CHILLIWACK TERRANE IN NORTHWEST WASHINGTON: NEOPROTEROZOIC-CAMBRIAN PASSIVE MARGIN BASEMENT, ORDOVICIAN-SILURIAN ARC INCEPTION

Devonian rocks in northwest Washington are part of an assemblage of mid-Paleozoic arc terranes extending from California to Alaska. Some of these terranes bear evidence of exotic origin whereas others apparently formed proximal to western Laurentia, posing a complex problem in unraveling the Paleozoic accretionary history of the Cordillera. In Washington, these rocks include the volcanic and sedimentary East Sound and Chilliwack Groups, and the plutonic Turtleback and Yellow Aster complexes, collectively making up the Chilliwack terrane. New zircon ages together with previous fossil ages and field relations indicate that the plutonic and volcanic rocks are mutually related as parts of the same arc complex, and that inception is as old as Late Ordovician to Silurian, older than most other parts of the mid-Paleozoic terrane assemblage. Basement to the arc complex is a passive margin assemblage of metamorphosed quartz sandstone and carbonate rock of the Yellow Aster Complex, bracketed in age by ca. 1000 Ma detrital zircons and 424-450 Ma arc intrusive rocks. This assemblage of quartzose paragneiss basement and overlying arc matches that of older parts of the extensive Yukon-Tanana terrane in the northern Cordillera and together these units define a broad outboard fragment of continental derivation. There is not definitive evidence that the paragneiss was derived from the western vs. eastern Laurentian margin or alternatively is a remnant of Australia following breakup of Rhodinia. However, zircon age patterns in the overlying arc show affinity to the miogeocline, consistent with published evidence in other terranes in the northern Cordillera for a connection to western Laurentia.