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Magma types and nature of micas in the plutonic complexes of northeastern Egypt

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The Ras Gharib Late Proterozoic to Early Paleozoic spectrum of plutonic complexes (Egypt) exhibit a variety of mica minerals. Within these complexes, three compositionally distinct mica groups are identified. Micas in the anorogenic peralkaline granites, which were developed from a hypersolvus felsic melt, are restricted to iron-rich siliceous annites and belong to type "A"-biotite (referring to biotite in anorogenic alkaline A-type suites). The substitutions Fe3+ Al, 3Fe2+ 2Al, Fe2+ Mg and (Fe 2+ + Si) 2Al are vital in producing these annitic compositions. In sharp contrast, biotites in the trondhjemitic pluton that crystallized from a peraluminous felsic melt, are siderophyllitic in composition and belong to type "P"-biotite (referring to biotite in peraluminous, including S-type suites). The substitutions Fe2+ Mg, along with the Tschermak's substitution [(Fe2+, Mg) + Si 2Al] are vital in producing siderophyllitic compositions. The substitution 2Al 3Mg (or Fe2+) also plays a role. The trondhjemite pluton contains late- to post-magmatic muscovite. Micas in the diorite-tonalite and the voluminous granodiorite-adamellite orogenic complexes (crystallized from calcic to calc-alkaline subsolvus magmas), cover a wide range of composition, are moderately-enriched in Mg and belong to type "C"-biotite (referring to biotite in calcic and calc-alkaline, mostly I-type suites). The nature of biotite and the magmatic stage of mica development were controlled in part by the various physiochemical conditions (including the behavior of volatiles) prevailed during the crystallization of these hypersolvus, dry, anorogenic magmas versus subsolvus, wet, orogenic magmatic systems.


Document Type: Research Article


Publication date: 2007-02-01

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