Magmatic osumilite in an ultrapotassic dyke, southern Peru; first occurrence

Osumilite-Mg, with 0..69 << Mg/(Mg + Fe) << 0.79, has been found in an ultrapotassic lamprophyric rock, 1.5 km NE of the Chili Hacienda, Puno Dept. S Peru. It occurs both as microlites (with up to 10.73 Si pfu) in the groundmass and as partial melting products of magnesian biotite phenocrysts incorporated from peraluminous granite spatially associated with the ultrapotassic rock. The P-T-fO (sub 2) conditions of osumilite deduced from the coexisting mineral assemblage (<< 1-5 kbar, 950-1050 degrees C and fO (sub 2) = WM) are in good agreement with the experimentally determined stability of this mineral. It is considered that osumilite in ultrapotassic rock is symptomatic of alumina-rich siliceous and magnesian melts.