METAMORPHIC EVOLUTION OF THE NORTHWEST WENATCHEE BLOCK, NORTH CASCADES, WASHINGTON

Detailed pressure-temperature-time paths combined with microstructural analyses provide new constraints on peak metamorphic conditions, differentiation between metamorphic events, and possible constraints on crustal loading mechanisms of the Napeequa Schist and Chiwaukum Schist of the North Cascades Crystalline Core in Washington State.

A new thermobarometric and petrographic dataset constrains both peak metamorphic conditions and pressure-temperature-time paths. Microstructural and mineralogical analyses indicate upper amphibolite to granulite facies assemblages with syn-tectonic to inter-tectonic peak metamorphic minerals (garnet and kyanite). The addition of 18 new thermobarometry samples yields a denser dataset which indicates that the baric gradient of the northern Wenatchee Block trends ESE from ~5 to ~11.0 kilobars over 15 kilometers. The resulting gradient is more orogen-parallel than that interpreted by previous workers. Thermobarometric results indicate that migmatization of the Napeequa Schist and Chiwaukum Schist occurred within granulite facies (7.8-9.7 kilobars, 580-700°C and 10.0 kilobars, >790°C, respectively), and that partial melting is likely a contributing factor to the migmatitic zones of both units.

Thermodynamic modeling of garnet chemical evolution with respect to fractionating bulk compositions (“isopleth thermobarometry”) yields detailed pressure-temperature-time paths of zoned garnets from both units. This technique has not been previously applied in the Crystalline Core and helps to characterize the polymetamorphic nature of the Core. Results from isopleth thermobarometry and petrographic analysis indicate at least two garnet-grade Barrovian metamorphic episodes in the Chiwaukum Schist and Napeequa Schist. Additionally, these episodes post-date at least one deformation within the Napeequa Schist. The pressure-temperature-time paths and structural relationships best support a slow thrusting/folding model as the dominant mechanism of crustal loading in the study area, although other models are supported by portions of the data.