Age, origin, and tectonic evolution of the Yellow Aster Complex: northwest Washington state

• Main Author: Hoffnagle, Eric Adam
• Format: Text
• Language: unknown
• Published: Western Washington University 2014
• Subjects: Broad Peak; Young Peaks; Yukon; Alaska
• Online Access: https://dx.doi.org/10.25710/mgxf-w646; https://cedar.wwu.edu/wwuet/356

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 refine and understand the significance of the depositional, magmatic, and metamorphic history of the Yellow Aster Complex. Field work and petrographic analyses reveal that the Yellow Aster Complex consists of calc-silicate and quartzo-feldspathic paragneisses with minor amounts of marble and quartzite. Leucocratic and mafic intrusions are also present throughout the terrane. Zircons from six Yellow Aster Complex samples were dated using U-Pb in order to evaluate the detrital provenance of the terrane. Sampling was conducted at two areas: Schriebers Meadow and Yellow Aster Butte. Two meta-sedimentary samples (YA-5, EAH-39) contain a range of Mesoproterozoic to Archean detrital zircon ages and younger grains from the Early Paleozoic. Sample YA-5 has two young peaks at 417 ± 3 Ma and 402 ± 7 Ma, while EAH-39 has a broad peak at ~428 Ma. Two other quartzose calc-silicate paragneisses have peak ages of 409 ± 5 (EAH-10b), 405 ± 4 Ma (EAH-11), and 386 ± 5 Ma (EAH-11). Sample EAH-03 is a pyroxene calc-silicate paragneiss with peak ages of 403 ± 8 Ma and 389 ± 6 Ma. Samples EAH-39 and YA-5 contain zircon ages within the North American magmatic gap (1.61-1.49 Ga) and Early Paleozoic (500-400 Ma) which are not consistent with ages from western North America, but rather similar to Baltica and the exotic Early Devonian Karheen Formation (Alexander terrane; northwest British Columbia - southeast Alaska). Peak ages from samples EAH-03, -10b and -11 are thought to be from grains locally derived from magmatic sources. 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. Characteristics of detrital zircon populations with a western Laurentian affinity include a significant number of 1800 Ma grains, no grains from the North American magmatic gap, and few grains younger than 1000 Ma. 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.