EVIDENCE FOR RECURRENT PALEOPROTEROZOIC AND MESOPROTEROZOIC MAGMATISM AND METAMORPHISM IN THE BOEHLS BUTTE-CLARKIA AREA, NORTH-CENTRAL IDAHO, USA

Evidence for recurrent plutonism and metamorphism in western Laurentia includes newly identified quartz diorite orthogneiss, dated at 1.86 Ga (U-Pb zircon LA-ICPMS), and garnet growth at ~1.47, 1.38, and 1.10-1.00 Ga (Lu-Hf dating). The orthogneiss is exposed between two masses of the younger (~1.78 Ga) Boehls Butte anorthosite in northern Idaho. Although it likely was basement to the 1.47-1.40 Ga Belt-Purcell Supergroup farther west, the orthogneiss may be allochthonous in the upper plate of a regional thrust fault and structurally above Belt-Purcell rocks and the anorthosite. The extent of basement rocks is unknown, but they could have a strike length of 100 km if other orthogneiss and associated schist in the region are Paleoproterozoic. This possibility is significant because it would be a considerable exposure of previously unrecognized crystalline basement to the Belt-Purcell Supergroup and provide additional constraints for continental reconstructions.

Lu-Hf geochronology has identified a rich history of garnet growth in the Proterozoic of western Laurentia. New garnet ages of 1.47 Ga (near Boehls Butte) and 1.08 Ga (Idaho star garnet near Clarkia) augment previously reported Lu-Hf ages of 1.38 Ga for garnet northwest of Clarkia in the southern Priest River complex and 1.1-1.0 Ga at Boehls Butte and near Clarkia (Sha et al., 2005; Zirakparvar et al., 2006). The 1.47 and 1.38 Ga garnet growth corresponds to previously dated magmatic events (Belt-Purcell sills and A-type granite magmatism of the East Kootenay orogeny, respectively). The 1.10-1.00 Ga garnet ages lack synchronous magmatism but add to the growing evidence of Grenville-age metamorphism in the northwest U.S. and southern British Columbia, Canada.