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

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ID: 308115
2010
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Abstract
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 region9s 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-Bonnechère 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.
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Authors George R. Dix, Sanjeev Sharma, Ihsan S. Al‐Aasm, Mario Coniglio, Robert L. Linnen, John Riva, A. Achab
Journal american journal of science
Year 2010
DOI
10.2475/05.2010.03
URL
https://ajs.scholasticahq.com/article/61812.pdf
Keywords Keywords not found

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