Geochemical and petrogenetic characterization of the Arroio Divisa Granitoids, Quitéria region, Rio Grande do Sul, Brazil

Abstract

The Arroio Divisa Granitoids (ADG), situated in the Quitéria region, eastern Sul-rio-grandense Shield, conform an elongate, NE-SW oriented body about 30 km long and 1 to 6 km wide. They are intrusive in Paleoproterozoic metatonalites, metagranodiorites, and tonalitic to dioritic gneisses at the northern border, whilst in the south they are intruded by Neoproterozoic granites and rhyolites. The ADG rocks are predominantly foliated granodiorites to granites, with medium- to coarse-grained equigranular textures, containing amphibole and biotite. Titanite, zircon, and apatite are accessory minerals. Dioritic to tonalitic rocks occur as mafic microgranular enclaves, bodies with metric dimensions, and synplutonic dikes, with interpenetrated and sinuous contacts, as usual for magma mingling products. Near the diorites, the mafic contents of the granitoids is increased. Centimeter- to meter-sized xenoliths of gneisses and metatonalites from the Arroio dos Ratos Complex, and of hornblende-biotite granodiorites correlated to the Cruzeiro do Sul Granodiorite are frequently observed. Where mingling and large xenoliths are abundant, the ADG granodiorites change their texture to heterogranular and porphyritic, in meter-wide zones. Magmatic foliation is marked by the shape orientation of plagioclase and biotite. Parallel to the magmatic foliation, a mylonitic one is developed with variable intensity and sinistral transcurrent movement. The steeply-dipping, ENE-striking structures are rotated towards NE strike at the eastern part of the body, where a regional cataclastic zone has controlled the emplacement of later intrusions. Quartz-mylonites and phylonites are found within the ADG along high-strain, low-temperature zones, sometimes hundred-meters wide. The ADG and associated mafic rocks show geochemical features that indicate their genetic relationship with medium to high-K magmas of tholeiitic affinity contaminated by crustal melts produced from garnet-bearing gneissic protoliths. The integrated interpretation of stratigraphic, tectonic and geochemical evidences indicates that the ADG and associated mafic rocks have formed during the early period of Neoproterozoic post-collisional magmatism in southernmost Brazil.