Petrologia de rochas hipabissais básicas relacionadas a lineamentos magnéticos no Escudo Sul-rio-grandense

Abstract

The Sul-rio-grandense Shield (SrgS) is the object of study in several works, mainly in the scope of structural geology. In this way, the main lineaments and faults that contributed to its formation and evolution are well studied, citing as an example the NE direction structures. However, NW direction structures and lineaments have received few detailed studies over time. In addition, structural highs are observed such as the Rio Grande Arc and magnetic anomalies arranged in the same direction. Several authors attribute to the NW structures an extensional character, with important function in the evolution of the Paraná Basin, acting as conduits for the volcanism of the Serra Geral Formation. In this context, the present work seeks to characterize geochemically the intrusive igneous rocks of SrgS related to magnetic anomalies of NW direction. In this way, it is also sought investigate the sources and processes related to magmatism and the identification of the magmatic series and the geotectonic environment. Thus, the present study will combine the magnetic anomalies of geophysical maps with the NW direction structural lineaments, to define target areas of the possible occurrence of these rocks. During the fieldwork, these delimited regions will be visited, in order to verify the presence or not of the igneous bodies, with collect of samples when it is present. Field work was performed to collect samples. As 13 samples collected receive initial treatment at the Unipampa campus in Caçapava do Sul, the Mineralogy and Petrology Laboratory and were sent to SGS GEOSOL's laboratories for whole rock geochemical analysis through ICP-MS (Inductively coupled plasma mass spectrometry) for the major elements and ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry), for trace elements and rare earth elements (REEs). Petrographically, the rocks were classified with olivine-gabbro/ dolerite and subordinate gabbro, they presented ophitic and sub- ophitic texture, they also presented texture variation, thin textures being in the edge of the bodies, while in the center thick textures. For geochemistry, the software GDKit 4.1 was used to make geochemical diagrams. As analyzes demonstrated, with toleitic affinity and calc-alkaline tendency, they show an increase of incompatible elements such as Th, Ba and K. It is also observed, negative anomaly trace elements of Nb, Ta and P, which indicates a source affected by previous subduction setting, common feature for continental flood basalts. Through tectonic environment discrimination diagrams, it can be verified that either the sample set assumes the intraplate character. Comparing these rocks with SrgS and Paraná Basin units, it1s possible to observe a great similarity in the geochemical signature between this sample set and sills of Fm. Serra Geral. So, it possible that the sample set, it is the dikes that are part of the Fm. Serra Geral, related to the plumbing systems of this province. Therefore, important NW structures, such as the Rio Grande Arch, acted in the geological evolution of Paraná Basin magmatic events, observed their NW orientation and magnetic anomalies overlap, forming part of the volcanism feeding systems of Fm. Serra Geral formation.