BALTIC, WESTERN LAURENTIAN, AND EARLY DEVONIAN ARC DETRITUS IN THE YELLOW ASTER COMPLEX OF NORTHWEST WASHINGTON STATE

The Yellow Aster Complex in northwest Washington is a basement portion of the Chilliwack composite terrane and appears throughout the western foothills of the North Cascades as fault bounded tectonic blocks (Brown, 1987; Brown et al. 2010). This study was undertaken in part to understand the significance of the depositional, magmatic, and metamorphic history of the Yellow Aster Complex which is dominantly composed of calc-silicate and quartzo-feldspathic paragneisses. Zircons from seven samples were dated using U-Pb in order to evaluate the detrital provenance of the terrane and timing of major geologic events. Three samples yield detrital zircon populations dominated by Middle to Early Devonian ages with peak ages that range from 409 to 386 Ma, and the grains are interpreted to be derived from local magmatic sources. Four other samples have detrital zircon populations with ages that range from Archean to Mesoproterozoic and a population of Early Paleozoic grains. Grains with ages within the North American magmatic gap (1.61-1.49 Ga) are found in two of the samples. These samples contain zircon ages that are not consistent with ages from western North America, but rather similar to Baltica and the exotic Early Devonian formations of the Alexander terrane (e.g. Karheen and Mathieson Channel formations). Previous work (Brown et al. 2010) concluded that the Yellow Aster Complex has similarities to parts of the Yukon-Tanana terrane, which has a western Laurentian affinity. In addition, Brown et al. (2010) identified two intrusions with ages of 418 Ma and 409 Ma, and suggested the occurrence of a pre-418 Ma metamorphic event. Based on the new detrital zircon ages of this study, the Yellow Aster Complex experienced sedimentation after 418 Ma and metamorphism after 386 Ma. The overall geology, paired with new zircon ages, suggests that the Yellow Aster Complex may have evolved from both Baltic and western Laurentia sourced detritus. This study also concludes that Baltic and western Laurentian sources preserved in the Yellow Aster Complex may have been caused by terrane amalgamation or sharing of a depositional basin prior to the formation of younger components of the Chilliwack composite terrane.