Geochemistry of Basalts: Evidence for formation of Dazhu Ophiolite, Tibet (China), in a Supra-Subduction Zone Environment

The Dazhu ophiolite of Tibet is characterized by a complete sequence of lithological assemblages comparable with the typical ophiolites worldwide. Geochemical studies show that basalts in the Dazhu ophiolite are LREE-depleted, low-K tholeiites and their ΣREE, the extent of REE depletion or enrichment and the REE distribution pattern are close to those of N-MORB (normal mid-oceanlc ridge basalt) flowever, relative to the typical MORB, their large ion lithophile elements (LILE) and high field strength elements (HFSE) have experienced varying degree of differentiation. Their trace element dish-ibulion patterns are different from those of the typical MORB. It is characterized by strong depletion of Nb and Ta, slight depletion of K and Ba, enrichment of Th and Kb, and relalively hlgh Th/Ta and La/Ta ratios, It is deduced that this ophiolite was formed in a supra-subduction zonc (SSZ), reprcsenting a relic of the occanlc crust of a small-sized spreading forearc basin.