Title : Using the monazite U-Th-Pb geochronology for dating the highly fractionated rare metal granites
Determining the absolute or relative age of the highly fractionated rare metal granites is one of the main challenges that face the petrochronologist, due to the significant effect of post-magmatic hydrothermal fluids during the evolution of these rocks. For example, most rare metal granites from different orogenic belts worldwide have anomalously high 87Rb/86Sr and 87Sr/86Sr due to the isotopic alteration during post-magmatic stage. Moreover, the reported zircons from these granites have extremely high U-concentration and intense metamictization, resulting in significant radiogenic Pb-loss, as well as common Pb contamination, along fractures and other imperfections, yielding imprecise to meaningless dates. Therefore, the accessory minerals such as monazite, thorite, titanite, columbite-tantalite and cassiterite were recently used as geochronometers to date and track the magmatic-hydrothermal process. CHIME (chemical Th–U-total Pb isochron method) dating method is used to date monazite in rare metal granites. This method required the precise measurement of Th, U and Pb in monazite by electron microprobe analyses (EMPA). The highly fractionated rare metal granites are widely distributed in the Eastern Desert of Egypt, which occupy the northmost tip of the Arabian Nubian Shield. Numerous methods have been applied to rare metal deposits on bulk-rocks (Rb-Sr and Sm-Nd) and minerals (Pb-Pb and U-Pb) such as zircon, columbite, tantalite, and cassiterite, However, ages obtained using these methods are inconsistent because post- to late-magmatic hydrothermal solutions and tectono-thermal events following magma emplacement have partially to completely reset the original mineral-rock isotopic system. Therefore, chemical dating" of monazite was used to constrain the timing of rare metal granitic magmatism and understand bulk-rock alteration associated with these highly evolved rare metal granites in the Arabian Nubian Shield. The results provide comparable ages agree with the timing of rare metal-rich intrusions (~ 600 Ma) related to the Pan-African orogeny.