ION MICROPROBE 207PB/206PB MONAZITE AGES FROM PRECAMBRIAN METAMORPHIC ROCKS OF SW MONTANA

Southwestern Montana contains a series of mountain ranges made up largely of metamorphosed Precambrian basement rock constituting the northern extent of the Wyoming Province. Previous work (Harms, et al., 2004) focused upon the Tobacco Root Mountains (TRM) established two different ages of metamorphism—the first at ~2450 Ma and a younger event, the Big Sky Orogeny, at 1720-1780 Ma (Cheney, et al., 2004). Work is being done to extend results from the TRM to surrounding mountain ranges in an effort to establish geographic extensions of the Big Sky Orogeny and continuity for Precambrian tectonic processes.

Seventeen rocks, predominantly quartzo-feldspathic gneisses and micaceous schists, from the Highland, Gravelly, Greenhorn, and Madison Ranges were collected for monazite dating. One hundred twenty-four 207Pb/206Pb spot ages have been obtained from these 17 samples using the IMS 1280 at the Northeast National Ion Microprobe Facility at WHOI. These Monazites are from a variety of textures, including those in the matrix as well as those occurring as inclusions within biotite, kyanite, staurolite, and garnet. Fifty-nine of these spot ages are between 1710 and 1790 Ma, initially indicating that, although the event at ~2450 Ma found in samples from the TRM is absent or masked in the surrounding mountain ranges, evidence of the Big Sky Orogeny is extensive. In addition, of the remaining 65 ages, 53 are between 1800 and 2000 Ma, suggesting a slightly older event preceding the Big Sky Orogeny. For comparison, monazite from four samples will also be dated on an electron microprobe, which provides more precise spatial resolution.

This work proposes possible parallels between the processes of formation of the TRM and its neighboring mountain ranges and continues the contextualization of the metamorphic history of the Wyoming Province as it was added to the North American craton.