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Transcript of craton amazonico
Geochemistry and isotopic constraints on the origin
of the mesoproterozoic Rio Branco ‘anorogenic’ plutonic suite,
SW of Amazonian craton, Brazil: high heat flow and
crustal extension behind the Santa Helena arc?
Mauro C. Geraldesa,*, Jorge S. Bettencourtb, Wilson Teixeirab, Joao B. Matosc
aTEKTOS–Faculdade de Geologia, Universidade do Estado do Rio de Janeiro, Rua Sao Francisco Xavier 524, Rio de Janeiro, CEP 20550-013, BrazilbInstituto de Geociencias, Universidade de Sao Paulo, Rua do Lago 562, Sao Paulo-SP, CEP 05508-900, Brazil
cDepartamento de Geologia, Universidade Federal de Mato Grosso. Av. Fernando Correia da Costa s/n, Cuiaba-MT, Brazil
Received 1 November 2002; accepted 1 May 2004
Abstract
The Rio Branco plutonic suite (RBS) occurs in the southwestern Amazonian craton, crops out in an area of 1500 km2, and is emplaced into
the ca. 1.79 Ga Alto Jauru terrane (Rio Negro/Juruena geochronological province). The RBS comprises basic (gabbro, diabase, and basalt)
and felsic (porphyritic and rapakivi granite) rocks. Hybrid rocks (monzosyenite) with rapakivi-like textures indicate commingling and
mixing among the basic and felsic magmas.
Silica contents range 45–47% in the basic rocks (metaluminous) and 69–71% in the felsic rocks (slightly peraluminous–metaluminous).
Lithogeochemical investigation also indicates higher contents of K2O, Rb, Zr, and Ba in felsic rocks, comparable with results reported
elsewhere for rapakivi granites. Trace element discrimination diagrams indicate that the RBS felsic and basic rocks have within-plate
signatures. In addition, the felsic rocks have strongly fractionated REE patterns that show marked negative Eu anomalies, probably due to
plagioclase fractionation. The basic rocks are similarly LREE enriched but display flatter patterns, characteristic of weakly fractionated
gabbros.
Single-grain IDTIMS U–Pb analyses yield an upper intercept age of 1427G10 (MSWDZ1.7) for magmatic zircon from a granophyre
of the RBS. This age contrasts significantly with an upper intercept age of 1471G8 Ma (with a concordant 207Pb/206Pb age of 1471G18 Ma)
obtained for zircon from a sample of the basic group. The latter rocks show positive 3Nd(1420) ranging from C1.2 to C1.9
(TDMZ1.86K1.82 Ga), which indicates mantle-derivation, whereas the felsic ones yield 3Nd(1420) values from C0.2 to K1.0
(TDMZ1.80K1.73 Ga), indicating some older crust in their source.
The RBS is interpreted to have formed at 1.47–1.42 Ga from a mixture of mantle source and crustal-derived magma. We propose high heat
flow and an extensional environment for the origin of the RBS as a response to the inboard Santa Helena arc (ca. 1.45–1.42 Ga) that
developed at the southwestern margin of the Amazonian craton at approximately the same time.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: Amazonian craton; Mesoproterozoic plutonism; Rapakivi granite; Synorogenic magmatism
Resumo
A Suite Intrusiva Rio Branco (SIRB) esta localizada no SW do craton Amazonico (Provıncia Rio Negro/Juruena), aflorando em area de
1500 kM2 e encaixada por rochas do terreno Alto Jauru de idade ca. 1.79 Ga. A suıte e composta por um grupo de rochas basicas
(gabros, diabasios e basaltos) e felsicas (granofiros e granitos rapakivi). Rochas hıbridas monzosienıticas com textura rapakivi indicam
processos de mistura entre magmas basicos e felsicos.
Journal of South American Earth Sciences 17 (2004) 195–208
www.elsevier.com/locate/jsames
0895-9811/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsames.2004.05.010
* Corresponding author. Tel.: C55-21-2587-7704; fax: C55-21-2254-6675.
E-mail address: [email protected] (M.C. Geraldes).
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208196
A porcentagem em peso de silica varia de 45 a 47% nas rochas basicas (de carater metaluminoso) e de 69 a 71% nas rochas felsicas (de
carater peraluminoso a metaluminoso). Analises quımicas tambem indicaram altos valores de K2O, Rb, Zr, Ba, nas rochas felsicas e os
valores de elementos tracos indicam ambiente tectonico intracratonico para estas rochas. Em adicao, as rochas felsicas apresentam padroes
enriquecidos de ETRL e anomalias negativas de Eu. As rochas basicas apresentam menor enriquecimento de ETRL e padroes mais
horizontalizados.
Analises U–Pb em monocristal de zircao fornecem idade (intercepto superior) de 1427G10 para as rochas felsicas e 1471G8 Ma para as
rochas basicas. Estas mostram valores positivos de 3Nd(1420) (C1.2 aC1.9) e TDMZ1.86K1.82 Ga, indicando derivacao mantelica, enquanto
as rochas felsicas apresentam valores de 3Nd(1420) entre C0.2 eK1.0 (TDMZ1.80–1.73 Ga), indicando importante contribuicao crustal para
sua fonte.
A SIRB provavelmente se formou ha ca. 1.47–1.42 Ga como resultado da mistura de fontes mantelicas e crustais em ambiente extensional,
onde o fluxo de calor necessario para gerar tal magmatismo provavelmente se originou a partir do ambiente de subduccao existente na borda
do craton onde estava se desenvolvendo o arco continental Santa Helena de idade entre 1.45–1.42 Ga.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
Ever since A-type granitoids were first recognized, the
class has been a focus of debate. The term A-type granites,
as defined by Loiselle and Wones (1979), does not fit the S,
I, and M classification scheme (Chapel and White, 1974;
White, 1979; Pitcher, 1982; Whalen et al., 1987), and a
chemically diverse group of granitoid rocks can be
contained within this designation. These granitoid rocks
may form in a variety of settings, not all of which are
confined to an anorogenic environment. Regardless, they
represent a group of mineralogically distinct and economi-
cally important granitoids distinguishable from those
normally included in the I, S, and M types.
According to Collins et al. (1982), all granitic plutons
with A-type affinities are intruded late in a magmatic
cycle or are generated by partial melting of the lower crust.
They commonly are associated with extensional regimes in
continental blocks but may occur in areas devoid of
orogenic tectonic activity. Collins et al. (1982) argue
against the hypothesis that envisages the production of
A-type melts by fractional crystallization of I-type melts for
two reasons. First, A-type melts are almost anhydrous, as
evidenced by the precipitation of only interstitial biotite and
amphibole crystals. Any fractionation from a felsic I-type
melt would lead to an anhydrous melt. Second, the low Rb
content and fairly high Sr content are not consistent with
production by extensive fractionation involving feldspar.
According to Eby (1990), if the A-type granitoids were
highly fractionated I-types, then the observed enrichment in
trace elements would be a function of the degree of
fractional crystallization, which is not observed.
For example, A-type granites exhibit chemical analyses
characterized by high SiO2, Na2OCK2O, Fe/Mg, F, Zr, Nb,
Ga, Sn, Y, and REE (except Eu) contents and low Ca,
Ba, and Sr.
The current definition of rapakivi granite simply
considers the rock type as an A-type granite characterized
by the presence of rapakivi texture (Haapala and Ramo,
1990, 1999; Ramo and Haapala, 1995). The magmatic
association of rapakivi granites is clearly bimodal (basic–
felsic), and hybrid intermediate members are interpreted to
result from the interaction of co-existing basic and felsic
magmas. Basic plutonic rocks seem abundant in the lower
parts of the rapakivi complexes, though some rapakivi
plutons do not appear to be associated with basic rocks,
which may be due to a relatively high erosion level or a lack
of associated basic rocks exposure. The rimming of
K-feldspar by Na-rich feldspar is perhaps the most
distinctive feature of rapakivi granites; however, this texture
also may be developed sporadically in other granites
(Dempster et al., 1991).
The origin of Proterozoic rapakivi granites is controver-
sial and unresolved. Hoffman (1989) postulates that these
mid-Proterozoic granites were generated by a mantle
superwell beneath a stationary supercontinent. In contrast,
Windley (1991, 1993) suggests that the early Proterozoic
Ketilidian rapakivi granites are postorogenic rocks gener-
ated by crustal melting deep within a thrust-thickened
orogen that had begun to undergo extensional collapse.
These rapakivi granites south of Greenland were formed
during the late stages of the Ketilidian orogeny, synchro-
nous with a period of extensional tectonics and low-pressure
granulite facies metamorphism (Dempster et al., 1991).
Anderson and Bender (1989) review anorogenic com-
plexes (1.4–1.5 Ga) extending across North America and
northeast into Labrador. These complexes comprise potas-
sic rapakivi granite, basic dyke swarms, charnockite, and
anorthosite formed during a long era dominated by local
extension. According to these authors, the rapakivi
generation model ties magmatism to heating in a
largely undepleted subcontinental mantle, the crustal
rise of mantle plumes, and the transfer of heat into
Proterozoic crust.
In Australia, major granitic intrusions (covering
5000 km2) in the Mounte Isa inlier have uniform geochem-
ical patterns (A-type), dated 1870–1840 Ma. The TDM
model source ages for these magmas are at least 200 Ma
older than their time of emplacement (Wyborn et al., 1988).
This model has important implications for petrogenesis,
because significant heating of the lower crust is required to
generate such large batholiths. Wyborn et al. (1988) suggest
that the generation of these granites is related to extensional
events accompanied by high heat flow.
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208 197
We address the age, source, and origin of the intracra-
tonic Rio Branco suite (RBS), intrusive into the Paleopro-
tezoic crust of the SW Amazonian craton. Our tools for this
study include whole-rock geochemistry, Sm–Nd isotopic
analysis, and U–Pb zircon dating based on fieldwork. In
addition, we discuss the origin of this type of ‘anorogenic’
granite using new insights reported in recent studies.
2. The Rio Branco suite
The SW Amazonian craton consists of several
NW–SE—trending belts that become younger to the
southwest, away from an Archean core (Teixeira et al.,
1989; Tassinari et al., 2000; Geraldes et al., 2001) (Fig. 1).
Nd isotopic data (Tassinari et al., 1996; Sato and Tassinari,
1997) indicate a major accretionary belt, the 2.0–1.8 Ga
Ventuari-Tapajos trending NW–SE, adjacent to the Archean
core. The next youngest is the Rio Negro-Juruena province
(RNJP), dominated by calc-alkaline granitic, granodioritic,
and tonalitic gneisses and migmatites. Limited Rb–Sr,
U–Pb, and Pb/Pb dating of these rocks yields primary
crystallization ages between 1.80 and 1.63 Ga, and isotopic
Fig. 1. Geologic sketch map, of Amazon craton showing the (1) Archean
core; (2) Maroni-Itacaiunas province; (3) Ventuari-Tapajos province;
(4) Rio Negro-Juruena province; (5) Rondonia-San Ignacio province;
(6) Sunsas, Aguapeı, and Nova Brasilandia belts; (7) Brasiliano-Pan African
belt (620–580 Ma); (8) Phanerozoic sedimentary rocks; (9) province limits;
and (10) national borders. Modified after Teixeira et al. (1989).
data suggest that RNJP rocks were formed during a juvenile
accretionary event. The RNJP basement is locally overlain
by undeformed, 1.7–1.6 Ga, felsic to intermediate volcanic
rocks (Teles Pires Group), which in turn are overlain by
1.6–1.4 Ga sedimentary rocks (Beneficiente Formation).
The RBS is located in the RNJP, which locally comprises
volcanic and plutonic rocks of the Alto Jauru terrane
(1.79–1.74 Ga) and younger (ca. 1.55 Ga) calc-alkaline
plutons of the Cachoeirinha suite (Fig. 2). West of the Alto
Jauru terrane area is the Santa Helena arc (200 km west
from the western limit in Fig. 2), dated 1.45–1.42 Ga
(Geraldes et al., 1997, 2001; Van Schmus et al., 1998, 1999)
and characterized by calc-alkaline plutonism interpreted to
reflect subduction of the ocean crust from W to E. The
youngest tectonic event in the SW Amazonian craton
includes the deformation that resulted in the 1.0–0.92 Ga
Aguapeı thrust.
The RBS outcrops occur in a 1500 km2 area, bordered to
the E by rocks of the Neoproterozoic Brasiliano orogenic
cycle and covered to the N by Cretaceous Parecis Group
Fig. 2. Geologic map of the Rio Branco and Araputanga region showing the
most important stratigraphic units. Modified after Leite et al. (1985);
Monteiro et al. (1986), and Carneiro et al. (1992). The set of 12 samples of
the RBS was taken along the profile in the NE sector of the map. The first
and last sample locations are as follows: Rb-01 (S15808.225 0,
W58807.0550) and Rb-12 (S15808.560 0, W58806.0160).
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208198
sedimentary rocks and to the S by Mesoproterozoic Aguapeı
Group sedimentary rocks (Fig. 2).
Studies of RBS rocks were carried out by Figueiredo
et al. (1974); Oliva (1979); Barros et al. (1982); Leite et al.
(1985), and Geraldes (2000). Barros et al. (1982) define the
RBS as composed of diabase, gabbro (at the base), and
subvolcanic rocks such as rhyolites and granophyric
granites (at the top). Barros et al. (1982) also report a
Rb–Sr isochron age of 1130G120 Ma and K/Ar ages of
818–1450 Ma. Leite et al. (1985) identify two units in the
RBS: basic rocks (quartz-diorites and gabbros) and felsic
rocks (monzonite, quartz–syenite, and syenite), which they
interpret as related to magmatic differentiation.
The best exposures of the RBS are observed around Salto
do Ceu (Fig. 2) in a 15-km continuous transect on a gravel
road. Along the transect, basic (samples Rb-01–Rb-05),
intermediate (Rb-06 and Rb-07), and felsic (Rb-08–Rb-12)
rocks were collected for chemical and isotopic studies.
The RBS basic rocks comprise gabbro, diabase, basalt,
and porphyritic basalt. Gabbros are isotropic, green, and
medium to coarse grained. Lath-shaped plagioclase, com-
monly fractured and saussuritized, is the dominant mineral.
Pyroxene is the second most abundant phase and exhibits a
needle-like euhedral to subhedral form.
The RBS felsic rocks comprise equigranular coarse-
grained to porphyritic granophyric granites composed of
microcrystals of alkali-feldspar, plagioclase, and quartz in
the groundmass. Phenocrysts consist of alkali-feldspar
(orthoclase) and subhedral perthite, sporadically altered to
sericite (mainly in the rims). Several alkali-feldspar
phenocrystals show plagioclase rims commonly altered to
sericite. Rare porphyritic quartz is observed with corrosion
borders. Interstitial amphibole (mostly altered to chlorite),
biotite, and Fe-oxides are the accessory minerals. Apatite,
zircon, and epidote are associated with hornblende and
biotite. The porphyritic granite shows a coarse-grained
matrix consisting of plagioclase, microcline, quartz, and
biotite. Microcline megacrysts are typically altered in the
borders and contain abundant quartz, biotite, and plagio-
clase inclusions.
Samples Rb-06 and Rb-07 exhibit centimeter-sized
alkali-feldspar crystals bordered by plagioclase. This
texture is commonly found at outcrop scale, where a
magmatic mingling texture occurs. Basic magmatic
enclaves are characterized by fusiform to rounded deci-
metric bodies of gabbro and basalt hosted by porphyritic
granite.
Petrographic studies of the rocks from Rio Branco
(Geraldes, 2000) indicate strong hydrothermal alteration
characterized by feldspar sericitization and pyroxene
uralitization. Amphibole cloritization is also observed in
felsic rocks. K-feldspar sericitization is indicated by the
presence of fine-grained sericite. Moreover, plagioclase in
basic rocks shows sericitization in its borders, as well as
along fractures and cleavage surfaces. In rocks with mixing
texture (Rb-06 and Rb-07), seritization occurs mostly in
the border of the plagioclase rather than in the K-feldspar
core. Felsic rocks exhibit indistinct sericitization in
plagioclase and K-feldspar, but in the K-feldspar pheno-
crystals, the alteration is mostly at the borders. In basic
rocks, round pyroxene is completely alterated to sericite,
chlorite, and probably calcite, and opaque minerals have
alteration only at the borders and/or fractures. Basic
minerals in felsic rocks similarly suffered hydrothermal
alteration, predominant in accessory minerals such as
amphibole and opaque phases, both of which were altered
to chlorite.
Geraldes (2000) reports O, H, and S stable isotope
analyses for RBS rocks and minerals. d18O values
range fromC5.4‰ to C5.6‰ (basic rocks) andC7.3‰ to
C9.0‰ (felsic rocks). The basic rocks have d18O values
closer to the mantle-derived rocks than do the felsic rocks,
which have d18O values characteristic of intermediate
crustal rocks. The d18O values for hybrid rocks (C8.3‰)
are consistent with a mixing process. In addition, in the
basic rocks, the dD values vary fromK83‰ to K92‰, and
the d34O S from C0.5‰ to C3.8‰; in the felsic
rocks, these values are, respectively, K83‰ to K88‰
and C1.1‰ to C5.2‰. The O, S, and H stable isotope
signatures of RBS rocks are coherent with a magmatic
source, which indicates that the hydrothermal solutions that
alterated these rocks probably represent late-stage
magmatic activity. In addition, the O isotopes signature of
the RBS indicates no evidence of metamorphic hydrother-
mal activity.
3. Field relationships of basic and felsic rocks
The hybrid rocks of the RBS suggest interaction between
felsic and basic magma where decimetric basic bodies are
included in felsic rocks. The basic xenoliths display
macroscopic porphyritic texture and chilled margins, with
phenocrysts of calcic plagioclase in a fine-grained matrix of
plagioclase, pyroxene, and magnetite. Microscopically,
magma interaction is suggested in felsic rocks by the
zoned plagioclase that shows a border reaction and by
K-feldspar enveloped by plagioclase. In basic rocks, zoned
plagioclase might be a consequence of magma commingling
(Vernon, 1983). In this context, felsic rocks exhibiting
rapakivi texture might indicate partially digested xenoliths
from basic magma. In this account, we use Sparks and
Marshall’s (1986) terminology, in which magma mixing
leads to homogeneous hybrid rocks, and magma mingling
produces inhomogeneous hybrid rocks. The textures
probably originated during rapid cooling (quenching) of a
basic magma with a high crystallization temperature in
contact with a cooler felsic magma (e.g. Hibbard, 1981),
as indicated by the granophyric texture in the most
differentiated parts of the felsic unit.
The hybrid features may be the result of brittle conditions
reached by the magma during solidification. Salonsaari’s
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208 199
(1995) leading-edge erosion due to enclave movement
through the host melt could explain chilled margins in some
enclave faces (Rb-06 and Rb-07), which vary from sharp/
crenulated to diffuse/veined. Salonsaari (1995) attributes
these features to the rupturing of brittle, chilled rims of
partially molten enclaves and the mingling of the two melts.
The presence of a rapakivi texture within the mixing
zone between basic and felsic rocks of the RBS may
indicate that the hybrid textures are related to the
incomplete mixture of both magma sources. Rapakivi
texture also can be formed during the subisothermal ascent
of crystal-saturated magma from mid- to high-crustal levels
(Eklund, 1993; Eklund and Shebanov, 1999). In such
conditions, partial dissolution of quartz and K-feldspar
megacrysts occurs, whereas plagioclase precipitates.
According to Dempster et al. (1991); Eklund and Shebanov
(1999), the exsolution of plagioclase from K-feldspar
ovoids appears to control the development of rapakivi-like
textures. In this hypothesis, plagioclase is the source for the
Na-feldspar mantles, exsolution may take place continu-
ously over a range of temperatures, and the growth of
plagioclase and mantles reflects periods of increased
mobility of the exsolved material. Dempster et al. (1991)
show, for rapakivi granite from south Greenland, that
though oxygen isotopes display a marked low-temperature
signature in the thinner plagioclase mantles, a low-
temperature Sr component dominates in both thick and
thin mantles. This finding may indicate that thicker mantles
include both high- and low-temperature components,
whereas thinner mantles may have formed only at low
temperatures. However, field relationship and chemical and
isotopic data indicate magma mixing for the origin of the
mantled K-feldspars in the case studied herein.
4. Analytical procedures
Major element analyses were carried out at the
Geochemistry Laboratory, Department of Mineralogy and
Geotectonics, University of Sao Paulo, using an ICO-ES
according to procedures described in Janasi et al. (1996).
Trace elements, including REE, were analyzed at
the ACTLAB (Toronto, Canada) using a neutron
activation routine.
For the U–Pb analyses, 20–30 kg of sample were crushed
and milled, and heavy minerals were concentrated in a
wiffley table at the University of Sao Paulo (Brazil).
Heavy liquids were used to separate zircon. U–Pb zircon
analyses were carried out in the Isotope Geochemistry
Laboratory (IGL), Department of Geology, University of
Kansas (USA). The less-magnetic fraction was abraded, and
handpicked single grains were spiked with 205Pb–235U
mixed tracer. Zircon grains were dissolved, and Pb and U
were separated using procedures modified after Krogh
(1973, 1982) and Parrish (1987). Zircon weight varied
from 0.001 to 0.005 mg. Isotopic ratios were measured
using a VG Sector multicollector mass spectrometer in
single collector mode with a Daly detector.
Pb isotope compositions were analyzed on single Re
filaments using silica gel and phosphoric acid. Uranium was
loaded with Pb in the same filament and analyzed as UO2C.
Radiogenic 208Pb, 207Pb, and 206Pb were calculated by
correcting for laboratory Pb blanks (7–17 pg total Pb during
the analyses) and nonradiogenic common Pb, after Stacey
and Kramer’s (1975) model for the approximate age of the
sample. The decay constants were 0.155125!10K9 yearK1
for 238U and 0.98485!10K9 yearK1 for 235U (Steiger and
Jager, 1977). Zircon data were regressed using Ludwig’s
(1998) ISOPLOT program. Uncertainties in concordia inter-
cept ages are given at the 2 sigma (s) level.
For the Sm–Nd analyses, rock powders were dissolved in
bombs at approximately 180 8C and spiked with 145Nd and144Sm. The REE were extracted using Patchett and Ruiz’s
(1987) method. Isotopic compositions were measured by a
VG Sector 5-collectors mass spectrometer at IGL. Sm was
loaded with H3PO4 on a single Ta filament and
typically analyzed as SmC in a static multicollector mode.
Nd was loaded with phosphoric acid on a single Re
filament with a thin layer of AGW-50 resin beads and
analyzed as NdC using the dynamic mode. Analyses of
BCR-1 during the period when our samples were analyzed
yielded NdZ29.44G0,70 ppm, SmZ6.77G0.21 ppm,147Sm/144NdZ0.13931G0.00071, and 143Nd/144NdZ0.512641G0.000007, which yields 3NdZ0.07G0. 12
(all at 1s). During the course of these analyses, Nd blanks
ranged 500–150 pg, with corresponding Sm blanks of
100–50 pg. Correction for blanks was insignificant for Nd
isotopic composition and generally insignificant for Sm–Nd
concentrations and ratios. Sm–Nd ratios are corrected to
within G0.5%, based on analytical uncertainties.
5. Geochemistry
Twelve whole-rock chemical results of the RBS felsic
and basic rocks are shown in Table 1. Silica contents range
45–47% in the basic group (Rb-01–Rb-05) and 69–71% in
the felsic group (Rb-08–Rb-12). The clear compositional
gaps between basic and felsic rocks are indicated in a
Harker diagrams (Fig. 3), where two distinct groups are
linked by hybrid rocks (Rb-06 and Rb-07) whose silica
contents are 60–61%. The diagrams indicate higher contents
of K2O, Rb, Zr, and Ba in felsic than in basic rocks
(elements with the same behavior are Nb, Sr, Hf, Ta, TI, and
Th). These chemical features are in agreement with the
results reported for granites in rapakivi granite complexes.
The rocks show higher Si, K, F, Rb, Ga, Zr, Hf, Th, U, Zn,
and REE (except Eu) and lower Ca, Mg, Al, P, and Sr
abundance than other granite types (Haapala and Ramo,
1990, 1999).
The alumina index for RBS samples is shown in Fig. 4.
Felsic samples vary from slightly peraluminous to
Table 1
Geochemistry results of Rio Branco unit
Sample RB-01 RB-02 RB-03 RB-04 RB-05 RB-06 RB-07 RB-08 RB-09 RB-10 RB-11 RB-12
SiO2 45.52 45.15 44.97 47.54 45.02 61.68 60.63 71.32 71.43 71.46 69.44 70.08
Al2O3 16.2 15.71 15.85 13.32 16.26 13.75 13.46 12.9 12.95 13.03 12.95 13.04
Fe2O3 13.73 13.9 13.86 16.43 13.43 9.64 9.66 3.95 3.71 3.86 4.25 3.64
MgO 6.95 6.1 7.06 4.36 7.12 2.04 2.05 0.56 0.43 0.46 0.36 0.31
CaO 8.76 8.74 8.77 5.56 8.74 3.85 3.86 0.88 0.92 0.91 1.18 1.12
Na2O 2.75 3.11 2.75 4.56 2.71 3.8 3.56 3.34 3.49 3.35 3.38 3.51
K2O 1.02 0.81 0.97 0.82 1.03 3.62 3.56 5.1 5.2 5.7 5.23 5.13
P2O5 0.31 0.49 0.31 0.62 0.33 0.32 0.33 0.05 0.04 0.04 0.05 0.04
MnO 0.19 0.2 0.19 0.23 0.19 0.14 0.15 0.09 0.07 0.08 0.09 0.07
TiO2 2.04 2.3 2.1 3.72 2.09 1.49 1.61 0.45 1.42 0.43 0.5 0.43
PF 2.71 1.63 2.6 2.41 2.69 1.18 1.49 1.11 1.1 0.92 1 0.81
Total 100.18 98.14 99.43 99.57 99.61 101.51 100.36 99.75 100.76 100.24 98.43 98.18
V 225 235 256 290 249 106 12 11 13 12
Cr 116 154 135 87 123 24 19 23 21 50
Co 58 52 57 55.1 56 37.9 29.3 22.1 62.7 34.3
Ni 133 107 146 54 150 23 14 18 14 65
Cu 67 94 62 50 60 29 11 14 12 13
Zn 103 105 121 194 117 119 96 82 94 106
Ga 20 20 21 23 20 23 22 22 22 22
Ge 1.4 1.4 1.8 1.5 1.5 1.6 1.6 1.6 1.4 1.7
As K5 70 K5 16 K5 5 6 K5 K5 K5
Rb 24.7 19.7 28 13.1 28 111.2 171.6 172.2 162.9 157.6
Sr 566.26 560.99 572 191.81 546 204.12 98.04 86.15 104.55 108.6
Y 25.8 32.7 26 103.8 26 71.4 119.1 85.9 83.7 123.5
Zr 138.7 139 132 281 136 431.6 554.1 554.1 585.5 519.6
Nb 12.8 10.6 11 22.3 11 28 38 36 38.4 38.1
Mo 1.3 9.6 2.6 2.1 2.2 2.4 1.8 2.1 2.4 1.9
In K0.1 0.1 K0.1 0.1 K0.1 0.2 0.2 0.2 0.1 0.3
Sn 2.6 4 1.7 9.7 7.8 19.7 16.9 15 14.2 97.9
Sb 2.63 1.33 0.29 6.73 0.73 0.73 1.35 11.74 0.53 8.93
Cs 1.7 2.2 1.8 0.1 1.4 1.3 1.7 1.4 1.4 1.1
Ba 382.5 575.3 354 273.9 371 1143.3 1488.2 1677.2 1555.9 1551.9
La 16.79 18.32 18.6 38.6 19.9 62.64 87.18 83.05 62.8 106.09
Ce 38.71 41.52 35 80.17 36.4 132.71 180.71 174.62 133.47 203.98
Pr 4.69 5.12 4.704 10.555 4.915 14.21 18.745 18.18 14.465 21.963
Nd 24.17 26.45 22.6 55.8 23.4 64.24 82.77 77.61 64.14 95.95
Sm 5.66 6.47 5.4 15.47 5.54 13.81 17.34 16.22 14.18 20.04
Eu 1.938 2.15 1.8 4.352 1.753 2.963 2.698 2.747 2.672 3.139
Gd 4.74 5.22 4.94 14.66 5 11.9 15.51 14.33 12.37 18.44
Tb 0.86 0.98 0.87 3.08 0.83 2.15 2.94 2.5 2.33 3.4
Dy 4.81 5.54 4.7 17.75 4.76 12.19 18.07 14.32 13.67 19.95
Ho 0.94 1.1 0.89 3.55 0.95 2.51 3.86 2.97 2.83 4.14
Er 2.63 3.18 2.53 9.38 2.55 7.38 11.62 9.03 8.65 12.32
Tm 0.345 0.423 0.35 1.209 0.367 1.051 1.721 1.323 1.307 1.793
Yb 2.36 2.84 2.19 7.62 2.31 7.24 11.25 9.04 8.99 11.73
Lu 0.337 0.414 0.341 1.14 0.355 1.057 1.674 1.323 1.293 1.745
M.C
.G
erald
eset
al.
/Jo
urn
al
of
So
uth
Am
erican
Ea
rthS
ciences
17
(20
04
)1
95
–2
08
20
0
Sam
ple
RB
-01
RB
-02
RB
-03
RB
-04
RB
-05
RB
-06
RB
-07
RB
-08
RB
-09
RB
-10
RB
-11
RB
-12
Hf
3.6
3.5
3.5
7.1
3.4
11
.51
4.4
14
.71
5.3
13
.6
Ta
1.0
40
.92
0.7
61
.68
0.8
93
.21
6.2
4.9
96
.67
.32
W1
8.4
10
5.3
22
23
.71
38
4.8
15
41
00
.23
15
.51
97
.5
Tl
0.1
20
.14
0.1
0.0
70
.08
0.4
20
.56
0.6
0.5
80
.59
Pb
K5
51
66
81
81
71
91
92
2
Th
1.6
71
.07
1.3
74
.09
1.3
61
2.2
18
.15
17
.65
14
.62
18
.67
U0
.66
0.3
30
.37
1.2
30
.39
3.3
75
.57
6.2
4.8
94
.88
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208 201
metaluminous, and basic unit samples are metaluminous.
These results align with the chemical characteristics for
rapakivi granites of Finland. According to Ramo and
Haapala (1995), rapakivi granites characteristically straddle
the metaluminous–peraluminous boundary.
One method of granite classification can be constructed
by statistical analyses of many trace element analyses of
granites from well-defined tectonic settings (Pearce et al.,
1984). This approach leads to discrimination diagrams
that can identify tectonic settings. Pearce et al. (1984)
propose that syncollisional, volcanic arc, ocean ridge, and
within-plate granites may be discriminated according to
Nb, Y, Ta, Yb, and Rb data. Chemical analyses of the
RBS felsic rocks plotted in a Rb versus YCNb tectonic
setting discrimination diagram (Fig. 5) and basic rocks
(Fig. 6) in a Zr/Y versus Zr diagram (Pearce and Norry,
1979) indicate a within-plate setting for the origin of both
rock types.
The felsic unit samples show REE-fractionated patterns
characterized by LREE enrichment, a strong negative Eu
anomaly (probably due to plagioclase removal in earlier
stages of magma ascent), and flat HREE. As shown in Fig. 7,
enrichment of REE contents is present in samples
Rb-08–Rb-12, which suggests that these rocks represent
various stages of magma fractionation, with Rb-08 as the
least evolved and Rb-12 the most. The REE patterns for
basic rocks are flatter than those for the felsic rocks. The
patterns of the basic samples are less fractionated between
BREE and LREE, and there are no Eu anomalies. The
progressive increase of the REE-contents in basic rocks
(with the exception of sample Rb-04, which presents
anomalous high REE values) without Eu anomalies suggests
the predominance of weak fractional crystallization of a
restricted magmatic series. Fractional crystallization simul-
taneously involving plagioclase, pyroxene, and magnetite
increases the total amount of REE in basaltic melts but does
not cause any significant LREE–HREE fractionation
(Fig. 8). Nevertheless, the absence of Eu-positive anomalies
in the basic rocks may be interpreted as a lack of
consanguinity between the magmas that formed the RBS
basic and felsic rocks.
6. Isotope data
U–Pb (single-grain) zircon geochronology was under-
taken on the felsic sample Rb-10 (20 kg) and the basic
sample Rb-04 (30 kg). Sample Rb-10 was processed to
concentrate heavy minerals, and a homogenous collection of
zircon grains was obtained. This collection consists of clear,
slightly caramel-colored grains, 50% of which have
biphasic (one gas and one liquid at room temperature)
fluid inclusions. Four single-zircon, fluid inclusion-free
grains were abraded and analyzed. The results yield an
upper intercept of 1423G10 Ma (Fig. 9), which we interpret
as the crystallization age of the felsic magma.
Fig. 3. Harker variation diagrams for oxides (K2O, Ca, MgO, and Fe2O3) and minor elements (Rb, Zr, Ba, and Cr) of the RBS mafic, hybrid, and felsic rocks.
Fig. 4. Alumina index of the RBS. The felsic rocks vary from slightly
peraluminous to metaluminous, and the basic rocks are metaluminous.
Fig. 5. Tectonic setting discrimination diagram (Pearce et al., 1984) for
RBS felsic rocks.
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208202
Fig. 6. Tectonic setting discrimination, diagram (Pearce and Norry, 1979)
for RBS basic rocks.Fig. 8. REE patterns for RBS basic rocks.
Fig. 9. Plot of zircon data for sample Rb-10. The upper intercept yields a
crystallization age of 1423G02 Ma. Uncertainty at 2-s.
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208 203
Five zircon grains obtained from sample Rb-04 were
analyzed, and the plotted results in the concordia diagram
(four fractions) yield an upper intercept age of
1471G18 Ma (Fig. 10). The grains are milky, and neither
the 001 faces nor the pyramidal ends are well defined. The
high MSDW (84) gives little confidence for this age, but a
concordant analysis (M(5) E in Table 2) indicates a207Pb/206 Pb age of 1471G18 Ma, which may be interpreted
as the crystallization age of the basic magma. Consequently,
zircon crystallization of basic magma took place 30–50
million years before felsic magma crystallization.
The U–Pb zircon ages are w300 Ma older than the
Rb–Sr whole-rock reference mixed line reported by Barros
et al. (1982) of 1130G72 Ma (87Sr/86SrinitialZ0.708) for the
RBS rocks. The Rb–Sr study is unreliable because the basic
and felsic rocks may have different sources. Recalculations
using only the felsic samples give an age of 1221G32,
200 Ma younger than the U–Pb ages, which indicates partial
resetting of the Rb–Sr ages. This resetting may be due to
younger events observed in the region, such as deformation
that produced the Aguapeı thrust (ca. 1000 Ma, Geraldes
et al., 1997).
Aliquots from the same powders used for whole-rock
major and trace element geochemistry were used for
Sm–Nd isotopic analyses (Table 3). The fractionation
factor (f) between Sm and Nd in the basic rocks varies
Fig. 7. REE patterns for RBS felsic rocks.
Fig. 10. Plot of zircon data for sample Rb-04. The upper intercept yields a
crystallization age of 1471G31 Ma. Uncertainty at 2-s.
Tab
le2
U/P
bre
sult
sfo
rsa
mp
les
Rb
-04
and
Rb-1
0
Fra
ctio
n*
Wei
gh
t(m
g)
20
7*/2
06
*
U(p
pm
)P
b(p
pm
)O
bse
rved
#R
atio
G2
SE
(%)†
Cal
cula
ted
ages
G2
SE
(Ma)
‡
206P
b/2
04P
b207*
Pb
/235U
206*
Pb/2
38U
207*
Pb
/206*
Pb
207*
Pb
/235U
206*
Pb
/238U
20
7*/2
06
*
Rb
-10
Gra
nop
hyr
eR
ioB
ranco
NM
(0)[
1]
0.0
02
13
33
85
20G
3.0
20
76
1.2
0G
0.2
43
57
1.1
7G
0.0
89
94
76
0.2
4G
0.9
80
14
13G
17
14
05G
16
14
24G
4.5
M(0
)[1
]0
.002
29
39
01
00
4G
3.6
03
36
0.7
9G
0.2
65
20
0.7
7G
0.0
98
54
52
0.1
8G
0.9
74
15
50G
12
15
16G
12
15
97G
3.3
M(1
)[1
]0
.003
29
97
81
52
5G
2.9
43
96
0.7
0G
0.2
37
79
0.6
8G
0.0
89
79
24
0.1
7G
0.9
69
13
93G
10
13
75G
09
14
21G
3.3
M(2
)[1
]0
.002
28
07
76
86G
2.9
68
79
0.8
2G
0.2
39
51
0.8
0G
0.0
89
90
09
0.1
8G
0.9
76
14
00G
11
13
84G
11
91
42
3G
3.4
Rb
-04
Ga
bb
roR
ioB
ran
co
M(5
)[2
]0
.005
44
09
28
86G
2.5
43
60
1.2
1G
0.2
00
64
51
.14G
0.0
91
94
31
0.4
0G
0.9
43
12
85G
16
11
79G
13
14
66G
7.6
M(5
)[1
]0
.003
49
71
37
61
6G
2.4
88
92
0.6
4G
0.2
04
26
80
.62G
0.0
88
37
10
.15
G0
.970
12
69G
08
11
98G
07
13
91G
03
M(5
)[1
]0
.002
51
71
08
56
9G
1.7
56
69
0.7
5G
0.1
54
67
80
.73G
0.0
82
36
96
0.1
8G
0.9
69
10
30G
07
92
7G
06
12
54G
3.6
M(5
)0
.004
78
82
07
99
8G
2.2
04
94
0.5
3G
0.1
87
30
80
.52G
0.0
85
37
66
0.1
1G
0.9
80
11
83G
06
11
07G
06
13
24G
2.1
M(5
)0
.002
19
35
94
94G
3.2
46
95
1.6
4G
0.2
55
52
21
.28G
0.0
92
16
08
0.9
7G
0.8
05
14
68G
23
14
67G
18
14
71G
18
NM
Zn
on
mag
net
ic;
MZ
mag
net
ic;
nu
mb
erin
par
enth
eses
ind
icat
esi
de
tilt
on
Fra
nz
sep
erat
or
at1
.5A
po
wer
;[1
]Zn
um
ber
of
gra
ins;
*d
eno
tes
radio
gen
icP
b;
†P
bco
rrec
ted
for
bla
nk
and
no
n-r
adio
gen
icP
b;
‡A
ges
giv
enin
Ma
usi
ng
dec
ayco
nst
ants
reco
mm
ended
by
Ste
iger
and
Jag
er(1
97
7);
ince
rtai
ns
inag
esar
e2s
.
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208204
from K0.31 to K0.32 (with the exception of Rb-04, with
fZK0.61); in the felsic rocks, f varies from K0.37 to
K0.41. The anomalous f value for sample Rb-04 probably
results from its high concentration of accessory minerals
with high concentrations of REE. In addition, the
concentration values of Nd and Sm for sample Rb-07,
obtained from neutron activation (15.5 and 55.8 ppm,
respectively; Table 1) and isotope dilution (11.2 and
89.82 ppm, respectively; Table 2), are anomalously high
(37.88% for Nd and 37.29% for Sm). Due to its
anomalously f value, chemical and Nd isotope data for
Rb-04 are not considered further.
3Nd(1420) values for basic rocks range from C1.2 toC1.9,
suggesting mantle source crustal rock contributions.
3Nd(1420) values for felsic rocks range from C0.2 to K1.0,
which suggest that they contain an important older crust
component. TDM ages of the basic rocks vary from
1.73–1.80 Ga, and TDM ages of felsic rocks are slightly
older, 1.81–1.89 Ga. 3Nd(0) values of the basic rocks
range from K8.3 to K10.4; of the felsic rocks, from
K13.1 to K15.2, thus indicating that basic and felsic rocks
have different Nd isotopic evolutions compared with the
depleted mantle over time (Fig. 11).
7. Discussion
Isotopic ages usually confirm a close temporal associ-
ation of basic rocks with rapakivi granites (Haapala and
Ramo, 1999). Even gabbroic and anorthosite rocks intruded
by granitic rocks (AMCG suites) yield isotopic ages usually
undistinguishable within experimental error. For example,
Haapala and Ramo (1999) report an U–Pb age gap of 5 Ma
between gabbroic and quartz–feldspar porphyry dykes in the
Ahvenisto complex. Similarly, there is a difference of
approximately 20 Ma (U–Pb ages) in the gabbroic rocks and
granites from the Salmi batholith in Russian Karelia
(Neymark et al., 1994).
The reported U–Pb geochronological data obtained in
single-zircon grains yields an age of 1471G18 Ma for the
basic rocks and 1427G10 Ma for the felsic rocks, both of
which may be interpreted as crystallization ages. There are
two suggestions to explain the U–Pb age gap of 50–30 Ma.
First, the zircon grains analyzed from the basic sample may
be xenocrysts. This hypothesis is not consistent with the
zircon shape, because the studied grains are characteristic of
basic rocks. Second, basic and felsic magmas may have
crystallized at different temperatures. In this case, the basic
magma solidified 50–30 Ma before the felsic magma, which
corroborates the commingling textures that correlate with
the incomplete mixing of the basic and felsic magmas due to
brittle conditions.
The TDM ages of the basic (1.86–1.82 Ga; 3Nd(1420)C1.2
to C1.9) and felsic (1.80–1.73 Ga; 3Nd(1420)C0.2 to K1.0)
rocks are similar to Sm–Nd data from the Alto Jauru terrane
and Cachoeirinha suite rocks (3NdC0.5 to K0.8, TDM ages
Table 3
Sm/Nd isotopic properties of rocks from RBS
Sample Rock Nd (ppm) Sm (ppm) 147Sm/144Nd 143Nd/144Nd E(Nd) tZ0 E(Nd)
t(U/Pb)
T(DM) Ma f
Rb-01 Gabbro 25.02 5.59 0.13511 0.511595 K0.78 1.85 1.75 K0.31
Rb-02 Gabbro 25.17 5.62 0.13497 0.512029 K10.41 1.24 1.8 K0.31
Rb-03 Gabbro 21.35 4.72 0.13380 0.511740 K9.96 1.91 1.73 K0.32
Rb-04 Porphyre
gabbro
89.82 11.29 0.07606 0.511632 K25.10 K2.33 1.86 K0.61
Rb-05 Basalt 20.62 4.73 0.13867 0.511711 K9.37 1.59 1.79 K0.30
Rb-06 Monzonite 64.33 12.69 0.11930 0.511913 K14.12 0.00 1.81 K0.39
Rb-07 Monzonite 58.69 12.04 0.12410 0.511758 K13.08 0.16 1.89 K0.37
Rb-08 Granite 81.12 16.08 0.11987 0.511501 K14.34 K0.33 1.84 K0.39
Rb-09 Porphyre
granite
74.28 14.49 0.11793 0.511602 K14.85 K0.49 1.85 K0.40
Rb-10 Porphyre
granite
69.55 13.40 0.11650 0.511605 K14.83 K0.20 1.86 K0.41
Rb-11 Porphyre
granite
58.09 11.91 0.12392 0.511869 K13.40 K0.13 1.84 K0.37
Rb-12 Porphyre
granite
92.76 18.17 0.11853 0.511639 K15.21 K0.96 1.89 K0.40
Fig. 11. 3Nd versus age plot of the RBS mafic and felsic rocks. Alto Jauru
greenstone belt and Cacheirinha Sm/Nd isotopic data are also plotted for
comparison (Geraldes et al., 2001). Felsic rocks are dark gray, basic rocks
are black, and host rocks are gray.
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208 205
2.05–1.75 Ga), as reported by Geraldes (2000). This Nd
isotope evidence indicates that the RBS country rocks,
represented by tonalite, granodiorite, and granite, probably
contributed to the source of the RBS rocks (Fig. 11). Creaser
et al. (1991) propose that at least some A-type magmas are
generated by melting of crustal igneous rocks of tonalitic to
granodioritic composition, as indicated by the pattern of
trace element ratios. Creaser et al. (1991) also suggest that
the partially melted lithosphere was originally produced by
continental margin or island-arc magmatism, similar to the
situation presented here.
If this hypothesis has merit, the felsic rocks of the RBS
may have three mixed sources. The oldest component is
represented by the 1.79–1.74 Ga Alto Jauru terrane rocks
(TDM 1.93–1.77 Ga), and the second component is the
1.57–1.52 Ga Cachoeirinha calc-alkaline rocks (TDM
1.79–1.75 Ga). We suggest that an obvious third source
for the RBS felsic rock protoliths is the mantle, as indicated
by the few positive 3Nd(1420) Values (C0.2 to K1.0).
In addition, the basic rocks may have resulted from
underplating magma, as indicated by the TDM values
(1.80–1.73 Ga), which indicate that the magma of these
rocks was generated from the mantle at that time and
intruded into the lower crust during crystallization
(1.47–1.42 Ga). The generation of the basic rocks from
underplating magma also may be explained by the low
3Nd(1420) values (C1.2 to C1.9); therefore, some crustal
rock participation is possible.
The Sm–Nd data of the RBS felsic rocks are similar to
those reported by Sato and Tassinari (1997), who interpret a
crustal accretion to the Amazonian craton at ca. 1.8 Ga, the
time of generation of the RNJP. Sato and Tassinari’s (1997)
report corroborates the hypothesis that the protolith of the
RBS basic rocks originated by underplating of the mantle-
derived basic magma formed at the mantle/crust boundary.
This magma probably was reactivated by heat flow due to
the ocean crust subduction of the Santa Helena magmatic
arc (1450 Ma), as speculated in Fig. 12. The thermal effect
on the basic magmas may have caused partial melting of the
lower crust (parent of the granites), which melted the base of
this crust and generated felsic magma and crystallization of
both basic and felsic rocks at the hypabyssal level. Mingling
of the basic and felsic magmas may have led to local
hybridization in intracrustal magma chambers (Salonsaari,
1995) and the origin of rapakivi granites of the Finnish
Jaala–litti complex.
An extensional geotectonic setting has been documented
for the rapakivi complexes of Finland (Haapala and Ramo,
1990) through the recognition of graben structures, crustal
Fig. 12. Summary of the tectonic setting for SW Mato Grosso at 1420 Ma. The formation of the Santa Helena magmatic arc occurred in the newly
accreted 1.79–1.74 Ga Alto Jauru greenstone belt and 1.58–1.52 Ga Cachoeirinha rocks. The RBS intruded the last two units and is coeval to the Santa
Helena magmatic arc.
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208206
thinning in rapakivi areas, and listric faulting. Previously,
no extensional features had been identified that might be
linked tectonically to the Rio Branco emplacement.
The RBS can be correlated with the Santo Antonio
intrusive suite observed in the Rondonian province. As
described by Bettencourt et al. (1999), seven distinct
episodes of rapakivi magmatism occurred in the Rondonia
Tin Province: (1) Serra da Providencia intrusive suite
(1606G24 to 1532G4.5 Ma); (2) Santo Antonio intrusive
suite (1406G32 Ma); (3) Teotonio intrusive suite
(1387G16 Ma); (4) Alto Candeias intrusive suite
(1347G4.7 Ma); (5) Sao Lourenco-Caripunas intrusive
suite (1314G13 to 1309G24 Ma); (6) Santa Clara intrusive
suite (1082G4.9 Ma); and (7) younger granites (998G5 to
991G14 Ma). Thus, felsic magmatism of the RBS is coeval
to an intrusive suite reported in the Rondonia Tin Province,
which may have important consequences for metal
exploratory models in the region.
8. The heating process and tectonic setting (lithospheric
versus astenospheric)
Ahall et al. (2000) explain Baltic shield growth during
the Middle Proterozoic, where juvenile crustal domains
were progressively built on or amalgamated to the
evolving continental margin. The stepwise growth was
approximately synchronous with inboard, episodic rapa-
kivi magmatism between 1.65 and 1.50 Ga. The data lead
Ahall et al. (2000) to propose a hybrid model to explain
the timing and spatial relationship of orogenesis and
episodic rapakivi magmatism in Baltica. According to
them, the 1.65–1.50 Ga rapakivi suites, generated in the
lower crust, may be due to decompressional melting
and coeval processes operating at the paleocontinental
evolving margin.
Ahall et al’s. (2000) hybrid synorogenic response model
may explain the origin of the RBS, which is very similar to
the rapakivi suites of Baltica. The heat convection in
the asthenosphere (source of the basic magma) may be
the result of the ocean crust subduction that was
synchronous with the development of the Santa Helena arc.
According to this hypothesis, the data reported here
indicate that there was thinning of the lithosphere, related to
the convecting asthenosphere and remobilization of the
underplating magma, which in turn led to crustal melting.
It follows that during an extensional event, there would be
widespread heating, metamorphism, and melting of the
crust, provided there was suitable source material
available to form felsic melts. Emplacement in the upper
crust requires a significant tectonothermal event; without it,
the heat needed to cause a melt of such a large scale could
not exist, not only for the generation of the melts, but
also for the prior formation of major A-type sources in the
lower crust.
9. Concluding remarks
The RBS can be considered an intraplate suite intrusive
into the Alto Jauru terrane. The results presented here, when
integrated with regional data, enable us to propose better
constraints on the sources and processes involved in the
genesis of this rapakivi complex.
Mantle extraction of the basic and felsic rock protoliths
took place, respectively, at 1.70–1.78 Ga and 1.84–1.89 Ga
(TDM ages), which matches the time span of the country
rocks (2.05–1.75 Ga). 3Nd(1420) values for the basic (C1.2 to
C1.9) and felsic (C0.2 to K1.0) rocks suggest that
protoliths of RBS rocks originated by magma underplating,
melting of the lower crust, and crystallization at the upper
crust, involving both mixing and commingling processes.
Felsic rocks of the RBS may have been derived from a
heterogeneous source comprised of the 1.79–1.74 Ga Alto
Jauru terrane rocks and the 1.57–1.52 Ga Cachoeirinha calc-
alkaline rocks. The RBS basic magma had a mantle source
with moderate contamination by the older crust.
U–Pb crystallization of basic and felsic magmas occurred
between 1.47 Ga and 1.42 Ga. The 50–30 million year
difference may be due to the presence of zircon xenocrysts
in sample Rb-10 or to the basic magma that solidified
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208 207
before the felsic magma. This hypothesis is corroborated by
the commingling textures, which are correlated with
the incomplete mixing of basic and felsic magmas in brittle
conditions.
U–Pb, Sm–Nd, and geochemical data of RBS rocks
provide a temporal correlation between the crustal growth of
the western margin (Santa Helena arc) and coeval distal
rapakivi anorogenic magmatism in the foreland. The RBS
rapakivi complex may represent a synorogenic response,
linked to the high heat flow in the asthenosphere, that
resulted from the subduction the ocean crust simultaneous
with the development of the Santa Helena arc.
Acknowledgements
This article was improved by suggestions from Profs.
Randy Van Schmus, Marcio Pimentel, and Roberto
Dall’Agnol. The manuscript was also improved by the
reviewers (Ignes Guimaraes and Charles Gower). This work
was sponsored by FAPESP Grant 1996-04819-7 to MCG
and FAPESP Grant 1996-12627-0 to WT. This article is a
contribution to IGCP-426: Granite Systems and Proterozoic
Lithospheric Processes.
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