The Composition And Geological Evolution Of Pre-cambrain Metamorphic Rocks In East Region Of East Kunlun Orgenic Belt

East Kunlun Orogenic Belt(EKOB)is a compound orogenic belt having a long geological history, overprinted by different orogenic phase and different type orogeny. There are numerous Precambrian metamorphic rocks such as Paleoproterozoic Baishahe Formation and Mesoproterozoic Xiaomiao Formation in EKOB, which are the important material record of the Precambrian tectonic evolution. Guided by the Plate Tectonic theory and continental dynamics and dominated by systematic studies on wild-field geological characteristic, petrology, petrologic geochemistry, mineralogy and zircon U-Pb isotope geochronology of the Precambrian metamorphic rock in the east section of EKOB, furthermore, on basis of which the paper rebuild the tectonic framework and discuss regional tectonic evolution process of the area in the Precambrian. At the same time, some new points have been achieved as follows.1、The Baishahe Formation located in Southern tectonic belt can be comparable with those in Northern tectonic belt, which belong to Eastern Kunlun Orogenic Belt(east section). It evolved to crystalline basement in the late paleoproterozoic,which records the continental crust vertical hyperplasia after forming continental nucleus of Kunlun area in Archaean. The Baishahe Formation exposured in the Gouli area of Northern tectonic belt is composed of gneiss, granulite, amphibolite, marble, with quartz schist, quartzite and so on interbedded, whose protolith is sandy mudstone, argillaceous sandstone, clayrock, carbonatite and basic volcanic rocks. The geochemical characteristics of the metasedimentary rocks indicate it formed in the continental island arc or active continental margin tectonic environment, which underwent the high amphibolite facies metamorphism, whose metamorphic temperature and pressure is T=684.1℃~ 713.3℃, p=0.9GPa ~ 1.0Gpa. While, the Baishahe Formation exposured in the Kekesha area of Southern tectonic belt consists of granulite, amphibolite, plagioclase amphibole schist, marble, with gneiss, quartz schist, quartzite and so on interbedded, which underwent the high amphibolite facies metamorphism, and whose protolith is sandy mudstone, argillaceous sandstone, sandstone, carbonatite, basic volcanic rocks and little acidic volcanic rocks. The geochemical characteristics of the meta-basic volcanic rocks indicate it formed in the continental island arc or active continental margin tectonic setting.2、The Xiaomiao Formation is a set of Mesoproterozoic amphibolite facies metamorphic strata. which are an important part of Precambrian basement exposed in the East Kunlun orogenic belt. The detailed field geological mapping and petrologic studies show that the Xiaomiao Formation is mainly composed of felsic metamorphic rocks,which can be subdivided into four units:quartzite unit,Al-rich gneiss unit,felsic gneiss unit and quartzite-arkose-quartzite unit. Minor basic metamorphic rocks are also included in the felsic metamorphic rocks. The Xiaomiao Formation records at least two stage low amphibolite facies metamorphism in Jingningian and Caledonian Period respectively, which also underwent dynamic metamorphism in Caledonian Period, contact metamorphism in Indo-sinian period and the green schist facies metamorphism in the later period. Petrologic and geochemical characteristics indicate that the protoliths of quartz-schist and Al-rich gneiss are pelitic rocks,and the protoliths of quartzite and arkose-quartzite are sandstones or greywackes or silicalites, and the protolith of amphibolite is tholeiite. The provenance of metasediment is characterized by mid-low weathering degree which belongs to the cold or drought zone(maybe tectonically active region). The provenance rocks are dominatly felsic rocks bearing few alkali-feldspar,and possiblely some basic volcanic rock. Taking all factors into account, the metasediment from the Xiaomiao Formation was formed in intra-continental extensional taphrogenic trough or rift basin, and the metabasite was formed in a continental rift environment during initial or late period,which suggested that the Xiaomiao Formation was formed in intra-continental rift environment.The zircon U-Pb isotopic ages spectrum of Xiaomiao Formation reveal three peak values(2.4Ga, 1.8Ga and 1.6Ga), which indicate that its material source mainly be Paleoproterozoic geological body and minor Archean substances, and underwent metamorphic events in 1.6Ga, formed in the Mesoproterozoic. The tectonic event record in zircon age of the Xiaomiao Formation is similar to the North China Block, it shows that the gathering and cracking events of Columbia super-continent also been record in the East Kunlun.3、A suite of granitic gneisses have been identified within the Paleoproterozoic Baishahe Formation, the Mesoproterozoic Xiaomiao Formation and Halagatu granitic intrusions in the Balong-Hatu area, East Kunlun Orogenic Belt. Zircon U-Pb dating of the granitic gneisses yielded magmatic crystallization ages of 854 Ma and 895 Ma.Some zircon of these sample yield younger age(433.8±6.2Ma), it’s reveal that the granitic gneisses suffer from metamorphism in the Early Paleozoic.They are characterized by high Si O2, Al2O3, and low Ti O2 contents. The A/CNK(1.09 ~ 1.14), A/NK(1.42 ~ 1.47) values and the mineral assemblage suggest that the granites are S-type and per-aluminous granite. The chondrite-normalized REE patterns of the samples show enrichment of LREE((La/Yb)N=13.01 ~ 13.84) and strong negative Eu anomalies(δEu=0.16 ~ 0.44). The primitive mantle-normalized trace element diagrams show enrichment of Rb and Th,and depletion of Ta, Nb, Sr, P and Ti. The geochemistry of the granitic gneisses resembles that of typical crust-derived syn-collisional granites.It is suggested that the granitic gneisses resulted from dehydration melting of the Paleoproterozoic Baishahe Formation, recording a collision between micro blocks in East Kunlun in Early Neoproterozoic period. It is a response of the assembly of Rodinia Super-continent in East Kunlun.4、Since the formation of Archean continental nucleus,East Kunlun orogenic belt formed the Precambrian metamorphic basement gradually and experienced multiple plate splitting and convergence events. Considering the composition and tectonic attribute of the Precambrian metamorphic rocks, the tectonic evolution of East Kunlun orogenic belt can be divided into five stages during the Precambrian.(1) The first tectonic evolution phase is the formation of continental nucleus in the Neo-archean.(2) After forming Neo-archean continental nucleus, the continental crust underwent vertical hyperplasia and evolved to crystalline basement in the late Paleoproterozoic.(3) From the Mesoproterozoic, East Kunlun began to breakup on the basis of the Paleoproterozoic crystalline basement. The Xiaomiao Formation is the important material records in East Kunlun during the Mesoproterozoic. It’s maybe a response of the cracking of Columbia Super-continent in East Kunlun.(4) In the Early Neoproterozoic, the magmatism records a collision event between micro blocks in East Kunlun in Early Neoproterozoic period. It is a response of the assembly of Rodinia Super-continent in East Kunlun.(5) East Kunlun began to breakup in the Late Neoproterozoic. It is a response of the cracking of Rodinia Super-continent in these area.