Study On The Geochemical Characteristics, Geochronology And Tectonic Significance Of Ophiolite And Related Volcanic Rocks Of Kekeerta Area, The South Margin Of East Kunlun Mountains

The A′nyemaqen-Buqingshan tectonic mélange belt in south margin of the Eastern Kunlun Mountains is the suture between the Eastern Kunlun block and the Bayan Har block,which the Buqingshan section is made up of different types, age, sizes of tectonic rocks. In the thesis, the North Kekeerta ophiolites, Sorth Kekeerta ophiolites and Herigougongma basalt/andesite was chosen for geology, petrology, geochemistry, zircon LA-ICP-MS U-Pb isotopic chronology and tectonic environment researching, which distributed in the Kekeerta area of the Buqingshan section. Some new advances and preliminary cognitions are proposed as follows.1、The Nouth Kekeerta ophiolites is composed of serpentinite and gneissose gabbro,which located in the north section of the Herigougongma trench. The serpentinite show as highly depleted mantle with high Mg#, MgO content and low CaO, Al2O3, TiO2 content,whose protolith is dunite, and show characteristics of metasomatism of rich LREE fluid/melt from subduction zone. Geochemistry study prove that gneissose gabbro is typical cumulate gabbro with high Mg# ranging from 78.11 to 84.01 and low REE content, and at the same time show MORB-like REE partitioning model. The zircon U-Pb age of gabbro from the ophiolite is 467±4Ma.Above all, The Nouth Kekeerta ophiolites is formed in the middle Ordovician and represents pieces of the Proto-Tethys ocean ocean crust + upper mantle under the background of subduction.2、The North Kekeerta ophiolites is composed of serpentinite, serpentinized lherzolite,gabbro and anorthosite, which located in the south section of the Herigougongma trench and the middle of Kekeerta trench. The serpentinite, protolith of which is lherzolite,and serpentinized lherzolite show as highly depleted mantle with high Mg#, MgO content and low CaO, Al2O3, TiO2, REE content. And HREE partial melting curve prove that the degree of partial meltingand of the serpentinite and serpentinized lherzolite is 15 ~ 35%, and the serpentinite and serpentinized lherzolite show characteristics of metasomatism of rich LREE fluid/melt from subduction zone that they are rich in LREE compared to the curve. Thegabbro and anorthosite show the same N-MORB geochemical characteristics. The zircon U-Pb age of gabbro from the ophiolite is 457±2Ma,and anorthosite is 452±9Ma. Above all,The North Kekeerta ophiolites is formed in the late Ordovician to early Silurian and also represents pieces of the Proto-Tethys ocean ocean crust + upper mantle under the background of subduction.3、The Herigougongma basalt/andesite is composed of basalt and andesite, which located in the middle of the Herigougongma trench, and the basalt and andesite of which show typical island-arc type calc-alkaline series of basalt geochemical characteristics. It represents the volcano-arc products of oceanic crust subduction of proto-Tethyan in the Middle Ordovician to Early-Middle Silurian, Combining with regional magmatic distribution characteristics.4 、 Considering the Regional geological data and combining the thesis, the A′nyemaqen-Buqingshan tectonic mélange belt is in the stage of ocean formation and evolution of the Proto-Tethys Ocean during the Early Paleozoic, and can be divided into four orogenic phase: 1) initial disintegration stage of the unified continent formed in Proterozoic from the end of Sinian to early Cambrian, 2) the oceanic crust formed stage from early Cambrian to late Cambrian, 3) the oceanic crust expansion and subduction of the oceanic crust stage from late Cambrian to late Silurian, 4) the oceanic crust withering away and collide-to-orogenesis stage from the end of late Silurian to Devonian.