Late Cretaceous and Early Tertiary Plutonism and Deformation in the Skagit Gneiss Complex, North Cascade Range, Washington and British Columbia
Late Cretaceous and early Tertiary plutonism and deformation in the Skagit Gneiss Complex, North Cascade Range, Washington and British Columbia RALPH A. HAUGERUD U.S. Geological Survey at Department of Geological Sciences, AJ-20, University of Washington, Seattle, Washington 98195 PETER VAN DER HEYDEN* Department of Geological Sciences, University ofBritish Columbia, Vancouver, British Columbia V6T2B4 Canada ROWLAND W. TABOR 1 ,, c „ . , c „ . „ , „ ,., . „.„„ ..„.. „ T,„v \ U.S. Geological Survey, Menlo Park, California 94025 JOHN o. SIALti ) ROBERT E. ZARTMAN U.S. Geological Survey, Denver, Colorado 80225 ABSTRACT The Skagit Gneiss Complex forms a more-or-less continuous ter- others, 1987a, 1987b, 1989). In the core of the range, in what appear to rane within the northern, more deeply eroded part of the North Cas- have been the most deep-seated rocks, direct evidence for the protolith age cade Range. The complex comprises abundant plutons intruded at and age(s) of metamorphism, migmatization, and deformation has been mid-crustal depths into a variety of metamorphosed supracrustal scarce. Relations of gneisses in the northern part of the Chelan block—the rocks of both oceanic and volcanic-arc origin. A plethora of syntec- region bounded by the Straight Creek, Ross Lake, and Entiat faults (Fig. tonic pegmatite, small plutons, and granitic dikes gives the complex a 1)—to surrounding less-metamorphosed rocks have been problematic (for migmatitic aspect. example, Davis and others, 1978; Hamilton, 1978; Whitney and U-Pb zircon ages from gneissic plutons within and near the McGroder, 1989). Skagit Gneiss Complex indicate magmatic crystallization between 75 These gneisses are a complex of metamorphosed granitoid plutons and 60 Ma.
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