SEDIMENTARY EVOLUTION AND PALAEOGEOGRAPHY OF THE QAMDO BLOCK IN XIZANG

The integration of the research results about Palaeozoic and Mesozoic sedimentary fa-cies,sedimentary associations,palaeogeography,biological communities and associations in-dicates that the Qamdo block is one of the continental masses in the Tethys Ocean interme-diate between Gondwana and Laurasia.The block has undergone five stages of evolution,i. e.(1)the Preeambrian crystalline basement represented by the Palaeo-to Mesoproterozoic Ningduo Group;(2)the Early Palaeozoic folded basement represented by the Qu'nyido Group;(3)the stable block,volcanic arc and back-arc basin represented by the Late Palaeo-zoic platform-type sediments;(4)Mesozoic foreland basin,and(5)Himalayan strike-slip pull-apart basin. The Late Palaeozoic sediments in the Qamdo block developed in an intracratonic shal-low-marine basin in the Caledonides.During the Mesozoic,the block experienced two tee-tonic cycles of Middle Triassic-Early Jurassic and Middle Jurassic-Late Cretaceous,and e-volved from an extensional rifted depression to a foreland basin depression.Till the Ceno-zoic(mainly in the Tertiary),the basin turhed into.a strike-slip pull-apart basin. The palaeontological communities and associations in the Qamdo block are mostly Yangtzephile or Huananphile,because:(1)in this block there are abundant endemic genera and species which are dominated by thermophilous Tethyan biotas,associated with coral reefs,Stromatopora reefs and sponge reefs;(2)the influence of glaciation and climate on Gondwanaland resulted in relatively frigid climates and incorporation of the cryophilic Gondwana biotic individuals into this area,and(3)due to the connection with the Tethyan marine realm,the biotas in this area comprise the individuals from the Yangtze,Northwest China and West Qinling areas in China,Eurasian continent,Soviet and West Europe.The Triassic biological associations are dominated by the Tethys-type,with subordinate cir-cum-Pacific individuals.In addition,the diversity and complexity of the sedimentary facies and palaeogeographic spatio-temporal distribution are also responsible for the mixing in the study area of the biotic individuals from other parts. The palaeogeographic changes of the Qamdo block indicate that during Devonian time the continent occupied the east and the ocean occupied the west of the block.The trans-gression prograded from west to east during Early to Late Devonian,resulting in the in-crease of the marine domain and decrease of the continental area.During the Early Car-boniferous,this area except the Qu'nyido zone was an open sea.Till the Late Permian,the continent became expanded in its scope in response to the uplifting and exposure of the Ri-woqe and Jomda zones.The block was characterized during different stages of the Car-boniferous and Permian by the continent and submarine rise,and by bathyal-abyssal slopes or basins on the eastern and western sides.The palaeogeographie framework of being high-er in the central part and lower at the edges of the block haye brought the evolution of the Mesozoic sediments under control.The ancient land and epicontinental seas predominated during the Early and Middle Triassic,and the eastern part of the block was in the bathyal and abyssal levels during the Middle Triassic.The ancient land passed into the shallow marine shelf during the Early and Middle Triassic.The block was uplifted giving rise to the presence of the terrigenous deltas during the late Late Triassic.The scope of the an-cient land became progressively wider during the Early to Middle Jurassic,and the trans-gression ceased throught the study area during the late Middle Jurassic,followed by the formation of the continent during the Late Jurassic to Cretaceous.