TARDI-MAGMATIC CLAYS IN THE MOST DIFFERENTIATED TERMS OF THE OCEANITE SERIES OF THE PITON DES NEIGES (REUNION ISLAND)
The syenitic rocks are characterized by a phaneritic texture with an exceptionally high intergranular primary macroporosity and the absence of perthitic exsolution in the feldspars. Such petrographic features are indicative of a rapid cooling which precluded the developments of subsolidus reactions usually observed in such intrusive rocks. Chlorite-like minerals crystallized during the latest stage of the fractional crystallization with quartz, carbonates and accessory minerals (zircon, monazite, rutile, apatite, pyrite). This observation is supported by isotopic microanalyses of O and C (ionic probe, CRPG, Nancy) of quartz, opaline, calcite and ankerite associated with these differentiated term. The low d13C values found in the carbonate crystals (-49 to -53‰) associated with d18O values ranging from 3 to 8 ‰, are indicative of juvenile origin of the mineralizing fluid.
We propose that a chemical transfer occurred at the end of the crystallization process, from the magmatic phases affected by post-magmatic reaction to the primary porosity, in a CO2-rich juvenile fluid and leading to the crystallization of tardi-magmatic clays. The proposed process is likely not specific of the particular case presented here and can be generalize to all of the alkaline series, so to the primitive crust of the telluric planets.
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