Partial Resetting Research Articles

40Ar/39Ar thermochronology of the Pan-African Damara Orogen, Namibia, with implications for tectonothermal and geodynamic evolution

Forty-three 40Ar/39Ar step-heating experiments on white mica, biotite, hornblende and whole rocks from the Damara Orogen, Namibia revise and refine the regional perspective on cooling, exhumation, and tectonic reactivation across the orogen. These data also document post-orogenic motion on major shear zones and the thermal effects of late-syn to post-tectonic granite intrusions. These data show regional cooling through temperatures of ∼300–350°C from 545 to 520Ma in the Central Kaoko Zone, 530–510Ma in the Eastern Kaoko Zone, and 495–480Ma in the Southern Zone of the Damara Belt. Discordant 40Ar/39Ar age spectra relate to (1) partial resetting of K-Ar isotopic system in micas by late-stage activity in the major Kaoko Belt shear zones such as the Purros and Three Palms Mylonite Zones, giving apparent ages of 467±6, 481±3 and ∼492Ma, and (2) thermal effects from granitic plutions particularly in the Ugab Domain of the Southern Kaoko Belt (500–475Ma) and in the granite-dominated Central Zone of the Damara Belt (490–460Ma). Phyllites of the lower grade flank regions of the orogen record apparent 40Ar/39Ar mica crystallization ages of ∼517Ma (Eastern Kaoko Zone), ∼526Ma (Northern Zone, Damara Belt), and ∼568–553Ma (Southern Foreland Zone, Damara Belt). The Southern Foreland samples, however, are maximum ages, due to variable influence of detrital mica. Rates of cooling vary between zones within the Kaoko Belt. In the Central Kaoko Belt hornblende, biotite and white mica apparent 40Ar/39Ar ages are similar, suggestive of rapid cooling through the 300–550°C temperature range, whereas in the Orogen Core or Western Kaoko Belt hornblende and biotite apparent ages are older than the white mica ages, because the white micas record the reactivation of the main structures. These data support models of diachronous deformation and metamorphism of the component parts of the orogen, and suggest that Pan-African orogenesis in the Damara is broadly bracketed between 580 and 500Ma with final movements through 480Ma, and spans through the terminal amalgamation of Gondwana in the Kuunga Orogeny through the initiation of subduction under the Gondwana continental margin.

Incipient charnockite formation in Kerala (S. India)—Constraints from geochronology and REE in zircon

Incipient charnockite in S. India have been intensively studied over the last three decades, resulting in a number of petrogenetic models based on the whole-rock and mineral chemical and isotopic data as well as fluid inclusion studies. The aim of our study is to date dehydration process and to detect phase transition effects on U–Th–Pb and REE systematic in zircon by comparing zircons separated from closely paired samples on either side of phase transition. We sampled incipient charnockites from number of localities from Kerala Khondalite Belt (KKB). Leptynitic gneisses and khondalite, two major lithologies of the KKB, are common hosts for the incipient charnockite. Three types of incipient charnockite were observed: (1) charnockite patches related to the leucosome formation in leptinitic gneisses, (2) elongated charnockitic patches cross cutting gneissic fabric in khondalite and (3) charnockite as margins to a pegmatitic dyke in porphyritic granite. Early results of examination of the zircons from incipient charnockite and hosting leptynitic gneiss in the Manali quarry (type 1) showed that oscillatory zoning in zircons are generally blurred and broad indicating recrystallisation of protolith zircon. Recrystallisation caused partial resetting of U–Th–Pb isotopic system resulting in large discordance of the obtained ages. Cl-dark metamorphic zircon rims majority of the grains. Zircons in charnockite have nearly identical internal structure however, CL-dark rims are much thicker with weak sector zoning. In addition some grains have two generations of identical rims. Two groups of CL-dark rims in leptynitic gneiss gave concordant ages of 520 ± 10 Ma and 582 ± 22 Ma. Zircon rims from charnockite split in two age groups as well: 503 ± 13 Ma and 555 ± 11 Ma. Younger and older generations of zircon rims from each lithology are coeval within age errors. Older generation of the zircon rims shows a HREE depleted pattern indicating closed system fractionation of the REE between zircon and garnet. Younger generation of zircon rims are most probably related to partial melting and leucosome formation therefore they date charnockite formation. However, depleted HREE pattern in these zircons are not consistent with open system partitioning.

The thermal evolution of Corsica as recorded by zircon fission-tracks

New zircon fission-track (ZFT) ages from Corsica record multiple thermal events that can be tied to the structural evolution of the western Mediterranean region. The Corsican zircons have a wide scatter of ZFT grain ages (243–14 Ma), which together define several age domains. Western Corsica consists largely of stable Hercynian basement characterized by ZFT ages in the range 161–114 Ma. We interpret these ages (Late Jurassic–Early Cretaceous) as the product of a long-lived Tethyan thermal event related to continental rifting and subsequent drifting during the separation of the European and African plates and the formation of the Liguro–Piemontese ocean basin. In contrast to Hercynian Corsica, Alpine Corsica (northeast Corsica) experienced widespread deformation and metamorphism in Late Cretaceous(?)–Tertiary time. Dated samples from Alpine Corsica range in age from 112 to 19 Ma and all are reset or partially reset by one or more Alpine thermal events. The youngest ZFT grain ages are from the northernmost Alpine Corsica and define an age population at ∼ 24 Ma that indicates cooling after Tertiary thermal events associated with the Alpine metamorphism and the opening of the Liguro–Provençal basin. A less well-defined ZFT age population at ∼ 72 Ma is present in both Alpine Corsica and Hercynian basement rocks. The thermal history of these rocks is not clear. One interpretation is that the ZFT population at ∼ 72 Ma reflects resetting during a Late Cretaceous event broadly synchronous with the early Alpine metamorphism. Another interpretation is that this peak is related to variable fission-track annealing and partial resetting during the Tertiary Alpine metamorphic event across central to north-eastern Corsica. This partial age resetting supports the presence of a fossil ZFT partial annealing zone and limits the peak temperature in this area below 300 °C, for both the affected pre-Alpine and Alpine units.

Fission-track thermochronology of the Oman Mountains continental windows, and current problems of tectonic interpretation

AbstractThe Oman Mountain belt offers a unique case study of an Alpine orogen where blueschist- and eclogite-facies metamorphism developed in continental rocks beneath an ophiolite in the absence of continental collision. We present hereafter the first systematic, fission-track (FT) study of the subophiolitic Jebel Akhdar and Saih Hatat windows in the central-southern Oman Mountains. Nineteen samples out of 28 yielded significant zircon and/or apatite FT results. All of the 9 samples from NE (internal) Saih Hatat, where metamorphism occurred at ca. 300°C < T < 550°C (low-grade blueschist to low-grade eclogite facies), yielded FT zircon ages at 66-70 Ma. Four samples from SW Saih Hatat (front of the Fe-Mg carpholite zone, ca. 280°C < T < 300°C) yielded 69-93 Ma dates, and 4 samples from Jebel Akhdar (ca. 250°C < T < 280°C) yielded 80-96 Ma dates, which reflects partial resetting of the zircon FT chronometre at T ≤ ca. 280°C. As eclogite metamorphism of the deepest Saih Hatat units is dated at ca. 80 Ma [Warren et al., 2003], we deduce an early cooling rate of 30°C/Ma for these units. Late thermal evolution is constrained by apatite FT length optimization analysis and includes, i) cooling under 60°C until 60-58 Ma (rate close to 10°C/Ma); and ii) heating over 80°C at ca. 20-8 Ma.The Oman Mountains tectonic building is currently accounted for by contrasting models involving one or two subductions. Taking into account the structural and geophysical data on the Saih Hatat metamorphic pile, we interpret the reported thermal evolution in the framework of the classical, one subduction model: i) Permian rifting and Mesozoic spreading; ii) late Cretaceous intra-oceanic subduction followed by continental margin subduction/ophiolite obduction; iii) late Late Cretaceous-Paleocene synorogenic exhumation of the subducted units through extrusion within the subduction channel (relative extension), then isostatic rebound and extensional inversion of the obduction thrust; and iv) Neogene compression resulting in rejuvenation of the basal thrust beneath the continental windows, which are considered to be parautochthonous.

Laser ablation 40 Ar/ 39 Ar dating of metamorphic fabrics in the Caledonides of north Ireland

In situ 40 Ar/ 39 Ar dating of two texturally distinct biotite generations within a phyllite from the Caledonian orogen of Ireland was performed using a high spatial resolution laser microprobe. In this sample, an older generation of biotite (Bt 1 ) is aligned subparallel to a steeply dipping crenulation foliation, which itself is overprinted by a regionally developed shallow-dipping foliation. The second biotite generation (Bt 2 ) occurs as randomly oriented porphyroblasts that overprint and therefore postdate both foliations. A dataset of 89 ages from a single thick section was acquired for six Bt 1 and eight Bt 2 crystals. The two biotite generations yielded two statistically distinct age distributions with Bt 1 maxima at c . 433 Ma and Bt 2 maxima at c . 417 Ma. A small number of analyses (11% Bt 1 and 26% Bt 2 ) from within crystals yielded ‘main population’ ages that conflict with the dominant age populations and relative chronology established by field relationships. Discordant analyses from the earlier Bt 1 crystals are younger (than the Bt 2 population) and probably reflect intracrystalline partial resetting during a later thermal perturbation. The discordant Bt 2 analyses are predominantly older (than the Bt 2 population) and are interpreted as indicating localized excess 40 Ar. Such dis cordant analyses indicate intracrystalline variation reflecting both excess 40 Ar (Bt 2 population) and 40 Ar loss (Bt 1 population), and were therefore discarded when determining geologically meaningful ages.

Platinum-Group Element and Re-Os Isotope Variations of the High-Grade Kilvenjarvi Platinum-Group Element Deposit, Portimo Layered Igneous Complex, Finland

The Kilvenjarvi deposit is a high-grade platinum-group element (PGE) deposit located in the basement below the Portimo Layered Igneous Complex, Finnish Lapland. It consists of a stockwork of massive Cu-Fe-Ni sulfide veins, up to a meter wide, which cut metamorphic banded gneiss, amphibolite, and granite. The veins are surrounded by intense sulfide disseminations that are accompanied by an alteration of the basement mineral assemblage to chlorite, calcite, orthoamphibole, monazite, and an unidentified hydrous ferroaluminous silicate. The veins locally carry in excess of 110 ppm PGE, and neighboring rocks in the basement that contain disseminated sulfides locally carry 25 ppm PGE. The PGE are hosted primarily by michenerite (PdBiTe) and sobolevskite (Pd[Bi,Te]), with only very few grains of merenskyite, sperrylite, vysotskite-braggite, paolovite, cooperite, and isomertieite. The massive sulfide veins host two distinct mineral assemblages that reflect an event of sulfur addition. The primary assemblage of chalcopyrite, pentlandite, and pyrrhotite is similar to that of common orthomagmatic sulfides and appears to represent the crystallization product from a sulfide melt. However, the primary minerals have been replaced extensively by a low-temperature secondary assemblage of chalcopyrite, pyrite, marcasite, Ni-bearing pyrite, violarite, and millerite. The disseminated sulfides in the basement lithologic units comprise only the secondary assemblage and are notably richer in chalcopyrite than the massive sulfide veins. The sulfides are associated with chlorite and calcite, indicating that they were likely introduced by carbon-rich (CO 2 or CH 4 ) hydrothermal fluids. The Kilvenjarvi deposit is extremely fractionated in PGE compared to unmineralized rocks and PGE occurrences within the Portimo Complex. Os and Ir concentrations are very low, and Pd/Ir ratios reach 2.5 × 10 5 . The massive and disseminated sulfides can be distinguished by their slightly different PGE patterns. Most notably, the massive sulfides are richest in Pd, and the disseminated sulfides are richest in Pt and Au. This fractionation appears to be consistent with an initial concentration in a magmatic sulfide melt followed by hydrothermal redistribution. The Re-Os isotopes have been significantly affected by Re mobility and cannot be used for dating. However, platinum-group mineral separates from the Kilvenjarvi deposit have nearly uniform isotopic compositions ( 187 Os/ 188 Os = 0.18–0.20) that are invariably higher than rocks from the Portimo Complex. Thus, it is evident that the Kilvenjarvi deposit had a significantly higher crustal contribution (possibly accounting for up to 18% of the Os) than parental magmas for the Layered series of the Portimo Complex. Contamination and fractional sulfide crystallization within the Portimo Marginal series may have been responsible for this isotopic signature. The deposit has some similarities to the various offsets associated with the Sudbury structure, Canada. A primary magmatic origin of the sulfides and PGE is evident from the mineralogy, geochemistry, and isotope geochemistry. However, the deposit has been subjected to extensive hydrothermal alteration involving carbon-rich fluids and resulting in a partial redistribution of the sulfides, PGE, and Re, extensive recrystallization, and a partial resetting of the Re-Os isotope system.

A high resolution record of multiple diagenetic events: Ultraviolet laser microprobe Ar/Ar analysis of zoned K-feldspar overgrowths

High spatial resolution UV laser Ar/Ar analyses were performed in situ on K-feldspar overgrowths from sandstone hosted Pb–Zn bodies from central Sweden. Two different populations of ages were detected—Precambrian–Cambrian (528 ± 10 to 567 ± 8 Ma) and late Silurian (403 ± 6 to 453 ± 8 Ma). These were both detected in single pore spaces, with the older population adjacent to the detrital feldspar whilst the younger population was always distal to the detrital grain. This younging of authigenic K-feldspar ages with distance from the detrital grain margin is discussed in terms of disturbance to the argon system within an authigenic overgrowth that is of a single age: (a) the overgrowths are Precambrian–Cambrian in age and the late Silurian population might be an artefact of argon-loss due to partial resetting, (b) the overgrowths are late Silurian and the Precambrian–Cambrian population might have resulted from excess argon uptake, and (c) the overgrowths are late Silurian but the Precambrian–Cambrian population is an experimental artefact arising from the analysis of overlapping detrital/authigenic feldspars. Diffusion modelling shows that the partial resetting scenario is a very unlikely cause for the observed age variation because it requires a very particular arrangement of sub-grain sizes. Excess argon was not detected in either the K-feldspar or quartz. The data are best interpreted as the ages of two discrete diagenetic events—an early burial diagenesis and a later tectonically induced fluid flow event relating to the collapse of the Caledonian orogen. Further, the use of 38Ar Cl/ 39Ar K as a proxy for Cl/K ratio highlights the physical boundaries between feldspar populations, where all other petrographic, SEM and CL analyses have failed to do so. This study shows that high spatial resolution UV laserprobe Ar/Ar analysis is a powerful tool for extracting age information from complex hydrothermal minerals as a means to investigating fluid flow events.

Chromitites of komatiitic affinity from the Archaean Nuggihalli greenstone belt in South India

Mineral chemistry and petrological data of chromites from chromitite bands in the N–S trending schist belt of Nuggihalli (southern Karnataka, India), belonging to the Dharwar craton of South India, are presented in this paper. Crystal chemical data indicate a komatiitic affinity of the chromitite. P–T calculations of the chromite-hosting peridotites yielded a pressure range of 13 to 28 kbar and temperatures ranging from 775 to 1080 °C; the oxygen fugacity (log fO2) varies from +0.5 to +1.6 above the QFM buffer. The P, T and fO2 data indicate that Nuggihalli chromitites crystallized in an environment akin to the upper mantle. The studied samples also show partial resetting; the lower temperatures ranging from 515 to 680 °C are ascribed to subsequent metamorphism of the area.

Resorption, growth, solid state recrystallisation, and annealing of granulite facies zircon—a case study from the Central Erzgebirge, Bohemian Massif

Zircon crystals have been investigated from a gneiss area in the Central Erzgebirge, Bohemian Massif, where small occurrences/lenses of granulites (and sometimes eclogites) are located within amphibolite facies gneisses. Geological relationships indicate that leucocratic quartzofeldspathic rocks within the granulite boudins represent melts, whereas garnet-rich melanocratic rocks are considered restites, derived through multiple extraction of the leucocratic melts. The morphology of zircon crystals is quite different in these two types of granulites, with rounding pointing to higher zircon resorption in garnet-rich rocks due to multiple interaction with melts. Extensive new zircon growth can be demonstrated within granulitic boudins on the basis of zircon morphology, internal zircon textures, and age data. The newly formed zircon grains and rims are non-cathodoluminescent and yield an age of about 340 Ma (secondary ionisation mass spectrometry (SIMS) and zircon evaporation). Raman microprobe investigations revealed heterogeneous metamictisation within zircon grains and pronounced damage of the zircon structure in U-rich non-cathodoluminescent zircon rims. The common reverse discordance of such non-cathodoluminescent zircon rims may be explained as a result of preferential Pb sputtering during SIMS analysis leading to apparent Pb excess, which seems to occur principally in the most damaged zircon domains. It is proposed from field relationships that surrounding gneisses, although equilibrated under amphibolite facies conditions, experienced the same metamorphic PT history as the granulites. These amphibolite facies rocks contain zircon distinct from the granulite zircon populations, with morphologies and internal textures typical for their protolith rocks implying only slight zircon changes due to HP overprint. New zircon growth is here mostly limited to the appearance of very rare new grains or, more commonly, to formation of thin overgrowths. The internal texture of these zircon crystals shows clear signs of solid state recrystallisation leading in most cases to a partial resetting of their U–Pb ages mainly due to Pb loss. In addition, U contents are slightly higher in CL-dark “rims” that we interpret to have formed mainly due to solid state recrystallisation. Most evaporation and SIMS ages vary between protolith rock age and 340 Ma, so that the interpretation of the 340 Ma age is not unambiguous. It could be explained as the age of zircon crystallisation from a U-rich and Th-depleted melt or alternatively as a resetting age due to solid state recrystallisation resulting from melt or fluid interaction with old zircon. It is difficult to determine at which stage of metamorphic evolution new zircon growth occurred in the granulitic boudins, and remains unclear that zircon growth occurred at peak metamorphic conditions.

Dysregulation of the Hypothalamic-Pituitary-Gonadal Axis with Menopause and Andropause Promotes Neurodegenerative Senescence

Senescence is characterized neurologically by a decline in cognitive function, which we propose is the result of degenerative processes initiated by the dysregulation of the hypothalamic-pituitary-gonadal (HPG) axis with menopause and andropause. Compelling epidemiologic evidence to support this assertion includes the increased prevalence of Alzheimer disease (AD) in women, the correlation of serum HPG hormones with disease and the decreased incidence, and delay in the onset of AD following hormone replacement therapy. Dysregulation of the axis at this time leads to alterations in the concentrations of all serum HPG hormones (decreased neuronal sex steroid signaling, but increased neuronal gonadotropin releasing hormone, luteinizing hormone, and activin signaling). Hormones of the HPG axis, receptors for which are present in the adult brain, are important regulators of cell proliferation and differentiation during growth and development. Based on this, we propose that dysregulated HPG hormone signaling with menopause/andropause leads to the abortive reentry of differentiated neurons into the cell cycle via a process we term "dyosis." Interestingly, the major biochemical and neuropathologic changes reported for the AD brain also are intimately associated with neuron division: altered AbetaPP metabolism, Abeta deposition, tau phosphorylation, mitochondrial alterations, chromosomal replication, synapse loss, and death of differentiated neurons. Recent evidence supports the premise that AD-related biochemical changes are likely the combined result of increased mitotic signaling by gonadotropins and GnRH, decreased differentiative and neuroprotective signaling via sex steroids, and increased differentiative signaling via activins. This results in a hormonal milieu that is permissive of cell cycle reentry but does not allow completion of metaphase. Partial resetting of the axis following administration of normal endogenous sex steroids delays the onset and decreases the incidence of AD. Ideally, supplementation with HPG hormones should mimic closely the serum concentrations of all HPG hormones in reproductive men and cycling women to prevent dyotic signaling and attempted neuron division.

Subduction of arc basaltic andesite: Implications for the tectonic history of the southern New England Fold Belt

Serpentinite mélange in the Peel-Manning Fault System, southern New England Fold Belt hosts tectonic blocks of varying size and lithology. One of the largest occurs at Woodsreef, northern New South Wales, as multiply deformed amphibole schists recording at least three phases of deformation (D1–D3). They have a peralkaline, composition (molecular Na2O+K2O = 0.083–0.189; Al2O3 = 0.046–0.130) and contain aegirine, quartz, albite, arfvedsonite and less common actinolite, neptunite, aenigmatite and bafertisite, as well as relict pre-D1 aegirine. The Zr/TiO2 and Nb/Y ratios suggest that the protoliths had originally basaltic to basaltic andesite composition, indicating that they have been altered to a rock of peralkaline composition. Chondrite normalised patterns ((Ce/Yb)N = 4.98–9.36), Hf/3–Th–Ta, Hf/3–Th–Nb/16 and Y/15–La/10–Nb/8 discrimination diagrams, and rock/MORB plots indicate that the protoliths had a calc-alkaline, arc composition. 143Nd/144Nd data (eNd(500) = 2.8–3.3; –4.8; eNd(400) = 2.0–2.5; –5.8) imply that they have been derived from isotopically mildly depleted and enriched sources. Single-grain 40Ar–39Ar dating of felsic and mafic fractions from one sample, produces ages of 1200–1100 Ma (albite-quartz vein) and 335.2 ± 2.1 Ma, 332 ± 3.7 Ma and 291 ± 3.7 Ma (arfvedsonite-rich matrix). The Neoproterozoic age obtained from the felsic fraction is attributed to the presence of excess 40Ar in quartz and albite, and the 291 Ma age to partial resetting. The 335–332 Ma ages are interpreted as cooling ages related to uplift after subduction. The basalts and basaltic andesites underwent alteration either within a volcanic pile or during subduction after collision with a ?west-dipping subduction zone at the proto-Gondwana margin. With continuing subduction, they were recrystallised and deformed in the deeper levels of the subduction zone, producing aegirine with up to 38% jadeite in solid solution. Replacement of the high-P assemblages by lower P/high-T arfvedsonite-bearing assemblages then took place during uplift at ca 334 Ma.

40Ar– 39Ar dating of detrital muscovite in provenance investigations: a case study from the Adelaide Rift Complex, South Australia

Detrital zircon ages are commonly used to investigate sediment provenance and supply routes. Here, we explore the advantages of employing multiple, complimentary techniques via a case study of the Neoproterozoic and Cambrian of the Adelaide Rift Complex, South Australia. Detrital muscovite Ar–Ar ages are presented from stratigraphic units, or equivalents, that have previously been the subject of U–Pb detrital zircon dating, and, in some cases, whole-rock Sm–Nd isotope studies. The zircon age ranges and whole-rock Sm–Nd isotope data suggest that early Neoproterozoic sediments from near the base of the Adelaide Rift Complex comprise a mixture of detritus derived from the adjacent Gawler Craton (Palaeoproterozoic to earliest Mesoproterozoic) and overlying Gairdner flood basalts. In contrast, detrital muscovites from this level have a broad scatter of Mesoproterozoic infrared (IR) laser total fusion Ar–Ar ages, while UV laser traverses indicate that the age spread reflects partial resetting by multiple heating events, rather than a mixture of sources. Younger Neoproterozoic sediments document replacement of the Gawler Craton by the more distant Musgrave and/or Albany–Fraser Orogens as the main provenance. The Cambrian Kanmantoo Group marks an abrupt change in depositional style and a new sediment source. The Kanmantoo Group have older Nd model ages than underlying strata, yet are dominated by near to deposition-aged (∼500–650 Ma) detrital zircons and muscovites, suggesting rapid cooling and exhumation of a tectonically active provenance region. Although this source remains uncertain, evidence points towards the distant Pan-African orogenic belts. Deposition in the Adelaide Rift Complex was terminated in the late Early Cambrian by the Delamerian Orogeny, and the results of previous detrital mineral dating studies from the Lachlan Fold Belt to the east are consistent with at least partial derivation of these sediments from reworked upper Adelaide Rift Complex (Kanmantoo Group), rather than a continuation of sediment supply from the Kanmantoo Group sediment source. More broadly, the data suggest a close link between basin formation and orogen exhumation, and we also speculate that mantle plumes have played a significant role in crustal evolution at this Palaeo-Pacific margin of Gondwanaland, challenging the notion that subduction zones are the principle sites of crustal growth and sediment provenance.

40Ar/ 39Ar thermochronologic constraints on deformation, metamorphism and cooling/exhumation of a Mesozoic accretionary wedge, Otago Schist, New Zealand

Structural thickening of the Torlesse accretionary wedge via juxtaposition of arc-derived greywackes (Caples Terrane) and quartzo-feldspathic greywackes (Torlesse Terrane) at ∼120 Ma formed a belt of schist (Otago Schist) with distinct mica fabrics defining (i) schistosity, (ii) transposition layering and (iii) crenulation cleavage. Thirty-five 40Ar/ 39Ar step-heating experiments on these micas and whole rock micaceous fabrics from the Otago Schist have shown that the main metamorphism and deformation occurred between ∼160 and 140 Ma (recorded in the low grade flanks) through 120 Ma (shear zone deformation). This was followed either by very gradual cooling or no cooling until about 110 Ma, with some form of extensional (tectonic) exhumation and cooling of the high-grade metamorphic core between 109 and 100 Ma. Major shear zones separating the low-grade and high-grade parts of the schist define regions of separate and distinct apparent age groupings that underwent different thermo-tectonic histories. Apparent ages on the low-grade north flank (hanging wall to the Hyde-Macraes and Rise and Shine Shear Zones) range from 145 to 159 Ma ( n=8), whereas on the low-grade south flank (hanging wall to the Remarkables Shear Zone or Caples Terrane) range from 144 to 156 Ma ( n=5). Most of these samples show complex age spectra caused by mixing between radiogenic argon released from neocrystalline metamorphic mica and lesser detrital mica. Several of the hanging wall samples with ages of 144–147 Ma show no evidence for detrital contamination in thin section or in the form of the age spectra. Apparent ages from the high-grade metamorphic core (garnet–biotite–albite zone) range from 131 to 106 Ma ( n=13) with a strong grouping 113–109 Ma ( n=7) in the immediate footwall to the major Remarkables Shear Zone. Most of the age spectra from within the core of the schist belt yield complex age spectra that we interpret to be the result of prolonged residence within the argon partial retention interval for white mica (∼430–330 °C). Samples with apparent ages of about 110–109 Ma tend to give concordant plateaux suggesting more rapid cooling. The youngest and most disturbed age spectra come from within the ‘Alpine chlorite overprint’ zone where samples with strong development of crenulation cleavage gave ages 85–107 and ∼101 Ma, due to partial resetting during retrogression. The bounding Remarkables Shear zone shows resetting effects due to dynamic recrystallization with apparent ages of 127–122 Ma, whereas overprinting shear zones within the core of the schist show apparent ages of 112–109 and ∼106 Ma. These data when linked with extensional exhumation of high-grade rocks in other parts of New Zealand indicate that the East Gondwana margin underwent significant extension in the ∼110–90 Ma period.

Hf-W システムにおける巨大衝突による部分平衡化の影響

Hf-W システムにおける巨大衝突による部分平衡化の影響

Neural dynamics and the fundamental mechanisms of event-related brain potentials.

Event-related potentials (ERPs) provide a critical link between the hemodynamic response, as measured by functional magnetic resonance imaging, and the dynamics of the underlying neuronal activity. Single-trial ERP recordings capture the oscillatory activity that are hypothesized to underlie both communication between brain regions and amplified processing of behaviorally relevant stimuli. However, precise interpretations of ERPs are precluded by uncertainty about their neural mechanisms. One influential theory holds that averaged sensory ERPs are generated by partial phase resetting of ongoing electroencephalographic oscillations, while another states that ERPs result from stimulus-evoked neural responses. We formulated critical predictions of each theory and tested these using direct, intracortical analyses of neural activity in monkeys. Our findings support a predominant role for stimulus-evoked activity in sensory ERP generation, and they outline both logic and methodology necessary for differentiating evoked and phase resetting contributions to cognitive and motor ERPs in future studies.

Stability of the Upper Nile drainage network (Ethiopia) deduced from (U–Th)/He thermochronometry: implications for uplift and erosion of the Afar plume dome

One of the best places to investigate the role of a mantle plume in creating topography at the onset of continental breakup is the Ethiopian volcanic province since it is the youngest and best preserved case of a large igneous province dissected by a system of rifts. In the center of the volcanic province, the northwestern Ethiopian plateau which surrounds the Afar depression displays the highest topography and experienced more than 1 km of surface uplift. Because hydrology and physiography are genetically linked, understanding the long-term evolution of these upper basins has implications regarding the morphotectonic evolution of the plateau as well as the paleo-hydrological evolution of the whole Nile River. We report the results of a combined thermochronological and morphological study aimed at understanding the long-term stability of the upper Blue Nile drainage network. Apatite and titanite He ages have been determined for crystalline basement samples collected below the lava pile, in various key sites of the drainage network. Titanite He ages, which range from 213 to 520 Ma, are in good agreement with published K-feldspar Ar–Ar data and most likely reflect post-Pan-African cooling of the Ethiopian basement. Apatite He ages, which range from 45 to 107 Ma, display a trend of decreasing age with increasing crustal depth in the Blue Nile gorge, which is interpreted as partial resetting of pre-existing ages due to basement burial because of the thick pile of flood lavas erupted 30 Ma ago. Simulations of apatite He age partial resetting suggest that erosion initiated in the Blue Nile canyon as early as 25–29 Ma ago, whereas erosion would have started after 11 Ma along the present retreat scarp. The early onset of erosion in the Blue Nile canyon suggests that: (i) volcanic and uplift-related tectonic divides represent long-lived (20–30 Myr) pre-breakup divides, and (ii) the elevated plateau physiography, which controls most of the present-day Nile hydrology, has existed since the Oligocene. We propose that the plateau represents the preserved part of a large uplifted dome, related to Afar plume impingement, and/or to massive underplating triggered by Oligocene continental flood basalt differentiation. In this case, the topographic development of the western Afar margin, which is related to extension and drift of the Arabian plate, would be mainly the result of the collapse of the Afar area from an initially elevated region.

Very fast exhumation of high-pressure metamorphic rocks with excess 40Ar and inherited 87Sr, Betic Cordilleras, southern Spain

In order to attempt to further constrain the age of the early Alpine tectonic evolution of the Mulhacén Complex and to explore the influence of inherited isotopes, micas from a small number of well-characterised rocks from the Sierra de los Filábres, with a penetrative tectonic fabric related to the exhumation of eclogite-facies metamorphic rocks, were selected for 40Ar/ 39Ar and Rb–Sr dating. A single phengite grain from an amphibolite yielded an 40Ar/ 39Ar laser step heating plateau age of 86.9±1.2 Ma (2 σ; 70% 39Ar released) and an inverse isochron age of 86.2±2.4 Ma with an 36Ar/ 40Ar intercept within error of the atmospheric value. Induction furnace step heating of a biotite separate from a gabbro relic in an eclogite yielded a weighted mean age of 173.2±6.3 Ma (2 σ; 95% 39Ar released). These ages are diagnostic of excess argon ( 40Ar XS) incorporation, as they are older than independent age estimates for the timing of eclogite-facies metamorphism and intrusion of the gabbros. 40Ar XS incorporation probably resulted from restricted fluid mobility in the magmatic rocks during their metamorphic recrystallisation. Rb–Sr whole-rock–phengite ages of graphite-bearing mica schists from Paleozoic rocks (Secano unit) show a dramatic variation (66.1±3.2, 40.6±2.6 and 14.1±2.2 Ma). An albite chlorite mica schist from the Mesozoic series of the Nevado–Lubrı́n unit has a whole-rock–mica–albite age of 17.2±1.9 Ma, which is within error of an 40Ar/ 39Ar plateau age published previously and of the youngest Rb–Sr age of the Paleozoic series obtained in this study. The significant spread in Rb–Sr ages implies that progressive partial resetting of an older isotopic system has occurred. The microstructure of the samples with pre-Miocene Rb–Sr ages reveals incomplete recrystallisation of white mica and inhibited grain growth due to the presence of graphite particles. This interpretation agrees with previously published, disturbed and slightly dome-shaped 40Ar/ 39Ar age spectra that may point similarly to the presence of an older isotope component. The progressively reset Rb–Sr system is a relic of Variscan metamorphism of the Paleozoic series of the Mulhacén Complex. In contrast, the origin of the ca. 17.2 Ma old sample from the Mesozoic series precludes any isotopic inheritance, in agreement with its pervasive tectono-metamorphic recrystallisation during the Miocene. Exhumation of the eclogite-facies Mulhacén Complex occurred in two stages with contrasting rates of about 22.5 mm/year during the early phase and 9–10 mm/year during the late phase; the latter with a cooling rate in the order of 330 °C/Ma.

SHRIMP U–Pb zircon dating of the host rocks of the Cannington Ag–Pb–Zn deposit, southeastern Mt Isa Block, Australia

SHRIMP U–Pb zircon ages are reported from a paragneiss, a pegmatite, a metasomatised metasediment and an amphibolite taken from the upper amphibolite facies host sequence of the Cannington Ag–Pb–Zn deposit at the southeastern margin of the Proterozoic Mt Isa Block. Also reported are ages from a middle amphibolite‐facies metasediment from the Soldiers Cap Group approximately 90 km north of Cannington. The predominantly metasedimentary host rocks of the Cannington deposit were eroded from a terrane containing latest Archaean to earliest Palaeoproterozoic (ca 2600–2300 Ma) and Palaeoproterozoic (ca 1750–1700 Ma) zircon. The ca 1750–1700 Ma group of zircons are consistent with sedimentary provenance from rocks of Cover Sequence 2 age that are now exposed to the north and west of the Cannington deposit. The metasedimentary samples also include a group of zircon grains at ca 1675 Ma, which we interpret as the maximum depositional age of the sedimentary protolith. This is comparable to the maximum depositional age of the metasediment from the Maronan area (ca 1665 Ma) and to previously published data from the Soldiers Cap Group. Metamorphic zircon rims and new zircon grains grew at 1600–1580 Ma during upper amphibolite‐facies metamorphism in metasedimentary and mafic magmatic rocks. Zircon inheritance patterns suggest that sheet‐like pegmatitic intrusions were most likely derived from partial melting of the surrounding metasediments during this period of metamorphism. Some zircon grains from the amphibolite have a morphology consistent with partially recrystallised igneous grains and have apparent ages close to the metamorphic age, although it is not clear whether these represent metamorphic resetting or crystallisation of the magmatic protolith. Pb‐loss during syn‐ to post‐metamorphic metasomatism resulted in partial resetting of zircons from the metasomatised metasediment.

Identification of partial resetting using De as a function of illumination time

Modern age samples from various depositional environments were examined for signal resetting. For 19 modern aeolian/beach samples all D e values obtained were <0.5 Gy , with ∼70% having D e <0.2 Gy . For 21 fluvial/colluvial samples, all D e values were <5 Gy with ∼80% being <1 Gy . D e as a function of illumination (OSL measurement) time ( D e( t)) plots were examined for all samples. Based on previous laboratory experiments, increases in D e( t) were expected for partially reset samples, and constant D e( t) for fully reset samples. All aeolian samples, both modern age and additional ‘young’ samples (<1000 years), showed constant (flat) D e( t) while all modern, non-zero D e, fluvial/colluvial samples showed increasing D e( t). ‘Replacement plots’, where a regenerated signal is substituted for the natural, yielded constant (flat) D e( t). These findings support strongly the use of D e( t) as a method of identifying incomplete resetting in fluvial samples. Potential complicating factors, such as illumination (bleaching) spectrum, thermal instability and component composition are discussed and a series of internal checks on the applicability of the D e( t) for each individual aliquot/grain level are outlined.

A Late Ordovician U?Pb age for the Troms? Nappe eclogites, Uppermost Allochthon of the Scandinavian Caledonides

The Scandinavian Caledonides contain the record of several high-pressure events reflecting distinct episodes of collision and subduction in the course of the global Caledonian plate reorganization process. In this study, the timing and speed of one of these events in the Tromso Nappe of the Uppermost Allochthon are detailed using multiple U–Pb geochronometers. This unit contains eclogites, the largest of which forms a whole mountain top, whereas many others occur as smaller lenses enclosed within a metamorphosed supracrustal sequence. A minimum age for the sedimentation is provided by a zircon age of 493 +5/−2 Ma for an eclogitized felsic intrusion. Formation of the eclogite, at pressures reaching 2.8 GPa, occurred at 452.1±1.7 Ma as evidenced by U–Pb in eclogitic zircon. Similar ages of 451–450 Ma are also provided by high-Al titanite in eclogite and titanite in leucosome veins, the latter of which was formed by partial melting during the exhumation of the eclogite. An age of 449 Ma for a rutile porphyroblast in another vein further confirms the rapidity of this high-pressure process. Matrix rutiles in two other eclogites yielded ages of 436 Ma and younger, probably indicating partial resetting during a subsequent metamorphic overprint. Lead isotopic compositions with high 207Pb/204Pb ratios are indicative of old crustal sources, thus supporting the previously proposed notion that the Uppermost Allochthon was derived from the Neoproterozoic margin of Laurentia.