PLUTONISM AT DIFFERENT CRUSTAL LEVELS OF ARCS: INSIGHTS FROM THE ~5-40 KM (PALEODEPTH) NORTH CASCADES CRUSTAL SECTION, WASHINGTON

The southern part of the crystalline core of the North Cascades (Cascades core) preserves a Cretaceous crustal section that facilitates evaluation of pluton construction, emplacement, geometry, composition, and relationships to deformation at widely different crustal levels (~5 to 40 km paleodepth) in a thick (> 60 km) continental magmatic arc. Intrusions range from large sheet-like and elliptical plutons that reflect regions of focused magmatism, to injection zones with variably concordant sheeted bodies of different sizes and orientations that record less focused magmatism. Both focused and non-focused systems occur at all crustal levels; the latter is relatively more abundant in the deeper crust. Plutons are typically either steep-sided bodies, some of which are internally sheeted, or initially subhorizontal (< 30⁰) bodies that are commonly thinner and tabular. The abundance of steep, vertically extensive intrusions, and sparsity of pluton roofs, contrasts with the widespread view that most plutons are subhorizontal tabular bodies and/or laccoliths or lopoliths. Pluton emplacement was dominated by vertical material transfer processes during regional arc-normal shortening. Downward ductile flow in narrow structural aureoles and stoping operated during emplacement of larger plutons at all crustal levels. Several plutons and thin sheets are localized in hinge zones of folds, but there is no direct relationship of magmatism to faults and shear zones. Regional strain was focused into amphibolite-facies host rocks next to some plutons after these bodies reached the solidus.

Plutons in the crustal section are dominantly tonalitic; gabbro-diorite and granodiorite are also present in many of the larger plutons at all crustal levels. Thus, the composition and many other features of intrusions are similar throughout the crustal section, but there are systematic changes. In deeper parts of the section: non-focused magmatism is more widespread; intrusions are more sheet-like and tend to have gentler contacts; and there is greater structural coupling between pluton and host rock. Magmatic rocks are abundant at all levels, but are most voluminous in the deep crust of the section.