Geochemistry and Petrography of Amphibolites from the Spuhler Peak Metamorphic Suite: Near the Branham Lakes Area, Tobacco Root Mountains, Southwestern Montana

Abstract

Located in the west-central Tobacco Root Mountains in southwestern Montana, the Archean-aged Spuhler Peak Metamorphic Suite (SPMS) of Gillmeister (1972) consists of lensoid amphibolite beds, more equant bodies of "wispy" amphibolite, pods of ultramafic rock, homblende-plagioclase gneisses, gamet-gedrite gneisses, quartzites, and sillimanite schists structurally overlying the Indian Creek Metamorphic Suite (ICMS) to the south and west and resting beneath the Pony-Middle Mountain Metamorphic Suite (PMMMS) to the north (Burger, 1969). The concern of this report, SPMS amphibolites from a variety of locations mostly within the area from Noble Peak and Mustard Pass in the northwest to Lower Branham Lake in the southeast have been analyzed for their major and trace element chemistry. As determined by Niggli trends and a variety of diagnostic geochemical criteria, the SPMS amphibolites are concluded to have been derived from an igneous and not sedimentary protolith. Tectonic discriminant diagrams based on elements generally considered immobile during metamorphism predominately indicate correspondence to modem-day ocean floor basalts (OFB), though some ambiguity exists. The plots of the SPMS amphibolites on the tectonic discrimination diagrams are somewhat dispersed as well, possibly due to composition changes caused by the relatively high grade of metamorphism that affected the amphibolites. This dispersion combined with a lack of detailed information regarding plate-tectonics in the Archean complicates the accuracy of interpreting the basalt-type for the SPMS amphibolites.