Geochemical constrains on the chromite petrogenesis of Yushigou opiolites in Qilian orogenic belt,West China

摘要

The Qilian orogenic belt is an important Early Paleozoic suture zone,in Western China.This belt includes several ophiolite belts outputing chromite.Chromite ore exsits at the bottom of the Yushigou fresh ophiolite.Geochemical research about chromites and mantle peridotites of Qilian orogenic belt was rare.In this paper,chromites and peridetites were sampled from Yushigou ophiolite for mineralogical,petrological and geochemical study in order to place some constraints on the petrogenesis of the chromite deposit in the context of tectonic evoution.The main results were as follows:The Al2O3(0.3-1.04wt.％) and CaO(0.01-0.5wt.％) content of the harzburgite are consistent with harzburgite being a residue of high extent of melt extraction,which is also corroborated by the average Fo(91.86) of olivine and the average Mg# of orthopyroxenes (92.5) and clinopyroxenes (93.24) in the harzburgite.The LREE-enriched in particular of the Pb and U-shaped REE patterns may reflect that the harzburgites are refertilized by a later percolating melt component.The range of spinel Cr# from podiform chromite deposit is wider than spinel Cr# of harzburgite and similar with dunite,which reveal that the dunite and chromite are both from Cr bearing melt.The main ore-host rocks are cumulative dunites refertilized by a later percolating melt component,we argued that podiform chromite ores resulted from the combined effects of magma and melt/rock interaction in the immature back-arc basin.