Zircon U-Pb geochronology, geochemistry, Sr-Nd isotopic compositions, and tectonomagmatic implications of Nay (NE Iran) postcollisional intrusives in the Sabzevar zone

Abstract

The mafic to felsic intrusive rocks of Nay (IRN) are located in the northeast of the central Iranian block. In this study, we presentnew major and trace element geochemistry, U-Pb zircon ages, and Sr-Nd isotopic data to discuss the origin of the IRN postcollisionalunits. The oldest units in the Nay area belong to Paleocene–early Eocene volcanic and pyroclastic series including basalt-andesite, latite,dacite, and tuff. These series are crosscut by subvolcanic and granitoid rocks with lithological composition varying from quartz gabbroto K-feldspar granite. The youngest igneous activity is represented by quartz monzodiorite dikes. Hornblende-biotite quartz monzonitefrom Nay granitoids was dated at 40 Ma (zircon U-Pb). The IRN rocks are metaluminous to peraluminous with high-K calc-alkalineand shoshonitic affinities. They display enrichment in light REEs [(La/Yb)N = 3.79–8.71] and LILEs (such as Ba, Th, Rb, U, and K), withdepletion in HFSEs (such as Nb, Zr, Y, and Ti). All rocks have negative Eu anomalies [(Eu/Eu*)N = 0.17–0.88] and relatively flat heavyREE patterns [(Gd/Yb)N = 1.12–1.69]. Granitoids have initial 87Sr/86Sr values from 0.7053 to 0.7061 and εNd values from –1.65 to –0.02calculated at 40 Ma. The geochemical composition of IRN rocks along with the low ISr and positive εNd values and mantle model ages of0.6–0.8 Ga indicate that two end-members, enriched mantle and a continental crust, were involved in the magma generation. We arguethat the Eocene IRN magmatism occurred as a postcollisional product by asthenospheric upwelling owing to the convective removal ofthe lithosphere during an extensional collapse of the central Iranian block.