PROVENANCE OF METASEDIMENTARY UNITS OF THE NORTH CASCADES CRYSTALLINE CORE, WA: RESULTS FROM A DETRITAL ZIRCON AND NEODYMIUM ISOTOPIC SURVEY

Exposed within the crystalline core of the North Cascades magmatic arc, WA are multiple bodies of metasedimentary rocks of various grade and unknown protolith. To understand the sediment provenance and how it was incorporated into the arc, detrital zircon and whole-rock Nd analyses are used as a signature to correlate metasedimentary material from the core to potential sediment sources in the flanking forearc and backarc basins and the accretionary wedge.

Representative metasedimentary samples from the Skagit Gneiss Complex, Little Jack unit, and Napeequa complex, as well as potential protolith material from the Methow basin, Western mélange belt, and Nooksack Formation were selected for detrital zircon U-Pb geochronology and preliminary Nd isotopic analyses. Detrital ages for protoliths for Skagit metasedimentary rocks fall into two groups: latest Cretaceous to Proterozoic or latest Cretaceous to Late Triassic. Two Skagit samples from the latter age group, a paragneiss and a phyllite, have a ε_Nd of 3.3 and 3.5, respectively, reflecting a relatively unevolved source. Detrital zircons from a Napeequa paragneiss (ε_Nd = 6.1) and a Little Jack phyllite are similar with peaks at ca. 210 and 245 Ma and ages up to the Late Permian. Outside of the core, a Western mélange belt sandstone, thought to be analogous to potentially subducted sediment, has large populations of Proterozoic and Mesozoic zircons as well as a ε_Nd = -9.3, thus requiring input from cratonic sources. In comparison, a Nooksack sandstone (ε_Nd = 2.6) and Methow sandstone (ε_Nd= 3.1) record only Early Jurassic to Late Cretaceous input and are likely derived from arc-related volcanic material.

Protolith variation reveals that the core is likely composite, incorporating backarc and accretionary wedge material. Metasedimentary rocks with latest Cretaceous to Late Triassic detrital ages have a similar signature to the Methow basin and may be a result of backarc underthrusting or burial. Samples with protolith detrital ages up to the Proterozoic match with the Western mélange belt, and may be the result of relamination of subducted sediment. These data can give insight as to how sediment is transferred to the mid- to lower crust in other similar arc settings, e.g., the modern Andes or ancient Fiordland.