MINERALOGY AND PETROGENESIS OF PHONOLITE-SERIES LAVAS FROM SUMACO VOLCANO, ECUADOR

Sumaco Volcano is a dormant edifice located in the rear-arc region of Ecuador. It is one of three volcanoes that comprise the rear-arc of the Northern Volcanic Zone and the only reported location in Ecuador where phonolite-series lavas are associated with arc magmatism. There are reports of possible eruptions between 1650 and 1895 and again in 1933 however there are no historic reports to support these hypotheses, and the ages of lava flows from Sumaco have not yet been determined. Samples were collected from the summit and southern flanks of the volcano and analyzed for major and trace elements. Electron microprobe analyses were conducted to determine mineral compositions. On the basis of total alkali content, samples are classified as basanite (43 wt. % SiO₂, 8 wt. % MgO) to tephriphonolite (53 wt. % SiO₂, 4 wt. % MgO). The mineral assemblage in the most mafic basanite lavas includes titanaugite, hercynite and accessory apatite; plagioclase is absent from these samples. The tephrites samples contain predominantly titanaugite but also plagioclase (An₈₈), and the most evolved samples contain titanaugite, plagioclase and hauyne. Hauyne is a sulfate mineral that forms under very oxidizing conditions, and typically at pressures less than 400 MPa. The combination of titanaugite and hauyne allows constraints to be placed on the conditions in the magma reservoir since experiments show that these minerals require high temperature, highly oxidizing conditions and relatively low pressures. The composition of the plagioclase crystals is consistent with experimental temperature of 1000C. The euhedral shape and hourglass zonation of the titanaugite crystals also indicates rapid crystallization in a shallow magma reservoir. Numerical models using trace element concentrations shows that the phonolite-series lavas from Sumaco can be related to each other by fractional crystallization of a parental magma that was formed by melting of either pyroxenite or a highly modified mantle wedge.