INDEX
ST4 - 12


Two attempts and one success - Meso-Neoproterozoic lithospheric extensional events in the São Francisco Craton



Tupinambá, M.1,2; Machado, N.2; Ragatky, D.1

 
1. Tektos - Geotectonic Research Group, Faculty of Geology, Rio de Janeiro State University, Brazil. tupi@uerj.br , ragatki@uerj.br

2. GEOTOP, Université du Quebec à Montréal, Canadá. machado.nuno@uqam.ca

 


 

Abstract


Crystallization and inherited U-Pb zircon ages between 1120 and 815 Ma from the São Francisco Craton and the surrounding Mantiqueira and Tocantins Provinces are distributed into three time spans. 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: Craton, Neoproterozoic, Mesoproterozoic

 


Its is expected that during the life time of a supercontinent, asthenospheric thermal circulation induces extensional processes that lead to final break-up. Therefore, fragments of broken supercontinents usually contain products of earlier extensional events and also of final break-up processes. Western Gondwana was formed by fragments of ancient Rodinia Supercontinent, namely the Amazon, São Francisco/Congo, Kalahari, Western Africa cratons and others of uncertain affinity as the Paraná (or Paranapanema) Block and smaller terranes. As a remnant of Rodinia, the São Francisco Craton contains rocks and structures related to Rodinia's lithospheric extension. In this paper, we present a compilation of U-Pb crystallization and inherited ages (and some Ar-Ar ages) from the São Francisco Craton and the Tocantins and Mantiqueira provinces between 1120 and 815 Ma (Fig. 1). The ages of the geological processes are grouped into three extensional events with distinct characteristics.

 

EVENT I (1120-1000 Ma) - The first basic magmatic event happened at the end of the Mesoproterozoic. Basic dikes were injected along the São-Francisco/Congo and Kalahari cratons between 1120 to 1000 Ma. In the São Francisco Craton, 1078±18 Ma and 1011±24 Ma Ar-Ar basic dikes were intruded into Paleoproterozoic granulites at Olivença and Ilhéus, respectively (Renne et al.,


1990). Probable correlative magmatic reservoirs for this magmatism were the intraplate anorogenic mafic-ultramafic layered complexes of Ipanema (1104±78 Ma) and Alvarenga (1035±160 Ma), both intruded into the Juiz de Fora paleoproterozoic granulites (Angeli et al., 2004). Basic intrusive magmatism is known at the same interval at the  Congo Craton in the Chela and Nosib groups (Renne et al., 1990), mafic magmatism at Sembe-Ouesso and Comba (Congo) and Ebolowa (Cameroon) (Correa-Gomes & Oliveira, 2000). In the Kalahari Craton, Renne et al. (1990) reported 1000 ± 20 Ma Ar-Ar magnetization ages in the Namaqua Province and compiled a data set of similar ages from five other provinces. At least in two of these sites the basic volcanism was followed by intrusive A-type peralkaline magmatism: the 998±6 Ma Noqui Granite of the West Congo Belt (Tack et al., 2001) and the 875±9 Ma Salto da Divisa Pluton in the Paleoproterozoic reworked basin in the Araçuaí Belt (Silva et al., 2002). Detrital zircons with ages in the 1120 to 1000 Ma range are present in several units of the São Francisco Paleocontinent passive margin: a) the Furnas Quartzite (Valeriano et al., 2004) and a feldspar garnet biotite schist (Piuzana et al., 2003) from the Araxá Group; b) quartzite from the Andrelândia Group at the interference zone between Brasília and Ribeira belts (Valeriano et al., 2004); c) quartzite from the Chapada do Acauã Formation


 


 

Figure 1: Ages of main geological events in the São Francisco Craton and Mantiqueira and Tocantins provinces between 1130 and 750 Ma. Basic magmatism (B): 1.layered mafic and ultramafic complexes; 2. volcanism

 (b-mafic; f-felsic); 3.mafic dykes and sills; Granitoids: 4. S-type; 5.I-type; 6.A-type;7.regional

metamorphism; 8.inherited zircon grains in plutonic rocks;9.detrital zircon grains in

metassedimentary rocks;10.error bar.Bibliographic references in the text.


 

 

 


(Pedrosa-Soares et al., 2000) and Jequitaí Formation (Butchwald et al., 1999) in the Araçuaí Belt; d) Palmital Sucession at the Búzios Domain of the Ribeira Belt (Schmitt et al., 2004). It is noteworthy that zircons from the Araxá Group schists present magmatic sector zoning that are typical of mafic rocks (Piuzana et al., 2003). It is possible to divide this event into two sub-events, based on the ages of the mafic magmatism: an older 1100-1070 and a younger 1030-1000 Ma sub-events. Metasediments from the Araxá and Andrelândia groups show detrital ages from both episodes and for the whole time interval of the Event I. The Macaúbas Group metasediments contain only detrital zircons from sources 1030-1000 Ma old.

 

EVENT II (930-900 Ma) - The second episode of basic magmatism ocurred during the Neoproterozoic Tonian Period in the São Francisco and Congo cratons between 930 and 900 Ma. In the West Congo Belt, Gangila continental flood basalts were produced between 930 and 920 Ma, while Mayumbian Group rhyolites and associated granites were emplaced at the same time (Tack et al., 2001). In the interior of the São Francisco craton, the Salvador basic dikes yielded an age of 924 Ma (Heaman, 1991). On the western São Francisco paleocontinent passive margin (Brasília Belt) metarhyolites from the Arenópolis Sequence yielded an age of 929±8 Ma (Pimentel et al., 2000) and an amphibolite of the Araxá Group yielded an older age, but with a larger error (959+46-45 Ma, Valeriano et al., 2004). To the interior of the São Francisco Craton, the Espinhaço Supergroup was injected by 906±2 Ma basic sills and dykes (Machado et al., 1989). Ages in the 930 to 900 Ma range for detrital zircons are only known in the Macaúbas Group of the Araçuaí Belt (Pedrosa-Soares et al., 2000) and are practically absent in other metasedimentary groups in the Brasília and Ribeira Belts (Valeriano et al., 2004; Piuzana et al., 2003; Schmitt et al., 2004). The presence of rocks of this event in the basement under the metassedimentary cover of the Araçuaí Belt is also revealed by inherited zircon grains of 938 and 913 Ma in younger Guarataia and São Vitor I-type granitoid plutons (Noce et al., 2000).

 

EVENT III (850-815 Ma) - During the Neoproterozoic Cryogenian Period basic magmatism affected the São Francisco Craton's passive margins between 850 and 815 Ma. It was registered at the Araçuaí Belt (Ribeirão da Folha mafic and ultramafic rocks, 816±72 Ma, Pedrosa-Soares et al., 1998), Ribeira Belt (848±11 Ma, amphibolite, Italva Klippe, Heilbron & Machado, 2003) and Brasília Belt (Bonfinópolis amphibolite, 838±20 Ma, Piuzana et al., 2003). In contrast with from the older events, this magmatism is not found in cratonic areas. Also, it was reported only on volcanic rocks, mainly MORB-type (Pedrosa-Soares et al., 1998, Piuzana et al., 2003). The predominance of oceanic rocks and the absence of continental magmatic rocks during this event are indicated by the lack of 850 to 815 Ma detrital zircons in the sedimentary basins. The exception is the Palmital Sucession at the Búzios Domain of the Ribeira Belt (Schmitt et al., 2004) that contains zircons of 817 to 798 Ma.

The Event III is probably related to the oceanic phase of Rodinia breakup, when the Goianides Ocean evolved between the São Francisco/Congo and Amazon Paleoplates and  the Adamastor Ocean formed between Kalahari and São Francisco/Congo Paleoplates. This event is also coeval with the evolution of several volcanic island arcs along active margins of the Goianides and Adamastor oceans. In the Brasília Belt, the Goiás Magmatic Arc was active since 862 Ma, and there are even older metavolcanic rocks of 899 Ma age (Pimentel et al., 2000). Metaluminous plutonites of presumably island-arc roots are also present at the Ribeira Belt on 811 Ma (tonalitic gneiss, Embu Domain, Cordani et al., 2002) and 792 Ma (Rio Negro Complex, Heilbron & Machado, 2003). Assimilation of rocks from Event III is testified by the presence of 808 and 845 Ma inherited zircons in younger Rio Negro Complex tonalites in the Ribeira Belt (Heilbron & Machado, 2003) and 839±9 Ma in the Brasilândia I-type Granite Pluton at the Araçuaí Belt (Noce et al., 2000).

 

DISCUSSION - Event I was widespread in cratonic areas (São Francisco, Congo and Kalahari), but volcanic rocks with this age are not known in Mantiqueira and Tocantins provinces. Detrital zircons are found in Araxá and Andrelândia Groups, and could result from the erosion of Event I's intrusive and extrusive rocks in cratonic areas. Event II was also registered at the São Francisco cratonic area, but also corresponds to: a) older plutonic and volcanic rocks of the Goiás Magmatic Arc; b) continental mafic and felsic volcanism at the West Congo Belt. From the rock types of the first two events (continental tholeiitic basalts and anorogenic granites) and the absence of these rocks in the Neoproterozoic sedimentary basins (present only as detrital products) it is possible to establish that they represent stages of lithospheric extension without formation of oceanic crust. They are possibly related to lithospheric extension and thinning along different trends that originates from a triple junction (Correa-Gomes et al., 2000, Martins-Neto, 1998). The final extension was not reached until the third event (850-815 Ma), with extrusion of MORB-type tholeiitic basalts. This last event could be related to ocean opening (Goianides and Adamastor Ocean). Alternatively, the presence of island arc volcanism during the same event could mean that those basalts could have been extruded in back-arc basins.

 

REFERENCES

 Angeli, N., Teixeira, W., Heaman, L., Moore, M., Fleet, M.E., Sato, K. 2004. Geochronology of the Ipanema layered mafic-ultramafic Complex, Minas Gerais, Brazil: evidence of extension at the Meso-Neoproterozoic time boundary. Int. Geol. Rev. 46:730-744.

Butchwaldt, R., Toulkeridis, T., Babinsky, M., Noce, C.M., Martins-Neto, M., Hecos, C.M. 1999. Age determination and age related provenance analysis of the Proterozoic glaciation event in central eastern Brazil. II South Am. Symp. Isotope Geol. Cordoba, p. 387-390.

Correa-Gomes, L. C., Oliveira, E. P. 2000. Radiating 1.0 Ga mafic dyke swarms of eastern Brazil an western Africa: evidence of post-assembly extension in the Rodinia Supercontinent? Gondwana Research, 3(3):325-332.

Heaman, L. 1991. U-Pb dating of giant radiating dyke swarms: potential for global correlation of mafic events. Intern. Symp. on mafic dykes. São Paulo, Brazil, ext. abst:7-9.

Heilbron, M. & Machado, N. 2003. Timing of terrane accretion in the Neoproterozoic-Eopaleozoic Ribeira orogen (se Brazil). Prec. Res. 125 (1-2): 87-112.

Machado, N.; Schrank, A.; Abreu, F.R.; Knauer, L.G.; Abreu, P.A. 1989. Resultados preliminares da geocronologia U-Pb na Serra do Espinhaço Meridional. In: An. 5º SGMG, Belo Horizonte, p.171-174.

Martins-Neto, M. 1998. Mantle plume, rifting and the Early Neoproterozoic glaciation in the São Francisco Craton and Araçuaí fold belt, southeastern Brazil. Int. Conf. on Basement Tectonics, 14. Ouro Preto. Abstracts: 32-34.

Noce, C.M., Macambira, M.J.B., Pedrosa Soares, A.C. 2000. Chronology of Neoproterozoic-Cambrian granitic magmatism in the Araçuaí Belt, eastern Brazil, based on single zircon evaporation dating. Rev. Bras. Geoc., 30(1):25-29.

Pedrosa-Soares, A.C., Vidal, P., Leonardos, O.H., Brito Neves, B.B. 1998. Neoproterozoic oceanic remnants in eastern Brazil: further evidence and refutation of an exclusively ensialic evolution for the Araçuai-West Congo Orogen. Geology, 26(6):519-522.

Pedrosa-Soares, A.C., Wiedemann-Leonardos, C.M. 2000. Evolution of the Araçuaí Belt and its connection to the Ribeira Belt, eastern Brazil. in Cordani, U.;Milani, E.J.; Thomaz Filho, A. & Campos, D. A. (ed.) - 2000 - Tectonic Evolution of South America, Rio de Janeiro, 31st International Geological Congress, 195-230.

Pimentel, M.M., Fuck, R.A., Jost, H., Ferreira Filho, C.F., Araujo, S.M. 2000. The basement of the Brasilia fold belt and the Goiás Magmatic Arc. In Cordani, U.G., Milani, E., Thomaz Filho, A., Campos, D.A. Tectonic evolution of South America. Rio de Janeiro, 2000: 195-229.

Piuzana, S., Pimentel, M.M., Fuck, R.A., Armstrong, R. 2003. SHRIMP U-Pb and Sm-Nd data for the Araxá Group and associated magmatic rocks: constraints for the age of sedimentation an geodynamic context of the southern Brasília Belt, central Brazil. Prec. Res 125:139-160.

Renne, P., Onstott, T.C., D'Agrella Filho, M.S., Pacca, I.G., Teixeira, W. 1990. 40Ar/39Ar dating of 1.0-1.1 Ga magnetizations from the São Francisco and Kalahari cratons: tectonic implications for Pan-African and Brasiliano mobile belts. Earth Plan. Sci. Lett. 101:349-366.

Schmitt, R.S., Trouw, R.J.A., Van Schmus, W.R., Pimentel, M.M. 2004. Late amalgamation in the central part of the West Gondwana: new geochronological data and the characterization of a Cambrian collisional orogeny in the Ribeira Belt (SE Brazil). Prec. Res. 133:29-61.

Silva, L.C., Armstrong, R., Noce, C.M., Carneiro, M.A., Pimentel, M., Pedrosa-Soares, A.C., Leite, C.A., Vieira, V.S., Silva, M.A., Paes, V.J.C., Filho, J.M.C. 2002. Reavaliação da evolução geológica em terrenos pré-cambrianos brasileiros com base em novos dados U-Pb SHRIMP, parte II: orógeno Araçuaí, Cinturão Mineiro e Cráton São Francisco Meridional. Rev. Bras. Geoc. 32(4): 513-528.

Tack, L., Wingate, M.T.D., Liegeos, J.P. Fernandez-Alonso,  M; Deblond, A. 2001. Early Neoproterozoic magmatism (1000-910 Ma) of the Zadinian and Mayumbian groups (Bas Congo): onset of Rodinia rifting at the west edge of the Congo Craton. Prec. Res. 110:277-306.

Valeriano, C.M., Machado, N., Simonetti, A., Valladares, C., Seer, H.J., Simões, S.A. 2004. U-Pb geochronology of the southern Brasília belt (SE-Brazil): sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana. Prec. Res. 130:27-55.