Petrology, Origin, and Tectonic Interpretation of an Archean Rock Package in the Ruby Range, Southwestern Montana

The Ruby Range of southwestern Montana contains Archean amphibolite- to granulite-facies metamorphic rocks belonging to three major units. The Cherry Creek Group is a meta-sedimentary unit dominated by concordantly interlayered units of marble, pelitic schist and gneisses, quartzite, calc-silicate gneisses, iron-formation, and amphibolites representing basaltic flows and sills. Structurally below this group is the Dillon gneiss, a quartzo-feldspathic gneiss of questionable origin. Possible protoliths include arkose, mudstone or siltstone rich in illite and quartz, mixed sediments and volcanics, and intrusive granite. The preferred origin is a synkinematic granitic intrusion with some interlayered sediments. Within this gneiss are several concordant layers of marble and amphibolite. Structurally below the Dillon gneiss is the pre-Cherry Creek Group. This group consists of biotite-quartz-plagioclase gneiss, biotite-garnet- quartz-plagioclase gneiss, sillimanite-garnet-quartz-plagioclase gneiss, hornblende-biotite gneiss, and hornblende gneiss. This unit is believed to be largely sedimentary in origin, with a greywacke protolith. Concordantly interlayered with these gneisses are basaltic amphibolites. Small pods of tectonically emplaced meta-ultramafic rock were emplaced into all these units prior to or during high-grade metamorphism. These rocks are believed to have been emplaced as serpentinized and partially serpentinized peridotites. A proposed tectonic model for the origin of this rock package consists of; 1) deposition of the pre-Cherry Creek unit as a basinal greywacke sequence, and possibly burial and metamorphis