Geologyical Characteristics And Tectonic Significance Of The Early Paleozoic Rock Blocks In Mayer Kangri Area,qiangtang Basin,Northern Tibet

The location of study area is around Mayer Kangri mountain,which is structurally located in the Middle of the Indosinian orogenic belt in southern Qiangtang,northern Tibet.The geological bodies involved in this orogenic belt is mainly Late Paleozoic blocks.In recent years,however,the early Paleozoic blocks with different properties have been found,and the study of the early Paleozoic blocks is still weak,thus the understanding differs.Based on the theory of the accretionary orogenic belt,a systematic study on the composition,distribution,deformation and geological evolution of the early Paleozoic rocks has been conducted.This study has great significance for the tectonic evolution of the Southern Qiangtang Basin.Base on structure,lithostratigraphy and isotope geochronology,three formation of complexes have been proposed in MayerKangri area: Ejiumai,Muaishan and Bensongcuo.Zircon dating of granitic gneisses from Ejiumai Complex and mylonite from Bensongcuo Complex yielded weighted average ages of 463.7±7.5Ma and 472.6±3.4Ma respectively,and SHRIMP dating results of detrital Zircons in metamorphic lithic sandstone from Mu`aishan Complex indicate that the maximum sedimentary ages of these two formations are 430.1±4.1Ma.These isotopic dating results show that the protoliths of these three formations were all formed in the early paleozoic era.Nautiloid and graptolite fossil analysis showed that the low-grade metamorphic detrital rocks in Tashishan and Wenquangou area are the lower Palaeozoic-Devonian.Petrographic and geochemical characteristics show that the protoliths of Ejiumai Complex and Bensongcuo Complex were formed in active continental margins.The Mu`aishan Complex has the early oceanic island characteristic.And the research of stratigraphy,depositional facies and fossil assemblage in Lower Palaeozoic slices from Tashishan and Wenquangou area indicates that they were both formed in Littoral and shallow oceanic environment.Aa correlating with regional stratigraphic fossils,magmatic events and age spectrum,it is considered that the Ejiumai Complex,located at the north of Longmucuo-Shuanghu Suture,is the metamorphic basement of the North Qiangtang Basin,which has the Gondwana affinity before the Middle Ordovician and Yangze affinity after that.The Mu`aishan Formation Complex,Bensongcuo Formation Complex,Tashishan lower Paleozoic and Wenquangou lower Paleozoic are all located at the south of the Longmucuo-Shuanghu Suture and have the Gondwana affinity.These early Paleozoic blocks lacated at the south of the Longmucuo-Shuanghu Suture can be divided into two parts: the lower part is subductional accretionary complex and is represented by Mu`aishan Complex.Rocks in the lower part commonly developed foliations of mineral differentiation(S1)and foliations of ductile shear deformation(S2)whith deformed base on S1.The upper part is the continental-margin sediment of the South Qiangtang,including the lower Paleozoic from Tashishan and Wenquangou area.In the upper part,the foliation(S1)was formed by the ductile deformation base on the bedding(S0).The upper and lower parts were separated by a thrust belt,in which the Bensongcuo Complex located.Base on structural analysis,it is considered that the NE and NNE compression reflected by S1 and S2 in subductional accretionary complex record the directions of the subduction of Longmucuo-Shuanghu Paleo-Tethys and the following collision in the late Triassic(~214Ma).Subsequently blocks are considered to have experienced a late Triassic to early Jurassic(196-189Ma)extensional detachment and late Cretaceous(~80Ma)crustal thickening and uplifting with subsequent(~70Ma)fast erosion.