Constraints on the timing of crustal imbrication in the central Trans-Hudson Orogen from single zircon 207Pb/206Pb ages of granitoid rocks from the Pelican thrust zone, Saskatchewan

The Pelican thrust is a major ductile high-strain zone in the Reindeer Zone, Trans-Hudson Orogen, northern Saskatchewan. It is interpreted as the main sole thrust separating stacked juvenile Paleoproterozoic allochthons and underlying Archean microcontinental crust in this central part of the orogen. Exposed nonmylonitic rocks in the footwall of the thrust consist of the Sahli monzocharnockite and the smaller, more highly retrograded MacMillan Point granite. Protomylonitic to ultramylonitic gneisses in the thrust zone derive from a variety of prethrust protoliths. A footwall "internal suite" mainly comprises quartzofeldspathic orthogneisses ("Q" gneisses) and high-grade migmatitic paragneisses. Hanging-wall "external suite" mylonitic gneisses include feldspar-porphyroclastic hornblendic grey gneisses probably derived from arc plutons, and laminated amphibolites derived from volcanic rocks. The overlying allochthon mainly comprises protoliths equivalent to those of the porphyroclastic orthogneisses and laminated amphibolites, together with interfolded and overlying Paleoproterozoic paragneisses of the Kisseynew domain. The Sahli monzocharnockite yields 207Pb/206Pb zircon and whole-rock Rb-Sr ages of ca. 2500 Ma, and the "Q" gneisses give 207Pb/206Pb zircon ages of up to ca. 2900 Ma, implying that most of the internal suite (footwall) mylonite protoliths are Archean. In contrast, external suite (hanging wall) porphyroclastic orthogneisses yield ca. 1880-1840 Ma 207Pb/206Pb zircon ages. Main, peak-metamorphic displacement on the Pelican thrust is interpreted to have occurred mainly between 1840 and 1820 Ma, as indicated by 207Pb/206Pb zircon ages from small, highly deformed synthrusting granite-pegmatite neosomal bodies in the thrust zone. Undeformed postcollisional granites and pegmatites were emplaced ∼1789 Ma. Total duration from arc development to completion of arc-continent collision was ∼100 Ma. The Pelican thrust zone may be similar in significance and style to younger, major, ocean closure related thrusts such as the Frontal Pennine thrust of the western Alps and the Main Mantle, Main Boundary, and Main Central thrusts of the Himalayas. As for the Pelican thrust, these displace oceanic rocks over older basement.published_or_final_versio

Crustal history of the Rae and Hearne provinces, southwestern Canadian Shield, Saskatchewan: constraints from geochronologic and isotopic data

The Cree Lake Zone (Hearne Province) and Western Granulite domain (Rae Province), in northern Saskatchewan, are contrasting parts of the variably reworked, (?) upper plate western hinterland of the Paleoproterozoic Trans-Hudson Orogen. Their junction is defined by the Virgin River Shear Zone, a part of the more extensive Snowbird "tectonic zone". Subsurface extension of the ∼ 2.0 Ga Thelon Orogen lies west of the Western Granulite domain. We present UPb zircon ages, SmNd and RbSr isotopic data from the Cree Lake Zone and Western Granulite domain, and from the Junction Granite which intrudes the Virgin River Shear Zone. Data from the Western Granulite domain indicate that prevalent granulite facies metamorphism occurred between ∼ 2.3 and 2.0 Ga, but is imposed on Archean protoliths ∼ 2.8-3.0 Ga old. Low amphibolite facies retrogression, which increases toward the Virgin River Shear Zone, probably postdates 2.1 Ga. Data from the Cree Lake Zone also yield Archean protolith ages as old as 3.0 Ga, and some rocks yield interpreted high-grade metamorphic overprint ages of ∼ 2.3-2.0 Ga. However, two samples yield data indicating "Hudsonian" high-grade metamorphic overprint ∼ 1.87 to 1.81 Ga. The Junction Granite has a UPb zircon age of 1.82 ± 0.30 Ga, but relict zircons and SmNd data show that it was derived by melting of Archean crust. High-grade mid-crustal thermal resetting in the Western Granulite domain may be coeval with the 1.9-2.0 Ga Thelon Orogen, although the data do not preclude an earlier event or events. Similar, ∼ 2.3-2.1 Ga reworking extends into the Cree Lake Zone, but high-grade "Hudsonian" (1.87-1.84 Ga) reworking is also evident in parts of this zone well to the west of the Wollaston domain, and may extend entirely across the zone to the Virgin River Shear Zone. Relative importance of the "Thelon" and "Hudsonian" overprints in Cree Lake Zone is unknown. Data from the Junction Granite and Western Granulite domain suggest that all recorded high-strain events in the Virgin River Shear Zone postdate ∼ 2.3 Ga and are thus unrelated to deep-crustal Archean granulite facies mylonites in the Tantato-Black Lake segment of the Snowbird tectonic zone, north of the Athabasca basin