IN SUSPECT TERRANE? SEDIMENT PROVENANCE OF THE LATE ARCHEAN PHANTOM LAKE METAMORPHIC SUITE, SIERRA MADRE, WYOMING

Archean supracrustal sequences in the Wyoming province are exposed in the eastern Owl Creek Mountains, the Wind River Range, Barlow Gap, the Rattlesnake Hills, the Medicine Bow Mountains, and Sierra Madre. Correlation between these sequences is difficult due to limited knowledge and small spatial distribution. A sequence of particular interest is the Phantom Lake Metamorphic Suite of the Sierra Madre, which includes the Jack Creek Quartzite, Silver Lake Metavolcanics, and the Bridger Peak Quartzite. A minimum age of the Phantom Lake Suite is established by cross-cutting relations with the 2710+10 Ma Spring Lake granodiorite (Premo and Van Schmus, 1989). The underlying Vulcan Mtn. Metavolcanics have yielded a U-Pb sphene age of 2691+7.7 Ma (Harper, 1997). The entire sequence is interpreted to be deposited on a quartzo-feldspathic orthogneiss.

There are two contrasting models for the origin of the Phantom Lake Metamorphic Suite. The first is that the rocks were deposited at or near the margin of the Wyoming craton (Houston et al., 1992). Alternatively, Chamberlain et al. (in press) suggest that the Phantom Lake Suite and its basement orthogneisses represent an exotic block accreted onto the Wyoming craton ca. 2620 Ma or younger. Field observations and Nd isotopic data do not definitively discriminate between either of these models, however the Sierra Madre orthogneisses have Late Archean Nd model ages of 2.9 Ga that distinguish them from orthogneisses in the central Wyoming Province that have Early Archean Nd model ages. The Phantom Lake Suite metasedimentary rocks also have Late Archean Nd model ages of 2.9 Ga, younger than model ages from most potentially correlative supracrustal sequences except for those preserved in the Rattlesnake Hills. The Rattlesnake Hills succession has been interpreted as back arc basin sediments tectonically juxtaposed onto much older basement gneiss (Fruchey, 2002). If the Phantom Lake Suite is related to the supracrustal sequence in the Rattlesnake Hills, it too may have been translated along the southern margin of the Wyoming craton.