The bilateral and nearly simultaneous obduction of Gaize (central Tibet) ophiolites at the Late Jurassic: constraints on the closure of Bangong-Nujiang suture

Systematic mapping of the Gaize area (central Tibet) across the middle-western Bangong-Nujiang suture and the Lhasa and Qiangtang blocks was conducted, during which ophiolite overthrust sheets in both northern Lhasa and southern South Qiangtang blocks were documented (Fig. 1A). Geochronological dating shows that these ophiolite sheets could have been transported bilaterally, both northward and southward, nearly simultaneously at the Late Jurassic (151~153 Ma). The Laguocuo ophiolite overthrust sheet is in northern Lhasa block south of Gaize county, and the electron microprobe analysis (EMPA) result shows that the newly-formed syntectonic minerals within the mica-quartz schist are phengites. Thermobaric conditions of those phengites are about 350±50 °C and 7~10 kb, and their 40Ar/39Ar plateau age is about 153 Ma. The counterpart ophiolite overthrust sheet in Cha’erkangcuo area is in southern South Qiangtang north of Gaize county and the EMPA result shows that the newly-formed syntectonic minerals within the mica-quartz schist are biotites with 40Ar/39Ar plateau ages of 151~153 Ma. Comprehensive analyses show that the Laguocuo and Cha’erkangcuo ophiolite mélanges are the north-to-south and south-to-norht obduction of the Early-Middle Jurassic Bangonghu-Nujiang ophiolites at the Late Jurassic respectively. The thermobaric conditions and age study of the Laguocuo and Cha’erkangcuo ophiolitie sheets show that there existed bilateral and nearly simultaneous obdution of Gaize ophiolites in middle-western Bangong-Nujiang ophiolitie belt and the north-to-south obduction remained dominant. The obduction could have been driven by external force, most probably by the flat-lying oceanic subduction of the Yarlung-Zangpo Neo-Tethys (Fig. 1B). Fig. 1. The simplified tectonic map of Tibetan Plateau (A) and schematic cross section showing the bilateral obdution of Gaize ophiolites (B)