U-Pb (Zircon) ages of metavolcanic rocks from the Itaiacoca Group: tectonic implications

Abstract

The main aim of this work is to present and discuss the U-Pb ages obtained for zircon grains from metavolcanic rocks of the Itaiacoca Group. The Itaiacoca Group is a metavolcano-sedimentary sequence, which occurs as a narrow belt between the Cunhaporanga granitic batholith to the northwest and the Itapirapuã shear zone to the south and southwest, which separates the sequence from the Três Córregos granite batholith and metasedimentary rocks of the Açungui Group. Geological studies of the southern part of the Itaiacoca belt led to the recognition of three units, represented (from base to top) by metawackes with an important volcanic component, metacarbonate, and metapelitic and metapsammitic rocks. The U-Pb geochronological analyses of zircon grains from two outcrops of metavolcanic rocks yield ages of 628 ± 18 Ma (SHRIMP) and 636 ± 30 Ma (conventional multi-grain analyses). These ages are quite close to the metamorphic event recorded in the Itaiacoca Group (628 - 610 Ma), suggesting a short interval between the formation of these rocks and closure of the basin. Furthermore, this volcanism is very close to the age of formation of the Três Córregos (630 Ma) and Cunhaporanga (590 Ma) granitic batholiths, admitted as associated with a probable magmatic arc. Such an isotopic pattern characterizes a Neoproterozoic tectonic scenario involving volcanism, metamorphism and granitic plutonism, interpreted here as the final stages in the evolution of the Itaiacoca Basin.