TECTONIC SIGNIFICANCE OF SHEETED PLUTONS IN THE WESTERN CORDILLERA: EVIDENCE FROM THE NORTH CASCADES, WASHINGTON

Steep-sided, vertically extensive sheeted plutons are important components of Cretaceous and Paleogene magmatic arcs in the western Cordillera from Baja California to SE Alaska. In the North Cascades, highly elongate plutons (aspect ratios up to 11:1 in map view) occupy a 25 km-wide, orogen-parallel zone in the central part of this arc. These plutons were emplaced at depths of ~ 25 km to < 5 km and intruded over a protracted interval, including 89-91 Ma, 72-78 Ma, and 46 Ma bodies. They are typified by early thin mafic sheets and subsequent larger tonalite and granodiorite sheets, and irregularly shaped masses. Geochemical modeling suggests these magmas formed by mixing of variable amounts of mafic, mantle-derived magma and crustal melt. Nd isotopes indicate that the crustal component was not derived from evolved continental crust. Epsilon Nd_t values of the plutons decrease over time from +5.2 to +2.5, and likely reflect either a decrease in mantle input as the arc matured or a change to more evolved crustal component. Protoliths of the amphibolite-facies host rocks are mainly a Triassic arc and older(?) oceanic assemblage. Host rock structures are dominated by folds and gently to moderately dipping foliation. The plutons discordantly cut these structures, except in < 1 km wide structural aureoles.

What controlled this distinctive, long-lived style of plutonism? Elsewhere in the Cordillera, several of the major sheeted zones (Peninsular Ranges batholith, western Idaho) sit along boundaries between oceanic and continental basement, and the tonalite sill complex of the Coast Mountains lies along a major terrane boundary and shear zone. In contrast, most of the Cascades plutons do not intrude terrane boundaries or major fault zones, and were not significantly influenced by host rock anisotropy at the level of emplacement. Similar processes and/or features in the source area were probably responsible for the consistent pluton geometries. A persistent linear zone of preferential melting may have existed in the lower crust and/or upper mantle, possibly controlled by inherited fertile compositions. Alternatively, steep arc-parallel, lower crustal anisotropy may have controlled magma ascent. Once early elongate intrusions formed, they likely constituted a preferential pathway for subsequent ascending magma.