Revista Brasileira de Geociências Miguel Tupinambá et al. 37(4 - suplemento): 87-91, dezembro de 2007

Meso-neoproterozoic lithospheric extensional events in the São Francisco Craton and its surrounding south American and African metamorphic belts: a compilation of U-Pb ages

Miguel Tupinambá1, Nuno Machado2 (In memoriam), Mônica Heilbron1 & Diana Ragatky1

Abstract Crystallization and inherited U-Pb zircon ages between 1120 and 815 Ma from basic rocks and related types the São Francisco craton and the surrounding Brasília, Ribeira, Araçuaí and West Congo belts are distributed into three extensional events. The first two events (1120-1000 and 930-900 Ma) are represented by mafic dykes and sills and restricted volcanic rocks. The third event (850-815 Ma) is characterized by extrusion of MORB-type tholeiitic basalts. The first two events are possibly related to lithospheric extension, and the last event could be related to ocean opening or extension in back-arc basins.

Keywords: São Francisco craton, Neoproterozoic, Mesoproterozoic.

Resumo Eventos distensionais meso-neoproterozóicos no Craton do São Francisco e seus cintu- rões metamorficos sul-americanos e africanos: uma compilação de idades U-Pb. Com base em ida- des U-Pb entre 1120 e 815 Ma de rochas básicas e tipos correlatos foram estabelecidos três eventos distensionais no Cráton do São Francisco e nas faixas Brasília, Ribeira, Araçuaí and West Congo. Os primeiros dois eventos (1120-1000 e 930-900 Ma) são representados por diques e sills máficos e rochas vulcânicas restritas. O terceiro evento (850-815 Ma) é caracterizado pela extrusão de basaltos toleíticos do tipo MORB. Os primeiros dois even- tos estão relacionados a processos de distensão da litosfera, enquanto que o último evento estaria relacionado à abertura oceânica ou distensão em bacias de back-arc.

Palavras-chave: Cráton do São Francisco, Neoproterozóico, Mesoproterozóico.

INTRODUCTION It is expected that during the to investigate extensional and intraplate processes, lifetime of a supercontinent, asthenospheric thermal we concentrate our compilation on basic plutonic and circulation induces extensional processes that lead to volcanic magmatic and anorogenic plutonic ages. As final break-up. Therefore, fragments of broken super- many of the cratonic and pericratonic areas are cov- continents usually contain products of earlier exten- ered by sediments, provenance and inherited magmatic sional and final break-up events. Western Gondwana U-Pb ages are also included. To complete the study we was probably formed by fragments of ancient Rodinia present some metamorphic and granitoid ages. Based Supercontinent, namely the Amazon, São Francisco/ on the overall data, three extensional events with dis- Congo, Kalahari, Western Africa cratons and others tinct characteristics are proposed and their tectonic of uncertain affinity as the Paraná (or Paranapanema) meaning is discussed. Block and smaller terranes (Fig. 1). As a Rodinia´s remnant, the São Francisco/ EXTENSIONAL EVENT I (1120-1000 Ma) The Congo craton contain rocks and structures related to its first extensional event took place at the end ofthe Mesoproterozoic lithospheric extension and break-up. Mesoproterozoic, during the Stenian period and is Until the Neoproterozoic Cryogenian period, it was a separated into two sub-events, based on the ages of portion of a larger continental paleoplate surrounded the mafic magmatism: an older 1100-1070 Ma and a by passive margins. Its western passive margins were younger 1030-1000 Ma sub-event. then tectonically inverted during the Ediacaran to Cam- In the craton´s interior, the older 1100-1070 brian Brasiliano/Pan African event to form the South- Ma sub-event is represented by the E-W Olivença ma- American Brasília, Ribeira and Araçuaí belts and the fic dykes swarm (Correa-Gomes & Oliveira 2000), in- African Congo belt (Fig. 1). truded into Paleoproterozoic granulites at 1078±18 Ma In this paper we present a compilation of U-Pb (Ar-Ar age, Renne et al. 1990) and the Ipanema mafic- crystallization and provenance ages (and some Ar-Ar ultramafic layered complex (1104±78 Ma Sm/Nd iso- and isochronic Sm-Nd ages) from the São Francisco/ chronic age, Angeli et al. 2004). Detrital zircons with Congo craton and its western belts (Fig. 2). In order ages within this range are present in the metassedimen-

1 - Tektos - Geotectonic Research Group, Faculty of Geology, Rio de Janeiro State University, Rio de Janeiro (RJ), Brazil. E-mail: diorito@ uol.com.br 2 - GEOTOP, Université du Quebec à Montréal, Montreal, Canada.

Arquivo digital disponível on-line no site www.sbgeo.org.br 87 Meso-neoproterozoic lithospheric extensional events in the São Francisco Craton and its surrounding south American and African metamorphic belts: a compilation of U-Pb ages

Figure 1 - Cratonic configuration in Western Gondwana. Tectonic vergence shown as black arrows. 1 - Brasilia Belt; 2 - Ribeira Belt; 3 - Araçuaí Belt; 4 - West Congo Belt.

tary rocks from the Brasília belt: the Furnas Quartzite EXTENSIONAL EVENT II (930-900 Ma) The (Valeriano et al. 2004) and a feldspar garnet biotite second extensional episode ocurred between 930 and schist (Piuzana et al. 2003) from the Araxá Group; a 900 Ma, during the Neoproterozoic Tonian Period. Its quartzite from the Andrelândia Group at the interference is registered in the interior of the São Francisco craton zone between Brasília and Ribeira belts (Valeriano et by: NW trending Salvador basic dikes Correa-Gomes al. 2004). It is noteworthy that zircons from the Araxá & Oliveira (2000) with a 924 Ma U-Pb baddleyite age Group schists present magmatic sector zoning that are (Heaman 1991); hypoabissal and intrusive mafic bod- typical of mafic rocks (Piuzanaet al. 2003). ies that crosscuts the Paleoproterozoic Minas and Es- The younger 1030-1000 Ma sub-event is regis- pinhaço supergroups at the Quadrilátero Ferrífero re- tered in the craton´s interior by the E-W trending Ilhéus gion (Silva et al. 1995); N60-70W tholeiitic dykes in mafic dykes swarm (Correa Gomes & Oliveira 2000) the southern part of the craton that do not crosscut the with an 1011±24 Ma Ar-Ar age (Renne et al. 1990) and Bambuí Group and yield a Sm-Nd isochronic age of the Alvarenga mafic layered complex (1035±160 Ma 984±110 Ma (Chaves 2001, Chaves & Correia-Neves Sm/Nd isochronic age, Paes 1999). In the Ribeira and 2005). Araçuaí belts, only detrital zircons from the younger At the Araçuaí Belt, the Espinhaço Supergroup sub-event were found: the Palmital Sucession at the was injected by E-W trending 906±2 Ma basic sills Búzios Domain of the Ribeira Belt (Schmitt et al. and dykes (Machado et al. 1989). In the Goiás Mag- 2004); quartzites from the Chapada do Acauã Forma- matic Arc at the Brasília Belt, metarhyolites from the tion (Pedrosa-Soares & Wiedemann-Leonardos 2000) basal Arenópolis Sequence yielded an age of 929±8 and Jequitaí Formation (Butchwald et al. 1999) from Ma (Pimentel et al. 2000) and an amphibolite of the the Macaúbas Group in the Araçuaí Belt. Araxá Group yielded an older age, but with a larger er- In the West Congo belt a transtensional rift ror (959+46-45 Ma, Valeriano et al. 2004). mechanism was active a this time, according to Tack Ages in the 930 to 900 Ma range for detrital et al. (2001) who investigated the peralkaline Noqui zircons are only known in the Macaúbas Group of the granite (999±7 Ma U-Pb zircon age) and coeval Pala- Araçuaí Belt (Pedrosa-Soares Wiedemann-Leonardos bala rhyolites of the Zadinian Group. These volcanic 2000) and are practically absent in other metasedimen- and plutonic rocks are located in the site suggested by tary groups in the Brasília and Ribeira Belts (Valeriano Correa-Gomes & Oliveira (1999, 2000) as the central et al. 2004, Piuzana et al. 2003, Schmitt et al. 2004). part of a giant triple junction with branches in the bra- The presence of rocks of this event in the basement un- zilian and african counterparts of the São Francisco/ der the metassedimentary cover of the Araçuaí Belt is Congo craton. also revealed by inherited zircon grains of 938 and 913 Ma in younger Guarataia and São Vitor I-type granitoid

88 Revista Brasileira de Geociências, volume 37 (4 - suplemento), 2007 Miguel Tupinambá et al.

Figure 2 - Ages of main extensional events in the São Francisco craton and Brasilia, Ribeira, Araçuaí and West Congo Belts between 1130 and 750 Ma. 1.layered mafic and ultramafic complexes; 2a. basic volcanism; 2b - felsic and acid volcanism; 3.mafic dykes and sills; Granitoids: 4.I-type; 5.A-type; 6. inherited zircon grains in plutonic rocks; 7.detrital zircon grains in metassedimentary rocks; 8.error bar. See text for details.

plutons (Noce et al. 2000). began with the extrusion of Palabala rhyolitic sills and The event was followed by the intrusion of flows. anorogenic plutonic rocks, as revealed by the 875±9 Ma Salto da Divisa Pluton intruded into the Paleopro- EXTENSIONAL EVENT III (850-815 Ma) During terozoic reworked basement of the Araçuaí Belt (Silva the Neoproterozoic Cryogenian Period basic magma- et al. 2002). In the West Congo Belt, Gangila conti- tism affected the São Francisco craton’s passive mar- nental flood basalts, Mayumbian rhyolites and coeval gins between 850 and 815 Ma. It was registered at the supracrustal microgranites were produced between Araçuaí Belt (Ribeirão da Folha mafic and ultramafic 928 and 900 Ma (Tack et al. 2001) in a large igneous rocks, 816±72 Ma, Pedrosa-Soares et al. 1998), Ribeira province generated in a transtensional rift system that Belt (848±11 Ma, amphibolite, Italva Klippe, Heilbron

Revista Brasileira de Geociências, volume 37 (4 - suplemento), 2007 89 Meso-neoproterozoic lithospheric extensional events in the São Francisco Craton and its surrounding south American and African metamorphic belts: a compilation of U-Pb ages

& Machado 2003) and Brasília Belt (Bonfinópolis am- are found in Araxá and Andrelândia Groups, and could phibolite, 838±20 Ma, Piuzana et al. 2003). In contrast result from the erosion of Event I’s intrusive and ex- with from the older events, this magmatism is not found trusive rocks in cratonic areas. The absence of mafic in cratonic areas. It is reported only on volcanic rocks, igneous rocks of this age in the surrounding belts could mainly MORB-type (Pedrosa-Soares et al. 1998, Piu- mean that these crustal regions did not represent exten- zana et al. 2003). The predominance of oceanic rocks sional passive margins at this time. and the absence of continental magmatic rocks during Event II is registered at the São Francisco cra- this event are indicated by the relatively lack of 850 tonic area, Goiás Magmatic Arc and West Congo Belt, to 815 Ma detrital zircons in the sedimentary basins. but 930-900 Ma mafic dikes are not known in the Neo- The exception is the Palmital Sucession at the Búzios proterozoic sedimentary basins in the Brasília, Araçuaí Domain of the Ribeira Belt (Schmitt et al. 2004) and or Ribeira belts. the Canastra quartzite in the Brasília Belt (Valeriano et From the rock types of the first two events al. 2004). (continental tholeiitic basalts and anorogenic granites) The Event III is probably related to the oceanic and the absence of these rocks in the Neoproterozoic phase of Rodinia breakup, when the Goianides Ocean sedimentary basins (present only as detrital products) evolved between the São Francisco/Congo and Amazon it is possible to stablish that they represent stages of Paleoplates and the Adamastor Ocean formed between lithospheric extension (including rift basin formation) Kalahari and São Francisco/Congo Paleoplates. This without formation of oceanic crust. They are possibly event is also coeval with the evolution of several vol- related to lithospheric extension and thinning along canic island arcs along active margins of the Goianides different trends that originates from a triple junction and Adamastor oceans. In the Brasília Belt, the Goiás (Correa-Gomes et al. 2000, Martins-Neto 1998). These Magmatic Arc was active since 862 Ma, and there are events (especially Event II) were probably related to even older metavolcanic rocks of 899 Ma age (Pimentel the rift stage of the Araçuaí basin (Uhlein et al. 1995, et al. 2000). Metaluminous plutonites of presumably Trompette 1994) that was later inverted during the island-arc roots are also present at the Ribeira Belt on Brasiliano Event. 811 Ma (tonalitic gneiss, Embu Domain, Cordani et al. The final extension was not reached until the 2002) and 792 Ma (Rio Negro Complex, Heilbron & Event III (850-815 Ma), with extrusion of MORB-type Machado 2003). Assimilation of rocks from Event III tholeiitic basalts. This last event could be related to is testified by the presence of 808 and 845 Ma inherited ocean opening (Goianides and Adamastor Ocean). Al- zircons in younger Rio Negro Complex tonalites in the ternatively, the presence of island arc volcanism during Ribeira Belt (Heilbron & Machado 2003) and 839±9 the same event could mean that those basalts could Ma in the Brasilândia I-type Granite Pluton at the Ara- have been extruded in back-arc basins. çuaí Belt (Noce et al. 2000). Acknowlegdments The authors thank the organizing DISCUSSION Records of the Event I are widespread comitee of the III Simpósio do Cráton do São Fran- in cratonic areas (São Francisco, Congo and even in cisco held in Salvador in 2005 for the opportunity to Kalahari craton, according to Renne et al. 1990), but expose and discuss relevant items about the evolution volcanic or plutonic rocks of this age are not known of cratonic areas and surrounding metamorphic belts. in the surrounding metamorphic belts. Detrital zircons The first author thanks CAPES-Brazil for finantial sup- port during his pos-doctoral stage in Canada.

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