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51.
长江中下游晚中生代中基性岩的铅同位素特征:富集地幔的证据 总被引:33,自引:3,他引:33
长江中下游东段庐枞、怀宁、繁昌、铜陵和宁芜地区的中基性岩属于碱性系列 ,具有高的U ,Th含量和Th/Pb ,U/Pb比值 ,分别平均为 2 .82× 10 -6,9.5 6× 10 -6和 0 .6 35 ,0 .184。样品的初始铅同位素 (130Ma)组成为 :(2 0 6Pb/ 2 0 4Pb) i=17.6 5~ 18.6 0 ,(2 0 7Pb/ 2 0 4Pb) i=15 .4 2~ 15 .5 0 ,(2 0 8Pb/ 2 0 4Pb) i=37.6 7~ 38.0 4。形成中基性岩的原始岩浆来源于富集的岩石圈地幔 ,具有EMⅠ和EMⅡ ,且以EMⅡ为主的特征。和长江中下游西段黄石地区以及大别地块西南部玄武岩的比较表明 ,长江中下游地区岩石圈地幔高的Th/Pb和U/Pb比值可能和俯冲板片析出流体的交代有关。晚中生代时期 ,华北板块岩石圈地幔以EMⅠ特征为主 ,华南板块岩石圈地幔以EMⅡ特征为主 ,岩石圈地幔性质的区域性分布与印支期扬子板块深俯冲事件密切相关。虽然中国东部新生代玄武岩因岩石圈大规模减薄表现了亏损特征 ,但残留的富集岩石圈地幔在中国东部新生代玄武岩的Pb同位素中仍有所反映 相似文献
52.
渤海湾老狼坨子海岸带^14C,^137Cs、^210Pb测年与现代沉积速率的加速趋势 总被引:10,自引:5,他引:10
通过对渤海湾西岸L剖面中同一关键层的碳酸钙和原生腹足类壳债权的^14C测年对比研究,确定了晚全新世的2800cal BP的层位及沉积速率0.045cm/a;8个样柱的^137Cs和^210Pbex强度、蓄积量揭示了距今约120年来堤后盐沼的平均沉积速率约为0.35cm/a,而面向开放海湾的潮坪上部则达到约2-3cm/a。研究区近1个世纪以来沉积速率的加速趋势,是渤海湾西岸泥质海岸带的特征之一。 相似文献
53.
The Bikou volcanic terrane is predominated by subalkaline tholeiitic lavas. Rock samples display lower initial ratios of Sr and Nd, 0.701248-0.704413 and 0.511080-0.512341 respectively. 207Pb and 208Pb are significantly enriched in the lavas. Most samples have positive εNd, which indicates that the magma was derived from EM-type mantle source, while a few samples with negative εNd indicate that there was contamination in the magma evolution. Magma differentiation is demonstrated by variations of LREE and LILE from depletion to enrichment. Additionally, normalized REE patterns and trace elements showed that lavas from the Bikou volcanic terrane have similar characteristics to those of basalts in arc settings caused by subduction and collision. Analyses showed that the Bikou volcanic terrane is a volcanic arc. New evidence proved that the Hengdan Group, north of the Bikou arc, is a turbidite terrane filling a forearc basin. Consequently, the Bikou volcanic terrane and the Hengdan turbidite terrane const 相似文献
54.
相山铀矿田成矿机理研究 总被引:44,自引:2,他引:44
本文重点研究了相山铀矿田的成矿时代及成矿环境 ,并剖析了典型矿床在垂向上物质成份的变化规律。研究结果表明 :相山矿田主要经历了两期铀矿化作用 ,第一期为铀 赤铁矿化阶段 ,成矿年龄为 1 1 5± 0 6Ma ;第二期为铀 萤石 水云母化阶段 ,成矿年龄为 99± 6Ma。两期成矿作用分别形成于不同的地质环境 ,第一期成矿作用主要与大规模火山塌陷及次火山岩侵位有关 ,第二期成矿作用则主要与因太平洋板块的松弛作用而形成的区域性伸展、裂解及中基性脉岩的活动有关。相山矿田的热液蚀变类型不仅在平面上存在东碱、西酸的演化趋势 ,而且在垂向上还存在上酸、下碱的演化规律。通过对相山矿田成矿机理的深入探讨 ,认为相山矿田是成矿元素多阶段富集、成矿热液多期叠加以及多种地质因素共同作用的产物。 相似文献
55.
56.
博格达山晚石炭纪造山活动的变形地质记录 总被引:13,自引:2,他引:13
主要由钙碱性火山岩、火山碎屑岩组成的博格达古岛弧是天山缝合造山带的重要组成部分 ,是一个发育较成熟的山链 ,其演化经历了晚古生代的韧性剪切收缩 ;中生代伸展调整及新生代再造山过程。晚古生代的造山活动在博格达山有很好的地质记录 ,并以显著的韧性剪切变形带的形成和发育同造山的褶皱构造为特点。剪切变形带内同构造的石英脉中的锆石U PbSHRIMP测年结果与山链中花岗岩、辉长岩年龄颇为一致 (311~ 316Ma) ,这个年龄反映在结束洋盆散聚、碰撞焊接的晚华力西期造山过程中 ,博格达古岛弧内存在一次虽不甚强烈 ,但又较为明显的构造岩浆事件 ,其成因可能与引起石炭纪大规模裂陆式喷发的深部断裂构造重新活动有关。 相似文献
57.
Axel K Schmitt Jan M Lindsay Shan de Silva Robert B Trumbull 《Journal of Volcanology and Geothermal Research》2003,120(1-2):43-53
High spatial resolution U–Pb dates of zircons from two consanguineous ignimbrites of contrasting composition, the high-silica rhyolitic Toconao and the overlying dacitic Atana ignimbrites, erupted from La Pacana caldera, north Chile, are presented in this study. Zircons from Atana and Toconao pumice clasts yield apparent 238U/206Pb ages of 4.11±0.20 Ma and 4.65±0.13 Ma (2σ), respectively. These data combined with previously published geochemical and stratigraphic data, reveal that the two ignimbrites were erupted from a stratified magma chamber. The Atana zircon U–Pb ages closely agree with the eruption age of Atana previously determined by K–Ar dating (4.0±0.1 Ma) and do not support long (>1 Ma) residence times. Xenocrystic zircons were found only in the Toconao bulk ignimbrite, which were probably entrained during eruption and transport. Apparent 238U/206Pb zircon ages of 13 Ma in these xenocrysts provide the first evidence that the onset of felsic magmatism within the Altiplano–Puna ignimbrite province occurred approximately 3 Myr earlier than previously documented. 相似文献
58.
Rosanna De Rosa Herv Guillou Roberto Mazzuoli Guido Ventura 《Journal of Volcanology and Geothermal Research》2003,120(3-4):161-178
A geochronological study of the Filicudi, Salina, Lipari and Vulcano Islands (Aeolian Archipelago) using the unspiked potassium–argon technique provides new age data which, combined with stratigraphic correlation, better constrain the temporal evolution of volcanism. The unspiked K–Ar age of the oldest exposed lavas on Filicudi, 219±5 ka, is significantly younger than the previous estimation of 1.02 Ma. In the general context of Aeolian volcanism, this new date suggests that the volcanism of the western sector of the Aeolian Archipelago is younger than previously thought. Geochronological data point out on the rapid transition from calc–alkaline to potassic volcanism. The distribution of the K–Ar ages within the Salina–Lipari–Vulcano group shows that the volcanism started on Lipari and propagated over time northward on Salina and southward on Vulcano. Geochronological and geophysical data suggest that the onset of volcanism in the central sector of the Aeolian Arc may be due to a mantle upwelling structure located below Lipari. A change in the style of the eruptions occurred in the Salina–Lipari–Vulcano system at about 100 ka from the present. Low-energy magmatic eruptions occurred between 188 and about 100 ka. From about 100 ka to the present, higher-energy eruptions and low-energy events due to magma–water interaction also occurred. This change in the style of activity, together with the appearance of evolved products (i.e. rhyolites) during the last 50 ka, is consistent with the formation of magmatic reservoirs located at shallower depth with respect to those of the 188–100-ka period. The new geochronological data and available petrological models reveal that a change in the deep source of the primary magmas occurred in a relatively short time interval. 相似文献
59.
Abderrahim Essaifi Alain Potrel Ramon Capdevila Jean-Louis Lagarde 《Comptes Rendus Geoscience》2003,335(2):193-203
The bimodal magmatism of central Jebilet is dated to 330.5+0.68?0.83 Ma by UPb dating on zircons. This age, similar to that of the syntectonic Jebilet cordierite-bearing granitoids, corresponds to the age of the local major tectonometamorphic event. The syntectonic plutonism of the Jebilet massif, composed of tholeiitic, alkaline, and peraluminous calc-alkaline series, is variegated. Magmas emplacement was favoured by the local extension induced by the motion along the western boundary of the Carboniferous basins of the Moroccan Meseta. The Jebilet massif exemplifies the activation of various magmas sources during an episode of continental convergence and crustal wrenching. 相似文献
60.
Bruno Dhuime Delphine Bosch Olivier Bruguier Renaud Caby Carlos Archanjo 《Comptes Rendus Geoscience》2003,335(15):1081-1089
The Borborema Province (BP) of northeastern Brazil is a complex crustal assemblage, which has undergone a polycyclic evolution during the Proterozoic. In the Piancó-Alto Br??gida belt, a metamorphosed leucosome vein inserted in amphibolites has a trace element pattern suggesting a T-MORB protolith. Apatites yield a REE pattern indicating growth in equilibrium with garnet, thus pointing to its metamorphic origin. UPb analyses yield an age of 540±5 Ma interpreted as a cooling age following amphibolite facies regional metamorphism associated with granitic emplacement at ca. 580 Ma. The resulting slow cooling rates (ranging from ca. 2.5 to 5 °C Ma?1) are consistent with underplating of mafic magmas, or crustal thickening caused by nappe stacking, as possible processes governing the metamorphic evolution of the BP. To cite this article: B. Dhuime et al., C. R. Geoscience 335 (2003). 相似文献