The Physicochemical Conditions for the Formation of Recent Basalts of Lokon Volcano, Sulawesi Island in the Pacific Ocean: Melt Inclusion Data

We studied phenocrysts and melt inclusions in order to evaluate physical and chemical conditions of petrogenesis of the Sulawesi Island (southwestern Pacific subduction belt) basalts. The compositions of the clinopyroxene and plagioclase phenocrysts hosted by basalts of the Lokon Volcano indicate two types of melts: primitive and differentiated. The primitive magmas produced high-anorthite plagioclases that are absent from the Lokon volcano andesites. The second type of melts produced clinopyroxenes and plagioclases of similar composition in basalts and andesites. Similarities in the compositions of homogeneous glasses of melt inclusions in minerals from the Lokon Volcano and other volcanoes of Sulawesi Island and the SW Pacific suggest similar P–T conditions for the formation of magmatic systems. In particular, inclusion data demonstrate that basaltic magmas of the Lokon and Soputan volcanoes are primitive melts (SiO 2 = 48.2–51.5 wt % and MgO up to 8 wt %), which is consistent with the contents of other oxides. These inclusions plot at the beginning of the “andesitic” trends in variation diagrams. Accordingly, these melts could be the primary melts for the entire magmatic system that formed andesitic to basaltic effusive rocks of the Tondono caldera and the Lokon and Soputan volcanoes. Based on the compositions of melt inclusions and host minerals in Lokon basalts, we concluded that clinopyroxene phenocrysts crystallized at two levels: 20.6–14.5 km (1150–1130°С) and 13–6 km (1145–1105°С), and plagioclase phenocrysts crystallized at 1160–1120°С. In general, our data on melt inclusions and phenocrysts indicate fundamental differences in the PTX parameters of the magmatic systems that formed basalts and andesites of Sulawesi Island. The basalt-hosted clinopyroxene phenocrysts crystallized from compositionally stable melts at slightly decreasing temperatures and a significant pressure drop during magma ascent. In contrast, crystallization of phenocrysts in the andesites occurred with delays in deep intermediate chambers, at constant pressures and a significant drop in temperature, which facilitated the processes of fractionation and evolution of melts.