Mazatzal–Labradorian-age (1.7–1.6 Ga) ductile deformation of the South Range Sudbury impact structure at the Thayer Lindsley mine, Ontario

Abstract

The Thayer Lindsley mine is located in the South Range of the Sudbury impact structure, near the contact between the 1.85 Ga Sudbury Igneous Complex (SIC) and the Paleoproterozoic Southern Province. Ni–Cu ore zones at the mine are strongly deformed within a southeast-dipping, lower amphibolite-grade shear zone, which offsets the contact between the SIC and Southern Province rocks. Numerous shear sense indicators, including shear bands, drag folds, and δ- and σ-type rotated porphyroclasts, consistently indicate south-over-north, reverse, dip-slip movement parallel to the mineral stretching lineation in the shear zone. The attitude, slip direction, and metamorphic grade of the shear zone are similar to those of the regional northeast-striking South Range Shear Zone that formed during post-impact, northwest-directed ductile contraction of the Sudbury impact structure. The South Range Shear Zone is generally interpreted as a ca. 1.9–1.8 Ga Penokean structure. Anhedral brown titanite grains from the Thayer Lindsley shear zone yield a mean 207Pb/206Pb Penokean age of 1815 ± 15 Ma. These grains are mantled by younger, syntectonic, colourless titanite, which have a mean 207Pb/206Pb age of 1658 ± 68 Ma. This younger age suggests that the South Range and Thayer Lindsley shear zones may have formed during a 1.7–1.6 Ga collisional tectonic event that is recorded along the southeast margin of Laurentia from the southwest USA. (Mazatzal Orogeny), through the mid-continent to Wisconsin, and as far northeast as Labrador (Labradorian Orogeny). 40Ar/39Ar analyses indicate post-tectonic thermal resetting of biotite occurred at 1477 ± 8 Ma during felsic plutonism across the Sudbury area.