Electronic Resource
Springer
Contributions to mineralogy and petrology
35 (1972), S. 215-225
ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00371216
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