Structural and metamorphic evolution of the Anta Gorda Antiform, Southern Ribeira Belt: testing the possible existence of an extensional metamorphic core complex

• Grant number: 15/26610-5
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Effective date (Start): April 01, 2016
• Effective date (End): December 31, 2016
• Field of knowledge: Physical Sciences and Mathematics - Geosciences - Geology
• Principal Investigator: Frederico Meira Faleiros
• Grantee: Maria Thereza Akemi Guimarães Yogi
• Host Institution: Instituto de Geociências (IGC). Universidade de São Paulo (USP). São Paulo , SP, Brazil

Abstract

The aim of the project is to determine whether there was an extensional event in the Anta Gorda Antiform capable of forming a metamorphic core complex. These complexes expose basement rocks in dome shapes covered by a succesion of metasedimentary rocks, which show decreasing metamorphic conditions from the bottom to the top of the sequence. Compared to the metasedimentary rocks, older and with higher metamorphic grade rocks occur in the basement nuclei, which had been exposed due to deep extensional zones. In this context, the metamorphic paths of the rocks involved will be associated to important decompressions that could be registred in the metamorphic minerals assemblages. Therefore, the study of metamorphism in sub-horizontal mylonitic zones is fundamental to distinguish compressional and extensional processes that had occured in depth, which structural and cinematic registers usually show ambiguous evidences. For this purpose, bibliographic review, field work, structural, petrographic and electron microprobe mineral chemistry analysis will be performed. The rocks that outcrop in the Anta Gorda Antiform belong to the Votuverava Group (Calymmian), and correspond to a succesion of metasedimentary rocks with metamorphic conditions variyng from greenschist to amphibolite facies. The paleoproterozoic basement (Rhyacian and Statherian) is called Tigre Nucleous and outcrops as gneissic and mylonitic elongated domes, interpreted as antiform structures. For those reasons, it is believed that the basement rock is exposed as a result of a extensional process that formed a metamorphic core complex. Deep extensional structures are not common in the Ribeira Belt and the recognition of those features is very important for scientific knowledge about the evolutionary history of this orogen.