Search

In the Appalachian Mountains of Vermont, USA, variably serpentinized ultramafic rocks mark the Ordovician Taconic orogenic suture zone. These ultramafic rocks provide evidence for several alteration events that occurred during Appalachian orogenesis. The largest of these ultramafic bodies occurs as a partially serpentinized meta-dunite located in East Dover, Vermont. Whole-rock X-ray fluorescence spectroscopy and electron microprobe data on variably serpentinized meta-dunite samples are interpreted with respect to several processes including fluid/melt-rock interaction in the mantle, serpentinization, and subsequent regional metamorphism. We report the first discovery of nickel arsenide minerals hosted in this meta-dunite, as well as rare occurrences of platinum-group mineral inclusions in chromitite. Although the platinum-group minerals and chromitite are rare, their occurrence and chemistry suggest that they formed by fluid/melt-rock interaction during partial melting events that produced the dunite, likely in a supra-subduction zone setting. Arsenic minerals are rare in un-serpentinized samples but are ubiquitous in highly serpentinized samples, which suggests that most of the arsenic was introduced into the ultramafic rocks during serpentinization. Whole-rock geochemical analyses also indicate that highly serpentinized samples contain the highest concentrations of arsenic. The discovery of arsenic minerals identifies a potential source to explain elevated arsenic in groundwater in Vermont, which is a serious health concern in places where wells have been drilled in serpentinite bedrock.

The Appalachian orogen has long been enigmatic because, compared to other parts of the Paleozoic orogens that formed following the subduction of the Iapetus Ocean, direct evidence for ultrahigh-pressure (UHP) metamorphism has never been found. We report the first discovery of coesite in the Appalachian orogen in a metapelite from the mid-Ordovician (Taconic orogeny) Tillotson Peak Complex in Vermont (USA). Relict coesite occurs within a bimineralic SiO2 inclusion in garnet. In situ elastic barometry and trace-element thermometry allow reconstruction of the garnet growth history during prograde metamorphism. The data are interpreted to indicate garnet nucleation and crystallization during blueschist- to eclogite-facies subduction zone metamorphism, followed by garnet rim growth at UHP conditions of > 28 kbar and > 530 ° C. Results provide the first direct evidence that rocks of the Appalachian orogen underwent UHP metamorphism to depths of > 75 km and warrant future studies that constrain the extent of UHP metamorphism.

Documenting ultrahigh-pressure (UHP) metamorphism in the geologic record is a key for understanding the evolution of plate tectonics on Earth. Characteristic UHP minerals like coesite and diamond that form during deep subduction are frequently replaced by their low-pressure polymorphs during exhumation. However, when entrapped as inclusions in resistant host minerals like garnet, coesite and diamond are shielded from external metamorphic fluids and may be preserved. Finding evidence for deep subduction processes in host garnets of large volumes of (partially) re-equilibrated crystalline rocks is challenging, time consuming, and often hampered by poor outcrop conditions due to weathering and soil formation. In contrast, by analyzing detrital garnet, natural processes such as erosion and sedimentary transport can sample garnet grains sourced from fresh as well as altered crystalline rocks located in the drainage area, enabling large crustal volumes to be screened using a comparatively low number of samples. Case-studies from the Western Gneiss Region of Norway (Schönig et al. 2018), the Saxonian Erzgebirge of Germany (Schönig et al. 2019, 2020), and the (U)HP terrane of eastern Papua New Guinea (Baldwin et al. 2021) demonstrate mineral inclusion analysis of detrital garnet to be a complementary and efficient tool in UHP research. This contribution gives a synopsis of the main findings from the three spatially, chronologically, and tectonically distinct UHP terranes studied, putting emphasis on the spatial extent of UHP metamorphism and lithologies involved.

The Caledonian orogen formed following Paleozoic subduction of the Iapetus Ocean and preserves evidence of ultrahigh-pressure (UHP) metamorphism and exhumation of crustal rocks from mantle depths. The Appalachian orogen similarly formed in the Paleozoic following subduction of Iapetus Ocean crust, but evidence for (U)HP metamorphism in exhumed Appalachian rocks has been challenging to identify. We present results from a metapelite from high-pressure rocks of the Tillotson Peak Complex in the northern Appalachians, which formed during the middle-Ordovician Taconic orogeny. This sample contained mm-cm scale garnet porphyroblasts that host abundant mineral inclusions. Confocal Raman microspectroscopy of inclusions in the rims of a garnet porphyroblast identified relic coesite, preserved as a bi-mineralic inclusion composed of coesite in α-quartz. Raman depth profiling and 2-dimensional mapping indicate the relic coesite is ~10 μm3, suggesting that mineralogical evidence of UHP metamorphism in the Appalachians may be preserved only as μm-scale inclusions contained in polymetamorphosed rocks. We applied quantitative WDS X-ray maps acquired with electron microprobe, quartz-in-garnet elastic thermobarometry, and Zr-in-rutile trace element thermometry to further constrain the metamorphic history of the coesite-bearing metapelite. Garnet zoning patterns in conjunction with elastic and trace element thermobarometry applied to co-entrapped mineral inclusions suggest that garnet nucleated at 14-15.5 kbar and 420-520 °C, and continuously crystallized to 15-19.5 kbar and 470-560 °C during subduction zone metamorphism. Peak metamorphic conditions based on the stability field of coesite and on Zr-in-rutile thermometry from inclusions in the garnet rims suggest UHP metamorphism at >28 kbar and 530 °C. UHP metamorphism of pelitic sediments within the Taconic paleo-subduction zone invite comparisons with similar UHP rocks in the Caledonian orogeny. Future studies of UHP metamorphism in the Appalachian orogen will focus on constraining: 1) the spatial and temporal scales of UHP metamorphism, 2) the retrograde/exhumation P–T path of the coesite-bearing metapelite, and 3) the P–T history of other nearby metamorphic units, such as the Tillotson peak metabasites, to evaluate if these units shared a similar metamorphic history.

Rock recycling within the forearcs of subduction zones involves subduction of sediments and hydrated lithosphere into the upper mantle, exhumation of rocks to the surface, and erosion to form new sediment. The compositions of, and inclusions within detrital minerals revealed by electron microprobe analysis and Raman spectroscopy preserve petrogenetic clues that can be related to transit through the rock cycle. We report the discovery of the ultrahigh-pressure (UHP) indicator mineral coesite as inclusions in detrital garnet from a modern placer deposit in the actively exhuming Late Miocene–Recent high- and ultrahigh-pressure ((U)HP) metamorphic terrane of eastern Papua New Guinea. Garnet compositions indicate the coesite-bearing detrital garnets are sourced from felsic protoliths. Carbonate, graphite, and CO(2) inclusions also provide observational constraints for geochemical cycling of carbon and volatiles during subduction. Additional discoveries include polyphase inclusions of metastable polymorphs of SiO(2) (cristobalite) and K-feldspar (kokchetavite) that we interpret as rapidly cooled former melt inclusions. Application of elastic thermobarometry on coexisting quartz and zircon inclusions in six detrital garnets indicates elastic equilibration during exhumation at granulite and amphibolite facies conditions. The garnet placer deposit preserves a record of the complete rock cycle, operative on

Exhumation of (ultra)high pressure [(U)HP] rocks by upper-plate divergent motion above an unbroken slab, first proposed in the Western Alps, has never been tested by numerical methods. We present 2D thermo-mechanical models incorporating subduction of a thinned continental margin beneath either a continental or oceanic upper plate, followed by upper-plate divergent motion away from the lower plate. Results demonstrate how divergent plate motion may trigger rapid exhumation of large volumes of (U)HP rocks directly to the Earth's surface, without the need for significant overburden removal by erosion. Model exhumation paths are fully consistent with natural examples for a wide range of upper-plate divergence rates. Exhumation rates are systematically higher than the divergent rate imposed to the upper plate, and the modeled size of exhumed (U)HP domes is invariant for different rates of upper-plate divergence. Major variations are instead predicted at depth for differing model scenarios, as larger amounts of divergent motion may allow mantle-wedge exhumation to shallow depth under the exhuming domes. The transient temperature increase, due to ascent of mantle-wedge material in the subduction channel, has a limited effect on exhumed continental (U)HP rocks already at the surface. We test two examples, the Cenozoic (U)HP terranes of the Western Alps (continental upper plate) and eastern Papua New Guinea (oceanic upper plate). The good fit between model predictions and the geologic record in these terranes encourages the application of these models globally to pre-Cenozoic (U)HP terranes where the geologic record of exhumation is only partly preserved.

Mineral compositions and textures of Late Miocene coesite eclogite from Tomagabuna Island were investigated to constrain P–T conditions during UHP metamorphism and subsequent exhumation. Two stages of garnet growth (core and rim), at eclogite-facies conditions, were documented. In addition to core garnet (I), peak assemblages include omphacite, coesite, phengite, and rutile. Rim garnet (II), amphibole, paragonite, plagioclase, quartz, and accessory biotite and spinel were formed after peak pressure conditions. Although a compositional gradient is present at the core–rim garnet interface, the garnet core has a relatively flat compositional profile, suggesting its crystallization in the coesite stability field. Together with minerals formed subsequent to peak pressure (UHP) conditions, the composition of the garnet rim indicates heating of the eclogite during its decompression into the amphibolite facies mineral stability field. Based on compositional zoning in garnet and application of diffusion modelling, we propose that eclogite-facies metamorphism of the mafic protolith and its host lithologies occurred at, or near, UHP conditions. The core garnet (I) formed at 650 °C at 8 Myr in the coesite stability field and was partially resorbed during the onset of exhumation. We infer that garnet rim growth at

McDougall distinguished himself as a leader in developing potassium-argon geochronology and its application to many Earth science issues, including the geomagnetic polarity timescale.

Experiments were conducted to quantify the temperature and pressure effects on the solubility of titanium in coesite. Powdered amorphous silica, titania (anatase), zirconia, and water were added to silver capsules and run in the coesite stability field (at 32, 35, and 40 kbar) from 700 to 1050 °C using a piston–cylinder apparatus. Crystallization of coesite, rutile, and zircon from silica-, titania-, and zircon-saturated aqueous fluids was confirmed by Raman spectroscopy. Cathodoluminescence images and electron microprobe measurements showed that coesite crystals are relatively homogenous. The Ti concentrations of coesite crystals are significantly higher than concentrations predicted using the Ti-in-quartz calibration (Wark and Watson in Contrib Mineral Petrol 152:743–754, 2006. https://doi.org/10.1007/s00410-006-0132-1 ; Thomas et al. in Contrib Mineral Petrol 160:743–759, 2010. https://doi.org/10.1007/s00410-010-0505-3 ). Titanium K-edge X-ray absorption near edge structure (XANES) measurements demonstrate that Ti4+ substitutes for Si4+ on fourfold tetrahedral sites in coesite at all conditions studied. A model was calibrated to describe the effects of pressure and temperature on the solubility of titanium in coesite by using a least-squares method to fit Ti concentrations in coesite to the simple expression: $$RT\ln X_{{{\text{TiO}}_{2} }}^{\text{coesite}} = - 55.068 + 0.00195 \times T\;({\text{K}}) - 1.234 \times P\;({\text{kbar}}) + RT\ln a_{{{\text{TiO}}_{2} }}^{\text{rutile}} ,$$ where R is the gas constant 8.3145 × 10−3 kJ/K, P is pressure in kbar, T is temperature in kelvin, $$X_{{{\text{TiO}}_{2} }}^{\text{coesite}}$$ is the mole fraction of TiO2 in coesite, and $$a_{{{\text{TiO}}_{2} }}^{\text{rutile}}$$ is the activity of TiO2 in the system referenced to rutile. Ti-in-coesite solubility can be used as a thermobarometer for natural samples when used in combination with another indicator of temperature or pressure, such as another thermobarometer in a cogenetic mineral (e.g. rutile) or other phase equilibria (e.g. graphite = diamond). Applications of the Ti-in-coesite thermobarometer to samples from the western Alps and Papua New Guinea are presented.

The thermal evolution of plutonic and metamorphic rocks in the upper crust may be revealed using fission-track (FT) analyses and other low-temperature thermochronologic methods. The segment of pressure–temperature–time–deformation (P-T-t-D) rock paths potentially constrained by FT data corresponds to the lower greenschist facies, prehnite–pumpellyite, and zeolite facies of metamorphic rocks and also includes regions where diagenetic alteration occurs. When plutonic and metamorphic rocks are exhumed, thermal perturbations caused by fluid alteration, and crystallisation below relevant closure/annealing temperatures at relatively shallow crustal depths, may preclude a simplistic interpretation of thermochronologic ages in terms of monotonic cooling. However, FT ages and track-length measurements provide kinetic data that allow interpretation of T-t paths, even in cases where assumptions based on bulk closure temperatures are violated. We show that geologically well-constrained sampling strategies, and application of multiple thermochronologic methods on cogenetic minerals from plutonic and metamorphic rocks, may provide the most promising means to document the timing, rates, and mechanisms of crustal processes. Case studies are presented for: (1) (ultra)high-pressure (U)HP metamorphic terranes (e.g., Papua New Guinea, Western Alps, Western Gneiss Region, Dabie–Sulu), (2) an extensional orogen (Transantarctic Mountains), (3) a compressional orogen (Pyrenees), and (4) a transpressional plate boundary zone (Alpine fault zone, New Zealand).

Since the first discovery of ultrahigh pressure (UHP) rocks 30 years ago in the Western Alps, the mechanisms for exhumation of (U)HP terranes worldwide are still debated. In the western Mediterranean, the presently accepted model of synconvergent exhumation (e.g., the channel-flow model) is in conflict with parts of the geologic record. We synthesize regional geologic data and present alternative exhumation mechanisms that consider the role of divergence within subduction zones. These mechanisms, i.e., (i) the motion of the upper plate away from the trench and (ii) the rollback of the lower plate, are discussed in detail with particular reference to the Cenozoic Adria-Europe plate boundary, and along three different transects (Western Alps, Calabria-Sardinia, and Corsica-Northern Apennines). In the Western Alps, (U)HP rocks were exhumed from the greatest depth at the rear of the accretionary wedge during motion of the upper plate away from the trench. Exhumation was extremely fast, and associated with very low geothermal gradients. In Calabria, HP rocks were exhumed from shallower depths and at lower rates during rollback of the Adriatic plate, with repeated exhumation pulses progressively younging toward the foreland. Both mechanisms were active to create boundary divergence along the Corsica-Northern Apennines transect, where European southeastward subduction was progressively replaced along strike by Adriatic northwestward subduction. The tectonic scenario depicted for the Western Alps trench during Eocene exhumation of (U)HP rocks correlates well with present-day eastern Papua New Guinea, which is presented as a modern analog of the Paleogene Adria-Europe plate boundary.

The 40 Ar/ 39 Ar technique applied to impact glass has been used to date both terrestrial and lunar impact events. The ability to utilize the 40 Ar/ 39 Ar technique rests on the assumption that impact glasses are closed to the loss of daughter product, 40 Ar ∗ , after formation. Diffusion experiments were performed on three Apollo 16 lunar impact glasses and yielded activation energies for 39 Ar of ∼17 to 20 kcal mol −1 and log 10 ( D 0 /a 2 ) values of −5.2 to −6.0 s −1 . The resulting diffusion coefficients are interpreted as minimum values and the Apollo 16 glass is probably some of the least retentive of lunar glasses, as the degree of non-bridging oxygen is at one end of the range in lunar glasses. At temperatures below the glass transition temperature (i.e., ∼660 °C), the data can be explained by volume diffusion from a single diffusion domain. Modeling shows that Apollo 16 composition glass could lose significant quantities of radiogenic argon ( 40 Ar ∗ ) (∼90–100% over 20–40 Myr assuming a diffusion domain size ( a ) of 75 μm) due to diurnal temperature variations on the lunar surface, although 40 Ar ∗ loss is highly sensitive to exposure duration and effective diffusion domain size. Modeling shows that loss from transient thermal events (e.g., heating to ∼200 °C for 10 2 yr duration) can also cause partial resetting of apparent 40 Ar/ 39 Ar ages. In small ( a = 75 μm) glasses a maximum of 50–60% of 40 Ar ∗ is lost over 4 Ga when buried to depths corresponding to temperatures of −15 °C. Results indicate that caution should be exercised in interpreting lunar impact glass 40 Ar/ 39 Ar ages, as the assumption of closed system behavior may have been violated, particularly in glasses with low fractions of non-bridging oxygen.

Field, microstructural, and 40Ar/39Ar thermochronologic data from the Louisiade Archipelago, the southern rifted margin of the Woodlark Basin in SE Papua New Guinea, document an accretionary wedge that formed during Early-Middle Miocene N-dipping subduction of the Australian margin and transpression along the Australian-Pacific plate boundary. Metasedimentary rocks of the Calvados Schist and the metagabbros that intrude them were metamorphosed at up to greenschist-facies conditions. Three tectonic foliations (S1–S3) are present and F1–F3 fold hinges plunge ESE or WNW, parallel to mineral, stretching, and intersection lineations. Fold vergence is dominantly to the SW, and top-to-the-SW thrusting of ultramafic rocks over the Calvados Schist is documented locally on Rossel Island. The data suggest progressive deformation associated with NNE-SSW shortening and ESE-WNW extension via dissolution-precipitation creep and, more locally, dislocation creep. 40Ar/39Ar step-heating analyses of three white mica separates yield Middle Miocene plateau or plateau-like segments that are affected by variable Pliocene argon loss and are interpreted as syntectonic mica growth during metamorphism and deformation followed by partial resetting just prior to the onset of seafloor spreading in the Woodlark Basin. A ∼12 Ma 40Ar/39Ar age from a dacite sill of the Panarora Volcanics provides a minimum age constraint for the termination of northward subduction and a maximum age for crosscutting brittle strike-slip faults. These data are critical to constraining the subduction-exhumation history of the world's youngest high-ultrahigh-pressure terrane and further support analogies between SE Papua New Guinea and the Early Oligocene Western Alps.

To understand zircon behavior as a function of bulk composition and metamorphic grade in the world's youngest (U)HP terrane, we report U-Pb SIMS spot-mode and depth-profiling analyses for quartzo-feldspathic gneisses. Zircons from two gneisses contain Cretaceous inherited cores, with μm sized metamorphic rims requiring depth profiling for reliable dating. Linear regression of the common-Pb uncorrected data for rims yield 206Pb/238U ages of 2.89 ± 0.29 Ma and 2.77 ± 0.99 Ma (concordia intercept ages at 95% confidence). The older age is within two-sigma error of previously reported 206Pb/238U ages on zircons from mafic eclogite within the gneiss, indicating that rims formed on inherited cores within host gneisses during eclogite facies metamorphism. At the (U)HP locality zircons from host gneiss lack inheritance and yield a 206Pb/238U age of 3.66 ± 0.13 Ma. These results are younger than previously reported 206Pb/238U ages on zircons from coesite eclogite within the gneiss, but are within error of the youngest reported LA-ICP-MS 206Pb/238U zircon ages on retrogressed mafic eclogite. We also report intragrain geochemical heterogeneity, indicated by zircon Hf, Y, and Ti variations in depth profiles which suggest chemical disequilibrium over the interval of zircon growth. Collectively, these results indicate that zircon recrystallization and new growth of zircon rims on relict grains occurred during eclogite facies metamorphism and during subsequent retrogression, but not at (U)HP conditions. Comparison between results from PNG and other (U)HP terranes bolsters previous suggestions that the PNG (U)HP terrane evolved rapidly.

The fission-track method has been applied for decades to quantify rates and timing of processes in the shallow crust. The most widely used approach, the external detector method, involves counting fission-tracks, a decay product resulting from the spontaneous fission of 238 U, and a paired set of induced tracks (parent proxy) from the thermal neutron irradiation of 235 U. We propose an alternate method of dating zircons that utilizes an electron microprobe to directly measure uranium concentration [U] and image the number of spontaneous fission-tracks or etch figures that intersect the surface of the crystal using an electron backscatter detector. The electron microprobe fission-track (EP-FT) method is used to date six zircon samples, four of which are widely used as standards: Fish Canyon Tuff, Peach Springs Tuff, Buluk Member of the Bakata Formation (tuff), Tardee Rhyolite, Mt. Dromedary Complex (hypabyssal granite), and Browns Park Formation (tuff). All samples yield ages that overlap within two standard deviations of published reference ages determined using other radiometric techniques (i.e., K/Ar, 40 Ar/ 39 Ar, and U/Pb) and also by the traditional fission-track external detector method. To correct for poorly constrained parameters such as the spontaneous fission decay constant, weight to volume percent conversions, etching efficiency, and selection bias, we calculate a preliminary Z proportionality factor of 4469 ± 661 (1σ). The EP-FT technique avoids the hazard and expense of thermal neutron irradiation, allows simultaneous chemical compositions to be determined, removes the step of counting an external detector manually, and will likely allow much higher track densities to be counted than would be normally possible with optical microscopy. The technique is ideal for dating moderate to high U zircons (>100 ppm U).

The Attic-Cycladic crystalline belt in the central Aegean region records a complex structural and metamorphic evolution that documents Cenozoic subduction zone processes and exhumation. A prerequisite to develop an improved tectono-metamorphic understanding of this area is dating of distinct P–T–D stages. To evaluate the geological significance of phengite ages of variably overprinted rocks, 40Ar/39Ar and Rb–Sr analyses were undertaken on transitional blueschist–greenschist and greenschist facies samples from the islands of Syros and Sifnos. White mica geochronology indicates a large age variability (40Ar/39Ar: 41–27 Ma; Rb–Sr: 34–20 Ma). Petrologically similar samples have either experienced greenschist facies overprinting at different times or variations in ages record variable degrees of greenschist facies retrogression and incomplete resetting of isotopic systematics. The 40Ar/39Ar and Rb–Sr data for metamorphic rocks from both islands record only minor, localized evidence for Miocene ages (c. 21 Ma) that are well documented elsewhere in the Cyclades and interpreted to result from retrogression of high-pressure mineral assemblages during lower pressure metamorphism. Field and textural evidence suggests that heterogeneous overprinting may be due to a lack of permeability and/or limited availability of fluids in some bulk compositions and that retrogression was more or less parallel to lithological layering and/or foliation as a result of, possibly deformation-enhanced, channelized fluid ingress. Published and new 40Ar/39Ar and Rb–Sr data for both islands indicate apparent age variations that can be broadly linked to mineral assemblages documenting transitional blueschist-to-greenschist- and/or greenschist facies metamorphism. The data do not record the timing of peak HP metamorphism, but may accurately record continuous (partial) resetting of isotopic systematics and/or (re)crystallization of white mica during exhumation and greenschist facies retrogression. The form of 40Ar/39Ar phengite age spectra are complex with the lowest temperature steps yielding Middle to Late Miocene ages. The youngest Rb–Sr ages suggest maximum ages of 20.6 ± 0.8 Ma (Syros) and 22.5 ± 0.6 Ma (Sifnos) for the timing of greenschist facies overprinting. The results of this study further accentuate the challenges of interpreting isotopic data for white mica from polymetamorphic terranes, particularly when mixing of populations and/or incomplete resetting of isotopic systematics occurs during exhumation. These data capture the full range of isotopic age variations in retrogressed HP rocks documented in previous isotopic studies, and can be interpreted in terms of the geodynamic evolution of the Aegean.

On the D’Entrecasteaux Islands, southeastern Papua New Guinea (PNG), a series of domes expose the youngest known ultrahigh-pressure (UHP) to high-pressure eclogites and associated gneisses. Abundant evidence of partial melting is preserved throughout the D’Entrecasteaux domes in the form of variably deformed leucosomes, dikes, and plutonic bodies. In order to understand (1) the timing of melt emplacement relative to the UHP metamorphism and (2) the complete exhumation and deformational history of the eclogites and surrounding gneisses, a series of leucosomes, dikes, and eclogites from the Mailolo dome in western Fergusson Island were analyzed by U–Pb ID-TIMS zircon geochronology. Zircons extracted from the eclogite at the UHP locality yield 206Pb/238U TIMS dates from 5.60±0.22 to 4.61±0.18 Ma. Zircons from three foliation-parallel leucosomes are interpreted to record melt crystallization from 3.49±0.01 Ma to 2.98±0.02 Ma. In comparison, weakly deformed granitic–granodioritic dikes that cut the leucosomes yield zircon 206Pb/238U dates of 2.48±0.01 Ma–2.420±0.002 Ma. Based on these new and previously published results, the eclogites may have undergone UHP metamorphism for several million years, from 7.1±0.7 Ma to 4.61±0.18 Ma; based on TIMS dates alone, a more restricted interval of 5.6–4.6 Ma is proposed. Following UHP metamorphism, the eclogite and gneisses were exhumed to lower crustal depths, intruded by foliation-parallel leucosomes by ca. 3.5–3.0 Ma and by ca. 2.45 Ma late discordant dikes. Multiple generations of leucosomes and dikes allow a timeline for the exhumation of the PNG UHP terrane to be constructed: mantle (∼100 km depths) to the base of the crust in ∼1 my, storage in the lower crust for ∼1 my, and final emplacement into the shallow crust ∼1 my later.

Hematite spherules, identified by the Opportunity Mars Exploration Rover (MER), have been interpreted as in situ evidence for past aqueous conditions on the Martian surface. Hematite has also been demonstrated as a reliable (U–Th)/He chronometer, although it is not widely used. In the absence of post-formational diffusive He loss, (U–Th)/He ages measured from Martian hematite spherules should yield the time since water was present on Mars. Using published morphologic constraints and He diffusion kinetics for hematite we model He diffusive loss to assess whether Martian hematite spherules will retain original (U–Th)/He ages during long residence times (4.0 Ga) at surface conditions (22 °C). Fractional loss calculations predict

The New Guinea region evolved within the obliquely and rapidly converging Australian and Pacific plate boundary zone. It is arguably one of the most tectonically complex regions of the world, and its geodynamic evolution involved microplate formation and rotation, lithospheric rupture to form ocean basins, arc-continent collision, subduction polarity reversal, collisional orogenesis, ophiolite obduction, and exhumation of (ultra)high-pressure metamorphic rocks. We describe the major onshore and offshore tectonic and geologic components, including plate boundaries, seismicity, faults, and magmatism, and we integrate these with emerging ideas about mantle dynamics to evaluate the Cenozoic tectonic evolution of New Guinea. Future research opportunities to resolve the mantle structure beneath New Guinea will enable mantle dynamics to be linked to lithospheric and surface processes. Virtually all plate tectonic and mantle processes have been active in the New Guinea region throughout the Cenozoic, and, as such, its tectonic evolution has global significance.

The Woodlark Rift of southeastern Papua New Guinea is among the fastest evolving plate boundaries on Earth. Rapid extension led to formation of metamorphic core complexes ahead of the westward-propagating Woodlark basin spreading center, but it is unknown whether all core complexes are still active. We assess the spatial pattern and recent history of rock and surface uplift in the subaerial portion of the Woodlark Rift using stream profile analyses on the D9Entrecasteaux Islands and eastern Papuan Peninsula. Most stream profiles are characterized by prominent convexities, or knickpoints, many of which occur at the heads of inner gorges and likely formed from transient stream erosion in the Quaternary due to an increase in rock uplift rate. The amount of transient incision and rock uplift (∼200–800 m) correlates with channel slopes below knickpoints normalized for drainage area. Recent erosion lags behind rock uplift; hence, the region of study has undergone net surface uplift, elevating a relict landscape upstream of knickpoints by an average of 450 m, and increasing basin relief by an average of 60%. On a local scale, stream slopes, transient incision, and uplifted relict stream channels provide independent quantitative evidence for patterns of active uplift in the D9Entrecasteaux Islands and Papuan Peninsula consistent with available geologic and thermochronologic data. Surface uplift increases from east to west on a regional scale, in a region of active lithospheric extension. Rising topography in thinned crust occurs over low-density asthenosphere, indicating that mantle buoyancy and flow contribute to active surface uplift and landscape evolution in the Woodlark Rift.

Jarosite 40 Ar/ 39 Ar ages can be used to date surface processes such as weathering and environmental transitions (i.e. aridification) on Earth and Mars. To better interpret jarosite ages from a thermochronological perspective, the diffusion kinetics of argon in jarosite were determined. Incremental fractional loss measurements indicate an activation energy (E) of 37.8 ± 1.5 kcal/mol and a log D o /a 2 of 5.68 ± 0.63 s − 1 corresponding to a closure temperature of 143 ± 28 °C, assuming a cooling rate 100 °C/Ma. Downward extrapolation of these parameters to Martian surface temperatures (≤ 22 °C) predicts 40 Ar/ 39 Ar age spectra using the least retentive E, D o /a 2 pairs predict that if held at 22 °C or less for 4.0 Ga, supergene jarosite would preserve original growth ages manifest as plateau ages consisting of > 95% of the gas release. Because of its susceptibility to mineralogical breakdown, 40 Ar/ 39 Ar ages on preserved Martian jarosite will reflect the time since water was present at a location that has since undergone aridification and remained hydrologically inactive and thermally quiescent.

High-pressure and ultra high-pressure (U)HP metamorphic rocks occur in many of the world's major orogenic belts, suggesting that subduction of continental lithosphere is a geologically important process. Despite the widespread occurrence of these rocks, relatively little is known about the timescales associated with (U)HP metamorphism. This is because most (U)HP terranes are tectonically overprinted and juxtaposed against rocks with a different history. An exception to this are the Late Miocene (U)HP metamorphic rocks found in active metamorphic core complexes (MCC) in the Woodlark Rift of southeastern Papua New Guinea. This region provides a rare opportunity to study the garnet Lu–Hf isotopic record of (U)HP metamorphism in a terrane that is not tectonically overprinted. In order to constrain the timing of garnet growth relative to the history of (U)HP metamorphism and the evolution of the Woodlark Rift, Lu–Hf ages were determined, in conjunction with measurements of Lu and major element zoning, for garnets from three metamorphic rocks. Garnets from the three samples yielded different ages that, instead of recording the spatial and temporal evolution associated with a single metamorphic event, provide information on the timing of three separate plate boundary events. The youngest Lu–Hf age determined was 7.1 ± 0.7 Ma for garnets in a Late Miocene coesite eclogite. The age is interpreted to record the time when a garnet-bearing partial melt of the mantle crystallized within subducted continental lithosphere at (U)HP conditions. The young Lu–Hf age from the coesite eclogite is in contrast to a 68 ± 3.6 Ma Lu–Hf age obtained on large (1–2 cm) garnet porphyroblasts, from within the Pleistocene amphibolite facies shear zone carapace bounding exposures of (U)HP rocks in the D'Entrecasteaux Islands. This older age records the growth of garnet in response to continental subduction and ophiolite obduction in the region north and east of Australia during late Mesozoic/early Cenozoic time. In order to relate the (U)HP metamorphic rocks to the larger-scale tectonic evolution of the Woodlark Rift, a third Lu–Hf age of 11.2 ± 2.1 Ma was determined for dynamically recrystallized garnet from the southeastern margin of the rift. While the tectonic implications of the Lu–Hf age from the southeastern-rifted margin are not fully known, this result suggests that the eastern and western parts of the Woodlark Rift might have a different metamorphic history. The garnet Lu–Hf results demonstrate that prior to rifting, the Woodlark Rift underwent a complex history of metamorphism involving garnet growth and recrystallization. The ability of the Lu–Hf system to date Late Miocene garnet growth, and at the same time preserve Latest Cretaceous garnets within Pleistocene shear zones in the Woodlark Rift demonstrate that Lu–Hf garnet geochronology is an essential component of investigations where the primary objective is documenting and understanding the full range of plate boundary processes recorded in metamorphic minerals.

The Woodlark Rift in Papua New Guinea hosts the world's youngest (2–8 Ma) eclogite-facies rocks and extensional deformation has played a key role in exhuming these (U)HP rocks at rates of > 20 mm/yr. During the Eocene Papuan arc-continent collision Australian Plate-derived continental rocks were subducted to (U)HP depths. There they remained for up to 30 m.y. until the Pliocene when asthenospheric circulation ahead of the west-propagating Woodlark spreading ridge introduced heat and fluids. This caused rocks to break away from the paleosubduction channel, recrystallize in the eclogite facies, and rise as Rayleigh–Taylor instabilities. The diapirs ascended adiabatically undergoing partial melting, which lowered their viscosity and increased buoyancy. (U)HP crust ponded near the Moho at ~ 2–4 Ma, thickening the crust to ~ 40 km (11 kb). Domal uplifts emerged above sea level, and these are still underlain by an unusually thick crust (> 26 km) for a rift that has stretched by factor of ~ 3 since 6 Ma. After ponding, they acquired a flat-lying foliation during amphibolite-facies retrogression. Vertical shortening accompanied the gravitationally driven outflow of ponded lower crust. The weak material was extended parallel to the rift margin, thinning ductilely by W k ~ 0.2), and was mechanically decoupled from – and orthogonal to – plate motion in the rift. Top-E shear fabrics suggest that this flow was westward, perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the Woodlark spreading ridge. At 100 °C per m.y. and were mechanically incorporated into the rift's upper crust. Final exposure was by normal faulting and minor erosion. Such exhumation may also apply to other (U)HP terranes where less evidence for Moho ponding is preserved.

The syntectonic continental conglomerates of the South‐Central Pyrenees record the late stages of thin‐skinned transport of the South‐Pyrenean Central Units and the onset of exhumation of the Pyrenean Axial Zone (AZ) in the core of the orogen. New magnetostratigraphic data of these syntectonic continental conglomerates have established their age as Late Lutetian to Late Oligocene. The data reveal that these materials were deposited during intense periods of tectonic activity of the Pyrenean chain and not during the cessation of the deformation as considered previously. The magnetostratigraphic ages have been combined with new detrital apatite fission track (AFT) thermochronology from AZ‐derived granite cobbles within the syntectonic conglomerates. Distribution of the granitic cobbles with different AFT ages and track lengths combined with their depositional ages reveal information on the timing and rate of episodes of exhumation in the orogen. Some AFT ages are considerably older than the AFT ages of the outcropping AZ granitic massifs, indicating erosion from higher crustal levels within the massifs than presently exposed or from completely eroded plutons. Inverse thermal modelling reveals two well‐defined periods of rapid cooling in the hinterland at ca. 50–40 and ca. 30–25 Ma, with another poorly defined cooling episode at ca. 70–60 Ma. The lowest stratigraphic samples experienced postburial annealing caused by the deposition of younger syntectonic sediments during progressive burial of the south Pyrenean thrust and fold belt. Moreover, samples from the deeper stratigraphic levels also reveal postorogenic cooling during the Late Miocene as a response to the excavation of the Ebro River towards the Mediterranean Sea. Our data strongly support previous ideas about the burial of the South Pyrenean fold and thrust belt by Late Palaeogene syntectonic conglomerates and their subsequent re‐excavation and are consistent with other thermochronological data and thermal modelling from the interior part of the orogen.

The Pyrenees Mountains of Spain and France are a collisional orogen cored by a south-vergent antiformal stack of Paleozoic basement thrust sheets (the Axial Zone). Thrusting accommodated shortening due to convergence between Iberia and Europe. Granitic plutons, intruded during the Hercynian orogeny, form an important component of the basement units. Thermochronology of granodioritic samples collected from a vertical [age–elevation] profile from the south-central Maladeta pluton provide new constraints on the timing of thrust faulting and rock exhumation of the Pyrenean orogen. The Maladeta massif lies within the Orri thrust sheet, presently occupying the immediate footwall of the Gavarnie thrust, a major Axial Zone Alpine-age thrust fault. New biotite and K-feldspar 40Ar/39Ar and apatite (U–Th)/He (AHe) data combined with previous apatite fission track (AFT) thermochronology and structural studies of the Maladeta pluton constrain the thermal evolution of this massif. Biotite weighted mean plateau and K-feldspar maximum 40Ar/39Ar ages from the highest elevations of the Maladeta pluton (2850 m) are ∼ 280 Ma, close to the age of intrusion, and are interpreted to date the timing of rapid cooling in the Hercynian. Subsequently the Maladeta massif remained close to the surface as indicated by the presence of unconformable Triassic continental sediments. K-feldspar 40Ar/39Ar multiple diffusion domain thermal models from two samples at lower elevations (1780 m and 1400 m) indicate heating initiated at ∼ 65 Ma, reaching maximum temperatures of 270 to 280 °C. These data are interpreted to record thrust burial of the Maladeta pluton to depths of ∼ 6–10 km in the footwall of the Gavarnie thrust. Burial and heating were followed by a period of relatively rapid cooling beginning at ∼ 50 Ma, recorded in three K-feldspar thermal models from samples at different elevations (2850 m, 1780 m, and 1400 m). AFT thermal models, in conjunction with the AFT and apatite AHe age–elevation profile interpretation reveal a lower temperature portion of the thermal history. These data indicate the onset of rapid cooling that began at 30–35 Ma, slowed dramatically at 25–30 Ma, and continued at a reduced rate until ∼ 15 Ma. Finally, AFT thermal models from the lowest elevation samples record rapid cooling in the Late Miocene–Pliocene, most likely due to re-excavation of the southern flank of the Pyrenean orogen, following burial by syn-tectonic conglomerates. The integrated thermochronologic data set from the age–elevation profile within the Maladeta pluton illustrates the potential for obtaining age constraints related to processes operating within the interior portions of convergent orogenic systems (e.g., the onset and/or duration of thrust fault activity) that are otherwise unobtainable.

Argon isotopic data is used to assess the potential of low-mass samples collected by sample return missions on planetary objects (e.g., Moon, Mars, asteroids), to reveal planetary surface processes. We report the first 40Ar/39Ar ages and 38Ar cosmic ray exposure (CRE) ages, determined for eleven submillimeter-sized (ranging from 0.06 to 1.2 mg) plagioclase-rich lithic fragments from Apollo 17 regolith sample 78461 collected at the base of the Sculptured Hills. Total fusion analysis was used to outgas argon from the lithic fragments. Three different approaches were used to determine 40Ar/39Ar ages and illustrate the sensitivity of age determination to the choice of trapped (40Ar/36Ar)t. 40Ar/39Ar ages range from ~4.0 to 4.4 Ga with one exception (Plag#10). Surface CRE ages, based on 38Ar, range from ~1 to 24 Ma. The relatively young CRE ages suggest recent re-working of the upper few centimeters of the regolith. The CRE ages may result from the effect of downslope movement of materials to the base of the Sculptured Hills from higher elevations. The apparent 40Ar/39Ar age for Plag#10 is >5 Ga and yielded the oldest CRE age (i.e., ~24 Ma). We interpret this data to indicate the presence of parentless 40Ar in Plag#10, originating in the lunar atmosphere and implanted in lunar regolith by solar wind. Based on a chemical mixing model, plagioclase compositions, and 40Ar/39Ar ages, we conclude that lithic fragments originated from Mg-suite of highland rocks, and none were derived from the mare region.

The D'Entrecasteaux Islands of south-eastern Papua New Guinea are active metamorphic core complexes that formed within a region where the plate tectonic regime has transitioned from subduction to rifting. While rapid, post 4 Myr exhumation and cooling of amphibolite and greenschist facies rocks that constitute the footwall of the crustal scale detachment fault system have been previously documented on Fergusson and Goodenough Islands of the D'Entrecasteaux chain, the timing of eclogite facies metamorphism in rocks of the footwall was unknown. Recent work revealed that at least one of the eclogite bodies formed during the Pliocene. We present combined in situ ion microprobe U-Pb age analyses of zircon from five variably retrogressed eclogite samples from Fergusson and Goodenough Islands that document Late Miocene-Pliocene (8-2 Ma) eclogite formation on these islands. Textural relationships and zircon-garnet rare earth element partition coefficients indicate that U-Pb ages constrain zircon crystallization under eclogite facies conditions in all samples. Results suggest westward younging of eclogite facies metamorphism from Fergusson to Goodenough Island. Present-day exposure of Late Miocene-Pliocene eclogites requires exhumation rates > 2.5 cm yr )1 .

In subduction zones, sediments, hydrothermally altered lithosphere, fluids, and atmospheric gases are transported into the mantle, where ultrahigh-pressure (UHP) metamorphism takes place. However, the extent to which atmospheric noble gases are trapped in minerals crystallized during UHP metamorphism is unknown. We measured Ar and Ne trapped in phengite and omphacite from the youngest known UHP terrane on Earth to determine the composition of Ar and Ne returned from mantle depths to the surface by forearc recycling. An (40)Ar/(39)Ar age [7.93 ± 0.10 My (1σ)] for phengite is interpreted as the timing of crystallization at mantle depths and indicates that (40)Ar/(39)Ar phengite ages reliably record the timing of UHP metamorphism. Both phengite and omphacite yielded atmospheric (38)Ar/(36)Ar and (20)Ne/(22)Ne. Our study provides the first documentation, to our knowledge, of entrapment of atmospheric Ar and Ne in phengite and omphacite. Results indicate that a subduction barrier for atmospheric-derived noble gases does not exist at mantle depths associated with UHP metamorphism. We show that the crystallization age together with the isotopic composition of nonradiogenic noble gases trapped in minerals formed during subsolidus crystallization at mantle depths can be used to unambiguously assess forearc recycling of atmospheric noble gases. The flux of atmospheric noble gas entering the deep Earth through subduction and returning to the surface cannot be fully realized until the abundances of atmospheric noble gases trapped in exhumed UHP rocks are known.

Low temperature thermochronologic techniques (e.g. apatite fission track (AFT) thermochronology and (U–Th)/He dating) constrain near-surface T–t paths and are often applied to uplift/denudation and landscape evolution studies. Samples collected in vertical profiles from granitic walls on either side of the Ferrar Glacier, southern Victoria Land, Antarctica were analyzed using AFT thermochronology and apatite (U–Th)/He dating to further constrain the lowest temperature thermal history of this portion of the Transantarctic Mountains. AFT central ages vary systematically with elevation and together with track length information define a multi-stage cooling/denudation history in the Cretaceous and early Tertiary. Apatite (U–Th)/He single grain age variation with elevation is not as systematic with considerable intra-sample age variation. Although many complicating factors (e.g., U- and Th-rich (micro)inclusions, fluid inclusions, variation in crystal size, α-particle ejection correction, zonation and α-particle ejection correction, implantation of He into a crystal or impediment of He diffusion out of a crystal, and 147Sm-derived α-particles) may contribute to age dispersion, we found that variation in single grain ages correlated with cooling rate. Samples that cooled relatively quickly have less variation in single grain ages, whereas samples that cooled relatively slowly (

The Pyrenees are a collisional mountain belt formed by convergence between the Afro–Iberian and European plates. Apatite fission track thermochronology from three vertical profiles along the ECORS seismic line constrain the exhumation history of the Pyrenean orogen and hence tectonic models for its formation. In the Eocene there is relatively uniform exhumation across the Pyrenees, but significantly more exhumation occurs on the southern flank of the axial zone in the Oligocene. The variation in exhumation patterns is controlled by a change in how convergence is accommodated within the Pyrenean double-wedge. Accommodation of thrusting on relict extensional features that leads to inversion dominated thrust stacking resulted in relatively slow exhumation in the Eocene. However, subsequent crustal wedging and internal deformation in the upper crust under the stacked duplex of antiformal nappes resulted in extremely rapid exhumation on the southern flank in the Oligocene. The Maladeta profile in the southern axial zone records extremely rapid Early Oligocene exhumation followed by dramatic slowing or cessation of exhumation in the middle Oligocene and the formation of an apatite partial annealing zone (PAZ). This PAZ has subsequently been exhumed 2–3 km since the Middle Miocene, supporting the observations of Coney et al. [J. Geol. Soc. London 153 (1996) 9–16] that the southern flank of the range was buried by ≤2–3 km of syntectonic conglomerates in the Oligocene and subsequently re-excavated from Late Miocene to Recent. The present-day topographic form of the Pyrenees is largely a relict of topography that formed in the Eocene and the Oligocene. Comparison with paleoclimatic records indicates that the Eocene–Oligocene exhumation patterns are controlled by tectonic forces rather than resulting from an orographic effect due to uplift of the Pyrenees.

Kohn et al. 2015 (this volume) present models of zircon dissolution and growth in hydrous MORB and metapelite bulk compositions under metamorphic conditions. They evaluate the propensity metamorphic minerals in equilibrium with zircon, to assess what minerals (besides zircon) affect whole rock Zr mass balance. Results indicate that mineral reactions over a range of metamorphic grades could play an important role in the dissolution and growth of metamorphic zircon, with Zr preferentially incorporated into melt, rutile, garnet, and hornblende with increasing temperatures and decreasing pressures.

Micas and potassium feldspars from the South Cyclades Shear Zone, Ios, Cyclades, Greece, yield varied and complex 40Ar/39Ar apparent age spectra. A correlation exists between 40Ar/39Ar apparent ages and the relative timing of different episodes of recrystallization and grain growth, as indicated by fabric and microstructural analysis. The 40Ar/39Ar apparent age spectra record the effects of variation in the degree of recrystallization and grain growth, and partial to complete resetting of argon systematics in potassium-bearing minerals during Hercynian (M0), Alpine (M1) and late Oligocene - early Miocene (M2) metamorphism. Deformation was strongly partitioned within the shear zone and this led to localized recrystallization and heterogeneous resetting of argon systematics within preexisting minerals. Modeling suggests the Oligo-Miocene thermal events were of insufficient magnitude and/or duration to completely reset the isotopic systematics in these samples. Our data lead to the concept of the argon partial retention and resetting zone (PRZ), defined as that portion of the crust where temperatures are insufficient to completely reset argon systematics within preexisting potassium-bearing minerals. Within the PRZ, some radiogenic argon in preexisting potassium bearing minerals will be outgassed and only partially retained. Tectonic exhumation of the PRZ involves movement on crustal-scale ductile shear zones, accompanied by strongly partitioned deformation and localized recrystallization. Recrystallization leads to resetting of argon systematics, and thus will result in heterogeneous 40Ar/39Ar age distributions within these ductile shear zones (e.g., in the South Cyclades Shear Zone).

Argon diffusion in mineral grains has been numerically modelled using P-T-t histories that may be relevant to multiply metamorphosed orogenic terranes and for rocks that have resided at high ambient temperatures in the Earth's crust for long durations. The MacArgon program1 generates argon concentration profiles in minerals assuming argon loss occurs via volume diffusion. It can be run on an Apple Macintosh computer, with arbitrary P-T-t histories used as input. Finite-difference equations are used in the calculation of 40Ar∗ concentration profiles across individual diffusion domains. The associated MacSpectrometer generates model spectra after a P-T-t history has been specified. The form of model 40Ar39Ar apparent age spectra suggests that considerable caution needs to be exercised in the use of the closure temperature concept and in the interpretation of the significance of plateaux observed in many 40Ar39Ar apparent age spectra, particularly in cases involving metamorphic rocks, where complex P-T-t histories might apply. Although modelled spectra cannot be directly compared to experimentally determined 40Ar39Ar age spectra, especially when hydrous phases are involved or in cases where loss of argon has not occurred via volume diffusion, they do provide insight into theoretically expected age spectra for samples that have experienced complex P-T-t histories.

Petrologic data, U-Pb analyses of zircon, and 40 Ar/ 39 Ar analyses were used to document the tectonothermal history of two mylonite zones and the temporal relationship of mylonite formation to movement on detachment faults in the Pinaleno-Jackson Mountain metamorphic core complex of southeastern Arizona. Mylonite formation in the Pinaleno Mountains is dated at ca. 29 and ca. 19 Ma. The later event possibly is synchronous with, and structurally related to, the most recent movement on the Black Rock detachment fault (at

The thermal and structural evolution of a Paleozoic thrust system in central Australia, the Arltunga Nappe Complex, has been examined through field mapping, petrographic, microstructural and chemical analyses, K/Ar and 40Ar/39Ar thermochronology, and multi-diffusion-domain (time-temperature) modeling of K-feldspar 40Ar/39Ar data. The Arltunga Nappe Complex contains three distinct tectonic units, bounded by regional faults and shear zones, that have undergone contrasting deformational and metamorphic histories. The cooling history of the three tectonic units has been constrained by K/Ar and 40Ar/39Ar isotopic age data from K-feldspars, micas, and amphiboles. Results are consistent with a piggyback history of crustal imbrication spanning over 100 m.y., with the hottest rocks uplifted and cooled first and with imbrication acting progressively toward the foreland. Implications for the intracratonic Paleozoic tectonics of central Australia are (1) uplift within the structurally highest tectonic unit was initiated before 400 Ma; (2) continued piggyback imbrication, footwall failure, and uplift and exhumation of the crust resulted in duplexing in the lowest structural unit at approximately 325–300 Ma; (3) the contrast in thermal history between the tectonic units indicates that once widely separated crustal segments were juxtaposed during thrusting above a hinterland dipping megathrust, resulting in ∼85 km of crustal shortening.

The D'Entrecasteaux Islands in eastern Papua New Guinea are composed of a number of active metamorphic core complexes which have been intruded by granodiorite plutons during their formation. The plutons do not appear to have been intruded by diapiric processes as previously suggested. Late, relatively undeformed plutons form flat-lying bodies which crosscut structural boundaries and are strongly discordant to core complex shear zones. Granodiorite magmatism and the development of the metamorphic core complexes have occurred in a linear zone which coincides with a zone of thick crust and rugged topography. It is proposed that plutonism facilitated deformation in ductile extensional shear zones which resulted in tectonic exhumation of deep crustal rocks and formation of the metamorphic core complexes. The source of the plutons is thought to be related to a linear zone of mantle upwelling beneath the islands related to the propagation of the Woodlark seafloor spreading center into continental crust. It is suggested that a localized heat source of this type, which can provide heat and magmatic material to the crust, is essential for the development of metamorphic core complexes.

[1] New electron backscatter diffraction measurements show that the Papua New Guinea (PNG) ultrahigh-pressure (UHP) terrane is dominated by rocks with weakly oriented quartz and feldspar and less abundant strongly oriented hornblende, clinopyroxene, and mica. Velocities measured at high pressures (600 MPa) show that VP is 5.8–6.3 km/s for gneiss samples, 6.5–7.7 km/s for amphibolite, and 7.7–8.2 km/s for eclogite and VS is 3.4–3.9 km/s for gneiss, 4.0–4.4 km/s for amphibolite, and 4.5–4.6 km/s for eclogite. Velocities and anisotropies calculated from mineral crystal preferred orientations (CPOs) are equivalent to within 5% of the measured values. The highest seismic anisotropy for the PNG terrane is in amphibolite at 8% and 7% for VP and VS, respectively. Calculations of seismic velocities at depth based on predicted mineral assemblages indicate that the exhuming UHP terrane is of dominantly mafic composition below ∼20 km depth. Anisotropy in the PNG terrane is expected to be quite low and is controlled by the orientation of the foliation. If observable, changes in anisotropy across the exhuming body may be used to differentiate among the different proposed mechanisms of UHP exhumation.

The D'Entrecasteaux Islands of eastern Papua New Guinea consist of a number of active metamorphic core complexes formed under an extensional tectonic setting related to sea-floor spreading in the west Woodlark Basin. The complexes are defined by mountainous domes (>2500 m high) of fault-bounded, high-grade metamorphic rocks (including eclogite facies) intruded by 2–4-Ma granodiorite plutons. Garnet–clinopyroxene exchange thermometers indicate that the temperature of equilibration of the eclogites was 730–900° C. The jadeite component of omphacite indicates minimum pressure of 21 kbar, suggesting depths of >70 km. The metamorphic rocks have undergone widespread retrogression to amphibolite facies. Retrogression of the metamorphic basement is associated with shearing and formation of the metamorphic core complexes. P–T conditions in the early stages of shear zone activity, determined using the garnet–biotite exchange thermometer and the GASP and GRIPS barometers, were 570–730° C and 7–11 kbar. A second phase of re-equilibration at much lower pressures appears to be related to the widespread intrusion of granodiorite plutons. One re-equilibrated gneiss indicated maximum temperature of 730° C at estimated pressures of approximately 4 kbar. This late, high-temperature metamorphism is also indicated by reactions involving the production of hercynite and corundum in aluminous gneisses and formation of sillimanite at the expense of kyanite. Two major episodes of granodiorite intrusion occurred during uplift and exhumation of the core complexes. Both closely coincide spatially with high-temperature metamorphic rocks, the onset of deformation in extensional shear zones and subsequent uplift of the metamorphic basement. These observations indicate a fundamental link between uplift and granodiorite intrusion during continental extension and the formation of the D'Entrecasteaux Islands metamorphic core complexes.

[1] The Transantarctic Mountains (TAM) define the western flank of the West Antarctic rift system. Cape Surprise, near the Shackleton Glacier in the central TAM, is the only location along the range's >3500 km length where upper Paleozoic Beacon Supergroup strata are down-faulted to near sea level. Previous studies have inferred a range front master normal fault accommodates extension and rock uplift across the TAM front in this region. The history of rock uplift is debated, suggested as early as Mesozoic, typically Cenozoic, or even Pliocene or younger. Structural observations, apatite fission track (AFT) thermochronology, and geomorphologic mapping undertaken within the TAM front east of the Shackleton Glacier indicate extension, faulting, and denudation was mostly late Eocene–late Oligocene and likely into the early Miocene. An exhumed AFT partial annealing zone is found at the coast and traced >50 km inland. The base of that exhumed partial annealing zone indicates denudation accelerated at ∼40 Ma near the coast and at 30–35 Ma on the inland side of the TAM front. Vertically offset AFT isochrones across the TAM front reveal a step-faulted architecture rather than a single master fault. The cumulative vertical offset of the 55 Ma isochrone is 2.3–2.7 km, compared to 2.4–2.6 km offset of Beacon strata, indicating that all significant normal faulting is Cenozoic, and not related to Mesozoic extension within the West Antarctic rift system. Denudation from

Thermochronologic, petrologic, and geochemical analyses of epidote-amphibolite-, amphilbolite-, eclogite-, and blueschist-facies blocks in serpentinite-matrix melanges from East San Benito Island and Cedros Island, west-central Baja California, provide constraints on the P-T-t history of this disrupted terrane. Results of {sup 40}Ar/{sup 39}Ar step heating experiments on minerals separated from these blocks vary according to metamorphic grade and indicate different P-T-t histories. Mid-Jurassic (160-170 Ma) epidote-amphibolite- and amphibolite-facies blocks were probably derived from oceanic crust and associated sediments that were metamorphosed during limitation of subduction. Coarse-grained blueschist blocks were likely metamorphosed during continued subduction in late Early Cretaceous time (that is, 115-100 Ma). Some epidote-amphibolite blocks are partially overprinted by blueschist-facies mineral assemblages and apparently record both metamorphic events. Fission-track analyses of apatites indicate that the blocks underwent significantly different post-metamorphic cooling histories. Epidote-amphilbolite and amphibolite blocks cooled below {approximately}100 {degrees}C form mid-Jurassic to Paleocene time; blueschist blocks cooled below {approximately}100 {degrees}C in Oligocene-Miocene time. In general, mafic blocks have trace-element concentrations and REE patterns characteristic of ocean-flood basalts. This study demonstrates that samples with very different P-T-t histories can evidently occur over relatively small length scales (<

The Acraman structure, located in the Gawler Range Volcanics, Gawler Craton, South Australia, is the largest probable meteorite impact structure known in Australia; it is notable also as the likely source of volcaniclastic ejecta within the Late Proterozoic Bunyeroo Formation located ∼ 300 km east of the impact site and within the correlative Rodda Beds in the Officer Basin ∼ 450 km to the northwest of the impact site. Conventional K/Ar analyses and 40Ar/39Ar total fusion and step heat experiments were performed on two samples of meltrock from the centre of the Acraman structure in an attempt to directly constrain the age of the impact event. The two samples of devitrified meltrock consist primarily of albite microlites in a hematitic devitrified matrix of cloudy K‐feldspar and quartz. Electron microprobe analyses of the samples reveal that the albite + potassium feldspar have nearly pure end‐member compositions (Ab99 and Or97) indicating they are likely low‐temperature authigenic phases. K/Ar, 40Ar/39Ar ...