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 present new major and trace element geochemistry, U-Pb zircon ages, and Sr-Nd isotopic data to discuss the origin of the IRN postcollisional units. 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 gabbro to K-feldspar granite. The youngest igneous activity is represented by quartz monzodiorite dikes. Hornblende-biotite quartz monzonite from Nay granitoids was dated at 40 Ma (zircon U-Pb). The IRN rocks are metaluminous to peraluminous with high-K calc-alkaline and 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), with depletion 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 heavy REE 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 –0.02 calculated at 40 Ma. The geochemical composition of IRN rocks along with the low ISr and positive εNd values and mantle model ages of 0.6–0.8 Ga indicate that two end-members, enriched mantle and a continental crust, were involved in the magma generation. We argue that the Eocene IRN magmatism occurred as a postcollisional product by asthenospheric upwelling owing to the convective removal of the lithosphere during an extensional collapse of the central Iranian block.