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).

http://www.elsevier.com/locate/jsames

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).


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


(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


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.


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.


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

.


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.


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.


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


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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