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

Abstract

Osumilite-Mg (0.69 < Mg/(Mg + Fe) < 0.79) has been discovered for the first time in an ultrapotassic rock. It occurs both as microlites (with up to 10.73 Si p.f.u.) in the groundmass and as partial melting products of magnesian biotite xenocrysts incorporated from peraluminous granites spatially associated with the ultrapotassic rock. The P-T-fO 2 conditions of osumilite deduced from the coexisting mineral assemblage (< 1-5 kbar, 950°C < T < 1050°C AND FO 2 = WM) are in good agreement with the experimentally determined stability of this mineral. It is concluded that osumilite in ultrapotassic rock would be symptomatic of alumina-rich siliceous and magnesian melts