Oldest Paleo-Tethyan ophiolitic melange in the Tibetan Plateau

An early Paleozoic ophiolitic melange has recently been documented in the west Gangma Co area, north-central Tibetan Plateau. It is composed of serpentinite, isotropic and cumulate gabbros, basalt, and plagiogranite. Whole-rock geochemical data suggest that these rocks were formed in an oceanic-ridge setting, and the depletion in Nb and Ti indicates their suprasubduction zone -nature. Furthermore, whole-rock epsilon(Nd)(t) (+3.5 to + 10.6) and zircon epsilon(Hf) (t) values (+11.4 to +14.5) suggest that these rocks were derived from a long-term depleted mantle source. These geochemical features, combined with the rock types and their field relationships, suggest the rocks represent an ophiolite suite. Zircon U-Pb dating of isotropic gabbros and plagiogranites yielded weighted mean ages ranging from 437 to 501 Ma, which makes the west Gangma Co ophiolitic melange the oldest-Paleo-Tethys ophiolitic melange in the Tibetan Plateau. The main Paleo-Tethys Ocean basin probably opened in the Middle Cambrian; it continued to grow throughout the Paleozoic and closed in the later Triassic. The Paleo-Tethys Ocean was formed by the breakup of the northern margin of Gondwana, with southward subduction of the proto-Tethys oceanic lithosphere along the northern margin of the supercontinent.