Abstract
The concentrically zoned Dahanib Alaskan-type complex in the Southern Eastern Desert (SED) of Egypt contains cumulate dunites associated with banded chromitites in the core, through harzburgites, wehrlites, and gabbroic rocks to the rim. Primary mineral inclusions hosted in chromian spinels of the banded chromitites are usually pargasite, orthopyroxene (Opx), aspidolite, phlogopite, edenite, olivine and PGE-bearing sulphides. The chromian spinel in Dahanib chromitites has a narrow range of Cr# (0.53–0.63) along with high Ti and Fe+3, like that in the host clinopyroxene-rich cumulate dunites. It resembles in chemistry to spinels in the Alaskan-type complex formed in an island arc setting. The early cumulus olivine in disseminated chromitites and host dunites (Fo88-91) shows a typical fractional crystallization trend. The interstitial clinopyroxene (Cpx) in chromitites is similar in chemical properties to Cpx crystallized from tholeiitic basalt melts, and traces a cumulate trend of arc-related Alaskan complexes. The peridotitic and gabbroic rocks contain primary amphiboles, which are pargasite and edenite with high Ti, Na and Al. The sulphide minerals are usually chalcopyrite, pentlandite and horomanite with subordinate Cu-pentlandite, cubanite, samaniite, heazlewoodite and pyrite. Pentlandite, horomanite and cubanite at temperatures between 900 and 1040 °C are rich in some platinum group elements (PGE: Os, Pt, Ir). They are classified into primary and secondary origins, which were crystallized from segregated sulphide droplets and from hydrothermal fluids at temperature < 760 °C during postmagmatic stage, respectively. The Dahanib banded chromitites and their mineral inclusions crystallized from hybrid melts, consisting of hydrous tholeiitic basalt melts mixed with slab-derived melts/fluids. They possibly formed by gravitational settling of semi-consolidated crystal mushes in a sub-arc lower-crustal magma chamber. Chemistry of primary minerals suggests that the Dahanib subarc deep-crustal magma is rich in Mg, Cr, Al, Ti, V, Ca, Sc and Na during the early magmatic stage, followed by increasing Fe, Mn, Ni, Co, Cu, S, Cs, B, Sr, and Pb with a progress of fractional crystallization.
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