Carbonate Microfacies Analysis And Geology Structural Significance Of The Riwanchaka Formation,Gangma Lake,Southern Qiangtang Of Tibet

The paper is based on the project of Special Investigation on Regional Geology of the Mesozoic Basin in South Qiangtang.The study area is in the Longmu Co-Shuanghu suture zone in the middle of the Qiangtang Basin in Tibet.The sample location is near Gangma Lake,north of Gaize County,Ali district,Tibet Autonomous Region.The Longmu Co-Shuanghu suture is generally considered as the product of the subduction of the Paleo-Tethys Ocean,so it has great significance for the evolution of the Paleo-Tethys Ocean.The paper is focused on the Lower Carboniferous Riwanchaka Formation.Combined the field observation and sections drawing with the microfacies analysis of carbonate rocks,10 types of carbonate microfacies were recognized and three sedimentary facies belts were divided,which were restricted platform facies,open platform facies and platform margin shoal facies.A total of 10 species of 8 genera of coral were identified,including Arachnolasma aichiapinense Fan,Arachnolasma chabugense Fan,Arachnolasma sp.indet.,Kueichouphyllum heishihkyuanense Yu,Kueichouphyllum sinense Yu,Axophyllum jamdaense Fan,Clisiophyllum cf.subimbricutum Thomson,Clisiophyllum sp.indet.,Dibunophyllum cf.turbinatum M.Coy,Dibunophyllum sp.indet.,Diphyphyllum gerzeense Fan,Neoclisiophyllum jamdaense Fan,Slimoniphyllum cf.retiforme Fan et S.J.Chen.The age of the fossil assemblage is the Lower Carboniferous period and the Riwanchaka Formation is a combination of warm-water type fossils,which shows that it has the affinity with the Yangtze block.Integrated with other sedimentary rocks and magmatic evidences in the study area,it is preliminarily considered that the Paleo-Tethys ocean was already a mature ocean basin in the Early Carboniferous.During the closing process,the fault block of Riwanchaka Formation fell into the suture zone near the continental slope,showing the characteristics of Melange of Accretionary Complex which include the Gonwana-affnity strata on the north and south sides of the study area but Eurasian-affnity in the middle part.