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.