INSIGHTS INTO HIGHLY ELONGATE, STEEPLY SHEETED ARC PLUTONS FROM INTRUSIONS IN THE SE COAST MOUNTAINS, BRITISH COLUMBIA AND WASHINGTON

Belts of steeply dipping, highly elongate plutons in Cordilleran continental magmatic arcs commonly consist of steep sheets of different compositions and textures. In the Coast Mountains-North Cascades Mesozoic to Paleogene arc, these belts include: the ~1,000-km-long, Paleocene tonalite sill complex; a 20-km-wide zone in the crystalline core of the North Cascades where highly elongate plutons intruded intermittently for >170 Ma; and a 260-km-long, <15-km wide belt of 250–110 Ma intrusions in the SE part of the Coast Mountains batholith. Our recent studies focus on the latter belt where plutons intrude Permian and Triassic arc rocks of Quesnellia on the east and are bounded by the >250–km-long, high-angle Pasayten fault on the west (e.g., Monger, 1989; Greig, 1992). Plutons in this belt generally young from NNW to SSE and vary in composition. In the north, the Triassic Lytton Complex ranges from gabbro to granodiorite, and contains xenoliths as far as 7 km from the host rock contact. Southward, the Lytton is intruded by the compositionally more homogeneous, ca. 157–123 Ma Eagle tonalite, and mappable Lytton xenoliths occur >40 km farther to the south. The eastern Eagle margin is a ~200-m-wide zone of m to 10s of m wide rafts of Quesnellia alternating with sheets of tonalite. Eagle and Lytton rocks are intruded by m to 2-km-wide sheets of 110 Ma, muscovite-bearing granotoids of the Fallslake Suite, and to the south the Cretaceous Okanogan Range batholith intrudes the Eagle pluton. The ~111 Ma, 1–5-km-wide, 75-km-long belt of hornblende tonalite and trondhjemite next to the Pasayten fault contrasts with more irregularly shaped bodies of the batholith to the east.

A major question is what localized intrusion of elongate plutons of different compositions for ~140 Ma in the Lytton-Eagle-Okanogan belt in contrast to coeval plutons with much lower aspect ratios to the east and west? Control by regional shear zones has been proposed for similar zones (e.g., tonalite sill complex, Main Donegal Granite), but no regional faults are associated with the North Cascades belt. The Pasayten fault may have localized intrusion, but earliest documented deformation is ~110–105 Ma, after almost all of the magmatism. A long-lived deep-crustal anisotropy may have controlled magmatism in the belt and the later position of the Pasayten fault.