Summary
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.
Zusammenfassung
Monazite aus amphibolitfaziellen metapelitischen Paragneisen der Abu Swayel Region in Südägypten (SW Eastern Desert) wurden mit der Mikrosonde analysiert, unter anderem um die Metamorphosealter der Gesteine auf dem Weg einer chemischen Th(U)-Pb Datierung annäherungsweise zu bestimmen.
Zwei Proben wurden untersucht, eine davon wies eine lei Überprägung unter grünschieferfaziellen Bedingungen auf. In jeder Probe wurden 18 bzw. 16 Monazite, jeweils in den Kornzentren, mit langer Zählzeit und mit auf 5 gm defokussiertem Strahl analysiert. Die aus den Analysen errechneten mittleren Th(U)-Pb Alter waren in beiden Proben praktisch gleich (636 10 Ma, 633 ± 10 Ma). Auch die Chemismen der Monazite erwiesen sich in beiden Proben als sehr ähnlich und ziemlich homogen (ThO2 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.%). Nur einzelne größere Körner ließen im BSE-Bild einen ganz schwachen konzentrischen Zonarbau mit größerer Helligkeit am Rand erkennen. Durch ein solches Korn aus der leicht retrograden Probe wurde ein chemisches Profil gelegt urn den Zonarbau zu quantifizieren bzw. die Altershomogenität zu testen. Die Randzone wies dabei leicht erhöhte Gehalte an U and Y auf. Die hohen Y Werte sprechen für eine Kristallisation unter hohen Temperaturen und gegen ein retrogrades Weiterwachsen. Die chemischen Modellalter waren im Kornzentrum im Schnitt etwas höher als am Kornrand (645±15, 633±16 Ma). Allfällige Zeitdifferenzen im Wachstum oder Bleiverlusteffekte waren allerdings innerhalb des methodischen Fehlers nicht sicher auflösbar. Die untersuchten Paragneise wurden bisher meist als Reste eines mittelproterozoischen, prä-panafrikanischen Kristallins angesehen. Die chemischen Modellalter für die Monazite bestätigen diese Ansicht aber nicht, sondern passen vielmehr gut zu anderen Metamorphosealtern von etwa 600 bis 650 Ma, welche in der letzten Zeit mehrfach entlang der Grenzzone zwischen dem Ostsahara-Kraton und dem ArabischNubischen Schild dokumentiert wurden. Die wahrscheinlichste Erklärung ist, daß diese Regionalmetamorphose durch das Andocken panafrikanischer Terrane an den Kraton verursacht wurde.
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Finger, F., Helmy, H.M. Composition and total-Pb model ages of monazite from high-grade paragneisses in the Abu Swayel area, southern eastern desert, Egypt. Mineralogy and Petrology 62, 269–289 (1998). https://doi.org/10.1007/BF01178032
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DOI: https://doi.org/10.1007/BF01178032