Discussion of construction processes for the São Sepé Granitic Complex, southernmost Brazil: geological and petrographic features

Abstract

Construction processes for the São Sepé granitic complex (SSGC) are discussed on the basis of geological and structural, field informations complemented by integrated studies of remote sensors (DEM-SRTM and ASTER images) and aeromagnetic maps for analyzing regional tectonic systems, and also by detailed petrographic investigation. The granitic complex, located in the west of the Sul-riograndense Shield (São Gabriel Block), represents the shallow level (250-160 MPa) of a vertical slow-production mush column controlled by N30-35°E and N55-60°W, pre-existents fault zones linked to post-collisional events of the Dom Feliciano Orogeny (640-620 Ma). Its formation occurred during transtensive tectonic event along N15-20°E fault zones, responsible for regional uplifting and extensive magmatic events at the end of the Neoproterozoic (570-560 Ma), as a reflex of some younger orogeny in the north. The SSGC is a moderate-size (473 km²), composite pluton composed of monzogranites in the center and syenogranites in the border, showing wide textural variations. The expansion of the magma system caused by volatile accumulation at the end of its formation, as registered by granitic cupola and wide felsic margins, has induced the reactivation of NW-SE fault zones. Consequently, this leads to infer some emplacement mechanisms involving roof uplifting. According to the model proposed, the SSGC was constructed through progressive and cyclic, mafic recharge at the bottom of a precursor syenogranite sill, which are put in evidence by features of hybridism in monzogranites. The result was the stratification of the resident magma, and the pluton started to crystallize separately: hybridized monzogranites at the base; and syenogranites from residual melts at the top, in which the concentration of F-H2 O-rich volatiles postponed the crystallization successively. Next episodes of mafic replenishments in a cold, partially crystallized intrusion resulted in re-heating leading to textural aging and thermal erosion of crystallized facies; the reactivation of crystal mushes with re-intrusion and co-mingling of hybridized magmas; and large thermal undercooling of the mafic pole. These features suggest the accumulation of highly mobile syenogranitic magmas on the top, which has built up one of the active magma chambers feeding of the coeval volcanism (Acampamento Velho Formation).