The Bol’shoi Semyachik volcanic massif, Kamchatka: Composition of the rocks and minerals, and petrogenesis

Abstract

This paper is the first attempt at a detailed description of petrographic, mineralogical, and geochemical features for the rock complex that makes the Bol’shoi Semyachik volcanic massif (BSVM). The massif was formed in three phases: the pre-caldera phase, the caldera-generating phase, and the post-caldera phase. The rocks vary in composition from moderately magnesian basalts (48.86–51.87 wt %, SiO2, 7.4–8.3 wt % MgO) to rhyolites (75.12 wt % SiO2, 3.86 wt % K2O). The pre-caldera phase was dominated by basaltic andesites and andesites. The composition of the pyroclastic deposits varies from rhyolite to andesite, thus providing evidence of a zonal structure of the upper crustal magma chamber beneath the caldera. The upper crustal chamber received more basalts after the eruption of ignimbrites. Practically all BSVM rocks are of hybrid derivation, as suggested by non-equilibrium mineral associations that are present in the rocks and by a complex zonality of phenocrysts. The variation in the major oxides and trace elements in volcanic rocks indicate a leading part played by fractional crystallization in the origin of the entire range of rocks in the area. The BSVM magnesian basalts have low concentrations of REEs and are depleted in high-charge elements, thus suggesting a relationship between the primary melts and a depleted mantle source of the N-MORB type. High concentrations of fluid-mobile large-ion elements (Cs, Rb, Ba, K, Pb, and Sr) in these shows that the magma generation involved fluids that separated from the subducted oceanic plate. We describe a conceptual model of the associated magmatic system.