Ultrapotassic rocks and xenoliths from South Tibet: Contrasting styles of interaction between lithospheric mantle and asthenosphere during continental collision

Widespread Miocene (24-8 Ma) ultrapotassic rocks and their entrained xenoliths provide information on the composition, structure, and thermal state of the sub-continental lithospheric mantle in southern Tibet during the India-Asia continental collision. The ultrapotassic rocks along the Lhasa block delineate two distinct lithospheric domains with different histories of depletion and enrichment. The eastern ultrapotassic rocks (89 degrees E-92 degrees E) reveal a depleted, young, and fertile lithospheric mantle (Sr-87/Sr-86 (t) = 0.704-0.707 [t is eruption time]; Hf depleted-mantle model age [T-DM] = 377-653 Ma). The western ultrapotassic rocks (79 degrees E-89 degrees E) and their peridotite xenoliths (81 degrees E) reflect a refractory harzburgitic mantle refertilized by ancient metasomatism (lavas: Sr-87/Sr-86 (t) = 0.714-0.734; peridotites: Sr-87/Sr-86 (t) = 0.709-0.716). These data integrated with seismic tomography suggest that upwelling asthenosphere was diverted away from the deep continental root beneath the western Lhasa block, but rose to shallower depths beneath a thinner lithosphere in the eastern part. Heating of the lithospheric mantle by the rising asthenosphere ultimately generated the ultrapotassic rocks with regionally distinct geochemical signatures reflecting the different nature of the lithospheric mantle.