title: Petrology and Structure of the Lookout Mountain - Little Devil Peak Area, North Cascades, Washington
Creators: name: Cary, Jeffrey A.
Publisher: name: Western Washington University
Year: 1990

description: <p>The focus of this study is on the protolith types, metamorphism and deformation of the Cascade River Schist and related rocks in the Lookout Mountain - Little Devil Peak area of the North Cascades, Washington.</p>
<p>Two distinctive protolith assemblages are recognized in the Cascade River Schist, a volcanic arc assemblage in the Lookout Mountain area and an oceanic/forearc assemblage in the Little Devil Peak area. The volcanic arc sequence consists from the bottom up of andesitic to rhyolitic tuffs and flows, coarse-grained volcaniclastics and fine grained pelites. The lower tuff/flow unit grades downward into and is transitional with the underlying Triassic Marblemount Meta-Quartz Diorite. Bulk rock chemistry and lithologic associations suggest an island arc tectonic setting. U/Pb zircon geochronology suggests a 220 Ma age for soda rhyolite tuff in the Cascade River Schist. U/Pb geochronology suggests that the Cascade River Schist and Marblemount Meta-Quartz Diorite are coeval, and contact relationships suggest that the two units are cogenetic.</p>
<p>In the Little Devil Peak area, the Cascade River Schist is composed of an oceanic/forearc assemblage consisting of ultramafite, amphibolite, quartzite, quartzofeldspathic schists and marble. This facies of the Cascade River Schist has been mapped as the Napeequa unit (Tabor et al., 1988). Intruding the Cascade River Schist are the 75 Ma Marble Creek Pluton and the Haystack Creek Pluton. High Al hornblende and magmatic epidote indicative of high-pressure crystallization (>6-10 Kb) are present in the intrusives, suggesting emplacement at deep crustal levels.</p>
<p>A low pressure - low temperature (400 ± 50° C; 3 i 1 Kb) greenschist facies metamorphism has affected the Marblemount Meta-Quartz Diorite and Cascade River Schist in the Lookout Mountain area. The Cascade River Schist, Marble Creek Pluton and Haystack Creek Pluton in the Little Devil Peak area underwent a high temperature - high pressure (600 ± 50° C; 9 ± 1 Kb) amphibolite facies metamorphism. The age of the amphibolite facies metamorphism is indicated by the 75 Ma age of the synmetamorphic Marble Creek Pluton. The age of the greenschist facies metamorphism is unknown, but is probably Late Cretaceous. Deformation of these rock units produced a northwesterly trending penetrative foliation with a subvertical dip. In the Lookout Mountain area, greenschist facies rocks contain a shallow, NW-plunging stretching and mineral lineation. Numerous rotational features associated with the deformation indicate that the kinematic regime during greenschist metamorphism was right-lateral, strike-slip.</p>
<p>The Entiat - Le Conte fault, a post metamorphic structure, trends northwesterly through the study area. It juxtaposes different facies of the Cascade River Schist, which were metamorphosed under disparate P/T conditions. Geobarometry suggests 15 km of post metamorphic dip-slip displacement along this structure.</p>
<p>This study has defined a mappable, stratigraphic sequence in the Late Triassic Cascade River Schist. It has also documented both high- and low-pressure regimes during Skagit metamorphism. Deformational fabric associated with the greenschist facies metamorphism suggests orogen-parallel, right-lateral, strike-slip shear.</p>