Petrographic, Radiometric and Paleomagnetic Studies for Some Alkaline Rocks, South Nusab El Balgum Mass Complex, South Western Desert, Egypt - 2014

Uranium (Nuclear Minerals) / Exploration, Mining, Marketing, & Acquisitions

Elsayed and two co-authors reported that Nusab El Balgum mass complex represents one of the alkaline igneous activities in the south Western Desert of
Egypt. Petrographic investigations defined some different rock types in south of the complex represented by alkaline volcanics (pyroclastics [rhyolitic crystal tuffs], spherulitic rhyolites, alkaline rhyolite dykes), sub-volcanic peralkaline granites and structurally controlled mylonitic volcanoclastics. These rocks recorded significant concentrations in terms of the two radioactive elements Th and eU, which displayed considerable spatial variations, especially within the peralkaline granites. The abundance of Th and eU is mainly related to favorable combination of structural and pos-magmatic hydrothermal conditions.

Paleomagnetic results give well-defined stable remanent magnetization directions of reliable VGP positions, which are presented and discussed in the context of the African APWP. Rock types, magnetization directions and VGP positions with the corresponding ages are as
follows:

1) Rhyolitic crystal tuffs; D/I = 340.0˚/−19.4˚, α95 = 6.8˚; VGP Lat./Long. = 51.4˚N/240.5˚E, A95 = 5.9˚ (Late Triassic).

2) Spherulitic rhyolites; D/I = 346.7˚/−6.6˚, α95 = 3.5˚; VGP Lat./Long. = 60.4˚N/237.0˚E, A95 = 3.0˚ (Late Triassic/Early Jurassic).

3) Alkaline rhyolite dykes; D/I = 341.3˚/16.7˚, α95 = 5.4˚; VGP Lat./Long. = 67.0˚N/262.8˚E, A95 = 4.2˚ (Early Jurassic).

4) Peralkaline granites; D/I = 173.3˚/−9.7˚, α95 = 3.6˚; VGP Lat./Long. = −70.5˚S/49.7˚E, A95 = 5.0˚ (Late Cretaceous).

5) Mylonitic volcanoclastics; D/I = 3.4˚/32.7˚, α95 = 4.4˚; VGP Lat./Long. = 83.6˚N/178.2˚E, A95 = 3.9˚ (Oligo-Miocene).

Except for the mylonitic volcanoclastics, these ages correspond to extrusion of the rocks and acquisition of primary thermal remanent magnetizations during the initial cooling. The formation events of the volcanic rocks are coincident with the most dynamic phases of Pangaea break-up, which occurred during the emplacement of the Central Atlantic Magmatic Province and the subsequent opening of the Central Atlantic Ocean.

The sub-volcanic peralkaline granites are coeval with anorogenic alkaline rocks in south of Egypt and these rocks formed due to alkaline melts in the asthenosphere. The mylonitic volcanoclastics reflect a secondary magnetization acquisition age, most likely, corresponding to local tectonic event where the rocks are genetically related to fault system in the area.