The Neoproterozoic Andrelândia group: Evolution from an intraplate continental margin to an early collisional basin south of the São francisco craton, Brazil

Abstract

New U–Pb and Lu–Hf zircon data for the Andrelândia Group are presented and discussed with the previously published data to propose an evolutionary model. The Andrelândia Group consists of two depositional sequences that record the geotectonic evolution from Tonian to Ediacaran times. Both sequences are part of the Andrelândia Nappe System, a fold-and-thrust system, generated in the 630-600 Ma interval, during the evolution of the Neoproterozoic convergence in the interference zone between the Southern Brasília and the Ribeira orogens, SE Brazil. The Andrelândia Group rocks record greenschist to granulite facies metamorphism related to this convergence. The lower sequence consists of paragneiss, metapelite, quartzite, metachert, calc-silicate and mafic rocks distributed in five lithofacies associations. They record shallow to deep water sedimentation coeval with mafic magmatism at the southern border of the São Francisco Craton. U–Pb data obtained from detrital zircon grains of deep water quartzites indicate a maximum depositional age of ca. 970 Ma. The U–Pb age peaks can be correlated to major events in the adjacent São Francisco paleocontinent. εHft isotopic signature obtained in this study shows a predominance of juvenile sources for the Mesoproterozoic and crustal reworking and juvenile additions for the Paleoproterozoic. This can be related to source rocks from the Espinhaço Supergroup and from the Mineiro Belt, respectively. We interpret our data together with previous published detrital zircon data and whole-rock εNdt signatures to suggest that the lower sequence of the Andrelândia Group is related to a passive margin of the São Francisco paleocontinent with a platformal and a deep water succession with roughly similar sources, indicating likely a resedimentation process. The upper sequence of the Andrelândia Group consists of a monotonous package of biotite schist interpreted as metamorphosed plagioclase sandstones and siltstones. The upper sequence covers the lower sequence units unconformably. Deposition occurred in a relatively deep and fast subsiding basin during the early collisional stage of the southern Brasília Orogen. Previously published U–Pb, Sm–Nd and Lu–Hf data for the upper sequence constrain likely sources in the southern São Francisco Craton and possibly in the early stages of the Neoproterozoic Socorro-Guaxupé arc, established at the eastern margin of the Paranapanema paleocontinent, which collided with the São Francisco paleocontinent during West Gondwana assembly.