Abstract

Dolomites in Lower and Middle Ordovician and Lower Silurian carbonates in Anticosti and Gaspe, respectively, are documented petrographically and geochemically to originate from early replacement and later void-filling phases, both derived from high temperature brines. These dolomite-rich intervals are interpreted as hydrothermal dolomites that formed during the early phases of foreland basin development along the continental margin of Laurentia; the Ordovician examples are related to the Taconian Orogeny whereas the Lower Silurian one is associated with the Salinic Orogeny.

The dolomitization of the Lower and Middle Ordovician carbonates on Anticosti platform occurred shortly after the demise of shallow marine carbonate sedimentation in early Late Ordovician, at a time of significant increase of subsidence rates and associated higher geothermal gradients along the entire continental margin of Laurentia. Two geochemically different fluids are documented; the differences could be related to more pronounced rock-water interactions for the Lower Ordovician carbonates. The dolomitization of the Lower Silurian carbonates on Gaspe Peninsula is demonstrated to be an early event, pre-Late Silurian, which occurred during a significant magmatic period and likely high geothermal gradients. The Lower Silurian hydrothermal dolomites record significant exchanges between the high temperature fluid and the mafic to ultramafic basement that supplied large volumes of Mg⁺² needed for this regional alteration.

This paper illustrates the tectonic link between active foreland basin and hydrothermal dolomite. Moreover, the efficiency of fault-controlled circulation related to upward thermal convection of dolomitizing fluids is possibly enhanced at times of significant magmatic and thermal episodes along the continental convergent margins.