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.     

New <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar age constraints on cooling and unroofing history of the metamorphic host rocks (and igneous intrusion associates) from the Bulfat Complex (Bulfat area), NE-Iraq

The Northern Zagros Suture Zone (NZSZ), formed as a result of the collision between Arabian and Sanandaj-Sirjan microplate, is considered as part of the Zagros orogenic belt. NZSZ is marked by two allochthonous thrust sheets in upward stacking order: lower and upper allochthon. The Bulfat complex is a part of the upper allochthon or “Ophiolite-bearing terrane” of Albian-Cenomenion age (97–105 Ma). Voluminous highly sheared serpentinites associated with ophiolites occur within this upper allochthon. In addition, the Gemo-Qandil Group is characterized by gabbroic to dioritic Bulfat intrusion with a crystallization age spanning from ~45 to ~?40 Ma, as well as extensive metapelites with contact to the Walash-Naupurdam metavolcanic rocks. Due to the deformation in the Sanandaj-Sirjan Zone along the eastern side of the Iraqi segment of NZSZ, the Gemo-Qandil Group was regionally metamorphosed during late Cretaceous (~?80 Ma). This tectono-compressional dynamics ultimately caused an oscillatory deformation against Arabian continental margin deposits as well. During these events, gabbro-diorite intrusion with high-grade contact metamorphic aureoles occurred near Bulfat. Thus, there is an overlap between regional and contact metamorphic conditions in the area. The earlier metamorphic characteristic can be seen only in places where the latter contact influence was insignificant. Generally, this can only observed at a distance of more than 2.5 km from the contact. According to petrographic details and field observations, the thermally metamorphosed metapelitic units of the metasediment have been completely assimilated, with only some streaks of biotite and relicts of initial foliation. They strongly resemble amphibolite-grade slices from the regional metamorphic rocks in the region. Metapelitic samples far from the intrusion give similar biotite cooling ages as the intrusive rocks. Thus, they may be affected by the same thermal event. ⁴⁰Ar/³⁹Ar dating of biotite in metapelite rocks of Bulfat by step-wise heating with laser gave average weighted isotopic ages of 34.78?±?0.06 Ma. This is interpreted as crystallization/recrystallization age of biotite possibly representing the time of cooling and uplift history of the Bulfat intrusion. Cooling and exhumation rates for the Bulfat gabbro-diorite rocks were estimated as ~?400 °C/Ma and ~?3.3 mm/year respectively. According to petrographic details, field observations and Ar/Ar dating concerning the contact metamorphism near Bulfat due to the gabbro-diorite intrusion, no significant deformation is visible during exhumation processes after the Paleogene tectono-thermal event, indicating that isotopic ages of 34.78?±?0.06 Ma could mark the timing of termination of the island arc activity in the Ophiolite-bearing terrane (upper allochthon).     

西天山北部晚古生代火山-浅侵位岩浆岩40Ar/39Ar同位素定年

西天山火山岩属玄武安山流纹岩建造,钙碱性(中钾、高钾)和橄榄玄粗岩系列。其^40Ar／^39Ar年龄范围为250～306Ma,以260～290Ma为高峰值。浅侵位岩浆岩属偏碱性、碱性岩石或高钾钙碱性系列岩石,^40Ar／^39Ar年龄范围为260～290Ma。综合Ar—Ar和Rb—Sr、U—Pb年龄资料,西天山地区火山—浅侵位岩浆活动高峰期发生在晚石炭世—早二叠世。^40Ar／^39Ar坪年龄资料还显示了晚二叠世、侏罗纪—早白垩世的热事件信息。对区内高钾钙碱性火山岩、橄榄玄粗岩和埃达克质岩石等富碱火成岩的年龄测定表明,本区构造格架的重大转折——由俯冲、碰撞造山的挤压,转向后碰撞的伸展、拉张——发生在石炭纪末—早二叠世(250～300Ma)期间。     

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.     

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.     

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.     

Laser probe <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating for quartz from auriferous quartz veins in the Shihu gold deposit,western Hebei Province,North China

The Shihu gold deposit is situated in the central shear zone of the Meso-Cenozoic Fuping metamorphic core complex in the middle-northern part of the Taihang Mountains. This kind of gold deposits named after ‘Shihu type’ widely occurs in this region. Gold-bearing quartz veins are their most important industrial orebodies. Detailed laser ⁴⁰Ar/³⁹Ar geochronology of the mineralized quartz veins in the study area reveals details of its tectono-magmatic history. The ⁴⁰Ar/³⁹Ar ages for the quartz are between 134.4 Ma and 155.9 Ma with a mean age of 141±4 Ma on the isochron line and 139±7 Ma on the inverse isochron line, respectively, which is interpreted to be the best estimate of the crystalline age of the quartz veins. The gold mineralization was most likely related to an underplating event that took place in the northern part of the Taihang Mountain at ca. 140 Ma. The timing of gold mineralization in the area is similar to those observed in other major gold-producing provinces in the NCC. This episode is simultaneous with those in the eastern NCC, indicative of a widespread late Yanshanian metallogenic event that was a response either to the subduction of the Izanagi-Pacific plate beneath eastern China or to the removal of the Early Cretaceous lithosphere in the eastern NCC. The ore-forming material sources may be the Early Precambrian metamorphic basement, instead of intermediate-acid rock bodies and dykes.     

Timing and duration of supergene mineralization at the Xinrong manganese deposit,western Guangdong Province,South China: cryptomelane <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating

Supergene Mn-oxide deposits are widely distributed in Guangxi, Guangdong, Yunnan, and Hunan Provinces, South China, accounting for 18% of the total Mn reserves in the country. Direct dating of supergene Mn enrichment, however, is lacking. In this paper, we present high-resolution ⁴⁰Ar/³⁹Ar ages of Mn oxides from the Xinrong Mn deposit, western Guangdong, to place numerical constraints on the timing and duration of supergene Mn enrichment. A total of ten cryptomelane samples, spanning a vertical extent of 67 m, were dated using the ⁴⁰Ar/³⁹Ar laser incremental heating technique, with seven samples yielding well-defined plateau or pseudo-plateau ages ranging from 23.48 ± 0.91 to 2.06 ± 0.05 Ma (2σ). One sample yields a staircase spectrum that does not reach a plateau; the spectrum, however, indicates the presence of two or more generations of Mn oxides in the sample, whose ages are best estimated at 22.34 ± 0.31 and 10.2 ± 0.86 Ma, respectively. The remaining two samples gave meaningless or uninterpretable results due to significant ³⁹Ar recoil and contamination by old phases. The ⁴⁰Ar/³⁹Ar data thus reveal a protracted history of weathering and supergene Mn enrichment that started at least in the end of the Oligocene or beginning of Miocene and extending into the latest Pliocene. Staircase-apparent age spectra, resulting from banded or botryoidal samples, yield an average growth rate of Mn oxides at 0.6–0.7 × 10⁻³ mm kyr⁻¹. The values indicate that a 1-mm grain of Mn oxides may host minerals precipitated during a time span of ca. 1.5 m.y., and accumulation of Mn oxides to form economic deposits under weathering environments may take millions of years. The distribution of weathering ages shows that the oldest Mn oxides occur on the top of the profile, whereas the youngest minerals are found at the bottom, suggesting downward propagation of weathering fronts. However, two samples located at the intermediate depths of the profile yield ages comparable with those occurring at the highest elevations. Such a complexity of age distribution is interpreted in terms of preferential penetration of Mn-rich weathering solutions along more permeable fault zones, or as a result of multi stages of dissolution and re-precipitation of Mn oxides. A synthesis of geochronological and geological data suggests that formation of the Xinrong deposit was a consequence of a combination of favorable lithological, climatic, and structural conditions. Because the climatic and structural conditions are similar among the provinces of South China during the Cenozoic, the geochronological results obtained at Xinrong may also have implications for the timing of supergene Mn enrichment throughout South China.     

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.     

Deep-seated pegmatites of the Emiytas mafic-ultramafic complex on Big Lyakhov Island,New Siberian Islands,and their age: <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar and SHRIMP data

Pegmatites of the Emiytas basic-ultrabasic metamorphic complex have a granodiorite-granite composition. Their genetic relations with the host amphibolites follow from the low initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7028 and from the P-T conditions (650°C and 10 kbar), which are close to those of the host rocks. Amphibole, biotite, and muscovite megacrysts analyzed by the ⁴⁰Ar/³⁹Ar method yielded plateau ages of 209.7 ± 0.9, 203.0 ± 0.9, and 178.5 ± 1.5 Ma, respectively. The former two minerals contain excess Ar, whereas the K-Ar system of the muscovite is undisturbed. The cooling of the complex to the closure temperature of this system was likely controlled by its exhumation to a shallower depth level. Zircons from the Emiytas pegmatites occur as polyhedral equant crystals with weakly contrasting sectorial zoning, very low concentrations of U (4–8 ppm in the enriched domains), and low Th/U ratios (0.002–0.003), which suggest that the mineral crystallized at significant depth in the presence of fluid. Tentative SHRIMP II measurements (five analyses) yielded a zircon age of 202 ± 17 Ma. The applying a specialized approach to the analysis of young low-U zircons on an ion probe is discussed. In spite of the small number of analyses, new geochronologic data leave no doubt that the crystallization age of the pegmatites is Late Triassic-Early Jurassic and invalidate earlier ideas that the Emiytas complex is Precambrian-Early Paleozoic. This conclusion makes the Emiytas amphibolites to be one of the various oceanic and suprasubduction complexes related to the Mesozoic South Anyui suture, which is important for reconstructions of the tectonic evolution of the East Siberian Arctic shelf.     

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.     

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.     

<Superscript>40</Superscript>Ar-<Superscript>39</Superscript>Ar age of a lava flow from the Bhimashankar Formation,Giravali Ghat,Deccan Traps

We report here a⁴⁰Ar-³⁹Ar age of 66.0 ± 0.9 Ma (2σ) for a reversely magnetised tholeiitic lava flow from the Bhimashankar Formation (Fm.), Giravali Ghat, western Deccan province, India. This age is consistent with the view that the 1.8–2 km thick bottom part of the exposed basalt flow sequence in the Western Ghats was extruded very close to 67.4 Ma.     

Tectono-thermal evolution of the India-Asia collision zone based on<Superscript>40</Superscript>Ar-<Superscript>39</Superscript>Ar thermochronology in Ladakh,India

New⁴⁰Ar-³⁹Ar thermochronological results from the Ladakh region in the India-Asia collision zone provide a tectono-thermal evolutionary scenario. The characteristic granodiorite of the Ladakh batholith near Leh yielded a plateau age of 46.3 ± 0.6 Ma (2σ). Biotite from the same rock yielded a plateau age of 44.6 ± 0.3 Ma (2σ). The youngest phase of the Ladakh batholith, the leucogranite near Himya, yielded a cooling pattern with a plateau-like age of ∼ 36 Ma. The plateau age of muscovite from the same rock is 29.8 ±0.2 Ma (2σ). These ages indicate post-collision tectono-thermal activity, which may have been responsible for partial melting within the Ladakh batholith. Two basalt samples from Sumdo Nala have also recorded the post-collision tectono-thermal event, which lasted at least for 8 MY in the suture zone since the collision, whereas in the western part of the Indus Suture, pillow lava of Chiktan showed no effect of this event and yielded an age of emplacement of 128.2 ±2.6 Ma (2σ). The available data indicate that post-collision deformation led to the crustal thickening causing an increase in temperature, which may have caused partial melting at the base of the thickened crust. The high thermal regime propagated away from the suture with time.     

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.