Hydrothermal dolomite in the Timiskaming outlier, central Canadian Shield: Proxy for Late Ordovician tectonic activity

The Timiskaming Paleozoic outlier on the central Canadian Shield preserves a thin distal portion of the once extensive Taconic deep-water shale succession that extended across eastern Laurentia. Within this succession, a thin (5 cm) lithic and phosphatic sandstone occurs a few meters above the top of the buried shallow-water carbonate platform, and contains abundant (40-60%) euhedral dolomite, rare fluorite, pyrite-cemented shale microbreccia, and crystal mosaics of subhedral dolomite with pyritic veinlets. The dolomite formed local cement, and precipitated very near the sediment-water interface as illustrated by a relatively uncompacted sandstone framework and a 3-dimensional fabric of clay particles trapped by dolomite growth. The majority of dolomite has planar crystal faces, with ferroan (4.1 ± 0.3 mol% FeCO 3 ) crystal cores and non-ferroan (0.9 ± 0.2 mol% FeCO 3 ) rims. Fluid inclusions, too small (<2 μm) for reliable microthermic analysis, are mostly liquid, which, in keeping with interpreted near-surface diagenesis, as well as temperature-controlled dolomite-crystal roughening models, may indicate that formation temperatures were no more than ∼60 to 80°C. δ 13 C PDB values (∼1.2 ‰) are similar to a Late Ordovician seawater composition, but δ 18 O PDB values (—4.8 to ―5.2 ‰) are too negative compared to the expected values for contemporary deep-marine dolomite. Combining the regional paleoceanographic framework with diagenetic constraints and revised models for Late Ordovician seawater temperature and δ 18 O compositions, the dolomite is interpreted to be a proxy for a low-temperature (at least ∼50°C) hydrothermal anomaly near the sediment-water interface. Fragments of shale microbreccia, pyrite veinlets, and subhedral dolomite, along with the region's structural history, allow speculation that this event coincided with local reactivation of a Precambrian fault, part of an ancestral fault system now manifest regionally by an extension of the Ottawa-Bonnechere Graben. Late Ordovician craton-interior tectonism, in a region previously considered tectonically stable at this time, is defined by local response of inherited Precambrian structure driven by, but distal to, Taconic orogenesis.