Eocene Kashmar granitoids (NE Iran): petrogenetic constraints from U-Pb zircon geochronology and isotope geochemistry

Abstract

Kashmar granitoids outcrop for ~100 km along the south flank of the Sabzevar ophiolite (NE Iran) and consist of granodiorite and monzogranite along with subordinate quartz monzonite, syenogranite and aplitic dikes. These granitoids intruded Early to Middle Eocene high-K volcanic rocks and can spatially be grouped into eastern and western granitoids. Five samples of granite have identical zircon U–Pb ages of ca. 40–41 Ma. The granitoids have quite high K2O (~1.3–5.3 wt.%) and Na2O (~1.1–4.6 wt.%) with SiO2 ranging between ~62 and 77 wt.%. They are metaluminous to peraluminous, calc-alkaline and I-type in composition. Their chondrite-normalized REE patterns are characterized by LREE enrichment and show slight negative Eu anomalies. Kashmar granitoids have low whole rock εNd (−0.43 to −2.3), zircon εHf values (−1.9 to +7.2), and somewhat elevated δ18O (+6.1 to +8.7‰) in the range of I-type granites. The Kashmar granitoids show Early Neoproterozoic zircon second stage Hf and bulk rock Nd model ages at ca. 500–1000 Ma (associated with ca. 640 Ma old inherited zircons). Bulk rock Nd–Sr isotopic modeling suggests that 10–20% assimilation of Cadomian lower crust by juvenile mantle melts and then fractional crystallization (AFC process) can explain the Sr–Nd isotopic compositions of Kashmar granitoids. Kashmar granitoids are products of crustal assimilation by mantle melts associated with extension above the subducting Neotethyan Ocean slab beneath SW Eurasia. Similar subduction-related extension was responsible for the flare-up of Eocene–Oligocene magmatism across Iran, associated with core complex formation in central Iran.