^40Ar／^39Ar dating of Daqingshan thrust

The Daqingshan thrust system is an important part of the western segment of the Mesozoic Yinshan-Yanshan orogenic belt. Therefore, much attention has been paid to the thrusts due also to their large-scale and entirely devel-oped structural elements and typical structural styles. Studies and progress have been concentrated on the ge-ometry, kinematics, dating and dynamic mechanism of the thrust and thrusting. To date, there have been no reliable published isotopic data, however, on the dating…     

^40Ar／^39Ar geochronological constraints on syn-orogenic strike-slip movement of Tan-Lu fault zone

Two phases of sinistral strike-slip ductile shear belts occur on the eastern margin of the Dabie orogenic belt. A muscovite ~(40)Ar/~(39)Ar plateau age of 128 Ma was obtained from mylonite in the later ductile shear zone. Three muscovite samples separated from mylonites of 3 localities in the earlier ductile shear belts yield ~(40)Ar/~(39)Ar plateau ages of 192.5±0.7 Ma, 189.7±0.6 Ma and 188.7±0.7 Ma, respectively. They are interpreted as cooling ages of the earlier sinistral strike-slip deformation. It is suggested that left-lateral displacement of the Tan-Lu fault zone started in a late stage of the collision orogeny in the Dabie-Sulu orogenic belt between the North and South China plates. Therefore, the earlier Tan-Lu fault zone was syn-orogenic strike-slip tectonics. The fault zone was used again for sinistral displacement during tectonic activities of peri-Pacific regime in Early Cretaceous. It is proposed that the fault zone occurred as a transform fault during the orogenic process.     

CO<Subscript>2</Subscript> gas emplacement age in the Songliao Basin: Insight from volcanic quartz <Superscript>40</Superscript>Ar-<Superscript>39</Superscript>Ar stepwise crushing

Due to a lack of suitable minerals, the gas/oil emplacement ages have never been accurately obtained before. CH₄-CO₂-saline- bearing secondary inclusions are found in quartz from the volcanic rocks of the Yingcheng Formation, the container rocks of the deep CO₂ gas reservoir in the Songliao Basin. The inclusion fluid was trapped into microcracks in quartz during the gas emplacement and accumulation, providing an optimal target for the ⁴⁰Ar-³⁹Ar stepwise crushing technique to determine the CO₂ gas emplacement age. ⁴⁰Ar-³⁹Ar dating results of a quartz sample by stepwise crushing yield a highly linear-regression isochron with an age of 78.4±1.3 Ma, indicating that the accumulation of the deep CO₂ gas reservoir in the Songliao Basin occurred in the late Cretaceous. This is the first time to report an exact isotopic age for a CO₂ gas reservoir, which indicates that the ⁴⁰Ar-³⁹Ar dating can serve as a new technique to date the oil/gas emplacement ages.     

Dating of sediment－hosted disseminated－type gold deposit by using ^40Ar／^39Ar laser probe： A case study of Changkeng large－scale gold deposit, China

Hydrothermal clay minerals that formed in the main metallogenic stage of the Changkeng disseminated-type gold deposit in central Guangdong were analyzed by the 40Ar/39Ar laser probe. The results give Ar-Ar isochron ages of 109.9 1.4 to 110.1 1.3 Ma, which are 30 to 50 Ma older than the age of silver ore bodies in the deposit. Both gold and silver mineralizations were not related to hidden granite batholith in the mining district, but probably related to evo-lution of sedimentary basins. This study shows that the 40Ar/39Ar laser probe is a sound dating tool applicable to sediment-hosted disseminated-type gold deposit.     

ISEA reversed event in the Cretaceous Normal Super-chron （CNS）： ^40Ar／^39Ar dating and paleomagnetic results

ThegeomagneticfieldwasfoundtobeofnormalpolarityforalongtimeduringtheCretaceousbyHelsleyandSteiner[1]andthiswaslaternamedasCretaceousNormalSuperchron(CNS)[2].TheCNSlastedalmost37Ma(120—83Ma).Severalabnormalgeologicalevents,suchasanoceanicanoxicevent,alargenumberofvol-canismandglobalclimaticwarmingduringtheCNShavebeenreported[3—11].Thishasledtomanyquestionsbeingraised.IsittruethattheEarthsmagneticfielddidnotreverseatallintheCNS?Ifitdidreverse,howmanytimesandwhendidthishappen?Istherea…     

<Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar isochron ages of lawsonite blueschists from Jiuquan in the northern Qilian Mountain,NW China,and their tectonic implications

Using laser ⁴⁰Ar/³⁹Ar dating method, we have gotten the metamorphic ages of lawsonite blueschist and epidote blueschist from Jiuquan, northern Qilian Mountain, NW China. The high quality laser ⁴⁰Ar/³⁹Ar dating of glaucophane from lawsonite blueschist gives an isochron age of 413±5 Ma. The isochron age obtained from phengite in epidote blueschist is 415±7 Ma. These data, combining with peak metamorphic P-T conditions and regional geological setting, allow us to infer that the lower limit of the ages of the prograde subduction metamorphism from lawsonite blueschist facies to epidote blueschist facies occurred at ca. 413–415 Ma, which also suggests that the formation of lawsonite blueschist in the northern Qilian Mountain maybe resulted from the corner flow in the cold subduction zone. This study shows that the final closing time of the northern Qilian remnant oceanic basin is about 413–415 Ma, which also represents the convergent age between the North China Craton and the Qaidam block.     

Lasing on pyroclastic rocks: A case study of <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating on Moshishan Group,eastern Zhejiang Province

It is difficult to date pyroclastic rocks, for almost all the dating methods, due to the multiple sources during their formation. ^40Ar/^39gAr incremental heating results on groundmass selected from the samples show that the age spectra are meaningless geologically. However, singe crystal total fusions of CO2 lasing on the sanidine separates could yield rational 4^40Ar/^39Ar results and distinguish their sources in this study. Timing on three formations of the Moshishan Group, after avoiding the exotic and altered grains by lasing on the single sanidine separate, was reported in this paper. The lowermost portion of the Chawan Formation gives an age of 113.7±0.3 Ma; the lower part of the Xishantou Formation was formed 116.4±0.4 Ma ago and the bottom of the Gaowu Formation took its shape at 118.4±0.4 Ma. These new ages are much younger than the previous ones, suggesting that these thick volcanic formations had been formed in very short durations.     

40Ar/39Ar同位素体系及其在地学上的应用

40Ar/39Ar同位素体系大体经历了3个技术阶段常规 40Ar/39Ar定年技术、40Ar/39Ar分步加热定年技术和40Ar/39Ar激光显微探针定年技术.研究表明可以通过大气中40Ar/39Ar比值随时间的变化窥探地球去气作用的时间和强度;40Ar/39Ar法不仅有助于研究岩浆冷却过程和花岗岩侵位机制等岩浆动力学的历史和壳幔演化,而且对研究热液成矿年龄也是很有效方法之一;在一定的地质条件下,沉积地层时代越老,40Ar/39Ar值越高;当今40Ar/39Ar年代法已为全球环境演变研究提供了一种强有力的工具.     

Ultra-violet laser probe measurement of ^40Ar／^39Ar age profile in phlogopite

Ultra-Violet Laser Ablation Microprobe (UVLAMP) extraction technique enables the direct investigation of Ar-Ar age prorde in crystals, and yields more information on rates and durations of geological process than conventional single time snapshots. Phlogopite flakes from lamprophyre at Pishan dyke in western Kunlun were dated by using an UV laser (λ= 213 nm) microprobe with spot analyses. The results show good agreement with those from the conventional ^40Ar/^39Ar step heating experiments. This indicates that the Ar isotopes are distributed homogenously in the phlogopite and the UVLAMP can be a powerful tool in the study of thermal history.     

氩同位素分析样品管的改造

随着质谱仪性能的改进,熔样系统成为影响Ar同位素分析精度的主要因素。将原来的单进样管改造成为多进样管,可以预先去气和测量热空白值,降低了非放射成因氩对测试结果的影响,这对于年轻火山岩的定年尤为重要;同时还大大节约了测试时间。长期的测试实践证明,熔样钼坩锅外面的石英管是影响热空白的主要因素。对石英管的形状进行了改造,使其与钼坩锅接触的石英材料减少到原来的大约1/5,进一步降低了热空白并减少了去气时间。通过对样品管的改造,提高了样品测试中放射成因氩的含量,标样多次测试结果具有很好的重现性和准确度。     

氩同位素分析样品管的改造

随着质谱仪性能的改进,熔样系统成为影响Ar同位素分析精度的主要因素。将原来的单进样管改造成为多进样管,可以预先去气和测量热空白值,降低了非放射成因氩对测试结果的影响,这对于年轻火山岩的定年尤为重要;1/5,进一步降低了热空白并减少了去气时间。通过对样品管的改造,提高了样品测试中放射成因氩的含量,标样多次测试结果具有很好的重现性和准确度。     

Further support for a Cretaceous age for the feathered-dinosaur beds of Liaoning,China:New 40Ar/39Ar dating of the Yixian and Tuchengzi Formations

We report new 40Ar/39Ar dating results obtained from total fusion and incremental-heating analyses of sanidine and biotite from three tuffs found interbedded within the fossil-bearing deposits of Liaoning, northeast China. The first is a new sample of the Bed 6 Sihetun tuff from the Yixian Formation, previously dated by our team as middle Early Cretaceous, and recently considered by Lo et al., partially reset due to metamorphism from a nearby basaltic sill. The second is the Yixian Bed 9 tuff from Hengdaozi considered by Lo et al. to be unaffected by metamorphism and whose age, based on total fusion 40Ar/39Ar dating of biotite, argues for a Jurassic age for the Yixian Formation.The third tuff is a previously undated tuff from the upper part of the underlying Tuchengzi Formation. Single crystal total fusion 40Ar/39Ar analyses of the Sihetun sanidine showed homogeneous radiogenic Ar, Ca/K ratios, excellent reproducibility and gave a mean age of 125.0 ± 0.18 (1SD) ±0.04 (SE) Ma. Single sanidine crystal total fusion 40Ar/39Aranalyses of the Hengdaozi tuff gave a mean age of 125.0 +0.19 (1SD) ± 0.04 (SE) Ma, which is indistinguishable from the Sihetun tuff. The Tuchengzi Formation tuff gave a mean age of 139.4 ± 0.19 (1SD) ± 0.05 (SE) Ma. Detailed laser incremental-heating analyses of biotite from Sihetun, Hengdaozi, and Tuchengzi tuffs show disturbed Ar release patterns and evidence of trapped argon components. We conclude from these analyses that the total fusion dates on biotite by Lo et al. are erroneously old and isotopic dating of both biotite and sanidine from tuffs of the Yixian Formation point to a middle Early Cretaceous age. The upper part of the Tuchengzi Formation can be referred to the Early Cretaceous.``     

Experiment of the ~(40)Ar/~(39)Ar dating of diagenetic illite in gas reservoirs

40Ar/39Ar dating of diagenetic illite has been performed to investigate gas reservoirs in the Sulige Gas Field of the northern Ordos Basin. A series of technical challenges were confronted, including illite purification, Ar recoil loss, and separation of diagenetic illite from detrital illite. Mineral growth ages for diagenetic illite were obtained by this experiment, from which the age of gas emplacement was deduced to be later than 169 Ma.     

The age of the Gaojiacun mafic-ultramafic intrusive complex in the Yanbian area,Sichuan Province： Geochronological constraints by U-Pb dating of single zircon grains and ^40Ar／^39Ar dating of hornblende

The Gaojiacun mafic-ultramafic intrusivecomplex in the Yanbian area, Sichuan Province, is a strati-form intrusive body that has undergone intensive magmaticdifferentiation. This intrusive body involves two magmaticaccumulating cycles. Systematic U-Pb dating of single zircongrains and ~(40)Ar/~(39)Ar dating of hornblende were conducted,and the results showed that the age of hornblende gabbro,which was formed at the main phase of intrusion of the Gao-jiacun intrusive complex, is 840±5 Ma, casting doubt on theconcept of "Yanbian Ophiolite". It is believed that the for-mation of the Gaojiacun intrusive complex seems to be re-lated to a super-mantl3 plume underneath the su-per-continent Rodinia. The above research results are helpfulfor us to get a better understanding of the characteristics ofNeoproterozoic tectonic evolution of the Yanbian area inSichuan Province.     

Potash-rich volcanic rocks and lamprophyres in western Shandong Province: 40Ar39Ar dating and source tracing

Highly precise 40Ar39Ar dating results demonstrate that the ages of potash-rich volcanic rocks in western Shandong Province are 114.7-124.3 Ma, and that of the lamprophyres is 119.6 Ma. The potash-rich volcanic rocks have relatively high (87Sr/86Sr)i ratios ( 0.708715-0.711418)and distinctly negative εNd values ( -11.47 - -17.54), and are enriched in radiogenic lead (206Pb/204pb=17.341-17.622,207pb/204Pb=15.525-15.538, 208Pb/204pb = 37.563-37.684).Similarly, the lamprophyres also have quite low εNd values ( -11.57 - -19.64). Based on the fact that the Sr, Nd and Pb isotopic compositions of potash-rich volcanic rocks are very consistent with that of the clinopyroxene separates, and by integrating comprehensive analyses of their tectonic settings,and extensive comparisons of the Sr, Nd isotopic compositions with that of the related simultaneous rocks, it is concluded that the potash-rich volcanic rocks and lamprophyres in western Shandong Province were most possibly derived from the partial melting of enriched mantle which was caused by source contamination and metasomatism of subducted continental crustal materials.``     

Laser step-heating ^40Ar/^39Ar dating on young volcanic rocks

An attempt was made to use CO_2 laser step-heating method to date the late Pleistocene basaltic groundmass (DF-2) from Tengchong volcanic field. Among the fourteen heating steps, ten define a good inverse isochron (MSWD = 1.4) with an age of 32.2 ± 7.1 ka (2 σ).The inverse isochron also shows that the initial argon isotopic ratio is 297.1±2.0 (2σ) which is the same as the atmospheric argon at the 2σ error level. Study indicates that it is a useful means to date young volcanic groundmass with low K content by using CO_2 laser step-heating ~(40)Ar/~(39)Ar method.     

40Ar retention in the terrestrial planets

The solid Earth is widely believed to have lost its original gases through a combination of early catastrophic release and regulated output over geologic time. In principle, the abundance of 40Ar in the atmosphere represents the time-integrated loss of gases from the interior, thought to occur through partial melting in the mantle followed by melt ascent to the surface and gas exsolution. Here we present data that reveal two major difficulties with this simple magmatic degassing scenario--argon seems to be compatible in the major phases of the terrestrial planets, and argon diffusion in these phases is slow at upper-mantle conditions. These results challenge the common belief that the upper mantle is nearly degassed of 40Ar, and they call into question the suitability of 40Ar as a monitor of planetary degassing. An alternative to magmatism is needed to release argon to the atmosphere, with one possibility being hydration of oceanic lithosphere consisting of relatively argon-rich olivine and orthopyroxene.     

Laser microarea analysis of40Ar-39Ar isochron age for Fanshan superlarge alunite deposit, Zhejiang Province, China

Laser micro area analysis 40 Ar 39 Ar isochron dating for pure alunite is first reported. Micro area 40 Ar 39 Ar isochron dating results revealed that the metallogenetic epoch of Fanshan superlarge alunite deposit is 74.50 Ma, and this age is 10-20 Ma later than that of regional volcanic activity. Moreover, the potential application of micro area analysis 40 Ar 39 Ar isochron dating method in metallogenetic epoch studies for nonmetal deposits is also discussed.     

<Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar geochronology constraints on hydrocarbon accumulation and destruction periods in the Bankeng paleo-reservoir in the southern margin of the middle Yangtze block

The exact determination of isotopic ages of hydrocarbon accumulation,reconstruction and destruction periods using traditional isotopic dating methods is complex because of the small numbers of minerals that correlate with hydrocarbons.The quantitative and direct study of hydrocarbon geochronology is therefore an important scientific problem for isotope geochronology and petroleum geology.This study obtains two isotopic ages from a quartz vein sample associated with bitumen in a reverse fault located in the ...     

~(40)Ar/~(39)Ar age evidence for Altyn fault tectonic activities in western China

As the northern boundary of the Tibetan Plateau theAltyn fault is a huge NEE strike-slip fault belt in the inner Asian continent. Its formation and evolution are closelyrelated to the uplift of the whole plateau and the mass es-cape to the east. In recent years the Altyn fault has be-come a hot point of geological study in the Tibetan Pla-teau. The formation age of the Altyn fault was argued for a long time. Some researchers emphasized that the Altyn fault had commenced since early Paleo…