Freestone dykes-an alkali-rich Jurassic dyke population in eastern Victoria

A newly discovered, roughly east-west-trending Early Jurassic (191 Ma) dyke population was studied in the Freestone Creek area, eastern Gippsland, Victoria. Twelve dykes, with individual thicknesses of 1-3 m are exposed in a 2 km-long creek section. Structural evidence suggests that the Freestone dykes were emplaced during north-northeast-south-southwest extension. The dykes are fine-grained, porphyritic, alkali-basaltic rocks with phenocrysts of pyroxene, kaersutitic amphibole (Mg 70-60), olivine (Fo_80-75) and plagioclase (An_65-35). Three chemically distinct populations of clinopyroxene are observed. All except one dyke contain abundant granitic xenoliths. The dykes have high total alkalies of 5.5 wt%; MgO content varies between 6.4 and 8 wt%. N-MORB normalised trace-element patterns are similar to those of OIB magmas. Elemental abundances also reveal similarities with the two alkali-basaltic dykes in the Tambo Crossing area and Newer Volcanics in western Victoria. The Freestone dykes however are distinguished by their potassium/sodium ratio, which is close to unity, an extensive enrichment in Ba, Sr, Rb, Nd, Ce and Nb, and depletion in Th, Ni and Cr compared to the other mafic rock types in the area. We suggest that the source for Freestone dykes was EMI-type mantle, geochemically similar to the source for the Newer Volcanics in western Victoria. Their age and position on the former southern edge of the Gondwana continental flood-basalt province suggests that these dykes may be related to the early stage of Gondwana breakup.