New Isotopic Constraints on the Age of the Halls Reward Metamorphics, North Queensland, Australia: Delamerian Metamorphic Ages and Grenville Detrital Zircons

The Halls Reward Metamorphics in North Queensland, Australia, consist mainly of mica schist, and have previously been regarded as a part of the Proterozoic craton, and separated from the Paleozoic orogenic zone on its eastern margin by the Tasman Tectonic Line. However, our Rb-Sr dating by whole-rock-muscovite isochron from the Halls Reward Metamorphics shows ca. 500 Ma. K-Ar dating of muscovite in the quartz-mica schist also shows the same age. Furthermore, U-Th-Pb chemical dating (CHIME) gives ca. 510 Ma crystallization age for monazite. These results suggest that the Halls Reward Metamorphics is not the part of the Proterozoic craton, but belongs to the early Paleozoic Delamerian orogenic belt extending the length of the eastern Gondwana margin. The location of the Tasman Tectonic Line should, therefore, be west of the Halls Reward Metamorphics. b chemical dating of detrital zircon from quartz-mica schists also shows an age distribution of peak at ca. 1000 Ma, with some older components. The presence of 1000 Ma (Grenvillian) detrital zircon from both geological units is consistent with the close proximity of Grenville-age source suggested by the AUSMEX continental reconstruction.