2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 25
Presentation Time: 8:00 AM-4:45 PM

Regional Geochemistry of the Dokhan Volcanics, Eastern Desert, Egypt

LONG, Leon E., Department of Geological Sciences, University of Texas, Austin, TX 78712 and HASSAN, Adel M., Department of Geology, Tanta University, Tanta, Egypt, leonlong@mail.utexas.edu

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 Analyses of major/minor oxides from 12 localities, compiled from published and unpublished sources, comprise a complete data set: 376 whole-rock samples, 4100 data.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.