Evidence of magmatic CO2-rich fluids in peraluminous graphite-bearing leucogranites from Deep Freeze Range (northern Victoria Land,Antarctica) |
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Authors: | Maria Luce Frezzotti Gianfranco Di Vincenzo Claudio Ghezzo Ernst A. J. Burke |
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Affiliation: | (1) Dipartimento di Scienze della Terra, Via delle Cerchia, 3, I-53100 Siena, Italy;(2) Instituut voor Aardwetenschappen, Vrije Universiteit, De Boelelaan 1085, NL 1081HV Amsterdam, The Netherlands |
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Abstract: | 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 (750°C and 3 kbar). The other types are post-magmatic: densities of type 2 and 3 inclusions suggest isobaric cooling at high temperature (700–550°C). Type 4 inclusions were trapped below 500°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°C) led to a continuous decrease offO2 can entering in the graphite stability field; at the same time, the feldspars reacted with CO2-rich fluids to give secondary fibrolite + graphite. Decrease ofT andfO2 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.Contribution within the network Hydrothermal/metamorphic water-rock interactions in crystalline rocks: a multidisciplinary approach on paleofluid analysis. CEC program: Human Capital and Mobility |
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