PETROLOGY AND GEOCHEMICAL ANALYSIS OF PELITIC GNEISSES, RUBY RANGE, SW MONTANA

Two structurally lower basement units in the Ruby Range of southwest Montana, the Dillon gneiss and the pre-Cherry Creek gneiss, were metamorphosed during the Big Sky orogeny, a collisional event that took place 1.8 to 1.7 Ga. In both units, the gneisses are diatexites that are segregated into paleosome and neosome. Neosome is characterized by relatively more quartz and feldspar and less biotite than the quartz and feldspar-poorer and biotite-richer paleosome. Samples of paleosome and neosome were collected from one unit in the Dillon gneiss and two units in the pre-Cherry Creek gneiss. In the Dillon gneiss, the assemblage of the paleosome is plagioclase + quartz + biotite + garnet + sillimanite + rutile. The neosome contains complexes of large, anhedral garnet grains associated with medium-grained biotite and sillimanite. These exist in a matrix of coarse-grained plagioclase, quartz and sillimanite. In the pre-Cherry Creek gneisses, neosome is dominated by quartz, feldspar, and coarse-grained garnet, with little associated biotite or sillimanite. Paleosome has quartz, feldspar, biotite, and deformed garnet grains associated with biotite, sillimanite, and cordierite. Among the pre-Cherry Creek paleosomes, there are variable amounts of sillimanite and cordierite associated with garnet ranging from none, to almost surrounding the garnet. Given the migmatitic character of the pelitic gneisses in this study, alkali feldspar would be expected in all of the neosome assemblages as a consequence of partial melting but, notably, these rocks contain no alkali feldspar. Possible causes for the absence of alkali feldspar include melt transport that removed alkali feldspar, or melt injection from a mafic protolith, enriching the neosome in plagioclase. It is also possible that the protolith was potassium poor and relatively enriched in calcium and magnesium, which could yield the melt assemblages observed.