Rare earth element geochemistry of Australian Paleozoic graywackes and mudrocks: Provenance and tectonic control

Abstract

In the Paleozoic graywackes of eastern Australia there is an increase in the total abundance of rare earth elements (REE), a light to heavy REE ratio and a decrease in the chondrite normalized Eu anomaly with the increase in SiO 2/Al 2O 3 and K 2O/Na 2O ratios. This is due to the change in the dominant source rocks from andesite to dacite to granite-gneiss and sedimentary rocks. Similarly, an increase in total REE and a decrease in Eu anomaly is also seen in the associated mudrocks with the increase in total clay content. This correlation in Paleozoic sedimentary rocks suggests that the REE characteristics of terrigenous sedimentary rocks reflect the depositional tectonic setting of the basin. The oceanic island-arc-type graywackes are characterised by their lower total REE abundance, only slight enrichment of LREE over HREE, and the absence of a negative Eu anomaly on chondrite-normalised plots. The continental island arc type graywackes are discriminated by their higher REE abundance and La/Yb ratio, the presence of a small negative Eu anomaly on the chondrite normalised plots and the significant enrichment of Eu compared to PAAS (Post-Archean Average Australian Shale). The sedimentary rocks deposited on Andean-type active continental margins, passive margins, platform and cratonic basins are all characterised by the high enrichment of LREE over HREE and the presence of a pronounced negative Eu anomaly on chondrite-normalised plots and can not be discriminated from each other by REE patterns alone.