Geochemistry and Genesis of the Yeba Volcanic Rocks in the Gangdise Magmatic Arc, Tibet

The Early Jurassic Yeba bimodal volcanic rocks occur among Lhasa, Dazi and Mozhugongka, with major rocks of metabasalt, basaltic ignimbrite, dacite, silicic tuff and volcanic breccia. SiO_2 contents in lava rocks are 41%-50.4% and 64%-69%, belonging to calc-alkaline basalt and dacite. One notable feature of basalt is its low TiO_2 contents, 0.66%- 1.01%, much lower than those of continental tholeiite. The REE contents of basalt and dacite are 60.3-135μg/g and 126.4-167.9μg/g respectively. Both rocks have similar REE and other trace element characteristics, with enriched LREE and LILE relative to HREE and HFS, similar REE plots and without Eu anomaly. The basalts have depleted Ti, Ta, Zr and Nb and slightly negative Nb and Ta anomalies, with Nb*=0.54-1.17 and an average of 0.84. The dacites have depleted P and Ti and also slightly negative Nb and Ta anomalies, with Nb*=0.74-1.06 and an average of 0.86.The e_ Nd(t), ( 87Sr/ 86Sr)_i values for basalts are 0.96-10.03 and 0.7043-0.7064. These values for dacites are (-1.42)-1.08 and 0.7038- 0.7049 respectively. Trace elemental and isotopic studies suggest that both basalt and dacite originated from partial melting of the mantle wedge at different degrees above subducted belt. The spinel lherzolite in upper mantle is likely the source rocks for the bimodal volcanic rocks, which might have undergone selective metasomasis of crustal fluids. Metamorphism at late stage made influence on the LILE contents. The Yeba bimodal volcanic rocks formed in temporal extensional environment in mature island arc settings, resulting from the Indosinian Gangdise magmatic arc.