Implications of Phase Equilibria and Chemical Parameters for the Origin of Archaean Ultramafic and Mafic Lavas

Abstract

Abstract Polybaric phase equilibria of komatiitic liquids along with possible upper mantle compositions in the pseudo-quarternary CaO-MgO-Al2O3-SiO2 (CMAS) system indicate that the parental rocks from which these liquids were derived were probably richer in CaO and hence in clinopyroxene than comparable source rocks. Available data indicate that the ultramafic and mafic liquids originated at c. 20–40 Kb (c. 60–120 km); a remaining problem relates to the availability of higher temperatures required (c. 1600–1700°C) to produce these liquids at such depths. Melting at deeper levels poses problems of quick transport which is essential for the preservation of the primitive character of these liquids. Consideration of major and minor oxide variation of the komatiitic suite indicates that the ultramafic members may have been the parental liquids to the basaltic and/or the pyroxenitic komatiites, and that fractionation of olivine and to a lesser extent clinopyroxene ± orthopyroxene may have controlled such an evolution. A comparative study is made of the textural and chemical features of Archaean komatiites, especially basaltic types, with those of non-cumulus and possibly primary or primitive picritic liquids of younger age. (1) Palaeozoic basaltic komatiites of the Rambler area, Newfoundland, are closely comparable in composition with similar rocks of the Archaean (mean of 9: MgO 14.21%, TiO2 0.16%, CaO/Al2O3, = 1, K2O 0.12%), although they lack the spinifex or quench textures. (2) Picritic rocks of the North Atlantic Province (such as Baffin Bay) and the Karroo Province (Nuanetsi) are characterised by skeletal forms of olivine indicating rapid cooling or quenching, akin to the quench textures of komatiites. Whereas the former have chemical affinity with komatiites, the latter are richer in K and Ti. (3) Deccan picritic basalts characteristically have high CaO/Al2O3, ratios (i. e. > 1), but unlike basaltic komatiites they are enriched in incompatible elements (e. g. Ti, K and P) and have evidently undergone much more heat loss before eruption because spinifex textures are not developed and olivine pheno-crysts are abundant. (4) Solomon Island picrites lack spinifex texture but are analogous to komatiites in chemistry (MgO > 10%, K2O Broad chemical similarity between Archaean and yomger basaltic komatiites and picritic rocks, especially of major elements, suggests that melting and crystallisation events that produced these rocks were repeated throughout earth history, whereas peridotitic komatiites are unique to the Arckaean implying the importance of early steep thermal gradients in their fornation.