Extrusive carbonatites: A brief review

49 known extrusive carbonatite occurrences are listed with brief details of their tectonic setting, structure, lithologies, associated silicate rocks, chemistry and presence or absence of included mantle materials. Half the occurrences appear to be related to tephra cones, tuff rings, diatremes and maars and the rest occur within strato-volcanoes. Pyroclastic carbonatitic rocks are present at all the localities, with carbonatite lava flows occurring at only 14 of them. The pyroclastic rocks, which include fallout tephra and deposits from pyroclastic surges and flows and products of phreatomagmatic eruptions, vary from rocks composed principally of carbonate to varieties with as little as 20% igneous carbonate. The most abundant silicate rocks associated with extrusive carbonatites are melilite-bearing rocks, nephelinite and/or ijolite, and phonolite and/or nepheline syenite; seven occurrences have no associated silicate rocks. 16 occurrences, most of them associated with small extrusive centres, contain mantle xenoliths or megacrysts, details of which are tabulated, with spinel lherzolite the most abundant rock type, but amphibole, phlogopite and garnet are also recorded. The lack of such materials in intrusive carbonatites may reflect their less energetic environment of emplacement. It is proposed that carbonatites are essentially of two types: (a) those rising energetically and rapidly from the mantle, which form small explosion craters, ash or tuff cones, or diatremes, have only low-volume associated silicate rocks, and entrain mantle debris, and (b) those which occur in strato-volcanoes, are associated with large volumes of silicate rocks and follow a more complex genesis, probably involving ponding and differentiation (separation from carbonate-bearing silicate magma) at higher levels in the mantle and/or crust. Most of the classic intrusive carbonatite complexes probably fall into the second category.     

陕西省华阳川铀铌铅矿床碳酸岩岩石学及地球化学特征

陕西省华阳川铀铌铅矿床是小秦岭成矿带中成矿特征最为独特的矿床,碳酸岩脉的破碎带是重要的成矿空间。未矿化的碳酸岩中矿物以方解石为主,其他矿物很少;发育铀矿化的碳酸岩脉中矿物种类繁多,大部分为方解石,其次为角闪石、金云母、榍石、褐帘石、铌钛铀矿、重晶石、磷灰石、石英、磁铁矿、碱性长石等矿物。碳酸岩的LREE含量异常高,δ13CV-PDB和δ18OV-SMOW值显示典型的火成碳酸岩特征。基于碳酸岩脉的Sr、Nd、Pb同位素比值(87Sr/86Sr-206Pb/204Pb、207Pb/204Pb-206Pb/204Pb-143Nd/144Nd-87Sr/86Sr)的关系图,初步判断华阳川铀铌铅碳酸岩脉是源于EMI的碱性硅酸盐-碳酸盐熔体-溶液结晶分异的产物。     

http://www.sciencedirect.com/science/article/pii/S1674987110000125

<正>Carbonatites are commonly related to the accumulation of economically valuable substances such as REE.Cu,and P.The debate over the origin of carbonatites and their relationship to associated silicate rocks has been ongoing for about 45 years.Worldwide,the rocks characteristically display more geochemical enrichments in Ba,Sr and REE than sedimentary carbonate rocks.However,carbonatite's geochemical features are disputed because of secondary mineral effects.Rock-forming carbonates from carbonatites at Qinling.Panxi region,and Bayan Obo in China show REE distribution patterns ranging from LREE enrichment to flat patterns.They are characterized by a Sr content more than 10 times higher than that of secondary carbonates.The coarse- and fine-grained dolomites from Bayan Obo H8 dolomite marbles also show similar high Sr abundance,indicating that they are of igneous origin.Some carbonates in Chinese carbonatites show REE(especially HREE) contents and distribution patterns similar to those of the whole rocks.These intrusive carbonatites display lower platinum group elements and stronger fractionation between Pt and Ir relative to high-Si extrusive carbonatite.This indicates that most intrusive carbonatites may be carbonate cumulates.Maoniuping and Daluxiang in Panxi region are large REE deposits.Hydrothermal fluorite ore veins occur outside of the carbonatite bodies and are emplaced in wallrock syenite.The fiuorite in Maoniuping has Sr and Nd isotopes similar to carbonatite.The Daluxiang fiuorite shows Sr and REE compositions different from those in Maoniuping.The difference is reflected by both the carbonatites and rock-forming carbonates,indicating that REE mineralization is related to carbonatites.The cumulate processes of carbonate minerals make fractionated fluids rich in volatiles and LREE as a result of low partition coefficients for REE between carbonate and carbonatite melt and an increase from LREE to HREE.The carbonatite-derived fluid has interacted with wallrock to form REE ore veins.The amount of carbonatite dykes occurring near the Bayan Obo orebodies may support the same mineralization model,i.e.that fluids evolved from the carbonatite dykes reacted with H8 dolomite marble,and thus the different REE and isotope compositions of coarse- and fine-grained dolomite may be related to reaction processes.     

The Serra da Graciosa A-type Granites and Syenites, southern Brazil: Part 2: Petrographic and mineralogical evolution of the alkaline and aluminous associations

The Serra da Graciosa Granites and Syenites comprise five distinct plutons in the Brasiliano/Pan-African Graciosa A-type Province, southern Brazil. Six petrographic series can be identified in these plutons: (1) Alkaline series 1, composed of amphibole-bearing alkali feldspar syenites with varied mafic mineralogy and quartz contents, from alkali feldspar syenites with calcic amphibole, clinopyroxene, olivine and allanite to alkali feldspar quartz syenites with sodic–calcic amphibole and chevkinite–perrierite and to alkali feldspar granites with sodic amphibole; (2) Alkaline series 2, characterized by amphibole-bearing alkali feldspar granites, with limited modal variations but amphibole compositions also varying from calcic to sodic; (3) Alkaline series 3, of limited occurrence, which includes alkali feldspar syenites with olivine and clinopyroxene and no amphibole; (4) Aluminous series 1, of widespread occurrence, with various petrographic facies of biotite granites with amphibole; (5) Aluminous series 2, characterized by alkali feldspar granites with biotite and only minor amphibole; (6) Monzodiorites, typically with biotite, calcic amphibole and augitic clinopyroxene, partially mingled with granitic magmas. The mafic minerals present are, in general, Fe-rich with correspondingly low Mg and Al contents. In Alkaline series 1, amphiboles crystallized in progressively more oxidizing and alkaline conditions, while in Alkaline series 2, the initial conditions were somewhat more oxidizing and shifted to reducing in the final stages. In Aluminous series 1 and Aluminous series 2, amphiboles are calcic and comparatively homogeneous. The amphiboles in the monzodioritic rocks, while also homogeneous, are more Mg-rich and show compositions quite distinct from the calcic varieties in the other associations, and this is also the case for clinopyroxene. Mg# in biotite decreases from the monzodioritic rocks to Aluminous series 1 and further to Aluminous series 2. Contrasting evolution of the various associations suggests that several coeval magmatic series are present in the Serra da Graciosa granites.     

REE mineralization in the carbonatites of the sung valley ultramafic-alkaline-carbonatite complex,Meghalaya, India

The Early Cretaceous Sung Valley Ultramafic-Alkaline-Carbonatite (SUAC) complex intruded the Proterozoic Shillong Group of rocks and located in the East Khasi Hills and West Jaintia Hills districts of Meghalaya. The SUAC complex is a bowl-shaped depression covering an area of about 26 km² and is comprised serpentinised peridotite forming the core of the complex with pyroxenite rim. Alkaline rocks are dominantly ijolite and nepheline syenite, occur as ring-shaped bodies as well as dykes. Carbonatites are, the youngest intrusive phase in the complex, where they form oval-shaped bodies, small dykes and veins. During the course of large scale mapping in parts of the Sung Valley complex, eleven carbonatite bodies were delineated. These isolated carbonatite bodies have a general NW-SE and E-W trend and vary from 20–125 m long and 10–40 m wide. Calcite carbonatite is the dominant variety and comprises minor dolomite and apatite and accessory olivine, magnetite, pyrochlore and phlogopite. The REE-bearing minerals identified in the Sung Valley carbonatites are bastnäsite-(Ce), ancylite-(Ce), belovite-(Ce), britholite-(Ce) and pyrochlore that are associated with calcite and apatite. The presence of REE carbonates and phosphates associated with REE-Nb bearing pyrochlore enhances the economic potential of the Sung Valley carbonatites. Trace-element geochemistry also reveals an enrichment of LREEs in the carbonatites and average ΣREE value of 0.102% in 26 bed rock samples. Channel samples shows average ΣREE values of 0.103 wt%. Moreover, few samples from carbonatite bodies has indicated relatively higher values for Sn, Hf, Ta and U. Since the present study focuses surface evaluation of REE, therefore, detailed subsurface exploration will be of immense help to determine the REE and other associated mineralization of the Sung Valley carbonatite prospect.     

Bayan Obo Carbonatites: Texture Evidence from Polyphase Intrusive and Extrusive Carbonatites

<正>Most of the so-called Bayan Obo fine-grained dolomite marbles collected from the main and east orebodies show a microporphyritic texture,namely the microphenocrysts are set in a very finegrained matrix,although nearly all of them have undergone recrystallization caused either by deformation or alteration.The texture seems likely to have maintained the original features.It is known that one of the most characteristic textures of volcanic rocks is the porphyritic texture,and the microporphyritic texture is a variety in which both the phenocrysts and the matrix are only distinguishable with the microscope.Therefore,the dolomite marbles in the main and east orebodies may be related to the extrusive carbonatites.In addition,there also occur some carbonatite sills and dykes with different textures at Bayan Obo.Thus,the Bayan Obo carbonatites are polyphase intrusive and extrusive carbonatites.     

Compositions of magmas,formation conditions,and genesis of carbonate-bearing ijolites and carbonatites of the Belaya Zima alkaline carbonatite complex,eastern Sayan

Based on the investigation of melt inclusions using electron and ion microprobe analysis, we estimated the composition, evolution, and formation conditions of magmas responsible for the calcite-bearing ijolites and carbonatites of the Belaya Zima alkaline carbonatite complex (eastern Sayan, Russia). Primary melt and coexisting crystalline inclusions were found in the nepheline and calcite of these rocks. Diopside, amphibole (?), perovskite, potassium feldspar, apatite, calcite, pyrrhotite, and titanomagnetite were identified among the crystalline inclusions. The melt inclusions in nepheline from the ijolites are completely crystallized. The crystalline daughter phases of these inclusions are diopside, phlogopite, apatite, calcite, magnetite, and cuspidine. During thermometric experiments with melt inclusions in nepheline, the complete homogenization of the inclusions was attained through the dissolution of a gas bubble at temperatures of 1120–1130°C. The chemical analysis of glasses from the homogenized melt inclusions in nepheline of the ijolites revealed significant variations in the content of components: from 36 to 48 wt % SiO₂, from 9 to 21 wt % Al₂O₃, from 8 to 25 wt % CaO, and from 0.6 to 7 wt % MgO. All the melts show very high contents of alkalis, especially sodium. According to the results of ion microprobe analysis, the average content of water in the melts is no higher than a few tenths of a percent. The most salient feature of the melt inclusions is the extremely high content of Nb and Zr. The glasses of melt inclusions are also enriched in Ta, Th, and light rare earth elements but depleted in Ti and Hf. Primary melt inclusions in calcite from the carbonatites contain a colorless glass and daughter phlogopite, garnet, and diopside. The silicate glass from the melt inclusions in calcite of the carbonatite is chemically similar to the glasses of homogenized melt inclusions in nepheline from the ijolites. An important feature of melt inclusions in calcite of the carbonatites is the presence in the glass of carbonate globules corresponding to calcite in composition. The investigation of melt inclusions in minerals of the ijolites and carbonatites and the analysis of the alkaline and ore-bearing rocks of the Belaya Zima Massif provided evidence for the contribution of crystallization differentiation and silicate-carbonate liquid immiscibility to the formation of these rocks. Using the obtained trace-element compositions of glasses of homogenized melt inclusions and various alkaline rocks and carbonatites, we determined to a first approximation the compositions of mantle sources responsible for the formation of the rock association of the Belaya Zima alkaline-carbonatite complex. The alkaline rocks and carbonatites were derived from the depleted mantle affected by extensive metasomatism. It is supposed that carbonate melts enriched in sodium and calcium were the main agents of mantle metasomatism.     

Geochemistry of an Island-Arc Plutonic Suite: the Uasilau-Yau Yau Intrusive Complex, New Britain, P.N.G

In the Uasilau-Yau Yau intrusive complex of central New Britain,Papua New Guinea, there is a compositional continuum in intrusiverock-types from gabbro to granodiorite and K-Ar mineral agesof the most mafic and most felsic components are not significantlydifferent (29?0.6 Ma versus 28.3?0.5 Ma, respectively). Tonaliteporphyry, the progenitor of porphyry copper mineralization inthe complex, represents a significantly younger intrusive eventat 24 Ma. Relatively calcic (An_95—50) plagioclase coresand salite to augite composition clinopyroxene are texturallyearly phases in the intrusive rocks. The main mafic mineral,calcic amphibole, generally has corroded clinopyroxene coresand may, like biotite, K-feldspar and quartz, generally be alate-stage, not a primary liquidus phase. Petrographic featuresindicate that the mafic minerals in the plutonic rocks crystallizedfrom melt, rather than being restite phases. The intrusive rocks cover an extensive silica range (45–75wt. per cent), do not exhibit simple straight-line variationon Harker diagrams for many elements (e.g. TiO₂, FeO, P₂O₅ andSr), and most are relatively depleted in incompatible traceelements (Rb, Zr, and REE). Major and trace element modellingsupports derivation of the complex by shallow level fractionalcrystallization dominated by removal of the phases calcic plagioclase,clinopyroxene, and magnetite from a parental magma closely resemblingrecent basaltic rocks in New Britain. The fact that the plutonicrocks are almost chemically indistinguishable from late Cainozoiccalc-alkaline volcanic rocks of New Britain supports fractionalcrystallization as a viable mechanism for generating these island-arcvolcanic rocks and indicates an analogous origin for the initialmagma. Granites, such as those of the Uasilau-Yau Yau intrusive complex,which are probably generated by partial melting of subductedoceanic crust or the overlying mantle, may be termed mantleor M-type granites. Documentation of the characteristics ofM-type versus normal I-type granites may enable the recognitionof M-type plutonic rocks in older, possibly more deeply erodedgeologic terrains. This would, by analogy to their volcanicequivalents, be very helpful in tectonic interpretations. Also,such plutonic rocks have known potential for Cu-Au mineralization.     

Calculated silica activities in carbonatite liquids

Carbonatite magmas precipitate silicates, in addition to the abundant carbonates, oxides, and phosphates. Calculated silica activities for equilibria involving silicates and a silica component in magmatic liquids predict specific assemblages for silicate and oxide phases in carbonatites. These assemblages provide tests of alternative sources (carbonatite magma, coeval silicate magma, or older rock) for silicate minerals in carbonatites. Quartz, feldspars, and orthopyroxene are unlikely to be primary magmatic phases in carbonatites, because the silica activity in carbonatite magmas is too low to stabilize these minerals. Zircon and titanite should be unstable relative to baddeleyite and perovskite, respectively, but they do occur in carbonatites. Liquids dominated by carbonate are strongly nonideal with respect to dissolved silica. Consequently, activity coefficients for a silica component in carbonatite liquids are >>1, so that small mole fractions of SiO₂ translate into silica activities sufficient to stabilize phlogopite, clinopyroxene, amphibole, monticellite, and forsterite, among other silicates. Examination of silicate mineral assemblages in carbonatites in the light of silica activity indicates that many carbonatites are contaminated by solid silicate phases from external sources but these xenocrysts can be discriminated from magmatic minerals.     

The Paleoproterozoic Montviel carbonatite-hosted REE–Nb deposit,Abitibi, Canada: Geology,mineralogy, geochemistry and genesis

The Montviel 250 Mt carbonatite-hosted REE–Nb deposit is hosted in a Paleoproterozoic alkaline suite located in the Sub-Province of Abitibi, in the Archean Province of the Superior. The alkaline intrusion consists of biotite clinopyroxenites, melano- to leucosyenites, a melteigite–ijolite–urtite series, riebeckite granite, a series of carbonatites and a carbonatite polygenic breccia. The carbonatite series includes silicocarbonatites, calciocarbonatites, rare magnesiocarbonatites, ferrocarbonatites and mixed carbonatites and are cut by a late, high-energy carbonatite polygenic breccia. Diamond drill hole assays and microscope observations indicate that Nb is hosted in pyrochlore from silicocarbonatite whereas the REE mineralization is mainly hosted in ferrocarbonatite, late mixed carbonatites and polygenic breccia, in REE-bearing carbonates and fluorocarbonate minerals. Diamond drill hole underground mapping and systematic assays have shed light on zones enriched in Nd and LREE with preferential Ba and Sr hydrothermal precipitation and zones enriched in Dy, Y and HREE displaying preferential F and P bearing hydrothermal precipitation. Petrographic observations, electron microprobe analyses, LA-ICPMS and X-ray diffraction were used to study the mineralization processes and to identify and quantify the REE-bearing burbankite–(Ce), carbocernaite–(Ce), ewaldite–(Y), huanghoite–(Nd), cordylite–(Ce), cordylite–(Nd), kukharenkoite–(Ce) and synchysite–(Ce). Most minerals are enriched in total LREE with values around 19.3 wt.%, have total MREE values around 2.2 wt.% and extremely variable total HREE values, with very high contents of Dy and Y averaging around 0.3 wt.% and 1.0 wt.%, respectively, and with total HREE reaching up to 10.0 wt.%. A paragenetic sequence is proposed that consists of: (1) a silicocarbonatite Nb stage, and (2) a calciocarbonatite stage, dominated by magmatism but accompanied by hydrothermal fluids, (3) a main ferrocarbonatite stage, dominated by episodes of Ba- and Sr-hydrothermalism and LREE mineralization, F- and P-hydrothermalism and HREE mineralization and evolved ferrocarbonatitic magmatism, (4) a renewed, mixed carbonatite magmatic stage with minor but increasing hydrothermalism, and (5) a terminal stage of fluid pressure buildup and explosion, leading to the creation of a HREE-enriched polygenic breccia. Globular melt inclusions of Ba–Cl–F (± Si–O) may indicate the presence and contribution of barium-bearing chlorofluoride melts during hydrothermal activity and mineralization of the carbonatite.     

鲁中碳酸岩成岩分析及其含矿性

鲁中隆起北缘分布有100多个燕山期的碳酸岩体.它们的分布受淄河断裂及禹王山断裂派生的次级断裂控制。主要有碳酸云母岩、云母碳酸岩及碳酸角砾岩三类碳酸岩,多呈岩床产出,另有两个角砾岩筒。根据产状、结构、构造、化学成分及矿物中包裹体等特征,认为是与金伯利岩有成因联系的岩浆产物。推测其深部可能存在一个较大的碳酸岩体,可能出现以稀有元素Nb为主的矿化。     

Characteristics of carbonatites from the northern part of the Korean Peninsula: A perspective from distribution,geology and geochemistry

The present study introduces the carbonatite in the northern part of the Korean Peninsula for the first time.Recent exploration and development of the phosphorus-bearing carbonate rocks in the area have accumulated new geological data which gave us an opportunity to study origin of the carbonate rocks.We conducted geological survey,geochemical analyses of trace elements and rare earth elements,and carbon and oxygen isotope analyses for the carbonatites from Ssangryong,Pungnyon,Yongyu and Puhung districts of the northern part of the Korean Peninsula.This research confirms that the phosphorus-bearing carbonate rocks are carbonatite originating from the mantle.The studied carbonatites are distributed at the junctions of ring and linear structures or around their margins and contain a greater amount of REEs,Y,and Sr than carbonate rocks.The carbonatites in Yongyu and Puhung area show evidence that they were formed from mantle plume generated at the lower mantle and display similar fractionation characteristics to carbonatites in Barrado Itapirapua in Brazil and Kalkfeld and Ondurakorume in Namibia.REE patterns of the carbonatites are typical of carbonatites and the carbon and oxygen isotope analyses demonstrate that the carbonatites were originated from mantle.The carbonatites from the northern part of the Korean Peninsula have a great potential for sources of REE,Y,PGE(platinum group elements),copper,and gold.     

The geochemistry of carbonatites and related rocks from two carbonatite complexes,south Nyanza,Kenya

Carbonatites, metasomatised country rocks, and carbonatitic calcite and magnetite have been analysed from two carbonatite complexes, Homa and Wasaki, W. Kenya.The carbonatites are all greatly Ce-earth enriched, contain abundant ‘carbonatitic’ trace elements (Sr, Ba, Nb and REE), and generally low concentrations of Cr, Co, Ni, Pb, Ga, Ge, Sn, Bi, Li and Mo. At both complexes early søvite is rich in Sr, and impoverished in other trace elements relative to the alvikites. The late-intruded melacarbonatites contain the greatest concentrations of Ba, REE, Fe and Mn.It is concluded that the accumulation of these elements in the later carbonatites is mainly due to fractionation of carbonates from carbonatite magma which was initially rich in ‘carbonatitic’ trace elements.     

Comments on: Carbonatites in a subduction system: The Pleistocene alvikites from Mt. Vulture (southern Italy) by d'Orazio et al., (2007)

[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.     

西藏类乌齐地区含镍碳酸岩的发现与地质意义

野外调查发现西藏类乌齐地区分布着众多的含镍碳酸岩体,呈十几米至几百米不等的岩株出露。岩体与超基性岩之间具明显的港湾状、枝状侵入接触、岩石不具层理、岩石表现出细粒结晶等粒结构、岩体内见超基性岩捕掳体等特征均表明其为火成碳酸岩岩体。在成分上以菱镁矿为主,岩石中MgO的含量为27.16%~31.75%,为镁质碳酸岩。岩石稀土元素配分曲线呈平缓右倾的轻稀土富集型,轻重稀土元素分馏现象明显,具有明显的Eu正异常和弱的Ce负异常;微量元素显示Rb、Th、Nb、Hf元素相对富集,K、U、Ti元素相对亏损。含镍碳酸岩可能是富CO2的深部流体在中下地壳对超基性岩交代的结果,具备寻找硫化镍矿床的有利条件。     

Calcio-carbonatite melts and metasomatism in the mantle beneath Mt. Vulture (Southern Italy)

At Mt. Vulture volcano (Basilicata, Italy) calcite globules (5–150 μm) are hosted by silicate glass pools or veins cross-cutting amphibole-bearing, or more common spinel-bearing mantle xenoliths and xenocrysts. The carbonate globules are rounded or elongated and are composed of a mosaic of 2–20 μm crystals, with varying optical orientation. These features are consistent with formation from a quenched calciocarbonatite melt. Where in contact with carbonate amphibole has reacted to form fassaitic pyroxene. Some of these globules contain liquid/gaseous CO₂ bubbles and sulphide inclusions, and are pierced by quench microphenocrysts of silicate phases. The carbonate composition varies from calcite to Mg-calcite (3.8–5.0 wt.% MgO) both within the carbonate globules and from globule to globule. Trace element contents of the carbonate, determined by LAICPMS, are similar to those of carbonatites worldwide including ΣREE up to 123 ppm. The Sr–Nd isotope ratios of the xenolith carbonate are similar to the extrusive carbonatite and silicate rocks of Mt. Vulture testifying to derivation from the same mantle source. Formation of immiscibile silicate–carbonatite liquids within mantle xenoliths occurred via disequilibrium immiscibility during their exhumation.     

Rare earth element partition between sphene,apatite and other coexisting minerals of the Kangerdlugssuaq intrusion,E. Greenland

Cumulus apatite, sphene, feldspar, amphibole and biotite from the pulaskite of the Kangerdlugssuaq alkaline intrusion have been analysed for rare earth elements (REE) by instrumental neutron activation analysis. The apatite is particularly rich in REE, contains 3.6% Ce and shows a steep, light REE-enriched, chondrite-normalised pattern. The other minerals have light REE enrichment but with sphene showing a peak at Ce on a chondrite-normalised plot. REE partition coefficient values show that the light REE are preferentially accommodated by apatite relative to sphene. The differences in these coefficients result from differences in the co-ordination of the REE in the two minerals.     

Unusual rocks and mineralisation in a new carbonatite complex at Kandaguba, Kola Peninsula, Russia

The paper presents the results from a reconnaissance investigation of carbonatites in a newly discovered alkaline complex in the Kola peninsula, Russia. The Kandaguba complex differs from other carbonatite plutons of the Kola alkaline province by (a) the absence of ultrabasic rocks, (b) widespread occurrence of nepheline-, cancrinite- and nepheline–cancrinite syenites and carbonatites and (c) presence of apatite–calcite ijolites and feldspar ijolites as separate intrusive phase. The Kandaguba carbonatites are notable for the predominance of late ferromagnesian varieties together with quartz and numerous accessory mineral species. The association of phosphates (monazite, gorseixite, goyazite, apatite), sulphides and tellurides (pyrite, sphalerite, galena, hessite), ilmenorutile, barite with quartz and ankerite is a remarkable feature of these carbonatites. The Kandaguba carbonatites are inferred to have been generated as the products of liquid immiscibility followed by differentiation of the carbonatite melt.     

辽宁省丹东地区“前震旦纪”侵入岩的锆石U-Pb 年代学、地球化学及其构造意义

本文对辽东南丹东地区部分原定前震旦纪侵入岩进行了锆石LA-ICP-MS U-Pb年代学和岩石地球化学研究,以便对其岩石成因和区域构造演化给予制约.详细的岩石学和岩相学研究表明,四个侵入岩体岩性分别为二长花岗岩、角闪石黑云母二长花岗岩、黑云母二长花岗岩和角闪辉长岩.所选样品中锆石均呈半自形-自形晶形、阴极发光图像显示具有成分生长环带或条痕状吸收特点,结合其高的Th/U比值(0.53 ～2.40)和亏损轻稀土元素(LREEs)、相对富集重稀土元素(HREEs)以及负Eu异常和正Ce异常等特征,表明其典型岩浆成因特点.锆石LA-ICP-MS U-Pb定年结果表明,它们均形成于早白垩世(峰期为126Ma).岩石学和地球化学分析结果显示,研究区早白垩世侵入岩具有双峰式岩浆作用特点,其中花岗质岩石SiO2=66.99％～73.13％,K2O=3.52％～4.67％,K2O/Na2O=1.01～1.43,MgO=0.45％～2.61％,Mg#值为29～56,Al2O3=13.97％ ～ 14.92％,富集LREEs和大粒子亲石元素(LILEs),亏损HREEs和高场强元素(HFSEs),暗示其具有I型花岗岩特点,岩浆应起源于下地壳的部分熔融;而角闪辉长岩SiO2=50.22％,MgO=10.61％,Mg#值为69,Cr(196×10-6)、Co(44.8×10-6)、Ni(73.4×10-6)含量较高,富集LREEs和LILEs,亏损HREEs和Nb、Ta、Ti等HFSEs,暗示其岩浆应起源于受俯冲流体交代的岩石圈地幔楔.结合区域地质资料表明,辽东南丹东地区早白垩世侵入岩应形成于类似弧后盆地的伸展环境,其形成应与古太平洋板块北西向向欧亚大陆之下的俯冲作用相联系.     

Enrichment of Rare Earth Elements during magmatic and post-magmatic processes: a case study from the Loch Loyal Syenite Complex, northern Scotland

Concern about security of supply of critical elements used in new technologies, such as the Rare Earth Elements (REE), means that it is increasingly important to understand the processes by which they are enriched in crustal settings. High REE contents are found in syenite-dominated alkaline complexes intruded along the Moine Thrust Zone, a major collisional zone in north-west Scotland. The most northerly of these is the Loch Loyal Syenite Complex, which comprises three separate intrusions. One of these, the Cnoc nan Cuilean intrusion, contains two mappable zones: a Mixed Syenite Zone in which mafic melasyenite is mixed and mingled with leucosyenite and a Massive Leucosyenite Zone. Within the Mixed Syenite Zone, hydrothermal activity is evident in the form of narrow altered veins dominated by biotite and magnetite; these are poorly exposed and their lateral extent is uncertain. The REE mineral allanite is relatively abundant in the melasyenite and is extremely enriched in the biotite–magnetite veins, which have up to 2 % total rare earth oxides in bulk rock analyses. An overall model for development of this intrusion can be divided into three episodes: (1) generation of a Light Rare Earth Element (LREE)-enriched parental magma due to enrichment of the mantle source by subduction of pelagic carbonates; (2) early crystallisation of allanite in melasyenite, due to the saturation of the magma in the LREE; and (3) hydrothermal alteration, in three different episodes identified by petrography and mineral chemistry, generating the intense enrichment of REE in the biotite–magnetite veins. Dating of allanite and titanite in the biotite–magnetite veins gives ages of c. 426 Ma, overlapping with previously published crystallisation ages for zircon in the syenite.