Was There a Jurassic Intra-Oceanic Arc in the Yarlung Zangbo Tethyan System?

Abstract:

It is still disputed topic on subduction history of the Neo-Tethys in the south Tibet. Aitchison et al. ( J. Geol. 2007, 115, 197–213) and Jain (Current Science, 2014, 106, 254-266) suggested that the there was a Jurassic intra-oceanic arc developed within the Neo-Tethys. However, Zhang et al. (JAES, 2013, 80, 34–55) documented that this arc should be a continental margin arc. Present study reports new geochronology and geochemistry of the Sangri Group volcanic rocks which has been believed to be typical products of the Neo-Tethys subduction in the south Tibet. The Sangri Group volcanic rocks are distributed along the southern edge of the Lhasa Terrane on the northern side of the Indus-Yarlung Zangbo suture zone. It consists of the Mamuxia and Bima formations and has long been considered to be Late Jurassic to Early Cretaceous in age. We have obtained for the first time zircon LA-ICP-MS U-Pb ages for the Bima Formation volcanic rocks in Sangri County, Tibet. Two samples yield zircon U-Pb ages of 195 ± 3 Ma and 189 ± 3 Ma, respectively. These data suggest that the volcanic rocks formed during the Early Jurassic and not the Late Jurassic-Early Cretaceous as previously reported. The volcanic rocks of the Bima Formation are composed dominantly of basalt and andesite enriched in LILEs and LREEs and depleted in HFSEs, showing typical characteristics of arc volcanic rocks. They also show positive ε_Nd (t) (3.19-7.02) values and low initial ⁸⁷Sr/⁸⁶Sr (0.703182-0.705489) ratios, similar to the MORB of the Indus-Yarlung Zangbo ophiolites, indicating that the volcanic rocks were derived from a depleted mantle wedge. The magmas of the Bima Formation volcanic rocks were thus generated by the northward subduction of Neo-Tethys as early as 195 Ma. Considering they were coeval with Late Triassic-Early Jurassic felsic intrusions within the Gangdese arc, we conclude two possibilities of the early Jurassic magmatic arc: (1) it was initiated at a juvenile continental margin arc during Late Triassic-Early Jurassic, (2) Sangri group volcanic rocks represented a remnant of an intra-oceanic arc in the Tethyan system, then emplaced into the southern margin of Lhasa terrane. Here we favor the second possibility but further investigation is necessary.