Osumilite-bearing metasedimentary gneiss occurs in the contact aureole of the Sand Hill Big Pond gabbronorite complex of Labradorian age in eastern Grenville Province, eastern Labrador, Canada. The osumilite (Os) occurs in association with cordierite (Cd), orthopyroxene (Opx), sillimanite (Sil), sapphirine (Sa), spinel (Sp), K-feldspar (Kf), plagioclase (Pl), phlogopite (Ph), hematite (Hm), magnetite (Mt), corundum (Co), and quartz (Q) in zones adjacent to the gabbronorite intrusion. The osumilite-in isograd is delineated at a distance of 1-3 km from the intrusive contact. The osumilite is characterized by enhanced Mg/(Mg + Fe) ratios (0.89-0.92) and relatively high K2O contents (4.13-4.78 wt. %). The compositional variation of the osumilite is best understood by the substitutions (Mg + Fe+ Mn) + Si = 2Al and (K + Na) + (Mg + Fe + Mn) = Al + vacancy. Symplectitic intergrowth of Opx-Cd-Kf-Q, which partly or completely replaced osumilite, is ubiquitously present. The stable osumilite-bearing assemblages (all with Hm, Mt, and Pl) deduced from petrographic features and from the phase relations in the KMAS system are Os-Sa-Cd, Os-Cd-Sil-Q, Os-Cd-Kf-Q, Os-Cd-Opx-Q, and Os-Opx-Kf-Q. The sapphirine-bearing assemblages are restricted to silica-deficient (quartz-absent) zones of the gneiss, which include Sa-Os-Cd, Sa-Sil-Cd, Sa-Opx-Sp, Sa-Sp-Cd, and Sa-Sp-Co. Orthopyroxene coronas mantling phlogopite reflect the breakdown reaction Ph + Q = Opx + Kf + vapor under extremely low water activity in the vapor. Petrogenetic grids in MAS, KMAS, and KMAS-H2O-CO2 appropriate to the mineral assemblages in the metasedimentary gneisses suggest that the changes of the mineral assemblages in the area studied reflect dehydration reactions Ph + Sil + Q = Cd + Kf + V and Ph + Q = Opx + Kf + V, and osumilite- and sapphirine-forming reactions Opx + Cd + Kf + Q = Os, Cd + Kf + Q = Os + Sil, Opx + Sil + Kf = Os + Sa, and Opx + Sil = Sa + Cd. Relatively high Mg/(Mg + Fe2+) (0.64-0.88) in the whole-rock composition and high oxygen fugacity (near hematite-magnetite buffer), together with exceptionally high temperature (almost-equal-to 1000-degrees-C) and extremely low water activity ( < 0.2) at peak metamorphic conditions might stabilize the osumilite- and sapphirine-bearing assemblages at middle or lower crustal levels. Relatively low water activity is probably caused by the relatively dry precursor, which had been metamorphosed at upper amphibolite facies metamorphic conditions before gabbronorite emplacement.
