SHRIMP U-Pb geochronology of authigenic xenotime and its potential for dating sedimentary basins

SHRIMP (Sensitive High-Resolution Ion MicroProbe) analytical procedures hav e been developed to enable dating of the small, early diagenetic xenotime o vergrowths that commonly occur on zircons in siliciclastic sedimentary rock s. The method will be particularly useful in Precambrian terranes, where di agenetic xenotime dating could play a role equivalent to biostratigraphic d ating in the Phanerozoic. Reliable Pb-207/Pb-206 data are more readily obta ined than Pb-206/U-238, which also favours application to the Precambrian. However, it is demonstrated that Pb-206/U-238 dating of larger overgrowths (> 10 mu m) is also viable and applicable to Phanerozoic samples. SHRIMP Pb /Pb geochronology of authigenic xenotime in an unmetamorphosed Palaeoproter ozoic sandstone in the Kimberley Basin has constrained diagenesis to a prec ision of +/- 7 Ma. In contrast, greenschist-facies metasediments of the Arc haean Witwatersrand Basin, South Africa, contain both authigenic and altera tion xenotime that record a complex history of growth from early diagenesis to the last major thermal event to affect the basin.