TECTONIC IMPLICATIONS OF DETRITAL ZIRCON GEOCHRONOLOGY AND NEODYMIUM ISOTOPES OF THE ARKOSIC PETROFACIES OF THE WESTERN MÉLANGE BELT, LAKE CHAPLAIN QUADRANGLE, WESTERN CASCADES, WASHINGTON
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.