SIMS Zircon U-Pb Ages and O Isotope Compositions of Gabbro from the Laguoco Ophiolite from Western Shiquanhe-Yonzhu Suture Zone (Tibet) and Their Geological Significance

The Shiquanhe-Laguoco-Yongzhu-Jiali ophiolite melange belt in Tibet includes the Laguoco Tso ophiolite as part of its western segment. Researchers studying the evolution of the Tibetan Plateau and the Tethys have taken a keen interest in the debated relationship between this ophiolite belt and the Bangong-Nujiang ophiolite belt. However, there is little research in this field. This study reports the rock geochemistry, SIMS (the secondary ion mass spectrometry) zircon U-Pb ages, and O isotope compositions of gabbro from the Laguoco ophiolite. Gabbro has substantial depletion in HFSEs (the high field strength elements) such as Th, Nb, Zr, and Hf, as well as exhibits comparatively high Mg-# (80.6 on average), and low K, N, P, and Ti contents. Therefore, it was possibly formed in an intra oceanic subduction island arc environment originating from similar to 30% partial melting of the depleted spinel lherzolite. The SIMS zircon U-Pb ages of the gabbro samples are 187 +/- 1.2 Ma and 189 +/- 2.1 Ma, and the weighted averages of delta O-18 are 5.24 parts per thousand +/- 0.12 parts per thousand and 5.34 parts per thousand +/- 0.13 parts per thousand, which are close to mantle-derived zircon delta O-18 (5.3 parts per thousand +/- 0.3 parts per thousand). These results suggest that during the Early Jurassic, the Laguoco ophiolite recorded the intra oceanic subduction of the northern branch of the Neo-Tethys.