Archean ocean-floor metamorphism in the North Pole area, Pilbara Craton, Western Australia

Abstract

Abstract Archean greenstones (ca. 3.49–3.46 Ga) in the North Pole Dome, Pilbara Craton, Western Australia, have undergone prograde metamorphism from prehnite-pumpellyite facies to greenschist-amphibolite facies. Observations of ∼3000 thin sections shows that a metamorphic aureole was restricted to within a few kilometers of the contact with the 3459 Ma North Pole Monzogranite. Three types of alteration were observed; carbonate, silicic and chlorite types. The distribution of the three types of alteration suggests that secondary chemical migration of elements occurred in the whole area, but that some samples were unaffected by this alteration. The southern North Pole area is outside of the contact metamorphic aureole, and is composed of five units from north (unit I) to south (unit V), dipping south about 30–70°. Metamorphism of the North Pole greenstones corresponds to the low-pressure type, which is broadly comparable to that inferred for ocean-floor metamorphism. Prehnite-pumpellyite (PP), transitional (TR), and greenschist (GS) facies prograde metamorphic zones occur from structural top to bottom within each of units. Metamorphic grade increases downward through the units and metamorphic isograds never cut unit boundaries.