FLUID INTERACTION IN EXTENSIONAL DETACHMENT SYSTEMS: 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 exhumed to upper crustal levels along low angle, brittle-ductile shear zones. The interaction of a bimodal fluid population provides an efficient convective cooling mechanism of the complex and the involvement of these fluids over the duration of activity of the detachment results in extensive fluid-rock interaction and isotopic exchange. The mechanisms and rates of fluid flow and the size and composition of the fluid reservoir ultimately control the deformation textures and mineral chemistry preserved. In order to understand the role of fluids on extensional detachment systems, we performed whole-rock analyses of micaceous quartzite to resolve variation in stable isotopes and conducted EMP analysis of white mica to understand the fluid influence on mica crystallization.

The Kettle metamorphic core complex of northeastern Washington, USA, presents exposure of a high-grade metamorphic core that was exhumed by an Eocene, east-rooted detachment system. Deformation was accommodated by and localized within an approximately 200 m continuous section of quartzite mylonite. High resolution sampling (up to 5 m) of this section provides an excellent opportunity to better understand the source of fluids and its influence on mica composition. Based on mica deuterium values, the fluid that participated in mica crystallization was determined to be meteoric in origin (ΔD around -110 per mil), and the interaction of this fluid with the quartzite mylonite results in large drops in quartz and mica oxygen isotopic 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. Trends of elemental exchange and patterns of isotopic variation can be compared and evaluated to resolve the style and mechanisms of fluid transport during exhumation of the core complex. These data and interpretations address the permeability of rocks deforming in the ductile regime and the thermal and mechanical consequences of fluid flow in extensional detachment systems.