Petrology and geochemistry of REE-rich Mafé banded iron formations (Bafia group,Cameroon)     

Charles Nkoumbou  Fuh Calistus Gentry  Jacqueline Tchakounte Numbem  Yolande Vanessa Belle Ekwe Lobé  Christin Steve Nwagoum Keyamfé 《Comptes Rendus Geoscience》2017,349(4):165-174

Archaean–Paleoproterozoic foliated amphibole-gneisses and migmatites interstratified with amphibolites, pyroxeno-amphibolites and REE-rich banded-iron formations outcrop at Mafé, Ndikinimeki area. The foliation is nearly vertical due to tight folds. Flat-lying quartz-rich mica schists and quartzites, likely of Pan-African age, partly cover the formations. Among the Mafé BIFs, the oxide BIF facies shows white layers of quartz and black layers of magnetite and accessory hematite, whereas the silicate BIF facies is made up of thin discontinuous quartz layers alternating with larger garnet (almandine–spessartine) + chamosite + ilmenite ± Fe-talc layers. REE-rich oxide BIFs compositions are close to the East Pacific Rise (EPR) hydrothermal deposit; silicate BIFs plot midway between EPR and the associated amphibolite, accounting for a contamination by volcanic materials, in addition to the hydrothermal influence during their oceanic deposition. The association of an oceanic setting with alkaline and tholeiitic magmatism is typical of the Algoma-type BIF deposit. The REE-rich BIFs indices recorded at Mafé are interpreted as resulting from an Archaean–Paleoproterozoic mineralization.  相似文献   

Lithological interpretation of crustal composition in the Fennoscandian Shield with seismic velocity data     

M. Kuusisto  I.T. Kukkonen  P. Heikkinen  L.J. Pesonen 《Tectonophysics》2006,420(1-2):283

In this study, we report the results of an investigation of lithological interpretation of the crust in the central Fennoscandian Shield (in Finland) using seismic wide-angle velocity models and laboratory measurements on P- and S-wave velocities of different rock types. The velocities adopted from wide-angle velocity models were compared with laboratory velocities of different rock types corrected for the crustal PT conditions in the study area. The wide-angle velocity models indicate that the P-wave velocity does not only increase step-wise at boundaries of major crustal layers, but there is also gradual increase of velocity within the layers. On the other hand, the laboratory measurements of velocities indicate that no single rock type is able to provide the gradual downward increasing trends. Thus, there must be gradual vertical changes in rock composition. The downward increase of velocities indicates that the composition of the crust becomes gradually more mafic with increasing depth. We have calculated vertical velocity profiles for a range of possible crustal lithological compositions. The Finnish crustal velocity profiles require a more mafic composition than an average global continental model would suggest. For instance, on the SVEKA'81 transect, the calculated models suggest that the crustal velocity profiles can be simulated with rock type mixtures where the upper crust consists of felsic gneisses and granitic–granodioritic rocks with a minor contribution of amphibolite and diabase. In the middle crust, the amphibolite proportion increases. The lower crust consists of tonalitic gneiss, mafic garnet granulite, hornblendite, pyroxenite and minor mafic eclogite. Assuming that these rock types are present in sufficiently extensive and thick layers, they would also have sufficiently high acoustic reflection coefficients for generating the generally well-developed reflectivity in the crust in the central part of the shield. Density profiles calculated from the lithological models suggest that there is practically no density contrast at Moho in areas of the high-velocity lower crust. Comparison of reflectors from FIRE-1 and FIRE-3 transects and the velocity model from SVEKA'81 wide-angle transect indicated that the reflectors correlate with velocity layering, but the three-dimensional structures of the crust complicate such comparisons.  相似文献   

Petrological and source region characteristics of ophiolitic hornblende gabbros from the Aksaray and Kayseri regions, central Anatolian crystalline complex, Turkey   总被引：1，自引：0，他引：1  

K. Kocak  F. Is&#x;k  M. Arslan  V. Zedef 《Journal of Asian Earth Sciences》2005,25(6):883-891

Many Neo-Tethyan ophiolitic bodies are well exposed as thrust-slices in Central Anatolia and are predominantly represented by massive hornblende gabbros, most of which are cut by Supra Subduction Zone (SSZ) plagiogranites. The allochthonous gabbros are distinct from their autochthonous counterparts, with their mineralogy including both igneous hornblende, relict diopside rimmed by replacement hornblende and their chemical composition corresponding mostly to gabbro rather than diorite.The results of major and trace element analyses of forty-two samples, and REE analyses of nine samples, indicate that the hornblende gabbros are SSZ-type and formed from a wet magma by high-degree partial melting of peridotite possibly coupled with contamination by predominantly neighbouring-slab derived fluids within an intra-oceanic back-arc basin. The mafic magmas then underwent high-level fractional crystallization involving titaniferous magnetite, diopside, tschermakite and possibly olivine. Emplacement was followed by extensive ocean–floor metamorphism, which has induced crystallization (or recrystallization) of chlorite, biotite, amphiboles and mobilisation of most of the major elements such as alkali and alkali earth elements, and some LREE.  相似文献   

云南宾川-永胜-丽江地区低钛玄武岩和苦橄岩的岩石成因与源区性质   总被引：3，自引：2，他引：3  

姜常义钱壮志  姜寒冰唐冬梅张蓬勃  朱士飞 《岩石学报》2007,23(4):777-792

云南宾川-永胜-丽江地区是峨眉山玄武岩厚度最大、喷发最早的地区,最主要的岩石类型是低钛和高钛玄武岩,并有少量摘要云南宾川—永胜—丽江地区是峨眉山玄武岩厚度最大、喷发最早的地区,最主要的岩石类型是低钛和高钛玄武岩,并有少量的苦橄质玄武岩、苦橄岩和麦美奇岩。大部分火山岩的岩石化学组成属于拉斑玄武岩系列,少数低钛玄武岩属碱性玄武岩系列。它们不同程度地富集大离子亲石元素和轻稀土元素,相对亏损重稀土元素,稀土元素分馏明显,显著亏损相容元素(Co,V,Cr,Ni)。陆壳物质对低钛玄武岩浆的混染程度明显大于对苦橄质岩浆的影响程度。而且混染作用对于Sr同位素和大离子亲石元素的影响程度明显大于对Nd同位素和稀土元素的影响程度。Nd和Sr同位素证明,混染物主要是下地壳变质岩,也有少量上部陆壳物质。未受混染的样品具有适度亏损的Nd、Sr同位素组成。低钛玄武岩和苦橄岩类岩石是不同原生岩浆分异演化的产物。低钛玄武岩的原生岩浆是高镁拉斑玄武岩浆,原生苦橄质岩浆以EM-55为代表(MgO= 16.56%)。此外,还有一种比EM-55更富镁的原生岩浆。高镁拉斑玄武岩浆分异过程中的主要分离结晶相/堆晶相是单斜辉石,并有少量斜长石。苦橄岩浆分异过程中的主要分离结晶相/堆晶相是橄榄石,并有少量单斜辉石。参考相关的实验岩石学成果,可以证明,地幔柱源区由两种岩石组成:一种是50%榴辉岩和50%橄榄岩反应形成的石榴石辉石岩,另一种是橄榄岩。在地幔柱绝热上升过程中,位于其轴部的石榴石辉石岩的熔融作用始于≈165km,主要的熔融作用发生于165～128km,持续到66km,熔融产物为苦橄岩浆。橄榄岩的部分熔融始于≈150km,持续到66km,熔融产物是比EM-55更富镁的岩浆。地幔柱头部的熔融作用始于≈100km,终止于66km,主要的熔融作用发生于尖晶石稳定域,熔融产物为低钛玄武岩浆。  相似文献   

广西合浦烟墩岭火山岩的K-Ar年代学和地球化学初步研究     

李霓樊祺诚  王团华孙谦 隋建立 《岩石学报》2007,23(6):1423-1430

广西合浦县烟墩岭火山是一座第四纪小型中心式喷发的火山,是我国华南火山在第四纪喷发活动的代表之一。烟墩岭火山岩岩性为粗面玄武岩,Mg~#值(59～69)中等偏高,而钛和碱质成分相对较高。烟墩岭火山岩及周边的烟头岭样品的稀土含量较为稳定,配分型式极其相似,几乎一样,REE 丰度模式呈稍陡的向 HREE 倾斜的轻稀土富集型。它们的微量元素原始地幔配分型式也很相似,暗示有共同的岩浆源区。烟墩岭火山岩的 Ba/Nb 比值变化范围远小于琼北和涠洲岛玄武岩,而 La/Nb 比值三者相近,都是低而稳定,与洋岛玄武岩数值接近,不相容元素 Nb/U-Th 图解也说明烟墩岭火山岩来自于类似于洋岛玄武岩的地幔源区。岩石学和地球化学化学证据都表明,烟墩岭地区火山岩虽不是原生地幔玄武岩浆的喷出物,但只经过低程度演化,未受到强烈的结晶分异的影响,也未受到地壳的混染。其~(87)Sr/~(86)Sr 和~(143)Nd/~(144)Nd 数据变化范围很小,具有低Sr 同位素比值和高 Nd 同位素比值的特点。本文采用火山岩 K-Ar 法测定了烟墩岭火山活动的时代,结合火山岩地质特征和前人资料,火山活动时代确定为第四纪早更新世,与北部湾涠洲岛早期火山活动及琼北早更新世多文岭期火山活动时代大致相当。  相似文献   

陕西省丹凤地区商丹断裂南侧糜棱岩的岩石学研究     

胡能高  安三元 《地球科学与环境学报》1989,(3)

以往都将研究区内商丹断裂南侧的岩石定为角岩化碎屑岩或火山岩,划归泥盆系青石垭组或加里东蛇绿岩套。笔者认为这些岩石是一套糜棱岩,不是一个独立的地层单位。对该区的长英质、富铝质、斜长角闪质和碳酸盐质四类糜棱岩,分别做了岩相学特征描述。通过化学成分、矿物成分、碎斑矿物特征和变余构造恢复了各种糜棱岩的原岩。并讨论了这些原岩的时代归属,认为这些原岩分别来自秦岭群、丹凤群和中泥盆统,说明有不同时代、不同成因的地质体卷入了这条韧性剪切带。  相似文献   

Estimation de la porosité dans un matériau argileuxPorosity assessments in a clay material     

Xavier Guillot  Muzahim Al-Mukhtar  Faiza Bergaya  Jean-Marie Fleureau 《Comptes Rendus Geoscience》2002,334(2):105-109

Porosity measurements on a clay material, carried out through various microscopic techniques, show a significant difference with the total porosity macroscopically estimated. Analyses have allowed to assign this difference, whose importance is linked to the hydromechanical stresses sustained by the material, to microporous spaces lower than 2 nm and to closed pore volumes, not accessible by the used techniques. Besides, the role of the drying methods of samples, which do not allow the removal of a part of the water, is highlighted. Its elimination would lead to irreversible microtextural modifications, prejudicial to the understanding of the macroscopic behaviour of the material. To cite this article: X. Guillot et al., C. R. Geoscience 334 (2002) 105–109.  相似文献   

内蒙锡盟新生代玄武岩的岩石学研究   总被引：6，自引：4，他引：6  

杨建军 《岩石学报》1988,4(2)

依照地层顺序和火山机构的完好程度,内蒙锡盟新生代玄武岩可粗略划分为三期。各期玄武岩又可分出若干层(次)。玄武岩由含量不等的橄榄石、单斜辉石和斜长石斑晶及基质组成。部分玄武岩含有二辉橄榄岩等深源包体。斑晶矿物均具有成分环带。矿物化学、岩石化学资料揭示出本区玄武岩从原生岩浆到进化岩浆的主要演化机制是以橄榄石为主的分离结晶作用,同时在部分进化岩浆中还存在岩浆混合作用。稀土元素资料还指出了一个形成本区原生岩浆以及派生岩浆的原生母岩浆的重要机制:上地幔渐进的部分熔融作用。  相似文献   

岩浆包裹体研究在火山岩岩石学中的地位     

夏林圻 《岩石学报》1987,3(2)

岩浆包裹体研究已经成为现代火山岩岩石学的一项分支学科。研究方面涉及:重溯火山岩浆结晶演化的热历史,提供有关火山岩浆沿下降液相线的成分数据;查明火山岩浆结晶演化过程中化学成分(包括挥发组分)的变迁规律;查明各种岩浆事件的性质(分离结晶、不混溶、混合、混染),及其发生发展的物理化学条件;帮助探索解决某些疑难问题,如下地壳—上地幔的性质及玄武岩浆起源、细碧岩—角斑岩系的成因、测定蚀变火山岩建造的年龄等。此外,岩浆包裹体的实验岩石学研究,以及利用岩浆包裹体阐明火山建造的含矿性,研究火山沉积作用、地热作用等,对于火  相似文献   

冀西北太古宙条带状麻粒岩的岩石学和地球化学特征   总被引：3，自引：2，他引：3  

沈其韩  耿元生 《岩石学报》1996,12(2):247-260

条带状麻粒岩是冀西北太古宙麻粒岩相带中特殊而重要的一种岩类，它由浅色条带和暗色条带组成，条带含量的比例变化不大。浅色条带由较多的斜长石、紫苏辉石、透辉石和一定量的石英组成，伴生少量小而细的磁铁矿；暗色条带由较多的紫苏辉石、透辉石和普通角闪石以及一定量的斜长石组成，伴生有较多较大的磁铁矿。少量黑云母在两种条带中均有所见。两种条带中的同种矿物的光性特征基本相同，而化学成分和稀土元素特征只有很少的差别。暗色条带为基性麻粒岩属ＴＨ２型拉斑玄武岩，而浅色条带的岩石为中酸性麻粒岩，相当英云闪长质类型。这种条带状的岩石形成于不同时期不同性质的岩浆作用和特定的构造环境。暗色条带的物源来自较早的（约２．８Ｇａ）亏损地幔，而浅色条带的物源来自较晚（约２．５Ｇａ）基性岩的部分熔融，与周围的大片区域性英云闪长岩的侵位大致同时。条带状麻粒岩不宜归入ＴＴＧ岩系  相似文献