EVIDENCE OF A LATE JURASSIC RIDGE SUBDUCTION EVENT: GEOCHEMISTRY AND AGE OF THE QUARTZ MOUNTAIN STOCK, MANASTASH INLIER, CENTRAL CASCADES, WASHINGTON

Previous studies of rocks in California and Oregon suggest a Late Jurassic ridge subduction event occurred. The geology of the Manastash inlier supports this Late Jurassic ridge subduction. The biotite hornblende granodiorite and biotite hornblende tonalite of the Quartz Mountain stock and its cupolas are located in the Manastash inlier, central Cascades, Washington. This stock and its cupolas intrude the low P/T metamorphosed Lookout Mountain Fm. turbidites – which are interpreted to have formed in a forearc depositional setting – and the suprasubduction zone Hereford Meadow amphibolite. A single crystal sensitive high resolution ion microprobe reverse geometry U–Pb age from stock zircons has an age of 157.4 ± 1.2 Ma (2σ). This age corroborates the published age from the stock, and is interpreted to be the crystallization age of the stock. New whole-rock major and trace element geochemistry suggests the granitoids from this stock were generated by shallow mantle melting in a subduction zone setting. Geochemistry also suggests the stock assimilated minor amounts of its wall rock, and the cupolas were generated from the same magmatic source as the main phase of the stock. The intrusion of the Quartz Mountain stock into the low P/T turbidites of the Lookout Mountain Formation and the suprasubduction zone Hereford Meadow amphibolite suggest that the magmas were generated by the subduction of a Late Jurassic spreading ridge. The geochemistry of the Quartz Mountain stock is similar to granitoids of the Taitao peninsula, Chile, which were generated by the subduction of the Chile ridge during the late Neogene.