Download petrology and isotope systematics of magma mushes

Revista Brasileira de Geociências
17(4):473-480, dezembro de 1987
PETROLOGY AND ISOTOPE SYSTEMATICS OF MAGMA MUSHES:
SOME PORPHYRITIC GRANITOIDS OF NORTHEASTERN BRAZIL
JUDE McMURRY*, LEON E. LONG** and ALCIDES N. SIAL***
ABSTRACT
More than 80 coarsely porphyritic granitoid plutons with K-feldspar megacrysts
(Itaporanga-type granites) intrude metarnorphic rocks of Northeastern Brazil. Textural evidence for
fIlter pressing and flow foliation indicates that these bodies were emplaced as viscous, crystal-laden
mushes. A representative Itaporanga-type pluton, the Monte das Gameleiras intrusion, consists of a
variety of rock types with Si02 ranging from 49.5 to 71.6 weight percent. Chemically and
petrographically, this hornblende-bearing pluton has !-type characteristics with some S-type affinities.
Whole-rock oxygen isotope data are in keeping with the distribution of 5180 for unaltered granitic
rocks. Similarly, REE data suggest a single magmatic process, at least for the more felsic rock types,
without significant post-intrusive effects. A 5-point, whole-rock Rb-Sr isochron for the Monte das
Gameleiras pluton gives t = 511 ± 26 Ma and an initial 87Sr/86Sr = 0.7099 ± 0.0004. This is
contrasted with disparate ages calculated from texturally similar bodies such as the Fazenda Nova
pluton, for which a 5-point whole-rock isochron gives t = 630 ± 24 Ma with initial 8 7Sr/86Sr = 0.7065
± 0.0005, and the type locality Itaporanga pluton, for which a 6-point isochron gives t = 625 ± 22 Ma.
RESUMO
Mais de 80 plutões graníticos grosseiramente porfuíticos, com megacristais de feldspato
potássico (granitos tipo Itaporanga), cortam rochas metamórficas do Nordeste do Brasil. Evidências
texturais de diferenciação por fIltragem e foliação de fluxo indicam que esses corpos foram intrudidos
como acumulações viscosas de cristais. Um plutão representativo do tipo Itaporanga, a intrusão de
Monte das Gameleiras, consiste em uma variedade de litotipos com Si02 variando de 49,5% a 71,6%
em peso. Química e petrograficamente, este plutão com hornblenda tem características do tipo-! e
algumas afinidades com o tipo-S. Dados de isótopos de oxigênio para rocha total estão de acordo
com a distribuição de 5018 para rochas graníticas inalteradas. Similarmente, dados de REE sugerem um processo magmático simples, pelo menos para os tipos de rochas mais félsicas, sem efeitos
pós-intrusivos significativos. Uma isócrona Rb-Sr de 5 pontos para rocha total, do plutão de Monte das
Gameleiras, fornece t = 511 ± 26 Ma e uma razão inicial 87Sr/86Sr = 0,7099 ± 0,0004. Esta contrasta
com as idades calculadas para corpos texturalmente semelhantes, tais como o plutão de Fazenda Nova,
onde uma isócrona Rb-Sr de 5 pontos para rocha total fornece t = 630 ± 24 Ma com razão inicial
87Sr/86Sr = 0,7065 ± 0,0005, e o plutão da localidade-tipo de Itaporanga, para o qual uma isócrona
de 6 pontos fornece t = 525 ± 22 Ma.
INTRODUCTION
Numerous plutons and dikes of a
wide variety of rock types intrude the polymetamorphosed
and deformed
shield rocks of Northeastern
Brazil.
Conspicuous among these is a group o.f porphyritic
granitoids with coarse, tabular phenocrysts of alkali
feldspar. At least 80 of these so-called "Itaporanga-type"
plutons are known; they are located where metamorphism
attained at least amphibolite
facies with associated
extensive migmatization (Almeida 1971, Brito-Neves &
Pessoa 1974). Most have received little more than
reconnaissance study. A thorough knowledge of their
petrology, however, is important to understanding the
nature of the lower crust from which the magmas
presumably were derived during the last major orogeny to
have affected this portion of Gondwanaland.
We have analyzed samples from a large Itaporanga-type
granitoid located about 80 km from the Atlantic coast on
the boundary between Paraíba and Rio Grande do Norte
states (Fig. 1). The Monte das Gameleiras pluton is an
enclave-rich, predominantly I-type granitoid (Chappell &
White 1974). Like other Itaporanga-type granitoids, it
appears to have been emplaced as a viscous, crystal-laden
magma derived from near-solidus partial melting with
incorporated mafic phases inéluding hornblende and biotite.
In addition,
a Rb-Sr study was made of another
Itaporanga-type igneous body in the vicinity of Fazenda
Nova, located 140 km west ofRecife (Fig. 1).
REGIONAL SETTING
These bodies are emplaced into
the Borborema tectonic province, a complex structural
mosaic of linear metasedimentary fold belts separated by
massifs of much older, previously deformed gneissic
basement (Almeida et aI. 1981). The final major orogeny in
this area was the Brasiliano Cycle, the metamorphic and
intrusive events of which date from about 700 to 500 Ma
and wich caused many of the older rocks to be thermally
overprinted (Wernick 1981, Brito-Neves et aI. 1974). At
this time, regional metamorphism
created rocks of
greenschist to amphibolite facies under low P/high T
conditions.
Isotopic ages cluster about several periods of thermal
activity within the Brasiliano Cycle (Brito-Neves et aI.
1974). Ages of 700 ± 20 Ma may correspond to early
thermal metamorphism
accompanied by folding. Peak
metamorphic conditions are well~documented by K-Ar ages
of about 630 ± 30 Ma. A younger cluster of intrusive and
* Department of Geological Sciences, University of Texas, Austin, TX 78713, USA. Current address: Department of Geosciences, Texas
Tech. University, Lubbock, TX 79409, USA
Department ofGeological Sciences, University ofTexas. Austin, TX 78713, USA
*** Departamento de Geologia, Universidade Federal de Pernambuco. CEP 50000, Pernambuco, PE, Brasil
**
Revista Brasileira de Geociências.
474
___
I"""
EXPLANATlON
(2]
o
• Faz.,.,da
R.elf'.
TERTlARY
PRECAMBRIAN
No,,",o
o
SEOIMENTARY
AOCKS
SEOIMENTAAY
ROCK$
METAMORPHIC
ROCKS:
•..
(2J SERIDO GROUP
Ei)
~'"
OUATERNARY
--
CAICÓ
IGNEOUS
FAULT.
--'-STATE
GROUP
INTRUSION
DASHED
WHEAE
INFERREO
BOUNDARY
Figure 1 - Location of study area, Northeastem Brazil
and generalized geologic map of the Monte das Carneleiras region (after Barbosa et alo 1974)
metamorphic
ages ranges between 570 and 540 Ma
Brasiliano activity ceased with post-tectonic intrusions
ending about 410 Ma ago (Long & Brito-Neves 1977).
PETROGRAPHY
OF THE MONTE
DAS GAMELEIRAS
The major rock types of the Monte das
PLUTON
Gameleiras pluton can be classified as:
1. medium-to-coarse-grained,
porphyritic
silicic rocks
(Itaporanga-type lithology);
2. fine-grained, equigranular granites sensu stricto;
3. dark, fine-grained, equigranular rocks (monzonites and
quartz monzodiorites).
ln addition, elongated mafic enclaves are common, as are
aplitic and pegmatitic dikes. Angular, metasedimentary
xenoliths are less abundant.
Porphyritic granitoid
Depending OFl the proportion of
megacrysts and matrix (which may be extremely variable
even on outcrop scale), the porphyritic rocks are granites,
granodiorites, quartz monzonites, or quartz monzodiorites;
for simplicity, these Itaporanga-type rocks are referred to as
"porphyritic granitoids".
Abundant tabular or (rarely) ellipsoidal megacrysts (1 to
10 cm long) of perthitic K-feldspar confer a striking texture
to the porphyritic granitoids. ln hand sample, these pale
pink or gray megacrysts have a faintly perceptible
concentric
zoning caused by oriented inclusions of
plagioclase near the center in addition to inclusions of
biotite, sphene, hornblende, and magnetite in the outer
layers of the megacrysts. Modal abundances of megacrysts
vary considerably throughout the pluton, but in general the
megacrysts constitute 25% to 40% of a given sample. ln
thin section, megacrysts commonly are bordered by a
Volume
17, 1987
narrow, 0.5 to 2 mm, anhedral rim af very-finc-grained
quartz, microcline, sericitized and turbid plagioclase, and
lobate myrmekite. Ellipsoid megacrysts have "tails" of
millimeter-sized
anhedral
quartz,
plagioclase,
and
microcline which pinch out into more mafic matrix several
millimeters from both ends of the megacryst.
The megacrysts are perthitic; many also have poorly
developed patches of microcline grid twinning. ln contrast,
K-feldspar in the matrix is equant, fine-grained, anhedral
microcline with sharply defined twinning.
Quartz, plagioclase, biotite, and hornblende dominate
the porphyritic granitoids. Quartz generally occurs in large,
anhedral, composite
pools. Plagioclase forms mostly
anhedral, equant to subhedral tabular crystals, the larger
crystals (1 to 5 mm) of which are faintly twinned and
concentrically zoned. Both biotite and hornblende are
intensely pleochroic: biotite Q = pale yellow-green and ~ = 'Y
= very dark green, and homblende Q = pale yellow-green, ~
= brown-green, and 'Y = intense blue-green. Slender crystals
of apatite are common inclusions in biotite and hornblende.
Other accessory phases include slender, metamict prisms of
allanite rimmed by epidote, magnetite, ilmenite rimmed by
sphene, and discrete, stubby prisms of apatite and zircon.
All of these minor phases occur in aggregates with biotite
and hornblende crystals.
Fine-grained granites
Many of the fine-grained granites
occur as crosscutting dikes in the porphyritic granitoids.
Other exposures, although massive and fairly extensive,
contain angular fragments of the porphyritic rocks. Their
overall texture is granitoid and equigranular, with an
average grain size less than 0.5 mm for most samples.
Plagioclase and quartz are subordinate to microcline in
these rocks. Scattered flakes of pleochroic green biotite
(less than 5% modally) impart a slight foliation.
Minor mafic phases include sphene, elongate zircon,
apatite (as íninute inclusions in plagioclase and as discrete
prisms), allanite rimmed with epidote, and rare Fe-Ti
oxides.
Dark Intrusives
The color index exceeds 40% for these
more mafic, equigranular rocks. Complexly twinned,
normally zoned plagioclase is the most abundant mineral.
Biotite and less abundant homblende exhibit the same
intense green pleochroism as do their counterparts in the
porphyritic
granitoids. Some hornblende crystals have
grainy cores of relict clinopyroxene.
Quartz and microcline are present only in minor
abundances. Quartz occurs as small, equant, individual
grains. Microcline is anhedral, equant, and seriate; some
crystals are rimmed by lobate myrmekite.
The accessory mineral suite of the dark intrusives
resembles that of the porphyritic granitoids. Sphene,
apatite, and Fe-Ti oxides are common inclusions in
homblende and also occur as discrete crystals. Zircon and
epidote-rimmed allanite are present in some samples.
Other rock types
Dark, lens-shaped stringers are
coínrnon as enclaves in the porphyritic granitoids. Modes
and mineralogy are similar to the dark intrusives, but some
enclaves also have ellipsoidal crystals of perthitic K-feldspar
which resemble more tabular megacrysts in the surrounding
granitoid. These large, cigar-shaped crystals are comparably
rimmed by very fine-grained quartz and feldspars, and they
Revista Brasileira de Geociências,
475
Volume 17. 1987
contain oriented rows of inclusions of slightly altered
plagioclase, just as the megacrysts do.
Large, angular xenoliths are far less common than the
elongated mafic enclaves. Some xenoliths are composite
blocks, including those in which amphibolite is in sharp
contact with pink, slightly porphyritic granite which in tum
veins the amphibolite, ali of which is entrained in
porphyritic granitoid.
Pegmatites with abundant K-feldspar, quartz, and biotite
are ubiquitous but are of minor importance volumetricaliy.
Narrow, even-sided, 2-t0-6 cm aplitic dikes crosscut ali
other lithologies.
TEXTURES
Tabular K-feldspar megacrysts in the porphyritic granitoids weather in high relief
from the matrix and accentuate the evidence that the Monte
das Gameleiras pluton was intruded as a viscous, crystal-laden
mush. In places, the megacrysts are aligned in faint swirls or
ductile shears; elsewhere, they are subparaliel or are so
consistently aligned that the rock has a gneissic appearance.
Concentrations of megacrysts form cumulate-like layers
which can be traced along strike for tens of meters. Filter
pressing appears to have operated on a smali scale to
produce some extremely megacryst-rich varieties of the
porphyritic granitoid where interstitial magma was injected
into narrow fissures in wall rock.
Contacts between the porphyritic granitoid and the dark
intrusive are complex, but the two rock types appear to
have been intruded over a brief time interval during which
both magmas "mingled" (Vemon 1983) and deformed
plasticaliy against each other. Some contacts are sharp and
planar; others grade into undulating, swirls of felsic and
mafic rock on the scale of a single outcrop. Elsewhere,
lobes of dark intrusive have intruded convex1y into the
porphyritic
granitoid.
Flow foliation and a curved
concentrated band of megacrysts at the contact signify that
the granitoid was plastically deformed by the intrusion.
MEGASCOPIC
MIC ROPROBE ANAl YSES
Table
1-
Chemical compositions
of
selected minerals were determined in Monte das Gameleiras
rock types
at the University
of Texas with a
three-spectometer
Applied Research Laboratories EMX
electron microprobe equipped with an energy-dispersive
detector. Computer programs appropriate for 15 kv
accelerating potential were used to calculate matrix
corrections (Bence & Albee 1968, Albee & Ray 1970) and
to convert the corrected data to weight percent oxides and
atomic proportions. Natural and synthetic mineral samRles
were used as primary and secondary standards.
Large, zoned plagioclase crystals in the
matrix of the porphyritic granitoid (Table 1) have cores
of calcic oligoclase (An24 to An27) which grade
monotonically
into rims of sodic oligoclase (An17 to
An2o). Plagioclase inclusions in alkali feldspar megacrysts
are AnIs on the average, but their outer parts are likewise
more sodic, and most are mantled by optically distinct
collars of albite.
Plagioclase in a dark-colored intrusive, GA-17, is zoned
from andesine cores (An34 to An36) to oligoclase rims
(An24 to An2S)' Note that there is mínimal compositional
overlap between plagioclase cores in the granitoid and
plagioclase rims in the dark intrusive (Table 1).
Plagioclase
Alkali
feldspar
The
proportion
of
KAlSi3 Os
component in alkali feldspar is consistently high in the
matrix microcline and in the megacrysts of the porphyritic
granitoid (Table 1). Microcline in the dark intrusive is also
comparably high.
Megacrysts are unzoned with respect to K, Na, or Ca,
but significant variation in Ba was detected in microprobe
traverses (Fig. 2). The Ba variations in the megacrysts
correspond to opticaliy distinguishable zones. For example,
the major portion of a typical megacrist in sample GA-5
contains concentric bands of oriented plagioclase inclusions
and has a high, consistent Ba concentration of about 5,000
pprn. The outer millimeter
of the megacryst
is
inclusion-free, and Ba concentration drops to about 3,000
Typical plagioclase and alkali te/dspar rnicroprobe analyses (oxides - W t %) Monte das Carneleiras pluton
---- --
3n.d.
.5
19.5
.0
5.2
0.2
n.d.
0.5
7.6
7of
.4
87.0
1.5
clase
clase
clase
n.d.
14.6
83.2
8.8
0.6
103.2
82.3
13.7
16.8
24.4
105.1
103.3
0.1
11.9
75.1
n.d.
14.00.0
1.9
98.5
0.1
8.4
103.8
1.1
101.0
03
84.6
2.9
100.4
1.0
63.4
7.4
0.0
1.2
15.5
0.9
99.9
8.3
74.3
24.9
0.1
n.d.
0.2
8.6ofof .
4.2
23.2
23.3
68.5
68.0
66.6
3.6
22.7
64.0
64.7
18.9
20.4
71.2
66.0
223
18.6
64.8
65.0
18.3
26.3
61.8
23.8
5.2
0.5
matrix
3.5
0.9
inchsion
90.7
91.4
0.8
Rim
Core
zoned
of
of
inclusion
zoned
Matrix
unzoned
Matrix
Core
9.7
100.5
80.4
19.1
104.8
9.9
0.2
nd.
15.2
0.8
35.9
103.2
62.8
0.5
"Mande"
Matrix
C
ore
0.7
(microcline)
K·feldspar
(microcline)
100.6
in
inmegamegacryst
crystal
K-feldspar
PlagioPlagioPlagiorimming
Megacryst
(perthitic)
llih
PlagioPlagioRock An 2,9
Mol.%
clase GA-15a
GA-l1GA-17
Dark Intrusive
matrix Granitoid
crystal
Porphyritic
476
Revista Brasüeira de Geociêru:ias. Volume
7,500
.
+
shell
+ ++ + +~
I""
[5,000
a.
core
++
I.t"""rlm
+
+1'+
+ +.. ++ + #+ +
+
matri.
+
•
••
+
+
GA-15o
o.
°0L--~--~4--~-----'8'-----'--+
12
Troverse (mm)
•
matri.
••
GA- 05
inclusion
4
---..t
b.
8
12
17, 1987
mafic enclaves, and metasedimentary xenoliths. Chemical
behaviour is more disparate for Al203, in which the content
for enclaves of ali types is markedly aberrant. Na2O data
are scattered, and MgO values are erratic.
Strontium in the dark-<:olored intrusive rocks exceeds
1,000 ppm (Table 6). The more felsic rocks contain from
600 to barely 100 ppm Sr, diminishing with increasing
Si02. Rubidium concentrations are low overali (107 to 172
ppm), with less variability and no distinct variation with
Si02 •
Field evidence indicates that the porphyritic granitoids
and
the
dark-<:olored
intrusives
were
intruded
contemporaneous1y
and that they have similar mineral
assemblages. However, divergent Al2 03, MgO, and Rb-Sr
isotopic data do not support the conclusion that the two
magma types are cogenetic. The connection between them,
if any, is uncertain and requires more detailed chemical and
isotopic study.
Troverse (mm)
Figure 2 - Electron microprobe traverses of alkali feldspar
megacrysts in porphyritic granitoid of the Monte das
Gameleiras pluton. Variations in Ba concentration correspond to optically discemible zones in the megacryst.
Ba concentration in matrix microcline plotted alongside
traverse for comparison
SYMIIOL8
+ GEOCHEMICAL
SAMPLE
o RB-SR
•
SAMPLE
VILLAGE
ppm. The outerrnost edge of the megacryst is a 'narrow,
anhedral rim characterized by very high, erratic Ba values
and by intricate, bulbous myrrnekite; both features have
been ascribed in other studies to late-stage hydrothermal
effects (Mehnert & Busch 1981, Phillips 1980). Microcline
in the matrix has variable Ba but is generaliy comparable to
megacryst rim and/or core values.
Hornblende and biotite
Amphibole in ali of the
analyzed rock types is magnesian hastingsitic hornblende
(terrninology after Leake 1978). Amphiboles in one sample
of porphyritic granitoid have edenitic cores, however
(Tab.2).
Biotite compositions are homogeneous throughout the
pluton (Tab. 2). It would be difficult to distinguish
between biotite from a porphyritic granitoid and from a
dark intrusive on the basis of chemical or optical properties.
WHOLE·ROCK
GEOCHEMISTRY
Seventeen samples
from the Monte das Gameleiras pluton, selected for broad
lithologic and geographic coverage (Fig. 3), were analyzed
at Sudene laboratory at the Universidade Federal de
Pernambuco.
Na and K were analyzed by flame
photometry, ali other major elements by atomic absorption
spectrophotometry,
and Rb and Sr were determined by
isotope dilution. Selected analyses are presented in tables 3
and 6.
As expected from the assortrnent of rock types, the
variation of Si02 in the Monte das Gameleiras pluton is
broad, ranging from 49.5 weight percent for a dark-colored
intrusive, to 64 to 69 weight percent for porphyritic
granitoids, to between 69 and 71.6 weight percent for
fine-grained,
nonporphyritic
granites (Table 3). The
distribution. of CaO, total FeO, and K20 versus Si02 is
systematic for ali samples, including dark-colored intrusives,
Figure
pluton
3 -
Sample location map, Monte dos Gameleiras
ISOTOPE
GEOCHEMISTRY
Oxygen
extractions were performed by reaction with flu'orine at the
University of Georgia, Athens. Isotopic analyses were made
using a VG Micromass 602C double-collector
mass
spectometer. A rose quartz standard was analyzed routinely
during the analyses as a monitor of perforrnance. Four
whole-rock samples of porphyritic granitoid and six of
fine-grained granite from the Monte das Gameleiras pluton
were selected for analysis. Most samples have rather
homogeneous 0180 values of about +8.3 to +8.6%0 but
several have anomalously low or high values to give a total
range of 0180 of +7.0 to +9.2%0 (tab. 4). The isotopic
signatures of theses anomalous samples do not appear to
be systematically
correlated
with whole-rock Si02,
geographic distribution, or rock type. The total range of
oxygen isotope values is similar to data from several other
porphyritic granitoids in Northeastern
Brazil such as
Itaporanga,
Conceição,
and Bodocó (Sial & Long
unpublished data).
Values of 018 O in quartz are higher than in feldspar and
whole rock, as expected if the oxygen isotope compositions
are essemtially at equilibriun (Tab. 4). Values of 6qz-bi for
OXYGEN
Uliw
Revista Brasikirade
Geociências,
Volume
Table 2 - Averaged values of hornblende and biotite
microprobeGA-14
analyses (oxides - Wt%) Monte das Gameleiras
GA-7b
GA-{)9
GA-{)8
GA-{)2
GA-{)6
CG·38
GA-05
Fine-grained
pluton (Total
Fe reported as FeO)
Porphyritic
Rod<
,
312.4
0..0.
0..0.
1.'
0..0.
1.'
9.4
0..'
0..0.
9.'
l.l
Rim
09'.2
..0.
9.2
11.5
1.2
0..2
1.4
1.8
935.1
14.9
0..2
Biolite
9.8
43.6
9.5
0..5
9.3
0..5
1.'
0..4
93.6
Homble,*
93.9
8Kllile
963
295.1
2l.l
18.1
0..1
0..0.
l.S
0..1
0..8
0.3
lU
11.2
0.3
0.0.
0..7
0..0.
0.3
0..0.
11.7
12.3
14.6
12.6
3>.9
41.7
0..6
8.9
0..5
U
0..'
0..5
8.'
14.6
8iotite
41.8
35.3
l.7
Homblende
0..6
41.0
10.6
6U
9.9
GA·lS.
Hombltnde
Homblende
933
19.4
J8.8
19.4
20..0.
18.9
18.3
0.0.
96.0.
CÁ-I'
GA.oS CoGraniloids
••
Daril-colored
Porphyritic::
intnuive
-U--
-r~
•••
D.rk<Olored
00..6
80..1
13.4
..2
.1
.2
9.8
9.8
0..4
14.4
13.5
0..7
2.5
53
99.9
GA.ol
GA-lSb
CÁ·17
GA-IS.
1.4
1.3
2.4
99.3
1.4
1.6
8.'
7.1
33.1
31.6
GA-Ib
0..1
0..0.
0..3
1..
0..0.
7.2
21.9
'.9
6.'
22.2
1.9
5.7
0..3
18.4
20.2
3.6
5.9
5.1
9.7
10.3
10.2
19.8
18.2
9.1
83
1.4Porphyritic:: Fint-paincd
30.8
0..9
49.S
2.7
99.2
100.1
GA.{)S
0..8
0..,
1.8
GA
CÁ-12.
GA.()ll
34.'
33.4
35.8
32.4
15.2
15.0
163
14.0
16.8
18.0
15.6
32.6
29.6
30.6
34.t
25.3
3.1
1.2
8.6
69.0
65.8
66.1
67.7
98.9
0..'
985
983
100.1
0..4
34.2
14.7
22.0
28.5
21.4
27.9
15.0
2.'
4.8
50.2
56.7
0..6
1.5
2.5
98.6
1.8
l.S
99.6
U
99.9
100.1
68.8
15.2
0..6
2.0.
0..'
1.0.
-173
2.2
2.'
0..5
2.2
2.8
3.3
'.8
5.0.
5.1
4.7
4.2
'.2
3.7
4.0.
3.5
3.8
8.1
0..1
2.1
12.6
16.5
0..2
5.4
7.'
5.0.
3.4
l.7
3.'.••• c::ndn~.
5.3
3.'
1.7
0..4
DA
2.0
0.0.
0..7
1.0.
0..0.
0..5
0..9
11.6
99.1
2.5
0..1
0..2
.,.,uloid
Table 4 - Oxygen isotope data (0100SMOW)
---- ---
++4.6
8.8
1.4
+
9.7
4.8
+8.3
9.2
7.0
Biotite
0.8
8.3
Whole·rock
+.9.6
8.6
+4.8
+ 8.0
7.9
Quartz
6qz-fs
Feldspar
6qz-bi
Sample 5.1
GA·la
GA-Ib
granites
granitoids
•
Table 3 - Representative chemical analyses (oxided - Wt%)
and nonn calculations (CIPW - mol%), Monte das Gameleiras pluton
Rock
477
17, 1987
pnite
samples GA-l and CB-38 provide apparent equilibrium
temperatures of 550°C and 500°C, respectively, indicating
redistribution of 180 during slow cooling. Minor changes in
biotite composition might be due to weathering, facilitated
by circulation of water along rock fractures, or to a
pervasive post-intrusive hydrothennal
effect. Whole-rock
Ó180 less than +100100 are consistent with an I-tipe magma
(O'Neil et aI. 1977), in agreement with the other evidence
for the character of the source of Monte das Gameleiras
magma.
RARE EARTH ELEMENT GEOCHEMISTRV
Trace
element and REE analyses were perfonned by a modified
version (Fryer 1977) of the thin-film X-ray fluorescence
procedure of Eby (1972) at the Memorial University of
Newfoundland, St. Johns. Four samples of porphyritic
granitoid and one of fme-grained granite for which there are
oxygen-isotope data were selected for analysis (Tab. 5). The
analytical erro r is estimated to be less than 10% for ali the
elements tabulated.
Trace element concentrations among the porphyritic
granitoid samples are similar to each other, with no
conspicuous depletions or enrichments (Tab. 5). Relative to
the granitoids, the fine-grained granite is enriched in Th and
Ba and depleted in Ni and V.
REE data from the felsic Monte das Gameleiras rocks
point to a single magmatic process without evidence for
significant post-intrusive events. There also does not
appear to be much chemical evidence for the observed
apparent "mingling" of felsic magma and more mafic
magma. Chondrite-nonnalized
abundance patterns for the
porphyritic granitoids are all similar (Fig. 4). Each sample
displays a steep, fractionated pattern with depleted heavy
rare earths. Such a depletion of HREE suggests the
likelihood of hornblende fractionation
and/or residual
hornblende or garnet in the magma source (Arth & Baker
1976, Frey et ai. 1978). It does not support the argument
that hornblende was added to a silicic magma by magma
mingling; unless the felsic magma was already so depleted in
HREE
that later
additions
are obscured,
adding
"extraneous" hornblende should enrich the rocks in HREE,
contrary to observation. A slight LREE enrichment among
the samples correlates with increasing silica contento
Negative Eu anomalies are modest, Eu/Eu * varying from
0.61 to 0.87, and are consistent with the "buffering
effects" of hornblende and plagioclase in the granitoids
(Noyes et aI. 1983). Whereas the porphyritic granitoid
samples exhibit a near-parallel trend, the more silicic,
rme-grained granite is substantially more enriched in LREE
and depleted in HREE relative to the most silicic
porphyritic granitoid. Apparently the LREE continued to
be concentrated in the evolving melt.
Rb-Sr GEOCHEMISTRV OF THE MONTE DAS GAMElEIRAS
PlUTON
Rubidium and strontium
were
analyzed in a variety of whole-rock samples by isotope
dilution at the University of Texas. Fifty to 80 sweeps to
the mass spectrum were recorded for each analysis (Tab. 6).
The coefficient of variation was less than ± 0.050/00 (20)
for ali of the measured isotope ratios.
A 5·point Rb-Sr isochron (Fig. 5) based on samples of
grafite and porphyritic granitoid from across the pluton
gives t = 511 ± 26 Ma, and initial 87Sr/86Sr) s,n 0.7099 ±
0.0004 (20 errors). Note that data points for dark-colored
intrusives fall distinctly below the isochron defined by the
data from the felsic rocks. The aberrant points were not
used in the isochron caIculation; if included, they produce
an age estimate of about 750 Ma, which is geologically
unreasonable given the timeframe of the Brasiliano orogeny.
478
Revista Brasi/eirade
Table 5 - Whole rock trace element abundances (ppm)
Monte das Gameleiraspluton
Geociências.
~OOO
MONTE
DAS GAMELEIRAS
O
O
Sample
Volume 17, 1987
23
10
420
4.5
5CG-38
14
17
15
278
2.7
3.2
5.0
5.5
118
78 GA-la
118
11
1.2
2127
1O0.3
24
GA-Q5
GA-lb
28
32
42
0.8
2.0
0.7
0.9
0.5
96GA-7b
31
30
489
603
27
49
167
43
0.0
4.9
0.6
34
O
19
16
120
622
59
42
45
26
73
66
134
137
13
315
289
0.9
46
7.5
83
7.8
1.8
1.9
2.8
2.5
0.2
5.6
1.5
1.3
1.1
5.1
524
1.4
1.2
0783
316
292
136
223
59
23
17
68
15
22
3.0
0.5
85.4
0.9
1.8
0.8
270
283
291
360
60
262
150
130
5.9
6.5
57
120
0.6
1,314
1,321
1,484
1,527
1,789
456
38
GA -Ia
GA -Ib
6
GA - 5
O
O
CG - 38
GA -7b
ou:~100
.....
aw
Wo::
w
u
•.~
10
Lo
C.
p,
Nd
Sm
Eu
Gd
Oy
Er
Yb
Figure 4 - Chondrite-normalized rare-earth datá, Monte das
Gameleiras pluton. Abundances used for REE normalization
are Leedey chondrite data (Masuda et al 1973) divided
by 1.20 (Sun &Hanson 1977, Taylor & Gorton 1977)
to make them comparable to average chondrite data
Rock type: 1 = frne-grained granite; 2 = porphyritic granitoid
Rb-Sr
GA-12a2
GA-172
PEAL-2
GA-08
GA-13b3
GA-13a
PEAL-3
GA-Q5
PEAL-4
PEAL-5
GA-13p3
(**)PEAL-1
(*) GA-02 OF
GEOCHEMISTRY
Table 6 - Rb-Sr isotope data
Sample
THE
FAZENDA
0301
0.7100
403
1_208
0.7187
0.572
603
120
0.7141
0.993
480
0.294
0.7086
1.197
335
1.761
282
0.7220
1.418
332
163
184
0_7202
0.7394
0.7362
122
0.7104
0.7126
169
0.991
500
0.7172
0.7165
0.7191
2094
3.560
149
1534
0.580
610
1,026
87Rb/86Sr
140
1,072
Rb
(ppm)
172
109
Sr
(ppm)
107
172
3343
1804
7534
165
0.586
87Sr/86Sr
NOVA
As part of a reconnaissance study, Rb-Sr data
were also obtained from another classic Itaporanga-type
pluton between 6 and 19 km east of Fazenda Nova,
Pernambuco, about 200 km SSW of Monte das Gameleiras
(Fig. 1). Unaltered porphyritie granitoid, consisting of
extremely coarse (up to 15 em) megaerysts of a1kali
feldspar set in a much fine-grained, generally mafie J:latrix,
was sampled from five roadside exposures. Unlike the
Monte das Gameleiras samples, the rocks from Fazenda
Nova are strongly sheared. A major lineament marked by a
mylonite belt as mueh as 1 km wide trends eastward aeross
Pernambuco, and a splay of this main shear zone extends
into the region of Fazenda Nova.
A whole-rock Rb-Sr isochron (Fig. 6, rabo 6)
eorresponds to t = 630 ± 24 Ma (20) and initial 8 7Sr/8 6 Sr =
0.7065 ± 0.0005 (x). Analytieal errors of less than ± 0.3%
were attributed to Rb/Sr ratios of the three samples
analyzed by isotope dilution. Much larger errors of ± 3%
were applied to Rb/Sr in samples PEAL-3 and -5, in which
Rb and Sr were analyzed by X-ray fluoreseence at the
Universidade de São Paulo. Nevertheless, the linearity of
data-point distribution is adequate and the best-fit line is a
true isoehron within the assigned errors (MSWD = 0.65).
Both this and the Monte das Gameleiras isoehron are
PLUTON
Corrected for mass discrimination assuming that 86Sr/88Sr
= 0.1194, Ã. = 1.42x10-11/year
2
Samp1e of "dark-colored intrusive", not included in isochron
calcu1ation
3 Samp1es of porphyritic granitoid from same outcrop; values averaged to a single point for isochron
4 Concentrations analyzed by XRF
(*) Monte das-Gameleiras
(**) Fazenda Nova
well-behaved in spite of the presenee of shearing (Fazenda
Nova) and megaseopie textures suggesting the "kneading
together" of a felsie, megaeryst-bearing magma and a more
mafie, hornblende- and biotite-bearing magma (Monte das
Revista Brasikirade
Volume 17,1987
Geociências,
0725
The porphyritic granitoids and dark
intrusives share similar mineral suítes and textures. If they
are genetically related, the porphyritic granitoid could have
resulted from mingling a silicic, partiy crystallized melt
with a more mafic, nearly liquid melt (Vemon 1983), or
the porphyritic rocks could result from an "unrnixing" of
silicic partial melt and refractory material from the source
region (restite: White & Chappell 1977). Chemical and
isotopic data at this point in our study support neither
hypothesis, however, and instead suggest that the dark
intrusives are quite distinct from the more felsic rocks
despite field evidence that the two were intruded
contemporaneously.
The Monte das Gameleiras pluton has some features
typical of I-type granitoids, including a wide range of SiOz
in varied rock types, b180 < +10%0, and a characteristic
mineral assemblage of hornblende,
biotite,
sphene,
magnetíte, and allanite. Nevertheless, the moderately high
initial 87Sr/86Sr of about 0.709, 0180 typically > +8%0,
and low CaO suggest that source rocks may have had a
metasedimentary
component.
The regularity of REE
abundance
pattems
and of Rb-Sr isotopic behavior
indicates a rather uniform history of magma differentiation
with no substantial post-intrusive
alteration.
Oxygen
isotope equilibrium compositions for mineral separates
support the conclusion that post-intrusive effects were
slighi. Intrusion
accompanied regional metamorphism
during the late stages of the Brasiliano Cycle.
The Monte das Gameleiras and Fazenda Nova intrusions
are examples of a large group of Itaporanga-type granitoids .
Intringuingly, the various bodies are characterized by a
wide range of compositions (metaluminous, peralkalic,
potassic), yet they share many of the same textural
features, including alkali feldspar megacrysts and evidence
for "kneaded"
magmas. Despíte
the composítional
variability, all appear to have been influenced by similar
differentiation
processes and intrusive conditions. The
regional extent and good exposure of these plutons
commends them for further study, with implications for
the origin of similar felsic bodies in Northeastern Brazil and
in "mountain roots" worldwide.
CONCLUSIONS
MONTE
I
a
DAS GAMELEIRAS
511
! 25
Mo
875r/865r l. '"0_7099!
0.0004
0720
li;
'"
.."-
~0115
0710
•
•
Oork-colo'ed
inl,usiwes
05
10
1.5
Figure 5 - Five-point whole-rock isochron, Monte das
Carneleiras pluton. Data points co"esponding to a sarnple
of dark intrusive and to a mafic enclave were not used
in the isochron calculation but are indicated on the figure
to illustrate that they are not colinear with data points
representing the more felsic rocks
0.74
FAZENDA
1=630!.
NOVA
24 Mo
815,/865,)." 0.7065' 0.0005
•
0.73
cii
'"
..
•
.....
r--W
072
CD
4
Figure 6 - Five-point whole-rock isochronfor the Fazenda
Nova suite. Squares indica te data obtained by XRF
Gameleiras). It is noteworthy that the 5-point Fazenda
Nova isochron, the 5-point Monte das Gameleiras isochron,
and a 6-point isochron from the type Itaporanga pluton
data)
(625 ± 24 Ma, cf. Sial & Long unpublished
correspond to somewhat diverse ages even though ali of the
plutons consist of distinctive Itaporanga-type lithology.
Acknowledgements
Earlier version of this manuscript
benefited from comments by Daniel S. Baker, Calvin G.
Barnes, Fred W. MacDowell, and Douglas Smith. We also
wish to thank Antonio Carlos Galindo, who provided
valuable guidance in the fiefd and supplied us with several
whole-rock samples for analysis. This research was
supported in part by National Science Foundation (NSF)
TraveI Grant 80LA/BOl to L.E. Long and by the Geology
Foundation of the University of Texas. A.N. Sial wishes to
thank Conselho Nacional do Desenvolvimento Científico e
Tecnológico (CNPq) for an one-year fellowship at the
Universities of Georgia, USA, and Memorial University of
Newfoundland, Canada, where the oxygen isotope and REE
analyses were carried out.
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MANUSCRITO 426
Recebido em 12 de março de 1987
Revisão aceita em 04 de junho de 1987
Na Terra não devem existir muitos lugares acessíveis, onde ainda seja possível examinar áreas nunea exploradas
Joseph Wood Krutch, 1958, Grand Canyon, today and a11itsyesterdays.
pelo homem.
William Sloan Assoe.