Geochemistry of clinopyroxene and chrome spinel in the Zhaheba peridotite, Eastern Junggar, Xinjiang, China and its chromitite metallogenesis

Zhaheba ophiolite is a significant ophiolite complex in Eastern Junggar, consisting mainly of peridiotite, basalt, layered gabbro, plagiogranite and chert. Among them, the peridiotite is mainly composed of harzburgite, herzolite, and minor dunite. The average contents of clinopyroxene in herzolite are Cr₂O₃ 1.11%, Al₂O₃ 4.77%, MgO 16.99%, CaO 21.84%, SiO₂ 50.00%, and the accessory mineral chromium spinel falling within the high-Al types has lower average contents Cr₂O₃ 40.35%、Cr^# 0.53 but higher Al₂O₃ 24.10%, MgO 13.23% and Mg^#(0.62), which indicate that it was formed during the extension of Zhaheba ocean in MOR environment. While the element contents of chrome spinels in massive chromites changes little with average contents of Cr₂O₃ 55.45%、Al₂O₃ 10.88%, MgO 11.98% and Mg^# 0.60, which fall in high-Cr type fields and belong to SSZ type of chromites. Clinopyroxenes in herzolite is characterized by typical structures of melted residue and melt-rock reaction, and orthopyroxene in harzburgite kept bastitic pseudomorphs and unaltered enstatites, but chrome spinel accessory minerals show corrosion features. The melting residual structure of monocline is a manifestation of the melting and chromium release of chromium-bearing minerals, and also a structural indicator of the partial melting degree of peridotite increasing and the evolution towards more magnesium rich direction, which may have limited contribution to the formation of Cr-rich chromite. The peridotite contains monoclinopyroxene, olivine and textures newly generated from melt-rock reaction. The role of fluid and volatiles in the melt-rock reaction cannot be ignored. Based on the above studies, the relationship among the content differences of Cr^#、Mg^# and Al₂O₃ of chromite and tectonic settings of ophiolite, and its influence factors are discussed. It is concluded that mantle convection and subduction of Kalamaili ocean led to the enrichment of the high-Al chrome spinels in MOR environment and forming of massive chromites.