Composition and total-Pb model ages of monazite from high-grade paragneisses in the Abu Swayel area, southern eastern desert, Egypt

Abstract

Monazites from high-grade metapelitic paragneisses from the southern Eastern Desert of Egypt (Abu Swayel area) were analysed with the electron microprobe mainly in an attempt to broadly constrain the metamorphic ages of the rocks by means of chemical Th(U)-Pb dating. Two samples were investigated, one showed weak signs of a greenschist facies overprint, the other one did not. For each sample, weighted average ages were calculated from long-time analyses of 18 (16) individual grains with a 5 μm beam placed in the grain centres. The average ages were almost the same (636± 10 Ma, 633±10 Ma). The monazites appeared chemically fairly uniform and homogeneous in both samples with ThO2 contents of ca. 3.3-4.5 wt.%, UO2 0.4-1.2 wt.%, La2O3 12–13 wt.%, Nd2O3 11-13 wt.%, Y2O3 1.8-2.6 wt.%. Some larger grains displayed a weak concentric zoning in the BSE image with increasing brightness near the rims. A microprobe traverse was laid across a zoned monazite from the slightly retrogressed sample. It was found that the U and Y contents were somewhat higher in the outer growth shell. The high Y contents at the rims argue for crystal growth under prograde temperature conditions and against a retrograde overgrowth. There appeared to be a tendency that the model ages become slightly younger towards the crystals rim (645±15 Ma in the core section versus 633±16 Ma in the rim section of the profile). However, the observed differences are interpreted as equivocal due to the limited resolution of EMP monazite dating. Clearly, the results do not support previous hypotheses, according to which Abu Swayel gneisses should belong to pre-Panafrican, mid-Proterozoic metamorphic sequences. Instead, the data accord with other 600-650 Ma metamorphic ages recently recognized near the contact of the East Sahara Craton and the Arabian Nubian Shield. The best interpretation is that high-grade metamorphism at that time occurred in connection with collisional crustal thickening, when a Panafrican terrane assembly was attached to the east Sahara Craton from the (present day) east. This event appears to be distinct from an earlier phase of high-grade regional metamorphism between ca. 700 and 750 Ma, which has been documented in other parts of the Arabian Nubian Shield.