Genesis of early cretaceous leucogranites in the Central Sanandaj–Sirjan zone, Iran: Reworking of Neoproterozoic metasedimentary rocks in an active continental margin

Abstract

New U–Pb ages and geochemical and isotopic data from leucogranites within the central Sanandaj–Sirjan zone (SSZ) of the Zagros orogen in Iran enable an evaluation of a migrating magmatic system in response to subduction of Neo-Tethyan lithosphere. The Esfajerd leucogranites, from the Golpaygan region of the Muteh–Golpaygan metamorphic complex, are composed mainly of tourmaline and two-mica leucogranites and are characterized by strongly peraluminous compositions. SIMS U–Pb zircon ages indicate that these rocks were emplaced over a 7 Myr period between 138 and 131 Ma. In multi-element variation diagrams, all samples display pronounced negative Eu anomalies, enrichment in light rare-earth elements and large-ion lithophile elements, and depletion in high-field-strength elements, Sr, and Ba. The tourmaline and two-mica leucogranites record positive zircon εHf values of 1.7–5.6 and 2.3–8.9, respectively, suggesting a predominantly Neoproterozoic crustal source. Zircon Hf–O isotopic compositions, together with the whole-rock geochemical data, show that both leucogranite types were produced by muscovite dehydration melting of metasedimentary sources under fluid-absent conditions, followed by fractional crystallization of the parental melt. The relatively high δ18O values (8.68–10.32‰) of the tourmaline leucogranites are consistent with their derivation from a psammite-dominated source, such as metagraywackes, whereas the wider range and higher maximum of the δ18O values (6.99–12.32‰) of the two-mica leucogranites correspond to mixed pelitic–psammitic sources, with little or no input of mantle-derived material. We suggest that the leucogranites were formed by melting of metasedimentary rocks during a stage of flat-slab subduction initiated by plate reorganization.