AGE CONSTRAINTS ON METASEDIMENTARY ROCKS NORTHWEST OF THE IDAHO BATHOLITH BASED ON DETRITAL ZIRCONS AND INTRUSIVE SILLS

Amphibolite-facies metasedimentary rocks north of Pierce, Idaho, have previously been correlated with the Revett and Wallace formations of the Belt Supergroup. Recent mapping by the Idaho Geological Survey identified significant lithologic differences from typical Belt units and the rocks were instead assigned to the Syringa metamorphic sequence. The age of the Syringa sequence is not known, but it may predate the Belt. Detrital zircon analyses by laser-ablation ICP-MS techniques at Washington State University confirm that at least some of the rocks north of Pierce do not belong to the Belt Supergroup. Quartzite near Bertha Hill that had been correlated with the Revett was found to contain a zircon population unlike those found in the Revett. The majority of the zircons are 1720-1850 Ma, more characteristic of the Syringa metamorphic sequence. Calc-silicate gneiss exposed northeast of Bertha Hill along the North Fork of the Clearwater previously correlated with the Wallace Formation was found to contain some detrital zircons that are significantly younger than the Belt Supergroup. These rocks contain numerous detrital grains in the 1000-1300 Ma range and even more abundant 1300-1500 Ma grains. The young grains are similar to Grenville-age zircons in the Buffalo Hump Formation in northeastern Washington. Two other localities (along Snake Creek and along Beaver Creek) do contain a Belt-like zircon population (numerous grains in the 1400-1550 Ma range and a peak at about 1580 Ma) and are probably not part of the Syringa metamorphic sequence as presently mapped. These results highlight the age complexity of basement rocks in northern Idaho and demonstrate the usefulness of laser-ablation ICP-MS techniques in distinguishing between the different basement terrains.

A granitic orthogneiss sill intruded into rocks mapped as Syringa metamorphic sequence near Dent, west-northwest of Pierce, was dated by SHRIMP methods. The sill, which is chemically similar to ~1380 Ma A-type orthogneiss intrusions near Elk City and Shoup, Idaho, has an intrusive age of 1378 ± 14 Ma. Rims on the zircons gave a metamorphic age of 86 ± 3 Ma. Although this intrusive age does not preclude the possibility that the metasediments at Dent are Belt age, they do provide a lower age limit, ruling out the Buffalo Hump Formation and younger units as a protolith.