Role of fluids in migmatites: CO 2 ‐H 2 O fluid inclusions in leucosomes from the Deep Freeze Range migmatites (Terra Nova Bay, Antarctica)

• Published in: Journal of Metamorphic Geology
• Main Authors: GIORGETTI, G., FREZZOTTI, M.‐L.E, PALMERI, R., BURKE, E. A. J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1996
• Subjects: Deep Freeze Range; Terra Nova Bay; Victoria Land; Antarc*; Antarctica
• Online Access: http://dx.doi.org/10.1111/j.1525-1314.1996.00307.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.1996.00307.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.1996.00307.x

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 x H2O c. 0.5). Distribution of fluid inclusions is controlled by host rock type: (1) CO 2 ‐H 2 O fluid inclusions occur only in graphite‐free leucosomes; (2) CO 2 –CH 4 ± H 2 O fluid inclusions are the most common type in leucosomes, and in graphite‐bearing mesosomes and gneiss; and (3) CO 2 –N 2 –CH 4 fluid inclusions are observed only in the gneiss, and subordinately in mesosomes. CO 2 –H 2 O mixtures (41% CO 2 , 58% H 2 O, 1% Nad mol.%) are interpreted as remnants of a synmig‐matization fluid; their composition and density are compatible P–T–a H2O conditions of migmatization ( c. 750° C, c. 4 kbar, x H2O c. 0.5). CO 2 ‐H 2 O fluid in graphite‐free leucosomes cannot originate via partial melting of graphite‐bearing mesosomes in a closed system; this would have produced a mixed CO 2 –CH 4 fluid in the leucosomes by a reaction such as Bt + Sil + Qtz + C ± H 2 O = Grt + Crd + Kfs + L + CO 2 + CH 4 . We conclude that an externally derived oxidizing CO 2 ‐H 2 O fluid was present in the middle crust and initiated anatexis. High‐density CO 2 ‐rich fluid with traces of CH 4 characterizes the retrograde evolution of these rocks at high temperatures and support isobaric cooling ( P–T anticlockwise path). In unmigmatized gneiss, mixed CO 2 –N 2 –CH 4 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.