Tectonic attribute of the Shiquanhe-Namco ophiolitic belt: Constraint from geochemistry of the island-arc basalts in the Asa mélange zone, central Tibet

The tectonic setting of the Shiquanhe-Namco ophiolitic belt in central Tibet is a key to understanding the Mesozoic tectonic evolution of the Meso-Tethys Ocean.Based on the analysis of major and trace element compositions of island arc basalts in the Asa mélange zone, central Tibet, its tectonic significance was discussed.Field observation and microscopic study indicate that they were formed in submarine eruption environment.Whole-rock geochemical analyses suggest that these rocks belong to calc-alkaline rocks, rich in light rare earth and Ba, Th, Pb large-ion lithophile elements, and depleted in Nb and Ta, with clear differentiation of light and heavy rare earth elements and flat distribution of heavy rare earth elements.Geochemical analysis shows that the rocks were originated from partial melting of depleted mantle metasomatized by melt of subducting sediments and formed under the oceanic island arc environment.Combined with previous work, it is suggested that there are some island arc basalts widely preserved in the Shiquanhe-Namco ophiolitic belt.The existence of the island-arc magmatic rocks within the Shiquanhe-Namco ophiolitic belt reveals that the Shiquanhe-Namco ophiolitic belt cannot be interpreted as an ophiolite in a single back-arc setting but contains magmatic products under various tectonic settings.The results show that the Shiquanhe-Namco ophiolite belt does not only retain the remains of backarc basin, but also contains the magmatic products of various genesis in the subduction environment.