Gondwana Research, V. 8, No. 3, pp. 1-16.
© 2005 International Association for Gondwana Research, Japan.
ISSN: 1342-937X
Gondwana
Research
Petrogenesis of A-type Granitoids from the Alto Moxoto and
Alto Pajeu Terranes of the Borborema Province, NE Brazil:
Constraints from Geochemistry and Isotopic Composition
Ignez P. Guimarães1, Adejardo F. Da Silva Filho1, Silvana C. Melo1 and Moacir B.
Macambira2
1
Departamento de Geologia, Universidade Federal de Pernambuco, Av. Prof. Moraes Rego S/N Cidade Universitária, Recife,
Pernambuco, Brazil, E-mail: [email protected]
2
Instituto de Geociências, Universidade Federal do Pará, Campus Universitário do Guamá, Rua Augusto Correa 01, CEP
66075-110, Belém, Brazil
(Manuscript received September 8, 2004; accepted April 2, 2005)
Abstract
A-type granitoids (~512 Ma) either intruded into Paleoproterozoic gneiss of the Alto Moxoto Terrane (Prata Complex
and Serra da Engabelada Pluton) or into Early Neoproterozoic metavolcanic metasedimentary sequence of the Alto
Pajeu Terrane (Serrote Santo Antonio Pluton), constitute a small proportion of Brasiliano (= Pan-African) granitoids in
the Central Tectonic Domain of the Borborema Province, northeastern Brazil. The Prata Complex consists of syenogranites,
monzogranites, mafic enclaves of diorites and norites. The felsic and mafic members are not genetically related through
fractionation. Mingling and mixing were extensive processes within the Prata Complex. The granites evolution appears
to have involved fractionation of alkali feldspar, biotite, apatite and sphene without significant wall-rock assimilation.
The Serra da Engabelada and Serrote Santo Antonio plutons consist of biotite syenogranites, with rare mafic enclaves.
The studied granitoids are dominantly metaluminous, characterized by Fe-rich biotite and Fe-hornblende. High total
alkalis, Y, Nb and REE and low CaO, MgO and Sr abundances and high FeO/(FeO+MgO) ratios characterize these
granitoids. Chondrite-normalized REE patterns show enriched LREE, moderate to strong negative Eu anomalies and
more or less flat heavy REE.
The studied granitoids and diabase from dykes and enclaves show negative εNd (512Ma) values (–14 to –10), high
incompatible elements such as LILE, HFSE and REE, suggesting important contribution of Paleoproterozoic crust. The
origin of the granites is thought to have involved partial melting of granodioritic or tonalitic lower crust. Such isotopic
signature of the diabase from the dykes also reflects a Paleoproterozoic enriched lithospheric mantle in the area. The
intrusion of the studied granitoids contemporary with sub-volcanic bimodal magmatism and deposition of many Cambrian
“pull-apart” basins in the north and central tectonic Domain of the Borborema Province, suggest intrusion during posttectonic relaxation of the Brasiliano orogeny following the assembly of West Gondwana.
Key words: A-type granitoids, Cambrian, Borborema Province, post-tectonic magmatism.
Introduction
A-type granitoids as defined by Loiselle and Wones
(1979) are anorogenic, characterized by high alkalis
(Na2O+K2O) contents, FeO/(FeO+MgO) ratios, Ga/Al,
Zr, Y, Nb, F, Cl and REE (except Eu), and low abundances
of CaO and MgO (Collins et al., 1982; Whalen et al., 1987).
They are also characterized by the presence of one or more
of such ferromagnesian minerals as annite-rich biotite,
ferro hastingsite, alkali amphibole and Na-pyroxene
(Collins et al., 1982; Whalen et al., 1987; Eby, 1990).
Landenberger and Collins (1996) argued that the elevated
F contents in A-type granites could be due to fractional
crystallization processes and as consequence, high-halogens
contents are not a major feature of A-type granite.
Because A-type granitoids can be emplaced at any time
during a tectonic - magmatic episode, the term anorogenic
should not be used to characterize A-type granitoids
(e.g., King et al., 1997; Barbarin, 1999; Wu et al., 2002;
Kebede and Koeberl, 2003; Mushkin et al., 2003).
Many models have been proposed to the origin of
A-type granitoids:
(1) Partial melting of deep crust of granulitic
metaigneous sources previously depleted in a hydrous
felsic melt (Collins et al., 1982; Clemens et al., 1986;
2
IGNEZ P. GUIMARÃES ET AL.
Whalen et al., 1987). This idea has been questioned by
Creaser et al. (1991); Landenberger and Collins (1996)
and Patiño Douce (1997), under the arguments that
melting of a wide range of crustal rocks produces
refractory granulitic residues that are depleted in alkalis
relative to alumina and in TiO2 relative to MgO. Remelting
of such residue cannot produces magmas with A-type (high
Na2O + K2O/Al2O3 and TiO2/MgO) characteristics.
(2) Partial melting of charnockitic lower crust, formed
as a residue from an earlier I-type magma extracted, at
temperature >900oC, in a subduction-related tectonic
setting (Landenberg and Collins, 1996).
(3) Dehydration melting of calc alkaline granitoids
(Anderson, 1983; Creaser et al., 1991). The experimental
data of Patiño Douce (1997) confirm that A-type granitic
melts can be generated in this way but, it is only possible
if melting takes place in the shallow crust. The necessary
high melting temperature requires the participation of hot
mafic magmas in the origin of A-type granites, may be
restricted to heat transfer or may entails shallow chemical
interactions between basaltic melts and crustal rocks
(Patiño Douce, 1997)
(4) Dehydration melting of amphibole- bearing
tonalite at 6-10 kbar, leaving behind a granulitic residue,
produces melts that resembles A-type granites, except for
their somewhat high Al 2O 3 contents (Skjerlie and
Johnston, 1992).
(5) Differentiation from basaltic magmas (Loiselle and
Wones, 1979; Eby, 1992; Turner et al., 1992; Beyth et al.,
1994),
(6) Small degree of partial melting of a felsic
infracrustal source region, with water and F contents
similar to those recorded in I-type granitoids source regions
(King et al., 1997).
Only few granitic intrusions with geochemistry
characteristics of A-type have been described in the Central
Tectonic Domain of the Borborema Province (Melo et al.,
1995; Melo et al., 1996). In this work, we provide
geochemical, geochronological and isotopic data for the
Prata Complex, Serra da Engabelada and Serrote Santo
Antonio plutons and discuss their sources.
Regional Geological Aspects
The Borborema Province comprises a large region in
northeastern Brazil, north of the São Francisco Craton
(Fig. 1A). In pre-drift reconstructions, this province is
adjacent to similar Pan- African and terranes in western
Africa; thus, the Borborema Province represents the
western part of a belt that occupies northern Gondwana
(Van Schmus et al., 1995).
The Brasiliano – Pan-African Orogeny within the
Borborema Province is characterized by the development
of shear zones hundreds of kilometers long, large amount
of granitic intrusions and metamorphism under high
temperature conditions (Bittar, 1999; Leite, 2001). The
majority of the granitoid intrusions had their emplacement
controlled by shear zones.
The Central Tectonic Domain (Van Schmus et al., 1995),
previously named Transversal Zone by Ebert (1970), was
interpreted by Santos (1995), Santos et al. (1997) and
Santos and Medeiros (1999), as the result of Brasiliano
accretion of exotic terranes. According to this terraneaccretion model, the studied granitoids are located
(Fig. 1A) in the Alto Moxoto and Alto Pajeú Terranes.
The Alto Moxoto Terrane is composed of metavolcanometasedimentary sequences, including a calc-alkaline
volcanic sequence of arc affinities and Paleoproterozoic
blocks (2.1–2.4 Ga) of tonalitic-granodiorítico
composition (Santos, 1995). Only few granitic intrusions
have been identified within the Alto Moxoto Terrane and
they are mainly intruded along the contact zone between
the Alto Moxoto Terrane, and either the Alto Pajeú and
Rio Capibaribe Terranes.
The Alto Pajeú Terrane comprises muscovite-biotite
gneisses, garnet-biotite schists, and metavolcanic rocks
intruded by Early Neoproterozoic granitic gneisses
(~ 950 Ma), deformed during the Brasiliano cycle, initially
by a transcurrent episode and later by extension (Santos
et al., 1997; Brito Neves et al., 2001). It is cut by large
granitic intrusions, with ages in the 512–644 Ma range
(Guimarães et al., 1999).
The timing of the peak of the metamorphism has not
been determined systematically in the Central Tectonic
Domain of the Borborema Province. In the Alto Pajeú
Terrane, Leite et al. (2000) reported an upper intercept
U-Pb zircon age of 972±4 Ma for orthogneisses intruded
by the Brasiliano Tabira pluton, and a concordant
sphene fraction from the same sample that is 612±9 Ma
old. Guimarães et al. (2004) demonstrated that the
intrusion of Timbaúba complex (644 Ma) was pre- to
syn-metamorphism. This evidence suggests that the
metamorphism took place between 612–640 Ma.
Narrow and elongated deposits of detritic sediments,
sandstones, arkoses and conglomerates occur to the west
of the Alto Pajeú Terrane. These sediments have been
interpreted as part of the Tacaratu Formation of the Jatobá
Basin, of Upper Silurian age (Veiga Júnior and Ferreira,
1990). Other small Paleozoic basins occur in the Central
Tectonic Domain (Betânia, Fátima, Carnaubeira,
Mirandiba, São José do Belmonte-Veiga Júnior and
Ferreira, 1990). The sediments cropping out in all of these
small basins have been interpreted as chrono-correlated
to the Tacaratu Formation. The absence of fossils in these
sediments make it difficult to date them. The Upper
Silurian age was estimated from lithologic correlation.
Gondwana Research, V. 8, No. 3, 2005
PETROGENESIS OF A-TYPE GRANITOIDS, NE BRAZIL
Several Cambrian pull-apart basins (Iara, Jaibaras graben
and Saíri), consisting of extension-related molasses
deposits have been described further northwest, at the
Ceará State (Fetter, 1999). The Mucambo granite that
flanks the Jaibaras Graben, the largest of these extensional
basins, yields U-Pb zircon crystallization age of 532±6 Ma
(Fetter, 1999).
Geological and Petrographic Aspects
The Prata Complex (350 km 2) is intruded into
Paleoproterozoic gneisses and migmatites from the Alto
Moxotó Terrane. Foliated fabrics occur near the external
contacts and major faults. In the southeast contact, a NW
fault associated to diabase dyke swarms penetrated the
complex. Along this fault, interactions between the diabase
and magmas of granitic composition are widely recorded
(Fig. 1B and Fig. 2A and 2B).
3
The Prata Complex encompasses many petrographic
facies: biotite porphyritic syenogranite (BSG), coarsegrained hornblende-biotite porphyritic syeno to
monzogranites (HBSMG), monzodiorite to qz-monzonite,
diorites and norites (Melo et al., 1996).
The HBSMG is composed of plagioclase, having oligoclase
composition, perthitic microcline showing locally plagioclase
mantling, amphiboles (hastingsite and Fe-edenite), anniterich biotite and subordinate allanite rimmed by epidote,
sphene, apatite and zircon. Enclave swarms of dioritic
composition are widespread. The dioritic enclaves show
crenulated contacts (Fig. 2A, B); enclose locally small volume
of the granitic magma and ovoid feldspar crystals (Fig. 2A),
acicular apatite, calcic zones in corroded plagioclase crystals,
poikilitic K-feldspar with biotite and plagioclase inclusions.
These textural features are compatible with magma mixing,
reflecting interactions between granitic and dioritic magmas.
Country rock enclaves are locally recorded (Fig. 2A).
Fig. 1A. Sketch geological map of the Central Tectonic Domain of the Borborema Province. The main shear zones and the tectonostratigraphic terranes
proposed by Santos et al. (1999): AMT=Alto Moxotó; APT=Alto Pajeú; RCT=Rio Capibaribe; PABB=Piancó Alto Brígida belt; SB=Sergipano
belt; PAT=Pernambuco-Alagoas; GJT=Grajeiro; SED=Seridó belt; JC=São José do Campestre; RP=Rio Piranhas; JG=Jaguaribeano;
CE=Ceara; MC=Medio Coreaú; RP=Riacho do Pontal. PWSZ and PESZ are the two branches (west and east) of the Pernambuco lineament
as proposed by Neves and Mariano, 1997). PSZ=Patos shear zone. 1–Paleozoic sedimentary basin. Gray square=study area.
Gondwana Research, V. 8, No. 3, 2005
4
IGNEZ P. GUIMARÃES ET AL.
Fig. 1B. Simplified geological map of the studied area. Modified from Santos et al. (2002) 1=dykes (a=diabase; b=riodacite; c=rhyolite);
2 – a=norites and b=the study granites (A=Prata Complex, north (A1) and South (A2) segments; B=Serra da Engabelada pluton;
C=Serrote Santo Antonio pluton); 3=Serra Branca granitoids (570Ma); 4– EoNeoproterozoic orthogneisses; 5–Mesoproterozoic
metasediments; 6–Paleoproterozoic metasedimentary sequence and orthogneisses; 7–Archean block. CSZ=Coxixola Shear Zone;
ConSZ=Congo Shear Zone; SJCSZ=São João do Cariri Shear Zone.
Enclave swarms of norites occur in an area of 6km2
close to the Prata Village. This intrusion of norite separates
the Prata Complex into north and south segments
(Fig. 1B). They are composed of plagioclase (labradorite),
hypersthene, salite and biotite. Olivine, hortonolite in
composition, was locally recorded. The crenulated and
corroded plagioclase crystals, showing calcic zone and
needles of apatite, reflect magma mixing processes
between basaltic and granitic magmas.
The south segment of the Prata Complex is composed
by biotite±hornblende syenogranite (BSG) cut by biotite±
garnet±fluorite syenogranite. Allanite occurs in the BSG
in modal amount as much as 5% and, it can reach up to
5 mm long. The diorites which form enclave swarms
within the BSG, aligned with the diabase dykes, contain
rounded and acicular (up to 10 mm long and 0.8 mm
wide) crystals of hypersthene, surrounded by augite and
hornblende. They also show hornblende-, biotite-mantled
quartz phenocrysts, acicular apatite, small clots of
hornblende+biotite and poikilitic K-feldspar, which are
features consistent with magma mixing processes between
the granitic and basaltic magmas. Monzodiorite to quartz
monzonite compositions are interpreted as a result from
these interactions.
Gondwana Research, V. 8, No. 3, 2005
PETROGENESIS OF A-TYPE GRANITOIDS, NE BRAZIL
5
The mineral phases of the studied rocks were analyzed by
microprobe at Instituto de Geociências, São Paulo University,
using a GEOL Super Probe, JXA-8600 model fitted with a
Link Systems energy dispersive detector. The operating
conditions were 15 Kv, with a specimen current of 20 mA.
with FeO/MgO ratios ranging from 5.23 to 13.61 while
biotite from the diorites has FeO/MgO ratios in the
3.26 to 4.24 range, and those from the norites have
FeO/MgO ratios in the 0.94 to 0.48 range. Flogopites
(FeO/MgO = 2.08) was recorded within the norites
(Sample IG-02n1). According to Abdel-Rahman (1994)
the FeO/MgO ratios in biotites define three
compositionally distinct fields: (1) Biotites from alkaline
anorogenic suites are iron-rich siliceous annite and show
FeO/MgO = 7.04, on average; (2) biotites from the
peraluminous granitoids, including the S-type, are
siderophyllite in composition (FeO/MgO ratio = 3.48, on
average), and (3) Biotites from the calc-alkaline granitoids
are moderately Mg-rich, with FeO/MgO ratio of 1.76 on
average. The FeO/MgO ratios in biotites from the Prata
Complex granites are similar to those from alkaline
anorogenic suites. The FeO/MgO ratios recorded in the
biotites of the diorites are similar to those recorded in
peraluminous granites. Because the diorites are not
peraluminous, the FeO/MgO ratios may reflect the
interactions between the granitic and basaltic magma.
Mica
Amphibole
Biotite in the granites from Prata Complex, Serra da
Engabelada and Serrote Santo Antonio is annite rich,
Amphiboles of the Prata Complex, were analyzed and
classified according to the scheme proposed by Leake
The Serra da Engabelada pluton (50 km2) is intruded
into Paleoproterozoic gneisses and migmatites from the
Alto Moxotó Terrane. The Serrote Santo Antonio pluton
(75 km2) intrudes an early Neoproterozoic metavolcanometasedimentary sequence of the Alto Pajeú Terrane. Both
plutons are constituted by coarse-grained, equigranular
biotite syenogranites. Biotite constitutes less than 10% of
the modal composition and it shows a long history of
crystallization within these granitoids, occurring as
inclusion within plagioclase and also as a later phase,
corroded by plagioclase, enclosing allanite, monazite and
zircon. Fluorite occurs as a later crystallized phase and
topaz was recorded locally.
Mafic Mineral Chemistry
(a)
(b)
Fig. 2. Photograph of diorite and granite petrographic facies within the Prata Complex, suggesting mingling processes. (a) Diorite (A) as enclaves,
showing crenulate and lobate contacts with the host granite (B) and containing enclave of the host granite (D), rounded crystals of alkali
feldspar and also enclave of the country rock (C). (b) Enclave of diorite (X) partially penetrated by vein of a hybrid facies (Z). The hybrid
facies also contain phenocrystals of alkali feldspar and enclaves of diorite and granite (Y).
Gondwana Research, V. 8, No. 3, 2005
6
IGNEZ P. GUIMARÃES ET AL.
et al. (1997). Fe2+ and Fe3+ were calculated according to
Holland and Blundy (1994).
Amphibole compositions in the granites range from
hastingsite to ferro-edenite. Temperature, total pressure
and pH2O are intensive parameters controlling biotite and
amphibole Fe/ (Fe+Mg) ratios (Wones, 1981) but, it is
fO2 that by far exerts the strongest control (Anderson and
Smith, 1995). With increasing fO2, the Fe/(Fe+Mg) ratio
markedly decreases in biotite and amphibole, independent
of the Fe/Mg ratio of the whole-rock. Low- fO2 granites
have amphibole Fe/(Fe+Mg) ratios exceeding the
0.40–0.65 interval. In the studied granites, the Fe/(Fe+Mg)
ratios range from 0.82 to 0.93, and Fe3+/(Fe2++Fe3+) ratios
from 0.06 to 0.12, reflecting low fO2 conditions. Amphibole
from the diorites, and the hybrid facies, monzodiorites to
qz- monzodiorites, show higher Mg contents, plotting in
the magnesio hastingsite and hastingsite field, with
Fe/(Fe+Mg) ratios ranging from 0.55 to 0.67, reflecting
high to intermediate fO2 conditions (Fig. 3).
The granitoids are metaluminous to slightly peraluminous
(Fig. 4), with ASI ranging from 0.82 to 1.2, show high
K2O contents (>4%), except the norites (<2%), with
K2O/Na2O ratios >1.0. Most of the samples plot within
the calc-alkaline field in the AFM diagram (Fig. 5).
However, some of the diorites and norites are Fe-rich,
plotting in the tholeiitic field. The granites show a trend
parallel to the AF side of the AFM diagram, reflecting
crystallization under low fO2 conditions.
SiO2 correlations with major oxides, including Al2O3,
MgO, CaO, K2O, Na2O, P2O5, TiO2 and some trace elements
(Ba, Sr, Nb and LREE) are shown in figure 6. The norites
and diorites do not define trends in the correlation
diagrams suggesting that they are not cogenetic. On the
other hand, the norites possess a relatively wide variation
in their major and trace elements, suggesting that their
Geochemistry
Major, minor and some trace elements (Ba, Sr, Y) were
analyzed by ICP-AES at ACME Laboratories (Canada). Rb,
Nb, Zr and REE were analyzed by X-Ray Fluorescence
Spectrometry at University of Pavia (Italy). Ta, Hf, Th and
REE in some samples were analyzed by INAA at ActLaboratories (Canada). The results were controlled by 01
covered international standard and 02 covered duplicated
samples for each batch of 20 samples. They are presented
in table 1.
Most granitoids (over 70% of analyzed samples) have
SiO 2>70%, indicating their highly evolved nature.
Fig. 3. Composition of amphiboles from the Prata Complex granitoids
in terms of Fe/(Fe+Mg) versus AlIV. Fields of fO2 and composition
of hornblende from Proterozoic anorogenic granites from North
America after Anderson and Smith (1995).
Fig. 4. Shand’s Index for the studied granitoids; fields after Maniar
and Piccolli (1989).
Fig. 5. The studied granitoids in the AFM diagram; fields after Irvine
and Baragar (1971).
Gondwana Research, V. 8, No. 3, 2005
PETROGENESIS OF A-TYPE GRANITOIDS, NE BRAZIL
7
Table 1. Representative whole–rock compositions of the Prata Complex, Serra da Engabelada and Serrote Santo Antonio plutons.
Sample
IG-38
IG-17A
IG-34D
STA-01
SE-15
IG-18
SiO2
TiO2
70.92
0.35
74.03
0.18
70.03
0.45
73.81
0.28
74.30
0.17
71.92
1.04
52.53
2.25
56.51
2.08
53,00
2,45
49.90
2.74
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
13.90
3.50
0.41
1.00
3.77
5.97
0.06
0.10
0.40
100.38
13.54
2.02
0.08
0.31
0.78
3.78
5.25
0.02
0.15
100.05
14.22
3.45
0.63
1.43
3.45
6.12
0.12
0.10
0.89
100.7
12.96
2.47
0.03
0.26
1.08
2.74
5.37
0.05
0.5
99.78
12.49 13.16
14.50 14.46
14.80
2.51
3.37
5.08
8.76
10.56
0.03
0.09
0.11
0.14
0.16
0.10
0.47
0.98
1.86
4.92
1.27
1.04
1.92
3.73
7.04
3.34
3.50
3.75
3.83
3.51
4.01
5.81
5.71
4.30
2.35
0.04
0.09
0.30
0.06
0.92
1.0
0.20
0.36
0.92
0.50
99.42 100.68 100.01 99.69 100.09
Trace-element compositions (ppm)
14.24
9.87
0.15
3.91
5.54
3.54
3.14
1.02
0.40
99.58
14,63
11,03
0,17
3,79
6,47
3,45
2,46
0,93
1,00
99,50*
15.26 18.80 12.90
11.90 10.43 14.03
0.17
0.14
0.14
4.22
7.30
9.62
6.47
7.71 11.17
3.90
3.39
2.20
2.05
1.56
0.77
1.00
0.71
0.07
1.20
0.54
0.63
99.14* 100.54 100.51
Ba
Sr
Rb
Th
Cr
Ni
Y
Nb
Zr
Hf
Ta
La
Ce
Nd
Sm
Eu
Gd
Tb
Dy
Er
Yb
Lu
Fe#
T°C(Zr)
430
70
170
nd
15
5
77
41
570
nd
nd
179
352
120
23.4
1.26
14.7
nd
12.2
5.6
4.53
nd
0.88
918
260
60
260
nd
<5
<5
66
nd
230
nd
nd
62
128
46
9.6
0.65
6.6
nd
6.4
3.4
4.04
nd
0.85
826
840
170
200
nd
<5
<5
54
36
550
nd
nd
99
194
75
15.7
1.93
9.5
nd
9.5
8.3
3.66
nd
0.83
914
875
140
185
25.8
15
20
35
23
290
7.8
nd
151
261
88
14.4
0.90
9.6
1.4
6.0
1.6
3.2
0.6
0.89
848
520
90
165
21.4
15
15
57
32
205
6.8
3.3
81
164
60
10.9
1.18
Nd
1.7
Nd
Nd
7.25
1.08
0.96
815
560
110
200
34
<5
<5
49
26
460
12.0
0.9
222
403
130
17.6
1.31
15.3
1.8
8.5
4.7
4.00
0.60
0.87
895
IG-12A IG-34Xdi IG-65Adi IG-34Bdi IG-36Ed IG-36Ld
66.98
0.68
60.97
1.36
830
1165
1030
1240
200
290
560
555
198
150
40
85
nd
nd
6.0
nd
<5
5
5
90
<5
<5
<5
30
62
59
48
56
35
37
25
38
640
795
490
620
nd
nd
10.2
nd
nd
nd
1.4
nd
194
96
75
88
289
195
140
175
98
82
63
76
20
18.1
12.3
16.5
2.08
2.84
3.42
3.06
11.6
12.4
nd
11.5
nd
nd
1.7
nd
10.73 10.6
nd
9.6
5.27
5.1
nd
4.6
4.81
4.84
5.3
4.02
nd
nd
0.76
nd
0.82
0.81
0.66
0.69
1133 1070
470
570
75
45
10,30
6.40
70
40
40
46
47
43
26
23
322
408
10,6
9.9
1,8
1.2
93
81
171
150.0
71
65
14,80
13.30
3,45
3.52
nd
nd
1,90
1.80
nd
nd
nd
nd
5,69
5.25
0,86
0.80
0.72 0.72
IG-05n
IG-01n
48.33
1.63
49.06
1.09
590
90
610
400
20
15
nd
0.20
130
220
110
130
34
21
19
6
310
73
nd
1.5
nd
0.3
31
40
72
9
37
6
8.9
2.33
2.16
0.80
7.12 nd
nd
0.50
6.5
nd
3.1
nd
2.89
2.23
nd
0.34
0.56
0.63
Samples IG=Prata Complex; STA=Serrate Santo Antonio; SE=Serra da Engabelada (Fig. 1B); nd=not analyzed; d=dolerite; n=norite; di=diorite
*=Ba as BaO added to the total. Total Fe as Fe2O3 . Fe#=FeO/(FeO+MgO). T°C(Zr)=Zircon saturation temperatures (Watson, 1987).
chemistry has been modified by interactions with the
granitic magmas. The studied granitoids (SiO2>66%) have
also high FeOt/(FeOt+MgO) ratios (>0.82), being
classified as Ferroan plutons and plotting within the
A-type (Fig. 7) granites field (Frost et al., 2001).
The granites from the Prata Complex, Serra da
Engabelada and Serrote Santo Antonio plutons are Y-rich
and Sr-poor compared to older granitoids, with similar
K2O and SiO2 contents, described in the Pajeú-Paraíba belt
(Guimarães et al., 2000), have variable Nb contents and
Y/Nb ratios in the 1.3 to 2.1 range (Fig. 6N). The low Rb
and Sr contents suggest fractionation of alkali feldspar
during the evolution of the granitic magmas.
The granites have REE chondrite-normalized patterns
(Sun, 1982) characterized by negative Eu anomalies
(Eu* = 0.27 to 0.33) and (Ce)N/(Yb)N ratios ranging from
6 to 27 (Fig. 8). The diorites show REE patterns similar to
Gondwana Research, V. 8, No. 3, 2005
those recorded in the granites, with smaller negative Eu
anomalies (Eu* = 0.5 to 0.67), while the norites, have
variable (Ce)N/(Yb)N ratios (Fig. 8), lack or have no
significant negative Eu anomalies.
The chondrite-normalized spiderdiagram patterns of
the granites, diorites and norites are shown in figures 9A
and 9B. The patterns of the granites are characterized
by deep trough at Ti, P, Sr and Ba, smaller troughs at Nb,
and LILE / HFSE ratios <10. These troughs may be related
to the fractionation of apatite (Sr e P), plagioclase (Sr)
and sphene (Ti). The probably fractionation of sphene
within the granitic magma is also supported by the
negative correlation between LREE (La+Ce) and SiO2
(Fig. 6L).
The studied granitoids plot within the A-type
granite fields in the Zr-Nb-Ce+Y versus FeO/Mg and
(K 2O+Na 2O)/CaO diagrams (Whalen et al., 1987)
IGNEZ P. GUIMARÃES ET AL.
Na2O (wt %)
Al2O3 (wt %)
8
SiO2 (wt %)
MgO (wt%)
CaO (wt %)
SiO2 (wt %)
SiO2 (wt %)
TiO2 (wt%)
K2O (wt %)
SiO2 (wt %)
SiO2 (wt %)
Fe2O3
P2O5 (wt %)
SiO2 (wt %)
SiO2 (wt %)
SiO2 (wt %)
Fig. 6. Variation diagrams for major and some trace elements in the studied granitoids.
Gondwana Research, V. 8, No. 3, 2005
PETROGENESIS OF A-TYPE GRANITOIDS, NE BRAZIL
9
Fig. 6. Contd.
(Figs. 10A and 10B). In the discriminant tectonic
diagrams of Pearce et al. (1984) and Pearce (1996), the
studied samples fall within the within the plate granite
field and post-collision granites field (Fig. 10C and 10D).
The Prata, Serra da Engabelada and Serrote Santo
Antonio granites are best characterized as A-type
granites (Whalen et al., 1987) or either alkaline postcollision type (Sylvester, 1989; Pearce, 1996); falling
within the A2 field (Fig. 10E) in the Nb-Y-Zr/4 diagram
(Eby, 1992), which reflect their derivation from crustal
sources.
Gondwana Research, V. 8, No. 3, 2005
Calculated Crystallization Temperatures
Zircon solubility can be empirically correlated with SiO2
content, the ratio of feldspar cations to aluminum in the
melt (K+Na+2Ca/Si.Al) and are systematically dependent
upon temperature (Watson, 1987; Watson and Harrison,
1983). The zircon saturation temperatures for the studied
granites with SiO2>70%, range from 815°C to 918°C
(Table 1). The occurrence of melt-precipitated zircon along
with zircon showing calculated age inheritance, as
recorded in the Prata granitoids, could imply that the
10
IGNEZ P. GUIMARÃES ET AL.
Fig. 7. The compositional range of the studied granitoids in the FeOtot/
(FeOtot + MgO) versus weight percent SiO2 diagram; fields of
ferroan and magnesian granitoids are from Frost et al. (2001).
538+23 Ma and 532+6 Ma respectively, which could be
considered identical within error, to the Rb-Sr age
presented by Melo et al. (1996).
The studied granites have T DM model age in the
1.8 Ga–2.1 Ga range and epsilon Nd (512 Ma) ranging
from –19.96 to –16.17 (Table 2). The diorites and diabases,
which occur as dyke swarms along the SE contact of the
Prata Complex, show similar isotopic signature (Fig. 11),
with epsilon Nd (512 Ma) values higher than those
recorded in the granites (–11.48 to –14.95). These data
suggest a Paleoproterozoic component involved in the
source of these granitoids which could be from the lower
crust. Melts of dioritic compositions can be generated
under temperatures higher than 1050°C (Rapp and
Watson, 1995), which are not expected to be reached in
the lower crust during orogenic events. Partial melting of
a Paleoproterozoic lithospheric mantle enriched in
incompatible elements is a good candidate as the source
of the diorites and diabase.
Norites show a significant range in their isotopic
composition, reflecting interactions with the Prata granitic
Fig. 8. Chondrite-normalized REE patterns (Sun, 1982) of the studied
granitoids
magma was oversaturated in zircon so that the calculated
temperature are above that of the magma.
Geochronology and Isotopic Geochemistry
Sm-Nd isotopic analyses were made at the Isotope
Geochemistry Laboratories, Kansas University, USA. The
methodology is described in the appendix. Rb-Sr isotopic
analyses were carried out in the Isotope Geological
Laboratories, Pará Federal University, Brazil. The results
of representative samples are presented in table 2.
There was a tentative to date these granitoids by U-Pb
zircon method. However, large zircon inherited component
present in these rocks did not allowed reliable results.
Rb-Sr whole-rock isochron (Melo et al., 1996) gave an
age of 512+30Ma. Granitoids with similar geochemistry
signature in the Central Tectonic Domain (Pereiro and
Serra da Velha Zuza – Guimarães et al., 1999) and in the
North Domain (Mucambo – Fetter, 1999) of the Borborema
Province, have U-Pb zircon ages of 543+6.7 Ma;
Fig. 9. Primitive mantle (Wood, 1979) normalized, trace element
abundance diagrams (spidergrams) for representative samples
of the studied granitoids (A) and norites, diabase dykes and
diorites (B).
Gondwana Research, V. 8, No. 3, 2005
PETROGENESIS OF A-TYPE GRANITOIDS, NE BRAZIL
11
Table 2. Rb-Sr and Sm-Nd Isotopic data of the granites and diorites (di) from the Prata Complex, Serra da Engabelada (SU) and Serrote Santo
Antonio (STA) ; diabases (d) and associated norites (n).
Sample
IG-36Ed
IG-36Ld
Rb(ppm)
Sr (ppm)
87
Rb/86Sr
87
Sr/86Sri
εSr(512 Ma)
Nd (ppm)
46
600
0.2200
0.706461
36
68.4
64
490
0.3847
0.707727
54
73.7
50
705
0.2046
0.706505
37
61.2
Sm (ppm)
Nd/144Nd
147
Sm/144Nd
εNd (today)
εNd (512 Ma)
TDM (Ga)
12.62
0.511380
0.1115
-17.12
-11.48
1.91
12.55
0.511202
0.1111
-20.62
-14.95
2.25
11.36
0.511262
0.1122
-19.39
-13.79
2.10
143
IG-65Adi
IG-01Gn
IG-05Cn
IG-34D
6.8
33.6
176
146
3.5140
0.713142
139
83.5
2.20
0.512211
0.1953
-4.75
4.74
******
6.81
0.511834
0.1225
-7.63
-2.70
1.33
13.3
0.510964
0.1043
-26.08
-19.60
2.25
magma. The less “contaminated” sample which is
LILE-poor and MgO- rich (7%–8%), show positive values
of εNd(512 Ma) (4.7), reflecting an asthenospheric mantle
source. The Nd signature of the norites, also suggest that
the isotopic signatures of the diorites and diabase do not
reflect homogenization with the enclosing granites. If so,
the norites should show the same behavior, since they
show clear evidence of interactions with the granitic
magma. Then, the isotopic signature of diabase and
diorites reflects the mantle isotopic signature, in the
Borborema Province.
Initial εSr (512 Ma) and εNd (512 Ma) isotopic
compositions of granites from Prata Complex and
associated diabase dykes are shown in figure 12, together
with the upper and lower crust fields. They show a wide
range in the εSr values (8.5 to 221.0), plotting in the IV.
The lowest εSr value was recorded in a granite sample
from the Prata north segment.
Petrogenesis
Field evidence points out the importance of magma
mixing and mingling in the evolution of the Prata
Complex. The occurrence of co-magmatic mafic and felsic
rocks could be interpreted as: (1) the felsic rocks been
generated by fractionation of mafic magmas, involving
significant fusion of the lower crustal rocks by the mafic
magmas; (2) the felsic and mafic rocks originated from
distinct sources and underwent mixing process and
subsequent fractionation.
The fractionation-dominated model could not be
applied to the formation of the felsic and mafic-studied
rocks, because they have distinct geochemistry signatures,
and the compositionally intermediate members of the
suite, represent mixing products between felsic and mafic
members, as it becomes evident by field and textural
relationships. However, chemical variations (CaO, K2O,
P2O5, TiO2, Na2O, Ba, Sr) indicate that magma mixing was
Gondwana Research, V. 8, No. 3, 2005
SU-15
STA-01
IG-84
IG-05di
IG-34Adi
148
76
5.6826
0.711996
115
63.2
184
135
3.8271
0.719430
221
97.8
249
255
2.8305
0.703927
8.9
70.6
39.9
89.2
11.5
0.511050
0.1101
-23.63
-17.92
2.39
14.6
0.511140
0.0901
-23.22
-16.17
1.97
12.2
0.510946
0.1043
-26.08
-19.96
2.44
7.7
0.511335
0.1163
-17.70
-12.36
2.05
15.0
0.511014
0.1020
-24.90
-18.6
2.30
not the only process involved (Fig. 6). Negative trends
presented by the granites in the CaO, P2O5, K2O, TiO2 and
(La+Ce) versus SiO2 diagrams suggest that fractionation
of apatite, alkali feldspar, sphene+allanite, took place
during the evolution of the granites from the Prata
Complex. The alkali feldspar fractionation is in agreement
with the positive trends recorded in the Rb/Sr versus Rb/
Ba diagram (Fig. 6M). Crystal-liquid fractionation process,
locally dominated by feldspar separation, is typically
significant in A-type granite systems, because of the high
temperatures and high proportion of melt that characterize
such melts during their emplacement (Clemens et al.,
1986; Collins et al., 1982; Eby, 1992; Dall’Ágnol et al.,
1999; Tollo et al., 2004). The deep troughs at Sr and Ti
recorded in the granites spidergrams (Fig. 9A) and REE
patterns showing deep negative Eu anomalies (Fig. 8)
could be also related to plagioclase and Fe-Ti oxides
residual or either the early extraction of plagioclase during
generation of calc-alkaline magmas.
The evolution of magmatic systems by fractionation,
combined with wall-rock assimilation, occurs in many
cases (Devey and Cox, 1987; Marsh, 1989). However, it
is important to establish the significance of this
assimilation to affect the chemical evolution of the studied
granitoids. Y/Nb ratios plotted against their SiO2 content
(Fig. 6N), show that within the granites and most of
diorites the Y/Nb ratios have no significant variation,
suggesting that wall assimilation was not significant
during the evolution of the granitic magmas. Norites, on
the other hand, show large variation of Y/Nb ratios, may
reflect their interactions with the granites.
The A-type studied granites have Sr-Nd isotopic
signatures similar to those recorded in the high-K calcalkaline and shoshonitic granitoids (Guimarães et al.,
2004) of the Central Tectonic Domain (580–590Ma).
Higher εNd values recorded in rock from the north
segment of the Prata Complex, suggest that an older
component, Paleoproterozoic or Archean, was possibly
12
IGNEZ P. GUIMARÃES ET AL.
involved in the source of rocks from this segment of the
Prata Complex. The Nd isotope and geochemistry also
show two distinct mantle-derived magmas involved in the
evolution of the Prata Complex: (1) diabase originated
from a metasomatized Paleoproterozoic lithosferic mantle,
which underwent mixing process with the granites,
generating the diorites and (2) the norites, showing more
primitive Nd signature and chemical composition
suggesting an asthenospheric mantle source. The norites
also underwent mingling process with the enclosing
granites.
(A)
(B)
(C)
(D)
Discussion
The Nd signatures of the granitoids from the Prata
Complex, Serra da Engabelada and Serrote Santo Antonio
plutons suggest a crustal component involved in the source
of these granitoids. The mineral chemistry of these granites,
suggest crystallization under low fO2 and low H2O.
High-K calc-alkaline and shoshonitic granitic
(SiO 2>66%) intrusions within the Central Tectonic
Domain of the Borborema Province have Nd signatures
similar to those recorded in the studied granitoids
(Guimarães et al., 2004), suggesting that they could share
(E)
Fig. 10. Trace elements for the studied granitoids in the tectonic discriminant
diagrams of: (A) and (B); Whalen (1987); (C) and (D) Pearce et al.
(1984); (E) Eby (1992).
Gondwana Research, V. 8, No. 3, 2005
PETROGENESIS OF A-TYPE GRANITOIDS, NE BRAZIL
Epsilon (Nd)
a similar source rock. However, they have a distinct chemistry,
distinct crystallization ages (580 Ma–590 Ma), and they
also crystallized under distinct fO2 and H2O conditions.
If one can consider that the high-K granitoids
(580-590 Ma) and the studied granitoids were originated
from the same source and, take under consideration the
classical granulite residual source model to generate the
A-type granites (Collins et al., 1982; Clemens et al., 1986;
Whalen et al., 1987), the studied granitoids could be
generated by melting of residual felsic granulite, from
which high-K granitoids (580 Ma-590 Ma) have been
previously extracted. In this model, the enrichment in Zr,
Nb, Y and REE is explained by either higher temperature
(>900°C) required for the second partial melting and/or
high F contents which promote a complexing effect,
increasing the HFSE contents in the melt (Collins et al.,
1982). However, most of the calculated Zr saturated
temperatures are lower than < 900°C, even considering
that they could be higher than that of the magma, due to
the presence of zircon showing calculated age inheritance.
Fluorite occurs locally, in the peraluminous granitoids. It
is in agreement with the experimental results of Dooley
and Patiño Douce (1996) and Patiño Douce and Beard
(1996) which show that high F contents in the source
favors Al enrichment in the melt, even if the source is not
oversaturated in alumina. Because fluorite is a later
crystallized phase and, it was only recorded in the later
intrusions, the increase F-contents in the melt may be due
to fractionation process. On the other hand, felsic granulite
residues, generated after the extraction of the high-K calcalkaline granitoids, are K2O depleted and their melting
could not generate melts with high K2O contents as
recorded in the study granitoids.
13
εNd
εSr
Fig. 12. εNd versus εSr correlation diagram for the studied granites, diorite
and diabase from the dykes. LC (lower crust) and UC (upper
crust) fields from Harmon et al. (1984).
The experimental data of Patiño Douce (1997) suggest
that incongruent melting of calc-alkaline granitoids, in
the shallow crust (P<4 Kbar) can produce A-type
metaluminous granites associated to anorthositic-mangeritic
rocks, which are the liquid and solid products. Such association
was not recorded in the area and, anorthosites within the
Central Tectonic Domain of the Borborema Province show
ages of ca. 1.7 Ga (Accioly et al., 2003).
Partial melting of felsic granodiorite and tonalite, under
vapor - absent conditions can generate A-type granite
magmas. The temperatures required for extraction of
magmas generated under vapor absent conditions are
higher than 900oC. Because the temperature estimated
for the study granitoids is lower, the heat required could
be provided by either hot mafic magmas in the source of
these magmas and/or high geothermal gradient associated
to crustal extension. The extensive field evidence of
magma mixing processes in the Prata Complex, suggest
that the participation of the hot mafic magmas was not
restricted to heat transfer, but it also entailed chemical
interaction with the crustal derived melts. In the Serra da
Engabelada and Serrote Santo Antonio plutons, mixing
processes were not recorded there, so the hot magma was
probably restricted to heat transfer.
-20
Summary and Conclusions
-30
-40
T (Ga)
Fig. 11. Nd isotopic composition of the studied granitoids. Isotopic
notations, model ages and reference mantle reservoirs are from
De Paolo (1988).
Gondwana Research, V. 8, No. 3, 2005
The Prata Complex, Serra da Engabelada and Serrote
Santo Antonio plutons, are about 100 Ma younger than
the calc alkaline granitoids described in the Alto Pajeú
Terrane (Guimarães et al., 2004). They show chemical
and mineralogical characteristics of within- plate granite,
were generated and emplaced in an extensional tectonic
environment and are associated with numerous dykes of
rhyolite and diabase.
14
IGNEZ P. GUIMARÃES ET AL.
Major and trace element modeling shows that the
studied granitoids were formed by fractional crystallization
of plagioclase, K-feldspar, apatite and allanite. In the Prata
Complex, field, petrographic and chemical evidence
indicate extensive magma mixing processes involving melt
resulted from partial melting of felsic granodiorite or
tonalite and either melt originated from enriched
subcontinental lithospheric mantle and melt generated
from astenospheric mantle which raised the norite
enclaves around the Prata Village. The negative εNd (512 Ma)
values (–14 to –10), high incompatible elements such as
LILE, HFSE and REE recorded in the diabase from dykes
and enclaves reflect a Paleoproterozoic- enriched
lithospheric mantle in the area.
The studied granitoids are contemporaneous with
sub-volcanic bimodal magmatism and deposition of many
Cambrian “pull-apart” basins in the north and central
tectonic Domain of the Borborema Province, suggesting
intrusion during post-tectonic relaxation of the Brasiliano
orogeny, following the assembly of West Gondwana.
Acknowledgments
This study was supported by FACEPE (Fundação de
Amparo a Ciência e Tecnologia do Estado de Pernambuco –
Grant No. APQ-0285-1.07/98) and CNPq (Council for
Scientific and Technological Development) Grant No.
475693/2001-9. I.P.G. is grateful for financial support
given by CAPES (Coordenação de Aperfeiçoamanto de
Pessoal de Ensino Superior – Gov. do Brasil), through the
Grant BEX0742/96, for a post-doctoral program at Kansas
University, USA. We wish to thank the following for the
analytical facilities: W.R. Van Schmus and Marianne Kozuch
(IGL-Kansas University-USA), Excelso Ruberti and Silvio
Vlach (São Paulo University). Constructive reviews by
journal reviewers Benjamim Bley Brito Neves, Alcides
Nobrega Sial and Victor Ramos were of great help in
improving the manuscript and are most appreciated.
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Appendix
Sm-Nd Isotope Determinations
Rock powders for Sm/Nd analyses were dissolved and the REE were
extracted using the general methods of Patchet and Ruiz (1987). Isotopic
compositions were measured with a VG Sector 5-collector mass
spectrometer. Sm was loaded with H3PO4 on a single Ta filament and
typically analyzed as Sm+ in the static multicollector mode. Nd was
also loaded with H3PO4 on a single Re filament having a thin layer of
AGW-50 resin beads and analyzed as Nd+ using the dynamic mode.
100 ratios were collected with a 1V 144Nd beam; this typically yields
internal precision of 10 to 20 ppm. External precision based on repeated
analyses of an internal standard is comparable at +30 ppm (1σ); all
analyses are adjusted for instrumental bias determined by measurements
of a internal standard for periodic adjustment of collector positions; on
this basis our analyses of La Jolla Nd average was 0.511870+0.000009.
Analysis of Rb-Sr Isotopes
50 mg of fine powdered rock samples and 87Rb/86Sr mixed spike
were dissolved with HF+HNO3+HClO4. Cation exchange columns used
Dowex AG50W-X8 resin and Rb and Sr were purified and separated.
Concentrated of Rb and Sr were loaded onto W filament, using TaCl5
activator with 0.25M H3PO4. Analysis of Rb and Sr were done with a
VG sector thermal ionization mass spectrometer model VG54E, at the
Pará - ISO Laboratories at the Pará Federal University.
Gondwana Research, V. 8, No. 3, 2005