The Shuksan Metamorphic Suite and Shuksan Thrust Mt Watson Area, North Cascades, Washington

Rocks of the Shuksan Metamorphic Suite exposed on Mt Watson are metamorphosed pillow basalts, basaltic flows (?), basaltic tuffs, carbonaceous sediments, Mn-enriched cherty sediments, rare calcareous sediments and rare ferromanganese nodules. Greenschist of the Suite records the following history: 1) early static hydrothermal metamorphism; 2) Early Cretaceous (about 125 ma ago) synkinematic blueschist-facies metamorphism at P~7 kb, T~350° C, Pfluid near Ptotal, XCO2 \u3c 0.1, with production of S1 and L1 ; 3) late-metamorphic isoclinal folding (F2) around axes commonly at high angles to L1; 4) L3 crenulation of S1. Phyllite of the Suite has been more strongly affected by D2, D3 and post-D3 events. This history appears typical for the Suite throughout NW Washington. West of Mt Watson the Shuksan Thrust is a 3 km+ thick tectonic melange. Most of the melange is little-recrystallized rocks of the Chilliwack Group(?). Aragonite and lawsonite are present in Chilliwack(?) rocks. Minor elements in the melange include probable Shuksan Suite rocks, metaquartz diorite and exotic well-recrystallized high P/T metavolcanic rocks. The relations between metamorphism of Chilliwack(?) rocks, melange deformation, and blueschist-facies Shuksan metamorphism are not known. A NNW-trending fault is inferred to pass between Mt Watson and Bacon Peak. Movement on the fault postdates D3 and is probably post-Paleocene in part. Recognition of the probable continuity of this fault with the Shuksan Thrust further N suggests that in the root zone the Thrust may be a series of en echelon NNW-trending structures