Cenozoic Mg-rich potassic rocks in the Tibetan Plateau: Geochemical variations, heterogeneity of subcontinental lithospheric mantle and tectonic implications

摘要

Cenozoic Mg-rich potassic rocks (MPRs; with MgO 5= 6 wt.%, K_2O/Na_2O ≥1) that are widespread in the Tibetan Plateau are commonly explained as the products of partial melting of subcontinental lithospheric mantle (SCLM) sources. This is because the MPR have characteristic geochemical features, including high contents of K_2O and other large ion lithophile elements, coupled with depletion in the high field strength elements, suggesting an enriched lithospheric mantle source metasomatized by earlier subduction events. In this paper, we report new data together with a synthesis of the Tibetan MPR. Relative to MPR from northern Tibet (i.e., the Songpan-Ganzi and Qiangtang blocks), MPR from southern Tibet (i.e., the Lhasa block) have obviously higher SiO_2, K_2O, Rb, Th and Zr contents, and higher K_2O/Na_2O (≥2) and Rb/Sr (>0.2) ratios, coupled with lower Al_2O_3, CaO, Na_2O, Sr and Ba/Rb (<12). In addition, MPR from southern Tibet show distinctly lower ~(143)Nd/~(144)Nd_((i)) and higher ~(87)Sr/~(86)Sr_((i)) isotopic ratios than those of the north. The geochemical variations suggest that MPR from southern Tibet have mantle source compositions and mineral phases different from those from northern Tibet, reflecting the heterogeneity in the SCLM beneath the plateau. The presence of such mantle heterogeneity, combined with the spatial and temporal changes reported in other types of Cenozoic lavas on Tibet, implies that the Tibetan Plateau experienced multiple-stage tectonic events and heterogeneous uplift processes at the surface. Although existing models such as convective removal and oblique subduction can explain some of the geological phenomena related to Tibetan formation, we suggest the integrated effect of multiple tectonic events to explain the spatial and temporal distribution of MPR and other Cenozoic lavas and associated topographic uplift.