Paleozoic Magmatic Rocks In The Western Kunlun Orogen Belt (Eastern Section) And Its Implications For Geological Evolution Of The Region

The Western Kunlun Orogen Belt(WKOB)is located in the northwestern margin of the Tibetan Plateau,which is the vital component of the Tibet Tethys and Central Orogenic Belt.A suite of early Paleozoic and Mesozoic granitoids occurs in the WKOB,and their ages and chemical compositions might provide important constraints on the tectonic evolution of the Proto-and Paleo-Tethyan oceans.In particular,the early Paleozoic granitic rocks are an important part of magmatic rocks in the WKOB.In this doctoral thesis,I mainly report the U-Pb ages as well as geology,petrology and geochemistry of some Paleozoic plutons,including Kashikashi pluton,Kayedi pluton,Yamen pluton,South Qimanyute pluton,Kuidaimai pluton,Alaleike pluton and Akesaiyin pluton.The Kashikashi pluton was emplaced during early-middle Ordovician at ~466 Ma.Major and trace element geochemical and isotopic data reveal that the magma was derived dominantly from partial melting of the Precambrian metasedimentary and metaigneous rocks.In addition,these rocks also display typical characteristics of continental arc-related igneous rocks,implying a subduction-related tectonic setting.The Kayedi pluton is peraluminous high-K calc-alkaline two-mica granites,was emplaced during early Late Ordovician at ~455 Ma.Major and trace element geochemical and isotopic data reveal that the magma was partial melting of the Precambrian metasedimentary rocks and enriched lithospheric mantle in the continental collision(overthrust)zone.The late Late Ordovician-Silurian granitoids in the eastern part of the WKOB could be divided into two groups,the high Sr/Y and the high-K granitoids.The Alaleike high Sr/Y granitoids were emplaced during late Late Ordovician at ~450 Ma.Major and trace element geochemical and isotopic data reveal that the magma was derived dominantly from partial melting of the juvenile lower crustal rocks under garnet amphibolite facies conditions.The Yamen high Sr/Y granitoids were emplaced during Silurian at ~432 Ma.Major and trace element geochemical and isotopic data reveal that the magma was derived dominantly from partial melting of the delamination lower crustal rocks under garnet amphibolite facies conditions.The plutons of Akesaiyin,South Qimanyute and Kuidaimai are high-K calc-alkaline granitoids,were emplaced during Silurian at 438~428 Ma.Major and trace element geochemical and isotopic data reveal that the magma was partial melting of 10%~40% the Precambrian metasedimentary and 60%~90% metaigneous rocks in the middle-lower crust at relatively low pressures without mantle materials.LA-ICP-MS and SHRIMP zircon U-Pb datings(3 samples)of the gabbros and basalts within the Qimanyute ophiolite suite show that they were emplaced during early Paleozoic period(494~483 Ma).Major and trace element geochemical and isotopic data reveal that the Qimanyute gabbros were formed in an oceanic island arc setting,which was derived dominantly from partial melting of 10%~30% shallow mantle.This result is of important significance for further understanding the early Paleozoic regional tectonics evolution in the WKOB.Based upon various analysis results in this study,in combination with previous regional geological data,a new tectonic evolution model for the WKOB is suggested.It is believed that the WKOB underwent complex early Paleozoic tectonic-magmatic evolution in this paper: early-middle Ordovician,the Proto-tethys ocean showed a double subduction;early Late Ordovician,the Proto-tethys ocean closed and the north Kunlun and south Kulun terrane collided;late Late Ordovician-Silurian,the WKOB began the post-collision stage.