Ampandrandava and similar phlogopite deposits in southern Madagascar: Derivation from a silicocarbonatitic melt of crustal origin

Abstract

The Ampandrandava phlogopite deposit, located near Beraketa in southern Madagascar, and others like it in the region, in both the Androyen and Anosyen domains, were created by the crystallization of pods and dikes of crustally derived silicocarbonatitic melt. The anatectic reaction seems to have involved regional marble with C and O isotopes shifted toward mantle values and, at Ampandrandava, in particular, input from an evaporitic sequence. This hypothesis accounts for the importance of magmatic anhydrite in the pegmatitic pods and the presence of chlorine in the fluorapatite and phlogopite. The pods and dikes were emplaced in phlogopite-bearing clinopyroxenite; the aggressive carbonate–sulfate melt digested the host locally, and the diopside, phlogopite and fluorapatite crystallized quickly owing to a vapor pressure quench rather than a thermal quench. The high-temperature anatectic event occurred after the final episode of regional metamorphism, at a time of exhumation of deep crust, tectonic quiescence accompanying post-orogenic collapse, and anorogenic magmatism once collision and deformation had ceased.