Chemical and oxygen isotopic compositions, age and origin of gem corundums in Madagascar alkali basalts

Madagascar is a large producer of gem corundum recovered from continental basaltic fields. The main mining areas are sapphire-bearing palaeoplacer deposits such as Ambondromifehy and Nosy Be in Antsiranana Province, northern Madagascar; Soamiakatra-Mandrosohasina in Antananarivo Province, central Madagascar; and Vatomandry district in Toamasina Province, eastern Madagascar. In Antananarivo Province, Soamiakatra is a primary deposit where ruby is found in metagabbro and pyroxenite xenoliths, brought-up to the upper crust by the Ankaratra volcanics. Petrographic studies indicate two different conditions of ruby formation, at the boundary of the eclogite domain (T~1100\ub0C, P~20kb) and granulite facies (T~1100\ub0C, P<15kb). In contrast, most of the sapphires in placer and paleoplacer have two origins: (i) differentiation of alkaline magma in chambers at the lower continental crust-mantle boundary (90% of the sapphires). They are related to syenite and anorthoclasite xenoliths in the basalts. These alkali-basalt hosts are linked with asthenosphere upwelling and E-W and N-S lithosphere thinning during Oligocene-Quaternary times. Zircons associated with the sapphires from Mandrosohasina and Ambatomainty sapphire deposits gave U-Pb ages at 7Ma. The sapphires have low \u3b418O values of 4.1\ub10.4\u2030 (n=8), within the range of sapphire in syenitic rocks. Chemical composition and mineral inclusions in sapphires, such as columbite-(Fe), tantalite-(Mn), pyrochlore group, samarksite group, uraninite and anorthoclase confirm their syenitic origin. (ii) metamorphic xenocrysts (10%) brought up by the same magma. Oxygen isotopic compositions of rubies from placer deposits, \u3b418O=3.1\ub11.1\u2030 (n=6) are typical of ruby in mafic and ultramafic rocks and 'plumasite' in mafic rocks (1.25<\u3b418O