The Neoproterozoic Abu Dahr ophiolite, South Eastern Desert, Egypt: petrological characteristics and tectonomagmatic evolution

Abstract

The Neoproterozoic Abu Dahr ophiolite, South Eastern Desert, Egypt, is one of the best preserved and least dismembered ophiolite successions in the Arabian-Nubian Shield. It contains a Penrose-type ophiolite sequence from mantle section below mafic crust upward to oceanic sedimentary cover overlying mafic volcanics, although the original magmatic (stratigraphic) contact between the mantle and crustal sections is disrupted by tectonism. The Abu Dahr ophiolite is metamorphosed under greenschist facies conditions, and low-temperature alteration is widespread. Petrography reveals that: (i) the mantle is homogenous, serpentinized, and dominated by harzburgite and less abundant dunite; (ii) the cumulate ultramafics are represented by wehrlite and pyroxenite; and (iii) the crustal section is represented by metagabbros, meta-anorthosites and metabasalts. The Abu Dahr serpentinized peridotites show high Mg# (0.92-0.93), with enrichment of Ni, Cr and Co, and depletion of Al 2 O 3 and CaO, and nearly flat and unfractionated REE chondrite-normalized pattern. Major and trace element characteristics of the Abu Dahr metagabbro and metabasalt (crustal section) indicate a tholeiitic to calc-alkaline affinity. Units of the crustal section have low-Nb and Zr concentrations, low Dy/Yb and relatively elevated La/Yb ratios, high U/Yb and Th/Yb ratios, and LREE enriched chondrite-normalized pattern. All of the Abu Dahr ophiolite units have trace-element signatures characterized by enrichment of LILE over HFSE. Rare and trace element patterns indicate a genetic link between the Abu Dahr mantle, cumulate ultramafics, and crust. Chromian spinel has survived metamorphism and is used as a petrogenetic indicator in the Abu Dahr serpentinized peridotites. The spinel is homogeneous with a limited composition, and shows high-Cr# (>0.6) combined with low-TiO 2 character (mostly <0.1 wt.%). The Abu Dahr ophiolite is interpreted as a fragment of depleted oceanic lithosphere that experienced high degrees of partial melting (up to 35 %) and originated in a fore-arc setting. Such interpretation contributes to the body of evidence suggesting that tectonomagmatic processes of the Neoproterozoic were largely similar to those of the Phanerozoic, implying little, or no significant, change in the geothermal regime of Earth since the Neoproterozoic.

Geochemistry and Tectonic Setting of Neoproterozoic Rocks from the Arabian-Nubian Shield: Emphasis on the Eastern Desert of Egypt

“…Figure 7a shows that Um Hassa greywackes of Hammamat molasses sediments have LREE-enriched chondritenormalized patterns similar to post-Archean Australian shale (PAAS) and UCC patterns [71]. Upper continental crust-normalized patterns for the Um Hassa greywackes reveal significant enrichment of Cr (234-434 ppm) and Ni (49-72 ppm) but depletions in Nb (4.1-7.7 ppm), Rb (33-63 ppm), and Th (3.64-8.92 ppm) relative to UCC values (35,20,25,112, and 10.7 ppm, respectively).…”

“…Most Gerf gabbros have REE patterns with a slight LREE enrichment and a small positive Eu anomaly, whereas the Gerf serpentinized peridotites have Large Ion Lithophile Elements (LILE)-depleted patterns. The Abu Dahr metagabbro and metabasalt have enrichment LILE and LREE enrichment, whereas serpentinized harzburgite and dunite are characterized by enrichment of LILE and nearly flat and unfractionated chondrite-normalized pattern indicating they originated by up to $30% partial melting of a spinel lherzolite mantle in a subarc setting [25].…”

“…El Bahariya [19] documents different occurrences of Neoproterozoic ophiolitic melanges in the CED of Egypt and classified the ophiolitic mélanges into: (i) tectonic mélange, (ii) olistostrome, and (iii) olistostromal mélange. Ophiolitic melanges are also recorded in different occurrences in the SED such as Atshan Ophiolite, Gerf, and Abu Dahr [25].…”

“…Generally, most samples of ophiolitic lavas are subalkaline and reveal tholeiitic affinities, together with minor calcalkaline characters, although subordinate, amount of boninites have been identified as in El Sid ophiolite. On the Ti-V tectonic setting discrimination diagram (Figure 3c), generally, ophiolitic metavolcanics and metagabbros of the ED of Egypt fall into two groups: (i) MORB ophiolites and (ii) fore arc or suprasubduction zone (SSZ) ophiolites (e.g., [20,25,39,[40][41][42][43][44][45][46]). The MORB affinity of metagabbros from Muweilih is documented for the first time by El Bahariya [43], and the whole Muweilih ophiolite sequence is mapped and recorded for the first time as MORB intact ophiolite by El Bahariya [20].…”

Suprasubduction-zone origin of the podiform chromitites of the Bir Tuluhah ophiolite, Saudi Arabia, during Neoproterozoic assembly of the Arabian Shield

“…In general, boninite is characteristic of intra-oceanic fore-arcs (e.g., Beccaluva et al, 2004;Dilek et al, 2008), supporting a fore-arc setting for the ultramafic rocks of Bir Umq. All the analyzed fresh Cr-spinel cores have low or negligible Fe 3+ # = Fe 3+ /(Fe 3+ +Cr+Al) < 0.03, which is characteristic of primary mantle-derived spinels (Hattori and Guillot, 2007;Gahlan et al, 2015;Obeid et al, 2016;Bernstein et al, 2013;Azer, 2014).…”