Metaconglomerados e rochas associadas do Grupo São Roque a norte da cidade de São Paulo, Brasil

Abstract

Resumo O Grupo São Roque caracteriza-se por rochas depositadas em ambiente marinho com atividade vulcânica submarina, as ocorrências da Formação Morro Doce são dominadas por metarcóseos e metarenitos feldspáticos com expressivas lentes de metaconglomerados, que formam uma seqüência considerada como unidade basal do Grupo São Roque. Rochas metavulcânicas ácidas e básicas intercaladas nesta seqüência constituem importante marcador tectônico e geocronológico, e foram usadas para determinar sua idade de deposição (1.75-1.79 Ga). O estudo petrográfico dos clastos graníticos dos metaconglomerados permitiu a identificação de quatro variedades petrográficas: biotita monzogranito porfirítico, monzogranito inequigranular, monzogranito equigranular e leucogranito inequigranular. O caráter comagmático entre os clastos é confirmado pelos dados geoquímicos e geocronológicos. Rochas metavulcânicas ácidas que ocorrem intercaladas a metarcóseos e metaconglomerados, na região do Morro do Polvilho, caracterizam-se por meta-traquidacitos e meta-riolitos porfiríticos. Com características geoquímicas típicas de magmatismo intraplaca, em especial baixo mg# (~20), altos teores de Zr (560-730 ppm), Y e Nb, além de baixo Sr (70-120 ppm), as rochas metavulcânicas ácidas do Grupo São Roque apresentam similaridades com as metavulcânicas ácidas da base do Supergrupo Espinhaço. Por outro lado, o corpo anfibolítico do Jaraguá, de idade ainda desconhecida, parece ser intrusivo na Formação Morro Doce tem características geoquímicas mais próximas de magmatismo de fundo oceânico, com baixas concentrações de elementos incompatíveis como Rb, Nb e Th, o que as aproximam das rochas metabásicas supracrustais da região de Pirapora do Bom Jesus. Palavras-chave:Grupo São Roque, metaconglomerados, ambientes tectônicos. Abstract Metacoglomerates and associated rocks of the São Roque Group, northern São Paulo city, Brazil.The São Roque Group is composed of rocks deposited in marine environment with coeval volcanic activity, the Morro Doce Formation is dominated by meta-arkose and feldspatic meta-sandstone with expressive metaconglomeratic lenses, which form a sequence regarded as the basal unit of São Roque Group. Metavolcanic acidic and basic rocks interspersed in this sequence are an important tectonic and geochronologic marker (1.75-1.79 Ga). The petrographic study of the granite pebbles from the Morro Doce Formation metaconglomerates allowed the identification of four petrographic varieties: porphyritic biotite monzogranite, inequigranular monzogranite, equigranular monzogranite and inequigranular leucogranite. The comagmatic character of these pebbles is confirmed by petrographic and geochemical data. Acid metavolcanic rocks interlayered with meta-arkose and metaconglomerates in the Morro do Polvilho region correspond to trachydacite and porphyritic meta-rhyolite. The meta-arkose shows geochemical affinities with metaconglomerate granitic pebbles, and differs from the acid metavolcanic rocks both in their geochemical signature and in its sedimentary fab...

Paleoproterozoic source contributions to the São Roque Group sedimentation: LA-MC-ICPMS U-Pb dating and Sm-Nd systematics of clasts from metaconglomerates of the Boturuna Formation

“…Given a 2.2 Ga magmatic crystallization age, Ɛ Nd(t) values are all negative and appear to cluster between values from -3.1 to -4.3; an exception is the inequigranular granite sample MD-01D that is characterized by a more negative value of -6.7. The metarkose framework is slightly less negative (Ɛ Nd(t) = -2.3), suggesting contribution from a distinct source, perhaps of mafic character, a feature also suggested by whole-rock chemistry (Henrique-Pinto and Janasi, 2010). Interestingly, the amphibolite clast does not seem to equate to this same source since its Ɛ Nd(t) (-3.6) at t = 2.2 Ga is within the same range for most of the granite clasts.…”

“…small lenses of basaltic trachyandesite within the metarkoses), and porphyritic meta-trachydacite (in the Morro do Polvilho region). The meta-trachydacites define a chemical signature typical of within-plate magmatism, with low mg#, high Zr, Y, Nb, and low Sr (Henrique-Pinto and Janasi, 2010).…”

“…The SRG itself is considered by some authors (Juliani, 1993;Martin, 2000) as a younger sequence, in view of its lower-grade metamorphism and proposed erosional contact marked by the presence, in metaconglomerates from its basal sequence (Boturuna Formation), of clasts and volcanic fragments, which are correlated to the Serra do Itaberaba Group (SIG). However, U-Pb zircon dating of acid and basic metavolcanic rocks from the Boturuna Formation has yielded consistently older ages (1790 ± 14 Ma; van Schmus et al, 1986;1750 ± 40 Ma;Oliveira et al, 2008), strongly suggesting that SRG in fact corresponds, at least in part, to an older sequence (Henrique-Pinto and Janasi, 2010).…”

Provenance and sedimentary environments of the Proterozoic São Roque Group, SE-Brazil: Contributions from petrography, geochemistry and Sm–Nd isotopic systematics of metasedimentary rocks

“…Our previous works (Henrique-Pinto and Janasi, 2010;Henrique-Pinto et al, 2012) investigated the paleoenvironment and provenance of metaconglomerates and associated metarkoses from exposures of the lower stratigraphic unit (Boturuna Formation) in a restricted area at the NW of the city of São Paulo. Here we use petrography, geochemistry and SmeNd isotope geochemistry as main instruments to extend that study to a broader area that encompasses a large portion of the SRD exposures, including samples of metasandstones.…”

Integrated geological map of the São Roque Domain, North of São Paulo City – Brazil

“…This work presents an integration of the previous maps in the western portion of the São Roque Domain (Main Map), using updated stratigraphic and geochronological data, and aided by some new field observations by the authors Henrique-Pinto & Janasi, 2010).…”