西藏羊八井白榴斑岩锆石U-Pb年代学和地球化学及其与裂谷发育的关系

The petrology, geochemistry, and U-Pb geochronology of the Yangbajain leucite porphyry collected from Rongniduo, in the Dangung-YYangbaaain rift (graben) in southern Tibet were studied. The leucite samples are porphyritic texture, with megacrysts of leucite (as large as 4cm to 6cm) and other small minerals (plagioclase, pyroxene and olivine). They are basaltic-like ( SiO ₂ =52% ∼ 54% ) , high alkalis (K₂O +Na₂O =9. 8% ∼ 12. 3% ) , potassic 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 (<5Eu = 0. 68 ∼0. 77). 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). This suggests that the rocks are not generated directly from mantle. And the rocks also show depleted in Sr-Nd isotopic , combined with other characteristics that containing olivine and pyroxene and basaltic-like ( SiO₂ =52% ∼54% ) , indicating that they did not result from partial melting of mantle directly , but they inherit the characteristics of the depleted mantle sources , and fractional crystallization occurred thereafter. The Yangbaaain leucite porphyry and Linzizong volcanic rocks are from different magma sources , magmatic evolutionary relationships do not exist between them. The zircon dating shows the rocks crystaUized at the age of 8. 2 ∼ 15. 4Ma (weighted average age is 10. 9 ± 1. 6Ma) , which provides a powerful evidence for the timing of the Dangxung-Yangba!ain rift (graben).