PÓMEZ BANDEADAS EN EL VOLCÁN CHIMPA: UN EJEMPLO DE CIZALLA EN EL CONDUCTO

ARNOSIO, M.; BUSTOS E.; BECCHIO RAÚL

Córdoba

Congreso; XIII CONGRESO DE MINERALOGÍA, PETROLOGÍA ÍGNEA Y METAMÓRFICA, Y METALOGÉNESIS; 2019

MINMET -AGA

Banded pumices at Chimpa volcano: an example of conduit shear process. Bandedpumices composed by a white, brown and black fraction were found in the ignimbritic deposits of Chimpa volcano. Mass-balance modeling using the Microsoft Excel workbook OPTIMASBA using mixing process was performed in order to evaluate the origin of each fraction. Phenocrysts modal percentage, mineral chemistry and glass compositions were used as inputs. Results shown that by adition of 20% of crystals (43% plagioclase, 28% hornblend, 23% biotite and 7% magnetite) to the rhyolitic melt, fraction brown glass is obtained (SSE 0.7; R 2  0.9998). By adding 23% crystals to the same liquid, black fraction glass is obtained (SSE 0.8; R 2  0.9997). The process was identified at Quilotoa and Pinatubo volcano and interpreted as shear in the conduit because of a mechanism of viscous dissipation and crystal grinding, acting as lubricant for high magnitude eruptions. However, at Chimpa volcano no fall deposits were identified and the banded pumices are distributed in all the ignimbritic sequence. So,banded pumices are no restricted to plinian paroxysm phases. This pumice typology also has been found at caldera related ignimbrites, therefore, this process is not exclusive of central type eruptions.