Early Jurassic and Late Cretaceous plagiogranites in Nain–Baft ophiolitic mélange zone in Iran: remnants of rift–drift and SSZ evolution of a Neotethyan seaway

Abstract

We have investigated the geology, geochemistry and geochronology of various plagiogranite intrusions that are spatially associated with ophiolitic subunits in a mélange terrain exposed within the Nain–Baft fault zone along the western boundary of the Central–East Iranian Microcontinent (CEIM). One group of plagiogranites is intrusive into foliated amphibolites, whereas the plagiogranites of the other group are intrusive into gabbros, dyke swarms and pillow lavas that are tectonically juxtaposed. U–Pb zircon dating of the first group of plagiogranites has revealed near-concordant crystallization ages of 176.94 ± 0.71, 179.84 ± 0.92 and 190.40 ± 2.1 Ma, indicating the early Jurassic timing of their emplacement. U–Pb zircon dating of the second group of plagiogranites has revealed a weighted average age of 92.34 ± 0.38 Ma, indicating that they were emplaced in the early Late Cretaceous. The Early Jurassic plagiogranites are low-K, sub-alkaline rocks with negative Eu anomalies, low Ti and high La and Ce contents. They display elevated ratios of large ion lithophile elements to high field strength elements. They were part of a magmatic system that facilitated the earliest rift–drift stages of a Neotethyan seaway development, and their magmas were produced by fractional crystallization of a mafic melt associated with asthenospheric upwelling beneath a rift system. The Late Cretaceous plagiogranites are low-K, sub-alkaline rocks with negative Eu anomalies, high Y and low Ta contents. They show light REE depletion relative to flat heavy REE patterns, and high Ba/Th ratios. Magmas of these plagiogranites were the products of partial melting of amphibolites in the subducting oceanic lithosphere in the same Neotethyan seaway. Thus, we posit that plagiogranite rocks in suture zones can be excellent trackers of geochemical, geochronological and magmatic evolution of various types of oceanic crust through rift–drift, seafloor spreading and subduction initiation stages within the same ocean basins in the past. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists