Year of Award
Master of Science (MS)
Department or School/College
Department of Geology
University of 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 metamorphism <3.1 b.y.) of this sequence to form an early basement, 2) deposition of the Cherry Creek Group in a stable shelf environment, possibly during a period of rifting, and 3) upper amphibolite grade metamorphism, deformation, migmatization of the pre-Cherry Creek Group, and concordant intrusion of the Dillon gneiss along the pre-Cherry Creek Group/Cherry Creek Group contact; all occurring 2.75 b.y. ago. A collision environment is preferred for this latest event.
Schaefer, Michael J., "Petrology, Origin, and Tectonic Interpretation of an Archean Rock Package in the Ruby Range, Southwestern Montana" (1986). Graduate Student Theses, Dissertations, & Professional Papers. 11076.
© Copyright 1986 Michael J. Schaefer