Microtextures, geochemistry and geochronology of authigenic xenotime: constraining the cementation history of a Palaeoproterozoic metasedimentary sequence

D.A. Vallini, Birger Rasmussen, B. Krapez, I.R. Fletcher, Neal Mcnaughton

    Research output: Contribution to journalArticlepeer-review

    39 Citations (Scopus)

    Abstract

    An approximate to 26 m thick unit of phosphatic sandstone and black shale (the Phosphatic Unit) in the Palaeoproterozoic Mount Barren Group of south-western Australia contains abundant authigenic xenotime crystals showing well-preserved diagenetic textures. Despite extensive regional deformation and thermal metamorphism, the peak of which occurred at approximate to 1205 Ma, the Phosphatic Unit was preserved as a low-strain envelope because of its pre-compaction carbonate and phosphate cementation. In situ U-Pb geochronology of xenotime reveals four discrete age populations at 1693 +/- 4, 1645 +/- 3, 1578 +/- 10 and 1481 +/- 21 Ma. When integrated with petrography, the age data place a timeframe on: (i) sediment deposition; (ii) phosphogenesis; (iii) diagenetic cement infilling; (iv) diagenetic pyrite formation; (v) secondary porosity generation; (vi) hydrocarbon migration; (vii) burial compaction; and (viii) hydrothermal alteration, up until peak thermal metamorphism. Xenotime growth at approximate to 1693 Ma occurred prior to compaction, whereas xenotime growth at approximate to 1645 Ma occurred during burial. Xenotime growth at approximate to 1580 Ma and at approximate to 1480 Ma appears to be the far-field record of thermotectonic events associated with intracontinental extension and magmatism recorded elsewhere in Australia. Geochemical analysis, integrated with geochronology, shows a systematic increase in MREE/HREE in xenotime crystals with decreasing age and with increasing stratigraphic depth. Coupled with a decrease in xenotime abundance and age with depth, it suggests that: (i) the main focus of porosity infilling was at the top of the Phosphatic Unit and progressed downwards over the > 200 Myr period of porosity infilling, and (ii) the changes in xenotime REE chemistry may be due to an influx of MREE from increasing amounts of dissolved apatite or changes, with respect to REE solubility, in the physiochemical nature of the fluids with burial depth.
    Original languageEnglish
    Pages (from-to)101-122
    JournalSedimentology
    Volume52
    Issue number1
    DOIs
    Publication statusPublished - 2005

    Fingerprint

    Dive into the research topics of 'Microtextures, geochemistry and geochronology of authigenic xenotime: constraining the cementation history of a Palaeoproterozoic metasedimentary sequence'. Together they form a unique fingerprint.

    Cite this