Indian cratonic mantle beneath northern Qiangtang in eastern Tibet ca. 11 Ma

Establishing the northern limit of Indian lithosphere under the Tibetan Plateau is key to understanding India−Asia convergence mechanisms. Its location is disputed due to conflicting seismic interpretations and in particular the inability to distinguish between the subducted Indian and thickened Tibetan lithospheres in tomographic images. We report the results of a new approach based on the petrology and geochemistry of Miocene (ca. 11 Ma) lamproites recently discovered in northern Qiangtang of the Himalayan-Tibetan orogen. They are clearly distinguished from other orogenic lamproites by lead isotope ratios plotting to the left of the geochron, and their Pb-Nd-Os isotopes indicate an Archean (>2.7 Ga) Indian cratonic mantle source. This is compelling evidence that Indian cratonic mantle underthrust northward for ∼600 km and had reached below northern Qiangtang in eastern Tibet at least 11 Ma. Such large-scale underthrusting of buoyant cratonic lithosphere resolves competing geophysical interpretations and plays an important role in shaping intraplate topography.