NEW THERMOBAROMETRIC ESTIMATES FOR THE WENATCHEE BLOCK, NORTH CASCADES CRYSTALLINE CORE, WASHINGTON

The metamorphic and plutonic rocks of the North Cascades Crystalline Core of Washington are the result of multiphase folding, faulting, and metamorphism related to a Mesozoic convergent margin along the North American Cordillera. The Core is divided by the Entiat fault into two structural blocks; the Chelan Block (to the NE) and the Wenatchee Block (to the SW). These blocks differ in their metamorphic and magmatic history, resulting in a steep, NW-trending baric gradient northeast of the fault and a gentle, NE-trending gradient in the southwest. Published models for the crustal thickening associated with the Mesozoic orogeny rely in part on sparse thermobarometric data in the Wenatchee Block that record a baric field gradient ranging from 3.9 kbar in the lower greenschist facies to 9.4 kbar in upper amphibolite facies. Peak temperatures range from 522 to 665 C° for these samples. New geothermobarometric tools such as the average PT method and new single-equilibrium geothermobarometers have made it possible to better constrain the peak conditions reached by samples of the Core and perhaps improve the precision and accuracy of these estimates. Published mineral assemblages and mineral chemistries for 5 samples from the Wenatchee Block were combined with new bulk-rock chemistries from XRF. Metamorphic mineral end-members and applicable activities were computed using AX (part of Thermocalc) and the average-PT Thermocalc technique was used to estimate peak conditions. Pseudosections were calculated as a secondary constraint on peak conditions. These new P-T data suggest that peak conditions in the Chiwaukum and Napeequa Schists of the Wenatchee Block are underestimated in pressure by as much as 2 kilobars. This implies a steeper gradient then presently established. This also suggests a different orientation of the gradient with higher pressure westward towards the Straight-Creek Fault and a more north-south orientation to the isobars.