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An early Paleozoic ophiolitic mélange 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 εNd(t) (+3.5 to +10.6) and zircon εHf(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 mélange the oldest Paleo-Tethys ophiolitic mélange 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. [ABSTRACT FROM AUTHOR]