Archaean Geochemistry

pp 47-72

Archaean Sedimentary Rocks and Their Relation to the Composition of the Archaean Continental Crust

  • S. M. McLennan
  • , S. R. TaylorAffiliated withResearch School of Earth Sciences, Australian National University

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The composition of post-Archaean terrigenous clastic sedimentary rocks is very uniform for several insoluble trace elements (e.g. REE, Th, Sc) and is thought to reflect the composition of the exposed upper continental crust. In contrast, Archaean sedimentary rocks display highly variable trace element abundances, but on average have lower levels of incompatible elements (Th, LREE), lower La/Yb ratios and lack the Eu depletion characteristic of post-Archaean sedimentary rocks. Early Archaean terrigenous sedimentary rocks also have high abundances of Cr and Ni. An examination of tectonic, sedimentological and trace element evidence indicates that all of the abundant Archaean igneous lithologies play a role in the provenance of Archaean sedimentary rocks. It is concluded that, although caution is warranted, information about the nature of the Archaean upper crust can be obtained from the sedimentary data. Systematic variations of trace element patterns (e.g. REE, La/Sc, Co/Th) in Archaean sedimentary rocks provide persuasive evidence for a dominant origin from the common Archaean bimodal mafic-felsic igneous suite. On average, a 1:1 mix is indicated for the Archaean upper crust. Such a model results in a considerably less differentiated composition compared to the present day upper crust. The general lack of Eu depletion in Archaean sedimentary rocks and evidence for garnet fractionation during the information of the felsic end member indicates that formation of both basic and felsic components of the Archaean crust occurred at mantle depths (>40 km). In contrast, the formation of the latest Archaean and post-Archaean upper crust took place by intra-crustal melting, forming K-granites and granodiorites with Eu depletion.