~(40)Ar/~(39)Ar热年代学现状与问题

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。     

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.     

桐柏——大别山主要构造热事件及40Ar/39Ar地质定年研究

桐柏——大别山是一条复合造山带。在其演化过程中曾经历了扬子旋回（1000Ma-761Ma）和加里东旋回（470Ma-401Ma）两个板块构造旋回的俯冲-碰撞造山作用，之后又经历了早、中华力西（357Ma-314Ma）的平移走滑和晚华力西（286Ma-261Ma）、印支期（224Ma-185Ma）、燕山期（130Ma-111Ma）逆掩或逆冲推覆的陆-陆叠覆造山作用。印支期的高压超高压变质岩系是在陆-陆叠覆造山作用下形成的。燕山期的造山不仅具显著的深层次构造岩浆作用特点，而且还伴随快速的隆升作用。     

Comparison of Conventional K–Ar and Ar/Ar Dating of Young Mafic Volcanic Rocks

K–Ar and ⁴⁰Ar/³⁹Ar ages have been measured on nine mafic volcanic rocks younger than 1 myr from the Snake River Plain (Idaho), Mount Adams (Washington), and Crater Lake (Oregon). The K–Ar ages were calculated from Ar measurements made by isotope dilution and K₂O measurements by flame photometry. The ⁴⁰Ar/³⁹Ar ages are incremental-heating experiments using a low-blank resistance-heated furnace. The results indicate that high-quality ages can be measured on young, mafic volcanic rocks using either the K–Ar or the ⁴⁰Ar/³⁹Ar technique. The precision of an ⁴⁰Ar/³⁹Ar plateau age generally is better than the precision of a K–Ar age because the plateau age is calculated by pooling the ages of several gas increments. The precision of a plateau age generally is better than the precision of an isotope correlation (isochron) age for the same sample. For one sample the intercept of the isochron yielded an ⁴⁰Ar/³⁶Ar value significantly different from the atmospheric value of 295.5. Recalculation of increment ages using the isochron intercept for the composition of nonradiogenic Ar in the sample resulted in much better agreement of ages for this sample. The results of this study also indicate that, given suitable material and modern equipment, precise K–Ar and ⁴⁰Ar/³⁹Ar ages can be measured on volcanic rocks as young as the latest Pleistocene, and perhaps even the Holocene.     

辽西义县组火山岩40Ar/39Ar、K-Ar法年龄测定

义县组为辽西地区广泛分布的陆相火山-沉积地层,义县火山旋回分为 4个亚旋回.含珍稀化石的湖相沉积层与第二亚旋回火山岩伴生.运用激光微区 40Ar/39Ar法、常规 40Ar/39Ar阶段升温测年法和 K- Ar法,对义县旋回火山岩进行了系统的年龄测定,结果表明,直接覆盖义县组底砾岩的义县旋回第一亚旋回第一小旋回玄武岩的 K- Ar年龄为 (133.3± 2.6) Ma、 (133.6± 2.6) Ma,激光微区 40Ar/39Ar法给出的相关性很好的 Ar- Ar等时线年龄为 (132.9± 4.5) Ma,第三、第四小旋回玄武岩样品 40Ar/39Ar阶段升温获得平坦的年龄谱线,坪年龄分别为 (130.6± 0.5) Ma、 (127.7± 0.2) Ma;第二亚旋回玄武岩和流纹质凝灰岩样品 Ar- Ar等时线年龄为 (126.1± 1.7) Ma、 (127.4± 1.3) Ma,第三亚旋回火山岩全岩 K- Ar年龄为 (124.4± 2.4)～ (124.9± 2.4) Ma.义县火山旋回发生的时间大致介于 120～ 135 Ma之间, 义县组的时代应为早白垩世.     

Intercalibration of the Hb3gr Ar/Ar dating standard

The ⁴⁰Ar/³⁹Ar dating technique is based on the knowledge of the age of neutron fluence monitors (standards). Recent investigations have improved the accuracy and precision of the ages of most of the Phanerozoic-aged standards (e.g. Fish Canyon Tuff sanidine (FCs), Alder Creek sanidine, GA1550 biotite and LP-6 biotite); however, no specific study has been undertaken on the older standards (i.e. Hb3gr hornblende and NL-25 hornblende) generally used to date Precambrian, high Ca/K, and/or meteoritic rocks.In this study, we show that Hb3gr hornblende is relatively homogenous in age, composition (Ca/K) and atmospheric contamination at the single grain level. The mean standard deviation of the ⁴⁰Ar^?/³⁹Ar_K (F-value) derived from this study is 0.49%, comparable to the most homogeneous standards. The intercalibration factor (which allows direct comparison between standards) between Hb3gr and FCs is R_FCs^Hb3gr = 51.945 ± 0.167. Using an age of 28.02 Ma for FCs, the age of Hb3gr derived from the R-value is 1073.6 ± 5.3 Ma (1σ; internal error only) and ± 8.8 Ma (including all sources of error). This age is indistinguishable within uncertainty from the K/Ar age previously reported at 1072 ± 11 Ma [Turner G., Huneke, J.C., Podosek, F.A., Wasserburg, G.J., 1971. ⁴⁰Ar-³⁹Ar ages and cosmic ray exposure ages of Apollo 14 samples. Earth Planet. Sci. Lett. 12, 19-35].The R-value determined in this study can also be used to intercalibrate FCs if we consider the K/Ar date of 1072 Ma as a reference age for Hb3gr. We derive an age of 27.95 ± 0.19 Ma (1σ; internal error only) for FCs which is in agreement with the previous determinations. Altogether, this shows that Hb3gr is a suitable standard for ⁴⁰Ar/³⁹Ar geochronology.     

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。     

Thermal history of the Ecstall pluton from Ar/Ar geochronology and thermal modeling

New ⁴⁰Ar/³⁹Ar thermochronology results and thermal modeling support the hypothesis of Hollister et al. (2004), that reheating of the mid-Cretaceous Ecstall pluton by intrusion of the Coast Mountains Batholith (CMB) was responsible for spatially variable remagnetization of the Ecstall pluton. ⁴⁰Ar/³⁹Ar ages from hornblende and biotite from 12 locations along the Skeena River across the northern part of the Ecstall pluton decrease with proximity to the Quottoon plutonic complex, the nearest member of the CMB to the Ecstall pluton. The oldest ⁴⁰Ar/³⁹Ar ages are found farthest from the Quottoon plutonic complex, and are 90 ± 3 Ma for hornblende, and 77.9 ± 1.2 Ma for biotite. The youngest ⁴⁰Ar/³⁹Ar ages are found closest to the Quottoon plutonic complex, and are 51.6 ± 1.2 Ma for hornblende, and 45.3 ± 1.7 Ma for biotite. No obvious relationship between grain size and age is seen in the Ecstall pluton biotites. Spatial trends in ⁴⁰Ar/³⁹Ar ages are consistent with model results for reheating by a thermal wall at the location of the Quottoon plutonic complex. Although no unique solution is suggested, our results indicate that the most appropriate thermal history for the Ecstall pluton includes both reheating and northeast side up tilting of the Ecstall pluton associated with intrusion of the Quottoon plutonic complex. Estimates of northward translation from shallow paleomagnetic inclinations in the western part of the Ecstall pluton are reduced to ∼3000 km, consistent with the Baja-BC hypothesis, when northeast side up tilting is accounted for.     

Calibration of GA1550 biotite standard for K/Ar and 40Ar/39Ar dating

The mineral separate GA1550 biotite has become an international standard for K/Ar and ⁴⁰Ar/³⁹Ar dating studies, although it was prepared as an intralaboratory standard at ANU to monitor tracer depletion from a gas pipette. It is one of a small number of samples that has been calibrated against ³⁸Ar tracers, some of which had been mixed with known amounts of atmospheric argon, so that a so-called primary calibration has been performed. By measuring GA1550 biotite against additional tracers from the same batch we have determined the radiogenic argon content of this sample as 1.342 (± 0.007) × 10^? 9 mol/g, and together with the measured K content of 7.645 (± 0.050) weight percent, we derive a best estimate for the K/Ar age as 98.5 ± 0.5 Ma, where the error is derived from averaging the ages determined relative to the ³⁸Ar tracer.     

单颗粒海绿石激光40Ar-39Ar定年的新突破

于地质记录中,并能独特地成为高质量的地层标记,故近250 Ma以来地质年表中的绝对年龄数据中40%是由海绿石提供的.然而大计量的海绿石样品提供的K-Ar年龄数据,趋于年轻,一般认为没有实际价值.但是对沉积物来说,它是唯一能够提供K-Ar年龄和Rb-Sr年龄数据的矿物相,对于缺乏具有可靠的高温矿物定年计的地层,海绿石能够给出其寄主沉积物的年龄(即年表界限),对于校正地质年表又十分重要.如何解决这一难题,1998年3月,加拿大多伦多大学P.E.Smith等[1]在Science上发表了一篇最新报导,利用单颗粒海绿石激光440Ar-39Ar定年技术,成功地解决了上述难题,并通过详细的年龄分布图,得出了短时期内,全球海平面变迁的情况,具有重大的突破性意义.     

Comparison between grain size and multi-mineral Ar/Ar thermochronology

In the case of volume diffusion, the closure temperature of a mineral is function of, among other factors, the characteristic diffusion dimension, which can be approximated by the grain size of the mineral analysed for grains smaller than or similar in size to the diffusion domains. The theoretical possibility of single mineral grain size thermochronology had been demonstrated empirically in earlier studies, mostly using biotite. In order to examine the potential of this method, it was tested alongside the widely used multi-mineral ⁴⁰Ar/³⁹Ar thermochronology. The sample comes from the granitic McLean pluton, in the south section of the Grenville orogeny. Seven grain size separates of biotite (ranging between 90 and 1000 μm), eight size fractions of amphibole (between 63 and 1000 μm), and three size fractions of K-feldspar (250-600 μm) were extracted and dated by the laser step-heating ⁴⁰Ar/³⁹Ar method. The total gas ages obtained behave as theoretically predicted, with increasing ages for increasing grain sizes, including for K-feldspar, but with the exception of the smallest and the largest grains for biotite and amphibole. The calculated cooling rates are ca. 0.7 °C/Ma for K-feldspar, ca. 2.5 °C/Ma for biotite, and ca. 11 °C/Ma for amphibole, corresponding very well to a monotonic cooling of the McLean pluton. A quick initial thermal re-equilibration with the cooler host-rocks is followed by a much slower cooling on a thermal path parallel to that of the Frontenac Terrain situated immediately to the southeast. The validity of the single mineral grain size thermochronology is demonstrated by comparison with the thermal evolution of the adjacent units and with the cooling history derived from a multi-mineral thermochronology, suggesting that it can be routinely used. The application of this method can be hampered by insufficiently low analytical uncertainties.     

Ar and Ar recoil loss during neutron irradiation of sanidine and plagioclase

The ⁴⁰Ar/³⁹Ar dating technique requires the activation of ³⁹Ar via neutron irradiation. The energy produced by the reaction is transferred to the daughter atom as kinetic energy and triggers its displacement, known as the recoil effect. Significant amounts of ³⁹Ar and ³⁷Ar can be lost from minerals leading to spurious ages and biased age spectra. Through two experiments, we present direct measurement of the recoil-induced ³⁹Ar and ³⁷Ar losses on Fish Canyon sanidine and plagioclase. We use multi-grain populations with discrete sizes ranging from 210 to <5 μm. One population consists of a mixture between sanidine and plagioclase, and the other includes pure sanidine.We show that ³⁹Ar loss (depletion factor) for sanidine is ∼3% for the smallest fraction. Age spectra of fractions smaller than ∼50 μm show slight departure from flat plateau-age spectrum usually observed for large sanidine. This departure is roughly proportional to the size of the grain but does not show typical ³⁹Ar loss age spectra. The calculated thickness of the total depletion layer d₀(sanidine) is 0.035 ± 0.012 (2σ). This is equivalent to a mean depth of the partial depletion layer (x₀) of 0.070 ± 0.024 μm. The latter value is indistinguishable from previous values of ∼0.07-0.09 μm obtained by argon implantation experiments and simulation results.We show that it is possible to adequately correct ages from ³⁹Ar ejection loss provided that the d₀-value and the size range of the minerals are sufficiently constrained. As exemplified by similar calculations performed on results obtained in a similar study of GA1550 biotite [Paine J. H., Nomade S., and Renne P. R. (2006) Quantification of ³⁹Ar recoil ejection from GA1550 biotite during neutron irradiation as a function of grain dimensions. Geochim. Cosmochim. Acta70, 1507-1517.], the d₀(biotite) is 0.46 ± 0.06 μm. The significant difference between empirical results on biotite and sanidine, along with different simulation results, suggests that for biotite, crystal structures and lattice defects of the stopping medium and possibly subsequent thermal degassing (due to ∼150-200 °C temperature in the reactor or extraction line bake out) must play an important role in ³⁹Ar loss.The second experiment suggests that ³⁷Ar recoil can substantially affect the age via the interference corrections with results that suggest up to ∼98% of ³⁷Ar can be ejected from the ∼5 μm grain dimension.Further investigation of silicates of various compositions and structures are required to better understand (and correct) the recoil and recoil-induced effects on both ³⁹Ar and ³⁷Ar and their influences on ⁴⁰Ar/³⁹Ar dating.     

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.     

南秦岭铧厂沟金矿床铬云母~(40)Ar/~(39)Ar年代学及其大地构造意义

南秦岭铧厂沟金矿床位于勉略缝合带以南的逆冲推覆带内,矿体呈透镜体或脉状产于新元古界蚀变细碧岩及泥盆系灰岩中,受控于近EW向叠瓦状逆冲断层及韧脆性剪切带。为了准确厘定其成矿时代,对矿区蚀变细碧岩型矿石和含矿石英脉中的铬云母进行了年龄测定。2件铬云母样品的~(40)Ar/~(39)Ar坪年龄分别为209.4±2.3 Ma和211.5±2.5 Ma,相应的等时线年龄211.4±3.6 Ma和215.3±3.9 Ma,与坪年龄在误差范围内一致。因此,铧厂沟金矿床的成矿年龄为212~209 Ma。结合铧厂沟金矿床的大地构造位置、矿床地质特征及成因类型,推测铧厂沟金矿床形成于扬子板块与秦岭微板块的碰撞过程,其成矿年龄代表了两板块发生碰撞的下限。     

Ar loss in experimentally deformed muscovite and biotite with implications for Ar/Ar geochronology of naturally deformed rocks

The effects of deformation on radiogenic argon (⁴⁰Ar^∗) retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite. Cylindrical cores, ∼15 mm in length and 6.25 mm in diameter, were drilled from granite collected from the South Armorican Massif in northwestern France, loaded into gold capsules, and weld-sealed in the presence of excess water. The samples were deformed at a pressure of 10 kb and a temperature of 600 °C over a period 29 of hours within a solid medium assembly in a Griggs-type triaxial hydraulic deformation apparatus. Overall shortening in the experiments was approximately 10%. Transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking, cracking, and grain segmentation of the micas.Infrared (IR) laser (CO₂) heating of individual 1.5-2.5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well-defined ⁴⁰Ar/³⁹Ar plateau ages of 311 ± 2 Ma (2σ). Identical experiments on single grains separated from the experimentally deformed granite yield results indicating ⁴⁰Ar^∗ loss of 0-35% in muscovite and 2-3% ⁴⁰Ar^∗ loss in biotite. Intragrain in situ ultraviolet (UV) laser ablation ⁴⁰Ar/³⁹Ar ages (±4-10%, 1σ) of deformed muscovites range from 309 ± 13 to 264 ± 7 Ma, consistent with 0-16% ⁴⁰Ar^∗ loss relative to the undeformed muscovite. The in situ UV laser ablation ⁴⁰Ar/³⁹Ar ages of deformed biotite vary from 301 to 217 Ma, consistent with up to 32% ⁴⁰Ar^∗ loss. No spatial correlation is observed between in situ⁴⁰Ar/³⁹Ar age and position within individual grains. Using available argon diffusion data for muscovite the observed ⁴⁰Ar^∗ loss in the experimentally treated muscovite can be utilized to predict average ⁴⁰Ar^∗ diffusion dimensions. Maximum ⁴⁰Ar/³⁹Ar ages obtained by UV laser ablation overlap those of the undeformed muscovite, indicating argon loss of <1% and an average effective grain radius for ⁴⁰Ar^∗ diffusion ?700 μm. The UV laser ablation and IR laser incremental ⁴⁰Ar/³⁹Ar ages indicating ⁴⁰Ar^∗ loss of 16% and 35%, respectively, are consistent with an average diffusion radius ?100 μm. These results support a hypothesis of grain-scale ⁴⁰Ar^∗ diffusion distances in undeformed mica and a heterogeneous mechanical reduction in the intragrain effective diffusion length scale for ⁴⁰Ar^∗ in deformed mica. Reduction in the effective diffusion length scale in naturally deformed samples occurs most probably through production of mesoscopic and submicroscopic defects such as, e.g., stacking faults. A network of interconnected defects, continuously forming and annealing during dynamic deformation likely plays an important role in controlling both ⁴⁰Ar^∗ retention and intragrain distribution in deformed mica. Intragrain ⁴⁰Ar/³⁹Ar ages, when combined with estimates of diffusion kinetics and distances, may provide a means of establishing thermochronological histories from individual micas.     

大气^40Ar／^36Ar端元值及其地质意义

月岩和陨石形成时捕获了太阳风中的Ａｒ同位素。作者对恩施陨石采用４０Ａｒ／－３６Ａｒ技术，并对Ｃｌ的诱发产物进行了校正后，获得了４６００Ｍａ前太阳风中４０Ａｒ／３６Ａｒ比值是０．８９±０．４４。它表明地球大气圈中４０Ａｒ／３６Ａｒ比值经历了从０．８９到２９５．５的演化过程，影响这一过程的主要因素是地球的去气作用。     

Ar degassing and the origin of the sialic crust

The degassing of radiogenic Ar⁴⁰ is defined as coherent if only the Ar⁴⁰ associated with parent K is degassed as K is transferred from the mantle to crust. Coherency predicts, for a 4.55 b.y. Earth, a sialic crust with 2.50 per cent K, using only the Ar content of the atmosphere and present crust (from a Hurley and Rand, 1969, age distribution). This is a maximum limit to K content of the sialic crust if the age of the Earth is no younger than 4.55 b.y. A K content of the sialic crust of 1.9 per cent (Holland and Lambert, 1972) implies an efficiency (E) less than 100 per cent for K transfer from oceanic basalt to sialic crust in subduction zones and/or some non-coherent (preferential) degassing of Ar from the mantle.K, Ar coherence for mantle differentiation to crust is supported however, by the agreement of the predicted oceanic He flux and radiogenic He-Ar ratios of volcanic gases with the observed limits if the best estimate of K, U, Th influx rates at oceanic ridges is used.Assuming K, Ar coherence, various sea-floor spreading rates as functions of time, and limiting K contents of the sialic crust, computed models give E and the portion of the sialic crust derived from melting oceanic basalt in subduction zones. Except for models with very high spreading rates in the Precambrian, they also predict that a significant part of the sialic crust was derived from vertical differentiation of the mantle, presumably early in Earth history. The results are in accord with Armstrong's model of an early sialic crust that is recycled to give a Hurley-type age pattern with the proviso that the ‘vertical’ sial K_υis formed early in Earth history for models with a high K_υcomponent.The coherent K, Ar models with preferred estimates of input parameters are also consistent with a limited mixing model (only old and new sial are equilibrated) for Sr isotopic evolution and the probable average $\text{Sr}{}^{87}\text{Sr}{}^{86}$ ratio now of the sialic crust.     

40Ar39Ar dating of glauconites: Measured 39Ar recoil loss from well-crystallized specimens

Nine glauconite samples with relatively high K concentrations and which appear to be well crystallized using normal X-ray diffraction techniques have been studied using the ${}^{40}\text{Ar}{}^{39}\text{Ar}$ method. The glauconite ${}^{40}\text{Ar}{}^{39}\text{Ar}$ apparent ages exceed their KAr, RbSr and, in most cases, stratigraphic ages by substantial amounts. ${}^{40}\text{Ar}{}^{39}\text{Ar}$ release spectra sometimes yield plateaus but these apparent ages have no geological significance. The results indicate that ³⁹Ar is lost by recoil from mineral grains during neutron irradiation, consistent with previously reported observations. The amount of ³⁹Ar loss was measured by isotope dilution for four samples and varied from 29% to 17%. In contrast, radiogenic ⁴⁰Ar is quantitatively retained during irradiation. The very fine blades which make up glauconite grains yield the mineral susceptible to large amounts of ³⁹Ar loss and unsuitable for ${}^{40}\text{Ar}{}^{39}\text{Ar}$ dating.     

U-Pb and Ar/Ar geochronological constraints on the exhumation history of the North Qinling terrane, China

The amphibolite facies grade North Qinling metamorphic unit forms the centre of the Qinling orogenic belt. Results of LA-ICP-MS U-Pb zircon, ⁴⁰Ar/³⁹Ar amphibole and biotite dating reveal its Palaeozoic tectonic history. U-Pb zircon dating of migmatitic orthogneiss and granite dykes constrains the age of two possible stages of migmatization at 517 ± 14 Ma and 445 ± 4.6 Ma. A subsequent granite intrusion occurred at 417 ± 1.6 Ma. The ⁴⁰Ar/³⁹Ar plateau ages of amphibole ranging from 397 ± 33 Ma to 432 ± 3.4 Ma constrain the cooling of the Qinling complex below ca. 540 °C and biotite ⁴⁰Ar/³⁹Ar ages at about 330–368 Ma below ca. 300 °C. The ages are used to construct a cooling history with slow/non-exhumation during 517– 445 Ma, a time-integrated cooling at a rate < 2.5 °C/Ma during the period of 445–410 Ma, an acceleration of cooling at a rate of 8 °C/Ma from 397 Ma to 368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma. The data show a significant delay in exhumation after peak metamorphic conditions and a long period of tectonic quiescence after the suturing of the North China and South China blocks along the Shangdan suture. These relationships exclude classical exhumation models of formation and exhumation of metamorphic cores in orogens, which all imply rapid cooling after peak conditions of metamorphism.     

^40Ar／^39Ar测年中的冷却年龄和变形年龄

^40Ar／^39Ar法年龄数据的解释与Ar同位素在矿物中的封闭温度有关。冷却年龄只能限制变质——变形事件发生的时间区间,不能真正代表地质事件的发生时间。因此,必须想办法得到变形年龄。本文从封闭温度概念出发,探讨了压力、冷却速率、有效扩散范围和几何形态、颗粒大小、动力学参数等对矿物封闭温度的影响,介绍了新近提出的“封闭窗”概念,并讨论了在什么情况下可以获得变形年龄。