Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185 Comunicações Geológicas (2014) 101, Especial I, 125-129 IX CNG/2º CoGePLiP, Porto 2014 ISSN: 0873-948X; e-ISSN: 1647-581X

1.38-1.30 Ga A-type granites related to the evolution of the Rondonian-San Ignacio orogenic system, SW Amazonian Craton, Brazil: a geochemical overview Granitos do tipo-A (1,38-1,30 Ga) relacionados com a evolução do sistema orogênico Rondoniano-San Ignacio, SW Craton Amazônico, Brasil: uma revisão geoquímica W. B. Leite Júnior1*, B. L. Payolla2, J. S. Bettencourt3, C. A. Tavares Dias1

© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP

Abstract: Two main groups of A-type or ferroan granitoids (1.38-1.30 Ga) related to the development of the Rondonian-San Ignacio Province (1.56-1.30 Ga) are recognized in the Rondônia Tin Province: alkalic metaluminous granitoids and alkali-calcic metaluminous granitoids with or without associated peraluminous granitoids. The first group includes the granitoids of the Teotônio Intrusive Suite, and the second one includes the granitoids of the Santo Antonio, Alto Candeias, and São Lourenço-Caripunas intrusive suite. These suites are interpreted to be related to episodic extensional regime in a within-plate tectonic setting. We believe that the alkalic metaluminous granitoids were formed by fractional crystallization of ferro-basalt parent magma, whereas the alkali-calcic metaluminous granitoids were crystallized from anatectic melts derived from quartzo-feldspathic sources in response to mafic underplating.

Keywords: Geochemistry, A-type granite, SW Amazonian Craton, Rondonian Tin Province. Resumo: Dois grupos principais de granitóides ferrosos ou tipo-A com idades entre 1,38-1,30 Ga e relacionados com o desenvolvimento da Província Rondoniana-San Ignacio (1,56-1,30 Ga) são reconhecidos na Província Estanífera de Rondônia: granitóides metaluminosos alcálicos e granitóides metaluminosos álcali-cálcicos com ou sem granitóides peraluminosos associados. Os granitóides do primeiro grupo são incluídos na Suíte Intrusiva Teotônio e os do segundo grupo ocorrem nas suítes intrusivas Santo Antônio, Alto Candeias e São Lourenço-Caripunas. Essas suítes são interpretadas como relacionadas a regimes extensionais episódicos em ambiente tectônico intraplaca. Nós entendemos que os granitóides metaluminosos alcálicos foram formados por cristalização fracionada a partir de magma original de composição ferro-basáltica, enquanto que os granitóides metaluminosos álcali-cálcicos foram cristalizados a partir de magmas anatécticos derivados da fusão de fontes de composição quartzo-feldspática em resposta a magma máfico underplating.

Palavras-chave: Geoquímica, Granito Tipo-A, SW do Cráton Amazônico, Província Estanífera de Rondônia.

1Instituto de Geociências e Ciências Exatas - Universidade Estadual Paulista (UNESP). Avenida 24A, 1515, CEP 13506-900, Rio Claro, São Paulo, Brasil. 2Eletrobras Eletronorte. SCN, Quadra 6, Conj. A, Bloco C, CEP 70.716-901, Brasília, Distrito Federal, Brasil. 3Instituto de Geociências, Universidade de São Paulo (USP). Rua do Lago, 562, CEP 05508-900, Cidade Universitária, São Paulo, São Paulo, Brasil. *Corresponding author / Autor correspondente: wleite@rc.unesp.br

1. Introduction

Proterozoic anorogenic granites in the Rondônia Tin Province (RTP), northern Brazil, were recognized in the late 1960’s and subsequently characterized as rapakivi granites. Bettencourt et al. (1999) identified seven rapakivi suites, which were temporarily correlated to the development of three tectonic provinces in the southwestern border of the Amazonian craton (Tassinari & Macambira, 1999): Rio Negro-Juruena (1.80-1.55 Ga), Rondonian-San Ignácio (1.50-1.30 Ga), and Sunsás-Aguapeí tectonic provinces (1.25-1.00 Ga).

In a recent review, Bettencourt et al. (2010a) characterized the Rondonian-San Ignacio province (RSIP; 1.56-1.30 Ga) in the southwestern margin of the Amazonian craton as an orogenic system, comprising an older complex accretionary orogen (1.56-1.34 Ga), and a terminal microcontinent-continent collisional orogen (1.34-1.32 Ga). In the RTP (northern sector of the RSIP) the Santo Antônio (SAIS; ~ 1.4 Ga), Teotônio (TIS; ~ 1.38 Ga), Alto Candeias (ACIS; 1.34-1.33 Ga) and São Lourenço-Caripunas (SLCIS; 1.31-1.30 Ga) intrusive suites are interpreted by Bettencourt et al. (2010a) as A-type granites related to the evolution of the RSIP. The SAIS and TIS are related to the rift stage (1.50-1.38 Ga), whereas the ACIS and SLCIS are related to the late to post-collisional stage (1.34-1.30 Ga).

The aim of this paper is to reveal the main lithogeochemical characteristics of the 1.38-1.30 Ga A-type granites related to the evolution of the Rondonian-San Ignacio orogenic system in the Rondônia tin Province.

2. Regional setting

Precambrian geology of the north-central Rondônia includes geological units with ages varying from 1.76 to 0.97 Ga. The older units are the Jamari Complex (1.76-1.73 Ga) that is composed of calc-alkaline tonalitic and dioritic gneisses, paleoproterozoic metavulcano-sedimentary (Mutum-Paraná and Igarapé Lourdes

Artigo Curto Short Article

126 W. Leite Júnior et al. / Comunicações Geológicas (2014) 101, Especial I, 125-129

formations: ~ 1.75 Ga) and metasedimentary (Quatro Cachoeiras Suite: 1.67-1.59 Ga) sequences, and rapakivi granites, charnockites and gabbros of the Serra da Providência Intrusive Suite (1.60-1.53 Ga) (Quadros & Rizzoto, 2007). These units are included in the Rio Negro-Juruena province (1.78-1.55 Ga) that is interpreted as a juvenile accretionary belt (Tassinari et al., 1996), with a terminal arc-continent collision at the time interval 1.67-1.63 Ga, and post-collisional magmatism from 1.60 to 1.53 Ga (Scandolara, 2006). Younger units are the A-type granites and related rocks of the Rio Crespo (RCIS; ~1.50 Ga), TIS (~1.38 Ga), SAIS (~1.37 Ga), ACIS (1.35-1.33 Ga), and SLCIS (1.31-1.30 Ga) (Bettencourt et al., 1999, 2010b; Payolla et al., 2012), which are interpreted to be related to the evolution of the Rondonian-San Ignacio province (1.56-1.30 Ga; Fig. 1) (Bettencourt et al., 2010a). The youngest units are the mafic dikes and sills of the Nova Floresta Formation (1.20 Ga), rapakivi granites and minor syenites of the Santa Clara (1.08-1.07 Ga) and Younger Granites of Rondônia (0.99-0.97 Ga) intrusive suites, and continental sediments of the Palmeiral Formation (<1.03 Ga) (Bettencourt et al., 1999; Quadros & Rizzotto, 2007), which are interpreted to be related to the evolution of the Sunsás-Aguapei province (1.20-0.95 Ga) (Teixeira et al., 2010).

3. Geology and petrography

The TIS and SAIS crops out in the northwestern sector of the RTP (Fig. 1). Bettencourt et al. (2010b) reported more precise ages for TIS (1.38-1.37 Ga) and SAIS (~ 1.37 Ga) intrusive suites that are more consistent to the field relations, where rocks of former suite host dykes of the later ones.

The TIS were described by Payolla (1994) in the Teotônio cataract area. Major units are massive coarse-grained ferrohedenbergite alkali-feldspar granite, banded medium-grained ferrohedenbergite alkali-feldspar granite, and pink coarse- to medium-grained quartz alkali-feldspar syenite with less common alkali-feldspar granite and syenogranite. The coarse grained and banded granites are cut by NE dipping, up to 2 meters wide, tabular bodies of fine– to medium-grained fayalite-ferrohedenbergite alkali-feldspar syenites, as well as synplutonic dykes of diorite, monzodiorite, and monzonite. The parallel arrangement of the tabular bodies and dykes defines a large scale banding in outcrops at Teotônio cataract. Late pink, fine-grained subsolvus monzogranites of the SAIS occur as SW dipping dykes cutting through the above rock types.

The SAIS is composed of two main granitic types: seriate to locally porphyritic biotite monzogranite and syenogranite with sparse rapakivi and anti-rapakivi textures and medium grained equigranular biotite monzongranite. Distinctive rock types of smaller areal extent include fine-grained hornblende-biotite quartz-monzonite, dyke-like bodies of hybrid rocks (monzogranite, quartz monzonite, and quartz monzodiorite), and synplutonic monzodioritic to dioritic and diabase dikes.

The ACIS crops out mainly in the Alto Candeias massif in the southern part of the RTP. The massif is oval-shaped with a general WNW-ESE trending area (up to 10.000 km2), and is partially covered by Nova Floresta and Palmeiral formations (Fig. 1). The internal structure of the massif is poorly known as well as the contacts with the basement rocks. According to Scandolara (2006) the granites of the northern boundary were affected by ductile shear zone, whereas only sparse ductile shear bands are observed internally. Two major types of rock associations are identified: granitic and charnockitic associations. The granitic association (~ 1.34 Ga; Bettencourt et al., 1999) is dominant in area and includes pinkish-gray and gray medium- to coarse-grained porphyritic and pyterlitic syenogranite, monzogranite and quartz-monzonite, with alkali-feldspar phenocrysts (up to 5 cm long), sometimes mantled by plagioclase, and hornblende and biotite as the main mafic minerals. Pinkish-gray medium- to fine-grained equigranular biotite (± hornblende) granites and aplites are observed in various places, mainly as dykes. The charnockitic association (~ 1.35 Ga; Payolla et al., 2012) occurs apparently as elongated body in the southeastern margin of the massif, as well as isolated areas in the granitic domain. Greenish-gray and reddish-brown medium- to coarse-grained porphyritic charnockites and quartz-monzonitic charnockites are the dominant petrographic varieties. The associated rocks are medium- to coarse-grained, slightly porphyritic quartz-monzodioritic charnockite and fine- to medium-grained slightly porphyritic dioritic charnockite.

The SLCIS crops out in a northeasterly direction homonymous massif (up to 3.000 km2) along the left bank of the Madeira River in the northwestern part of the RTP (Fig. 1). The São Lourenço-Caripunas massif is composed of plutonic, subvolcanic and volcanic rocks mainly of granitic composition (syenogranites, alkali-feldspar granites and rhyolites). Pink fine- to medium-grained equigranular granites are apparently dominant in the Caripunas region, besides pink porphyritic granite, pyterlite, and minor wiborgite (Leite Júnior et al., 2011). They contain biotite and hornblende as the main mafic minerals, but clinopyroxene and fayalite occur in some even-grained varieties. In the São Lourenço area, pink porphyritic hornblende-biotite syenogranites and biotite-hornblende quartz-syenites, and pink to cream even-grained biotite granites are common, besides brown rhyolite porphyry, and minor gabbro (Leite Júnior et al., 2013).

4. Geochemistry

Major elements were analyzed by XRF at the São Paulo State University in Rio Claro, São Paulo, Brazil, while the trace-elements and REE were determined by ICP-MS at Acme Analytical Laboratories Ltd. in Vancouver, Canada, both using a lithium borate fusion method.

The granites of all suites are mainly metaluminous to slightly peraluminous rocks (A/CNK= 0.87-1.04), excepting some high-SiO2 granites (>75.0 wt %) of the

1.38-1.30 Ga A-type granites, Rondônia, Brazil 127

SLCIS that are peraluminous in character (A/CNK1.10). The K2O/Na2O ratios vary from 1.1 to 3.0 revealing a distinct potassic affinity for the granites.

The granites and some associated rocks (syenites, quartz monzonites, monzodiorites and charnockites) of the suites were evaluated on the geochemical classification scheme for granitic rocks based on major-element chemistry proposed by Frost et al. (2001). In a plot of FeOtot/(FeOtot+MgO) vs SiO2 all the samples lie in the ferroan field (Fig. 2a). In the MALI diagram (Na2O+K2O-CaO vs SiO2), the TIS samples plot in the alkalic field, the SAIS and SLCIS samples in the alkali-calcic field, whereas the granites and charnockites of the ACIS are alkali-calcic and calc-alkalic, respectively. Some granites of TIS, ACIS and SLCIS showing >75 wt.% SiO2 plot in the calc-alkalic field (Fig. 2b).

Dall’Agnoll & Oliveira (2007) proposed a distinction between A-type granites and calc-alkaline granites based on some oxide ratios. In a plot of CaO/(FeOt+MgO+TiO2) vs CaO+Al2O3 all the samples fall in the A-type granites field (Fig. 2c), while in the FeOt/(FeOt+MgO) vs Al2O3/(K2O/Na2O) diagram, that discriminate oxidized and reduced A-type granites, the SAIS is oxidized, ACIS is reduced, SLCIS shows both characters, and the TIS samples fall out of the fields, but the high FeOt/(FeOt+MgO) ratios (>0.90) suggest a reduced nature (Fig. 2d).

In the FeO*/MgO vs Zr+Nb+Ce+Y diagram of Whalen et al. (1987) all the samples lie in the A-type field (Fig 2e),

and according to Nb-Y-3Ga diagram (Eby, 1992) the SAIS, ACIS, and SLCIS samples plot in the A2-type granite field, whereas TIS samples in the A1-type granite field (Fig. 2f). All the samples when plotted in the Rb vs Y+Nb tectonic discrimination diagram of Pearce (1996) fall in the within-plate granite field (Fig. 2g).

The rare earth elements (REE) patterns are shown in Figure 2h. The granites of the SAIS, ACIS and SLCIS exhibit very similar patterns, showing low to moderate concentrations of REE (~10 to 400 times the chondrite), low to moderate enrichment of the light REE (LREE) relative to the heavy REE (HREE) (LaN/YbN= 2.2-13.2), and moderate to strong Eu anomalies (Eu/Eu*= 0.54-0.06) (Fig. 2h), whereas the granites and syenites of the TIS are richer in LREE contents (~200 to 2000 times the chondrite), and show higher enrichment of the LREE over the HREE (LaN/YbN= 9.4-31.8) and moderate Eu anomalies (Eu/Eu*= 0.50-0.15) (Fig. 2h). No fractionation of the middle REE (MREE) relative to the HREE is shown by SAIS and SLCIS granites and quartz monzonite (GdN/YbN= 0.43-1.45), while a low depletion of the HREE relative to the MREE is observed in the TIS and ACIS granites and syenites (GdN/YbN= 1.11-3.09). Associated rocks as monzonites, monzodiorites (TIS and SAIS) and charnockites (ACIS) exhibit also low to moderate concentration of REE with respect to chondritic values (~30 to 300 times), low to moderate enrichment of the LREE relative to the HREE (LaN/YbN= 4.0-8.2), and none to moderate Eu anomalies (Eu/Eu*= 1.0-0.53) (Fig. 2h).

Fig. 1. Simplified geologic map of the Rondônia Tin Province (RTP) and adjacent areas (modified after Bettencourt et al., 1999). Fig. 1. Mapa geológico simplificado da província Estanífera de Rondônia (RTP) e áreas adjacentes (modificado de Bettencourt et al., 1999).

128 W. Leite Júnior et al. / Comunicações Geológicas (2014) 101, Especial I, 125-129

Fig. 2. TIS, SAIS, ACIS and SLCIS granites and associated rocks compositions plotted on granite classification diagrams of Frost et al. (2001) (a and b), Dall’Agnoll & Oliveira (2007) (c and d); Whalen et al. (1987) (e), Eby (1992) (f), Pearce (1996) (g), and chondrite-normalized rare earth elements composition (Boynton, 1984) (h). TIS= green triangles; SAIS= purple crosses; ACIS= red circles; SLCIS= blue X. Gray shadow in a) and b): A-type fields (Frost et al., 2001). Fig. 2. Composições de granitos e rochas associadas das SIT, SISA, SIAC e SISLC plotadas nos diagramas de classificação para granitos de Frost et al. (2001) (a e b), Dall’Agnol & Oliviera (2007) (c e d), Whalen et al. (1987) (e), Eby (1992) (f), Pearce (1996) (g), e composições dos elementos de terras raras normalizados para o valor do condrito (Boyton, 1984) (h). Triângulos verdes= SIT; cruzes roxas= SISA; círculos vermelhos= SIAC; X azuis= SISLC. Em cinza nas figuras a e b, campos dos granitos tipo A (Frost et al., 2001).

5. Discussion

Two main varieties of A-type or ferroan granitoids related to the development of the RSIP are recognized in the RTP, according to the Frost & Frost (2011) classification: alkalic metaluminous granitoids and alkali-calcic metaluminous granitoids with or without peraluminous granitoids. The first one includes the granitoids of the TIS, and the second one includes the granitoids of the SAIS, ACIS, and SLCIS.

The alkalic metaluminous granitoids of the TIS are mainly alkali-feldspar granites, although the rocks associated with them cover a wide range of silica content. Diorite is the most silica-poor rock of this suite and it grades to monzodiorite, monzonite, syenite, alkali feldspar syenite, and quartz-alkali feldspar syenite. The granites with highest silica contents (> 73 wt. %) show an alkali-calcic composition.

The alkali-calcic metaluminous granitoids and associated rocks of the SAIS, ACIS, and SLCIS show also a range of silica contents. Basic rock (diabase or gabbro) is the most silica-poor rock of the SAIS and SLCIS, indicating a bimodal character to these suites, whereas ferroan calc-alkalic charnockites, including the most silica-poor dioritic charnockites, are the associated rocks in the ACIS. High silica (> 75 wt. %) granites of the ACIS and SLCIS show, respectively, calc-alkalic and calc-alkalic peraluminous compositions. The latter ones are spatially related to the greisen-type tin deposits.

These suites show timing of crystallization (1.38-1.30 Ga) and tectonic setting consistent with an anorogenic

period and intraplate setting (TIS and SAIS: 1.38-1.37 Ga), and post-collision (ACIS: 1.35-1.34 Ga) and post-orogenic (SLCIS: 1.31-1.30 Ga) periods and intraplate setting related to the development of the Rondonian-San Ignacio orogeny (1.37-1.34 Ga) (Bettencourt et al., 2010a; Rizzotto et al., 2013).

These suites represent multiphase granitoid plutons. Some of them are bimodal in character, or include intermediate rocks as minor facies, synplutonic dykes, and enclaves and only one is clearly related to subvolcanic rocks and primary tin deposit. We believe that the alkalic metaluminous granitoids were formed by fractional crystallization of ferro-basalt parent magma, whereas the alkali-calcic metaluminous granitoids were crystallized from anatectic melts derived from quartzo-feldspathic sources in response to mafic underplating.

6. Conclusion

Alkalic metaluminous granitoids and alkali-calcic metaluminous granitoids are the main varieties of A-type or ferroan granitoids in the RTP that are timing related to the RSIP. They constitute multiphase granitoid plutons that show bimodal character or include rocks of intermediate composition. Only one pluton is clearly related to subvolcanic rocks and primary tin deposit. The alkalic metaluminous granitoids were formed by fractional crystallization of ferro-basalt parent magma, whereas the alkali-calcic metaluminous granitoids were crystallized from anatectic melts derived from quartzo-feldspathic sources in response to mafic underplating.

1.38-1.30 Ga A-type granites, Rondônia, Brazil 129

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