Can Nb-enriched arc basalts result from mixing of boninite and ocean island basalt magmas? A case study from Jurassic gabbroic rocks of western Iran (Sanandaj-Sirjan Zone)

The origin of Nb-rich basalts—specifically high-Nb basalts (HNB) and Nb-enriched basalts (NEB)—is a key issue in subduction-related igneous petrology. The gabbroic rocks from the Alvand Plutonic Complex (ALPC) in the Neo-Tethyan Sanandaj-Sirjan Zone of Iran, with zircon U–Pb crystallization ages of 165 ± 2 Ma, provide a valuable natural setting for investigating these magmas. The trace element chemistry of gabbroic clinopyroxene and amphibole indicates crystallization from a parental melt with similarities with both HNB and OIB-type alkaline magmas. Major and trace element data show that the clinopyroxene compositions lie between those of OIB-like melts and highly depleted ultramafic boninitic rocks (HDUR) found in the same plutonic belt. Similarly, bulk-rock gabbros that are the intrusive equivalent of basalts span a compositional range from HNB to NEB and are transitional between OIB and HDUR end-members. We propose that the ALPC gabbros formed by mixing between an OIB-type magma, sourced from upwelling asthenosphere through a slab window, and a highly depleted boninitic melt sourced from partial melting of a depleted mantle wedge. This mixing model contrasts with the widely accepted adakitic model, which involves melting of a mantle wedge previously metasomatized by slab-derived melts. Our findings highlight the overlooked role of boninitic magmas in generating Nb-rich basalts and suggest that variable proportions of OIB and boninitic components can produce the compositional spectrum observed in HNB and NEB worldwide.