The late Neoproterozoic Pan-African low-grade metamorphic ophiolitic and island-arc assemblages at Gebel Zabara area, Central Eastern Desert, Egypt: Petrogenesis and remote sensing - Based geologic mapping

Abstract

The late Neoproterozoic Pan-African basement rocks of the Gebel Zabara area, Central Eastern Desert of Egypt is of particular interest where the medium-grade Hafafit gneisses juxtapose the low-grade ophiolitic and island-arc assemblages along major structural contacts. The area comprises important mineral deposits including gold, base metal vein-type deposits as well as emerald and talc. In the current work, various image processing algorithms including Principal Component Analyses (PCA), Minimum Noise Fraction (MNF) and Decorrelation Stretch (DS) were proposed and applied on Landsat-8, ASTER and Sentinel-2 data to delineate and discriminate the different rock units. Several ASTER mineral indices are successfully used in the mapping process to detect the alteration zones, which are mainly confined to the NNW-SSE and N-S trending structures. The petrogenesis studies for the different rock unites in the study area indicate that the ophiolitic assemblage include serpentinite-talc carbonate, metagabbros complex and metabasalts. Serpentinite rocks represent the hydrated low temperature form of a harzburgite precursor, while the talcose serpentinites evolved via SiO2 aqueou metasomatic alteration. Geochemically, the ophiolitic rocks exhibit N-MORB tholeiitic affinity and imply a back-arc setting. The island-arc rock assemblages encompass calc-alkaline meta-andesite, metadacite and metarhyolite, which imply a normal continental island-arc setting, i.e. less mature than the proper active continental margin. Apart from serpentinites, the ophiolitic and island-arc assemblages experienced regional greenschist facies metamorphism. The ophiolitic metagabbros exhibit the metamorphic assemblage: actinolite + albite (An < 5%)/oligoclase (An 19–20%) + chlorite + epidote + quartz. The coexistence of albite and oligoclase may indicate either: (1) disequilibria association, or (2) a possible greenschist-amphibolite facies transition. The P-T metamorphic condition of the metagabbros was constrained at about 5 kbar and 430 to 452°C.