The Late Jurassic high-Mg andesites in the Daru Tso area, Tibet: Evidencefor the subduction of the Bangong Co-Nujiang River oceanic lithosphere.

This paper reports zircon U-Pb and Hf isotopic data of major and trace elements for the andesites of Jienu Group from the Daru Tso area in the northern Lhasa block. LA-ICP-MS U-Pb zircon analyses suggest that the andesite were formed at 163.3±1.7Ma, and that the volcanic rocks were formed in early Late Jurassic period. Geochemical studies show Daru Tso andesites have typical geochemical characteristics similar to the high-Mg andesite (HMA). They are characterized by high MgO content, Mg# values, and low TFeO/MgO ratios. They are enriched in LREE in the chondrite-normalized REE patterns with negative Eu anomalies, and are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs). Zircon εHf(t) values are from -8.5 to-6.7. Geochemical features of the Daru Tso high-Mg andesites are considered to have been generated by the subduction of Bangong Co-Nujiang River oceanic lithosphere, and can be genetically regarded as resulting from partial melting of subduction sediment melts that had been metasomatized by mantle peridotite under the condition of oceanic crust subduction. They were formed at the tectonic setting of the active continental margin (Andeans-type) associated with Bangong Co-Nujiang River oceanic crust subduction. It is therefore held that the Late Jurassic Jienu Group volcanic rocks in the Daru Tso area are the products of subduction of the Bangong Co-Nujiang River oceanic lithosphere. These results provide direct evidence of the volcanic rocks for the subduction of the Bangong Co-Nujiang River Tethys Ocean.