Age boundaries of formation of the Tomtor alkaline-ultramafic pluton: U-Pb and 40Ar/39Ar geochronological studies

This paper presents the results of geochronological studies of the Tomtor alkaline-ultramafic pluton, one of the largest Nb, Y, Sc, and TR deposits. A new scheme of its magmatism is given. The current K-Ar and Rb-Sr ages of different igneous rocks of the Tomtor pluton range from 800 to 250 Ma [Zaitsev et al., 1992; Frolov et al., 2003]. Such dispersion is probably related to the intense carbonatization of the rocks. The U-Pb zircon and ⁴⁰Ar/³⁹Ar mica ages indicate two stages of the formation of the pluton (700 and 400 Ma), which agrees well with the age of cycles of rift-related tectonogenesis of the Siberian platform.     

K-Ar法年龄标准物质ZGC粗面岩K和40 Ar*含量及年龄的定值结果

为了满足K-Ar和Ar-Ar法定年的需要,年代学工作者研制了一个用于中新生代定年的K-Ar法年龄标准物质——ZGC粗面岩。该粗面岩采自广东省南海市走马营第三纪火山岩。~(40)Ar/~(39)Ar定年结果表明,~(40)Ar~*在矿物晶格中保存均匀稳定,年龄谱平坦,~(39)Ar析出量高达97.9%。证明该粗面岩结晶以后未受过热扰动,完好地保持了~(40)K-~(40)Ar~*同位素计时的化学封闭体系。坪年龄为52.8±0.3 Ma,总气体年龄为52.9±0.8 Ma。~(36)Ar/~(40)Ar-~(39)Ar/~(40)Ar反等时线年龄为52.5±0.4 Ma,~(40)Ar/~(36)Ar初始值为296.6±4.2,此值与(~(40)Ar/~(36)Ar)。大气氩丰度比(295.5±0.5)一致,表明样品不含过剩氩。国内12个实验室参加了ZGC粗面岩K含量和~(40)Ar~*含量的定值分析。经统计学方法检验,结果显示全部定值数据服从正态分布并具等精度。当置信概率为0.95时,~(40)Ar~*和K含量的相对标准偏差都小于1%。~(40)Ar~*和K含量分析的认定值和不确定度分别为:~(40)Ar~*=4.199±0.022×10~(-10)mol/g,K=4.576±0.028%,由此计算得K-Ar年龄为52.2±0.5 Ma。根据国家一级标准物质技术规范的相关要求,经统计学方法检验在0.05显著性水平下,K和~(40)Ar~*的F分布小于F临界值,说明该标准物质是均匀的。t检验法表明,该标准物质具有良好的稳定性,~(40)Ar~*和K含量在有效期内不会发生显著性变化。ZGC粗面岩粒度为0.3～0.7mm,重量为850 g,缩分成最小样品单元100瓶,每瓶8.5 g。可供K-Ar和Ar-Ar法实验室使用43年。     

海绿石 K-Ar、40Ar/39Ar 年代学研究

海绿石是与沉积岩同时形成的高钾层状硅酸盐自生矿物，其 K-Ar、40Ar/39Ar 年龄存在失真现象。笔者认为，引起年龄失真的主要原因是由于该矿物结构中含有可膨胀层。通过测量可膨胀层含量及中子活化过程中39Ar的丢失率，使可对海绿石常规 K-Ar、40Ar/39Ar 年龄进行校正，得到其年龄校正公式。     

湖北省随州三里岗地区二长花岗岩Rb-Sr、40Ar/39Ar同位素年龄

三里岗二长花岗岩与花山蛇绿混杂岩中的基性火山岩呈侵入接触关系,其年龄的确定可解决花山蛇绿混杂岩中基性岩形成年龄的上限问题. 本文对三里岗二长花岗岩分别进行了Sm、Nd同位素研究及Rb-Sr、40Ar/39Ar同位素年龄测定.3个全岩样品的Nd模式年龄平均值为1 064±105 Ma;12个全岩样品的Rb-Sr等时线年龄为422±53(2σ)Ma;二长花岗岩中所含角闪石矿物的40Ar/39Ar坪年龄为141.4±0.3 Ma,等时线年龄为142±2 Ma.这些结果暗示了花山蛇绿混杂岩中基性火山岩的形成年龄不可能晚于422 Ma.     

U-Pb and 39Ar/40Ar dating and Sm-Nd and Pb-Pb isotopic study of the Kalguty molybdenum-tungsten ore-magmatic system, Southern Altai

The Kalguty ore-magmatic system comprises two intrusive complexes: the Kalguty granite-leucogranite complex and Eastern Kalguty complex of dikes and small intrusions. U-Pb dating of individual zircon grains from granites of the main intrusive phase demonstrated that the crystallization age of small grains of magmatic habits and outer rims of large grains is almost concordant and is 216 ± 3 Ma. Ar-Ar isotope study shows that the K-Ar system of biotites from granites of the main phase within the Kalguty ore field was disturbed (radiogenic Ar was partially lost) and gave an age of 202 ± 1 Ma. The Ar-Ar dating of muscovites from intraore and postore dikes of the Eastern Kalguty complex devoid of signatures of postmagmatic recrystallization and superimposed greisenization gave similar ages of 205–201 Ma. This date is considered as the emplacement age of the Eastern Kalguty dikes and associated complex W-Mo-Bi-Be ore mineralization. Sm-Nd and Pb-Pb isotopic study of granites, ongonites, and elvans of the Kalguty ore-magmatic system and host rocks shows that these systems were closed. For example, recalculation of Nd isotopic ratios for corresponding ages of crystallization of magmatic systems (216 and 205 Ma) shows that ?_Nd(T) values decrease from ?1.9 to ?3.5 ... ?5.08 with transition from granite-leucogranite to subvolcanic granite porphyry, ongonite, and elvan dikes with corresponding increase of model ages of protoliths from 1.0 to 1.25 Ga. Lead isotopic ratios for leaching residues of whole-rock samples of all rock varieties (²⁰⁶Pb/²⁰⁴Pb = 18.305–18.831; ²⁰⁷Pb/²⁰⁴Pb = 15.527–15.571) are plotted well below the line of average crustal lead evolution according to the Stacey-Kramers model.     

40Ar/39Ar and Rb-Sr geochronology of the Qingyang batholith,Anhui Province,China

The geochronology and genesis of the Qingyang batholith were investigated using⁴⁰Ar/³⁹Ar and Rb-Sr isotopic techniques. The Qingyang is a composite batholith consisting of two major rock types granodiorite and granite in the Yangtze fold belt.⁴⁰Ar/³⁹Ar spectra for biotite and amphibole separates are internally concordant. The concordance of the minerals and spectra indicate no thermal disturbance of the ages, and rapid cooling of the rocks. The granodiorite has an age of 137.6±1.4 m.y. and the granite 122.7±1.2 m.y. Whole-rock Rb-Sr analysis yields ages consistent with the⁴⁰Ar/³⁹Ar dates. Thus, the Qingyang batholith was formed in two major stages in the late Jurassic and early Cretaceous. The batholith is not Triassic as was previously proposed. Special⁴⁰Ar/³⁹Ar analysis of two granodiorite samples has precisely documented a 1.0 m.y. apparent age difference between these samples. Several factors could account for this difference, but different emplacement times seem most convincible. The granodiorite and granite show little variation in initial⁸⁷Sr/⁸⁶Sr ratio (about 0.7085). The high initial Sr ratios suggest that the magmas were formed by anatexis of older crustal materials.     

Whole-rock geochemical,U-Pb and Sm-Nd isotope characteristics of the Dongueni Mont nepheline syenite intrusion,Mozambique

The Dongueni Mont nepheline syenite intrudes migmatitic paragneisses and siliciclastic metasediments of the Barue Complex, Mozambique. This study reports the whole-rock geochemical, U-Pb and Nd isotopic data of the nepheline syenite. The ferroan and alkalic geochemical characteristics are typical of alkaline rocks formed in a within-plate setting. The strong depletion in high field strength elements(HFSEs)(e.g. Ba, Nb, P,and Ti) and enrichment in large ion lithophile elements(LILEs)(e.g. Rb, Th, K, and Pb) are consistent with magmatism in a continental alkaline magmatic province associated with intracontinental rifting. Zircon U-Pb data yielded crystallization ages from 498 ± 19 to 562± 14 Ma,consistent with the Pan-African Orogeny and the inherited zircons yield an age of 1040 Ma, which supports the presence of a Mesoproterozoic crust. Theε_(Nd)(t) values from the nepheline syenite samples range from-15.1 to-16.1 and the T_(DM)values from 1.77 to 1.67 Ga, which indicate that the initial nepheline syenite magma formed from a tholeiitic or mantle source in a within-plate setting with crustal assimilation.     

阿尔金铅、钕、锶、氩、氧同位素研究及其早期演化

根据铅、钕、锶、氩、氧等同位素体系及其年代学研究，阿尔金在其早期演化阶段中存在２７９０—２５９０、１８８３—１７０４、５７５和４７０—４２０Ｍａ４个构造热事件，各自具有不同的性质和时代意义。     

A K-Ar and 40Ar/39Ar study on white micas from the Brixen Quartzphyllite,Southern Alps

The Brixen Quartzphyllite, basement of the Southern Alps (Italy), consists of metasediments which had suffered progressive deformation and low grade metamorphism (p _max4 kbar, T _max375±25° C) during the Palaeozoic. It has been excavated by pre-Permian erosion, buried again beneath a pile of Permo-mesozoic to Cainozoic sediments (estimated T _max150° C), and is now exposed anew due to late Alpine uplift and erosion. The behavior of the K-Ar system of white micas is investigated, taking advantage of the narrow constraints on their thermal history imposed by the geological/stratigraphic reference systems.The six structurally and petrographically differing samples come from a single outcrop, whose position is roughly two kilometers beneath the Permian land-surface. White mica concentrates from five grain size fractions (<2 , 2–6 , 6–20 , 20–60 , 60–75 ) of each sample have been analyzed by the conventional K-Ar method, four selected concentrates additionally by the ⁴⁰Ar/³⁹Ar stepwise heating technique; furthermore, Ar content and isotopic composition of vein quartz were determined.The conventional ages of the natural grain size fractions (20–60 , 60–75) are in the range 316±8 Ma, which corresponds to the ⁴⁰Ar/³⁹Ar plateau age of 319.0±5.5 Ma within the error limits. The finer grain size fractions yield significantly lower ages, down to 233 Ma for fractions <2 . Likewise low apparent ages (down to 83 Ma) are obtained for the low temperature ⁴⁰Ar/³⁹Ar degassing steps.There is no correlation between microstructural generation of white mica prevailing in the sample and apparent age. This favours an interpretation of the 316±8 Ma values as cooling age; progressive deformation and metamorphism must be respectively older and their timing cannot be resolved by these methods. The data preclude any significant influence of a detrital mica component as well as of excess argon.The lower ages found for the fine grain-size fractions (respectively the low-T degassing steps) correspond to a near-surface period (p-T-minimum); the values are geologically meaningless. The effect is interpreted to result from partial Ar loss due to reheating during Mesozoic-Cainozoic reburial. A model based on diffusion parameters derived from the outgassing experiments and Dodson's (1979) equation yields a closure temperature of 284±40 °C for a cooling rate of 18° C/Ma. Furthermore, this model suggests that the observed argon loss of up to 5% may in fact have been induced by reheating to 150 °C for 50 Ma.     

Dating of deformation phases using K-Ar and 40Ar/39Ar techniques: results from the northern apennines

Paleozoic to Oligocene metasedimentary rocks present in the Alpi Apuane region of the Northern Apennines, Italy, have been sequentially deformed during a Tertiary progressive deformation. In an attempt to date the individual deformation episodes, over 50 conventional K-Ar and 11⁴⁰ Ar/³⁹Ar incremental gas release analyses have been carried out on fine grained white micas separated from samples whose structural settings were well known. Mineralogy, X-ray diffractometry, and thin-section analyses indicate that the constituent muscovite and phengite formed under metamorphic conditions of 3–4 kbars and 300–400°C during all deformational phases. Pre-existing micas were variably crenulated during each subsequent deformational phase. Both K-Ar and ⁴⁰Ar/³⁹Ar analyses were carried out on 0.6-2μm, 2–6 μm and 6–20 μm size separates of the phengitic white mica. Although the K-Ar apparent ages range from 11 to 27 Ma and are consistent with available stratigraphic constraints, the ⁴⁰Ar/³⁹Ar age spectra display variable internal discordancy. These isotopic data indicate that: (1) both the K-Ar and ⁴⁰Ar/³⁹Ar total-gas ages decrease as the degree of crenulation increases; (2) the K-Ar and ⁴⁰Ar/³⁹Ar total-gas ages decrease as grain size decreases; (3) for each sample, characteristics of the ⁴⁰Ar/³⁹Ar age spectra depend upon grain size, with fine sizes yielding discordant patterns which systematically increase in apparent age from low to high temperature and (4) phengitic micas associated with earliest structures yield generally older ages than micas associated with later structures.The isotopic results are interpreted to indicate that the major deformation phase (D₁) occurred at approximately 27 Ma with subsequent pulses ending by c. 10 Ma. These results may be combined with finite strain data to suggest that the region was deformed at strain rates between 10⁻¹⁵ and 10⁻¹⁴ s⁻¹. A 27 Ma age indicates Mid-Oligocene initiation of plate tectonic activity in the Western Mediterranean and concomitant deformation in the Northern Apennines.     

Magmatic-hydrothermal evolution of the Cretaceous Duolong gold-rich porphyry copper deposit in the Bangongco metallogenic belt,Tibet: Evidence from U-Pb and 40Ar/39Ar geochronology

The Duolong gold-rich porphyry copper deposit was recently discovered and represents a giant prospect (inferred resources of 4–5 Mt fine-Cu with a grade of 0.72% Cu; 30–50 t fine-gold with a grade of 0.23 g/t Au) in the Bangongco metallogenic belt, Tibet. Zircon SHRIMP and LA-ICP-MS U–Pb geochronology shows that the multiple porphyritic intrusions were emplaced during two episodes, the first at about 121 Ma (Bolong mineralized granodiorite porphyry (BMGP) and barren granodiorite porphyry (BGP)) and the second about 116 Ma (Duobuza mineralized granodiorite porphyry (DMGP)). Moreover, the basaltic andesites also have two episodes at about 118 Ma and 106 Ma, respectively. One andesite yields an U–Pb zircon age of 111.9 ± 1.9 Ma, indicating it formed after the multiple granodiorite porphyries. By contrast, the ⁴⁰Ar/³⁹Ar age of 115.2 ± 1.1 Ma (hydrothermal K-feldspar vein hosted in DMGP) reveals the close temporal relationship of ore-bearing potassic alteration to the emplacement of the DMGP. The sericite from quartz-sericite vein (hosted in DMGP) yields a ⁴⁰Ar/³⁹Ar age of 115.2 ± 1.2 Ma. Therefore, the ore-forming magmatic-hydrothermal evolution probably persisted for 6 m.y. Additionally, the zircon U–Pb ages (106–121 Ma) of the volcanic rocks and the porphyries suggest that the Neo-Tethys Ocean was still subducting northward during the Early Cretaceous.     

Age and thermal history of the Maksyutov metamorphic complex: 40Ar/39Ar evidence

The ⁴⁰Ar/³⁹Ar method with stepwise heating was used to date phengite and glaucophane in the contact zone of garnet glaucophanite an omphacite-garnet rock (eclogite) from the lower unit of the Maksyutov metamorphic complex. The correlation of the measured age and the sizes of the phengite flakes indicates that the behavior of radiogenic Ar in them was controlled by the mechanisms of volumetric diffusion. Taking into account the fact that all of the rocks have the same thermal history, the dates most close to the age of metamorphism are those of the largest phengite flakes from garnet glaucophanite: 392 Ma. The age values obtained on phengite from an omphacite-garnet rock sampled at the maximum distance from the contact are equal to 378 Ma and correspond to the time when the rocks cooled to temperatures below 350°C. The results of numerical simulations indicate that the metamorphic age is no younger than 400 Ma, and the linear cooling rate can be estimated at 3.40 ?0.75/+1.24°C/m.y. The maximum values of the phengite ages are consistent with the dates of glaucophane from three rock samples: 389–411 Ma.     

云开群硅质岩的40Ar/39Ar年龄及其地质意义

粤西云开群浅变地层年代的确定对于研究华南地壳演化有重要意义。以往的同位素年龄值多测自其中的顺层火成岩，故对其他质意义有不同的理解。本文首次测得了云开群中硅质岩的40Ar/39Ar年龄，从而提供了云开群的沉积年代。     

40Ar/39Ar ages in deformed potassium feldspar: evidence of microstructural control on Ar isotope systematics

Detailed field and microstructural studies have been combined with high spatial resolution ultraviolet laser ⁴⁰Ar/³⁹Ar dating of naturally deformed K-feldspar to investigate the direct relationship between deformation-related microstructure and Ar isotope systematics. The sample studied is a ~1,000 Ma Torridonian arkose from Skye, Scotland, that contains detrital feldspars previously metamorphosed at amphibolite-facies conditions ~1,700 Ma. The sample was subsequently deformed ~430 Ma ago during Caledonian orogenesis. The form and distribution of deformation-induced microstructures within three different feldspar clasts has been mapped using atomic number contrast and orientation contrast imaging, at a range of scales, to identify intragrain variations in composition and lattice orientation. These variations have been related to thin section and regional structural data to provide a well-constrained deformation history for the feldspar clasts. One hundred and forty-three in-situ ⁴⁰Ar/³⁹Ar analyses measured using ultraviolet laser ablation record a range of apparent ages (317-1030 Ma). The K-feldspar showing the least strain records the greatest range of apparent ages from 420-1,030 Ma, with the oldest apparent ages being found close to the centre of the feldspar away from fractures and the detrital grain boundary. The most deformed K-feldspar yields the youngest apparent ages (317-453 Ma) but there is no spatial relationship between apparent age and the detrital grain boundary. Within this feldspar, the oldest apparent ages are recorded from orientation domain boundaries and fracture surfaces where an excess or trapped ⁴⁰Ar component resides. Orientation contrast images at a similar scale to the Ar analyses illustrate a significant deformation-related microstructural difference between the feldspars and we conclude that deformation plays a significant role in controlling Ar systematics of feldspars at both the inter- and intragrain scales even at relatively low 'bulk' strains. The data show that Ar loss and trapping within the deformed K-feldspars reflects the presence of a deformation-induced population of small diffusion domains in combination with 'short-circuit' diffusion along deformation-induced defects. The complex history of microstructures induced in the K-feldspars during their cooling, alteration, erosion and sedimentation do not appear to be as significant as deformation-induced microstructures in controlling the distribution of apparent ages at the grain scale.     

40Ar/39Ar dating of cleavage formation in tuffs during anchizonal metamorphism

⁴⁰Ar/³⁹Ar incremental-release analyses were carried out on whole-rock and constituent white mica (illite)-rich size fractions (0.63–1 to 6.3–20 m) within two very-low grade, penetratively cleaved metatuffs of contrasting anchizonal metamorphic grade (northeastern Rheinisches Schiefergebirge, Federal Republic of Germany). One sample from the upper anchizone displays internally concordant ⁴⁰Ar/³⁹Ar spectra with plateau ages ranging between ca. 316 and 325 Ma. These are similar to conventional K-Ar ages determined for the whole-rock and size fractions. Together the isotopic results suggest that cleavage formed at ca. 320 Ma during a concomitant very-low grade metamorphism. This is consistent with biostratigraphic controls which suggest that metamorphism and cleavage formation occurred during the Westphalian.A metatuff sample from the middle anchizone records more internally discordant ⁴⁰Ar/³⁹Ar age spectra with total-gas ages ranging from 366 to 372 Ma. These are ca. 35–45 Ma older than corresponding conventional K-Ar ages, indicating marked recoil-loss of ³⁹Ar occurred during irradiation. Transmission electron microscopy reveals that white mica grains within size fractions from the upper anchizone sample have clearly defined, straight edges whereas those within the middle anchizone samples are embayed and diffuse. This results in an increase in surface/volume ratio and therefore greater susceptibility for recoil-loss of ³⁹Ar in the middle anchizone sample. Grain-edge morphology appears to be a major factor in determining the extent of recoil-loss of ³⁹Ar during ⁴⁰Ar/³⁹Ar analysis of fine-grained size fractions.     

柴北缘锡铁山韧性剪切带的基本特征及其形成时代

在柴北缘锡铁山地区元古宇达肯大坂群与奥陶系滩涧山群之间发育一条呈NW向展布的基底韧性剪切带。本文对该韧性剪切带中的宏观构造(鞘褶皱、不对称褶皱、塑性流变组构、S-C组构、旋转碎斑系、构造透镜体、粘滞型石香肠构造)和糜棱岩微观构造(云母鱼构造、压力影构造、动态重结晶、雪球状旋转构造、波状消光和云母扭折)特征作了详细的研究。对锡铁山基底韧性剪切带中花岗质糜棱岩进行白云母Ar-Ar同位素测年,获得坪年龄为398±4 Ma,等时线年龄为399±4 Ma,二者基本一致,代表了锡铁山基底韧性剪切带的形成时间。综合上述特征判断,锡铁山韧性剪切带是一条由元古宇达肯大坂群向SW斜向逆冲所形成的右行逆冲韧性剪切带,该韧性剪切带与早古生代柴达木地块向NE斜向俯冲碰撞造山作用有关,并可能导致了超高压变质体折返到浅部地壳层次。同样,该韧性剪切带的存在,也对锡铁山铅锌矿起到了十分重要的后期构造控制作用。     

庐山变质核杂岩伸展拆离和岩浆作用的年代学约束

庐山变质核杂岩是华南中生代岩石圈减薄事件的浅层响应。研究伸展拆离和岩浆作用在其形成过程中的相互关系,对于认识地壳浅层的伸展减薄过程和变质核杂岩的形成机制具有重要的意义。在宏观和显微构造研究的基础上,分别对基底拆离断层和玉京山岩体进行了40Ar/39Ar年代学和锆石U-Pb年代学研究,为伸展拆离和岩浆作用提供了年代学约束,主要获得了以下认识:(1)变质核杂岩核部玉京山花岗岩体的锆石LA-ICP-MSU-Pb年龄为(133.0±2.1)Ma,代表了变质核杂岩核部岩浆活动的启动时间。(2)基底拆离断层下盘韧性剪切带内的白云母40Ar/39Ar年龄(140.4±1.7)Ma代表了庐山变质核杂岩的形成时间,也代表了庐山地区岩石圈伸展减薄的时间,即岩石圈伸展减薄始于早白垩世。(3)庐山变质核杂岩是中生代不同尺度的伸展拆离和岩浆活动共同作用的结果,在地壳浅层的具体形成过程中伸展拆离早于岩浆侵位,基底拆离断层的减压效应为深部岩浆的上升侵位创造了条件,岩浆侵位造成山体的隆升,又引发了盖层的重力滑脱。