Contact metamorphism of layered carbonate-shale sequences in the Oslo Rift. II: Migration of isotopic and reaction fronts around cooling plutons

Abstract

Intrusion of felsic plutons of Permian age into Lower Paleozoic shale-carbonate sequences caused extensive contact metamorphism in the Oslo Rift. Oxygen and carbon isotope data from both carbonates and shales are presented to demonstrate that the progress of metamorphic reactions in the contact aureole around the Drammen granite was, to a large extent, controlled by the infiltration of water-rich fluids of a magmatic oxygen and with a carbon isotope signature. Highly integrated fluid fluxes were confined to shale-rich lithologies; massive limestones were relatively impermeable to fluid flow. The spatial distribution of isotopic and reaction fronts in the aureole and coupled 18O 13C isotope variations of carbonates from interlayered shale-carbonate units fits well to a 1-D transport model, with fluid flow away from the intrusive contact; whereas the same isotope variations in limestones containing thin shale intercalations do not. The high fluid fluxes estimated from 1-D models (> 2000 m 3/m 2) require either focused flow of magmatic fluids in the study area or large-scale convective flow, possibly involving fluids of meteoric origin. Limited kinetic broadening of the oxygen isotopic front indicates fluid fluxes of < 10 −8 m/s. Fluid flow must have persisted for > 10 4 yr to have moved the oxygen front 1000 m from the intrusive contact. Simple 2-D transport models, involving dominant fluid flow along aquifers or fractures and involving diffusion as well as advection, explain most of the coupled 18O 13C trends reported for carbonates from contact aureoles.