Major Nb/Ta Fractionation Recorded in Garnet Amphibolite Facies Metagabbro

Mobilities and fractionations of high-field-strength elements, especially Nb and Ta within a subducting slab, are important for deciphering the formation of the continental crust (CC). Here we report geochemical results on an epidote garnet amphibolite facies metagabbro body in the Tongbai-Dabie orogenic belt, central China. Our samples were hydrated during prograde metamorphism of the Triassic plate subduction. Major minerals such as amphibole, garnet, rutile, and ilmenite and garnet amphibolite bulk rocks show varied and overall lower Nb/Ta and/or Zr/Hf ratios than the continental crust. Magma differentiation might have contributed to variations of Zr/Hf but not those of Nb/Ta, suggesting major Nb/Ta fractionations during plate subduction. LA-ICPMS in situ trace element analyses of amphibole and especially rutile grains exhibit obvious chemical zonations. Typically, the rutile cores are usually small with higher Nb and Ta concentrations and lower Nb/Ta ratios compared to the thick rims. Chemical and fabric characteristics of the zonations may be explained by diverse external fluid activities: the gabbro first absorbed low Nb/Ta fluids that were released during blueschist to amphibolite transformation in deeper portions of the subducting slab, followed by acquiring external fluids with elevated Nb/Ta released during amphibolite to eclogite transformation. Our results imply that fluids with low Nb/Ta released during blueschist to amphibolite transformation can be transferred to cold regions within a subducted plate and also to the mantle wedge through fluid-rock reaction. Such regions are more easily melted during further subduction, especially in the early history of the earth, providing a plausible explanation for the low Nb/Ta in the CC.