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1 Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, People's Republic of China
2 Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland
3 Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, People's Republic of China
4 Laboratoire de Géochronologie et UA 1278, Université de Bretagne Occidentale, 29287 Brest, France
5 Academy of Geological Survey of Qinghai Province, Xining 810012, People's Republic of China
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 mafic 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 
Nd values (–2.09 to –4.58); high 87Sr/86Sr (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 87Sr/86Sr ratios and highest 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 (
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.
Key Words: adakite • lower crust • subduction • volcanic rock • Cenozoic • Tibet
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