Abstract

Metamafic orthoderivate rocks, characterized by MORB signatures and paleoproterozoic ages occur in Cajamar region central east part of Sao Paulo State, Brazil. They are associated to philites, metasandstones and metalimestones of Sao Roque Group volcanosedimentary sequence, Southeastern portion of the Ribeira Belt. For the metamafic rocks the assemblage of metamorphic peak is represented by actinolite-albite-clinozoisite-chlorite typical of low-grade regional metamorphism, greenschist facies. To have a good characterization of the metamorphic P and T parameters of metamorphism microprobe analyses of main minerals were performed. Metamorphism did not destroy completely the primary mineralogy of original basalt and microgabbro, and augite crystals are registered in many places, preserved together with metamorphic assemblage, that indicate for the igneous temperatures of crystallization values between 900 and 1100 o C. Textures of mineral equilibrium for the peak paragenesis, are not always present in the studied samples because of the superposition of retrometamorphic events caused by regional shearing process. For this reason it becomes difficult the application of geothermometers and geobarometers. Where the equilibrium is reached, in the occurrences in the south of the area, temperature and pressure determinations indicate values close to 487 ± 42 o C and 8,2 ± 2.0kbar, respectively. Other values for P and T lower than that of the peak were also obtained in central and north portion of the area and they are: 382 o C for T e 5,7kbar for P (point 45) and 341 o C for T and 4,1kbar for P (point 77) and can indicate a metamorphic event of retrometamorphism. As a concluding remark one can say that the exumation process in this area occurred by isothermal decompression

Highlights

  • Geothermobarometric characterization of metabasic rocks of Cajamar region (SP): São Roque Group, Ribeira Belt

  • Metamafic orthoderivate rocks, characterized by MORB signatures and paleoproterozoic ages occur in Cajamar region central east part of São Paulo State, Brazil

  • They are associated to philites, metasandstones and metalimestones of São Roque Group volcanosedimentary sequence, Southeastern portion of the Ribeira Belt

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Summary

Introduction

Geothermobarometric characterization of metabasic rocks of Cajamar region (SP): São Roque Group, Ribeira Belt. A associação de pico metamórfico é representada por actinolita-albita-epidotoclorita, típica de metamorfismo regional da fácies dos xistos verdes.

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