Structural, petrological and U–Pb SHRIMP geochronological study of the western Beaverlodge domain: Implications for crustal architecture, multi-stage orogenesis and the extent of the Taltson orogen in the SW Rae craton, Canadian Shield

The Beaverlodge domain is a polydeformed Precambrian terrane in the southwestern Rae craton whose rocks variably record the effects of Archean tectonism and four major Paleoproterozoic tectonic events: the 2.5–2.3 Ga Arrowsmith orogeny, the 1.99–1.93 Ga Taltson orogeny, the ca. 1.90 Ga Snowbird orogeny and the ca. 1.87–1.80 Ga Hudsonian orogeny. Structural–metamorphic analysis and in situ U–Pb SHRIMP zircon and monazite geochronology reported herein demonstrate that this domain consists of two main structural levels variably exposed due to complex interference folding: a lower level comprising mainly high-grade Archean (ca. 3.0 and 2.7–2.6 Ga) metaplutonic and metasedimentary gneisses and a higher level comprising mainly ca. 2.33 to <2.17 Ga supracrustal rocks of the Murmac Bay Group at more variable metamorphic grade. Whereas the older package underwent thermal and/or metamorphic events in the Archean (ca. 2.57 Ga) and early Paleoproterozoic (ca. 2.34 Ga; Arrowsmith orogeny), rocks at higher structural levels bear no record of these events, recording only younger (1.94–1.90 Ga) metamorphism interpreted to reflect two further stages of orogenesis. The first stage, correlated with Taltson orogeny, involved tight to isoclinal folding (D1/2) that led to development of the composite ESE-trending (S0/S1/S2) transposition foliation followed by more open folding (D3). The absence of Taltson-age plutonic rocks, except for low-volume crustal melts, suggests that this cycle was driven by crustal thickening, consistent with a clockwise P–T–t path derived for one sample. The high degree of strain along the contact between Archean rocks and the Murmac Bay Group, along with local structural discordance, indicates some degree of translation of cover rocks over basement during this stage. Pelitic rocks at higher structural levels (low- to medium-grade zone) underwent prograde metamorphism to middle amphibolite facies whereas rocks at deeper structural levels (high-grade zone) attained upper amphibolite facies conditions. Monazite data bracket prograde metamorphism (and related garnet growth) between 1939 ± 6 Ma and 1934 ± 5 Ma. The second stage, dated most precisely at 1906 ± 7 Ma, involved refolding of D1 to D3 structures about NE-trending axes (F4) and coeval dextral shearing and is correlated with Snowbird orogenesis. Rocks of the low- to medium-grade zone were held at roughly the same crustal level whereas rocks in the high-grade zone were uplifted via dextral–oblique displacement along domain-bounding shear zones, instigating decompression (garnet to cordierite) reactions. This study demonstrates that after <2.17 Ga deposition of the upper Murmac Bay Group, rocks along the entire west-southwestern Rae margin, from the Northwest Territories/northern Alberta to the Snowbird tectonic zone, were affected by Taltson (1940–1930 Ma) orogenesis. The Taltson tectono-metamorphic cycle then appears to have transitioned, as rocks underwent further shortening and uplift in a dextral transpressive regime, into Snowbird tectonism at ca. 1910–1900 Ma. Snowbird orogenesis is therefore best viewed as part of a semi-continuous 1940–1900 Ma cycle of tectonic burial and exhumation, with its inception at ca. 1910 Ma triggered by a change in boundary conditions, as yet not fully understood, but consistent with arrival of the Hearne craton proposed by others. While the Beaverlodge domain has many features in common with the Tantato domain, future work needs to explore why rocks in the Tantato domain lack evidence for Arrowsmith orogeny and failed to record the cycle of 1.94–1.93 Ga tectonic burial.

► First U–Pb SHRIMP data for southwestern Rae craton in the Athabasca region, Saskatchewan. ► First evaluation of crustal-scale architecture; two structural levels, contrasting histories. ► Lower level, Archean and Arrowsmith orogenesis; higher level, Taltson and Snowbird orogenesis. ► Distinction between Taltson (1940–1930 Ma) and Snowbird (1910–1900 Ma) events. ► Linkage to structural fabric elements, P–T history and regional structural–tectonic evolution.