昌都地块沉积演化与古地理

SEDIMENTARY EVOLUTION AND PALAEOGEOGRAPHY OF THE QAMDO BLOCK IN XIZANG

关键词:  沉积演化  古地理  多岛屿的洋  地块群

Key words:  sedimentary evolution  palaeogeography  archipelagic ocean  block group

摘要:

通过对昌都地块古生代至中生代沉积岩相、沉积组合、古地理及生物群落与生物的深入研究及现有成果,提出了沉积地质与构造演化的五个阶段:以古中元古代的宁多群为代表的前寒武纪结晶基底形成阶段;以青泥洞群为代表的早古生代褶皱基底形成阶段;晚古生代台地型沉积为代表的稳定地块及火山弧至弧后盆地形成、演化阶段;中生代前陆盆地形成发展阶段及喜山期走滑拉分盆地发展阶段^[1]。同时。表明了介于冈瓦纳和劳亚两个超级大陆之间的特提斯,是个多阶段发育的具多岛屿的洋,其间存在一系列的地块群,昌都地块便是其中之一。

Abstract:

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.