The Magmatic Record in the Peridotites from Yushigou, Qilian Orogen and the Petrogenesis of the Ophiolite-Type Chromitites

The origin of ophiolite-type chromites remain poorly understood despite the great effort over the years. We have sampled podiform chromites and its host peridotites at certain intervals in Yushigou ophiolite, which is well-known for its high-Cr chromites in the Early Paleozoic Qilian suture zone. The very high MgO and low SiO₂ content of these serpentinized peridotites reflect that they are too depleted to be residues of partial melting. Both the chromites and peridotites may have undergone multi-processes of melt refertilization. Compared with global abyssal peridotites, their spinels have very high Cr^# (>65). Furthermore, the uniform chemical characteristics of chromite-hosted silicate mineral inclusions suggest that the orebody may have formed in the suprasubduction zones setting. With irregular changes of spatial distribution in Cr^# from 77.2 to 43.9 and TiO₂ from 0.34% to 0.06%, the spinels in host peridotites show that formation of the high-Cr chromites have a boninitic melt affinity instead of originating from the host peridotites. The hydrous mineral inclusions only occur near the outer edge of chromian spinels and silicate minerals may provide evidence for our hypothesis: chromite ore formation results from interaction of hydrous melt with harzburgitic ambience during ascent.