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| Titel |
Mineral chemistry and geochemistry of the Late Neoproterozoic Gabal Abu Diab granitoids, Central Eastern Dessert, Egypt: Implications for the origin of rare metal post-orogenic A-type granites |
| VerfasserIn |
Mabrouk Sami, Theodoros Ntaflos, Esam S. Farahat, Awaad F. Ahmed, Haroun A. Mohamed |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104070
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| Publikation (Nr.) |
 EGU/EGU2015-5393-3.pdf |
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| Zusammenfassung |
| The Neoproterozoic Gabal Abu Diab pluton is a part of the Arabian Nubian shield (ANS)
continental crust and located in the Central Eastern Desert (CED) of Egypt. It constitutes
multiphase granitic pluton intruded into granodiorite and metagabbro-diorite rocks with sharp
and nonreactive contacts. Based on field observations, colors, structural variations and
petrographic investigations, this granitic outcrop consists of an inner core of two-mica
granite (TMG) followed outward by garnet bearing muscovite granite (GBMG) and
albite granite (AG). Petrographical study indicated that medium to coarse-grained
TMG is dominated by K-feldspar (Or88-98), quartz, plagioclase (albite, An0-7),
muscovite and biotite with hypidiomorphic texture. With exception the appearance of
garnet and the disappearance of biotite the GBMG resembles the TGM, while AG is
leucocratic without any mafic mineral. The main accessories are zircon, Nb and
Ta-bearing rutile, columbite, ilmenorutile, ilmenite, magnetite and apatite. This
mineralogical similarity and the existence of columbite group minerals (CGM) in
all granitoids, indicates a cogenetic relationship. Microprobe analyses reveal that,
besides the CGM, rutile and ilmenite are the main repository phases for Nb-Ta-Ti.
Columbite-(Mn) exists as individual subhedral crystals (up to 100μm in size) or intimate
intergrowth with Nb-bearing rutile and/or ilmenite. The CGM are represented mostly by
columbite-(Mn) with Ta/(Ta+Nb) and Mn/(Mn+Fe) ratio ranging from 0.02-0.08 and 0.4-0.9,
respectively suggesting extreme degree of magmatic fractionation. Rutile contains
significant amounts of Ta (up to 4 wt.% Ta2O5) and Nb (up to 22 wt.% Nb2O5).
Biotites are phlogopite-annite in composition (Ann47-60Phlog40-53,on average) and
are enriched with AlIV that characterize peraluminous granites. Garnets contain
60-69 mol.% spessartine and 28–36 mol.% almandine where, the ratio of spessartine
and almandine together exceeds 95 mole percent, similar to garnet occur within
A-type granite worldwide. According to Zhang et al., 2012, the garnet crystallized
at the expense of biotite from the MnO-rich evolved melt after fractionation of
biotite, plagioclase, K-feldspar, zircon, apatite, and ilmenite. The granitoids are alkali
feldspar granites showing distinct geochemical features and most likely, belong to
the post-orogenic younger Egyptian granitoids. They are peraluminous A-type
alkaline rocks but they have lower Fe2O3, MgO, MnO, CaO, TiO2, P2O5, Sr, Ba, V,
and higher SiO2, Na2O, K2O, Nb, Ta, U, Zr, Th, Ga/Al and Rb than the typical
rocks of this type. The positive correlation between Ba and Sr, and the negative
correlation between Rb and K/Rb reveal fractional crystallization of alkali feldspar. The
similarity in most geochemical characteristics suggests that Abu Diab granitoids are
genetically related to each other and extremely enrichment in incompatible elements
such as Nb and Ta, indicating that they crystallized from extremely differentiated
magmas.
References:
Zhang, J., Ma, C. and She, Z., 2012. An Early Cretaceous garnet-bearing metaluminous
A-type granite intrusion in the East Qinling Orogen, central China: Petrological,
mineralogical and geochemical constraints. Geoscience Frontiers 3 (5), 635-646. |
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