Mineralogy, geochemistry, tectonic environment and origin of granodiorite in the east of Bideshk (Urumieh-Dokhtar magmatic zone)

Granitoids are one of the most abundant and common igneous rocks in the continental crust and they formed the world's largest batholiths. They are widely distributed in Precambrian to Cenozoic orogenic belts (e.g., Raymond, 2002), but some are formed in non-orogenic zones (Blatt et al., 2006). Because much of the continental crusts in orogenic belts are composed of granitoids, they are of particular importance in explaining the petrologic processes in orogenic belts.Cenozoic magmatism of Urmieh-Dokhtar magmatic arc is intruded by Oligo-Miocene plutonic rocks in some regions (Arvin et al., 2004). An outcrop of Oligo-Miocene granites is found in Zafarghand area in the southeast of Ardestan in Isfahan Province. Sarjoughian et al. (2018), Aminoroayaei Yamini et al. (2017), Sadeghian and Ghaffary (2011), Khalatbari Jafari et al. (2016), and Ghalamghash et al. (2019) suggested that this magmatism is a result of lower crust melting during mantle wedge metasomatism, occurred by Neo-Tethys subduction.This study aims to investigate the Oligo-Miocene granodiorites of East Bideshk, which is exposed in the central part of the Urumieh-Dokhtar magmatic arc in the northeast of Isfahan city. Despite the tectonomagmatic importance of this pluton in completing the geological history of Urumieh-Dokhtar magmatic arc, there are no comprehensive petrological studies performed on Bideshk granitoid. Thus, this study considered the mineralogy, geochemistry, tectonic environment, and origin of this granodiorite.

Microprobe analysis of the minerals was performed using a CAMECA SX 100 model with 15 kV accelerator voltage and 20 nA current at Stuttgart University, Germany. The Minpet software was also used to calculate the structural formula of the minerals and plot the diagrams. Intact and less altered rock samples are selected for geochemical analysis of major, trace, and rare earth elements by ICP-MS and ICP-OES methods in the geochemical laboratory of ALS Chemex in Ireland. The LOI values are also obtained by the gravimetric method. Fe2+ and Fe3+ are calculated based on the method by Middlemost (1989). Abbreviations for minerals are from Whitney and Evans (2010).

The Eocene Granodiorite - diorite rocks outcrop in the east of Bideshk area, in the northeast of Isfahan, and along the Urumieh-Dokhtar magmatic zone. According to lithological studies, they are mainly composed of granodiorite with predominant texture are granular, granophyre, and porphyroid.The major minerals are plagioclase, quartz, K-feldspar, hornblende, and biotite. Accessory minerals include magnetite, and apatite. Calcite and chlorite are the secondary minerals. Embayed plagioclases and quartz with rounded margins and plagioclases with oscillatory zoning, sieved texture, and dusty rims show non-equilibrium conditions during magma mixing. The composition of calcic amphiboles in these rocks is actinolite- tremolite, hornblende, and magnesio- hornblende. Plagioclases in the rocks of the east of Bideshk are andesine to labradorite in composition, and some show oscillatory zoning. Thermobarometry results indicate pressure, temperature, and crystallization depth decrease from the core (average ~ 3.01-3.63 kb, 685-732℃) of hornblende crystals. Geochemical investigations show that this granitoid is metaluminous, calc-alkaline, and I-type. The primitive mantle and the chondrite- normalized patterns of Bideshk whole-rock samples show enrichment of LREE against HREE. It is in accordance with magmatism in a subduction zone. Geochemical diagrams and variation in Rb content relative to Nb can also indicate a subduction-related magma source and mantle wedge metasomatism in the east of Bideshk, during Neo-Tethys subduction beneath central Iran.