Geochemistry of a silicified, felsic volcaniclastic suite from the early Archaean Panorama Formation, Pilbara Block, Western Australia: an evaluation of depositional and post-depositional processes with special emphasis on the rare-earth elements

Abstract

Many elements in a suite of pervasively silicified, dacitic volcaniclastic and volcanic rocks from the early Archaean Panorama Formation, Warrawoona Group, Western Australia, were strongly depleted or enriched relative to those in equivalent unaltered units during widespread, post-depositional metasomatism. The present compositions of the altered rocks are dominated by Al 2O 3, K 2O, and SiO 2 and lie on or close to a quartz-sericite mixing line. The Al contents were probably inherited from the original dacitic rocks while Si, some K and possibly Rb may have been introduced during metasomatism. The elements Mg, Ca, Na and, to a lesser extent, Fe, Mn, and Sr, have been depleted relative to Al during alteration whereas silica has been greatly enriched. The high and variable Ba concentrations suggest that Ba, like Si, was introduced during silicification. The REE and Th were relatively immobile during alteration, and were diluted during silicification in the same proportion as Al. Most of the rocks represent detrital sand- and silt-sized felsic tephra, debris eroded from felsic volcanic rocks or ash-fall deposits that underwent a short interval of transport and deposition dominated by physical rather than chemical processes. A few siltstones contain clay minerals that probably formed by the chemical weathering of Warrawoona Group basalts. The siltstones have lower Th and La concentrations, lower La Lu ratios, and higher Sc and Cr concentrations than felsic rocks in the suite. Regional alteration pervasively silicified the original sediments shortly after deposition. The replacement mineral assemblage indicates a maximum temperature for hydrothermal alteration of about 350°C. Because of the lack of chemical weathering and the low temperature of post-depositional alteration, REE abundances in volcaniclastic rocks of the Panorama Formation reflect their felsic volcanic provenance and are interpreted to have preserved a relict magmatic signature. The lack of a negative Eu anomaly and high La Lu ratio (chondrite-normalized ratio is 7 to 39) suggest that magmas of the Panorama Formation formed by direct melting of an eclogite residue without subsequent crystallization of feldspar during the commencement of cratonization of the Pilbara Block. This origin differs from the origin of some other silicic volcanic rocks of the Duffer Formation, representing earlier felsic volcanic activity of the Warrawoona Group, that were produced by fractional crystallization from basaltic parents. This suggests a polygenetic origin for felsic magmas of the Warrawoona Group.