The Blue Hills Intrusive Complex in southern Namibia - relationships between carbonatites and monticellite picrites

Geochemical and petrological investigations were carried out on ultrabasic silicate and carbonatite magmatic rocks from the Blue Hills Intrusive Complex (southern Namibia). From detailed field mapping and geomagnetic traverses we propose a model for the complex build-up of this hypabyssal laccolith. The bulk comprises mica-olivine-carbonate picrite, with numerous small offshoots into the surrounding Nama shales which are metamorphosed at the contacts to the intrusive rock. The mica-olivine-carbonate picrite is intruded by small lenses and massive layers of monticellite picrite. Locally, a phlogopite-carbonate picrite occurs, that cuts through both the monticellite picrite and the mica-olivine-carbonate picrite. Late carbonatite dykes and sills and a carbonate-phlogopite-apatite pegmatite represent the last stage of the Blue Hills magmatism. Phlogopite-carbonate picrites are regarded as the parental and nearly primary magmas. They may be formed by partial melting in the dolomite stability field at a depth of 100 to 110 km and at a temperature of 1100-1250 degrees C. Fractional crystallisation of mostly olivine led to formation of the volatile-rich, mica-olivine-carbonate picrites, which were the first to intrude into the present position of the Blue Hills Intrusive Complex. Part of the phlogopite-carbonate picrite magma intruded undifferentiated, while part of it suffered devolatilisation and oxidation on its way to the surface which led to formation of the monticellite picrites. Both magma types intruded the hypabyssal mica-olivine-carbonate picrite. The late carbonatites most probably are residual liquids, i.e. the result of almost complete solidification of magmas Like phlogopite-carbonate picrites. The carbonatitic liquids were removed from the crystal mush by filter pressing. Pegmatitic veins are the late stage residual fluids or melts from within the Blue Hills body.