Petrografia e litogeoquimica das rochas da parte Oeste do Alto de Salvador, Bahia

Abstract

PETROGRAPHY AND LITHOGEOCHEMISTRY OF THE ROCKS OFTHE WESTERN PART OF SALVADOR HIGH, BAHIA STATE, BRAZIL The Salvador city is located in the Salvador-Esplanada belt, which can be subdivided into three major domains: The Recôncavo sedimentary basin, the coastal plain and the Salvador high. The Salvador high can be sub-divided into two provinces, separated by the Iguatemi Fault. In the block located to the west of this fault metamorphic rocks in the granulite facies dominate, whereas to the east metamorphic rocks in the amphibolite facies are present. This paper is focused in the western province where it is possible to identify the following rock types: (i) alumino-magnesian granulites, basic granulites and quartzites with garnetorthopyroxene and (ii) orthoderived metamorphic rocks composed of tonalitic and charnoenderbitic granulites, including enclaves of metapiroxenites and metagabbros. Concerning the nature of the protolites, it is assumed that the alumino-magnesian granulites, are restites from the fusion of pelitic rocks, which are usually associated with light-colored garnet bearing granites, considered as the liquids resulting from that fusion. The basic granulites and quartzites with garnet-orthopyroxene associated with the aluminomagnesian granulites are thought to be the product of the metamorphism of basalts and cherts. With respect to the orthoderived protolites, the petrochemistry has shown that the piroxenitic enclaves were derived from Mg-rich tholeiitic magmas and the gabbroic ones have had their origin from either tholeiitic or transitional tholeiitic/calc alkaline magmas. The tonalitic and the charnoenderbitic granulites, were both derived from calc-alkaline low-K and high-K magmas, respectively. The rocks were deformed both in the ductile and brittle states. In the first case, recumbent folds were refolded, originating upright isoclinal folds. In the second case fractures with several orientations were formed. Many of these fractures were filled either with metamorphic and non-metamorphic monzosienogranites and mafic dikes. Same times mixture between mafic and felsic magmas result in commingling structures. Despite the small number of geochronological data it is possible to infer that the granulitic protolytes are archaean (SHRIMP age of ± 2,5 Ga, obtained in the center of a zircon from a tonalitic granulite) and that the granulitization (± 8kbar – 850oC) took place during the paleoproterozoic (SHRIMP age of ± 2,0 Ga, obtained from the periphery of the zircons).