Gebel Dahanib, Egypt: A Late Precambrian Layered Sill of Komatiitic Composition

Gebel Dahanib is a well preserved, late Precambrian layered mafic-ultramafic sill about 1.5 km thick, forming a prominent peak in the Southeastern Desert of Egypt. Dunites, harzburgites and thin chromite layers at the base grade upward through pyroxenites to layered gabbros and rare anorthosite at the top. Peridotitic dykes from the main body, unsheared margins, and ubiquitous cumulate textures all suggest magmatic rather than tectonic emplacement.

Olivine ranges from Fo ₉₄ to Fo ₇₆ , orthopyroxene from En ₉₀ to En ₇₉ , and abundant clinopyroxene is typically low Cr diopside or diopsidic augite. Plagioclase is exceptionally calcic (An ₉₄ -An ₈₃ ) and typically is 10% more calcic at a given coexisting olivine composition than plagioclase in the Skaergaard and Stillwater intrusions. Assimilation of the country rock is common along the borders, and chilled margins are not developed. The bulk composition of the sill, estimated from whole rock analyses weighted according to outcrop areas, is SiO ₂ =50.1%, A1 ₂ O ₃ = 8.9%, MgO=19.9%, CaO=12.1%, K ₂ O=0.06%, TiO ₂ =0.2%. REE contents in the original magma were less than four times chondritic abundances. The chemical and mineralogical characteristics of the Dahanib sill suggest comparisons with basaltic or pyroxenitic komatiites.