Rainbow Range

The Rainbow Range is a late Miocene shield volcano 30 km in diameter and covering 370 km2. The flanks are gently dipping and composed of hawaiite, mugearite, trachyte and comenditic flows, the lowest of which are apparently continuous with adjacent plateau lavas. There is a central complex of small domes, short thick flows, and small intrusive bodies. A small area on the north flank of the volcano has been mapped in detail by Bevier (1981, Fig. 2) which indicated four igneous episodes with a comenditic trachyte unit at the base, followed upwards by mugearite, comendite and hawaiite units. Comendite lavas account for 75% of the lavas on the north flank of the volcano with individual flows up to 90 m thick in valley fills. The comendites consist of 5-10% phenocrysts of sanidine (Or 35-37), sodic ferrohedenbergite, fayalite and rare arfvedsonite in a groundmass of alkali feldspar, quartz, aegirine, arfvedsonite, aenigmatite, magnetite and ilmenite. Comenditic trachytes have 5-10% phenocrysts of anorthoclase (Or 25-27) and hedenbergite, in a groundmass of alkali feldspar, quartz, aegirine, arfvedsonite, aenigmatite, magnetite, ilmenite and a little glass. Bevier (1981) gives rock and mineral analyses including strontium isotope data.