Silicic Xenoliths from Andesite Lava of Kudryavy Volcano (Iturup Island, Kuril Arc): Constraints on conditions of Metamorphism and Protolith Origin

Abstract

Andesites erupted by Kudryavy Volcano (Iturup Island, Kuril Arc, Russia) enclose porous vitreous Si-rich aluminous buchite xenoliths strongly depleted in Mg and Ca. The buchite samples have pseudoporphyritic microstructures and consist of abundant (up to 75 %) high-silica K-Al glass and high-temperature-low-pressure phases: <200 μm anhydrous high-temperature silicate and oxide minerals including cordierite (X_Mg = 15–26 %), hercynite, tridymite, and intermediate members of the magnetite–ulvöspinel solid solution series, as well as minor plagioclase (An₄₁Ab₅₃Or₆ – An₁₅Ab₇₀Or₁₅), anorthoclase (An_7-14Ab_66-70Or_19–25), ilmenite, and rare Fe-bearing mullite, Fe-bearing corundum, cristobalite, intermediate members of the Fe³⁺₂TiO₅ – (Mg,Fe²⁺)Ti₂O₅ –Fe²⁺Ti₂O₅ solid solution series, and chalcopyrite. The textures and mineral chemistry of the buchitic xenoliths evidence of bulk melting at temperatures of ∼1000–1100 °C and subsequent quenching. Major- and trace- element data indicate that the host andesitic magma did not interact with the anhydrous silicate melts due to the high viscosity of molten xenolith material and the brevity of the thermal impact. The volcanic protoliths bear signatures of strong sulfate alteration of silicic and advanced argillic facies within the magmatic-hydrothermal system of the volcano conduit. Judging by the features of their mineralogy and chemistry, the sampled buchites originated from hydrothermally altered volcanic rocks which subsequently underwent extremely fast pyrometamorphism.