Zircon U-Pb geochronology, Sm-Nd and Pb-Pb isotope systematics of Ediacaran post-collisional high-silica Acampamento Velho volcanism at the Tupanci area, NW of the Sul-Rio-Grandense Shield, Brazil

Carlos Augusto Sommer,Evandro Fernandes De Lima,Carla Joana Santos Barreto,Miguel Ângelo Stipp Basei,Ruy Paulo Philipp,Vinicius Matté,Felipe Padilha Leitzke,Rommulo Vieira Conceição,Jean Michel Lafon

doi:10.1590/2317-4889201720170064

Abstract

ABSTRACT: We present new U-Pb zircon ages and Sm-Nd-Pb isotopic data for volcanic and hypabyssal acid rocks from the northernmost exposure of the Acampamento Velho Formation in the NW portion of the Sul-Rio-Grandense Shield, Brazil. The first volcanic episode, grouped in the high-Ti rhyolites from the Tupanci hill, shows age of 579 ± 5.6 Ma, which is in agreement with the post-collisional Acampamento Velho Formation volcanism in the Bom Jardim Group of the Camaquã Basin. A poorly constrained age of 558 +/- 39 Ma was obtained for rhyolites from the low-Ti group at the Picados Hill, which may indicate a younger acid volcanism, or a greater time span for the volcanism of the Acampamento Velho Formation in southernmost Brazil. Regarding magmatic sources, Sm/Nd isotopic data coupled to Pb isotopes and a review of trace element geochemistry indicate different amounts of Paleoproterozoic (Dom Feliciano, Pinheiro Machado Suite) to Neoproterozoic (Rio Vacacaí terrane) lower crust melting. Our data, coupled with literature data, contribute to a better understanding of the stratigraphic evolution for the Neoproterozoic post-collisional volcanic successions of the Camaquã Basin in the Sul-Rio-Grandense Shield.

Highlights

The Sul-Rio-Grandense Shield (SRGS) in southernmost Brazil is constituted by rocks related to the Brasiliano/PanAfrican orogenic cycle emplaced in a Paleoproterozoic metamorphic basement (Hartmann et al 1999, Soliani et al 2000, Nardi et al 2008)
New U-Pb ages obtained in zircon grains from high-Ti rhyolites from the Acampamento Velho Formation (AVF) at the NW portion of the SRGS indicate an event at 579.1 +/- 5.6 Ma and are in good agreement with reported ages for the time span of the AVF volcanism in the Bom Jardim Group of the Camaquã Basin (CB)
A second U-Pb age of 558 Ma was obtained for rhyolites from the low-Ti group at the Picados Hill, with a high uncertainty associated (+/- 39 Ma) probably due to the high degree of alteration of the zircon grains

Summary

Introduction

The Sul-Rio-Grandense Shield (SRGS) in southernmost Brazil is constituted by rocks related to the Brasiliano/PanAfrican orogenic cycle emplaced in a Paleoproterozoic metamorphic basement (Hartmann et al 1999, Soliani et al 2000, Nardi et al 2008) The magmatism in this era was developed in two main phases: arc-related environment, with ages from 850–700 Ma; and post-collisional environment (Liégeois 1998), with ages from 650–535 Ma, marked by magmatism along transcurrent shear zones (Fernandes et al 1995, Babinski et al 1997, Bitencourt & Nardi 2000, Chemale Jr. 2000, Hartmann et al 2000, 2007, Nardi & Bitencourt 2009). The sedimentary sequences in the Camaquã Basin evolve from shallow marine in the Maricá Formation to lacustrine and alluvial in the Bom Jardim and Camaquã groups (Paim et al 2000)

Results

Discussion

Conclusion