Geochemical characteristics of some Archaean greenstone suites of the Yilgarn structural province,Australia

Geochemical results representing 225 full major element analyses, mostly by XRF methods, are evaluated. This study is complementary to papers presented by the author and colleagues at the Archaean Symposium of the Geological Society of Australia in Perth in May 1971. Only certain new analyses are given in tabulation, but the variation diagrams presented are based on many additional analyses published in works referred to in the text. The emphasis of this study is a consideration of the relationship of the pre-metamorphic Archaean sills to the volcanics of the same greenstone belts: to consider the original nature of magmas differentiated after emplacement: and to more closely define the pre-metamorphic intrusives.A contrast is seen between magnesia-rich basic and ultrabasic eruptives, represented amongst both sills and volcanics, and the abyssal, K-poor tholeiite suites of the monotonous pillow lava sequences studied in detail by Hallberg (1970, 1972). A similar contrast is evident, though less emphatically, among the post-metamorphic dykes. Residually iron and alkali enriched suites are evident among the pre-metamorphic and post-metamorphic eruptives. The possibility of some degree of gradation between magnesia-rich and tholeiite suites cannot be discounted: they are intimately intermingled in space and time, and amongst intrusive and volcanic sequences. Alkali/silica plots after Macdonald and Katsura show that, whether or not the magnesia-rich rocks are regarded as tholeiitic or komatiitic (following Anhausser, Viljoen and Viljoen), no alkali igneous rocks are represented. Spilites are of no more than extremely rare occurrence. FMA plots indicate that among the pre-metamorphic volcanics and intrusives, there are eruptives, suites and differentiated bodies that resulted from accession of a number of contrasting differentiated melts or magmas from deep in the crust, possibly to suffer further differentiation in situ after emplacement. That is the parental stem to both differentiated and undifferentiated sills, and to volcanics, was a product, in many cases, of an earlier deep-seated differentiation process. A diagram based on solidification indices illustrates this, and shows that the same applies to postmetamorphic dyke intrusions.These results are discussed with reference to the komatiites of the Barberton Mountain Land, South Africa. The resemblance between komatiite analyses and olivine tholeiites of Nockolds is stressed, and it is suggested that the highly magnesian greenstone eruptive suites of South Africa and Western Australia might be regarded as an extension of the abyssal tholeiite association, rather than as a discrete compartment in petrology. The anti-uniformitarian, one-burst connotation afforded to these suites is discussed.