METAMORPHIC EVOLUTION OF THE WENATCHEE BLOCK, SUIATTLE RIVER REGION, NORTH CASCADES, WASHINGTON

The Crystalline Core (CC) of the North Cascades in Washington represents the southeastern extent of the Coast Plutonic Complex (CPC). The CPC is the heart of the collision between the Insular and Intermontane Superterranes and represents an excellent region for understanding metamorphism in a convergent margin. The CC is divided by the Entiat fault into two structural blocks; the Chelan Block (to the NE) and the Wenatchee Block (to the SW). Sparse microprobe and kinematic data of the two blocks have spawned controversy as to the method of crustal thickening and subsequent metamorphism. Whether metamorphism is caused by loading of orogen-normal thrust sheets (thrust loading) or large igneous bodies (magma loading) is important to understanding the tectonic evolution of the CC, and by extension, convergent margin processes in general.

Metamorphic mineral end-members and applicable activities were computed from mineral chemistry data from previous work in the Wenatchee Block as well as newly collected specimens. Multiple-equilibria methods (average-PT using Thermocalc) were applied to these data to estimate peak metamorphic conditions. Garnet inclusions were analyzed to constrain portions of the prograde Pressure-Temperature (P-T) path.

Results suggest a mechanism other than stacking of thrust sheets as the primary mechanism for metamorphism. Reassessment of previous mineral chemistry data using current multiple-equilibria methods indicates that estimates of pressure and temperature were underestimated by as much as 2 standard deviations. These new P-T conditions, combined with thermobarometry on new specimens, suggest that peak metamorphic pressure increases irregularly in a SE direction. Previously published isobaric gradients illustrate a broader, gentle gradient, whereas our findings show steep portions in the Wenatchee Block. Consequently, the isobar geometry is inconsistent with thrust-loading, as P-T points suggest a non-linear and non-orogen-parallel isobar pattern. Preliminary P-T paths based on inclusion thermobarometry are counter-clockwise, indicating that heating occurred prior to peak metamorphic conditions. This evidence is more consistent with a magma-loading model for regional Barrovian metamorphism in the Wenatchee Block.