Geochemical and Isotopic Constraints on the Origin of the Jurassic Dolerites of Tasmania

The Jurassic dolerites of Tasmania represent a small part ofthe widespread Mesozoic magmatism of Gondwana, and possess someof the most extreme isotopic and elemental signatures observedin tholeiitic rocks. This study documents the remarkable uniformityof the magma in major, trace element and isotopic compositionat the time of emplacement. Many of the trace element and isotopiccharacteristics compare more closely with those of continentalcrustal rocks than with typical mantle-derived materials, indicatingthe dominance of a crustal component in the petrogenesis ofthese tholeiites. Difficulties in reproducing the trace elementsignatures of the dolerites using calculations involving crustalassimilation, and the unreasonable isotopic compositions requiredof the contaminant, make such models unlikely. We argue that a more likely explanation involves the introductionof a small quantity of sediment (3 wt.%) into a depleted mantlesource by the process of subduction. It is envisaged that thetrace element and isotopic compositions of the mantle sourcewere overprinted by the crustal signatures owing to the lowabundance of incompatible trace elements originally present.These crustal signatures were subsequently inherited by magmasproduced during partial melting.