Journal: |
Gondwana Research
ELSVIER
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Volume: |
82
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Abstract: |
The Neoproterozoic Korab Kansi mafic-ultramafic intrusion is one of the largest (100 km2) intrusions in the
Southern Eastern Desert of Egypt. The intrusion consists of Fe-Ti-bearing dunite layers, amphibole peridotites,
pyroxenites, troctolites, olivine gabbros, gabbronorites, pyroxene gabbros and pyroxene-hornblende gabbros,
and also hosts significant Fe-Ti deposits, mainly as titanomagnetite-ilmenite. These lithologies show rhythmic
layers and intrusive contacts against the surrounding granites and ophiolitic-island arc assemblages. The wide
ranges of olivine forsterite contents (Fo67.9-85.7), clinopyroxene Mg# (0.57–0.95), amphibole Mg# (0.47–0.88),
and plagioclase compositions (An85.8-40.9) indicate the role of fractional crystallization in the evolution from ultramafic
tomafic rock types. Clinopyroxene (Cpx) has high REE contents (2–30 times chondrite) with depleted
LREE relative to HREE, like those crystallized from ferropicriticmelts generated in an island-arc setting. Melts in
equilibrium with Cpx also resemble ferropicrites crystallized from olivine-rich mantle melts. Cpx chemistry and
its host rock compositions have affinities to tholeiitic and calc-alkaline magma types. Compositions of maficultramafic
rocks are depleted in HFSE (e.g. Nb, Ta, Zr, Th and U) relative to LILE (e.g. Li, Rb, Ba, Pb and Sr) due
to the addition of subduction-related hydrous fluids (rich in LILE) to the mantle source, suggesting an islandarc
setting. Fine-grained olivine gabbrosmay represent quenchedmelts approximating the primarymagma compositions
because they are typically similar in assemblage and chemistry as well as in whole-rock chemistry to
ferropicrites. We suggest that the Korab Kansi intrusion crystallized at temperatures ranging from ~700 to
1100 °C from ferropicritic magma derived from melting of metasomatized mantle at b5 Kbar. These hydrous
ferropicritic melts were generated in the deep mantle and evolved by fractional crystallization under high ƒO2
at relatively shallow depth. Fractionation formed calc-alkaline magmas during the maturation of an island arc
system, reflecting the role of subduction-related fluids. The interaction of metasomatized lithosphere with upwelling
asthenosphericmelts produced the Fe and Ti-rich ferropicritic parentalmelts that are responsible for precipitating
large quantities of Fe-Ti oxide layers in the Korab Kansi mafic-ultramafic intrusion. The other factors
controlling these economic Fe-Ti deposits beside parentalmelts are high oxygen fugacity, water content and increasing
degrees ofmantle partial melting. The generation of Ti-rich melts and formation of Fe-Ti deposits in few
layered intrusions in Egypt possibly reflect the Neoproterozoic mantle heterogeneity in the Nubian Shield. We
suggest that Cryogenian-Tonian mafic intrusions in SE Egypt can be subdivided into Alaskan-type intrusions
that are enriched in PGEs whereas Korab Kansi-type layered intrusions are enriched in Fe-Ti-V deposits.
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