Zircon xenocrysts in Tibetan ultrapotassic magmas: Imaging the deep crust through time

Zircons entrained in mantle-derived magmas offer a unique opportunity to identify cryptic magmatic episodes in the deep crust and thus to image lithospheric thickening and crustal evolution. We investigated zircon xenocrysts from mantle-derived ultrapotassic rocks in southern Tibet to evaluate their potential as a probe of crustal evolution. Similar age (Proterozoic-Paleozoic) distributions of these zircons and those in the Lhasa terrane detrital spectra demonstrate the continental origin of xenocrysts with high U/Yb. Time-progressive variations in zircon epsilon(Hf)(t) reveal three major magmatic pulses ca. 90, 50, and 20 Ma, suggesting significant crustal growth in the Lhasa terrane at those times. This is consistent with major mantle inputs previously documented from surface rocks in the Lhasa terrane. Increasing Dy-N/Yb-N and U/Yb since ca. 55 Ma are interpreted to reflect progressive crustal thickening in response to the India-Asia convergence. Zircon xenocrysts with varying U-Pb ages and heterogeneous Hf isotopes indicate assimilation of Lhasa terrane crust in the genesis of ultrapotassic magmas.