Evolution cristallochimique des amphiboles dans la série granitique de Fort-Trinquet (Mauritanie) |
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| Authors: | Jacques Fabriès Georges Rocci |
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| Institution: | (1) Laboratoire de Minéralogie, Muséum National d'Histoire Naturelle, Paris, France;(2) Laboratoire de Pétrologie, Université de Nancy I, France |
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| 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 NaxSiCaxAlIV.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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