Does second phase content control the evolution of olivine CPO type and deformation mechanisms? A case study of paired harzburgite and dunite bands in the Red Hills Massif, Dun Mountain Ophiolite

Abstract

We have examined the microstructures and crystallographic preferred orientations (CPOs) of six “paired” harzburgite and dunite samples from a small (~ 1 km2) area in the Ellis Stream Complex, Red Hills Massif, Dun Mountain ophiolite belt in New Zealand. Here, banded peridotites with isoclinal folds, E-W striking vertical foliations and vertical lineations are inferred to result from shear on vertical planes that overprint previous mantle structures. The samples contain five different olivine CPOs: typically known as A, C, D, E and AG-types. The co-located harzburgite and dunite of each pair show different CPOs (e.g., A vs E type). In each pair, dunite generally has larger grain size and stronger CPO. At two localities, the dunite has layers with different CPOs (A and D/E types) and grain sizes, corresponding to variations in spinel content. Paired samples were collected from nearby (a few cm maximum) parallel layers with no pinch and swell structures to suggest strain partitioning and dynamically recrystallized grain long axes parallel to lineation in both. Thus we infer that the deformation conditions (e.g., temperature, pressure) and kinematics were the same for the harzburgite and dunite in each pair. It is most feasible that second phase content (orthopyroxene and/or spinel) is the primary factor controlling the CPO type at a single locality. Additionally, we observed a change of the CPO types and a decrease in CPO strength from the eastern to the western part of the Ellis Stream Complex. We infer this relates to decreasing strain. The different CPOs of harzburgites and dunites suggest that the second phase content exerts a significant control on olivine CPO development.