The geology and geochemistry of the Hohonu Batholith and adjacent rocks, North Westland, New Zealand.

Abstract

The Hohonu Batholith lies within the Buller terrane, immediately adjacent to the Alpine Fault and inland from Hokitika and Greymouth on the West Coast of the South Island of New Zealand. Detailed mapping has identified ten distinct granitoids intruded into Greenland Group metasediments. Four geochemical suites are recognized within the Hohonu Batholith. Palaeozoic magmatism in the batholith is represented by the Summit Granite, which yields a Palaeozoic (381.2 Ma) age and displays affinites with granitoids of the Karamea Suite of Tulloch (1988a). The informal name Summit Granite suite is used to describe this pluton. The Summit Granite has acted as country rock and is intruded by two Cretaceous plutons. The poorly constrained Mount Graham Granite may also belong within the Summit Granite suite. The Hohonu Batholith is dominated by the mid-Cretaceous (114-109 Ma) I-type Hohonu Super-suite, which is considered to encompass the previously defined Rahu Suite of Tulloch (1988a). The Hohonu Super-suite is characterized by relatively restricted radiogenic isotopic compositions with Sr(110) = 0.7062 to 0.7085 and εNd(110) = -4.4 to -6.1, and represents melting of a complex source combining depleted mantle-derived material, similar in composition to the source of the Early Cretaceous Separation Point Suite, and a complex, heterogeneous and largely unconstrained lower continental crustal component. A model is proposed whereby the Hohonu Super-suite was generated following the collapse and thinning of Western Province crust previously over thickened by the generation of the Median Tectonic Zone volcanic arc and its subsequent collision with the Western Province. Collapse of the over thickened crust is believed to be a consequence of the cessation of subduction along the Pacific Margin of the New Zealand portion of Gondwana and the subsequent removal of compressional forces maintaining crustal thickening. Rapid isothermal uplift of the thickened crustal root resulted in partial melting of the lower crust. Ambient temperatures in the lower crust were also raised by mafic underplating associated with isothermal uplift and adiabatic melting of the underlying mantle. Emplacement of the Hohonu Super-suite in an extensional environment is indicated by the intimate relationship between the Rahu Suite Buckland Granite and the Paparoa Metamorphic Core Complex, and the development of the extensional sedimentary basins of the Pororari Group. This extensional event is considered to predate and be unrelated to the separation of Australia and New Zealand and opening of the Tasman Sea. Two suites are recognized within the Hohonu Super-suite in the Hohonu Batholith; the Te Kinga Suite and the Deutgam Suite. Geochemical contrasts between these two suites are attributed to melting at differing crustal depths, at varying water activities, and in equilibrium with different residual assemblages. The relatively mafic, meta1uminous, I-type compositions of the Deutgam Suite are ascribed to dehydration melting in equilibrium with an amphibolitic (plagioclase + amphibole) residue. Residual plagioclase retains Sr, Al2O3, Na2O and Eu and results in the low concentrations of these elements which characterize this suite. In contrast, the peraluminous high silica compositions of the Te Kinga Suite are attributed to water-saturated to under saturated melting in equilibrium with an eclogitic (garnet + amphibole residue) at greater depths in the crust. Residual garnet produces the HREE-depleted nature of the suite, and a lack of residual plagioclase contributes to the characteristically higher Sr, Al2O3, Na2O and Eu contents of the Te Kinga Suite. Late Cretaceous magmatism in the Hohonu Batholith is represented by the French Creek Suite. This suite comprises the composite French Creek Granite, which displays geochemical and petrographic features typical of A-type granitoids, and associated hypabyssal rhyolite dikes. The alkaline magmatism of the French Creek Suite and the closely associated Hohonu Dike Swarm are intimately linked to extension during the opening of the Tasman Sea. The Hohonu Dike Swarm consists of predominately doleritic dikes, with subordinate camptonites and rare phonolites, concentrated on the Hohonu Ranges and Mount Te Kinga. Field evidence indicates that the Hohonu Dike Swarm and French Creek Granite are, at least partially, contemporaneous. The age of this activity is constrained by an 81.7 Ma SHRIMP age for French Creek Granite and is contemporaneous with the generation of the first oceanic crust in the Tasman Sea. A strong WNW-ESE trend within the Hohonu Dike Swarm parallels the line of Australia New Zealand break-up, and the alkaline compositions of both the dikes and the French Creek Granite are characteristic of emplacement into an anorogenic extensional environment. Consequently strong links are indicated between the opening of the Tasman Sea and genesis of the Hohonu Dike Swarm and French Creek Granite. Geochemical data are consistent with generation of French Creek Granite by prolonged fractionation of plagioclase and mafic phases from saturated and oversaturated members of the Hohonu Dike Swarm. Approximately 20% crustal contamination is also required to produce the isotopic compositions of French Creek Granite from the relatively depleted compositions of the Hohonu Dike Swarm. Amphibolite-facies paragneisses, orthogneisses and metabasites of the Granite Hill Complex can be confidently correlated with similar rocks of the Fraser Complex. The dominance of metabasaltic rocks, distinct isotopic compositions and preliminary zircon inheritance studies indicate these gneisses are unlikely to represent metamorphic equivalents of the Greenland Group and intrusive granitoids as proposed for the Charleston Metamorphic Complex. Possible correlatives of the Fraser and Granite Hill Complexes may occur in Fiordland. Poorly exposed Tertiary rocks along the north-west margin of the Hohonu Ranges are briefly described. These rocks are considered to represent material incorporated in a major fault zone along which the batholith has been uplifted and exposed during recent compression across the Alpine Fault.