Australia: The Land Where Time Began

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A Sillimanite-Bearing Quartzite from Mt Narryer, Western Australia with a High Abundance of Grains and Age-distribution of Detrital Zircons from the Early Archaean

In this paper Pidgeon & Nemchin report the results of their SHRIMP U-PB analyses carried out on a population of zircons from a sample of quartzite containing sillimanite from the Narryer sedimentary succession in the Narryer Terrane of the northwestern Yilgarn Craton. The detrital zircons define 2 distinctive age groups, an older group from 4,000 - 4,280 Ma, and a younger group from 3,750 3,250 Ma. The abundance of about 12 % present in the older group zircons greatly exceeds the abundance in the first discovery reported of ancient zircons in a quartzite from the Narryer metasediments, being equivalent to the abundance of >3,900 Ma zircons in metaconglomerate sample W74 from Jack Hills, which is confirmed by new measurements that are reported in this paper. According to Pidgeon & Nemchin most analyses of detrital populations of zircons from Narryer and Jack Hills are discordant. Strong recent loss of Pb dominates the analyses of zircons from Jack Hills, whereas a more complex history was found in the analyses Narryer zircons which have experienced at least 1 Pb loss event, which was possibly associated with high-grade metamorphism that occurred at about 2,700 Ma, following which there was further disturbance of the U-Pb systems in relatively recent times. A complex provenance is suggested for the ancient zircons by the age distributions of the older, >3,900 Ma, zircons from the Narryer and Jack Hills samples being different, though the number of analyses is limited and many of the zircon analyses are discordant. The distribution of ages for the zircons in the younger population from Mt Narryer is similar to that reported for zircons from the surrounding Meeberrie Gneiss, which supports previous suggestions that zircons from the gneisses or their precursors were a major contributor to the suite of detrital zircons. The strong age peak from 3,600-3,750 Ma in the population of zircons from Narryer is lacking in the younger zircon population from Jack Hills sample W74, and conversely the strong age group of zircons at about 3,500-3,350 Ma in the population from Jack Hills is represented only weakly in the Narryer zircon population. When comparing zircon populations from quartzite populations at other locations in the Yilgarn Craton the age distributions of the zircon populations from Narryer and Jack Hills are taken for benchmarks.

Sources & Further reading

  1. Pidgeon, R. T. and A. A. Nemchin (2006). "High abundance of early Archaean grains and the age distribution of detrital zircons in a sillimanite-bearing quartzite from Mt Narryer, Western Australia." Precambrian Research 150(34): 201-220.

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 07/01/2015
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading