The barents sea magmatic province: Geological-geophysical evidence and new <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dates

Resulting from study of the geological structure of the Franz Josef Land and Svalbard archipelagoes, this work presents new 17 ⁴⁰Ar/³⁹Ar age datings for basalts taken during coastal expeditions in 2006–2010. Radiological age determination for intrusive units (sills) located in the western part of Nordensciold Land (Spitzbergen Island) has been made for the first time. In relation to use of the interpretation results of marine geological-geophysical data, the distribution peculiarities and time ranges for Jurassic-Cretaceous basic magmatism within the studied regions of the Barents Sea continental margin and within the Arctic as a whole are discussed.     

Compositional controls on <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar ages of zoned mica from a rare-element pegmatite

A high spatial resolution Ar–Ar dating study of compositionally zoned micas using UV laser extraction has been used to investigate the effect of composition and compositional boundaries upon argon diffusion in mica. The crystals are characterised by muscovite cores and Li–F–Rb–Cs-rich lepidolite rims produced by the interaction of a residual melt-vapour phase with the original muscovite during the late stages of pegmatite crystallisation. Single crystals exhibit dramatic intra-grain age variations, with a maximum range of 2,880–2,117 Ma. Backscattered electron images combined with UV-laser analyses reveal that apparent ages younger than 2,600 Ma are directly associated with the compositionally distinct rims, suggesting that Ar diffusion was faster in the lepidolite than in muscovite. Although it is not possible to be fully quantitative using the present data, it seems that argon diffusion rates in lepidolite are similar to those in biotite. Major rearrangements of the crystal structure needed to accommodate high concentrations of lithium, fluorine and large interlayer cations (Rb⁺, Cs⁺) in muscovite account for lower argon retentivity of the crystal rims. In most cases the age profiles show no disturbance at the compositional boundary, despite the transition from a di-octahedral to tri-octahedral mica, indicating that in general the boundaries do not act as fast diffusion pathways. However, in some cases there is a clear drop in ages at the compositional boundary, which acts as an effective sub-grain boundary. The difference between the two types may be related to the internal structure of the compositional boundaries or their degree of contact with the grain boundary network of the rock.     

First <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar age determinations on the Malkhan granite-pegmatite system: Geodynamic implications

The Malkhan granite-pegmatite system located in Central Transbaikalia, in the southwestern portion of the Malkhan-Yablonovy structure-formational zone of the Caledonian folding comprises two granite massifs (Bolsherechensk and Oreshny) and a miarolitic pegmatite field of the same name, which adjoins the Chikoi deep-seated fault and Lower Cretaceous Chikoi rift depression in the north. The first ⁴⁰Ar/³⁹Ar data were obtained on porphyritic biotite granites of the Oreshny massif and on K-feldspar, muscovite, and lepidolite from the Oktyabrskaya pegmatite vein. According to these data, the age of the granitepegmatite system is 123.8–127.6 Ma, which is consistent with the age of Lower Cretaceous rocks from the Chikoi depression. The intimate spatial relationship and isochronism between the Chikoi depression and the Malkhan granite-pegmatite system are strongly suggestive of a rift regime that affected its evolution, thus highlighting the need to regard the evolution of this system as being intimately related to depression development. Such a model can easily be realized within the framework of the concept of a metamorphic core complex, which was used to explain the nature of Transbaikal-type rift depressions and conjugate granite-gneiss swells.     

The performance of the Noblesse multi-collector noble gas mass spectrometer for <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar geochronology

Noblesse multi-collector noble gas mass spectrometer is specially designed for multi-collection of Ar isotopes with different beam sizes, especially for small ion beams, precisely, and hence is perfectly suitable for ⁴⁰Ar/³⁹Ar geochronology. We have analyzed widely used sanidine, muscovite, and biotite standards with recommended ages of ~ 1.2–133 Ma, with the aim to assess the reliability of Noblesse for ⁴⁰Ar/³⁹Ar dating. An ESI MIR10 30W CO₂ laser was used for total fusion or incremental heating samples. Extracted gases were routinely purified by four SAES NP10 getters (one at ~ 400 °C and others at room temperature). A GP50 getter and a metal cold finger cooled by liquid N (? 196 °C) were also attached for additional purification if necessary. The Ar isotopes were then measured by Noblesse using Faraday or multiplier according to the signal intensities. Over a period of 1.5 months 337 air calibrations produced a weighted mean ⁴⁰Ar/³⁶Ar of 296.50 ± 0.08 (2σ, MSWD = 4.77). Fish Canyon sanidine is used to calculate J-values, which show good linear relationship with position in irradiation. The age of four mineral standards (Alder Creek sanidine, Brione muscovite, Yabachi sanidine, and Fangshan biotite) are within error of the accepted ages. Five Alder Creek sanidine aliquots yielded an age range of 1.174–1.181 ± 0.013 Ma (2σ) which broadly overlaps the established age of the standard and the uncertainty approaches those of the foremost Ar/Ar laboratories in the world. The weighted mean ages of four Brione muscovite aliquots (18.75 ± 0.16 Ma, 2σ), five Yabachi sanidine aliquots (29.50 ± 0.19 Ma, 2σ), and three Fangshan biotite aliquots (133.0 ± 0.76 Ma, 2σ) are consistent with the recommended values of these standards, and the uncertainties are typical of modern Ar/Ar laboratories world-wide.     

In-situ UV-laser <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar geochronology of a micaceous mylonite : an example of defect-enhanced argon loss

Muscovite and biotite from a crustal-scale mylonite zone (Pogallo Shear Zone, southern Alps) were investigated using furnace step-heating and in-situ UV-laser ablation ⁴⁰Ar/³⁹Ar geochronology. Undeformed muscovite porphyroclasts yield ⁴⁰Ar/³⁹Ar plateau ages of 182.0ǃ.6 Ma, whereas in-situ UV-laser ablation ⁴⁰Ar/³⁹Ar dating and furnace step-heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger. The range of ⁴⁰Ar/³⁹Ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra-grain scale. These microstructurally controlled segments are bound by either first-order lattice discontinuities, sub-microscopic structural defects and/or zones of high defect density, which create variable length-scales for intragranular argon diffusion. The observed deformational microstructures within muscovite and biotite acted as intra-grain fast diffusion pathways in the slowly cooled mylonitic rocks. Therefore, the high-spatial resolution ⁴⁰Ar/³⁹Ar data record the initial and final closure to argon diffusion over a time span of about 60 Ma.     

Interpreting high-pressure phengite <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar laserprobe ages: an example from Saih Hatat,NE Oman

New single grain fusion and core-rim ⁴⁰Ar/³⁹Ar laserprobe phengite data from the Saih Hatat high-pressure terrane in NE Oman show that individual samples yield a range of apparent ages which is similar to that previously reported from across the entire terrane. The majority of the determined ages are older than the previously reported U-Pb zircon peak metamorphic age. Core to rim age variations within individual grains range from no discernible difference across the grain to grains with older cores, or, rarely, older rims; some samples manifest all three patterns. Numerical diffusion modelling shows that due to the peak temperature of ca. 550°C, the measured apparent ages cannot be explained by simple cooling or by partial retention of crystallisation or detrital ages in an open system. The age variability is better explained by spatially and temporally variable open or closed system behaviour at the mm-cm scale coupled with pervasive and heterogeneously distributed excess argon. Anomalously old eclogite phengite ⁴⁰Ar/³⁹Ar ages are due either to internally derived ⁴⁰Ar inherited from a K-bearing precursor, or externally derived ⁴⁰Ar distributed by grain boundary fluids. Mica-rich schists within the eclogite boudins yield younger phengite ages, suggesting excess argon was absent or diluted. Pelites hosting the eclogite appear to have been affected by later fluid ingress during deformation and greenschist-facies overprint and yield very variable ages commonly with apparently older rims on younger cores. The grain- and sample-scale age variations measured in Saih Hatat indicate that the grain boundary network in eclogite pods was not an efficient transfer pathway for argon transport, whereas the grain boundary network in the surrounding pelites acted as a more efficient pathway on the timescale of the metamorphic cycle.     

40^Ar／39^Ar Ages of Auriferous Quartz Veins from the Fengyang and Zhangbaling Regions and Their Geological Significance

Geotectonically the Fengyang and Zhangbaling regions belong to the North China craton and the Dabie-Sulu oragene, respectively. Neo-Archean gneiss and amphibolite and metamor-phosed sea-facies sodic volcanic rocks axe the main outcrops in the two regions, respectively. The Zhangbaling terrane strike-skipped along the Tancheng-Lujiang fault zone in Mesozoic and Cenozo-ic eras and got close to the Fengyang terrane. Mesozoic Yanshanian intrusions occur broadly in thetwo regions. Gold-beating quartz veins occur in the metamorphic rocks in the Fengyang region and in the granodiorite and metamorphosed sea-facies sodic volcanic rocks in the Zhanghaling region.Generally, the formation of the auriferous quartz veins involved three stages. At the first stage,gold-poor sulfide quartz veins were formed; at the second stage gold-rich quartz sulfide veins wereformed; and at the third stage gold-poor barite and/or carbonate veins were formed. The 40^Ar/29^Ar step-heating plateau ages of the first-stage and the second-stage quartz aggregates from the Zhuding, Maoshan and Shangeheng gold deposits range between 116.1 0.6 Ma and 118.3 0.5 Ma and are pretty close to their least apparent ages and isoehronal ages, respectively. All plat-eau, least apparent and isoehronal ages range between 113.4 0.4 Ma and 118.3 0.5 Ma,which are considered as the formation age range of the quartz. It is reasonable and reliable to takethe 40^Ar/39^Ar age range of the quartz as the formation age range of gold-bearing quartz veins onthe basis of spatial relationship between gold-bearing quartz veins and their country rocks. Thegold deposits in the two regions were formed in Aptian, Cretaceous, when the Tancheng-Lujiangfault zone moved as a normal fault with slightly right-lateral strike-skip, was extensional and expe-rienced very strong magnmtic process. It is shown that the magnmtic hydrothermal fluid is a veryimportant part of the gold ore-forming hydrothermal fluid in the Fengyang and Zhanghaling re-gions. The formation of the gold ore deposits in the Fengyang and Zhanghaling regions had genetic relations with the extensional movement of the Tancheng-Lujiang fault zone and magmatic activities and took place under the extensional dynamic condition in Late Cretaceous. Therefore, the exten-sional movement of the Tancheng-Lujiang fault zone presented the energy and space for magmatic and gold ore-forming processes.     

Detailed <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of geologic events associated with the Mantos Blancos copper deposit,northern Chile

The ⁴⁰Ar/³⁹Ar geochronological method was applied to date magmatic and hydrothermal alteration events in the Mantos Blancos mining district in the Coastal Cordillera of northern Chile, allowing the distinction of two separate mineralization events. The Late Jurassic Mantos Blancos orebody, hosted in Jurassic volcanic rocks, is a magmatic-hydrothermal breccia-style Cu deposit. Two superimposed mineralization events have been recently proposed. The first event is accompanied by a phyllic hydrothermal alteration affecting a rhyolitic dome. The second mineralization event is related to the intrusion of bimodal stocks and sills inside the deposit. Because of the superposition of several magmatic and hydrothermal events, the obtained ⁴⁰Ar/³⁹Ar age data are complex; however, with a careful interpretation of the age spectra, it is possible to detect complex histories of successive emplacement, alteration, mineralization, and thermal resetting. The extrusion of Jurassic basic to intermediate volcanic rocks of the La Negra Formation is dated at 156.3 ± 1.4 Ma (2σ) using plagioclase from an andesitic lava flow. The first mineralization event and associated phyllic alteration affecting the rhyolitic dome occurred around 155–156 Ma. A younger bimodal intrusive event, supposed to be equivalent to the bimodal stock and sill system inside the deposit, is probably responsible for the second mineralization event dated at ca. 142 Ma. Other low-temperature alteration events have been dated on sericitized plagioclase at ca. 145–146, 125, and 101 Ma. This is the first time that two distinct mineralization events have been documented from radiometric data for a copper deposit in the metallogenic belt of the Coastal Cordillera of northern Chile. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Argon retention properties of silicate glasses and implications for <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar age and noble gas diffusion studies

Sized aggregates of glasses (47–84 wt% SiO₂) were fused from igneous-derived cohesive fault rock and igneous rock, and step-heated from ~400 to >1,200 °C to obtain their ³⁹Ar diffusion properties (average E=33,400 cal mol^?1; D _o=4.63×10^?3 cm² s^?1). At T<~1,000 °C, glasses containing <~69 wt% SiO₂ and abundant network-forming cations (Ca, Fe, Mg) reveal moderate to strong non-linear increases in D and E, reflecting structural modifications as the solid transitions to melt. Extrapolation of these Arrhenius properties down to typical geologic T-t conditions could result in a 1.5 log₁₀ unit underestimation in the diffusion rate of Ar in similar materials. Numerical simulations based upon the diffusion results caution that some common geologic glasses will likely yield ⁴⁰Ar/³⁹Ar cooling ages rather than formation ages. However, if cooling rates are sufficiently high, ambient temperatures are sufficiently low (e.g., <65–175 °C), and coarse particles (e.g., radius (r) >~1 mm) are analyzed, glasses with compositions similar to ours may preserve their formation ages.     

Results of <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of phlogopites from kelyphitic rims around garnet grains (Udachnaya-Vostochnaya kimberlite pipe)

⁴⁰Ar/³⁹Ar dating of phlogopite from kelyphitic rims around garnet grains from the Udachnaya–Vostochnaya kimberlite pipe in the Sakha (Yakutia) Republic (Russia) revealed that when this mineral has contact with a kimberlite melt its age corresponds (within error limits) to that of the formation of the kimberlite pipe, thus indicating that the method may be used for dating kimberlites and related rocks. In mantle xenoliths, kelyphitic phlogopites rimming garnet grains partially lose radiogenic Ar, which results in a complex age spectrum. Rejuvenation of the K/Ar system in them is determined by the thermal impact of the kimberlite melt on captured rocks.     

Mesozoic rift magmatism in the North Sea region: <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar geochronology of Scanian basalts and geochemical constraints

More than 100 volcanic necks composed of basanites and melanephelinites occur in Scania, southern Sweden, at the junction of two major tectonic lineaments, the Phanerozoic Sorgenfrei-Tornquist Zone (STZ) and the Proterozoic Protogine Zone. New ⁴⁰Ar/³⁹Ar isotope analyses of whole rock fragments of nine selected basalt necks suggest that the Mesozoic alkaline volcanism in the Scanian province commenced earlier than previously reported and comprised three separate volcanic episodes that span a total period of ca. 80 Myr: a first Jurassic (191–178 Ma), a second at the Jurassic/Cretaceous boundary (ca. 145 Ma), and a final middle Cretaceous episode (ca. 110 Ma). The new results allow for precise time correlations between eruption events in the Scanian and those in the North Sea volcanic provinces. The older, early Jurassic event in Scania is largely synchronous with that in the Egersund Basin and the Forties field whereas the event at ca. 145 Ma is correlated with activity in the Central Graben. These volcanic episodes also correlate in age with Kimmerian tectonic activity. Volcanic activity in the middle Cretaceous period has also been dated in the triple junction in the North Sea and offshore in the Netherland Sector. The correlation of basalt volcanism in Scania with the Egersund nephelinites strongly suggest that volcanism was triggered by repeated tectonic activity along the STZ. Geochemical data of alkaline mafic rocks in the Scanian and the North Sea volcanic provinces imply that different provinces have largely unique geochemical signatures in favour of a heterogeneous mantle in the North Sea volcanic region. However, basalts of different generations in one and the same province cannot be readily separated on the basis of geochemistry, suggesting that the same lithospheric mantle was the source of repeated volcanism over time in each province. The data suggest a low degree of melting of a volatile-bearing mantle lherzolite enriched in incompatible elements with the exception of the Forties basalts in the rift centre, produced by larger degree of melting and evolved by fractional crystallization.     

Variation of illite/muscovite <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar age spectra during progressive low-grade metamorphism: an example from the US Cordillera

⁴⁰Ar/³⁹Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from ⁴⁰Ar/³⁹Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 °C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.05–2 μm should have closure temperatures between 250 and 350 °C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful ⁴⁰Ar/³⁹Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite ⁴⁰Ar/³⁹Ar age spectra at different stages of low-grade metamorphism and also illuminate the transformation of illite to muscovite. At Frenchman Mtn., NV, where the Cambrian Bright Angel Formation is at zeolite facies conditions, illite/muscovite ⁴⁰Ar/³⁹Ar data suggest a detrital muscovite component with an apparent age ≥967 Ma. The correlative Carrara Fm. is at anchizone conditions in the Panamint and Resting Spring Ranges of eastern California, and in these locations, illite/muscovite ⁴⁰Ar/³⁹Ar data suggest an early Permian episode of subgreenschist facies metamorphism. The same type of data from equivalent strata at epizone conditions (greenschist facies) in the footwall of the Bullfrog/Fluorspar Canyon detachment in southern Nevada reveals a period of slow-to-moderate Late Cretaceous cooling.     

Integrated stratigraphy and <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar chronology of the Early to Middle Miocene Upper Freshwater Molasse in eastern Bavaria (Germany)

A detailed integrated stratigraphic study was carried out on middle Miocene fluvial successions of the Upper Freshwater Molasse (OSM) from the North Alpine Foreland Basin, in eastern Bavaria, Germany. The biostratigraphic investigations yielded six new localities thereby refining the OSM biostratigraphy for units C to E (sensu; Heissig, Actes du Congres BiochroM’97. Mem Trav EPHE, Inst Montpellier 21, 1997) and further improving biostratigraphic correlations between the different sections throughout eastern Bavaria. Radioisotopic ages of 14.55 ± 0.19 and 14.88 ± 0.11 Ma have been obtained for glass shards from the main bentonite horizon and the Ries impactite: two important stratigraphic marker beds used for confirming our magnetostratigraphic calibration to the Astronomical Tuned Neogene Time Scale (ATNTS04; Lourens et al. in Geologic Time Scale 2004, Cambridge University Press, 2004). Paleomagnetic analysis was performed using alternating field (AF) and thermal (TH) demagnetization methods. The AF method revealed both normal and reverse polarities but proofs to yield unreliable ChRM directions for the Puttenhausen section. Using the biostratigraphic information and radioisotopic ages, the magnetostratigraphic records of the different sections are tentatively correlated to the Astronomical Tuned Neogene Time Scale (ATNTS04; Lourens et al. in Geologic Time Scale 2004, Cambridge University Press, 2004). This correlation implies that the main bentonite horizon coincides to chron C5ADn, which is corroborated by its radioisotopic age of 14.55 Ma, whereas the new fossil locality Furth 460, belonging to OSM unit E, probably correlates to chron C5Bn.1r. The latter correlation agrees well with the Swiss Molasse locality Frohberg. Correlations of the older sections are not straightforward. The Brock horizon, which comprises limestone ejecta from the Ries impact, possibly correlates to C5ADr (14.581–14.784 Ma), implying that, although within error, the radioisotopic age of 14.88 ± 0.11 Ma is somewhat too old. The fossil localities in Puttenhausen, belonging to the older part of OSM unit C, probably coincide with chron C5Cn.2n or older, which is older than the correlations established for the Swiss Molasse. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

Metallogenic features of Miocene porphyry Cu and porphyry-related mineral deposits in Ecuador revealed by Re-Os, <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar,and U-Pb geochronology

Mineralization and alteration events at ten Miocene porphyry Cu and porphyry-related epithermal mineral deposits in southern, central, and northern Ecuador were dated by means of molybdenite Re-Os, biotite and alunite ⁴⁰Ar/³⁹Ar, and titanite U-Pb geochronology. Most of these hydrothermal events show a spatio-temporal correlation with porphyry intrusion emplacement as constrained by zircon U-Pb ages. The total age range for these events spans the 23.5–6.1 Ma period, without displaying systematic along- or across-arc age distribution trends. While epithermal deposits tend to be spatially associated with volcanic rocks of a similar age, porphyry Cu deposits in Ecuador are frequently spatially associated with deeper-seated basement units and batholith-scale precursor intrusive systems assembled over ≥5 m.y. time periods. In most cases, formation of the porphyry Cu deposits is related to the youngest magmatic (-hydrothermal) event in a given area, postdating batholith construction at a regional scale. The majority of Miocene deposits occurs in southern Ecuador where areally extensive, post-mineralization (late Miocene to recent) volcanic sequences with the potential to conceal mineralization at depth are lacking. Only few Miocene deposits occur in northern-central Ecuador, where they mainly crop out in the Western Cordillera, west of the productive present-day volcanic arc. The surface distribution of post-mineralization arc volcanism reflects along-arc variations in subducting slab geometry. Porphyry Cu and epithermal deposits in Ecuador define a Miocene metallogenic belt broadly continuous with its coeval counterpart in northern-central Peru. Although both belt segments were formed in an overall similar tectonomagmatic and metallogenic setting, their respective metal endowments differ significantly.