Early sulfide saturation in the Gangdese lower crustal arc magmas induced by assimilation of ancient crustal materials

The segregation of sulfide may be an essential step during the differentiation of arc magmas at the base of matured arc crust, but causes for the saturation of sulfide in lower crustal arc magmas remain enigmatic. Three suites of sulfide-rich and one suite of sulfide-poor mafic–ultramafic hornblende-rich cumulates in the lower crust of the Gangdese arc, which represents an Andean-type convergent margin prior to the Indian–Asian collision, were studied here. Petrographic observations integrated with whole-rock and zircon elemental geochemistry, as well as Sr–Nd–Pb–Hf–O isotopic systematics, suggest that sulfides in the Gangdese lower crustal cumulates, mostly occurring as droplet, formed earlier than most cumulus silicates (e.g. clinopyroxene and hornblende) and magnetite, and that the relatively early saturation of sulfide was likely to be caused by the assimilation of reduced metamorphic basement of the Gangdese arc. Given the evident Cu deficit in most intermediate–felsic Gangdese arc magmas (MgO < 4 wt%), dominantly enriched Sr–Nd–Hf isotopic compositions demonstrate that the Gangdese arc magmas underwent variable degree of crustal assimilation in general, implying the segregation of sulfide promoted by assimilation of reduced crustal materials should be pervasive in the lower crust. Accordingly, the juvenile Gangdese lower crust is characterized by the simultaneous enrichment in both mafic silicates and sulfides, and subsequent foundering of the sulfide-rich mafic–ultramafic lower crust may generate an arc crust depleted in Fe, Mg and Cu.