The Petrogenesis Of Paleozoic Granitoids In The North Margin Of Qaidam Basin And Their Orogenic Response

As the product of crust-mantle interaction, granites contain abundant information about geological processes. So, it's not easy to discuss Paleozoic granites and their orogenic response without detailed study in North Margin of Qaidam Basin. Meanwhile , the initial Sr ,Nd isotope ratios and Nd isotopic model age are used to trace the source rock of granites, but the precision estimation of these parameters have not been attached enough importance to. Also, there is no formula can be used to calculate errors of these parameters.Based on above reasons, the purposes of the research are to build a reliable geochronological framework of Paleozoic granitoid in study area, to distinguish their genetic types and tectonic settings, and to analyses their orogenic response; to derive a set of formulas for calculating errors of the paramers mentioned above; to ascertain if North Qaidam belongs to North China Block or Yangtze Block (South China). In order to achieve these goals, field geological surveying, laboratory testing, comprehensive analysis and research are adopted as a way for the studying.Research results and conclusions are as follows:1) Granitic magmatic activities during Paleozoic in North Margin of Qaidam Basin can be divided into five stages, from early to late , the first stage is from 475Ma to 460Ma, or from the end of Early Ordovician to Middle Ordovician; the second stage from 450Ma to 425Ma, or from the end of late Ordovician to Early Silurian; the third stage from 410Ma to 395Ma, equivalent to Early Devonian; the fouth stage from 380Ma to 370Ma, equals to Late Devonian; the last stage from 275Ma to 260Ma, equivalent to Middle Permian.2) Aolaoshan, Datouyanggou, Saishitengshan and Tuanyushan plutons belong to metaluminous calc alkaline series; Northwest Daqaidan, Yematan, Xitieshan, Sanchagou and Qaidamshan plutons attributed to peraluminous high-k calc-alkaline series; Luliangshan pluton belongs to strongly peraluminous calc-alkaline and high-k calc-alkaline series.3) The source rock of these plutons: basic rock (basalt ) for Aolaoshan and Tuanyushan plutons; Middle-Proterozoic Shaliuhe Group for Xitieshan and Yematan plutons; Early-Proterozoic Dakendaban Group for Chaidamushan and Lvliangshan plutons; mainly Dakendaban Group and a part of basic rock for Northwest Dachaidan and Datouyanggou plutons; the intermixture of metamorphic basic rock and sedimentary for Sanchagou and Saishitengshan plutons.4) Nd isotopic model ages show that the basement of North Qaidam is similar to that of Yangtz Block, Genetic types of granites in the area: Qaidamshan pluton and Xitieshan pluton are S-type granites; Lvliangshan pluton is post-collision SP type; Daqaidamxibei and Yematan plutons are I-S-type granites, the other plutons belong to I-type granite. Additionally, different genetic type granites can occur in the same stage of magmatasm.5) The orogenic responses of granitic magma activities during Paleozoic in North Margin of Qaidam Basin are: deep subduction and ecologite-facies metamorphism of the oceanic crust, and volcanic arc (Tanjianshan Group) formed at the first stage; Qaidam Block was subducted beneath Oulongbuluke Block and undergone a high-pressure granulite facies metamorphism to form Lvliangshan high-pressure basic granulite, upwelling mantel material caused the partial melting of crust material at the second stage; post-collision stage, crustal thickening reached to the top and ready to thinning, partial melting produced within thickened lower crust at the third stage; continuous crustal thinning , intramontane basins , rift basins and molasses - continengtal volcanic rock formations, upwelling mantel thermal flow lead to partial melting of the crust at the fourth stage; Paleo-Tethys Ocean was closed, the second subduction in North Qaidam Basin resuled in the activity of I-type granitic magma again.6) Formulas for error calculating about the initial Sr, Nd isotope ratios and Nd isotopic model age are theoretically reliable, and the application of these formulas in this research are also successful. The result of the application indicates that the error of Nd isotopic trace parameters is less than that of Sr isotopic trace parameters. By the way, the crystallization thermometer of rutile is more suitable than that of zircon.