The Magmatic Record In The Peridotites From Yushigou, Qilian Orogen And The Petrogenesis Of The Ophiolite-type Chromitites

The origin of ophiolite-type chromitites remain poorly understood despite the great effort over the years. We have sampled podiform chromitites and its host peridotites at certain intervals in Yushigou ophiolite, which is well-known for its high-Cr chromitites in the Early Paleozoic Qilian suture zone. Based on the researches in mineralogy and geochemistry, the magmatic records in the peridotites and the petrogenesis of the ophiolite-type chromitites are discussed in this thesis.The peridotites are extensively serpentinized with anhydrous whole-rock totals< 90%, with 2%-8% spinel. The silicate mineral inclusions such as olivine, clinopyroxene etc. occur in spinel from chromitites and its host peridotites. A few hydrous phases occur near the outer edge of chromian spinels.Similared with the mantle peridotites in suprasubduction zones, the high-MgO (46.23%-51.10%) and low-AI2O3 (0.01%-2.02%) and CaO (0.09%-0.56%) content (normalization of the data to 100 wt% on an anhydrous basis), high MgO/SiO2 (1.06-1.24) of the peridotites reflect that they are too depleted to be residues of partial melting. The low REE abundances of peridotites have the charactristic of LREE enriched and HREE depleted, showing varying degrees of Nb negative anomalies and Cs-U-Pb positive anomalies. There is no consistent correlation between LREE and LOI.While the significant correlations of LREE with HFSE suggest that enrichments don’t result from the serpentinization. The peridotites may have undergone hydrous melt refertilization in the suprasubduction zones setting.Compared with global abyssal peridotites, the spinels of host peridotites near the orebody have very high Cr* (>65). Futhermore, the uniform chemical characteristics of chromite-hosted silicate mineral inclusions (Al2O3-opx:0.36%-0.43%, Al2O3-Cpx: 0.70%-1.00%, TiO2-Amp:0.17%-0.33%) suggest that the orebody may have formed in the suprasubduction zones setting. With irregular changes of spatial distribution in Cr# from 78.0 to 45.1 and TiO2 from 0.31% to 0.07%, the spinels in host peridotites show that formation of the high-Cr chromitites have a boninitic melt affinity instead of originating from the host peridotites. Water in the arc magma play an important role in the formation of podiform chromitites.The subsolidus re-equilibration processes severely affect the Fe-Mg distribution in silicate mineral inclusions and host chromites. Such Fe-Mg exchange can also occurred between the chromitites and its directly surrounding rock. In addition to the aforementioned geochemical characteristics that also owned by the suprasubduction zones mantle peridotites, the higher-Mg1value, higher- Cr content in the host peridotites can be regarded as indicators to identify the orebody from general large-scale harzburgites or dunites nearby.