UNRAVELLING THE WESTERN CLEARWATER COMPLEX: NEW INSIGHTS FROM U-PB GEOCHRONOLOGY

The Clearwater complex is a complexly deformed and poly-metamorphosed block of Precambrian meta-igneous and meta-sedimentary rocks located in north-central Idaho. New U-Pb zircon analyses from meta-igneous and meta-sedimentary samples from the western part of the complex serve as the basis for a new architectural understanding of the area. U-Pb zircon data from a fine-grained quartz- and feldspar-rich orthogneiss from Marble Creek record ages at 2.7-2.6 Ga and 1.86 Ga. The two populations may be interpreted as a 1.86 Ga intrusion within pre-existing 2.7-2.6 Ga basement. These two populations match the dominant igneous age populations preserved elsewhere throughout the Clearwater complex. This sample lends new evidence supporting previously proposed (Reid, 1973) old Precambrian basement within the Marble Creek area of the complex. Some debate remains, however, regarding the genesis of this sample. Analysis of magmatic zircons from a hornblende-biotite tonalite from the nearby Glover Creek area to the south yield an age of 1.86 Ga. This sample and similar granitic intrusions throughout the Clearwater complex serve as tracers for identifying the extent of pre-Belt basement rock. Detrital zircon ages from a clean quartzite in the lower Marble Creek area yield two distinct age peaks at 1.8 Ga and 2.9-2.4 Ga. This age distribution is similar to that of the Fort Steele Formation, a likely correlative in Canada that was previously unrecognized within the complex. An earlier assignment of Belt-Purcell Ravalli Group appears unlikely. Probability density plots of detrital zircon ages from a mica-rich quartzite in the upper Marble Creek area show dominant age populations between 1800-1600 Ma with a smaller population of 2.6-2.4 Ga grains. This age distribution is consistent with published results from the Belt-Purcell Prichard Formation and extends the area of the Prichard Formation within the western part of the complex.