MINERALOGIC AND BULK CHEMICAL COMPOSITION OF PROTEROZOIC MYLONITIC GARNET LEUCOGNEISS IN THE RUBY RANGE, MONTANA

Paleoproterozoic mylonitic garnet leucogneiss (MGL) is widespread in the Ruby Range of southwest Montana. The specific mineralogic and bulk chemical composition of the MGL has not previously been studied, although its age coincides with the timing of the Beaverhead orogeny in the northwestern Wyoming Province at 2.45 Ga. This tectonic event is poorly understood due to pervasive metamorphic overprinting by the younger Big Sky orogeny at 1.78 Ga, however, the lack of overprinting in the MGL means that it contains valuable information regarding the Beaverhead orogeny. The MGL is found discontinuously and at multiple horizons, occurring within the Dillion Gneiss unit and the structurally lower section of the Christensen Ranch suite. Cross-cutting contacts of the MGL through the gneissic fabric of the Dillon gneiss and metasupracrustal layers of the Christensen Ranch suite indicate that the MGL has an intrusive protolith, while the leucocratic composition of the rock suggests a continental crustal melt as the source.

The MGL is composed of quartz, feldspar, and garnet, and has an average bulk SiO₂ content of 75.3%. Additionally, muscovite, monazite, ilmenite, and zircon occur as minor accessory minerals. Feldspar makes up more than 50% of the MGL. Both plagioclase and alkali feldspar are present, although the ratio of plagioclase to alkali feldspar differs significantly on all scales: from outcrop to outcrop, across a single outcrop, and within a hand sample. Feldspar grain size ranges from 0.1 to 10 mm, with the larger feldspars likely reflecting the intrusive protolith of the MGL. Garnet constitutes 0 to 25% of the MGL, and ranges in size from 0.1 to 5.0 mm. Smaller garnet grains are typically euhedral and inclusion free, whereas larger grains are typically subhedral and poikiloblastic with inclusions constituting up to 30% of a grain. Determination of specific mineral compositions of garnet and feldspar in the MGL, accomplished through petrographic microscopy, electron dispersive spectrometry (EDS), and bulk chemical analyses, support correlations with possible source material and metamorphic conditions that created the MGL – with results that lead to a better constrained understanding of the Beaverhead orogeny.