Queensland Geological Record 2009/03
Joint GSQ-GA NGA geochronology project New England Orogen and Drummond Basin, 2008
This report presents new SHRIMP U-Pb zircon results for 10 samples from the New England Orogen and Drummond Basin carried out under the auspices of the National Geoscience Agreement (NGA) between Geoscience Australia and the Geological Survey of Queensland during the 2007-2008 financial year. A companion report completed under the same agreement contains the results of 11 new SHRIMP U-Pb zircon results from the Mount Isa region. The data and age interpretations contained in these reports are available in the OZCHRON database, maintained by Geoscience Australia.
New SHRIMP U-Pb zircon ages of ~360Ma for the Silver Hills Volcanics at the Glen Eva Pit (200701501) near Mount Coolon and Bimurra Volcanics south of Conway Station (200701503) have extended our knowledge of the timing and temporal distribution of cycle one volcanic rocks of the Drummond Basin. This is a significant contribution as all known economic epithermal gold-silver mineralisation in the rummond basin is hosted by cycle one volcanics and volcaniclastics.
SHRIMP U-Pb zircon dating of the Eurimbula Granite (200701526) and Agnes Water Volcanics (2007015250) in the Bundaberg region has shown that these two units have similar ages of ~227Ma. This finding supports the genetic link proposed for these two units and coupled with field evidence, suggests that the Eurimbula Granite intruded its own volcanic pile within a relatively short period following the eruption of the Agnes water Volcanics. Additionally, these new data support a major Late Triassic magmatic event highlighted by previous geochronology).
The Jibbinbar Granite in southern Queensland was formerly classified by Olgers as a part of the Late Permian Ruby Creek Granite. Recently however, based on geochemical and mineralogical grounds, it has been redefined as belonging to the Mount You You Supersuite. SHRIMP U-Pb zircon dating of the Jibbinbar Granite (200701539) has shown that it crystallised at ~298Ma, confirming the reclassification of this granite as distinct from the Ruby Creek Granite. The crystallisation ages of other undeformed granites in the Texas region which also intrude accretionary wedge sediments (i.e. Bullaganang Granite, ~291Ma and the Greymare Granodiorite, ~280Ma), were recently used as possible minimum age constraints for the timing of the Texas megafold event. The new age reported here for the Jibbinbar Granite potentially places an even older minimum age for the timing of megafolding in the Texas region.
The Mingimarny Granite is the most northerly pluton exposed in the New England Batholith. It is a leucogranite and, given its location in the far north-west of the batholith and the relatively old SHRIMP ages yielded by other leucogranites (Bullaganang and Jibbinbar Granites) in the western part of the batholith, the Mingimarny Granite may have also been expected to yield a comparatively old age. Instead it is one of the youngest units dated in the western part of the batholith, and is of similar age to the Boxwell Granodiorite (~257Ma) exposed farther south in the Texas area. The results indicate that the New England Batholith contains leucogranites of several discrete ages.
During the Late Permian, the New England region of northern New South Wales and Southern Queensland underwent a major period of explosive silicic volcanism. During this time, the mainly terrestrial Wandsworth Volcanic Group was emplaced. SHRIMP U-Pb zircon dating of an ignimbrite from the Wallangarra Volcanics (200701541) has a crystallisation age of 253±2.4Ma. This result helps constrain the timing of Wandsworth Group volcanism and is well within the ~254Ma SHRIMP and TIMS ages defined for other units of the Wandsworth Volcanic Group.
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