SUBDUCTION MODEL FOR THE ORIGIN OF SOME DIAMONDS IN THE PHANEROZOIC OF EASTERN NEW-SOUTH-WALES

Eastern New South Wales has produced over 500 000 carats of diamonds, mostly from the Copeton-Bingara area in the north. A local derivation is indicated by their distinct tribal character and lack of surface da mage, while carbon isotopic values and composition of mineral inclusio ns are unlike those in diamonds from conventional diamond-producing ar eas, for example Yakutia (Siberia), Kapvaal (South Africa), or Argyle (Western Australia). The eastern New South Wales tectonic setting is a lso unlike that for 'conventional' diamonds, involving a Phanerozoic s equence of accreted subduction terranes, with relatively thin hot crus t. A subduction diamond model is developed to explain the origin and g eology of eastern New South Wales diamonds. This model involves prolon ged subduction of mature oceanic crust, resulting in the development o f a low-temperature metamorphic window into the diamond stability fiel d within the downgoing slab at half the depth required by conventional models. The diamonds are preserved at depth by termination of subduct ion, and brought to the surface by obduction or by entrainment in suit able magmas. The type of diamond formed depends on the original source rock (diamond affinity is shown in brackets): carbonaceous-rich and c arbonaceous-poor trench sediments (CaFeMgMn-silicate, eclogitic), grap hite-poor basalt and gabbro (eclogitic) and Iherzolite (peridotitic). The model predicts that the New South Wales diamonds are young (Phaner ozoic) and that their features, age, associated minerals and geographi c distribution relate to New South Wales tectonic provinces. The subdu ction diamond model extends the range of indicator minerals to include macro-diamond, corundum, Na-garnet, with a new series of carrier magm as (basanite, nephelinite, leucitite). Unsuccessful previous explorati on in New South Wales, based on conventional models, has produced some positive results when reinterpreted in terms of the subduction diamon d model. A reevaluation of diamond potential of eastern New South Wale s is warranted.