EPISÓDIOS INTRUSIVOS NO ARCO DE PONTA GROSSA, DETERMINADOS ATRAVÉS DE UM ESTUDO PALEOMAGNÉTICO

Abstract

This work presents paleomagnetic data on the mafic dyke swarm associated with the tectonic structure known as the Ponta Grossa Arch. It is generally accepted that the dykes were emplaced when this structure were already well developed, during the Lower Cretaceous. The existing faults and fractures were then filled with magmatic material, mainly of basic composition giving rise to the so called Ponta Grossa Swarm. For the paleomagnetic analyses, 127 dykes were sampled which are widely distributed in the whole area of the Ponta Grossa Arch. These dykes cut the Parana Basin sediments, mainly Paleozoic in age, as well as the crystalline basement. About 400 hand-samples and cylinders were collected and oriented by both magnetic and sun compasses. The magnetic stability was investigated by both thermal and alternate magnetic field demagnetization methods. The characteristic remanent magnetization of each sample was identified through Zijderveld diagrams. Normally 3 specimens from each sample were submitted to the proper fields or temperatures. The mean magnetization direction of each dyke was calculated by averaging the data from the corresponding specimens. Statistic parameters associated to the means were calculated by Fisher's statistics (1953). Paleomagnetic directions recorded within each of the areas where the dykes are concentrated: Fartura, Sapopema, Telemaco Borba, Curitiba and Guapiara, show significant differences suggesting that the dykes were emplaced in different intrusive phases. A statistical comparison of the magmatic phases from different areas indicate that frequently not all the areas were active at the same time. It is suggested that the Ponta Grossa Arch was affected by nine main intrusive episodes, from which five took place during a normal polarity geomagnetic interval, three took place a during reversed interval and one occurred when the geomagnetic field presented anomalous directions. This latter episode was only recorded in Guapiara and Fartura. In general each episode affected two or more areas. However, one of the normal field episodes was identified only in Fartura and one of the reversed field episodes affected only Curitiba.