The geochemistry of komatiites and basalts from the Deadman Hill area, Munro Township, Ontario, Canada

Abstract

A sequence of Archean komatiites (> 18 wt.% MgO), komatiitic basalts (10–18 wt.% MgO), high-Mg tholeiites (6–10 wt.% MgO), and high-Fe tholeiites (< 8 wt.% MgO) is exposed in the Deadman Hill area of Munro Township, Ontario, Canada. Major- and trace-element analyses of 28 samples are used to assess their petrogenetic significance. The use of molecular proportion ratio plots shows the samples have maintained their primary SiO2, FeO*, MgO, TiO2, Al2O3, Ni, Cr, Zr, Y, and V contents. Secondary redistribution of Na2O, K2O, Rb, Sr, Ba, and, in some samples, CaO has occurred.Covariation in both major- and trace-element data suggests the komatiites are primary melts that equilibrated with a harzburgite residua at pressures of 3–6 GPa. Garnet did not have a major role in their petrogenesis or in the petrogenesis of spatially related komatiitic basalts and high-Mg tholeiites. Major- and trace-element variation in komatiitic basalts with 17–12 wt.% MgO requires that they be partial melts in equilibrium with clinopyroxene at pressures of < 3 GPa. They are unrelated to the komatiites. Both lower degree partial melting of the same source as lavas with 17–12 wt.% MgO and crystal fractionation of clinopyroxene from liquids with ~12 wt.% MgO can model the evolution of less magnesian komatiitic basalts and high-Mg tholeiites. The Zr/Y and Zr/Ti ratios of the high-Fe tholeiites indicate that they are unrelated to the komatiites, komatiitic basalts, and high-Mg tholeiites and were derived by partial melting of a garnet lherzolite source.