EXTENSIONAL COLLAPSE OF A SMALL OROGENIC PLATEAU AND THE PRODUCTION OF VOLUMINOUS POST-COLLISIONAL GRANITE: AN EXAMPLE FROM THE NEOPROTEROZOIC ARAÇUAÍ OROGEN, EASTERN BRAZIL

The Neoproterozoic Araçuaí belt of eastern Brazil, a region similar in size to the Altiplano of the Andes, is one of many Brasiliano/Pan African orogens formed during the assembly of Gondwanaland. Erosion provides an oblique cross-section of the orogen, with deeper levels exposed in the east and shallower levels in the west, revealing processes involved in the development and collapse of a small orogenic plateau. The Araçuaí orogen records closure of a Red Sea-type basin between the São Francisco craton (SFC) and the Congo craton. The southern part of this basin was floored by oceanic crust, so closure involved subduction and arc magmatism yielding plutons that date between 625 and 575 Ma. The northern part was floored by stretched continental crust, so closure resulted in inversion of an ensialic rift. We suggest that this collision overall led to formation of a 200 km-wide orogenic plateau. About 35 million years after cessation of subduction, metapelitic gneisses underwent biotite dehydration melting at about 750°C and 5 kbar, yielding voluminous S-type granites which intruded between 540 to 535 Ma. The present outcrop of these intrusions covers an area over 20,000 km2, suggestive of widespread extensional collapse and rapid exhumation of the orogen. Between 520 and 500 Ma, four suites of post-tectonic granites were intruded. These include I-type (ca. 520 and 498 Ma) and S-type (ca. 513 and 502) granites. The S-type granites appear to have formed in thermal/fluid aureoles around bodies of I-type granite. The youngest S-type leucogranites in the area are localized on the western margin of the intrusive belt, and may be related to a discrete extensional shear zone in which shear was accommodated by formation of crenulation cleavage. The huge volume of post-subduction igneous activity in the Araçuaí orogen emphasizes that significant melting can occur beneath even a small orogenic plateau. This melting may be associated with delamination and collapse accompanying thickening formed in response to protracted post-subduction shortening (in this case, driven by collision of Amazonia with the northwestern margin of the SFC), or it may be due to heating of the lithosphere after supercontinent assembly, if assembly prevents escape of mantle heat.