Jurassic-Cretaceous Radiolaria In The Middle And Eastern Yarlung-Zangbo Suture Zone,Xizang(Tibet) And Its Geological Significance

Detailed field work was carried out over the past several years along the Yarlung-Zangbo Suture Zone(YZSZ)in Xizang.Totally five sections were measured,including three sections(the Laogongshan Section,the Jinlang Section,the Zouxue Section)of the eastern segment of the YZSZ in Zedong area and two sections(the Qunrang I Section,Qunrang II Section)of the middle part of the YZSZ in Xigaze area.172 siliceous rocks and cherty samples were collected from these sections,161 radiolarians species belonging to 68 genera were recognized and have been described systematically in this thesis.Amang them,28 species belonging to 19 genera from the Laogongshan Section,42 species belonging to 25 genera from the Jinlang Section,54 species belonging to 29 genera from the Zouxue Section,62 species belonging to 25 genera from the Qunrang ? Section and 60 species belonging to 29 genera from the Qunrang? Section.Based on the analyses of the radiolarian fauna from the Jinlang Section,three assemblages have been established,including Laxtorum(?)jurassicum Assemblage(Aalenian),Quarticella ovalis Assemblage(late Bajocian)and Stichocapsa robusta Assemblage(middle Bathonian)in ascending order.Compared with the radiolarian zones and subzones in west Tethys,these assemblages can be well correlative to UAZones 1-2,UAZone 4 and UAZone 6.Three radiolarian assemblages have been recovered from the cherty block and one radiolarite sample from the Laogongshan Section in Zedong area.They can be correlated with UAZone 13,UAZone 17 and UAZone 22 in the west Tethys,which indicated the age of cherty blocks is from middle-upper Tithonian to Aptian.The biostratigraphic characteristics of radioraian fauna reveal that the sequence of cherty blocks has been overturned.Moderately to well-preserved radiolarians have been extracted from greyish green and purplish-red bedded cherts from the Jinlang Section in Zedong area.Three assemblages including Laxtorum(?)jurassicum Assemblage(Aalenian),Quarticella ovalis Assemblage(late Bajocian)and Stichocapsa robusta Assemblage(middle Bathonian)were established in ascending order.These assemblages can be well correlative to the Middle Jurassic UAZones(Baumgartner et al.,1995)in west Tethys and in central Japan and provide reliable age data for the Middle Jurassic stratigraphic correlation of the pelagic sediments along the YZSZ.Abundant and rather well-preserved radiolarians were extracted from the sequences of dark grey cherts which are closed to the ophiolitic rocks from the Zouxue Section in Zedong area.The radiolarian fauna can be compared with UA 1-11 which indicated the age of cherts was late Barremian to early Albian.The characteristics of radiolarian fauna and geochemical analyses reveal that the depositional environment of chert sequences of Zouxue section is probably similar to the Chongdui Formation in Xigaze area,which seems to be a transition zone between continental margin and open ocean basin.The fauna of the third layer of the lamellar cherts and siliceous mudstone from the Qunrang I Section in Xigaze area is distributed in the UA 1-9,which can be roughly compared with the"Turbocapsula Zone" established by O'Dogherty(1994).The age is from late Barremian to Aptian.The two samples in the fifth layer can be compared with UA 1-5 and UA 6-11 respectively,and the ages are late Barremian-early Aptian and late Aptian.The age of the purple-red siliceous intercalation in the sixth basaltic volcanic lava is also concentrated in Aptian.According to their geochemical data,the sedimentary environment of the cherty sequences from Qunrang ? Section was probably in a location which little further from continent margin rather than a deep-water sedimentary basin.The assemblage found in the purpish-red radiolarite from the Qunrang ? Section have been shown a lower Barremian to Aptian in age.The major elements and the rare earth elements all reflect that these cherts formed in a deep-water environment near the margin of continent where terrestrial material had obvious effect to them.Based on the radiolarian biostratigraphic research works and geochemistry characteristics,this research will provide an important timing evidence for constrain on the ophiolite belt and improve the better understand the evolution of the Neothys Ocean during Jurassic-Cretaceous.