MELT INJECTION IN THE SWAKANE BIOTITE GNEISS, NORTH CASCADES CORE: IMPLICATIONS FOR MELTING AND DIKE EMPLACEMENT IN DEEP CRUST

The Swakane Biotite Gneiss (SBG) is the deepest exposed unit of the North Cascades Crystalline Core (NCCC), a Cretaceous to Paleogene magmatic arc (Miller 2000). Barometric data indicate peak pressures of 10.5-12 kbar (Valley 2001) suggesting exhumation from 40 km depth. Crustal melts in the form of thin dikes, and larger sheet-like bodies are present in the deepest exposed areas of the SBG. Although the veins were injected, they were most likely derived from a local source, and did not travel far from the source. Timing of injection is constrained by a U-Pb age of 68 Ma (Mattinson 1972), obtained from a deformed leucocratic dike in the Swakane, and is presumably a maximum age for injection. All leucocratic dikes and host rock structures are cut by mafic dikes, dated at ~48 Ma. During this period, the NCCC was undergoing transpression in a SW-NE direction, resulting in a maximum principal stress (s1), subhorizontal, in the same direction (Paterson et al., in review). More than 2/3 of these leucocratic veins cut the pervasive foliation at an angle greater than 15°, and show varying degrees of deformation (TTN shear, boudinage, folding, shearing, brittle faulting, etc.). We have identified 4 distinct sets of leucosomes, forming a network of veins with a conjugate pattern. Statistical analysis of this conjugate pattern reveals a s 1 value parallel to the regional stress field. Emplacement of these leucocratic melts was not controlled by the local anisotropy, but by the regional stress field during transpression.