Characteristics Of Paleozoic Sedimentary Formation In Central Eastern Gangdese,Tibet

Tibetan plateau has been considered as the cradle of new geological research by geologists over long periods.The study area in central eastern Gangdese is located in the west of Brahmaputra grand canyon,the east of Yangbajin fracture,and the north and south boundary is between Jia Li fracture and the Yarlung zangbo river.The area is the focus that scholars pay close attention to,in which there are a lot of different types of sedimentary formation.Based on the sedimentary petrology,stratigraphic sequence,tectonic and paleogeography and other advanced theory,the author studies the sedimentary characteristics in view of deficiencies in previous study of late Paleozoic sedimentary construction.The author takes the way of closely linked to the sedimentary rocks and sedimentary facies,and take sedimentary characteristics as clues for study.This article is completed by participating in "the national mineral resources potential evaluation project".Late Paleozoic strata in the study area from old to new are as follows:Nuocuo group,Laigu group,Wuluong group,Luobadui group,Mengla group and Xima group.According to preliminary studies suggest that the lithological characteristics of Nuocuo group are as follows:the bottom of the formation is composed of dark-grey mid-thick layered metamorphic fine sandstone,middle shallow gray crystalline limestone,marble and slate;the middle part of the formation is composed of dark-grey sandy slate and conglomeratic metamorphic fine-grained sandstone,bedded in thick green gray andesitic welded tuff and crystalline limestone;the upper part of the formation is composed of pale-grey slate and silty slate,bedded in dark gray thick crystalline limestone and a small amount of gray mid-thick bedded marble;Sedimentary facies is composed of marina-shallow sea,continental shelf edge of slope and carbonate platform.The lithology of Laigu group is characterized by containing gravel slate of a combination of clastic rocks and a small amount of carbon rocks.The lower part of the formation is composed of light gray-gray thick layer-thick layer of coarse-and fine-grained quartz sandstone,appearing in the thick layer of fine grained lithic quartz sandstone,gray thick layer of fine grain of argillaceous lithic quartz greywacke;the upper part is dominated by grey thick layered metamorphic fine-grained quartz sandstone and dark grey toner sandy slate with repetitive rhythm in the vertical.The Lagu group is composed of marina-neritic facies,neritic shelf clastic facies,deep-water shelf,turbidite facies,conglomeratic facies and carbonate platform facies of five types of sedimentary facies.The lithological characteristics of WuluLong group are as follows:the lower part is composed of thin layers mudstone and marl rhythm layer which is in contact with the underlying Lagu group integration;the central part is calcareous mudstone with hundred meters thick;the upper part is mainly composed of a film of shale,sand,shale or thin layer marl rocks.The sedimentary facies is mainly littoral one.The Luobaudui group is mainly composed of a set of shallow Marine facies carbonate rocks.The Lithology of the formation is grey-light grey mid-thick bedded limestone,biological limestone and bioclastic limestone with hoar dolomitic limestone and amaranth thin limestone.The group is mainly composed of shallow sea shelf and platform edge facies.The lithological characteristics of Mengla group are as follows:the lower part is schistose metamorphic fine-grained quartz sandstone with phyllitic and silty slate;the middle part is phyllitic and silty slate bedded in schistose metamorphic fine-grained quartz sandstone,the upper part is composed of metamorphic sandstone,mud gravel fine grained quartz sandstone and conglomeratic with phyllitic and silty slate bedded.The sedimentary facies of Mengla group is mainly composed of marina-neritic facies and shallow sea facies.The Xima group is mainly composed of a set of marine facies clastic rocks and a small amount of carbonate rocks,The lithology of the formation is conglomeratic metamorphic complex sandstone,fine siltstone,slate and phyllite,and with a small amount of limestone or lens bedded.The sedimentary facies of Xima group is composed of the shelf edge slope sandwiched in the carbonate platforms.Although the sedimentary deposit is less in study area,but pb-zn deposits are mainly skarn type which is hydrothermal metasomatic type,such as the Per-mian Luoba group providing good ore wall rock.