Near-solidus delta 18 O depletion in a Ta-Nb-bearing albite granite; the Beauvoir Granite, France

Abstract

The Beauvoir albite-topaz-lepidolite granite has been intersected through an interval of 700 m in a fully cored drill hole at Echassieres, in the Massif Central (France). The Beauvoir granite is a southerly inclined sheet, bearing late Hercynian hololeucocratic Ta-Nb-Li-Be mineralization, that intrudes mica schists. The drill hole intersects the schists from the surface to a 100-m depth and again at 800 m at the bottom of the hole. Three facies of the granite have been defined on the basis of petrographic and geochemical studies--B1 (100-500 m), B2 (500-750 m), and B3 (750-790 m). Trace element and Sr isotope geochemistry indicate the cogenetic nature of the three facies. Ta contents increase fairly regularly from [asymp]30 to [asymp]250 ppm from the base to the top of the sheet.Previously analyzed samples from exploration drill holes in the apical varieties of the granite (B1, B2) gave scattered delta 18 O values ranging from 4.6 to 8.6 per mil. The delta 18 O value was measured in whole rocks and mineral separates from the deep drill hole samples. These samples were carefully selected to be unaffected by hydrothermal and low-temperature alteration. The B1 facies and the upper part of the B2 facies are characterized by the presence of two generations of quartz, an early globular form (Q g ) and a late interstitial quartz (Q i ). The results show a decrease in delta 18 O values from depth upward. In the B3 granite, the average values are normal for leucogranites: delta 18 O of interstitial quartz = 11.6 per mil and of mica = 9.3 per mil, whereas in the B1 granite they are 5.3 per mil for interstitial quartz and 4.5 per mil for mica. The values for the B2 granite are intermediate between those for the B1 and B3 facies. Oxygen isotope temperatures are 570 degrees + or - 50 degrees C, similar to experimentally obtained solidus temperatures. The delta D values of lepidolite and whole rock from fresh samples from the drill hole range, respectively, from -91 to -66 and -76 to -60 per mil compared with SMOW. The wt percent of H 2 O in whole rock ranges from 0.23 to 0.76. No correlation exists between the delta D values of lepidolite or whole rock and the delta 18 O values of minerals, or the depth or wt percent of H 2 O. These results are interpreted as evidence of interaction, in the liquidus-solidus interval, between the magmatic system and external water of meteoric origin. This water probably originated in the convective system of the hydrothermal fluids related to the penecontemporaneous emplacement of the nearby Colettes granite.Globular quartz from the B1 facies shows delta 18 O values similar to quartz in the lower part of the drill hole (delta 18 O = 8.5-11ppm). This similarity confirms that the three granitic facies are cogenetic and constrains the time of interaction with the external fluid to a period after the crystallization of the early globular quartz. Topaz is also an early crystallizing mineral that contains fluid inclusions of magmatic origin. A variation of 5.0 to 10.1 per mil in the delta 18 O values of topaz downward in the borehole agrees with a high-temperature isotopic equilibrium between quartz and lepidolite and provides evidence of early interaction between the upper part of the Beauvoir granite with external fluids during the magmatic and late magmatic stages. This early interaction shown by the isotopic characteristics of magmatic mineralogy and by the isotopic temperatures took place across the liquidus-solidus interval.