江西庐山中生代构造事件的40Ar/39Ar同位素年龄研究

江西庐山"星子变质核杂岩"中发育有大量中生代花岗岩,这些花岗岩中具有强烈的韧脆性剪切变形,典型的新生变质矿物为绢云母.通过对庐山海会花岗岩(其形成年龄为122～130 Ma)中动力变质矿物绢云母进行40Ar/39Ar法同位素测年,分别获得104.22 Ma±0.18 Ma和104.95 Ma±0.22 Ma的坪年龄值,以及104.7 Ma土0.36 Ma和105.03Ma±0.8 Ma的等时线年龄值,这表明江西庐山地区在130～104 Ma期间经历了从伸展构造环境向左行走滑构造环境的转变.     

PLC自动控制系统在常规40Ar/39Ar同位素定年系统纯化部分中的应用

对北京大学常规40Ar/39Ar同位素定年实验室纯化系统进行了自动化改造。在明确系统控制要求的基础上,设计了一种以PLC(可编程逻辑控制器)为控制核心对纯化系统进行控制的方法。自动化改造包括硬件和软件两部分:硬件部分的改造是,首先将手动阀门更换为适合系统要求的自动阀门,然后根据纯化系统对自动阀的控制要求对PLC的I/O口进行分配,并设计相关的外围电路;软件部分的改造是,利用辅助寄存器的相关位实现自动阀的互锁和对自动阀实现时序控制。纯化系统的自动化可以节省人力,提高实验仪器的利用效率,并降低人为因素对实验结果的影响,同时为实验室的进一步自动化改造奠定了基础。     

年轻火山岩激光40Ar/39Ar定年中的n值与MSWD值

激光熔蚀⁴⁰Ar/³⁹Ar等时线法是第四纪年轻火山岩样品精细定年的新方法, 该方法处于逐步完善过程中。针对怎样判定获得的年龄结果是否可靠的问题, 通过对数据处理过程中所用参数的分析, 认为样品属性是决定年轻火山岩精细定年结果质量和可靠性的关键因素, 与样品测试数n值相关的加权均方差MSWD(mean square weighted deviation)是判断同一样品各测试点的数据是否符合正态分布, 能否用来客观地评价测试结果, 进而评估样品是否适合进行相关测试, 结果是否可靠的重要指标。其次, 属于同一真实年龄值样品的MSWD在99.7%置信区间的取值范围可通过计算加以确认, 并提出对于表面年龄和等时线年龄数据, 必须定量地考察相应的MSWD值, 判定在具有足够样品测试数(足够大的n值)的情况下, 是否符合概率统计的正态分布, 确定样品是否对应同一年龄真实值。在MSWD值达不到理想值1.0的情况下, 可以利用表面年龄概率密度曲线, 区分测试样品内部包含的非同源或非等时或封闭不好体系的数据, 再结合MSWD与n值关系, 适量地去除部分偏离正态分布的测试数据, 识别出可能归属多个年龄真实值的测试数据组, 并以此计算不同数据组的反等时线年龄, 获得年轻火山岩喷发的准确年龄。     

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 .     

华南武夷山早古生代构造事件的^40Ar／^39Ar同位素年龄研究

华南武夷山地区广泛发育一套强烈韧性剪切变形的中、高级变质岩系,其新生矿物白云母非常普遍.对武夷山南麓政和县蝴碟街大理岩和北麓弋阳县慈竹片麻状花岗岩中的动力变质矿物白云母进行40Ar/39Ar法同位素测年,分别获得391±3Ma和421±8Ma的坪年龄值,以及385±5Ma和422±6Ma的等时线年龄值,时代相当于志留纪末到泥盆纪初.结合地质证据,认为武夷山地区存在过一期早古生代沉积作用和早古生代末期的构造热事件.至少有一期的强烈韧性剪切变形和糜棱岩化作用是该构造事件所导致     

二道沟矿床绢云母的39Ar/40Ar年龄及其地质意义

通过对绢云母３９Ａｒ－４０Ａｒ坪年龄的研究,得到二道沟金矿床的成矿年龄为１４０±２．８Ｍａ;从年代方面证实了金矿化与对面沟花岗闪长岩无直接关系,而与火山岩和次火山岩有密切的关系;该矿床是一岩浆热液为主要来源而混入有一部分大气降水的浅成低温热液型金矿床     

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.     

排山楼金矿床40Ar-39Ar年龄及其地质意义

采用Ar-Ar同位素测年法对辽宁排山楼金矿床矿化阶段形成的钾长石和似斑状花岗岩中的钾长石进行了测试,获得矿化阶段形成钾长石40Ar-39Ar坪年龄为(116.69±1.15)Ma,等时线年龄为(116.97±1.39)Ma,以及似斑状花岗岩中原生钾长石40Ar-39Ar坪年龄为(126.71±2.03)Ma,等时线年龄为(124.00±4.32)Ma的数据.前者代表了该矿床的形成年龄,即该矿床形成于早白垩世,后者表明金矿化不早于(126.71±2.03)Ma.这一成矿年龄与华北陆块北缘大部分金矿床的成矿年龄一致,表明华北陆块北缘绝大多数金矿床形成于中国东部中生代动力学体制发生转折的时期.     

西藏措勤地区布嘎寺正长岩体正长石的^40Ar-^39Ar年龄

西藏措勤地区布嘎寺正长岩体地处冈底斯带西段,空间上与大面积中新世钾质火山岩共生,是以中-中粗粒或似斑状霓辉正长岩为主要岩性的中深成侵入体.岩体中正长石的40Ar-39Ar坪年龄和等时线年龄分别为(27.14±0.05) Ma和(27.15±0.04) Ma,代表岩体的结晶年龄.这一年龄数据表明布嘎寺正长岩是整个冈底斯带目前发现的时代最早的后碰撞超钾质碱性系列岩浆岩,暗示拉萨地体内新生代后碰撞陆内钾质-超钾质岩浆作用至晚应始于27 Ma前,且很可能从大约30 Ma前已经发生,而不是目前认为的25 Ma前.该结论对高原南部岩石圈演化及整个高原后碰撞岩浆事件的对比研究提供了更为可靠的时空定位.     

^40Ar-^39Ar geochronology of Cenozoic Linzizong volcanic rocks from Linzhou Basin, Tibet, China, and their geological implications

Whole-rock and mineral separate Ar-Ar dating was carried out for the Linzizong volcanic rocks at Linzhou Basin in Tibet to constrain the time span of volcanism and the corresponding stratigraphic sequence. Sampling was based on detailed geologic mapping and stratigraphic sequence of Dianzhong, Nianbo, Pana Formations, systematically from the bottom to near the top. The results indicate that the Linzizong volcanic rocks erupted from Paleocene to middle of Eocene (64.43--43.93 Ma). Among them, the Pana Formation formed from ca. 48.73 to 43.9 Ma, the Nianbo Formation around 54 Ma and the Dianzhong Formation from 64.4 to 60.6 Ma. In combination with evidence from the geochemical characteristics of the volcanic rocks, and from stratigraphy in southern Tibet, it is postulated that the age of the lowest member in the Dianzhong Formation of the Linzizong volcanic rock, which overlies unconformably the Late Cretaceous Shexing Formation, likely corresponds to the inception of the collision between Indian and Asian continents in southern Tibet.     

Laser step-heating ^40Ar/^39Ar dating on young volcanic rocks

An attempt was made to use CO_2 laser step-heating method to date the late Pleistocene basaltic groundmass (DF-2) from Tengchong volcanic field. Among the fourteen heating steps, ten define a good inverse isochron (MSWD = 1.4) with an age of 32.2 ± 7.1 ka (2 σ).The inverse isochron also shows that the initial argon isotopic ratio is 297.1±2.0 (2σ) which is the same as the atmospheric argon at the 2σ error level. Study indicates that it is a useful means to date young volcanic groundmass with low K content by using CO_2 laser step-heating ~(40)Ar/~(39)Ar method.     

冀北赤城红旗营子群黑云斜长片麻岩的黑云母40Ar/39Ar年龄及其地质意义

探讨冀北赤城红旗营子群是否经历了印支期的构造热事件.通过对冀北赤城红旗营于群黑云斜长片麻岩样品进行的黑云母40Ar/39 Ar年龄测定,获得其坪年龄和等时线年龄分别为(238.63±1.44) Ma和(238.77±1.71) Ma,两者基本一致.此年龄数据是黑云斜长片麻岩在晓古生代角闪岩相区域变质作用之后所经历印支期构造热事件的记录.综合分析冀北岩浆作用及其地质年代学特征,认为它可能与区域上同时期的酸性岩浆活动有关,是后者为此次构造热事件提供了必要的热源.     

ISEA reversed event in the Cretaceous Normal Super-chron （CNS）： ^40Ar／^39Ar dating and paleomagnetic results

ThegeomagneticfieldwasfoundtobeofnormalpolarityforalongtimeduringtheCretaceousbyHelsleyandSteiner[1]andthiswaslaternamedasCretaceousNormalSuperchron(CNS)[2].TheCNSlastedalmost37Ma(120—83Ma).Severalabnormalgeologicalevents,suchasanoceanicanoxicevent,alargenumberofvol-canismandglobalclimaticwarmingduringtheCNShavebeenreported[3—11].Thishasledtomanyquestionsbeingraised.IsittruethattheEarthsmagneticfielddidnotreverseatallintheCNS?Ifitdidreverse,howmanytimesandwhendidthishappen?Istherea…     

40Ar/39Ar ages and stable isotopes of supergene jarosite from the Baiyin VHMS ore field,NE Tibetan Plateau with paleoclimatic implications

Supergene jarosite is widely distributed in weathering profiles derived from hypogene sulfide ores.Precise40Ar/39Ar dating and determination of hydrogen and oxygen isotopes of supergene jarosite can not only place tight constraints on the timing and history of continental weathering and supergene enrichment of sulfide ore deposits,but also shed significant insights into paleoclimatic conditions and tectonic uplift responsible for the weathering.In this paper,we present a preliminary study of40Ar/39Ar geochronology and stable isotope geochemistry of supergene jarosite from the early Paleozoic Zheyaoshan volcanic-hosted massive sulfide deposit of the Baiyin ore field located in the eastern portion of the Qilian Mountain along the northeastern(NE)margin of the Tibetan Plateau in an attempt to better understand the climatic conditions prevailed during the pervasive weathering in the region.Jarosite from the Zheyaoshan mining area occurs either as gravels within alluvial deposits over low-relief flat-top hills or cm-wide veins crosscutting weathered sulfide ores or their wall rocks.The gravels consist of massive jarosite aggregates with tabular crystal morphology and contain 8.21 wt%–8.31 wt%K2O on average.Two jarosite grains extracted from the gravels(08-02-2 and 08-02-5)yield well-defined40Ar/39Ar plateauages of 37.1±0.3 and 41.2±0.4 Ma(2r),respectively.They have dD values of-133%and-156%,and d18OSO4values of 2.6%and 2.5%.The vein-type jarosite consists of hexagonal pyramid crystals and contains much lower K2O,averaging at 2.44 wt%–2.72 wt%.Two jarosite veins(06-16-3 and 06-16-4)yield similar plateau ages of 3.2±0.1 and 3.3±0.1 Ma(2r),respectively.These jarosite veins have dD values of-158%and-160%,and d18OSO4values of 2.8%and 2.5%.The40Ar/39Ar ages indicate that sulfide ores of the Zheyaoshan deposit must have been brought to the surface since the late Eocene,and subsequently subjected to prolonged oxidation and supergene enrichment.The precipitation and preservation of jarosite in weathering profiles indicate that arid–semiarid climatic conditions must have been prevailed since the late Eocene.The hydrogen and oxygen isotopes of the jarosite further point to variation of climatic conditions in the area,with the Pliocene being slightly cooler relative to the late Eocene.Given that weathering profiles overlying sulfide ore deposits are widely distributed along the Qilian Mountain,systematic studies of40Ar/39Ar geochronology and stable isotope geochemistry of jarosite-group minerals from these profiles would provide valuable information on the timing and evolution of regional tectonic uplift,paleoclimatic changes,and supergene mineralization in this mountainous belt along the NE edge of the Tibetan Plateau.     

Ultra-violet laser probe measurement of ^40Ar／^39Ar age profile in phlogopite

Ultra-Violet Laser Ablation Microprobe (UVLAMP) extraction technique enables the direct investigation of Ar-Ar age prorde in crystals, and yields more information on rates and durations of geological process than conventional single time snapshots. Phlogopite flakes from lamprophyre at Pishan dyke in western Kunlun were dated by using an UV laser (λ= 213 nm) microprobe with spot analyses. The results show good agreement with those from the conventional ^40Ar/^39Ar step heating experiments. This indicates that the Ar isotopes are distributed homogenously in the phlogopite and the UVLAMP can be a powerful tool in the study of thermal history.     

~(40)Ar/~(39)Ar age evidence for Altyn fault tectonic activities in western China

As the northern boundary of the Tibetan Plateau theAltyn fault is a huge NEE strike-slip fault belt in the inner Asian continent. Its formation and evolution are closelyrelated to the uplift of the whole plateau and the mass es-cape to the east. In recent years the Altyn fault has be-come a hot point of geological study in the Tibetan Pla-teau. The formation age of the Altyn fault was argued for a long time. Some researchers emphasized that the Altyn fault had commenced since early Paleo…