Topaz solid solution in the F-rich granitic rocks from Blond (NW Massif Central, France)
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ORIGINAL PAPER
Topaz solid solution in the F-rich granitic rocks from Blond (NW Massif Central, France) Mohamed Soufi 1 & Toufik Remmal 1 & Iz Eddine EL Amrani El Hassani 2 & Samira Makhoukhi 1 Received: 3 February 2018 / Accepted: 3 July 2018 # Saudi Society for Geosciences 2018
Abstract The Blond peraluminous leucogranitic stock, which is a part of the Limousin in Massif Central of France, ranges in composition from two micas leucogranite to topaz–albite leucogranite which are associated to topaz microgranitic dyke (ongonite). The topazbearing granitic rocks recognized in this area appear as minor and late components of the massif. Analysis of topaz features within this facies allows the setting of a typological classification based upon the relationships between habits, chemical composition, and the genesis conditions of this mineral. The primary, magmatic nature of topaz has been inferred by microtextural investigations, whereas electron microprobe analyses indicate that topaz lies entirely within the space defined by natural F-OH topaz solid solution series at moderate pressures. The F-content ranges from 14.39 to 20.14 wt.% and OH/OH + F ratio ranges from 0.04 to 0.31. Two separate generations of magmatic topaz, including an early-stage medium-F topaz and a latestage high-F topaz, are shown. These characteristics could be related to high HF concentration in the melt, while the effect of temperature on the F-content of topaz as it was demonstrated experimentally can be reviewed. Keywords Blond granitic stock . Massif Central of France . Hercynian belt . Topaz-bearing granitic rocks . Topaz
Introduction The Blond granitic stock is located in the High Limousin, 30 km northwest of Limoges (North-West French Massif Central). The massif, 17 km long and 5 km wide, constitutes a leucogranitic area of approximately 60 km2. It intrudes to the north the metamorphic Upper Gneiss Unit and to the south the late Devonian Cieux-Vaulry biotite–cordierite granite. The stock is bounded to the west by the N135°E Oradour fault. Structural analysis from the right side of this fault provides strong evidence of syntectonic emplacement and crystallization of the Blond massif (Soufi 1988; Chevremont et al. 1992; Soufi et al. 2015). The whole-rock Rb–Sr isochron indicates an age of 301 ± 5 Ma for the Blond granite (Duthou 1977). Recently, an emplacement 40Ar/39Ar age of 313 Ma obtained
* Mohamed Soufi [email protected] 1
Faculty of Sciences Aïn Chock, Hassan II University of Casablanca, B.P. 5366, Maârif, Casablanca, Morocco
2
Department of Earth Sciences, Scientific Institute, Mohammed V-Agdal University, Rabat, Morocco
on muscovite was proposed by Alexandrov et al. (2002), corresponding to the late Hercynian (Variscan) geological history of the Limousin. Previous data have shown Blond granite as homogeneous biotite and muscovite granite (Barbier 1967, 1968). A geochemical study by Burnol (1974a) considers the granite as a rare-element (Li-rich) granitic pluton. At Richemont, to the north of Blond granite, a rhyolitic dyke is
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