Fig. 6

CIA ternary diagram plotting 11 samples (unfilled circles) range between 61 and 78% from laminated mudstones in the Urapunga 4 core. Elemental data from XRF analysis. Correction of CIA for K-metasomatism (illitization)⁴⁸ following the dashed line (a) from the average upper crust value (black square) identifies a range of CIA values for these same samples up to 87%. Data from Tosca et al.³⁶ (crosses) show a lower range of values between 51 and 61% that were corrected to 72–78% (c). The sample with highest corrected CIA value (from 107 m depth) was used to evaluate the effects of replacement and authigenic clay formation (Fig. 3). The CIA value for the restored detrital composition from Fig. 3c (filled circle) shows a lower CIA value (b) resulting from the addition of kaolinite and illite as replacement of biotite and feldspar. Average upper Archean upper crust composition identified by Condie et al.⁶² and adapted by Tosca et al.³⁶ for correction for metasomatism