Crystalline Klippen in the Index District, Cascade Range, Washington

Abstract

At Mt. Baring, Gunn Peak, and Merchant Peak in the Cascade Range 40 miles east of Seattle, two klippen of hornblende-rich migmatitic rocks overlie a sequence of weakly metamorphosed argillites, quartzites, greenstones, and Permian marbles. The klippen are composed of schistose amphibolites which underwent partial static granitization into migmatites. During and after their tectonic emplacement, the migmatites were extensively sheared and brecciated. The underlying sedimentary and volcanic rocks are here described as the Gunn Peak Formation (restricted) and the Barclay Creek Formation, a new formation of possible Permian age. These rocks were metamorphosed in the greenschist facies; then, during cataclastic metamorphism, slabs of Barclay Creek argillite became tectonically intercalated between blocks of migmatite. Small ultramafic intrusions were emplaced near and at the base of the klippen. Emplacement of the klippen probably preceded deposition of the Late Cretaceous and Paleocene Swauk Formation. After cataclastic metamorphism, both migmatites and underlying rocks underwent contact metamorphism ranging from greenschist to pyroxene-hornfels facies; this is attributed to the intrusion of two Tertiary quartz diorite batholiths east and west of the klippen. Similar hornblende-rich, coarse crystalline bodies occur in association with weakly metamorphosed and unmetamorphosed rocks along a 175-mile zone west of the Mesozoic crystalline core of the northern Cascades. These crystalline rocks, like the ultra-mafic rocks, may be shreds of pre-Middle Devonian basement dragged up from deeper levels of the crust during the emplacement of one of several large Cretaceous overthrusts which formed west of the Cordilleran plutonic belt.