THE RECORD OF EARLY CRUSTAL EVOLUTION PRESERVED IN DETRITAL ZIRCONS FROM MOUNT MURCHISON METASEDIMENTARY ROCKS, WESTERN AUSTRALIA

The Mt. Narryer and Jack Hills metasedimentary belts of the Narryer Terrane of Western Australia have been the subject of intense study for almost thirty years because they contain ca. 4.35 Ga detrital zircons, which are the oldest minerals known on Earth. These rocks also contain numerous younger populations of Hadean and Archean detrital zircons of diverse provenance, preserving a rich archive of information on early crustal evolution. Largely ignored has been a third major sequence of metasedimentary rocks in the Narryer Terrane, located at Mt. Murchison, 27 km south-southeast of Mt. Narryer and 98 km southwest of the Jack Hills. The detrital zircon population at Mt. Murchison can provide insights into the extent of Hadean sources in the Narryer Terrane, and whether magmatic events that produced detrital zircons in this region were episodic or continuous during the Archean.

The Mt. Murchison metasedimentary belt is 5 km long and 2 km wide, and contains fuchsitic quartzite, bedded coarse-grained quartzite, glassy quartzite, and quartz pebble conglomerate that appear similar in appearance to mature clastic units at Mt. Narryer and the Jack Hills. We have determined the U-Pb ages of detrital zircons in a sample of fuchsitic quartzite from Mt. Murchison using laser ablation-inductively coupled plasma mass spectrometry. Forty-one detrital zircon grains in the sample are concordant within 10%. Two-thirds of the grains have ²⁰⁷Pb/²⁰⁶Pb ages that fall within two populations: 3.20 ± 0.05 Ga (34%) and 3.42 ± 0.05 Ga (32%). There is a minor age peak (15%) at 3.54 ± 0.04 Ga. The oldest grain has a ²⁰⁷Pb/²⁰⁶Pb age of 3646 ± 18 Ma (2s); the youngest grain is 3054 ± 14 Ma (2s).

The detrital zircon population of the Mt. Murchison sample differs from those of quartzites and conglomerates from Mt Narryer and the Jack Hills in the lack of grains older than 3.7 Ga, and the presence of a large population of 3.2 Ga grains. On the other hand, the 3.4 Ga age peak in the Mt. Murchison sample is also a prominent detrital zircon age population in the Jack Hills, and 3.5 Ga sources contributed to all three metasedimentary belts. This suggests that while there was significant age heterogeneity in the detrital sources of the Narryer Terrane, there were also some common sources that linked the paleodrainage systems of the Mt. Murchison, Mt. Narryer and Jack Hills areas.