MINERALOGIA DO DIQUE DE TONINHAS, UBATUBA, LITORAL NORTE DO ESTADO DE SÃO PAULO: FELDSPATOS

CELSO DE BARROS GOMES

Resumo


Electron microprobe analyses for Na, K, and Ca are given for twenty feldspar crystals from the Mesozoic Toninhas dike which crops out near Ubatuba, along the north shoreline of the State of São Paulo. The rocks collected at different distances from the contact zone show very sharp textural, mineralogical, and chemical variations. At the central parts of the dike they are coarser grain-size and have a granophyric composition. Chemical data for 456 grains indicate that about 70% of the analyzed plagioclases crystals have an andesitic composition. As given by 129 analyses, orthoclase with an Or content ranging from 66 to 99% seems to be the main potassic phase in the alkali feldspar group. Some general considerations on the genesis of the Toninhas rocks are also taken by using chemical data of the feldspars.

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