Mobilization of Ti–Nb–Ta during subduction: Evidence from rutile-bearing dehydration segregations and veins hosted in eclogite, Tianshan, NW China

Field evidence from the western Tianshan subduction complex in northwestern China indicates that the high field strength elements Ti, Nb, and Ta were mobilized and thereby fractionated from Zr and Hf during the dehydration process that transformed blueschist into eclogite. Both a segregation with a depletion halo, thought to represent initial mobilization during dehydration, and a transport vein, indicative of the long distance transport were investigated. In each case, centimeter-sized rutile grains grew as needle-like crystals in the segregation and as prismatic crystals in the vein. Within the host rock of the segregation, the Ti contents of garnet and omphacite, the modal abundances of rutile and titanite and the bulk rock Ti, Nb, and Ta contents decrease towards the segregation. These observations are consistent with transport of Ti, Nb, and Ta from the host rock into the segregation. Textural and geochemical data for the eclogite-facies vein minerals indicate that Ti–Nb–Ta-rich fluids were transported over long-distances (at minimum meter-scale) during fracture-controlled fluid flow. Complex forming ligands (e.g., Na–Si–Al polymers and F−) may have enhanced the solubility of Ti, Nb, and Ta in the fluid. Changes in fluid composition (e.g., XCO2) may both precipitate rutile and fractionate Ti, Nb, and Ta from LILE and REE.