Paper No. 142-6
Presentation Time: 10:15 AM
TECTONIC IMPLICATIONS OF DETRITAL ZIRCON GEOCHRONOLOGY AND NEODYMIUM ISOTOPES OF THE ARKOSIC PETROFACIES OF THE WESTERN MÉLANGE BELT, LAKE CHAPLAIN QUADRANGLE, WESTERN CASCADES, WASHINGTON
SAUER, Kirsten, Department of Geological Sciences and Engineering, University of Nevada, Reno, 1664 N. Virginia Street, Reno, NV 89557, DRAGOVICH, Joe D., Washington State Department of Natural Resources, Division of Geology and Earth Resources, 1111 Washington St SE, MS 47007, Olympia, WA 98504-7007, MACDONALD Jr., James H., Marine & Ecological Sciences, Florida Gulf Coast University, 10501 FGCU Blvd South, Ft. Myers, FL 33965, FRATTALI, Christina L., Washington State Department of Natural Resources, Geology and Earth Resources Division, 1111 Washington St SE, PO Box 47007, Olympia, WA 98504-7007, ANDERSON, Megan, Geology Dept, Colorado College, 14 E. Cache La Poudre St, Colorado Springs, CO 80903, DUFRANE, S. Andrew, Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building, Edmonton, AB T6G 2E3, Canada and GORDON, Stacia M., Department of Geological Sciences, University of Nevada- Reno, 1664 N. Virginia Street, Reno, NV 89557
The Western mélange belt (WMB) is an accretionary complex of marine origin located in the North Cascades, WA associated with Jurassic to Cretaceous oblique subduction along the North American continental margin. The WMB consists of weakly metamorphosed clastic and igneous rocks with rare marble and serpentinite. The clastic rocks are separated into three distinct petrofacies by Jett and Heller (1988): arkosic, lithic, and chert-rich. The well-bedded turbidites of the arkosic facies have a two-mica granitic provenance with up to 20% K-feldspar. The age and provenance of this mélange has implications for the timing of terrane accretion and thrusting events during the Late Cretaceous Cordilleran forearc region.
U-Pb dates of detrital zircons (n = 129) from a prehnite-pumpellyite facies WMB arkose sandstone from the Lake Chaplain quadrangle are dominantly Mesozoic in age (~59%), with a minor Paleozoic population (~3%) and a significant Meso- to Paleoproterozoic presence (~38%). The youngest population (n = 21) of detrital grains has ages ranging from 69 to 78 Ma and a weighted-mean average of ca. 74 Ma. Two previous detrital zircon studies, located to the south and southeast of the sample site, provided minimum ages of 87 Ma (Brown, 2012) and 96 Ma (Dragovich et al., 2009) respectively. Thus, the 74 Ma age substantially reduces the minimum age of the WMB. This new minimum depositional age moves the maximum age of thrust emplacement for the WMB from 87 Ma (Brown, 2012) to 74 Ma. Whole rock Nd (εNd = -9.3) and petrographic evidence suggests an evolved, true granite provenance for the arkosic petrofacies of the WMB. However, true-granite sources of appropriate age are absent within the North Cascades range. Potential sources include the Idaho Batholith, which would require northward translation of the WMB from deposition to its current position. Overall, detrital zircon, Nd isotopic, and petrographic results require an evolved, granitic source for the arkosic petrofacies as well as a younger age estimate for the northwest Cascades thrust system.
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