The Composition And Geological Evolution Of Pre-Devonian Metamorphic Rocks Of Dulan, Qinghai

The metamorphic terrane of pre-Devonian in Dulan region of Qinghai province was important part of central orogen in China. The tectonic location situated between the North China, Tarim and Yangtze blocks, that is the join position of the Qinling, Qinlian and Kunlui orogen. There were abundant geological records of assembly and break-up of the Rodinia supercontinent in Meso-and Neoproterzoic that were preserved in Qulan region. The studying mission is to find out the composition and the tectonic evolution of pre-Devonian metamorphic terrane in Dulan and the relation with the Rodinia supercontinent reconstruction. It would be based on the field geological survey. The geological events group is main thread and by means of isotopic geo-chronology and other knowledge. The metamorphic terranes of pre-Devonian in Dulan have been subdivided into three units from north to south, it is according to geological survey and integrating study, which the scale is 1:250,000 and 1:50,000, and based on sedimentary construction, magma movement, metamorphism and structure deformation, Analysis to the information about geophysics and faults boundary. The units include Olongbluck block, Shaliuhe UHP collision belt and Qaidam block, so the regional tectonic framework has been found. Olongbluck block was composed of Delingha Gneiss, Mehe Gneiss, Dakaidaban Group in the Paleoproterozoic, Wandonggou Group in the Mesoproterozoic and Quanji Group in the Nanhua-Sinian. The Shaliuhe UHP collision belt was constituted by Shaliuhe Group, granodioritic gneiss, tonalitic gneiss, moyitic gneiss in the Meso-and Neoproterzoic, eclogite, Tanjianshan Group and ophiolite in early Paleozoic. Qaidam block include Jinshuikou Grorp and sillimanite-cordierite granitic gneiss. The former Dakaidaban Group has been disassembled and confirmed. It has been considered that one part is metamorphic volcanic-sedimetary rock series in the Paleoproterozoic, the othe part is metamorphic clasolite in the Mesoproterozoic, which is Shaliuhe Group(or Yuqia Group) found by us in this geological survey. It provide new data for establishing tectonic framework of northern magin of Qaidam basin. At the same time, abundant granitic gneiss of the Paleoproterozoic have been found in former Dakaidaban Group, they underwent metamorphism and deformation together in the Paleoproterozoic, especially anatexis in 1900Ma. Quanji Group was unconformity above Dakaidaban Group, which is a strata in magnesoiu-carbonate rock and glacial conglomerate predominatingly. Their main rock type are glutenite, quatzite, sandshale, dolostone and tillite which have not undetwent metamorphism. The formed age is 760Ma from volcanic rocks in the bottom by isotopic geochronology. The age is approximately correspond with Rodinia break-up in Yangtze block, South Australia and America et al.. this is most persuasion Rodinia break-up evidence found in northern magin of Qaidam basin. Based up on geological, lithological, geochemical and geochronological characteristics of granitic gneiss in the proterozoic, we found out that the granitic gneiss in Paleoproterozoic including Delingha Gneiss and Mehe Gneiss, the granitic gneiss in the Mesoproperozoic consist of Luliangshan Gneiss, Caishigou Gneiss and Xitieshan Gneiss. Delingha Gneiss is biotite-adamellitic gneiss, belong to ultra-aluminium rock series, and the tectonic setting is intracontinent, the forming age is 2366±10Ma from single zircon U-Pb data. Mehe Gneiss consist of dark-gray, banding, fine grain-size, biotite-hornblende-plagioclase gneiss and meta-tonalite, the forming age is 2348±43Ma. It indicated that northern of Dulan region had underwent a large scale magma movement at 2300-2400Ma in Olongbluck block. The gneisses in Meso-and Neoproterzoic forming ages are 1190±39Ma,928±18Ma 和842Ma by TIMS and SHRIMP zircon U-Pb isotopic dating method. The petrochemistry and geochemisitry indicated that their origin environmental is magma-arc and collision orogenics type. It showed a large scale continent assembly in 1200-800Ma in this area. The early magma movement (1200-1000Ma) was consistent with Rodinia in the world, but the later period magma movement (1000-800Ma) was lagged the other block in the whole world. It has very vivid character of China block and important actual significance for studying the position of China block in the globe tectonics. Eclogites were distribute in the granitic gneisses and Shaliuhe Group with lenticle or banding like. They could be subdivided into five kinds, representative eclogite, hornblende eclogite, coesite-bearing muscovite-hornblende eclogite, kyanite eclogite and degrading eclogite. The petrochemical and geochemical characteristics were indicated that the eclogites could be subdivided into three types which were high-Ti, middle-Ti and low-Ti. Studying metamorphic minerals and metamorphism, the eclogite metamorphism underwent three stages: epidote amphibolite facies, HP eclogite facies and high-amphibolite facies. The peak temperature of eclogite facies was 650±20℃, the pressure could reached 24-28kbar. The eclogites have been determined using high precise SHRIMP and TIMS zircon U-Pb isotopic dating method, giving four group ages of 763Ma, 550-560Ma, 465-500Ma and 434Ma, the age of 465-500Ma was reflected to main collision period, so we think that the forming age of eclogites was early Paleozoic. Studying by rock paragenesis assemblage and geochemistry, the protolith of eclogites were confirmed that could be basic volcanic rocks or basic dykes,a part of eclogites belong to Shaliuhe Group, the other part of eclogites had the characteristics of island arc to ocean ridge which could be the remnant of oceanic crust or volcanic arc in early Paleozoic. Accordingly, the forming tectonic setting and mechanism of eclogites have been further discussed.