Key incompatible element ratios for metabasalts from the Early Permian Croisilles and Patuki Volcanics, South Island, New Zealand, provide constraints on the tectonic environment in which their precursor ophiolitic protolith formed. Two geochemically distinct metabasalt suites are recognised among both the Croisilles and Patuki rocks. Metabasalts from one suite exhibit the geochemical signatures of depleted tholeiites from oceanic spreading ridges (N-type MORB) or spreading centres in evolved back-arc basins. These N-type tholeiites show high Zr/Nb ratios (Zr/Nb = 37-62) and are depleted in light rare earth elements (LREE)((La/Y)N<1)._x000d_>
Plagiogranites cogenetic with the basic rocks imply their origin at a slow-spreading ridge._x000D_
Metaigneous rocks with closely similar trace element characteristics form much of the Early Permian Dun Mountain Ophiolite Belt (DMOB) in South Island. The Croisilles and Patuki tholeiites were probably derived from the same Lower Permian seafloor sequence as the Dun Mountain ophiolite. The second metabasalt suite comprises mainly pillow lavas with low Zr/Nb ratios (Zr/Nb = 6-7) and moderately fractionated REE distributions ((La/Y)N = 3-14). These rocks show greatest affinities with transitional to alkaline within-plate (WPA) basalts, such as those from ocean islands, seamounts or fracture zones. Geochemically analogous rocks are rarely recognised among marginal basin basalts. The diversity of trace element ratios indicates that spreading which gave rise to the Croisilles and Patuki volcanics, together with the DMOB, is unlikely to have occurred in a back-arc setting, where a component of depleted mantle from above a subduction zone would have been involved in magma genesis._x000D_
The data imply that the Benioff Zone related to development of the nearby Brook Street Permian volcanic arc in New Zealand, was westward dipping.