Geology, geochemistry and petrogenesis of post-collisional adakitic intrusions and related dikes in the Khoynarood area, NW Iran

Abstract

The Khoynarood area is located in the northwest of Iran, lying at the northwestern end of the Urumieh–Dokhtar volcano-plutonic belt and being part of the Qaradagh–South Armenia domain. The main intrusive rocks outcropped in the area have compositions ranging from monzonite–quartz monzonite, through granodiorite, to diorite–hornblende diorite, accompanied by several dikes of diorite–quartz diorite and hornblende diorite compositions, which were geochemically studied in order to provide further data and evidence for the geodynamic setting of the region. The SiO2, Al2O3 and MgO contents of these rocks are about 58.32–68.12%, 14.13–18.65% and 0.68–4.27%, respectively. They are characterized by the K2O/Na2O ratio of 0.26–0.58, Fe2O3+MnO+MgO+TiO2 content about 4.27–13.13%, low Y (8–17ppm) and HREE (e.g., 1–2ppm Yb) and high Sr contents (750–1330ppm), as well as high ratios of Ba/La (13.51–50.96), (La/Yb)N (7–22), Sr/Y (57.56–166.25), Rb/La (1.13–2.96) and La/Yb (10–33.63), which may testify to the adakitic nature of these intrusions. Their chemical composition corresponds to high-silica adakites, displaying enrichments of LREEs and LILEs and preferential depletion of HFSEs, (e.g., Ti, Ta and Nb). The REE differentiation pattern and the low HREE and Y contents might be resulted from the presence of garnet and amphibole in the solid residue of the source rock, while the high Sr content and the negative anomalies of Ti, Ta and Nb may indicate the absence of plagioclase and presence of Fe and Ti oxides in it. As a general scenario, it may be concluded that the adakitic rocks in the Khoynarood were most likely resulted from detachment of the subducting Neo-Tethyan eclogitic slab after subduction cessation between Arabian and Central Iranian plates during the upper Cretaceous–early Cenozoic and partial melting of the detached slab, followed by interactions with metasomatized mantle wedge peridotite and contamination with continental crust.