Major Nb/Ta Fractionation Recorded in Garnet Amphibolite Facies Metagabbro

Abstract

AbstractMobilities 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...