GEOLOGY AND GEOCHEMISTRY OF THE DE ROUX UNIT AND POSSIBLY CORRELATIVE TECTONOSTRATIGRAPHIC TERRANES WITHIN THE CASCADE MOUNTAINS, WASHINGTON

Pre-Cenozoic tectonostratigraphic terranes are important components of the central Cascades, Washington. These terranes are critical to the understanding of the Mesozoic tectonic development of western North America; however, they have received little study. They include, in part, the De Roux unit, western mélange belt and Russell Ranch complex. The undated De Roux unit consists of penetratively deformed greenschist facies rocks that are principally slates, phyllites, metachert, and massive & pillowed flows, and minor graywacke, tuff, limestone serpentinite and rare amphibolite, metadacite & mategabbro. The major and trace element geochemistry of the De Roux unit (n=23) includes calc-alkaline (CA), within-plate basalt (WPB), transitional normal-mid-ocean ridge basalt-island arc tholeiite (N-MORB-IAT), and lesser enriched- and normal-mid-ocean ridge basalt (E-MORB; N-MORB) affinities. A water lain tuff is mafic.

The western mélange belt and Russell Ranch complex have similar metamorphic facies, and metaigneous & metasedimentary rock types as the De Roux unit. The western mélange belt is penetratively deformed, volcanically derived sandstones are more common than the De Roux, and sparse fossils give Jurassic-Early Cretaceous ages, with rare Permian limestone. A U-Th-Pb zircon age from a metatonalite is ~150-160 Ma. LA-ICP-MS detrital zircon dating from sandstone indicates a youngest age population of ~96 Ma, with a significant population at ~147 Ma. Sparse geochemistry of greenstone from the western mélange belt includes 3 IAT and 1 MORB, and 2 tuffs are CA. The Russell Ranch complex is less deformed then the De Roux and arkosic sandstones are more common. Radiolarians from chert are Late Jurassic-Early Cretaceous and a limestone clast in a breccia is Permian. Sparse geochemistry of greenstone from the Russell Ranch complex includes 9 WPB and 2 MORB. Deformed plutonic rocks of the Indian Creek complex are steeply faulted against the Russell Ranch complex; two U-Pb zircon ages from these plutonic rocks are ~154 Ma.

We suggest that the De Roux unit, western mélange belt and Russell Ranch complex are correlative. The structure, lithologies, metamorphic grade, and ages of these terranes suggest that they may be both olistostromal and tectonic in origin; and formed within an accretionary wedge.