Chromite in Komatiites, II. Modification during Greenschist to Mid-Amphibolite Facies Metamorphism

Abstract

Chromite compositions in komatiites are influenced by metamorphic chromite is highly susceptible to modification during processes, particularly above 500°C. Metamorphosed chromite is early hydrous alteration and subsequent prograde metasubstantially more iron rich than igneous precursors, as a result of morphism of host rocks. This modification is the subject Mg–Fe exchange with silicates and carbonates. Chromite metaof this paper. morphosed to amphibolite facies is enriched in Zn and Fe, and Metamorphic modification of chromite has been disdepleted in Ni, relative to lower metamorphic grades. Relative cussed extensively in the literature in the context mainly of proportions of the trivalent ions Cr, Al and Fe are not ophiolitic or ‘alpine ultramafic’ complexes (Onyeagocha, greatly modified by metamorphism up to lower amphibolite facies, 1974; Ulmer, 1974; Evans & Frost, 1975; Hoffman & although minor Fe depletion occurs during talc–carbonate alWalker, 1978; Loferski & Lipin, 1983; Kimball, 1990; teration at low temperature. Significant Al is lost from chromite Burkhard, 1993) and in a few studies of komatiitic rocks cores above 550°C, as a result of equilibration with fluids in (Bliss & MacLean, 1975; Donaldson, 1983; Gole & Hill, equilibrium with chlorite. Elevated Zn content in chromite is 1990), and in a detailed study of the Pechenga intrusions restricted to rocks with low (metamorphic) Mg/Fe ratios, and is (Abzalov, 1998). These studies have highlighted two the result of introduction of Zn during low-temperature alteration, important effects. First, chromites become rimmed and with further concentration and homogenization during prograde progressively replaced by chromian magnetite or ‘fermetamorphism. Cobalt and Mn also behave similarly, except where ritchromit’. Second, chromite core compositions become carbonate minerals are predominant in the metamorphic assemblage. progressively modified during prograde metamorphism Chromite at amphibolite facies is typically extensively replaced by as a result of exchange of components with surrounding magnetite. This is the result of incomplete metamorphic reaction silicate minerals (Evans & Frost, 1975; Abzalov, 1998). between chromite and chlorite-bearing silicate assemblages. MagThis paper examines the nature and magnitude of these netite compositions at the inner chromite–magnetite boundary are effects in komatiites from a variety of localities in the indicators of metamorphic grade. Eastern Goldfields Province of the Archaean Yilgarn Block in Western Australia (Fig. 1). Brief locality descriptions are given in Appendix B.