Geochemistry and geochronology of the amphibolite blocks in ophiolitic mélanges along Bangong-Nujiang suture, central Tibet

Amphibolites occur as blocks within serpentinite-matrix mélange at Dong Tso and Lagkor Tso in the Gertse area of central Tibet, where they are associated with the Bangong-Nujiang suture (BNS). Geochemical data shows amphibolites have similar MORB or arc-like MORB geochemical characteristics. Lagkor Tso amphibolites show greater arc affinity, whereas those from Dong Tso amphibolites are more MORB-like. These characteristics are similar to basalts from continental back-arc basin settings. Hornblendes from the amphibolites dated by the 40Ar/39Ar single-grain laser fusion technique show similar metamorphic ages ∼170–177 Ma, which is interpreted to represent the timing of the main metamorphic event experienced in this region. We suggest that the western and middle sectors of the BNS contain the remnants of a short-lived back-arc basin (the Bangong-Nujiang ocean), which partly opened and closed in the Middle Jurassic.