Petrology, Geochemistry And Exsulutions In Eclogite And Its Tectonic Implications Of Western Peng Lake Ophiolite, Tibet

Western Peng Lake ophiolite belongs to the middle segment of the Bangong-Nujiang suture. It is a good ophiolitic accumulate section occurring in Lake Area, northern Tibet. Base on predecessor research result and detailed field investigation of Western Peng Lake ophiolite, this paper study the petrology, geochemistry and exsolutions in eclogites of this ophiolite, Furthermore discuss the geological implication of exsolutions in eclogites.The research about petrography of the ophiolite indicated the rock type of Western Peng Lake ophiolite include pyroxene-bearing dunite, accumulative pyroxene peridotite, (olivine) gabbro, eclogite and (pyroxene-bearing) plagioclasite. The rock type is mainly accumulative rock. The accumulus minerals include olivine, clinopyroxene and plagioclase. Chromite is the secondary accumulus mineral occurring in ultramafic rock in the bottom of the accumulative sequence. The sequence of fractional crystallization of these minerals is chromite→olivine→clinopyroxene→plagioclase, which correspond to the rocks sequence of dunite→pyroxeneperidotite→olivine gabbro→gabbro or/and eclogite→pyroxene-bearing plagioclasite→plagioclasite. Fractional crystallization of the magma reflects rocks are abundant in Mg and Fe early while rich in Al and Ca later.Gabbros in Western Peng Lake ophiolite mostly undergo eclogite phase metamorphism and transform to eclogite. Pyroxeneperidotite also have rock type of bearing garnet. Exsolutions are found in many minerals in Pyroxeneperidotite, eclogite, gabbro and even plagioclase. These rocks are characterized by cataclastic deformation. Exsolutions in rocks imply that these rocks had ever existed in the deep mantle and suffer from ultra-high pressure metamorphism and multi-stage structure deformation.The composition of rocks of Western Peng Lake ophiolite is similar to that of the representative ophiolitic accumulative rocks in the world. West Peng Lake ophiolite was produced by fractionation of tholeiitic basalt magma. The Harker diagram and AlO₃-CaO-MgO diagram of the rocks well reflect the process of magma accumulation. The evolution of magma displays a trend which element enrichment in rocks vary from MgO and Fe₂O₃ to Al₂O₃ and CaO.The distribution of trace and rare earth elements of the rocks are consistent each other. It indicates same genesis of these rocks. Nb and Ta content of rocks are extremely depletion. REE content is very low. Distribution of REE of rocks are flat type with LREE lightly depletion. Geochemistry characteristics of Western Peng Lake ophiolite indicate it is SSZ type ophiolite and formed in a spreading center of back arc basin.Exsolution structure is ubiquitous in ophiolitic eclogites. The exsolutions include garnet, clinopyroxene, orthopyroxene, alumchromite and albite. The exsolution structure is characteristic by garnet and orthopyroxene exsolutions in host clinopyroxenes and clinopyroxene exsolutions in host garnets. According to the characteristics of exsolution structure and exsolution relationship three stages of clinopyroxene-garnet pair are identified.Composition of the exsolutions measured by electron microprobe and energy spectrum shows host clinopyroxenes in eclogite are diopsides. Clinopyroxenes exsolutions in host clinopyroxenes and host garnets include diopsides and augites. Orthopyroxene exsolutions are enstautes. The composition of garnet exsolutions vary slightly. They are all garnet with nearly 100% pyrope.Eclogite P-T path research implies Western Peng Lake ophiolite had ever existed at deep mantle about 150 km under the surface and suffered from ultra-high metamorphism. Decrease of temperature and pressure of rocks result in multi-stage and multi-type exsolutions in subsequent process of returning to the surface. According the study above we conclude that oceanic crust subducting to deep mantle after closing of Tethys induces the oceanic rocks occur ultra-high metamorphism. Subsequently, Collision of two land block between the Tethys produced a force which squeezed out the oceanic mass in wedge area of the supra-subduction zone and make the oceanic rocks return to the surface.