Evolution of a Plutonic Complex, Okanogan Range, Washington
Plutonic events in the Toats Coulee Creek region of the Okanogan Range, Washington, are mainly determined by a textural and structural study of the rocks, although several genetic relationships between plutons are based on chemical data. Several isotopic dates have confirmed relative ages determined by geologic means and provide an approximate picture of the distribution of the periods of tectonism, metamorphism, plutonism, and erosion in the Mesozoic Era. Late Triassic major orogeny resulted in Barrovian (?) regional dynamotnermal metamorphism of Permian (?) eugeosynclinal sediments and volcanics to various quartzose, pelitic, and calcareous schists and amphibolites. The Chopaka gabbroic and ultramafic intrusives and the earliest member of the granitic Toats Coulee Magma Series were involved in this metamorphism at the present relative level with respect to the metamorphic unit, and were converted into a variety of gneisses. The mafic and ultramafic portions of the Chopaka Intrusive Complex were emplaced together sometime between the Permian and Late Triassic. Emplacement of large discordant members of the Toats Coulee Magma Series (new name) represents a loss of continuity in the regional metamorphic belt and a culmination of Late Triassic erogenic events. Following a period of major erosion, plutons of the Horseshoe Basin Magma Series were emplaced at much shallower depths in the Early to Middle Cretaceous. Widespread, but only locally intense, tectonic activity at this time resulted in deformation of the older rock assemblage, converting portions of some Toats Coulee series plutons into cataclastic gneisses and providing structural weaknesses into which members of the younger series intruded. Injection of some of these magmas into zones of active cataclasis and recrystallization produced hybrid gneisses that are partly migmatitic. The leucocratic gneiss member of the series, correlating with the Boulder Creek Gneiss, is characterized by schlieren-like distribution of mafic minerals and textures that are not typically magmatic, and may be a result of simultaneous intrusion and crystallization of magma nearing a “minimum” composition. Such a gneiss-forming process is believed to be consistent with shallow emplacement and with proximity to the unmetamorphosed Methow-Pasayten Cretaceous sedimentary rocks to the west. The Horseshoe Basin Magma Series, including the leucocratic gneisses, is Early to Middle Cretaceous, which points to a period of nearly simultaneous erosion, sedimentation, and pluton emplacement.