Continental margin

Abstract: The graywackes of Paleozoic turbidite sequences of eastern Australia show a large variation in their trace element characteristics, which reflect distinct provenance types and tectonic settings for various suites. The tectonic settings recognised are oceanic island arc, continental island arc, active continental margin, and passive margins. Immobile trace elements, e.g. La, Ce, Nd, Th, Zr, Nb, Y, Sc and Co are very useful in tectonic setting discrimination. In general, there is a systematic increase in light rare earth elements (La, Ce, Nd), Th, Nb and the Ba/Sr, Rb/Sr, La/Y and Ni/Co ratios and a decrease in V, Sc and the Ba/Rb, K/Th and K/U ratios in graywackes from oceanic island arc to continental island arc to active continental margin to passive margin settings. On the basis of graywacke geochemistry, the optimum discrimination of the tectonic settings of sedimentary basins is achieved by La-Th, La-Th-Sc, Ti/Zr-La/Sc, La/Y-Sc/Cr, Th-Sc-Zr/10 and Th-Co-Zr/10 plots. The analysed oceanic island arc graywackes are characterised by extremely low abundances of La, Th, U, Zr, Nb; low Th/U and high La/Sc, La/Th, Ti/Zr, Zr/Th ratios. The studied graywackes of the continental island arc type setting are characterised by increased abundances of La, Th, U, Zr and Nb, and can be identified by the La-Th-Sc and La/Sc versus Ti/Zr plots. Active continental margin and passive margin graywackes are discriminated by the Th-Sc-Zr/10 and Th-Co-Zr/10 plots and associated parameters (e.g. Th/Zr, Th/Sc). The most important characteristic of the analysed passive margin type graywackes is the increased abundance of Zr, high Zr/Th and lower Ba, Rb, Sr and Ti/Zr ratio compared to the active continental margin graywackes.

Abstract: Volcanic are basalts can usually be distinguished from basalts erupted in other settings by their selective enrichments in LIL elements and also, in some cases, in light rare-earth elements and P. Ibis enrichment component is independent of the nature of the overlying mantle wedge and is assumed to be subduction-derived. However, basalts erupted at active continental margins carry an additional geochemical component, which is not observed in basalts front oceanic island arcs. This component, which contains Nb, Ta, Zr and Hf in addition to the elements mentioned above, cannot realistically have a subduction or crustal origin and is thus assumed to be derived from trace element enriched metasomatized sub-continental lithosphere. The relative contributions of the subduction and lithosphere components to the composition of active continental margin basalts can be calculated from their MORB-normalized geochemical patterns. In a typical basalt, from Central Chile, the subduction zone can be shown to make the dominant contribution to its content of Rb, K, Ba, 71 and Sr, whereas the lithosphere contributes the greater proportion of its content of Ce, Sm and P and all its content of Ta, Nb, Zr, Hf, Ti, Y and Yb.