Abstract

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.

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