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Role of fluids in migmatites: CO 2 ‐H 2 O fluid inclusions in leucosomes from the Deep Freeze Range migmatites (Terra Nova Bay, Antarctica)
Author(s) -
GIORGETTI G.,
FREZZOTTI M.L.E,
PALMERI R.,
BURKE E. A. J.
Publication year - 1996
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1996.00307.x
Subject(s) - geology , migmatite , metamorphism , gneiss , geochemistry , fluid inclusions , grossular , anatexis , partial melting , metamorphic rock , crust , paleontology , quartz
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.