Late Cretaceous and Early Palaeocene intermediate-felsic intrusions from the Maizhokunggar region, southern Lhasa, Tibet: Implications for the geodynamic transition from oceanic subduction to continental collision

The tectonic transition from oceanic subduction to continental collision is a fundamental process during orogenesis, yet the nature of such transition and associated deep geodynamic processes for the Tibetan Plateau are still controversial. The long-lived magmatism of the Gangdese Belt of southern Tibet provides an excellent opportunity to study the tectonic transition from subduction of the Neo-Tethys oceanic lithosphere to the continental collision between Indian and Asian plates. We present zircon U-Pb-Hf isotopes, whole-rock geochemistry, and Sr-Nd isotopes for the intrusions from Maizhokunggar, southern Lhasa Terrane. The zircon U-Pb dating indicates that two episodes of magmatism occurred in the Maizhokunggar region, ca. 70 Ma diorites and ca. 60 Ma granitoids. The ca. 70 Ma diorites are medium-K calc-alkaline and characterized by pronounced high-field-strength element depletion and light rare earth element enrichment, similar to arc-type rocks. They have homogeneous (Sr-87/Sr-86)(i) ratios (0.70378-0.70424) and epsilon(Nd)(t) values (+2.6 to +3.4), and epsilon(Hf)(t) (+5.9 to +13.0) with relatively young depleted mantle model ages (307-762 Ma), suggesting that they were generated from the juvenile mafic lower crust induced by the underplating of basaltic magmas from depleted mantle wedge. The ca. 60 Ma granitoids, Kajiacun (KJC) granodiorites, and Qulongxue (QLX) and Bangdacun (BDC) monzogranites are high-K series and metaluminous to weakly peraluminous. The KJC granodiorites with high Sr/Y ratios (25-43) are adakitic and were likely derived from partial melting of a thickened lower crust. The BDC monzogranites have more evolved Sr-Nd-Hf isotopes and older Nd-Hf model ages than the QLX monzogranites. These geochemical variations were mainly controlled by the fractionation crystallization with assimilation of ancient crustal materials. We propose that at ca. 70 Ma, the subduction of the Neo-Tethys oceanic slab triggered the partial melting of the juvenile lower crust and formed the diorites with arc affinity, and then the initial collision (> 60 Ma) between the Indian and Asian plates caused locally thickened lower crust, resulting in adakitic granodiorites. Our study sheds light on the lithospheric scale magma evolution from the deep mantle to the shallow crust during the critical geodynamic transition to collision, and early uplift.