Role of fluids in migmatites: CO2-H2O fluid inclusions in leucosomes from the Deep Freeze Range migmatites (Terra Nova Bay, Antarctica)

Abstract

The metasedimentary sequence of the Deep Freeze Range (northern Victoria Land, Antarctica) experienced high-T/low-F metamorphism during the Cambro-Ordovician Ross orogeny. The reaction Bt+Sil+Qtz=Grt+Crd+Kfs+melt was responsible for the formation of migmatites. Peak conditions were c. 700-750°C, c. 3.5-5 kbar and xH2O c. 0.5). Distribution of fluid inclusions is controlled by host rock type: (1) CO2-H2O fluid inclusions occur only in graphite-free leucosomes; (2) CO2-CH4±H2O fluid inclusions are the most common type in leucosomes, and in graphite-bearing mesosomes and gneiss; and (3) CO2-N2-CH4 fluid inclusions are observed only in the gneiss, and subordinately in mesosomes. CO2-H2O mixtures (41% CO2, 58% H2O, 1% NaCl mol.%) are interpreted as remnants of a synmigmatization fluid; their composition and density are compatible P-T-aH2O conditions of migmatization (c. 750°C, c. 4 kbar, xH2O c. 0.5). CO2-H2O fluid in graphite-free leucosomes cannot originate via partial melting of graphite-bearing mesosomes in a closed system; this would have produced a mixed CO2-CH4 fluid in the leucosomes by a reaction such as Bt+Sil+Qtz+C±H2O=Grt+Crd+Kfs+L+CO2+CH4. We conclude that an externally derived oxidizing CO2-H2O fluid was present in the middle crust and initiated anatexis. High-density CO2-rich fluid with traces of CH4 characterizes the retrograde evolution of these rocks at high temperatures and support isobaric cooling (P-T anticlockwise path). In unmigmatized gneiss, mixed CO2-N2-CH4 fluid yields isochores compatible with peak metamorphic conditions (c. 700-750°C, c. 4-4.5 kbar); they may represent a peak metamorphic fluid that pre-dated the migmatization.