TIMESCALES OF MIGMATIZATION AND MELT CRYSTALLIZATION IN THE SKAGIT GNEISS, NORTH CASCADES

The Skagit Gneiss, North Cascades of Washington and SW British Columbia, has long served as a crucible for the understanding of mid-crustal melting and the origin of migmatites, yet the timing of melt crystallization remains uncertain. To understand the relationship between metamorphism and partial melting, crustal flow, and exhumation in the Cretaceous-Paleocene continental arc, we determined the age and duration of anatectic migmatization using the chemical abrasion (CA-TIMS) technique on zircon from leucosomes in the Skagit Gneiss. The Skagit Gneiss contains metasedimentary and meta-igneous rocks that reached peak conditions of >9-10 kbar and >700 ºC, and previous studies have identified leucosomes formed by both anatectic and subsolidus processes. Three well-studied outcrops from a 10 km long transect perpendicular to strike and containing a variety of leucosome textures (e.g., stromatic/discordant; fine-grained/pegmatitic) were targeted for geochronology. The results reveal two main pulses of melt crystallization: I) 68 to 60 Ma, and II) 53 to 51 Ma. Within individual outcrops, the leucosomes record a narrow range of crystallization ages. Zircon in trondhjemitic leucosomes from a metapelitic migmatite at the west end of the transect yield Group I ages, and the mesosome (sillimanite-garnet-biotite gneiss) contains zircons as young as 60 Ma. Zircon in leucosomes from the easternmost outcrop yield ages solely from Group II. A biotite gneiss from this outcrop contains zircon as young as 47 Ma. The outcrop in the middle of the transect gave ages from both groups: 65 Ma and 53 Ma. A garnet-bearing gneiss from the third outcrop yielded ages ranging from 160 to 93 Ma. In addition, a dike that cuts across the leucosomes and gneiss contains inherited zircons and a zircon as young as 46 Ma. The 53 Ma age is concentrated in coarse-grained leucosomes, but in general, there does not appear to be characteristic compositions or fabrics for each zircon age population. Our new geochronological results indicate that migmatites were formed during both Late Cretaceous metamorphism as well as during Eocene transtension and exhumation of the arc. The data further suggests that metamorphism proceeded from west to east over 20 million years.