Zhongwu Lan

New xenotime ages obtained from the Paleoproterozoic Kimberley Group, NW Australia: implications for regional hydrothermal events

New xenotime ages are obtained from the Paleoproterozoic King Leopold Sandstone of the Kimberley Group, NW Australia. The concordant age of 1679 ± 13 Ma can be equated with the ca 1.7 Ga xenotime age within errors obtained from the Warton and Pentecost Sandstones of the same group. These ages are much younger than the robust SHRIMP ages (1790 ± 4 Ma) obtained from zircons from the dolerite dyke intrusions, and thus indicate that the xenotime growths recorded in the Kimberley Group are unlikely to be diagenetic in origin but may have resulted from the post-depositional hydrothermal event. The overgrowth of xenotime recorded from these three units of the Kimberley Group was probably caused by the same hydrothermal event that took place ca 1.7 Ga ago in Kimberley. Radiogenic Pb loss occurred at ca 325 Ma, which could have been caused by the 400–300 Ma Alice Springs Orogeny. This Pb loss event was responsible for the discordant U–Pb ages from 1.6 to 1.1 Ga.

A new SIMS zircon U–Pb date from the Ediacaran Doushantuo Formation: age constraint on the Weng'an biota

As a well-known phosphatized Lagerstatte, the Ediacaran Weng9an biota in central Guizhou Province of South China contains diverse acanthomorphic acritarchs, algal thalli, tubular microfossils as well as various spheroidal fossils. These fossils provide crucial palaeontological evidence for the radiation of multicellular eukaryotes after the termination of the Neoproterozoic global glaciation. While the Weng9an biota is generally considered as early Ediacaran in age on the basis of phosphorite Pb–Pb isochron ages ranging from 572 Ma to 599 Ma, the reliability and accuracy of these age data have been questioned and some geologists have proposed that the Weng9an biota may be younger than 580 Ma instead. Here we report a SIMS zircon U–Pb age of 609 ± 5 Ma for a tuffaceous bed immediately above the upper phosphorite unit in the Doushantuo Formation at Zhangcunping, Yichang, South China. Litho-, bio- and chemostratigraphic correlations suggest that the upper phosphorite unit at Zhangcunping can be well correlated with the upper phosphorite unit at Weng9an, which is the main horizon of the Weng9an biota. We therefore conclude that the Weng9an biota could be as old as 609 ± 5 Ma.

Paleodictyon from a nearshore paleoenvironmental setting in the Guadalupian (Middle Permian) of the Carnarvon Basin, Western Australia

Additional material of Paleodictyon from the Guadalupian (Middle Permian) Mungadan Sandstone (Kennedy Group) of the Carnarvon Basin, Western Australia, is documented here. The Mungadan Sandstone was deposited in a nearshore depositional setting as evidenced by the lithofacies, ichnofacies and fossil assemblage. The Carnarvon Basin specimens agree well with the morphology of Paleodictyon. SEM images of a mesh string/burrow show abundant framboids, which are interpreted as the result of decomposition of organic remains of the tracemakers. The Carnarvon Basin specimens provide an additional example that tracemakers of Paleodictyon live in much broader range of habitats than commonly assumed, ranging from deep sea to shoreface/nearshore settings. Broad spatiotemporal distributions suggest that Paleodictyon traces could have been made by different organisms with similar behaviours during the Phanerozoic.

Hydrothermal origin of the Paleoproterozoic xenotime from the King Leopold Sandstone of the Kimberley Group, Kimberley, NW Australia: Implications for a ca 1.7 Ga far-field hydrothermal event

A mineral chemical study by electron microprobe microanalyser was carried out to decipher the origin of xenotime overgrowths from the King Leopold Sandstone, the Kimberley Group, NW Australia. Results show that the King Leopold Sandstone xenotime overgrowths are characterised by LREE depletion and MREE–HREE enrichment. The King Leopold Sandstone xenotime overgrowths also show higher Zr content and lower U, Fe, and Lu contents, suggestive of hydrothermal origin rather than diagenetic origin, as previously claimed. The intrusive contact relationships between the Hart Dolerite and King Leopold, Warton and Pentecost sandstones suggest that xenotime overgrowths from the latter two units should also be hydrothermal in origin. The differing U and Th concentrations suggest a different chemical pore fluid would have been responsible for the formation of xenotime overgrowths from the King Leopold, Warton and Pentecost sandstones as the depth of siliciclastic sediments reached the zone of xenotime precipitation. The ca 1.7 Ga ages of xenotime overgrowths from the King Leopold Sandstone can be equated with the ca 1.7 Ga xenotime overgrowth ages obtained from the Warton and Pentecost sandstones of the same group and the ca 1.7 Ga zircon U–Pb ages from the Dingo Granite in northern Australia, and together they are consistent with the age of Capricorn orogeny. As such, the ca 1.7 Ga ages point to a common hydrothermal event possibly associated with the low temperature far-field Dingo Granite intrusion during Capricorn orogeny in the Paleoproterozoic Australia. The Paleozoic Alice Springs Orogeny resulted in radiogenic lead loss and thus is likely responsible for the 1.6–1.3 Ga discordant ages from the King Leopold, Warton and Pentecost sandstones.