Geochemistry of dike rocks in Dun Mountain Ophiolite, Nelson, New Zealand

Basaltic dikes of the Lee River Group (Permian) at Roding River and in the Red Hills ultramafic body, both parts of the Dun Mountain Ophiolite, were analysed for major and trace elements. The dikes in the Lee River Group change into massive and pillowed lava towards the uppermost part of the sequence. The dikes in the Red Hills occur parallel to the layering of host ultramafic rocks. The Red Hills ultramafic body corresponds to the mantle-crust transition zone. The age of Red Hills dikes has been determined at c. 280 Ma. Hence, the Lee River Group and Red Hills dike complexes are about the same age. There are contrasting differences, however, between major and trace element compositions in those two dike rock groups. Lee River Group basalts show an increase in FeO*/MgO with increase in SiO2; high field strength elements and REE concentrations are higher in the Lee River Group than those in the Red Hills basalts; and epsilon Nd values are +7.5 to +8.3, suggesting the source to be depleted mantle. On the other hand, Red Hills basalts do not show an increase in FeO*/MgO with increase in SiO2, and are relatively enriched in large ion lithophile elements such as Ba and Sr, depleted in high field strength elements and REE, and epsilon Nd values (+6.2 and +7.1) of plagiogranites associated with the Red Hills basalts are lower than those in the Lee River Group basalts. Lee River Group basalts have MORB or back-arc basin basalt affinities, whereas Red Hills basalts have island arc related characteristics. The new set of geochemical data is consistent with the Red Hills ultramafic body being a section of residual uppermost mantle previously described. The Dun Mountain Ophiolite is here suggested to have been derived from an island arc related tectonic environment following the extraction of MORB in a back-arc basin at 280 Ma.