U-Pb zircon chronology, geochemical and Sr-Nd isotopic composition of Mesozoic-Cenozoic granitoids in the SE Lhasa terrane: Petrogenesis and tectonic implications

摘要

Whole-rock geochemistry, Sr-Nd isotope and zircon U-Pb isotope data are reported for seven granitoid intrusions from the eastern Lhasa terrane. Our zircon U-Pb data exhibit three periods of magmatism: 164-125 Ma, 83 Ma, and 71-45 Ma. Granitoids from the Middle Jurassic to Early Cretaceous (164-125 Ma) display evolved Nd isotope composition, with ε_(Nd)(t) = -8.6 to -15.5 and T_(2DM) = 1-6 to 2.2 Ga. In contrast, the Late Cretaceous (83 Ma) granitoids with adakitic characteristics (high Sr/Y ratios) display less evolved Nd isotopic composition, with ε_(Nd)(t) = -0.3 to -3.0 and T_(2DM) = 0.9 to 1.1 Ga. Geochemical and Sr-Nd isotopic data indicate that the Middle Jurassic to Early Cretaceous granitoids were derived from partial melting of Proterozoic crustal basement and the Late Cretaceous adakitic granitoids were derived from partial melting of over-thickened lower crust Geochemical and Sr-Nd isotopic data of granitoids from the Latest Cretaceous to Eocene period (71-45 Ma) reveal that they result from diverse magma sources including both Juvenile and reworked mature crustal materials. Our compilation of new and published data from the eastem Lhasa terrane show a marked variation in Sr/Y ratios and a step change in Sr-Nd isotope compositions during the Late Cretaceous. We suggest that the Middle Jurassic to Early Cretaceous granitoids resulted from the northward Neo-Tethyan ocean slab subduction. The increasing Sr/Y ratios from the Middle Jurassic to Late Cretaceous granitoids and northeastward migration of arc magmatism in the eastem Lhasa terrane during the interval ca. 125-95 Ma are attributed to the shallowing angle of subduction of the Neo-Tethyan ocean slab. The Late Cretaceous magmatism in the eastem Lhasa terrane probably resulted from the Neo-Tethyan mid-ocean ridge subduction and subsequent delamination of the arc root The Latest Cretaceous to Eocene granitoids could be interpreted as a magmatic response to roll-back and/or break-off of the subducted Neo-Tethyan slab.