Geologic Studies in Alaska by the U.S. Geological Survey, 1998 U.S. Geological Survey Professional Paper 1615 New Geochronological Evidence for the Timing of Early Tertiary Ridge Subduction in

• Main Authors: Southern Alaska, C. Bradley, All Parrish, William Clendenen, Daniel Lux, Paul W. Layer, Matthew Heizler, D. Thomas Donley
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Kodiak; Alaska
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.550.6012; http://alaska.usgs.gov/staff/geology/bradley/pubs/2000_Bradley_timing_ridge_sub.pdf

We present new U/Pb (monazite, zircon) and 4 0 ~ r / 3 9 ~ r (biotite, amphibole) ages for 10 Tertiary plutons and dikes that intrude the Chugach-Prince William accretionary complex of southern Alaska. The Sanak pluton of Sanak Island yielded ages of 61.1k0.5 Ma (zircon) and 62.7k0.35 (biotite). The Shumagin pluton of Big Koniuji Island yielded a U/Pb zircon age of 61.1k0.3 Ma. Two biotite ages from the Kodiak batholith of Kodiak Island are nearly identical at 58.3k0.2 and 57.3k2.5 Ma. Amphibole from a dike at Malina Bay, Afognak Island, is 59.3k2.2 Ma; amphibole from a dike in Seldovia Bay, Kenai Peninsula, is 57.0k0.2 Ma. The Nuka pluton, Kenai Peninsula, yielded ages of 56.0k0.5 Ma (monazite) and 54.2k0.1 (biotite). Biotite plateau ages are reported for the Aialik (52.2k0.9 Ma), Tustumena (53.2k1.1 Ma), Chernof (54.2k1.1 Ma), and Hive Island (53.4k0.4 Ma) plutons of the Kenai Peninsula. Together, these new results confirm, but refine, the previously documented along-strike diachronous age trend of near-trench magmatism during the early Tertiary. We suggest that this event began at 61 Ma at Sanak Island, 2-4 m.y. later than previously supposed. An intermediate dike near Tutka Bay, Kenai Peninsula, yielded a hornblende age of 11522 Ma. This represents a near-trench magmatic event that had heretofore gone unrecognized on the Kenai Peninsula; correlative Early Cretaceous near-trench plutons are known from the western Chugach Mountains near