CONSTRAINING METAMORPIHC P/T CONDITIONS FROM PRECRAMBRIAN CORUNDUM-BEARING ROCKS OF THE RUBY RANGE IN SW MONTANA

The northern border of the Wyoming province in southwest Montana has been obscured due to metamorphic events at 2.45 Ga and 1.78 Ga. Along this border lies a Laramide uplift, the Ruby Range, that exposes a Precambrian basement of quartzofeldspathic gneiss along with metasupracrustal rocks. The Dillon Gneiss, a suite at least in part of probable meta-igneous origin, is the largest unit of the Ruby Range. Conditions of metamorphism in the Dillon Gneiss have been hard to constrain because it consists mostly of quarztofeldspathic gneiss. Biotite-garnet-corundum-bearing gneiss within the Dillon Gneiss, however, can be used to constrain metamorphic pressure and temperature.

Eight samples were collected from a 0.5 m thick biotite-garnet gneiss horizon within the Dillon Gneiss, discontinuously exposed along strike for ~400 meters. The biotite-garnet gneiss contains 1-2 cm long lenses of quartz-feldspar neosome and is immediately adjacent to a mylonitic leucogneiss, suggesting high-grade metamorphic conditions. The mineral assemblage in these samples consists of biotite-garnet-plagioclase-orthoclase +/- sillimanite, spinel, corundum, and muscovite. The corundum in these samples occurs in modes of 5-10 percent and in all cases with spinel and sillimanite.

Petrographic and SEM-EDS mineral composition analyses constrain the pressure and temperature of metamorphism of the corundum-bearing gneiss, determined in the context of a metamorphic mineral assemblage diagram (pseudosection)—tailored specifically to the bulk composition of the samples. Constraining the pressure and temperature conditions of metamorphism is crucial for determining the relationship between the Dillon Gneiss and Wyoming province during its process of unification with Laurentia—giving insights into the growth of continents.