HR: 14:00h
AN: V23B-01    [Abstracts]
TI: Nature of Phyllosilicates and Magma Types in the Plutonic Complexes of NE Egypt
AU: * Abdel-Rahman, A
EM: arahman@aub.edu.lb
AF: American University of Beirut, Department of Geology, Bliss Street, P.O.Box 11-0236, Beirut, , Lebanon
AB: The Late Proterozoic plutonic complexes of northeastern Egypt belong to a number of magma types of variable tectonic regimes, and exhibit a variety of phyllosilicate minerals. The results show that the phyllosilicates define three compositionally distinct groups. Phyllosilicates 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). Such Fe-enriched, Al-depleted mica compositions suggest that the substitutions Fe³⁺ = Al, 3Fe²⁺ = 2Al, Fe²⁺ = Mg and (Fe²⁺ + Si) = 2Al are vital in producing annitic compositions in the peralkaline granites. In sharp contrast, phyllosilicates 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 Tschermaks substitution (Fe²⁺, Mg) + Si = 2Al are vital in producing siderophyllitic compositions in the trondhjemite pluton. The substitution 2Al = 3Mg (or Fe²⁺) also plays a role when referring to a possible solid solution between phlogopite (or annite) end-member and muscovite end-member. This pluton also contains late- to post-magmatic muscovite. Phyllosilicates 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, and belong to type C-biotite (referring to biotite in calcic and calc- alkaline, mostly I-type suites). The nature of micas and the magmatic stages of their development were controlled 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.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3620 Mineral and crystal chemistry (1042)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly