Character of fluids associated with hydrothermal alteration and metamorphism of Palaeoproterozoic submarine volcanic rocks, Baffin Island, Nunavut, Canada

The Bravo Lake Formation on central Baffin Island, Canada formed in a Palaeoproterozoic (∼1.9 Ga) intrabasinal submarine volcanic rift, the rocks of which preserve mineral assemblages formed during submarine hydrothermal alteration, amphibolite facies regional metamorphism, and retrograde metamorphism. Both stable isotope and cation exchange geothermometry of co-existing mineral assemblages indicate preservation of hydrothermal alteration at ∼300 °C, amphibolite facies metamorphism at ∼590 °C, and retrograde metamorphism at ∼300 °C. Fluid isotopic compositions calculated from δ18O and δD values measured from minerals suggest that three isotopically distinct fluids interacted with the Bravo Lake Formation during its evolution: (1) Palaeoproterozoic seawater infiltration during hydrothermal alteration; (2) regional metamorphic fluids at low water–rock ratios; and (3) retrograde fluids indicating increasing water–rock ratios. Hydrothermal fluids have δ18O and δD values near 0‰, similar to modern seawater values. The similarity in the isotopic composition of Palaeoproterozoic and modern seawater is consistent with the buffering of the isotopic composition of ocean water by hydrothermal circulation at mid-ocean ridges by similar processes as operate today. The results of this study show that the isotopic composition of prior fluid events can be preserved in amphibolite facies submarine mafic volcanic rocks if there is minimal penetrative structural deformation and dry prograde metamorphism.