Geochronology And Geochemistry Of Volcano Rocks From The Luobusha Conglomerates In Southeast Tibet And Their Implications

The Yarlung Tsangpo suture zone marks the important tectonic boundary between India and Asia which record the evolution of the Tibet Plateau. In order to providing more data for depicting the evolution of the Tibet Plateau and collision between Indian Plate and Eurasian Plate, This dissertation reports the data of zircon U-Pb geochronology, zircon Lu-Hf isotopes, bulk-rock major element, trace element and Sr-Nd-Pb isotopes of the volcanic rocks inter-bedded in the Luobusa conglomerates, in the eastern portion of the suture zone to the SE of Lhasa terrane, Tibet.Luobusa conglomerates, a molasses unit, is a thick succession of coarse clastic strata and a depositional contact upon Gangdese batholith surface can be observed. After field investigation the author confirmed that the existence of significant changes in the sediment source, reflecting the tectonic activities along the sides of the suture zone. Near at the bottom of the conglomerates among dacite-tuffs,the author found that there is a small amount of dacite, layered output.LA-ICP-MS zircons U-Pb data show that volcanic rocks interbedded with the conglomerates was erupted at 18.7 ± 0.2Ma ago along with the sedimentation of Luobusa conglomerates,and the granite just underlying the conglomerates was emplaced at the age of 29.9±0.4Ma. Considering the geological relations, the author argue that somewhere between 18.7Ma ~ 29.9Ma Luobusa conglomerates began deposition.The volcanic rocks belong to shoshonite-like series, all the samples are enriched in LREE and LILE but depleted in HREE and high field strength elements, with weak negative Eu anomalies. Moreover, the volcanic rocks belong to adakite-like derived from partial melting of the thickened lower crust, with very high content of Sr(average 405 × 10-6), very low content of Y(average 10.05×10-6) and ratio of Sr/Y=36.40 ~43.21. With εNd(t)=-2.58~-3.36,depleted mantle model age TDM(Ga)= 0.84Ga~0.91 Ga, bulk SrNd-Pb isotope composition show has characteristics of juvenile crust, and zircons Lu-Hf isotope composition consistent with the bulk Sr- Nd-Pb isotope composition, indicating that depleted mantle provided materials for the source of the volcanic rocks, and neo-tethys ocean crust played a role in the development of the volcanic rocks.The volcanic rocks interbedded with luobusa conglomerates having the trend of tethys ocean crust subduction suggest that at 18.7±0.2Ma the subduction of Indian plate didn’t reach this region. Comparing with the ultrapotassic rocks in the west of the Lhasa terrane, This dissertation speculate that the velocity of Indian plate subduction is much big.