Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 2
Presentation Time: 1:20 PM


LEE, Robert1, LARSON, Peter2, VERVOORT, Jeffery D.3 and KNAACK, Charles3, (1)Department of Geology, Washington State Univ, Pullman, 99164-2812, (2)Geology Department, Washington State Univ, Pullman, WA 99163, (3)Department of Geology, Washington State Univ, Pullman, WA 99164-2812,

            Laser-ablation (LA) ICP-MS U-Pb dating of zircon has been applied to provide new insights into the regional setting of magmatic events in north-central Idaho.  During the Cretaceous, Paleozoic to Mesozoic oceanic island-arc rocks of the Wallowa-Seven Devils (WSD) terrane were accreted on to the North American craton.  This juxtaposition, and subsequent granitic magmatism associated with accretion, defines the Idaho suture zone. The region was subsequently intruded by late Cretaceous magmas of the Idaho batholith.  Samples were collected along the Clearwater River for U-Pb analysis to help constrain the chronology of emplacement.  Two tonalite-trondhjemite samples were collected immediately west of the suture zone between Greer and Kamiah.  Previous studies have shown that these samples are generally characterized by low initial 87Sr/86Sr ratios. Three samples collected east of the suture zone along the South Fork of the Clearwater River include two granitic rocks of the Idaho batholith and one sample of biotite schist from the Elk City metamorphic sequence.  Zircons were separated from each sample and analyzed by LA-ICPMS at Washington State University.  Analytical procedure includes a repetition rate of 10 Hz, 40 micron spot size, and 30 second total run time with 100 sweeps from 202Hg to 238U.  Twenty grains (with an average of three spots per grain) were analyzed for each sample.  The tonalite-trondhjemite samples yielded ages of ~113 Ma with early Cretaceous inheritance.  The two Idaho batholith samples have crystallization ages that cluster around 85 and 87 Ma, but with clear evidence of inherited Mesozoic and Mesoproterozoic ages.  Zircons from the biotite schist have dominantly Mesoproterozoic ages of its sedimentary precursor.  The results suggest two syn- to post-accretion magmatic events during the Cretaceous in this region:  intrusion of tonalite-trondhjemite magmas into the WSD west of the suture zone during the early Cretaceous and later intrusion of granitic rocks of the Idaho batholith into Precambrian sediments during the late Cretaceous.