Pillow form morphology of selected Neoproterozoic metavolcanics in the Egyptian Central Eastern Desert and their implications

This study discusses the features of well-preserved pillow forms of seven Neoproterozoic metavolcanic occurrences found in the Central Eastern Desert of Egypt and compares them to modern and ancient tectonic environments. The aim is to present a mode of origin model, parent magma composition and eruption environment of these lavas.Categorized on size and shape, three different pillow types have been recognized: elongated pillows (type A), in the Muweilih and Wadi Beririq; bulbous pillows (type B) in Wadi Ghadir, Gabal Ghadir and Wadi Um Seleimat; and spherical pillows (type C) in Wadi Kareim and Gabal Semna.Absent in the studied pillow forms are the tubular type pillow, connection between pillows, and radial crack features. The most prominent features of the studied pillows are flattening, tight packing with development of chilled margins, small size, and the abundance of vesicular structure. These features indicate their accumulation at high initial temperature and hydrostatic pressure, low viscosity, and rapid eruption when compared to modern mid-oceanic ridge pillows.The significant features of the studied pillow forms show they differ from pillows of the modern mid-oceanic ridge environments, and are akin to pillows found in ancient marginal basin environments (especially types A and B). The small size of type C pillows is considered a by-product of the felsic composition, which suggests formation in an island-arc environment. The deduced paleotectonic environments are supported by geochemical data and the resulting regional implication is that the central part of the Eastern Desert of Egypt could represent a SE subducted arc-back-arc system.