Regional Geochemistry of the Dokhan Volcanics, Eastern Desert, Egypt

The Dokhan Volcanics, a prominent Precambrian basement unit in the Eastern Desert of Egypt, were erupted during a late compressional phase transitioning into extensional. Isolated exposures, which cannot be correlated, are scattered along a 1000-kilometer N-S distance. Rb-Sr whole-rock isochron ages and U-Pb zircon ages are ~0.61 Ga, with one high outlier and no regional age trend. Analyses of major/minor oxides from 12 localities, compiled from published and unpublished sources, comprise a complete data set: 376 whole-rock samples, 4100 data. For 3439 analyses of 35 trace elements, data sets are incomplete and largely non-overlapping. There are negligible mineral chemical data.

Dokhan Volcanics range from basalt to rhyolite (45 to 79% SiO₂) and, although typically regarded as andesitic, according to a TAS classification they are 29% rhyolite but only 19% andesite sensu stricto. Wt% SiO₂ is mildly bimodally peaked at ~62% and ~74% both regionally and for several individual suites. On an AFM diagram the data plot in a tholeiitic trajectory. Harker diagrams exhibit well-correlated trends with similar negative slopes, displaced in a vertical sense (i.e., at a specified value of SiO₂, values of TiO₂, MgO, CaO, or P₂O₅ differ according to local suite). Planes whose axes are SiO₂ (felsic index) vs. MgO (mafic index) vs. TiO₂, P₂O₅, FeO*, etc. are fitted to chemical data. Within one standard deviation, best-fit planes accommodate a remarkable 90% of the total range of coordinated variations among SiO₂, MgO (>1 wt%), TiO₂, P₂O₅, and FeO*. Dokhan Volcanics are a vast unified chemical system, created from melting and differentiation of a uniform source. Following deep erosion, rocks in various stages of differentiation progress are preserved in local eruptive centers. N-MORB-normalized trace element plots for basaltic samples exhibit negative-slope arrays. According to the Ce/Y indicator, contemporary crustal thickness was ≥40 km.