青海玉树哈秀岩体成因及^40Ar/^39Ar年代学研究

哈秀石英闪长岩体出露于西金乌兰金沙江缝合带西段,对其进行成因和侵位时代研究可以对沿西金乌兰金沙江缝合带发生的板块俯冲、碰撞事件提供同位素年代制约。用主元素判别法判定其为造山花岗岩类(IAG型),形成于板块俯冲阶段。角闪石的40Ar/39Ar测年结果表明其侵位时代约为216.4Ma,是印支晚期岩浆活动的产物。哈秀岩体的成因和时代研究表明,沿金沙江缝合带,迟至晚三叠世,板块碰撞作用仍然没有开始,洋壳的俯冲作用仍在继续。     

西金乌兰——金沙江缝合带西段印支期石英闪长岩LA-ICP-MS锆石U-Pb年龄与地球化学特征

青海省玉树巴塘一带的达叶隆巴、尕兴隆石英闪长岩体出露于西金乌兰—金沙江缝合带西段,呈不规则长带状侵位于西金乌兰群中。通过锆石U-Pb年代学及岩石地球化学等研究,探讨其形成构造环境。结果表明:两个岩体中的石英闪长岩均形成于印支期[(211.3±1.1)Ma、(211.05±0.83)Ma],且具有相似的岩石地球化学特征,属钙碱性准铝质岩石系列,岩石富集Rb、K、Ba、La、Nd、Sm等大离子亲石元素(LILE),相对亏损P、Zr、Ti等高场强元素(HFSE),Mg、Fe含量较高,P2O5较低,TiO2含量较低,Na2O含量较高,K2O含量低,K2O/Na2O比值小于1。综合研究表明:达叶隆巴、尕兴隆石英闪长岩是形成于俯冲背景下的岛弧花岗岩,因此可以初步确定在西金乌兰—金沙江缝合带玉树巴塘一带,洋壳的俯冲作用至少持续到211.05Ma(晚三叠世末),这一结果为西金乌兰—金沙江缝合带西段发生的板块运动过程提供了新的证据与时限约束。     

青海省俄昌公仁地区尕笛考组火山岩地球化学特征及构造环境

俄昌公仁地区地处西金乌兰—金沙江构造混杂岩带与北羌塘—昌都地体的交汇部位。文章通过对乌丽—开心岭尕笛考组火山岩岩石学、主量元素、稀土元素、微量元素等地质地球化学研究表明:尕笛考组火山岩为典型的钙碱性岩石组合,推断尕笛考组火山岩属于与板块俯冲作用有关的岩浆岛弧带的产物,形成于与板块俯冲作用有关的岛弧构造环境;锆石U-Pb同位素年代为258.5 Ma±4.6 Ma,属晚二叠世早期。     

大兴安岭北部壮志林场花岗岩年代学特征及其大地构造意义

新林—喜桂图断裂带是大兴安岭西北部重要的NE向断裂带,也代表了额尔古纳和兴安地块的结合带。本文通过对大兴安岭北部典型的环二库蛇绿混杂岩地区出露的壮志林场花岗岩的岩石学、地球化学特征及年代学进行研究,探讨其地质意义,为新林—喜桂图断裂带北段地质演化提供基础资料。研究表明,壮志林场花岗岩遭受了强烈糜棱岩化,其年代学研究显示,岩浆锆石LA-ICP-MS U-Pb加权平均年龄为488.9±2.6Ma,表明其形成时代为晚寒武世;该岩体糜棱岩化新生矿物绢云母~(40)Ar/~(39)Ar坪年龄是151.5±1.2 Ma,代表了韧性变形年龄。岩石具有高硅、富钾、富铝,轻稀土元素富集、重稀土元素亏损,富集大离子亲石元素和亏损高场强元素的特征,形成于后造山背景,代表了额尔古纳地块与兴安地块碰撞拼合结束后的产物,其韧性变形年龄代表了新林—喜桂图断裂带北段晚侏罗世受到了蒙古—鄂霍茨克洋演化过程的影响。     

辽宁丹东地区侏罗纪花岗岩的变形时代：^40Ar／^39Ar年代学制约

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。     

冈底斯带门巴花岗岩同位素测年及其对新特提斯洋俯冲时代的约束

本文应用单颗粒锆石SHRIMP U—Pb法和角闪石^40Ar／^39Ar法，对出露在冈底斯构造带上的门巴花岗岩体进行了同位素测年，时代分别为207Ma和215Ma，表明它们形成于晚印支期。地质与地球化学研究显示该时期形成的岩体主要为黑云角闪花岗闪长岩和黑云母二长花岗岩，它们形成于岛弧环境，与其南侧墨竹工卡一带出露的晚三叠世钙碱性火山岩（叶巴组）共同构成与板块俯冲有关的特征性岩石组合，揭示新特提斯洋的俯冲时代应早于晚三叠世。     

内蒙古乌拉特中旗扎嘎乌苏岩体LA-ICP-MS锆石U-Pb年龄、地球化学特征及其地质意义

内蒙古乌拉特中旗中蒙边境地区的扎嘎乌苏岩体主要由闪长岩、石英闪长岩、花岗闪长岩和黑云母二长花岗岩组成,岩体及围岩普遍发育面理南倾的糜棱岩带。通过LA-ICP-MS锆石U-Pb测年,获得扎嘎乌苏岩体中黑云母二长花岗岩的结晶年龄为450.3±3.7Ma(MSWD=2.8),表明扎嘎乌苏岩体的形成时代为晚奥陶世。扎嘎乌苏岩体中不同类型岩石全岩微量和稀土元素组成总体相似,均呈现轻稀土元素相对于重稀土元素强烈富集,大离子亲石元素Ba、Sr等富集,高场强元素Nb、Ta、Ti相对亏损,高Sr,低Y、Yb,高Sr/Y、La/Yb值等特征,具有埃达克岩和岛弧岩浆岩的地球化学特征。这些新的年代学和岩石地球化学数据,为索伦山地区晚奥陶世古亚洲洋俯冲消减提供了可靠的证据。     

甘孜-理塘蛇绿混杂岩带晚三叠世构造演化——来自理塘地区勇杰岩体地球化学、年代学的制约

甘孜-理塘蛇绿混杂岩带是西南三江构造带的重要组成部分,经历了古特提斯构造体系的演化,形成完整的沟-弧-盆体系。义敦岛弧属甘孜-理塘弧盆系范畴,位于甘孜-理塘缝合带西侧。对义敦岛弧勇杰岩体开展详细的岩石学、地球化学、年代学研究,为甘孜-理塘洋盆晚三叠世构造演化研究提供新的证据。义敦勇杰岩体黑云母二长花岗岩和花岗质细晶岩的锆石UPb年龄分别为214.2±1.4Ma和206.2±1.8Ma,整体属高钾钙碱性弱过铝质花岗岩;稀土元素配分曲线具轻稀土元素相对富集、重稀土元素相对平坦的特征,负Eu异常明显;微量元素表现为大离子亲石元素Rb、Ba、Th、U、K相对富集,高场强元素Nb、Ta、P、Ti亏损的特征,显示勇杰岩体黑云母二长花岗岩明显具俯冲型花岗岩的特征,而花岗质细晶岩具有碰撞型花岗岩的特征。总体反映晚三叠世甘孜-理塘洋盆从俯冲至碰撞的地球动力学背景,应属理塘蛇绿混杂岩带碰撞造山过程的产物。     

浙南加里东期龙泉岩体年代学、地球化学特征及其地质意义

龙泉岩体是浙南地区少有的加里东期花岗质岩体.岩石学、 地球化学和年代学研究显示,龙泉岩体由花岗岩-二长花岗岩组合(花岗岩类)和英云闪长岩-奥长花岗岩-花岗闪长岩组合(TTG)组成,两类岩石组分普遍富集大离子亲石元素Rb、Ba、Th、U、K,亏损高场强元素Nb、Ta、P、Ti,显示岛弧岩浆岩的特征.TTG岩石的地球化学特...     

内蒙古北山地区小红山钒钛磁铁矿区双峰式侵入岩——岩石学、年代学、地球化学特征及其地质意义

对内蒙古北山地区小红山钒钛磁铁矿区内侵入岩的岩石学、锆石U-Pb年代学和全岩地球化学资研究显示,该岩体岩性为辉长岩和花岗岩,LA-ICP-MS锆石U-Pb测年结果表明成岩年龄分别为431.1±2.4 Ma和424.9±2.3 Ma,形成时代相近,属同一构造岩浆作用事件. 该套侵入岩的SiO₂含量呈双峰式,稀土、微量元素特征表明它们具有岛弧岩浆岩地球化学特征,其源区可能受到了俯冲流体交代作用的影响. 地球化学特征指示小红山花岗岩源于下地壳物质在高温条件下的部分熔融,辉长岩为富集地幔部分熔融的产物. 结合区域地质背景及构造判别,认为小红山中晚志留世双峰式岩浆组合是北山洋南向俯冲诱导大陆边缘伸展环境下的产物.     

多宝山斑岩铜矿区水热蚀变矿物的激光显微探针40Ar/39Ar定年

在利用脉冲显微激光探针⁴⁰Ar/³⁹Ar定年技术测定标准样品的J值和年龄适用范围的基础上,研究了黑龙江省嫩江县北部多宝山斑岩铜矿区水热成因矿物的年代学。结果表明,高钾含量的水热蚀变矿物适合于激光探针的表面微区测定,并且可以克服水热蚀变矿物中的³⁹Ar “反冲”丢失对年龄结果的影响。矿区的岩浆-水热事件主要有两期,分别为253-220Ma和184-162Ma,前者代表了成矿时代,后者反映出由成矿期后岩浆活动所引起的水热叠加事件。     

Age and origin of charoitite,Malyy Murun massif,Siberia, Russia

Сharoitite consists of gem-quality mineral charoite and subordinate quartz, aegirine, K-feldspar, tinaksite, canasite, and some other minerals. This rock type is known only from one locality in the world associated with the Early Cretaceous (131.3 ± 2.4 Ma, K–Ar age) Malyy Murun syenite massif, Siberia, Russia. Although charoitite mineralogy is well known, there is disagreement whether it reflects metasomatic or magmatic activity. In order to understand when the charoitites formed we attempted to date it by ⁴⁰Ar/³⁹Ar incremental step-heating and laser ablation techniques. Our results show that the fibrous structure of water-bearing charoite does not retain radiogenic argon. Laser ablation ⁴⁰Ar/³⁹Ar for K-feldspar and tinaksite from the charoitite yielded several age clusters even from the same mineral grain. The oldest cluster of 134.1 ± 2.9 Ma for the K-feldspar agrees with the age of the Malyy Murun syenites. The youngest age of 113.3 ± 3.4 Ma for charoitite K-feldspar overlaps with the youngest of published K–Ar ages (112 ± 5 Ma) for one K-feldspar sample of the Malyy Murun syenite. Tinaksite is characterized by a similar spread of ages (from 133.0 ± 3 Ma to 115.7 ± 4.3 Ma) within a single grain. We suggest that charoitites originated due to the interaction of metasomatic agents derived from the Malyy Murun magma and country rocks. Timing of magma emplacement and charoitite crystallization is reflected by the older cluster of ages, whereas the younger ages are due to a secondary process.     

New radiometric dating of the dykes from the Hurd Peninsula, Livingston Island, South Shetland Islands

Fifteen new K–Ar ages in the range of 79–31 Ma are partially confirmed by three ⁴⁰Ar/³⁹Ar plateaus and isochron data of 64.9±0.4, 55.5±0.1 and 52.8±0.6 Ma. The new geochronological data reveal a much more detailed picture of the subduction imprint in the Hurd Peninsula. Using cutting relationships, the dyke emplacement history is divided into four episodes. The Late Cretaceous–Paleocene dykes in the range of 80–60 Ma are related to the main magmatism in Livingston Island and most likely reflect the final stages of subduction of the proto-Pacific oceanic crust. The Early Eocene dykes (56–52 Ma) fill the gap in volcanic activity 70–50 Ma ago. They are the only magmatic event manifested at this time in the region. The 45–42 Ma dykes may be related to the intrusion of the Barnard Point tonalite. Three samples of Oligocene age appear to represent the last igneous activities on the Hurd Peninsula prior to the opening of the Bransfield Strait.     

滇西腾冲更新世粗面安山岩Ar-Ar年代学、地球化学特征及其构造意义

腾冲芒棒地区广泛发育新近纪以来的火山岩,本文首次对该区粗面安山岩进行的高精度40Ar/39Ar同位素定年,其坪年龄分别为2.1±0.1Ma和2.0±0.1Ma,代表其喷溢时代为早更新世。岩石地球化学特征研究显示,粗面安山岩属高钾钙碱性系列,富集大离子亲石元素、亏损高场强元素,具有弧火山岩与大陆板内火山岩的双重属性,推测其形成于后造山环境。综合研究表明,腾冲地块更新世火山岩的形成与印度-欧亚板块俯冲-碰撞造山隆升后的伸展垮塌、富集地幔部分熔融作用密切相关,而印支陆块向东南逃逸以及腾冲弧形走滑断裂带(龙川江断裂带、大盈江断裂带)的活化促使了该期火山岩的喷发与侵位。     

40Ar/39Ar evidence for the timing of Paleoproterozoic gold mineralisation at the Sandpiper Deposit,Tanami region,northern Australia

At the Sandpiper gold deposit in the Tanami region of northern Australia sericite is intimately intergrown with arsenopyrite in gold-bearing quartz veins and breccias, suggesting sericite crystallisation synchronous with gold-bearing fluid flow. This ore-stage sericite yields a ⁴⁰Ar/³⁹Ar plateau age of 1785 ± 32 Ma (2σ including both analytical and systematic uncertainties). Recalculation using revised and more precise values for the ⁴⁰K decay constants and the age of the Fish Canyon Sanidine standard shifts the age to 1794 ±12 Ma (2σ including all known uncertainties). Given the possibility of post-mineralisation isotopic resetting this age can be conservatively interpreted as a minimum constraint on the timing of gold deposition although, given local geological relationships and estimates for the argon retentivity of white mica, we consider complete isotopic resetting to be unlikely. The preferred interpretation is, therefore, that the sericite ⁴⁰Ar/³⁹Ar age indicates the timing of gold mineralisation. Thesericite age accords with a limited dataset of ²⁰⁷Pb/²⁰⁶Pb xenotime ages of ca 1800 Ma from other gold deposits in the Tanami region, interpreted as mineralisation ages. The agreement between independently derived ages from several gold deposits lends support for a widespread gold-mineralising event at ca 1800 Ma in the Tanami region.     

Amphibole Ar/Ar Geochronology from the Okcheon Metamorphic Belt, South Korea and its Tectonic Implications

An understanding of the Okcheon Metamorphic Belt (OMB) in South Korea is central to unraveling the tectono-metamorphic evolution of East Asia. Amphibole-bearing rocks in the OMB occur as calcsilicate layers and lenses in psammitic rocks, in the psammitic rocks themselves, and in the mafic volcanic layers and intrusives. Most amphiboles fail to show ⁴⁰Ar/³⁹Ar plateau ages; those that do have ages ranging from 132 to 975 Ma. The disturbed age pattern and wide variation in ⁴⁰Ar/³⁹Ar ages can be related to metamorphic grade, retrograde chemical reactions, excess Ar and amphibole composition. The oldest age (975 Ma) can be interpreted either as an old igneous or metamorphic age predating sedimentation or a false age caused by excess Ar. The youngest age of 132 Ma and the disturbed age pattern found in amphiboles from rocks located close to Jurassic granitoids are the result of retrograde thermal metamorphic effects accompanying intrusion of the granitoids. Some medium- or coarse-grained amphiboles in the calcsilicates are aggregates of fine-grained crystals. As a result, they are heterogeneous and prove to be readily affected by excess Ar. A disturbed age pattern in amphiboles from the calcsilicates occurring in the high-grade metamorphic zone may also be the product of excess Ar. On the other hand, the disturbed pattern of amphiboles present in the calcsilicates from the low-grade metamorphic zone could arise from both excess Ar and mixed ages. However, amphiboles from psammitic rocks and some calcsilicates in the high-grade metamorphic zone and in intrusive metabasites display real plateau ages of 237 to 261 Ma. The temperature conditions in the high-grade metamorphic zone were higher than the argon closing temperature for amphibole, and the amphiboles in this zone give plateau ages only when they are homogeneous in composition, lack excess Ar, and have not been thermally affected by intrusion of the granitoids. The unmodified ⁴⁰Ar/³⁹Ar ages prove rather younger than the age of the Late Paleozoic metamorphic event of 280 to 300 Ma, but they are close to muscovite K-Ar ages of 263 to 277 Ma. These ⁴⁰Ar/³⁹Ar amphibole ages are interpreted as the time of cooling that followed the main regional, intermediate-P/T metamorphic climax. The results demonstrate that interpretation of ⁴⁰Ar/³⁹Ar amphibole ages in an area subjected to several metamorphic events can be accomplished only by undertaking a thorough tectono-metamorphic study, accompanied by detailed chemical analysis of the amphiboles.     

Ar/Ar thermochronology of the Kampa Dome, southern Tibet: Implications for tectonic evolution of the North Himalayan gneiss domes

Structural and thermochronological studies of the Kampa Dome provide constraints on timing and mechanisms of gneiss dome formation in southern Tibet. The core of Kampa Dome contains the Kampa Granite, a Cambrian orthogneiss that was deformed under high temperature (sub-solidus) conditions during Himalayan orogenesis. The Kampa Granite is intruded by syn-tectonic leucogranite dikes and sills of probable Oligocene to Miocene age. Overlying Paleozoic to Mesozoic metasedimentary rocks decrease in peak metamorphic grade from kyanite + staurolite grade at the base of the sequence to unmetamorphosed at the top. The Kampa Shear Zone traverses the Kampa Granite — metasediment contact and contains evidence for high-temperature to low-temperature ductile deformation and brittle faulting. The shear zone is interpreted to represent an exhumed portion of the South Tibetan Detachment System. Biotite and muscovite ⁴⁰Ar/³⁹Ar thermochronology from the metasedimentary sequence yields disturbed spectra with 14.22 ± 0.18 to 15.54 ± 0.39 Ma cooling ages and concordant spectra with 14.64 ± 0.15 to 14.68 ± 0.07 Ma cooling ages. Petrographic investigations suggest disturbed samples are associated with excess argon, intracrystalline deformation, mineral and fluid inclusions and/or chloritization that led to variations in argon systematics. We conclude that the entire metasedimentary sequence cooled rapidly through mica closure temperatures at  14.6 Ma. The Kampa Granite yields the youngest biotite ⁴⁰Ar/³⁹Ar ages of  13.7 Ma immediately below the granite–metasediment contact. We suggest that this age variation reflects either varying mica closure temperatures, re-heating of the Kampa Granite biotites above closure temperatures between 14.6 Ma and 13.7 Ma, or juxtaposition of rocks with different thermal histories. Our data do not corroborate the “inverse” mica cooling gradient observed in adjacent North Himalayan gneiss domes. Instead, we infer that mica cooling occurred in response to exhumation and conduction related to top-to-north normal faulting in the overlying sequence, top-to-south thrusting at depth, and coeval surface denudation.     

琼北火山岩激光40Ar/39Ar定年研究

新生代以来,雷琼地区多次、大量地喷发了一系列火山岩。前人主要基于K-Ar法对此划分了期次。本文采用激光40Ar/39Ar年代学方法,对琼北火山岩区进行了精细定年研究。低本底激光40Ar/39Ar法能够对低钾含量,极少量样品(毫克级)进行精细测定,非常适合极年轻火山岩的定年工作。结果显示的火山岩激光40Ar/39Ar法高质量数据表明琼北火山喷发活动时限跨越1.3～0.052Ma。在比较了表观年龄与等时线年龄差异之后,本文给出了年龄推荐值。正如测试数据所显示,本地区新生代火山岩普遍存在40Ar和36Ar过剩的问题,此时只有等时线年龄才代表喷发的真实年龄。     

藏北羌塘中部绒玛地区蓝片岩岩石学、矿物学和40Ar/39Ar年代学

藏北羌塘中部沿龙木错-双湖-线出露一条低温高压变质带,目前已有多处蓝片岩的报道.然而,除冈玛错地区产有典型的蓝闪石外,多数地区并没有典型蓝闪石的报道.绒玛蓝片岩位于羌塘中部高压变质带的中段,是该带中规模最大、保存最好的蓝片岩,对蓝片岩进行了详细的岩石学和矿物学研究,钠质角闪石主要为蓝闪石、青铝闪石、钠闪石和镁钠闪石.对蓝片岩中蓝闪石和多硅白云母进行了40Ar/39Ar定年,获得了227.3±3.8Ma和215±1.5Ma的坪年龄,分别代表蓝片岩快速俯冲消减和俯冲作用结束开始折返抬升的时代.绒玛蓝片岩岩石学、矿物学和40Ar/39Ar年代学研究为羌塘中部高压变质带的研究提供了新的资料.     

Neoproterozoic tectonothermal evolution of the Central Eastern Desert, Egypt: a slow velocity tectonic process of core complex exhumation

Regional cooling in the course of Neoproterozoic core complex exhumation in the Central Eastern Desert of Egypt is constraint by ⁴⁰Ar/³⁹Ar ages of hornblende and muscovite from Meatiq, Sibai and Hafafit domes. The data reveal highly diachronous cooling with hornblende ages clustering around 580 Ma in the Meatiq and the Hafafit, and 623 and 606 Ma in the Sibai. These ⁴⁰Ar/³⁹Ar ages are interpreted together with previously published structural and petrological data, radiometric ages obtained from Neoproterozoic plutons, and data on sediment dynamics from the intramontane Kareim molasse basin. Early-stage low velocity exhumation was triggered by magmatism initiated at 650 Ma in the Sibai and caused early deposition of molasses sediments within rim synforms. Rapid late stage exhumation was released by combined effect of strike-slip and normal faulting, exhumed Meatiq and Hafafit domes and continued until 580 Ma. We propose a new model that adopts core complex exhumation in oblique island arc collision-zones and includes transpression combined with lateral extrusion dynamics. In this model, continuous magma generation weakened the crust leading to facilitation of lateral extrusion tectonics. Since horizontal shortening is balanced by extension, no major crustal thickening and no increase of potential energy (gravitational collapse) is necessarily involved in the process of core complex formation. Core complexes were continuously but slowly exhumed without creating a significant mountain topography.