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| Titel |
Mineralogy and geochemistry of the Late Neoproterozoic rare metal granitoids of Gabal El-Ineigi pluton, Northern Arabian-Nubian Shield, Egypt |
| VerfasserIn |
Mabrouk Sami, Theodoros Ntaflos, Haroun A. Mohamed, Esam S. Farahat, Awaad F. Ahmed |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250123770
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| Publikation (Nr.) |
 EGU/EGU2016-3075.pdf |
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| Zusammenfassung |
| Gabal El-Ineigi granitoid pluton is situated in the Central Eastern Desert of Egypt
and is considered as one of the good examples of the fluorite bearing rare metal
granites in the Arabian Nubian Shield (ANS). It constitutes a multiphase pluton
consists of porphyritic syenogranites (SG) and coarse to medium grained highly
evolved alkali-feldspar granites (AFG) intruded into the older granodiorites and
metagabbro-diorite rocks. Petrographic features indicate that quartz, K-feldspar
(perthite, Or97−99), plagioclase (albite, An0−6) and biotite are the major mineral
phases of both granitic types with subordinate muscovite that is observed only in the
AFG. Columbite, rutile, fluorite, zircon and thorite are the significant accessory
minerals in the AFG while, allanite is exclusively encountered in the SG. Mineral
chemistry study reveals that Nb-Ta-Ti-bearing oxides [columbite-group minerals
(CGM)] and Nb-bearing oxides (ilmenorutile) represent the most common Nb-Ta host
in the AFG. The CGM are represented mostly by complex zoned columbite-(Fe)
and rarely by yttrocolumbite-(Y), with Mn/(Mn+Fe) ratios ranging from 0.17 to
0.31. Xenomorphic fluorite (F=46-51 wt%) is commonly filling the spaces between
the major mineral phases and sometimes host rare metal minerals, e.g. columbite
and thorite. Euhedral zoned allanite (Ce-Nd) is the common REE bearing mineral
encountered in the SG. Geochemically, Gabal El-Ineigi granitoids are metaluminous
(A/CNK= 0.95-0.99) related to post-collosional A2-type granites. The late phase
AFG have distinctive geochemical features typical of rare-metal granites. They are
highly fractionated calc-alkaline granitoids characterized by high Rb, Nb, Y, U and
many HFSE contents, and extremely low Sr and Ba contents (4-35 and 13-18 ppm,
respectively). Moreover, their REE patterns show pronounced negative Eu anomalies
(Eu/Eu*=0.03 - 0.05) and tetrad effect (TE1,3=1.16 and 1.42), implying extensive
fractionation via fluid-rock interaction that characterize the late magmatic stage
differentiation. On the other hand, the SG are remarkably enriched in Sr and Ba (58-76
and 299-422 ppm, respectively) and show a relatively invariable enrichment in
light rare earth elements (LREEs). The extremely high 87Sr/86Sr (1.7491-2.5376)
ratios of the AFG is almost related to the disturbance in the Rb-Sr isotope system at
the proposed high temperature magma-fluid interaction. On the other hand, the
Sr-Nd isotope systematics of the SG are characterized by a high initial 87Sr/86Sr
ratios of 0.7087-0.7109 and ɛNd(t) values of +4.7 to +5.8, with depleted mantle
model ages (tDM) between 0.82 and 0.98 Ga, implying a juvenile magma source of
Neoproterozoic age, typical of other granitoids from the ANS continental crust. |
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