The Structural and Metamorphic evolution of cretaceous high-P granulites, Fiordland, New Zealand

Fiordland is located southwest of South Island of New Zealand. The field area of this thesis is in northern Fiordland, at the boundary of pristine arc rocks (Median Tectonic Zone) and a belt of Paleozoic paragneisses and orthogneisses of variable age that represent the metamorphosed paleo-Pacific Gondwana margin

Patterns of strain localization in heterogeneous, polycrystalline rocks – a numerical perspective

The spatial and temporal patterns of strain localization in materials with pre-existing heterogeneities are investigated via a series of two-dimensional numerical models. Models include (i) a dynamic feedback process, to simulate rheological weakening in response to the transition from non-linear flow (dislocation creep) to linear flow (diffusion creep/grain boundary sliding), and (ii) a time dependent strengthening process, counteracting the weakening process. Different load bearing framework geometries with 20% weak component are used to evaluate the impact of geometry on the strength of the material and its ability to localize strain into an interconnected weak layer (IWL). Our results highlight that during simple shear, if dynamic weakening with or without strengthening feedbacks is present, strain is quickly localized into an IWL, where an increasing proportion of weak material increases the interconnections between the IWLs, thereby increasing the anastomosing character of the shear zones. We establish that not only bulk strain localization patterns but also their temporal patterns are sensitive to the dominance of the weakening or strengthening process. Consequently, shear zones are dynamic in time and space within a single deformation event. Hence, the pattern of finite strain can be an incomplete representation of the evolution of a shear zone network

Hornblendite delineates zones of mass transfer through the lower crust

Geochemical signatures throughout the layered Earth require significant mass transfer through the lower crust, yet geological pathways are under-recognized. Elongate bodies of basic to ultrabasic rocks are ubiquitous in exposures of the lower crust. Ultrabasic hornblendite bodies hosted within granulite facies gabbroic gneiss of the Pembroke Valley, Fiordland, New Zealand, are typical occurrences usually reported as igneous cumulate hornblendite. Their igneous features contrast with the metamorphic character of their host gabbroic gneiss. Both rock types have a common parent; field relationships are consistent with modification of host gabbroic gneiss into hornblendite. This precludes any interpretation involving cumulate processes in forming the hornblendite; these bodies are imposter cumulates. Instead, replacement of the host gabbroic gneiss formed hornblendite as a result of channeled high melt flux through the lower crust. High melt/rock ratios and disequilibrium between the migrating magma (granodiorite) and its host gabbroic gneiss induced dissolution (grain-scale magmatic assimilation) of gneiss and crystallization of mainly hornblende from the migrating magma. The extent of this reaction-replacement mechanism indicates that such hornblendite bodies delineate significant melt conduits. Accordingly, many of the ubiquitous basic to ultrabasic elongate bodies of the lower crust likely map the ‘missing’ mass transfer zones

Oceanic Zircon Records Extreme Fractional Crystallization of MORB to Rhyolite on the Alarcon Rise Mid-Ocean Ridge

The first known occurrence of rhyolite along the submarine segments of the mid-ocean ridge (MOR) system was discovered on Alarcon Rise, the northernmost segment of the East Pacific Rise (EPR), by the Monterey Bay Aquarium Research Institute in 2012. Zircon trace element and Hf and O isotope patterns indicate that the rhyolite formed by extreme crystal fractionation of primary mid-ocean ridge basalt (MORB) sourced from normal to enriched MOR mantle with little to no addition of continental lithosphere or hydrated oceanic crust. A large range in zircon ϵHf spanning 11 ϵ units is comparable to the range of whole rock ϵHf from the entire EPR. This variability is comparable to continental granitoids that develop over long periods of time from multiple sources. Zircon geochronology from Alarcon Rise suggests that at least 20 kyr was needed for rhyolite petrogenesis. Grain-scale textural discontinuities and trace element trends from zircon cores and rims are consistent with crystal fractionation from a MORB magma with possible perturbations associated with mixing or replenishment events. Comparison of whole rock and zircon oxygen isotopes with modeled fractionation and zircon-melt patterns suggests that, after they formed, rhyolite magmas entrained hydrated mafic crust from conduit walls during ascent and/or were hydrated by seawater in the vent during eruption. These data do not support a model where rhyolites formed directly from partial melts of hydrated oceanic crust or do they require assimilation of such crust during fractional crystallization, both models being commonly invoked for the formation of oceanic plagiogranites and dacites. A spatial association of highly evolved lavas (rhyolites) with an increased number of fault scarps on the northern Alarcon Rise might suggest that low magma flux for ∼20 kyr facilitated extended magma residence necessary to generate rhyolite from MORB

Mass transfer in the lower crust: Evidence for incipient melt assisted flow along grain boundaries in the deep arc granulites of Fiordland, New Zealand

Knowledge of mass transfer is critical in improving our understanding of crustal evolution, however mass transfer mechanisms are debated, especially in arc environments. The Pembroke Granulite is a gabbroic gneiss, passively exhumed from depths of >45 km from the arc root of Fiordland, New Zealand. Here, enstatite and diopside grains are replaced by coronas of pargasite and quartz, which may be asymmetric, recording hydration of the gabbroic gneiss. The coronas contain microstructures indicative of the former presence of melt, supported by pseudosection modeling consistent with the reaction having occurred near the solidus of the rock (630–710°C, 8.8–12.4 kbar). Homogeneous mineral chemistry in reaction products indicates an open system, despite limited metasomatism at the hand sample scale. We propose the partial replacement microstructures are a result of a reaction involving an externally derived hydrous, silicate melt and the relatively anhydrous, high-grade assemblage. Trace element mapping reveals a correlation between reaction microstructure development and bands of high-Sr plagioclase, recording pathways of the reactant melt along grain boundaries. Replacement microstructures record pathways of diffuse porous melt flow at a kilometer scale within the lower crust, which was assisted by small proportions of incipient melt providing a permeable network. This work recognizes melt flux through the lower crust in the absence of significant metasomatism, which may be more common than is currently recognized. As similar microstructures are found elsewhere within the exposed Fiordland lower crustal arc rocks, mass transfer of melt by diffuse porous flow may have fluxed an area >10,000 km2

Melt-present shear zones enable intracontinental orogenesis

Localized rheological weakening is required to initiate and sustain intracontinental orogenesis, but the reasons for weakening remain debated. The intracontinental Alice Springs orogen dominates the lithospheric architecture of central Australia and involved prolonged (450–300 Ma) but episodic mountain building. The mid-crustal core of the orogen is exposed at its eastern margin, where field relationships and microstructures demonstrate that deformation was accommodated in biotite-rich shear zones. Rheological weakening was caused by localized melt-present deformation coupled with melt-induced reaction softening. This interpretation is supported by the coeval and episodic nature of melt-present deformation, igneous activity, and sediment shed from the developing orogen. This study identifies localized melt availability as an important ingredient enabling intracontinental orogenesis

Chemical Signatures of Melt–Rock Interaction in the Root of a Magmatic Arc

Identification of melt–rock interaction during melt flux through crustal rocks is limited to field relationships and microstructural evidence, with little consideration given to characterising the geochemical signatures of this process. We examine the mineral and whole-rock geochemistry of four distinct styles of melt–rock interaction during melt flux through the Pembroke Granulite, a gabbroic gneiss from the Fiordland magmatic arc root, New Zealand. Spatial distribution, time-integrated flux of melt and stress field vary between each melt flux style. Whole-rock metasomatism is not detected in three of the four melt flux styles. The mineral assemblage and major element mineral composition in modified rocks are dictated by inferred P–T conditions, as in sub-solidus metamorphic systems, and time-integrated volumes of melt flux. Heterogeneous mineral major and trace element compositions are linked to low time-integrated volumes of melt flux, which inhibits widespread modification and equilibration. Amphibole and clinozoisite in modified rocks have igneous-like REE patterns, formed by growth and/or recrystallisation in the presence of melt and large equilibration volumes provided by the grain boundary network of melt. Heterogeneities in mineral REE compositions are linked to localisation of melt flux by deformation and resulting smaller equilibration volumes and/or variation in the composition of the fluxing melt. When combined with microstructural evidence for the former presence of melt, the presence of igneous-like mineral REE chemical signatures in a metamorphic rock are proposed as powerful indicators of melt–rock interaction during melt flux

Experimental replacement of zircon by melt-mediated coupled dissolution-precipitation causes dispersion in U-Pb ages

Zircon geochronology provides critical information on the rates and durations of geological processes and enables researchers to explore deep time. However, some zircon datasets show a continuum of concordant ages (‘smear’) without well-defined age populations. These age smears are typically interpreted to represent variable loss of radiogenic Pb or protracted geological events lasting tens of millions of years. Coupled dissolution-precipitation replacement of zircon has been suggested as one process that may produce these complex age datasets. Here, we react fragments of the well characterised Mud Tank zircon standard with natural intermediate and mafic melts (0.9 GPa, 1100–1180°C) to test if short-term exposure to a melt can modify the geochronological patterns of zircon. Our observations show that within a short duration (18 h to 3.5 days), most Mud Tank zircon fragments display microstructural and/or chemical evidence for modification by dissolution at fragment boundaries along with partial replacement by coupled dissolution-precipitation processes. The replaced zircon domains have U–Pb ages that smear over one hundred million years, between 764–647 Ma, illustrating variable mobility and redistribution of the U and Pb isotopes. Our experiments demonstrate that zircon modified by coupled dissolution-precipitation replacement may not faithfully record the age or duration of geological events and that investigation of zircon microstructure in high-resolution backscattered electron, cathodoluminescence imaging and/or Raman mapping is needed to better understand complex zircon geochronological datasets

Reconstruction of the Talkeetna intraoceanic arc of Alaska through thermobarometry

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): B03204, doi:10.1029/2007JB005208.The Talkeetna arc is one of two intraoceanic arcs where much of the section from the upper mantle through the volcanic carapace is well exposed. We reconstruct the vertical section of the Talkeetna arc by determining the (re)crystallization pressures at various structural levels. The thermobarometry shows that the tonalites and quartz diorites intruded at ∼5–9 km into a volcanic section estimated from stratigraphy to be 7 km thick. The shallowest, Tazlina and Barnette, gabbros crystallized at ∼17–24 km; the Klanelneechena Klippe crystallized at ∼24–26 km; and the base of the arc crystallized at ∼35 km depth. The arc had a volcanic:plutonic ratio of ∼1:3–1:4. However, many or most of the felsic plutonic rocks may represent crystallized liquids rather than cumulates so that the liquid:cumulate ratio might be 1:2 or larger. The current 5- to 7-km structural thickness of the plutonic section of the arc is ∼15–30% of the original 23- to 28-km thickness. The bulk composition of the original Talkeetna arc section was ∼51–58 wt % SiO2.Funded by NSF EAR-9910899