UNDERSTANDING FLUID-ROCK INTERACTION DURING CORE COMPLEX EMPLACEMENT: STABLE ISOTOPE VARIATION AND WHITE MICA CHEMISTRY, KETTLE DETACHMENT, WASHINGTON, USA

Extensional detachment systems are fundamental processes controlling crustal reorganization following orogenic collapse whereby high grade rocks are rapidly exhumed to upper crustal levels along low angle, brittle-ductile shear zones. Field and laboratory observations illuminate pervasive fluid interaction within these detachment systems; the activity of a fluid as an agent of heat and mass transfer promotes localization of deformation and is fundamental to the late stages of core complex emplacement. Thermal convection provides an efficient cooling mechanism of the complex and results in extensive fluid-rock interaction and isotopic exchange. Deformation textures, mineral chemistry, and reaction progress are largely dependent on the mechanisms of fluid flow, size and composition of the fluid reservoir, and residence time during which a fluid phase interacts with the actively deforming mylonite. To resolve the character of this syndeformational fluid, we performed whole-rock analyses of micaceous quartzite to resolve variation in stable isotopes and conducted EMP analysis of white mica to understand mica crystallization and retrograde reaction progress.

The Kettle metamorphic core complex of northeastern Washington, USA, presents exposure of a high-grade metamorphic core that was exhumed by an east-rooted, Eocene detachment system. High resolution sampling of a 200 m quartzite section provides opportunity to better understand the source of fluids and its influence on mica composition. Mica deuterium values indicate that the fluid that participated in mica crystallization was meteoric in origin (δD ~ -110 ‰), and the interaction of this fluid with the quartzite results in large drops in quartz and mica oxygen isotope values. Major element variation of white mica populations throughout the vertical extent of the detachment and intragrain, major element zonation patterns complement these isotope results and contribute to understanding the growth history and recrystallization of mica. These data and interpretations address the permeability of rocks deforming in the ductile regime and the thermal, mechanical, and chemical consequences of fluid flow in extensional detachment systems.