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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |