Some subcalcic clinopyroxenites from Salt Lake Crater,Oahu, and their petrogenetic significance

Some inclusions from Salt Lake Crater are essentially single-phase subcalcic clinopyroxenites whose original clinopyroxenes, prior to extensive unmixing, were tschermakitic subcalcic varieties with compositions close to Ca₃₄Mg₅₄Fe₁₂. In addition to copious amounts of orthopyroxene, very minor garnet and spinel also were exsolved from the subcalcic clinopyroxenes.The genesis of the garnet pyroxenite suite at Salt Lake Crater has been examined in terms of three models, namely: (i) cumulates from alkali basaltic magmas; (ii) fractional fusion of basanitic garnet clinopyroxenite; and (iii) anatexis of upper mantle lherzolites. Field, mineralogical, chemical and experimental data collectively favour model (iii) and indicate that the nodules are genetically unrelated to their nephelinitic hosts. The Salt Lake garnet pyroxenites can be closely equated with the garnet pyroxenites in magmatictype layers in certain alpine-type ultramafic massifs and they are also similar to many garnet pyroxenite xenoliths in alkaline volcanics from other localities.Liquids produced by anhydrous partial melting of spinel Iherzolite at pressures of approximately 20 kb commonly have picritic chemistries. The crystallization behaviour of picritic liquids at elevated pressures ( 20 kb) indicates that the initial crystallization products may be either essentially single-phase subcalcic clinopyroxenites (with minimal high pressure fractionation) or a range of olivine-aluminous orthopyroxene-aluminous subcalcic clinopyroxene-garnet-(spinel) assemblages with variable 100 Mg/(Mg+Fe) ratios (when fractionation has been operative). All these assemblages may be subsequently modified by subsolidus exsolution and recrystallization.