Petrology and Sm–Nd dating of the Genina Gharbia Alaskan-type complex (Egypt): Insights into deep levels of Neoproterozoic island arcs

• The Genina Gharbia Alaskan-type complex crystallized at the crust–mantle boundary
• Cumulus mineral–melt interaction and the evolution of hydrous magma
• A Neoproterozoic island arc lasted slightly less than 100 Ma to build-up.
• Comparison between Archean, Neoproterozoic and Phanerozoic island arcs

The deep levels of Neoproterozoic island arcs are poorly known due to limited accessibility. The Genina Gharbia Alaskan-type complex (south Eastern Desert, Egypt) is the remains of a magma chamber that crystallized at the base (crust–mantle boundary) of a mature Neoproterozoic island arc. The rock assemblage comprises hornblende-bearing harzburgite, lherzolite, pyroxenite, norite and gabbro. All lithologies show cumulus texture with evidence of extensive cumulus mineral–melt interactions. Clinopyroxenes from all lithologies have similar rare earth element (REE) patterns with slight medium-rare earth element (MREE) enrichment. Hornblendes are slightly enriched in MREE and light rare earth elements (LREE). Island arc signatures are indicated by high contents of large ion-lithophile elements and low concentration of high field-strength elements. Positive initial εNd (+ 5.7 to + 7.0) and Nd model ages (963 ± 81 Ma) are consistent with the Genina Gharbia magma being extracted from a depleted mantle source. Modeling of estimated parental magma indicates 10% partial melting of a 90% depleted mantle source with a 10% (MORB + sediments)-derived fluid, commencing in the spinel stability field (< 85 km). Relative to Phanerozoic arcs, the Neoproterozoic arcs were more hydrous, had low oxidation states and probably lasted shorter time to build-up. The hydrous nature of the sub-Arabian–Nubian Shield mantle and the long-life of the arcs are among reasons responsible for the vast crustal growth during the Pan-African Orogeny throughout the Gondwana.

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