Late Archaean/Early Proterozoic CO2 streaming through the lower crust and geochemical segregation

Abstract

Considerable evidence exists for large‐scale migration of CO2‐rich fluids through the lower continental crust in the Late Archaean and Early Proterozoic, with transfer of heat producing elements and H2O to higher levels. The evidence manifest in the large charnockitic terrains of this age includes deformation‐related charnockitic alteration of gneisses, nearly ubiquitous CO2‐rich fluid inclusions in minerals, depletion of large‐ion‐lithophile elements (LILE), particularly Rb, relative to typical upper crustal rocks, similar depletion of 18O in paragneisses and petrographic and geochemical evidence of open‐system replacement of amphibole gneiss by charnockite. A conveyor‐belt upper mantle source appears to have been necessary for prolonged CO2 supply in high‐grade crustal metamorphism, most plausibly by deep subduction of marine carbonate under continental interiors. This plate tectonic mode seems consonant with crustal thickening and orogenic patterns inferred for most charnockitic terrains and with CO2 transport mechanisms constrained by experimental petrology.