal cause of the
proportionally, but more strongly in the LREE, and regional syntectonic retrogression of staurolite and
REE patterns similar to the common leucogranite of andalusite in rocks outside the migmatite front (Solar
& Brown, 1999).the Phillips pluton may be the result (Figs 9 and 12).
816
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
Fig. 12. REE concentrations normalized to the composition of metasedimentary rock 95-97 (representative of the presumed protolith) as
explained in text. The heavy dashed line represents the composition of 95-97.
melt loss, in this case leaving a more felsic cumulate
CONCLUSIONS than the more residual migmatites.
In Maine, metamorphism and granite crystallization Exposed migmatites preserve evidence in cumulate
are contemporaneous; and syntectonic magma ascent leucosomes of the flow network that drained these rocks
was controlled by deformation and development of of an evolved melt. We postulate that migmatites similar
strain fabrics. Mineral assemblages and geochemical to those exposed represent the source of common leuco-
data are consistent with muscovite dehydration melting, granite in the Phillips pluton. Thus, the Phillips pluton
and suggest that migmatite leucosomes and smaller- may be connected at depth to granites similar to those
volume granites represent cumulate rocks (±residual found in the migmatites of the TAD and WAD in the
material and some retained fractionated melt) that manner described by Brown & Solar (1999) where the
complement common leucogranite in the Phillips pluton. heterogeneous migmatites and granites in ACZs formed
The residual geochemistry of stromatic migmatite and in the cores of thermal antiforms developed during
diatexite relative to the metasedimentary rocks, and regional contraction. However, the relation between mig-
the ‘pinch-and-swell’structure of granite sheets, are matite leucosomes, smaller-volume granites and leuco-
consistent with melt loss from those rocks, perhaps granite in plutons is not straightforward.
driven by the accommodation of deformation. Rare In Maine, we have demonstrated that migmatite
leucosomes and smaller-volume granites are cumulateschlieric granites suggest some melt redistribution before
817
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
Fig. 13. Model protolith-normalized REE patterns of two-component mixtures of residual biotite and leucosome compared with REE patterns
of granite from stromatic migmatite (a) and diatexite (b) as explained in text. Data used for the model are given in Table 8. Also plotted are
typical normalized REE patterns for common accessory minerals in granites (from Bea, 1996).
rocks, whereas the leucogranite of the adjacent Phillips compositions without entrained residue or modification
by fractional crystallization is naive. The popular notionpluton is consistent with the putative liquid lost from
these rocks. Any expectation that migmatite leucosomes that migmatites represent ‘failed’granites should be
and leucogranite in plutons should show simple melt reconsidered in the light of multiple syntectonic
processes, and there is no a priori reason to suppose
Fig. 14. Schematic structure section of an orogenic system based upon
the Maine example (after Brown & Solar, 1999). The plane of section
is sub-vertical, drawn along the line of section A–A′in Fig. 2. The
continuous line labeled ‘solidus’corresponds to the migmatite front
that marks the margin of the TAD and WAD (Fig. 2). Dashed lines
are boundaries between structural domains (see Figs 1 and 2 for an
explanation of other symbols). The migmatite front, which tracks the
solidus, was progressively extended into shallower parts of the orogenic
system by advection of material during contractional thickening, in-
cluding the sequential ascent of granite melt (e.g. Brown & Solar,
1999). Crystallization of the granites at the solidus exsolves a water-
rich volatile phase that we postulate was responsible for widespread
generation of retrograde muscovite in migmatites and retrogression of
staurolite and andalusite in subsolidus rocks.
818
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
Table 8: Model effect of residual biotite contamination of leucosome REE compositions
Stromatic migmatite 95-103 Diatexite 95-35
host host leuco leuco +leuco +granite whole whole rock leuco leuco +leuco +granite
recalculated 2·5% Bt 5% Bt sheet rock recalculated 15% Bt 25% Bt cylinder
to 100% Bt 95-103 to 100% Bt 95-49
ppm
La 65·0 108 19·621·824·022·148·997·86·14 19·929·127·3
Ce 126 210 39·243·547·742·295·2 190 12·238·956·747·5
Pr 14·524·24·57 5·06 5·55 4·51 10·521·01·45 4·38 6·34 5·09
Nd 56·994·917·719·621·617·140·480·85·94 17·224·719·0
Sm 12·320·54·08 4·49 4·90 3·79 8·85 17·71·54 3·96 5·58 3·60
Eu 1·29 2·15 1·08 1·11 1·13 1·81 1·55 3·10 1·63 1·85 2·00 1·10
Gd 9·52 15·93·68 3·99 4·29 2·84 7·26 14·51·99 3·87 5·12 2·50
Tb 1·59 2·65 0·73 0·78 0·83 0·41 1·14 2·28 0·37 0·66 0·85 0·36
Dy 8·97 15·04·86 5·11 5·37 2·18 6·21 12·42·28 3·80 4·81 1·86
Ho 1·69 2·82 1·02 1·07 1·11 0·39 1·17 2·34 0·42 0·71 0·90 0·33
Er 4·35 7·25 2·92 3·03 3·14 0·94 3·05 6·10 1·02 1·78 2·29 0·83
Yb 3·72 6·20 2·61 2·70 2·79 0·82 2·91 5·82 0·73 1·49 2·00 0·78
Lu 0·56 0·93 0·40 0·41 0·43 0·12 0·45 0·90 0·10 0·22 0·30 0·13
Host, melt-depleted host; leuco, leucosome. Stromatic migmatite 95-103 melt-depleted host contains >60% Bt; diatexite 95-
35 whole rock contains >50% Bt.
that the process of crustal melting and the evolution REFERENCES
of leucogranites will be any less complex than for Ague, J. J. (1994). Mass transfer during barrovian metamorphism of
other magma suites. pelites, south–central Connecticut, I: evidence for composition and
volume change. American Journal of Science 294, 989–1057.
Allibone, A. H. & Norris, R. J. (1992). Segregation of leucogranite
microplutons during synanatectic deformation: an example from the
Taylor Valley, Antarctica. Journal of Metamorphic Geology 10, 589–600.
ACKNOWLEDGEMENTS Ashworth, J. R. (1985). Introduction. In: Ashworth, J. R. (ed.) Migmatites.
Glasgow: Blackie, pp. 1–35.
We wish to thank those who have helped us in the
Ayers, M. & Harris, N. (1997). REE fractionation and Nd-isotope
field and in the interpretation of the chemical data, disequilibrium during crustal anatexis: constraints from Himalayan
in particular E. Sawyer, T. Rushmer, A. B. Thompson, leucogranites. Chemical Geology 139, 247–267.
G. Hanson, S. McLennan, J. Hurowitz, T. Rasbury, Bea, F. (1996). Residence of REE, Y, Th and U in granites and crustal
S. Hemming and P. Tomascak. G.S.S. is grateful to protoliths; implications for the chemistry of crustal melts. Journal of
the ‘Dirt Group’of the Department of Geosciences at Petrology 39, 521–552.
Benn, K., Cruden, A. R. & Sawyer, E. W. (1998). Preface. Journal of
SUNY Stony Brook for guidance concerning the quality
Structural Geology 20,Special Issue: Extraction, Transport and Emplacement
and interpretation of geochemical data. Of course, any of Granitic Magmas.
misconceptions and errors are our own. We thank T. Blundy, J. D. & Wood, B. J. (1991). Crystal-chemical controls on the
Rushmer, C. Barnes and an anonymous referee for partitioning of Sr and Ba between plagioclase feldspar, silicate
thorough reviews and critical comments, and Sorena melts, and hydrothermal solutions. Geochimica et Cosmochimica Acta 55,
Sorensen for her extensive and helpful editorial com- 193–209.
Bradley, D. C. (1983). Tectonics of the Acadian orogeny in New
ments. We acknowledge partial support of this work
England and adjacent Canada. Journal of Geology 91, 381–400.
from the Department of Geology, University of Mary-
Bradley, D. C., Tucker, R. D., Lux, D. R., Harris, A. G. & McGregor,
land, the Department of Geological Sciences, Virginia D. C. (1998). Migration of the Acadian orogen and foreland basin
Tech, the Department of Geosciences, SUNY Stony across the northern Appalachians. US Geological Survey, Open-File Report
Brook, the Geological Society of America Research 98-770,79pp.
Grants Program and NSF Grant EAR-9705858 (to Breaks, F. W. & Moore, J. M. (1992). The Ghost Lake batholith,
Superior Province of northwestern Ontario—a fertile, S-type,
M.B.).
819
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
peraluminous granite–rare-element pegmatite system. Canadian Min- Cullers, R. L., Bock, B. & Guidotti, C. (1997). Elemental distributions
and neodymium isotopic composition of Silurian metasediments,
eralogist 30, 835–875.
western Maine, USA: redistribution of rare earth elements. Geochimica
Brearley, A. J. & Rubie, D. C. (1990). Effects of H
2
O on the dis-
et Cosmochimica Acta 61, 1847–1861.
equilibrium breakdown of muscovite +quartz. Journal of Petrology
Deer, W. A., Howie, R. A. & Zussman, J. (1997). Rock Forming Minerals,
31, 925–956.
Volumes 1, 3 and 4. Harlow, UK: Longman.
Brown, M. (1973). The definition of metatexis, diatexis and migmatite.
De Yoreo, J. J., Lux, D. R., Guidotti, C. V., Decker, E. R. & Osberg,
Proceedings of the Geological Association 84, 371–382.
P. H. (1989). The Acadian thermal history of western Maine. Journal
Brown, M. (1994). The generation, segregation ascent and emplacement
of Metamorphic Geology 7, 169–190.
of granite magma: the migmatite-to-crustally derived granite con-
D’Lemos, R. S., Brown, M. & Strachan, R. A. (1992). Granite magma
nection in thickened orogens. Earth-Science Reviews 36,83–130.
generation, ascent and emplacement within a transpressional orogen.
Brown, M. & Rushmer, T. (1997). The role of deformation in the
Journal of the Geological Society, London 149, 487–490.
movement of granite melt: views from the laboratory and the field.
Eusden, J. D., Jr & Barreiro, B. (1988). The timing of peak high-
In: Holness, M. B. (ed.) Deformation-enhanced Fluid Transport in the Earth’s
grade metamorphism in central–eastern New England. Maritime
Crust and Mantle. London: Chapman and Hall, pp. 111–144.
Sedimentation and Atlantic Geology 24, 241–255.
Brown, M. & Solar, G. S. (1998a). Shear zones and melts: positive
Evansen, M. N., Hamilton, P. J. & O’Nions, R. K. (1978). Rare-earth
feedback in orogenic belts. Journal of Structural Geology 20, 211–227.
abundances in chondritic meteorites. Geochimica et Cosmochimica Acta
Brown, M. & Solar, G. S. (1998b). Granite ascent and emplacement
42, 1199–1212.
during contractional deformation of convergent orogens. Journal of Friend, C. R. L., Brown, M., Perkins, W. T. & Burwell, A. D. M.
Structural Geology 20, 1365–1393. (1985). The geology of the Qo
ˆrqut granite complex north of Qo
ˆrqut,
Brown, M. & Solar, G. S. (1999). The mechanism of ascent and Godtha
˚bsfjord, southern West Greenland. Bulletin Grønlands Geologiske
emplacement of granite magma during transpression: a syntectonic Undersøgelse 151,43pp.
granite paradigm. Tectonophysics 312,1–33. Govindaraju, K. (1994). Compilation of working values and sample
Brown, M., Averkin, Y. A., McLellan, E. L. & Sawyer, E. W. (1995). descriptions for 383 geostandards. Geostandards Newsletter, Special Issue
Melt segregation in migmatites. Journal of Geophysical Research 100,18,15–53.
15655–15679. Grocott, J., Brown, M., Dallmeyer, R. D., Taylor, G. K. & Treloar,
Brown, M. A., Brown, M., Carlson, W. D. & Denison, C. (1999). P. J. (1994). Mechanisms of continental growth in extensional arcs:
Topology of syntectonic melt flow networks in the deep crust: an example from the Andean Plate Boundary Zone. Geology 22,
inferences from three-dimensional images of leucosome geometry in 391–394.
migmatites. American Mineralogist 84, 1793–1818. Guidotti, C. V. (1963). Metamorphism of the pelitic schists in the
Chamberlain, C. P. & England, P. C. (1985). The Acadian thermal Bryant Pond quadrangle, Maine. American Mineralogist 48, 772–791.
history of the Merrimack Synclinorium in New Hampshire. Journal Guidotti, C. V. (1989). Metamorphism in Maine: an overview. In:
of Geology 93, 593–602. Tucker, R. D & Marvinney, R. G. (eds) Studies in Maine Geology,
Chappell, B. W. (1996). Compositional variation within granite suites Volume 3: Igneous and Metamorphic Geology. Maine Geological Survey,
of the Lachlan Fold Belt: its causes and implications for the physical pp. 1–19.
state of granite magma. Transactions of the Royal Society of Edinburgh: Guidotti, C. V. (1993). Textural aspects of high T–low Ppoly-
Earth Sciences 88, 159–170. metamorphism in the Rangeley Area, western Maine: general im-
Chappell, B. W., White, A. J. R. & Wyborn, D. (1987). The importance plications for studies of Acadian metamorphic rocks in New England.
of residual source material (restite) in granite petrogenesis. Journal of Geological Society of America, Northeastern Section Meeting, Abstracts with
Petrology 28, 1111–1138. Programs 25, A21.
Clemens, J. D. & Mawer, C. K. (1992). Granitic magma transport by Hanson, G. N. (1980). Rare earth elements in petrogenetic studies of
fracture propagation. Tectonophysics 204, 339–360. igneous systems. Annual Review of Earth and Planetary Sciences 8, 371–406.
Cocks, L. R. M., McKerrow, W. S. & van Staal, C. R. (1997). The Harris, N. B. W. & Inger, S. (1992). Trace element modelling of pelite-
margins of Avalonia. Geological Magazine 134, 627–636. derived granites. Contributions to Mineralogy and Petrology 110,46–56.
Collins, W. J. & Sawyer, E. W. (1996). Pervasive granitoid magma Harris, N. B. W., Inger, S. & Massey, J. (1993). The role of fluids in
transfer through the lower–middle crust during non-coaxial com- the formation of High Himalayan leucogranites. In: Treloar, P. J.
pressional deformation. Journal of Metamorphic Geology 14, 565–579. & Searle, M. P. (eds) Himalayan Tectonics. Geological Society, London,
Collins, W. J. & Vernon, R. H. (1991). Orogeny associated with Special Publications 74, 391–400.
anticlockwise P–T–tpaths: evidence from low-P, high-Tmeta- Harris, N., Ayres, M. & Massey, J. (1995). Geochemistry of granitic
morphic terranes in the Arunta Inlier, central Australia. Geology 19,melts produced during the incongruent melting of muscovite: im-
835–838. plications for the extraction of Himalayan leucogranite magmas.
Couture, R. A., Smith, M. S. & Dymek, R. F. (1993). X-ray fluorescence Journal of Geophysical Research 100, 15767–15777.
analysis of silicate rocks using fused glass discs and a side-window Holdaway, M. J., Guidotti, C. V., Novak, J. M. & Henry, W. E. (1982).
Rh source tube: accuracy, precision and reproducibility. Chemical Polymetamorphism in medium- to high-grade pelitic metamorphic
Geology 110, 315–328. rocks, west–central Maine. Geological Society of America Bulletin 93,
Crock, J. G. & Lichte, F. E. (1982). Determination of rare earth 572–584.
elements in geologic materials by inductively couple argon plasma/ Holdaway, M. J., Mukhapodhyay, B., Dyar, M. D., Guidotti, C. V. &
atomic emission spectrometry. Analytical Chemistry 54, 1329–1332. Dutrow, B. L. (1997). Garnet–biotite geothermometry revised: new
Cruden, A. R. (1998). On the emplacement of tabular granites. Journal Margules parameters and a natural specimen data set from Maine.
of the Geological Society, London 154, 853–862. American Mineralogist 82, 582–595.
Cullers, R. L., Long-Tsu, Y., Sambhudas, C. & Guidotti, C. V. (1974). Holk, G. J. & Taylor, H. P., Jr (1997).
18
O/
16
O homogenization of the
Rare earth elements in Silurian pelitic schists from N.W. Maine. middle crust during anatexis: the Thor–Odin metamorphic core
complex, British Columbia. Geology 25,31–34.Geochimica et Cosmochimica Acta 38, 389–400.
820
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
Hubbard, M. S., West, D. P., Jr, Ludman, A., Guidotti, C. V. & Lux, Patin
˜o Douce, A. E. & Harris, N. (1998). Experimental constraints on
D. R. (1995). The Norumbega fault zone, Maine: a mid- to shallow- Himalayan anatexis. Journal of Petrology 39, 689–710.
level crustal section within a transcurrent shear zone. Atlantic Geology Paterson, S. R. & Miller, R. B. (1998). Magma emplacement during
31, 109–116. arc-perpendicular contraction. Tectonics 17, 571–856.
Hutton, D. H. W. & Reavy, R. J. (1992). Strike-slip tectonics and Paterson, S. R., Fowler, T. K., Jr & Miller, R. B. (1996). Pluton
granite petrogenesis. Tectonics 11, 960–967. emplacement in arcs: a crustal-scale exchange process. Transactions
Icenhower, J. & London, D. (1995). An experimental study of element of the Royal Society of Edinburgh 87, 115–123.
partitioning among biotite, muscovite and coexisting peraluminous Power, G. M. (1993). Geochemical differences between the Cadomian
silicic melt at 200 MPa (H
2
O). American Mineralogist 80, 1229–1251. granites of Mancellia and the St. Malo migmatites, Armorican
Icenhower, J. & London, D. (1996). Experimental partitioning of Rb, Massif, France. Journal of the Geological Society, London 150, 465–468.
Cs, Sr and Ba between alkali feldspar and peraluminous melt. Pressley, R. A. & Brown, M. (1999). The Phillips Pluton, Maine, USA:
American Mineralogist 81, 719–734. evidence of heterogeneous crustal sources and implications for granite
Johannes, W. (1988). What controls partial melting in migmatites? ascent and emplacement mechanisms in convergent orogens. Lithos
Journal of Metamorphic Geology 6, 451–465. 49, 335–366.
Kretz, R. (1983). Symbols for rock-forming minerals. American Min- Rapp, R. P. & Watson, E. B. (1986). Monazite solubility and dissolution
eralogist 34, 345–368. kinetics: implications for the Th and LREE chemistry of felsic
Lagache, M. & Carron, J. P. (1982). Zonation des e
´le
´ments en traces magmas. Contributions to Mineralogy and Petrology 94, 304–316.
au cours de la croissance des cristaux dans les bains silicate
´s: Ratcliffe, N. M., Hames, W. E. & Stanley, R. S. (1998). Interpretation
l’exemple de Rb, Cs, Sr and Ba dans le syste
`me Qz–Ab–Or–H
2
O. of ages of arc magmatism, metamorphism, and collisional tectonics
Geochimica et Cosmochimica Acta 46, 2151–2158. in the Taconian orogen of western New England. American Journal
LeFort, P., Cuney, M. Deniel, C., France-Lanord, C., Sheppard, S., of Science 298, 791–797.
Upreti, B. N. & Vidal, P. (1987). Crustal generation of Himalayan Robinson, P., Tucker, R. D., Bradley, D., Berry, H. N., IV & Osberg,
leucogranite. Tectonophysics 134,39–57. P. H. (1998). Paleozoic orogens in New England, USA. GFF 120,
Ludman, A. (1998). Evolution of a transcurrent fault system in shallow 119–148.
crustal metasedimentary rocks: the Norumbega fault zone, eastern Rubie, D. C. & Brearley, A. J. (1987). Metastable melting during the
Maine. Journal of Structural Geology 20,93–107. breakdown of muscovite +quartz at 1-kbar. Bulletin de Mine
´ralogie
Mahood, G. & Hildreth, W. (1983). Large partition coefficients for 11 0, 533–549.
trace elements in high-silica rhyolites. Geochimica et Cosmochimica Acta Rushmer, T. (1996). Melt segregation in the lower crust: how have
47,11–30. experiments helped us? Transactions of the Royal Society of Edinburgh:
Marchildon, N. & Brown, M. (2001). Melt segregation in late-tectonic Earth Sciences 87,73–83.
migmatites: an example from the Onawa contract aureole, Maine, Rutter, E. H. (1997). The influence of deformation on the extraction
U.S.A. Physics and Chemistry of the Earth (in press). of crustal melts: a consideration of the role of melt-assisted granular
Menhert, W. D. (1968). Migmatites and Origin of Granitic Rocks. Amsterdam: flow. In: Holness, M. (ed.) Deformation-enhanced Melt Segregation and
Elsevier. Metamorphic Fluid Transport. Mineralogical Society Series 8,82–110.
Miller, C. F. & Mittlefehldt, D. W. (1982). Depletion of light rare- Sawyer, E. W. (1994). Melt segregation in the continental crust. Geology
earth elements in felsic magmas. Geology 10, 129–133. 22, 1019–1022.
Miller, R. B. & Paterson, S. R. (1999). In defense of magmatic diapirs. Sawyer, E. W. (1998). Formation and evolution of granite magmas
Journal of Structural Geology 21, 1161–1173. during crustal reworking, the significance of diatexites. Journal of
Milord, I., Sawyer, E. W. & Brown, M. (2001). Formation of diatexite Petrology 39, 1147–1167.
migmatite and granite magma during anatexis of semi-pelitic meta- Sawyer, E. W. (1999). Criteria for the recognition of partial melting.
sedimentary rocks: an example from St. Malo, France. Journal of Physics and Chemistry of the Earth (A) 24, 269–279.
Petrology 42, 487–505. Schumacher, J. C., Hollicher, K. T., Robinson, P. & Tracy, R. J. (1990).
Mittlefehldt, D. W. & Miller, C. F. (1983). Geochemistry of the Progressive reactions and melting in the Acadian metamorphic high
Sweetwater Wash pluton, California: implications for ‘anomalous’
of central Massachusetts in southwestern New Hampshire, USA. In:
trace element behavior during differentiation of felsic magmas.
Ashworth, J. R. & Brown, M. (eds) High-temperature Metamorphism and
Geochimica et Cosmochimica Acta 47, 109–124.
Crustal Anatexis. New York: Unwin Hyman.
Montel, J. M. (1993). A model for monazite/melt equilibrium and
Sevigny, J. H. (1993). Monazite controlled Sm/Nd fractionation in
application to the generation of granitic magmas. Chemical Geology
leucogranite: an ion microprobe study of garnet phenocrysts. Geo-
110, 127–146.
chimica et Cosmochimica Acta 57, 4095–4105.
Nabelek, P. I. (1999). Trace-element distribution among rock-forming
Sevigny, J. H. Parrish, R. R. & Ghent, E. D. (1989). Petrogenesis of
minerals in Black Hills migmatites, South Dakota: a case for solid-
peraluminous granites, Monashee Mountains, southeastern Ca-
state equilibrium. American Mineralogist 84, 1256–1269.
nadian Cordillera. Journal of Petrology 30, 557–581.
Nabelek, P. I. & Glascock, M. D. (1995). REE-depleted leucogranites,
Smith, H. A. & Barreiro, B. (1990). Monazite U–Pb dating of staurolite
Black Hills, South Dakota: a consequence of disequilibrium melting
grade metamorphism in pelitic schists. Contributions to Mineralogy and
of monazite-bearing schists. Journal of Petrology 36, 1055–1071.
Petrology 105, 602–615.
Nash, W. P. & Crecraft, H. R. (1985). Partition coefficients for trace
Solar, G. S. (1999). Structural and petrological investigations in the
elements in silicic magmas. Geochimica et Cosmochimica Acta 49, 2309–
Central Maine belt, western Maine, with special reference to the
2322.
migmatites. Ph.D. thesis, University of Maryland, College Park.
Patin
˜o Douce, A. E. (1999). What experiments tell us about the relative
Solar, G. S. & Brown, M. (1999). The classic high-T–low-Pmeta-
contributions of crust and mantle to the origin of granitic magmas.
morphism of west–central Maine, USA: is it post-tectonic or syn-
In: Castro, A., Fernandez, C. & Vigneresse, J.-L. (eds) Understanding
tectonic? Evidence from porphyroblast–matrix relations. Canadian
Granites. Integrating New and Classical Techniques. Geological Society, London,
Special Publications 168.Mineralogist 37, 289–311.
821
JOURNAL OF PETROLOGY
VOLUME 42 NUMBER 4 APRIL 2001
Solar, G. S. & Brown, M. (2000). The classic high-T–low-Pmeta- APPENDIX
morphism of west–central Maine, USA: is it post-tectonic or syn- Analytical methods
tectonic? Evidence from porphyroblast–matrix relations.—Reply.
Canadian Mineralogist 38, 1007–1026. After removal of weathered material, >0·5 kg of each
Solar, G. S. & Brown, M. (2001). Contrasting patterns of strain and specimen (depending upon homogeneity and grain size)
kinematic partitioning of flow during oblique contraction in the was reduced to powder at the University of Maryland
Central Maine belt, northern Appalachian orogen, USA. Journal of using a SPEX Industries shatterbox with an alumina
Structural Geology 22 (in press). grinding chamber and puck [see Tomascak (1995) for
Solar, G. S., Pressley, R. A., Brown, M. & Tucker, R. D. (1998). details]. Approximately 200 mg of each powdered speci-
Granite ascent in convergent orogenic belts: testing a model. Geology
men was fused at Washington University in St Louis,
26, 711–714.
Stromgard, K. E. (1973). Stress distribution during formation of bou- Missouri, and analyzed by XRF (Couture et al., 1993;
dinage and pressure shadows. Tectonophysics 16, 215–248. R. Couture, analyst; data in Tables 1–4). Thirty-two
Stu
¨we, K., Sandford, M. & Powell, R. (1993). Episodic metamorphism of the powders were processed at Washington State
and deformation in low-pressure, high-temperature terranes. Geology University, for REE composition primarily, using a
21, 829–832. fusion–dissolution method adapted from Crock & Lichte
Taylor, S. R. & McLennan, S. M. (1985). The Continental Crust: its (1982), and analyzed by ICP-MS using a Sciex Elan
Composition and Evolution. An Examination of the Geochemical Record Preserved model 250 instrument (C. Knaack, analyst; data in Tables
in Sedimentary Rocks. Oxford: Blackwell Science.
5 and 6).
Thompson, A. B. & Tracy, R. J. (1976). Model systems for anatexis of
pelitic rocks. II, facies series melting and reaction in the system
CaO–KAlO
2
–NaAlO
2
–Al
2
O
3
–SiO
2
–H
2
O. Contributions to Mineralogy
and Petrology 70, 429–438.
Analytical uncertainty and correlation
Tomascak, P. B. (1995). The petrogenesis of granitic rocks in south-
western Maine. Ph.D. thesis, University of Maryland, College XRF
Park. Precision of XRF data is estimated from results for
Tomascak, P. B., Krogstad, E. J. & Walker, R.J. (1996). Nature of the standard GRD8083B, analyzed 36 times (18 runs) with
crust in Maine, USA: evidence from the Sebago batholith. Con-
each set of analyses of unknown specimens of this study.
tributions to Mineralogy and Petrology 125,45–59.
Vernon, R. H. & Collins, W. J. (1988). Igneous microstructures in The means of standard values compared favorably with
migmatites. Geology 16, 1126–1129. reference values (Solar, 1999). The 2uncertainty is
Wark, D. A. & Miller, C. F. (1993). Accessory mineral behavior during >1% of the value for SiO
2
,Al
2
O
3
and FeO∗,>1·5–6%
differentiation of a granite suite: monazite, xenotime and zircon in the of the value for MgO, CaO, Na
2
O and K
2
O, >2·5%
Sweetwater Wash pluton, southeastern California, U.S.A. Chemical of the value for TiO
2
, and >8 and >31% of the value
Geology 10,49–67. for MnO and P
2
O
5
, respectively. The high uncertainty
Watson, E. B. & Harrison, T. M. (1983). Zircon saturation revisited: for P
2
O
5
is due to drift in concentration of the standard
temperature and compositional effects in a variety of crustal magma
disk over time (R. Couture, personal communication,
types. Earth and Planetary Science Letters 64, 295–304.
Watt, G. R. & Harley, S. L. (1993). Accessory phase controls on the 1999); however, given the low concentration of P
2
O
5
in
geochemistry of crustal melts and restites produced during water- samples (<0·30 wt %; Tables 1–4), this uncertainty in
undersaturated partial melting. Contributions to Mineralogy and Petrology the standard translates to <0·1 wt % in samples. The
114, 550–566. 2uncertainties for trace elements are in the range 1–5
Watt, G. R., Burns, I. M. & Graham, G. A. (1996). Chemical char- ppm for Nb, Zr, Y, Sr, Rb, Pb, Ga, Zn, Ni and Co, 7
acteristics of migmatites: accessory phase distribution and evidence ppm for V, 13 ppm for Cr and 17 ppm for Ba. Sn was
for fast melt segregation rates. Contributions to Mineralogy and Petrology below detection limit in the standard.
125, 100–111.
Univariate and multivariate tests of the significance
Weinberg, R. F. (1999). Mesoscale pervasive felsic magma migration:
of XRF whole-rock data to represent the geochemical
alternatives to dyking. Lithos 46, 393–410.
West, D. P., Jr (1999). Timing of displacements along the Norumbega variation of the various rock groups, and a discussion of
fault system, south–central and south–coastal Maine. In: West, D. these tests based upon arguments of Power (1993) have
P., Jr & Ludman, A. (eds) Norumbega Fault System of the Northern been given by Solar (1999). Solar (1999) determined that
Appalachians. Geological Society of America, Special Paper 331, 167–distributions of major element oxide data, and of trace
178. element data, are normal and asymmetric for the chosen
West, D. P., Jr & Hubbard, M. S. (1997). Progressive localization of metasedimentary rocks, stromatic migmatite and di-
deformation during exhumation of a major strike-slip shear zone: atexite samples of these populations.
Norumbega fault zone, south–central Maine. Tectonophysics 273,
185–202.
West, D. P., Jr, Guidotti, C. V. & Lux, D. R. (1995). Silurian orogenesis ICP-MS
in the western Penobscot Bay region, Maine. Canadian Journal of Precision of ICP-MS data is estimated from results for
Earth Sciences 32, 1845–1858.
standard BCR-P, analyzed 24 times (12 runs) over a
Wickham, S. M. (1987). The segregation and emplacement of granite
magmas. Journal of the Geological Society, London 144, 281–297. 4 month period of 1994. The means of standard values
822
SOLAR AND BROWN POSSIBLE SOURCE OF LEUCOGRANITE IN PLUTONS?
compare well with reference values of Govindaraju this uniform anomaly, and because we cannot find a
mineralogical explanation to account for such a uniform
(1994). The 2uncertainty is <5% with the exception
anomaly across different rock types, we have left Tm off
of Th (19%), U (19%), Pb (6·5%), Cs (6%), Ta (5·4%)
our REE plots.
and Eu (5%). The larger uncertainties for Th and U
translate to Ζ5 and 2 ppm, respectively, in samples of
this study (Tables 5 and 6). The 2uncertainties for the Correlation between XRF and ICP-MS data
REE are smaller than the plotted symbol size in the Six trace elements were analyzed by both XRF and ICP-
normalized plots. Three specimens were re-fused as a MS (Nb, Y, Sr, Rb, Pb and Ba). Thirty-one of these
check on reproducibility [schlieric granite 95-215 (whole 192 data (six elements by 32 specimens) are in >10%
rock), 95-215 leucosome, stromatic migmatite 96-286 disagreement, but only eight translate to a difference of
(whole rock)], and these data are listed in Tables 5 >10 ppm. Twenty-two of these 31 data show a lower
and 6. As a result of difficulties in determining the concentration in the XRF analyses ( Tables 5 and 6).
concentration of thulium that stem from its low abund- The most common disagreement is found in Y (17 of
ance and mono-isotopic nature, complicated by its low the 32 specimens), but only five of those are different by
abundance in chondrites (S. M. McLennan & S. R. >10 ppm, and none is different by >20 ppm. The
Hemming, personal communication, 2000), the measured next most common disagreement is found in Pb (six
Tm concentrations of all 32 of our specimens produced specimens), none different by >10 ppm. We find from
uniform slight positive anomalies on chondrite-nor- the comparison that the correlation between the XRF
and ICP-MS data is within reasonable uncertainty.malized plots ( Tm/Tm∗=1·06 ±0·02). Because of
823
