Journal of South American Earth Sciences 31 (2011) 383e396
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Journal of South American Earth Sciences
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UePb SHRIMP data constraints on calc-alkaline granitoids with 1.3e1.6 Ga Nd TDM
model ages from the central domain of the Borborema province, NE Brazil
I.P. Guimarães a, *, A.F. Silva Filho a, C.N. Almeida b, M.B. Macambira c, R. Armstrong d
a
Departamento de Geologia, UFPE, Brazil
Departamento de Geologia, UFRJ, Brazil
c
Paraíso-Instituto de Geociências, UFPA, Brazil
d
Australian National University, Australia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 September 2010
Accepted 3 March 2011
The Tabira, Itapetim and Timbaúba granitoids are intruded into metasedimentary sequences and Cariris
Velhos (Tonian) orthogneisses from the Central Domain of the Borborema Province, NE Brazil. They have
UePb SHRIMP ages of 593 7 Ma; 615 9 Ma and 616 5 Ma respectively. The studied granitoids have
zircon cores inherited from the protholith, with a large number of analyses showing 206Pb/238U ages
ranging from 950 to 1200 Ma. Oscillatory zoning typical of magmatic zircon is common, although it is
faint in some inherited cores.
The studied granitoids are calc-alkaline and show Nd TDM model ages ranging from 1.30 to 1.56 Ga and
3Nd (600 Ma) ranging from 2.40 to 5.34. These values are similar to those recorded in the country
rocks. The lowest values of 3Nd (600 Ma) were recorded in enclaves of dioritic composition. Nd and UePb
SHRIMP data suggest a significant participation of the metasedimentary rocks in the protholith of these
granitoids. The Mesoproterozoic Nd TDM model ages recorded in the studied granitoids are interpreted as
the result of a hybrid source involving melting of metagraywackes, metamafic rocks of Tonian ages and/
or biotite e bearing orthogneisses (Cariris Velhos Orthogneisses). The resulted melting was modified by
mingling with juvenile Brasiliano melts, diorite in composition.
The Timbaúba granitoids intrusions are coeval with high-T metamorphism and flat-lying foliation
forming event in an intracontinental setting, during the Brasiliano convergence and contractional
deformation. The Itapetim Pluton was emplaced in the convergence - lateral escape setting and the
Tabira granitoids were intruded after the flat-lying foliation event, representing sin transcurrent
intrusions.
Our data show that within the Central Domain of the Borborema Province, granitoids with similar
petrographic and geochemical compositions can have distinct ages and be intruded in distinct tectonic
regimes.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Granitoids
Brasiliano
Source rock
UePb SHRIMP data
r e s u m o
Os granitóides Tabira, Itapetim e Timbaúba intrudem metassedimentos e ortognaisses Cariris Velhos do
Domínio Central da Província Borborema. Dados geocronológicos foram obtidos pela sistemática U-Pb
SHRIMP em zircão fornecendo idades de 593 7 Ma, 615 5 Ma, e 616 5 Ma respectivamente. Os
granitóides estudados apresentam núcleos de zircões herdados das rochas encaixantes, com um grande
número de análises apresentando idade 206Pb/238U variável entre 950 e 1200 Ma. Zonação oscilatória
típica de zircões de origem magmática é comum, apesar de fraca em alguns zircões herdados. Núcleos
com idades paleoproterozóicas não foram observados.
Os granitóides abordados apresentam idades modelos de Nd (TDM) variando entre 1.30 e 1.56 Ga., e
valores de 3Nd(600 Ma) variando entre -2.4 e -5.4, semelhante aos registrados nas rochas encaixantes. Os
valores mais baixos de 3Nd (600Ma) foram registrados nos enclaves dioríticos. Dados de Nd e de U-Pb em
zircão por SHRIMP sugerem uma participação significativa de rochas metassedimentares no protólito
destes granitóides. As idades modelos TDM mesoproterozóicas registrada nos granitóides estudados
* Corresponding author.
E-mail address: [email protected] (I.P. Guimarães).
0895-9811/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsames.2011.03.001
384
I.P. Guimarães et al. / Journal of South American Earth Sciences 31 (2011) 383e396
refletem magmas gerados a partir de fonte híbrida envolvendo metagrauvacas, rochas metamáficas de
idade Toniana e/ou biotita ortognaisses (ortognaisses Cariris Velhos), os quais interagiram parcialmente
com magma juvenil Brasiliano (dioritos).
As intrusões dos granitóides do Complexo Timbaúba são sincrônicas a metamorfismo de alta temperatura e evento formador de foliação de baixo angulo, durante a convergência Brasiliana, em um
ambiente incontracontinental. Os granitóides do Complexo Itapetim foram intrudidos durante períodos
de extensão da convergência e, os granitóides do Complexo Tabira representam intrusões sin
trancorrência.
Os dados apresentados mostram que no Domínio Central da Provincia Borborema, granitóides com
caracterícticas petrográficas e geoquímicas semelhantes podem ter idades distintas e serem intrudidos
em diferentes regimes tectônicos.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
The EdiacaraneCambrian orogenesis (Brasiliano) in the Borborema Province, northeastern Brazil, is characterized by a large
number of granitic intrusions coeval with the development of largescale shear zones and metamorphism under high-temperature
conditions. U/Pb (TIMS) zircon data for granite intrusions from the
Central Domain of the Borborema Province yielded ages that suggest
more than 100 Ma of intrusive magmatism (Guimarães et al., 2004).
The Brasiliano intrusive magmatism in the central - eastern part of
the Central Domain of the Borborema Province, was divided by
Guimarães et al. (2004) into four groups: 1) medium to slightly
high-K calc-alkaline I-type granitoids (between 640 and 600 Ma); 2)
high-K calc-alkaline granitoids and shoshonitic granitoids, associated with K-diorites, intruded between 590 and 581 Ma; 3) alkaline
post-collisional granitoids intruded at ca 570 Ma and 4) A-type,
post-orogenic, extension-related granites, associated with subvolcanic bimodal magmatism (540e520 Ma).The granitoids from
group 1 are intruded into metasedimentary sequence and show
Mesoproterozoic TDM model ages. The others three groups of granitoids have mostly Paleoproterozoic TDM model ages, except some
plutons from group 2 which are intruded along the contact between
metasedimentary rocks,with Mesoproterozoic Nd TDM model ages
and Paleoproterozoic orthogneisses such as those described by
Sampaio et al. (2003).
The studied granitoids correspond to the calc-alkalic magmatic
epidote (mEp) e bearing granitoids of Sial (1990).
Zircon has been used to investigate a range of crustal processes.
Zircon is highly refractory at the earth surface and it can record
stages of growth and dissolution that can observe through cathodoluminescence (CL) and back-scattered electron imaging (BSE).
These features associated to geochemical and isotopic data can
provide insights into the protholith composition and thermal
histories.
In this work, we present and discuss the UePb SHRIMP zircon
data from three granitic intrusions of group 1 of Guimarães et al.
(2004) (Fig. 1). These results are compared with previous conventional UePb ages and are used to infer the possible source rocks of
the plutons.
2. Geological aspects of the Borborema province
The Borborema Province (Almeida et al., 1981) comprises
a structural province formed as a result of the convergence of the
Amazonian, West African-São Luis and São Francisco e Congo
cratons during the assembly of west Gondwana (w600 Ma). It
consists of gneissic and migmatitic basement complexes, mostly
formed during the Palaeoproterozoic (2.0e2.2 Ga - Transamazonian/Eburnean orogeny), including minor Archaean blocks,
partially covered by Mesoproterozoic to Neoproterozoic metasedimentary and metavolcanic rocks (Van Schmus et al., 1995;
Fetter, 1999; Brito Neves et al., 2001; Kozuch, 2003). In addition
to the Transamazonian/Eburnean orogeny, the Borborema Province
was affected by the Cariris Velhos event (w1.0 Ga) and the Brasiliano orogeny (650e550 Ma). This latter affected the entire province and was responsible for low- to high grade metamorphism,
a great abundance of granitic intrusions, and development of
continental-scale transcurrent shear zones. The Brasiliano orogeny
was responsible by the actual architecture of the Borborema
Province, although none can deny the importance of the Transamazonian/Eburnean orogeny have been important as a crustforming event (Van Schmus et al., 1995; Neves et al., 2004; Neves
et al., 2006; Neves, among others).
The E-W shear zones divide the Borborema Province into three
segments, referred to as North, Central and South Domain by Van
Schmus et al. (1995). The studied area is located in the Central
Domain, which was previously named the Transversal Zone by
Ebert (1970). It is limited by the Patos shear zone to the north,
Pernambuco shear zone to the south, Afogados da Ingazeira shear
zone to the west, and the coastal area to the east (Fig. 1a).
A Brasiliano evolution of the Borborema Province controlled by
accretion of exotic terranes was proposed by many authors (Santos,
1995; Santos and Medeiros, 1999). Each tectonic domain was
segmented into many terranes. According to the terrane model, the
central - east part of Central Domain was segmented into: Alto
Pajeú Terrane (APT), Alto Moxotó Terrane (AMT) e Rio Capibaribe
Terrane (RCT).
The APT comprises muscoviteebiotite gneisses, garnet-biotite
schists, marble and metavolcanic rocks. These sequences have been
divided into Riacho Gravatá and São Caetano complexes (Tonian
ages e Kozuch, 2003) and Caroalina Complex. The Riacho Gravatá
and São Caetano complexes are cut by early Neoproterozoic granitic
plutons, now augen-gneisses (Kozuch, 2003). These rocks were
deformed during the Brasiliano cycle, initially by a compressional
episode top to NW, followed by transcurrent episode with top to
the NW tectonic transport later by extension (Brito Neves et al.,
2001; Neves, 2003; Guimarães et al., 2004).
The AMT is composed of metavolcano-metasedimentary sequences, including a calc-alkaline volcanic sequence of arc affinity and
Paleoproterozoic blocks (2.1e2.4 Ga) of tonalitic to granodioritic
composition, and by the Sertânia Complex (Santos, 1995). These
rocks show Brasiliano deformation similar to those recorded in the
APT.
The RCT is constituted by a Paleoproterzoic basement, Mesoproterozoic orthogneiss of granitic composition as well as anorthositic intrusions (Accioly et al., 2000; Sá et al., 2002) and later
Neoproterozoic (Surubim Complex) sequences of schist and gneiss
with intercalations of marble, metavolcanic (dominantly mafic) and
calc-silicate rocks. Detrital zircons constrain the deposition of this
supracrustal sequence to be younger than 665 Ma (Neves et al.,
2006). The RCT was affected by the Brasiliano deformation as the
others listed terranes above. The high-temperature metamorphism
I.P. Guimarães et al. / Journal of South American Earth Sciences 31 (2011) 383e396
385
Fig. 1. a) Schematic geologic map of the Central Domain of the Borborema Province. PWSZ and EPSZ are the two branches (west and east) of the Pernambuco shear zone; PSZ, Patos
shear zone; AISZ e Afogados da Ingazeira shear zone; SSZ e Solidão shear zone; JBSZ e Juru-Belem shear zone; CCSZ ¼ Campina Grande shear zone; CSZ e Congo shear zone.
1 ¼ Panerozoic covers; 2 e Brasiliano granitoids; 3 e Brasiliano gabbros (Av e Alto Vermelho, Jb ¼ Jabitacá); 4 e Neoproterozoic supracrustals and orthogneisses; 5 e Occurrences of
eclogites (Beurlen et al., 1992); 6 e Paleoproterozoic to Archean basement; 7 e shear zone. b) Simplified geologic map of the Timbaúba complex. CSZ e Camutanga shear zone;
ISZ ¼ Itambé shear zone; CNSZ - Cruzeiro do Nordeste shear zone. 1 e Timbauba complex; 2 e Surubim complex; 3 e Sertânia complex. c) Simplified geologic map of the Itapetim
Complex. VMSZ e Vaca Morta shear zone; CNSZ - Cruzeiro do Nordeste shear zone; AISZ ¼ Afogados da Ingazeira shear zone. 1 e Itapetim Complex; 2 e Tonian orthogneiss; 3 e São
Caetano complex; 4 - Paleoproterozoic orthogneiss. d) Simplified geologic map of the Tabira pluton. BJSZ e Belem Juru shear zone; AISZ ¼ Afogados da Ingazeira shear zone; 1 Tabira
pluton; 2 e Tonian orthogneiss; 3 São Caetano complex; 4 - Paleoproterozoic orthogneiss.
coeval with formation of a flat-lying foliation in basement and
supracrustal rocks has been dated at 626 15 Ma (Neves et al., 2006).
Several authors (Mariano et al., 2001; Guimarães and Brito
Neves, 2004; Neves, 2003 Neves et al., 2006 and references
therein) have questioned the terrane accretion model, suggesting,
instead, continuity between the proposed terranes since the Paleoproterozoic. Due this controversy, in this work we use the nongenetic terms to designate the supracrustal sequences and
orthogneisses of the Central Domain: Alto Pajeú, Alto Moxotó and
Rio Capibaribe belt (Fig. 1a).
The Brasiliano metamorphic peak conditions for the west part of
the Alto Pajeú Belt (São Caetano Complex) was estimated by Bittar
(1999) at P ¼ 4.4 kbar 1.0 kbar (geobarometer of Hodges and
Spear, 1982) and T ¼ 700 C (geothermometer of Spear, 1981).
These data are similar to those obtained by Coutinho (1994) for the
garnet-biotite gneisses from the São Caetano Complex, based on
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the equilibrium of the metamorphic assemblage, and using the
Thermocalc program (P ¼ 4.7 kbar 1.6 kbar, T ¼ 703 C 101 C)
and those obtained by Leite et al. (2000) for the Alto Pajeú Belt close
to the Tabira pluton intrusion. Silva et al. (2005), using the Thermocalc program, estimated the metamorphic peak conditions in
the Rio Capibaribe Belt (Surubim Complex) at P ¼ 8.0 0.9 kbar and
T ranging within the 645e750 C interval.
It is important to establish the timing of the peak of the metamorphism in order to assess its possible relationship with the
studiedplutonism. Within the Central Domain of the Borborema
Province, Leite et al. (2000) reported an upper intercept UePb
zircon age of 972 4 Ma for orthogneisses intruded by the Brasiliano Tabira pluton and a concordant sphene fraction from the same
sample giving the age of metamorphism at 612 9 Ma. Neves et al.
(2006) reported magmatic zircons with LA-ICP-MS UePb ages of
626 15 Ma and 625 24 Ma from leucosome of a migmatitic
paragneiss and from a felsic layer of banded orthogneiss respectively plus a lower intercept age of 619 36 Ma from a deformed
granodiorite dated at 2097 5 Ma crystallization age. Neves et al.
(2009) date by LA-ICP-MS the peak metamorphic conditions at
623 6 Ma in the Surubim complex, and at 600 22 Ma at Rio Una
metasedimentary complex in the South Domain (PEAL Domain).
The available data suggest the age of the Brasiliano high-temperature metamorphism in the Central and South domains occurred
within the interval 600e625 Ma.
3. Geology of the studied plutons
The Tabira pluton and Timbaúba complex comprise porphyritic
to equigranular epidote-bearing biotite, hornblende mesocratic
granodiorites to monzogranites. The Timbaúba granitoids were
deformed under high-T conditions. Rounded to elliptical microgranular enclaves are common. Amphibole-rich clots, surrounded
by coarse-grained amphibole and biotite are hosted by granodiorite
and monzogranite. Quartz chunks, up to 10 cm long, were also
observed enclosed by the granitoids of the Timbaúba complex.
Amphibole-rich clots and quartz chunks are features shared by
many granitoid intrusions of similar ages and compositions in other
belts of the Borborema Province (Silva Filho et al., 1997; Mcreath
et al., 1998; Sial et al., 1998), the so-called Conceição type granitoids of Almeida et al. (1967). Partial melting of the mafic enclaves is
rather common within the Timbaúba Complex (Fig. 2a).
The Timbaúba complex is composed by four plutons. The largest
one is intruded along the east branch of the dextral Cruzeiro do
Nordeste shear zone. All plutons are intruded into the Surubim
metasedimentary complex of the Rio Capibaribe Belt, which includes
garnet-bearing biotite gneiss intercalated with marble, metavolcanic
mafic rocks and quartzite, showing Nd TDM model ages in the
1.52e1.68 Ga interval (Guimarães et al., 2004). The Tabira pluton
intrudes ca. 0.95 Ga orthogneisses (Leite et al., 2000) and metasedimentary rocks of the São Caetano Complex showing Nd model ages in
the 1.4e1.8 Ga range (Kozuch, 2003) from the Alto Pajeú belt.
Flat-lying foliation crossed by high-angle foliation is recorded in
the Timbaúba granitoids and in their country rocks, suggesting that
the emplacement of these granitoids is related to the peak of
regional metamorphism and associated with a flat-lying foliation
forming event.
Dikes of dioritic composition associated with the Timbaúba
complex showing migmatization, as well as migmatization recorded in some areas of the largest pluton (Fig. 2b) suggest that the
Timbaúba complex was intruded before or during the Brasiliano
migmatization. Large sphene grains, up to 2 cm long, are observed
in the leucosome, within the largest pluton of the Timbaúba
Complex.
The Tabira granitoids cut the flat-lying regional foliation and
represent sin transcurrent intrusions (Araújo et al., 1997).
The Itapetim complex is constituted by five plutons: the largest
one has a sigmoid-like shape (Fig. 1c), two are elongated, and the
others two have rounded outlines. All plutons are intruded in
between two sinistral NE-SW- striking shear zones: the Vaca Morta
Fig. 2. Dioritic enclave partially melted (a) and migmatization (b) within the Timbaúba complex. c) Mafic enclaves in the coarse-grained monzogranite, showing lobate contacts
within the Itapetim complex.
I.P. Guimarães et al. / Journal of South American Earth Sciences 31 (2011) 383e396
387
Fig. 3. a) Shand’s Index for the studied granitoids. Fields after Maniar and Piccoli (1989). b) The compositional range of the studied granitoids in the FeOtot/FeOtot þ MgO) vs.
weight percent SiO2 diagram. Fields are from Frost et al. (2001).
4. Geochemistry
The studied granitoids show SiO2 ranging from 58.19 to 71.00
wt%, with enclaves showing SiO2 variation within the
44.79e58.39 wt% interval. SiO2 values of <56 wt% were recorded
in some mafic enclaves of all studied plutons. The alkali contents
are high (>6 wt %) and K2O/Na2O ratios <1. They are metaluminous to slightly peraluminous with A/CNK 1.1 (Fig. 3a), claalkaline and plot within the magnesian series field in the FeOtot/
(FeOtot þ MgO) versus SiO2 diagram (Fig. 3b), reflecting hydrous,
oxidizing magmas (Frost and Lindsley, 1991). According to the
classification scheme proposed by Barbarin (1999), the Timbaúba
and Tabira granitoids are classified as ACG (Amphibole-rich Calcalkaline Granitoids) and the Itapetim granitoids are classified as
KCG (K-rich and K-feldspar porphyritic Calc-alkaline Granitoids).
Chondrite normalized REE patterns are characterized by fractionated patterns with (Ce/Yb)N > 30 and absence of significant Eu
anomalies (Fig.4).
Trace element distribution patterns (Fig. 5) normalized to the
chondrite are characterized by no significant troughs at Sr, less
pronounced Ti troughs, and lower Y, Yb, and Nb values. All the
studied granitoids are enriched in LILE (large ion lithophile
elements) compared to HFSE (high field strength elements), which
is a general characteristic of calc-alkaline granitoids. The granitoids
trace element distribution pattern is characteristic of calc-alkaline
granitoids Thompson et al., 1984; Fowler et al., 2008).
In trace elements discriminant diagrams (Pearce et al., 1984;
Pearce, 1996) the studied granitoids plot within the volcanic arc
granitoid field (Fig. 6a) and within the post-collision granitoids field
(Fig. 6b). The post-collision granites are the most difficult to classify,
since some show subduction-like mantle sources with many
characteristics of volcanic arc granites, and others show withinplate granite character (Pearce, 1996).
5. Isotope geochemistry - SmeNd and RbeSr data
SmeNd and RbeSr analyses were performed at the Isotope
Geochemistry Laboratory at the University of Kansas. The results
are shown in Table 1.
5.1. SmeNd and RbeSr isotope determinations
About 300 mg of rock powder was placed into 23 ml Teflon with
a 149Sm/150Nd mixed spike and/or 87Rb/84Sr mixed spike. The
samples were pre- on a hot plate with 1 ml pure 7N HNO3 þ 3 ml
pure 48% HF, in order to reduce the amount of silica in the sample,
and the solution was dried. To the dried samples were added 1 ml
pure 7N HNO3 þ 5 ml pure 48% HF. The cups were capped and the
assembly was placed in stainless steel jackets for bombing. The
1000
Itapetim
Timbauba
Tabira
100
Rock/Chondrite
and Cacimba Nova shear zones. They cut Cariris Velhos orthogneisses (w950 Ma; Kozuch, 2003) and metasedimentary rocks with
Nd TDM model ages ranging from 1.3 to 1.8 Ga (Kozuch, 2003;
Guimarães et al., 2004). The Cariris Velhos orthogneisses show
Brasiliano flat-lying foliation and are locally migmatized. Detailed
structural data from Torres (2001), suggest that the Itapetim
Complex comprises sin transcurrent intrusions, evolved in the early
stage through dyking ascent mechanism associated to extensional
shear. The sigmoid-like shape of the complex was developed later,
associated to pure regional shear (Torres, 2001).
The Itapetim complex is constituted by epidote - bearing
porphyritic monzogranites, with large perthitic microcline and
plagioclase phenocrysts (up to 7 cm long) in a matrix of biotite,
hornblende, microcline plagioclase, and quartz. Swarms of diorite
enclaves are quite common (Fig. 2c). Late dikes of granodioritic
composition, showing magmatic foliation and layering, cut the
complex. Guimarães and Silva Filho (2000) based on Al-content in
calcic amphiboles from the monzogranites and using the Schmidt’s
calibration (1992) defined solidification pressures within the
6.6e6.9 0.6 kbar intervals and temperatures of plagioclaseeamphibole equilibrium ranging from 695 C to 748 C for the
granitoids of the Itapetim complex.
The studied granitoids show euhedral to subhedral epidote
grains with or without allanite cores and features of magma mixing
processes as rapakivi texture, acicular apatite and mafic enclaves
showing crenulate to lobate contact with the host granite. The
mafic enclaves can be rounded to slightly angled shape and they are
cut by veins of quartz monzonite in a net-veined or stockwork
pattern. Close to the contact with the country rocks the mafic
enclaves occur as elongated strip.
10
1
La Ce Pr Nd Sm Eu Gd
Tb Dy Ho Er Tm Yb Lu
Fig. 4. Chondrite normalized REE plots for the studied granitoids. Chondrite values
from Sun (1980).
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I.P. Guimarães et al. / Journal of South American Earth Sciences 31 (2011) 383e396
a 1000
Itapetim
Timbauba
Tabira
WPG
Nb (ppm)
100
10
Fig. 5. Trace element distribution (spidergrams) of the studied granitoids normalized
to the chondrite values suggested by Thompson (1982).
VAG + COLG
5.2. SmeNd and RbeSr results
The Itapetim, Timbaúba and Tabira granitoids show SmeNd TDM
model ages ranging from 1.30 to 1.56 Ma, and 3Nd (600 Ma) values
1
b
1000
100
10
1
Y(ppm)
2000
1000
Syn-COLG
WPG
Rb (ppm)
samples were put in a 180 C oven for 03 days. The samples were
dried on a hot plate. 6 ml of pure 6N HCL was added and the bomb
assemblies were placed again in the 180 C oven overnight, to
convert fluoride compounds into Chlorides.
Cation exchange columns used Dowex AG50W-X8 200e400
mesh resin and Rb, Sr, and the bulk rare-earth elements were
purified and separated. The rare-earth elements were passed
through anion exchange columms containing EIChrom LN-Spec
100e200 mesh resin, following procedures of Patchet and Ruiz
(1987).
Analysis of Rb, Sr, Sm and Nd used the VG Sector thermal ionization mass spectrometer, with Faraday collectors. Rb samples
were loaded onto single Re filament, with purified water and were
analyzed as Rbþ in single collector mode. Sr samples were loaded
onto Ta filaments using 0.25 M H3PO4 and were analyzed as Srþ in
dynamic multicollector mode. Sm samples were loaded with H3PO4
on a single Ta filament and typically analyzed as Smþ 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
Ndþ using the dynamic mode.
Internal precision for Sr was maintained using the 88Sr peak to
collect 50e100 ratios at a signal of 1 V, or 150 ratios for an intensity
of 0.5 V. Internal precision for Nd used the 144Nd peak to collect 100
ratios at a signal intensity of 1 V. These yielded a typical internal
precision of 10e20 ppm at 2s. External precision based on repeated
analyses of an internal standard is comparable at 30 ppm (1d); 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.
ORG
100
POG
10
VAG
1
1
ORG
10
100
1000
Y + Nb (ppm)
Fig. 6. The studied granitoids in the tectonic discriminant diagrams of (a) Pearce et al.
(1984) and b) Pearce (1996).
ranging from 2.4 to 5.37 (Fig. 7) and 3Sr (600 Ma) values ranging
from 50 to 110. The 3Nd values of the studied granitoids are lower
compared to younger granitoids intruded within the paleoproterozoic orthogneisses of the Central Domain (Guimarães et al., 2004).
The Nd TDM model ages are insert within the interval of Nd TDM
model ages recorded in the metasedimentary and orthogneisses
country rocks (1.4e1.68 Ga). However, the dioritic enclaves show
higher 3Nd values compared to the host granites. Among the
studied granitoids, the Itapetim Complex show the highest 3Nd
(600 Ma) values (2.69 to 3.6) while the Timbaúba pluton show
the lowest 3Sr (600 Ma) values (50e55) (Table 1).
Table 1
Summary of representative SmeNd and RbeSr isotopic results.
Sample
Nd (ppm)
Sm (ppm)
147
ITP-20
ITP-46
ITP-03
TP-03
TP24
96-207a
TI-12
TI-11
TI-01
13.19
32.30
48.42
34.06
45.46
41.18
49.90
44.14
32.20
2.46
6.14
10.29
6.78
7.90
7.86
8.78
8.53
5.92
0.1129
0.1150
0.1284
0.1240
0.1054
0.1155
0.1064
0.1069
0.1112
Sm/143Nd
143
Nd/144Nd
0.512102
0.512157
0.512232
0.512203
0.512038
0.512057
0.512053
0.512049
0.512085
3Nd (today)
3Nd (0.6Ga)
TDM(Ma)
Rb (ppm)
Sr (ppm)
87
10.5
9.27
7.92
8.49
11.71
11.34
11.42
11.48
10.78
3.6
3.12
2.69
2.65
4.70
5.11
4.72
5.37
4.23
1402
1367
1447
1372
1430
1520
1410
1562
1423
174
221
191
nd
89
119
119
124
135
253
216
286
nd
492
503
672
624
741
2.0039
2.9651
1.9301
nd
0.5210
0.6870
0.5139
0.5774
0.5290
ITP e Itapetim Complex; TP e Tabira pluton; TI e Timbaúba Complex. nd ¼ Not determined.
a
Tabira data from Kozuch (2003).
Rb/86Sr
87
3Sr (0.6 Ga)
0.729642
0.734897
0.726033
nd
0.714178
0.715290
0.711735
0.712602
0.711980
110
73
68
nd
84
80
50
55
52
Sr/86Sr
I.P. Guimarães et al. / Journal of South American Earth Sciences 31 (2011) 383e396
Fig. 7. Nd isotopic composition of the studied granitoids. Isotopic notations, model
ages and reference mantle reservoirs are from De Paolo (1988).
The granitoids of the Itapetim show higher 3Nd compared to the
others studied granitoids, defining a trend compatible with
a mixture between mantle component, similar to the Jabitacá
gabbro (crystallization age of 606 6 Ma e Kozuch, 2003), and the
Alto Vermelho (crystallization age of 619 9 Ma e Kozuch, 2003)
gabbro (Fig. 8) with either Cariris Velhos orthogneisses (crystallization ages of 920e970 Ma; Kozuch, 2003; Brito Neves, et al., 2001)
or supracrustal rocks.
The Tabira granitoids show 3Nd values similar to those recorded
in the Timbaúba granitoids. However, they show higher 3Sr values
suggesting a major contribution of a crustal component, which can
be either, the Tonian orthogneisses and/or supracrustal rocks
(Fig. 8).
6. Geochronology UePb zircon data
Previous geochronological data using UePb TIMS zirconshow
ages ranging from 645 5 Ma (Timbaúba), 638 5 Ma (Itapetim),
to 624 2 Ma (Tabira), when forced to zero (Guimarães et al.,
2004). Ages in the same time span, have been obtained by
Kozuch (2003) and Brito Neves et al. (2003) in other calc-alkaline
granitoids, the so-called Conceição type (Almeida et al., 1967) of the
Borborema Province.
389
of a greater number of grains and more representative population
statistics.
The raw data were corrected for non-radiogenic Pb using 204Pb
as a monitor and the composition of Broken Hill Pb (a common
contaminant in Australian labs) to correct for non-radiogenic 206Pb,
207
Pb, and 208Pb (“204-corrected” data; cf. Stern, 1997). The data
were also processed to refine the common Pb correction by
assuming concordance between the 207Pb/235U and 206Pb/238U ages
(so-called “207-corrected” data), which sometimes yields more
precise and more accurate results for young zircons.
Only 204-corrected results are used in this work. 206Pb/238U
ages were preferred for zircons younger than 1200 Ma due to
general concordance of the results and better precision, while
207
Pb/206Pb ages are preferred for samples with ages greater than
1200 Ma because they are more representative of the true ages for
older zircons.
The 207Pb/235U and 206Pb/238U data from 204-corrected analyses
were plotted on conventional concordia diagrams using the software ISOPLOT (Ludwig, 2001) to show the distribution of the
detrital populations over the full age range of all grains. Probability
distributions of 204-corrected 206Pb/238U ages for grains younger
than 1200 Ma were calculated for three samples, also using ANU
software “NOBLE”.
6.2. SHRIMP UePb results
6.2.1. The Timbaúba Complex
Most zircon grains from the analyzed sample are subhedral and
some are euhedral. Their main characteristics are the presence of
overgrowth showing oscillatory zoning typical of magmatic
growth. Metamorphic overgrowth can be seen in some grains, as
thin discontinuous white (due to low U) rims. Almost all zircon
grains have inherited cores (Fig. 9a), and eventually oscillatory
zoning. Some cores are smaller, generally darker in CL images than
their overgrowths and show greater rounding and truncation of
internal zoning.
Twenty five zircon spots were analyzed in the sample from the
Timbaúba Complex (Table 2a). Fourteen analyzed rims give a nice
cluster of data, defining a concordia age of 616 5 Ma (Fig. 10). Two
cores show some Pb-loss and were excluded from age calculation.
10
1
2
3
Nd
6.1. SHRIMP UePb analyses
The SHRIMP RG (Sensitive High Resolution Ion Microprobe)
from the Research School of Earth Sciences of the Australia National
University was used to measure UePb ages on zircons from the
studied granitoids. The results are shown in Table 2aec.
Zircon grains from each sample were picked and mounted in
epoxy disks and polished with 3 mm and 1 mm diamond to give
a scratch-free surface. Each sample was previous photographed in
both transmitted and reflected light and scanning electron microscope (SEM) Cathode Luminescence (CL) images were made for all
grain mounts. Individual analytical spots for the SHRIMP analyses
were typically about 20 mm in diameter. The analyses were carried
out in core-free and overgrowth-free areas of individual crystals.
Analytical methods and data reduction followed those detailed
by Williams (1998) and Stern (1997) using the FC1 standard
(1099 Ma; Paces and Miller, 1993). In order to optimize the number
of grains analyzed, each grain was mass-scanned four times and the
standard analyzed after every group of seven sample grains. This
yields somewhat lower precision for each grain, but allows analysis
4
5
6
7
5
Mantle Array
0
Av
Jb
I-type
-5
S-type
-10
-15
Sr
-20
-50
0
50
100
150
200
250
300
350
Fig. 8. 3Nd(t) versus 3Sr (t) diagram for granitoids from the Itapetim Complex. SN
(Sierra Nevada granitoids) S-Type and I-type granitoids (Australian), fields from
Harmon et al. (1984). Supracrustal (1), Tonian orthogneiss (2), Paleoproterozoic
basement (3) and Brasiliano gabbros (4; Av-Alto Vermelho and Jb e Jabitacá) samples
are from Kozuch (2003). 5 e Itapetim complex (open triangle e monzogranites; grey
triangle e hybrid rocks and closed triangle e diorite); 6 e Tabira pluton; 7 e Timbaúba
complex.
390
I.P. Guimarães et al. / Journal of South American Earth Sciences 31 (2011) 383e396
Table 2a
Summary of SHRIMP UePb zircon data. Sample ITA 01 e Complexo Itapetim.
Grain. Spot
1.1
1.2
2.1
2.2
3.1
3.2
4.1
4.2
5.1
5.2
6.1
7.1
7.2
8.1
8.2
9.1
9.2
10.1
11.1
11.2
12.1
12.2
13.1
13.2
14.1
15.1
%
206
Pbc
0.32
1.41
0.22
0.93
0.19
0.19
0.24
0.06
0.87
0.00
0.50
0.07
0.11
0.06
0.22
0.68
0.27
0.00
0.00
0.08
0.00
0.02
0.63
0.30
1.97
0.16
Ppm U
3542
709
3401
1291
678
821
417
839
584
554
788
613
412
164
1346
769
860
313
2308
1345
545
686
2282
900
1207
751
Ppm Th
518
299
281
1081
96
138
190
56
233
77
224
191
26
30
138
149
107
128
308
338
169
345
78
55
1333
103
232
Th/238U
0.15
0.44
0.09
0.86
0.15
0.17
0.47
0.07
0.41
0.14
0.29
0.32
0.06
0.19
0.11
0.20
0.13
0.42
0.14
0.26
0.32
0.52
0.04
0.06
1.14
0.14
Ppm
206
Pb*
382.0
43.1
326.0
68.6
60.3
66.4
56.5
72.9
58.1
49.3
67.3
79.4
41.0
16.5.0
111.0
78.2
62.7
27.4
200.0
147.0
78.7
97.0
205.0
82.9
99.0
64.5
(1)206Pb/
238
UAge
760
435
681
383
633
579
943
621
701
636
608
905
706
716
590
716
523
625
620
773
1001
982
637
655
577
613
(1)207Pb/
206
PbAge
7
4
7
4
6
6
9
6
7
7
6
54
7
8
6
7
5
7
6
7
10
10
6
7
6
6
The eleven analyzed cores are variable in age showing 206Pb/238U
ages at w1200 Ma, 940e976 Ma and 871e878 Ma. The spots with
distinct ages show Th/U ratios ranging from 0.58 to 0.25. One spot
shows Th/U ratio ¼ 0.2.
Zircon detrital grains with 208Pb/238U ages of 1200 Ma,
940e976 Ma and 871e878 Ma have been described in the São Caetano (Guimarães et al., 2010) and Surubim metasedimentary
complexes (Neves et al., 2006, 2009). The metasedimentary rocks of
the Surubim Complex are the country rocks of the Timbaúba
Complex. Some analyzed detrital zircons from the Surubim Complex,
(Neves et al., 2006, 2009), show 206Pb/238U Paleoproterozoic ages,
which were not recorded in the Timbaúba complex. The absence of
detrital zircon with Paleoproterozoic ages within the Timbaúba
Complex could be related to either absence or scattered contribution
of Paleoproterozoic source in the protholith of the Timbaúba
Complex. If there was some Paleoproterozoic contribution we failed
to find it within the grains, due to the limited number of data.
6.2.2. The Itapetim Complex
The analyzed sample shows variable zircon populations. The
most common zircon population comprises shorter to slightly
elongated grains (aspect ratios up to 2:1). Zircon grains comprise
cores and rims with the magmatic component usually zoned, but in
some instances the zoning can be quite weak. In some grains,
overgrowths with oscillatory zoning grew over preexisting
corroded crystals. One population is characterized by elongated
716
665
798
621
615
620
939
609
881
621
621
947
716
734
626
917
598
602
615
894
949
952
645
619
767
616
10
48
8
33
19
21
18
14
38
16
26
26
19
26
16
27
24
21
7
12
12
11
18
22
47
17
%Disc
6
35
15
38
3
7
0
2
20
2
2
4
1
2
6
22
12
4
1
14
6
3
1
6
25
0
(1)207Pb*/
206
Pb*
%
(1)207Pb/
235
U
%
0.0632
0.0617
0.0657
0.0605
0.0603
0.0605
0.0704
0.0601
0.0684
0.0605
0.0605
0.0706
0.0632
0.0638
0.0606
0.0696
0.0598
0.0600
0.0603
0.0688
0.0707
0.0708
0.0612
0.0604
0.0648
0.0603
0.49
2.20
0.36
1.50
0.87
0.96
0.86
0.63
1.80
0.72
1.20
1.30
0.87
1.20
0.75
1.30
1.10
0.96
0.34
0.58
0.57
0.54
0.85
1.00
2.20
0.80
1.0900
0.5940
1.0090
0.5107
0.8590
0.7830
1.5270
0.8380
1.0840
0.8640
0.8250
1.4690
1.0100
1.0330
0.8020
1.1280
0.6980
0.8410
0.8399
1.2100
1.6390
1.6060
0.8760
0.8910
0.8360
0.8300
1.1
2.5
1.1
1.9
1.4
1.4
1.4
1.2
2.1
1.3
1.6
6.5
1.4
1.7
1.3
1.7
1.5
1.5
1.1
1.2
1.2
1.2
1.3
1.5
2.5
1.3
(1)206Pb/
U
%
Err
corr
0.1251
0.0698
0.1114
0.0612
0.1032
0.0939
0.1574
0.1011
0.1149
0.1036
0.0989
0.1508
0.1158
0.1175
0.0959
0.1176
0.0846
0.1018
0.1010
0.1274
0.1681
0.1645
0.1039
0.1069
0.0936
0.0998
1.0
1.1
1.1
1.1
1.1
1.0
1.1
1.0
1.1
1.1
1.1
6.4
1.1
1.2
1.0
1.1
1.1
1.1
1.0
1.0
1.1
1.1
1.0
1.1
1.1
1.1
0.900
0.429
0.945
0.574
0.775
0.736
0.785
0.856
0.500
0.840
0.653
0.980
0.779
0.700
0.813
0.626
0.697
0.760
0.949
0.871
0.881
0.892
0.766
0.735
0.429
0.795
238
(aspect ratios up to 3:1), subhedral to euhedral zircon grains with
oscillatory zoning (Fig. 9b).
Twenty six zircon spots were analyzed from a sample of the
Itapetim Complex (thirteen rims and thirteen cores) and the results
are shown in Table 2b. Most zircon grains show cores and rims
usually zoned, but in some instances the zoning can be quite weak.
Inheritance was recorded in many grains, with 206Pb/238U ages
within the 940e1000 Ma and 706e760 Ma intervals. Cores with
ages within the 940e1000 Ma interval have Th/U ratios in the range
0.52 to 0.32, which suggest igneous source. The provenance of
these xenocrystic zircons are probably the Cariris Velhos orthogneisses which have UePb TIMS zircon crystallization ages ranging
from 970 to 920 (Kozuch, 2003; Santos et al., 2010) and the
supracrustal of either the São Caetano Complex or the Riacho
Gravatá Complex (Kozuch, 2003; Guimarães et al., 2010).
The cores of zircon grains showing 206Pb/238U ages ranging from
716 to 760 Ma have Th/U ratios of 0.19 and 0.15 respectively.
Igneous rocks with such ages have not been recorded within the
Central Domain of the Borborema Province. However, similar ages
in detrital zircons have been reported in metasedimentary rocks of
the Surubim Complex (Neves et al., 2006) and in the eastern part of
the São Caetano Complex (Guimarães et al., 2010). Igneous rocks
with crystallization ages ranging of w715 Ma are known in the
Sergipano Belt in the Southern Domain of the Borborema Province
(Oliveira et al., 2010) and in the São Francisco Craton (Conceição
et al., 2009).
Table 2b
Summary of SHRIMP UePb zircon data. Sample CT-02 e Pluton Tabira.
1.1
3.2
3.1
4.1
4.2
7.1
7.2
8.1
0.05
0.03
3.84
0.27
2.34
0.20
0.07
0.06
2187
412
212
601
202
195
483
1090
2220
181
53
64
71
56
155
87
1.05
0.45
0.26
0.11
0.37
0.30
0.33
0.08
181.0
35.5
17.7
50.0
16.4
23.2
40.2
86.0
593
616
574
595
569
837
595
566
6
6
7
6
7
9
6
6
587
643
545
592
560
836
600
602
9
22
130
43
120
37
22
13
1
4
5
0
2
0
1
6
0.0595
0.0611
0.0584
0.0597
0.0588
0.0670
0.0599
0.0600
0.4
1.0
6.1
2.0
5.6
1.8
10.0
0.6
0.7913
0.8450
0.7500
0.7950
0.7480
1.2790
0.7990
0.7589
1.1
1.5
6.2
2.3
5.8
2.1
1.5
1.2
0.0964
0.1003
0.0931
0.0966
0.0923
0.1386
0.0967
0.0918
1.0
1.1
1.2
1.1
1.3
1.2
1.1
1.0
0.931
0.733
0.199
0.475
0.222
0.557
0.735
0.868
I.P. Guimarães et al. / Journal of South American Earth Sciences 31 (2011) 383e396
391
Table 2c
Summary of SHRIMP UePb zircon data. Sample Ti-01 - Timbaúba Complex.
1.1
1.2
2.1
3.1
4.1
5.1
6.1
7.1
8.1
9.1
10.1
11.1
12.1
13.1
14.1
15.1
16.1
17.1
18.1
19.1
20.1
0.04
–
0.04
0.17
0.05
0.00
0.07
0.00
0.14
0.00
0.16
0.12
0.10
0.06
0.18
0.00
0.00
0.05
0.07
0.11
0.26
372
70
361
250
288
266
290
51
194
216
284
215
179
256
255
183
130
537
52
176
206
167
26
165
118
129
112
141
26
108
104
140
86
72
116
75
93
73
84
21
52
50
0.46
0.39
0.47
0.49
0.46
0.44
0.50
0.53
0.58
0.50
0.51
0.41
0.42
0.47
0.30
0.52
0.58
0.16
0.41
0.31
0.25
32.4
9.79
30.8
21.7
23.7
22.7
25
6.32
15.8
18.3
35.7
22.5
15.8
22.3
22.3
15.8
17.5
45.7
9.08
15.6
17.3
623
976
611
621
589
611
616
871
583
606
878
738
633
621
623
617
940
609
1194
634
600
7
13
6
7
6
7
7
12
7
7
11
8
7
7
7
7
11
6
16
7
7
621
939
614
611
606
610
637
961
625
600
900
762
604
604
621
617
985
629
1215
623
637
20
31
28
30
22
21
22
35
29
24
27
24
42
35
29
24
20
16
26
29
48
0
4
0
2
3
0
3
9
7
1
2
3
5
3
0
0
5
3
2
2
6
0.0605
0.0704
0.0603
0.0602
0.0601
0.0602
0.0609
0.0711
0.0606
0.0599
0.0690
0.0646
0.0600
0.0600
0.0605
0.0604
0.0720
0.0607
0.0807
0.0605
0.0609
0.9
1.5
1.3
1.4
1.0
1.0
1.0
1.7
1.4
1.1
1.3
1.2
2.0
1.6
1.3
1.1
1.0
0.7
1.3
1.3
2.2
0.8460
1.5860
0.8270
0.8390
0.7930
0.8250
0.8430
1.4180
0.7910
0.8140
1.3890
1.0810
0.8540
0.8380
0.8470
0.8370
1.5580
0.8300
2.2660
0.8630
0.8190
1.5
2.1
1.7
1.8
1.5
1.5
1.5
2.2
1.8
1.6
1.8
1.6
2.3
2.0
1.7
1.6
1.6
1.3
2.0
1.8
2.5
0.1014
0.1635
0.0994
0.1011
0.0957
0.0994
0.1003
0.1447
0.0947
0.0986
0.1459
0.1214
0.1032
0.1012
0.1015
0.1005
0.1570
0.0991
0.2035
0.1034
0.0975
1.1
1.4
1.1
1.1
1.2
1.1
1.1
1.5
1.2
1.2
1.3
1.2
1.2
1.1
1.1
1.2
1.2
1.1
1.5
1.2
1.2
0.770
0.680
0.640
0.634
0.752
0.760
0.738
0.653
0.652
0.728
0.698
0.712
0.528
0.570
0.652
0.722
0.779
0.819
0.735
0.674
0.463
Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Standard calibration was 0.30% (not included in above errors but required
when comparing data from different mounts). 1) Common Pb corrected using measured 204Pb.
One spot analyzed in an overgrowth with faint of oscillatory
zoning of a grain with core showing 206Pb/238U age of 905 54 Ma
(spots 7.2 and 7.3 in Table 2a), yielded a 206Pb/238U age of
706 7 Ma and a Th/U ratio of 0.06. The core has a near oval shape,
suggesting that this grain represents contribution from a metasedimentary country rocks.
Rims and most of the analyzed cores showing oscillatory zoning
yielded 206Pb/238U ages within the 655e613 Ma interval (Table 2b)
and have low Th/U ratios (0.04e0.15). Only two analyzed grains
(one spot in the core and one in the rim) show Th/U ratios higher
than 0.2. Zircons with Th/U ratios lower than 0.2 are often
considered to be metamorphic in origin (Williams and Claesson,
1987). However, oscillatory zoning recorded in most of these
overgrowths and subhedral to euhedral shape of the crystals argue
against a metamorphic origin. The low Th/U ratios in these zircons
are associated with high-U contents and medium to low Th
contents. In the analyzed sample, modal allanite and monazite
were recorded. Therefore the low Th contents in the analyzed
Fig. 9. Cathodoluminescence pictures of zircons from sample a) Ti-01 e Timbaúba complex; b) BJ-01 Itapetim complex and c) CT-02 e Tabira pluton.
392
I.P. Guimarães et al. / Journal of South American Earth Sciences 31 (2011) 383e396
Fig. 10. Concordia plot for SHRIMP 204-corrected, zircon analyses from the Timbaúba complex sample (Ti-01).
zircon appear to reflect simultaneous magmatic crystallization of
zircon, allanite, and/or monazite rather than metamorphism,
during the evolution of the Itapetim magmas.
The age pattern is complicated due to inheritance in the cores
and some serious discordance in the high-U rims (spot 2.2). The
main group of data for the magmatic zircons plots on concordia
(Fig. 11), but there is some scatter as a result of discordance,
probably through Pb-loss. The age for these data was calculated in
two ways: (1) an upper intercept age of 615.5 8.7 Ma (N ¼ 13;
MSWD ¼ 0.2; probability ¼ 0.994), and (2) a weighted mean
207
Pb/206Pb age on the least discordant points gives 615.9 9 Ma
(N ¼ 10; MSWD ¼ 0.15; probability ¼ 0.998). The two calculated
ages are very similar and 616 9 Ma can be interpreted as the
crystallization age of the Itapetim granitoids. This age is younger
than that determined by Guimarães et al. (2004) using UePb zircon
TIMS methodology (638 5 Ma).
common Pb blows out the error ellipses as the error uncertainty is
propagated through to the final value (e.g. spots 3.1and 4.2).
Only two analyzed spots show Th/U ratios lower than 0.2. The
others spots, excluding one with 206Pb/238U age of 837 10 Ma,
show Th/U ratios ranging from 0.26 to 0.45, and only one show even
higher Th/U ratio ¼ 1.05. These variations in the Th/U ratios for
spots with similar 206Pb/238U age appear to reflect more than one
stage of zircon crystallization during the magma evolution.
One grain shows core with 206Pb/238U age of 837 10 Ma (%
disc ¼ 0). Zircon grains with such ages have been described in the
São Caetano Complex. It suggests some contribution of the metasediments from the São Caetano Complex in the protholith of the
Tabira Pluton.
The age was defined by a group of three analyses (spots 1.1; 4.1;
7.2) that combine to give a concordia age of 593 7 Ma
(MSWD ¼ 0.98) (Fig. 12).
6.2.3. Tabira Pluton
Eight spots (03 rims and 05 cores) from five zircon grains were
analyzed from a sample of the Tabira pluton (Table 2c). The zircon
grains are highly variable in terms of shape and size (Fig. 9c). Some
grains are euhedral and elongated with aspect ratios up to 4:1. In
general they comprise crystals with zoned margins or tips and
a central area that may be zoned, but occasionally altered, (spot 8.1). It
is difficult to tell whether or not these are inherited cores, or simply
earlier parts of the crystals growth history. The analyses of cores/rims
show that they are probably the same age, and that the “cores” can
have high common Pb and can have lost Pb. In some grains, the high
7. Discussions
The UePb zircon SHRIMP data presented in this paper revealed
ages younger than those obtained using the UePb zircon TIMS-ID
methodology for the studied granitoids (Guimarães et al., 2004;
Kozuch, 2003).
The youngest age recorded in the Tabira Complex could be
interpreted as due to a limited number of data, because the granitoids of the Borborema Province with similar petrographic and
geochemical signatures have crystallization ages older than 600 Ma.
Granitoids with such composition are coeval with a flat e lying
Fig. 11. Concordia plot for SHRIMP 204-corrected, zircon analyses from the Itapetim complex sample (BJ-01).
I.P. Guimarães et al. / Journal of South American Earth Sciences 31 (2011) 383e396
393
Fig. 12. Concordia plot for SHRIMP 204-corrected, zircon analyses from the Tabira pluton sample CT-02.
foliation event related to the Brasiliano Orogeny which attained its
climax at 630e600 Ma, and was followed by Transcurrent/transpressional deformation, after 600 Ma (Neves et al., 2006, 2009).
However, as reported by Araújo et al. (1997) the Tabira granitoids cut
the regional flat-lying foliation and represents sin transcurrent
intrusions. It suggests that the younger age obtained in this work
(593 7 Ma) represents the crystallization age of the Tabira
granitoids and do not reflect a limited number of data.
The Timbaúba and Itapetim granitoids are broadly contemporaneous to gabbros, which have UePb TIMS ages in the 607e619 Ma
range and 3Nd (600 Ma) ranging from 1.58 to 0.82 (Kozuch,
2003). They have ages similar to those recorded for the peak of
a high-temperature metamorphism coeval with formation of a flatlying foliation forming event associated with the convergence
between the São Francisco/Congo Craton and Western African
Craton. Field relationships show that migmatization took place
during and after intrusion of the Timbaúba granitoids, and finished
before the intrusion of the high-K to shoshonitic granitoids with
ages w592 Ma (Guimarães et al., 2004), in the east part of the
Central domain.
Inherited zircon cores with 208Pb/238U ages of 120 Ma,
940e976 Ma and 871e878 Ma and similarities between the Nd TDM
model ages recorded in the studied granitoids and those from the
country rocks, strong suggest contribution from the São Caetano
and Surubim metasedimentary rocks and/or Cariris Velhos
orthogneisses in the protholiths of the studied granitoids. These
inheritances explain the slightly older crystallization ages defined
by TIMS-ID analysis. The absence of Paleoproterozoic detrital zircon
grains rule out important contribution from Paleoproterozoic
basement.
The studied granitoids are calc-alkaline, show a volcanic arc
(VAG) geochemical signature (Pearce et al., 1984; Pearce, 1996) and
they are classified as magnesian granitoids (Frost et al., 2001). The
origin of the granitoids of magnesian series has been interpreted as
broadly subduction related (Frost et al., 2001). However, evidences
for Brasiliano subduction of oceanic lithosphere have not been
found in the Central Domain. Just north of the Timbaúba complex
intrusion, lenses of mafic-ultramafic rocks enclosed by orthogneisses of granodioritic composition have been described by
Almeida et al. (2005, 2009) as retroeclogites following a NE-SW
high magnetic trend, which broadly coincide with the Congo shear
zone. However, the high pressure metamorphism has been date by
SmeNd methodology using garnet and whole rock at 1870 27 Ma
(Almeida et al., 2005).
The volcanic arc signature can be interpreted as 1) the result of
interaction between mantle-derived sources and crust which tends
to move the granite composition towards the volcanic arc field
(Pearce, 1996); 2) inherited from the source and 3) closure of small
oceans.
Magmas derived from purely metasedimentary or biotite e
bearing metaigneous sources are strongly peraluminous, SiO2erich
(>72 wt %) and depleted in Ca, Mg and Fe (Patiño Douce and
Johnston, 1991; Patiño Douce and Beard, 1996; Gardien et al.,
1995). Melts derived from amphibolites of normal composition
are also dominantly peraluminous because of the low alkali content
(Patiño Douce, 1995). Thus, the metasediments from the São Caetano and Surubim complexes, as well as the Cariris Velhos biotite e
bearing orthogneisses cannot be the protholith of the studied
granitoids, which have SiO2 < 71 wt%, are dominantly metaluminous and are not depleted in Ca, Mg and Fe. A hybrid source
consisting of anhydrous high-Al tholeiitic basalt and biotite gneiss
is a good candidate as a source rock, because the experimental
products (Patiño Douce, 1995) at pressure 1.0 GPa (Fig. 13a,b)
approach the composition of the studied granitoids. This crustal
source could be the metagraywackes from the São Caetano and
Surubim complexes, which represents a mixture of Paleoproterozoic enriched crust, metavolcanic rocks and orthogneisses of
Tonian ages (Guimarães et al., 2010; Neves et al., 2006), whilst the
Sr and Nd isotopic data also highlight some contribution of
a mantle component. The mantle component in the case of the
Tabira and Itapetim granitoids have isotopic composition similar to
those recorded in gabbros from the Alto Pajeú Belt as the Alto
Vermelho and Jabitacá gabbros (Fig. 1b). On the other hand,
contribution of a depleted-mantle component appears to be
involved in the protolith of the Timbaúba granitoids as suggested
by the low LILE contents recorded in the most primitive diorites
associated with the complex.
The lowest 3Sr (600 Ma) values recorded in the granitoids of the
Timbaúba complex appear to reflect more contribution of metavolcanic mafic rocks in the protholith of these granitoids. This
hypothesis is supported by the field data ie, intercalations of metamafic rocks are common in the Surubim complex, close to its
contact with the Timbaúba granitoids. However, the low 3Sr
(600 Ma) values can also be associated to a greater contribution of
depleted - mantle component in the protholith of these granitoids
and/or interactions with juvenile Brasiliano melts.
The Itapetim granitoids are classified as Itaporanga- type
(Almeida et al., 1967), porphyritic high-K calc-alkaline granitoids,
which comprises the KCG of Barbarin (1999). Within the Central
Domain, Itaporanga e type granitoids have ages within the
592e580 Ma interval and with rare exception, Paleoproterozoic
SmeNd TDM model ages.
According to Barbarin (1999), the high-K calc-alkaline granites
and granodiorites (KCG) are present in various geodynamic
394
I.P. Guimarães et al. / Journal of South American Earth Sciences 31 (2011) 383e396
a
8
=1
=2
Al2O3/(MgO+FeO)
6
1.5
1.5
4
Biotite melts
calc-alkaline 1.2
melts
Amphibole melts
1.2
0.7
0.5
2
0.5
0.0
b
0.5
0.7
1.0
1.0
CaO/(MgO + FeO)
8
1.5
=1
=2
Granitoids), which according to Barbarin (1999) are invariably
emplaced above subduction zones, forming vast batholiths, elongated parallel to the trench in the active continental margins.
However, as discussed before, the Tabira granitoids are not
subduction related, they were emplaced after the flat-lying foliation associated to the Brasiliano convergence.
The Timbaúba Complex is intruded far away from the Itapetim
and Tabira granitoids and it is coeval with the mainly Brasiliano
event of migmatization. Southwestern from the Timbaúba intrusion, Neves et al. (2006) reported metamorphic age of 619 36 Ma
associated to a flat-lying deformation event, similar to the crystallization age of the Timbáuba granitoids. It suggests that the
Timbaúba granitoids intrusions occurred during the peak of
regional metamorphism. The closure of a small ocean could result
in the development of a continental arc, and the Timbaúba granitoids could be intruded in such tectonic setting. If so, the ocean
crust should be completely subducted and younger mantle magmatism should reflect metasomatism with such ages. However, the
younger diorites in this region have Paleoproterozoic Nd TDM model
ages. As one can see, it is difficult to prove that the tectonic setting
of the Timbaúba granitoids is a volcanic arc.
8. Conclusions
6
Al2O3/(MgO+FeO)
1.5
Biotite melts
calc-alkaline
1.5
melts
1.2
4
1.2
1.0
1.0
2
0.5 0.7
0.9
0.7
0.5
Amphibole melts
1.0
1.1
1.2
1.3
1.4
Al2O3/(CaO+Na2O+K2O)
1.5
1.6
Fig. 13. Plots of Al2O3/(MgO þ FeO) against CaO/(MgO þ FeO) (a) and molar Al2O3/
(CaO þ Na2O þ K2O) in (b) for the monzogranites (filled circle) and granodiorites (open
triangle), with SiO2 > 65%, from the studied granitoids. Samples with SiO2 < 65% were
excluded to limit the effects of interactions with the dioritic melts. 1 - represent the
compositions of melts, resulting from experimental fluid-absent partial melting of
a mixture of 50wt% of biotite gneiss þ HAOT (high-Al olivine tholeiitic melt) and
2)- metapelites þ HAOT at pressures of 0.7, 1.0, 1.2 and 1.5 GPa (Patiño Douce, 1995).
Fields for melts were derived from experimental partial melting of amphibolites
(amphibole melts), metapelites and biotite gnaeisse (biotite melts). The calc-alkaline
melts field is based on calc-alkaline granites data from the literature (Patiño Douce,
1995).
environments, and they indicate more a variation of the tectonic
regime than geodynamic environments. The KCG can occur either
during the relaxation phases of a collision event, or marking the
transition from a compressional to a tensional regime (e.g Lameyre
et al., 1980; Barbarin, 1999). The KCG become largely dominant just
before the completely stop of the convergence geodynamic (e.g.
Brown et al., 1980; Barbarin, 1999). The Itapetim Complex
comprises sin transcurrent extensional intrusions (Torres, 2001),
coeval with extension e related gabbros. This evidence associated
to the high-K calc-alkaline geochemical signature could suggest
that the Itapetim granitoids were intruded during relaxation phases of the Brasiliano convergence and contractional deformation.
The Timbaúba and Tabira granitoids show petrographic and
geochemical features of ACG (Amphibole-rich Calk-alkaline
The available UePb SHRIMP data associated to Nd and Sr
isotopic compositions indicate that the Mesoproterozoic Nd TDM
model ages recorded in the studied granitoids are the result of
a mixture involving a hybrid source consisting of biotite gneiss
and high-Al toleiitic basalt ie., the Surubim (Timbaúba Complex)
and São Caetano complexes and Tonian biotite- bearing orthogneisses (Tabira and Itapetim complexes) that mingling with
Brasiliano juvenile melt. These mixing processes can also respond
for the VAG geochemical signature recorded in the studied
granitoids.
The similarities of ages and P and T conditions between the
regional metamorphic peak and crystallization of the Timbaúba
granitoids suggest that these granitoids were intruded during the
peak of high-T metamorphism conditions, synchronous to
a tangential deformation event. In the Timbaúba area, field relationships suggest that anatexis of the country rocks is contemporaneous with or postdates the intrusion of the Timbaúba Complex.
Lack of migmatization within the Itapetim and Tabira granitoids
suggests that within the Central Domain of the Borborema Province
the Brasiliano basin closures happened from west to east.
The Timbaúba granitoids were intruded during the Brasiliano/
Pan-African convergence and contractional deformation in an
intracontinental setting. The Itapetim granitoids were probably
emplaced in the Brasiliano convergence - lateral escape setting and
the Tabira granitoids are coeval with a transcurrent event.
Our data suggest that the association between granitoids types
and geodynamic environments, as proposed by Barbarin (1999),
cannot be applied to the Brasiliano granitoids of the Central Domain
of the Borborema Province, because granitoids resulted from mixed
origin (crust þ mantle e ACG) with similar petrography and
chemistry can have distinct ages and be intruded in distinct
tectonic regimes.
Acknowledgments
IPG thanks CAPES for a post-doctoral scholarship (BEX 41270702). A grant from the PRONEX/FACEPE program (APQ-0479-1.07/
06) made possible SHRIMP analyses at the ANU, Canberra, Australia.
We are grateful to Dr. Sergio Pacheco Neves and two anonymous
reviewers for comments and suggestions.
I.P. Guimarães et al. / Journal of South American Earth Sciences 31 (2011) 383e396
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