G of Neeochemistry and petrogenesisoproterozoic Nuqara Dokhan volcanics, Central Eastern Desert, Egypt

The Neoproterozoic Nuqara Dokhan volcanics are distinguished into two main groups: a) intermediate volcanics that comprised basaltic andesites, andesites and their associated pyroclastics, b) felsic volcanics, consisting of dacite, rhyodacite, rhyolites and ignimbrites.. Their major and trace elements have  a wide ranges in which SiO2 (76-54%), CaO (8.95-0.2%), TiO2 (1.6-0.4%), Sr (97-52 ppm), Zr (40-160 ppm), Cr (231-4 ppm), and Ni (57-1 ppm). The fractional crystallization relationships as well as low-P fractionation is indicated by increasing of incompatible elements as passing from basaltic andesite through andesite to rhyolite series. This conclusion is again confirmed by the low contents of  P2O5 in the felsic Dokhan volcanics (P2O5=0.01-0.17; x=0.06). and which was used as argument against their formation by crustal anatexis. The intermediate Dokhan volcanics contains low MgO (2.25-5.34, x= 33.6), Cr (77-99; x=11) and Ni (18.55-57.23; x=34.85) precluding a primary mantle melt for these rocks. Moreover, the Dokhan volcanics are petrogenetically related as shown by their more or less similar average ratios such as K/Rb (intermediate=500, felsic=462), Rb/Zr (intermediate=0.19, felsic=0.28).

The Nuqara Dokhan volcanics are enriched in LILE relative to the HFSE and have tendency toward calc-alkaline subduction-related magmatism. However, the intimate association of Dokhan volcanics and post-orogenic A-type granites, as well as the bimodal dykes, in space and time is the argument against their formation under compressional tectonic regime and thus were developed in an extensional that follow the cessation of subduction.

The obtained mineral chemistry of the studied Dokhan volcanics reveals the following points: :i. The anorthite content of plagioclase is from An₆₁ to An₅₅ in the basaltic andesites, from An₄₁ to An₂₈ in the andesites and from An₁₆ to An₄ in the rhyolites; ii. The decrease in An contents of plagioclase as passing from basaltic andesites through andesites to rhyolites, together with their variation vs. Al2O3, K2O and Zr suggesting that fractional crystallization plays significant role in their formation; iii .Alkali feldspars have sanidine composition suggest a high temperature for their host lava flows; iv. Clinopyroxenes have augite composition, illustrated the subalkaline nature for the host lava flows and their crystallization temperature in the range 1100-1300^oC; and vi. Amphiboles have mainly tschermakitic hornblende to magnesio-hornblende composition and suggest the low-pressure crystallization of the andesitic flows.