Discovery of the boninite series volcanic rocks in the Bangong Lake ophiolite mélange, western Tibet, and its tectonic implications

The boninite series volcanic rocks, mainly com- posed of basaltic andesite, andesitic lava breccia and andesite porphyrite, were recognized for the first time in the Bangong Lake ophiolite mélange, western Tibet. These rocks have a strong boninitic affinity, with high SiO2 (55.61%—59.23%, weight percent), MgO (6.63%—13.08%, 9.13% on average (weight percent)), Al2O3/TiO2 ratios (36—54), Mg# (0.61— 0.74), Ni (116 ppm on average) and Cr (354 ppm on average), low TiO2 (0.23%—0.39%, weight percent), and strong LILEs enrichment relative to the depleted HFSEs. C1-chondrite normalized (La/Gd)N and (Gd/Yb)N ratios of about 1.70 and 0.83, respectively, produce prominent “U-shaped” nor- malized REE (rare earth element) patterns. Such a close compositional affinity to boninite indicates that these volcanic rocks were formed in a forearc setting produced by the intra-oceanic subduction.