LEVANTAMENTO GEOLÓGICO E PETROGRÁFICO NA ILHA DO MONTE DE TRIGO (LITORAL NORTE DE SÃO PAULO, BRASIL)

JOSÉ MOACYR VIANNA COUTINHO, GERALDO CONRADO MELCHER

Resumo


The small island of Monte de Trigo, 10 km offshore of northern São Paulo coast has been geologically and petrographycally studied. The island is part of an alkaline massif in which theralites and foyaites are the major lithologies and the only plutonic types represented. Hypabissal types arc always dykes cutting indifferently the two major plutons. A large variety of alkaline lamprophyres are here represented especially by monchiquites but there arc other meso and melanocratic rocks such as syenobasalts and microshonkinites as well as leucocratic dyke rocks; phonolite and rhomb porphyry. One thick dyke occurrence is actually a magmatic trachitic breccia containing fragments of theralitic and foyaitic parentage as well as gneissie or granitic material not to be seen anywhere else in the island. Geological and petrographical descriptions are given and many modal and three chemical analyses are furnished but there are not enough elements to draw petrogenetic conclusions.

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