Age and petrogenesis of Na-rich felsic rocks in western Iran: Evidence for closure of the southern branch of the Neo-Tethys in the Late Cretaceous

Abstract

Intermediate to felsic volcanic and granitic rocks with high Na2O concentrations (5.2–9.1wt.%) are widely distributed in the Harsin area along the Zagros thrust zone in western Iran. Most of these rocks are classified as low-potassium tholeiite, display affinity with oceanic plagiogranite and contain somewhat high Na content and low concentrations of K2O, Al2O3, Rb, Sr and Ca. Thus, we prefer to apply the term Na-rich felsic rocks to this complex. U–Pb dating yielded ages of 94.6±2.7Ma (2σ) from baddeleyite and 95.0±2.4Ma (2σ) from zircon grains, indicating that the complex crystallized in the Late Cretaceous. Based on the mineral compositions, the crystallization occurred at low pressures (mostly <2kbar) and low temperatures (<750°C). High initial ratios of 143Nd/144Nd (0.51288–0.51304) and positive values of ɛNd(t) (+7.0 to +11.5) are consistent with those of mid-oceanic ridge basalt (MORB). During collision of the Arabian plate and Biston-Avoraman block in the Late Cretaceous, an increasing geothermal gradient was responsible for partial melting of altered mafic rocks and for producing the Na-rich felsic rocks in the Harsin area. The presence of these types of rocks along the main Zagros fault indicates local collisions. These collisions were caused by southwestward subduction under the Arabian plate in the southern branch of the Neo-Tethys. This event was the first stage of the Zagros collision, which was followed by collision of the Arabian and Iranian plates during the Eocene through Neogene.