Geochronology, Geochemistry And Petrogenesis Of The Yangbajing Leucite Porphyry, Tibet

The Leucite porphyry from Yangbajing as the research object, the article use the petrographic observation, electron microprobe analysis, main/trace elements analysis,whole rock Sr-Nd isotope and zircon U-Pb isotopic dating to fill the information gaps.Generally believed that the post-collisional potassic-ultrapotassic magmatic is eruption gradually from west to east in the Lhasa block, and this article identifies new ultrapotassic volcanic rocks in the central Lhasa block, near the Yangbajing rift. The use of zircon U-Pb dating to determine the age of these rocks are 8~15Ma, and adjacent areas have been studied such as, Majiang(10.1~21.29Ma), Yangying(10.7~11Ma), have the similar age characteristics. They are basaltic-like(SiO2 =52%~54%), high alkalis(K2O+Na2O=9.8%~12.3%), ultrapotassic rocks, which belongs to tephriphonolite in TAS diagram. The rocks are enriched in light rare earth element([La/Yb]N =19~20), with slightly Eu nagetive anormaly(δEu = 0.68~0.76).They show enrichment in large-ion lithophile elements(LILEs, Rb, Th, and U),depleted in high field strength elements(HFSEs, Nb, Ta, P, and Ti), and depleted in MgO, and other compatible elements(Cr, Co and Ni). In addition, the low Ce / Pb ratios(6.71~8.18) and low Nb/U(3.1~4.4), also indicating the join in of the subduction. High Th / La(0.47~0.50) and Th/Nb(0.62~0.65), and low Ba/La ratios,indicating the partial melting of subducted sediments when its components into the mantle. The most diagnostic signatures for the involvement of continent-derivedmaterials are the superchondritic Zr/Hf(45.5~49.2) and elevated Hf/Sm values(0.81~0.91) in the ultrapotassic rocks.Therefore, think Yangbajing leucite porphyry source area is likely to be fertile mantle which joined by the subduction of the crust and may also contain amphibole. The zircon dating shows the rocks crystallized at the age of 8.2~15.4 Ma(weighted average age is 10.9 ±1.6Ma),which provides a powerful evidence for the timing of the Dangxung-Yangbajain rift(graben).