Geochemical Constrains On The Chromite Ore Petrogenesis Of Yushigou Opiolites In Qilian Orogenic Belt, West China

The Qilian orogenic belt is an important early Paleozoic suture zone, in west China, which includes several ophiolite belts with chromite ore. Chromite ore exsits at the bottom of the Yushigou ophiolite. Geochemical constrains on ore genesis of chromites and peridotites of Qilian orogenic belt was not conducted. In this paper, chromites and peridetites of Yushigou ophiolite were sampled for mineralogical, petrological and geochemical study to reveal the petrogenesis of the chromite deposit in the context of tectonic evoution. The main results were as follows:1. The contents of Al2O3(0.3-1.04wt.%) and CaO(0.01-0.5wt.%) of the harzburgite are consistent with residuary harzburgite after high extent of melt extraction, which is also corroborated by the average Fo(91.26) of olivine and the average Mg# of orthopyroxenes (91.4) and clinopyroxenes (93.1) in the harzburgite. The LILE-enriched in particular of the Pb and U-shaped REE patterns may reflect that the harzburgites are refertilized by a later percolating melt component.2. The olivine Fo value and clinopyroxene Al2O3 content range at 89.7-91.0 and 1.45-3.51wt.%, respectively. The dunite samples are characteristiced by rich large ion lithophile element (LILE) in particular of the Rb,U and Pb, and depleted Ba, Sr and Hf. Combined with LREE-enriched REE patterns and cumulate teture of the olivine, we can concluded that the chromite riched dunites are cumulate crystallized from a mantle melt and refertilized by a later percolating melt component.3. The TiO2 content (0.01wt.%) and spinel Cr# of the harzburgite are similar with the SSZ harzburgit. Considering the MORB and IAT features of related pillow basalt, we concluded that the podiform chromite deposit may had been formed in an initial back-arc basin.4. The range of spinel Cr# for podiform chromite deposit is similar with spinel Cr# of dunite, which reveal that the dunite and chromite are both from Cr bearing melt. Podiform chromite ores resulted from the combined processes of magmatism and melt/rock interaction.Maybe the Cr of podiform chromite ore mainly came from the partial melting of mantle peridotites.