The Jurassic dolerites of Tasmania represent a small part of the widespread Mesozoic magmatism of Gondwana, and possess some of the most extreme isotopic and elemental signatures observed in tholeiitic rocks. This study documents the remarkable uniformity of the magma in major, trace element and isotopic composition at the time of emplacement. Many of the trace element and isotopic characteristics compare more closely with those of continental crustal rocks than with typical mantle-derived materials, indicating the dominance of a crustal component in the petrogenesis of these tholeiites. Difficulties in reproducing the trace element signatures of the dolerites using calculations involving crustal assimilation, and the unreasonable isotopic compositions required of the contaminant, make such models unlikely.We argue that a more likely explanation involves the introduction of a small quantity of sediment (≤3 wt.%) into a depleted mantle source by the process of subduction. It is envisaged that the trace element and isotopic compositions of the mantle source were overprinted by the crustal signatures owing to the low abundance of incompatible trace elements originally present. These crustal signatures were subsequently inherited by magmas produced during partial melting.