Insights on the Mesoarchean to Neoproterozoic evolution of the western Campo do Meio Shear Belt, SE Brazil, using structural, aerogeophysical and U-Pb zircon data

Abstract

The Campo do Meio Shear Belt (CMSB) is an approximately 200 × 25 km transcurrent area along the south-southwest margin of the São Francisco Craton. A comprehensive review of geology was performed in the western CMSB, where rocks from the Campos Gerais and Petúnia Complexes are predominant. The Campos Gerais Complex is relatively well positioned in the geotectonic context of the region. It corresponds to an Archean granite-greenstone belt lithological association related to the south-southwest part of the São Francisco Craton. The Petúnia Complex, in contrast, is unraveled in this contribution through the integration of aerogeophysical data from the western Campo do Meio Shear Belt (CMSB) with new fieldwork observations and zircon U–Pb (LA-MC-ICP-MS) dating.

The Petúnia Complex has been restricted to two Mesoarchean gneissic units – Petúnia and Ribeirão da Conquista Gneiss, with crystallization ages of 2993 ± 11 Ma and 2870 ± 9 Ma, respectively. Although the ages are broadly coincident with those found in the Campos Gerais Complex, three characteristics suggest an evolutionary independent scenario (pre-S2) during the Archean for the Petúnia and Campos Gerais Complexes: i) the sharp contrast in gamma-ray emissions between these complexes; ii) the absence of komatiite affiliation in the meta-ultramafic rocks from the Petúnia Complex; and, iii) the absence of mafic dykes in the Petúnia Complex. These two complexes also can be distinguished by the frequency of Paleoproterozoic granitoid intrusions. The Petúnia Complex has only one occurrence, the Bom Jesus da Penha Metagranite, with a crystallization age of 2004 ± 11 Ma. On the other hand, several Paleoproterozoic granitoid intrusions (2.0–1.9 Ga) have been found in the Campos Gerais Complex. These Orosirian granitoids are restricted to CMSB and younger than those in the Mineiro Belt. We suggest that these Orosirian granitoids may represent remnants of a collision event that occurred after 2.1 Ga, leading to the integration of the Petúnia and Campo Gerais Complexes into the Columbia paleocontinent. During the Neoproterozoic, the evolution in the CMSBinvolved the cratonic basement and units related to the Southernmost Brasília Orogen. This set configures a tectonic mélange that underwent progressive deformation in a non-coaxial, partitioned transpressional regime.