MECHANISMS OF MAGMA ASCENT: IMPLICATIONS OF SHEET TIPS IN THE ENTIAT PLUTON, CASCADES CRYSTALLINE CORE, WASHINGTON

Examining the propagation margins or tips of sheeted intrusions is a test of ascent mechanisms that predicts extensive fracturing, faulting, and limited crystal-plasticity at a dike tip and a ductile aureole at a diapir tip. The mid-crustal 73 Ma Entiat pluton is an example of an extensively sheeted pluton emplaced during active contraction. The sheets are parallel to the Cascades arc and normal to the direction of regional shortening, an orientation oblique to that predicted for dikes intruded during arc-normal shortening. The Chipmunk Creek tip is a well exposed finger of tonalite of the Entiat pluton intruded into amphibolite-facies meta-plutonic rocks. The tonalite has a strong magmatic foliation and a weak gently plunging lineation. The foliation is folded in concordance with folds within the host rock along an axis trending 320° and plunging 12°. The tip lies within the hinge of a host rock fold where the sheet margins are roughly parallel to the axial plane. Numerous sheets of Entiat separated by screens of orthogneiss mark the tip contact. Fractures and small faults are not more abundant and do not show a consistent pattern in front of the tip.The Mad River tip is a well-exposed internal contact marked by granite intruded into tonalite. This tip has a much wider radius of curvature and lower length-to-width ratio than the Chipmunk Creek tip. Both the granite and the host tonalite have a strong magmatic foliation and a moderately plunging lineation. The magmatic fabric is not deflected across the sharp contacts suggesting it records regional strains. This fabric is overprinted by widespread low-temperature solid-state deformation defined by chloritization, kinking, and micro-fracturing that is too low temperature to be induced by an ascending hot dike or diapir. The distribution of fractures and faults does not reflect dike propagation. Although the two tips show significant differences, the concordance of a magmatic fabric across contacts and lack of a damage zone in front of the tips suggests that diking is probably not an important means of ascent for these sheets.