Nb/Ta fractionation produced by amphibole and titanite differentiation during arc magma formation: a case study from the Yabulai batholith, Alxa Block

Niobium (Nb) and tantalum (Ta) are geochemically similar high-field-strength elements, and their Nb/Ta ratio is a key tracer of crust-mantle differentiation. The compositional resemblance between arc magmas and continental crust points to arc magmatism as a major driver of Nb-Ta systematics during crust formation. This study examines Nb-Ta fractionation during the magmatic evolution of the Early Permian (～276.2-277.9 Ma) Yabulai batholith (gabbro-granitoid) in the Alax Block. Through integrated whole-rock geochemistry, Sr-Nd-Hf isotopes, and in situ mineral chemistry, we reconstruct the differentiation processes shaping this arc-related batholith. The Yabulai granitoids are I-type granites, derived from the fractional crystallization of coeval basaltic magmas represented by the associated Yabulai gabbros. Whole-rock geochemical data reveals a progressive decrease in Nb/Ta ratios in Yabulai Batholith, from 20.0 in gabbro to 2.41 in granite. Petrographic and geochemical evidence indicates that this trend is primarily controlled by the fractional crystallization of amphibole and titanite, which strongly influence Nb-Ta partitioning during magmatic evolution. Our results support that the subchondritic Nb/Ta ratios in arc granitoids, and by extension the continental crust, are mainly controlled by the crystallization and removal of amphibole- and titanite-rich cumulates, which may constitute a significant, previously overlooked Nb-rich reservoir in the crust.