Geochronology And Geochemistry Of The Volcanic Rocks In Shexing Group, Lin Zhou Basin, Tibet

There are a large number of Mesozoic and Cenozoic magmatic rocks distributedin the southern and central Lhasa terrane. The issue concerning their petrogenesis andgeological setting are one of the hottest ones among the Tibetan magmatism research.This dissertation reports the data of zircon U-Pb geochronology, bulk-rock majorelement, trace element and Sr-Nd-Pb isotopes of the volcanic rocks inter-bedded inthe late Cretaceous Shexing formation sedimentary rocks, in Linzhou basin to the NEof Lhasa in South Lhasa terrane, Tibet, aiming to provide more petrology andgeochemistry answers for these questions.The volcanic rocks in Linzhou Basin in Shexing formation include two types,basalts and basaltic andesites, which are inter-bedded within the sand rocks. Thewhole rocks K-Ar isotope dating results show that the volcanic rocks at ca.70Maalong with the sedimentation of Shexing formation.The volcanic rocks show relatively consistent geochemical characteristics. Thebasalt belong to alkaline to high potassium calc-alkaline series, while the andesite arehigh potassium calc-alkaline rocks. All the samples are enriched in light REE anddepleted in heavy REE, most samples show weak negative Eu anomalies; withenriched Th, U, and Pb, and depleted Nb, Ta, Ti, and Sr. Basic rocks have strongerenriched feature compare to intermediate rocks. All samples show continental arcenvironmental in trace element geochemical features. The basalts and basalticandesites are depleted in Nd isotopes (εNd(t)=+0.4~+4.4), with young model age(TDM<600Ma), suggested that they are from juvenile crust. This is also the generalisotopic feature showing by other magmatic rocks in the southern Lhasa Terrane,which reflects the significant mantle contribution to the source regions of Shexingvolcanic rocks.The basalts of Shexing formation show geochemical similarity with continentalmarginal arc basalts, accompanied with significant contributions from sub-slabasthenosphere mantle. The andesites, for the first time, is recognized in this study, as adakitic rocks, which originated from the partial melting of subducted oceanic crust.The data reported here, combined with regional geological background of thesouthern Tibet, suggest that the basalt of Shexing formation can be viewed as theproducts of partial melting of the enriched mantle wedge modified by fluids and meltsduring the subduction of Neo-Tethys oceanic lithosphere, which was induced by slabroll-back since ca.70Ma. Therefore, the existence of ca.70Ma basalt in southernTibet, formed during Neo-Tethys oceanic subduction, suggests that the initial collisionbetween India and Asia is not earlier than ca.70Ma.