DECOUPLING OF NB-TA AND TI IN ARC MAGMATISM: A CASE STUDY OF THE YANGZHUANG GRANITE PORPHYRY IN WEST JUNGGAR, XINJIANG, CHINA
The Nb-Ta-Ti depletion in island-arc magmatic rocks is typically interpreted to result from partial melting, leaving residual ilmenite and rutile (minerals typically enriched in Nb and Ta) during partial melting. Melting of rutile and ilmenite would likely cause Nb-Ta enrichment in the generated magma, but it fails to explain the enrichment of Nb and Ta and coexisting depletion of Ti in the YGP.
Previous research revealed that in Ti-poor systems, highly incompatible elements including Nb and Ta can be transferred into the mantle wedge by crystallization of amphibole from the ascending fluids generated by dehydration of subducted slab. Located in the Xuemisitan volcanic belt, the studied area has experienced two stages of southward subduction, followed by a third northwestward ridge subduction. The mantle wedge, therefore, was extensively metasomatized by dehydration of the subducting slab. Nb and Ta were strongly enriched in the mantle wedge where Nb- and Ta-rich and Ti-poor amphibole crystallized during the long-term metasomatism. Therefore, we infer that enhanced heat flux brought by the Late Carboniferous ridge subduction decomposed amphibole in the mantle wedge to generate Nb and Ta-rich and Ti-poor melt to produce the YGP.