THE FIELD CHARACTERISTICS AND PETROLOGY OF ARCHEAN AND PROTEROZOIC KOMATIITES

A review of the geological literature, augmented by recent field and laboratory studies, reveals that komatiites constitute an igneous suite made up of volcanic and hypabyssal rocks ranging in composition from dunite and peridotite to basalt or magnesian andesite. Ultramafic komatiites (MgO > 20 wt. % anhydrous) are rich in olivine and occur as spinifex-textured, massive or pillowed lava flows, as bedded volcaniclastic rocks, and as small dykes and sills. Mafic komatiites (MgO < 20 wt. %) have, as their predominant minerals, olivine, pyroxene (augite or pigeonite) and plagioclase. They occur as massive flows with brecciated or pyroxene spinifex-textured tops, or as pillow lavas. Also common are layered ultramafic-mafic flows and sills. Ultramafic and mafic komatiites char acteristically occur in Archean greenstone belts, where in several areas dunitic flows and large dunitic intrusions are the hosts for nickel sulfide deposits. Ultramafic komatiites are unknown in Proterozoic and Phanerozoic terranes, but mafic komatiite lavas, and layered ultramafic-mafic flows and sills, are common in the Proterozoic Cape Smith fold belt of Quebec, and pillow lavas with compositions similar to mafic komatiites occur in Paleozoic ocean-floor sequences in Newfoundland.