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

A newly discovered, roughly east-west-trending Early Jurassic (191Ma) dyke population was studied in the Freestone Creek area, eastern Gippsland, Vict oria. Twelve dykes, with individual thicknesses of 1-3 m are exposed in a 2 km-long creek section. Structural evidence suggests that the Freestone dyk es were emplaced during north-northeast-south-southwest extension. The dyke s are fine-grained, porphyritic, alkali-basaltic rocks with phenocrysts of pyroxene, kaersutitic amphibole (Mg# 70-60), olivine (Fo(80-75)) and plagio clase (An(65-35)). Three chemically distinct populations of clinopyroxene a re observed. All except one dyke contain abundant granitic xenoliths. The d ykes have high total alkalies of 5.5 wt%; MgO content varies between 6.4 an d 8 wt%. N-MORB normalised trace-element patterns are similar to those of O IB magmas. Elemental abundances also reveal similarities with the two alkal i-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, R b, Nd, Ce and Nb, and depletion in Th, Ni and Cr compared to the other mafi c rock types in the area. We suggest that the source for Freestone dykes wa s EMI-type mantle, geochemically similar to the source for the Newer Volcan ics in western Victoria. Their age and position on the former southern edge of the Gondwana continental flood-basalt province suggests that these dyke s may be related to the early stage of Gondwana breakup.