Petrologic and geochemical constraints on the origin of Astaneh pluton, Zagros orogenic belt, Iran

Abstract

The Astaneh plutonic complex consists of a series of granitoid rocks ranging in composition from quartzdiorites to monzogranites and evolving from metaluminous to weakly peraluminous compositions. They belong to the high-K calc-alkaline series, having features of typical Andean-type cordilleran granitoids. Trace and rare-earth elements distribution patterns for the Astaneh rocks indicate a distinctive depletion in Nb, Sr, Ba, P and Ti relative to other trace elements and a greater enrichment in LILE compared to HFSE. These geochemical characteristics suggest the participation of an important recycled (sedimentary?) component in the source region of the granitoids. They have Sr initial isotopic ratios in the range 0.7078–0.7084 and negative eNd values of 5.39 to 6.13 for a time of generation of 170 Ma. There is a genetic link between quartz-diorites and granodiorites, the dominant rock types of the Astabeh intrusion. Direct melting or fractionation from a diorite source is very unlike. It is proposed that the Astaneh parental Qtd-diorite magmas were produced by the partial melting of a mixed source, dominantly composed of amphibolites and sediments, that was formed during subduction of Neo-Tethyan oceanic crust below the Iranian microcontinent during Middle Jurassic times.