40Ar/39Ar Geochronology and stable isotope geochemistry of Late-Hercynian intrusions from north-eastern Iberia with implications for argon loss in K-feldspar

The cooling history of Hercynian calc-alkalic, post-kinematic plutonic intrusions of the Montnegre massif (NE Spain) has been determined by ⁴⁰Ar/³⁹Ar analysis of two hornblendes, four biotites and eight K-feldspars (Kfs). The hornblendes have ⁴⁰Ar/³⁹Ar total fusion ages of 291Dž Ma and define the magmatic cooling of the basic and oldest structural intrusions. The biotites from the acid and intermediate rocks have ⁴⁰Ar/³⁹Ar plateau ages of 285Dž Ma, which date cooling of the intrusions through argon closure in biotite. The ⁴⁰Ar/³⁹Ar ages of the K-feldspars vary widely, ranging from 276-191 Ma. A correlation between K-feldspar ⁴⁰Ar/³⁹Ar total fusion age and several other features such as structural state, microstructures indicated by obliquity and, to some degree, optically visible perthites, is consistent with post-crystallisation partial argon loss in the K-feldspars. The 'D values of the biotites also correlate with age and chlorite contents, but this is not so for the '¹⁸O values of either feldspar or quartz. We infer that most microtextural changes occurred during cooling of the batholith, but a possibly Mesozoic, late disturbing hydrothermal event of weak intensity and with only minor fluid circulation must have occurred. This event provoked significant argon loss in the most structurally complex K-feldspars and is recorded in the hydrogen, but not oxygen, isotope data.     

福建紫金山矿田深部似斑状花岗闪长岩锆石SHRIMP U-Pb和单矿物~(40)Ar/~(39)Ar年龄及其地质意义

通过对福建紫金山矿田深部与成矿作用有关的主期似斑状花岗闪长岩3组锆石SHRIMP U-Pb和2组角闪石、钾长石~(40)Ar/~(39)Ar测年,获得锆石~(206)Pb/~(238()U加权平均年龄为101.8±1.5 Ma(n=34,MSWD=1.0),代表紫金山矿田深部与成矿作用有关的主期似斑状花岗闪长岩的成岩年龄;同时获得角闪石~(40)Ar/~(39)Ar冷却年龄为100±11 Ma、102.2 Ma,钾长石的~(40)Ar/~(39)Ar冷却年龄为96.3±1.7 Ma、98.5 Ma。依据矿物封闭温度理论,估算紫金山矿田深部与成矿作用有关的主期似斑状花岗闪长岩由锆石结晶至角闪石40Ar/39Ar体系封闭、再到钾长石~(40)Ar/~(39)Ar体系封闭的岩石冷却速率分别是40.7~67.1℃/Ma、116.9~216.3℃/Ma,显示岩石的冷却速率较大;由古地温梯度推算主期似斑状花岗闪长岩结晶(101.8±1.5 Ma)至钾长石~(40)Ar/~(39)Ar体系封闭(96.3±1.7 Ma)期间岩体隆升剥露了约3 km,暗示地壳在这一时期发生了快速隆升剥蚀作用。紫金山矿田深部似斑状花岗闪长岩锆石206Pb/238U年龄佐证了紫金山矿田深部存在一个大岩基,并约束了紫金山矿田斑岩型矿床的成矿时代,单矿物的~(40)Ar/~(39)Ar年龄为矿区的隆升剥露研究提供新资料。     

The Late Triassic Central Patagonian Batholith: Magma hybridization, 40Ar/39Ar ages and thermobarometry

The Late Triassic Central Patagonian Batholith is a key element in paleogeographic models of West Gondwana just before to the break-up of the supercontinent. The preexisting classification of units of this batholith was mainly based on isotopic and geochemical data. Here we report the results of field mapping and petrography, backed up by three new ⁴⁰Ar/³⁹Ar biotite ages, which reveal previously unnoticed relationships of the rocks in the batholith. Based on the new information we present a reorganization of units where the batholith is primarily formed by the Gastre and the Lipetrén superunits. The Gastre Superunit is the oldest magmatic suite and is composed of I-type granites which display evidence of felsic and mafic magma interaction. It is formed by 4 second-order units: 1) equigranular hornblende–biotite granodiorites, 2) porphyritic biotite–hornblende monzogranites, 3) equigranular biotitic monzogranites and 4) hornblende quartz-diorites. Emplacement depth of the Gastre Superunit is bracketed between 6 and 11 km (1.8–3 kbar), and the maximum recorded temperatures of emplacement are comprised between 660 and 800 °C. The recalculated Rb/Sr age is 222 ± 3 Ma and the porphyritic biotite–hornblende monzogranites yielded a ⁴⁰Ar/³⁹Ar age in biotite of 213 ± 5 Ma. On the other hand, the Lipetrén Superunit is made up by fine-grained biotitic monzo- and syenogranites that postdate magma hybridization processes and intrude all the other units. The recalculated Rb/Sr age for this suite is identical to a ⁴⁰Ar/³⁹Ar age in biotite extracted from one of its monzogranites (206.4 ± 5.3 and 206 ± 4 Ma, respectively). This and the observed textural features suggest very fast cooling related to a subvolcanic emplacement. An independent unit, the “Horqueta Granodiorite”, which has previously been considered as the record of a Jurassic intrusive stage in the Central Patagonian Batholith, gave a ⁴⁰Ar/³⁹Ar age in biotite of 214 ± 2 Ma. This and the reexamination of available isotopic data allow propose that this granodiorite unit is part of the Late Paleozoic intrusives in the region. The Late Triassic Central Patagonian Batholith is overlain by 190–185 Ma volcano-sedimentary rocks, suggesting that it was exposed sometime between the latest Triassic and earliest Jurassic times, roughly coeval with a major accretionary episode in the southwestern margin of Gondwana.     

Effects of progressive mylonitization on Ar retention in biotites from the Santa Rosa mylonite zone,California, and thermochronologic implications

The Santa Rosa mylonite zone developed predominantly from a granodiorite protolith in the eastern margin of the Peninsular Ranges batholith. A wide variation in K−Ar biotite dates within the zone is shown to reflect the times of cooling through closure temperatures whose variability is chiefly a result of deformation-induced reduction in grain size. We suggest that such variation generally may be exploited to place constraints on the timing of deformation episodes. Previous workers have shown that deformation in the Santa Rosa mylonite zone involved the formation of mylonites and an imbricate series of low-angle faults which divide the area into structural units. Field work, petrographic studies, and TEM analysis of deformation mechanisms in biotite show that the granodiorite mylonite, the lowermost structural unit, formed below the granodiorite solidus. The granodiorite mylonite varies from protomylonite to ultramylonite, with regions of high strain distributed heterogeneously within the zone. Previously reported biotite and hornblende K−Ar dates from the granodiorite protolith below (82–89 Ma) and the Asbestos Mountain granodiorite above (61–68 Ma) the mylonite zone indicate dramatically dissimilar thermal histories for the lowermost and uppermost structural units. Other workers' fission track dates on sphene, zircon, and apatite from the granodiorite mylonite and the Asbestos Mountain granodiorite suggest thermal homogenization and rapid cooling to ∼100° C by ca 60 Ma. Within and adjacent to the mylonite zone, K−Ar dates on 5 samples of biotite from variably deformed granodiorite range from 62–76 Ma; dates are not correlated with structural depth but clearly decrease with degree of deformation and concomitant grain size reduction. ⁴⁰Ar/³⁹Ar incremental heating analyses of biotite from the granodiorite protolith reveals no evidence of excess argon and produces a relatively flat age spectrum. ⁴⁰Ar/³⁹Ar incremental heating analysis of biotite from the granodiorite mylonite discloses discordance consistent with ³⁹Ar recoil loss. K analysis of samples, allowing K−Ar dates to be calculated, is therefore recommended as an adjunct to ⁴⁰Ar/³⁹Ar step heating analysis in rocks that have experienced similar deformation. During mylonitization, biotite grain size reduction through intracrystalline cataclasis results in estimated grain dimensions as small as 0.05 μm locally within porphyroclasts as large as 1 mm. Because biotite compositions are relatively Uniform (Fe/[Fe+Mg+Mn+Ti+Al^VI]=0.47–0.52) and show no systematic variation with grain size, compositional dependence of activation energy and diffusivity can be eliminated as sources of variation in Ar retention. Ar closure temperatures, calculated with appropriate diffusion parameters for the observed grain sizes, are in the range ∼220–280° C and define a cooling curve consistent with a thermal history intermediate between those of the granodiorite protolith below and the Asbestos Mountain granodiorite above the mylonite zone. Changes in the slope of the cooling curve indicate that the main deformation episode initiated at or above ca 330° C (∼80 Ma), above the closure temperature for thermally activated diffusion of argon in biotite, and continued to a minimum of ca 220–260° C (∼62 Ma).     

西昆仑库地韧性剪切带的40Ar/39Ar年龄

西昆仑库地以南有一套变质变形较强的岩系,前人依照区域对比关系将其划为前寒武的古老基底。对西昆仑早期构造演化的论述均基于该观点,但没有提供确凿的同位素年代学证据。笔者通过野外观察、室内研究,确认库地以南的变质变形岩系是大型韧性推覆剪切作用的产物。通过对新生变质矿物角闪石和黑云母单矿物的⁴⁰Ar/³⁹Ar年龄分析,确定剪切变质年龄为426-451Ma,说明库地的变质变形岩系是形成于早古生代晚期的一条大型韧性剪切带,这对于解释西昆仑的早期构造演化具有重要意义。     

Timing of the Yuchiling giant porphyry Mo system, and implications for ore genesis

The Yuchiling Mo deposit is a recently discovered giant porphyry system in the East Qinling Mo belt, China. Its apparent causative intrusion, i.e., the Yuchiling granite porphyry, is the youngest intrusion (phase 4) of the Heyu multiphase granite batholith, which was emplaced between 143 and 135 Ma. New robust constraints on the formation of the Yuchiling porphyry Mo system are provided by combined zircon U–Pb, biotite ⁴⁰Ar/³⁹Ar, and molybdenite Re–Os dating. Zircon grains from the Mo-mineralized granite porphyry yield weighted ²⁰⁶Pb/²³⁸U age of 134.0?±?1.4 Ma (n?=?19, 2σ error, MSWD?=?0.30). Magmatic biotite from the same sample yield a ⁴⁰Ar/³⁹Ar plateau age of 135.1?±?1.4 Ma (2σ error), and an inverse isochron age of 135.6?±?2.0 Ma (n?=?7, 2σ error, MSWD?=?10.8), which are effectively coincident with the zircon U–Pb age within analytical error. Three pulses of mineralization can be deduced from the molybdenite Re–Os ages, namely: ～141, ～137, and ～134 Ma, which agree well with the zircon U–Pb ages of granitic phases 1, 2, and the Yuchiling porphyry (phase 4), respectively. These well-constrained temporal correlations indicate that Mo mineralization was caused by pulses of granitic magmatism, and that the ore-forming magmatic-hydrothermal activity responsible for the Yuchiling porphyry Mo system lasted about 8 Ma. The Yuchiling Mo deposit represents a unique style of porphyry Mo system formed in a post-collision setting, and associated with F-rich, high-K calc-alkaline intrusions, which differ from convergent margin-associated porphyry Mo deposits.     

U‐Pb Geochronological and Thermochronological Time–Temperature Constraints of 40Ar/39Ar Hornblende Reference Material HB3gr

Hornblende from the Lone Grove Pluton, Llano Uplift, Texas, has served as an irradiation reference material in ⁴⁰Ar/³⁹Ar studies for decades. In order to evaluate the apparent age bias that currently exists between the U‐Pb and ⁴⁰Ar/³⁹Ar systems, zircon and titanite were dated by isotope dilution‐thermal ionisation mass spectrometry (ID‐TIMS) from the same rock from which the hornblende ⁴⁰Ar/³⁹Ar reference material HB3gr is derived. Zircon U‐Pb data indicate initial crystallisation at 1090.10 ± 0.16 Ma (2s), a date that is 1.7% older than the accepted K‐Ar date (1072 ± 14 Ma, 2s) for HB3gr; an offset that exceeds the typical 0.5–1% bias between the two systems, though remaining within uncertainty due to the large uncertainties in the ⁴⁰K decay constant. Zircon data are presented using both EARTHTIME tracers ET535 and ET2535 and are statistically indistinguishable. Single grain titanite analyses range between 1082 ± 0.75 and 1086 ± 0.81 Ma (2s) and are interpreted to record the subsequent cooling following crystallisation at rates between 30 and 50 °C Ma^?1. This is supported by the observation that hornblende ⁴⁰Ar/³⁹Ar dates corrected for decay constant bias are resolvably younger than the zircon U‐Pb date and in good agreement with titanite U‐Pb dates, permitting the conclusion that both titanite U‐Pb and hornblende ⁴⁰Ar/³⁹Ar systems provide a record of cooling.     

福建紫金山矿田罗卜岭铜钼矿化斑岩锆石LA-ICP- MS U-Pb年龄及成矿岩浆高氧化特征研究

罗卜岭斑岩铜钼矿床是紫金山Cu-Au-Mo浅成低温-斑岩矿田内新近发现的大型斑岩铜钼矿床,本文在岩芯及光薄片系统观察的基础上,分析了矿化斑岩锆石LA-ICP-MS U-Pb年龄及锆石Ce4/Ce3+比值.罗卜岭赋矿斑岩体可分为两期,早期为角闪黑云母花岗闪长斑岩及黑云母花岗闪长斑岩,晚期为黑云母花岗闪长斑岩.早期角闪黑云母花岗闪长斑岩和黑云母花岗闪长斑岩锆石LA-ICP-MS U-Pb年龄分别为103.7±1.2Ma,MSWD=0.33和103.0±0.9Ma,MSWD=1.00;晚期黑云母花岗闪长斑岩锆石LA-ICP-MS U-Pb年龄为97.6±2.1Ma,MSWD=6.00.罗卜岭成矿斑岩基质普遍发育硬石膏,两期成矿斑岩锆石都具较高的Ce4 +/Ce3平均值,在630 ～770之间,高于区内非成矿花岗岩锆石的Ce4+/Ce3+平均值(182 ～577),显示罗卜岭斑岩矿床成矿岩浆具有高氧逸度的特征.据罗卜岭斑岩矿床的形成时代、高氧逸度岩浆特征,结合华南地区中生代构造背景,我们初步认为罗卜岭斑岩矿床的形成可能和中生代古太平洋向北西西方向俯冲有关.     

An 40Ar/39Ar investigation of the contact effects of a dyke intrusion,Kapuskasing Structural Zone,Ontario

An ⁴⁰Ar/³⁹Ar study on biotite and hornblende from baked tonalite gneiss adjacent to a 14-m-wide Kapuskasing dyke in the Chapleau Block of the Kapuskasing Structural Zone employed resistance-furnace step-heating, of bulk-mineral separates, single-grain laser step-heating, and laser spot dating of individual grains in thick section. All three methods yielded concordant results for the biotite, indicating that it has been uniformly overprinted at ca. 2.05 Ga. This date is probably a close estimate of the age of the dyke intrusion. The argon data from the hornblende are more difficult to interpret. All three methods suggest that the hornblende was partially overprinted and then affected by a later influx of excess argon from the external environment. Laser spot dates record an apparent age gradient, extending radially outward from 2.25 Ga in the grain cores to 2.4–2.7 Ga in the grain rims. Corresponding ³⁷Ar/³⁹Ar values show no systematic variation across the hornblende, although ³⁸Ar/³⁹Ar ratios display a similar pattern to the age distribution, with lower values in the cores than in the rims. The bulk separate and single grain step-heating spectra mimic a volume diffusion Ar-loss profile that would appear to be inconsistent with the laser-spot dates. Three possible interpretations to explain this apparent discrepancy may involve either experimental difficulties, an effective radius of diffusion which is much smaller than the actual grain radius, or an effective radius of diffusion that is of the order of the grain radius. It is clear that for the ⁴⁰Ar/³⁹Ar dating of samples that have experienced complex geologic histories, it is important to utilize both resistance-furnace and laser techniques.     

Argon retentivity of hornblendes: A field experiment in a slowly cooled metamorphic terrane

Hornblende from samples of amphibolite and granitic gneiss, collected within a single outcrop in the central Adirondacks, yield significantly different ⁴⁰Ar*/³⁹Ar_k dates of 948 ± 5 and 907 ± 5 Ma. Assuming that this terrane cooled slowly following high-grade metamorphism and that the samples have experienced the same thermal history, the difference in dates apparently reflects a corresponding difference in blocking temperature for diffusion of radiogenic argon in these hornblende samples.The Fe/(Fe + Mg + Mn) of the hornblende samples are 0.8 and 0.6, the higher ratio corresponding to the younger ⁴⁰Ar*/³⁹Ar_k date. Transmission electron microscopy observations indicate that both hornblende samples are homogeneous and devoid of any exsolution, but contain zones of fibrous phyllosilicates ~0.1 to 2 μm wide parallelling (100) and (110). These alteration zones probably formed during post-metamorphic cooling as a result of the migration of fluids through the hornblendes, and are obvious pathways for argon escape from hornblende. As these features are more abundant in the hornblende sample with the younger⁴⁰Ar*/³⁹Ar_k date and higher Fe/(Fe + Mg + Mn), they may influence the argon blocking temperature by effectively partitioning the hornblende grains into diffusion domains of varying size.Biotite from the granitic gneiss yields an ⁴⁰Ar*/³⁹Ar_k date of 853 ± 2 Ma, with a mildly discordant stepheating spectrum that in part reflects the degassing of submicroscopic inclusions precipitated during alteration of the host biotite. Plagioclase from the amphibolite yields a ⁴⁰Ar*/³⁹Ar_k integrated date of 734 ± 3 Ma. All the ⁴⁰Ar*/³⁹Ar_k data are consistent with postmetamorphic cooling rates of 1° to 5°C/Ma.     

西藏谢通门县雄村铜金矿主要地质体的40Ar/39Ar年龄及地质意义

雄村铜金矿Ⅱ号矿体在2007-2008年取得了重大的找矿突破,详细的地质编录成果表明,矿体同样受含眼球状石英斑晶的角闪石英闪长玢岩和角闪石英闪长玢岩控制.含眼球状石英斑晶的角闪石英闪长玢岩和角闪石英闪长玢岩的锆石U-Pb年龄已经确定(164～177 Ma),因此,雄村铜金矿不同地质体的云母类、长石类矿物Ar-Ar同位素年龄的测定显得尤为关键.文章通过对穿切I号矿体的黑云母花岗闪长岩、云煌岩脉的黑云母Ar-Ar同位素测年,结合其他研究者的成果,得到一组十分重要的年龄数据.穿插矿体的黑云母花岗闪长岩中的黑云母(样号6187-335)坪年龄(46.96±0.42)Ma,穿插矿体的无矿化的云煌岩(5053-324.4)(Cu含量0.0551%,Au含量0.034 g/t,Ag0.6 g/t)中的黑云母给出了一个较好的似坪年龄,加权平均年龄为(49.59±0.58)Ma.结合其他研究者测定的中侏罗世侵位的角闪石英闪长玢岩(不含矿)[锆石U-Pb年龄为(177.1±2.0)Ma]中黑云母的At-At同位素年龄为(48.57±0.31)Ma;含矿凝灰岩围岩[锆石U-Pb年龄为(176±5)Ma,MS3VD=0.63}的蚀变绢云母Ar-Ar年龄为(47.07±0.30)Ma;似伟晶岩中长石的At-At年龄为(47.62±0.7)Ma,认为不同形成时代、不同产出空间、不同矿化程度的地质体的云母类、长石类矿物的Ar-Ar同位素年龄的一致性,反映了后期岩浆热事件对中侏罗世早期形成的地质体和矿体的黑云母氩同位素体系产生了较强的扰动或置换.谢通门大岩基黑云母花岗闪长岩的侵位致使各地质体发生显著的退变质,形成典型的角岩化带,这种退变质的时限在46-48 Ma之间的始新世lutetian期,进而认为各地质体中云母类矿物的~(40)Ar/~(39)Ar同位素年龄不能作为成矿年龄.     

内蒙古长山壕金矿区花岗岩同位素年代学研究及地质意义

长山壕金矿床是近年来在内蒙古中西段找到的一特大型金矿床。尽管金矿化在中元古界白云鄂博群变质沉积岩内呈层状、似层状和透镜状产出,但是与各类花岗岩类侵入岩具有密切空间分布关系。本次研究采用LA-ICP-MS方法分别对有关花岗岩类侵入岩进行了系统年代学研究,花岗斑岩和二长花岗斑岩样品的锆石U-Pb年龄值分别为290.9±2.8Ma (MSWD=1.4)和287.5±1.9Ma (MSWD=2.4);2件黑云母花岗岩样品的年龄值分别为267.9±1.2Ma (MSWD=0.95)和274.0±2.3Ma (MSWD=1.4)。根据上述同位素年龄数据,同时结合金矿区野外地质调查和岩(矿)相学证据,可以认为,长山壕金矿区花岗斑岩和二长花岗斑岩及相关金矿床的形成时间为早二叠世早期,并且遭受到早二叠世晚期-中二叠世早期构造-岩浆活动的叠加改造,矿区切穿含金矿脉黑云母花岗岩体的存在就是很好的例证。强烈的中酸性岩浆作用为金矿床的形成提供了动力、热力和物质来源,初步研究结果表明,长山壕金矿床是海西期构造-岩浆作用及相关流体活动的产物,属于与侵入岩有关的中温热液脉型金矿床。     

Timing of Cadomian and Variscan tectonothermal activity,La Hague and Alderney,North Armorican Massif: Evidence from 40Ar/39Ar mineral ages

The La Hague region of northwest France exposes Palaeo-Proterozoic Icartian gneisses which were reworked and intruded by calc-alkaline plutonic rocks during the Cadomian Orogeny (about 700–500 Ma). ⁴⁰Ar/³⁹Ar mineral cooling ages have been determined to clarify the timing of the regional metamorphism of orthogneisses and the emplacement of quartz diorite plutons in this region. Metamorphic amphiboles within Icartian gneisses display discordant ⁴⁰Ar/³⁹Ar apparent age spectra interpreted to result from limited Variscan (about 350–300 Ma) overprinting of intracrystalline argon systems which initially cooled through post-metamorphic hornblende closure temperatures during the Cadomian at about 600 Ma. Igneous hornblendes from the weakly foliated Jardeheu and Moulinet quartz diorites record isotope correlation ages of 599 ± 2 and 561 ± 2 Ma, respectively. Igneous hornblende and biotite from foliated quartz diorite on the nearby Channel Island of Alderney record isotope correlation ages of about 560 Ma. The results imply that metamorphic and plutonic events in the La Hague-Alderney region were approximately contemporaneous with those recorded on Guernsey and Sark, which are thus likely to have formed part of the same tectonic block during the Cadomian Orogeny.     

利源复式花岗岩锆石SHRIMP U-Pb定年研究

利源复式花岗岩体位于粤赣二省交界部位,处在"粤北乐昌-连平钨锡铅锌成矿带"的南东段,主要由中粒斑状黑云母二长花岗岩(主体)和细粒黑云母钾(二)长花岗岩(补体)组成,为弱过铝质高钾钙碱性系列的岩石。应用高精度的锆石SHRIMPU-Pb法,获得前者的成岩年龄为(227.2±4.4)Ma(MSWD=3.4),属于印支期。结合岩石地球化学特征及区域成矿特征分析,认为补体花岗岩可能形成于燕山早期。     

甘肃花牛山东钾长花岗岩40Ar/39Ar同位素年龄及其地质意义

对甘肃花牛山东复式花岗岩体中钾长花岗岩的钾长石进行了详细的⁴⁰Ar/³⁹Ar同位素年龄测定,11个加热阶段所获数据构成-条相关关系非常好的直线,其对应的等时线年龄为194.25±1.96 Ma(2σ).⁴⁰Ar/³⁶Ar初始值为288.87±2.17(2σ),接近于尼尔值(295.5).鉴于该岩体形成之后未受到明显的构造-岩浆活动或其它热事件的影响,因此,194 Ma代表了钾长花岗岩钾长石的形成时代.花牛山东钾长花岗岩是燕山早期构造-岩浆活动的产物.由此推断,花牛山地区构造-岩浆活动时间不是印支期,更不是海西期,而是中生代燕山早期.     

辽宁鞍山地区东山杂岩带3.3~3.1 Ga期间的岩浆作用——锆石SHRIMP U-Pb定年

报道了鞍山地区东山杂岩带奥长花岗岩和二长花岗岩的锆石SHRIMP U-Pb年龄。中粗粒奥长花岗岩中岩浆锆石的年龄为3329 Ma ± 22 Ma (MSWD=9.6),存在年龄为3687~3784 Ma的残余锆石。细粒奥长花岗岩和二长花岗岩中岩浆锆石的年龄分别为3141 Ma ± 8 Ma (MSWD=1.5)和3142 Ma ± 5 Ma (MSWD=0.35)。研究表明,约~3.3 Ga和3.1 Ga是鞍山地区2个重要的地壳演化阶段。     

Nature and timing of the Solonker suture of the Central Asian Orogenic Belt: insights from geochronology and geochemistry of basic intrusions in the Xilin Gol Complex,Inner Mongolia,China

The Solonker suture zone of the Central Asian Orogenic Belt (CAOB) records the final closure of the Paleo-Asian Ocean. The nature and timing of final collision along the Solonker suture has long been controversial, partly because of an incomplete record of isotopic ages and differing interpretations of the geological environments of key tectonic units. The Xilin Gol Complex, consisting of strongly deformed gneisses, schists and amphibolites, is such a key tectonic unit within the CAOB. Lenticular or quasi-lamellar amphibolites are dispersed throughout the complex, intercalated with biotite–plagioclase gneiss. Both rock types experienced amphibolite-facies metamorphism. The protolith of the amphibolite is a basic rock that intruded into the biotite–plagioclase gneiss at 319 ± 4 Ma based on LA-ICPMS zircon U–Pb dating. The basic intrusion was sourced from a modified magma that experienced crystal fractionation and was admixed with slab-derived fluids. The slab-derived fluids, which formed during Early Paleozoic oceanic subduction along the north-dipping Sonidzuoqi–Xilinhot subduction zone, mixed with the magma source and produced subduction-related geochemical signatures superimposed on volcanic arc chemistry. After Early Paleozoic oceanic subduction and arc-continent collision, a transient stage of extension occurred between 313 and 280 Ma in the Sonidzuoqi–Xilinhot area. Deformation and recrystallization during the switch from compression to extension and reheating by the later magmatic intrusions reset the isotope systems of minerals in the Xilin Gol Complex, recorded by a 312.2 ± 1.5 Ma biotite ⁴⁰Ar/³⁹Ar age from biotite–plagioclase gneiss, a 309 ± 12 Ma zircon intercept age and a 307.5 ± 3.5 Ma hornblende ⁴⁰Ar/³⁹Ar age from amphibolites in the complex. There was an arc/forearc-related marine basin at the southern margin of the Xilin Gol Complex during the Permian. The closure of the oceanic basin led to Late Paleozoic–Middle Triassic north-dipping subduction beneath the Xilin Gol Complex and induced the amphibolite-facies metamorphism of the complex. The final suturing of the Solonker zone occurred from 269 to 231 Ma. This latest amphibolite-facies metamorphism with pressures of 0.31–0.39 GPa and temperatures of 620–660 °C was recorded at 263.4 ± 1.4 Ma to the Xilin Gol Complex, as indicated by the hornblende ⁴⁰Ar/³⁹Ar age from the amphibolites, as well as several zircon ages of 260 ± 3–231 ± 3 Ma. The Xilin Gol Complex documented the progressive accretion of a single, long-lived subduction system at the southern margin of the south Mongolian microcontinent from the Early Paleozoic (~452 Ma) to Middle Triassic (~231 Ma). The CAOB shows protracted collision prior to final suturing.     

K-Ar法地质年龄标准物质ZBJ角闪石的定值结果

为了满足K-Ar定年中K和⁴⁰Ar^*分析的质量监控及Ar-Ar法样品在反应堆照射时中子通量监测的需要,我国氩同位素年代学工作者研制了一个K-Ar法年龄标准物质ZBJ角闪石,它采自北京房山花岗闪长岩体。它的⁴⁰Ar-³⁹Ar阶段加热分析结果表明:⁴⁰Ar^*在矿物晶格中保存均匀稳定,年龄谱平坦,³⁹Ar析出量高达97%。这些证据充分表明该黑云母结晶以后未受过热扰动,40K-⁴⁰Ar^*同位素计时体系封闭良好。坪年龄为133.3±0.6Ma,总气体年龄为134.4±1.4Ma,³⁶Ar/⁴⁰Ar-³⁹Ar/⁴⁰Ar反等时线年龄为133.2±0.8Ma,⁴⁰Ar/³⁶Ar初始值为297.6±4.8,此值与(⁴⁰Ar/³⁶Ar)a大气氩丰度比(295.5±0.5)处于同一范围,表明样品不含过剩氩。这几个年龄值的一致性,说明该样品具有良好的均匀性和稳定性,它作为K-Ar和Ar-Ar法地质年龄标准物质是适合的。ZBJ角闪石均匀性检验结果表明:在0.05显著性水平下经统计学方法检验,证明K和⁴⁰Ar^*的F分布值小于F临界值,说明该样品是均匀的。国内8个实验室参加了ZBJ角闪石K含量和⁴⁰Ar^*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都服从正态分布并具等精度。在置信概率为0.95时,⁴⁰Ar*和K含量的相对标准偏差都小于1%。两个特性量值定值分析结果的一致值(认定值)和不确定度分别为:⁴⁰Ar^*=(2.464±0.018)×10^-10mol/g,K=(1.027±0.008)%,K-Ar年龄(标准值)=133.3±1.5Ma(2σ)。此标准物质纯度为98.1%,粒度为0.15～0.30mm,总重量为740g,缩分成最小样品单元共100瓶,每瓶7.4g,可供我国K-Ar和Ar-Ar法同位素年代学实验室使用37年。     

内蒙古赵井沟铌钽矿床燕山期成矿:来自LA-MC-ICP-MS独居石、锆石U-Pb和黑云母40Ar-39Ar年龄的证据

内蒙古武川县赵井沟铌钽矿是新近取得勘查突破的一例大型铌钽矿床.矿体赋存岩石为含天河石钠长花岗岩,其次为花岗细晶岩、云英岩及天河石花岗伟晶岩.运用LA-MC-ICP-MS法对矿区钠长花岗（细晶）岩脉、正长花岗岩中的独居石、锆石进行了U-Pb同位素年龄测定,同时对石英-黑云母脉中的黑云母进行了40Ar-39Ar定年.4件独居石的LA-MC-ICP-MS U-Pb年龄分别为124±2 Ma（MSWD=2.9,n=27）、124±3 Ma（MSWD=2.0,n=46）、121±1 Ma（MSWD=3.3,n=43）和124±2 Ma（MSWD=3.2,n=12）,1件锆石的LA-MC-ICP-MS U-Pb年龄为125±1 Ma（MSWD=1.6,n=18）;黑云母40Ar-39Ar坪年龄为133.84±0.79 Ma（MSWD=3.31）,40Ar/36Ar-39Ar/36Ar等时线年龄为134.55±0.79 Ma（MSWD=1.93）,反等时线年龄为134.58±0.80 Ma（MSWD=1.99）,这些年龄限定赵井沟铌钽矿形成于早白垩世.综合分析表明,燕山期是内蒙古地区重要的铌钽成矿期,赵井沟铌钽矿是燕山期伸展体制下构造岩浆活动的产物.      

40^Ar／39^Ar Ages of Auriferous Quartz Veins from the Fengyang and Zhangbaling Regions and Their Geological Significance

Geotectonically the Fengyang and Zhangbaling regions belong to the North China craton and the Dabie-Sulu oragene, respectively. Neo-Archean gneiss and amphibolite and metamor-phosed sea-facies sodic volcanic rocks axe the main outcrops in the two regions, respectively. The Zhangbaling terrane strike-skipped along the Tancheng-Lujiang fault zone in Mesozoic and Cenozo-ic eras and got close to the Fengyang terrane. Mesozoic Yanshanian intrusions occur broadly in thetwo regions. Gold-beating quartz veins occur in the metamorphic rocks in the Fengyang region and in the granodiorite and metamorphosed sea-facies sodic volcanic rocks in the Zhanghaling region.Generally, the formation of the auriferous quartz veins involved three stages. At the first stage,gold-poor sulfide quartz veins were formed; at the second stage gold-rich quartz sulfide veins wereformed; and at the third stage gold-poor barite and/or carbonate veins were formed. The 40^Ar/29^Ar step-heating plateau ages of the first-stage and the second-stage quartz aggregates from the Zhuding, Maoshan and Shangeheng gold deposits range between 116.1 0.6 Ma and 118.3 0.5 Ma and are pretty close to their least apparent ages and isoehronal ages, respectively. All plat-eau, least apparent and isoehronal ages range between 113.4 0.4 Ma and 118.3 0.5 Ma,which are considered as the formation age range of the quartz. It is reasonable and reliable to takethe 40^Ar/39^Ar age range of the quartz as the formation age range of gold-bearing quartz veins onthe basis of spatial relationship between gold-bearing quartz veins and their country rocks. Thegold deposits in the two regions were formed in Aptian, Cretaceous, when the Tancheng-Lujiangfault zone moved as a normal fault with slightly right-lateral strike-skip, was extensional and expe-rienced very strong magnmtic process. It is shown that the magnmtic hydrothermal fluid is a veryimportant part of the gold ore-forming hydrothermal fluid in the Fengyang and Zhanghaling re-gions. The formation of the gold ore deposits in the Fengyang and Zhanghaling regions had genetic relations with the extensional movement of the Tancheng-Lujiang fault zone and magmatic activities and took place under the extensional dynamic condition in Late Cretaceous. Therefore, the exten-sional movement of the Tancheng-Lujiang fault zone presented the energy and space for magmatic and gold ore-forming processes.