Dyke swarms of the Paraná triple junction, southern Brazil

Abstract

This work intends primarily to survey the field, mineralogical and petrographic characters of the mafic dykes which occur on a stretch of 650 km along the Southeastern coast of Brazil, between the city of Sao Sebastiao, and the island of Santa Catarina. New chemical and geochronological data are also presented. The coastal dyke swarms are envisaged as the northern and southern arms of a plume-generated triple junction system centered on the Parana State coast, and related to the initial opening of the South Atlantic. Mafic magma intruded as dyke swarms along three directions: N-S (the southern arm, along the Parana-Santa Catarina coast), NW-SE (Ponta Grossa arch) and NE-SW (the northern arm along the Sao Paulo coast). Fifty two dykes, almost all tholeiitic diabases, were mapped and sampled along the south arm coast. The Ponta Grossa arch dykes are chiefly composed of tholeiitic diabases and lesser intrusions of andesitic to rhyolitic composition. Over 240 dykes were sampled and identified along the north arm west of Sao Sebastiao. Lamprophyres are here abundant, followed by diabases, microdiorite porphyries and lesser amounts of trachy-andesite, carbonatite and Precambrian dykes. Special attention was given to the study of lamprophyres, their field appearance relative abundance, mineral and chemical composition, enclaves and relations to neighboring alkaline intrusions.