Southward subduction of the Bangong-Nujiang Tethys Ocean: insights from ca. 161–129 Ma arc volcanic rocks in the north of Lhasa terrane, Tibet

The Meso-Tethys Ocean was mainly represented by the Bangong-Nujiang suture zone between the South Qiangtang and Lhasa terranes in the Tibetan plateau. The subduction polarity of this Tethys Ocean during the Mesozoic is still being debated and it has been suggested to be northward or bidirectional subduction. A series of volcanic rocks, including andesite, dacite, rhyolite and volcaniclastic rocks, are documented in the Xinji area, north of Lhasa terrane, Tibet. These samples yielded zircon U–Pb ages between 161 and 129 Ma, which represent the emplacement age of the volcanic rocks. The volcanic rocks show typical arc calc-alkaline signatures, with strong depletion in Nb, Ta and Ti, enrichment in Rb, Th, U and Pb. These features are indicative of subduction-related arc magmatism. The Jurassic andesite shows high Mg# values, and high Th but low Sr contents, and it is interpreted as a result of interaction of subduction sediment-derived melting with mantle wedge peridotite. The Early Cretaceous andesite, dacite and rhyolite show similar geochemical features and are suggested to be formed by melting of ancient crustal materials with contribution of mantle-derived magma, following by varying degrees of fractional crystallization and assimilation. Finally, we suggest that the Xinji volcanic rocks represent an Andean-type magmatic arc along the north of Lhasa terrane. They were produced by southward subduction of the Bangong-Nujiang Tethys Ocean. Consequently, a bidirectional subduction model was suggested for the evolution of the Bangong-Nujiang Tethys Ocean during the Middle Jurassic to Early Cretaceous.