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1.
The Siriwasan carbonatite-sill along with associated alkaline rocks and fenites is located about 10 km north of the well-known Amba Dongar carbonatite-alkaline rocks diatreme, in the Chhota Udaipur carbonatite-alkaline province. Carbonatite has intruded as a sill into the Bagh sandstone and overlying Deccan basalt. This resulted in the formation of carbonatite breccia with enclosed fragments of basement metamorphics, sandstone and fenites in the matrix of ankeritic carbonatite. The most significant are the plugs of sövite with varied mineralogy that include pyroxene, amphibole, apatite, pyrochlore, perovskite and sphene. REE in sövites is related to the content of pyrochlore, perovskite and apatite. The carbon and oxygen isotopic compositions of some sövite samples and an ankeritic carbonatite plot in the “mantle box” pointing to their mantle origin. However, there is also evidence for mixing of the erupting carbonatite magma with the overlying Bagh limestone. The carbonatites of Siriwasan and Amba Dongar have the same Sr and Nd isotopic ratios and radiometric age, suggesting the same magma source. On the basis of available chemical analyses this paper is aimed to give some details of the Siriwasan carbonatites. The carbonatite complex has good potential for an economic mineral deposit but this is the most neglected carbonatite of the Chhota Udaipur province.  相似文献   

2.
In the Amba Dongar diatreme, “ferrocarbonatite” is not a single unit of late differentiate of calciocarbonatite magma but it is a family with variation on field occurrence, mineralogy and chemistry of each unit. The family includes dikes of ankeritic carbonatites (phase I and II), plugs of ankeritic carbonatite within sövite ring dike, dikes of sideritic carbonatite in ankeritic carbonatite plug and rödberg veins. Their intrusive relations are very clear in the field and each phase has characteristic mineralogy and trace and REE geochemistry. According to the nomenclature suggested by Harmer and Gittins (1997) majority of “ferrocarbonatites” of Amba Dongar plot in field of “ferruginous calciocarbonatite” and only siderite and rödberg plot in the field of “ferrocarbonatite”. Within these family members, their trace and REE show clear increase from early phase to last phase of sideritic carbonatite. The present short communication discusses various aspects of “ferrocarbonatites”.  相似文献   

3.
A carbon and oxygen isotope survey based on 42 samples from the Amba Dongar carbonatite complex of Gujarat, India, indicates that the magmatic differentiation series sövite → alvikite → ankeritic carbonatite is beset with a distinct isotope trend characterized by a moderate rise in 13C coupled with a sizeable increase in 18O. From an average of −4.6 ± 0.4 ‰ [PDB] for the least differentiated (coarse) sövite member, δ13C values slowly increase in the alvikite (−3.7 ± 0.6 ‰) and ankeritic fractions (−3.0 ± 1.1 ‰), whereas δ18O rises from 10.3 ± 1.7 ‰ [SMOW] to 17.5 ± 5.8 ‰ over the same sequence, reaching extremes between 20 and 28 ‰ in the latest generation of ankeritic carbonatite. While an apparent correlation between δ13C and δ18O over the δ18O range of 7–13 ‰ conforms with similar findings from other carbonatite complexes and probably reflects a Rayleigh fractionation process, the observed upsurge of 18O notably in the ankeritic member is demonstrably related to a late phase of low-temperature hydrothermal activity involving large-scale participation of 18O-depleted groundwaters. As a whole, the Amba Dongar carbonatite province displays the characteristic 13C/12C label of deep-seated (primordial) carbon, reflecting the carbon isotope composition of the subcontinental upper mantle below the Narmada Rift Zone of the Indian subcontinent.  相似文献   

4.
J. Gittins  R.E. Harmer  D.S. Barker   《Lithos》2005,85(1-4):129-139
The concept of compositional bimodality in carbonatites has become widely accepted and has been used to impose restrictions on the composition of carbonatite magmas. We agree that mineralogical bimodality exists in carbonatites (most are either calcitic or dolomitic/ankeritic), but we argue that there is no compositional bimodality. The idea of bimodality is based on the interpretation of a variety of element distribution diagrams which were compiled only from chemical analyses in which SiO2 is < 10 wt.%. All others were rejected. Even with such a restricted data set the case for compositional bimodality is extremely weak, but the inclusion of analyses with higher SiO2 content destroys it completely. Yet these more siliceous compositions must be included, for many carbonatites contain substantial amounts of Fe–Mg silicates which are an essential part of the magmatic mineralogy of the rocks. They account for much of the Mg in carbonatites that are otherwise calcitic. Many such carbonatites contain well in excess of 10 wt.% SiO2. Supporters of the bimodality concept argue that liquids having compositions between calcite and dolomite can precipitate neither calcite nor dolomite because the minimum on the solid solution loops in the system calcite–dolomite permits only a carbonate of intermediate composition. Therefore, it is argued, liquids of such intermediate composition cannot be parental to calcitic and dolomitic carbonatites; their parent magmas must be calcitic and dolomitic. This deduction is incorrect. It is well established that dolomitic liquids have calcite as the liquidus phase over substantial temperature intervals, and that this is followed by dolomite precipitation. Mixed calcite–dolomite carbonatites are explicable in this way. Therefore, dolomitic liquids can be parental to calcitic carbonatites. However, dolomitic carbonatites cannot crystallize from a calcitic liquid. We suggest that intermediate composition carbonatite magmas are probably common. Bimodality in carbonatites is solely mineralogical, not compositional.  相似文献   

5.
The carbonatites of the Ilmeny-Vishnevogorsky Alkaline Complex (IVAC) are specific in geological and geochemical aspects and differ by some characteristics from classic carbonatites of the zoned alkaline-ultramafic complexes. Geological, geochemical and isotopic data and comparison with relevant experimental systems show that the IVAC carbonatites are genetically related to miaskites, and seem to be formed as a result of separation of carbonatite liquid from a miaskitic magma. Appreciable role of a carbonate fluid is established at the later stages of carbonatite formation. The trace element contents in the IVAC carbonatites are similar to carbonatites of the ultramafic-alkaline complexes. The characteristic signatures of the IVAC carbonatites are a high Sr content, a slight depletion in Ba, Nb, Та, Ti, Zr, and Hf, and enrichment in HREE in comparison with carbonatites of ultramafic-alkaline complexes. This testifies a specific nature of the IVAC carbonatites related to the fractionation of a miaskitic magma and to further Late Paleozoic metamorphism. Isotope data suggest a mantle source for IVAC carbonatites and indicate that moderately depleted mantle and enriched EMI-type components participated in magma generation. The lower crust could have been involved in the generation of the IVAC magma.  相似文献   

6.
火成碳酸岩及其风化产物是全球战略性关键金属稀土元素(REE)和铌(Nb)的主要来源。因此,对关键金属在火成碳酸岩中的超常富集机理研究具有重要的科学意义。研究表明成矿碳酸岩常常与碱性杂岩体存在密切的时空联系,因而母岩浆应属于碳酸盐化的硅酸盐岩浆,并以霞石岩岩浆为主。针对碳酸岩关键金属矿床的成岩成矿过程,已有实验发现母岩浆在地壳内的演化过程中,既可以通过分离结晶作用,也可以通过液态不混溶作用形成碳酸岩。然而,更加接近自然样品的多组分体系的实验均表明液态不混溶作用总是先于碳酸盐矿物分离结晶作用。因此,液态不混溶作用对关键金属成矿过程有着不可忽视的作用。尽管如此,已有不混溶实验表明当碳酸盐熔体和硅酸盐熔体发生不混溶之后,关键金属REE与Nb总是优先分配到硅酸盐熔体(碱性岩)中,但是在成矿杂岩体中,REE与Nb是高度富集在碳酸岩中。虽然不混溶实验表明REE与Nb在碳酸盐-硅酸盐熔体中的分配系数与含水量有关,即与熔体的聚合程度有关,但是绝大部分成矿碳酸岩成矿过程一般并不富水,所以碳酸岩中REE和Nb等关键金属元素超常富集的机理并不明确。因此未来的研究应重点关注在碳酸岩演化的过程中,除了水以外,其他配体对于关键金属元素在不混溶硅酸盐-碳酸盐熔体之间分配系数是否有影响,从而找到控制碳酸岩中关键金属成矿的关键。  相似文献   

7.
F. Stoppa  C. Principe  P. Giannandrea 《Lithos》2008,103(3-4):550-556
[D'Orazio, M., Innocenti, F., Tonarini, S., Doglioni, C., 2007. Carbonatites in a subduction system: the Pleistocene alvikites from Mt. Vulture (southern Italy). Lithos 98, 313–334] describe a new finding of alvikite Ca-carbonatite at Vulture. They stress its importance as being the first carbonatite to be discovered in a subduction environment. They suggest that this rock is different from the other Italian carbonatites, considered as ‘rocks sharing a carbonatitic affinity’, which are radiogenic and chemically diluted by addition of sedimentary limestone. They note that Vulture ‘alvikite’ is not diluted and is very unradiogenic with respect to other Italian carbonatites. However, they maintain that Vulture ‘alvikite’ carbonate is derived from subducted limestones. We present an account of the field relationships relating to the above-mentioned rocks, setting the geological and petrographic records straight and describing pyroclastic rocks. We did not find that these rocks are formed from alvikite dykes or lava, but instead recognised them to be a continuous blanket of ‘flaggy’, welded tuff. We found that the rocks consist of physically separated melilitite and carbonatite juvenile lapilli settled into a carbonatite ash matrix form the rock. We disagree with the geochemical interpretation of the rock by [D'Orazio, M., Innocenti, F., Tonarini, S., Doglioni, C., 2007. Carbonatites in a subduction system: the Pleistocene alvikites from Mt. Vulture (southern Italy). Lithos 98, 313–334], and are particularly concerned by their conclusion of its carbonate origin. We remark on the rock's geodynamic assignment in the frame of an extensional tectonic setting, also referring to the other Italian carbonatite occurrences. We reject any ad hoc modified subduction as a direct source of Vulture and Italian carbonatites.  相似文献   

8.
Occurrence of carbonatite is reported from the Munnar area, Kerala, where an alkali granite-syenite-carbonatite association is seen emplaced along the intersection zone of the Attur and Kerala fault-lineaments. The carbonatites are of two varieties, a calcite-rich sovite and a very coarse grained, calcite and dolomite bearing alvikite. Higher levels of SiO2, Al2O3 and CaO are characteristic of these as compared to the composition of typical carbonatites. The transition element levels are high whereas the incompatible elements show lower values. The low Sr values, lower amount of apatite and absence of rare metal minerals preclude a primary carbonatite magma. The associated syenite and alkali granite have higher K2O, K2O/Na2O, K/Rb, K/Ba and transition element levels. Petrochemical features suggest the rock association to be a result of separation of an immiscible fraction of less viscous carbonate liquid during cooling and ascent from a more viscous polymerized alkali silicate phase. The pre-requisites for melt equilibration and liquid immiscibility were achieved through volatile degassing related to crustal warping and rifting. The unique alkaline association of Munnar, which shows spatial relationships with deep-seated faults as well as a probable triple-point junction, is suggested to be a signature of late Precambrian alkaline magmatism which manifested in the Indian shield as a precursor to the rifting of the continental margin.  相似文献   

9.
A great deal of data is available on the geochemistry of the carbonatite family as well as diverse sedimentary, metamorphic and endogenic carbonate rocks. The distinctive geochemical features of carbonatites are expressed first of all in their simultaneous enrichment in Sr, Ba, ree and V. Since the carbonatite family is related both to alkaline petrogenesis and a mantle origin, these associations allow distinction of carbonatites from the large variety of other carbonate rocks. At the same time, carbonatites associated with different types of alkaline rocks under different geologic-tectonic settings differ in contents of a number of elements (Sr, Ba, Nb, ree, Pb, Zn, P). These differences permit the geochemical classification of various rock associations of carbonatites. This geochemical classification is of practical interest because the ore productivity of the various associations is different.  相似文献   

10.
Identification of the Late Mesozoic carbonatite province in Central Asia is herein discussed. Its regional extent and distribution is investigated, and the areas with manifestations of carbonatite magmatism are described. It is shown that they were developed in terranes with heterogeneous and heterochronous basements: Siberian (Aldan Shield) and North China cratons; Early Paleozoic (Caledonian) and Middle–Late Paleozoic (Hercynian) structures of the Central Asian fold belt (Transbaikal and Tuva zones in Russia; Mongolia). Irrespective of the structural position, the carbonatites were generated within a relatively narrow time interval (150–118 Ma). The geochemical (Sr, LREE, Ba, F and P) specialization of carbonatites of the province is reflected in their mineral composition. Some rocks of the carbonatite complexes always include one or more distinctive minerals: fluorite, Ba–Sr sulfates, Ba–Sr–Ca carbonates, LREE fluorocarbonates, or apatite. Compared to counterparts from other age groups (for example, Maimecha–Kotui group in North Asia), these carbonatites are depleted in Ti, Nb, Ta, Zr and Hf. It is shown that the Sr and Nd isotope composition of carbonatites correlates with the geological age of the host crust. Rocks of carbonatite complexes associated with cratons are characterized by the lowest εNd(T) and highest ISr(T) values, indicating that their formation involved an ancient lithospheric material. Carbonatite magmatism occurred simultaneously with the largest plateau basalts 130–120 Ma ago in rift zones in the Late Mesozoic intraplate volcanic province of Central Asia. This interval corresponds to timing of global activation of intraplate magmatism processes, suggesting a link of the carbonatite province with these processes. It is shown that fields with the carbonatite magmatism were controlled by small mantle plumes (“hot fingers”) responsible for the Central Asian mantle plume events.  相似文献   

11.
岩浆(型)碳酸岩研究进展   总被引：19，自引：0，他引：19  
主要从岩石学，矿物学，岩石分类，C，O，Sr同位素，碳酸岩与矿化的关系等各方面对（碱性）碳酸岩的研究进行了较为全面的总结，并结合近20年来实验岩石等，流体包裹体研究，CO2^- H2O－NaCl流体体系的性质的研究，对碳酸岩岩浆的来源及成因，岩浆－热液的演化进行了分析和探讨，碳酸岩形成至少经历了三个阶段，即岩浆阶段，岩浆期后阶段（气相碳酸岩／岩浆热液阶段），交代碳酸岩阶段，而作为与碳酸岩在空间和成因上有密切联系的基性，超基性岩，碱性岩杂岩体，则经历了碳酸岩成岩阶段以前的岩浆不混熔作用，结晶分异作用，岩浆结晶作用以及碳酸岩形成之后的围岩蚀变（霓长岩化）作用。  相似文献   

12.
Petrogenetic studies of carbonatites are challenging, because carbonatite mineral assemblages and mineral chemistry typically reflect both variable pressure–temperature conditions during crystallization and fluid–rock interaction caused by magmatic–hydrothermal fluids. However, this complexity results in recognizable alteration textures and trace-element signatures in the mineral archive that can be used to reconstruct the magmatic evolution and fluid–rock interaction history of carbonatites. We present new LA–ICP–MS trace-element data for magnetite, calcite, siderite, and ankerite–dolomite–kutnohorite from the iron-rich carbonatites of the 1.3 Ga Grønnedal–Íka alkaline complex, Southwest Greenland. We use these data, in combination with detailed cathodoluminescence imaging, to identify magmatic and secondary geochemical fingerprints preserved in these minerals. The chemical and textural gradients show that a 55 m-thick basaltic dike that crosscuts the carbonatite intrusion has acted as the pathway for hydrothermal fluids enriched in F and CO2, which have caused mobilization of the LREEs, Nb, Ta, Ba, Sr, Mn, and P. These fluids reacted with and altered the composition of the surrounding carbonatites up to a distance of 40 m from the dike contact and caused formation of magnetite through oxidation of siderite. Our results can be used for discrimination between primary magmatic minerals and later alteration-related assemblages in carbonatites in general, which can lead to a better understanding of how these rare rocks are formed. Our data provide evidence that siderite-bearing ferrocarbonatites can form during late stages of calciocarbonatitic magma evolution.  相似文献   

13.
Carbonatites that are hosted in metamorphosed ultramafic massifs in the roof of miaskite intrusions of the Il’mensky-Vishnevogorsky alkaline complex are considered. Carbonatites have been revealed in the Buldym, Khaldikha, Spirikha, and Kagan massifs. The geological setting, structure of carbonatite bodies, distribution of accessory rare-metal mineralization, typomorphism of rock-forming minerals, geochemistry, and Sr and Nd isotopic compositions are discussed. Dolomite-calcite carbonatites hosted in ultramafic rocks contain tetraferriphlogopite, richterite, accessory zircon, apatite, magnetite, ilmenite, pyrrhotite, pyrite, and pyrochlore. According to geothermometric data and the composition of rock-forming minerals, the dolomite-calcite carbonatites were formed under K-feldspar-calcite, albite-calcite, and amphibole-dolomite-calcite facies conditions at 575–300°C. The Buldym pyrochlore deposit is related to carbonatites of these facies. In addition, dolomite carbonatites with accessory Nb and REE mineralization (monazite, aeschynite, allanite, REE-pyrochlore, and columbite) are hosted in ultramafic massifs. The dolomite carbonatites were formed under chlorite-sericite-ankerite facies conditions at 300–200°C. The Spirikha REE deposit is related to dolomite carbonatite and alkaline metasomatic rocks. It has been established that carbonatites hosted in ultramafic rocks are characterized by high Sr, Ba, and LREE contents and variable Nb, Zr, Ti, V, and Th contents similar to the geochemical attributes of calcio-and magnesiocarbonatites. The low initial 87Sr/86Sr = 0.7044?0.7045 and εNd ranging from 0.65 to ?3.3 testify to their derivation from a deep mantle source of EM1 type.  相似文献   

14.
This work reviews the character and origin of primary and supergene economic deposits of niobium associated with carbonatites. The Brazilian supergene deposits account for about 92% of the total worldwide production of Nb, with the primary St. Honoré carbonatite and other sources accounting for only for 7 and 1%, respectively. The emphasis of the review is upon the styles of Nb mineralization and the geological factors which lead to economic concentrations of Nb-bearing minerals. Primary economic deposits of Nb are associated principally with carbonatites found in diverse types of plutonic alkaline rock complexes. Primary magmas are principally those of the melilitite, nephelinite and aillikite clans. Although many primary niobium deposits are associated with carbonatites, ijolites and syenites in the same alkaline complexes can also contain significant Nb mineralization in the form of niobian titanite and diverse Nb–Zr-silicates (marianoite-wöhlerite); these potential sources of Nb have not as yet been explored or exploited. Primary Nb deposits can be regarded as large tonnage, low grade (typically < 1 wt.% Nb2O5) disseminated ore deposits. Niobium is hosted principally by diverse Na–Ca–U-pyrochlores, ferrocolumbite and fersmite. Every actual, and potential, primary Nb deposit is unique with respect to the varieties of pyrochlore present; extent of replacement by other minerals; and degree of alteration by deuteric/hydrothermal fluids. Within a given occurrence individual petrographically-defined units of carbonatite contain distinct suites of pyrochlore. Bulk rock analysis for Nb gives no indication of the style of mineralization and provides no information of use regarding beneficiation of the ore. Evaluation of any Nb deposit requires extensive definition drilling and detailed mineralogical studies. Primary Nb deposits result from the early crystallization of Nb-bearing minerals in magma chambers followed by crystal fractionation, magma mixing, and redistribution of Nb-minerals by density currents. Supergene Nb deposits occur in laterites formed by extensive weathering of primary carbonatites. The process results in the decomposition of apatite and magnetite, removal of soluble carbonates and physical concentration of resistant primary pyrochlore. Intense lateritization results initially in the replacement of primary pyrochlores by supergene, commonly Ba, Sr, K or Pb-bearing pyrochlores, and ultimately complete decomposition of pyrochlore and formation of Nb-bearing rutile, brookite, and anatase. The Nb contents of the laterites can be enriched up to 10 times or more above those of the primary carbonatite. Commonly, pyrochlores in laterites are fine grained and intimately intergrown with hematite, goethite and minerals of the crandallite group. The different styles of mineralization of primary and secondary Nb deposits require different methods of ore beneficiation.  相似文献   

15.
This paper studies the petrology of K-alkaline lamproite-carbonatite complexes, which are widespread in Siberia. They are exemplified by the Murun and Bilibino massifs in West and Central Aldan. In these massifs, the entire range of differentiates was first found, from K-ultrabasic-alkalic rocks through basic and intermediate ones to alkali granites and unique residual calc-silicate rocks (benstonite Ba-Sr carbonatites and charoite rocks). Also, intrusive equivalents of lamproites occur in these massifs, and the Murun massif was probably formed from highly differentiated lamproite magmas. In many K-alkaline complexes, silicate and silicate-carbonate magma layering takes place. Stages of magmatism are described for both massifs. Binary and ternary petrochemical diagrams exhibit the same compositional trend from early to late rocks.In this paper, lamproites are considered from the chemical point of view; their diagnostic properties are described in terms of chemical and mineral composition. From geological, petrological, and geochemical data, formational analysis of alkaline complexes was performed, four formational types of world lamproites were first identified, and diamond content criteria were developed for them.The carbonatite problem was studied from the petrological point of view, and four formational types of carbonatites were identified using geological, geochemical, and genetic criteria. It has been suggested that for dividing carbonatite complexes into four formational types the following criteria be used: the alkalinity type (Na or K) of alkalic rocks in the complex and the time when the carbonatite liquid separates from silicate melts in different stages of primary magma differentiation. These linked parameters influence the ore content type of carbonatite complexes.A formation model for K-alkaline carbonatite complexes is given, and the Tomtor alkaline carbonatite massif with tuffaceous rare-metal ores is described to prove that they have ore reserves. The geochemistry of C, O, Sr, and Nd isotopes shows that K-alkaline complexes, depending on their geotectonic setting, can originate from three types of mantle sources: depleted mantle, enriched mantle 1 (EM1), and enriched mantle 2 (EM2). It is concluded that ore-bearing ultrabasic-alkaline complexes of lamproites and carbonatites can melt out of different types of mantle, whose composition only slightly influences their ore content. Apparently, the main factors are the low degree of selective mantle melting (less than 1%) and plumes supplying fluid and alkaline components, which stimulate this melting. Later on, the processes important for the accumulation of ore and trace elements are long-term magma differentiation and its layering during crystallization.  相似文献   

16.
白云鄂博碳酸岩型REE-Nb-Fe矿床是世界上最大的稀土矿床。稀土矿石产于整个白云石碳酸岩体和部分脉状碳酸岩中。对比世界上20余个火成碳酸岩地区的特征后发现,白云鄂博地区完全具备国外火成碳酸岩区的地质特征。在岩石、矿石组合上,本区也发育一套碳酸岩+超基性岩+碱性基性岩(含基性熔岩)+碱性岩+稀土矿石+铁矿石组合;在矿物组合上,以白云石为主,方解石次之,伴生一套碱性闪石、长石、霓石、磷灰石、萤石、磁铁矿、稀土矿物组合;在全岩化学成分、微量元素、稀土元素和Sr、Nd、Pb、C、O同位素上,这些岩石具有一定的亲缘关系,有着共同的来源;在岩体的形态与岩石组构上,它们以岩席、岩筒和脉岩的形式出现,并发育有强烈的熔离作用与流动构造;在区域构造上,发育隐伏穹窿构造、岩筒构造和巨型断裂汇聚构造。综合分析上述特征表明:白云鄂博地区具有中元古代破火山机构的痕迹,赋矿白云石碳酸岩体则是顺层侵入的火成碳酸岩体,东矿、主矿可能是一个火山颈构造控矿,而赋矿白云石碳酸岩体西南侧的苏木图矿床则是隐伏岩筒构造控矿。  相似文献   

17.
Melt inclusions in clinopyroxenes of olivine foidite bombs from Serra di Constantinopoli pyroclastic flows of the Vulture volcano (Southern Italy) were studied in detail. The rocks contain abundant zoned phenocrysts and xenocrysts of clinopyroxene, scarce grains of olivine, leucite, haüyne, glass with microlites of plagioclase and K-feldspar. The composition of clinopyroxene in xenocrysts (Cpx I), cores (Cpx II), and in rims (Cpx III) of phenocrysts differs in the content of Mg, Fe, Ti, and Al. All clinopyroxenes contain two types of primary inclusion-pure silicate and of silicate-carbonate-salt composition. This fact suggests that the phenomena of silicate-carbonate immiscibility took place prior to crystallization of clinopyroxene. Homogenization of pure silicate inclusions proceeded at 1 225 – 1 190°C. The composition of conserved melts corresponded to that of olivine foidite in Cpx I, to tephrite-phonolite in Cpx II, and phonolite-nepheline trachyte in Cpx III. The amount of water in them was no more than 0.9 wt.%. Silicate-carbonate inclusions decrepitated on heating. Salt globules contained salts of alkali-sulphate, alkali-carbonate, and Ca-carbonate composition somewhat enriched in Ba and Sr. This composition is typical of carbonatite melts when decomposed into immiscible fractions. The formation of sodalite-haüyne rocks from Vulture is related to the presence of carbonate-salt melts in magma chamber. The melts conserved in clinopyroxenes were enriched in incompatible elements, especially in Cpx III. High ratios of La, Nb, and Ta in melts on crystallization of Cpx I and Cpx II suggest the influence of a carbonatite melt as carbonatites have extremely high La/Nb and Nb/Ta and this is confirmed by the appearance of carbonatite melts in magma chamber. Some anomalies in the concentrations and relatives values of Eu and especially Ga seems typical of Italian carbonatite related melts. The mantle source for initial melts was, most likely, rather uniform, undepleted and was characterized by a low degree of melting and probable presence of garnet in restite.  相似文献   

18.
The magmatic heritage of carbonatites can be identified on the basis of a combination of geological criteria such as, their mode of occurrence, the nature of associated igneous rocks, the presence of minerals of igneous origin, fenitization, characteristic trace element contents and isotopic composition. Late Proterozoic Samalpatti carbonatites were studied in view of these criteria, and were found to contain metamorphic minerals that normally form under thermal metamorphic conditions and which have unusual chemical compositions. A combination of criteria points clearly to a magmatic origin for these carbonatites. Field relations indicate that the dominant modes of intrusion of carbonatite into the encompassing pyroxenites and syenites include small dykes, veins, or lenses. The igneous nature of these carbonatites has been described elsewhere and chemically they are classified as calico-carbonatites. Currently, very little is known about the metamorphic textures and mineralogy observed in the Samalpatti carbonatites. In this study, several metamorphic minerals are reported including diopside, grossularite, vesuvianite, K-feldspar and wollastonite, and a hornfelsic texture is described. These mineral phases and texture characterize thermal metamorphism under low pressure and high temperature (LP-HT) metamorphic conditions (650°_750°C) or metasomatism aided by hot-fluid advection. The metamorphic nature of minerals reported is also confirmed by electron microprobe study. The Samalpatti carbonatite samples show much lower values of characteristic trace elements (P, Sr, Ba, Zr, Nb, Th, Y and REEs) than average concentrations for magmatic carbonatite. Stable isotopic (d13C and d18O) compositions of Samalpatti carbonatites do not fall in the primary igneous carbonatite (PIC) domain. The petrological and chemical signatures of these carbonatites suggest metasomatism in conjunction with fluid advection. Such a metasomatic process may drastically change the chemistry of the rocks in addition to enrichment of heavier stable isotopes. During this metasomatic process, characteristic elements would be dissolved in the high d18O fluid, and together with Rayleigh fractionation would contribute to enhanced concentrations of 13C and 18O in Samalpatti carbonatites.  相似文献   

19.
陕西华阳川碳酸岩地质学和岩石学特征及其成因初探   总被引：6，自引：0，他引：6  
本文提供了陕西华阳川碳酸岩产状、规模形态、空间分布、围岩蚀变和主要造岩矿物特征,以及相互关系等方面的研究资料,初步论证了华阳川碳酸岩的成因。  相似文献   

20.
The brevity of carbonatite sources in the mantle: evidence from Hf isotopes   总被引：5，自引：0，他引：5  
Hf, Zr and Ti in carbonatites primarily reside in their non-carbonate fraction while the carbonate fraction dominates the Nd and Sr elemental budget of the whole rock. A detailed investigation of the Hf, Nd and Sr isotopic compositions shows frequent isotopic disequilibrium between the carbonate and non-carbonate fractions. We suggest that the trace element and isotopic composition of the carbonate fraction better represents that of the carbonatite magma, which in turn better reflects the composition of the carbonatitic source. Experimental partitioning data between carbonatite melt and peridotitic mineralogy suggest that the Lu/Hf ratio of the carbonatite source will be equal to or greater than the Lu/Hf ratio of the carbonatite. This, combined with the Hf isotope systematics of carbonatites, suggests that, if carbonatites are primary mantle melts, then their sources must be short-lived features in the mantle (maximum age of 10–30 Ma), otherwise they would develop extremely radiogenic Hf compositions. Alternatively, if carbonatites are products of extreme crystal fractionation or liquid immiscibility then the lack of radiogenic initial Hf isotope compositions also suggests that their sources do not have long-lived Hf depletions. We present a model in which the carbonatite source is created in the sublithospheric mantle by the crystallization of earlier carbonatitic melts from a mantle plume. This new source melts shortly after its formation by the excess heat provided by the approaching hotter center of the plume and/or the subsequent ascending silicate melts. This model explains the HIMU-EMI isotope characteristics of the East African carbonatites, their high LREE/HREE ratios as well as the rarity of carbonatites in the oceanic lithosphere.  相似文献   

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