Correlation of the Tonga Formation and the Chiwaukum Schist, North Cascades, Washington: Implications for Late Cretaceous orogenic mechanisms

The Tonga Formation of the North Cascades, Washington, consists of weakly deformed, bedded feldspathic graywacke and shale metamorphosed at chlorite to staurolite grade. Analysis of protolith lithology, Rb‐Sr isotopic signature, and plutonic, metamorphic, and deformational evolution in the Tonga Formation suggests that this unit is correlative with the Chiwaukum Schist of the Cascades crystalline core. The Tonga Formation occurs on the flank of the Late Cretaceous orogen, whereas high‐grade Chiwaukum Schist is exposed in the core of the orogen. Metamorphic pressure, as reflected by the distribution of metamorphic minerals and thermobarometry, increases continuously northward in the Tonga Formation from the chlorite zone (<4 kbar) to the staurolite zone (≈ 7 kbar) and then to the kyanite zone (≈ 7 kbar) in the Chiwaukum Schist. The findings reported herein do not support the previously advanced concept that the Cascades crystalline core represents the Easton blueschist terrane that was converted into higher‐temperature metamorphic rock by thermal relaxation after cessation of subduction. The results of this study suggest a relatively simple two‐stage metamorphic history for the Chiwaukum Schist represented by 90–93 Ma shallow contact metamorphism, overprinted by Harrovian metamorphism that increased sharply in pressure from south–west to northeast and reached peak conditions after 90 Ma, all events occurring in a plutonic/magmatic arc setting.