Metamorphism of the Wenatchee Ridge orthogneiss: a combined application of geochronology and phase equilibrium modeling

Abstract

Pressure and temperature estimates from phase diagram models indicate that the Wenatchee Ridge orthogneiss is the deepest exposed segment of the crust in the Nason terrane of the Cascades crystalline core, WA. Peak metamorphic P-T conditions of 8-11.5 kbar and ~600-700 ºC exceed predictions for the Chiwaukum Schist and Mt. Stuart batholith to the west-southwest and are compatible with a gradient of increasing P and T from the southernmost Nason terrane to the NRMG and WRO. Wenatchee Ridge orthogneiss zircon U-Pb ages (88.9-84.8 Ma) are identical to other zircon ages, interpreted as ages of intrusion, from the Pear Lake orthogneiss. Additional similarities in external grain morphology and internal zoning suggest an igneous origin to the Wenatchee Ridge orthogneiss. The P-T results and ages support the interpretation of the Wenatchee Ridge orthogneiss origin as one of many intrusive bodies within the Nason Ridge Migmatitic Gneiss, all of which intruded the Chiwaukum Schist. In addition, the results are compatible with a proposed southwest oriented thrust loading resulting in 1) older metamorphic ages and 2) elevated pressure and temperature conditions in the northeastern Nason terrane. (Published By University of Alabama Libraries