PARTIAL MELTING AND MELT COLLECTION AND TRANSPORT IN THE SWAKANE TERRANE, NORTH CASCADES CRYSTALLINE CORE, WA

The Swakane Terrane recorded the highest pressure and temperature conditions (650-725 ^oC/10-12 kbar) in the North Cascades Crystalline Core (NCCC), yet it has no arc-related magmatism and is one of the youngest metamorphic units, with a protolith age of ca. 73 Ma (Matzel et al., 2002).  Following burial, the Swakane Terrane was subjected to partial melting producing in situ stromatic migmatite that is crosscut by peraluminous, often pegmatitic dikes, which have a U-Pb zircon age of 68 Ma (Mattinson, 1972).  Melting occurred under fluid present conditions while the Swakane was buried to ca. 35 km at a rate of 6-8 mm/year (Matzel et al., in review).  Initial melt migration occurred parallel to the dominant foliation during which time strain gradients controlled migration from intergranular melt pockets to relatively lower strain sites.  With increasing melt volume, melt migrated through dikes that consistently crosscut the generally subhorizontal anisotropy in the Swakane Terrane.  These melts were subsequently intruded into higher levels of the Swakane Terrane, as cm scale dikes and sills, and 10 to 100 meter sheets and stocks.  In several domains in the Chelan Block a conjugate pattern of dikes and sheet injections indicates a maximum principal stress that is subhorizontal and trending SW-NE.  This direction is consistent with previous assertions of Farallon/North American plate margin kinematics, and strain fields determined from the geometry of regional folds, lineations, and kinematic indicators in the NCCC.  In a separate domain in the Wenatchee Block, dike injection appears to have occurred in a random fashion and the majority of the dikes cut foliation at moderate to high angles, suggesting that a buoyancy contrast between melt and host rock was the dominant driving force for melt ascent.  Our study shows that tectonic stress and buoyancy controlled the emplacement of these peraluminous bodies, and local anisotropies such as foliation, folds, and shear zones had minimal effects on melt ascent.