The Ediacaran Post-collisional Dokhan Volcanics

Abstract

The term “Dokhan Volcanics” refers to thick varicolored successions of pyroclastics and lava flows of mainly andesitic to rhyolitic composition commonly in association with felsic ignimbrites. Their type locality is Gabal Dokhan in the North Eastern Desert (NED), from which they acquired their name. The Dokhan volcanic successions cover a limited area of the exposed Precambrian basement in Egypt, and are largely exposed north of latitude 26° N in the northern and central territories of the Eastern Desert, and also in southern Sinai. In contrast, they are never reported in the Western Desert. The most abundant lava varieties among Dokhan Volcanics are andesites, dacites, rhyodacites, and rhyolites with infrequent intercalations of andesitic basalt, basalt, and trachyte. Nonetheless, many Dokhan volcanic sequences are dominated by pyroclastics rather than lavas. The Dokhan Volcanics are commonly overlain and/or interbedded with the molasses-type Hammamat sediments, constituting vast volcano sedimentary successions. The Dokhan Volcanics define a broad spectrum of compositions from basalts to high-silica rhyolites (43–78 wt% SiO2). Successions including basaltic lava flows are contended to be either basalt-rhyolite compositional continuum with no evident compositional gaps or bimodal suites. They are typically medium- to high-K calc-alkaline, although some low-K tholeiitic basalts and basaltic andesites have been reported. Dokhan Volcanics fluctuate between peraluminous and metaluminous, but never being peralkaline. Their characteristic trace element features are their obvious enrichment in large ion lithophile elements (LILE) K, Ba, Rb and Th relative to high field strength elements (HFSE) Zr, Nb, Y, Ti and P, and elevated LILE/HFSE ratios, strongly positive Pb anomalies and Nb–Ta depletions relative to MORB. Subordinate adakitic lavas are documented in some Dokhan volcanic successions in the NED and Sinai, and are geochemically characterized by low MgO ( 15 wt%), Sr (>650 ppm), Sr/Y (>40) and La/Yb (>20). The isotopic signature data (whole-rock Rb–Sr and Nd-Sm and SIMS 18O of zircons) suggest derivation of the Dokhan Volcanics magma from a depleted mantle source that underwent only trivial contamination by older continental crust during their magmatic evolution. Dokhan Volcanics have dual geochemical characteristics of orogenic arc and anorogenic within-plate magmas, and are concluded to be erupted during post-collisional extensional collapse which was controlled largely by lithospheric delamination and slab breakoff. The recently published precise U–Pb zircon ages of Dokhan Volcanics of the North Eastern Desert and Sinai reveal their eruption within the 590–630 Ma time span. The parental magma of the Dokhan Volcanics was highly likely generated by partial melting of primitive crustal rocks affiliated to older accreted arc crust of the juvenile Arabian-Nubian Shield. It is assumed that crystal fractionation was linked with crustal assimilation during their evolution.