Pre-collisional, Tonian (ca. 790 Ma) continental arc magmatism in southern Mantiqueira Province, Brazil: Geochemical and isotopic constraints from the Várzea do Capivarita Complex

Abstract

This paper focuses on the pre-collisional mature arc magmatism (ca. 790 Ma) recorded in orthogneisses from the Várzea do Capivarita Complex (VCC), southern Mantiqueira Province (PM), Brazil. The complex comprises ortho- and paragneisses tectonically interleaved during a transpressive high grade regime (ca. 650 Ma), possibly related to oblique collision. The VCC orthogneisses are metaluminous to peraluminous calc-alkaline rocks, with high 87Sr/86Sr(i) ratios from 0.71628 to 0.72509 and εNd(790) values from −7.19 to −10.06. The VCC magmatism is correlated with other ca. 800 Ma arc sequences from southern PM, as the Porongos Metamorphic Complex (PMC) metavolcanic rocks and the orthogneisses from Cerro Bori (CB), Uruguay. All associations show signatures typical of accretionary orogens, TDM and Meso to Paleoproteroic inheritance ages, and strong evidence of crustal assimilation/contamination. Their high K contents, and the tendency to move toward the post-collisional field in geotectonic diagrams suggest that they were generated in thick-crust, mature arc environments. In contrast, the CB sequence exhibits a less mature continental-arc character, suggestive of thinner crust or shorter distance to the active margin. VCC and CB orthogneisses, and part of the PMC metavolcanic rocks may be interpreted as part of the same magmatism, or at least as fragments of similar magmatic arcs. However, VCC magmatism is distinct from continental arc sequences in the São Gabriel Block (ca. 700–750 Ma). Isotope signatures for this younger magmatism indicate major contribution of Neoproterozoic juvenile sources, with only little amounts of reworked, old continental crust. Geochemical and Sr-Nd signatures presented in this paper suggest that at least part of the PMC metavolcanic rocks are the protoliths of the VCC orthogneisses. This, together with the isotope evidence of similarity between VCC and PMC igneous and sedimentary fractions, corroborates the hypothesis that the VCC and PMC are partly expressions of the same context, wherein the magmatic and sedimentary activity occurred in a single continental arc environment. Binary mixing models point to evolved EM II mantle reservoir as source for the VCC magmatism. A Paleoproterozoic TTG association (ca. 2.0 Ga) from the Arroio dos Ratos Complex (ARC) seems to be the main crustal contaminant of the VCC magmatism. Together with the small inheritance contribution at ca. 2.0, this suggests that the crust melted at ca. 790–800 Ma was predominantly like the ARC. Our models also indicate that in addition to crustal assimilation processes, fractional crystallization was important during the VCC magmatism evolution.