Magmatic plumbing system beneath a fossil continental arc volcano in western Tianshan (NW China): Constraints from clinopyroxene and thermodynamic modelling

Abstract

Determining the processes of magma ascent from magma source to the surface is paramount to understand the evolution of magmatic plumbing systems. Here, we perform systematic petrological investigations and thermodynamic modelling on the early Carboniferous calc-alkaline basalt in the Xinyuan region of western Tianshan. As the most common phenocryst in basalt, clinopyroxene with diverse textures and zoning patterns can be classified into 6 types. Type 1 clinopyroxene, occurring throughout the whole basaltic sequence is unzoned or normally zoned. Type 2 to type 6 clinopyroxenes, only observed in the late erupted basalt, show disequilibrium textures including resorbed cores, sieve textures, oscillatory zoning, and patchy zoning. These textural and compositional features of clinopyroxene suggest that fractional crystallization controlled the early stage of basaltic magma evolution, and the late emitted basaltic magmas might be formed by combined processes of fractional crystallization, recycling of early-formed crystals, repeated recharge of hot, primitive magmas with subsequent mixing, and convection of residing magmas at depth. Thermobarometric calculations and thermodynamic modelling with the rhyolite-MELTS software suggest at least two magma storage zones: a deep magma chamber at ∼22–25 km depth and a shallow magma chamber at ∼ 2–3 km depth. Chromite and clinopyroxene with Mg# values of 77–84 crystallized from melts with ∼1 wt % H2O at QFM-1 conditions in the deep magma chamber. Plagioclase (An = 59-70) started to co-crystallize with Fe-rich clinopyroxene (Mg# = 74–78) in the shallow magma chamber in more oxidized conditions (close to the QFM buffer).