Friday, April 15, 2016

Evidence From Brazil of the PaleoProterozoic Paranapanema Paleocontinent

U–Pb age of detrital zircon from the Embu sequence, Ribeira belt, Se Brazil


Duffles et al


The Embu Complex crops out in the southeastern part of São Paulo state in Brazil and consists mainly of Neoproterozoic metasedimentary rocks belonging to the interference zone between the Brasilia and Ribeira fold belts. We present U–Pb ages (LA-IPCMS) supported by cathodoluminescence images for detrital zircon from these rocks in order to constrain the formation and evolution of the source area of this complex.

In this paper we choose to individualize within the Embu Complex a Paleoproterozoic basement, a metasedimentary sequence, the Embu Sequence, and Neoproterozoic intrusive bodies, now largely orthogneisses. Six distinct rock types were recognized in the Embu Sequence: biotite gneiss, biotite schist, quartzite, amphibolite, calc-silicate rock and talc schist. The sequence is strongly deformed by several deformation phases and metamorphosed into amphibolite facies. An important shear zone, the Buquira Shear zone, separates the Embu Complex from the Socorro-Guaxupé Nappe, to the north, considered to be part of the upper plate of the southern Brasília belt collision. However, our mapping showed that the shear zone cuts obliquely into the Embu Complex, diminishing considerably its regional importance. Three samples were separated for dating, a schist north of the shear zone and two quartzites south of the shear zone. The results show that the ages of detrital zircon of these samples are mainly Paleoproterozoic, with a minor Archean component. The youngest igneous zircon grain from the schist produced an age of about 1000 Ma, indicating the maximum sedimentation age for the sequence. No significant difference appears between the samples north and south of the shear zone, reforcing the interpretation that this zone is only of local importance. Important metamorphic overgrowth of zircon in the age range 670–640 is present in all three samples; it is interpreted as related to the installment of a continental magmatic arc in the Socorro-Guaxupé Nappe and in the Embu Complex. A second metamorphic pulse between 621 and 599 Ma is only apparent in one sample. According to the literature this was the period of continental collision between the Paranapanema and São Francisco paleocontinents leading to the formation of the southern Brasília belt. Possibly the fact that the analyzed samples are localized some 30 km away from the suture, in the upper plate, explains that the increase in temperature during the collision in this area was insufficient to grow significant new zircon. A third metamorphic pulse with ages of 576–574 Ma, possibly related to collision in the Ribeira belt, is only detected in three analyses of metamorphic rims in the schist sample. Earlier reported ages of about 790 Ma, both for igneous crystallization and metamorphism in a more westerly part of the Embu Complex, are only recognized in one analysis of metamorphic overgrowth (794 ± 24 Ma) from the schist sample, and two probably hybrid analyses of 830 ± 19 Ma and 767 ± 14 Ma from the northern quartzite sample. It is concluded that the results show that the detritic zircon is probably derived from the Paranapanema Paleocontinent.

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