Gabbro Akarem mafic-ultramafic complex, Eastern Desert, Egypt: a Late Precambrian analogue of Alaskan-type complexes |
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Authors: | H M Helmy M M El Mahallawi |
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Institution: | (1) Geology Department, Faculty of Science, Minia University, Minia, Egypt, EG |
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Abstract: | Summary ?Gabbro Akarem is a Late-Precambrian concentrically-zoned mafic-ultramafic 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.
Mafic 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 mafic to the ultramafic
units. Spinel has a wide range of Cr# and Mg# ratios. The various rock units define a fractionation trend. The mafic rocks
are slightly LREE-enriched relative to the ultramafic 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 stratification; 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 mafic magma was
partly crystallized while the unsolidified core ultramafic 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 mafic-ultramafic
intrusions.
Received April 24, 2001; revised version accepted November 20, 2001 |
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