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1.
The French Massif Central (FMC) represents the whole West European Variscan (WEV) belt, in terms of both the geodynamic evolution and the metallic content. Thus, a study of the metallogenic evolution of the FMC may elucidate the conditions that allow the mineralisation of a collision belt, since recent collision belts, e.g. the Himalayas or the Alps show that mineralisation does not necessarily result from the collision process. The Palaeozoic history of the FMC is divided into three geodynamic stages unevenly involved from the metallogenic view point. The Eo-Variscan stage (Cambrian to Silurian) was not important; the Meso-Variscan stage (Devonian-Early Carboniferous) was of limited importance; and most of the mineralisations formed during the Neo-Variscan stage (Late Carboniferous-Early Permian). In addition, some more mineralisation was produced during the Mesozoic because of the thermal reactivation linked with the Alpine orogenies. The Eo-Variscan stage (Cambrian-Silurian) corresponded to the pre-collision history, marked at the WEV belt scale by a fragmentation of the northern Gondwana (immature crust evolved from the Late Proterozoic Cadomian orogeny), up to the break-up of the crust and the formation of oceanic basins (Cambrian-Ordovician), followed by their resorption by subduction during the Silurian. In the FMC, no subduction-related magmatism is known (being rare at the WEV belt scale), and consequently subduction-related mineralisation, e.g. porphyry copper, is unknown in the WEV belt. Although some ophiolitic remnants are known, they never display Cyprus-type VMS deposits, nor massive podiform chromitites. Beside platformal sedimentary deposits on passive margins, the only deposits formed during the Eo-Variscan stage were of the SEDEX type, linked with the early rifting of the Gondwanian crust. The Meso-Variscan stage (Devonian-Early Carboniferous) corresponded to the collision proper, with the formation of crustal-scale nappe structures and the intrusion of collision-related peraluminous granites. Although these granites were enriched in rare metals they did not yield significant hydrothermal mineralisation, due to the great depth of their emplacement, as the similar granites in the Himalayas. However, they were a source of rare metals (in particular, uranium) for later mineralisation events. At the WEV belt scale Devonian distensive events are coeval with the collision. They were recorded by the formation of sedimentary basins of limited time and space extent, corresponding to the splitting of the continental crust (up to formation of oceanic domains in many cases), and were characterised by a bi-modal (“spilite-keratophyre”) volcanism. These basins formed in transtensional (or pull-apart) settings along major strike-slip faults, a peculiarity of the Variscan collision belt (which may conveniently be described as a “strike-slip orogen”). In such basins, many deposits linked with the volcanic thermal energy were formed: SEDEX deposits of the Meggen-type, iron deposits of the Lahn-Dill-type and VMS base metal deposits, the latter being the only ones known in the FMC (Brévenne deposits). The Neo-Variscan stage corresponded to the “hypercollision” and was characterised by a shift from compressional tectonics (late thickening of the crust during the Sudetian event and long-lasting dextral strike-slip tectonics along NW-SE to NE-SW fault zones) towards extensional tectonics (“basin and range” of the Late Stephanian-Early Permian), as well as by high heat flows, recorded by LP-HT metamorphism, extensive granitisation and granulitisation of the lower crust. These characteristics record the development of a lithospheric delamination process. In response to the energetic input released by this process, numerous hydrothermal deposits were formed in the FMC, as well as in the whole WEV belt, during the Neo-Variscan stage. These are mainly: (1) high-temperature granite-centered tungsten deposits, mainly associated with cordierite-bearing high level intrusions of Namurian-Westphalian age; (2) rare metal granites (and the associated hydrothermal tin mineralisations), resulting from fluid-induced low-degree partial melting of the middle crust in relation with the devolatilisation of the granulitised lower crust; (3) shear-zone hosted gold and antimony deposits, related to crustal-scale hydrothermal circulation, triggered by the transition to extensional tectonics at about 300 Ma; and (4) uranium deposition in extensional settings related to the Early Permian distension. The Post-Variscan mineralising events recorded the renewal of thermal flows in the lithosphere linked with early Alpine events (mainly the Trias-Lias distension in the Tethyan realm and the middle Cretaceous opening of the Bay of Biscay in the Pyrenean realm). They resulted in low-enthalpy geothermal systems, leading to a variety of deposits, mainly: (1) F-Ba districts, reworking F and Ba from Late Variscan granites and ignimbrites; (2) a major uranium deposit (Lodève), reworking uranium from the Permian Lodève basin; and (3) Zn-Pb districts of the MVT-type. Finally, the mineralisation of the Variscan collision belt is mainly the consequence of the Neo-Variscan lithospheric delamination process. By contrast, the absence of such a process in collision belts like the Himalayas or the Alps is the key of them being devoid of mineralisation. It appears that the mechanical energy released by the collision itself is not sufficient to mobilise and concentrate the trace elements involved in the metallogenic processes. Received: 1 September 1998 / Accepted: 3 February 1999  相似文献   

2.
近年发现,太平洋和印度洋的深海盆地中存在大量富含稀土的深海沉积物。主要类型为多金属软泥、沸石黏土和远洋黏土,其中的全稀土含量(∑REY,∑REE+Y)为400×10-6~2000×10-6,最高可达6600×10-6,重稀土含量(HREE)已达到或超过中国南方离子吸附型矿床的重稀土品位两倍以上,是潜在的新型稀土资源,具有重要的经济价值。目前不少学者对富稀土的深海沉积物进行了大量地球化学及部分矿物学的工作,认为多金属软泥中的稀土元素多赋存于与海底热液作用有关的铁锰氧化物和氢氧化物中,而沸石黏土和远洋黏土中稀土元素的富集则与磷酸盐的混入密切相关,其稀土元素主要存在于与磷灰石成分相当的生物鱼骨屑中。深海黏土的北美页岩标准化稀土配分模式与海水相似,表明其中的稀土元素主要来自于海水,REY富集成矿可能主要受控于磷灰石早期成岩阶段,期间稀土元素未发生分异。尽管近些年对深海沉积物中的稀土元素研究取得了不少成果,但是,对于沉积物中的稀土富集机制及影响因素等问题仍然需要更加深入的研究。作为稀土资源大国,为了争取我国在国际海底稀土资源竞争中的话语权,维护中国的稀土利益,中国应加紧开展相关的稀土资源勘查和潜力评价。  相似文献   

3.
辽宁赛马碱性岩体早年因产铀矿而闻名,该岩体主要由响岩、霞石正长岩和异霞正长岩组成,其中铀、锆和稀土等元素矿化主要集中于异霞正长岩岩浆阶段。异性石是异霞正长岩中特征的锆-稀土矿物,主要分为两期,晚期异性石表现出更加富集Nb、REE等高场强元素的特点。早期异性石经历了一系列的热液蚀变,根据蚀变强弱程度,蚀变矿物组合可分为:(1)异性石+钠锆石+霓石±钠沸石;(2)异性石+钠锆石+锆石+钠沸石±霓石;(3)异性石假晶,假晶主要由残余异性石+钠锆石+锆石+钠沸石+霓石+钾长石+铈硅磷灰石组成。相比于岩浆锆石,蚀变组合中次生锆石具有富Ca、Al、Fe的特点,与异性石本身化学成分和流体性质密切相关。通过对异性石及其蚀变组合的精细矿物学研究,我们得知假晶的形成可能是异性石"溶解-再沉淀"的结果,致使假晶形成的流体至少包括:(1)占主导的富Na(±K)、Al、F的自交代流体;(2)少量晚期富Ca流体。假晶中次生锆石和铈硅磷灰石的结晶说明了Zr和REE等高场强元素的热液活动性,自交代碱性流体和富Ca流体在此过程中起到"搬运"和"提纯"的作用,这对认识碱性岩稀有、稀土成矿机制具有重要的指示意义。  相似文献   

4.
This paper discusses the observations of the Atmospheric Surface Layer (ASL) parameters during the solar eclipse of August 11th, 1999. Intensive surface layer experiments were conducted at Ahmedabad (23‡21′N, 72‡36′E), the western part of India, which was close to the totality path. This rare event provided by nature is utilised to document the surface layer effects during the eclipse period using measurements of high frequency fluctuations of temperature, tri-axial wind components as well as mean parameters such as temperature, humidity, wind speed and subsoil temperature. Analysis showed that during the eclipse period, the turbulence parameters were affected leading to the suppression of the turbulence process, the main dynamic process in the atmospheric boundary layer, while the mean parameters showed variations within the natural variability of the observational period. The spectra of the wind components and temperature indicated decrease in spectral power by one order in magnitude during the eclipse period. The rate of dissipation of turbulent kinetic energy is found to decrease by more than one order during the eclipse period. The stability parameter showed a change from unstable to stable condition during the period of eclipse and back to unstable condition by the end of eclipse  相似文献   

5.
针对赣南风化淋积型稀土矿中稀土元素的分异现象,通过研究该稀土矿的主要矿物成分——高岭石和埃洛石-7的矿物特征及其在不同条件下的吸附特征,探讨了该类型稀土矿的成矿机制和稀土元素分异机理。高岭石和埃洛石-7吸附稀土元素的能力受体系p H值以及金属阳离子钾、钠的的影响,且埃洛石-7吸附稀土元素的能力高于高岭石。横向对比实验结果表明当体系中赋存有K+时,两种粘土矿物对稀土元素的吸附呈现出分化趋势,其中高岭石主要吸附中-重稀土元素,埃洛石-7反之,因此风化淋积型稀土矿风化过程中释放出的K+可能会使高岭石和埃洛石-7呈现出差异吸附特征,进而反过来影响稀土矿中稀土元素的配分。  相似文献   

6.
Following previous publication of major–minor elementdata, this paper presents rare earth element (REE) data forheterogeneous (chemically zoned) garnets belonging to the peridotitesuite of mantle xenoliths from the Jagersfontein kimberlitepipe, South Africa. The rim compositions of the garnets in thehighest temperature–pressure (deepest) deformed peridotitesshow a typical megacryst-like pattern, of very low light REE(LREE) increasing through the middle REE (MREE) to a plateauof heavy REE (HREE) at c. 20 times chondrite; these compositionswould be in equilibrium with small-volume melts of the mid-oceanridge basalt (MORB) source (asthenosphere). With decreasingdepth the garnet rims show increasing LREE and decreasing HREE,eventually resulting in humped relative abundance patterns.A set of compositions is calculated for melts that would bein equilibrium with the garnet rims at different depths. Theseshow decreasing relative abundance of each REE from La to Lu,and the La/Lu ratio of the melts increases with decreasing depthof formation. Modelling of the effects of crystal fractionationshows that this process could largely generate the sequenceof garnet rim and melt compositions found with decreasing depth,including the humped REE patterns in high-level garnets. Consideringthe behaviour of major–minor elements as well as REE,a process of percolative fractional crystallization is advocatedin which megacryst source melts percolate upwards through peridotitesand undergo fractionation in conjunction with exchange withthe peridotite minerals. The initial megacryst melt probablyincludes melt of lithospheric origin as well as melt from theMORB source, and it is suggested that the process of percolativefractional crystallization may form a variety of metasomaticand kimberlitic melts from initial megacryst melts. Repeatedmetasomatism of the lower lithosphere by such differentiatingmelts is suggested by consideration of garnet core compositions.Such metasomatism would progressively convert harzburgites tolherzolites by increasing their CaO content, and this may accountfor the fact that the Cr-rich diamond–garnet harzburgiteparagenesis is commonly preserved only where it has been encapsulatedin diamonds. KEY WORDS: cratonic lithosphere; garnet zoning; mantle xenoliths; megacryst magma; metasomatic melt  相似文献   

7.
本文对华北克拉通北缘东段辽宁北部法库地区东小陵岩体、前旧门岩体、胡家屯岩体及柏家沟岩体进行了岩相学、地球化学、锆石U-Pb定年以及Lu-Hf同位素研究,以此制约古亚洲洋东段演化过程。岩相学特征表明,本文所研究岩体主要为花岗质岩石,普遍遭受了后期的动力变质作用改造。锆石测年结果显示,东小陵岩体及前旧门岩体形成于中二叠世（264.6±5.9Ma、262.8±3.5Ma）,胡家屯岩体及柏家沟岩体分别形成于晚二叠世（257.7±3.1Ma）及早三叠世（248.2±1.5Ma）。岩石地球化学表明,东小陵岩体及胡家屯岩体皆属于准铝质-弱过铝质、高钾钙碱性A型花岗岩,形成于造山后伸展环境;前旧门岩体属于准铝质-弱过铝质、钙碱性-高钾钙碱性高分异I型花岗岩,形成于火山弧环境;柏家沟岩体属于准铝质-弱过铝质、高钾钙碱性-钾玄岩性I型花岗岩,形成于同碰撞造山环境。研究区花岗质岩体皆富集大离子亲石元素（LILEs）和轻稀土元素（LREEs）,并且亏损高场强元素（HFSEs）和重稀土元素（HREEs）,结合Lu-Hf同位素特征,认为其原始岩浆应受到了俯冲流体交代的岩石圈地幔的影响。综合前人研究,本文认为在中二叠世-早三叠世期间,研究区经历了古亚洲板块的俯冲、闭合过程。  相似文献   

8.
Brecciated mafic+ultramafic plutonic rocks of the East Taiwan Ophiolite occur as detritus and slide blocks in the Pliocene Lichi Mélange. These plutonic rocks have been subjected to two stages of post-magmatic recrystallization: (I) pre-brecciation ridge-type metamorphism attended by high-grade greenschist and rare amphibolite facies physical conditions; and (II) later off-axis metamorphism under zeolite to lowest greenschist facies conditions that postdated brecciation, submarine talus accumulation and deposition of associated pelagic sediments. The effects of the earlier ridge metamorphism are the main concern of this paper. (I) Dominant antigorite together with chlorite and talc in some ultramafics suggests that these rocks recrystallized at T>350 ° C. The primary compositions of gabbroic calcic plagioclase have been modified from An 45–70 to An 13–38, and the igneous clinopyroxenes and hornblendes partly replaced by actinolite+chlorite. Stable mineral assemblages in the metagabbros are thus ∼oligoclase+actinolite+chlorite±very rare epidote+sphene, and intermediate plagioclase +actinolite+chlorite+sphene. Amphibolites are less common and consist of more calcic plagioclase (An 25–49)+hornblende. The presence of assemblages transitional between greenschist and amphibolite facies for basaltic compositions is suggestive of very low-pressure thermal metamorphism such as would be appropriate to the crustal portions of an oceanic spreading center. (II) The occurrence of vein albite+actinolite+ chlorite near the base of the brecciated plutonic sequence and vein prehnite+laumontite in the upper part suggests that the brecciated plutonic rocks were later feebly retrograded under conditions of the greenschist and zeolite facies respectively-probably some distance removed from the thermal regime of a mid-oceanic ridge. The East Taiwan Ophiolite probably represents the western termination of the Philippine Sea lithospheric plate. Portions of this oceanic crust and underlying mantle were incorporated in the Lichi Mélange of the Coastal Range of eastern Taiwan as a consequence of antithetic faulting and erosion. This process evidently accompanied east-directed underflow of the Asiatic (South China Sea) plate.  相似文献   

9.
磷矿伴生稀土元素是获取稀土资源的重要途径。我国磷块岩型稀土矿分布广,稀土含量高,具有综合回收价值,是仅次于独立稀土矿床的伴生稀土资源。本文主要研究云南安宁磷矿中稀土元素分布规律和赋存状态,并比较了磷矿石中稀土元素总量与磷含量的关系,结果表明磷矿石中稀土氧化物总量为72×10-6～1 050×10-6,与磷含量呈一定的正相关关系。另外通过光学显微镜及电子显微镜观察发现,安宁磷矿中缺乏独立的稀土矿物,只在部分海绿石中找到了独立的稀土矿物(可能为独居石和褐帘石)。LA-ICP-MS分析结果表明,胶磷矿单矿物稀土元素含量在770×10-6～920×10-6之间,而白云石单矿物稀土元素含量均低于34×10-6,石英单矿物的稀土元素平均含量为180×10-6。由于部分独立的稀土矿物的存在,海绿石矿物中稀土元素总量可高达2 947.27×10-6～3 159.87×10-6。综合分析结果表明,安宁磷矿中稀土元素主要以类质同像的...  相似文献   

10.
磷灰石可以记录和保存岩浆和热液活动的信息。可可托海伟晶岩型稀有金属矿床磷灰石发育,为研究该矿床伟晶岩成岩成矿过程提供了优良的条件。已有对可可托海伟晶岩型稀有金属矿床磷灰石的研究集中在其稀土元素特征,较少讨论其对伟晶岩成岩成矿过程的制约。本文选取可可托海伟晶岩型稀有金属矿床富矿伟晶岩脉(3号脉)和相对贫矿伟晶岩脉(1号、2b号和3a号脉)中的磷灰石作为研究对象,进行磷灰石岩相学和地球化学研究。岩相学分析表明,磷灰石主要与钠长石、石英、白云母、锰铝榴石等伴生。EPMA分析显示,磷灰石F含量为3.67%～4.41%,Cl含量小于0.67%,较低的Cl含量表明伟晶岩熔体出溶的流体Cl含量较低;大部分磷灰石MnO含量为4.67%～8.71%,但2b号脉磷灰石MnO含量变化较大(1.23%～14.28%),这是由于2b号脉磷灰石具有分带结构,暗示其遭受后期热液作用,促使磷灰石溶解-再沉淀,导致MnO含量发生较大变化。LA-ICP-MS分析显示,贫矿伟晶岩脉磷灰石的稀土元素含量较低(180×10~(-6));相反,富矿伟晶岩脉磷灰石的稀土元素含量较高( 700×10~(-6)),并具有明显的四分组效应(TE_(1-3)平均值为1.7)。1号脉和3a号脉磷灰石均显示轻稀土元素富集,反映其形成过程中有含Cl热液的参与。3号脉磷灰石显示强烈Eu负异常和Ce正异常,而2b号脉磷灰石显示强烈Ce负异常和中等Eu负异常,这种Eu、Ce异常的差异可能与岩浆-热液阶段大量流体出溶密切相关。磷灰石的沉淀将导致热液中HF含量的降低,促使磷灰石周围铌钽矿结晶和Nb、Ta进入磷灰石中。可见,在伟晶岩形成过程中,磷灰石并非保持稳定,其分带结构和主微量成分变化记录了后期热液活动,暗示后期热液活动对伟晶岩的成矿具有重要作用。  相似文献   

11.
Metalliferous black slates, which were locally exploited for their low-quality coal, are distributed in the Late Proterozoic to Paleozoic Okcheon Metamorphic Belt (OMB) of the Korean Peninsula. The mineralogy of the fine matrix is dominated by either quartz–(Ba, V)–mica–graphite or Quartz-Ba-feldspar–graphite. Polycrystalline submillimeter ellipsoids and elongate lenses aligned along foliations and veinlets are scattered through the fine matrix. Both ellipsoids and veinlets include many minor minerals containing rare elements: titanite, apatite, allanite, rutile, polycrase, barite, uraninite, xenotime, armenite, zircon, molybdenite, and sphalerite. Large graphite–apatite ellipsoids (nodules) with widths of several centimeters also occur in the highly carbonaceous black slates. Goldmanite occurs locally as porphyroblasts. The maximum rare element contents are: Ba 9.7%, V 2.04%, Mo 0.13%, U 0.11%, Cr 0.33%, Cu 254 ppm, Ni 479 ppm, Zn 607 ppm, Y 255 ppm, platinum-group element (PGE) + Au 309 ppb, and carbon 57%. The occurrence of the black slates and their elemental abundances suggests that most of the rare elements were accumulated from seawater in an oxygen-poor environment. However, the high Ba content of the OMB black slates indicates some hydrothermal input into an organic-rich basin. Although metamorphism and multiple deformations prevent a direct temporal and spatial correlation, metal abundances and a close association with graphite–apatite nodules and low-quality coal suggest that the OMB black slates are metamorphosed analogues of the Early Cambrian Ba–V deposits hosted by the black shales in the South China Block.  相似文献   

12.
Eight ferromanganese crusts (Fe-Mn crusts) with igneous and sedimentary substrates collected at different water depths from the Afanasiy-Nikitin Seamount are studied for their bulk major, minor and rare earth element composition. The Mn/Fe ratios < 1.5 indicate the hydrogenetic accretion of the Fe-Mn hydroxides. These Fe-Mn crusts are enriched in Co (up to 0.9%, average ∼ 0.5%) and Ce. The Ce-content is the highest reported so far (up to 3763 ppm, average ∼ 2250 ppm) for global ocean seamount Fe-Mn crusts. In spite of general similarity in the range of major, minor, and strictly trivalent rare earth element composition, the dissimilarity between the present Fe-Mn crusts and the Pacific seamount Fe-Mn crusts in Co and Ce associations with major mineral phases indicates inter-oceanic heterogeneity and region-specific conditions responsible for their enrichment. The decrease in Ce-anomaly (from ∼ 8 to ∼ 1.5) with increasing water depth (from ∼ 1.7 km to ∼ 3.2 km) might suggest that the modern intermediate depth low oxygen layer was shifted and sustained at a deeper depth for a long period in the past.  相似文献   

13.
High-alumina basalts from seven High-Andean stratovolcanoes (37 °30′S to 41 °S) have major and trace element (including rare earth elements, REE) that are consistent with derivation by partial melting (typically 10–15%) of garnet-free peridotite followed by fractional crystallization of olivine and pyroxene. High-alumina basalts from two stratovolconoes require significantly lower degrees of melting (<5%) or melting of an incompatible, element-enriched source. However, a poorly understood feature of all of these basalts-and calc-alkaline rocks in general-is the mechanism for causing their low TiO2 and heavy REE content relative to oceanic basalts. Further north in Chile (33 °–34 °S and 21 °–22 °S) amphibole-bearing andesites have REE abundances consistent with derivation from a garnet-bearing source such as incompatible, element-rich eclogite (e.g., Franciscan eclogites) or garnet peridotite. The marked petrological and geochemical changes along strike of the Andes are probably related to the varying nature of the subduction process; e.g., dips of the downgoing slab varying from 10 to 30 °.  相似文献   

14.
In order to ascertain the forms in which uranium is present in ores of the Melovoe rare metal sedimentary deposit of uranium and rare earth minerals (South Mangyshlak), we investigated a series of typical ore samples that were collected earlier; both the uranium content and the total content of rare earth metals in them lay within 0.1–0.3%. The study was carried out by analytical electron microscopy using transmission electron microscopy and scanning electron microscopy, electron microdiffraction, and microprobing. It was ascertained that both uranium and rare earth elements are present in ore mostly associated with biogenic phosphate in the form of natural minerals, such as uraninite, ningyoite, coffinite, autenite, and churchite. Iron hydroxides and graphitized organic matter are present in some samples. It is assumed that the co-occurrence of uranium and rare earth elements, which is nontypical for the sedimentary process, resulted from secondary epigenetic processes and alternation of reducing and oxidizing environmental conditions.  相似文献   

15.
In the Brévenne Series (NE Massif Central), a low-grade bimodal association of metabasalts and metarhyolites is exposed, together with intrusive trondhjemite bodies. Zircon U-Pb dating constrains their magmatic emplacement at 366 ± 5 Ma and 358 ± 1 Ma, respectively. The metabasalts are characterized by a distinct enrichment in incompatible elements (e.g. Th and LREE) and positive ɛNdi (from +5 to +8). Combined isotope and trace element systematics rule out crustal contamination of mafic melts as a suitable cause of the LILE (large ion lithophile element)-enrichment. Rather, a mixing process between a component similar to mid ocean ridge basalts and an enriched end-member with ɛNdi > +5 is suggested. An enriched-mantle source of ocean island basalt affinity is precluded by the relative depletion of high field strength elements, especially Nb which shows negative anomalies in chondrite-normalized patterns. On the contrary, a subduction-related origin for the LILE enrichment would be more consistent. It may be inferred that arc-like melts [enriched in Th and LREE (light rare earth elements) and depleted in Nb, with ɛNdi > +5] were produced through partial melting of a depleted-mantle source, to which a small amount of crustally derived component had been added. The metarhyolites are enriched in LILE, and have a close genetic relationship with the metabasalts, as evidenced by their high ɛNdi (from +4.7 to +6.8). Although the chemical evidence remains ambiguous, it is suggested that fractional crystallization, accompanied by subordinate assimilation, is the petrogenetic process most consistent with the data. The trondhjemites are isotopically distinct from the metarhyolites. Their ɛNdi values (from −1.0 to +2.2) reflect an important contribution of continental crust to their genesis, and disprove their inferred cogenetism with the felsic volcanics. A review of modern environments in which such bimodal suites are exposed, shows that settings involving incipient rifting of a volcanic arc fringing a continental margin, or built upon young, thin continental crust might provide suitable analogues. Geodynamic reconstructions are complicated by subsequent tectonic events which disrupted the initial patterns, and by Mesozoic-Cenozoic sedimentary cover. However, this subduction-related magmatism enlarges the growing body of evidence for southward subduction processes until the Late Devonian during the evolution of the northern flank of the European Variscides. As a general implication, it is suggested that the combined use of the Sm-Nd system with incompatible elements relatively resistant during alteration and low-grade metamorphism (REE, Th, Zr, Nb) may provide diagnostic criteria for recognizing the tectonic setting of bimodal metaigneous suites in ancient orogenic belts. Received: 21 April 1997 / Accepted: 30 June 1997  相似文献   

16.
The chromite deposit of the Paleoproterozoic Ipueira–Medradosill is hosted in a single, thick (5–8 m), massive layer,which sets severe constraints for the origin of chromitites.It is divided from bottom to top into: (1) a Marginal Zone (5–20m); (2) an Ultramafic Zone (<250 m) consisting of duniteand harzburgite that host the chromitite layer, in which intercumulusamphibole is important and more abundant toward the top; (3)a Mafic Zone (<40 m). The parental magma was large ion lithophileelement and light rare earth element enriched and high fieldstrength element depleted. Sm–Nd isotopic compositionsare consistent with a 2 Ga age, but suggest a variable initialNd isotopic composition that correlates with the abundance ofamphibole. The more negative  相似文献   

17.
曹磊  苏茂荣  周飞  李磊  杨宝宏 《地质通报》2019,38(4):632-642
内蒙古格勒敖包位于兴蒙造山带内,对该区奥陶纪岩浆岩的研究,有助于进一步认识该造山带早古生代的构造格局及其演化。正长花岗岩和斑状二长花岗岩样品LA-ICP-MS锆石年龄为441.6±3.7～454.9±4.6Ma,形成时代为奥陶纪。经地球化学分析,岩体为高钾钙碱性系列;铝饱和指数显示其为过铝质花岗岩。岩石富集大离子亲石元素(LILEs)Rb、Th、U、K,亏损Ba、Sr;富集高场强元素(HFSEs)La、Ce、Nd、Zr,亏损Nb、P、Ti;稀土元素总量为38.7×10~(-6)～120.23×10~(-6),轻稀土元素较重稀土元素分异明显,分馏程度较高,表现出较弱的负Eu异常。综合区域地质资料,认为格勒敖包奥陶纪岩浆源区为上地壳物质的部分熔融,其形成环境为弧后盆地,在古亚洲洋向北俯冲之后经历了岛弧岩浆作用旋回,为弧后伸张阶段的产物。  相似文献   

18.
 The uranium deposits in the basin of Franceville (Gabon) host the only natural fission reactors known in the world. Unique geological conditions favoured a natural fission reaction 2 Ga ago. This was detected by anomalous isotopic compositions of uranium and rare earth elements (REE), which are produced by the fission reaction. In total, 16 reactor zones were found. Most of them are mined out. The reactor zone of Bangombé, is only 10–11 m below the surface. This site has been influenced by surface weathering processes. Six drill cores have been sampled at the site of the reactor zone of Bangombé during the course of the study and only one drill core (BAX 08) hit the core of the reactor. From these data and previous drilling campaigns, the reactor size is estimated to be 10 cm thick, 2–3 m wide and 4–6 m long. The migration of fission products can be traced by the anomalous isotope ratios of REE because of the fission process. The 149Sm/147Sm ratio close to the reactor zone is only 0.28 (normal: 0.92) because of the intense neutron capture of 149Sm and subsequent transmutation, whereas 147Sm is enriched by the fission reaction. Similar changes in isotopic patterns are detectable on other REE. The isotope ratios of Sm and Nd of whole rock and fracture samples surrounding the reactor indicate that fission-genic REE migrated only a few decimetres above and mainly below the reactor zone. Organic matter (bitumen) seems to act as a trap for fission-genic REE. Additional REE-patterns show less intense weathering with increasing depth in the log profile and support a simple weathering model. Received: 26 November 1999 · Accepted: 2 May 2000  相似文献   

19.
We have observed apparent decoupling of the Re–Os andSm–Nd isotopic systems in sulphide-saturated magmas thatsuggests that bulk two-component or assimilation–fractionalcrystallization (AFC) mixing modelling, based on Re–Osisotopic data, is inappropriate for chalcophile isotopic systemsin turbulent sulphide-saturated magmas. This behaviour is observedin three Palaeoproterozic layered mafic–ultramafic intrusionsin the Halls Creek Orogen of Western Australia. All intrusionsclearly have a basaltic parental magma based on primitive olivineand spinel compositions. The intrusions are light rare earthelement enriched and define a narrow range of initial Nd-isotopicsignatures (  相似文献   

20.
Abundant ferroan, metaluminous granitoids (970–950 Ma) emplaced at the end of the Sveconorwegian collisional orogeny (1130–900 Ma) are dominated by intermediate to silicic compositions with rare mafic facies. Both 73% fractional crystallization of an amphibole-bearing gabbroic cumulate substracted from the parent mafic composition and 30% non-modal batch melting of an amphibolitic source equivalent in composition to the mafic facies produce a monzodioritic liquid with appropriate trace element composition. A better fit is obtained for the partial melting process. Both processes could have occurred simultaneously to produce mafic cumulates and restites. As there is no evidence for large volumes of dense mafic rocks in the Sveconorwegian upper crust, these dense mafic rocks were probably produced in the lower crust. Formation of these granitoids, thus, contributed to the vertical stratification of the Proterozoic continental crust and also to the transfer of water from the lower crust to the surface.  相似文献   

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