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The late Neoproterozoic Dahanib mafic-ultramafic intrusion, South eastern Desert, Egypt: Is it an Alaskan-type or a layered intrusion?

Azer, Mokhles K. and Gahlan, Hisham A. and Asimow, Paul D. and Al-Kahtany, Khaled M. (2017) The late Neoproterozoic Dahanib mafic-ultramafic intrusion, South eastern Desert, Egypt: Is it an Alaskan-type or a layered intrusion? American Journal of Science, 317 (8). pp. 901-940. ISSN 0002-9599.

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In Egypt, mafic-ultramafic complexes have been classified into three major types: incomplete ophiolite sequences; Alaskan-type intrusions, concentrically-zoned bodies formed in a subduction arc environment; and layered intrusions, vertically-zoned bodies intruded in post-collisional tectonic environments and rift-related bodies associated with the opening of the Red Sea. We present new field work, geochemical data, mineral chemistry and interpretations for the late Neoproterozoic Dahanib mafic-ultramafic intrusion in the South Eastern Desert of Egypt (northernmost Arabian–Nubian Shield, ANS). The Dahanib intrusion shows no evidence of metamorphism or deformation, with excellent preservation of intrusive contacts, well-preserved textures and primary mineralogy. Field relations indicate that it is younger than the surrounding metamorphic rocks and syn-tectonic granitoids. The intrusion is composed of a basal suite of ultramafic rocks (dunite, lherzolite, wehrlite and pyroxenite) and an overlying suite of mafic rocks (olivine gabbronorite, gabbronorite and anorthosite). It displays evident layering of modal abundance, visible directly in outcrop, as well as cryptic layering discernible through changes in mineral compositions. The western and eastern lobes of the Dahanib intrusion occur in the form of a lopolith with readily correlated layers, especially in the upper mafic unit. The present-day dip of the layering decreases from the ultramafic units into the mafic sequence. Structural and compositional relations show that the ultramafic units are cumulates from a high-Mg tholeiitic parent magma emplaced at deep crustal levels and evolved via fractional crystallization rather than any kind of residual mantle sequence. Fo content of olivine and Mg# of pyroxenes display a systematic decrease from ultramafic to mafic rocks, well-correlated with whole-rock Mg#. Spinels in ultramafic samples vary from Cr-rich to Al-rich and have Mg# and Fe^(3+)# similar to spinels from typical stratiform complexes and clearly different from those found in ophiolitic and Alaskan-type complexes. Although the mafic and ultramafic units are clearly related and can be derived from common parent magma, they were not emplaced coevally; rather, they represent different pulses of magma. The Dahanib mafic–ultramafic intrusion does not display any features that convincingly identify it as a typical Alaskan-type body, particularly the lack of clinopyroxenite and hornblendite, rarity of primary hornblende, and the notable abundance of orthopyroxene and plagioclase in its rocks. Our results confirm that it is more akin to a layered mafic-ultramafic intrusion with a multistage evolution. It was emplaced into a stable post-orogenic cratonic setting, with a trace element signature indicating contamination of the mantle source by previous subduction events.

Item Type:Article
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URLURL TypeDescription DOIArticle
Azer, Mokhles K.0000-0002-3711-8630
Asimow, Paul D.0000-0001-6025-8925
Additional Information:© 2017 by American Journal of Science. We are indebted to Geological Science Department, National Research Centre, Egypt for their support. Special thanks are paid to King Saud University, Deanship of Scientific Research, Research Group No. RG-1436-036 for their support. PDA is supported in part by the US National Science Foundation geoinformatics program, award number EAR-1550934. MKA's Post-doctoral mission to the Division of Geological and Planetary Sciences, California Institute of Technology (Caltech), USA, was supported by the Cairo Initiative of the US Agency for International Development. MKA is indebted to Professor George Rossman and Dr. Michael Baker, California Institute of Technology, for their kind assistance during his post-doctoral mission in Caltech. Special thanks to Dr. Chi Ma for his help with the microprobe analyses.
Funding AgencyGrant Number
National Research Centre (Egypt)UNSPECIFIED
King Saud UniversityRG-1436-036
U.S. Agency for International DevelopmentUNSPECIFIED
Subject Keywords:Neoproterozoic, Arabian-Nubian Shield, Eastern Desert, layered intrusion, post-collision
Issue or Number:8
Record Number:CaltechAUTHORS:20171102-083956298
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Official Citation:Mokhles K. Azer, Hisham A. Gahlan, Paul D. Asimow, and Khaled M. Al-Kahtany The late Neoproterozoic Dahanib mafic-ultramafic intrusion, South eastern Desert, Egypt: Is it an Alaskan-type or a layered intrusion? Am J Sci October 2017 317:901-940; doi:10.2475/08.2017.02
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:82875
Deposited By: Tony Diaz
Deposited On:02 Nov 2017 17:04
Last Modified:09 Mar 2020 13:18

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