Geology of the southern portion of the Greenhills ultramafic complex

Exposed along a three kilometre stretch of coastline on the southern extremities of the South Island, New Zealand are a suite of calc-alkaline to tholeiitic ultramafic and gabbroic rocks which form the southern portion of the Greenhills Ultramafic Complex (GUC). This complex consists of a layered series of dunite, wehrlite, olivine-clinopyroxenite and gabbro of Earliest Triassic age (247Ma), which intrude Lower Permian meta-sedimentary lithologies of the Greenhills Group. Accompanying the intrusion of the complex is a narrow contact metamorphic aureole which decreases rapidly in grade from pyroxene-hornfels facies metamorphism, directly adjacent to the body, to regional prehnite-pumpellyite facies metamorphism, with distance from the contact. The layered series of the GUC is stratigraphically divisible into an upper gabbroic portion of both non-cumulate and cumulate gabbroic rocks, and a lower ultramafic portion of dunite, wehrlite and olivine-clinopyroxenite. The lower ultramafic portion shows well-developed accumulate structures and textures that are typical of stratiform cumulate intrusions. Widespread slumping in the layered series in addition to discrete zones of intense brecciation, faulting, and multiple phases of dyke injection indicate recurring conditions of instability during the evolution of the complex. Textural, mineralogical, and chemical evidence suggests that two gabbro suites comprise the upper gabbroic portion. Namely, a cumulate suite (Shipwreck Gabbro) that is closely related to the lower ultramafic portion, and a non-cumulate (Barracouta Point Gabbro) suite, which is thought to have crystallised from a mixed magma. Whole rock chemistry of the layered series indicates a clear magmatic fractionation trend through dunite to gabbro, consistent with chemical fractionation from a basaltic parental magma. This trend is characterised by a systematic decrease in magnesium content with a concordant increase in silica, aluminium, calcium, and alkalis. A similar fractionation trend is exhibited by the evolution of the primary mineral phases olivine, clinopyroxene and plagioclase through the layered series. The theory that the GUC may have been derived by dry partial melting of the mantle wedge is supported by the similarity in trace element chemistry between the GUC and N-type Mid Ocean Ridge Basalt (MORB). Similarly, the trace element chemistry correlates well with recent basalts and basaltic andesites from the Tonga-Kermadec Island Arc, indicating that present day active ocean-ocean island arc subduction zones may serve as closely representative models for the evolution of remnant arcs such as that inferred for the GUC. The development of a strong tholeiitic to calc-alkaline island arc chemistry in the GUC is typical for magmatic bodies throughout the Brook Street Terrane, which are thought to represent the remnant of an ancient island arc system. A comparison of chemistry between the GUC and that of the Blashke Islands Alaskan-type intrusion from SE Alaska, indicates that these two bodies have been de1ived by fractional crystallisation of a closely similar parental magma, and thus, the GUC can be classified as a Alaskan-type Intrusion. The Greenhills Ultramafic Complex was produced as the result of crystal settling during fractional crystallisation of a basaltic parental magma produced by dry melting of the mantle wedge in an ocean-ocean island arc subduction zone. Modification of the layered body by magmatic slumping, mingling and brecciation and faulting depict recurring conditions of instability within the pluton which is considered typical of island arc subduction zones