Paper No. 8
Presentation Time: 10:20 AM


LONG, Leon E., Department of Geological Sciences, Jackson School of Geosciences, University of Texas, Austin, TX 78712 and HASSAN, Adel M., Department of Geology, Tanta University, Tanta, Egypt,

Dokhan Volcanics (~0.6 Ga) crop out sporadically along more than 700 km of the Eastern Desert of Egypt, which is part of the Arabian-Nubian Shield, one of the largest tracts of juvenile crust on Earth. Data points from 840 Dokhan whole-rock chemical analyses populate every compartment of a TAS diagram, from rare basalt, to abundant andesite through rhyolite (and trachytic counterparts). Plagioclase, clinopyroxene, amphibole, biotite, K feldspar, titanite, and quartz (but not olivine) comprise the major minerals. Matrix calculations, using chemical compositions of whole rocks and constituent minerals, provide modal abundances and endmember compositions (e.g., ab/an in plagioclase, and hd/dp in clinopyroxene). On average, Dokhan Volcanics are classic continental crust.

Pearce Element Ratios (PER), which are also derived by matrix calculations, afford further insight into the origin of Dokhan Volcanics. A PER diagram tests whether a postulated set of minerals, present in any relative abundance including zero abundance, can account for all chemical compositions of a suite of whole-rock samples. A successful diagram is a precise linear array of data points with slope (m) = 1, and non-zero y-intercept. An incorrect set of hypothesized minerals generates an unsuccessful diagram in which m ≠ 1, or the data array is a “blob.”

PER diagrams based upon the above list of minerals are successful, but equally so is a simpler postulate of ab + an + fa + fo + hd + dp + ti. Together with qz component, this composition of generic basalt is both a necessary and sufficient primary composition from which to generate the vast array of Dokhan chemical compositions. Amphibole, biotite, and K feldspar, having real existence, are not necessary and more than sufficient. Olivine, which is a “phantom” mineral, is necessary, having once existed in deep magma chambers.

Earlier in this region, subduction had occurred with vergence away from the continent (very rare in the modern Earth). Subducting continental crust jammed the process; slab breakoff ensued, followed by upwelling of hot asthenosphere into the hinge zone, there to experience decompression melting, magma differentiation and mixing, and recharge by primary magma, creating the Dokhan Volcanics. Chemical discrimination diagrams are consistent with this origin.