Rocky Mountain Section - 64th Annual Meeting (911 May 2012)
Paper No. 6-7
Presentation Time: 10:30 AM-10:45 AM


GUEVARA, Victor1, BALDWIN, Julia1, CROWLEY, James L.2, LEWIS, Reed S.3, and FOSTER, David A.4, (1) Dept. of Geosciences, University of Montana, 32 Campus Dr #1296, Missoula, MT 59812-1296,, (2) Department of Geosciences, Boise State University, Boise, ID 83725, (3) Idaho Geological Survey, University of Idaho, PO Box 443014, Moscow, ID 83844-3014, (4) Department of Geological Sciences, University of Florida, Gainesville, FL 32611

The Clearwater metamorphic core complex in northern Idaho is comprised of polymetamorphosed Precambrian rocks. While the presence of basement intrusive rocks in the Clearwater complex has long been known, only recent geochronological data have shed light on their ages and spatial extent. The ages of metasedimentary rocks in the Clearwater complex remain cryptic, however, as direct correlation with ~1450 Ma Belt Supergroup lithologies is nearly impossible due to the obliteration of sedimentary structures through multiple metamorphic events, and it is possible that these rocks are exposures of previously unknown basement. LA-ICPMS U-Pb detrital and igneous zircon geochronology, coupled with 1:24,000-scale mapping, have identified exposures of pre-Belt quartzite, as well as 1870 ± 9 Ma orthogneiss and surrounding basement rocks that were hitherto unknown. Probability density plots of zircon ages from the feldspar-poor quartzites show a dominant zircon population near ~1800 Ma, with smaller Neo- and Mesoarchean peaks, and thus suggest correlation with the pre-Belt Neihart Quartzite. The quartzites lie stratigraphically above feldspathic schist that was intruded by the 1870 Ma orthogneiss. The contact between the quartzite and the schist therefore represents the boundary between basement and an overlying sedimentary package. The orthogneiss is similar in age to other meta-igneous rocks related to arc magmatism along the southern margin of the Archean Medicine Hat block in the Great Falls Tectonic Zone. This previously unknown basement complex provides a new opportunity to study Proterozoic magmatism and metamorphism, as well as aid in supercontinent reconstructions. Further analysis of these rocks, including Hf-analyses of the zircons, will lead to a better understanding of Archean-Paleoproterozoic provinces of the northern Rockies as well as the evolution of the Belt Basin.

Rocky Mountain Section - 64th Annual Meeting (911 May 2012)
General Information for this Meeting


Session No. 6
Paleoproterozoic Orogeny and Mesoproterozoic Enigmas: Constraints on the Formation, Assembly, and Evolution of the Precambrian Rocks in the Rockies I
Hotel Albuquerque: Alvarado A&B
8:30 AM-12:00 PM, Wednesday, 9 May 2012

Geological Society of America Abstracts with Programs, Vol. 44, No. 6, p. 9

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