ORIGIN OF DOKHAN VOLCANICS, EASTERN DESERT OF EGYPT, AS REVEALED BY PEARCE ELEMENT RATIOS

Scattered exposures of Neoproterozoic Dokhan Volcanics are distributed along the Eastern Desert, from Sinai to southernmost Egypt and beyond. According to a TAS classification, Dokhan Volcanics range from very rare basalt, through about equally abundant andesite, dacite, and rhyolite (or their trachytic counterparts). Our chemical database consists of 18,022 analyses of 780 whole-rock samples from 22 localities, plus 245 analyses of clinopyroxene, hornblende, and biotite.

In Pearce Element Ratio (PER) diagrams, the x- and y-axes are constructed of ratios whose common denominator is the molar abundance of a “conserved,” or non-participatory (incompatible) element. Numerators are linear combinations of molar abundances of other rock-forming elements. PER diagrams test whether members of an igneous suite could have been related by fractionation or sorting of minerals of specified stoichiometry, and they are not subject to the familiar complicating effects of closure (element abundances summing to 100%). Matrix algebra is used to calculate the numerator coefficients in a complex PER diagram whose slope (m) by convention is 1. The linear data array of a valid PER diagram does not project through (0,0).

Chemical data from all localities provide excellent linear arrays (n = 22, m = 0.994 ± 0.007, r = 0.99994 ± 0.00005) if we postulate that fractionation of plagioclase + olivine + Ti-Fe oxide + clinopyroxene + hornblende + biotite (compositions of the latter three constituents determined from mineral analyses) operated to create the entire range of rock chemistries. Any other combination of minerals results in a diffuse scatter of PER data, or an array whose m ≠ 1.

However, olivine is absent in Dokhan Volcanics. The data argue for at least two stages of development in which “phantom olivine” was present in deeper magma chambers that fed the subvolcanic chambers. Dokhan Volcanics comprise a unified geochemical system.