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1.
东喜马拉雅构造结岩体冷却的~(40)Ar/~(39)Ar年代学研究   总被引:2,自引:1,他引:1  
对采自东喜马拉雅构造结核心地段雅鲁藏布大峡谷地区的13件标本中的20件矿物样品进行了系统的常规^40Ar/^39Ar年代学研究。数据显示,样品的(^40Ar/^39Ar)i值均接近尼尔值(295.5±5),且绝大部分样品的坪年龄与其反等时线年龄在误差范围内一致。从数据统计结果来看,所测样品的^40Ar/^39Ar年龄大都集中在1.3Ma和2.5Ma左右,表明南迦巴瓦地区在上新世中期和更新世早期均经历了快速冷却抬升事件。本次测试的样品采自不同的高程及不同的构造单元,且样品原岩的成因及岩性各异,但沿着大峡谷由北向南不同地段的样品的不同矿物(角闪石、黑云母、白云母、钾长石)的^40Ar/^39Ar年龄相近,而同一样品中不同矿物的^40Ar/^39Ar年龄大小又并非完全按照矿物对氩同位素体系的封闭温度高低来分布,表明该地区在上新世以来的岩体冷却速率很大,以致该地区的矿物对氩同位素体系的封闭过程与处于缓慢冷却环境中的封闭过程明显不同。以本文报道的数据估算,南迦巴瓦地区的岩体在最近3Ma以来的冷却速率达120~240℃/Ma,岩体抬升速率达3.4—6.9mm/a。  相似文献   

2.
对取自云南三江地区碧罗雪山与临沧岩体和西藏冈底斯岩带曲水岩体的黑云母、斜长石、钾长石与角闪石进行了^40Ar/^39Ar坪年龄测定。结果表明三个岩体的形成对代分别为420、234和43.4Ma。通过热历史与封闭温度研究表明碧罗雪山与临沧岩体在85—90Ma对经历了相同的热动力作用事件,代表了巽他次板块与欧亚板块的碰撞时间。曲水岩体在40-44Mt期间发生了快速隆起(2.6mm/a),则标志着印度与欧亚板块的碰撞时间。  相似文献   

3.
迁安紫苏花岗岩的~(40)Ar/~(39)Ar年龄谱   总被引:1,自引:1,他引:1  
对采自河北省迁安县水厂地区的紫苏花岗岩中的黑云母和紫苏辉石进行了~(40)Ar/~(39)Ar年龄测定,分别给出了18.7亿年和19.6亿年的~(40)Ar保存年龄。这两种矿物的年龄谱的视年龄的梯度变化表明,紫苏花岗岩形成后是缓慢冷却的。3.9亿年左右的一次热事件,造成了放射成因~(40)Ar的丢失。根据热历史和封闭温度的研究,从27亿年(侵入到该区紫花岗岩中的花岗闪长岩的锆石U-Pb年龄)到19.6亿年,紫苏花岗岩岩体的抬升速率为6.5m/Ma,但从19.6亿年到18.7亿年,其抬升速率高达111m/Ma,具有明显的构造抬升作用。  相似文献   

4.
豫西蛇尾秦岭群角闪石^40Ar/^39Ar年龄谱及其地质意义   总被引:2,自引:2,他引:2  
河南西峡县北部蛇尾地区秦岭群变质基性岩中,普通角闪石~(40)Ar/~(39)Ar快中子活化年代测定获得了353.1±0.2Ma的坪年龄。平坦的年龄坪表明,加里东热事件一直延续到早海西阶段,随着秦岭群自北而南逆冲到古生界二郎坪群所导致的快速隆起抬升、温度急剧冷却才终结。  相似文献   

5.
郯庐断裂带肥东段走滑运动的40Ar/39Ar法定年   总被引:32,自引:2,他引:32       下载免费PDF全文
郯庐断裂带在华北与华南板块碰撞之后是否发生过大规模左行平移及其准确的时间,仍然是存在着争议的重要问题。郯庐断裂肥东段地表出露了大规模的、北北东走向的韧性剪切带。野外构造、显微构造与石英C轴组构分析皆指示为左旋走滑韧性剪切带。糜棱岩中变形矿物组合与矿物变形行为指示该韧性剪切带形成于中—高绿片岩相环境。通过对该段走滑糜棱岩中角闪石与黑云母的40Ar/39Ar年代学研究,本次工作中获得了一个角闪石(N14)的40Ar/39Ar坪年龄为143.3±1.3Ma(早白垩世初),据变形温度判断其代表了该断裂带左行平移中的变形年龄。该糜棱岩(N14)中新生的黑云母40Ar/39Ar坪年龄为134.1±0.6Ma,而同一采场中另一糜棱岩(N13)中新生黑云母40Ar/39Ar坪年龄为130.3±0.6Ma,皆指示了左行平移活动的冷却年龄。另外4处走滑糜棱岩中黑云母的40Ar/39Ar坪年龄分别为137.2±0.8Ma(N47)、135.6±0.6Ma(N17)、124.8±0.7Ma(N21)和125.9±0.4Ma(N22),也都属于冷却年龄,反映了该走滑韧性剪带内的不均匀冷却现象。由此变形年龄与冷却年龄可以判断该断裂带的左行平移持续时间不超过6Ma左右。由N14糜棱岩中的角闪石与黑云母坪年龄得到该处的平均冷却速率为21.7℃/Ma,属于较快速的冷却。本次40Ar/39Ar测年结果,更可靠地证实了郯庐断裂  相似文献   

6.
本文以浙江长兴地区P—T界线上的粘土岩为研究对象,对其中的成岩混层I/S进行了K—Ar和Ar—Ar测年实验研究,结果表明,沉积岩中成岩混层I/S的K—Ar年龄在没有碎屑含钾矿物混入的情况下,比其对应的地层时代要年轻。成岩混层I/S的常规^40Ar/^39Ar阶段升温测年一般情况下得不到年龄坪,只有当类蒙皂石层的含量为零时(即没有膨胀层的成岩伊利石),才能形成平坦的年龄谱。^39Ar的核反冲丢失不仅发生在矿物表面,也发生在矿物内部,最高可达48%左右,0.2μm粒级可能是^39Ar核反冲丢失量的拐点,同时也表明,用快中子照射硅酸盐样品,核反冲距离可以达到0.2μm。  相似文献   

7.
辽东半岛出露大量片麻状花岗岩,其锆石U-Pb年龄为180~157Ma,该套岩石的最大特点表现在岩石明显经历了早期上盘向NW推覆和后期近EW向伸展作用的韧性改造,然而其变形时代却一直未得到确定。本文以丹东市西南部黑沟二云母二长花岗岩岩体为例,通过激光^40Ar/^39Ar年代学研究探讨该岩体的变形时代。激光^40Ar/^39Ar定年结果表明,黑沟岩体经历的早期推覆、挤压事件发生在~143Ma,而后期地壳伸展作用则发生在121~113Ma,并且该区在早白垩纪期间经历了快速冷却、抬升过程。从而表明在晚侏罗纪-早白垩纪期间(143~113Ma)辽东半岛经历了区域构造体制变革。结合前人大地构造研究成果,本文认为辽东半岛晚侏罗纪-早白垩纪NW向推覆、挤压事件是古太平洋板块向欧亚大陆俯冲作用的结果,而晚期(早白垩纪)的地壳伸展事件是古太平洋板块俯冲作用的转向和变速、华北东部岩石圈较薄以及挤压后地壳松弛等综合作用的结果。  相似文献   

8.
大青山逆冲推覆构造下盘发育一套泥质板岩,它们是由二叠系泥岩、粉砂质泥岩和粉砂岩、细砂岩遭受逆冲推覆变形引起的动力变质作用改造形成的,发育一组透入性劈理构造.通过岩石薄片鉴定,沿劈理面形成绢云母、白云母和绿泥石等同构造新生矿物.对其中新生白云母矿物进行了激光微区40Ar/39Ar测年,获得了(121.6±1.6)Ma的等时线年龄.野外地质证据表明了大青山逆冲断层发生过两期逆冲推覆变形作用,早期发生在印支期,晚期发生在燕山期.(121.6±1.6)Ma年龄代表燕山期逆冲推覆事件发生时间.  相似文献   

9.
Reater  A 李才 《世界地质》1993,12(4):49-55
在较低级变质泥质岩石中利用全岩的K—Ar和~(40)Ar/~(39)Ar分析方法来测定劈理形成年龄时,由于矿物样品的特殊性和碎屑颗粒间潜伏氩的存在,使测定工作变得复杂。复杂的矿物影响可通过仔细分辨K/Ca和~(40)Ar/~(39)Ar的年龄光谱加以解决。然而,碎屑的影响可能继续存在,甚至在具有渗透性裂开的变质泥质样品中(全岩和白云母,粒级均在0.4~0.63μm之间)也不例外,直到浅变质带(或浅带)的边缘这种影响才被消除。伴有浅变质裂开的凝灰岩夹层其~(40)Ar/~(39)Ar和K—Ar法测定的全岩劈理年龄与地层古生物学测定的劈理年龄相一致,这说明K—Ar和~(40)Ar/~(39)Ar法不受碎屑的影响。在粒级上与上述浅变质凝灰岩相区别,富含白云母的另一种凝灰岩具有下面特征,颗粒间界限明显,K—Ar和~(40)Ar/~(39)Ar全气体年龄与上述凝灰岩一致,不随颗粒的大小发生变化。近带高变质凝灰岩由边界不清的白云母颗粒组成,它们连续记录了~(40)Ar/~(39)Ar全气体年龄,其中~(40)Ar/~(39)Ar值比K—Ar值高10%~15%(全岩粗碎屑颗粒),这一差异是由于在辐射中受反冲气体~(39)Ar的影响,并表明颗粒的边界形态(有效表面积/体积)在很大程度上控制了~(39)Ar的反冲。  相似文献   

10.
对东秦岭松树沟蛇绿岩残片中超镁铁质岩中的辉石巨晶化学成分的系统研究 ,表明其具有高压变质成因的特点 ;采用 4 0 Ar/ 39Ar快中子活化法得其高温坪年龄为 833.8± 4 Ma,等时线年龄为 84 8.2± 4 Ma。考虑到辉石对 Ar的封闭温度(6 2 0℃ )明显低于该岩片早期塑性变形变质温度 (≥ 80 0℃ ) ,因此该年龄不代表形成辉石巨晶的峰期高压变质时代 ,而代表了该岩体变质后初始抬升的冷却事件。这一年龄值与区内蛇绿岩中的镁铁质变质岩获得的变质年龄 (983± 14 0 Ma)在误差范围内大体一致 ,暗示松树沟蛇绿岩片可能整体经历了俯冲至下地壳深度遭受高压变质后再抬升的地质演化过程  相似文献   

11.
The paragenic minerals plagioclase,perthite,biotite,hornblende and pyroxene in acid-granite alkali-granite,monzonite and volcanic rocks collected from seven areas different in thermal history have been determined.On the basis of 14 plateau age spectra and isochron ages of paragenic miner-als in conjunction with the observed mineral textures ,the suitability of plateau age spectra of plagioclase and perthite and their thermo-chronological significance are discussed in this paper.The results indicate that undisturbed feldspar gives satisfactory^40Ar-^39Ar plateau ages in consis-tence with those of paragenic minerals.This means that feldspars from the undisturbed area are suita-ble for ^40Ar-^39Ar dating .On the other hand,the age spectra of feldspars as well as of biotite,pyroxene and hornblende affected by tectonic or thermodynamic events appear unsmooth at varying temperatures,thus complicating their interpretation.Feldspars may give an emplacement age of a rock and /or that of the latest thermodynamic event,depending on the intensity of the event and the retentivity of Ar in the mineral.  相似文献   

12.
王松山 《地质科学》1987,(4):364-373
40Ar/39Ar年龄谱是研究陨石冲击事件的重要资料。根据对55块陨石40Ar/39Ar冲击年龄和陨石暴露年龄的分析,发现陨石的冲击年龄与陨石类型之间存在对应关系。据此,将陨石冲击事件划分为九期。其中3900-4000Ma、470-540Ma和小于65Ma是陨石母体的三个重要演化阶段。阶段Ⅰ、Ⅱ和Ⅲ(冲击年龄大于30亿年)主要涉及高钙型无球粒陨石。所有球粒陨石的冲击年龄均小于30亿年。陨石暴露年龄因类型而异,铁陨石最大,石铁陨石次之,石陨石最小。  相似文献   

13.
根据伊利石K-Ar测年法研究,确定鄂尔多斯盆地存在两期与构造运动有关的成岩作用事件:即与早期燕山运动有关的侏罗纪(170—160Ma)和与晚期加里东运动有关的志留-泥盆纪(420-370Ma)成岩作用时代。为了揭示碎屑物质对K-Ar年龄的影响和伊利石成岩作用的机制,本文提出了一个通过观察K-Ar年龄随深度变化的趋势来进行判断的模式。在K-Ar年龄小于地层时代的条件下,K-Ar年龄与深度呈正相关,或者很相近的K-Ar年龄与深度变化无关均标志着没有或很少有碎屑物质的影响;同时,前者指示逐渐埋藏条件下的成岩作用,而后者则反映短暂的热事件引起的成岩作用事件。  相似文献   

14.
 Ultra-high pressure eclogite/amphibolite grade metamorphism of the Dora Maira Massif in the western Alps is a well established and intensively studied event. However, the age of peak metamorphism and early cooling remains controversial. The 40Ar-39Ar step-heating and laser spot ages from high pressure phengites yield plateau ages as old as 110 Ma which have been interpreted as the time of early cooling after the high pressure event. Recent U/Pb and Sm/Nd results challenge this assertion, indicating a much younger age for the event, around 45 Ma, and hence a radically different timing for the tectonic evolution of the western Alps. In a new approach to the problem, samples from the undeformed Hercynian metagranite, Brossasco, were studied using an ultra-violet laser ablation microprobe technique for 40Ar-39Ar dating. The new technique allowed selective in situ analysis, at a spatial resolution of 50 μm, of quartz, phengite, biotite and K-feldspar. The results demonstrate the frequent occurrence of excess argon with high 40Ar-36Ar ratios (1000–10000) and a strong relationship between apparent ages and metamorphic textures. The highest excess argon ratios are always associated with high closure temperature minerals or large diffusion domains within single mineral phases. The best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event, then mixed with an atmospheric component during rapid cooling and retrogression, producing a wide range of argon concentrations and 40Ar/36Ar ratios. Step-heating analysis of minerals with this mixture would produce linear arrays on a 36Ar/40Ar versus 39Ar/40Ar correlation diagram, leading to geologically meaningless plateau ages, older than the true closure age. In the present case, some ages in the range 60–110 Ma could be explained by the presence of excess argon incorporated around 40–50 Ma ago. Similar results found in other high pressure terrains in the Alps may reconcile the argon geochronometer with other systems such as Rb/Sr, U/Pb or Sm/Nd. This study therefore calls for an increasing use of high resolution in situ sampling techniques to clarify the meaning of 40Ar/39Ar ages in many high pressure terrains. Received: 6 January 1994/Accepted: 4 April 1995  相似文献   

15.
K-feldspar from the late Miocene Capoas Granite on Palawan in The Philippines appears to contain highly retentive diffusion domains that are closed to argon diffusion at near-solidus temperatures during cooling of this ~7 km-diameter pluton. This is an important result, for K-feldspar is commonly considered not retentive in terms of its ability to retain argon. Closure temperatures for argon diffusion in K-feldspars are routinely claimed to be in the range ~150–400°C but the release of 39Ar from irradiated K-feldspar during furnace step-heating experiments in vacuo yields Arrhenius data that imply the existence of highly retentive core domains, with inferred closure temperatures that can exceed ~500–700°C. These high closure temperatures from the Capoas Granite K-feldspar are consistent with the coincidence of 40Ar/39Ar ages with U–Pb zircon ages at ca 13.5 ± 0.2 Ma. The cooling rate then accelerated, but the rate of change had considerably slowed by ca 12 Ma. Low-temperature (U–Th)/He thermochronology shows that the cooling rate once again accelerated at ca 11 Ma, perhaps owing to renewed tectonic activity.  相似文献   

16.
吴林  杨列坤  师文贝  王非 《地质科学》2010,45(3):905-916
Ar同位素体系定年矿物的封闭温度范围广,并且可以获得650℃~150℃温度段的非线性冷却历史,因此~(40)Ar/~(39)Ar热年代学成为研究地质体热演化历史过程中最有效的工具之一。但是由于矿物中Ar同位素扩散机制还有一些问题没有清楚地被认识,因此在一定程度上制约了~(40)Ar/~(39)Ar热年代学的发展。本文介绍了目前应用最广泛的两种~(40)Ar/~(39)Ar热年代学模式:多重扩散域模式和多路径扩散模式,讨论了它们的发展现状、存在的问题、可能解决的方法以及今后的发展方向。  相似文献   

17.
对来自二郎坪岩群的白云母、角闪石用阶段升温法进行了~(40)Ar/~(39)Ar年龄谱研究,结合二郎坪岩群地质特征,认为白云母近似直线年龄港的评年龄(111.2Ma)及角闪石稳定评年龄(121.5Ma)代表了二郎坪岩群最后一次热事件的时代;角闪石与白云母封闭温度的不同以及角门石的成分与结构环带导致二者坪年龄的差异,二郎坪岩群在121.5~111.2Ma曾发生区域变质作用,整个秦岭造山带在此期间仍处于构造活动期。  相似文献   

18.
长乐—南澳断裂带变形火成岩的U-Pb和40Ar/39Ar年龄   总被引:11,自引:1,他引:11  
长乐—南澳断裂带中同构造变形的花岗闪长岩主岩和闪长岩包体的锆石UPb法和角闪石40Ar/39Ar法定年结果分别为~130Ma和~95Ma。锆石UPb年龄代表了变形花岗质岩石的形成年龄,也表明长乐—南澳断裂带至少在此时已经开始活动。角闪石40Ar/39Ar年龄不能代表长乐—南澳断裂带的活动年龄,它记录的是与大规模、未变形的早白垩世晚期(相当于石帽山群)火成活动相关的热变质事件的时间。  相似文献   

19.
A polymetamorphic complex in the Arunta Block, Central Australia, which has undergone metasomatism involving introduction of Rb and loss of Sr has been investigated by 40Ar/ 39Ar step and total-degassing techniques in an attempt to date five metamorphic events (M1–M5) identified microstructurally. The complex isotopic data and apparent age spectra obtained have been interpreted in the light of constraints provided by an associated tectonometamorphic study, together with independent geochronological controls from the immediate study area and elsewhere in the Arunta Block. This has led to the delineation of ages of 1,780–1,820 M.y. for M1, 1,690–1,720 M.y. for M2, 1,450–1,500 M.y. for M3, 1,030–1,060 M.y. for M4 and 370–510 for M5. The polyphase M5 event is correlated with the Alice Springs Orogeny, previously accorded younger age limits of 330–370 M.y. The latter are interpreted to reflect closure of mica K/Ar systems as uplift continued subsequent to the close of the event, possibly culminating in minor high-level brittle deformation and the development of pseudotachylytes in the study area.Apparent age spectra recorded in this study do not all conform with generally accepted patterns of behaviour of mineral systems which have undergone polythermal histories. Two hornblendes have survived subsequent high temperature metasomatic reheating without loss of argon or resetting of their ages. Their resistance to argon loss may be attributable to low water contents, due to their growth under anhydrous granulite facies conditions, and may reflect a relationship between argon loss from hornblende and the water content of hornblende, which decreases with increasing metamorphic grade (Kostyuk and Sobolev 1969). Other samples display false plateaux in their apparent age spectra and in three cases the spectra are indistinguishable from those of undisturbed spectra.The age data recorded in this study suggest that during basement reactivation loss of argon is mainly confined to the vicinity of shear zones, implying that diffusion of argon from minerals in the deep crust may be dependent more on deformation than on reheating. However, thermal activity may also be associated with active shear zones, through the medium of superheated fluids streaming up the shear zones from below. The wide spread of K/Ar and 40Ar/39Ar ages (330–510 M.y.), recorded for the Alice Springs Orogeny in the Arunta basement, may reflect such controls.  相似文献   

20.
Mineral assemblages in the Dinggyê area of southern Tibet (28°N; 88°E) provide new insights regarding the poorly understood Eohimalayan metamorphic event in the eastern Himalayan orogen. Major element partitioning thermobarometry of pelitic rocks indicates temperatures of 750–830 K at depths of 14±3 km, consistent with the presence of kyanite, sillimanite, and andalusite schists in the area. Laser and resistance furnace40Ar/39Ar analyses of hornblendes from intercalated amphibolites yield closure ages of 25 Ma. Overlap between the probable range of Ar closure temperatures for these hornblendes and the metamorphic temperatures estimated through thermobarometry suggests that Eohimalayan metamorphism in the Dinggyê area occurred in Late Oligocene time, no more than about 10 million years before the main or Neohimalayan phase of metamorphism in Early to Middle Miocene time. Muscovite, biotite, and K-feldspar40Ar/39Ar ages indicate an important episode of rapid cooling between 16 and 13 Ma, which is interpreted as a signature of tectonic denudation related to movement on N-dipping extensional structures of the South Tibetan detachment system.  相似文献   

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