Geochronometry of the Eagle Plutonic Complex and the Coquihalla area, southwestern British Columbia

Abstract

New U–Pb, K–Ar, and Rb–Sr dates from the Eagle Plutonic Complex and adjacent map units place timing constraints on intrusive and deformational events along the southwestern margin of the Intermontane Belt. U–Pb zircon minimum dates for Eagle tonalite and gneiss (148 ± 6, 156 ± 4, and 157 ± 4 Ma) document previously unrecognized Middle to Late Jurassic magmatism and syn-intrusive deformation along the eastern margin of the Eagle Plutonic Complex and the southwestern margin of the Intermontane terrane. Widespread mid-Cretaceous (Albian–Cenomanian) resetting of K–Ar and Rb–Sr isotopic systematics in Jurassic and older rocks is coeval and cogenetic with emplacement of plutons of the Fallslake Plutonic Suite (110.5 ± 2 Ma, U–Pb) which crosscut Jurassic plutons and structures but were themselves ductilely deformed along the Pasayten fault during sinistral, east-side-up, reverse displacement. K–Ar and Rb–Sr cooling dates for the Fallslake Suite of ca. 100 Ma, including dates from mylonites along the Pasayten fault, suggest that uplift, cooling, and unroofing of the Eagle Plutonic Complex occurred in mid-Cretaceous time along the Pasayten fault. Regional geologic evidence suggests that this thermal and unroofing event affected much of the southwest margin of the Intermontane Belt. Initial 87Sr/86Sr ratios and U–Pb geochronometry for the Fallslake Plutonic Suite suggest that it was derived, in part, from preexisting and relatively nonradiogenic Paleozoic to Mesozoic crust. K–Ar dating of several stocks demonstrates widespread Early Eocene plutonism in the Coquihalla area, and dating of the Needle Peak pluton indicates plutonism continued into Middle Eocene time.

Jurassic and Cretaceous plutonic and structural styles of the Eagle Plutonic Complex, southwestern British Columbia, and their regional significance

“…The field relations and structural setting described herein provide the framework for interpreting the geochronometry reported in Greig et al (1992). That the field relations are in agreement with geochronometric and other data demonstrates that the Eagle Plutonic Complex provides an excellent example of the use of plutons to date regional deformation (cf.…”

“…Camsell proposed the name "Eagle granodiorite" for plutonic rocks in the Britton (formerly Eagle) Creek valley, near the confluence with Illal Creek. His "type" locality, however, is a Late Paleocene or Early Eocene granodiorite stock (Greig 19896;Greig et al 1992). Cairnes (1924) first noted compositional variety within Eagle plutonic rocks, such as muscovite-rich varieties and pegmatites, and more mafic granitic rocks along the western flank of the Eagle Complex.…”

East-derived strata in the Methow basin record rapid mid-Cretaceous uplift of the southern Coast Mountains batholith

“…In Washington state, the western Okanogan Range is bounded by the Pasayten fault to the west and includes Jurassic and Cretaceous plutons (including the Similkameen pluton; Petö 1974) that range in age from 166 to 154 Ma (Rb-Sr dates, consistent with K-Ar dates; Petö and Armstrong 1976) and from 114 to 110 Ma (U-Pb dates; Grieg et al 1992;Hurlow 1993). Hurlow and Nelson (1993) concluded that the 114-110 Ma plutons of the Okanogan Range represent an older, southerly segment of the late Albian volcanic rocks of the Spences Bridge Group in British Columbia, and defined the Okanogan -Spences Bridge arc as a >300 km long northnorthwest-trending Early Cretaceous magmatic arc intruded into the western margin of the Intermontane superterrane.…”

“…Alternatively, sinistral oblique motion on the Pasayten fault has been interpreted as transpressional along a steeply dipping reverse fault (Grieg 1992). Grieg et al (1992) suggest that the ductile strain in the 110.5 ± 2 Ma Fallslake plutonic suite of the Eagle Plutonic complex in southern British Columbia developed during sinistral, east-side-up reverse displacement, with northeast-dipping foliation indicating a northeast-dipping orientation for the Pasayten fault during transpressional ductile shear.…”

Lithofacies control in detrital zircon provenance studies: Insights from the Cretaceous Methow basin, southern Canadian Cordillera

“…The southern Canadian Cordillera includes Mississippian to Jurassic oceanic mélange of the Cache Creek terrane, Triassic to Jurassic volcanic arc assemblages of Quesnellia, late Paleozoic oceanic rocks of the Slide Mountain terrane, and pericratonic rocks of the Kootenay terrane (Monger et al, 1982). These terranes are intruded by an extensive suite of Late Triassic to early Tertiary plutonic rocks; major pulses of magmatism occurred in Late Triassic-Early Jurassic (210-187 Ma), Middle to Late Jurassic (180-148 Ma), and middle to Late Cretaceous (130-85 Ma) time (Armstrong et al, 1988;Greig et al, 1992;Woodsworth et al, 1992;Ghosh et al, 1995).…”