HF CHARACTERIZATION OF DETRITAL ZIRCONS FROM THE METASEDIMENTARY ROCKS OF THE NORTH CASCADES CRYSTALLINE CORE, WA: IMPLICATIONS FOR SEDIMENT INCORPORATION INTO ARC SYSTEMS

Multiple bodies of metasedimentary rocks of unknown provenance are exposed within the crystalline core of the North Cascades magmatic arc, WA. In order to understand how and when sediment was incorporated into the arc, the Hf isotope composition was analyzed from well-characterized detrital zircons in order to link these rocks to potential protolith units located in the forearc, backarc, and accretionary wedge. Metasedimentary samples were collected from the migmatitic arc basement, including Skagit Gneiss Complex and the oceanic Napeequa Complex, and the greenschist facies, marine Little Jack unit, located to the east of the crystalline core. Detrital zircon age patterns from Skagit Gneiss samples divide it into two groups: rocks that contain Proterozoic ages (group 1) versus samples that only revealed ages as old as the Late Triassic (group 2). In group 1 samples, Proterozoic zircons yield εHf_(t) of -1.5 to +9.5 (t = 1.3–1.8 Ga), whereas Jurassic and Early Cretaceous grains have juvenile Hf isotope compositions (+3.8 to +9.4), and Late Cretaceous to Eocene zircons record varied εHf_(t) values (-23.0 to +9.5). For the group 2 Skagit rocks, εHf_(t) values fall between +4.4 to +14.2 (t = ca. 60–200 Ma), and there are no patterns between age and Hf. In comparison, Napeequa and Little Jack zircons have Hf isotope compositions that vary from εHf_(t) = +9.7 to +12.5 and +11.1 to +13.7, respectively (t = ca. 200–250 Ma). 

Triassic zircons from the Napeequa and Little Jack units are the most juvenile and agree with previous interpretations that these rocks are related to oceanic units with minimal continental input. The juvenile εHf_(t) and U-Pb ages of Proterozoic zircons from the Skagit Gneiss metasediments indicate the Belt and Windermere supergroups as a possible source. The unevolved Hf isotope composition of Jurassic to mid-Cretaceous Skagit Gneiss zircons indicate that these grains were eroded from an active magmatic arc and deposited in an adjacent basin(s). The latest Cretaceous to Eocene zircons likely represent melt-crystallized and/or metamorphic grains and the range of εHf_(t) values are a result of zircon alteration and/or dissolution-reprecipitation. These results, with the U-Pb data, link Skagit metasedimentary rocks to mid-Cretaceous arc-related basins and indicate these rocks reached mid-crustal depths by ca. 75 Ma.