Petrology of the Colíder Group volcanic successions in the northernmost Mato Grosso, Brazil: A contribution to the knowledge of the felsic volcanism of the Alta Floresta Gold Province

Abstract

The Colider Group ignimbrites and rhyolites, located between Guarantã do Norte and Serra do Cachimbo, Brazil derives from a felsic magmatism with a wide spatial distribution at the Alta Floresta Gold Province (AFGP). The geological mapping and petrographic studies allow distinguishing three volcanic successions of the Colider Group at the study area, named Lower (LS), Intermediate (IS) and Upper (US). The LS characterizes by its abundance in pyroclastic density current deposits, such as ignimbrites and surge deposits, with subordinated effusive rocks. The US consists of effusive rhyolites and trachytes and, subordinately, rheomorphic ignimbrites. The magmas that generated the volcanic rocks of the LS is of a subsolvus character. The US rocks present, dominantly, a mineralogy indicating a hypersolvus character. This characteristic can be indicating a variation of the fluid pressure conditions within the magma chambers that generated this magmatism. Regarding the chemical composition of the volcanic rocks from the two successions, we can point out that both have a metaluminous to slightly peraluminous character and affinity with A2-type magmas. The zircon U-Pb LA-ICPMS isotopic data of a LS rhyolite indicate a crystallization age of 1810 ± 9 Ma, interpreted as the Lower Succession age. One of the hypotheses that may be suggested to the generation of the volcanic successions is that the petrographic, lithochemical, and geochronological contrasts indicate the occurrence of two distinct magmatic events, one of ∼1.80 Ga and another of ∼1.76 Ga. The second hypothesis is that both successions belong to the same magmatic event, which occurred from 1.81 Ga to 1.76 Ga.