SIGNIFICANCE OF FABRIC DEVELOPMENT IN THE GABBROIC RIDDLE PEAKS PLUTON, NORTH CASCADES, WASHINGTON

The gabbroic Riddle Peaks pluton is the largest (~ 40 km2) mafic pluton in the Cascades core, a Cretaceous and Paleogene continental magmatic arc (e.g. Tabor et al., 1989) dominated by tonalitic plutons. The Riddle Peaks pluton is a hornblende gabbro that is mostly layered on the cm to meter scale, though a massive phase is present at the northern and southern tips of the pluton. Included within the pluton are numerous amphibolite and schist xenoliths of the Triassic Holden Assemblage that range in size from 10s of cm to 10s of meters.

A variety of syn-magmatic and high temperature sub-solidus structures were formed in the gabbro and include: NW-striking, moderate to steeply NE-dipping, magmatic and high-temperature sub-solidus foliations; moderate to steeply NE-plunging lineations and folds; NW-striking ductile shear zones; and NNW-striking magma filled fractures. In the layered phase, magmatic and sub-solidus foliations are commonly discordant to the layer boundaries, generally £ 20° in a consistent clockwise direction, and are commonly deflected along cm-scale ductile shear zones developed along layer contacts. Locally, folds of the magmatic foliation have axial planes oriented similarly to the northwest strike of the axial planes of folds in the host rock and host rock xenoliths. The type and orientation of structures implies emplacement occurred during overall regional contactional tectonism.

It is interesting to compare the syn-emplacement deformation of the Riddle Peaks gabbro with the deformation recorded in the extensively sheeted, dominantly tonalitic Cardinal Peaks pluton. These plutons are adjacent to each other and field relations suggest that they were emplaced synchronously. This is further supported by preliminary U/Pb zircon data that date both plutons at ~76-77 Ma. Studies of the Cardinal Peak pluton (Miller and Paterson, 2001) show that the magmatic structures largely record regional contractional deformation, analogous to the more mafic Riddle Peaks pluton. We thus suggest that the Riddle Peaks pluton may provide insights into how large bodies of mafic magma deform during emplacement in a contractional arc.