Abstract

Major and trace elements of metasediments of the granulite facies Narryer and the amphibolite facies Jack Hills metasediments have been used to devise plots which enable the original sedimentary components of these rocks to be unravelled, despite their metamorphic state. These plots overcome the predominant effect of quartz which ranges up to 100% and can be used to study other metasediment suites. Six components controlled the composition of the clastic metasediments (1) kaolinite, (2) illite-muscovite, (3) chlorite–montmorillonite, (4) dolomite, (5) quartz, (6) resistant heavy minerals such as rutile, zircon, chromite and monazite. Identifying the first four of these components enables the trace element composition associated with the components to be estimated. Feldspar was largely destroyed by chemical weathering but there is a suggestion that traces of it formed a minor seventh component in the Narryer rocks. There was little or no mixing of the original kaolinite-bearing and illite–muscovite-bearing samples, indicating two distinct sources, although both sets of samples contain >4 Ga zircons, suggesting a third zircon source. The old zircons may have been derived from re-worked sediment or metasediment as they occur in the matrix and in the quartzite clasts of conglomerates. Using Bhatia's criteria, a passive margin is the likely site of deposition. Calc alkaline tholeiites in the Narryer succession are intrusive and did not contribute to the sediments which were derived from extremely heavily weathered granites, komatiites, schists and reworked sandstones or quartzites.

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