The Study Of The Ophiolite And Chromite Deposit In Luliangshan Luofengpo,Qinghai Province

Luliangshan chromite is located in the tectonic zone of the North Qaidam, its geological tectonic evolution is a microcosm of the structural zone’s evolution of North Qaidam.Outcropping ultramafic rocks can be divided into two parts:the metamorphic peridotites and the ultramafic cumulates.The chromite ore body is associated with the ultramafic cumulates.The ophiolite in the survey area is mostly composed of ultramafic rocks, gabbro as well as Tanjianshan volcanic rocks. In general, ultramafic rocks, gabbro and Tanjianshan volcanic rocks distributed in proper sequence from the west to the east, but there is a lack of ultramafic rocks and gabbro in some of this area. The output form of gabbro is dykes, and it was intrusive when contacted with ultramafic rocks and Tanjianshan volcanic rocks. The ultrabasic rocks and Tanjianshan volcanic rocks were often contacted with the NW-trending fault.Through the analysis of chrome spinel’s Cr-Mg# graph, we determined that chromite in Luliangshan ultrabasic rocks did not fall into the classic abyssal peridotites area, but fell on the position of the layered intrusions. The growth of the chromite was associated with the high-pressure phase cumulates which marked by pyroxenite. It was impossible to be formed from a low depleted mantle below the Moho.It should be formed from a highly depleted mantle. The associated growth of chromite and the high-pressure phase cumulates belonged to the PPG series ophiolite, and formed in the island arc or fore-arc basin environment. Thus it can be seen that the ophiolite formed from a tectonic setting which is related to the environment of island arc, and it should belong to the SSZ-type ophiolite.Luliangshan Luofengpo chromite deposit metallogenic rock piled rock series included multiple depositional cycles, and pointed out that the complex rock was not formed from a single differentiation, they had a relationship with the repeated pouring in of the new melt in an open magma chamber. In short, no matter from the wild macroscopic characteristics or from the microscope trait, or even from the mineral’s chemical features, it can be estimated that Luliangshan Luofengpo chromite deposit belonged to the earliest magmatic deposits, and it was a product of the magmatic fractionation crystallization.When chromite intruded into the thickened lithosphere after its subduction, the temperature would be up to 1123～1158℃ (1139.54℃ on average), and the mantle pressure was 3.355GPa to3.474GPa (3.404GPa on average), while the oxygen fugacity was FMQ-1.02～FMQ-2.09log units (FMQ-2 141og units on average).The research of Luliangshan Luofengpo chromite with compositional zoning suggested that the ultramafic cumulates had undergone a thermal path of temperature decreasing and oxygen fugacity increasing. It can be inferred that the ultramafic cumulates has experienced the process of high temperature and high mantle pressure (lower oxygen fugacity), and due to tectonic exhumation movement, it was thrust to the crustal surface where they were subjected to regional metamorphism (higher oxygen fugacity).