Abstract
The precambrian postorogenic pluton of the Fort-Trinquet area (Northern Mauretania) is composed by a series of granitic rocks in which amphiboles are the characteristic mafic minerals. Twenty six amphiboles have been separated and chemically analysed; the optical properties and the unit-cell data are also given.
The crystallographic and chemical differences between these minerals reflect the variations in bulk composition of the host-rocks. In the plutonic suite, two igneous trends have been recognized: 1) a granitic trend (quartz monzonite-adamellite-granite) and 2) a syenitic trend (syenite-quartz bearing syenite-alkali granite). The amphiboles of the first trend belong to the tremolite-hastingsite series; they become richer in hastingsite mole with increasing proportion of quartz and decreasing Mg/(Mg+Fe) ratio in the rocks. The granitoids of the second trend are characterized by the occurrence of two amphiboles: a primary prismatic green-coloured actinolitic hornblende generally surrounded by a dark blue rim of riebeckite composition; the riebeckite may also form some acicular crystals associated to needles of stilpnomelane. These ≪coexisting≫ amphiboles would result from autometasomatic reactions which affected the rocks of the syenitic trend and which gave rise to the late alkali pegmatites where the constituent is a low arfvedsonitic riebeckite. The major substitutions involved in that amphibole transformation are Nax R3+→Cax R2+ and NaxSi→CaxAlIV.
Comparison with experimental data allows to estimate the physical conditions during the emplacement and the tardimagmatic evolution of this granitoid series.
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Fabriès, J., Rocci, G. Evolution cristallochimique des amphiboles dans la série granitique de Fort-Trinquet (Mauritanie). Contr. Mineral. and Petrol. 35, 215–225 (1972). https://doi.org/10.1007/BF00371216
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DOI: https://doi.org/10.1007/BF00371216