COMPOSITION AND TOTAL-PB MODEL AGES OF MONAZITE FROM HIGH-GRADE PARAGNEISSES IN THE ABU-SWAYEL AREA, SOUTHERN EASTERN DESERT, EGYPT

Monazites from high-grade metapelitic paragneisses from the southern E astern Desert of Egypt (Abu Swayel area) were analysed with the electr on microprobe mainly in an attempt to broadly constrain the metamorphi c ages of the rocks by means of chemical Th(U)-Pb dating. Two samples were investigated, one showed weak signs of a greenschist facies overp rint, the other one did not. For each sample, weighted average ages we re calculated from long-time analyses of 18 (16) individual grains wit h a 5 mu m beam placed in the grain centres. The average ages were alm ost the same (636+/-10 Ma, 633+/-10 Ma). The monazites appeared chemic ally fairly uniform und homogeneous in both samples with ThO2 contents of ca. 3.3-4.5wt.%, UO2 0.4-1.2 wt.%, La2O3 12-13 wt.%, Nd2O3 11-13 w t.%, 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 m icroprobe traverse was laid across a zoned monazite from the slightly retrogressed sample. It was found that the U and Y contents were somew hat higher in the outer growth shell. The high Y contents at the rims argue for crystal growth under prograde temperature conditions and aga inst 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 p rofile). However, the observed differences are interpreted as equivoca l due to the limited resolution of EMP monazite dating. Clearly, the r esults do not support previous hypotheses, according to which Abu Sway el gneisses should belong to pre-Panafrican, mid-Proterozoic metamorph ic sequences. Instead, the data accord with other 600-650 Ma metamorph ic ages recently recognized near the contact of the East Sahara Craton and the Arabian Nubian Shield. The best interpretation is that high-g rade metamorphism at that time occurred in connection with collisional crustal thickening, when a Panafrican terrane assembly was attached t o the east Sahara Craton from the (present day) east. This event appea rs to be distinct from an earlier phase of high-grade regional metamor phism between ca. 700 and 750 Ma, which has been documented in other p arts of the Arabian Nubian Shield.