Arc magmatism at different crustal levels, North Cascades, WA

Abstract

The mechanisms of magma ascent and emplacement inferred from study of intrusive complexes have long been the subject of intense debate. Current models favor incremental construction, but much of this work has been focused on a single crustal level. However, the study of magmatism throughout the crust is critical for understanding how magma ascends through and intrudes surrounding crustal material. I present new field, geochronologic and geochemical data from three intrusive complexes emplaced at a range of crustal depths in the Cretaceous North Cascades magmatic arc. Integration of geological mapping and high-precision U-Pb TIMS geochronology allows me to demonstrate variable styles of intrusion in different complexes: Assembly of the Black Peak intrusive complex occurred via a series of small (<1 km 3) magmatic increments from ca. 91.7 Ma to 86.8 Ma. My data indicate each of these increments were emplaced and solidified without major assimilation of country rock. The Seven- Fingered Jack intrusive complex, emplaced around ~20-25 km, preserves a similar record of intrusion between ca. 91.8 Ma and 90.5 Ma. Significant compositional variability and antecrystic zircons suggest that the Seven-Fingered Jack represents the remnants of mid-crustal magmatic conduit. Geochronology from the deep-crustal (~25-30 km) Tenpeak intrusive complex, intruded between ca. 92.2 Ma and 89.5 Ma, suggests that plutons comprising the complex were assembled rapidly (<300 ka). These intrusive complexes represent different parts of an arc system, including deep- and shallow-crustal intrusions and a magmatic conduit. I propose a model where increments of magma migrated through the crust in magmatic conduits that were active multiple times. These conduits focused rising magma and served as a crustal filter. Geochronologic and isotopic variability between the Tenpeak and Black Peak intrusive complexes are likely a result of this filtering process and the vastly different conditions in the deep and shallow crust.