Chronology And Geochemistry Of Magmatic Rocks In Xainza At Central Lhasa Terrane, Tibet

There are manny Mesozoic volcanic rocks and associated granitoids in the north and central Lhasa area, their petrogenesis and geological setting is is still ongoing debate up to now. This dissertation reports the data of chronology, bulk-rock major element, trace element and Sr-Nd-Hf isotopes of the magmatic rocks in the Xainza region of Central Lhasa Terrane, Tibet, aiming to provide more answers for the solving of this question.The magmatic rocks in the Xainza region include two types, intrusive rocks and volcanic rocks. The intrusive rocks include granodiorite, gabbroic enclaves and dioritic enclaves. The volcanic rocks mainly are basalt - andesite– dacite in Cretaceous Zeno group, and dacite have a majority, basalt are less.The result of zircon U-Pb LA-ICP-MS and SHRIMP dating indicate: the age of host granodiorite is 113 Ma and dioritic enclave is 111 Ma; the age of two dacite sample which located northwest of Waang Lake are 117 Ma and 111 Ma respectively, and the age of one dactie sample which located sourtheast of Xainza town is 112 Ma, and the age of two dacite sample which located the west shore of Geren Lake are both 114 Ma. All the age of the samples are Early Cretaceous, we collect the ages of the present Mesozoic magmatic rocks, and the statistical result indicates that the magmatic flare-up at～113 Ma occurred in the central and northern Lhasa Terrane.The geochemistry feature of the Xainza magmatic rocks divided into significant two groups. All the intrusive rocks and most of the volcanic rocks are metaluminous, medium- to high-K calc-alkaline rocks. All the samples are enriched in light REE and depleted in heavy REE,most samples show well-developed to weak negative Eu anomalies; Rb, Th, U are enriched and Ba, Nb, Ta, Ti, Sr are depleted. Intermediate-acidic rocks have stronger enriched and depleted feature compar to intermediate-basic rocks. Most intermediate-acidic rocks display enriched Sr-Nd isotopic feature (εNd (t) values are -13.6～-9.1) which may represent the anatexis of the ancient Central Lhasa basement, and most intermediate-basic rocks display relative depleted feature (εNd (t) values are -4.1～﹢0.6) which may represent the ancient crust basement that mixed with mantle materials or juvenile crust materials. All the zircon Hf isotopic samples are intermediate-acidic rocks, all samples indicate negativeεHf (t) values (-13.9～-3.7) and relative old Hf modal ages (1440～2057 Ma). Combined with regional Hf isotopic date, central Lhasa Terrane have an Archean crust basement.The petrogenesis of the magmatic rocks in Xainza are juvenile crust or mantle materials mixing with the old crust. The Xainza magmatic rocks and the great Early Cretacesous magmatism at～113 Ma in the north and central Lhasa Terrane are formed in a setting of south subducton of Bangong-Nujiang sea floor and following slab rollback and break-off which caused the rising asthenosphere materials The basic rocks in Xainza magmatic rocks generally have the geochemical affinity of within-plate basalts, this feature is consistent with the modal of slab break-off.