Origin of Cenozoic alkaline potassic volcanic rocks at KonglongXiang, Lhasa terrane, Tibetan Plateau: Products of partial melting of a mafic lower-crustal source?

It is generally believed that Cenozoic potassic and ultrapotassic volcanic rocks of the Tibetan Plateau were generated by partial melting of enriched mantle. This paper reports new data regarding the elemental and isotopic compositions of the KonglongXiang volcanic rocks (KVR; K2O/Na2O > 2 and low MgO contents) in the middle–western Lhasa terrane, Tibetan Plateau. Ar–Ar isotope dating of a sample from the KVR yields an age of 21.3 Ma, indicating that the KVR lavas erupted during the Early Miocene. The KVR consists mainly of trachyte and rhyolite. A comparison of the KVR with Cenozoic potassic volcanic rocks of the Tibetan Plateau and the results of melting experiments involving K-rich volcanic rocks, as well as other K-rich volcanic rocks from throughout the world, reveals that the KVR trachytes originated from a mafic lower crust containing K-rich minerals rather than from a mantle source. The KVR rhyolites were probably derived from the middle–upper crust. Based on age data, compositional data, and the regional tectonic setting, we propose that the formation of the KVR and associated N–S grabens was triggered by Early Miocene E–W regional extension in the area of the southern Tibetan Plateau.