Geochemical and isotopic signatures, and zircon U[sbnd]Pb ages of the oldest known intrusive rocks associated with porphyry Cu deposits in the central Urumieh–Dokhtar magmatic arc, Iran

Abstract

The Zafarghand pluton, having ~10 km2 of outcrop exposure, lies within the central Urumieh–Dokhtar magmatic arc (UDMA) in Iran, of ~34 Ma age. Zafarghand plagioclase is albitic, with amphibole and chlorite being the distinctive mafic minerals. Although productive porphyry Cu deposits (PCDs) occur nearby, Zafarghand and its immediate neighboring plutons are sub-productive. Values of SiO2 and K2O span a narrow compositional range from 69 to 73 wt% and 2.2 to 3.5 wt%, respectively, corresponding to granodiorite or granite and calc-alkaline in affinity. εNd(t) values are ranging from ~0 to +1.2, a diagnostic range for the mantle-dominant melts. The SrNd isotopic values are significantly lower and higher than those of slab melting- and crust melting adakites, respectively. The 34 Ma age of Zafarghand plutons make them to be the oldest Cu porphyry deposits yet to be found in the UDMA, in which mineralization had occurred during a mature, late stage of subduction (pre-collisional). We propose multi-stage refertilizing of Cu in lithospheric mantle beneath the UDMA, negligibly during Eocene to Early Oligocene slab roll-back accompanied by middle to rather high fO2 context, and dominantly, during the Middle–Late Miocene slab break-off, the latter significantly tapping into influxing of asthenosphere mantle melt. Buffering of fO2 at a sufficiently high level (and not partial melting of slab or thickened crust) is the most plausible explanation for forming Late Eocene–Oligocene sub-productive (–and non-adakitic) Zafarghand Cu-bearing plutons.