Geology of the Mafic-Ultramafic Mata Grande Complex, São Sepé, RS

Abstract

The Mata Grande Mafic-Ultramafic Complex (MGC), located in São Sepé municipality, NW portion of the Sul-rio-grandense Shield, is a 5 km² layered intrusion bounded by normal faults southwest with the gneisses of the Cambaí Complex, southeast with magnesian schists and serpentinites of the Arroio Lajeadinho Complex and in north with the sedimentary rocks of the Paraná Basin. Three rock unities of cumulates are described in this paper: Mafic Unit (MU), Ultramafic Unit (UMU) and Transicional Unit (TU). The preserved primary structures are a compositional/textural millimetric to centimetric layering and an outcrop-scale or regional intercalation. The MU rocks crystallized from an accumulation of plagioclase crystals and, in less proportion, olivine crystals, and also from intercumulus phases, representing 24 % - 41 % of the interstitial trapped liquid in the accumulation. In TU, plagioclase and olivine accumulation occurred in very close proportions, with less trapped liquid proportion (~ 15 %). The UMU samples show olivine principal accumulation, intercumulus plagioclase and trapped liquid (20 %). All MGC units are affected by contact metamorphism caused by the São Sepé Granite, under albite-epidote hornfels and hornblende hornfels temperature conditions. The subsolidus deformation features indicate a compaction process during the stages of final crystallization and cooling.