Search

A suite of Neoproterozoic island arc volcanic and plutonic rocks are exposed in the Hamash area of the south Eastern Desert of Egypt. The volcanic assemblage includes metabasalt, a group of andesites with subordinate basalts (the Hamash Volcanics), and felsic volcanic porphyry. The plutonic rocks include a diorite-tonalite series, fine-grained granite, and granite porphyry. The volcanic rocks are generally sub-alkaline and feature a strong depletion in Nb and Ta. The metabasalt belongs to the low-K tholeiitic series and has nearly flat rare-earth element patterns. Its compositional characteristics indicate generation by partial melting of a depleted mantle source in an immature oceanic island arc setting. On the other hand, the basalt, andesite and felsic volcanic porphyry have low- to medium-K calc-alkaline affinity and are enriched in light rare-earth elements and large ion lithophile elements. Their parental magmas likely formed by fluid-assisted partial melting of a less depleted mantle source with major subduction input, in a mature island arc setting. The andesite cannot be derived from the basalt by fractional crystallization; their parental magmas likely represent different degrees of melting underneath crust of different thickness. Estimation of the arc crustal thickness during formation of these rocks yields ~ 5 km for the metabasalt, ~20 km for the Hamash basalt, and ~ 30 km for the Hamash andesite, consistent with a progression in maturity of the arc over time. The geochemical characteristics of the diorite-tonalite series suggest that it represents an intrusive equivalent of the medium-K calc-alkaline Hamash andesite, and the fine-grained granite represents residual liquid after continued fractionation of the parental magma of the andesite and diorite-tonalite. We present thermodynamic fractionation models that constrain the conditions of fractionation for the andesite, diorite-tonalite, and fine-grained granite suite. The compositional similarity of the granite and rhyolite porphyries, especially their SiO2 and Al2O3 contents and Y/Nb ratios, indicate generation by partial melting of similar crustal sources, although greater assimilation of upper crust is evident in elevated Rb/Sr ratios and K2O and light rare-earth element concentrations in the granite porphyry. We conclude that the magmatic rocks of the Hamash area represent an arc crustal section that records the evolution of a Neoproterozoic oceanic island arc. The arc was later caught up in the assembly of the northern Arabian-Nubian Shield.

Large outcrops of ultramafic rocks are common in the Central Eastern Desert (CED) of Egypt and represent residual mantle sections and cumulates of dismembered Neoproterozoic oceanic lithosphere. This study integrates field observations, petrography, geochemistry and mineral chemistry to investigate ultramafic rocks of the dismembered ophiolitic rocks at four areas distributed from east to west across the CED, with the aim of gaining a better understanding the tectonic settings and mantle conditions under which the ultramafic rocks formed. The ultramafic masses studied are composed of serpentinized peridotites, serpentinites and pyroxenites, and occasionally host podiform chromitites. Preserved textures, relict primary minerals and whole-rock compositions indicate that the protoliths of these ultramafic rocks were dominated by harzburgites. The Al2O3- and CaO-depleted nature of residual harzburgites, the high Fo and NiO contents of primary olivines and the high Cr# and low TiO2 contents of fresh Cr-spinels suggest that these ultramafic rocks are remnants of depleted to highly depleted forearc mantle of suprasubduction zone (SSZ) system. Temperatures from olivine-spinel geothermometry (668–778 °C) and Al-in-orthopyroxene geothermometry (984 °C) estimated for residual harzburgites are comparable to those reported for modern forearc mantle peridotites. The range of oxygen fugacity [ΔlogƒO2 (FMQ)] values (−3.01 to +0.32) calculated for the residual harzburgites reveals heterogeneity in the Neoproterozoic mantle oxidation state and suggests that SSZ mantle was not consistently more oxidized than MOR mantle. The podiform chromitites of different ultramafic masses were formed following the interaction of residual mantle peridotites with forearc basaltic and boninitic melts, implying the evolution of Neoproterozoic SSZ system from subduction initiation to more mature island arc stage. Integration of the analyses carried out in this study shows a Neoproterozoic SSZ system where slab-derived melts evolve in composition and oxidation state from early reduced forearc basalt in the east to more oxidised boninitic melts in the west.

The medium- to high-K calc-alkaline subduction-related Dokhan volcanic rocks of the Wadi Hamad area record two flare-up events during the history of a Neoproterozoic continental island arc, an early pulse dominated by andesite and a later pulse dominated by dacite. The trace element abundances and (La/Yb)_n ratios of the basalts, andesites and dacites all overlap, contrary to the expectations of a single fractional crystallization trend, suggesting the need for three distinct parental magmas for the three groups. The parental magmas of the basalts and andesites were generated by variable degrees of partial melting of subduction-metasomatized mantle, whereas that of the dacites requires mixing of mantle and crustal melts. The andesites and dacites both evolved mostly through fractionation of clinopyroxene and plagioclase, accompanied by apatite and Fe-Ti oxides in the more evolved dacitic rocks. Oscillatory and reverse zoning in clinopyroxene and plagioclase indicates that magma replenishment and mixing played a role in the genesis of the basalts and andesites. Depth-sensitive geochemical parameters show that the earlier andesites evolved at deeper levels in the arc crust compared with the later dacites, but not so deep as to stabilize garnet as a fractionating phase. Estimated values of the arc crust thickness indicate that the crust thickened from ∼35 km to ∼50 km from the time of basalt eruption to the time of andesite eruption, probably implying a high rate of crustal growth. The estimated arc crust thickness during the later subduction flare-up is slightly less than that of the earlier one despite ongoing magmatic addition, implying that arc crust delamination began before the production of the later dacites. The subduction-related geochemical characteristics of the Dokhan volcanic rocks suggest that the terminal collision between East and West Gondwana in the tip of the Nubian Shield occurred at ∼600 Ma.

The ophiolitic metabasalts (pillowed and sheeted dikes) of Wadi Ghadir area, Eastern Desert, Egypt, were analyzed for their major, trace and rare earth elements, Nd and Sr isotopes and the chemistry of their plagioclase, amphibole and chlorite was also reported. Geochemically these rocks range from tholeiitic basalt to basaltic andesite. The generally low MgO, Cr and Ni and high Zr contents are consistent with derivation of these rocks from an evolved magma. The high TiO2 contents (mostly between 1.76% and 2.23%) classify Wadi Ghadir ophiolitic metabasalts as MORB ophiolite. The chondrite-normalized REE patterns of most samples display small LREE-enrichment with (La/Yb)n ranging from 1.44 to 2.56. The MORB-normalized spider diagram shows variable LILE abundances, which are either similar to or enriched relative to MORB, and most samples display small Nb depletion. The abundances of some LILE (Ba, Rb and K) as well as Na and Si were modified by post-magmatic seafloor hydrothermal alteration. Enrichment of the least mobile LILE (Th & U) indicates that Wadi Ghadir ophiolitic metabasalts are akin to C (contaminated)-MORB. These geochemical characteristics are similar to BABB modified by contamination. Wadi Ghadir metabasalts have low initial Sr ratios (0.7010–0.7034) which are similar to those of MORB, while their eNd(t) values (+7.7–+4.5) are either more or less positive than the value of depleted mantle (DM). The more positive eNd(t) values indicate DM source for these basalts, while the less positive eNd(t) values reflect the involvement of slightly older component in Wadi Ghadir ophiolite. We suggest that the parent magma of metabasalts was contaminated by slightly older material, most probably oceanic-arc crustal rocks, which caused enrichment in LREE, and by analogy LILE, but did not significantly affect Nd isotopic systematics or modify Sr isotopes. Such contaminated MORB character also revealed by other ophiolitic metavolcanics in the Central Eastern Desert, contrasting the N-MORB character of the Gerf ophiolite in the South Eastern Desert. Moreover, the present work suggests the increase of the degree of contamination of the ophiolitic metabasalts from south to north in the Central Eastern Desert.