2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 6
Presentation Time: 8:00 AM-12:00 PM


LEWIS, Reed S., Idaho Geological Survey, University of Idaho, Moscow, ID 83844, BURMESTER, Russell F., Geology Department, Western Washington Univ, Bellingham, WA 98225-9080, OSWALD, Peter, Department of Geol. Sciences, Univ. of Idaho, Moscow, ID 83844 and VERVOORT, Jeff, School of Earth and Environmental Sciences, Washington State University, P.O. Box 642812, Pullman, WA 99164, reedl@uidaho.edu

U-Pb dating by LA-ICPMS of detrital zircons (DZs) from amphibolite-grade metasedimentary rocks in north-central Idaho is revealing patterns of sediment supply and minimum ages for their protoliths that require substantial revision to geologic maps and history. The 1470-1400 Ma Belt-Purcell Supergroup extends unambiguously as far south as the Benton Creek-Kelly Forks fault north of Pierce, Idaho. Most high-grade rocks immediately south of the fault have enough grains younger than the age of the lower Belt Prichard Fm. to suggest that they were derived from upper Belt units. Farther south, biotite-feldspar-quartz paragneiss and feldspathic quartzite (primarily Elk City and Golden sequences) intruded by 1380 Ma A-type anorogenic granite, now augen gneiss, extend in Idaho from near Moscow discontinuously to Elk City and SE past Shoup. DZ ages from these rocks range from 1800 to 1400 Ma with a peak at 1700 Ma and subsidiary older populations. Like the rocks north of Pierce, they are likely derived from upper Belt units and not the Prichard Fm. Neoproterozoic feldspar-poor quartzite, schist, and calc-silicate rocks of the Syringa metamorphic sequence may be unconformable on the Elk City and Golden sequences. We have found Neoproterozoic grains as young as 680 Ma in rocks of the Syringa northwest and southwest of Lowell, confirming ages reported by Lund and others (2005) near Lowell. A prominent population at 1800 Ma is common, and a population around 1100 Ma found in the southernmost sample is similar to that found in the Buffalo Hump Fm. in NE WA (Ross and others, 1992). The northernmost sample (Bertha Hill quartzite north of Pierce) lacks young grains but otherwise has a similar age spectrum. Given the northwestward decrease of young grains in the Syringa, the Bertha Hill quartzite may also be Neoproterozoic. The Neoproterozoic rocks near Lowell, the Elk City and Golden sequences, and the 1380 Ma augen gneiss are bounded on the northeast by the Trans-Idaho discontinuity (Yates, 1968), a thrust fault proposed by Skipp (1987), and the Orofino shear zone (McClelland and Oldow, 2004). We suspect that juxtaposition of Belt rocks with the others by contraction across this zone was facilitated by prior extensional faulting during Neoproterozoic rifting and crustal thinning, perhaps on structures that were active during Belt deposition.