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1.
<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. 相似文献
2.
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. 相似文献
3.
Francesco Stoppa Adrian P. Jones Victor V. Sharygin 《Central European Journal of Geosciences》2009,1(2):131-151
Vulture volcano displays a wide range of mafic to alkaline, carbonate-, and/or CaO-rich volcanic rocks, with subvolcanic and plutonic rocks together with mantle xenoliths in pyroclastic ejecta. The roles of magmatic volatiles such as CO2, S, and Cl have been determined from compositions and trapping temperatures of inclusions in phenocrysts, which include the Na-K-Ca-carbonate nyerereite within melilite. We surmise that this alkali carbonate crystallised from an appropriate carbonatitic melt at relatively high temperature. Carbonatitic metasomatic features are traceable throughout many of the mantle xenoliths, and various carbonatitic components are found in the late stage extrusive suite. There is no evidence that alkali carbonatite developed as a separate magma, but it may have been an important evolutionary stage. We compare the rare occurrence of nyerereite at Vulture with other carbonatites and with an unaltered kimberlite from the Udachnaya pipe. We review the evidence at Vulture for associated carbonatitic metasomatism in the mantle, and we suggest that low viscosity alkali carbonatitic melts may have a primary and much deeper origin than previously considered. 相似文献
4.
碳酸岩的地质地球化学特征及其大地构造意义 总被引：20，自引：0，他引：20
从已知碳酸岩的地质产状、岩石学特征、Nd-Sr-Pb-O-C同位素及痕量元素地球化学特征数据,结合高温高压实验岩石学资料,论述了其地幔源区的物质成分、交代过程软流圈地幔部分熔融机制和碳酸岩岩浆的演化模型。碳酸岩既可以产生于拉张岩石圈构造背景,也能够产生于挤压而派生的引张岩石圈构造背景。前者以产于裂谷环境、与硅酸不饱和过碱性岩构成环状碳酸岩—碱性杂岩为特征,主要由起源于软流圈地幔的霞石质超基性—基性岩浆经液态不混溶作用而形成;后者产于碰撞造山过程中派生的引张岩石圈断裂带,以单一的透镜状、条带状和似层状碳酸岩体为标志,直接由导源岩石圈富集地幔的低程度部分熔融作用而产生的碳酸岩浆侵入或喷发所形成。 相似文献
5.
火成碳酸岩的实验岩石学研究及对地球深部碳循环的意义 总被引：3，自引：0，他引：3
火成碳酸岩是地表出露较少的幔源岩石之一。实验岩石学研究表明碳酸盐化的橄榄岩和循环的地壳物质(如碳酸盐化榴辉岩或泥质岩)的低程度(<1%)部分熔融均可以产生碳酸岩质的熔体,其中碳酸盐化泥质岩具有最低的熔融温度且更加富碱质、CO2和不相容元素;富CO2的霞石质等硅酸盐岩浆也可以通过不混溶或分离结晶作用产生碳酸岩,用于解释碳酸岩在空间中常与碱性硅酸岩的共生关系。由于碳酸岩熔体具有极低的粘度和高的活性,形成后在上升过程中会将二辉橄榄岩转变为异剥橄榄岩,是引起地幔交代作用和地幔地球化学不均一性的重要介质之一。实验表明在俯冲作用过程中,大多数的碳酸盐在位于岛弧之下的含水熔融并不分解而是被带入到深部地幔并且稳定存在,含碳地幔的熔融又会形成碳酸岩质的熔体,这说明俯冲循环物质可能对碳酸岩的成因也起着重要的作用。然而,对于碳酸岩的初始熔体成分、岩浆演化、地幔交代作用、成矿特征以及碳从地球深部返回到地表的途径和过程等都存在着很大的争议。我国火成碳酸岩出露相对较多,分布广泛,因此,加强我国碳酸岩以及伴生硅酸岩的成因研究,同时开展与碳酸岩相关的实验岩石学工作,不仅可以检验现有的成因理论,而且有助于提高我国对火成碳酸岩的研究水平;由于其特殊的成因背景,还可为许多存在很大争议的重大地质事件提供新的科学依据。 相似文献
6.
Comments on: Carbonatites in a subduction system: The Pleistocene alvikites from Mt. Vulture (southern Italy) by d'Orazio et al., (2007) 总被引：1，自引：1，他引：0
[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
7.
WOOLLEY A. R.; BARR M. W. C.; DIN V. K.; JONES G. C.; WALL F.; WILLIAMS C. T. 《Journal of Petrology》1991,32(6):1143-1167
Carbonatites occur in the Uyaynah area, United Arab Emirates,within a tectonic window of oceanic metasediments beneath theSemail Ophiolite Complex at the northern end of the Oman Mountains.The carbonatites form conformable layers, pods, and lenses,up to 10 m thick, associated with deep-sea sediments that includeradiolarian cherts and pillow lavas. Although disrupted duringdeformation, the carbonatites are internally undeformed andcontain abundant lapilli, indicating an extrusive, subaerialorigin, probably on a small volcanic island. The lapilli comprisecalcite, abundant Sr-bearing apatite and iron oxides in a calcitematrix. Alkali amphiboles, biotite, chlorite, and allanite wereproduced as a result of metamorphism but otherwise the primaryigneous carbonatite mineralogy remains. One distinct group ofrocks also contains primary calcic amphibole and biotite. Niobiantitanite, strontian baryte, and albite are also present, andrare earth elements are held in abundant allanite-(Ce) and raremonazite-(Ce). Analyses of 26 rocks define two groups whichare distinguishable in the field. One group consists of silico-carbonatitescontaining primary calcic amphibole, biotite, and abundant allanite,and is chemically similar to average intrusive ferrocarbonatite.The other group corresponds chemically to average intrusivecalciocarbonatite. Unlike other extrusive carbonatites describedin the literature, the Uyaynah carbonatites have higher incompatibleelement abundances than average intrusive carbonatites and arenotably rich in rare earth elements (up to 2?6 wt.%) and P2O5(average > 7 wt.%). The two carbonatite groups are consideredto be related magmatically one to the other, and the presenceof rare ferroan chromian spinel is interpreted as evidence fora direct mantle origin. 相似文献
8.
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. 相似文献
9.
V.S. Samoilov 《Journal of Geochemical Exploration》1991,40(1-3)
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.
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
11.
N.V. Vladykin 《Russian Geology and Geophysics》2009,50(12):1119-1128
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
12.
This paper reviews the results of investigations of melt inclusions in minerals of carbonatites and spatially associated silicate rocks genetically related to various deep-seated undersaturated silicate magmas of alkaline ultrabasic, alkaline basic, lamproitic, and kimberlitic compositions. The analysis of this direct genetic information showed that all the deep magmas are inherently enriched in volatile components, the most abundant among which are carbon dioxide, alkalis, halides, sulfur, and phosphorus. The volatiles probably initially served as agents of mantle metasomatism and promoted melting in deep magma sources. The derived magmas became enriched in carbon dioxide, alkalis, and other volatile components owing to the crystallization and fractionation of early high-magnesium minerals and gradually acquired the characteristics of carbonated silicate liquids. When critical compositional parameters were reached, the accumulated volatiles catalyzed immiscibility, the magmas became heterogeneous, and two-phase carbonate-silicate liquid immiscibility occurred at temperatures of ≥1280–1250°C. The immiscibility was accompanied by the partitioning of elements: the major portion of fluid components partitioned together with Ca into the carbonate-salt fraction (parental carbonatite melt), and the silicate melt was correspondingly depleted in these components and became more silicic. After spatial separation, the silicate and carbonate-silicate melts evolved independently during slow cooling. Differentiation and fractionation were characteristic of silicate melts. The carbonatite melts became again heterogeneous within the temperature range from 1200 to 800–600°C and separated into immiscible carbonate-salt fractions of various compositions: alkali-sulfate, alkali-phosphate, alkali-fluoride, alkali-chloride, and Fe-Mg-Ca carbonate. In large scale systems, polyphase silicate-carbonate-salt liquid immiscibility is usually manifested during the slow cooling and prolonged evolution of deeply derived melts in the Earth’s crust. It may lead to the formation of various types of intrusive carbonatites: widespread calcite-dolomite and rare alkali-sulfate, alkali-phosphate, and alkali-halide rocks. The initial alkaline carbonatite melts can retain their compositions enriched in P, S, Cl, and F only at rapid eruption followed by instantaneous quenching. 相似文献
13.
Bayan Obo Carbonatites: Texture Evidence from Polyphase Intrusive and Extrusive Carbonatites 总被引：5，自引：0，他引：5
<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. 相似文献
14.
Summary ?A new occurrence of carbonatites associated with intrusive ijolite and syenite has been discovered within the Hawasina Complex
underlying the Semail Ophiolite Complex at the southern part of the Rawda-Masfut ridge, Northern Oman Mountains. The carbonatites
occur as dikes and sills with lengths of several hundreds of meters and range in composition from calciocarbonatites to ferruginous
calciocarbonatites. The carbonatites intruded the ijolite and the associated radiolarian cherts of the Early Cretaceous Sid’r
Formation. The close spatial association of carbonatite, ijolite, syenite and radiolarian cherts along with geological, petrographical
and geochemical data indicates that these rocks are of intra-oceanic origin. Petrological and field relationships between
the carbonatite and associated alkaline silicate intrusives from the Masfut area are consistent with the carbonatites being
generated as derivative magmas through liquid immiscibility. They appear to represent magmas related to the volcanism associated
with regional crustal extension that preceded the genesis of the Semail Ophiolite.
Received April 19, 2001; revised version accepted February 18, 2002 相似文献
15.
At Oricola (Aquila-Abruzzo, Italy) carbonatite is associated with phonolitic foidite tuff. The Oricola carbonatite contains fresh silicate glass of kamafugitic foidite composition which, compared with carbonate, shows similar trace element patterns but lower concentrations. As a whole, the mineralogy of the Oricola rocks matches that of the neighbouring Grotta del Cervo kamafugitic foidite and carbonatitic foidite and is in the range of the Intramountain Ultralkaline Province (IUP) of melilitites and carbonatites of Italy.
The IUP carbonatites and kamafugitic foidites definitely form intra-outcrop conjugate pairs. All these co-eruptive rocks have parallel trace element patterns, namely REE, which implies a dilution–concentration relationship among them but always with higher contents in primary calcite. Based on current textural and compositional criteria, as well as comparable experimental data, we attribute this feature to liquid immiscibility dominant over crystal fractionation at crustal pressure. This relatively late immiscibility phenomenon is superimposed on co-magmatic features shown by inter-outcrop conjugate rock couples. In fact if we consider San Venanzo kamafugite and Polino Ca-carbonatite, or Grotta del Gervo kamafugite and Oricola Ca-carbonatite, we note couple by couple that they are chemical heteromorphs erupted in isolation in different place. The REE distribution is another distinctive feature of these couples and shows a marked crossover at MREE level. A fact we interpret as produced by near mantle-solidus immiscibility. After this early phenomenon the two members of the couple can erupt in a near primary state carrying mantle xenoliths or undergo some evolution including settling out mantle xenoliths and crystals and fractionation and finally exsolve a carbonatitic residuum by immiscibility. 相似文献
16.
《Gondwana Research》2000,3(1):39-53
Alkaline magmatism during the late Proterozoic is an important event in the northern part of the South Indian granulite terrain. A number of alkaline plutons comprising saturated syenite and ultramafic rocks often associated with carbonatite are found localized along NEHYPHEN;SW trending lineaments, which are considered as deep crustal fractures. Along one such lineament, the alkaline complexes of Yelagiri, Sevattur and Samalpatti have intruded into the country rocks comprising epidote hornblende gneiss. The isotope characteristics and geochemistry of Yelagiri and Sevattur plutons are examined in this paper. Whole rock Rbhyphen;Sr isochron ages of the Yelagiri and Sevattur syenites are 757±32 Ma and 756±11 Ma respectively. The close spatial relationship, similarities in age, mineralogical and geochemical characteristics of these plutons strongly suggest their close genetic relationship. The initial Sr and Nd isotope ratios of the Sevattur carbonatites suggest their derivation from an alkali metal and LREE enriched mantle source. However, the silicate rocks of the Yelagiri and Sevattur plutons have distinctly different isotopic characteristics from this enriched mantle source. Combined geochemical and isotopic characteristics of these silicate rocks indicate that silicate rocks of both plutons are derived independently from isotopically different sources from those of carbonatites. Moreover, comparison with the isotopic characteristics of Archean crustal rocks in South India indicates that the source regions of both silicate rocks are lowerhyphen;crustal portions, which are deeper than any other crustal portion exposed in South India, or isotopically metasomatized crustal portions by volatile influx from carbonatite. 相似文献
17.
Iron isotope compositions of carbonatites record melt generation, crystallization, and late-stage volatile-transport processes 总被引：2，自引：0，他引：2
Clark M. Johnson Keith Bell Brian L. Beard Aaron I. Shultis 《Mineralogy and Petrology》2010,98(1-4):91-110
Carbonatites define the largest range in Fe isotope compositions yet measured for igneous rocks, recording significant isotopic fractionations between carbonate, oxide, and silicate minerals during generation in the mantle and subsequent differentiation. In contrast to the relatively restricted range in δ56Fe values for mantle-derived basaltic magmas (δ56Fe?=?0.0?±?0.1‰), calcite from carbonatites have δ56Fe values between ?1.0 and +0.8‰, similar to the range defined by whole-rock samples of carbonatites. Based on expected carbonate-silicate fractionation factors at igneous or mantle temperatures, carbonatite magmas that have modestly negative δ56Fe values of ~ ?0.3‰ or lower can be explained by equilibrium with a silicate mantle. More negative δ56Fe values were probably produced by differentiation processes, including crystal fractionation and liquid immiscibility. Positive δ56Fe values for carbonatites are, however, unexpected, and such values seem to likely reflect interaction between low-Fe carbonates and Fe3+-rich fluids at igneous or near-igneous temperatures; the expected δ56Fe values for Fe2+-bearing fluids are too low to produced the observed positive δ56Fe values of some carbonatites, indicating that Fe isotopes may be a valuable tracer of redox conditions in carbonatite complexes. Further evidence for fluid-rock or fluid-magma interactions comes from the common occurrence of Fe isotope disequilibrium among carbonate, oxide, silicate, and sulfide minerals in the majority of the carbonatites studied. The common occurrence of Fe isotope disequilibrium among minerals in carbonatites may also indicate mixing of phenocyrsts from distinct magmas. Expulsion of Fe3+-rich brines into metasomatic aureols that surround carbonatite complexes are expected to produce high-δ56Fe fenites, but this has yet to be tested. 相似文献
18.
We report here, for the first time, on the new finding of extrusive calciocarbonatite (alvikite) rocks from the Pleistocene Mt. Vulture volcano (southern Italy). These volcanic rocks, which represent an outstanding occurrence in the wider scenario of the Italian potassic magmatism, form lavas, pyroclastic deposits, and feeder dikes exposed on the northern slope of the volcano. The petrography, mineralogy and whole-rock chemistry attest the genuine carbonatitic nature of these rocks, that are characterized by high to very high contents of Sr, Ba, U, LREE, Nb, P, F, Th, high Nb/Ta and LREE/HREE ratios, and low contents of Ti, Zr, K, Rb, Na and Cs. The O–C isotope compositions are close to the “primary igneous carbonatite” field and, thus, are compatible with an ultimate mantle origin for these rocks. The Sr–Nd–Pb–B isotope compositions, measured both in the alvikites and in the silicate volcanic rocks, indicate a close genetic relationship between the alvikites and the associated melilitite/nephelinite rocks. Furthermore, these latter products are geochemically distinct from the main foiditic-phonolitic association of Mt. Vulture. We propose a petrogenetic/geodynamic interpretation which has important implications for understanding the relationships between carbonatites and orogenic activity. In particular, we propose that the studied alvikites are generated through liquid unmixing at crustal levels, starting from nephelinitic or melilititic parent liquids. These latter were produced in a hybrid mantle resulting from the interaction through a vertical slab window, between a metasomatized mantle wedge, moving eastward from the Tyrrhenian/Campanian region, and the local Adriatic mantle. The occurrence of carbonatite rocks at Mt. Vulture, that lies on the leading edge of the Southern Apennines accretionary prism, is taken as an evidence for the carbonatation of the mantle sources of this volcano. We speculate that mantle carbonatation is related to the introduction of sedimentary carbon from the Adriatic lithosphere during Tertiary subduction. 相似文献
19.
Tapira is an alkaline silicate–carbonatite complex belonging to the kamafugite-carbonatite association in the Late-Cretaceous Alto Paranaíba Igneous Province (APIP). It is dominated by coarse-grained plutonic rocks (bebedourite – a phlogopite-, apatite-, and perovskite-rich clinopyroxenite – with subordinated dunites, wehrlites, carbonatites and phoscorites). The plutonic rocks are crosscut by fine-grained ultramafic alkaline rocks (phlogopite picrites, bebedouritic dikes) and fine-grained carbonatites. Both types of dike-rocks show petrographic evidence of the coexistence of immiscible silicate and carbonatite liquids, such as carbonate ocelli present in the silicate rocks and, more rarely, silicate ocelli within carbonatites. A detailed geochemical study of the rock types in the complex, with emphasis on the fine-grained varieties, showed that whilst some rocks may be related to each other through crystal fractionation (e.g. phlogopite picrites and bebedouritic dikes), others display anomalous trace-element behaviour that cannot be readily explained by the fractionation of a particular phase or combination of phases. We interpret such anomalous geochemical signatures as produced by silicate–carbonate liquid immiscibility, on the basis of available experimental data on partition coefficients between coexisting immiscible liquids. The immiscibility signatures comprise: (a) decoupling of geochemical pairs, such as Nb–Ta and Zr–Hf; (b) rotation of REE patterns, which cross over the patterns of the primitive liquids; and (c) matching and opposite enrichment-depletion trace elements relationships in spider diagrams of conjugate immiscible liquids. We suggest that, once established, such geochemical signatures are very difficult to erase during the subsequent petrogenetic evolution processes, which may result in superimposed conflicting signatures. 相似文献
20.
N.V. Vladykin 《Russian Geology and Geophysics》2013,54(4):416-435
Earlier, a belt of alkali-granite plutons and a carbonatite province were discovered in the South Gobi Desert, Mongolia. The Lugingol pluton of pseudoleucitic syenites with carbonatites was assigned to the alkali-granite belt. However, new dating showed that it is 40 Myr younger than the Khan-Bogdo pluton and a large fault separates it from the alkali-granite belt. In the same part of the South Gobi Desert, a dike series of alkaline K-shonkinites with a rare-metal carbonatite vein was found by V.I. Kovalenko west of the Lugingol pluton, near Mt. Baruun Hasar Uula, and a dike series of alkali and nepheline syenites was found by us northeast of the Lugingol pluton. These data give grounds to distinguish an intrusive complex of K-alkaline shonkinites and leucitic syenites with Late Paleozoic REE-bearing carbonatites. Thus, three alkaline-rock complexes of different ages are distinguished in the South Gobi Desert. We present refined geological maps of these complexes. The plutons of all three complexes are deposits of trace elements (REE, Nb, Zr, Y, P). The chemical composition of the silicate rocks of the complex, rare-metal agpaitic pegmatites, and carbonatite and apatite rare-metal ores was considered in detail. Shonkinites from Mt. Baruun Hasar Uula and the Mountain Pass mine (United States) and their carbonatites, along with the Lugingol carbonatites, belong to a single association of K-alkaline rocks and carbonatites, as evidenced by their identical chemical, mineral, and geochemical rare-metal compositions. Rare-earth element patterns and spidergrams show similarities and differences between the rare-metal rocks of three complexes as well as paragenetic differences between their rare-metal minerals. A rare process is described—the amorphization of rare-metal minerals, related to their high-temperature crystallization in a medium with abnormal silica contents of the Khan-Bogdo pegmatites. The parental magmas of the alkali-carbonatite complexes were generated from the EM-2 contaminated mantle that had undergone recycling, whereas the parental magmas of the Khan-Bogdo agpaitic alkali granites were produced from depleted mantle. 相似文献