Formation conditions of andesites of Sulawesi Island (Indonesia)

Research subject. Andesitic complexes of the Tondono caldera and Lokon-Empung volcano locate din the northeastern part of the Sulawesi Island (Indonesia).Aim. To determine the petrogenesis conditions of andesites in the northeast Sulawesi Island based on detailed studies of volcanic rocks of the Tondono caldera and Lokon-Empung volcano.Materials and methods. We studied volcanic rock samples collected by I.Yu. Safonova. To determine the petrogenesis conditions of andesites, conventional petrochemical, geochemical and mineralogical methods were used. Melt inclusions were also studied using a MIRA 3 LMU scanning microscope equipped with Aztec Energy XMax 80 system of microanalysis, and a Horiba LabRam HR800 Raman spectroscope. The PT-parameters of crystallization were estimated from the data on the composition of melt inclusions using approaches reported by K.D. Putirka, F. Yavuz and D.K. Yıldırım.Results. The Sulawesi andesites represent tholeiitic and calc-alkaline island-arc magmas. Pyroxenes phenocrysts crystallized from melts that evolved with accumulation of alkalis and silica. Plagioclase phenocrysts crystallized from the felsic magmas, which are characterized by a decreasing role of alkalis. The composition of volcanic glass of the mesostasis suggests participation of felsic melts with a very high content of alkalis. The compositions of minerals and glasses in inclusions and in the mesostasis allowed us to estimate PT-parameters of the petrogenesis of the andesites. The phenocrysts of pyroxene crystallized in two intermediate magma chambers at depths of 27.6–14.6 and 11.3–7.2 km and temperatures ranging from 1150 to 970ºС. The phenocrysts of plagioclase crystallized at 930–910 and 900–890ºС. The microcrystals (laths) of plagioclase in the mesostasis crystallized at lower temperatures of 875–865 and 840–810ºС.Conclusions. The andesites of the Sulawesi Island were derived from tholeiitic to calc-alkaline melts compositionally similar boninites. The compositions of the glasses in melt inclusions and mesostasis showed three types of compositionally different parental magmas, which produced the Sulawesi andesites. The phenocrysts of pyroxenes crystallized from these melts in two magma chambers at depths of 27.6 to 7.2 km and at temperatures of 1150 to 970ºС. The phenocrysts and laths of plagioclase crystallized at lower temperatures of 930 to 810ºС.