Hydrothermal Dolomitization of Upper Ordovician Limestone, Central-East Canada: Fluid Flow in a Craton-Interior Wrench-Fault System Likely Driven by Distal Taconic Tectonism

Narrow (tens of meters) vertical bodies of dolomite replace a nonporous Upper Ordovician (lower Chatfieldian) limestone along the leading limb of an asymmetric faulted anticline well inboard (∼300 km) of the northern Appalachian orogen, central-east Canada. Patterns of geometry, texture, and geochemistry suggest that dolomitizing fluids were focused (possibly along microfractures) within a paleosinistral transpressive stress field generated with reactivation of an underlying Neoproterozoic fault system. Strike-slip failure of the developing anticline resulted in fracture- and fault-controlled fluid flow from which precipitated saddle dolomite. A paragenetic succession of Fe-poor planar-e to ferroan planar-s dolostone marks the peak phase of replacement dolomitization. A subsequent increase (20%–30%) in porosity created through local dissolution of relict limestone was partially occluded by ferroan planar dolomite that, geochemically, is similar to later fracture-fill saddle dolomite. Replacement dolomitization is associated with a slight rise in Sr-isotope ratios (to 0.71085) from Late Ordovician–early Silurian marine signatures. Isotope (C, O, Sr) signatures support influx and mixing of burial fluids that had interacted with local Mg-rich (gabbro, anorthosite, syenite) crystalline basement with a background fluid similar to Late Ordovician seawater or dissolved marine limestone. Dolomitization predated maximum burial in the Late Paleozoic. Fluid inclusion and isotope paleothermometry suggest that dolomitizing temperatures (100°–120°C) were 20°–30°C warmer than associated with prior limestone diagenesis. The dolomitized limestone is an archival record of structure, hydrology, and heat flux that best fits with Taconic tectonism in the latest Ordovician through earliest Silurian.