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Author(s):
Caue Rodrigues Cioffi
Total Authors: 1
Document type: Doctoral Thesis
Press: São Paulo.
Institution: Universidade de São Paulo (USP). Instituto de Geociências
Defense date:
Examining board members:
Mario da Costa Campos Neto; Valdecir de Assis Janasi; Renata da Silva Schmitt
Advisor: Mario da Costa Campos Neto
Abstract

The Southern Brasilia Orogen has been interpreted as the result of the Ediacaran collision between the active margin of the Paranapanema plate and the passive margin of the São Francisco paleocontinent. The collision generated a pile of sub-horizontal nappes with a southwest-dipping tectonic wedge and tectonic transport towards the east-northeast. Archean and Paleoproterozoic migmatitic orthogneisses, that represent the orogen basement, occur in a NE-SW trending tectonic window between the two domains of the Socorro Guaxupé Nappe. This window is approximately 350 km long and 15 - 75 km wide. These orthogneisses can be divided into two main tectonic domains: (1) a Paleoproterozoic domain represented by the Pouso Alegre Complex and (2) an Archean domain that comprises the Amparo, Serra Negra, Heliodora and Minduri complexes. The Pouso Alegre Complex is comprised mainly of migmatitic orthogneisses of tonalitic to granodioritic composition with igneous crystallization ages from 2.15 to 2.08 Ga and juvenile isotopic signatures (Nd-Hf). The geochemical signatures suggest a continental arc margin or an evolved accreted oceanic arc as the favored tectonic setting for the Pouso Alegre Complex. The Pouso Alegre Complex is interpreted as the orogenic counterpart of the Mineiro Belt arc system. The Pouso Alegre Complex and the juvenile suites of the Mineiro Belt represent a major continental crust generation event at the southern edge of the São Francisco paleocontinent, between 2.35 and 2.08 Ga, during a period considered to have relatively low preservation rates of juvenile rocks on a global scale. Therefore, the data presented in this thesis supports the idea that there is a bias in the juvenile rock record and consequently in the crust preservation models because of the small dataset for South America. The Archean complexes in the basement of the southern Brasília Orogen show a well-defined period of TTG-type magmatism between 2.96 and 3.00 Ga. An additional period of Neoarchean high-K granitoid magmatism at ca. 2.76 Ga is interpreted to mark the transition from TTG-type to high-K granitoid magmatism in the area. The Mesoarchean igneous crystallization ages presented in this study lie within the southern São Francisco craton \"magmatic gap\" and suggest that these Archean complexes are exotic to the Archean crust of the São Francisco craton. These complexes are separated from the São Francisco craton Archean crust by the Paleoproterozoic Pouso Alegre Complex / Mineiro Belt arc system. These Archean complexes are interpreted as Archean microcontinents that were accreted to the southern São Francisco paleocontinent. The timing of accretion is not well constrained but most likely occurring during the Paleoproterozoic after development of the Pouso Alegre Complex arc-related suites. The Neoproterozoic collisional events were responsible for intense deformation and metamorphism in upper amphibolite facies associated with partial melting. The presence of tonalitic leucosomes with peritectic hornblende suggests the occurrence of water-fluxed melting reactions. Maximum P-T conditions of ca. 670°C and 9.5 kbar were obtained by THERMOCALC average P-T thermobarometry for the Pouso Alegre Complex. These results are in good agreement with the average temperatures obtained by Zr-in-titanite thermometry of ca. 700°C. Zircon and titanite U-Pb geochronology constrain the age of the upper amphibolite facies metamorphism between 620 and 616 Ma. Titanite recrystallized rims and intrusive leucogranites have ages of ca. 607 Ma that are interpreted as the age of the post-metamorphic peak decompression related to the exhumation of the Pouso Alegre Complex. (AU)

FAPESP's process: 12/24933-3 - GEOLOGICAL EVOLUTION OF MIGMATITES FROM THE INFRASTRUCTURE OF SOUTHERN BRASÍLIA BELT NAPPE SYSTEM
Grantee:Caue Rodrigues Cioffi
Support type: Scholarships in Brazil - Doctorate