U-Pb ages and aluminium concentrations of colourful sapphire-related zircon megacrysts from Far North Queensland:  Merlot, Rosé and Champagne

Charlotte M Allen1, Erica J Porter1 and Mitchell DeBruyn2

1Central Analytical Research Facility, Institute for Future Environments, Queensland University of Technology, Brisbane, 2School of Mechanical, Medical and Process Engineering, Faculty of Science, Queensland University of Technology, Brisbane

Detrital sapphires are commonly found with megacrystic zircon but it is only co-location that links these free grains.  If related, zircon can constrain the circumstances by which nature creates corundum. Trail and co-workers suggested that zircons from high-Al-activity magmas (peraluminous ones) have Al contents > 4ppm, a step-jump up from Al contents from zircons from metaluminous magmas.  Aluminium is generally not considered to crystallize in zircon because of radius and charge but was included in the 24 isotopes analytical list. Twenty-seven 25 micron laser ablation spots were targeted on 3 colour groups from detrital zircon megacrysts associated with sapphires of Scot’s Camp near Undara National Park, a Lava Plains locality.  Three spots on each of three grains in each colour group were analysed by Agilent 7900 ICPMS.  Zircons were cathodoluminescence and backscatter imaged. Reported dates are concordant 206Pb/238U ages using Temora as calibrant with no Th-disequilibrium correction. Trace element concentrations (TE) are based on NIST 610 with Si as internal standard and assumed 15.22 wt% concentration.

Champagne grain #1, wholly, proved to be older than the rest at 2716 Ma (1stdev, n=3) but its cathodoluminescence, morphology and TE are not distinctive. Two other Champagne spots gave 5.5 and 4.1 Ma, ages older than Merlot grains which are consistent at 2.60.2 Ma (n=7). Rosé grains give the same age (2.90.5 Ma, n=5).

TE among the 9 grains have somewhat grain-specific compositions which is true of aluminium.  Merlot, Rosé and Champagne have average Al contents of 98, 2619 and 3323 ppm (1stdev), respectively.  The 271 Ma grain (Champagne#1) contains variable Al (4 to 78 ppm) whereas #2 is consistent at 812 ppm (n=3).  Compare these averages to our reference zircons and sources: Temora, (quartz-monzodiorite), Plešovice, (alkalic granulite), and 91500, a detrital megacryst provisionally associated with syenite (?). Respectively, they give 1.81.0, 8935 and 7.72.4 ppm Al.  The Lava Plains megacrysts, indeed, were generated from Al-rich sources.

Sapphire-associated zircons tend to be TE-poor as in a 13.6 Ma example from Mt Weldborough, NE Tasmania. Uranium is <100 ppm in Champagne and Rosé but Merlot has U from 200 to 1250 ppm save one and thus gives the more precise age. Except for Al and P, Merlot has greater TE contents.  Zircon classification based on Yb, U, and Hf indicate these are continental and/or kimberlitic zircon types. Features of note are small to zero Eu anomalies and positive Ce anomalies of 100-300 for Merlot and Rosé but 10s for Champagne (method after Blundy and Wood). These Lava Plains zircons are decidedly not-mantle-like, particularly in their Zr/Hf (>>37, the chondritic value).

The dominant age of 2.6 Ma is in the range of the oldest Lava Plains eruption ages, however, to find a morphologically non-distinctive Permian megacryst among them means that nature has repeated herself in providing the odd conditions to generate both sapphire and zircon megacrysts, crystals that have been brought to the surface by alkalic basalts of similar or much younger age. Permian is a common age among Queensland sapphire-related zircons.


Biography

Charlotte is the Director of the Central Analytical Research Facility, IFE.  She is interested in zircons, their ages and trace elements.  As collateral material from a process engineering problem, Mitch identified the zircons as interesting, and Erica named, imaged and analyzed them. Potential reference materials?

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