Evidence of magmatic CO2-rich Fluids in Peraluminous Graphite-Bearing Leukogranites from deep Freeze Range (Northern Victoria Land, Antarctica)

Fine-grained peraluminous synkinematic leuco-monzogranites (SKG), of Cambro-Ordovician age, occur as veins and sills (up to 20-30 m thick) in the Deep Freeze Range, within the medium to high-grade metamorphics of the Wilson Terrane. Secondary fibrolite + graphite intergrowths occur in feldspars and subordinately in quartz. Four main solid and fluid inclusion populations are observed: primary mixed CO2 + H2O inclusions + Al2SiO5 +/- brines in garnet (type 1) ; early CO2-rich inclusions (+/- brines) in quartz (type 2) ; early CO2 + CH4 (up to 4 mol%) +/- H2O inclusions + graphite + fibrolite in quartz (type 3); late CH4 + CO2 + N2 inclusions and H2O inclusions in quartz (type 4). Densities of type 1 inclusions are consistent with the crystallization conditions of SKG (almost-equal-to 750-degrees-C and 3 kbar). The other types are post-magmatic: densities of type 2 and 3 inclusions suggest isobaric cooling at high temperature (almost-equal-to 700 -550-degrees-C). Type 4 inclusions were trapped below 500-degrees-C. The SKG crystallized from a magma that was at some stage vapour-saturated; fluids were CO2-rich, possibly with immiscible brines. CO2-rich fluids (+/- brines) characterize the transition from magmatic to post-magmatic stages; progressive isobaric cooling (T< 670-degrees-C) led to a continuous decrease of f(O2), entering in the graphite stability field; at the same time, the feldspars reacted with CO2-rich fluids to give secondary fibrolite + graphite. Decrease of T and f(O2) can explain the progressive variation in the fluid composition from CO2-rich to CH4 and water dominated in a closed system (in situ evolution). The presence of N2 the late stages indicates interaction with external metamorphic fluids.