Integration of mineralogical, geochemical and ASD reflectance spectra data for characterization and evolution of Neoproterozoic serpentinites and listwaenites at Wadi Kalalat area, South Eastern Desert of Egypt

Abstract

The present research focuses on the application some remote sensing techniques, mostly ASD FeldSpec combined with principal component analysis (PCA) and fusion color composite (FCC), to discriminate different Neoproterozoic shield rocks at the Wadi Kalalat area, Egypt. In addition, geochemical characteristics are utilized for better understanding of geodynamic evolution of ophiolitic serpentinites in the area as the oldest rock unit. Also, the present research records two varieties of listwaenite (calcic and silicic) at a new locality in the Eastern Desert of Egypt. The ophiolitic serpentinites ± listwaenites, as well as the island-arc metagabbro-diorite complex, crop out at both flanks of the Wadi Kalalat main course while the arc-related fresh peridotite and olivine gabbro of the Gabal El-Motaghairat intrusion crop out at the northern flank of the wadi. Mineralogically, partially serpentinized peridotite relics contain ferroenstatite (12.49 wt% FeO, 28.6 wt% MgO), diopside, green transparent spinel (pleonsate) and Cr-spinel. Chemistry of Cr-spinel indicates freshness of a picotite composition (4.34–9.88 wt% MgO, 17.87–19 wt% Al₂O₃, 42.48–44.14 wt% FeO, 23.16–26.07 wt% Cr₂O₃). Trails of fine magnetite form at cores of chrysotile veinlets contemporaneous with the formation of mesh-textured massive serpentinite. Coarse magnetite disseminations and Cr-magnetite as the peripheral rim of zoned Cr-spinel are connected to antigorite formation at upper greenschist-lower amphibolite serpentinization condition. Feldspec reflectance bands in the range 0.45–2.5 μm, combined with results from PCA and FCC, discriminate serpentinites and their related listwaenites from other rock units in the area, and with perfect discrimination of El-Motaghairat mafic-ultramafic arc intrusion from the island arc rocks and the Batuga granite pluton. Magnetite content, serpentine mineralogy, contents of metal-OH, Mg²⁺, Fe²⁺, Fe³⁺, in addition to fresh phases such as olivine and clinopyroxene are the controlling factors for the lithological discrimination on the basis of reflectance spectroscopy. From the geodynamic point of view, and based on contents of SiO₂, Al₂O₃, MgO, CaO and LILEs in the Wadi Kalalat serpentinites, the mantle peridotite was emplaced as a depleted wedge in a mid-ocean ridge (MOR) setting rather than in a supra-subduction zone (SSZ) setting. Our findings point out to the fact that listwaenitization is a hydrothermal process that lead to drastic depletion of transitional metals (Cr, Ni, V, Co) from the serpentinite precursor.