The geochemistry, origin, and tectonic significance of rodingites from the dun mountain ultramafics, d’urville island, New Zealand

Numerous rodingite dikes and rodingitised tectonic blocks occur within serpentinised harzburgite of the Dun Mountain Ultramafics, D’Urville Island, New Zealand. The rodingites formed during progressive metasomatic alteration of a wide range of mafic to silicic ophiolitic rocks, including metavolcanics and argillite, as well as gabbro and plagiogranite. Petrographic, mineralogical, and geochemical data distinguish between two types of rodingite which represent different degrees of metasomatic alteration. These are (1) coarse-grained rodingites that often preserve mineralogical features of their parent rocks, and (2) intensely altered, fine-grained rodingite pods which are bordered by distinctive chloritised serpentinite reaction zones. The two rodingite types differ in their degree of Ca and Al enrichment, and Si, total Fe and alkali depletion, as well as in their extent of oxidation, hydration or dehydration, and abundances of incompatible elements and transition metals. These differences are related to progressively changing mineralogies in the rodingite reaction zones that were imposed by chemically evolved fluids associated with serpentinisation of the enclosing ultramafic rocks. Despite alteration, the rodingites show geochemical affinities with light rare earth element (LREE) depleted lavas and intrusives comprising the Patuki Volcanics at D’Urville Island. No rodingitised samples of an earlier formed suite of LREE-enriched Patuki tholeiites have been found. Rodingitisation of the D’Urville rocks occurred during emplacement of the Dun Mountain Ultramafics in a continental crust environment, and the metasomatic processes are unrelated to early amphibolite and greenschist forming ocean-floor metamorphism of the Patuki Volcanics. In this respect, the D’Urville rodingites differ from other rodingites which have been dredged from the ocean floor

Genesis of the Permian ophiolites and ultramafics in the Dun Mountain terrane, New Zealand

The origin of the Patuki ophiolite complex and the Dun Mountain ultramafics of Middle Permian age in Nelson, New Zealand is discussed. The Patuki volcanics have strong affinities with tholeiites of mid ocean spreading ridges transitional to alkaline basalts, similar to those from fracture zones and ocean islands. The Dun Mountain Group, of serpentinites, peridotites, dunites, gabbros, intruded the upper Patuki Group as a linear series of aligned pods and sheets. They carried up with them tectonic inclusions of older rock, which were metasomatised, and altered wall-rock. The rocks did not act as a floor (and alleged source) for the Patuki, but partly post-dated and were emplaced into Patuki. -Author

The Patuki intrusive suite: closed-system fractionation beneath a slow-spreading ridge

A wide range of mafic and ultramafic rock types, together with cogenetic silicic plagiogranites, form a structurally coherent intrusive sequence within the Patuki Volcanics at south D'Urville Island, New Zealand. In addition, gabbroic rocks comprise abundant tectonic inclusions in highly-sheared, concordant serpentinite bands which intrude the Patuki suite. Chemical evidence suggests many of the gabbros, including those in which recrystallization has obliterated original textures, represent magmatic cumulates and indicates extensive closed-system fractionation analogous to that known to occur beneath slow-spreading mid-oceanic ridges. Dyke intrusion occurred throughout the generation of the suite. An early stage of spreading is suggested by the anomalously low thickness of the sequence, the non-sheeted nature of the dyke suite and chemical characteristics of the lavas which comprise the extrusive component of the ophiolite

Oceanic ridge metamorphism of the Patuki Volcanics, D'Urville Island, New Zealand

Metamorphic assemblages in ophiolitic rocks of the Patuki Volcanics are grouped into four facies types which represent recrystallization under widely diverse P-T conditions. Prehnite-pumpellyite and greenschist facies assemblages characterize the lavas, metasediments, dykes and high-level gabbros which have undergone extensive spilitization. Hydrothermal alteration affecting the lavas is strongly non-isochemical and is related to the degree of crystallinity of the original rock. Epidote-amphibolite facies assemblages are developed in completely recrystallized rocks in shear zones. Metamorphism of Patuki intrusive rocks was at a higher grade but less extensive and more nearly isochemical than in the lavas. Early-formed amphibolite facies assemblages characterize the metacumulate suite and many cross-cutting dykes. Greenschist facies assemblages are common in late-stage intrusives and disequilibrium textures are often observed, reflecting a series of reactions at progressively lower temperatures in a tectonically active environment. Hydrothermal alteration affecting the 1- to 2-km-thick Patuki ophiolite sequence implies a steep geothermal gradient (at least 250-350°C/km). Metamorphism at a slow-spreading mid-oceanic ridge is indicated and the sequential nature of metamorphic reactions occurring at the ridge site is demonstrated

Discrimination between ophiolitic metabasalts, north d’urville island, New Zealand

Whole-rock minor and trace element abundances, relict clinopyroxene compositions and petrographic features discriminate between 2 metabasalt units within the Patuki Volcanic complex, north D'Urville Island, New Zealand. The majority of the Patuki metabasalts show P, Zr, Nb, and light rare earth element (LREE) depletion and exhibit tholeiitic differentiation characteristics. They show close similarities to many abyssal tholeiites produced at modern spreading ridges (Ntype ocean ridge basalts). Pristine relict clinopyroxene preserves Ti-poor endiopside and endiopsidic augite compositions somewhat enriched in Cr, and shows systematic iron enrichment. A smaller proportion of the metabasalts are enriched in Nb, LREE, and related hygromagmatophile elements, but retain phenocryst assemblages and some trace element abundances similar to ocean-floor basalts. They contain Ca- and AI-rich titaniferous salites. These metabasalts are atypical of ophiolitic metabasalts and of metabasalts formed at normal ridge segments and show closest similarity to largeion- lithophile element enriched, ocean-floor tholeiites (E-type mid-ocean ridge basalts). At north D’Urville Island, the N-type metabasalts form an upper unit composed of massive and more rarely pillowed flows, and the group of E-type metabasalts form a lower unit composed of pillow lava and rare tabular bodies of ophitic spilite, associated with volcanic breccia. Interbedded marine sediments include sandstone, argillite, mudstone and minor tuffs, and ophiolitic breccia The trace element data indicate that the 2 suites of Patuki metabasalts at north D’Urville Island are derived from geochemically distinct mantle sources, similar to sources from which N- and E-type midocean ridge basalts originate. The data imply mantle heterogeneity beneath a poorly evolved Middle Permian seafloor