| As the residue of the oceanic lithosphere, ophiolitic melange is an important evidence to determine the existence of the ancient plate boundary,which lithological units can be correspondence with the various spheres of modern oceanic lithosphere one by one. Chromite hosted in the ophiolitic melange rock is one of desperately short mineral in our country. Therefore, the study of ophiolitic melange rock in research area has important theoretical and prospecting meanning.Bangong Lake-Nujiang suture zone adjacents to Qiangtang block in the northern, connects to the northern margin of Lhasa block to the southward and to the eastward connects to the ophiolite developing in Suoxian-Dingqing-JiaYu Bridge zone,and links to Gaize-Bangong Lake ophiolite belt in the western.The work area is located in the middle of the ophiolite belt, southwest of Amdo County.the Dongqiao chromite is a large deposit developing in the mantle peridotite which is outcropping fully and located in the middle of Bangong Lake-Nujiang ophiolite zone,which It is in higher degree research in present in our country.The geochemical characteristics of ophiolitic melange rock indicading that it has the characteristics of the tholeiitic basalt,and formats at the ridge expansion environment. This article has also explored the the Dongqiao chromite bed forming material sources and the mineralization process by the use of the geochemical characteristics of the deposit and geological features. The knowledge and results of the paper is as follows:1.Through field geological survey, the paper finds east Qiao rock has metamorphic peridotite, magnesium iron ultramafic cumulate complex rock, gabbro-diabase wall, basic lava and silicalite5complete rock assemblage sequences, which fit for typical ophiolites of the combination features and provide important evidence for research on tectonic properties of public lake-Nujiang stitching.2.Comparing between petrochemistry and geochemistry, the ophiolite formation environment of the study area are researched in detail. In (FeO) MgO-(FeO) and (FeO) MgO-TiO2figure, most points are in the oceanic tholeiite and island arc tholeiite, as well as in their overlapping range. All these results indicate that the ophiolitic rocks form in the seafloor spreading environment of the ocean ridge.By analyzing mineral composition and chemical composition of Dongfeng chromite ophiolites, the paper studies the mineral deposits of mineral sources. Mineral compositions are mainly for mantle peridotite in epiphytic chrome spinel, olivine and rhombic pyroxene peridotite, while chemical constituents evolve from rich magnesium to rich iron aluminum, which concluded mineralization material originates from the primitive mantle.4.Trough more detailed research on geological background and geological features of Dongfeng chromite, discussed the Dongfeng ore metallogenic stages and metallogenic pattern, and combined the results of previous studies, the paper puts forward the three stage metallogenic model:(1) the early spreading of ocean environment of ore preconcentration: due to mantle plume diapiric rise, reduced stress increases partial melting degree and releases the ore-forming elements forming accessory minerals, which makes chromite to initial enrichment;(2) above the ocean subduction zone (SSZ) between the main mineralization arc extensional environment:As a carrier of the glass security quality and dunite melt, Cr melts and then becomes enrichment;(3) tectonic uplift stage advanced mineralization:later formed chrome spinel fills in the tectonic fissures, and with orogeny tectonic uplift, it thrusts in a set of third conglomerate. |
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