Research Abstract
Podiform chromitites hosted in serpentinites (after harzburgite and dunite) and
talc-carbonate rocks from the Abu Meriewa–Hagar Dungash district (MHD), Eastern
Desert of Egypt, together with metagabbros, pillow metavolcanics, and metasediments,
form an ophiolitic mélange formed during the Neoproterozoic Pan-African Orogeny. The
chromitites show massive, disseminated, and nodular textures. Chromite cores in chromitites
have high and restricted ranges of Cr# (0.65–0.75) and Mg# (0.64–0.83), implying
primary compositions not affected by metamorphism. Therefore, they are used as reliable
indicators of parent magma composition and tectonic affinities of these highly metamorphosed
rocks. On the contrary, the altered rims are high-Cr, low-Fe3+ spinel (rather than
ferritchromit) enriched in Cr, Fe, and Mn, and depleted in Al and Mg (Cr# = 0.75–0.97,
Mg# = 0.29–0.79), due to equilibration with interstitial silicates during regional metamorphism
up to transitional greenschist–amphibolite facies at about 500–550°C. The primary
chromite compositions suggest derivation from a high-Mg tholeiitic, to possibly boninitic,
parental magma in a supra-subduction zone (arc–marginal basin) environment, similar to
the spatially associated metavolcanic rocks. The MHD chromitites are most probably
formed by melt–rock interaction mechanisms. The high Cr# of the investigated chromites
suggests high degrees of partial melting of a depleted harzburgite source by interaction
with primitive basaltic melt of deeper origin followed by mixing. Such Cr-rich chromites
are common in chromitites from the Eastern Desert of Egypt, implying broad thermal
anomalies, possibly linked to an important geodynamic feature of the Arabian–Nubian
Shield (ANS) evolution. This could revive interest in models that involve asthenospheric
uprise, related to plume interaction or most probably due to oblique convergence of arc
terranes during early evolution of the ANS.
