Abstract
Field relations, petrography and chemical composition constrain old and young volcanic sequences for the formation of the Neoproterozoic-Early Cambrian Nuqara rock belts. These volcanics are a part of a post-subduction and extensional-related magmatic event in east Egypt. The two volcanic sequences differ in phenocryst mineralogy (hydrous vs anhydrous assemblages) and whole-rock major and trace element chemistry. Their eruptions were punctuated by occasional volcaniclastic deposits that generated fall, flow or reworked suites compositionally identical to the lava flows. The model proposed for their origin involves contrasting ascent paths and differentiation histories through crustal columns with different thermal and density gradients.
The old volcanic sequence covers a compositional range from basalt through andesite to rhyolite (51-73 wt% SiO2) of the medium to high-K calc-alkaline series. They show lower contents of K2O, Al2O3 and of several other incompatible trace element abundances and ratios than those of younger volcanics with comparable degree of evolution. The evolution of the whole old volcanic spectrum was governed mainly by crystal/melt fractionation of Ol, Cpx, Plag, Fe-Ti oxides and Ap in the intermediate varieties, and of Cpx, Plag, Fe-Ti oxides and Ap in the felsic varieties. At each stage of evolution, fractional crystallization was accompanied by variable degree of crustal contamination. The young volcanic sequence involves basaltic andesite and andesite rock suites and defines high-K tholeiitic series. Replenishment, tapping and subsequent fractional crystallization processes (RTF) within strongly zoned magma chamber led to the generation of tholeiitic melts. These young volcanics become enriched in LILEs and moderately compatible elements with time and this indicates that magma mixing become more important during the lifespan of the volcanic centers.
The source of the old and young volcanic magmas at Nuqara area was probably a metasomatised fertile asthenospheric mantle, where the long period of subduction (>300 Ma) in eastern Egypt prior to the onset of these volcanic magmatisms resulted in LILEs enrichment of the subcontinental mantle lithosphere. It is inferred that the old calc-alkaline melts came from a garnet-and amphibole-bearing mantle at deeper depth, modified by a subduction component, whereas the tholeiitic melts came from enriched phlogopite-bearing spinel lherzolite at shallower depth.
The contrasts in phase assemblages and differentiation trends reflect contrasting polybaric fractionation histories from two parental magmas. This polybaric evolution can account for the chemical discontinuity between the old and young volcanic magmas erupted on Nuqara area.