Named for exposures along Mission Creek, Omak Lake 15' quad. Type locality designated as exposures in road-gravel quarry about 0.5 mi north of Route 10A and about 3 mi east of Omak, SW/4 sec.32 T34N R27E, Omak Lake 15' quad, Okanogan Co, WA. Crops out in quad and extends into adjoining Tonasket and Disautel 15' quads. Is one of two formally named units in mylonitic orthogneiss that crops out in broad belt northeast of Omak Lake, Mission Creek Granodiorite and French Valley Granodiorite. Mission Creek is composed of porphyritic biotite granodiorite that contains conspicuous pink to gray K-feldspar megacrysts. Field evidence indicates that Mission Creek and French Valley Granodiorites were emplaced during or after high-grade metamorphism of Tonasket Gneiss, but before mylonitization. K-Ar determination on biotite from Mission Creek Granodiorite yielded 47.2 +/-1.4 Ma (Fox and others, 1976). Provisionally assigned Cretaceous or Eocene age.

Source: GNU records (USGS DDS-6; Menlo GNULEX).