CLINOPYROXENE PHENOCRYSTS (WITH GREEN SALITE CORES) IN TRACHYBASALTS: IMPLICATIONS FOR TWO MAGMA CHAMBERS UNDER THE KOKCHETAV UHP MASSIF, NORTH KAZAKHSTAN

Abstract

Clinopyroxene phenocrysts in the Kokchetav trachybasalts are variable in composition and textures. Two distinctive cores are recognized: diopside cores and green salite cores. The diopside cores with Mg# of 80-90 are mantled by colorless salite rims with Mg# of 70-80. The green salite cores have especially low Mg# (<70) but high Al and Ti contents. A Mg-rich band (Mg#=82-90) usually occurs between a green salite core and its rim, and/or between a colorless salite mantle and its rim. Dissolution surfaces are observed on all textural variants. Two magma chambers are needed to explain the observed clinopyroxene phenocrysts. A deep chamber at about 120 km in the upper mantle in which diopside cores crystallized, and a shallow chamber at depths of less than 40 km in which diopside cores were resorbed and overgrown by salite rims or mantles. Magma mixing in the shallow chamber is responsible for the formation of dissolution surfaces between the diopside bands and the colorless salite mantles. The dissolution surfaces on the diopside cores formed in the shallow chamber as a result of pressure decrease. This magma evolution scenario is complicated by the occurrence of the crustal-origin green salite cores in diopsides. These green cores likely represent the relics of continental materials, which were captured in the deep chamber and partially re-melted. Our observations indicate that subducted continental materials were returned to the Earth's surface as a result of magmatism. This study therefore provides direct evidence of a link between subducted continental materials (slab) and magmatism in this orogenic belt.