A new structural and tectonic interpretation of the western part of the Shuksan blueschist terrane, northwestern Washington

Abstract

The Shuksan Metamorphic Suite (SMS) is a blueschist terrane exposed in the northwest Cascades of Washington. Along its eastern flank, the SMS is known from previous studies to consist of meta-MORB and pelitic and metalliferous metasedimentary rocks that resemble ocean-floor deposits. In the western part of the SMS, however, the metasedimentary rocks lack metalliferous enrichment and contain interbeds of mafic to felsic lithic: tuff and volcanic-derived sandstone. Relatively small, isolated bodies of meta-igneous rock in this area comprise a variable suite of arc-related mafic to felsic plutonic and volcanic rocks. These have been assigned previously to units other than the SMS, postmetamorphic faulting being invoked to explain their emplacement. In the South Chuckanut, Lyman, and Bowman Mountain areas, however, the SMS metasediments contain clasts derived from the nearby meta-igneous units, metamorphic structures have a similar orientation in both the metasedimentary and metaigneous rocks, and the metamorphic facies in the metasedimentary and meta-igneous units are the same. These observations require (1) reassignment of the meta-igneous units to the Shuksan Suite, (2) rejection of previous interpretations of the contact between the metasedimentary and meta-igneous rocks as being an expression of either the Shuksan thrust or Haystack thrust, and (3) broadening of the tectonic setting of deposition of Shuksan protolith rocks to include an arc component. A maximum protolith age for the western part of the SMS of 163 ± 2 m.y. is established by a U/Pb zircon age of metaquartz diorite from Bowman Mountain. We infer deposition of the SMS protolith in a Late Jurassic marginal basin behind a west-facing arc.