华南双桥山群和河上镇群凝灰岩中的锆石SHRIMP U-Pb年龄—对江南新元古代造山带演化的制约     

高林志  杨明桂  丁孝忠  刘燕学  刘训  凌联海  张传恒 《地质通报》2008,27(10)

通过西澳科庭大学离子探针中心的远程测试,在双桥山群横涌组和安乐林组斑脱岩中获得大量锆石,其SHRIMP U-Pb加权平均年龄为831 Ma±5 Ma(横涌组)、829 Ma±5 Ma（安乐林组）,在河上镇群上墅组中获得加权平均年龄767 Ma±5 Ma。锆石SHRIMP U-Pb年龄表明华南地区广为发育的双桥山群应归入新元古界,该年龄为标定双桥山群在地层柱中的位置提供了准确的年代学依据。  相似文献   

武夷山南段前寒武纪浅变质岩系的划分与时代     

张利民 《地层学杂志》1996,20(1):16-22

武夷山南段前寒武纪地层分布较广。从下至上分为楼子坝群、丁屋岭组、南岩组和黄连组。过去因缺乏可靠的微古植物化石证据和同位素地质年龄资料,对其时代归属分歧颇大。为此,作者对该区的前寒武系进行了系统的微古植物取样、分析,参考最近的Nd同位素模式年龄资料,认为:楼子坝群为前震旦系,大致相当于蓟县系与青白口系;丁屋岭组为下震旦统;南岩组和黄连组为上震旦统;江西东南部的"下寒武统炭质板岩"应归入老虎塘组,属上震旦统。  相似文献   

Radiocarbon redating of contaminated samples from a tropical volcano: the Mansion ‘Series’ of St Kitts,West Indies     

D. D. Harkness  M. J. Roobol  A. L. Smith  J. J. Stipp  P. E. Baker 《Bulletin of Volcanology》1994,56(5):326-334

Heavy rainfall and dense vegetation on tropical volcanoes produce abundant carbonized wood in pyroclastic deposits, in addition to easy contamination of this wood by root systems and soluble humic material. Because the physical nature of the charcoal varies, some samples are more prone to contamination. Two independent studies of the same volcano, Mt Liamuiga on St Kitts in the Lesser Antilles, sometimes using samples from the same carbonized tree, yielded a systematic difference in radiocarbon ages. An exchange of samples and a re-investigation of three physically distinct types of charcoal yielded the following results. Rare, hard, dense charcoal, lacking contamination, which had yielded a spurious age of 2860 years bp, was redated at 1845±58 years bp. Common soft, friable charcoal with good cellular structure proved to be susceptible to contamination. A field decontamination technique utilized by one group seems significant as it yields older ages than when only routine laboratory pre-treatment was used, indicating that the latter technique only partly removes the dried and hard residue produced by the decomposition of modern plant rootlets. A previous date of 24870 years bp obtained from powdery charcoal in a horizon beneath the Mansion Series contradicted ages older than 41000 years bp from common friable charcoal in the lower Mansion Series. The soft powdery charcoal was re-investigated using a sample collected a few centimeters from the original, although field decontamination of this sample was not possible, more extensive laboratory treatment yielded an age of ca. 43000 years bp, again proving that routine laboratory pretreatments are inadequate. A revised geochronology for the Mansion Series is described and a cautionary discussion is presented for the benefit of investigators using radiocarbon ages to date volcanic deposits.  相似文献   

陆丰南地区断拗转换界面厘定及其地质意义     

阙晓铭  雷永昌  张向涛  邱欣卫 《地质学报》2022,96(11):3943-3954

珠江口盆地经历裂陷阶段向拗陷阶段的转变，同期沉积地层记录了该构造变革和沉积演化过程，为厘定断拗转换界面提供了可能。基于地震反射、沉积旋回特征，识别出陆丰南地区恩平组内部存在全区可追踪对比的构造沉积转换面——T72界面，也是区分上下恩平组界面。对陆丰13洼洼间隆钻井LFA开展上下恩平组碎屑锆石U- Pb 定年、重矿物组分、元素分析，提出恩平组T72界面上下地层物源体系发生转变。结合区域沉积充填特征，表明陆丰南地区T72界面为断- 拗转换界面。具体表现为：下恩平组沉积期湖盆继承文昌期裂陷盆地特征，湖盆水体较深，物源主要为盆地内部的中生代基底凸起，沉积层序明显受盆内低凸起控制，在盆内低凸起四周发育一系列近源的辫状河三角洲，井区主要接受来自惠陆低凸起物源;上恩平组沉积期间，湖盆进入断拗过渡期，湖盆扩大、水体变浅，盆内低凸起四周的断裂活动减弱，沉积层序逐渐向盆内低凸起超覆，盆外太古宙—古生代物源组分供给不断加强，近源的三角洲砂体逐渐向远源的浅水三角洲砂体转化，同时发育宽广的滨浅湖环境，原供给物源的惠陆低凸起为水体淹没，井区接受了北部隆起外的物源。  相似文献   

甘孜- 理塘洋可能存在北向俯冲：来自松潘- 甘孜道孚- 炉霍岩浆岩的证据     

卢雨潇  杨经绥  许志琴  连东洋 《地质学报》2022,96(7):2380-2402

松潘- 甘孜造山带巨厚的三叠系复理石沉积盖层给探讨其基底性质、俯冲- 碰撞过程和深部岩浆作用增添了难度，使得带内广泛出露的花岗质岩体和少量镁铁质岩体成为解开松潘- 甘孜造山带构造演化谜团的重要研究对象。锆石U- Pb定年结果表明道孚花岗闪长岩形成于223. 5~217. 4 Ma，炉霍二长岩结晶年龄为219. 4 Ma，辉长岩为218. 9 Ma，均属于晚三叠世岩浆活动的产物。化学成分研究结果表明，花岗质岩石都表现出I型花岗岩特征，其中炉霍二长岩具有较高的Ba、Sr含量，相对较高且均一的εHf(t)值（-3. 69~-1. 65），表明其可能来源于富集玄武质新生下地壳的熔融。道孚花岗闪长岩具有分散的εHf(t)值（-13. 51~0. 41），野外和地球化学特征指示其形成于幔源熔体与古老壳源熔体的混合。辉长岩在微量元素蜘蛛图上具有类似的模式，不同程度富集Ba、Sr、Pb、Th和U元素，出现Nb、Ta、Ti元素亏损，具有岛弧玄武岩的特征，来源于流/熔体交代的地幔楔部分熔融。综合区域已有资料，我们对甘孜- 理塘洋的演化历史提出新的认识，认为甘孜- 理塘洋不仅仅存在向南俯冲的可能性，同时也具有北向俯冲的历史，晚三叠世道孚- 炉霍岩浆岩的形成受控于甘孜- 理塘洋向北边松潘- 甘孜地块俯冲的背景之下。  相似文献   

秦岭南缘勉略带构造属性及晚古生代地质背景:来自碎屑锆石U-Pb年代学的制约     

李小兵  裴先治  李佐臣  李瑞保  裴磊  高峰  王盟 《岩石学报》2021,37(5):1444-1468

勉略构造带作为秦岭造山带内重要的构造边界,关于其构造属性及晚古生代以来的地质背景,一直是学术界争论的焦点。碎屑锆石U-Pb年代学在限定地层单元的最大沉积年龄、研究区域构造岩浆事件及约束构造地质背景等方面行之有效。基于此,通过对勉略带内五郎坪北侧两河口变沉积地层和侵入其中的变形花岗岩脉体进行LA-ICP-MS锆石U-Pb年代学研究。获得2件变形花岗岩脉的结晶年龄均为406±1Ma。碎屑锆石主年龄谱分别为422～456Ma和558～826Ma,峰值年龄为441Ma和771Ma、813Ma,次级年龄谱分别为942～1495Ma和1658～2981Ma,峰值年龄不明显。依据最小一组碎屑锆石的峰值年龄(441Ma),和侵入其中的变形花岗岩脉(406±0.6Ma),限定该变沉积地层形成时代为406～441Ma(S_1-D_1)。碎屑锆石年龄谱显示该套变沉积地层物质来源较为复杂,其中秦岭造山带及扬子板块北缘早古生代、新元古代岩浆岩为其提供了74%±的物源,古老变质基底为其提供了26%±的物源。通过与区域上已有资料对比,认为勉略构造带内晚古生代沉积地层形成环境与邻区大致相同,且本次所获得的变沉积岩碎屑锆石年龄谱也与邻区泥盆系相似。综合认为,勉略构造带与邻区在晚古生代应属同一构造环境,晚古生代"勉略海盆"应当包括整个南秦岭。  相似文献   

The age of the Kagenfels granite (northern Vosges) and its bearing on the intrusion scheme of late Variscan granitoids     

J. C. Hess  H. J. Lippolt  B. Kober 《International Journal of Earth Sciences》1995,84(3):568-577

In the Saxothuringian part of the Vosges (France), a first series of Variscan plutonic rocks (diorites to granites) has been intruded by several younger granites. Rocks of both the older generations have been cross-cut by the late orogenic Kagenfels granite. The averages of the hitherto published mineral ages of the earlier rock generations are 331 and 334 Ma, respectively, whereas Rb-Sr and K-Ar dates around 290 Ma have been reported for the Kagenfels granite. Because of the unlikely large age hiatus, a redetermination of the intrusion age of the Kagenfels granite formation appeared to be irrevocable. The newly obtained mineral ages on the Kagenfels granite (K-Ar and ⁴⁰Ar/³⁹Ar biotite ages as well as single zircon radiogenic ²⁰⁷Pb/²⁰⁶Pb data: 331 ± 5 Ma) are about 40 Ma older than the previous results. They are interpreted as giving the time of emplacement of the Kagenfels granite during the latest Visan. The mineral ages of the earlier plutonic rocks in this part of the Variscan Orogeny in all probability are not significantly different from their ages of intrusion. Therefore the age concordance of all three granitoid generations constrains a rather narrow time interval of orogenic magmatism close to the Lower-Upper Carboniferous boundary.  相似文献   

Exposure ages from relict lateral moraines overridden by the Fennoscandian ice sheet     

Derek Fabel  David Fink  Jon Harbor  Arjen P. Stroeven 《Quaternary Research》2006,65(1):136-146

Lateral moraines constructed along west to east sloping outlet glaciers from mountain centred, pre-last glacial maximum (LGM) ice fields of limited extent remain largely preserved in the northern Swedish landscape despite overriding by continental ice sheets, most recently during the last glacial. From field evidence, including geomorphological relationships and a detailed weathering profile including a buried soil, we have identified seven such lateral moraines that were overridden by the expansion and growth of the Fennoscandian ice sheet. Cosmogenic ¹⁰Be and ²⁶Al exposure ages of 19 boulders from the crests of these moraines, combined with the field evidence, are correlated to episodes of moraine stabilisation, Pleistocene surface weathering, and glacial overriding. The last deglaciation event dominates the exposure ages, with ¹⁰Be and ²⁶Al data derived from 15 moraine boulders indicating regional deglaciation 9600 ± 200 yr ago. This is the most robust numerical age for the final deglaciation of the Fennoscandian ice sheet. The older apparent exposure ages of the remaining boulders (14,600-26,400 yr) can be explained by cosmogenic nuclide inheritance from previous exposure of the moraine crests during the last glacial cycle. Their potential exposure history, based on local glacial chronologies, indicates that the current moraine morphologies formed at the latest during marine oxygen isotope stage 5. Although numerous deglaciation ages were obtained, this study demonstrates that numerical ages need to be treated with caution and assessed in light of the geomorphological evidence indicating moraines are not necessarily formed by the event that dominates the cosmogenic nuclide data.  相似文献   

Temperature–composition–time (T–X–t) data from authigenic K-feldspar: An integrated methodology for dating fluid flow events     

Darren F. Mark  John Parnell  Simon P. Kelley  Sarah C. Sherlock 《Journal of Geochemical Exploration》2006,89(1-3):259

Integration of fluid inclusion analysis with high spatial resolution Ar–Ar dating of K-feldspar cements has been used to resolve and reconstruct palaeo-fluid flow. Fluid inclusion analysis allows discrimination of distinct cement phases, thereby identifying discrete episodes of fluid flow. Ar–Ar dating of the same cements via high spatial resolution laserprobe establishes absolute age constraints on the framework previously constructed. Integration of these two datasets yields temperature–composition–time data.  相似文献   

The bimodal rift-related Juscelândia volcanosedimentary sequence in central Brazil: Mesoproterozoic extension and Neoproterozoic metamorphism     

Renato Moraes  Reinhardt A. Fuck  Mrcio Martins Pimentel  Simone M.C.L. Gioia  Maria H.B.M. de Hollanda  Richard Armstrong 《Journal of South American Earth Sciences》2006,20(4):287-301

The Barro Alto Complex and Juscelândia volcanosedimentary sequence are exposed in the central part of the Neoproterozoic Brasília belt of central Brazil. The former is a large (approximately 150 km long), boomerang-shaped, mafic-ultramafic, layered complex formed by two different intrusions metamorphosed under granulite facies. These rocks are tectonically overlain by rocks of the Juscelândia volcanosedimentary sequence, represented mainly by biotite-gneiss and amphibolite, or amphibolite facies metamorphic equivalents of rhyolite and basalt, respectively. New SIMS U–Pb zircon data and Sm–Nd isochron data presented herein help clarify the igneous and metamorphic evolution of the Juscelândia volcanosedimentary sequence, as well as its relationship with the Barro Alto Complex. Zircon grains from two biotite gneisses were analyzed by SIMS (SHRIMP) and indicate Mesoproterozoic dates, approximately 1.28 Ga, interpreted as the time of bimodal volcanism in a tectonic setting transitional between a continental rift and an ocean basin. Metamorphism is constrained by Sm–Nd garnet-whole-rock isochrons for garnet amphibolite and pelitic schists of the Juscelândia sequence, as well as for clinopyroxene-garnet amphibolite and garnet granulite of the Barro Alto Complex, which give ages between 0.74 and 0.76 Ga, in agreement with SIMS dates for metamorphic zircon rims. These new data are significant, because they establish that a single metamorphic event affected both the Barro Alto Complex and the Juscelândia sequence. Based on these new data, we present a modified tectonic model for the Brasília belt.  相似文献