Komatiitic and Iron-rich Tholeiitic Lavas of Munro Township, Northeast Ontario

Abstract

Munro Township, in the Archean Abitibi greenstone belt of northeast Ontario, contains volcanic and hypabyssal rocks of two magma series: (1) an Fe-rich tholeiitic series of basaltic to dacitic lava flows (3–10 m thick), layered peridotite-pyroxenite-gabbro flows (120 m thick), and layered sills (700 m thick); (2) an ultramafic-mafic komatiitic series, comprising discrete lava flows of peridotitic to andesitic composition (1–17 m thick), layered peridotite-gabbro flows (120 m thick), and layered sills (500 m thick). The komatiitie lavas form a succession about 1000 m thick that is both underlain and overlain by thicker successions of tholeiitic volcanic rocks. Three types of komatiite are recognized: peridotitic, pyroxenitic, and basaltic komatiites. The most ultramafic are peridotitic cumulates rich in forsteritic olivine (Fo89–94), at the bases of flows and sills. Many less mafic peridotitic komatiites (MgO: 20–30 per cent), which typically form the upper parts of flows and the marginal parts of small intrusions, exhibit spinifex textures indicative of their formation from ultrabasic liquids. Pyroxenitic komatiites (MgO: 12–20 per cent) also may contain olivine, but are dominated by clinopyroxene, usually in spinifex textures. Basaltic komatiites (MgO <12 per cent) are composed mainly of clino-pyroxene and plagioclase or devitrified glass, rarely in spinifex texture and more commonly in equigranular textures. Peridotitic komatiite with roughly 30 per cent MgO appears to represent a parental liquid from which the more ultramafic komatiites formed by accumulation of olivine, and the less mafic types were derived by fractionation of olivine, joined and finally succeeded in later stages by clinopyroxene and plagioclase. Komatiites of Munro Township share many of the characteristics of the komatiites from the Barberton Mountain Land, South Africa (Voljoen & Viljoen, 1969a and b), but lack the high CaO/Al2O3 ratios that distinguish the Barberton rocks. The Munro komatiites are identical in this respect to ultramafic volcanic rocks in Australia, Canada, Rhodesia, and India. It is proposed that the definition of the term komatiite be broadened so that it includes all members of this ultramafic-mafic rock series, not only those from Barberton Mountain Land. The proposed criteria are: (1) highly ultramafic compositions in noncumulate lavas; (2) unusual volcanic structures such as spinifex texture and polyhedral jointing; (3) low Fe/Mg ratios at given Al2O3 values or high CaO/Al2O3 ratios; low TiO2 at given SiO2; and high MgO, NiO, and Cr2O3.