NEWLY RECOGNIZED PRE-BELT QUARTZITE IN THE SOUTHERN PRIEST RIVER COMPLEX

Pre-Belt basement rocks in the Priest River complex south of Coeur d’Alene (ID) consist of a variety of highly deformed orthogneisses and spatially related quartzites in a large structural culmination. The quartzites, herein named the Razorback quartzite from its prominent location along the crest of Red Hog Ridge, are white to grey orthoquartzites with accessory graphite, tourmaline, and mica, along with trace amounts of fibrolite. They occur along the flanks of the culmination, where they appear to separate the pre-Belt orthogneisses from the surrounding Hauser Lake Gneiss (meta-Belt), and as large continuous bodies (up to 5 km in length) surrounded by orthogneiss. The quartzite bodies are generally parallel to the mylonitic lineation within the core of the culmination. Although the contacts between the quartzites and orthogneisses are poorly exposed, the quartzites are not intruded by any dikes or sills, nor do any metasedimentary xenoliths exist in the orthogneisses. We interpret these quartzites to be resting unconformably on pre-Belt basement. The quartzites in the core of the culmination are interpreted as remnants of the flanking quartzites preserved in tight synforms along the crest of the culmination in these highly deformed rocks.

Preliminary detrital zircon U-Pb geochronology on the Razorback quartzite, performed at Washington State University, reveal main populations of ages in the Neoarchean (2.70-2.59 Ga) and Paleoproterozoic (1.87-1.73 Ga), with a scattering of Paleo- to Mesoarchean ages (3.4-2.8 Ga). This age pattern is similar to what has been identified in the Neihart and Gold Cup quartzites.

Our initial interpretation correlates the Razorback quartzite to the Gold Cup quartzite at Priest River (ID) and possibly the Neihart quartzite of Montana. The Gold Cup is a graphite-bearing orthoquartzite sitting unconformably between the Archean-aged Pend Oreille gneiss and the overlying Hauser Lake Gneiss. More field work and study is needed to test the Gold Cup-Neihart correlations. If correct, the Razorback quartzite bolsters claims of a regional pre-Belt or basal Belt aged quartzite across much of this part of Laurentia with important implications for Paleoproterozoic paleogeography and plate tectonic reconstructions.