Geochronological, geochemical and Sr-Nd-Hf isotopic composition of the Late Cretaceous-Eocene granitoids and mafic rocks in the SE Lhasa terrane: petrogenesis and tectonic implications

Abstract:

The Late Cretaceous-Eocene granitoids and mafic rocks are widely exposed in the Motuo tectono-magmatic belt (southeast Lhasa terrane). LA-ICP-MS U-Pb zircon dating shows that the granitoids have magma crystallization ages of 83 and 71-45 Ma, and the mafic rocks have magma crystallization ages of 69-50 Ma. The 83 Ma granitoids exhibit adakitic characteristics (high Sr/Y ratios) and display weakly evolved Sr-Nd-Hf isotopic composition (whole-rock I_Sr=0.7060 to 0.7065, ε_Nd(t)=-3.0 to -0.3, and zircon ε_Hf(t)=-4.2) . The 71-45 Ma granitoids have slightly variable Sr-Nd-Hf isotopic composition (whole-rock I_Sr=0.7056 to 0.7100, ε_Nd(t)=-4.0 to +3.0, and zircon ε_Hf(t)=-4.0 to +10.8). The chemical composition of the mafic rocks is comparable with low-MgO high-alumina basalts to basaltic andesites. These mafic rocks also have weakly evolved Sr-Nd-Hf composition (whole-rock I_Sr=0.7064 to 0.7086, ε_Nd(t)=-3.4 to +1.2, and zircon ε_Hf(t)=-3.8 to +6.4). Whole-rock geochemical and Sr-Nd isotopic and zircon Hf isotopic data suggest that the 83 Ma adakitic granitoids were derived from partial melting of over-thickened lower crust, the 71-45 Ma granitoids result from diverse magma sources including both juvenile and reworked mature crustal materials, and the mafic rocks were derived from partial melting of metasomatised lithospheric mantle. Our compilation of new and published data from the eastern Lhasa terrane granitoids show a marked variation in Sr/Y ratios and a step change in Sr-Nd isotope compositions during the Late Cretaceous. Besides, a synthesis of available whole-rock Sr-Nd isotopic data on lithospheric mantle-derived basalts provides an insight into the different secular evolution of the lithospheric mantle beneath the central part and the eastern part of the southern Lhasa terrane. In the central part of the southern Lhasa terrane, the Paleocene to Eocene basaltic rocks exhibit variable Sr-Nd isotopic composition. However, Sr-Nd isotopic composition of the Motuo Late Cretaceous-Eocene mafic rocks argue that they were derived from partial melting of an relatively homogeneous and depleted lithospheric mantle. We propose that the Late Cretaceous delamination resulted in the replacement of ancient lithospheric mantle by the juvenile homogeneous lithospheric mantle in the eastern Lhasa terrane.