Postcollisional potassic and ultrapotassic rocks in southern Tibet: Mantle and crustal origins in response to India–Asia collision and convergence

Abstract

Postcollisional potassium-rich magmatism in southern Tibet provides an important insight into the deep processes inducing accelerated plateau uplift and associated geologic episodes during early Miocene. As a major outcropping of postcollisional magmatic rocks in the southwestern Tibet, ultrapotassic and potassic volcanism in Xungba basin occurred coevally at ∼23Ma, consisting of bimodal-like volcanic sequence. The mantle-derived ultrapotassic rocks (Group 1) are latites and marked by both mantle and crustal geochemical signatures, similar to other younger (19–10Ma) ultrapotassic rocks elsewhere in the southern Tibet. The high Th/Yb ratios, low Ba/La and Hf/Sm ratios observed in ultrapotassic rocks strongly imply derivation from a metasomatized lithospheric mantle regions enriched by inputs of pelagic sediment and carbonate during previous Tethyan oceanic subduction, while their relatively high SiO2 and Dy/Yb, low Ni/MgO and CaO/Al2O3, and convex upward 87Sr/86Sr–δ18OV-SMOW relationship suggest additional contributions of overthickened lower crust and ancient basement except for enriched mantle sources. The potassic rocks (Group 2), which underlie Group 1 rocks, are intermediate to silicic and exhibit adakitic geochemical affinities with high Sr/Y and La/Yb ratios, and low Y and Yb concentrations. The fingerprint of overthickened lower crust identified both from ultrapotassic and potassic rocks may be an important feature for magmatism occurred under the background of continental collision. And the bimodal volcanic sequence of Xungba postcollisional magmatism may further corroborate that the removal of lower part of over-thickened lithospheric mantle at depth have triggered an extension setting associated with Miocene plateau uplift.