Geology Department, Faculty of Science, Minia University, Minia, Egypt. Received ... southern part of the Eastern Desert of Egypt, e.g, Gabbro Akarem (Carter,.
Mineralogy and Petrology (2003) 77: 85±108 DOI 10.1007/s00710-001-0185-9
Gabbro Akarem ma®c-ultrama®c complex, Eastern Desert, Egypt: a Late Precambrian analogue of Alaskan-type complexes H. M. Helmy and M. M. El Mahallawi 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 contamination. 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®cultrama®c intrusions.
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Introduction Alaskan-type zoned ma®c-ultrama®c complexes are characterized by a concentric arrangement of rock types, i.e., dunite core enveloped by olivine clinopyroxenite, magnetite hornblende clinopyroxenite and hornblende gabbro. Such complexes have been described from Alaska, the Urals of Russia, Australia, Canada, Colombia, Venezuela and the United States (Taylor, 1967; Snoke et al., 1981; Nixon et al., 1990; Kepezhinskas et al., 1993; Tistl et al., 1994; Fershtater et al., 1997). Alaskan-type complexes are small in size, elliptical or rounded in shape and located along lineaments that in some cases are more than 1000 km long (e.g., Alaska, Urals, and Colombia-Ecuador). In addition to these geologic features, these complexes share many petrologic and structural characteristics, such as the dominance of olivine, clinopyroxene, hornblende and the scarcity of orthopyroxene and plagioclase. Although different parent magma compositions are suggested for different intrusions, there is a general belief that all magmas are of mantle origin with no signi®cant crustal contamination. Distinctive internal structures include unconformities, troughs, modal layering and syn-depositional faults. Several concentrically zoned ma®c-ultrama®c complexes are known in the southern part of the Eastern Desert of Egypt, e.g, Gabbro Akarem (Carter, 1975), Abu Hammamid (Hafez et al., 1991) and Genina Gharbia. These complexes form small elliptical bodies (3.5±11 km long and 0.5±2.5 km wide) and are located (Fig. 1A) along major NE-SW trending fracture zones in the Eastern Desert (Garson and Shalaby, 1976). Two of these intrusions (Genina Gharbia and Gabbro Akarem) host sub-economic Cu±Ni-PGE mineralization (Helmy et al., 1995; Sharara et al., 1999; Helmy and Mogessie, 2001). Detailed information concerning the petrogenesis of these concentrically zoned ma®c-ultrama®c complexes is essential for development and application of successful exploration strategies. The objectives of the present paper are: (1) to document the geology, petrology and geochemistry of the Gabbro Akarem ma®cultrama®c complex, (2) to compare the different geological, petrological and geochemical characteristics of the complex with other Alaskan-type complexes, and (3) to build a genetic model of the intrusion. Geological setting The Gabbro Akarem intrusion is located 130 km east of Aswan in the South Eastern Desert (Lat. 24 010 N, Long. 34 080 ), Egypt (Fig. 1A). This area is a part of the Proterozoic Shield comprising metasediments, metavolcanics, granitoids and ultrama®c rocks. A review of the regional geology of the Nubian shield is given in Helmy and Mogessie (2001). The most important structural features developed during the Late Precambrian in the Eastern Desert are ENE deep fracture zones and NW-SE fault zones (Najd Fault System). The ENE-trending deep fracture zones are prominent geologic features in the south Eastern Desert extending for more than 100 km. Garson and Shalaby (1976) believe that the ma®c-ultrama®c rocks were intruded along these ENE-trending structures. The Najd Fault System is a younger complex set of intracontinental strike-slip faults (Shackleton, 1994); it is well developed in the north Eastern Desert and was formed at 620±540 Ma (Stern, 1985).
Gabbro Akarem ma®c-ultrama®c complex
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Fig. 1. A Location map of the concentrically-zoned intrusions in the Eastern Desert of Egypt. Transverse tectonic fractures from Garson and Shalaby (1976). B Geological map of the Gabbro Akarem area, Eastern Desert, Egypt (Carter, 1975)
The Gabbro Akarem ma®c-ultrama®c intrusion consists of an eastern and a western body, which are 1.5 km apart. They intruded meta-semipelites and quartz diorite-granodiorite complex (Fig. 1B). The meta-semipelites are represented by various types of schists and gneisses. Foliation of the schists and gneisses strikes ENE and the dip steepens from low angle to vertical approaching the southern contact of the ma®c-ultrama®c body. These rocks are frequently garnetiferous
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within 50 m of the intrusion. Plagioclase hornblendite and pyroxenite dikes from the ma®c-ultrama®c body locally intrude into the gneisses. The contact between the ma®c-ultrama®c intrusion and the older meta-semipelites is sharp as indicated by the ®eld observations and geophysical data. The Gabbro Akarem intrusion comprises olivine-plagioclase hornblendite, plagioclase hornblendite and peridotite. Plagioclase hornblendite is the major rock type with signi®cant amounts of olivine-norite and peridotite (14%). Hornblendite veins commonly develop close to the margin of the intrusion. The contact between the plagioclase hornblendite and the rocks in the core is sometimes gradational with development of olivine-rich rocks (olivine-plagioclase hornblendite) in contact with the peridotite core. Sul®de-bearing dunite (dunite pipes) which forms the center of the peridotite masses is enveloped by sul®de-poor lherzolite. Boulders and rafts from the ma®c rocks are commonly encountered in the peridotites. Quartz diorite-granodiorite forms low relief masses. Field and petrographic observations suggest that this association is part of the calc-alkaline (G2) granites. Diorite-granodiorite dikes are observed on the surface and in drill cores intersecting the plagioclase hornblendite. Similar quartz diorite-granodiorite rocks from the Dahanib Alaskan-type complex, 80 km to the southeast of Gabbro Akarem (El Gaby, person. communication, 1999) were emplaced at about 710 Ma (Dixon, 1981). Both the gneisses and the ma®c-ultrama®c intrusion were highly dissected by faults that trend roughly NW (Fig. 1B). Petrography Ma®c rocks Plagioclase hornblendite: consists mainly of hornblende, plagioclase, orthopyroxene with minor olivine, biotite and sul®des. Hornblende occurs either as small size (0.7 cm) crystals de®ning small-scale layering. Plagioclase occurs as small crystals (3 mm) locally included in clinopyroxene. Euhedral crystals of spinel are commonly included in olivine. Clinopyroxene and orthopyroxene form cumulus crystals. Exsolution intergrowths are commonly present in the large pyroxene crystals. Pargasitic hornblende crystals occur in equilibrium with the olivine-spinel-enstatite-diopside assemblage. Olivine-hornblende clinopyroxenite: Diopside forms large (3±4 mm) subhedral to anhedral cumulus crystals. Exsolution lamellae parallel to (100) are characteristic of clinopyroxene. Olivine forms large (2±3 mm) subhedral crystals with characteristic undulose extinction. Magnesio- and pargasitic hornblende form large euhedral crystals in equilibrium with the olivine-clinopyroxene-spinel assemblage (Fig. 2). Orthopyroxene forms discrete crystals showing thin exsolution lamellae parallel to (100). Hornblende-plagioclase pyroxenite: This rock consists of olivine, clino- and orthopyroxenes, hornblende and plagioclase. Olivine occurs as large (>5 mm) porphyritic and medium-grained (
