Late Cretaceous crustal growth in the Gangdese area, Southern Tibet: petrological and Sr-Nd-Hf-O isotopic evidence from Zhengga diorite-gabbro

Recent studies of Gangdese granitic magmatism demonstrate a mantle contribution to crustal growth in southern Tibet during the Jurassic-Early Eocene. However, the specific mechanism for adding such juvenile crust has been disputed owing to a lack of reliable evidence for contemporaneous deep mantle geodynamic processes. Here, we report on the Zhengga diorite-gabbro suite from the Gangdese area. They consist of plagioclase (labradorite and anorthite), amphibole, clinopyroxene, biotite and minor magnetite, epidote, zircon and apatite. LA-ICP-MS zircon U-Pb dating for two samples gives a ca. 94Ma age for the Zhengga intrusive rocks, i.e., the Late Cretaceous. Apart from one high-SiO₂ (52.2wt.%) diorite sample with slightly high K₂O (1.72wt.%) and initial ⁸⁷Sr/⁸⁶Sr (0.7068) and low ε_Nd(t) (-5.6) values, the gabbro samples are geochemically characterized by low SiO₂ (39.8-50.1wt.%) and K₂O (0.3-1.1wt.%), strongly negative Nb-Ta and positive Sr anomalies, and uniform initial ⁸⁷Sr/⁸⁶Sr (0.7043-0.7048). The gabbros can be divided into two groups: Group I gabbros with relatively low total rare earth element (REE), Rb and SiO₂ contents and positive Eu anomalies, and Group II gabbros with slightly higher total REE, Rb and SiO₂ contents and negative Eu anomalies. The Group I gabbros have ε_Nd(t) (+1.7 to +4.1), and zircon ε_Hf(t) (+6.5 to +11.1) and δ¹⁸O (5.89 to 7.24‰) values, which are slightly different to those of the Group II gabbros (-0.2 to +2.0, +2.9 to +6.5 and 6.24 to 7.05‰). Trace element compositions of amphibole and clinopyroxene grains suggest that the Zhengga mafic magmas contained a significant fluid-transported component, probably released from subducted oceanic lithosphere. We suggest that the parental magmas of the Zhengga gabbros were generated by the hydrous partial melting of lithospheric mantle metasomatized by sediment melts/fluids. The Group I gabbros were likely generated by the fractional crystallization of olivine or clinopyroxene from such parental magmas, with insignificant crustal contamination, whereas the Group II gabbros were probably produced by assimilation and fractional crystallization (AFC) processes from mafic magmas that were geochemically similar to the Group I gabbros. Pre-collisional underplating of mantle-derived mafic magmas likely played an important role in crustal growth and supplied the source materials for some late Late Cretaceous-Cenozoic granitoids of the Gangdese batholiths. This study also demonstrates that the hydrous partial melting of mantle wedge triggered by the dehydration in a subduction setting has a capacity to create significant volumes of juvenile continental crust.