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The U–Pb isotopic signature of titanite collected across an exhumed refractory lower crustal block within the Albany–Fraser Orogen, Australia, records thermal overprints not apparent in a suite of other U–Pb chronometers. This helps to reconcile a dichotomy within the geochronological record of two adjacent zones within the orogen. The zircon U–Pb record for the older Biranup Zone preserves widespread overprinting at 1225–1140 Ma (Stage II), whereas the younger Fraser Zone records an older 1330–1260 Ma (Stage I) tectonothermal event. Titanite in the Fraser Zone also predominantly records a U–Pb age of 1299 ± 14 Ma, reflecting the interval of closure to radiogenic Pb mobility. Nonetheless, small titanite grains reveal subsequent overprinting with a mean reset age of 1205 ± 16 Ma. By contrast, titanite from metasedimentary rocks within the adjacent Biranup Zone principally record U–Pb ages of 1200–1150 Ma, interpreted as dating cooling after prolonged Stage II metamorphism. Interestingly, titanite also preserves domains with old apparent ages. These domains have a statistically significant association with lower U content and also indicate reduced Sm/Yb ratios and are interpreted to have lost U but acquired HREE (e.g. Yb) more rapidly than MREE (e.g. Sm). The old apparent ages are interpreted as artefacts of a Stage II U redistribution process, leading to unsupported radiogenic Pb. In addition, titanite grain size has a strong effect on the preservation or resetting of metamorphic U–Pb ages. Thermochronological modelling based on apparent age versus grain size relationships indicates that complete resetting of small titanite grains requires overprinting temperatures of 695–725 °C during Stage II in the Fraser Zone. This result is similar to estimates from the Biranup Zone based on phase equilibrium modelling that indicates pressures and temperatures of 6.5–8.5 kbar and 675–725 °C. An in situ U–Pb analysis strategy for titanite that targets a range of grain sizes has the potential to reveal differential resetting and place important controls on thermal history.

The last British-Irish Ice Sheet (BIIS) had extensive marine-terminating margins and was drained by\ud multiple large ice streams and is thus a useful analogue for marine-based areas of modern ice sheets.\ud However, despite recent advances from investigating the offshore record of the BIIS, the dynamic history\ud of its marine margins, which would have been sensitive to external forcing(s), remain inadequately\ud understood. This study is the first reconstruction of the retreat dynamics and chronology of the western,\ud marine-terminating, margin of the last (Late Midlandian) BIIS. Analyses of shelf geomorphology and core\ud sedimentology and chronology enable a reconstruction of the Late Midlandian history of the BIIS west of\ud Ireland, from initial advance to final retreat onshore. Five AMS radiocarbon dates from marine cores\ud constrain the timing of retreat and associated readvances during deglaciation. The BIIS advanced without\ud streaming or surging, depositing a bed of highly consolidated subglacial traction till, and reached to\ud within ~20 km of the shelf break by ~24,000 Cal BP. Ice margin retreat was likely preceded by thinning,\ud grounding zone retreat and ice shelf formation on the outer shelf by ~22,000 Cal BP. This ice shelf\ud persisted for �2500 years, while retreating at a minimum rate of ~24 m/yr and buttressing a >150-km\ud long, 20-km wide, bathymetrically-controlled grounding zone. A large (~150 km long), arcuate, flattopped\ud grounding-zone wedge, termed here the Galway Lobe Grounding-Zone Wedge (GLGZW), was\ud deposited below this ice shelf and records a significant stillstand in BIIS retreat. Geomorphic relationships\ud indicate that the BIIS experienced continued thinning during its retreat across the shelf, which led\ud to increased topographic influence on its flow dynamics following ice shelf break up and grounding zone\ud retreat past the GLGZW. At this stage of retreat the western BIIS was comprised of several discrete,\ud asynchronous lobes that underwent several readvances. Sedimentary evidence of dilatant till deposition\ud suggests that the readvances may have been rapid and possibly associated with ice streaming or surging.\ud The largest lobe extended offshore from Galway Bay and deposited the Galway Lobe Readvance Moraine\ud by

The contribution of the Greenland and West Antarctic ice sheets to sea level has increased in recent decades, largely owing to the thinning and retreat of outlet glaciers and ice streams1, 2, 3, 4. This dynamic loss is a serious concern, with some modelling studies suggesting that the collapse of a major ice sheet could be imminent5, 6 or potentially underway7 in West Antarctica, but others predicting a more limited response8. A major problem is that observations used to initialize and calibrate models typically span only a few decades, and, at the ice-sheet scale, it is unclear how the entire drainage network of ice streams evolves over longer timescales. This represents one of the largest sources of uncertainty when predicting the contributions of ice sheets to sea-level rise8, 9, 10. A key question is whether ice streams might increase and sustain rates of mass loss over centuries or millennia, beyond those expected for a given ocean–climate forcing5, 6, 7, 8, 9, 10. Here we reconstruct the activity of 117 ice streams that operated at various times during deglaciation of the Laurentide Ice Sheet (from about 22,000 to 7,000 years ago) and show that as they activated and deactivated in different locations, their overall number decreased, they occupied a progressively smaller percentage of the ice sheet perimeter and their total discharge decreased. The underlying geology and topography clearly influenced ice stream activity, but—at the ice-sheet scale—their drainage network adjusted and was linked to changes in ice sheet volume. It is unclear whether these findings can be directly translated to modern ice sheets. However, contrary to the view that sees ice streams as unstable entities that can accelerate ice-sheet deglaciation, we conclude that ice streams exerted progressively less influence on ice sheet mass balance during the retreat of the Laurentide Ice Sheet.

Reconstructions of former ice sheets and glaciers provide important palaeoglaciological information about their behaviour in response to climate changes. Glacial trimlines record both the margin positions and palaeo ice thickness, enabling the production of empirically constrained 3-Dimensional reconstructions. However, the literature review into the characteristics, interpretation, and use of glacial trimlines here presented shows that these features have been under-utilised and are poorly described in the existing literature, with a confusing terminology currently in use. A new classification scheme and terminology for trimline identification and interpretation is developed to better facilitate further research into these common features of glacierised and formerly glaciated landscapes.

Characterization of host rocks is an essential step towards understanding the genesis and history of sedimentary rock-hosted mineral deposits. Abra is a sedimentary rock-hosted polymetallic deposit, located within the Mesoproterozoic Edmund Basin, in the central Capricorn Orogen, Western Australia. Stratigraphic analysis of the Abra host rock, coupled with detrital zircon geochronology and geochemistry, revealed that the host comprises four distinct units that correlate with four formations of the Edmund Basin (Irregully Formation, Gooragoora Formation, Blue Billy Formation, Kiangi Creek Formation). Detrital zircon U-Pb geochronology constrains the maximum depositional age of the Irregully Formation to 1626 Ma, and 1484 Ma for the Kiangi Creek Formation. The detrital zircon ages compared with available geochronological data on the mineralization, highlight an age gap of c. 100 Ma between the deposition of the basal succession, which hosts the primary base metals deposit, and a second mineralization event in the topmost sedimentary succession. The lowest formations accumulated just after the 1680–1640 Ma Mangaroon Orogeny. Moreover, the Gooragoora and Blue Billy formations exhibit a distinct shift of sediment provenance from the west (Gascoyne Complex) to the south (Yilgarn Craton granites), coincident with extensional tectonic reactivation of the northern Yilgarn Craton margin. Tectonic reactivation is interpreted to have promoted basin deepening and local hydrothermal fluid circulation, which is inferred to have enhanced mineralization in the Abra polymetallic deposit.

Aluminous metapelitic granulites from the central Namaqua Sector of the Namaqua–Natal Metamorphic Province, South Africa, host partly retrogressed hercynite–quartz assemblages. These assemblages record peak P–T conditions of 860–890 °C and 5–5.5 kbar, estimated via thermodynamic modelling and ternary feldspar thermometry. In-situ U–Pb geochronology and geochemistry of anatectic zircon and monazite constrains this event as part of the main Namaqua Orogeny affecting the terrane at c. 1040–1035 Ma. Detrital zircon age spectra span from about 1860 to 1100 Ma, peaking at c. 1800–1700 and 1400–1300 Ma, suggesting detritus was sourced from nearby Paleoproterozoic- and Mesoproterozoic terranes. Thin sections reveal narrow monomineralic coronae of garnet, cordierite or sillimanite separating formerly coexisting hercynite and quartz. Calculated chemical potential relationships show that coronae have developed in response to very minor, near-isobaric cooling shortly after the thermal peak, involving limited diffusion down gradients of μFeO–μMgO. Corona formation occurred in distinct compositional domains, controlled by neighbouring porphyroblasts such as garnet and/or sillimanite. Isolated hercynite–quartz pairs distal from porphyroblasts developed thin rinds of sillimanite, requiring no additional compositional gradients. Cordierite inclusions in garnet, limited melt production and extraction and corona-textures are consistent with an anticlockwise evolution, involving burial and concomitant heating to near-ultrahigh temperature conditions followed by near-isobaric cooling at mid-crustal depths. The hercynite–quartz equilibria record the highest metamorphic temperature affecting the Namaqua Sector, requiring a crucial component of advective heating via voluminous pre- to syn-metamorphic felsic-charnockitic magmatism of the Spektakel Suite. Namaqua-aged granulite facies metamorphism at the terrane scale was driven by a combination of mafic underplating and radiogenic heating. Calculated temperature–enthalpy profiles suggest that advective heating locally increased the suprasolidus enthalpy budget by a third which allowed peak temperatures to be some 50 °C higher than would otherwise have been achieved.

This work was supported by the UK Natural Environment Research Council consortium grant: BRITICE‐CHRONO NE/J009768/1. The TCN analyses were undertaken at the NERC Cosmogenic Isotope Analysis Facility (allocation 9155/1014). During the Last Glacial Maximum, Donegal in north‐west Ireland functioned as an independent centre of ice dispersal that separated and fed into the Donegal Bay Ice Lobe (sourced in the Irish Midlands) to the south and the Hebrides/Malin Sea Ice Stream to the north. We report geochronological data that demonstrate marked contrasts in the timing and rate of deglaciation in northern and southern Donegal. In northern Donegal, which occupied an inter‐ice‐stream/lobe location, decoupling from the Hebrides/Malin Sea Ice Stream resulted in formation of a marine embayment along the north coast by ∼22–21 ka, and subsequent slow (∼4 ± 1 m a−1) climatically driven inland retreat of the ice margin to mountain source areas by ∼17 ka. By contrast, in southern Donegal, which lay near the axis of the Donegal Bay Ice Lobe, deglaciation was delayed until ∼18 ka following readvance of ice to a moraine in outer Donegal Bay. The ice margin subsequently underwent net retreat, apparently uninterrupted by readvances, at a net rate of ∼ 18 ± 6 m a−1. A mean terrestrial cosmogenic nuclide age of ∼15.0 ka obtained for samples from the foothills of the Blue Stack Mountains in south‐east Donegal indicates that ice persisted in valley heads and cirques at the beginning of the Lateglacial Interstadial, suggesting that these and nearby mountains supported the last remnants of the Irish Ice Sheet before complete deglaciation of Ireland, and that almost all the shrinkage of the ice sheet in this sector occurred under stadial conditions before the onset of interstadial warming at ∼14.7 ka. Publisher PDF

A pilot palaeomagnetic study was conducted on the recently dated with in situ SHRIMP U-Pb method at 1134 ± 9 Ma (U-Pb, zircon and baddeleyite) Bunger Hills dykes of the Mawson Craton (East Antarctica). Of the six dykes sampled, three revealed meaningful results providing the first well-dated Mesoproterozoic palaeopole at 40.5°S, 150.1°E (A95 = 20°) for the Mawson Craton. Discordance between this new pole and two roughly coeval poles from Dronning Maud Land and Coats Land (East Antarctica) demonstrates that these two terranes were not rigidly connected to the Mawson Craton ca. 1134 Ma. Comparison between the new pole and that of the broadly coeval Lakeview dolerite from the North Australian Craton supports the putative ~40° late Neoproterozoic relative rotation between the North Australian Craton and the combined South and West Australian cratons. A mean ca. 1134 Ma pole for the Proto-Australia Craton is calculated by combining our new pole and that of the Lakeview dolerite after restoring the 40° intracontinental rotation. A comparison of this mean pole with the roughly coeval Abitibi dykes pole from Laurentia confirms that the SWEAT reconstruction of Australia and Laurentia was not viable for ca. 1134 Ma.

The extraction of geological lineaments from digital satellite data is a fundamental application in remote sensing. The location of geological lineaments such as faults and dykes are of interest for a range of applications, particularly because of their association with hydrothermal mineralization. Although a wide range of applications have utilized computer vision techniques, a standard workflow for application of these techniques to mineral exploration is lacking. We present a framework for extracting geological lineaments using computer vision techniques which is a combination of edge detection and line extraction algorithms for extracting geological lineaments using optical remote sensing data. It features ancillary computer vision techniques for reducing data dimensionality, removing noise and enhancing the expression of lineaments. We test the proposed framework on Landsat 8 data of a mineral-rich portion of the Gascoyne Province in Western Australia using different dimension reduction techniques and convolutional filters. To validate the results, the extracted lineaments are compared to our manual photointerpretation and geologically mapped structures by the Geological Survey of Western Australia (GSWA). The results show that the best correlation between our extracted geological lineaments and the GSWA geological lineament map is achieved by applying a minimum noise fraction transformation and a Laplacian filter. Application of a directional filter instead shows a stronger correlation with the output of our manual photointerpretation and known sites of hydrothermal mineralization. Hence, our framework using either filter can be used for mineral prospectivity mapping in other regions where faults are exposed and observable in optical remote sensing data., 17 pages, 10 figures, 2 tables

The timescales and P–T conditions recorded by granulite facies metamorphic rocks permit inferences about the geodynamic regime in which they formed. Two compositionally heterogeneous cordierite–spinel‐bearing granulites from Vizianagaram, Eastern Ghats Province (EGP), India, were investigated to provide P–T–time constraints using petrography, phase equilibrium modelling, U–Pb geochronology, the rare earth element (REE) composition of zircon and monazite, and Ti‐in‐zircon thermometry. These ultrahigh temperature (UHT) granulites preserve discrete compositional layering in which different inferred peak assemblages are developed, including layers bearing garnet–sillimanite–spinel and others bearing orthopyroxene–sillimanite–spinel. These mineral associations cannot be reproduced by phase equilibrium modelling of whole rock compositions, indicating that the samples became domainal on a scale less than that of a thin section, even at UHT conditions. Calculation of the P–T stability fields for six compositional domains within which the main rock‐forming minerals are considered to have attained equilibrium suggests peak metamorphic conditions of ~6.8–8.3 kbar at ~1000°C. In most of these domains, the subsequent evolution resulted in the growth of cordierite and final crystallization of melt at an elevated (residual) H2O‐undersaturated solidus, consistent with

International audience; The Celtic Sea contains the world's largest continental shelf sediment ridges. These megaridges were initially interpreted as tidal features formed during post-glacial marine transgression, but glacigenic sediments have been recovered from their flanks. We examine the stratigraphy of the megaridges using new decimetric-resolution geophysical data correlated to sediment cores to test hypothetical tidal vs glacial modes of formation. The megaridges comprise three main units, 1) a superficial fining-upward drape that extends across the shelf above an unconformity. Underlying this drape is 2), the Melville Formation (MFm) which comprises the upper bulk of the megaridges, sometimes displaying dipping internal acoustic reflections and consisting of medium to coarse sand and shell fragments; characteristics consistent with either a tidal or glacifluvial origin. The MFm unconformably overlies 3), the Upper Little Sole Formation (ULSFm), previously interpreted to be of late Pliocene to early Pleistocene age, but here shown to correlate to Late Pleistocene glacigenic sediments forming a precursor topography. The superficial drape is interpreted as a product of prolonged wave energy as tidal currents diminished during the final stages of post-glacial marine transgression. We argue that the stratigraphy constrains the age of the MFm to between 24.3 and 14 ka BP, based on published dates, coeval with deglaciation and a modelled period of megatidal conditions during post-glacial marine transgression. Stratigraphically and sedimentologically, the megaridges could represent preserved glacifluvial features, but we suggest that they comprise post-glacial tidal deposits (MFm) mantling a partially-eroded glacial topography (ULSFm). The observed stratigraphy suggests that ice extended to the continental shelf-edge.

Uncertainties in future sea level projections are dominated by our limited understanding of the dynamical processes that control instabilities of marine ice sheets. A valuable case to examine these processes is the last deglaciation of the British-Irish Ice Sheet. The Minch Ice Stream, which drained a large proportion of ice from the northwest sector of the British-Irish Ice Sheet during the last deglaciation, is well constrained, with abundant empirical data which could be used to inform, validate and analyse numerical ice sheet simulations. We use BISICLES, a higher-order ice sheet model, to examine the dynamical processes that controlled the retreat of the Minch Ice Stream. We simulate retreat from the shelf edge under constant "warm" surface mass balance and subshelf melt, to isolate the role of internal ice dynamics from external forcings. The model simulates a slowdown of retreat as the ice stream becomes laterally confined at a "pinning-point" between mainland Scotland and the Isle of Lewis. At this stage, the presence of ice shelves became a major control on deglaciation, providing buttressing to upstream ice. Subsequently, the presence of a reverse slope inside the Minch Strait produces an acceleration in retreat, leading to a "collapsed" state, even when the climate returns to the initial "cold" conditions. Our simulations demonstrate the importance of the Marine Ice Sheet Instability and ice shelf buttressing during the deglaciation of parts of the British-Irish Ice Sheet. Thus, geological data could be used to constrain these processes in ice sheet models used for projecting the future of our contemporary ice sheets.

We present LA-ICP-MS U-Pb monazite and zircon geochronology, trace element chemistry and phase equilibria forward modelling to constrain the P-T-t evolution of the Bunger Hills, East Antarctica. Metasedimentary rocks in the Bunger Hills record evidence for a protracted metamorphic history during the Mesoproterozoic. Taken in isolation, zircon and monazite ages suggest an extremely long duration of high-temperature conditions (ca. 200 Myr). Calculated P-T models indicate metamorphism involved medium pressures of 5.5-7.1 kbar and high to ultrahigh temperatures of 800-960 °C, and that the P-T path likely tracked along a down-pressure to isobaric cooling trajectory. Integrating trace element data from zircon, monazite and garnet indicates that, despite the spread in U-Pb ages, peak metamorphism essentially occurred over the interval ca. 1220-1180 Ma. The age and conditions of Mesoproterozoic metamorphism are consistent with the high-grade metamorphic evolution proposed previously for Stage-2 of the Albany-Fraser Orogeny in southwestern Australia. The P-T-t conditions are interpreted to reflect extension, potentially associated with unloading and exhumation of a collisional orogen following Stage-1 of the Albany-Fraser Orogeny. This is the first study to integrate geochronology, trace element chemistry and P-T modelling to constrain the metamorphic evolution of the Bunger Hills and to interpret these constraints within the context of the now separate terranes of the Musgrave-Albany-Fraser Orogen. The three-way approach adopted in this study demonstrates that zircon and monazite may grow and modify through a number of processes. An integrated petrochronologic approach is therefore essential for investigations on high-grade terranes. Refereed/Peer-reviewed

Exhumed eclogitic crust is rare and exposures that preserve both protoliths and altered domains are limited around the world. Nominally anhydrous Neoproterozoic anorthositic granulites exposed on the island of Holsnoy, in the Bergen Arcs in western Norway, preserve different stages of progressive prograde deformation, together with the corresponding fluid-assisted metamorphism, which record the conversion to eclogite during the Ordovician-Silurian Caledonian Orogeny. Four stages of deformation can be identified: 1) brittle deformation resulting in the formation of fractures and the generation of pseudotachylites in the granulite; 2) development of mesoscale shear zones associated with increased fluid–rock interaction; 3) the complete large-scale replacement of granulite by hydrous eclogite with blocks of granulite sitting in an eclogitic ‘matrix’; and 4) the break-up of completely eclogitised granulite by continued fluid influx, resulting in the formation of coarse-grained phengite-dominated mineral assemblages. P–T constraints derived from phase equilibria forward modelling of mineral assemblages of the early and later stages of the conversion to eclogite document burial and partial exhumation path with peak metamorphic conditions of ~ 21–22 kbar and 670–690 °C. The P–T models, in combination with existing T–t constraints, imply that the Lindas Nappe underwent extremely rapid retrogressive pressure change. Fluid infiltration began on the prograde burial path and continued throughout the recorded P–T evolution, implying a fluid source that underwent progressive dehydration during subduction of the granulites. However, in places limited fluid availability on the prograde path resulted in assemblages largely consuming the available fluid, essentially freezing in snapshots of the prograde evolution. These were carried metastably deeper into the mantle with strain and metamorphic recrystallization partitioned into areas where ongoing fluid infiltration was concentrated. This article is protected by copyright. All rights reserved.

HL was funded by a NERC PhD studentship (NE/I528050/1) at Queen Mary University of London, the Queen Mary Postgraduate Research Fund, and an Arctic Field Grant from the Research Council of Norway through the University Centre in Svalbard (UNIS). SL acknowledges funding from the Westfield Trust. JCY was funded by a Carlsberg Foundation grant (2007/01/0383). A Royal Geographical Society Peter Fleming Award and a grant from the National Geographic Committee for Research and Exploration to Andy Hodson helped fund some of the fieldwork. Some surge-type glaciers on the High-Arctic archipelago of Svalbard have large glacitectonic composite ridge systems at their terrestrial margins. These have formed by rapid glacier advance into proglacial sediments during the active surge phase, creating multicrested moraine complexes. Such complexes can be formed during single surge advances or multiple surges to successively less-extensive positions. The few existing studies of composite ridge systems have relied on detailed information on internal structure and sedimentology to reconstruct their formation and links to surge processes. However, natural exposures of internal structure are commonly unavailable, and the creation of artificial exposures is often problematic in fragile Arctic environments. To compensate for these issues, we investigate the potential for reconstructing composite ridge system formation based on geomorphological evidence alone, focusing on clear morphostratigraphic relationships between ridges within the moraine complex and relict meltwater channels/outwash fans. Based on mapping at the margins of Finsterwalderbreen (in Van Keulenfjorden) and Grønfjordbreen (in Grønfjorden), we show that relict meltwater channels that breach outer parts of the composite ridge systems are in most cases truncated upstream within the ridge complex by an inner pushed ridge or ridges at their ice-proximal extents. Our interpretation of this relationship is that the entire composite ridge system is unlikely to have formed during the same glacier advance but is instead the product of multiple advances to successively less-extensive positions, whereby younger ridges are emplaced on the ice-proximal side of older ridges. This indicates that the Finsterwalderbreen composite ridge system has been formed by multiple separate advances, consistent with the cyclicity of surges. Being able to identify the frequency and magnitude of former surges is important as it provides insight into the past behaviour of surge-type glaciers and, if absolute dating is possible, allows for the assessment of surge-type glacier response to climate change on decadal to centennial timescales. Although further investigations into the internal structure of these deposits should be sought where possible, our study demonstrates that geomorphology could be an invaluable tool for reconstructing the formation of composite ridge systems. Postprint

Geomorphological mapping is a well-established method for examining earth surface processes and landscape evolution in a range of environmental contexts. In glacial research, it provides crucial data for a wide range of process-oriented studies and palaeoglaciological reconstructions; in the latter case providing an essential geomorphological framework for establishing glacial chronologies. In recent decades, there have been significant developments in remote sensing and Geographical Information Systems (GIS), with a plethora of high-quality remotely-sensed datasets now (often freely) available. Most recently, the emergence of unmanned aerial vehicle (UAV) technology has allowed sub-decimetre scale aerial images and Digital Elevation Models (DEMs) to be obtained. Traditional field mapping methods still have an important role in glacial geomorphology, particularly in cirque glacier, valley glacier and icefield/ice-cap outlet settings. Field mapping is also used in ice sheet settings, but often takes the form of necessarily highly-selective ground-truthing of remote mapping. Given the increasing abundance of datasets and methods available for mapping, effective approaches are necessary to enable assimilation of data and ensure robustness. This paper provides a review and assessment of the various glacial geomorphological methods and datasets currently available, with a focus on their applicability in particular glacial settings. We distinguish two overarching ‘work streams’ that recognise the different approaches typically used in mapping landforms produced by ice masses of different sizes: (i) mapping of ice sheet geomorphological imprints using a combined remote sensing approach, with some field checking (where feasible); and (ii) mapping of alpine and plateau-style ice mass (cirque glacier, valley glacier, icefield and ice-cap) geomorphological imprints using remote sensing and considerable field mapping. Key challenges to accurate and robust geomorphological mapping are highlighted, often necessitating compromises and pragmatic solutions. The importance of combining multiple datasets and/or mapping approaches is emphasised, akin to multi-proxy approaches used in many Earth Science disciplines. Based on our review, we provide idealised frameworks and general recommendations to ensure best practice in future studies and aid in accuracy assessment, comparison, and integration of geomorphological data. These will be of particular value where geomorphological data are incorporated in large compilations and subsequently used for palaeoglaciological reconstructions. Finally, we stress that robust interpretations of glacial landforms and landscapes invariably requires additional chronological and/or sedimentological evidence, and that such data should ideally be collected as part of a holistic assessment of the overall glacier system.

The Southern Granulite Terrane in southern India preserves evidence for regional-scale high to ultrahigh temperature metamorphism related to the amalgamation of the supercontinent Gondwana. Here we present accessory mineral (zircon and monazite) geochronological and geochemical datasets linked to the petrological evolution of the rocks as determined by phase equilibria modelling. The results constrain the duration of high to ultrahigh temperature (> 900 °C) metamorphism in the Madurai Block to be c. 40 Ma with peak conditions achieved c. 60 Ma after the formation of an orogenic plateau related to the collision of the microcontinent Azania with East Africa at c. 610 Ma. A 1D numerical model demonstrates that the attainment of temperatures > 900 °C requires that the crust be moderately enriched in heat producing elements and that the duration of the orogenic event is sufficiently long to allow conductive heating through radioactive decay. Both of these conditions are met by the available data for the Madurai Block. Our results constrain the length of time it takes for the crust to evolve from collision to peak P–T (i.e. the prograde heating phase) then back to the solidus during retrogression. This evolution illustrates that not all metamorphic ages date sutures.

Madagascar occupies an important location within the East African Orogen (EAO), which involves a collection of Neoproterozoic microcontinents and arc terranes lodged between older cratonic units during the final assembly of the supercontinent Gondwana. The detrital zircon record of Proterozoic metasedimentary rock packages within the Antananarivo Domain (the Ambatolampy, Manampotsy, Vondrozo, Itremo and Ikalamavony Groups) is used to identify pre-orogenic tectonic affiliations of the region—affiliations that control interpretations of the evolution of the Mozambique Ocean and the formation of this part of central Gondwana. Here we focus on the Ambatolampy Group, a previously poorly known group of high-grade siliciclastic metasedimentary rocks, and compare these data to similar data from the other sedimentary packages in central Madagascar (the Vondrozo, Sofia, Manampotsy, Itremo, Molo, Ikalamavony, Androyen, Ambodiriana, Iakora and Maha Groups). New U–Pb (SHRIMP) zircon data for the Ambatolampy Group yield detrital age maxima of ~ 3000 Ma, ~ 2800–2700 Ma, ~ 2500 Ma, ~ 2200–2100 Ma and ~ 1800 Ma. The youngest near-concordant detrital zircon age is 1836 ± 25 Ma, which we suggest represents the maximum depositional age of the Ambatolampy Group, in contrast to younger ages reported elsewhere. The detrital spectra are very similar to those from the Itremo Group and we suggest that they correlate with each other. Metamorphic zircons and zircon rims constrain the minimum depositional age to be ~ 540 Ma; the interpreted age of metamorphism. New δ 18 O data (SHRIMP-SI) and hafnium (MC-LA–ICP–MS) isotopic data complement the U–Pb data and provide new constraints on the age, geochemistry and provenance of the metasedimentary rocks. We suggest that central Madagascar contained a Mesoproterozoic (to possibly Tonian) siliciclastic sedimentary basin, within which the Ambatolampy Group, and the Itremo and Maha Groups (and possibly part of the Iakora Groups) were deposited. Later sedimentary systems are represented by the Ikalamavony, Ambodiriana, Manampotsy and Molo Groups.

An important question in petrology is whether the production of granite magma in orogens is a closed-system process with respect to mass input from the mantle. This is commonly addressed by inversion of geochemical data from upper crustal granites, but a complementary approach is to assess the kinship of residual granulites and associated granites in exhumed orogenic crust. Here we report geochemical data for a suite of contemporaneous metasedimentary granulites and granites from the Eastern Ghats Province, India, part of a Grenville-age orogen. The prograde metamorphic evolution involved increasing temperature ( T ) and pressure ( P ) to a metamorphic peak at >1000 °C at ∼0.7 GPa, followed by slow close-to-isobaric cooling. Variations in the composition of granites are interpreted to be due to local processes, including fractionation during melting or crystallization, and/or peritectic mineral entrainment. The Nd and Sr isotope compositions of the granites can be matched by mixing between different granulites, suggesting that they may have been derived solely from sedimentary protoliths leaving behind granulite facies residues. However, by including geochemical data from an adjacent area to the north, it becomes clear that an increasingly important mass input from the mantle was involved in granite genesis from southwest to northeast in the Eastern Ghats Province, as confirmed by modeling assimilation–fractional crystallization between an exemplar mantle-derived melt at 1000 Ma and the residual granulites. The extreme peak metamorphic temperature and P – T evolution suggest extended lithosphere that relaxed thermally to its former thickness during slow cooling. We postulate that the spatial variation in mantle input to the granites was related to changing feedback between the rates of extension and flux of mantle melt.

The ~ 120,000 km2 Musgrave Province forms part of a continuous Musgrave–Albany–Fraser mid-late Mesoproterozoic orogenic system that transects central and southern Australia, and continues into formerly contiguous Antarctica. Voluminous felsic magmatic rocks that intruded over the interval 1330–1150 Ma, corresponding globally to the Grenvillian timeline, dominate the Musgrave Province. However, rare but widely distributed metapelitic granulites contain peak metamorphic mineral assemblages comprising garnet + sillimanite ± quartz ± spinel. These are variably overprinted by coronae and/or symplectites of cordierite-bearing assemblages such as cordierite + spinel + magnetite ± plagioclase ± garnet. Petrologic forward modelling and Zr-in-rutile thermometry indicates these peak mineral assemblages developed at thermally extreme conditions of approximately 1000 °C and ca. 7–8 kbar. These ultra-high temperature (UHT) conditions appear to have prevailed throughout the Musgrave Province, across an approximate 600 km strike distance. The retrograde P–T evolution was characterised by modest decreases in pressure during the initial high temperature segment of the cooling path, suggesting that the crust was not significantly thickened as a result of tectonism. Combined SIMS (SHRIMP) and LA–ICP–MS U–Pb geochronology constrains the total range of metamorphic monazite growth/recrystallisation ages span 1263–1111 Ma with most individual samples spanning an age range of ≥ 80 Myr. The total age span implies approximately 150 Myr of perturbed thermal conditions during the Musgrave Orogeny. Our data requires that monazite is extremely resistive to isotopic resetting, even when exposed to extreme thermal conditions for long (≥ 80 Myr) periods. The thermal conditions, large regional footprint and long timescale of metamorphism and magmatism classify the Musgrave Province as a large, hot orogen.

Gondwana amalgamated along a suite of Himalayan-scale collisional orogens, the roots of which lace the continents of Africa, South America, and Antarctica. The Southern Granulite Terrane of India is a generally well-exposed, exhumed, Gondwana-forming orogen that preserves a record of the tectonic evolution of the eastern margin of the East African Orogen during the Ediacaran-Cambrian (circa 600–500 Ma) as central Gondwana formed. The deformation associated with the closure of the Mozambique Ocean and collision of the Indian and East African/Madagascan cratonic domains is believed to have taken place along the southern margin of the Salem Block (the Palghat-Cauvery Shear System, PCSS) in the Southern Granulite Terrane. Investigation of the structural fabrics and the geochronology of the high-grade shear zones within the PCSS system shows that the Moyar-Salem-Attur shear zone to the north of the PCSS system is early Paleoproterozoic in age and associated with dextral strike-slip motion, while the Cauvery shear zone (CSZ) to the south of the PCSS system can be loosely constrained to circa 740–550 Ma and is associated with dip-slip dextral transpression and north side-up motion. To the south of the proposed suture zone (the Cauvery shear zone), the structural fabrics of the Northern Madurai Block suggest four deformational events (D1–D4), some of which are likely to be contemporaneous. The timing of high pressure-ultrahigh temperature metamorphism and deformation (D1–D3) in the Madurai Block (here interpreted as the southern extension of Azania) is constrained to circa 550–500 Ma and interpreted as representing collisional orogeny and subsequent orogenic collapse of the eastern margin of the East African Orogen. The disparity in the nature of the structural fabrics and the timing of the deformation in the Salem and the Madurai Blocks suggest that the two experienced distinct tectonothermal events prior to their amalgamation along the Cauvery shear zone during the Ediacaran/Cambrian.

Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure–temperature–time (P–T–t) evolution of the Nagercoil Block at the southernmost tip of peninsular India. An inferred peak metamorphic assemblage of garnet, K-feldspar, sillimanite, plagioclase, magnetite, ilmenite, spinel and melt is consistent with peak metamorphic pressures of 6–8 kbar and temperatures in excess of 900 °C. Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 °C. SHRIMP U–Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palaeoproterozoic crystallisation ages. New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation, and the minimum age of the metamorphic peak, to around 560 Ma. The data suggest two stages of monazite growth. The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite. Relatively REE-depleted rims, which have a pronounced negative europium anomaly, grew during melt crystallisation along the retrograde path at around 535 Ma. Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer, supporting a long-lived high-grade metamorphic history. The metamorphic conditions, timing and duration of the implied clockwise P–T–t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.

The deformation of monazite in the polymetamorphic Sandmata granulite complex in India has been investigated by electron backscattered diffraction and sensitive high-resolution ion microprobe (SHRIMP). Quantitative microstructural analyses document the development of deformation twins in {100}, {001}, and ![Graphic][1] orientations; low-angle (

New data for intrusive meta-igneous rocks from the Southwest terrane of the Sor Rondane Mountains confirm the view that this is a juvenile oceanic arc terrane, with the main phase of subduction-related magmatic activity around 995–975 Ma. Younger magmatism (960–925 Ma) is more varied: a high Sr/Y (‘adakitic’) suite is present, as well as high-Ti mafic dykes, and one sample of A-type granite. This is interpreted as reflecting the end of subduction. The occasional presence of Archaean inherited zircons suggests proximity of Sor Rondane to an older continental nucleus from which detritus was shed. Although the ‘meta-igneous sector’ appears to be unique in representing a juvenile oceanic arc terrane, igneous ages and isotopic compositions around 1000–900 Ma suggest a broad coherence between outcrops ranging from Schirmacher Oasis (11° E) to Yamato Mts (35° E). This area seems unrelated to the slightly older, and isotopically and geochemically more enriched Mesoproterozoic rocks of central and western Dronning Maud Land. A closer relation appears to exist with Sri Lanka-India-Madagascar during the earliest Neoproterozoic than with southern Africa.

In this paper we undertake a quantitative analysis of the dynamic process by which ice underneath a dry porous debris layer melts. We show that the incorporation of debris-layer airflow into a theoretical model of glacial melting can capture the empirically observed features of the so-called Østrem curve (a plot of the melt rate as a function of debris depth). Specifically, we show that the turning point in the Østrem curve can be caused by two distinct mechanisms: the increase in the proportion of ice that is debris-covered and/or a reduction in the evaporative heat flux as the debris layer thickens. This second effect causes an increased melt rate because the reduction in (latent) energy used for evaporation increases the amount of energy available for melting. Our model provides an explicit prediction for the melt rate and the temperature distribution within the debris layer, and provides insight into the relative importance of the two effects responsible for the maximum in the Østrem curve. We use the data of Nicholson and Benn (2006) to show that our model is consistent with existing empirical measurements.

The use of rare earth element (REE) partition coefficients is an increasingly common tool in metamorphic studies, linking the growth or modification of accessory mineral geochronometers to the bulk silicate mineral assemblage. The most commonly used mineral pair for the study of high-grade metamorphic rocks is zircon and garnet. The link from U–Pb ages provided by zircon to the P-T information recorded by garnet can be interpreted in relation to experimental data. The simplistic approach of taking the average REE abundances for zircon and garnet and comparing them directly to experimentally derived partition coefficients is imperfect, in that it cannot represent the complexity of a natural rock system. This study describes a method that uses all the zircon analyses from a sample, and compares them to different garnet compositions in the same rock. Using the most important REE values, it is possible to define zircon–garnet equilibrium using an array rather than an average. The array plot describes partitioning between zircon and garnet using DYb and DYb/DGd as the defining features of the relationship. This approach provides far more sensitivity to mineral reactions and diffusional processes, enabling a more detailed interpretation of metamorphic history of the sample. This article is protected by copyright. All rights reserved.

A persistent problem in the study of garnet geochemistry is that the consideration of major elements alone excludes a wealth of information preserved by trace elements, particularly the rare-earth elements (REEs). This is despite the fact that trace elements are generally less vulnerable to diffusive resetting, and are sensitive toa broader spectrum of geochemical interactions involving the entire mineral assemblage, including the growth and/or dissolution of accessory minerals. We outline a technique for the routine acquisition of high-resolution 2D trace element maps by LA-ICP-MS, and introduce an extension of the software package XMapTools for rapid processing of LA-ICP-MS data to visualise and interpret compositional zoning patterns. These methods form the basis for investigating the mechanisms controlling geochemical mobility in garnet, which are argued to be largely dependent on the interplay between element fractionation, mineral reactions and partitioning, and the length scales of intergranular transport. Samples from the Peaked Hill shear zone, Reynolds Range, central Australia, exhibit contrasting trace element distributions that can be linked to a detailed sequence of growth and dissolution events. Trace element mapping is thus employed to place garnet evolution in a specific paragenetic context and derive absolute age information by integration with existing U–Pb monazite and Sm–Nd garnet geochronology. Ultimately, the remarkable preservation of original growth zoning and its subtle modification by subsequent re-equilibration is used to ‘see through’ multiple superimposed events, thereby revealing a previously obscure petrological and temporal record of metamorphism,metasomatism, and deformation. Refereed/Peer-reviewed

Bed topography is a critical boundary for the numerical\ud modelling of ice sheets and ice–ocean interactions.\ud A persistent issue with existing topography products for the\ud bed of the Greenland Ice Sheet and surrounding sea floor is\ud the poor representation of coastal bathymetry, especially in\ud regions of floating ice and near the grounding line. Sparse\ud data coverage, and the resultant coarse resolution at the ice–\ud ocean boundary, poses issues in our ability to model ice\ud flow advance and retreat from the present position. In addition,\ud as fjord bathymetry is known to exert strong control\ud on ocean circulation and ice–ocean forcing, the lack\ud of bed data leads to an inability to model these processes\ud adequately. Since the release of the last complete Greenland\ud bed topography–bathymetry product, new observational\ud bathymetry data have become available. These data can be\ud used to constrain bathymetry, but many fjords remain completely\ud unsampled and therefore poorly resolved. Here, as\ud part of the development of the next generation of Greenland\ud bed topography products, we present a new method for constraining\ud the bathymetry of fjord systems in regions where\ud data coverage is sparse. For these cases, we generate synthetic\ud fjord geometries using a method conditioned by surveys\ud of terrestrial glacial valleys as well as existing sinuous\ud feature interpolation schemes. Our approach enables the capture\ud of the general bathymetry profile of a fjord in north-west\ud Greenland close to Cape York, when compared to observational\ud data. We validate our synthetic approach by demonstrating\ud reduced overestimation of depths compared to past\ud attempts to constrain fjord bathymetry. We also present an\ud analysis of the spectral characteristics of fjord centrelines using\ud recently acquired bathymetric observations, demonstrating\ud how a stochastic model of fjord bathymetry could be parameterised\ud and used to create different realisations.

Meta-igneous lithologies of the Cullivoe inlier in NE Yell, Shetland, have tonalite–trondhjemite–granodiorite (TTG) chemistry and yield U–Pb zircon crystallization ages of c. 2856 – 2699 Ma. Formation was coeval with protoliths of the Lewisian Gneiss Complex and the time of major Neoarchaean crustal growth in the North Atlantic Craton. The adjacent metasedimentary Yell Sound Group accumulated between c. 1019 and c. 941 Ma. The Cullivoe inlier and the Yell Sound Group were metamorphosed at c. 944 – 931 Ma, the former preserving granulite-facies mineral assemblages inferred to be of this age. Similar-aged metamorphic events recorded in other Laurentian metasedimentary successions in the North Atlantic region are attributed to development of the Valhalla orogen along the Rodinia margin. Ordovician (482 ± 30 Ma) and Silurian (428 ± 16 Ma) thermal rejuvenation resulted from successive phases of the Caledonian orogeny during closure of the Iapetus Ocean. The mechanism by which the Cullivoe inlier was emplaced into its current structural setting is uncertain. Either its western or eastern boundary is a major tectonic break, probably an early ductile thrust. However, this is now cryptic as a result of the Caledonian ductile reworking. Supplementary material: SIMS and LA-ICP-MS analytical data, statistical analyses and major trace element analytical data are available at https://doi.org/10.6084/m9.figshare.c.3575723.

Ultrahigh temperature (UHT) granulites in the Eastern Ghats Province (EGP) have a complex P–T–t history. We review the P–T histories of UHT metamorphism in the EGP and use that as a framework for investigating the P–T–t history of Mg–Al-rich granulites from Anakapalle, with the express purpose of trying to reconcile the down pressure-dominated P–T path with other UHT localities in the EGP. Mafic granulite that is host to Mg–Al-rich metasedimentary granulites at Anakapalle has a protolith age of c. 1580 Ma. Mg–Al-rich metasedimentary granulites within the mafic granulite at Anakapalle were metamorphosed at UHT conditions during tectonism at 960–875 Ma, meaning that the UHT metamorphism was not the result of contact metamorphism from emplacement of the host mafic rock. Reworking occurred during the Pan-African (c. 600–500 Ma) event, and is interpreted to have produced hydrous assemblages that overprint the post-peak high-temperature retrograde assemblages. In contrast to rocks elsewhere in the EGP that developed post-peak cordierite, the metasedimentary granulites at Anakapalle developed post-peak, generation ‘2′ reaction products that are cordierite-absent and nominally anhydrous. Therefore, rocks at Anakapalle offer the unique opportunity to quantify the pressure drop that occurred during so-called M2 that affected the EGP. We argue that M2 is either a continuation of M1 and that the overall P–T path shape is a complex counter-clockwise loop, or that M1 is an up-temperature counter-clockwise deviation superimposed on the M2 path. Therefore, rather than the rocks at Anakapalle having a metamorphic history that is apparently anomalous from the rest of the EGP, we interpret that other previously studied localities in the EGP record a different part of the same P–T path history as Anakapalle, but do not preserve a significant record of pressure decrease. This is due either to the inability of refractory rocks to extensively react to produce a rich mineralogical record of pressure decrease, or because the earlier high-P part of the rocks history was erased by the M1 loop. Irrespective of the specific scenario, models for the tectonic evolution of the EGP must take the substantial pressure decrease during M2 into account, as it is probable the P–T record at Anakapalle is a reflection of tectonics affecting the entire province. This article is protected by copyright. All rights reserved.

The partitioning of rare earth elements (REE) between zircon, garnet and silicate melt was determined using synthetic compositions designed to represent partial melts formed in the lower crust during anatexis. The experiments, performed using internally heated gas pressure vessels at 7 kbar and 900–1000 °C, represent equilibrium partitioning of the middle to heavy REE between zircon and garnet during high-grade metamorphism in the mid to lower crust. The DREE (zircon/garnet) values show a clear partitioning signature close to unity from Gd to Lu. Because the light REE have low concentrations in both minerals, values are calculated from strain modelling of the middle to heavy REE experimental data; these results show that zircon is favoured over garnet by up to two orders of magnitude. The resulting general concave-up shape to the partitioning pattern across the REE reflects the preferential incorporation of middle REE into garnet, with DGd (zircon/garnet) ranging from 0.7 to 1.1, DHo (zircon/garnet) from 0.4 to 0.7 and DLu (zircon/garnet) from 0.6 to 1.3. There is no significant temperature dependence in the zircon–garnet REE partitioning at 7 kbar and 900–1000 °C, suggesting that these values can be applied to the interpretation of zircon–garnet equilibrium and timing relationships in the ultrahigh-T metamorphism of low-Ca pelitic and aluminous granulites.

The limitations of conventional thermobarometry and petrogenetic grids for determining the peak P – T conditions of granulites are well known. These limitations have been overcome during the past decade with the calibration of single mineral thermometers, particularly Al-in-orthopyroxene, Zr-in-rutile and Ti-in-zircon, and the increased use of P – T pseudosection thermobarometry. Most recent studies of ultrahigh temperature (UHT) granulites (those formed at >900 °C) have used one or other of these methods to argue for peak metamorphic temperatures up to or beyond 1000 °C. Since models for the thermal evolution of orogens generally do not predict such extreme temperatures it is important to confirm their veracity. Here we combine in a single study single mineral thermometry with P – T pseudosection thermobarometry to provide a robust determination of peak temperature and tight constraints on the retrograde P – T path for one UHT granulite locality in the Eastern Ghats Province. This is the first study to apply the most recent update of the internally consistent thermodynamic dataset of Holland and Powell (2011) and the re-parameterized a – x models of White et al. (2014) and Wheller and Powell (2014) to UHT granulites. For two samples, we report Zr-in-rutile temperatures of >1000 °C and Ti-in-zircon temperatures of ∼900 °C, supported by Al-in-orthopyroxene temperatures of ∼900 °C, that correspond closely to those estimated using P – T pseudosections for conditions at the thermal peak and at the solidus on the retrograde P – T path, respectively. The P – T path is counter-clockwise in common with other UHT granulite localities in the Eastern Ghats Province. By demonstrating that UHT metamorphism at T > 1000 ° C is real we provide a robust constraint that must be met by geodynamic models for the development of ultrahot orogens.

Coupled U-Pb and Hf isotopic analysis of detrital zircons from metasedimentary rocks of the Southern Granulite terrane (India) provides provenance information that helps unravel their paleotectonic position before Gondwana amalgamated. The metasedimentary packages of the Salem block (southernmost extension of Dharwar craton) record a restricted juvenile late Archean to early Paleoproterozoic (2.7–2.45 Ga) source provenance and epsilon Hf values between +0.3 and +8.8. Similar late Archean juvenile crust is found throughout the Dharwar craton and represents a likely source for the Salem block metasedimentary rocks. By contrast, the metasedimentary rocks of the Madurai block (south of the Salem block) show a predominantly Archean to Paleoproterozoic provenance (3.2–1.7 Ga) in the northern part of the Madurai block and a largely late Meso protero zoic to Neoproterozoic provenance (1.1–0.65 Ga) in the southern part of the Madurai block. Collectively, the Madurai block metasedimentary rocks record a mixture of reworked Archean and Paleo proterozoic sources, as well as juvenile Paleo proterozoic, late Mesoproterozoic, and evolved Neoproterozoic sources. These detrital signatures best fi t the combined basement ages of the Congo-Tanzania-Bangweulu block and central Madagascar (Azania), thus linking the tectonic evolution of the southernmost tip of India to these domains throughout much of the Proterozoic. The diachroneity of metamorphic ages obtained from the rims of Madurai block detrital zircons attests to their poly-metamorphic history that is different from that of the Salem block. The contrasting metamorphic and depositional histories between the Salem and Madurai blocks place them on opposite sides of the Mozambique Ocean until the latest Neoproterozoic when they came together to form Gondwana. Refereed/Peer-reviewed

Elevated heat flow and mafic magmatism during lithospheric extension have often been invoked as a mechanism to drive high-temperature low-pressure metamorphism that produces granulite facies mineral assemblages. Typically, however, evidence of the contemporaneous heat source, such as coeval mafic magmatism, is absent. In this study, we present pressure–temperature (P–T) pseudosection analysis combined with U–Pb isotopic data from zircon and monazite that constrain both the conditions and timing of granulite facies metamorphism in the Fraser Range Metamorphics of the Albany-Fraser Orogen in southern Western Australia. These results also elucidate the extremely rapid timing of, sequentially, deposition of sedimentary protoliths, mafic magmatism, partial melting, and metamorphism within the Fraser Zone during the Mesoproterozoic. The youngest detrital zircons, together with the magmatic ages of intrusive rocks, constrain the depositional age of the protoliths to the Fraser Range Metamorphics to between 1334 and 1293 Ma. Peak metamorphic conditions at c. 1290 Ma were c. 850 °C at pressures of 7–9 kbar. Peak metamorphism was followed by a period of isobaric cooling at pressures of c. 9 kbar. U–Pb zircon ages from leucosomes and metamorphic overgrowths in the metapelitic rocks indicate crystallization of partial melts at 1290 Ma, essentially coincident with the emplacement of mafic rocks at 1292 Ma. In situ analyses of both matrix hosted monazite and monazite inclusions in garnet yield ages between 1285 and 1268 Ma, with no significant age difference between monazite in the two textural positions. Cooling of the Fraser Zone below the Rb-Sr biotite closure temperature (∼400 °C) occurred at 1260 Ma. Cooling and strengthening of the Fraser Zone rendered it less susceptible to subsequent tectonic events that affected rocks to the north and south of this resistant lozenge. Only rare geochronological evidence of later events can be resolved in recrystallised monazite rims dated at 1234 ± 17 Ma.

Understanding the retreat behaviour of past marine-ice sheets provides vital context to accurate assessment of the present stability and long-term response of contemporary polar-ice sheets to climate and oceanic warming. Here new multibeam swath-bathymetry data and sedimentological analysis are combined with high resolution ice-sheet modelling to reveal complex landform assemblages and process-dynamics associated with deglaciation of the British-Celtic Ice Sheet (BCIS) within the Irish Sea Basin. Our reconstruction indicates a non-linear relationship between the rapidly receding Irish Sea Ice Stream, the largest draining the BCIS, and the retreat of outlet glaciers draining the adjacent, terrestrially based ice sheet centred over Wales. Retreat of Welsh ice was episodic; superimposed over low-order oscillations of its margin are asynchronous outlet re-advances driven by catchment-wide mass balance variations that are amplified through migration of the ice cap's main ice-divide. Formation of large, linear ridges which extend at least 12.5 km offshore (locally known as sarns) and dominate the regional bathymetry are attributed to repeated frontal and medial morainic deposition associated with the re-advancing phases of these outlet glaciers. Our study provides new insight into ice-sheet extent, dynamics and non-linear retreat across a major palaeo-ice stream confluence zone, and has ramifications for the interpretation of recent fluctuations observed by satellites over short-time scales across marine-sectors of the Greenland and Antarctic ice sheets.

Zircons from two paragneisses (from Mount Sones and Dallwitz Nunatak) and one orthogneiss (from Gage Ridge) in the Tula Mountains, Napier Complex (East Antarctica) were analyzed for U-Pb age, oxygen isotopes, REEs and by scanning ion imaging. A large number of zircons from all samples are reversely discordant. Mount Sones zircons show an age range from 3.0 Ga to 2.5 Ga and underwent high-grade metamorphism at both ∼2.8 Ga and 2.5 Ga. Zircons from Dallwitz Nunatak record detrital ages between 3.5 Ga and 2.5 Ga. Zircons from Gage Ridge record multiple age groups, with concordant data between 3.6 Ga and 3.3 Ga and reversely discordant data that form a discrete ∼3.8 Ga population. All of the grains show evidence of Pb mobility during metamorphism. Ion imaging of zircons reveals Y and U zonation, characteristic of magmatic zircon, together with a micro-scale patchy distribution of 206Pb and 207Pb that does not correspond to either growth zonation or crystal imperfections. Some of these patches yield 207Pb/206Pb ages >4 Ga, whereas others yield ages younger than the magmatic crystallization age. Reversely discordant data are the result of ancient Pb mobilization, which is independent of the degree of metamictisation, oxygen isotope and REE content of the zircons. This mobilization can result in spurious ages and was most likely caused by polymetamorphism under anhydrous conditions; that is two high-grade events; one poorly defined at ∼2.8 Ga and the other ultra-high temperature (UHT) metamorphism at 2.5 Ga.

Mega-scale glacial lineations (MSGLs) are the largest flow parallel bedforms produced by ice sheets and are formed beneath rapidly-flowing ice streams. Knowledge of their characteristics and genesis is likely to result in an improved understanding of the rate at which ice and sediment are discharged by ice sheets, but there is little consensus as to how they are formed and there are few quantitative datasets of their characteristics with which to formulate or test hypotheses. This paper presents the results of a remote sensing survey of ∼46,000 bedforms on the Dubawnt Lake palaeo-ice stream bed, focussing on a central transect of 17,038 that includes highly elongate bedforms previously described as MSGLs. Within this transect, lineations exceed 10 km in length (max. >20 km) and 23% have elongation ratios >10:1 (max. 149:1). Highly elongate features are interspersed with much shorter drumlin-like features, but longer bedforms are typically narrower, suggesting that their length develops more quickly than, or at the expense of, their width. Bedforms are broadly symmetrical in plan-form and have a preferred lateral spacing of 50–250 m, which implies a regular, rather than random, pattern of corrugations. Comparison with drumlins reveals that the more attenuated MSGLs simply extend the ‘tail’ of the distribution of data, rather than plotting as a separate population. Taken together, this supports the idea of a subglacial bedform continuum primarily controlled by ice velocity, but existing hypotheses of MSGL formation are either not supported, or are insufficiently developed to explain our observations. Rather, we conclude that, under conditions of rapid ice flow, MSGLs attain their great length relatively quickly (decades) through a probable combination of subglacial deformation, which attenuates ridges, and erosional processes that removes material from between them.

Mega-scale glacial lineations (MSGLs) are highly elongate, subglacial landforms produced beneath zones of fast-flowing ice. While qualitative data on their morphology have existed for several decades, studies of their composition and sedimentology are comparatively rare. Sediment exposures along the course of the Finnie River in Nunavut, northern Canada, provide a window into the internal stratigraphy and sedimentology of MSGLs formed by the Dubawnt Lake Palaeo-Ice Stream during regional deglaciation of the Laurentide Ice Sheet. Stratigraphic sections record evidence for an initial advance of ice into the study area followed by ice sheet recession and deposition of glacifluvial and glacilacustrine outwash. Subsequently, the Dubawnt Lake Palaeo-Ice Stream overrode and reworked this outwash subglacially forming an ‘MSGL till’. This till comprises a sandy, red diamicton facies, forming the core of the MSGL ridges and containing variably deformed lenses, stringers and rafts of outwash. The sedimentology of this diamicton is consistent with an origin as a glacitectonite and hybrid till formed by a combination of non-pervasive subglacial sediment deformation and lodgement. Facies variations from stratified to massive diamicton reflect, in turn, variations in strain and subglacial transport distance. The occurrence of stratified glacifluvial sediments within these ridges and the well-preserved nature of many of the sandy inclusions within the diamicton imply relatively short transport distances and incomplete mixing. MSGLs under the Dubawnt Lake Palaeo-Ice Stream formed through a combination of subglacial erosion and deposition. This included non-pervasive, subglacial sediment deformation and the reworking of pre-existing sediment depocentres during streaming flow. These results highlight the importance of sediment supply to MSGL formation with the presence of abundant pre-existing sediments which were subsequently overridden being critical to lineation formation.

We provide a mechanistic explanation for observed metrics for drumlins, which represent their sizes and shapes. Our explanation is based on a concept of drumlin growth occurring through a process of instability, whereby small amplitude wave forms first grow as ice slides over a bed of deformable sediments, followed by a coarsening process, in which the wavelength as well as the relief of the drumlins continues to grow. The observations then provide inferences about the growth process itself.

The exposed residual crust in the Eastern Ghats Province records ultrahigh temperature (UHT) metamorphic conditions involving extensive crustal anatexis and melt loss. However, there is disagreement about the tectonic evolution of this late Mesoproterozoic–early Neoproterozoic orogen due to conflicting petrological, structural and geochronological interpretations. One of the petrological disputes in residual high Mg–Al granulites concerns the origin of fine-grained mineral intergrowths comprising cordierite + K-feldspar ± quartz ± biotite ± sillimanite ± plagioclase. These intergrowths wrap around porphyroblast phases and are interpreted to have formed by the breakdown of primary osumilite in the presence of melt trapped in the equilibration volume by the melt percolation threshold. The pressure (P)–temperature (T) evolution of four samples from three localities across the central Eastern Ghats Province is constrained using phase equilibria modelling in the chemical system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO). Results of the modelling are integrated with published geochronological results for these samples to show that the central Eastern Ghats Province followed a common P–T–t history. This history is characterized by peak UHT metamorphic conditions of 945–955 °C and 7.8–8.2 kbar followed by a slight increase in pressure and close-to-isobaric cooling to the conditions of the elevated solidus at 940–900 °C and 8.5–8.3 kbar. In common with other localities from the Eastern Ghats Province, the early development of cordierite before osumilite and the peak to immediate post-peak retrograde reaction between osumilite and melt to produce the intergrowth features requires that the prograde evolution was one of contemporaneous increasing pressure with increasing temperature. This counter-clockwise (CCW) evolution is evaluated for one sample using inverse phase equilibria modelling along a schematic P–T path of 150 °C kbar−1 starting from the low P–T end of the prograde P–T path as constrained by the phase equilibria modelling. The inverse modelling is executed by step-wise down temperature reintegration of sufficient melt into the residual bulk chemical composition at the P–T point of the 1 mol.% melt isopleth at each step, representing the melt remaining on grain boundaries after each prograde drainage event, to reach the melt connectivity transition (MCT) of 7 mol.%. The procedure is repeated until a plausible protolith composition is recovered. The result demonstrates that clastic sedimentary rocks that followed a CCW P–T evolution could have produced the observed mineral assemblages and microstructures preserved in the central Eastern Ghats Province. This study also highlights the role of melt during UHT metamorphism, particularly its importance to both chemical and physical processes along the prograde and retrograde segments of the P–T path. These processes include: (i) an increase in diffusive length scales during the late prograde to peak evolution, creating equilibration volumes larger than a standard thin section; (ii) the development of retrograde mineral assemblages, which is facilitated if some melt is retained post-peak; (iii) the presence of melt as a weakening mechanism and the advection of heat by melt, allowing the crust to thicken; and (iv) the effect of melt loss, which makes the deep crust both denser and stronger, and reduces heat production at depth, limiting crustal thickening and facilitating the transition to close-to-isobaric cooling.

Along the coast of Peninsular India, the Eastern Ghats expose a deep crustal section through a composite Proterozoic orogenic belt. To quantify the late Mesoproterozoic–early Neoproterozoic pressure (P)–temperature (T)–time (t) evolution of the Eastern Ghats Province, new SHRIMP U–Pb zircon and monazite age data from multiple localities are reported and integrated with the results of phase equilibria modelling. Samples of residual granulite, migmatite and enderbite yield a spread of weighted mean 207Pb/206Pb zircon and monazite ages between ca 970 and ca 930 Ma. Based on ranges of spot ages from several samples, the late prograde to peak ultrahigh temperature (UHT) metamorphism (counter-clockwise evolution (CCW) to T >950 °C at P >8 kbar) and initial cooling is interpreted to have occurred between ca 1130 and ca 970 Ma. Regionally extensive enderbite and charnockite magmas were emplaced into the hot, suprasolidus crust around the time of peak metamorphism. For the residual granulites and migmatites the retrograde P–T–t path is characterized by close-to-isobaric cooling to the variable but elevated solidi for different samples. Weighted mean ages between ca 970 and ca 930 Ma in several samples are interpreted to record the timing of crystallization of melt trapped by the percolation threshold in each of these samples. Two additional weighted mean ages of ca 980 Ma (from Korhonen F.J., Saw, A.K., Clark, C., Brown, M., Bhattacharya, S., 2011. New constraints on UHT metamorphism in the Eastern Ghats Province through the application of phase equilibria modelling and in situ geochronology. Gondwana Research 20, 764–781) extend this range back in time by 10 My. The variability in the calculated weighted mean ages across the region is interpreted to be due mainly to differences in the temperature of the elevated solidus from sample to sample, suggesting a slow cooling rate of ∼1 °C/My during the retrograde stage of this long-lived UHT metamorphism. These results demonstrate that the Eastern Ghats Province sustained UHT conditions (T >900 °C) for ≫50 My, and perhaps for as long as 200 Ma from ca 1130 to 930 Ma, during a single CCW tectono-metamorphic event.

Ion microprobe analyses of garnet porphyroblasts from three separate splays of the mid-crustal Walter-Outalpa shear zone, Curnamona Province, South Australia, indicate homogeneous δ18O values of

In the Caledonides of northwestern Ireland there is little evidence for the crystalline basement to the Dalradian Supergroup, though ascertaining its nature is a critical prerequisite for palaeogeographical reconstructions. In Donegal a lamprophyre intrusion has sampled material with isotopic composition, age and fabrics that permit the investigation of the hidden basement of this region. U–Pb and Lu–Hf isotopes in concert with structural analysis are used to elucidate the origin of three morphologically distinct zircon populations from the lamprophyre. The first of these comprises c . 828 to c . 1660 Ma rounded inherited grains. The second population, of 476 ± 3 Ma large xenocrystic grains, displays oscillatory zoned cores embayed by cross-cutting overgrowths, implying a multi-stage magmatic genesis. The third population, of 437 ± 5 Ma high aspect ratio euhedral grains, is interpreted to date the magmatic crystallization of the lamprophyre. Hf isotopic data from the Silurian and Neo- to Mesoproterozoic zircons yield crustal residence ages of 1.0–2.3 Ga. However, the Ordovician grains have ϵHf i values of c . −20 and model ages of 2.7–2.8 Ga. These data are consistent with the presence of either Archaean crust or an underthrust Ordovician magmatic rock with an Archaean source component, at depth in NW Ireland. Felsic xenoliths, interpreted to be derived from the country rock Dalradian metasediments, exhibit a pre-entrainment foliation and are randomly oriented relative to the foliation within the lamprophyre matrix, indicating deformation prior to c . 437 Ma, probably related to the Grampian orogeny. The host lamprophyre contains a magmatic foliation parallel to a foliation in the country rock, implying synkinematic emplacement during regional NW–SE compression at 437 Ma. Supplementary material: Details of the analytical methods are available online at www.geolsoc.org.uk/SUP18614.

Subglacial bedforms preserved in deglaciated landscapes record characteristics of past ice–sediment flow regimes, providing insight into subglacial processes and ice sheet dynamics. Individual forms vary considerably, but they can often be grouped into coherent fields, typically called flow-sets, that reflect discrete episodes of ice flow. Within these, bedform size–frequency distributions (predominantly height, width and length) are currently described by several statistics (e.g., mean, median, and standard deviation) that, arguably, do not best capture the defining characteristics of these populations. This paper seeks to create a better description based upon semi-log plots, which reveal that the frequency distributions of bedform dimensions (drumlin, mega-scale glacial lineation, and ribbed moraine) plot as straight lines above the mode (ϕ). This indicates, by definition, an exponential distribution, for which a simple and easily calculated, yet statistically rigorous, description is designed. Three descriptive parameters are proposed: gradient (λ; the exponent, characterising bedforms likely least affected by non-glacial factors), area-normalised y-intercept (β0; quantifying spatial density), and the mode (ϕ). Below ϕ, small features are less prevalent due to i) measurement: data, sampling and mapping fidelity; ii) possible post-glacial degradation; or iii) genesis: not being created sub-glacially. This new description has the benefit of being insensitive to the impact of potentially unmapped or degraded smaller features and better captures properties relating to ice flow. Importantly, using λ, flow sets can now be more usefully compared with each other across all deglaciated regions and with the output of numerical ice sheet models. Applications may also exist for analogous fluvial and aeolian bedforms. Identifying the characteristic exponential and that it is typical of ‘emergent’ subglacial bedforms is a new and potentially powerful constraint on their genesis, perhaps indicating that ice–sediment interaction is fundamentally stochastic in nature.

Zircon is arguably the most commonly used geochronometer, but the reliability of ages obtained requires a full understanding of processes that might compromise the integrity of its U-Pb systematics. Here we present the results of a multifaceted ion microprobe study of zircon grains from the Napier Complex, East Antarctica, a region affected by pervasive high-temperature metamorphism at 2.5 Ga, and from which previous zircon geochronological interpretations have been problematic. Both U-Pb spot analysis (∼15 μm) and high spatial resolution (∼2 μm) scanning ion imaging of Pb isotopes have been applied in an attempt to quantify the effects of metamorphism. Spot analyses spread along concordia yielding 207 Pb/ 206 Pb ages from 2.5 Ga to 3.9 Ga, with the oldest grains reversely discordant. Ion images of uranogenic Pb reveal a surprising micrometer-scale patchy distribution that is unrelated to crystal morphology or damage. The 207 Pb/ 206 Pb ratios within these subdomains correspond to apparent zircon ages as old as 4.2 Ga. These are interpreted as artifacts of ancient redistribution of radiogenic Pb, a process that can generate meaningless ages, and are not relicts of ancient (including Hadean) zircon. Scanning ion imaging thus facilitates identification of unsupported radiogenic Pb and enables testing of the validity of old ages from zircon known to have a long and complicated history.

We present an 8000-year history spanning 650km of ice margin retreat for the largest marine- terminating ice stream draining the former British-Irish Ice Sheet. Bayesian modelling of the geochronological data shows the ISIS expanded 34.0-25.3ka, accelerating into the Celtic Sea to reach maximum limits 25.3-24.5ka before a collapse with rapid marginal retreat to the northern Irish Sea Basin (ISB). This retreat was rapid and driven by climatic warming, sea-level rise, mega-tidal amplitudes and reactivation of meridional circulation in the North Atlantic. The retreat, though rapid, is uneven, with the stepped retreat pattern possibly a function of the passage of the ice stream between normal and adverse ice bed gradients and changing ice stream geometry. Initially, wide calving margins and adverse slopes encouraged rapid retreat (! 550m a " 1 ) that slowed (! 100m a " 1 ) at the topographic constriction and bathymetric high between southern Ireland and Wales before rates increased (! 200m a " 1 ) across adverse bed slopes and wider and deeper basin configuration in the northern ISB. These data point to the importance of the ice bed slope and lateral extent in predicting the longer-term (>1000 a) patterns and rates of ice-marginal retreat during phases of rapid collapse, which has implications for the modelling of projected rapid retreat of present-day ice streams. Copyright # 2013 John Wiley & Sons, Ltd.