Late Cretaceous subduction-related magmatism on the southern edge of Sabzevar basin, NE Iran

Abstract

Despite detailed geological studies on the Late Cretaceous Sabzevar ophiolites, the non-ophiolitic, subduction-related igneous rocks of NE Iran are still enigmatic. This paper focuses on the Late Cretaceous, non-ophiolitic Sabzevar magmatic rocks, and presents both precise U–Pb ages and a tectonomagmatic scenario for the genesis of these igneous rocks. A thick sequence of Late Cretaceous acidic volcanic rocks associated with sandstones, shales and pelagic limestones dominates the northern parts of the Lut block. This sequence is intruded by a series of shallow mafic to felsic intrusions. New zircon U–Pb results on the intrusive and extrusive rocks show ages of 75–101 Ma. These ages show a pulsed magmatism in NE Iran, which lasted for c. 25 myr. The whole-rock geochemistry of the igneous rocks shows suprasubduction-zone-related geochemical signatures, represented by negative anomalies in high field strength elements and enrichment in large ion lithophile elements. The initial 87Sr/86Sr ratios and eNd(t) values of the extrusive rocks range from 0.70411 to 0.70628 and from +5.9 to +7.4 respectively, and for intrusive rocks are in the range of 0.70423–0.70579 and +5.8 to +7.2. High eNd(t) values for these rocks confirm that their melts were derived from a depleted-mantle source. Both geochemical and isotopic data indicate that the genesis of these rocks was related to the partial melting of mid-ocean ridge basalt-type slab or depleted-mantle wedge sources during the northward movement of the Sabzevar oceanic slab beneath the southern edge of Eurasia (Turan block) in Late Cretaceous time.