Proterozoic gneisses of the Malton Complex, near Valemount, British Columbia: U–Pb ages and Nd isotopic signatures

Abstract

Proterozoic gneisses of the Malton Complex in the vicinity of Valemount, British Columbia, occur in a series of lithologically and structurally complex, fault-bounded slices of crystalline basement and interleaved cover. Gneisses of the Malton Complex span the Southern Rocky Mountain Trench and underlie the western part of the Rocky Mountain fold and thrust belt and the eastern part of the Omineca Belt of the Canadian Cordillera. Structural and stratigraphic relationships indicate that they formed the basement upon which an enigmatic quartzite unit and the Late Proterozoic Windermere Supergroup were deposited.The Yellowjacket and Bulldog gneisses, on the east side of the Rocky Mountain Trench, have yielded four U–Pb zircon crystallization ages of ca. 1870 Ma, with εNd(T) values of −2.6 to −3.4. The Hugh Allan gneiss, also on the east side of the trench but separated from the Yellowjacket gneiss by a major thrust fault, includes leucocratic granite gneiss having a zircon U–Pb age of [Formula: see text], which has intruded an older (undated) lithologically heterogeneous assemblage of gneiss. The basal Windermere succession of the Valemount region is inferred to be younger than ca. 740 Ma, since these intrusions are not found within the Late Proterozoic stratified rocks.Augen granitoid orthogneiss of the Malton Range on the west side of the Rocky Mountain Trench has been dated as [Formula: see text] using zircons. A second sample yields data suggesting an age between 2050 and 2100 Ma, but its interpretation is uncertain because of scatter in analyses and possible zircon inheritance. The latter sample has an εNd(T) at 1990 Ma of −2.6. Nd model ages for the Malton, Yellowjacket, and Bulldog samples range from 2.45 to 2.56 Ga, indicating that the igneous protoliths were derived from a source that probably had some component of Archean crustal material involved.The U–Pb ages and Nd model ages are quite similar to those of rocks underlying portions of Alberta and the western Canadian Shield, specifically the Fort Simpson terrane, the Great Bear magmatic zone, and parts of the Thelon–Taltson arc. This evidence, as well as structural and stratigraphic arguments, links the Malton Complex gneisses with those of the Canadian Shield, precluding their derivation by large-magnitude displacements from the southwestern United States. Structural analysis indicates that they restore to locations 100–200 km southwest of their present exposure.Structural, stratigraphic, and isotopic data indicate that the Southern Rocky Mountain Trench is not a suture.

Orogenic pulses in the Alberta Rocky Mountains: Radiometric dating of major faults and comparison with the regional tectono-stratigraphic record

“…Mississippian, Permian, and Triassic ages associated with inferred tectonothermal events, albeit infrequent and scattered, have been reported from the Windermere Supergroup and basement rocks in southern Omineca (Mortensen et al, 1987;Murphy et al, 1995;Crowley et al, 2000;Millonig et al, 2012) and the adjacent western Rocky Mountain foldand-thrust belt (McDonough and Parrish, 1991;McMechan and Roddick, 1991). Mica formed during these events could have been recycled in the Western Canada sedimentary basin from domes and arches during the "platformal" stage and from the orogen or forebulge during the "foreland" stage of basin evolution.…”

“…Such mica could have been recycled into strata of the Western Canada sedimentary basin through erosion of Precambrian rocks exposed in domal and arch structures developed during the early, middle (ca. 500 Ma, 360-340 Ma; Millonig et al, 2012), or late (McDonough andParrish, 1991) Paleozoic tectonothermal events that affected the outboard side of the Western Canada sedimentary basin. For example, the Neoproterozoic Windermere Supergroup uplifted in the Peace River Arch (index map in Fig.…”

Determining the Lifespan of Hydrothermal Systems Using Thermochronology and Thermal Modeling

“…Single‐grain AHe ages range between 53.3 and 8.1 Ma (Fraser et al., 2021; Figure 2b). Inverse thermal history modeling of this sample using QTQt (Gallagher, 2012), alongside published high temperature 40 Ar‐ 39 Ar and zircon U‐Pb constraints (McDonough & Parrish, 1991; Pirie, 2000), provides the background thermal history which suggest a linear cooling rate of 6.2 °C/Myr between 52 and 0 Ma (Figure 6; Figure S3 in Supporting Information S1). This sample exhibits AHe and AFT ages nearly double those derived from 77 to 15 and the lack of accelerated cooling or rehearing in thermal histories implies they have not experience reheating and record the regional erosional history…”

Resolving the Cenozoic History of Rock Exhumation Along the Central Rocky Mountain Trench Using Apatite Low‐Temperature Thermochronology

“…To the west of the RMT, in the Monashee Mountains, is the MGC (Figure 3b). The MGC is the northernmost exposure of Precambrian basement (Figure 3b) (Campbell, 1968), and was transported along Early Cretaceous thrust faults and subsequently folded (McDonough & Parrish, 1991; Murphy, 2007; Simony & Carr, 2011). To the east of the RMT, in the westernmost Rocky Mountains, are four smaller gneiss bodies (Figure 3b) (Campbell, 1968; Price & Mountjoy, 1970).…”