Abstract
This contribution deals with new geology, petrography, and bulk-rock/mineral chemistry of the poorly studied ultramafics of Wadi Ibib–Wadi Shani (WI–WS) district, South Eastern Desert, Egypt. These ultramafics are dismembered ophiolitic rocks that can be subdivided into serpentinites and serpentinized peridotites. Primary minerals such as olivine and pyroxene are absent in serpentinites, but relics of them occur in serpentinized peridotites. Pseudomorph after olivine is indicated by common hourglass textures with less mesh, whilst schistose bastites reflect a pyroxene pseudomorph. Chromite can be subdivided into Cr-spinel and Al-spinel. Cr-spinel ranges from chromite to magnesochromite in composition, whereas Al-spinel belongs to the spinel field. Cr-spinel includes YCr (Cr/(Cr+Al+Fe+3), YAl (Al/(Al+Cr+Fe+3), and YFe+3 (Fe+3/(Fe+3+Al+Cr), similar to forearc peridotite, whilst Al-spinel is more similar to abyssal peridotite, and may be formed during inanition of subduction processes in proto forearc environments. The main secondary minerals are tremolite, talc, and chlorite—which is subdivided into pycnochlorite and diabantite—and their temperature ranges from 174 to 224 °C. The examined rocks had undergone high partial melting degrees (>25%), as indicated by the Cr# of their unaltered cores (Cr-spinel, >0.6), whole rocks (Al2O3, SiO2, CaO, and MgO), trace and REEs, depleted Na2O, Al2O3, and Cr2O3 of clinopyroxene, and high forsterite content ((Fo = 100 Mg/Mg + Fe), av. 95.23 mol%), consistent with forearc settings.
Highlights
Neoproterozoic ultramafic rocks are commonly distributed in central and southern sectors of the Egyptian Eastern Desert (ED) in the northern Nubian Shield
The Nubian Shield is considered the western side of the Arabian Shield, both of which are juvenile crusts forming the Arabian–Nubian Shield (ANS) of the northern East African Orogeny (EAO) [1]
Orthopyroxene is replaced by serpentine, chlorite, tremolite, and magnetite, forming schiller structures (Figure 3f)
Summary
Neoproterozoic ultramafic rocks are commonly distributed in central and southern sectors of the Egyptian Eastern Desert (ED) in the northern Nubian Shield. The field and geochemical signatures of the ultramafic magma provide clues as to the different tectonic settings They are either older, dismembered ophiolitic fragments—which are partially or completely metamorphosed to serpentinite, mostly consisting of chromitites and magnesites of mid-ocean ridges or supra-subduction settings—or younger, unmetamorphosed ones [7,8]. Ophiolites are slices of the oceanic lithosphere thrust on the continental plates, which help in recognizing the tectonic processes in the mantle [12]. Ophiolites are slices of the oceanic lithosphere thrust on the continental plates, w2hoifch help in recognizing the tectonic processes in the mantle [12]. FFiigguurree 22.. (aa) Thrust contact between serpentinites and metasediments, and (bb) pod of talc–carbonate within serpentinites aalloonngg tthhee sshheeaarr zzoonnee
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