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Decoding near-concordant U-Pb zircon ages spanning several hundred million years: recrystallisation, metamictisation or diffusion?


Halpin, JA and Daczko, NR and Milan, LA and Clarke, GL, Decoding near-concordant U-Pb zircon ages spanning several hundred million years: recrystallisation, metamictisation or diffusion?, Contributions to Mineralogy and Petrology, 163, (1) pp. 67-85. ISSN 0010-7999 (2012) [Refereed Article]

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Copyright 2011 Springer-Verlag

DOI: doi:10.1007/s00410-011-0659-7


In situ isotopic (UPb, LuHf) and trace element analyses of zircon populations in six samples of the intrusive Mawson Charnockite, east Antarctica, emphasise complex zircon behaviour during very high-grade metamorphism. The combination of geochemical data sets is used to distinguish xenocrysts and identify a population of primary igneous zircon in situations where UPb data spread close to concordia over a few hundred Myr. The population is filtered to exclude grains with: (1) UPb ages [2% discordant, (2) anomalous trace element-content (Th, U, Y, REE) and (3) outlying Hf-isotopic values. Rare metamorphic-type grains were also excluded. Upon filtering the population, minimum emplacement ages for each sample were determined using the oldest grain(s). This approach improves upon age determinations in complex data sets that use weighted mean or isochron methods. Our results suggest that the Mawson Charnockite was emplaced episodically at c. 11451140 Ma, c. 10801050 Ma and c. 985960 Ma. Core-outer core-rim and core-rim textures were identified but are not correlated with UPb ages. We establish that recrystallisation (mainly of zircon rims) must have occurred shortly following igneous crystallisation and that metamictisation/cracking is a Paleozoic to Recent event. Therefore, intra-zircon diffusion in a high-T, highstrain environment during Meso-Neoproterozoic orogenesis is inferred to have caused the extensive UPb isotopic disturbance. Charnockitic magmatism prior to c. 1,000 Ma has not previously been recorded in the Mawson region and indicates that orogenesis may have commenced c. 150 Myr earlier than previously thought. Correlations with similar aged rocks in adjacent regions have implications for supercontinent reconstructions.

Item Details

Item Type:Refereed Article
Keywords:Rayner Orogen; Mawson Charnockite; East Antarctica; Hf-isotopes; Trace elements; Pb-loss; Laser Ablation-Inductively Coupled Plasma Mass Spectrometer
Research Division:Earth Sciences
Research Group:Geology
Research Field:Geochronology
Objective Division:Expanding Knowledge
Objective Group:Expanding knowledge
Objective Field:Expanding knowledge in the earth sciences
UTAS Author:Halpin, JA (Dr Jacqueline Halpin)
ID Code:75911
Year Published:2012
Web of Science® Times Cited:78
Deposited By:Centre for Ore Deposit Research - CODES CoE
Deposited On:2012-02-20
Last Modified:2017-04-11

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