This report is concerned with the bedrock geology of a 1,000-km2 area in the central part of the Okanogan Range in north-central Washington. Most of the rocks underlying this area were formed under conditions of high-grade amphibolite-facies metamorphism and/or plutonism.

The metamorphism, accompanied by metasomatic granitization and anatexis, occurred between Late Triassic and late Early Cretaceous time. Plutonic rocks in the area are grouped into three temporal suites: (1) an early premetamorphic suite represented by inclusions of mafic to granitic basement rock in gneisses of a trondhjemitic batholith which also is believed to have served as basement to supracrustal rocks deposited in a late Paleozoic-early Mesozoic geosyncline; (2) a synmetamorphic suite of gneisses, including both concordant and discordant intrusions; and (3) a post-metamorphic suite of massive rocks which generally are discordant. The igneous rocks range in composition from peridotitic to granitic; trondhjemites and granodiorites are most abundant. Many of these units are somewhat leucocratic for their respective compositions. Younger intrusive rocks are more felsic than older ones. After the plutonic rocks had been deroofed, probably in early Tertiary time, the eastern part of the area was covered by quartz-keratophyre flows. These flows, widely scattered diatremes, and other hypabyssal intrusives are the youngest bedrock elements.

Structures generated during metamorphism and plutonism generally trend northwest and indicate lateral compression. Some younger (middle and late Tertiary?) structures followed the established northwest grain, but most formed athwart it. These structures represented a basin-and-range type block-faulting episode in the region—a period of presumed lateral extension. The strike of these fault-block structures seldom varies more than a few degrees from north.

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