What do ~10 000 whole rock geochemical analyses tell us about Australian proterozoic intraplate igneous activity?
[back to top] Wyborn et al. (1997) have shown that in most major Proterozoic magmatic provinces the dominant Sr-depleted, Y-undepleted I-(granodioritic) types can be divided into three groups which show a time progression in geochemistry. The oldest group (Group 1) at 1870-1850 Ma comprises restite-rich suites. Group 2, emplaced at 1840-1800 Ma is a low-Ca type that shows evidence of magmatic fractionation. The youngest group (Group 3) is enriched in incompatible elements and comprises three subgroups: Subgroup 31, dated at around 1800-1780 Ma, has very high values of Zr, Nb and Y; Subgroup 32, usually emplaced between 1760 and 1650 Ma, is enriched in F and has variable amounts of Y, Zr, and Nb; and Subgroup 33, emplaced from 1640 to 1500 Ma, is more oxidised with a wide range in SiO2 values. A simple explanation for the geochemical evolution from Groups 1 to 3 is that as the temperature in the source region increases, the magma is dominated first by minimum melt, then by biotite break
