The geochemistry, origin, and tectonic significance of rodingites from the Dun Mountain Ultramafics, D'Urville Island, New Zealand

Abstract

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.

Keywords:

• rodingite
• metasomatism
• Patuki Volcanics
• supergene processes
• vesuvianite
• hydrogrossular
• pyroxene group.