Olivine ‘leucitites’, their xenolith and megacryst suites, Hoskings Peaks, north Queensland

Abstract

Olivine ‘leucitite’ lavas and vent breccias at Hoskings Peaks, McLean Province, north Queensland, contain abundant and unusual xenoliths and xenocrysts derived from an inhomogeneous, metasomatised and veined mantle source. Xenoliths include common spinel lherzolite, rare spinel‐amphibole harzburgite, and a single partly recrystallised garnet‐chromian spinel clinopyroxenite. Megacrysts of titanian phlogopite, and colourless diopsidic clinopyroxene are most abundant, together with dark green sodian clinopyroxene, sodian anorthoclase, and few zircon. Xenocrysts include abundant pyrope and disaggregated lherzolite minerals, metamorphic almandine, corundum, apatite and oxides. Colourless clinopyroxene and titanian phlogopite similar to the abundant megacrysts are preserved as a single coarse xenolithic aggregate; the chemistry of similar megacrystic clinopyroxene indicates a high‐pressure origin. This xenolith assemblage is interpreted as part of an olivine‐poor metasomatic mantle vein, the selective melting of which, together with an oxide, apatite, and zircon, can produce major and trace element compositions close to olivine ‘leucitites’ in the present suite, without involvement of significant lherzolitic wall rock. Repeated minor degrees of partial melting and crystallisation within mantle veins (vein‐located ‘zone refining'), may progressively fractionate such melts and enrich incompatible elements, alkalis and alumina, producing extreme mineral compositions in peripheral mantle veins. Coarse xenocrysts such as zircon, dark green sodian clinopyroxene, sodian anorthoclase and corundum, may represent disaggregated relics of strongly differentiated (?higher level) mantle vein assemblages. Evolution of the olivine ‘leucitites’ towards strong alkali enrichment is confirmed by patches of pegmatoid differentiate containing barian sanidine, sodian clinopyroxene, alkali amphibole, phlogopite, and magnesian ilmenite. Combined trends for Australian leucitite suites gives a general leucitite evolutionary trend also towards alkali and silica enrichment.