关键词: 铬铁矿, 蛇绿岩, 弧后盆地, 扎河坝

Abstract: The tectonic environment of the formation of the Zaheba ophiolite, an important component of the Aermantai ophiolite belt in East Junggar, Xinjiang, remains highly debated. In order to determine the physicochemical condition and tectonic environment of the formation of the Zhaheba ophiolite in Xinjiang, fresh unaltered chromites were selected for electron microprobe analysis. The result shows that chromites contain uniform compositions of Cr2O3 (52.62% to 56.66%), Al2O3 (7.87% to 12.69%) and TFeO  (16.70% to 19.56%), with Cr# values of 73.58-83.80, Mg# values of 55.30-63.09, Yfe values of 4.02-8.00 and Fe2+#values of 36.91-44.70. They are mainly  chromites and a few high-iron spinel-rich chromites. The estimated average temperature and pressure for crystallization of the chromite are 1 307.84 ℃ and 4.08 GPa, respectively, inferring a formation depth of approximately 126.34 km. It is inferred that the Zhaheba chromite-hosting peridotite could be derived from the garnet lherzolites in the depleted upper mantle with mantle oxygen fugacities of 0.14 to 1.21 log units (average 0.59 log units) and degrees of partial melting of the mantle from 22.67% to 23.23% (average 22.91%). Therefore, the chromites show the characteristics of a super-subduction zone, and we suggest that the Zhaheba ophiolite was formed in the intra-oceanic back-arc basin in a subduction zone.

Key words: chromite, ophiolite, intra-oceanic back-arc basin, Zhaheba