Teores de potássio, urânio, tório e taxa de produção de calor radiogênico no embasamento adjacente às bacias sedimentares de Camamu e Almada, Bahia, Brasil

Abstract

ABSTRACT.The bedrock adjacent to Camamu and Almada sedimentary basins is characterized mainly by rocks of granulite and amphibolite facies, with archean and paleoproterozoic ages, which belong to orogen Itabuna-Salvador-Curaçá. The units in major proportion in this context are the metatonalites associated with basic and metamonzonites belonging to Itabuna belt. In smaller area occur the Teolândia granite and the Moenda granodiorite associated with the Ipiaú band amphibolites, the charnockites and charnoenderbites of Jequié bloc, the neoproterozoic sienites and the mafic dikes. The K, U and Th contents of the rocks vary from 0,02 to 6,33% for K, from < 0, 2 to 9,10 ppm for U and from < 0, 4 to 64,38 ppm for Th. These contents are higher in the charnockites, Moenda granodiorite, Teolândia granite and sienites, intermediate in the metatonalites and metamonzonites and lower in the basic granulites. The heat production rates are higher in the lithologies where K, U and Th are also higher, varying from 0,58 to 5, 57 µW m -3 . The coverage areas of such lithologies are, however, small compared with that of the metatonalitic granulites, metamonzonitic granulites and sienites where the rates vary from 0,10 to 1, 44 µW m -3 , 0,23 to 5, 55 µW m -3 and 0,60 to 2, 24 µW m -3 , respectively. In this case, the heat production rates vary from 0,10 to 1, 44 µW m -3 . The basic granulites have the smaller rates, from 0,06 to 0, 36 µW m -3 . The observation of the lithologies in the margins of the two basins suggest that, in the bedrock under the younger sediments, may predominate the metatonalites, followed by the metamonzonites, with some significant participation of sienites in the Almada basin. In those lithologies, the volumetric heat production rates, with one standard deviation range, are 0, 41 ± 0, 30 µW m -3 for metatonalites, 0, 71 ± 0, 57 µW m -3 for metamonzonites and 1, 20 ± 0, 51 µW m -3 for sienites.Keywords: gamma-ray spectrometry, heat production in rocks, K, U and Th in rocks. RESUMO.O embasamento adjacenteàs bacias sedimentares de Camamu e Almadaé caracterizado, principalmente, por rochas das facies granulito e anfibolito, com idades arqueanas e paleoproterozoicas, pertencentes ao orógeno Itabuna-Salvador-Curaçá. As unidades em maior proporção neste contexto são os granulitos metatonalíticos associados aos granulitos básicos e metamonzoníticos pertencentes ao cinturão Itabuna. Em menorárea, tem-se o granito de Teolândia e o granodiorito de Moenda associado aos anfibolitos da banda de Ipiaú, os charnockitos e charnoenderbitos do bloco Jequié, os sienitos neoproterozóicos e os diques máficos. Os teores de K, U e Th nessas rochas variam de 0,02 a 6,33% para o K, < 0, 2 a 9,10 ppm para o U e < 0, 4 a 64,38 ppm para o Th. Esses teores são mais altos nos charnockitos, no granodiorito de Moenda, no granito de Teolândia e nos sienitos, têm valores intermediários nos metatonalitos e metamonzonitos e são mais baixos nos granulitos básicos. As taxas de produção de calor nessas rochas são mais altas onde os teores de K, U ...

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