Obtaining diagenetic ages from metamorphosed sedimentary rocks: U-Pb dating of unusually coarse xenotime cement in phosphatic sandstone

The depositional age of nonfossiliferous, metamorphosed sedimentary rocks is commonly bracketed between the age of the youngest detrital mineral and the age of the oldest metamorphic mineral. The technique of dating diagenetic xenotime by ion microprobe can provide robust minimum ages for sediment deposition. However, in most cases, xenotime is only a few microns (mum) in size and rarely exceeds 10 mum, the minimum size for in situ ion microprobe analysis. Phosphatic sandstone in the greenschist facies Mount Barren Group, in southwestern Australia, contains unusually abundant xenotime occurring as exceptionally coarse (200 mum) pore-filling cement that nucleated on detrital zircon grains. The optimum environmental site for the formation of the cement was sand beds within a black shale condensed section. Analysis of xenotime by sensitive high-resolution ion microprobe yields two age populations, 1696 +/- 7 Ma for cement adjacent to detrital zircon grains, and 1646 +/- 8 Ma for outer zones. Preserved textures show that initial xenotime growth was early diagenetic, establishing the ca. 1700 Ma age as a proxy for the depositional age of the Mount Barren Group. The younger age (ca. 1650 Ma) is regarded as burial related. The xenotime data reduce considerably the previous limits on the age of the succession, i.e., between ca. 1850 Ma (youngest zircon population) and ca. 1200 Ma (peak metamorphism). The significant achievement of our results is establishing that early diagenetic xenotime retains its physical form and U-Pb isotopic age despite greenschist-facies metamorphism and penetrative deformation.