Cenozoic K-rich adakitic volcanic rocks in the Hohxil area, northern Tibet: Lower-crustal melting in an intracontinental setting

It is generally accepted that the Cenozoic potassic volcanic rocks of northern Tibet were derived from a lithospheric mantle source. Here we report new chronological, geochemical, and isotopic data for the Miocene (ca. 18-15 Ma) K-rich adakitic volcanic rocks from the Hohxil area of the Songpan-Ganzi block in northern Tibet. We contend that these rocks were generated by partial melting of the malic lower crust, in an intracontinental setting unrelated to subduction of oceanic crust. The Hohxil rocks exhibit high Sr/Y and La/Yb ratios, high Sr and La contents, but low Yb and Y concentrations, similar to adakites formed by slab melting associated with subduction. However, their relatively low epsilon(Nd) values (-2.09 to -4.58); high Sr-87/Sr-86 (0.7072-0.7075), Th/U, Th/Ba, and Rb/Ba ratios; and distinctive potassium enrichments (K2O > Na2O) are very different from the composition of typical adakites. In addition, those K-rich adakitic rocks with the highest SiO2 contents (> 61 wt%) exhibit the lowest Sr-87/Sr-86 ratios and highest epsilon(Nd) values and are the oldest Cenozoic volcanic rocks exposed in the Songpan-Ganzi block, suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma. Taking into account the composition of lower-crustal mafic xenoliths in Tibet, as well as the tectonic and geophysical evidence, we conclude that the Hohxil adakitic magmas were produced by partial melting of amphibole-bearing eclogites with a K-rich mafic bulk composition, in the lower part (>=similar to 55 km) of the thickened northern Tibetan crust. Partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward-subducted continental lithosphere.