Mineralogy and Petrology 2003) 77: 85±108 DOI 10.1007/s00710-001-0185-9 GabbroAkaremma®c-ultrama®ccomplex, EasternDesert,Egypt:aLatePrecambrian analogueofAlaskan-typecomplexes H.M.Helmy and M.M.ElMahallawi Geology Department, Faculty of Science, Minia University, Minia, Egypt Received April 24, 2001; revised version accepted November 20, 2001 Published online June 20, 2002; # Springer-Verlag 2002 Summary Gabbro Akarem is a Late-Precambrian concentrically-zoned ma®c-ultrama®c intrusion located along a major fracture zone trending NE-SW in the Eastern Desert of Egypt. It intruded low-grade metasedimentary rocks, and has a contact metamorphic aureole a few meters wide. This intrusion comprises a dunite core enveloped by clinopyroxene hornblende-bearing lherzolite, olivine-hornblende clinopyroxenite and plagioclase hornblendite. The contacts between the rock types are gradational. They have cumulate textures and the observed crystallization sequence is: olivine   cotectic spinel)- orthopyroxene Opx)-clinopyroxene Cpx)-hornblende. Ma®c minerals from the core of the intrusion are highly magnesian, a consistent increase in the Mg# of olivine from 69 to 87), Opx from 62 to 89), Cpx from 85 to 96) and hornblends from 62 to 88) is observed from the ma®c to the ultrama®c units. Spinel has a wide range of Cr# and Mg# ratios. The various rock units de®ne a fractionation trend. The ma®c rocks are slightly LREE-enriched relative to the ultrama®c units and chondrites. In many aspects, the Gabbro Akarem intrusion is similar to Alaskan-type complexes. Mineralogical and geochemical data suggest that the different rock units were fractionated from a hydrous picritic magma with no apparent crustal contami- nation. A petrogenetic model involving a rapid rise of hydrous mantle magma along a major fracture zone is proposed. Extensive fractional crystallization led to magma chamber strati®cation; internal circulation and strong vertical stretching up the center of the rapidly rising diapir increased the rate of magma ascent towards the core. Due to cooling and high viscosity the marginal ma®c magma was partly crystallized while the unsolidi®ed core ultrama®c magma continued its ascent. As a result, different mineral phases crystallized at different pressure-temperature paths. Field relations, geophysical, petrological and experimental studies support this model which explains many of the characteristics of the Gabbro Akarem and some other concentrically zoned ma®c- ultrama®c intrusions.