Добірка наукової літератури з теми "K-Ar and 40Ar/39Ar geochronology"

K-Ar and more recently the40Ar/39Ar variant are well established dating methods. The40Ar/39Ar method requires irradiation with neutrons, posing some complications that are greatly outweighed by the benefits. The40Ar/39Ar method is particularly powerful due to the availability of internal reliability criteria, the ability to analyze single crystals, and the amenability of the analyses to automation.40Ar/39Ar dating has the capability for unsurpassed precision and is applicable to the broadest range of geologic environments and time scales of any radioisotope dating technique. For chronostratigraphic applications,40Ar/39Ar is most important in the Cenozoic, becoming progressively less useful into the early Phanerozoic due to alteration and loss of radiogenic argon. Precision and accuracy of40Ar/39Ar dating have been improved considerably in recent years, but an uncertainty of about 1% in the decay constant for40K, probably mainly in the electron capture decay branch, still limits accuracy at about this level. Inconsistent use of standards (neutron fluence monitors) and attribution of variable ages to standards is still a source of confusion, but straightforward recalculation procedures can overcome the underlying problems provided that appropriate standards are used.

Abstract. The decay of 40K to the stable isotopes 40Ca and 40Ar is used as a measure of time for both the K-Ca and K-Ar geochronometers, the latter of which is most generally utilized by the variant 40Ar∕39Ar system. The increasing precision of geochronology has forced practitioners to deal with the systematic uncertainties rooted in all radioisotope dating methods. A major component of these systematic uncertainties for the K-Ar and 40Ar∕39Ar techniques is imprecisely determined decay constants and an incomplete knowledge of the decay scheme of 40K. Recent geochronology studies question whether 40K can decay to 40Ar via an electron capture directly to ground state (ECground), citing the lack of experimental verification as reasoning for its omission. In this study, we (1) provide a theoretical argument in favor of the presence of this decay mode and (2) evaluate the magnitude of this decay mode by calculating the electron capture to positron ratio (ECground/β+) and comparing calculated ratios to previously published calculations, which yield ECground/β+ between 150–212. We provide support for this calculation through comparison of the experimentally verified ECground/β+ ratio of 22Na with our calculation using the theory of β decay. When combined with measured values of β+ and β− decay rates, the best estimate for the calculated ECground/β+ for 40K yields a partial decay constant for 40K direct to ground-state 40Ar of 11.6±1.5×10-13 a−1 (2σ). We calculate a partial decay constant of 40K to 40Ar of 0.592±0.014×10-10 a−1 and a total decay constant of 5.475±0.107×10-10 a−1 (2σ), and we conclude that although omission of this decay mode can be significant for K-Ar dating, it is minor for 40Ar∕39Ar geochronology and is therefore unlikely to have significantly biased published measurements.

The Lake George antimony mine was at one time North America’s largest producer of antimony. Despite being widely known for the antimony mineralization, the deposit also hosts a range of styles of mineralization such as multiple generations of W-Mo bearing quartz veins as well as a system of As-Au bearing quartz–carbonate veins. In situ U-Pb zircon geochronology, using LA ICP-MS, of the Lake George granodiorite yielded a weighted mean 206Pb/238U age of 419.6 ± 3.0 Ma. Step heating of phlogopite separated from the lamprophyre dykes produced a 40Ar/39Ar plateau segment date of 419.4 ± 1.4 Ma. Single molybdenite crystal analysis for Re-Os geochronology was conducted on two W-Mo-bearing quartz veins, which cross-cut altered granodiorite and altered metasedimentary rocks and yielded two dates of 415.7 ± 1.7 Ma and 416.1 ± 1.7 Ma respectively. 40Ar/39Ar geochronology of muscovite from alteration associated with Au-bearing quartz–carbonate veins yielded one representative plateau segment date of 414.1 ± 1.3 Ma. The dates produced in this study revealed that the different magmatic–hydrothermal events at the Lake George mine occurred over approximately a 10-million-year period at the end of the Silurian and the start of the Devonian following the termination of the Acadian orogeny.

New U/Pb and 40Ar/39Ar isotopic dating in the Babine porphyry copper district of central British Columbia documents three distinct magmatic events at 107–104, 85–78, and 54–50 Ma. The earliest event involved emplacement of rhyolite domes into submarine volcanic rocks of the Rocky Ridge Formation. The rhyolite domes and related dacitic to basaltic volcanic rocks gave a U–Pb age of 107.9 ± 0.2 Ma and an 40Ar/39Ar age of 104.8 ± 1.2 Ma. The rhyolites, which were previously mapped as Eocene, are reinterpreted to be part of a previously unrecognized mid-Cretaceous cauldron subsidence complex. The regionally extensive Late Cretaceous magmatic event is also recognized in the Babine district and is represented by 40Ar/39Ar ages of 85.2 ± 2.8 and 78.3 ± 0.8 Ma on two Bulkley intrusions, one of which has associated porphyry copper mineralization. The final magmatic event is the most widespread and involved emplacement of the Babine intrusions and formation of numerous porphyry copper deposits including the Bell and Granisle past producers. Twenty-one new 40Ar/39Ar isotopic ages for these intrusions and coeval andesites of the Newman Formation have a narrow range from 53.6 ± 0.9 to 49.9 ± 0.6 Ma, whereas previous K–Ar isotopic dating had a possible range of 15 Ma. The mid-Cretaceous, Late Cretaceous, and Eocene magmatic suites in the Babine district are interpreted to be part of a long-lived volcano-plutonic complex that was the site of periodic magmatism and porphyry copper mineralization over a 60 Ma time period. This complex may have evolved within a zone of extension (pull-apart basins) situated between dextral strike-slip faults that were active during periods of rapid oblique plate convergence.

Abstract. In this study, we provide 40Ar / 39Ar geochronology data from a suite of variably deformed rocks from a region of low-grade metamorphism within the Cambro–Ordovician Delamerian Orogen, South Australia. Low-grade metamorphic rocks such as these can contain both detrital minerals and minerals newly grown or partly recrystallised during diagenesis and metamorphism. Hence, they typically yield complex 40Ar / 39Ar age spectra that can be difficult to interpret. Therefore, we have undertaken furnace step heating 40Ar / 39Ar geochronology to obtain age spectra with many steps to allow for application of the method of asymptotes and limits and recognition of the effects of mixing. The samples analysed range from siltstone and shale to phyllite and contain muscovite or phengite with minor microcline as determined by hyperspectral mineralogical characterisation. Whole rock 40Ar / 39Ar analyses were undertaken in most samples due to their very fine-grained nature. All samples are dominated by radiogenic 40Ar, and contain minimal evidence for atmospheric Ca- or Cl-derived argon. Chloritisation may have resulted in limited recoil, causing 39Ar argon loss in some samples, which is especially evident within the first few percent of gas released. Most of the age data, however, appear to have some geological significance. Viewed with respect to the known depositional ages of the stratigraphic units, the age spectra from this study do appear to record both detrital mineral ages and ages related to the varying influence of either cooling or deformation-induced recrystallisation. The shape of the age spectra and the degree of deformation in the phyllites suggest the younger ages may record recrystallisation of detrital minerals and/or new mica growth during deformation. Given that the younger limit of deformation recorded in the high-metamorphic-grade regions of the Delamerian Orogen is ca. 490 Ma, the ca. 470 to ca. 458 Ma ages obtained in this study suggest deformation in low-grade shear zones within the Delamerian Orogen may have persisted until ca. 20–32 million years after high-temperature ductile deformation in the high-grade regions of the orogen. We suggest that these younger ages for deformation could reflect reactivation of older structures formed both during rift basin formation and during the main peak of the Delamerian orogeny itself. The younger ca. 470 to ca. 458 Ma deformation may have been facilitated by far-field tectonic processes occurring along the eastern paleo-Pacific margin of Gondwana.

Miles Canyon basalt is an informal term used to describe numerous exposures of young alkaline olivine basalt flows in southern Yukon. The volcanic rocks are part of the Northern Cordilleran volcanic province. K-Ar and Ar-Ar whole-rock dates indicate that the Miles Canyon succession of flows at the Whitehorse Rapids are clearly Late Miocene in age (ca. 8.4 Ma). The largest exposure of the Miles Canyon basalt occurs at the Alligator Lake volcanic complex where two nearly concordant Pliocene Ar-Ar dates indicate eruption at ca. 3.2 Ma. K-Ar analyses from other sites yield dates of 2.4 and 7.1 Ma and indicate an episodic Neogene volcanic history for the region. There is no evidence of Quaternary or postglacial volcanism. The dates are older than assumed by previous workers, and in some cases the K-Ar dates are strongly discordant from Ar-Ar determinations. More accurate Ar-Ar determinations may result from the method's ability to select smaller amounts of better material for analysis. Excess 40Ar was not encountered. As a result, the accuracy of any single or several discordant K-Ar determinations for Neogene subaerial volcanic rocks, particularly low-K rocks such as basalts, should be questioned and resulting interpretations made with caution. Models accounting for the eruption of the Northern Cordilleran volcanic province lavas have typically relied upon extension along north-trending faults that were generated by stresses along the continental margin. However, we consider a slab window model which better accounts for the initiation and distribution of northern Cordilleran Neogene volcanic activity.

Laser heating 40Ar/39Ar geochronology provides high analytical precision and accuracy, mum-scale spatial resolution, and statistically significant data sets for the study of geological and planetary processes. A newly commissioned 40Ar/39Ar laboratory at CPGeo/USP, São Paulo, Brazil, equips the Brazilian scientific community with a new powerful tool applicable to the study of geological and cosmochemical processes. Detailed information about laboratory layout, environmental conditions, and instrumentation provides the necessary parameters for the evaluation of the CPGeo/USP 40Ar/39Ar suitability to a diverse range of applications. Details about analytical procedures, including mineral separation, irradiation at the IPEN/CNEN reactor at USP, and mass spectrometric analysis enable potential researchers to design the necessary sampling and sample preparation program suitable to the objectives of their study. Finally, the results of calibration tests using Ca and K salts and glasses, international mineral standards, and in-house mineral standards show that the accuracy and precision obtained at the 40Ar/39Ar laboratory at CPGeo/USP are comparable to results obtained in the most respected laboratories internationally. The extensive calibration and standardization procedures undertaken ensure that the results of analytical studies carried out in our laboratories will gain immediate international credibility, enabling Brazilian students and scientists to conduct forefront research in earth and planetary sciences.

The Laozuoshan gold deposit, located in the central part of the Jiamusi Massif, is hosted by the contact zone between granitic complex and Proterzoic strata. In this study, we present the results of geochronology and geochemistry of ore-related granodiorite and diorite porphyry, and hydrothermal sericite 40Ar/39Ar dating. The granodiorite and diorite porphyry in the Laozuoshan gold deposit are calc-alkaline and high-K (calc-alkaline) series, which are enriched in LREE and LILE and depleted in HFSE, with no depletion of Eu. The geochronology data show that zircon U–Pb ages of the granodiorite and diorite porphyry are ∼262 Ma and ∼105 Ma, respectively. The sericite 40Ar/39Ar ages are ∼194 Ma and ∼108 Ma. On the basis of previous researches, ore geology and geochronology studies show that the Laozuoshan gold deposit underwent at least two gold mineralization events. We suggest that the first one, which was related to skarnization, resulted from the collision between the Jiamusi and Songnen Massifs in Late Permian. The subsequent gold mineralization resulted from the subduction of the paleo-Pacific Plate in Early Cretaceous.

John Tuzo Wilson coined the term “plate” in plate tectonics. He is famous for inventing transform boundaries, hot spot tracks, and the Wilson cycle of ocean birth, growth, and decline. Less well remembered is his work in the 1950s on tectonic and radiometric age provinces of the Canadian Shield, as part of which he fathered U/Pb geochronology in Canada. This work gave strong support to the notion of continental growth through accretion of successively younger terranes onto an ancient cratonic core. The present paper reviews how paleomagnetism can trace the motions of continents to test Wilson’s ideas. Continental accretion often involves deep burial of one of the colliding elements through subduction or crustal underplating; such was the case with the Grenville orogen and its subprovinces in their Proterozoic accretion onto the Laurentian craton. The resulting heating and metamorphism erases most pre-collisional magnetic information but adds something new: the possibility of following the post-metamorphic uplift and cooling history, in time and space. The time element is provided by a new form of isotopic geochronology, thermochronometry, which provides dates for specific minerals together with the temperatures at which they became closed to isotopic migration. U/Pb dating of sphene is one method used; another is the 40Ar/39Ar variant of K/Ar dating applied to hornblende, micas, and feldspars, which have a wide range of Ar closure temperatures. The two specific Grenville studies described deal with parallel uplift histories determined by 40Ar/39Ar dating and by magnetics for the accreted terranes of the Central Metasedimentary Belt in Ontario and with the paleomagnetic detection of the post-1240 Ma closing of a small ocean between the Elsevir terrane and Laurentia during the Grenvillian orogeny.

As part of the wider European GTS Next project, I propose new constraints on the ages of the Late Cretaceous, derived from a multitude of geochronological techniques, and successful stratigraphic interpretations from Canada and Japan. In the Western Canada Sedimentary Basin, we propose a new constraint on the age of the K/Pg boundary in the Red Deer River section (Alberta, Canada). We were able to cyclostratigraphically tune sediments in a non-marine, fluvial environment utilising high-resolution proxy records suggesting a 11-12 precession related cyclicity. Assuming the 40Ar/39Ar method is inter-calibrated with the cyclostratigraphy, the apparent age for C29r suggests that the K/Pg boundary falls between eccentricity maxima and minima, yielding an age of the C29r between 65.89 ± 0.08 and 66.30 ± 0.08 Ma. Assuming that the bundle containing the coal horizon represents a precession cycle, the K/Pg boundary is within the analytical uncertainty of the youngest zircon population achieving a revised age for the K/Pg boundary as 65.75 ± 0.06 Ma. The Campanian - Maastrichtian boundary is preserved in the sedimentary succession of the Horseshoe Canyon Formation and has been placed ~8 m below Coal nr. 10. Cyclostratigraphic studies show that the formation of these depositional sequences (alternations) of all scales are influenced directly by sea-level changes due to precession but more dominated by eccentricity cycles proved in the cyclostratigraphic framework and is mainly controlled by sand horizons, which have been related by autocyclicity in a dynamic sedimentary setting. Our work shows that the Campanian - Maastrichtian boundary in the Western Canada Sedimentary Basin coincides with ~2.5 eccentricity cycles above the youngest zircon age population at the bottom of the section and ~4.9 Myr before the Cretaceous - Palaeogene boundary (K/Pg), and thus corresponds to an absolute age of 70.65 ± 0.09 Ma producing an ~1.4 Myr younger age than recent published ages. Finally, using advances with terrestrial carbon isotope and planktonic foraminifera records within central Hokkaido, Northwest Pacific, sections from the Cretaceous Yezo group were correlated to that of European and North American counterparts. Datable ash layers throughout the Kotanbetsu and Shumarinai section were analysed using both 40Ar/39Ar and U-Pb methods. We successfully dated two ash tuff layers falling either side of the Turonian - Coniacian boundary, yielding an age range for the boundary between 89.31 ± 0.11 Ma and 89.57 ± 0.11 Ma or a boundary age of 89.44 ± 0.24 Ma. Combining these U-Pb ages with recent published ages we are able to reduce the age limit once more and propose an age for the Turonian - Coniacian boundary as 89.62 ± 0.04 Ma.

This thesis presents a 40Ar/39Ar geochronological analyses of biotite from thin ductile shear zones in Paleoproterozoic granulite-facies gneisses from the Ellesmere-Devon crystalline terrane, Nunavut, Canada. The gneisses are part of the Paleoproterozoic Thelon tectonic zone. U-Pb dates of zircon show that the gneisses have magmatic protolith ages ranging from 2007–1958 Ma. The quartzofeldspathic gneisses in southeast Ellesmere Island display centimeter-scale E- to NE-striking sinistral and dextral mylonite zones offsetting pegmatitic dikes that are the last stage of ductile deformation of the basement rocks. Samples were taken from nearshore outcrops at Hayes Fiord, Pim Island, NE of the Leffert Glacier and NW of Cape Isabella. Biotite clusters replace orthopyroxene as the result of post-granulite facies metamorphism in the gneisses. Biotite in mylonitic and ultramylonitic fabrics is found as flattened clusters and also as individual crystals defining shear bands related to mylonitization. Eight samples were dated, including biotite from five mylonites, one deformed pegmatite, one tonalite and muscovite from a pegmatite. Major element X-ray maps demonstrate that the biotite is chemically homogenous. Backscattered electron images and electron dispersive spectroscopy via scanning electron microscopy confirm that biotite lacks intercrystalline layering with other K phases. Step-heating analysis of mica at the University of Vermont yielded Paleoproterozoic 40Ar/39Ar ages. The apparent age spectra form plateau ages in all but one mylonite sample. Biotite from a protomylonite was 2051 ± 26 Ma, older than the protolith ages obtained from U-Pb zircon geochronology, and most likely indicates excess Ar. Pegmatitic muscovite was 1977 ± 35 Ma. Biotite dates range from 1874 ± 13 Ma to 1838 ± 14 Ma for the five mylonites without excess Ar. Biotite dated from ductile shear zones signals the latest deformation in the basement, which was active as early as 1887 Ma.

L'évolution des îles volcaniques résulte d'interactions complexes entre croissance des volcans et processus de destruction (éruptions explosives, glissements de terrain, érosion fluviale, altération). Les changements climatiques peuvent influencer ces processus sur des échelles variées. A l’échelle événementielle, des précipitations intenses peuvent engendrer des épisodes érosifs extrêmes. Sur le long-terme, des variations des taux d'altération, sensibles aux précipitations et à la température, peuvent impacter la fertilité des sols et le cycle global du carbone.Les îles volcaniques des Açores offrent un cadre idéal pour étudier ces interactions, avec une grande importance scientifique et sociétal, en particulier dans le contexte actuel du réchauffement climatique. Au centre de l'Atlantique Nord, elles sont influencées par des facteurs climatiques majeurs. Ces îles ont eu des pulses d'activité volcanique au cours du dernier million d'années (Ma), une période marquée par d’importants changements climatiques liés aux cycles glaciaires-interglaciaires. Si les variations des conditions globales ont été bien documentées sur cette période, la reconstruction paléoclimatique aux échelles locales/régionales reste un défi. Les paléosols (PSs) sont des anciens sols inclus dans le registre géologique. Leur géochimie fournit des informations précieuses sur les conditions paléo-environnementales, et la géochronologie des produits volcaniques les encadrant permet leur contrainte temporelle.Dans ce travail, nous avons reconstruit les conditions moyennes annuelles de précipitation (MAP) et de température de l’air (MAAT) aux Açores au cours du dernier Ma. Deux proxies basés sur la composition en éléments majeurs des PSs ont été utilisés : l’indice d'altération CIA-K et l’argilosité, tous deux validés dans d'autres milieux volcaniques. La datation précise des unités volcaniques par K-Ar sur mésostase séparée (laves) et par ⁴⁰Ar/³⁹Ar sur monocristaux de feldspath potassique (dépôts pyroclastiques), révèle des «pulses» d‘altération sur quelques milliers d’années, notamment après les terminaisons glaciaires (MIS 21, 19, 11, 9e, 5e et 1). La géochimie des PSs montre des changements environnementaux rapides et des MAATs (12-28 ᵒC) en accord avec les données de température marine de surface établies à partir d’archives océaniques. Cette concordance indique une étroite relation océan-atmosphère. Ces «pulses» suggèrent en outre des phases d'affaiblissement de l'anticyclone des Açores, permettant aux courants d'air humides d'atteindre des secteurs plus au sud.Les taux moyens de formation des sols (3-180 mm/kyr) ont été influencés par la structure et la texture du substrat rocheux. Des PSs se sont formés lors de MAPs plus faibles dans les dépôts pyroclastiques que dans les coulées de lave (seuils de ~500 et ~800 mm/an). Cette différence supporte une cinétique favorisée par la fragmentation et une surface spécifique élevée. L’altération accrue en surface et le long des discontinuités géologiques de sub-surface peut avoir favorisé l'érosion par glissements de terrain. Des MAPs élevées (jusqu'à 1500 mm/an) sont notamment obtenues autour du stade interglaciaire de l’Eemien, qui coïncide avec l’initiation d’un glissement complexe sur le flanc Sud de Pico. Des MAPs intenses ont pu accélérer l’infiltration des eaux et favoriser les interactions hydromagmatiques. L’augmentation associée de pression interstitielle a ainsi pu déclencher la mise en mouvement du flanc le long de failles listriques toujours actives. Les conditions actuelles aux Açores sont plus humides et légèrement plus chaudes qu’au cours du dernier Ma, ce que pourrait favoriser le détachement du flanc externe de Pico, avec des conséquences potentiellement drastiques.Plus généralement, l’altération accrue favorise l’évolution rapide du paysage sur de telles îles et engendre des flux élémentaires et une absorption de CO₂ atmosphérique croissantes, ce qui a impacts locaux, régionaux et globaux
Landscape evolution on volcanic islands is driven by complex interactions between volcano growth and destruction by a variety of processes (explosive eruptions, landslides, riverine erosion, weathering). Major climate changes, may impact the dynamics of degradation processes at different spatial and temporal scales. For example, extreme rain can produce an immediate hydrological response causing important destruction. Changes in weathering rates, sensitive to precipitation and temperature, can trigger changes in soil fertility but also modify global carbon cycling.The Azores volcanic islands provide an ideal setting to study these interactions, with both scientific and societal significance, especially in the context of ongoing global warming. Located in the Central North Atlantic, they are under the influence of major climatic drivers. Most of them had pulses of volcanic activity over the past 1 Myr, a period characterized by high-amplitude glacial-interglacial transitions with major climatic changes. While global climatic variations have been relatively well-studied for this period, reconstructing the atmospheric paleoclimate and its effects at local/regional scales remains challenging. Paleosols (PSs) are fossil soils formed by weathering at surface, and later incorporated into the geological record. Their geochemistry provides valuable insights into past environmental conditions, while the geochronology of volcanic products “bracketing” PSs allows their temporal constraint.In this work, we reconstructed mean annual precipitations (MAP) and air temperature (MAAT) over the last 1 Myr in the Azores region through a combined geochemical-geochronological study of PSs. Two proxies based on PSs’ major element were used: the weathering index (CIA-K) and the Clayeyness, both validated in other volcanic settings. The precise dating of volcanic units by either unspiked K-Ar on lava flow groundmass separates or ⁴⁰Ar/³⁹Ar on single K-feldspar of trachytic fallout evidence “pulses” of soil-formation within only a few kyr. This occurred especially after glacial terminations (MIS 21, 19, 11, 9e, 5e and 1), under wet and warm conditions. Fast paleoenvironmental changes were recorded in PSs’ geochemistry, and MAAT reconstructions (12-28 ᵒC) agree with previously published Sea Surface Temperatures, pointing to a tight ocean-atmosphere teleconnection. Those “pulses” suggest sustained weakening phases for the Azores High, allowing humid air currents (Westerlies) to reach further to the south.Our data also show contrasted rates of vertical soil development (3-180 mm/kyr). Weathering was favored by the structure and texture of parental materials, as PSs formed under lower MAP in pyroclastic deposits than in lava flows (~500 and ~800 mm/yr thresholds). This highlights the influence of fragmentation on weathering’s kinetics due to higher specific surface area. Enhanced weathering at surface and along geological discontinuities may have promoted mechanical weakening, favoring erosion and landslides. Notably, high MAPs (up to 1500 mm/yr) obtained around the Eemian interglacial stage are coincident in time with the initiation of a large slide complex on the southern flank of Pico. Intense precipitation may have led to increased water infiltration favoring enhanced hydromagmatic interactions. Drastic increase in pore pressure may then have triggered the initiation of the flank movement along listric faults that are still active. Current conditions in the Azores are wetter and slightly warmer than during the last Myr. Increased infiltration along faults could partly control subsequent movement and yield to detachment of the outer flank of Pico, with potentially dramatic consequences.More generally, present temperature and humidity increase on volcanic islands points to intense weathering, resulting in fast landscape evolution, increased lixiviation and elementary export and high atmospheric CO₂ uptaking, with local, regional and global impacts

The complete consumption of the oceanic domain of a tectonic plate by subduction into the upper mantle results in continent subduction, although continental crust is typically of lower density than the upper mantle. Thus, the sites of former oceanic domains (named suture zones) are generally decorated with stratigraphic sequences deposited along continental passive margins that were metamorphosed under low-grade, high-pressure conditions, i.e., low temperature/depth ratios (< 15°C/km) with respect to geothermal gradients in tectonically stable regions. Throughout the Mesozoic and Cenozoic (i.e., since ca. 250 Ma), the Mediterranean realm was shaped by the closure of the Tethyan Ocean, which likely consisted in numerous oceanic domains and microcontinents. However, the exact number and position of Tethyan oceans and continents (i.e., the Tethyan palaeogeography) remains debated. This is particularly the case of Western and Central Anatolia, where a continental fragment was accreted to the southern composite margin of the Eurasia sometime between the Late Cretaceous and the early Cenozoic. The most frontal part of this microcontinent experienced subduction-related metamorphism around 85-80 Ma, and collision-related metamorphism affected more external parts around 35 Ma. This unsually-long period between subduction- and collision-related metamorphisms (ca. 50 Ma) in units ascribed to the same continental edge constitutes a crucial issue to address in order to unravel how Anatolia was assembled. The Afyon Zone is a tectono-sedimentary unit exposed south and structurally below the front high-pressure belt. It is composed of a Mesozoic sedimentary sequence deposited on top of a Precambrian to Palaeozoic continental substratum, which can be traced from Northwestern to southern Central Anatolia, along a possible Tethyan suture. Whereas the Afyon Zone was defined as a low-pressure metamorphic unit, high-pressure minerals (mainly Fe-Mg-carpholite in metasediments) were recently reported from its central part. These findings shattered previous conceptions on the tectono-metamorphic evolution of the Afyon Zone in particular, and of the entire region in general, and shed light on the necessity to revise the regional extent of subduction-related metamorphism by re-inspecting the petrology of poorly-studied metasediments. In this purpose, I re-evaluated the metamorphic evolution of the entire Afyon Zone starting from field observations. Low-grade, high-pressure mineral assemblages (Fe-Mg-carpholite and glaucophane) are reported throughout the unit. Well-preserved carpholite-chloritoid assemblages are useful to improve our understanding of mineral relations and transitions in the FeO-MgO-Al2O3-SiO2-H2O system during rocks’ travel down to depth (prograde metamorphism). Inspection of petrographic textures, minute variations in mineral composition and Mg-Fe distribution among carpholite-chloritoid assemblages documents multistage mineral growth, accompanied by a progressive enrichment in Mg, and strong element partitioning. Using an updated database of mineral thermodynamic properties, I modelled the pressure and temperature conditions that are consistent with textural and chemical observations. Carpholite-bearing assemblages in the Afyon Zone account for a temperature increase from 280 to 380°C between 0.9 and 1.1 GPa (equivalent to a depth of 30-35 km). In order to further constrain regional geodynamics, first radiometric ages were determined in close association with pressure-temperature estimates for the Afyon Zone, as well as two other tectono-sedimentary units from the same continental passive margin (the Ören and Kurudere-Nebiler Units from SW Anatolia). For age determination, I employed 40Ar-39Ar geochronology on white mica in carpholite-bearing rocks. For thermobarometry, a multi-equilibrium approach was used based on quartz-chlorite-mica and quartz-chlorite-chloritoid associations formed at the expense of carpholite-bearing assemblages, i.e., during the exhumation from the subduction zone. This combination allows deciphering the significance of the calculated radiometric ages in terms of metamorphic conditions. Results show that the Afyon Zone and the Ören Unit represent a latest Cretaceous high-pressure metamorphic belt, and the Kurudere-Nebiler Unit was affected by subduction-related metamorphism around 45 Ma and cooled down after collision-related metamorphism around 26 Ma. The results provided in the present thesis and from the literature allow better understanding continental amalgamation in Western Anatolia. It is shown that at least two distinct oceanic branches, whereas only one was previously considered, have closed during continuous north-dipping subduction between 92 and 45 Ma. Between 85-80 and 70-65 Ma, a narrow continental domain (including the Afyon Zone) was buried into a subduction zone within the northern oceanic strand. Parts of the subducted continent crust were exhumed while the upper oceanic plate was transported southwards. Subduction of underlying lithosphere persisted, leading to the closure of the southern oceanic branch and to subduct the front of a second continental domain (including the Kurudere-Nebiler Unit). This followed by a continental collisional stage characterized by the cease of subduction, crustal thicknening and the detachment of the subducting oceanic slab from the accreted continent lithosphere. The present study supports that in the late Mesozoic the East Mediterranean realm had a complex tectonic configuration similar to present Southeast Asia or the Caribbean, with multiple, coexisting oceanic basins, microcontinents and subduction zones.
Kontinentale Subduktion resultiert aus dem Abtauchen des ozenanischen Gebiets einer tektonischen Platte in den Oberen Erdmantel. Dies geschieht obwohl die kontinentale Erdkruste normalerweise eine geringere Dichte besitzt als der Obere Erdmantel. Die Lage ehemaliger ozeanischer Gebiete (auch als Suturzonen bezeichnet) ist dementsprechend durch stratigraphische, sedimentäre Gesteinsabfolgen gekennzeichnet, die entlang des passiven Kontinentalrandes abgelagert wurden. Anschließend wurden diese Gesteine unter niedrigen Temperaturen und hohem Druck umgewandelt, auch niedrig-gradige Hochdruckmetamorphose genannt. Während der gesamten Zeitspanne des Mesozoikums und Känozoikums (seit etwa 250 Millionen Jahren bis heute) wurde der mediterrane Raum durch die kontinuierliche Schließung des Tethyschen Ozeans (dem heutigen Mittelmeer) geprägt, der vermutlich in zahlreichen kleineren Ozeanen und Mikrokontinenten aufgeteilt war. Dennoch bleiben die genaue Anzahl und Lage der tethyschen Ozeane und Kontinente (die Paläogeographie der Tethys) bis heute umstritten. Das ist insbesondere der Fall in West- und Zentral-Anatolien, wo im Zeitraum zwischen der Oberen Kreide (vor 98 bis 65 Mio. J.) und dem unteren Känozoikum (vor 65 bis 40 Mio. J.) ein kontinentales Fragment am südlichen Kontinentalrand der Eurasischen Platte angelagert wurde (auch als Akkretion bezeichnet). Der vorderste Bereich von diesem Fragment erfuhr vor etwa 85-80 Millionen Jahren eine metamorphe Umwandlung, die mit den Prozessen der fortschreitenden Subduktion assoziiert werden können. Hingegen wurden die hinteren Bereiche erst später vor ca. 40-30 Mio. J. durch die Kollison der zwei Platten metamorph überprägt. Die ungewöhnlich lange Zeitspanne von etwa 40-50 Mio. J. zwischen den metamorphen Prozessen der Subduktion und der Kollision, stellt eine entscheidende Frage zum Verständnis der Entstehung von Anatolien dar. Die Afyon Zone repräsentiert hierbei eine tektonisch-beanspruchte sedimentäre Gesteinseinheit, die in einer strukturell tieferen Position bezüglich des frontalen metamorphen Hochdruckgürtels liegt und südlich von ihm anzutreffen ist. Die Afyon Zone besteht aus mesozoischen sedimentären Einheiten (250 bis 65 Mio. J. alt), die auf präkambrischem (älter als 545 Mio. J.) bis paläozoischem Untergrund (bis vor 250 Mio J.) abgelagert wurden, und vom nordwestlichen bis zentralen Anatolien, entlang der vermutlichen Tethys-Suturzone, verfolgt werden können. Obwohl die Afyon-Zone als eine niedrig-temperierte metamorphe Gesteinseinheit bezeichnet wird, wurde in letzter Zeit von Vorkommen von Hochdruckmineralen (v.a. Eisen(Fe)-Magnesium(Mg)-Karpholith in metamorphen Sedimenten) im zentralen Bereich berichtet. Diese neuen Erkenntnisse stellen die bisherigen Interpretationen zur tektonisch-metamorphen Entstehung der gesamten Region in Frage, insbesondere der der Afyon-Zone. Deshalb war eine erneute gründliche Überarbeitung und Untersuchung der wenig studierten metamorph-überprägten Sedimentgesteine in diesem Gebiet notwendig. Deshalb, überarbeitete ich die metamorphe Entwicklung der gesamten Afyon Zone, beginnend mit intensiver Geländearbeit und -beobachtungen. Mineralvergesellschaftungen aus Karpholith und Glaukophan, die unter niedrigen Temperaturen und hohem Druck entstanden sind, wurden in der gesamten Gesteinseinheit gefunden. Guterhaltene Mineralvergesellschaftungen aus Karpholith und Chloritoid sind nützlich für das Verständnis unter welchen Temperatur- und Druck-Bedingungen die Gesteine in die Tiefe gelangen (prograde Metamorphose). Durch die Untersuchungen von Gesteinsgefügen und der Eisen-Magnesium-Verteilung zwischen den Mineralien Karpholith und Chloritoid lassen sich Aussagen zu der Bildungstemperatur und dem Druck dieser Minerale machen. Dafür benutzte ich eine verbesserte Datenbank mit Mineraleigenschaften, die mir die Modellierung von Temperatur und Druck erlaubte und im Einklang mit den chemischen und mikroskopischen Beobachtungen steht. Es ergab sich, dass die Karpholith-haltigen Gesteine in der Afyon-Zone einen Temperaturanstieg von 280 zu 380°C (bei einer Tiefe von 30-35 km) erfahren haben. Um noch bessere Aussagen über die Entstehung zu treffen, wurden auch radiometrische Datierungen an Proben aus der Afyon-Zone, sowie an zwei weiteren Sedimentgesteinseinheiten (Ören- und Kurudere-Nebiler-Einheit aus SW Anatolien) gemacht. Für die Altersbestimmung benutzte ich die weitverbreitete 40Ar-39Ar Datierungsmethode an Hellglimmer-Mineralien in den Karpholith-haltigen Gesteinen. Temperatur und Druck können auch bestimmt werden, wenn man den Übergang von einer Mineralvergesellschaftung zu einer anderen Vergesellschaftung beobachtet. Dies gilt zum Beispiel für den Übergang von einer Karpholith-haltigen Zusammensetzung zu einer Quartz-Chlorit-Glimmer und Quartz-Chlorit-Chloritoid Mineralvergesellschaftung wenn tief subduzierte Gesteine wieder nach oben gelangen (Exhumation). Damit lassen sich die radiometrischen Alter den metamorphen Prozessen zu bestimmten Temperaturen und Drücken zuordnen. Mit diesen Erkenntnissen lassen sich die Afyon-Zone und die Ören-Einheit einem Hochdruck-Gebirgsgürtel in der späten Kreidezeit zuordnen, während die Kurudere-Nebiler Einheit durch die mit der Subduktion in Verbindung stehende Metamorphose vor ca. 45 Mio. J. beeinflusst wurde. Später wurde diese Einheit durch die Metamorphose, resultierend aus der Kollision vor 26 Mio. J., abgekühlt. Die Ergebnisse dieser und anderer Arbeiten erlauben es die Anlagerung von Kontinenten in West-Anatolien besser zu verstehen. Es wird gezeigt, dass mindestens zwei (im Gegensatz zu vorher einem) voneinander unabhängige Ozeanarme während der Subduktion von 92 bis 45 Millionen Jahren geschlossen wurden. Zwischen 85-80 und 70-65 Millionen Jahren, wurde ein schmales kontinentales Gebiet (welches die Afyon-Zone beinhaltet) in die Subduktionszone hineingzogen. Teile der subduzierten kontientalen Kruste kamen wieder an die Oberfläche (Exhumation), während die obere ozeanische Platte südwärts transportiert wurde. Die anhaltende Subduktion im oberen Bereich des Erdmantels (Lithosphäre) führte zu der Schließung des südlichen Ozeanarms und zu der Subduktion des zweiten kontinentalen Gebietes (welches die Kurudere-Nebiler-Einheit beinhaltete). Darauf folgte die kontinentale Kollisionsphase unter dem Ausklingen der Prozesse der Subduktion, der Krustenverdickung und der Abtrennung der subduzierten ozeanischen Platte von der akkretionierten kontientalen Lithosphäre (auch als Delamination bezeichnet). Die hier präsentierte Arbeit unterstüzt die Annahme das während der Oberen Kreidezeit das Ost-Mediterrane Gebiet tektonsich komplex angeordnet war, vergleichbar mit dem heutigen Südost-Asien oder der Karibik, mit ihren vielen gleichzeitig existierenden ozeanischen Becken, Mikrokontinenten und Subduktionszonen.

Ce travail de thèse présente le développement et la mise au point d'un procédé original pour la datation par la technique 40_Ar/39_Ar. Le cœur de ce système, un spectromètre de masse utilisant la collection simultanée des cinq isotopes de l'argon, est aujourd'hui unique à l'échelle internationale. Tout d'abord testé et validé sur des échantillons géologiques standard, ce système a permis de réaliser, dans sa configuration actuelle, des analyses de grande qualité sur les roches volcaniques de l'Ethiopie, avec une reproductibilité analytique de l'ordre de 0. 2%. Il est maintenant communément admis que les grands traps basaltiques sont en étroite relation avec les grandes déchirures continentales. C'est notamment le cas du point chaud Afar localisé dans un contexte extensif de point triple. Un des objectifs de cette thèse est d'apporter, à la lumière de 28 et 68 nouvelles données acquises, respectivement, en 40_Ar/39_Ar et K/Ar, des contraintes temporelles sur l'épanchement du point chaud et sur les prémices de l'ouverture continentale. Pour cela, des laves et des granites d'ouverture ont été échantillonnées sur les trois zones géographiques aujourd'hui désunies, c'est-à-dire sur les hauts plateaux éthiopiens, yéménites et somaliens, ainsi que le long de leurs escarpements aux marges de la dépression Afar. Nos résultats attestent de l'édification rapide des grands traps (en moins de 1 Ma, pour l'essentiel de la pile volcanique) et d'une initiation du volcanisme autour de 30. 2 Ma. Les prémices de l'ouverture apparaissent en conséquence totalement synchrones de cette phase, comme l'indique la mise en place de granites, dès 30 Ma. Les résultats de cette étude sont repris et discutés de manière plus générale, afin de préciser et de proposer un schéma global d'évolution depuis 30 Ma
We present here a new analytical system for 4O_Ar/39_Ar dating that relies onto an original 180ʿ sector multiple collection mass spectrometer with five faraday cups. Results of preliminarily experiments undertaken on geological standard minerals highlight the good behavior of this new instrument for step heating analyses. The age reproducibility of successive steps lead to analytical errors lower than 0. 2 %. Such a system appears today unique on the international plan for 40_Ar/39_Ar routine dating. This new penta-collector MS enabled us to perform highly reproducible 40_Ar/39_Ar dating of Ethiopian volcanics. It is now admitted that continental flood basalt volcanism has a strong relationship with continental break-up. The Ethiopian-Afar plume has been linked to the early opening of the Afar depression, where the propagation of Red Sea and Gulf of Aden within the depression is still an ongoing process. The purposes of this study are to constrain the emplacement and duration of the trap series, and to reconstruct the eruptive chronology of the earlier opening stages, since the initiation of the fracturation. Twenty-eight 40_Ar/39_Ar and 68 K/Ar new ages data have been performed on volcanic series and granitic bodies, sampled on the plateaus (Ethiopian, Yemenite, Somalian) and along their boundaries on the Afar margin. Our results support that the onset of traps volcanism took place around 30 Ma, with a duration lower than 1 Myr. The emplacement of granitic bodies as early as 30 Ma argues for a synchronous initiation of the opening. Finally, all results obtained in this study on both plateau and rifted margin areas helped us to propose a global scheme for the evolution of the Afar depression since 30 Ma

Pour poursuivre le développement de la géothermie de haute-enthalpie dans l’arc insulaire des Petites Antilles il faut d’une part identifier de nouvelles zones potentielles et d’autre part mieux comprendre le fonctionnement des réservoirs géothermaux. Dans ce cadre, l’objectif de cette thèse est de caractériser l’évolution spatio-temporelle de l’hydrothermalisme au niveau de la plaque supérieure de l’arc en Guadeloupe au travers d’une analyse pluridisciplinaire (études structurales, géochimiques, minéralogiques, pétrologiques et géochronologiques) des complexes volcaniques les plus anciens de l’archipel. Au Nord de la Basse-Terre nous identifions un métamorphisme d’arc de type sub-Schistes Verts. L’état thermique standard de la croûte d’arc en Guadeloupe ainsi établit indique une transition fragile-ductile située à 3-4 km de profondeur. Un paléo-réservoir exhumé, identifié au Sud de l’île de la Basse-Terre, illustre un métamorphisme hydrothermal de type Schistes-Verts synchrone du développement de couloirs de schistosité et révèle la présence d’horizons verticaux et latéraux de transfert de fluides hydrothermaux situés à 2-3 km de profondeur. En comparant l’âge du volcanisme et la datation Ar-Ar des phases hydrothermales de haute-température, nous estimons une durée maximale de fonctionnement du paléo-réservoir à 650 ka. Enfin, la découverte, l’analyse et la datation K-Ar d’une nouvelle brèche hydrothermale, au coeur du système géothermal actif, met en évidence un système épithermal distal relié à l’activité volcanique de la Soufrière. Nos résultats conduisent à un modèle conceptuel de fonctionnement d’un réservoir géothermal de haute-énergie en contexte d’arc actif
To further develop high-enthalpy geothermal energy in Lesser Antilles arc, it is necessary to identify possible new key targets, and to better understand the modes of fluids and heat transfers in geothermal reservoirs. The objective of this work is thus to characterize the spatio-temporal evolution of hydrothermalism at the upper plate of the arc in Guadeloupe with a multidisciplinary approach (combined structural, geochemical, mineralogical, petrological and geochronological analyses) focussed on the oldest volcanic complexes of the archipelago. In the north of Basse-Terre Island reveals an arc metamorphism developed under sub-Greenschist facies. The standard thermal state of the arc crust in Guadeloupe thus established shows a brittle-ductile transition located at depths of 3 to 4 km. An exhumed geothermal paleo-reservoir, identified at the south of Basse-Terre Island, indicates a hydrothermal metamorphism developed under Greenschist facies synchronous with the development of schistose corridors and attests for both vertical and lateral hydrothermal fluid transfers at depths between 2 and 3 km. Comparison of the age of the volcanic activity and Ar-Ar dating of the high-temperature hydrothermal phases allows us to estimate a maximum operating time of the paleo-reservoir at 650 ka. Finally, the discovery, the analysis and the K-Ar dating of a new hydrothermal breccia, within the active geothermal system, reveals a link between current geothermal activity and volcanic activity of the Soufrière, interpreted as a distal epithermal system. Our results lead to a conceptual model for the operation of a high-energy geothermal reservoir in the context of an active arc

Abstract The 40Ar/39 Ar method of dating rocks has its foundations in the potassium-argon (K/Ar) isotopic dating method, a widely used technique for measuring numerical ages on minerals and rocks. Since the K/Ar method was developed more than 40 years ago, it has been applied to a diverse range of geological samples to help elucidate many important geological problems of local, regional, or global significance. Particularly notable successes, dependent largely upon dating by the K/Ar method, include the development of the geomagnetic polarity time scale and the numerical calibration of the Phanerozoic geological or relative time scale.

Abstract Up to this point, our focus has largely been on describing the materials that are appropriate for 40 Ar/39Ar dating and presenting the technical details required to produce reliable isotope ratio measurements. In this and subsequent chapters, we discuss the methods used to interpret 40Ar/ 39 Ar data in terms of geologic history. Depending upon the physical environment, 40 Ar/39Ar dates may be interpreted as rock forming ages or in terms of thermal closure to argon loss. In certain cases, interpretation of results using these models is quite straightforward, but in others, analysis artifacts and natural complications tend to obscure the age and/or thermal history information. Even in these latter cases, however, there are correction schemes we can apply to help overcome problems. In this chapter, we introduce the two graphical devices that have proved most useful in interpreting step heating results, the age spectrum and isochron. We then describe various artifacts of sample preparation and analysis that can reduce the clarity of the chronological information and discuss how these effects can be avoided or minimized. We include a discussion of the effects of excess argon on age interpretation and the steps that can be taken to see through this contamination.

This contribution examines the current state of knowledge regarding the postmetamorphic cooling of most Archean to Paleoproterozoic tectonic elements of the northern Canadian Shield. This was achieved by building an extensive data set of over 2000 combined K-Ar and 40Ar/39Ar cooling ages. The Churchill Province (amalgamated Rae and Hearne cratons) shows a general west to east cooling pattern illustrated by two large-scale thermochronological profiles.