YAVAPAI-MAZATZAL DETRITAL ZIRCON AGES IN ACCRETED ROCKS IN SOUTHERN ALASKA: LOOKING FOR POTENTIAL SOURCE ROCKS IN THE NORTH AMERICAN CORDILLERA

Preliminary detrital zircon studies from the Yakutat Group, part of a thick Late Cretaceous to Paleocene accretionary wedge complex now located in southern Alaska, reveal a robust suite of Precambrian zircons with ages typically associated with the Yavapai-Mazatzal province of SW Laurentia. The basement of the Yakutat terrane is composed of Eocene basalt and Late Cretaceous and Paleocene flysch and mélange units of the Yakutat Group. The mélange unit has a mudstone matrix containing sandstone phacoids and clasts of deformed granitoids, pillow basalt, and marble; some of the deformed granitoid blocks are up to 1 km in the long dimension. The flysch unit is defined by coherent turbidites composed predominantly of arkosic sandstone and shale with minor conglomerate. Maximum depositional ages (MDAs) of 14 sandstone samples from the flysch and mélange units range from 60.4 ± 1.8 Ma to 101.1 ± 2.7 Ma with the majority of samples (n=11) between 60-72 Ma. Grain age distributions from these rocks are nearly identical with prominent Phanerozoic peaks at 74 Ma, 91 Ma, 154 Ma, and 188 Ma and Precambrian peaks at 1375 and 1710 Ma. εHf (t) values from the Phanerozoic zircons range from -31.1 to 12.1, but the range from the two Precambrian populations is much narrower from -3.4 to 12.7 with the majority (84%) above CHUR (εHf = 0).

The Phanerozoic grain-age distributions from the Yakutat Group align with typical ages from the Coast Plutonic Complex of British Columbia and have peak ages that occur throughout much of the North American Cordillera. The extremely negative εHf values suggest that some of the zircons came from Cretaceous plutons that were derived in part from highly evolved Precambrian crust. In contrast, the Precambrian grain-age distributions and their juvenile εHf values is unusual for Alaska, and appear to correlate well with known ages and juvenile isotopic composition of the Yavapai-Mazatzal province of SW Laurentia. An outstanding question is whether there are other potential sources in the North American Cordillera for these distinctive Precambrian populations of zircons? One possible candidate explored here is the Belt Supergroup of Montana, Idaho and British Columbia, which has similar ages but typically more evolved εHf values.