Metamorphic evolution of Archean hyaloclastites, Noranda area, Quebec, Canada. Part I: Comparison of Archean and Cenozoic sea-floor metamorphism

Detailed petrographic comparison of Archean and Cenozoic hyaloclastites and pillows permits us to outline an analogous history of sea-floor metamorphism. The Cenozoic hyaloclastites have been replaced by palagonite (hydrated and oxidized glass), smectites, zeolites, calcite and, locally, by silica. Present minerals in Archean hyaloclastites formed during a phase of load metamorphism, but relict textures permit one to recognize textures of palagonite, former zeolites and smectites, followed by replacement by chlorite, carbonate, and silica. Most pillows of Cenozoic and Archean age consist of three zones: a border zone consisting of palagonitized or chloritized glass; a rim defined by the presence of feldspar spherulites; and a microlitic core. Chloritization, carbonatization, and silicification follow the growth of feldspar spherulites.The evidence indicates that Archean and Cenozoic sea-floor metamorphism took place under identical conditions. In particular Archean palagonite is highly oxidized.