REAVALIAÇÃO DA EVOLUÇÃO GEOLÓGICA EM TERRENOS PRÉ-CAMBRIANOS BRASILEIROS COM BASE EM NOVOS DADOS U-PB SHRIMP, PARTE I: LIMITE CENTRO-ORIENTAL DO CRÁTON SÃO FRANCISCO NA BAHIA

Abstract

This paper discusses new U-Pb SHRIMP zircon data for 12 key-exposures of several geological units exposed at the eastern border of the Sao Francisco Craton. The samples represent mostly Archean basement units within the Paleoproterozoic Eastern Bahia Belt (Orogen). Samples were collected along several E-W transects trying to more accurately assess the areal distribution of the Archean policyclic basement of the Sao Francisco Craton and to identify the limits of Paleoproterozoic metamorphic overprint resulting from the development of the Eastern Bahia Orogen. Owing to the policyclic evolution and/or high grade metamorphic conditions which most of the rock units investigated have undergone, zircon morphology and the U-Pb analytical data exhibit very complex patterns. These are characterized by a combination of inheritance, partial resetting and new zircon growth during high grade metamorphism. As a consequence, very careful and detailed analyses of cathodoluminescence imagery were required to allow distinction between inheritance, newly melt-precipitated zircon and partially reset zircons, as well as between the ages of magmatic and metamorphic events. Except for one unit (sample LH 44), which present crystallization age of ca 3000 Ma - interpreted, therefore, as the eastern extension of the Serrinha Craton – the others are ascribed to two major age groups at ca. 2870-2500 Ma and ca . 2200?-2030 Ma. The former group includes orthogneisses with crystallization ages between ca. 2870-2500 Ma, which have been mapped and interpreted, in its major extension, as juvenile Paleoproterozoic arc (Itabuna and Salvador-Curaca belts/domains). The new data presented in this study, however, indicate that these orthogneisses represent a multi-episodic collage of primitive Archean orogenic arcs, which gave rise to the Archean basement of that part of the Sao Francisco Craton. All the investigated zircon populations were extensively recrystallized at ca. 2080-2050 Ma as a result of the Eastern Bahia Belt collision. The isotopic signature and the metamorphic assemblage are suggestive of recrystallization in very deep crustal levels, under high P and T conditions. A similar metamorphic regime is recorded in the western Jequie Block (sample LC 61), so far interpreted as a cratonic area which has been stable since the Archean. Accordingly, the Eastern Bahia Belt was characterized as the deep roots of a collisional orogen, with discrete newly accreted magmas between ca . 2200? and ca. 2090 Ma - herein ascribed to the Western Bahia Orogen which was formerly referred to as the Itabuna, Salvador-Curaca and Salvador-Esplanada belts.