Metasomatic effects related to channelled fluid streaming through deep crust: fenites and associated carbonatites (In Ouzzal Proterozoic granulites, Hoggar, Algeria)

Journal of Metamorphic Geology - Tập 14 Số 6 - Trang 763-781 - 1996
Serge Fourcade1, Jean-Robert Kiénast2, K. Ouzegane3
1Géosciences Rennes, University of Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France (email: [email protected])
2Laboratoire de Pétrologie, University of Paris VI-VII, URA-CNRS 736, Tour 26–0, 4 Place Jussieu, 75252 Paris Cedex 05, France
3Institut des Sciences de la Terre, U.S.T.H.B., B.P. 9, Dar el Beida, Algiers, Algeria

Tóm tắt

ABSTRACTThe In Ouzzal granulitic unit (IOGU) consists predominantly of felsic orthogneisses most of which correspond to granitoids emplaced during the Archaean, plus metasediments, including olivine‐spinel marbles, of late Archaean age. All units were metamorphosed at granulite facies during the Eburnean (2 Ga). The stable isotope signature of the marbles (δ13C=–0.8 to –4.2‰/PDB; δ18O = 7.9 to 18.9‰/SMOW) does not record a massive streaming of C‐bearing fluids during metamorphism. Most of the isotopic variation in the marbles is explained in terms of pregranulitic features. Metasomatic transformation of granulites into layered potassic syenitic rocks and emplacement of carbonate veins and breccias occurred during retrogressive granulite facies conditions. The chemistry of these rocks is comparable with that of fenites and carbonatites with high contents of (L)REEs, Th, U, F, C, Ba and Sr but, with respect to these elements, a relative depletion in Nb, Ta, Hf, Zr and Ti. The isotopic compositions of Nd (ɛNd(T)=–6.3 to –9.9), of Sr (87Sr/86Sr(T)= 0.7093–0.7104), and the O isotopic composition of metasomatic clinopyroxene (δ18O = 6.9 to 8‰), all indicate that the fluid had a strong crustal imprint. On the basis of the C isotope ratios (δ13C =–3.5 to –9.7‰), the fluid responsible for the crystallization of carbonates and metasomatic alteration is thought to be derived from the mantle, presumably through degassing of mantle‐derived magmas at depth. Intense interaction with the crust during the upward flow of the fluid may explain its chemical and isotopic signatures. The zones of metasomatic alteration in the In Ouzzal granulites may be the deep‐seated equivalents of the zones of channelled circulation of carbonated fluids described at shallower levels in the crust.

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Journal of Metamorphic Geology

Tập 14 Số 6

763-781

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