Petrology and Structure of the Cascade River Schist, in the Sibley Creek Area, Northern Cascades, Washington

The focus of this study is on the protolith types, metamorphism and structure of the Cascade River Schist in the Sibley Creek area of the North Cascades, Washington. The two general lithologic packages are an arc and ocean floor unit termed the Cascade River and Napeequa units, respectively, by Tabor and others (in press). The Cascade River unit has a recognizable stratigraphy which is inverted and repeated across strike from the southwestern to northeastern sides of the field area, apparently as a result of macroscopic synclinal folding. The Napeequa unit lies on top of the Cascade River unit, in the center of the sync line, probably as a result of thrust faulting. Metamorphic zones and facies in the field area increase from the biotite zone of the greenschist facies, west of the Le Conte fault, to the garnet and staurolite-kyanite zones of the epidote amphibolite facies east of the fault. The greenschist facies is estimated to be metamorphosed at pressures of 3-5 Kb, and temperatures of 450-500 C and the amphibolite facies at 8-10 kb and 600-700 C. Structural control of the greenschist-anphibolite facies transition by the Le Conte fault is suggested by the first appearance of the garnet and oligoclase with the fault, as well as a P-T differential across the fault, which suggests considerable dip-slip displacement across the fault. The metamorphic structures in the field area consist of: 1) a synmetamorphic first deformation characterized by steeply dipping S-tectonites containing weakly defined down-dip mineral lineations and flattened conglomerate clasts, and 2) a late-metamorphic and commonly retrogressive second deformation characterized by steeply dipping L-Stectonites containing a sub-horizontal stretching lineation, prolate spheroidal conglomerate clasts, and dextral kinematic indicators. Later post-metamorphic structures are folds and faults related to forceful enplacement of the 73 Ma Hidden Lake pluton and Eocene strike-slip offset along the Le Conte fault.