Petrological and geochemical characteristics of Kangqiong serpentinites in the central-western part of Bangong-Nujiang suture zone, Tibet

Kangqiong serpentinites, exposed in the central-western part of Bangong-Nujiang suture zone, constitute the ultrabasic end member of Kangqiong ophiolitic melange. Serpentinites in group A have relatively high Al2O3(4.23%) and Ca O(3.48%) contents, relatively depleted LREEs, their trace element normalized partition curves are similar to those of the global abyssal peridotites,whereas serpentinites in group B have lower Al2O3(0.61%) and Ca O(0.27%) values and slightly more enriched LREEs with negative Eu and Ce anomalies, and their trace element normalized partition curves resemble those of Izu-Bonin-Mariana forearc mantle peridotites, with both of them enriched in LILE(Rb, Cs) and HFSE(U, Nb). Estimation reveals that protoliths of group A were the mantle residue from partial melting of asthenospheric mantle, and protoliths of group B underwent higher-degree partial melting, indicating that they at least have experienced two stages of partial melting. It is preliminarily held that protoliths of group A were the mantle residue from partial melting of asthenospheric mantle in the forearc environment, characterized by depletion of LREE but enrichment of U and Nb because of MOR-like melt-rock interaction, and that protoliths of group B resulted from partial re-melting of group A in the intra-ocean subduction zone and became more reluctant to melting and more enriched in LREE and HFSE(U, Nb) because of boninitic melt-rock interaction, thus belonging to forearc mantle peridotites. Both protoliths of group A and group B transformed into serpentinites resulting from serpentinization, and eventually remained in the suture zone while the ancient ocean collided and closed. They are thus the rock records of intra-ocean subduction of the Bangong-Nujiang Neo-Tethys Ocean.