Whole-rock Geochemistry and Mineral Chemistry of the Late Cretaceous Dacites in SW Sabzevar: constrain on their origin and tectonic setting

A thick sequence of Upper Cretaceous volcano-sedimentary rocks are cropped out on the northern edge of Central Iran, southwest of Sabzevar. The volcanic rocks are dacite/rhyolite and are associated with pyroclastic rocks, including agglomerate, breccia and tuff. Dacites are the thickest felsic volcanic unit. They show felsitic porphyry, glomero-porphyry and microlitic-porphyry textures and are composed of clinopyroxene, plagioclase and quartz. Plagioclases show sieve and zoning textures. Whole-rock and clinopyroxene chemistries reveal the tholeiitic nature of these rocks. Primitive mantle-normalized spider diagrams of these rocks show negative anomalies of Nb, Ti and positive anomalies of Pb and K, which are characteristic features of the subduction environment magmas. Plot of the samples in different tectonic setting discrimination diagrams indicates their close relationships to the island arc subduction environments. The results of the clinopyroxene chemistry also indicate their augitic composition. Application of geothermobarometry diagrams show temperature ranges between 850 ºC to 950 ºC and pressures less than 5 kb for crystallization of these rocks. The results of this study show that the tholeiitic magma that formed these rocks has been generated by partial melting of a spinel lherzolitic mantle source in an island arc subduction zone during the closure of the Sabzevar Neo-Tethyan oceanic basin in the late Cretaceous.subduction zone during the closing of the Sabzevar Neo-Tethyan oceanic basin in the late Cretaceous.