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

Abstract

The 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.

A review of Archaean and Paleoproterozoic evolution of the In Ouzzal granulitic terrane (Western Hoggar, Algeria)

“…2) occur that are systematically associated with fenites at their contact with the granulites (Ouzegane, 1987;Ouzegane et al, 1988;Bernard-Griffiths et al, 1988;Carpena et al, 1988;Fourcade et al, 1996). These rocks display some very particular features, differing from usual carbonatites by the lack of feldspathoids and the abundance of wollastonite which coexists with calcite and quartz.…”

“…In the area of Tekhamalt, the dolomitic marbles in contact with charnockites (alkali-granitic orthogneisses) develop Al-Mg-Ca granulites with diopside-anorthitespinel. Fourcade et al (1996) interpret these rocks as resulting from decarbonatization, i.e. paleoskarns, based on their high W and Ag contents (300 ppm and 325 ppb, respectively).…”

“…Accessory minerals include titanite, allanite, fluorite, magnetite and grossulariteandradite garnet. In the field, a relative chronology comprising several stages can be established for the emplacement of the carbonatite complex Fourcade et al, 1996). Two carbonatite generations are distinguished.…”

“…The identification of the protoliths of the dolomitic marbles was established by Fourcade et al (1996), based on the chemical composition and isotopic characteristics of these marbles. The high Al and Mg contents of the dolomitic marbles could be due to the addition of a chloritic component derived from the hydrothermal alteration of a mafic/ultramafic source during carbonate sedimentation.…”

Possible fluid–rock interactions on differentiated asteroids recorded in eucritic meteorites

“…Moreover, cryptic or modal metasomatism induced by carbonate-rich fluids are well known in terrestrial systems. The fingerprints of these fluids are exemplified by a high mobility of some elements and deposition of carbonate phases (e.g., Fourcade et al, 1996). If CO-CO 2 rich fluids have circulated into eucrites, Fe-rich carbonates (or their decomposition products) should be expected.…”