Continent stabilisation by lateral accretion of subduction zone-processed depleted mantle residues; insights from Zealandia

To examine how the mantle lithosphere stabilises continents, we present a synthesis of the mantle beneath Zealandia in the SW Pacific Ocean. Zealandia, Earth's “8th continent”, occurs over 4.9 M km2 and comprises a fore-arc, arc and back-arc fragment rifted from the Australia–Antarctica Gondwana margin 85 Myr ago. The oldest extant crust is ∼500 Ma and the majority is Permian–Jurassic. Peridotitic rocks from most known locations reveal the underpinning mantle to comprise regional domains varying from refractory (Al2O3 < 1 wt%, olivine Mg# > 92, spinel Cr# up to 80, Pt/Ir < 1) to moderately depleted (Al2O3 = 2–4 wt%, olivine Mg# ∼90.5, spinel Cr# < ∼60). There is no systematic distribution of these domains relative to the former arc configuration and some refractory domains underlie crust that is largely devoid of magmatic rocks. Re-depletion Os model ages have no correlation with depletion indices but do have a distribution that is very similar to global convecting mantle. Whole rock, mineral and isotopic data are interpreted to show that the Zealandia mantle lithosphere was constructed from isotopically heterogeneous convecting mantle fragments swept into the sub-arc environment, amalgamated, and variably re-melted under low-P hydrous conditions. The paucity of mafic melt volumes in most of the overlying crust that could relate to the depleted domains requires melting to have been followed by lateral accretion either during subduction or slab rollback. Recent Australia–Pacific convergence has thickened portions of the Zealandia mantle to >160 km. Zealandia shows that the generation of refractory and/or thick continental lithosphere is not restricted to the Archean. Since Archean cratons also commonly display crust–mantle age decoupling, contain spinel peridotites with extreme Cr# numbers that require low-P hydrous melting, and often have a paucity of mafic melts relative to the extreme depletion indicated by their peridotitic roots, they too may – in part – be compilations of peridotite shallowly melted and then laterally accreted at subduction margins

Determining relative bulk viscosity of kilometre-scale crustal units using field observations and numerical modelling

Though the rheology of kilometre-scale polymineralic rock units is crucial for reliable large-scale, geotectonic models, this information is difficult to obtain. In geotectonic models, a layer is defined as an entity at the kilometre scale, even though it is heterogeneous at the millimetre to metre scale. Here, we use the shape characteristics of the boundaries between rock units to derive the relative bulk viscosity of those units at the kilometre scale. We examine the shape of a vertically oriented ultramafic, harzburgitic-lherzolitic unit, which developed a kilometre-scale pinch and swell structure at mid-crustal conditions (~ 600 °C, ~ 8.5 kbar), in the Anita Shear Zone, New Zealand. The ultramafic layer is embedded between a typical polymineralic paragneiss to the west, and a feldspar-quartz-hornblende orthogneiss, to the east. Notably, the boundaries on either side of the ultramafic layer give the ultramafics an asymmetric shape. Microstructural analysis shows that deformation was dominated by dislocation creep (n = 3). Based on the inferred rheological behaviour from the field, a series of numerical simulations are performed. Relative and absolute values are derived for bulk viscosity of the rock units by comparing boundary tortuosity difference measured on the field example and the numerical series. Our analysis shows that during deformation at mid-crustal conditions, paragneisses can be ~ 30 times less viscous than an ultramafic unit, whereas orthogneisses have intermediate viscosity, ~ 3 times greater than the paragneisses. If we assume a strain rate of 10⁻ ¹⁴ s⁻ ¹ the ultramafic, orthogneiss and paragneiss have syn-deformational viscosities of 3 × 10²², 2.3 × 10²¹ and 9.4 × 10²⁰ Pa s, respectively. Our study shows pinch and swell structures are useful as a gauge to assess relative bulk viscosity of rock units based on shape characteristics at the kilometre scale and in non-Newtonian flow regimes, even where heterogeneity occurs within the units at the outcrop scale

The Anita Peridotite: A Study of Subcontinental Lithospheric Mantle Emplaced into the Lower Crust of Northern Fiordland, New Zealand

Accompanying CD contains two spreadsheets of data: Sheet 1 contains all microprobe and SEM mineral chemistry data collected for the project, arranged by mineral and presented as both wt.% oxides and calculated cations. Data were collected at the University of Otago, Macquarie University, and the University of Potsdam. Sheet 2 contains all LA-ICP-MS trace element data collected for the thesis, arranged by mineral. Reported major elements were also determined by LA-ICP-MS but are approximate only. All data were collected at the University of Otago Centre for Trace Element Analysis.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

The Anita Peridotite: A Study of Subcontinental Lithospheric Mantle Emplaced into the Lower Crust of Northern Fiordland, New Zealand

Accompanying CD contains two spreadsheets of data: Sheet 1 contains all microprobe and SEM mineral chemistry data collected for the project, arranged by mineral and presented as both wt.% oxides and calculated cations. Data were collected at the University of Otago, Macquarie University, and the University of Potsdam. Sheet 2 contains all LA-ICP-MS trace element data collected for the thesis, arranged by mineral. Reported major elements were also determined by LA-ICP-MS but are approximate only. All data were collected at the University of Otago Centre for Trace Element Analysis.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

New P–T and U–Pb constraints on Alpine Schist metamorphism in south Westland, New Zealand

Metamorphic mineral compositions of a staurolite-bearing greyschist from the middle reaches of the Moeraki River valley in south Westland reveal peak equilibration at c. 558 ± 50 °C and c. 6.1 ± 1.2 kbar. Two c. 83 Ma U–Pb monazite age populations from the cores of monazite-apatite-allanite-epidote corona structures in mylonitised schists from near Fox Glacier confirm that Alpine Schist metamorphism occurred during the Late Cretaceous. The published spread in Late Cretaceous metamorphic ages indicates that metamorphism was diachronous or was a protracted event. Further dating is required to pin down the cryptic transition into the Jurassic–Early Cretaceous metamorphosed Otago Schist, but the Alpine Schist must extend at least 11 km east of the Alpine Fault in south Westland and overprint the suture between the Pounamu and Rakaia terranes. The P–T–t results imply that the Late Cretaceous crust represented by portions of the Alpine Schist was probably of similar thickness to that beneath the Southern Alps today, but with dehydration and partial melting occurring near the base. The crust under Westland and Otago may be dry and therefore strong

The architecture of long-lived fault zones: insights from microstructure and quartz lattice-preferred orientations in mylonites of the Median Tectonic Line, SW Japan

Abstract We combine field mapping with quartz microstructure and lattice preferred orientations (LPO) to constrain the mechanisms and spatio-temporal distribution of deformation surrounding the Median Tectonic Line (MTL), SW Japan. In the study area, the MTL occurs either as a narrow gouge zone or as a sharp contact between hanging-wall quartzofeldspathic mylonites to the north and footwall pelitic schists to the south. Along the northern margin of the MTL, there exists a broad zone of mylonitic rocks, overprinted by cataclastic deformation and a damage zone associated with brittle deformation. The mylonitic shear zone is dominated by coarse-grained protomylonite up to ~ 100 m from the MTL, where fine-grained ultramylonite becomes dominant. We observe a systematic variation in quartz LPO with distance from the MTL. In protomylonites, quartz LPOs are dominantly Y-maxima patterns, recording dislocation creep by prism slip at ~ 500 °C. Closer to the MTL, we observe R- and Z-maxima, and single and crossed girdles, reflecting dislocation creep accommodated by mixed rhomb and basal slip, likely under cooler conditions (~ 300 °C–400 °C). Some ultramylonite samples yield weak to random LPOs, interpreted to result from the influx of fluid into the shear zone, which promoted deformation by grainsize-sensitive creep. Following cooling and uplift, deformation became brittle, resulting in the development of a narrow cataclasite zone. The cataclasite was weakened through the development of a phyllosilicate foliation. However, healing of fractures strengthened the cataclasites, resulting in the development of anastomosing cataclasite bands within the protomylonite

The Anita Peridotite, New Zealand: Ultra-depletion and Subtle Enrichment in Sub-arc Mantle

The orogenic Anita Peridotite in Fiordland, SW New Zealand, provides an opportunity to examine the composition of a large block of upper mantle exhumed from beneath a Cretaceous arc. This little-studied 1 km x 20 km massif is dominated by spinel-facies harzburgite and dunite. Olivine Mg# of 92-93, spinel Cr# of 70, orthopyroxene with low Al2O3, and extremely depleted whole-rock geochemical characteristics indicate that the peridotite body experienced > 30% melt extraction, probably within the spinel facies. Mineral compositions show some similarity to those of cratonic peridotitic mantle. Rare Cr-rich amphibole suggests that the peridotite has been subsequently re-enriched. Distinctive, coupled Eu and Sr anomalies in the amphiboles, which can be subdivided into three groups, are interpreted to show that they formed by hydration of metasomatic clinopyroxene-plagioclase aggregates. Measured amphibole Sr-87/Sr-86 (0 center dot 705-0 center dot 706), I mu Nd ( +6 center dot 3 to + 11 center dot 1), Pb-208/Pb-204 (37 center dot 8-38 center dot 9) and I mu Hf ( +5 center dot 6 to 36 center dot 9) indicate that the metasomatic agent, which caused crystallization of clinopyroxene and plagioclase, had an isotopic composition similar to ocean island basalt. On the basis of isotopic data and mineral chemistry, the enriched nature of the peridotite is interpreted to have been caused by percolation of small volumes of a mafic silicate melt. Additional evidence for the passage of such melts is the rare occurrence of hornblendite veins and orthopyroxene hornblendite dykes. This peridotite body therefore preserves evidence of extreme melt depletion and the passage of silicate melts and hydrous fluids within the sub-arc mantle

Rheological weakening of olivine + orthopyroxene aggregates due to phase mixing, Part2: Microstructural development

International audienceTo understand the processes involved in phase mixing during deformation and the resulting changes in rheological behavior, we conducted torsion experiments on samples of iron-rich olivine plus orthopyroxene. The experiments were conducted at a temperature, T, of 1200°C and a confining pressure, P, of 300 MPa using a gas-medium, deformation apparatus. Samples composed of olivine plus 26% orthopyroxene were deformed to outer radius shear strains up to γ ≈ 26. In samples deformed to lower strains of γ ≲ 4, elongated olivine and pyroxene grains form a compositional layering. Already by this strain, mixtures of small equant grains of olivine and pyroxene begin to develop and continue to evolve with increasing strain. The ratios of olivine to pyroxene grain size in deformed samples follow the Zener relationship, indicating that pyroxene grains effectively pin the grain boundaries of olivine and inhibit grain growth. Due to the reduction in grain size, the dominant deformation mechanism changes as a function of strain. The microstructural development forming more thoroughly mixed, fine-grained olivine-pyroxene aggregates can be explained by the difference in diffusivity among Me (Fe or Mg), O, and Si, with transport of MeO significantly faster than that of SiO2. These mechanical and associated microstructural properties provide important constraints for understanding rheological weakening and strain localization in upper mantle rocks