Decoding magma storage and pre-eruptive processes in the plumbing system beneath early Carboniferous arc volcanoes of southwestern Tianshan, Northwest China

The volcanic rocks, consisting of basalt, basaltic andesite, andesite, dud tuff with volcanic clastic sedimentary rocks in the Dahalajunshan region represent an early Carboniferous continental arc formed during subduction of the Paleo-southern Tianshan ocean. The basalt is characterized by a phenocryst assemblage of clinopyroxene and plagioclase, while basaltic andesite and andesite contain variable amounts of clinopyroxene, orthopyroxene, plagioclase, and Fe-Ti oxides phenocrysts. Clinopyroxene, the most common phenocryst in the studied rocks, has a large variation of compositions and displays multiple zoning patterns, suggesting complex magmatic processes in different magma storages beneath the Dahalajunshan region. A deep reservoir at depths of >28 km is suggested by clinopyroxene with Me values of 84-88 and an intermediate reservoir is suggested by clinopyroxene with Mg# values of 75-79. The shallow reservoir is crystal-rich as indicated by the presence of crystal clots consisting of Fe-rich clinopyroxene (Mg# < 75), orthopyroxene (Mg# = 65-76), plagioclase, and Fe-Ti oxides. Petrological observations, combined with the whole-rock geochemical and Sr-Nd isotopic data, suggest that the studied basalt mainly formed by fractional crystallization in both deep and intermediate magma reservoirs, while basaltic andesite and andesite might be produced via open-system processes of crystal fractionation, recycling of early-formed crystals, interaction of colder magmas with hotter, recharging magmas, and crustal contamination in the shallow reservoir. This study suggests the importance of textures and compositions of erupted minerals to decode the pre-eruptive histories of magmatic plumbing systems beneath fossil arc volcanoes. (C) 2018 Elsevier B.V. All rights reserved.