Tectonic significance of the Dongqiao ophiolite in the north-central Tibetan plateau: Evidence from zircon dating, petrological, geochemical and Sr-Nd-Hf isotopic characterization

摘要

The Dongqiao ophiolite occurs in the central segment of the Bangong-Nujiang suture zone, in north central Tibet, China. It is still debated on the tectonic setting of the Dongqiao ophiolite despite after more than 30 years' studies. The Dongqiao ophiolite has a complete section of a typical ophiolite, composed of harzburgite, dunite, layered and isotropic gabbros, pillow and massive basalts, as well as radiolarian chert. Whole-rock geochemical analyses show that harzburgite displays a broad U-shaped REE pattern and has a fore-arc affinity, whereas basalts show affinities of E-MORB, OIB and IAB. The basalts were probably formed in different tectonic settings, that is, mid-ocean ridge, oceanic island and island arc. The gabbros and basalts are characterized by positive epsilon(Nd)(t) (+1.6 to +6.7) and epsilon(Hf)(t) (+8.1 to +13.9) values. Zircon U-Pb dating yielded ages of 188 +/- 1 Ma for the layered gabbro and 181 +/- 1 Ma for the amphibole gabbro. The new ages and the published age data of the Dingqing and Dong Co ophiolites led us to conclude that the Bangong-Nujiang Ocean existed from the Late Triassic to Early Cretaceous. The new geochemical data also suggested that the Dongqiao ophiolite was a typical SSZ-type ophiolite formed in an initial fore-arc oceanic basin. Fore-arc ophiolites are probably widely distributed along the BangongNujiang suture zone. If so, the Tethys Ocean of the Bangong-Nujiang area probably existed as a fore arc oceanic basin during the Late Triassic to Early Jurassic. (C) 2015 Elsevier Ltd. All rights reserved.