Origin of the post-orogenic dyke swarms of Saharan Metacraton, at Qaret El-Maiyit-Bir Safsaf area, southwest Egypt: Constraints on the magmatic–tectonic processes at the end of the Precambrian

Abstract

Deep in the Western Desert of Egypt, in the southernmost part, between Qaret El-Maiyit and Bir Safsaf, swarms of dykes cut through the Neoproterozoic rocks. This area lies halfway between the juvenile crust of the Arabian-Nubian Shield (ANS) and the Gebel Kamil terrains, close to the border with Libya. Acidic dykes include rhyolites and trachy-dacites. Intermediate dykes include trachy-andesite, basalt-trachy-andesite, and basalt-andesite, while basic dykes consist of basalt. Felsic dykes are more numerous and younger compared to the mafic (intermediate and basic) dykes. Felsic dykes trend mostly run to the northeast and northwest, while mafic dykes mainly run to the northwest, less frequently to the eastwest. Acidic and intermediate dykes show elevated REE concentrations (up to 164 and 203 ppm, respectively) with highly fractionated patterns (av. (La/Lu)_N = 20.56 and 18) and moderately fractionated HREEs (av. (Gd/Lu)_N = 2.5 and 3.2) and LREEs (av. (La/Sm)N = 5.7 and 3.6 respectively). The basic dyke samples exhibit modest REE concentrations (up to 112 ppm), weakly fractionated patterns (av. (La/Lu)_N = 8), and mildly to weakly fractionated HREE (av. (Gd/Lu)_N = 2.3) and LREEs (av. (La/Sm)_N = 2.5) patterns. There are no recognizable Eu anomalies in the dyke samples. The magma ascended in an extensional setting and the geochemical features indicate a subduction mode, possibly originating from the Atmur-Delgo suture zone in northern Sudan or by mantle delamination during the early Neoproterozoic. Both the felsic and mafic dykes are derived from a mafic calc-alkaline melt and show fractionation on a single downward line, indicating a genetic relationship. The mafic dykes were formed by partial melting of an enriched mantle source (about 10%) that started at 2.7–3.0 GPa and a solidus temperature of about 1420 °C. The enrichment of the mantle melt beneath the Saharan metacraton, which is the origin of the studied melts, could be related to mantle delamination.