Partial melting of the mantle-crust transition zone, northern Tibet: Evidence from the Cenozoic volcanics

Neogene volcanic rocks in the Belog Co area, Qiangtang, northern Tibet, are represented by a typical intermediate-basic and intermediate alkaline rock association and the main rock type is latite-trachyte. Their chemical composition is characterized by SiO2=52%-62%, Al2O3 > 15%, Na2O/K2O > 1 and MgO < 3.30%. In addition, the volcanic rocks are LREE-enriched with LREE/HREE=10-13, (La/Yb)(N)=15-19, and show a weak negative Eu anomaly with delta Eu=0.71-0.89. The close relationship between Mg-# and SiO2 and co-variation of the magmatophile elements and ultra-magmatophile elements such as La/Sm-La and Cr-Tb indicate that this association of volcanic rocks is the product of comagmatic fractional crystallization. The rock association type and lower Sm/Yb values (SmfYb=3.23-3.97) imply that this association of volcanic rocks should originate from partial melting of spinel lherzolite in the lithospheric mantle. On the other hand, the weak negative Eu anomaly and relative depletion in Nb, Ta and Ti reflect the features of terrigenous magma. So the Neogene Belog Co alkaline volcanic rocks should be the production of partial melting of the special crust-mantle transition zone on the Qinghai-Tibet plateau.