Geologic, petrographic and tectonic background

According to the detailed petrographic framework of Rakotondrazafy et al. (1996), the rocks containing the corundum crystals, such as the anorthite-hibonite-spinel-corundum calcsilicates occur as zones at the contact between marbles and granitic or titanite-bearing scapolite-clinopyroxene skarns.

Different stages of metamorphism can be summarized as follows:

Stage 1

Metasomatic transformation of the rocks due to CO2-rich fluids at relatively high temperatures and medium pressures (850°C and 5 kilobar). Peraluminous and calcium-rich segregations made up of meionite, spinel and corundum are formed within titanite-bearing scapolite-clinopyroxene skarns. Concentrations of REE and Ti were low enough to preclude the crystallization of hibonite and since the quartz activity was below the zircon-baddeleyite-buffer, baddeleyite (Zr02) rather than zircon (ZrSiO4) was formed during this phase. Typical, stable minerals are: aluminous diopside, carbonate, scapolite, sphene or spinel, thorianite, corundum, baddeleyite.