Abstract
Peridotitic xenoliths in Cenozoic volcanic rocks around Otago and the Dun Mountain Ophiolite in the South Island of New Zealand offer a window into the lithospheric mantle beneath the Zealandia continent. We selected peridotites from the Red Hills Massif, Dun Mountain Ophiolite, and mantle xenoliths from East and West Otago areas to reconstruct their microstructural evolution.
Oriented sample “pairs” of harzburgite and dunite from a small area (1 km2) are of the Red Hills Massif (Ellis Stream Complex) contain five of the six most common types of olivine crystallographic preferred orientations (CPO; A, C-E and AG except B-type). In each pair, the dunite has a larger grain size and a stronger CPO than the harzburgite. The CPO type in the dunite is different to that in the harzburgite in each pair. Across the area there is an increase in CPO strength from east to west. The secondary dominant orthopyroxene in harzburgite inhibits both olivine grain size coarsening and CPO development. Similarly spinel content in dunite corresponds to local variations in grain size and CPO. Variations across the area relate to a gradient in strain and/or deformation kinematics.
Mantle xenoliths from both East and West Otago areas have no clear foliation and lineation to use as a CPO reference frame. We use the most typical orthopyroxene (100) [001] CPO as a reference frame. Each of the CPOs is rotated so that orthopyroxene (100) is oriented as foliation and [001] as lineation. This enables better inter-comparison of CPOs among xenoliths.
East Otago samples have coarse average olivine grain size (565-800 μm). Olivine CPOs have concentrated [100]OL and [010]OL, and the [100]OL is (sub)parallel to [001]OPX (i.e., coherent olivine and orthopyroxene CPOs). We infer that these CPOs are generated by dislocation creep at high temperature (~1000-1100 °C) and probably with melt present. The differential stresses calculated by using a subgrain piezometer are 3-6 MPa. Harzburgites have similar orthopyroxene and clinopyroxene CPOs while lherzolites do not, probably because clinopyroxene in harzburgite is generated by carbonate metasomatism from orthopyroxene and the new-generated clinopyroxene inherits the orthopyroxene CPO. Clinopyroxene lamellae in orthopyroxene are straight are not strongly deformed, suggesting that major deformation occurred before Cenozoic cooling prior to Alpine Fault initiation.
The samples from West Otago are more complicated. Mantle xenoliths are divided into coarse-grained and protomylonitic harzburgites. In coarse-grained samples, CPO types change from a range of CPOs to dominant axial-[100] (maxima in [100], girdles in [010] and [001]) type, with grain size increasing. Olivine and orthopyroxene Mg# and spinel Cr# are higher and orthopyroxene Al2O3%) lower in coarser samples. Plotting olivine and orthopyroxene grain sizes on a Zener parameter figure is consistent with ~1000 ℃ recrystallisation temperature. Olivine and orthopyroxene CPOs are mostly incoherent, which might relate to melt percolation. Protomylonites have much finer grain size, generated by subgrain rotation (SGR) during dislocation creep. Four-grain junctions and weaker CPOs are used to infer operation of grain boundary sliding in the protomylonites. Olivine CPOs are all [010] clustered and mostly incoherent with orthopyroxene CPOs. The deformation temperature is around ~800 ℃, with estimated ~10-12 s-1 strain rates. Where samples contain remnant coarse grains the CPOs of coarse-grained and protomylonitic material do not align, suggesting that they developed in different episodes with different kinematics. The protomylonites are interpreted as the result of the Alpine Fault movement.
Seismic parameters calculated from CPO data shows that individual coarse-grained samples from East Otago have higher seismic anisotropies than those from West Otago (at both ambient and in-situ conditions), due to higher olivine content. Protomylonitic samples have the lowest seismic anisotropies. The seismic properties of each sample suite are averaged to compare with present day geophysical measurements. As the olivine CPOs and orthopyroxene CPOs are coherent in East Otago the average properties do not depend on a choice of reference frame for samples. In contrast, West Otago xenoliths have incoherent CPOs and choice of reference frame for averaging makes a significant difference. The East Otago seismic data fit current geophysical data well. To fit well, West Otago data must use the orthopyroxene CPO reference frame.
The seismic anisotropy data allow speculation about the large-scale mantle structures in the Lithosphere beneath Otago. To explain the seismic data, olivine [100] needs to be sub-horizontal and parallel to NNW-SSE fast polarization directions with [010] sub-horizontal and perpendicular to these directions. This is consistent with the East Otago lithospheric mantle containing a fossil vertical shear zone with NNW-SSE strike slip motion. The West Otago data is more complicated and requires a range of differently orientated deformation zones.