EVIDENCE FOR SUB-HORIZONTAL FLUID FLOW DURING PROGRADE METAMORPHSIM IN THE NORTH CASCADES CRYSTALLINE CORE

The movement of fluids through the lower crust is a key control in the evolution of the lithosphere. Current models in the literature suggest that fluids generated during prograde devolitilization in the crust are (a) either quickly relocated vertically in the crust through fractures when permeability no longer supports fluid fluxes, or (b) the fluid’s path is determined by the effective permeability in the crust. The latter model suggests a wide range of possibilities for the movement of fluids. Traditional metamorphic petrology combined with T-X(CO₂) pseudosections constructed for a set of rocks from the North Cascade Mountains constrain fluid flow to a sub-horizontal direction and thus it was likely controlled by existing permeability during prograde metamorphism.

This study examines two well-exposed bodies of marble located in the Nason terrane northwest of Leavenworth, WA. The principal rocks exposed in the study area are the Wenatchee Ridge Orthogneiss, the Chiwaukum Schist, and its altered and migmatized counterpart, the Nason Ridge Migmatitic Gneiss (NRMG). The bodies of Grt-Qtz-white mica±Zo marble occur within the NRMG and Chiwaukum Schist near the structurally deepest, though not highest grade, part of the Nason terrane. Kilometer scale, NW-SE trending folds define the regional structure of the study area.

Relating mineralogy to T-X(CO₂) pseudosections provides constraints on X(CO₂) for each specimen. Fluid composition ranges from 0.02 to 1.0 mole fraction CO₂ over the study area. Statistical analysis shows a X(CO₂) increasing most steeply along a line trending 36º ± 23º and inclined 25° above horizontal. Based on these data, fluids were moving sub-horizontally and, based on textures and conditions, pervasively through the study area during peak conditions. Late-stage fluid flow at lower temperatures and pressures is recorded in heavily mineralized retrograde veins where flow was focused and also by sericitization and pyrite. Results support a permeability-driven model for synmetamorphic fluids in the lower crust.