Podiform chromite and associated ophiolitic rocks in West Junggar, Xinjiang, NW China

Two main types of ophiolites occur in the West Junggar Mountains: PTG (peridotite-troctolite-gabbro) and PPG (peridotite-pyroxenite-gabbro) lineages. The former is exemplified by the Hongguleleng and Sartouhai ophiolites associated with high-Al chromite deposits, and the latter occurs in Tangbale and Saleinuohai and is related to high-Cr chromite deposits.; The mantle sequences in the Hongguleleng and Sartouhai ophiolites consist of harzburgite, lherzolite, pl lherzolite and dunite. The crystallization sequence in the cumulates and mantle dikes (veins) is ol (+sp)-pl-py. In contrast, the Tangbale and Saleinuohai mantle sequences contain more abundant dunite and opx-poor harzburgite, and less lherzolite with the absence of pl lherzolite.; Although post-magmatic processes (such as subsolidus reequilibration, metasomatism and alteration) affect the compositions of minerals, compositional differences in the West Junggar ophiolites are predominantly controlled by partial melting. The Tangbale and Saleinuohai ophiolites were formed by higher degrees of partial melting (depletion) of the mantle than the Hongguleleng and Sartouhai ophiolites.; Harzburgite and lherzolite in the mantle sequences are considered as residues left after various degrees of partial melting. Dunite envelopes around chromite segregations and dunite found in the transition zone might be generated by magmatic accumulation processes but the remainder of dunites in the mantle sequences are either magmatic or residual in origin.; The primary silicate inclusions in chromite were entrapped during the precipitation of chromite rather than formed by post-magmatic entrapment. Compositions of the silicate inclusions depended on: (1) P, T and composition of the magma; (2) subsolidus reequilibration; and (3) post-magmatic hydration or alteration.; Chemical differences between the two types of segregated chromite are principally due to the degree of partial melting.; Post-magmatic processes also modified the compositions of chromite. Subsolidus reequilibration between chromite and silicates gave rise to the movement of Mg and Ni into silicates and Fe and Mn into chromite. Two processes, metasomatism due to the penetration of late mafic melt and hydrothermal alteration, resulted in an increase in {dollar}rm Fesp{lcub}2+{rcub}, Fesp{lcub}3+{rcub},{dollar} Ti, Mn, Cr# and YFe{dollar}sp{lcub}3+{rcub}{dollar} and a decrease in Al, Mg, Ni and Mg# in chromite.; Based on comparison with other ophiolites in the world and abyssal peridotites, it is proposed that the Tangbale and Saleinuohai ophiolites were formed in fore-arc regions but the Hongguleleng and Sartouhai ophiolites were probably produced in back-arc basins.; The dynamic mini-magma chamber model explains the coexistence of magmatic-textured chromite segregations and residual peridotites. The episodic influx of aqueous liquid and/or fractionated magma into mini-magma chambers probably caused chromite-dominant crystallization. Convection processes in mini-magma chambers might account for the fractionation of magma, and overgrowth and reequilibration of chromite with the melt. After the solidification of the mini-magma chambers in residual peridotites, chromite segregations underwent mantle deformation and were later emplaced onto the continents along with the residual peridotites. (Abstract shortened by UMI.)...