GEOCHEMISTRY OF METAIGNEOUS ROCKS FROM THE WESTERN MÉLANGE BELT, LAKE CHAPLAIN, SNOQUALMIE, AND SULTAN 7.5 MINUTE QUADRANGLES, WESTERN CASCADES, WASHINGTON: EVIDENCE FOR A PREDOMINANTLY VOLCANIC ARC SETTING

The western mélange belt (WMB) is a mostly low to medium grade and penetratively deformed accretionary complex. It consists of meta-argillite, metasandstone, metaigneous rocks, and metachert, with phyllite, slate and rare marble and serpentinite. U-Th-Pb zircon age from a metatonalite is 150-160 Ma. Detrital zircons from 3 metasandstones have youngest average age populations of 74 Ma, 87 Ma, and 96 Ma, respectively. Fossil ages are predominantly Late Jurassic to Early Cretaceous. Rare metachert is Early Jurassic and preserved fossils in marble are Permian. North of our study area the mélange is extremely disrupted; however, in our study area it consists of large-scale blocks with intact well bedded strata in most areas.

Previously published WMB geochemistry include arc (n = 5) and MORB (n = 2) affinities. New whole rock geochemical data for 16 metaigneous samples furthers this interpretation of the original tectonic setting of the WMB. Metagabbros were excluded from most plots due to the high Al and Eu values indicative of cumulate compositions. Metaigneous samples predominantly have high Th, low Nb/Yb, and volcanic arc chondrite- and N-MORB normalized patterns. They also plot in fields defined by modern volcanic arcs on discrimination diagrams. Metatrondhjemite and two metatuffs are magnesian, calcic, and metaluminous. Another metatuff is peraluminous and may be the product of assimilation. The metatuff have Nb/Yb ratios that are higher than all other arc affinity samples. Five of the metaigneous samples have calc-alkaline affinities, while the rest are tholeiitic. Geochemistry suggests that the metaigneous samples in our study area primarily originated in a Cretaceous to Jurassic volcanic arc setting. The calc-alkaline and tholeiitic affinities and diversity of the Nb/Yb ratios suggest a complex arc system that may have evolved over time. This is supported by sandstone petrography, detrital zircon age populations, and metasedimentary geochemistry. One greenstone has high Nb/Yb, low Hf, and likely originated in an oceanic island setting. The predominantly arc setting, with lesser within-plate and MORB affinities, is consistent with geochemistry from the likely correlative De Roux unit and Russell Ranch complex, Washington.