Magmatic processes in the plumbing system of the Cenozoic ultrapotassic volcanic rocks in the southeastern Tibetan Plateau

Abstract Although great effort has been put into characterizing the sources of the Cenozoic ultrapotassic volcanic rocks in the southeastern Tibetan Plateau, insufficient attention has been paid to the magmatic processes in the magma plumbing systems of these rocks. We filled this gap by analyzing clinopyroxene (Cpx) phenocrysts of these rocks from the Wozhong area for major and trace elements and Sr isotopes. Based on textural and Mg# variations, Cpx phenocrysts were classified into five subtypes: type 1 has three zones (core, mantle, and rim) with decreasing Mg# outward (normal zoning); type 2 has two zones (core and rim) with normal zoning; type 3 has three zones with reverse zoning; type 4 has a homogeneous core and an oscillatory-zoned rim; and type 5 has fine oscillatory zoning. Estimated crystallization temperatures and pressures for the different types/zones are variable at grain scale, suggesting that the crystallization of a single crystal took place at various depths. The prevalence of oscillatory and reverse zoning, parallel trace-element patterns, and overlapping chemical-isotopic compositions of the Cpx phenocrysts with relatively high Mg# (87–93) indicate that magma recharge and mixing took place in the magma plumbing system. All of the Cpx data together show two different compositional evolution trends that are consistent with fractional crystallization and melt-mush reactions induced by magma recharge in the magmatic plumbing system. Trace elements and Sr isotopes of the most primitive Cpx (Mg# 87–93) support the interpretation that the parental magma for the Wozhong ultrapotassic volcanic rocks was originally derived from a metasomatized mantle source.