The Anita Peridotite: A Study of Subcontinental Lithospheric Mantle Emplaced into the Lower Crust of Northern Fiordland, New Zealand
The Anita Peridotite is an orogenic peridotite emplaced within a lower crustal ductile shear zone in Fiordland, south-western New Zealand. The unit has undergone little alteration and may preserve primary mantle mineralogy and microstructures. This massif has received relatively little attention compared to well-studied orogenic peridotites in Europe, and so the first-order goal of this thesis is to provide a thorough description of the unit using modern analytical techniques, allowing inferences to be made regarding the nature of the upper mantle. Whole-rock and mineral compositions revealed a highly refractory composition, similar to cratonic or supra-subduction zone lithospheric mantle, indicative of a large degree of melt extraction. Amphibole occurs in the peridotites, as well as in veins and dykes, and formed during subtle enrichment by a melt and/or fluid. A trace element and isotopic study reveals that amphibole in the peridotites likely formed through hydration of plagioclase + clinopyroxene aggregates, which crystallised during flow of a silicate melt with a composition similar to ocean island basalts. Melting is interpreted to have taken place in the sub-arc mantle, where silicate melts and hydrous fluids are known to be common. Platinum-group element concentrations record melt depletion and have not been disturbed during metasomatism, although evidence of Re mobility is observed. Re-depletion ages were calculated using Re-Os isotopic ratios, producing a range of ages from -0.23 to 1.53 Ga. Parts of the Anita Peridotite are therefore more than a billion years older than any exposed Zealandia crust. Isotopic heterogeneity is explained by invoking mixing between depleted and fertile mantle, followed by later melting. The large Os isotopic heterogeneity is similar to that displayed by oceanic lithosphere, suggesting that Zealandia may be underlain by accreted oceanic mantle. The peridotites preserve fine recrystallised grain sizes even in monomineralic zones suggesting rapid exhumation. Thermodynamic modelling of peridotites constrains the cooling history but cannot be used to constrain pressure changes. Instead, adjacent metapelitic rocks are investigated and shown to record equilibration under sillimanite-grade conditions followed by burial and the growth of kyanite and high-Ca garnet rims at the base of the crust. The metapelites were recrystallised with the peridotites, with quartz recrystallising by grain boundary migration. The timing of exhumation is estimated as 104 Ma, concomitant with regional extension. Models for incorporation of peridotite into the crust are proposed, with the favoured model involving ductile extrusion during transpressive deformation within the Anita Shear Zone. Olivine and orthopyroxene microstructures and lattice-preferred orientations (LPO) record changing deformation mechanisms during shearing in the lithospheric mantle and crust. Olivine porphyroclasts record movement of dislocations on slip systems characteristic of hydrous conditions, while fine matrix grains have random LPO and lack internal structure, leading to the interpretation that they deformed by grain size sensitive (GSS) processes. Rare pods of protomylonite record dislocation creep deformation in the lithospheric mantle. The transition to GSS creep in the mylonites caused strain weakening and localisation across the entire massif. Fine grain sizes were maintained by phase mixing, potentially through transport of components along grain boundaries.
xvii, 269 pages A4
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MULTIPOLYGON (((167.5098228312 -44.5245951017851,167.909721891112 -44.5431707600917,167.892030632743 -44.7445935472102,167.490758235026 -44.7258877381384,167.5098228312 -44.5245951017851)))
Czertowicz, Thomas (Tom), “The Anita Peridotite: A Study of Subcontinental Lithospheric Mantle Emplaced into the Lower Crust of Northern Fiordland, New Zealand,” Otago Geology Theses, accessed January 21, 2021, http://theses.otagogeology.org.nz/items/show/626.