JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 2
PAGES 251–276
2001
Crystal Accumulation and Shearing in a
Megacrystic Quartz Monzonite:
Bodocó Pluton, Northeastern Brazil
J. McMURRY1∗
DEPARTMENT OF GEOSCIENCES, TEXAS TECH UNIVERSITY, LUBBOCK, TX 79414, USA
RECEIVED APRIL 28, 1999; REVISED TYPESCRIPT ACCEPTED MAY 5, 2000
The Bodocó pluton, typical of numerous felsic intrusions in northeastern Brazil that are characterized by blocky megacrysts of
K-feldspar, consists mainly of porphyritic coarse-grained quartz
monzonite (SiO2 58–70 wt %) and is reversely zoned from a
granitic margin to a quartz monzodioritic core. There is little
variation in mineral composition throughout the pluton, despite a
range of variation in mineral proportions. Isotopic characteristics
also are homogeneous, with 18Oquartz between +9·3 and +9·8‰
and initial 87Sr/86Sr within limits of >0·7056–0·7063. Petrogenetic modelling indicates that in situ crystal accumulation processes, accompanied by the upward migration of a crystal-poor felsic
melt, can account for many of the observed chemical and isotopic
features, petrographic textures, and spatial relationships of rock
types. Localized shearing associated with regional ductile deformation
produced extensive kilometre-wide bands of strongly foliated megacrystic quartz monzonite intruded by mafic dykes. Shear-related
magma mingling and/or mixing were localized post-emplacement
differentiation processes, particularly at the upper level of the intrusion
and in quartz monzonite border units along the southeast margin.
Key factors in current models of granitic magmatism
include the role of underplating by mantle-derived basaltic magma in promoting partial melting and chemical
differentiation of the lower crust and the role of largescale geodynamics in the segregation and emplacement
of magma (Hildreth, 1981; Glazner, 1991; Tikoff &
Teyssier, 1992; Bergantz & Dawes, 1994). After a melt
has been generated in the crust, a number of processes
can affect its homogenization, differentiation, segregation,
and transport. These processes include restite unmixing
(Chappell et al., 1987; Chappell & White, 1992), fractionation, and mixing or mingling either with other
crustal melts or with mantle-derived melts (DePaolo et
al., 1992). If a magma is generated by one set of processes
at one location but solidifies elsewhere, its evolution is
further complicated by other processes associated with
its transport and emplacement—such as intrusion-related
mingling of two or more magma types, differentiation
by crystal accumulation and melt migration, multiple
reintrusion, and supersolidus and subsolidus mineral reactions—that are likely to obscure the earlier history.
In some plutonic bodies, however, textures or mineral
assemblages are preserved that provide a broader framework for the interpretation of chemical and isotopic data.
The Bodocó pluton in northeastern Brazil is a distinctive
example of a magma that spent most of its syn- and postemplacement history as a crystal-rich mush in a region
that was undergoing shear-related deformation, and as
such it provides an excellent opportunity to study the
relationship between crystallinity, geodynamics, and magmatic processes.
The Bodocó pluton is a conspicuously porphyritic
quartz monzonite characterized by blocky megacrysts of
perthitic K-feldspar, generally 2–5 cm in length, in a
coarse-grained matrix of plagioclase, hornblende, biotite,
and quartz (Fig. 1). Many of the megacrysts have megascopic oscillatory zoning. Although texturally unusual,
the Bodocó pluton is not unique. At least 80 similar
intrusions have been identified in northeastern Brazil
∗Present address: Atomic Energy of Canada Limited, Whiteshell
Laboratories, Pinawa, Man. R0E 1L0 Canada. Telephone: (204)
753-2311. Fax: (204) 753-2455. E-mail: [email protected]
 Oxford University Press 2001
KEY WORDS:
accumulation; Brazil; megacryst; petrogenesis; shearing
INTRODUCTION
JOURNAL OF PETROLOGY
VOLUME 42
FEBRUARY 2001
sheared rock types, abundant microgranitoid enclaves,
rapakivi overgrowths of plagioclase on K-feldspar, and
large-scale reverse zoning in which the intrusion has a
felsic outer margin and a more mafic central region—
have been cited as circumstantial evidence for magma
mixing (Hackspacher et al., 1987; Jardim de Sá et al.,
1987; Neves & Vauchez, 1995b), although the chemical
and isotopic data do not require such a process (McMurry
et al., 1987). The Bodocó pluton was selected for detailed
analytical and field study for two reasons. First, it is
a well-exposed example of a typical Itaporanga-type
granitoid. The extent and similarity of these megacrystic
intrusions throughout northeastern Brazil suggest that
the processes and conditions that generated them were
not localized but are more indicative of widespread and
consistent processes at depth. Second, the importance of
post-emplacement processes is indicated in a structurally
high part of the Bodocó pluton, where late-stage shearing
appears to have promoted melt migration and magma
mingling in the final stages of differentiation. The localized shearing, which can be related structurally to
nearby regional-scale shear zones, may provide a general
insight into the relationship between geodynamics and
petrological diversity in many felsic intrusions, including
the Itaporanga-type granitoids.
Fig. 1. Typical texture of megacrystic quartz monzonite, Bodocó
pluton. Large rectangular light-coloured crystals are K-feldspar; most
of the smaller and more equant light-coloured crystals are plagioclase.
Dark grains are glomerocrystic intergrowths of hornblende and biotite.
(Note the faint oscillatory zoning in the twinned megacryst.) Side of
compass: 7 cm.
(e.g. Brito Neves & Pessoa, 1974; Mariano & Sial, 1990;
Neves & Vauchez, 1995b), where they are termed
‘Itaporanga-type’ granitoids (Almeida, 1971). In general,
these bodies are associated with the Brasiliano–PanAfrican orogeny (>700–500 Ma), the last widespread
tectonothermal event to have affected northeastern Brazil
(Almeida et al., 1981). The Brasiliano orogeny was initiated by the intrusion of a suite of mafic dykes and
by early thermal metamorphism associated with folding
(Brito Neves et al., 1974; Almeida et al., 1981), indicative
of a tectonic environment in which basaltic underplating
and dyking regionally heated and softened the lower
crust (Hildreth, 1981). The climax of the orogeny, about
630 ± 30 Ma, was characterized by regional high-temperature low-pressure metamorphism during which a
transpressional shear regime produced upright and inclined folds accompanied by voluminous intrusions of
felsic magma ( Jardim de Sá et al., 1987; Corsini et al.,
1991; Vauchez & Egydio-Silva, 1992). Later stages of
the orogeny were characterized by intrusion of small,
isolated bodies of leucogranite, followed by regional cooling (Long & Brito Neves, 1977) and by the aligned
intrusion of peralkaline syenite dykes (Sial & Long, 1978;
Ferreira & Sial, 1987; Sial et al., 1987).
The textures and field relations of many of the
Itaporanga-type granitoids—including mingled and
NUMBER 2
TECTONIC SETTING
The pluton is located in the Borborema tectonic province
in northeastern Brazil (Almeida et al., 1981; Vauchez et
al., 1995). The defining structural features and rock
types of the tectonic province were developed during the
Brasiliano–Pan-African orogeny. Dextral shear zones,
some of which extend for hundreds of kilometres, divide
the tectonic province into a mosaic of metasedimentary
fold belts separated by older crystalline basement (Fig.
2). Major axes of folds conform to the regional trend
within each fold belt.
The Bodocó pluton forms an elongated northeastoriented intrusion 35 km in length on the western margin
of the Cachoeirinha–Salgueiro Fold Belt, midway between the Patos Shear Zone and the West Pernambuco
Shear Zone (Fig. 2). It intruded along a major contact
between fine-grained felsic gneisses and schists of the
Uauá Group and biotite–sillimanite schists of the Salgueiro Group (Dantas, 1974). The West Pernambuco
Shear Zone, 45 km south of the pluton, terminates in a
splayed, fanlike structure that passes at a high angle
into the Cachoeirinha–Salgueiro Fold Belt. Vauchez &
Egydio-Silva (1992) concluded that during Brasiliano-age
deformation, the strain that accommodated the northern
block of the West Pernambuco Shear Zone was transferred into hot, ductile continental crust, where the
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CRYSTAL ACCUMULATION AND SHEARING IN BODOCÓ PLUTON
Fig. 2. Major shear zones and fold belts of the Borborema tectonic province. Inset (after Neves & Vauchez, 1995b) shows location of the study
area in relation to regional Brasiliano-age tectonic features and igneous activity. CSFB, Cachoeirinha–Salgueiro Fold Belt; WPSZ, West
Pernambuco Shear Zone; PSZ, Patos Shear Zone.
transpressional forces resulted in northeast-trending folding, stretching, and more localized strike-slip faulting.
Such localized deformation is indicated in the Bodocó
pluton by several northeast-oriented, kilometre-wide
bands of strongly foliated megacrystic quartz monzonite
that traverse the length of the pluton (Fig. 3). There is
little or no evidence for fault-related deformation in
the metamorphic country rocks along strike with these
internal shear zones; metamorphic foliations near the
contact generally conform to the shape of the pluton
margins. Measurement of small structural features in
megacrystic rocks is difficult (Bateman, 1989) but the
shapes and overall lack of consistent orientation of mafic
enclaves suggest that the pluton in general is undeformed.
With the exception of enclaves in the localized shear
zones and enclaves near pluton margins—which are
elongate in plan view, have near-vertical dips and appear
flattened parallel to the contact—most enclaves are
roughly circular in plan view. Similar localization of
shearing has been observed for other Brasiliano-age plutons, where it has been attributed to rheological heterogeneities introduced by the intrusion and solidification
of high-temperature magma (Tommasi et al., 1994; Neves
& Vauchez, 1995a).
The Bodocó pluton was exposed by erosion during the
Cretaceous, at which time it became covered by silty red
clays and evaporite deposits of the Araripe Basin. These
sedimentary rocks now form a regionally extensive topographic high, the Araripe Plateau (Fig. 2), that conceals
the northwest margin and most of the central third of
the pluton.
ROCK TYPES
With few exceptions, the Bodocó pluton is characterized
by a mineral assemblage of plagioclase, K-feldspar, hornblende, biotite, and quartz, with accessory titanite, apatite, Fe–Ti oxides, zircon, allanite, and clinopyroxene.
The megacrystic portion of the pluton is reversely zoned,
from a granitic and granodioritic eastern margin to a
quartz monzodioritic core (Fig. 3). Most of the pluton
consists of quartz monzonite, of which several varieties
can be distinguished on the basis of texture. Mafic dykes,
which are common in the localized shear zones, and mafic
enclaves, which are common throughout the pluton,
generally range in composition from diorite to monzonite.
Megacrystic quartz monzonite
253
Approximately 80% of the Bodocó pluton has typical
Itaporanga-type characteristics. Blocky, subhedral megacrysts of K-feldspar, many of which are >3–5 cm in
length, are dispersed in a matrix of plagioclase and of
JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 2
FEBRUARY 2001
Fig. 3. Geological map of the Bodocó pluton, with main rock types and textural distinctions.
glomerocrystic hornblende and biotite, with minor quartz
and titanite (Fig. 1). In some locations the tabular megacrysts are unevenly distributed in clusters and layers.
Where a shear-related foliation is not superimposed,
megacrysts either are unoriented or else are subparallel
in curvilinear trains of crystals. There is almost no interstitial K-feldspar, but where present it consists of finegrained anhedral microcline with sharply developed grid
twinning. In contrast, the K-feldspar megacrysts are
coarsely perthitic microcline, with patchy and faint grid
twinning. Their megascopic shape is rectangular and
subhedral, but microscopically they have rounded corners
and irregular edges that are rimmed by an anhedral
intergrowth of very fine-grained plagioclase, microcline,
quartz, biotite, hornblende, and lobate myrmekite.
Rarely, megacrysts have plagioclase overgrowths. Many
megacrysts have oscillatory zoning consisting of concentric albitic exsolution lamellae, visible in hand specimen, that outline a nested set of euhedral growth faces
(Fig. 4).
Plagioclase forms weakly twinned, slightly sericitized
subhedral crystals <1 cm in length. Some grains have
ellipsoidal cores fretted with biotite inclusions. Pleochroic
blue–green hornblende and dark green biotite occur
mainly as large glomerocrysts (maximum dimension
2–3 cm), the interiors of which typically contain several
large (0·5 cm) anhedral hornblende crystals. Small biotite
flakes rim the outside of the glomerocrysts and form finegrained, anastomosing intergrowths with fine-grained plagioclase between glomerocrysts. Some hornblende grains
have patchy cores of clinopyroxene or pale green amphibole rimmed by very fine-grained Fe–Ti oxides.
Quartz occurs as irregularly shaped equant to slightly
elongated pools of several composite grains. Accessory
minerals titanite and apatite are associated almost exclusively with hornblende and biotite in the glomerocrysts.
Titanite is present as slightly pleochroic (pink to tan)
subhedral crystals and as secondary fine-grained blebs
adjacent to biotite. Apatite occurs as discrete stubby
prisms associated with other mafic minerals in the glomerocrysts and as slender euhedral inclusions in biotite
and hornblende. Fe–Ti oxide minerals are conspicuously
rare, occurring for the most part as small grains within
or adjacent to hornblende.
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CRYSTAL ACCUMULATION AND SHEARING IN BODOCÓ PLUTON
Fig. 4. Oscillatory zoning in K-feldspar from the Bodocó pluton. (a)
Concentric growth faces in a weathered megacryst (centre of photo).
Diameter of lens cover: 50 mm. (b) Photomicrograph of fine-scale
oscillatory zoning, now preserved as very fine-grained albitic exsolution
lamellae, in a K-feldspar phenocryst from porphyritic quartz monzonite.
Field of view: 5 mm.
Fig. 5. Foliated megacrystic quartz monzonite. (Note the small mafic
enclave below the ruler that has been disrupted by shearing.) Length
of ruler: 15 cm.
Monzonite dykes and enclave swarms
The foliated rocks in the kilometre-wide shear zones are
intruded by mafic dykes several metres wide and tens of
metres long, with a pronounced vertically dipping
foliation marked by hornblende and biotite. The dykes
typically are composite, with two or more intermingled
textural or modal variants of monzonite, monzodiorite,
and quartz monzodiorite (Fig. 6), and they generally
have sharp contacts with the foliated megacrystic rocks.
Some dykes have ptygmatically folded felsic veinlets.
The kilometre-wide shear zones also contain swarms of
elongated mafic enclaves in which the orientation of the
enclaves is parallel to the strike of the foliation in the
deformed megacrystic rocks.
Foliated megacrystic quartz monzonite
Ductile (solid-state) deformation fabrics in megacrystic
quartz monzonite are observed in the kilometre-wide
shear zones that parallel the regional northeast structural
trend and the direction of elongation in the pluton
(Fig. 3). The K-feldspar megacrysts define the foliation,
although the amount of deformation varies on the scale
of tens of centimetres (Fig. 5). The foliated megacrysts
appear either flattened and elongated or lens-shaped,
and most have pressure shadows of optically continuous
microcline on their terminations. Few large felsic mineral
grains, including megacrysts, are fractured, but strained
and undulatory extinction is common in them. Quartz
forms ribbon-like composite grains. There is some mortar
texture of very fine-grained feldspar, quartz, biotite, and
chlorite in narrow spaces between the megacrysts, but
otherwise there is little petrographic evidence for cataclastic deformation.
Heterogeneous upper unit
At least one of the kilometre-wide shear zones on the
northwest side of the pluton intersects a structurally
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FEBRUARY 2001
Fig. 6. Composite mafic dyke intruding foliated megacrystic quartz
monzonite. Side of compass: 7 cm.
higher zone exposed near the top of the 300 m escarpment
of the Araripe Plateau (Fig. 3). The rocks in this upper
region include complexly intermingled and heterogeneous mafic–felsic rocks that are texturally distinct
from the remainder of the pluton (Fig. 7). In contrast to
the megacrystic rocks in the lower part of the shear
zone, there is no textural evidence for ductile (solidstate) deformation, but disequilibria textures are common.
Many of the mafic rocks contain embayed and rounded
K-feldspar megacrysts with rapakivi overgrowths, and
they have xenocrystic pods (1–5 mm) of quartz that are
rimmed either by hornblende or by fine-grained felsic
minerals. The mafic rocks have abundant nebulitic swirls
and diffuse patches of fine-grained interstitial microcline.
Where such K-feldspar is present, the adjacent hornblende crystals are coarser, more abundant, and more
euhedral than elsewhere in the rock. Apatite forms abundant acicular inclusions in fine-grained plagioclase and in
fine-grained interstitial K-feldspar, but not in the rounded
K-feldspar megacrysts.
The more felsic portions of the heterogeneous rocks
are streaked with nebulitic mafic patches, and many
feldspar crystals are mantled by rapakivi overgrowths.
The cores of plagioclase crystals are lozenge-shaped
and fretted with biotite. Most K-feldspar is fine-grained,
anhedral microcline. Megacrysts of K-feldspar are generally smaller and less abundant than elsewhere in the
pluton but locally some are very large, exceeding 15 cm
in length. In this upper part of the pluton there are
also a few isolated outcrops of undeformed equigranular
granite and megacrystic granite that do not seem to have
interacted with mafic magma during crystallization.
Quartz monzonite border units
Two non-megacrystic varieties of quartz monzonite are
exposed in the southern part of the pluton (Fig. 3).
The outermost border unit is a sheared medium-grained
Fig. 7. A heterogeneous rock from upper unit of pluton, with felsic
and mafic phases distributed diffusely throughout the rock. Pencil at
left of photo: 10 cm.
quartz monzonite in which a pronounced foliation is
marked by slender, aligned phenocrysts of translucent
and slightly iridescent dark grey microcline with thick
overgrowths of opaque white microcline (Fig. 8). The
phenocrysts have sharply defined oscillatory zoning like
that in the megacrystic rocks, but the zoning does not
extend into the overgrowths. In the most strongly foliated
samples, the overgrowth material is thicker at the terminations than on the sides of the phenocrysts, and it
pinches out and merges with interstitial K-feldspar in the
matrix. The foliation is accentuated by layered segregations of dark-coloured hornblende and biotite, giving the rock a layered appearance. On the scale of tens
of centimetres, the foliation splays into a plumose, semiradiating, sigmoidal pattern of C- and S- structures
that elsewhere has been ascribed to shearing of an
incompletely crystallized magma (Ramsey, 1982; Blumenfeld & Bouchez, 1988). In this ‘plumose’ border unit,
few of the individual minerals except the K-feldspar
phenocrysts have a shape-preferred orientation. There is
no evidence of intracrystalline deformation, suggesting
that deformation ceased while melt was still present, so
that the final stages of crystallization did not occur during
shearing.
The second and more interior border unit of quartz
monzonite, termed ‘porphyritic’ quartz monzonite in Fig.
3 to discriminate it from the plumose unit, is almost
identical in appearance to the main, megacrystic rocks
except that it has much smaller K-feldspar phenocrysts,
which rarely exceed 1 cm in length, and the remainder
of the mineral assemblage is also proportionately finer
grained. Relict cores of clinopyroxene in hornblende are
slightly more common in this border unit than in the
megacrystic rocks. The porphyritic quartz monzonite is
generally unfoliated, except along a transitional contact
with the plumose quartz monzonite, where sheared bands
256
McMURRY
CRYSTAL ACCUMULATION AND SHEARING IN BODOCÓ PLUTON
Fig. 8. ‘Plumose’ foliation in the outermost quartz monzonite border
unit. Aligned K-feldspar phenocrysts and layered aggregates of hornblende and biotite define a semi-radiating foliation over a distance of
tens of centimetres. Diameter of lens cover: 50 mm.
of the porphyritic quartz monzonite alternate with
sheared bands of plumose quartz monzonite on a scale
of tens of centimetres. Foliated plagioclase and K-feldspar
crystals in the porphyritic quartz monzonite from this
area are broken, bent, or lens-shaped, suggesting that
the porphyritic magma had a higher crystal:melt ratio
than the plumose quartz monzonite magma when the
two were intermingled. Perthitic K-feldspar phenocrysts
with sharply defined oscillatory zoning have been broken
across the concentric rings and subsequently overgrown
by unzoned K-feldspar rims. Minor areas of very finegrained plagioclase and biotite suggest grain-size reduction as a result of shearing (Blumenfeld & Bouchez,
1988).
Other rock types
Microgranitoid mafic enclaves of a variety of sizes, shapes,
and compositions are common throughout the pluton
and have the same set of minerals as the other rock
types. Most enclaves are an equigranular intergrowth of
plagioclase, hornblende, biotite, and K-feldspar, with
accessory titanite and some quartz. Many have glomerocrysts of hornblende (± ragged cores of clinopyroxene), biotite, and titanite that are more resistant
to weathering than the surrounding matrix, giving the
enclaves a speckled grey-and-black appearance in outcrop, and many have rounded, coarse-grained plagioclase
crystals with biotite-fretted cores. Some of the enclaves,
particularly the more silicic compositions, have anhedral
pods of quartz that are rimmed either by anhedral
hornblende and microcline or by very fine-grained plagioclase and biotite. Many enclaves contain abundant
Fig. 9. Bottom portion of two large mafic enclaves, transected by an
aplite dyke, in a vertical exposure of megacrystic quartz monzonite.
rounded or ellipsoidal K-feldspar megacrysts, including
one example of an enclave in megacrystic quartz monzonite in which the portion of the megacryst in the quartz
monzonite is euhedral and tabular, but the portion of
the megacryst protruding into the enclave is rounded
and has an optically continuous overgrowth of very finegrained hornblende and biotite.
Most enclaves are roughly discoid or slightly elongate
and bulbous in plan view, and they range in size from
several centimetres in diameter to tens of centimetres.
Large enclaves exposed in cross-section typically have
rounded tops and narrow with depth, terminating in a
tattered fringe of mafic enclave ‘fingers’ (Fig. 9). The
enclaves in the quartz monzonite border units tend to be
proportionately smaller and finer grained than enclaves in
the megacrystic rocks, and, unlike in the interior of the
pluton, the border unit mafic enclaves generally are
elongated in plan view and flattened parallel to the pluton
margin. Oriented swarms of texturally and compositionally diverse, elongated enclaves are locally conspicuous. In these swarms, few of the enclaves are in
direct contact with each other but instead are separated
by thick, nearly monomineralic seams of either pink
megacrystic K-feldspar or very coarse-grained hornblende.
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Water (SMOW). Precision of analyses, including extraction procedures, was about 0·1‰. Microprobe analyses were performed using a JEOL JXA-733 electron
microprobe, with an accelerating voltage of 15 kV and
a beam current of 20 nA. All compositions were determined by wavelength-dispersive analysis using natural
and synthetic standards.
Narrow (10–20 cm), fine-grained, late-stage monzonite
and monzodiorite dykes are the least abundant mafic
rock type. Small aplite dykes that crosscut all other rock
types are ubiquitous, but the pluton contains almost no
pegmatite.
GEOCHEMISTRY
Geochemical analysis was based on 85 whole-rock
samples from the pluton and from adjacent country rocks.
Sample locations are indicated in Fig. 3. (Sample numbers
correspond to the location number; multiple samples
from a single location are designated by A, B, C, etc.)
A representative selection of the geochemical data is
presented in Table 1. A larger dataset, including unpublished data, was used to construct Figs 10–12.
Methods
To provide chemically representative data, all samples,
including megacrystic rocks, were at least an order of
magnitude greater in volume than their coarsest crystals
(McMurry, 1995). The largest samples were crushed and
divided repeatedly with a Jones splitter, after which a
fraction of the sample was powdered in a tungsten carbide
shatterbox mill. Whole-rock powders were prepared for
chemical analysis by fusing the sample with lithium
metaborate flux and dissolving in a dilute HNO3 solution.
Major oxide data were obtained by atomic absorption
spectra-photometry (AA), except for Na and K, which
were determined by flame photometry, and P, which
was determined by inductively coupled plasma spectraphotometry (ICP). Trace element data for Ba, Sr, Y, and
Zr were obtained by ICP and for Rb by flame emission
for the entire suite of samples. Additional trace element
data [rare earth elements (REE), Rb, Sr, Ba, Zr, Sc, Cr,
Co, Cs, Hf, U, Th, and Ta] were obtained for 15 samples
by instrumental neutron activation analysis (INAA). Rb
isotopic ratios were measured on a 30 cm radius, 60°
sector field, solid-source mass spectrometer using a 3·5 kV
ion beam acceleration. Strontium isotopic ratios were
measured on an automated Finnegan-MAT 261 sevencollector, solid-source mass spectrometer. Corrections for
Sr mass fractionation were applied assuming 86Sr/88Sr =
0·1194. The decay constant used in isochron calculations
was 1·42 × 10−11 a−1. Oxygen for isotopic analysis was
liberated from whole-rock and quartz samples in an
extraction line using ultrapure fluorine gas according to
methods described by Mariano et al. (1990) and was
converted to CO2 by reaction with a hot graphite rod.
The ratio 18O/16O was measured from CO2 gas on
a triple-collector, dual gas inlet Finnegan-MAT mass
spectrometer. Analyses were normalized to a rose quartz
standard for comparison with Standard Mean Ocean
Major oxides
The plumose quartz monzonite border unit and most
samples of the megacrystic rocks have a limited range of
SiO2, from about 60 to 64 wt % (Table 1). The porphyritic
quartz monzonite border unit has slightly lower SiO2
(58–60 wt %) than the megacrystic rocks that it resembles
texturally. Most mafic dykes and mafic enclaves have
SiO2 between 52 and 56 wt %. A few of the dykes are
as silicic as many of the quartz monzonites but contain
xenocrystic quartz. The entire plutonic suite is metaluminous in the sense of Shand (1951), with the exception
of several aplites and one felsic upper-zone granite (61F),
which are slightly peraluminous. Major oxides correlate
linearly with SiO2 on variation diagrams for the various
rock types (Fig. 10), with the exception of Al2O3, for
which data are scattered for the more mafic rock types,
and Na2O and K2O, for which data are scattered but
high throughout. Spatial variation of several major oxides—particularly SiO2, MgO, and CaO—delineates
more finely the reverse chemical zoning of the megacrystic
rocks, from felsic east and west margins to a more mafic
central region.
Trace elements
Zirconium proved to be the most effective single chemical
discriminator of the three main textural varieties of quartz
monzonite in the pluton (Fig. 11a). Quartz monzonite
from the plumose border unit has high Zr (330–480 ppm
for most samples), in contrast to the megacrystic quartz
monzonite, which for the most part has between 250
and 350 ppm Zr. Samples of the porphyritic border unit
generally have <200 ppm Zr, and in this respect they
resemble a subgroup of low-Zr samples of megacrystic
quartz monzonite and quartz monzodiorite from the
central region of the pluton. Mafic dykes are bimodal
with respect to Zr, containing either <100 ppm Zr or
250–300 ppm Zr. Concentrations of Zr are highly variable in mafic enclaves (Fig. 11a).
Rubidium concentrations are generally limited to between 95 and 175 ppm, regardless of rock type (Fig.
11a). Most of the mafic dykes sampled have low Rb
(<120 ppm), but Rb in mafic enclaves varies from 100
to 260 ppm.
258
259
16A
18A
28A
40A
48A
57A
63A
64A
qz monzonite∗
qz monzonite∗
qz monzonite
qz monzonite
qz monzonite
qz monzonite
qz monzonite
qz monzonite
67A
71A
qz monzodiorite
qz monzodiorite
qz monzodiorite
75B
qz monzonite
qz monzonite
22D
29A
38A
52A
75A
qz monzonite
qz monzonite
qz monzonite
qz monzonite
qz monzonite
‘Plumose’ border unit
22C
74A
qz monzonite∗
Porphyritic border unit
73A
59A
qz monzonite
68B
13A
qz monzonite
70A
11C
qz monzonite
qz monzonite
6A
qz monzonite∗
5A
77A
granite
qz monzonite
76A
granite
granodiorite
56A
granite
Megacrystic units
BOD-
rock type
61·50
61·34
61·29
58·71
63·76
59·39
57·89
59·94
61·38
60·63
63·67
60·70
64·52
62·65
63·39
62·47
63·56
61·42
63·12
61·15
61·91
62·74
60·93
62·13
63·28
66·36
67·27
65·82
66·30
SiO2
Sample wt%
Map unit and
0·69
0·61
0·73
0·79
0·67
0·78
0·66
0·78
0·93
0·92
0·78
0·81
0·68
0·74
0·70
0·76
0·67
0·65
0·70
0·72
0·73
0·67
0·78
0·66
0·56
0·51
0·53
0·56
0·59
TiO2
15·80
16·26
15·87
17·92
15·25
16·12
16·09
16·28
15·11
15·81
15·69
15·89
15·55
15·49
15·78
15·50
15·61
15·81
15·93
15·69
15·43
15·52
15·72
15·99
16·05
15·31
15·52
15·14
14·70
Al2O3
1·32
1·49
1·49
1·09
1·33
1·46
1·42
1·41
2·80
2·20
1·40
1·76
1·64
1·54
1·59
2·48
1·29
1·31
1·52
1·74
1·37
1·39
1·80
1·33
1·47
1·65
0·91
1·66
1·33
Fe2O3
2·80
2·78
2·59
2·49
2·54
3·34
3·74
3·46
2·89
3·49
2·86
3·37
2·69
2·91
2·77
2·94
2·82
2·63
2·65
2·95
3·02
2·74
3·02
2·59
1·99
2·09
2·49
2·32
2·51
FeO
2·74
2·83
2·64
2·55
2·45
2·93
3·48
3·25
3·35
3·50
2·53
3·17
2·37
2·47
2·39
2·74
2·23
2·18
2·36
2·59
2·52
2·18
2·92
2·28
1·70
1·79
1·61
1·94
1·92
MgO
3·69
3·83
3·41
3·97
3·27
3·64
4·45
4·39
4·82
5·24
3·89
4·21
3·80
3·81
4·08
3·88
3·39
3·83
3·72
4·22
4·14
3·66
4·61
3·61
2·88
2·98
2·83
3·27
3·05
CaO
3·72
3·86
3·85
4·69
3·77
3·89
3·52
3·79
3·93
4·23
4·13
3·57
4·01
4·00
3·96
3·88
3·95
4·03
4·07
3·99
3·99
3·84
4·21
3·96
3·83
4·25
3·99
3·89
3·65
Na2O
Table 1: Representative whole-rock chemical analyses, Bodocó pluton
5·69
5·61
5·87
5·01
5·80
5·35
5·41
5·04
3·71
3·75
4·53
5·03
4·53
3·79
4·84
4·75
5·14
5·10
5·11
4·89
4·61
5·37
4·53
5·45
6·08
4·28
5·03
4·64
5·25
K 2O
0·44
0·44
0·43
0·48
0·37
0·51
0·53
0·51
0·60
0·60
0·45
0·51
0·47
0·50
0·48
0·50
0·46
0·45
0·47
0·49
0·46
0·41
0·49
0·41
0·42
0·42
0·37
0·43
0·40
P2O 5
98·4
99·0
98·2
97·7
99·2
97·4
97·2
98·9
99·5
100·4
99·9
99·0
100·3
97·9
100·0
99·9
99·1
97·4
99·7
98·4
98·2
98·5
99·0
98·4
98·3
99·6
100·6
99·7
99·7
Total
174
144
188
147
158
153
132
141
107
107
118
123
116
125
125
128
143
120
116
123
125
134
108
122
156
140
156
132
150
Rb
ppm
1273
1592
1313
2431
1257
1405
1585
1574
1259
1407
1335
1327
1236
1072
1290
1266
1267
1454
1419
1466
1269
1325
1381
1437
1360
1087
1047
1100
1097
Sr
22
16
17
13
19
24
21
23
21
23
16
28
20
15
19
19
18
16
21
19
24
17
18
19
16
15
18
21
16
Y
360
329
368
285
384
210
131
134
161
89
264
164
273
192
292
150
281
292
266
340
244
268
334
264
264
273
242
281
263
Zr
2691
3260
2845
3906
2491
3143
3827
3281
1906
2129
2482
3035
2251
1877
2544
2675
2708
3124
2935
2969
2511
3068
2561
3464
3524
1832
1994
1961
2467
Ba
McMURRY
CRYSTAL ACCUMULATION AND SHEARING IN BODOCÓ PLUTON
59·47
60A
61B
monzonite
qz monzodiorite
260
19A
19B
qz monzodiorite
qz monzodiorite
24B
granite
63B
12A
monzodiorite
monzonite
∗Foliated (shear zone).
10A
56C
diorite
diorite
73·43
59·26
55·51
53·13
53·66
68·23
67·46
0·21
0·52
1·90
1·21
1·09
0·36
0·70
0·09
1·46
0·73
1·43
0·88
1·18
1·10
1·09
0·80
0·97
0·82
0·44
19·59
15·86
15·20
17·46
16·13
15·13
14·08
16·03
17·00
14·48
17·58
17·12
17·46
15·80
18·59
16·75
16·14
13·28
14·26
14·15
14·74
Al2O3
2·04
2·25
2·69
1·83
0·78
1·00
0·42
2·84
1·42
2·15
1·90
2·66
2·10
2·20
1·57
1·56
1·97
1·02
0·33
0·78
1·07
Fe2O3
2·13
5·33
5·77
4·91
1·19
2·27
0·54
5·38
3·76
5·36
3·29
4·70
4·77
5·40
3·25
4·69
3·57
1·58
1·07
1·04
1·23
FeO
1·23
4·55
6·77
4·06
0·79
0·93
0·67
5·33
2·47
7·19
2·72
4·59
3·77
5·77
2·47
3·40
2·87
1·06
0·61
0·62
2·55
MgO
2·67
5·94
6·35
5·26
1·84
3·03
1·04
7·01
4·84
6·40
4·77
6·14
5·85
6·70
3·75
5·50
4·65
2·09
1·40
1·40
1·82
CaO
5·50
4·01
4·44
4·33
4·31
3·64
4·25
3·64
4·27
2·81
4·19
4·67
4·29
3·73
5·03
4·28
4·13
3·67
4·28
3·70
4·03
Na2O
5·28
3·72
3·05
4·87
4·89
4·21
4·67
3·94
3·47
5·21
4·95
3·85
4·13
3·97
4·80
4·03
4·43
4·06
4·58
5·06
5·11
K 2O
0·31
1·04
0·75
0·74
0·19
0·21
0·04
0·87
0·46
0·86
0·48
0·63
0·67
0·67
0·44
0·69
0·59
0·24
0·11
0·14
0·22
P2O 5
98·5
100·1
99·4
98·2
98·7
98·6
99·2
100·1
99·4
98·5
99·5
99·2
99·6
99·2
98·2
98·9
98·6
97·6
96·7
96·7
100·3
Total
113
135
174
147
114
150
132
96
95
190
104
132
102
110
163
104
109
113
123
190
158
Rb
1616
1825
787
1631
1284
364
269
1469
1082
1358
2127
1513
1482
1246
1667
1620
1355
740
690
388
676
Sr
29
23
32
27
8
9
4
30
18
31
18
22
21
21
14
24
23
11
9
10
12
Y
707
69
80
32
219
201
87
24
276
19
306
56
43
39
292
48
76
181
95
193
153
Zr
2675
3402
614
3199
2661
802
691
3658
2560
2911
5134
3041
3793
3006
2915
3285
2827
1753
1825
1360
1682
Ba
NUMBER 2
Mafic enclaves
8A
13C
granite
granite
53·58
60·94
52·63
58·72
53·66
55·45
53·85
57·51
57·07
69·84
0·32
0·34
TiO2
ppm
VOLUME 42
Aplite dykes
12B
21B
monzonite
monzonite
16C
48B
monzodiorite
monzodiorite
2B
16B
monzodiorite
monzodiorite
Mafic dykes
70·12
62A
62E
granite
granite
69·18
69·51
61E
61F
granite
granite
Heterogeneous (upper) unit
BOD-
rock type
SiO2
Sample wt%
Map unit and
Table 1: continued
JOURNAL OF PETROLOGY
FEBRUARY 2001
McMURRY
CRYSTAL ACCUMULATION AND SHEARING IN BODOCÓ PLUTON
Fig. 10. Major oxide variations with respect to SiO2.
Strontium and barium concentrations vary systematically and are high throughout the pluton (Fig.
11b), though typical of Itaporanga-type granitoids (Sial
et al., 1987). In the megacrystic rocks, Sr ranges from
1000 to 1500 ppm. Concentrations in the porphyritic
border unit are slightly higher overall (1400–1800 ppm).
The plumose border unit has 1250–1650 ppm Sr, with
the exception of one anomalously high value (>2400
ppm). Strontium concentrations in the mafic dykes and
enclaves vary widely, up to 2150 ppm, and do not
correlate well with SiO2 compared with most other
elements. In the southwestern portion of the pluton, Ba
exceeds 4000 ppm in one small, late-stage monzonite
dyke (Fig. 11b). The metamorphic country rocks adjacent
to this part of the pluton are intruded by several small
clinopyroxene-bearing syenite dykes and by a clinopyroxene-bearing quartz monzonite that also are very
enriched in Ba, up to 1·4 wt % (Fig. 11b). The consistently
high Ba and Sr concentrations suggest that elevated
values of these elements may be hydrothermal rather
than magmatic in origin. Contrary to what might be
expected from pervasive hydrothermal alteration, however, the gneisses and schists that host the intrusions have
low Ba and Sr (Fig. 11b).
Concentrations of REE, normalized to chondritic
abundances, correspond to those observed for other
Itaporanga-type plutons in northeastern Brazil (Sial et al.,
1981; Sial, 1987). Analysed samples of megacrystic quartz
monzonite, of quartz monzonites from the border units,
of two mafic dykes and a mafic enclave are all strongly
enriched in light REE (LREE) and plot parallel to each
other within a narrow field (Fig. 12). None of these
samples has a europium anomaly. Analysed samples with
slightly different REE signatures include a monzonite
dyke that has elevated heavy REE (HREE), an aplite
that has lower REE than the other samples, and an
equigranular granite from the upper part of the pluton
that has a slight Eu anomaly.
Mineral compositions
Regardless of rock type, most plagioclase in the pluton
is unzoned or weakly zoned oligoclase, An17–21 (Table 2).
However, plagioclase crystals from the porphyritic border
unit have pronounced normal zoning, with cores in the
compositional range An37–25 and oligoclase rims (An18–22).
Phenocrysts in some of the heterogeneous rocks in the
upper part of the pluton are reversely zoned (An22 rims
and An17 cores).
Microprobe analyses of K-feldspar in the megacrystic
quartz monzonite as well as other rock types indicate
that the proportion of albite component in solid solution
is generally >8–9% (Table 2). K-feldspar phenocrysts
in both of the quartz monzonite border units are zoned
with respect to Ba, with BaO consistently >0·6 wt %
higher in phenocryst cores than in rims. Zoned Ba
concentration has been used qualitatively to justify a
261
JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 2
FEBRUARY 2001
magmatic origin for megacrysts in felsic rocks (Mehnert &
Busch, 1981; Vernon, 1986), including other Itaporangatype granitoids (McMurry et al., 1987). However, megacrysts in the Bodocó pluton lack sharply defined Ba
zoning, with non-systematic BaO variations of between
1·0 and 1·3 wt % in a traverse of a typical megacryst
(Table 2), perhaps as a result of Ba diffusion during slow
cooling.
All Bodocó amphiboles are members of the calcic
amphibole group and are further characterized as hornblende, magnesio-hornblende, or edenite [terminology
and classification after Leake (1978)] (Table 3). Several
mafic dykes and enclaves have low-Si amphiboles, with
compositions clustering near the edenitic hornblende and
ferro-edenitic hornblende boundary. One mafic enclave,
56C, contains actinolitic hornblende.
Biotite is relatively homogeneous (Table 3), with most
samples having Fe/(Fe + Mg) values of 0·3–0·4. In
general, TiO2 content in biotite is highest in the cores of
coarse-grained flakes, in biotite inclusions in the ‘fretted’
interiors of large plagioclase grains, and in mafic enclaves.
Intensive variables
Fig. 11. Trace element variation: (a) Rb and Zr; (b) Sr and Ba.
Fig. 12. Chondrite-normalized (Haskin, 1979) values for rare earth
elements. Most rocks analysed (megacrystic samples 48A, 56A, 63A,
71A, 76A; quartz monzonite border unit samples 75A, 75B; mafic dyke
and mafic enclave samples 16B, 48B, and 63B) plot parallel to each
other within the narrow shaded field.
In the pelitic schists of the Salgueiro Group adjacent to
the pluton, the paragenesis of staurolite plus sillimanite,
biotite, and pseudomorphs of garnet suggests a regional
minimum pressure during the peak of Brasiliano-age
metamorphism of 3–4 kbar, depending on the amount
of spessartine originally in the garnet (Miyashiro, 1973).
The generally undeformed Bodocó pluton must have
been intruded after the peak metamorphic event, but
this regional pressure estimate is in general agreement
with an estimate of pluton emplacement depth of 3 ±
1 kbar based on Al-in-hornblende geobarometry (Hollister et al., 1987). Because the pluton has variable amphibole
rim compositions, the hornblende geobarometer produces a range of pressure estimates.
During most of its crystallization history, the Bodocó
magma was probably undersaturated in H2O. Petrographic evidence for undersaturation is provided by the
supersolidus reaction of clinopyroxene and melt to form
hornblende, an equilibrium that requires >3–3·5 wt %
H2O at upper- to middle-crustal levels (Eggler, 1972). The
assemblage of quartz, ferromagnesian silicates, euhedral
titanite, and magnetite is characteristic of a magma with
a relatively high oxygen fugacity, although the ratio of
Mg/(Mg + Fe) in hornblende, 0·30–0·50, is less than
expected for these relatively oxidizing conditions (Wones,
1989).
The textural relationship between hornblende and
biotite in the Bodocó samples, in which granoblastic
hornblende typically forms the centre of glomerocrysts
and biotite occurs around the outer edges, suggests that
262
BOD-
rock type
28A
qz monzonite
263
monzodiorite
62C
62D
62D
granite
‘mixed’ texture
‘mixed’ texture
Heterogeneous (upper) unit
56C
56C
diorite
diorite
Mafic enclaves
48B
75B
75B
qz monzonite
qz monzonite
Mafic dykes
33D
33D
qz monzonite
qz monzonite
Porphyritic border unit
52A
52A
qz monzonite
qz monzonite
‘Plumose’ border unit
56A
56A
granite
granite
Megacrystic units
Plagioclase feldspar
Sample
Map unit and
62·34
62·45
63·18
61·76
61·89
63·63
61·66
63·38
58·56
62·21
61·51
63·17
62·24
63·39
63·81
SiO2
22·35
23·28
22·73
23·80
23·86
24·11
24·68
23·27
26·70
23·50
23·05
22·70
23·18
23·88
22·26
Al2O3
Concentration (wt %)
0·11
0·05
0·11
0·19
0·10
0·06
0·04
0·05
0·10
0·07
0·01
0·05
0·11
0·16
0·12
FeO
3·53
4·63
3·88
4·78
4·73
4·04
5·23
3·86
7·82
4·41
3·81
3·46
3·96
3·93
3·66
CaO
9·70
9·00
9·50
8·80
8·99
9·22
8·63
9·51
7·08
9·01
9·27
9·44
9·02
8·95
9·31
Na2O
Table 2: Representative feldspar microprobe compositions
0·11
0·18
0·21
0·41
0·14
0·18
0·09
0·14
0·19
0·31
0·16
0·15
0·16
0·25
0·27
K2O
0·07
0·31
0·18
0·17
0·30
0·45
0·34
0·32
0·34
0·29
0·32
0·36
0·14
0·22
0·09
SrO
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
BaO
98·1
99·6
99·6
99·7
99·7
101·2
100·3
100·2
100·5
99·5
97·8
99·0
98·7
100·6
99·4
Total
16·5
21·7
17·5
22·3
22·0
19·2
24·7
18·0
37·2
20·8
20·7
16·5
19·1
19·0
17·5
An
%
core
rim
matrix
core
rim
matrix
core
rim
core
rim
core
rim
core
core
rim
Position
McMURRY
CRYSTAL ACCUMULATION AND SHEARING IN BODOCÓ PLUTON
BOD-
rock type
1·63
Na2O
12·25
K2O
0·30
SrO
0·97
BaO
97·2
Total
71A
264
granite
Heterogeneous (upper) unit
diorite
Mafic enclaves
monzonite
62C
56C
21B
64·63
64·65
64·65
63·78
64·25
63·96
64·30
64·68
64·52
63·99
65·12
18·71
18·63
18·96
18·43
18·88
18·87
19·14
18·75
18·60
18·95
19·15
19·17
19·04
0·07
0·07
0·07
0·05
0·04
0·07
0·02
0·08
0·06
0·11
0·08
0·08
0·08
0·01
0·00
0·01
0·00
0·00
0·00
0·00
0·00
0·03
0·00
0·02
0·04
0·02
1·06
0·77
0·79
1·04
0·81
0·78
0·71
0·90
1·10
0·90
1·20
1·80
1·34
14·16
13·95
13·52
14·64
14·52
15·42
13·54
13·25
13·48
14·48
13·74
13·06
13·89
0·29
0·02
0·25
0·24
0·48
0·15
0·15
0·25
0·11
0·24
0·23
0·36
0·34
1·23
1·00
0·73
1·32
1·67
1·09
1·36
0·75
1·15
1·28
1·03
1·30
0·66
99·3
99·1
98·7
99·3
100·2
100·2
99·1
98·4
98·9
99·7
100·3
100·1
99·1
d = 5 mm
matrix
matrix
matrix
core
rim
core
rim
d = 20 mm (rim)
d = 18 mm
d = 15 mm
d = 12 mm (core)
d = 9 mm
NUMBER 2
Mafic dykes
75B
75B
qz monzonite
qz monzonite
64·10
64·05
VOLUME 42
Porphyritic border unit
75A
75A
qz monzonite
qz monzonite
‘Plumose’ border unit
71A
qz monzodiorite
qz monzodiorite
71A
71A
qz monzodiorite
qz monzodiorite
71A
71A
qz monzodiorite
qz monzodiorite
Traverse:
0·05
CaO
d = 0 mm (rim)
0·07
FeO
Position
71A
18·45
Al2O3
An
%
qz monzodiorite
63·82
SiO2
Concentration (wt %)
Megacrystic units
Alkali feldspar
Sample
Map unit and
Table 2: continued
JOURNAL OF PETROLOGY
FEBRUARY 2001
265
47·28
46·29
51·61
45·85
50·53
45·19
45·37
37·95
37·34
37·67
37·33
37·50
36·41
37·22
37·66
‘Plumose’ border unit
qz monzonite
75A
qz monzonite
75A
Porphyritic border unit
qz monzonite
33D
qz monzonite
75B
21B
48B
48B
24A
28A
71A
71A
Mafic dykes
monzonite
monzodiorite
monzodiorite
Mafic enclaves
monzodiorite
Biotite
Megacrystic units
qz monzonite
qz monzodiorite
qz monzodiorite
Porphyritic border unit
qz monzonite
33D
qz monzonite
33D
Mafic enclaves
monzodiorite
monzodiorite
monzodiorite
Heterogeneous (upper) unit
granite
62C
granite
62C
24A
63B
63B
46·00
48·67
48·38
44·84
28A
56A
56A
71A
Hornblende
Megacrystic units
qz monzonite
granite
qz monzodiorite
qz monzodiorite
37·50
37·52
43·33
SiO2
BOD-
rock type
2·67
1·59
3·47
2·02
3·43
1·50
2·16
1·91
3·55
3·01
1·59
0·53
0·82
0·85
0·15
1·10
0·78
1·35
1·36
1·11
1·16
1·23
TiO2
Concentration (wt %)
Sample
Map unit and
14·36
14·82
14·45
14·70
14·52
14·44
14·59
14·47
13·84
13·84
8·91
5·14
8·50
8·72
4·00
7·57
6·70
7·48
7·25
5·93
5·89
8·43
Al2O3
17·91
18·56
20·36
19·68
16·83
17·27
17·40
16·94
17·51
17·04
18·05
12·77
17·11
17·05
13·17
16·13
14·99
15·36
15·49
15·05
14·25
16·74
FeO∗
0·23
0·27
0·17
0·22
0·27
0·19
0·23
0·24
0·21
0·20
0·27
0·29
0·31
0·30
0·36
0·29
0·30
0·30
0·38
0·42
0·39
0·33
MnO
12·88
13·30
10·79
12·39
13·41
13·72
13·30
14·30
13·24
13·62
9·85
15·28
12·01
11·81
15·22
12·34
13·33
12·89
12·61
13·57
13·73
12·18
MgO
0·00
0·00
0·01
0·00
0·00
0·00
0·01
0·02
0·00
0·01
11·20
12·05
11·53
10·60
12·13
11·36
11·34
11·17
11·43
11·48
11·59
11·58
CaO
Table 3: Representative hornblende and biotite microprobe compositions
0·11
0·09
0·09
0·07
0·06
0·07
0·08
0·06
0·09
0·10
1·71
1·06
1·48
1·30
0·67
1·60
1·27
1·84
1·64
1·02
1·55
1·78
Na2O
9·88
10·03
9·67
9·31
9·48
9·74
9·75
9·58
9·52
9·84
1·19
0·55
0·93
0·93
0·32
0·84
0·87
0·97
0·89
0·63
0·73
1·10
K2O
0·16
0·13
0·35
0·53
0·43
0·16
0·13
0·28
0·23
0·25
0·05
0·00
0·07
0·03
0·03
0·03
0·05
0·06
0·06
0·03
0·01
0·05
BaO
0·04
0·05
0·11
0·10
0·06
0·07
0·08
0·07
0·11
0·13
0·16
0·03
0·04
0·04
0·05
0·06
0·08
0·09
0·11
0·04
0·10
0·09
Cl
0·59
0·63
0·25
0·66
0·72
0·41
0·34
0·63
0·47
0·44
0·42
0·29
0·54
0·15
0·00
0·33
0·45
0·38
0·23
0·21
0·23
0·09
F
96·3
97·0
96·1
96·9
96·9
94·9
95·6
96·5
96·1
96·2
95·9
98·1
97·7
96·9
97·6
97·0
96·7
97·5
97·0
97·8
97·6
98·2
Total
core
rim
core
inclusion
core
rim
inclusion
rim
core
rim
core
rim
rim
rim
core
rim
rim
core
core
Position
McMURRY
CRYSTAL ACCUMULATION AND SHEARING IN BODOCÓ PLUTON
1·08
1·23
1·17
7·34
6·77
6·84
6·69
21B
48B
48B
24A
266
0·17
0·32
0·38
0·28
0·21
0·23
0·20
0·22
0·25
0·21
1·83
2·25
1·87
1·95
1·45
1·88
1·50
1·79
1·70
1·94
1·69
1·71
5·61
5·60
5·64
5·62
63B
63B
62C
62C
5·67
5·57
33D
24A
5·66
5·68
71A
2·38
2·36
2·40
2·39
2·43
2·33
2·32
2·34
2·38
2·34
Al-IV
0·24
0·19
0·15
0·22
0·18
0·27
0·27
0·12
0·08
0·21
Al-VI
0·18
0·30
0·38
0·23
0·40
0·25
0·17
0·34
0·40
0·21
Ti
2·33
2·25
2·09
2·48
2·61
2·20
2·20
2·14
2·20
2·11
Fe2+
2·26
2·65
2·69
3·29
2·76
3·30
2·85
2·97
2·71
3·02
2·97
2·83
Mg
0·03
0·03
0·03
0·03
0·02
0·03
0·02
0·03
0·03
0·03
Mn
0·17
0·09
0·09
0·06
0·13
0·01
0·15
0·08
0·14
0·12
0·12
0·15
Ti
2·97
2·89
2·97
2·78
2·46
3·00
3·11
3·05
2·97
3·18
Mg
1·86
1·69
1·76
1·87
1·82
1·86
1·78
1·80
1·81
1·83
1·80
1·84
Ca
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
Ca
0·04
0·00
0·00
0·01
0·00
0·00
0·06
0·00
0·00
0·06
0·01
0·02
Na
0·01
0·01
0·03
0·03
0·02
0·01
0·01
0·01
0·01
0·02
Ba
0·08
0·27
0·20
0·10
0·15
0·10
0·13
0·16
0·15
0·07
0·14
0·11
Fe2+
0·03
0·03
0·02
0·02
0·03
0·02
0·02
0·03
0·03
0·02
Na
0·03
0·04
0·04
0·03
0·03
0·04
0·04
0·04
0·04
0·04
0·05
0·04
Mn
1·92
1·90
1·80
1·79
1·89
1·88
1·89
1·89
1·83
1·82
K
0·47
0·38
0·41
0·27
0·47
0·17
0·47
0·35
0·50
0·38
0·28
0·45
Na
A
0·01
0·01
0·02
0·03
0·03
0·02
0·02
0·03
0·03
0·02
Cl
Anions
0·22
0·17
0·17
0·09
0·15
0·06
0·17
0·15
0·21
0·13
0·11
0·15
K
0·30
0·28
0·34
0·31
0·12
0·16
0·20
0·21
0·22
0·30
F
0·00
0·02
0·09
0·00
0·00
0·02
0·00
0·01
0·04
0·00
0·00
0·00
Ca
16·02
15·89
15·83
15·93
15·76
15·83
15·92
15·85
15·80
15·93
cations
Total
15·68
15·58
15·67
15·36
15·62
15·25
15·64
15·51
15·75
15·51
15·39
15·61
sum
NUMBER 2
33D
5·66
5·62
28A
71A
Si
Biotite: cations per 22 oxygen atoms
1·31
0·67
0·20
0·20
Fe2+
M4
Cation
VOLUME 42
∗Total Fe.
0·49
1·10
7·51
6·90
33D
1·09
0·93
1·27
0·85
0·83
75B
7·07
6·91
75A
75A
7·15
6·73
56A
71A
6·92
7·17
28A
Al-VI
Si
Al-IV
M1/M2/M3
Tetrahedral
Amphibole: cations per 23 oxygen atoms
56A
BOD-
Sample
Table 3: continued
JOURNAL OF PETROLOGY
FEBRUARY 2001
McMURRY
CRYSTAL ACCUMULATION AND SHEARING IN BODOCÓ PLUTON
much of the biotite may not have crystallized directly
from a melt but instead formed by supersolidus reaction
between hornblende and residual liquid. In most igneous
rocks, magmatic biotite is less magnesian than coexisting
hornblende, and so the KD value [(XMgbio/XFebio)/(XMghbl/
XFehbl)] for biotite–hornblende pairs would be expected
to be <1·0 (Mason, 1985; Speer, 1987). In the Bodocó
pluton, hornblende–biotite pairs from megacrystic quartz
monzonite (five analyses) have average KD values of
about 1·03. However, biotite–hornblende pairs from the
porphyritic quartz monzonite border unit (two analyses)
have KD values of 0·66–0·69, suggesting a magmatic
origin for at least some biotite in the pluton. Magmatic
biotite also is indicated by euhedral biotite inclusions in
K-feldspar megacrysts and in some plagioclase cores.
ISOTOPE DATA
Rb–Sr analyses
Thirty-five whole-rock samples were selected for Rb–Sr
isotopic analysis, including metamorphic and igneous
country rocks (Table 4). In addition, mineral separates
were analysed from four samples. Age estimates for the
pluton are poorly defined by Rb–Sr geochronology,
ranging from about 559 to 593 Ma depending on the
subset of samples used for the calculations. Fitting an
isochron to the data is hampered by the consistently high
concentrations of common Sr in most of the whole-rock
samples, which cause 87Rb/86Sr to cluster between 0·14
and 0·45.
The range of possible initial 87Sr/86Sr for individual
samples was bracketed by back-calculating 87Sr/86Sr on
the basis of crystallization ages of 559 Ma and 593 Ma.
Initial 87Sr/86Sr values range in general from 0·7057 to
0·7063 if t0 = 559 Ma, or from 0·7056 to 0·7062 if t0 =
593 Ma (Table 4). The similarity of initial 87Sr/86Sr
between the quartz monzonite samples and the various
mafic rock types precludes an isotopic test for mixing
(McMurry & Long, 1995).
Additional information about initial 87Sr/86Sr is available from apatite separates (Table 4). Apatite contains
negligible Rb, but Sr substitutes easily for Ca in the
apatite structure. Apatite can acquire radiogenic Sr during subsolidus cooling (Creaser & Gray, 1992), but measurements of present-day 87Sr/86Sr in apatite give the
upper limit of initial 87Sr/86Sr in the melt at the time of
apatite crystallization. The estimates of initial 87Sr/86Sr
for most rocks in the pluton compare favourably with
the present-day 87Sr/86Sr value of 0·7060 obtained from
apatite in a megacrystic quartz monzonite. The maximum possible initial 87Sr/86Sr for this apatite, had it
crystallized at any time during the interval between 559
and 593 Ma, would have been 0·70592.
Oxygen isotopes
Oxygen isotope data were obtained for 20 of the same
samples that were analysed for Rb–Sr isotopes (Table
4). Most analyses were based on mineral separates of
quartz, which is relatively resistant to post-crystallization
isotopic exchange or alteration. In addition, whole-rock
18O values were determined for several samples, including the quartz-poor mafic samples.
The 18O values in the Bodocó samples display little
variation. Values of 18O in quartz range from +9·3 to
+9·8‰, regardless of rock type or location within the
pluton. The one exception is quartz from aplitic sample
13-C, which has 18O = +13·0‰. Compared with the
remainder of the plutonic suite, this aplite also has
anomalous 87Rb/86Sr and 87Sr/86Sr (Table 4). The wholerock 18O values of Bodocó samples also show little
variation, from +6·8 to +7·2‰. The difference in 18O
between whole-rock and quartz measurements from the
same rock suggests that the pluton has experienced some
low-temperature isotopic exchange with meteoric water.
This is not surprising, given that the pluton has been
exposed at or near the Earth’s surface at least since the
Cretaceous. Quartz 18O values in metamorphic country
rocks are notably higher than those of the plutonic rocks,
ranging from +11·0‰ in a felsic gneiss to +16·0‰ in
a pelitic schist.
EMPLACEMENT AND
DEFORMATION
Many characteristics of the Bodocó pluton, and of Itaporanga-type rocks in general, appear to have developed
during ascent and emplacement of the magma. Mafic
microgranitoid enclaves, which are likely to have resulted
from mingling of small amounts of a mafic magma with
a more voluminous felsic one (Vernon, 1983), are widely
and rather evenly distributed throughout the pluton,
suggesting that they had been transported some distance
from the point where they were first incorporated. Side
views of enclaves appear to record a sense of upward
motion (Fig. 9). The complex oscillatory zoning observed
in many K-feldspar megacrysts and phenocrysts, now
preserved as concentric exsolution lamellae (Fig. 4), originally marked compositional differences that may have
developed either in the region of melt generation or in
response to changes in temperature, pressure, and local
melt chemistry during ascent (Hibbard, 1995).
On the basis of the preserved mineral assemblages in
the pluton and in the surrounding metamorphic host
rocks, it appears that the Bodocó pluton intruded as a
relatively oxidized magma at pressures of >3 kbar. The
contact between the schists of the Cachoeirinha–
Salgueiro Fold Belt and the crystalline basement gneisses
of the Uauá Group may have provided a path for the
267
JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 2
FEBRUARY 2001
Table 4: Isotopic analyses, whole rocks and mineral separates
Map unit and
rock type
Sample
BOD-
Megacrystic units
qz monzonite
qz monzonite
qz monzonite
granite
qz monzonite
apatite
biotite
hornblende
K-feldspar
qz monzodiorite
granodiorite
qz monzonite
qz monzonite
granite
13A
18A
48A
56A
63A
63A-apa
63A-bio
63A-hbl
63A-ksp
67A
68B
70A
71A
76A
Border units
qz monzonite
qz monzonite
qz monzonite
qz monzonite
qz monzonite
87
Rb/86Sr
87
Sr/86Sr
Initial 87Sr/86Sr
Est. A∗
Est. B†
0·23170
0·28784
0·25238
0·39282
0·29123
0·00960
48·85354
0·33997
0·27787
0·22294
0·34983
0·28283
0·25551
0·35482
0·70793
0·70846
0·70927
0·70930
0·70823
0·70600
1·07643
0·70786
0·70802
0·70773
0·70881
0·70805
0·70771
0·70862
0·7061
0·7062
0·7073
0·7062
0·7059
0·7060
0·7060
0·7071
0·7060
0·7058
0·7060
0·7060
0·7058
0·7057
0·7058
38A
52A
74A
75A
75B
0·41436
0·25732
0·22950
0·38707
0·31558
0·70937
0·70815
0·70799
0·70911
0·70860
Mafic dykes
monzodiorite
monzonite
monzodiorite
qz monzodiorite
monzonite
monzodiorite
2B
12B
16B
19A
21B
48B
0·26913
0·14320
0·25973
0·26595
0·42313
0·25292
Mafic enclaves
amphibolite
monzonite
diorite
apatite
biotite
plagioclase
hornblende
monzodiorite
9C
12A
56C
56C-apa
56C-bio
56C-plg
56C-hbl
63B
Rb‡
(ppm)
Sr‡
(ppm)
18O (‰)§
Quartz
+ 9·5
+ 9·4
0·7058
0·7059
0·7057
0·7056
0·7056
108
122
116
148
126
4
683
10
183
103
129
118
111
133
1338
1222
1325
1092
1250
1110
33
86
1900
1326
1060
1201
1247
1078
0·7061
0·7061
0·7062
0·7060
0·7061
0·7059
0·7060
0·7061
0·7058
0·7059
179
138
123
169
153
1245
1541
1549
1257
1393
0·70828
0·70685
0·70806
0·70840
0·70907
0·70800
0·7061
0·7057
0·7060
0·7063
0·7057
0·7060
0·7060
0·7056
0·7059
0·7062
0·7055
0·7059
163
100
110
94
190
132
1737
2008
1216
1019
1295
1509
0·53762
0·19147
0·68142
0·00250
32·99697
0·02493
0·00226
0·22057
0·71083
0·70772
0·71160
0·70610
0·95526
0·70643
0·70980
0·70884
0·7066
0·7062
0·7062
0·7063
0·7061
0·7058
0·7071
0·7070
155
101
184
0·7
717
14
13
134
832
1524
781
860
63
1599
83
1745
+ 9·3
Heterogeneous (upper) unit
qz monzodiorite
61B
granite
61F
granite
62E
0·19004
1·49038
0·45191
0·70737
0·71785
0·70967
0·7059
0·7060
0·7061
0·7058
0·7053
0·7059
107
186
111
1617
361
708
+ 9·7
+ 9·6
Aplite dykes
granite
granite
8A
13C
1·45676
1·24090
0·71899
0·72102
0·7074
0·7111
0·7067
0·7105
131
149
260
348
+ 9·4
+ 13·0
Country rocks
biotite schist
qz monzonite
qz–fsp gneiss
4A
37A
34A
1·37470
0·23870
0·44515
0·71807
0·70782
0·72738
0·7071
0·7059
0·7238
0·7065
0·7058
0·7236
77
163
56
162
1961
366
+ 16·0
+ 9·5
+ 11·0
∗Back-calculated assuming t0 = 559 Ma.
†Back-calculated assuming t0 = 593 Ma.
‡Analyses by isotope dilution.
§Relative to Standard Mean Ocean Water (SMOW).
268
Whole rock
+ 9·4
+ 9·4
+ 9·6
+ 9·3
+ 9·6
+ 7·2
+ 9·5
+ 9·8
+ 9·5
+ 9·6
+ 7·0
+ 7·2
+ 7·1
+ 6·8
+ 7·0
+ 7·2
+ 7·2
McMURRY
CRYSTAL ACCUMULATION AND SHEARING IN BODOCÓ PLUTON
ascent of the magma from depth, particularly if this
contact was a zone of weakness during regional-scale
shearing.
The two border units of quartz monzonite are likely to
have been emplaced approximately contemporaneously
with each other, given that in places they appear to have
been physically combined by shearing before crystallization was complete in either unit. The plumose unit
responded relatively fluidly during shearing, with little
intracrystalline deformation, and retained some melt that
did not crystallize until after deformation ceased. The
porphyritic quartz monzonite had a greater crystal:melt
ratio than the plumose quartz monzonite and experienced
some cataclastic deformation during shearing.
The main, megacrystic unit was emplaced as a crystalrich magma, as indicated by exposures on the eastern
margin of the pluton where relatively rapid cooling
adjacent to metamorphic country rocks preserved an
early emplacement texture of large euhedral phenocrysts
of hornblende, plagioclase, and megacrystic K-feldspar
in a relatively high proportion of interstitial felsic melt.
Elsewhere in the pluton, textures suggest slower cooling
rates. The rocks have a coarse-grained matrix with little
or no interstitial K-feldspar, and hornblende occurs as
anhedral glomerocrysts that have undergone supersolidus
reaction with melt to form biotite. The development of
concentric exsolution lamellae that accentuate oscillatory
zoning in the K-feldspar crystals also is attributed to slow
cooling, as is the presence of microcline throughout the
pluton (Hibbard, 1995).
The second distinct stage in the evolution of the Bodocó
pluton was marked by localized deformation of the
megacrystic quartz monzonite to produce kilometre-wide
bands of strongly foliated quartz monzonite corresponding to the overall direction of elongation in the
pluton. The foliated rocks have elongated or lens-shaped
K-feldspar megacrysts with pressure shadows, as well as
ribbon quartz and other textures indicative of solid-state
deformation. The deformation was accompanied by the
intrusion of mafic dykes. Individual dykes are texturally
and chemically composite monzonites and quartz monzodiorites that may represent mingling of several mafic
magmas during ascent in the fracture zone. Some of the
mafic dykes appear to have been intruded as synplutonic
dykes (Pitcher, 1991), before the quartz monzonite was
completely crystallized, and became disrupted by shearing to form large swarms of oriented mafic enclaves.
Most of the pluton had crystallized before or during
the localized shearing event that produced the foliated
megacrystic quartz monzonite, but during this same
interval the upper part of the pluton appears to have
contained a phenocryst-poor high-SiO2 melt. The shearrelated mafic dykes that propagated vertically through
the foliated megacrystic quartz monzonite encountered
the felsic melt at this upper level, where it appears that
the two crystal-poor melts mingled to form texturally
heterogeneous mafic and felsic rocks with many disequilibria textures. The local shearing ceased before these
hybridized magmas crystallized, and so they did not
develop a preferred orientation or cataclastic textures.
CHEMICAL DIFFERENTIATION
MODELS
Field relationships suggest that the evolution of the
Bodocó pluton can be separated into two stages—
widespread processes related to magma generation and
ascent, and localized processes related to post-emplacement, in situ differentiation. Major oxide and trace
element data were used to test three petrogenetic models
for the differentiation of the Bodocó magma, with a
particular emphasis on the evolution of the main megacrystic rocks. Models should be consistent with observed
chemical characteristics such as the isotopic homogeneity
of the pluton, variations in trace element chemistry, the
reverse zoning of the main megacrystic rocks (from a
felsic outer margin to a more mafic core), and the
uniformity of mineral compositions despite a range of
bulk-rock compositions from granite to quartz monzodiorite. Developing a model that can account for the
observed features is complicated by the fact that specific
characteristics can be produced by more than one magmatic process. For example, reverse chemical zoning of
a pluton has been cited as evidence for magma mixing
(e.g. Wiedemann et al., 1987), for flow separation (e.g.
Speer et al., 1989), and for autointrusion of a fractionated
magma, with the deeper and more mafic layers intruded
last (Nabelek et al., 1986; Nironen & Bateman, 1989).
Fractional crystallization
The isotopic uniformity of samples among a wide range
of bulk compositions suggests that the pluton could have
originated by the fractional crystallization of a single
melt. Mafic dyke and megacrystic quartz monzodiorite
samples were used as possible starting magma compositions to test the hypothesis. The evolution of the
plutonic suite was modelled in a relatively simple manner
by subtracting proportions of fixed mineral compositions
in a sequence of intermediate parent–daughter steps and
evaluating the quality of the linear regression in each
step. Mass balance calculations were performed based
on the major oxide chemistry of the samples (Bryan et
al., 1969). A fractionation step was considered statistically
acceptable if the sum of the squares of the residuals (r2)
was <1·000. The compositions of mineral phases removed
were based on microprobe data for the relevant rock
269
JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 2
FEBRUARY 2001
Table 5: Crystal fractionation modelling trials
Trial
Step no.
Parent
Daughter
melt
melt
r2
Mineral phases removed (wt %)
plag
hbl
A
1
19B
16C
0·670
—
unsuccessful
16C
67A
14·685
—
unsuccessful
16C
73A
18·518
B
1
67A
63A
0·068
13·3
10·8
2
63A
57A
0·034
2·9
5·3
3
57A
77A
0·192
5·8
biot
apa
tit
0·6
0·6
0·4
0·1
6·8
0·8
mgt
16·7
1·0
0·5
0·1
4
77A
61F
0·156
11·6
8·1
1·5
0·4
0·9
C
1
19B
61B
0·082
7·3
13·8
9·6
1·2
1·3
0·3
2
61B
60A
0·048
11·7
4·1
4·2
0·7
0·1
0·9
—
unsuccessful
60A
61E
2·068
3
61E
62A
0·905
9·0
0·7
0·1
types, and the predicted proportions of minerals removed
were compared with modal data.
Results of several representative fractionation trials are
listed in Table 5 and are illustrated in Fig. 13a. Not all
combinations tested are reported here. The quality of
the statistical fit for individual modelling steps varied
greatly, even when the two end-members were relatively
similar to each other. Although it was possible to model
step-wise the parent-to-daughter fractional crystallization
of two of the most mafic dyke samples (Trial A in Table
5), no acceptable transitional steps were found to model
the evolution from these samples to the composition of
less mafic dykes (e.g. sample 41A or 19A) or to the
composition of the megacrystic quartz monzodiorites (e.g.
67A, 71A). The best statistical results for the fractional
crystallization model were obtained by using 67A, the
most mafic of the megacrystic quartz monzodiorite
samples, as the starting parent, and terminating the
sequence with a late-stage equigranular granite (61F)
from the upper region of heterogeneous rocks (Trial B
in Table 5).
The fractional crystallization simulations generated
from major oxide data were tested with trace element
data (Rb, Sr, Ba, Zr, and Y) as a function of the degree
of crystallization of the parent magma. For Rayleigh
fractionation, definitive concentration trends should be
produced for each element depending on its bulk distribution coefficient. Such characteristic trends were indicated for a few, but not all, of the trace elements in
any of the modelled combinations of steps.
Crystal accumulation and melt migration
Differentiation by crystal accumulation and melt migration is based on the assumption that the starting
magma is not completely liquid but consists instead of a
melt phase and many entrained phenocrysts. Chemical
differentiation occurs when the melt phase physically
separates from a framework of crystallizing solid phases,
leaving behind phenocrysts and some trapped melt. Modelling by Srogi & Lutz (1996) indicated that the migration
of melt from a crystallizing magma can produce large
differences in bulk composition, even over short distances
within a pluton, with little change in mineral or isotopic
composition.
To test the hypothesis that the megacrystic Bodocó
rocks were derived by a process of crystal accumulation
and melt migration, granite sample 77A from the northeastern margin of the pluton (Fig. 3) was selected to be
the most representative of an undifferentiated starting
magma. As noted previously, rocks from this margin are
the only megacrystic samples that contain abundant
interstitial K-feldspar in addition to euhedral K-feldspar
megacrysts, and they have euhedral hornblende phenocrysts instead of glomerocrystic hornblende. The
groundmass consists of relatively fine-grained, equigranular quartz, plagioclase, and K-feldspar. Acicular
apatite crystals, indicative of rapid cooling, are abundant
as inclusions in the groundmass plagioclase crystals. The
various textures suggest that a porphyritic magma was
emplaced at this contact, and it contained a high proportion of melt that solidified relatively quickly.
Mass balance calculations similar to those used for the
fractionation model were performed to determine if the
composition of an evolved cumulate (represented by
various megacrystic rocks from the interior of the pluton)
could be simulated by removing silicic melt from the
starting magma (Fig. 13b). The composition of the separated melt phase was represented in the modelling
270
McMURRY
CRYSTAL ACCUMULATION AND SHEARING IN BODOCÓ PLUTON
silicic melt also contained minor K-feldspar and hornblende, which represent a few phenocrysts entrained in
the migrating liquid (Table 6). Concentrations of Ba,
Rb, and Sr for the phenocrysts were estimated from data
in Tables 2, 3 and 4.
Magma mixing
Mafic microgranitoid enclave textures, intermingled rock
types in dykes and in shear zones, disequilibrium textures
in some minerals, and large-scale reverse zoning in the
pluton suggest that there may be a continuum of magma
mixing and mingling in the Bodocó rocks. Mass balance
calculations were used to test the hypothesis that magma
mixing could have played a key role in the differentiation
of either the main megacrystic rocks or the heterogeneous
rocks in the upper level of the intrusion. In the mixing
simulations, various proportions of mafic and felsic endmembers were combined to form intermediate daughter
compositions (Fig. 13c). The same statistical criteria were
applied as were used for the other two mass balance
models. The results were tested with trace element data
using the same procedure that was used for the accumulation–migration model; in this case the proportions
of end-members and their trace element compositions
were used to predict the trace element concentrations of
the mixed daughter.
Magma mixing trials for the megacrystic rocks used
various megacrystic granite and granodiorite samples
from the northeastern contact as the felsic end-member,
on the assumption that these samples contained a high
proportion of felsic melt when they were emplaced.
Several shear-zone dyke samples were tested as mafic
end-members. Statistically satisfactory results for mixing
were obtained for a number of megacrystic samples,
some of which are reported in Table 7. In many cases
the observed and estimated trace element concentrations
differed by <15%, and in a number of simulations the
differences were <5% for all four trace elements.
Fig. 13. Selected results of differentiation modelling, illustrated by
variation of CaO with respect to SiO2 in Bodocó samples: (a) fractional
crystallization; (b) crystal accumulation and melt migration; (c) magma
mixing.
by an equigranular granite (61F) from the region of
heterogeneous rocks in the upper part of the pluton. A
trial was considered acceptable if the sum of the squares
of the residuals (r2) was <1·000 (Table 6). Results were
tested independently with trace element data. The proportions of melt phase and cumulate phase, as determined
from the mass balance modelling, were used to predict
the concentrations of trace elements (Rb, Sr, Zr, Ba)
expected in the starting magma. The estimates then were
compared with the measured concentrations of trace
elements in the starting composition, 77A, and the discrepancy was expressed as the per cent difference between
the predicted and measured concentrations.
The accumulation and migration modelling produced
linear regressions of good quality for almost every sample
tested (Table 6). In addition, the trace element data
agreed well with the predicted proportions of melt phase
and cumulate phase. Moreover, values of r2 typically
<0·100 were achieved by assuming that the segregated
Discussion of differentiation models
Although the petrogenetic modelling indicates that fractional crystallization could have produced some compositional variation in the pluton, in general this process
fails to simulate the observed range of bulk-rock variation
with respect to major elements and trace elements. In
fractional crystallization, changes in the composition of
the melt are accompanied by changes in the composition
of the minerals crystallizing in equilibrium with a particular fraction of the melt; however, variations in mineral
composition are minor in the Bodocó pluton despite a
range of bulk-rock types. In contrast, crystal accumulation
and melt migration is chemically feasible, at least for
271
JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 2
FEBRUARY 2001
Table 6: Crystal accumulation and melt migration modelling trials
r2
Starting
Cumulate
Separated
Additional
magma∗
phase†
melt
phenocrysts‡
Discrepancy§ (%)
Rb
Sr
Ba
Zr
77A
=
80% 76A
+
20% 61F
0·273
−8·1
−8·3
−7·6
9·0
77A
=
66% 57A
+
34% 61F
0·063
2·0
−7·9
12·5
3·6
77A
=
63% 70A
+
37% 61F
0·168
−8·2
−11·7
−3·5
0·7
77A
=
62% 16A
+
38% 61F
0·172
−0·5
−7·4
21·4
−1·0
77A
=
60% 59A
+
40% 61F
0·130
−5·7
−9·0
1·7
−2·8
77A
=
60% 40A
+
40% 61F
0·069
−6·4
−4·4
15·2
−2·3
77A
=
58% 64A
+
42% 61F
0·067
−2·4
−13·0
2·6
3·5
77A
=
58% 11C
+
42% 61F
0·320
−3·6
−4·7
29·6
−3·2
77A
=
56% 18A
+
44% 61F
0·105
−1·5
−16·0
0·4
−8·5
77A
=
51% 63A
+
49% 61F
0·257
1·6
−20·2
−29·3
1·8
77A
=
51% 28A
+
49% 61F
0·043
−0·3
−10·0
9·7
11·0
77A
=
48% 13A
+
52% 61F
0·124
−3·2
−17·8
−3·1
7·5
77A
=
46% 73A
+
54% 61F
0·171
2·0
−21·5
7·0
−25·8
77A
=
65% 6A
+
35% 61F
1·318
7·7
−2·9
38·4
−1·3
77A
=
49% 6A
+
46% 61F
0·381
5·6
−18·7
18·3
−9·6
77A
=
57% 68B
+
43% 61F
0·400
−1·9
−25·8
−17·0
−20·5
77A
=
56% 68B
+
39% 61F
0·056
−2·2
−19·3
−24·8
1·5
77A
=
58% 48A
+
42% 61F
0·182
−4·1
−4·3
19·2
3·3
77A
=
52% 48A
+
46% 61F
0·083
−3·7
−10·7
12·8
−0·6
77A
=
43% 71A
+
57% 61F
0·732
−1·1
−27·1
−20·0
−25·9
77A
=
44% 71A
+
50% 61F
0·219
−1·8
−17·9
3·0
−30·8
77A
=
40% 67A
+
60% 61F
0·390
0·6
−24·2
−16·5
−37·3
77A
=
40% 67A
+
55% 61F
0·086
0·4
−17·4
1·1
−41·1
+
+
4% Hbl
5% K-fsp
+
2% Hbl
+
6% K-fsp
+
5% K-fsp
∗Starting magma consists of melt plus crystallized mineral phases.
†Accumulated phenocrysts and some felsic melt.
‡Phenocrysts entrained in the melt when it separated from cumulates, but removed before the melt phase reached its final
destination.
§Discrepancy in trace element concentrations between measured values in BOD-77A and values estimated from the modelled
proportions of cumulate and melt phases.
the main megacrystic rocks. Most differentiation of the
magma in such a case would be produced by varying
degrees of separation of melt phase and phenocrysts after
emplacement. The isotopic and REE homogeneity of
the pluton also suggests that differentiation occurred
either from a single magma source or from magmas
that became thoroughly mixed at depth. The extent of
supersolidus reaction between hornblende and biotite in
glomerocrysts, the lack of sharply defined Ba zoning in
megacrysts, and the development of concentric exsolved
exsolution lamellae in the megacrysts suggest a slow
crystallization rate for most of the Bodocó pluton, which
also would facilitate melt migration (Srogi & Lutz, 1996).
Although the relatively uniform oxygen isotope and
initial Sr ratios for the intrusion as a whole do not
require that widespread mixing played a major role in
the differentiation of magma in the pluton, the process
nevertheless could account for some of the observed
variations in bulk-rock composition in the main megacrystic rocks, particularly the reverse zoning in the central
portion of the pluton. For the heterogeneous rocks from
the upper part of the pluton, it is noteworthy that the
petrogenetic modelling obtained a much better match
statistically for mixing processes (e.g. 60A in Table 7) than
for fractionation (Trial C in Table 5). This observation is
in good agreement with field evidence for local mixing.
Petrographic textures such as acicular apatite inclusions
in interstitial feldspar, ellipsoidal and biotite-fretted plagioclase crystals, rapakivi and anti-rapakivi overgrowths
in feldspar, and hornblende rims on rounded quartz
inclusions are common in the heterogeneous rocks, and
all are textures described as possible evidence for mixing
272
273
72% 77A
77% 77A
67% 77A
75% 77A
80% 77A
45% 5A
67% 56A
20% 61F
+
+
+
+
+
+
+
+
28% 19B
23% 19B
33% 19B
25% 19B
20% 19B
55% 48B
33% 48B
80% 61B
=
=
=
=
=
=
=
=
=
=
=
=
Mixed
60A
63A
67A
70A
64A
63A
59A
18A
60A
73A
16A
68B
daughter
0·065
0·202
0·626
0·117
0·219
0·285
0·220
0·195
0·143
0·427
0·545
0·135
r2
121
144
136
144
141
136
142
139
128
128
139
127
Rb
1377
1234
1323
1132
1152
1186
1145
1164
1158
1276
1189
1217
Sr
Estimated (ppm)
2906
2656
2500
2328
2406
2542
2380
2454
3073
3082
2783
2477
Ba
77
195
153
198
188
170
191
182
87
161
211
197
Zr
Trace element comparison for daughter
109
128
107
116
125
128
118
125
109
123
134
125
Rb
1355
1266
1407
1236
1290
1266
1335
1269
1355
1327
1325
1072
Sr
Measured (ppm)
2827
2675
2129
2251
2544
2675
2482
2511
2827
3035
3068
1877
Ba
76
150
89
273
292
150
264
244
76
164
268
192
Zr
11
13
27
24
13
6
20
11
17
4
4
2
Rb
2
−3
−6
−8
−11
−6
−14
−8
−15
−4
−10
14
Sr
Discrepancy∗ (%)
3
−1
17
3
−5
−5
−4
−2
9
2
−9
32
Ba
3
1
30
72
−27
−36
13
−28
−25
14
−2
−21
Zr
∗Discrepancy in concentrations between measured values in daughter and values estimated from mixing the modelled proportions of parent compositions.
54% 56A
30% 61F
+
+
46% 16C
76% 56A
+
70% 16C
67% 5A
+
24% 16C
parent
33% 16C
Felsic
parent
Mafic
Mass oxide mass balance
Table 7: Magma mixing modelling trials
McMURRY
CRYSTAL ACCUMULATION AND SHEARING IN BODOCÓ PLUTON
JOURNAL OF PETROLOGY
VOLUME 42
of two physically and chemically disparate magmas (Hibbard, 1981).
CONCLUSIONS
Like other Itaporanga-type intrusions in northeastern
Brazil, the Bodocó pluton is texturally varied and distinctive in appearance, but mineralogically and isotopically it is relatively homogeneous. There is a wide
range of variation in the proportions of mineral phases,
resulting in a range of bulk-rock chemistries, but little
variation in specific mineral compositions. Isotopic ratios
of 18Oquartz between +9·3 and +9·8‰ and initial 87Sr/
86
Sr within limits of >0·7056–0·7063 are compatible
with, but not limited to, a source that included a mantlederived component modified by a crustal component.
This source region seems reasonable in the context of
magmatism associated with the Brasiliano orogeny, in
which the intrusion of mantle-derived dioritic dykes appears to have heated and softened the lower and middle
crust, which subsequently was heated, deformed, and
subjected to voluminous intrusions of felsic magmas,
including those of the Itaporanga-type granitoids. The
fine-scale oscillatory zoning that is common in K-feldspar
megacrysts from these granitoids also is suggestive of an
origin at depth, perhaps in a convecting magma chamber,
or of exposure to varying physical and chemical conditions during ascent.
Slow cooling may have obscured some of the original
chemical and isotopic distinctions of the Bodocó
magma(s), but it appears that many of the characteristics
of the pluton that developed after emplacement can be
attributed to differentiation of the magma by crystal
accumulation and melt migration. Megacrystic rocks
from the eastern margin of the pluton indicate that the
main Bodocó magma was intruded as a granitic or
granodioritic melt containing large euhedral phenocrysts
of hornblende, plagioclase, and K-feldspar. At these
margin localities, the melt phase was retained in situ and
crystallized as a granular matrix of anhedral plagioclase,
quartz, and K-feldspar. Interstitial minerals, particularly
quartz and K-feldspar, are much less common in the
remainder of the megacrystic rocks. If we assume that
the Bodocó magma was emplaced as a crystal-rich melt,
much of the variation in bulk-rock compositions across
the pluton can be reproduced by a petrogenetic model
in which progressively greater proportions of felsic melt
separate from the phenocrysts. Observed trace element
concentrations also correlate well with the modelled
cumulate and melt phases. Such a model also reproduces
the observed reverse chemical zoning, if we assume that
the hotter interior of the pluton crystallized more slowly,
and consequently there was more opportunity for the
silicic melt phase to migrate away than in the outer,
NUMBER 2
FEBRUARY 2001
more rapidly crystallizing regions. Slow cooling rates
are also indicated by textures such as glomerocrystic
hornblende and biotite, perthitic megacrysts with concentric exsolution lamellae, and coarse-grained matrix
minerals.
The pluton as a whole is relatively undeformed, except
in several elongated kilometre-wide shear zones in which
the megacrystic rocks underwent localized ductile shearing transverse to dextral strip-slip faulting in the regionalscale Patos and West Pernambuco Shear Zones. The
localized intraplutonic shear zones, which host numerous
composite mafic dykes, also appear to have provided a
pathway for the felsic and relatively crystal-poor melt
phase that separated from the crystallizing megacrystic
magma. This melt migrated to a structurally higher
portion of the pluton, where it mingled with mafic magma
in a shear zone to produce heterogeneous rocks that
have numerous examples of disequilibria textures associated with the mixing of two chemically and thermally
distinct magmas, such as rapakivi overgrowths on feldspar, xenocrystic quartz rimmed by hornblende, and
acicular apatite inclusions in diffuse patches of interstitial
feldspar.
ACKNOWLEDGEMENTS
Helpful reviews and discussions of this work were provided by Rod Metcalf, Calvin Miller, Alain Vauchez,
Calvin Barnes, Leon Long, and George Clark. A. N. Sial
and the Projeto Geociências of the FADCT/FINEP
supplied invaluable logistical support. Cleto de Oliveira
Cavalcanti and Elias Barbosa e Silva assisted in the field.
The ICP analyses of trace elements were performed by
Melanie Barnes. The study was assisted by a grant-inaid of research from Sigma Xi.
274
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