Geochemical and Sr–Nd–Hf isotopic evidence for Cenozoic partial melting of mantle beneath Natanz, Central Iran

Abstract

The products of Cenozoic continental arc magmatism in Iran provide an outstanding natural laboratory for investigating subduction-related processes. Here we present whole rock, SrNd isotopic, zircon U–Pb–Hf age, and mineral composition data for Cenozoic intrusive rocks from the Natanz area, central Urumieh–Dokhtar magmatic arc (UDMA), with a view to re-evaluate spatial–temporal variations in arc magmatism, magma petrogenesis and mantle source characteristics. The suite investigated in this study ranges from less evolved (gabbro) to highly evolved (granite) rocks with calc-alkaline affinity and an age range from 23.2 to 19.7 Ma. Their εNd(t) values vary from −0.32 to 2.73, initial 87Sr/86Sr values range from 0.7049 to 0.7075, and zircon εHf(t) values range from −2.2 to +8.9. Combined modelling of trace element and isotopic data suggests that the most mafic Natanz magmas could have formed through ∼5% melting of a source comprising 99% depleted mantle +1% sediment, with the melts modified subsequently by ∼3% contamination by Cadomian upper crustal components. We postulate that Natanz magmatism originated during a flare-up event instigated by retreat of the subducted slab prior to or in conjunction with the collision between the Arabian and Eurasian plates. The spatial–temporal associations in central UDMA (i.e., 33° 30′ N to 34° N) mirror three distinct flare-up episodes that occurred between 57 and 36 Ma, 26–15 Ma, and < 10 Ma; these events have not been reported previously from the northwestern and southeastern portions of the UDMA. Our results argue against any lull in magmatic activity in central UDMA during Eocene to Miocene times.