The cooling history of Silurian to Cretaceous alkaline ring complexes, south Eastern Desert, Egypt, as revealed by fission-track analysis

Abstract

Forty fission-track ages of apatite, zircon and sphene, and nine horizontal “confined” track-length distribution patterns in apatite have been used to establish the cooling history of nine Silurian to Late Cretaceous alkaline ring complexes which intrude Precambrian basement in the southern Eastern Desert of Egypt. Zircon or sphene fission-track ages were determined from three complexes for whichK/Ar andRb/Sr ages on the same samples were also available, these ages are concordant and are interpreted as emplacement ages resulting from rapid cooling following high level crustal intrusion into relatively thick volcanic piles.

Average apatite ages for each of the eight ring complexes range from 33 to 167 m.y. Track-length distribution patterns for apatites taken together with their ages invite subdivision into two groupings. Those complexes yielding Early Oligocene apparent apatite ages suggest cooling from the total track annealing zone followed by a relatively lengthy residence near the base of the partial annealing zone whereas those with Late Cretaceous ages indicate cooling from a shallower level in the partial annealing zone. Variations in cooling history resulted from differential uplift between fault-bound blocks. One block, that containing the Late Cretaceous Abu Khruq complex, was relatively stable and the different degrees of partial resetting recorded in apatites of this complex are attributed to the thermal effect of localised Tertiary dyke intrusion.

Fission-track analysis in combination with geologic data indicates that in the south Eastern Desert of Egypt a phase of uplift commenced in Late Oligocene time and was accompanied by paleogeothermal gradients of ca. 40–50°C/km. Uplift was more pronounced (at least 2–2.5 km) in areas within about 100 km from the present Red Sea coast. This uplift, which is viewed as part of a broader regional tectonism related to the opening of the Red Sea, occurred along a northwest fracture pattern and was controlled by pre-existing lines of weakness in the basement complex.