Petrological studies of neoproterozoic serpentinized ultramafics of the nubian shield: spinel compositions as evidence of the tectonic evolution of egyptian ophiolites

Abstract

The mafic-ultramafic rocks of the Gabal El-Degheimi area, Central Eastern Desert of Egypt, are parts of an ophiolitic section. The ophiolitic rocks are dismembered and tectonically enclosed within, or thrust over, island arc assemblages. Serpentinites, altered slices of the upper mantle, represent a distinctive lithology of the dismembered ophiolites. Some portions of the serpentinized rocks contain fresh relicts of primary minerals such as chromian spinel and olivine. The abundance of bastite and mesh textures suggests harzburgite and dunite protoliths, respectively, for these serpentinites. Some fresh cores of chromian spinel are rimmed by ferritchromite and Cr-magnetite. The development of alteration rims around chromian spinel cores indicates their formation during prograde alteration and under oxidizing conditions during lower amphibolite facies metamorphism. Fresh chromian spinels are characterized by high contents of Cr2O3 (48.92-56.74 wt. %), Al2O3 (10.29-20.08wt. %), FeO (16.24-28.46 wt. %) and MgO (4.89-14.02 wt. %), and very low TiO2 contents (<0.16 wt. %). The analyzed fresh chromian spinels have high Cr# (0.62-0.79) characteristic of spinels in mantle peridotite that has undergone some degree of partial melting. The data presented here suggest that the mantle peridotites of the Gabal El-Degheimi area are similar to forearc peridotites of suprasubduction zone environments.

Mineralogy and chemical aspects of some ophiolitic metaultramafics, central Eastern Desert, Egypt: Evidences from chromites, sulphides and gangues

“…The aluminium chromites of the studied three areas display podiform composition when plotted on Cr 2 O 3 versus Al 2 O 3 (Bonavia, Diella, & Ferrario, ). This is comparable to ophiolitic chromitites from the central Eastern Desert of Egypt (Abdel‐Karim et al, ; Ahmed et al, ; Azer, ) (Figure c).…”

“…(Bonavia, Diella, & Ferrario, 1993). This is comparable to ophiolitic chromitites from the central Eastern Desert of Egypt (Abdel-Karim et al, 2014;Ahmed et al, 2001;Azer, 2014) ( Figure 5c).…”

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).…”

“…The most accepted mechanism for the formation of podiform chromitite is melt-peridotite interaction and subsequent mixing (e.g. González-Jiménez et al, 2011, 2014Arai and Miura, 2016;Khedr and Arai, 2017;Azer et al, 2019). Through this mechanism, the formation of chromitite pods begins with interaction between ascending melt and overlying refractory mantle harzburgite through which the melt percolates in large conduits.…”

Alteration of mélange-hosted chromitites from Korydallos, Pindos ophiolite complex, Greece: evidence for modification by a residual high-T post-magmatic fluid

“…It is commonly referred to as a petrogenetic indicator since its chemistry is believed to be strongly dependent on the degree of mantle melting and the compositional signature of the partial melt generated (e.g., Dick and Bullen 1984;Proenza et al 1999;Hellebrand et al 2001;Kamenetsky et al 2001;Zhou et al 2005;Ahmed et al 2012;Azer 2014;Xiong et al 2014). Due to its refractory nature it is highly resistant to long-standing post-magmatic processes, such as hydrothermal alteration, and to metamorphism.…”