Plutonism and orogeny in north-central Washington: Timing and regional context

Abstract

Bedrock in north-central Washington comprises (1) weakly to moderately metamorphosed eugeosynclinal rocks of Permian, Triassic, and possibly Jurassic ages, (2) high-grade polymetamorphic rocks-gneiss, schist, and amphibolite-that are, at least in part, more highly metamorphosed derivatives of the rocks of the previous category, (3) Mesozoic and Cenozoic plutonic rocks, and (4) Cenozoic lavas and continental sedimentary deposits.A review of the radiometric ages from north-central Washington suggests a complex history of plutonism and metamorpP.ism beginning in Late Triassic and extending to Eocene time. However, ages of coexisting minerals from single samples typically are moderately to highly discordant. The discordance reaches a maximum along a zone flanking the Okanogan gneiss dome on the west. This dome forms the southwestern extremity of the Omineca crystalline belt, a north-trending orogenic subprovince about 250 km (150 mi) long and 55 km (35 mi) wide in British Columbia and Washington characterized by the presence of Shuswap (Monashee Group) terrane, gneiss domes, and allied metamorphic rocks.The Okanogan gneiss dome and the Shuswap are believed to be products of metamorphism and deformation, in part at least, ofLate Cretaceous age. The development of the gneiss domes indicates that metamorphism within the Omineca reached sufficient intensity at depth to cause incipient anatexis and mobilization of the infrastructure. The discordance west of the gneiss dome and the Omineca belt is attributed to weak thermal metamorphism that developed above and west of the zone of more intense metamorphism.Ages from within the Okanogan gneiss dome range from Late Cretaceous (lead-uranium, zircon) to Eocene (K-Ar, biotite and muscovite; fission track, apatite). Their discordance is attributed to slow cooling after the climax of metamorphism in the Late Cretaceous.Orogeny and plutonism in the north-central area of Washington began almost simultaneously during the Late Triassic with folding of the Permian and Triassic bedded rocks and their intrusion by the 195-million-year-old Loomis pluton. Thermal events subsequent to Late Triassic cannot be linked with specific orogenic deformation, except for deformation associated with the hypothesized Late Cretaceous metamorphism and mobilization of the Shuswap and the gneiss domes within it. This event is temporally associated with westward-directed overthrusting along the Shuksan thrust to the west, attributed to mid-Cretaceous orogeny, and with eastwarddirected overthrusting to the east along the Cordilleran thrust belt, attributed to the Laramide orogeny, which ended in the Late Cretaceous or early Tertiary. The Shuswap terrane and associated gneiss domes appear to occupy the axial zone between convergent thrusts that show an aggregate crustal contraction of possibly 250 km (150 mi).These relations suggest a genetic model as follows: The Permian volcanic and pyroclastic rocks of the region were probably deposited in island-arc and back-arc basins located east of an east-dipping sub...

Paleogeographic evolution of the United States Pacific Northwest during Paleogene time

“…In northern Washington the westward shift of the arc apparently began a little earlier and perhaps migrated more quickly than in Oregon, with widespread magmatism occurring from the Okanogan region to the North Cascades at about 50 Ma (Fig. 2b) (Fox et al 1977;Frizzell and Vance 1983;Taboret al 1984;Armstrong 1978). Arc magmatism has been restricted to the Cascade Range of Washington and Oregon for at least the past 35 Ma.…”

“…Kettle, Okanogan, Priest River, Bitterroot, Pioneer, Albion Armstrong ( ), Fox et al (1977, Chase et al (1978), metamorphic core complexes, Republic graben Cheney (1980), Hyndman (1980), Miller (1980), Rhodes and Cheney (1981), Harms and Price (1983) …”

Kettle dome and related structures of northeastern Washington

“…The map of Fox (1970) suggests that the Klondike Mountain Formation may exist in the Okanogan Valley. East of the Columbia River the mafic, olivine-bearing flows that locally lie above the Sanpoil immediately east of long.…”

Tectonic setting and U‐Pb geochronology of the Early Tertiary Ladybird Leucogranite Suite, Thor‐Odin ‐ Pinnacles Area, Southern Omineca Belt, British Columbia

“…The Upper Crustal Zone is generally at lower grade (chlorite +/-biotite +/-staurolite) than the predominantly highgrade (sillimanite -potassium feldspar -melt or sillimanitemuscovite) Middle Crustal Zone, although there are some local exceptions (Figure 3). More importantly, metamorphism in the upper plate is Mesozoic and, in general, cooling of pre-Eocene rocks below the K-Ar closure temperature of micas and hornblende occurred in the Mesozoic, whereas rocks in the Middle Crustal Zone were at high grade in the Late Cretaceous and Paleocene and cooled in the early Tertiary [Fox et al, 1977;Mathews, 1981 (Figure 1). North of Revelstoke on the eastern margin of Frenchman Cap, the Columbia River fault is interpreted as a predominantly brittle structure [Lane, 1984;Lane et al, 1989].…”