Magmatic affinities and evolution of Mount Telaga Rano, Mount Sahu and Mount Onu, North Maluku: analyses of phenocrysts chemical composition

Abstract

Mount Talaga Rano, Mount Sahu and Mount Onu are volcanoes in Telaga Rano volcanic complex, North Maluku. Lava of Mount Telaga Rano showed andesitic composition with magmatic temperature of 920°C-1170°C, contains plagioclase (An36-85), enstatite, augite, pigeonite, magnetite, ulvospinel and olivine (Fo64-73) phenocrysts in the groundmass of microliths of plagioclase, volcanic glass, opaque minerals and pyroxene. Lava of Mount Sahu showed the andesitic and basaltic composition. The andesitic magma has temperature in the range between 946°C and 1007°C, whereas the basaltic magma has temperature in the range between 1192°C and 1330°C. The andesitic lava of Mount Sahu has plagioclase (An29-67), enstatite, augite, magnetite and ulvospinel phenocrysts in the groundmass of microliths of plagioclase, volcanic glass, opaque minerals and pyroxene, whereas the basaltic lava contains augite, pigeonite, plagioclase (An30-91), olivine (Fo56-77) and magnetite in the groundmass of microliths of plagioclase, volcanic glass pyroxene and opaque minerals. Moreover, lava of Mount Onu showed the andesite composition with magmatic temperature of 970°C-1090°C, contains plagioclase (An25-89), enstatite, a small amount of ferrosilite, augite, pigeonite, magnetite, ulvospinel and olivine (Fo51-72) phenocrysts in the groundmass of microliths of plagioclase, volcanic glass, opaque minerals and pyroxene. The chemical composition of olivine, pyroxene and plagioclase phenocrysts have been used to identify the magmatic affinities and evolution of those volcanoes. Clinopyroxene phenocrysts of the lavas show quadrilateral component variations which are characterized by low Cr, Al and Ti contents. The pyroxene data indicate that the magmas from those volcanoes have magmatic affinities with calc-alkaline to transitional series. Furthermore, we suspected that there was iron enrichment during the magma evolution. Based on the mineralogical identification of phenocrysts, magma evolution within each magma was controlled by fractional crystallization, associated with magma mixing. The more primitive magma injection greatly affected the composition of the magma and it could turn into more basaltic composition.