Sector-zoned clinopyroxene in alkaline basalts: insights from the Cenozoic Gelaoshan volcano, Datong Volcanic Field, North China Craton

Sector-zoned clinopyroxene records varying degrees of magma undercooling (ΔT = T_liquidus − T_{crystallisation}), which can be utilized to track the dynamics of magma plumbing systems. Nevertheless, the mechanism controlling element partitioning among different sectors are poorly constrained. The alkaline basalts erupted at Gelaoshan volcano in the Datong Volcanic Field (DVF) are characterized by the presence of sector-zoned clinopyroxene phenocrysts. Here we combined petrography and mineral chemistry to represent the partitioning of elements within sector-zoned clinopyroxene and their effect on the P-T estimation based on Cpx-only thermobarometer. Our results show that sector-zoned clinopyroxene indicates an undercooling degree of ∼ 36 °C. Through the trend of the different elemental partition coefficients of each pyroxene from core to rim, we had shown that in addition to undercooling, the trace element partitioning is also affected by primitive magma replenishment, diffusion rates of various elements within the clinopyroxene lattice and the mechanisms of charge balance. Moreover, sector-zonation affects the application of Cpx-only thermobarometers, and    {-111} sectors could give reliable estimation results because of the clinopyroxene-melt equilibrium test. Hence, based on the composition of the    {-111} sectors, we estimated the P-T conditions of Gelaoshan Cpx. Our results indicate the existence of two magma chambers within the volcanic plumbing system beneath Gelaoshan, one is at ∼ 25 km, with the other at ∼ 15 km.