Australia: The Land Where Time Began

A biography of the Australian continent 

Narryer Gneiss Complex, Western Australia – SHRIMP U-Pb Geochronology

It has been confirmed by SHRIMP U-Pb Geochronology of the Narryer Gneiss Complex, Western Australia of the Yilgarn Craton that there are abundant gneisses, 3,300-3,730 Ma,  from the Early Archaean in its eastern half, having found gneisses with ages of 3,000 Ma or less in the western half. The gneisses of Early Archaean age in the southern part of the Narryer Gneiss Complex, that has been studied more intensively, are divided into the Eurada Gneiss Association, where the oldest rocks are 3,490 Ma, and the Nookawarra Gneiss Association, the oldest rock of which are 3,730 Ma. Only the oldest components in these associations are tectonic, with all the younger components of both associations being of granodioritic or granitic composition. Different histories are shown by these associations until granites and pegmatites were emplaced at 3,280-3,300 Ma, at which time they might have been tectonically juxtaposed. Both of these gneiss associations have been found to be intercalated with the Narryer Supracrustal Association, which contains sediments which are rich in quartz and many detrital zircons. Mt Narryer and Jack Hills are formed from the largest exposures of this association, where of the detrital zircons that are present a small proportion is of an age that is greater than 4 Ga. The age populations of detrital zircons vary from sample to sample. These age populations have been shown by analysis to have possibly been derived from rocks of the Nookawarra and Eurada Gneiss Associations in varying populations, apart from zircons of 3,800-4.280 Ma age, the source of which is still unknown.

Intrusion of several generations of granite sheets, intercalation with the Narryer Supracrustal Association and folding that occurred between 2,750 Ma and 2,620 Ma, which was followed by movement on subvertical shear zones, either at the end of the Archaean or the in the Proterozoic,   have obliterated the original relationships between the gneiss associations of the Early Achaean. The events occurring in the Late Archaean coincide with similar events throughout the Yilgarn Craton, and are believed to be concomitant with craton growth, probably by the assembly with terranes, probably in several distinct episodes.  

Sources & Further reading

1. Nutman, A. P., P. D. Kinny, W. Compston and I. S. Williams (1991). "SHRIMP U-Pb zircon geochronology of the Narryer Gneiss Complex, Western Australia." Precambrian Research 52(3–4): 275-300.