Abstract

The record of metamorphism, deformation, and melting in the North Cascades continental arc provides insights into the timing and mechanisms of extensional unroofing that followed crustal thickening. The Skagit Gneiss (North Cascades) is composed of variably deformed 90–45 Ma tonalitic to granitic intrusive rocks. These lithologies and metasedimentary rocks are migmatitic. Zircon and monazite from different textural varieties of migmatite from three outcrops along an east-west transect were analyzed using the isotope dilution–thermal ionization mass spectrometry (ID-TIMS) technique. The data reveal two main migmatization pulses: (1) 68–63 Ma and (2) 53–47 Ma. In the westernmost locality, leucosome zircon yields group 1 dates, and mesosome (sillimanite-garnet-biotite gneiss) zircon is as young as 61 Ma. Leucosome zircon in the easternmost outcrop yields only group 2 dates, and biotite gneiss contains 47 Ma zircon. The third outcrop has leucosome dates from both groups: 65 Ma and 53 Ma. Monazite from leucosomes and mesosomes likely records the timing of prograde metamorphism (ca. 69 Ma) and later synkinematic ± fluid-mediated growth (49–46 Ma). These results indicate that partial melting occurred during 71–61 Ma metamorphism and lasted through 46 Ma deformation associated with exhumation. The results and field observations suggest the presence of ductilely deforming crust from 68 to 46 Ma. The similarity between the youngest U-Pb dates in the migmatites and cooling and basin-filling ages suggests a link between ductile and brittle processes over a range of structural levels in the arc, from a zone of crustal melting, flow, and migmatite crystallization to Earth's surface.

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