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

摘要

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 epsilon Hf values of 1.7-5.6 and 23-8.9, 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 delta O-18 values (8.68-1032%.) 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 delta O-18 values (6.99-1232%.) 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. (C) 2019 Elsevier B.V. All rights reserved.