SHRIMP DETRITAL ZIRCON AGES FROM THE YELLOWJACKET, APPLE CREEK AND GUNSIGHT FORMATIONS, SALMON RIVER MOUNTAINS, CENTRAL IDAHO

SHRIMP U-Pb ages of 60-grain detrital zircon samples in Mesoproterozoic strata of the Yellowjacket, Apple Creek and Gunsight Formations, Salmon River Mountains, Idaho demonstrate linkage between these strata and the upper part of the 1470 to 1400 Ma Belt Supergroup of Montana. All three samples analyzed have similar detrital zircon signatures, with over 70% of the grains giving near-to-concordant 207Pb/206Pb ages between 1640 and 1780 Ma. A few grains (5%) in all samples are Archean and early Paleoproterozoic (2400 to 2600 Ma); a small population (less than 10%) is between 1540 and 1640 Ma (the ‘non-North American grains’ of Ross and Villeneuve, 2003, GSAB), with a minor component (less then 5%) of syn-Belt grains between 1400 and 1500 Ma. The youngest detrital grains overlap in age or are younger than the 1454 ±9 Ma ash bed in the upper Helena Formation in Glacier National Park (Evans et al., 2000, CJES). The youngest concordant grains are ~1440 to 1410 Ma suggesting that the all strata were deposited in this ~30 m.y. time interval. The detrital zircon signatures are similar to those of the upper Belt Missoula Group from Montana, and from the Gunsight and Swauger formations of the Beaverhead Mountains. The signatures are distinct from the lower Belt Prichard Formation, which contains a major component of 1540 to 1640 Ma grains, and which lacks the 1640 to 1780 Ma grains. These data suggest the Yellowjacket, Apple Creek and Gunsight Formations are part of a composite clastic wedge with abundant fine-grained 1640 to 1780 Ma magmatic zircons. These grains may have been derived from a composite Yavapai-Mohave juvenile island-arc system, with grains transported northward by a large river system and deposited as a vast alluvial/shallow subaqueous fan in the southwestern Belt Basin. Alternatively, they may represent unroofing of the subjacent Paleoproterozoic basement to the Belt basin, estimated to have formed as juvenile crust 1750 to 1800 Ma (Foster and Fanning, 1997, GSAB).