The ultrabasic lava flows of Mashhad, North East Iran

Abstract

SummaryUltrabasic and basic lavas are interbedded with metamorphosed Lower-Carboniferous sediments in the northern slope of the Alborz mountains, NE Iran. In the outcrop area at least 15 individual units of ultrabasic lava have been observed. Flow units range in thickness from a few metres up to about 70 m. The inner parts of the flow units are holo-crystalline, showing a poikilitic texture with rounded small crystals of serpentinized olivine surrounded by large crystals of clinopyroxene (‘wehrlitic facies’). The upper portions of the thicker units are olivine-free, and pyroxene, sometimes accompanied by brown hornblende, is set in a groundmass of fine-grained epidotized plagioclase (‘doleritic facies’). In the upper and lower margins of flows the groundmass is devitrified to chlorite and tremolite. Small-scale differentiation and igneous lamination is observable in transition zones between the wehrlitic and doleritic ‘facies’. The upper doleritic facies and other individual basic units have a tholeitic chemistry. In contrast, the chemical composition of wehrlitic rocks (which predominate amongst the exposed rocks in the area) is comparable with Archaean ultrabasic lava flows in Canada and southern Africa.