GEOTECTONIC MODELS AND GEOLOGIC EVOLUTION OF THE HIGH-GRADE GNEISS TERRANES OF JUIZ DE FORA (MG), BRAZIL

JOSÉ RENATO NOGUEIRA, ASIT CHOUDHURI

Resumo


The orthogneisses of the Ribeira Belt, in the Juiz de Fora area, underwent a sequence of geologic processes during times previous to the Brasiliano orogeny that generated this belt. The present study intends to recover and to reconstitute the geological evolution related to these processes. The units are distributed in two main tectonic domains (JFTD and ANDTD) that were subdivided in three structural domains with specific characteristics. The metamorphic studies point to the existence of two high-grade metamorphic peaks associated to two different tectonic events. The last event is related to the Brasiliano orogeny, with ages around 610-490 Ma and this affected the orthogneisses of the basement as well as Andrelândia Group supracrustal rocks, deposited between 1.0 and 0.6 Ga. The metamorphism related to the Transamazonian orogeny, dated at 2.2-1.9 Ga, affected only the basement orthogneisses. Peak P-T conditions for these events are considered to be 6-8 kb/750-800o C and 4-6kb/800-850oC respectively. It is proposed that the tectonic evolution for the Transamazonian orogeny could result in crustal thickening by continuous subcontinental accretion (vertical) and consequent thinning of the lithospheric mantle, and generating this way a continental magmatic arc. The resulting anomalous heat flow would elevate the continental geotherms causing extensive partial melting and differentiation of the lower crust generating preferentially tonalitic magma, and later passing on to more granitic compositions. These magmas would ascend in the crust, reworking older segments (Archean?) in upper crustal levels. The proposed geological evolution involves extensional processes with magmatic intra and underplating, during the Transamazonian orogeny, that provided CO2 -rich fluids and the necessary heat for the granulite metamorphism. The final stage of the Transamazonian orogeny witnessed crustal thinning aided by erosion, which resulted in rifting of the upper crust, recorded by the intracratonic São João del Rei basin metasedimentary rocks. After a long period of quiescence, compressive continental collision processes took place during the Brasiliano orogeny and this juxtaposed the supracrustals and basement rocks. Intense anatetic processes in this period, removed large amounts of H2O from the system, enabling the metasedimentary rocks to attain granulite facies.

Palavras-chave


High-grade gneiss; Geotectonic; Transamazonian; Orogeny.

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