DETERMINING PRECAMBRIAN CRUSTAL ARCHITECTURE OF THE NORTHWEST MARGIN OF THE ARCHEAN WYOMING PROVINCE IN THE RUBY RANGE, SW MONTANA: MAPPING OF THE MINE GULCH 7.5' QUADRANGLE

The Ruby Range in southwestern Montana is a fundamental location within the Archean Wyoming province for examining Precambrian crustal architecture. The study area covers the Mine Gulch 7.5’ quadrangle, part of the greater Dillon 30’x60’ quadrangle. Mapping is in collaboration with the Montana Bureau of Mines and Geology and is supported by the USGS EDMAP program. Rocks in this region are comprised of metasedimentary sequences which are likely to preserve geology that records Early Proterozoic tectonic processes at the margin of the Wyoming craton. Recent petrological and geochronological work in the region suggests a previously unrecognized fault or unconformity may exist in the map area which separates Proterozoic metasedimentary rocks from Archean crystalline basement gneisses. Additionally, the Ruby Range is host to world-class talc deposits and detailed mapping provides information regarding the regional structural geology and talc occurrences.

The Precambrian rocks of the Ruby Range previously were divided into three northeast-trending units. From youngest to oldest, these units include the Christensen Ranch Metasedimentary Suite (CRMS), the Dillon gneiss, and the volumetrically minor Pre-Cherry Creek Suite. Within the CRMS and Dillon gneiss, zircon and monazite geochronological analyses have identified two distinct metamorphic age populations; these include an older population at 2.55-2.45 Ga and a younger population at 1.8-1.7 Ga. New data shows that a subset of these supracrustal rocks may not have experienced an older 2.45 Ga history, and instead may have been deposited on the cratonic margin post-2.45 Ga. Therefore, mapping of the Mine Gulch quadrangle allows for the determination of the structural relationship between the CRMS and the Dillon gneiss in addition to establishing deformational histories for both units. In particular, previous work identified a mylonitic garnet leucogneiss which occurs at this contact and cuts across both the CRMS and Dillon gneiss. Thus, determining the petrogenesis of this unit and its structural relationship to surrounding lithologies is crucial to understanding the nature of the boundary between the two major Precambrian units in the Ruby Range.