THE MESOPROTEROZOIC VOLCANO-SEDIMENTARY SERRA DO ITABERABA GROUP OF THE CENTRAL RIBEIRA BELT, SÃO PAULO STATE, BRAZIL: IMPLICATIONS FOR THE AGE OF THE OVERLYING SÃO ROQUE GROUP

Abstract

One of the fundamental problems to understanding the evolution of volcano-sedimentary sequences in southeastern Brazil is constraining their depositional ages. Brasiliano tectonic and metamorphic overprinting either obscured or destroyed primary features, such as unconformities, as well as other geologic relationships. This problem is exemplified by the Serra do Itaberaba and São Roque groups, where the lack of data about the timing of their deposition has prevented resolution of proposed one- and two-stage geotectonic/depositional models. Recent U-Pb zircon data obtained from metavolcanic rocks in the São Roque Group indicate that it was deposited between 628 and 607 Ma. New U-Pb zircon data of 1395 + 10 Ma for a metandesite in the basal Morro da Pedra Preta Formation (Serra do Itaberaba Group) indicate the maximum age for the beginning of the deposition of the pelites overlying MORB-like basalt. A metarhyolite of the upper unit, the Nhanguçu Formation, contains two zircon populations. One yielded an age of 619 + 3 Ma, which defines the crystallization age of the rock, and the other an age of 1449 + 3 Ma, interpreted as inherited xenocrystal grains from older units of the Serra do Itaberaba Group. The younger metarhyolite was affected only by the S 2 foliation, generated during the Brasiliano orogenesis, whereas the Middle Proterozoic metavolcano-sedimentary sequence records additional metamorphic and deformational events, confirming the presence of two different geotectonic cycles.