Sr-Nd-Hf isotopic constraints and U-Pb geochronology of the Arroio Pedrado Gneisses, Dom Feliciano Belt, Brazil: A 680 Ma shoshonitic event in the final stages of The Piratini Arc evolution

Abstract

In this work we present whole-rock and isotopic (Sr-Nd-Hf) chemistry data and zircon U-Pb geochronology from new occurrences of the Arroio Pedrado Gneisses, located near the Brazil/Uruguay border in the southeastern sector of the Dom Feliciano Belt. These monzogranitic orthogneisses are intermediate, metaluminous, with SiO2 concentration ranging from 59.22 to 65.37 wt%, Na2O/K2O ratios < 1, and Ce/Yb and Th/Yb ratios typical of shoshonitic associations. The studied samples are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE), resulting in a high LILE/HFSE ratios (LaN/LuN ratios between 17 and 33). These features, together with significant negative Ta and Nb anomalies and low Y and Yb concentrations, fit within the characteristics of subduction-related magmatism. The whole-rock isotopic study demonstrates that the samples have intermediate 87Sr/86Sr(i) ratios (0.7095–0.7112), negative εNd(t) values (−3.7 to −4.2) and Nd model ages of 1.48–1.61 Ga. The studied zircons have a wide range of εHf(t) values (−13.44 to −15.16), suggesting a crustal influence in their generation. The geochemistry and isotope characteristics indicate that these shoshonites are derived from the partial melting of a thickened lower crust, which was affected by subduction-related fluids/melts. This magma underwent significant fractional crystallization – K-feldspar, plagioclase, biotite, and amphibole, and accessory minerals such as allanite and Fe-Ti oxides – in oxidative conditions during magma ascent. The LA-ICP/MS U-Pb zircon dating reveals that the Arroio Pedrado Gneisses were emplaced during the Cryogenian, as suggested by a concordia age of 680 ± 2.9 Ma interpreted as the igneous protolith crystallization age. The metamorphic age of 660 ± 2.8 Ma (zircon rims) is interpreted as the amphibolite-facies peak which marks an early collisional event during the convergence of the Río de la Plata Craton and African-related terranes. The Arroio Pedrado Gneisses are here interpreted as representing the latest magmatic event in the construction of the allochtonous Andean-type Piratini Arc, whose evolution is constrained between ca. 780 (initial magmatism) and 650 Ma (continental collision) during the West Gondwana assembly.