ISOTOPIC AND ELEMENTAL ANALYSIS OF SERPENTINITES TO DETERMINE THE TECTONIC HISTORY OF THE DUN MOUNTAIN OPHIOLITE BELT, NEW ZEALAND

This study investigates serpentinites from the Dun Mountain Ophiolite Belt (DMOB), New Zealand, to identify the origin of their protolith, subsequent serpentinization history, and tectonic evolution. We use major and trace element geochemistry of whole rocks and relict clinopyroxene (CPX), as well as stable isotope ratios (δ¹⁸O and δD) of mineral separates on twelve samples from the Dun Mountain Ophiolite (DMO) and four samples from the associated Patuki and Croisilles Mélanges to determine tectonic history. Petrographic analysis and elemental profiles indicate that the protolith of the serpentinites is spinel harzburgite. Overall, DMO samples have relatively low rare earth element (REE) concentrations, flat to U-shaped chondrite-normalized REE patterns, and fluid mobile element ratios consistent with formation in a mantle wedge setting. A few DMO samples have comparatively higher REE concentrations and differing FME ratios, suggesting possible melt-rock interaction and a mid-ocean ridge (MOR) origin. In contrast, mélange samples have relatively high incompatible trace element and REE concentrations compared to typical depleted MORB-mantle. The bulk rock serpentinites exhibit flat REE patterns and do not show pronounced Eu anomalies. These data are consistent with a protolith originating from a MOR. δ¹⁸O values of serpentine grains range from -6.5 to -8‰ and -2.7 to -4.7‰ for DMO and mélange samples, respectively. Bulk rock δD values range from -41 to -92‰, indicating that most samples have undergone some post-emplacement alteration by meteoric water. These preliminary findings suggest that harzburgite comprising the proto-Patuki and Croisilles mélanges likely formed at a MOR, serpentinized in an abyssal setting, and then accreted to form the mélange belts. As subduction progressed, the system transitioned into a supra-subduction zone (SSZ), where proto-DMO harzburgites serpentinized in the mantle wedge. This serpentinite-derived interpretation of the DMO’s evolution reinforces studies that reach similar conclusions based on tectonics, geochronology, and geochemistry of igneous rocks of the DMO (e.g., Jugum et al., 2019).

Jugum, E. Stewart, J. M. Palin, N. Mortimer, R. J. Norris, W. M. Lamb, 2019. Geological Society, London, Memoirs 49, 75-92. doi: 10.1144/m49.