ORTHO-PYROXENE MINETTES AND LAMPROITES - THEIR STATUS AND GENETIC SIGNIFICANCE

Orthopyroxene is a normal if rare, constituent of some minettes and la mproites. It is sometimes partly xenocrystic, but mostly authigenic, i .e. separated directly from lamprophyric magmas, as can be shown when orthopyroxene lamprophyres are compared with the experimental phase di agrams of appropriate systems. The orthopyroxene-bearing lamprophyres represent modified mantle-derived magmas characterized by a high MgO, high SiO2 speciation which was acquired in different ways in individua l regions. The crystallization of orthopyroxene lamprophyres was not c ontrolled by particular pressure - temperature conditions. Compared wi th clinopyroxene potassic lamprophyres the orthopyroxene lamprophyres represent more primitive magmas. Orthopyroxene minettes can also be al tered by a post-magmatic hydrothermal inflow, which occasionally incre ases their CaO content to the level normal in clinopyroxene minettes. If this had happened before magma consolidation, clinopyroxene instead of orthopyroxene would have crystallized. Orthopyroxene minettes are well defined mineralogically and chemically and may be related to othe r types in the minette series.