Upper Paleozoic komatiites near Mashhad, NE Iran

Abstract

We report the discovery of upper Paleozoic komatiites from a location near Mashhad in NE Iran. Like Archean komatiites, they erupted as differentiated lava flows with olivine spinifex−textured upper portions and olivine cumulate lower portions. They are associated with komatiitic basalts and undifferentiated ultramafic units and are intercalated with clastic and carbonate sedimentary rocks. Using the compositions of samples with randomly oriented spinifex and the forsterite contents of olivine, we estimate that the parental magmas contained >20 wt% MgO. As such, they represent only the second known occurrence of post-Archean komatiite. Their geochemical compositions show relatively low Al/Ti and depleted heavy rare earth elements, indicating that they belong to the Al-depleted variety. This is the only reported post-Archean example of this type of komatiite. The parental magmas probably formed by moderate degrees of melting (∼20%) of a hydrous peridotite source at a depth of ∼180 km and erupted into a subduction zone at the margin of the Paleo-Tethys Ocean.