Petrological Study of the Meta-ophiolite from the North Qaidam UHP Metamorphic Belt and Its Geological Implications

The North Qaidam ultra-high pressure (UHP) metamorphic belt is a typical early Paleozoic continental orogenic belt of subduction and collision. Two meta-ophiolite cross-sections in Dulan area is among the oceanic relicts before the continental subduction. Taking two meta-ophiolite cross-sections from Yematan and Shaliuhe as examples, the authors described the research progress in this area from aspects of petrology, geochemistry and geochronology, and discussed their geological significance as well. The Yematan meta-ophiolite cross-section consists of eclogite exposed in gneiss, garnet pyroxenite and serpentinized peridotite. Eclogite witnessed UHP metamorphism in k-cymrite and exsolution muscovite in omphacite. The peak metamorphism conditions are P=2.9-3.2 GPa, T=631-687 ℃. Protoliths of the eclogites resemble N-MORB and E-MROB geochemically, and zircon U-Pb age of metamorphic geochronology is of 445-457 Ma. The Shaliuhe meta-ophiolite cross-section consists of eclogites, harzburgites and rodingite. Coesite is preserved in eclogite with the peak metamorphic conditions of P=2.7-3.4 GPa and T=610-725 ℃. Geochemical information of eclogite indicates that the protoliths include cumulate gabbro and E-MORB while the harzburgite is abyssal peridotite and the high Mg^# in the second-generation olivine indicates that it underwent a subduction metamorphism. Zircon U-Pb of the protolith age is at about 520 Ma and the metamorphic age is at 440～445 Ma. Detailed studies of these two meta-ophiolite cross-sections are of great help to understand the transition process from the oceanic subduction to the continental subduction and collision.