SHRIMP U–Pb dating of diagenetic xenotime in the Stirling Range Formation, Western Australia: 1.8 billion year minimum age for the Stirling biota

The Stirling Range Formation in southwestern Australia contains discoidal fossils previously linked to the late Neoproterozoic Ediacaran biota and possible trace fossils interpreted to have been made by vermiform, mucus-producing, motile organisms. The age of the sedimentary succession was recently reported to be between 2.0 and 1.2 billion years old, based on U–Pb geochronology of detrital zircon and metamorphic monazite, respectively. Ion microprobe U–Pb geochronology of xenotime in sandstones from the same succession produces three sets of radiometric data. Detrital xenotime grains yield ages between 3120 and 2100 Ma (consistent with the detrital zircon data; between 3460 and 1960 Ma), whereas compositionally distinct xenotime overgrowths yield a weighted mean 207Pb/206Pb age of 1800 ± 14 Ma for the oldest population and 1662 ± 15 Ma for a younger population. The new data show that the Stirling Range Formation is more than 1.8 billion years old, some 600 million years older than the minimum defined by metamorphic monazite. The results show that the various megascopic fossils reported in the succession are between 2.0 and 1.8 billion years old, emphasizing their already controversial nature. If our interpretation of the fossils is correct, then motile multicellular (or possibly syncytial) organisms had already emerged in the late Paleoproterozoic, simultaneously with the first large alga-like multicellular eukaryotes.