西藏南部羊应乡后碰撞火山岩40Ar/39Ar年龄及其地质意义

通过西藏羊应乡地热田4个高钾火山岩样品进行的快中子40Ar/39Ar 阶段升温定年结果,并结合其岩石地球化学研究数据,确定该地热田后碰撞火山作用发生在(10.73±0.09)～(11.40±0.11)Ma之间,形成与加厚下地壳,构造环境与岩石圈地幔减薄有关.通过对青藏高原南部冈底斯地区新近纪火山岩同位素年龄的展布的进一步分析,认为后碰撞高钾火山岩在该区广泛存在,并有由西向东逐渐年轻的趋势.     

福建省新生代玄武岩40Ar/39Ar年代学及地质意义

对福建省沿海至内陆出露的4件新生代玄武岩进行⁴⁰Ar/³⁹Ar年龄测试,对玄武岩中橄榄石斑晶进行电子探针测试;其中洋底和柳城玄武岩首次获得精确的⁴⁰Ar/³⁹Ar年龄数据.结果显示,闽西明溪⁴⁰Ar/³⁹Ar年龄数据为(1.44±0.11)Ma,柳城为(17.35±0.93)Ma;闽中洋底玄武岩年龄为(11.71±0.44)Ma;闽东厦门玄武岩年龄为(17.0±0.36)Ma.结合前人年龄数据,福建省新生代玄武岩整体表现出从闽东至闽西年龄逐渐减小的趋势.橄榄石斑晶电子探针数据显示闽东至闽西玄武岩碱性程度逐渐增加,可能指示岩浆熔融程度由沿海至内陆逐渐减少,岩石圈厚度由东向西逐渐增厚.最新获得的闽西柳城玄武岩⁴⁰Ar-³⁹Ar年龄为(17.35±0.93)Ma,与东部沿海玄武岩相似,推测可能是太平洋板块北西向俯冲时引起的岩浆扰动作用形成的.     

西藏南部羊应乡后碰撞火山岩40Ar/39Ar年龄及其地质意义

通过西藏羊应乡地热田 4个高钾火山岩样品进行的快中子4 0 Ar/39Ar阶段升温定年结果 ,并结合其岩石地球化学研究数据 ,确定该地热田后碰撞火山作用发生在 (10 .73± 0 .0 9)～ (11.4 0±0 .11)Ma之间 ,形成与加厚下地壳 ,构造环境与岩石圈地幔减薄有关 .通过对青藏高原南部冈底斯地区新近纪火山岩同位素年龄的展布的进一步分析 ,认为后碰撞高钾火山岩在该区广泛存在 ,并有由西向东逐渐年轻的趋势 .     

大同旧高山火山岩岩石学特征及构造意义

为了厘定山西大同旧高山火山岩的时代归属,探讨旧高山火山岩形成的构造环境及其构造背景,笔者对旧高山火山岩进行了野外地质调查、岩矿鉴定和主、微量元素地球化学特征研究,认为旧高山火山岩形成于早白垩世,为一套高钾钙碱性系列的中基性火山岩,火山作用以宁静式喷发为主;火山岩形成于大陆板内裂谷大地构造环境,岩浆来源于地幔与下地壳物质部分熔融的混合;火山岩的形成受控于晚中生代古太平洋板块向欧亚大陆俯冲的动力学背景。本结果对研究华北克拉通西北部的破坏具有重要意义。     

五台山-恒山地区繁峙玄武岩岩石学特征及构造意义

为了探讨滹沱河地区新生代的陆壳演化特征,在野外地质调查的基础上,开展了繁峙玄武岩的岩相学、岩石主微量元素地球化学研究。结果表明,繁峙玄武岩为一套钠质亚碱性-碱性系列的基性火山岩,形成于大陆板内裂谷的板块构造环境;繁峙玄武岩的岩浆来源于地幔物质的部分熔融与下地壳组分部分熔融的混合;繁峙玄武岩形成于太平洋板块向欧亚板块俯冲的动力学背景,标志在始新世-渐新世,滹沱河裂陷开始发育。深入探讨繁峙玄武岩的岩浆作用特征、物质来源和岩浆作用的构造环境,对揭示五台山-恒山地区的陆壳演化特点,具有重要的意义。     

西藏措勤地区典中组火山岩地球化学特征及构造背景

措勤盆地位于西藏冈底斯构造-岩浆岩带的西段北侧,该区古新世火山活动十分强烈,形成了一套厚度大于1 000 m的中酸性火山岩地层,火山岩K-Ar和Rb-Sr同位素年龄为63.9～61.0 Ma,区域上可与古近纪林子宗群下部层位相对比,归为古新统典中组.火山岩岩石化学、地球化学特征分析表明,该火山岩系属高钾钙碱性-钙碱性系列,轻稀土富集,负Eu异常明显.与原始地幔相比,微量元素Rb,Ba,K,Th,U富集;Ti,P,Sr,Ta(Nb)亏损.锶、氧同位素组成显示岩浆来源与陆壳物质关系密切,推测是来自于俯冲带的幔源基性岩浆与陆壳重熔酸性岩浆以不同比例混合所形成.综合研究认为,这套火山岩形成于陆缘弧构造环境,与喜马拉雅特提斯洋壳向北俯冲消减作用有关.     

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.     

国内期刊亮点

正青藏高原东北缘白银矿田表生黄钾铁矾40Ar/39Ar年代学和H-O同位素地球化学的初步研究中国科学院地质与地球物理研究所陈蕾和李建威对青藏高原东北缘祁连山地区白银矿田折腰山块状硫化物矿床风化壳中表生黄钾铁矾进行了40Ar/39Ar年代学和H-O同位素地球化学的初步研究,为祁连     

藏北新生代火山岩系列的地球化学及成因

自始新世（45Ma)印度板块和欧亚板块碰撞支来,藏北新生代火山活动相当频敏,在空间上自南向北随时间早晚依次形成了羌塘、可可西里和喀喇昆仑-玉门3个火山岩带,依岩石组合和地球化学特征可划分为5个火山岩系列：（1）钠质碱性玄武岩系列;（2）白榴石碧玄岩-响岩系列;（3）高钾钙碱性系列;（4）可可西里钾玄岩系列;（5）喀喇昆仑-玉门钾玄岩系列,通过对各系列火山岩成因的研究,可以作为了解新生代以来青藏高原岩石圈物质组成、壳幔结构以及高压隆升机制的重要窗口。     

冀北凤山晚古生代闪长岩-花岗质岩石的成因:岩石地球化学、锆石U-Pb年代学及Hf同位素制约

冀北凤山地区侵位于前寒武纪岩石组合中的晚古生代的闪长岩-花岗质岩石,具有富Na,高Sr,低Y和重稀土元素等地球化学特征,Sr/Y在37.15～151.22之间变化,绝大多数样品均显示正Eu异常(一个样品例外),Eu·在0.92～1.53之间变化.LA-ICP-MS锆石U-Pb同位素测年和锆石Hf同位素分析表明闪长岩(JB6024)和二长花岗岩(样品JB6037-1)分别形成于315±2.8Ma和306.6±6Ma,即该区晚古生代存在两个岩浆作用幕.地球化学、全岩Sr-Nd同位素和锆石Hf同位素研究揭示凤山晚古生代闪长岩是EMI型富集岩石圈地幔部分熔融形成的岩浆与古老下地壳部分熔融形成的长英质岩浆混合作用结果,而花岗质岩石则是闪长质岩浆发生分离结晶作用的残余岩浆同化上部地壳岩石形成的.     

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

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/³⁹Ar analyses 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.     

^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 thermochronology of two ductile shear zones from Yiwulüshan, West Liaoning Region: Age constraints on the Mesozoic tectonic events

Two ductile shear zones trending EW and NNE respectively not only controlled the tectonic framework of the northern North China, but also constrained the geodynamic background for gold mineralization in this region. Field observations and microstructural analyses reveal that the EW trending ductile shear zones are mainly contributed to dextral compressional deformation resulting from top-to- the-southeast oblique thrust shearing, whereas the NNE trending ones are genetically related to sinistral strike-slip and extensional faulting. One sample from the former yielded an 40Ar-39Ar plateau age of (219±4) Ma (Bi) and two samples from the latter gave 40Ar-39Ar plateau ages of (116±2) Ma (Bi) and (127±3) Ma (Bi). These ages provide constraints on the top-to-the-southeast oblique thrusting event occurring in Late Triassic and the sinistral extensional and strike-slip faulting event which occurred in Early Cretaceous.     

The 40Ar/39Ar age record of formation and uplift of the blueschists and eclogites in the western Tianshan Mountains

The 40Ar/39Ar ages indicate that the eclogite facies rocks of the Hasiate slice in the western Tianshan Mountains were formed at the early stage of Devonian (401 Ma) and had been uplifted to the greenschist facies tectonic level in the middle stage of Devonian (381 Ma). The formation and uplift of the blueschists of the Akesayi slice are constrained to the late stage of Devonian (370─364 Ma). The different tectonic slices in the high-pressure metamorphic belt have experienced the different uplift history.     

^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.     

^40Ar/^39Ar dating for Cenozoic kamafugite from western Qinling in Gansu Province

A suite of Cenozoic ultra-potassic volcanic rocks and carbonatites are widespread along the Lixian-Dang- chang areas of western Qinling in Gansu Province. There are abundant mantle-derived xenoliths within these ultra-potassic volcanic rocks. Systemmatic …     

The40Ar/39Ar age record of formation and uplift of the blueschists and eclogites in the western Tianshan Mountains

The⁴⁰Ar/³⁹Ar ages indicate that the eclogite facies rocks of the Hasiate slice in the western Tianshan Mountains were formed at the early stage of Devonian (401 Ma) and had been uplifted to the greenschist facies tectonic level in the middle stage of Devonian (381 Ma). The formation and uplift of the blueschists of the Akesayi slice are constrained to the late stage of Devonian (370-364 Ma). The different tectonic slices in the high-pressure metamorphic belt have experienced the different uplift history.     

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.``     

40Ar-39Ar thermochronology of two ductile shear zones from Yiwulüshan, West Liaoning Region: Age constraints on the Mesozoic tectonic events

Two ductile shear zones trending EW and NNE respectively not only controlled the tectonic framework of the northern North China, but also constrained the geodynamic background for gold mineralization in this region. Field observations and microstructural analyses reveal that the EW trending ductile shear zones are mainly contributed to dextral compressional deformation resulting from top-to-the-southeast oblique thrust shearing, whereas the NNE trending ones are genetically related to sinistral strike-slip and extensional faulting. One sample from the former yielded an ⁴⁰Ar–³⁹Ar plateau age of (219± 4) Ma (Bi) and two samples from the latter gave ⁴⁰Ar–³⁹Ar plateau ages of (116± 2) Ma (Bi) and (127±3) Ma (Bi). These ages provide constraints on the top-to-the-southeast oblique thrusting event occurring in Late Triassic and the sinistral extensional and strike-slip faulting event which occurred in Early Cretaceous.     

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.``