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1.
Chemical analyses of over seventy lavas or dykes containing spinel lherzolite inclusions of high pressure mineralogy, show that most host magmas are of alkali olivine basalt or basanite composition with relatively rare olivine nephelinites, and olivine melilitites. The 100 Mg/Mg+Fe++ ratios of host magmas display a strong maximum at about Mg70 consistent with partial melting of source peridotite with olivine of Fo88–90. In contrast to these primary magmas, there occur some host magmas with 100 Mg/Mg+Fe++<60 and with chemical compositions resembling those of classical hawaiite, mugearite, and nepheline benmoreite magmas. It is inferred that these magmas have been produced by crystal fractionation, within the upper mantle, of parental basanites or alkali olivine basalts. The presence of kaersutitic hornblende xenocrysts accompanying the lherzolite inclusions, and the nature of the chemical variation between associated basanites and nepheline benmoreites suggests that crystal fractionation has been dominated by kaersutitic hornblende, together with olivine and, in some cases, probably clinopyroxene. The mantle-derived nepheline benmoreite magmas also show similarities to some plutonic nepheline syenites.  相似文献   

2.
U. Kramm  L. N. Kogarko 《Lithos》1994,32(3-4):225-242
Nd and Sr compositions of the highly evolved agpaitic nepheline syenites and associated ijolites and carbonatites from the Khibina and the Lovozero alkaline centres define three magma sources. Isotopes of the voluminous nepheline syenites and ijolites of Khibina intrusions III, IV, V, VI and VII as well as of nepheline syenites of Lovozero lie on the Kola Carbonatite Mixing Line which is close to the “mantle array” defined by the components “bulk earth” and “prema” on a Sr---Nd plot. The Khibina carbonatites and associated silicate rocks of intrusion VIII, which have more radiogenic Sr, did not evolve from the same parent magma as the nepheline syenites.

Isotopic constraints exclude a pre-enrichment of Rb, Sr, Sm and Nd in the lithospheric mantle below Kola over more than 10 Ma prior to the crystallization of the magmas. A formation of the melts involving major participation of the Precambrian crust of the Baltic Shield is also excluded.

The lack of significant Eu anomalies in the Lovozero nepheline syenites gives evidence that the agpaitic magmas in the Kola region did not form from basaltic liquids by fractional crystallization of plagioclase or anorthoclase at crustal levels. A formation from nephelinite or nepheline benmoreite magmas at mantle pressures is more likely, possibly by dynamic flow crystallization.

Enrichment factors suggest that large-ion lithophile and high field-strength elements as Ta, La, Nb and Zr, which are highly concentrated in the agpaites, were scavenged from mantle volumes of some 100,000 km3. An enrichment of these elements prior to magma formation may have been performed by volatile transfer.

The well-defined whole-rock isochrons of the Khibina III–VII and the Lovozero agpaites of c. 370 Ma date the magma separation for the different intrusion, if these melts are cogenetic and formed by fractional crystallization in a Khibina and a Lovozero magma chamber. If, however, Rb and Sr were collected by a process of volatile transfer, and the initial Sr isotopic compositions of the two distinguished agpaite suites are, therefore, averages of the sampled mantle volumes, the Rb---Sr whole-rock isochron ages of c. 370 Ma would date this process of element collection. The concordance of the whole-rock ages with the mineral ages of Khibina and Lovozero samples is then further evidence for the short period between magma genesis, intrusion and crystallization.  相似文献   


3.
Malignites from the Poohbah Lake complex of northwestern Ontario, Canada are melanocratic cumulates. Cumulus pyroxene and apatite are poikilitically enclosed in a groundmass of large plates of intercumulus orthoclase and nepheline. Nepheline-feldspar fingerprint-like intergrowths occur. Nephelines are commonly zeolitized and pyroxenes altered to aggregates of biotite and/or garnet by deuteric alteration. Pyroxenes are weakly zoned from Di71 Hd18Ac11 to Di63Hd22Ac15, and are similar to the least evolved pyroxenes of other alkaline rocks. Nephelines all have compositions within the Morozewicz-Buerger convergence field and feldspars have a limited compositional range from Or88 to Or95. Perthites are absent.Inconsistancies in the usage of the terms malignite and juvite are discussed and it is considered that a non-genetic petrographic classification of nepheline syenites leads to the obscuration of a group of potassic nepheline syenites, characterized by the presence of nepheline plus orthoclase which are typically associated with saturated to over-saturated alkaline rocks, contain pseudo-leucite or nepheline-orthoclase intergrowths, are emplaced in mobile belts and are not associated with rocks of the ijolite-carbonatite suite.A genetic classification of nepheline syenites is suggested and it is proposed that; (1) mafic-rich nepheline syenites be referred to as mela-nepheline syenites (sensu lato) rather than as malignites; (2) the term malignite be used for magmatic potassic nepheline syenites characterised by the presence of nepheline plus a single potassium-rich feldspar (orthoclase or microcline) and devoid of exsolution perthite under subsolvus conditions; (3) the metasomatic malignites and juvites of ijolite-carbonatite complexes be referred to as varieties of fenites.  相似文献   

4.
The petrogenesis of Abu Khruq, an 89 Ma alkaline ring complex of eastern Egypt which is composed of alkali gabbros and both silica over- and undersaturated syenites, has been investigated. Major and trace element relationships and Nd and Sr isotope data are consistent with formation of the gabbros from an alkaline mafic magma that experienced extensive fractionation, and all syenites from a felsic derivative of this melt. The parental magma had an 87Sr/86Sr of 0.7030 and an 143Nd/144Nd of 0.512750 (Nd = +4.4) indicating derivation from a depeleted mantle source. The initial 143Nd/144Nd ratios are: 0.512721 to 0.512748 for the gabbros, 0.512739 to 0.512750 for the alkali syenites and trachytes, 0.512717 to 0.512755 for the nepheline syenites, and, 0.512706 to 0.512732 for the quartz syenites. In contrast, analyzed Precambrian granites from eastern Egypt have generally lower 143Nd/144Nd ratios (ranging from 0.51247 to 0.51261 or Nd = -0.8 to 1.7, for 89Ma); their Nd model ages range from 775 to 935 Ma and suggest there was no significant input of pre-Pan-African crust in their formation. Among Abu Khruq rocks, 143Nd/144Nd ratios indicate that the quartz syenites formed by open-system, crustal contamination processes whereas the nepheline syenites experienced little or no contamination. Modeling shows that contamination occurred at various stages, affecting both mafic and more evolved compositions with input of about 20% crustal Nd for the most contaminated samples. The degree of contamination is related to the silica saturation of the quartz syenites. Simplified modeling of magma evolution within Petrogeny's Residua System demonstrates the ability of AFC processes to cause a critically undersaturated magma to evolve across the feldspar join and produce oversaturated rocks. The oversaturated syenites at Abu Khruq were produced in this manner whereas the nepheline syenites formed by fractionation without similarly large degrees of contamination. The results have broad implications for the formation of subvolcanic complexes in continental settings beyond the important production of silica oversaturated compositions from crustal interaction. They underscore the importance of crustal interactions in the formation of the various lithologies. Such interactions occur at various stages in the evolution of the magmas and, as such, are not strictly coupled with fractional crystallization. While previous study of Abu Khruq has demonstrated extensive hydrothermal alteration of O and Sr isotopes, the present work shows that the Nd isotope ratios were not significantly affected and thus reflect magmatic signatures. This feature combined with relatively small corrections for initial ratios emphasizes the utility of Nd isotopes for petrogenetic studies.  相似文献   

5.
云南个旧碱性岩体主要的岩石类型有碱性正长岩和似长石正长岩,其中,似长石正长岩中出现大量似长石矿物霞石、方钠石和碱性暗色矿物。本文根据矿物成分及特征,将这些似长石正长岩进一步划分为黑榴霞石方钠正长岩、霞石方钠正长岩、霞石正长岩及方钠霞石正长岩4类。岩石地球化学结果表明,4类岩石的地球化学行为整体表现出过碱质岩的特征,K2O+Na2O含量很高,为钾玄岩系列,同时表现出钾质的特点。分异指数高,呈现高度分异演化特点。稀土元素变化大,轻重稀土元素分异明显,富集轻稀土元素,亏损重稀土元素。微量元素富集大离子亲石元素Th、U及Zr、Hf等高场强元素,亏损Ba、Sr大离子亲石元素,而P和高场强元素Ta、Ti亏损,同时Cr、Co、Ni含量非常低,具有中等的负Eu异常和微弱的负Ce异常。研究表明,似长石正长岩在岩浆演化过程中表现出明显的分离结晶作用特征,且岩浆起源温度较高,约为835℃,起源较深。个旧似长石正长岩为A型岩套A1亚型,结合构造判别图解,认为其可能来自角闪石或者金云母相矿物存在的富集地幔,形成于燕山晚期伸展的构造背景,岩浆在较高温度下高度结晶分异,并在侵位过程中伴随陆壳成分的混染。  相似文献   

6.
Sushina nepheline syenite gneisses of Early Proterozoic North Singhbhum Mobile Belt (NSMB), eastern India suffered regional metamorphism under greenschist-amphibolite transitional facies condition. The Agpaitic Sushina nepheline syenite gneisses consist of albite, K-feldspar, nepheline (close to Morozewicz-Buerger composition), aegirine, biotite, epidote, piemontite, sodalite, cancrinite, natrolite and local alkali amphibole. Accessory phases include zircon, hematite, magnetite, rare pyrochlore and occasional eudialyte and manganoan calcic zirconosilicates. Mineral chemistry of albite, K-feldspar, nepheline, aegirine, alkali amphibole, natrolite and zirconium silicate minerals are described. The detailed textural features together with chemical data of some minerals indicate metamorphic overprint of these rocks. A new reaction is given for the genesis of metamorphic epidote. Metamorphic piemontite suggests greenschist facies metamorphism under high fO2 (Hematite-Magnetite buffer). Up to 15.34 mol% of jadeite component in aegirine suggests that the metamorphic grade of the nepheline syenite gneiss reached at least to greenschist-amphibolite transitional facies or higher. Nepheline geothermometry suggests temperature of metamorphism <500 °C, which is consistent with greenschist facies metamorphism of surrounding chlorite-biotite-garnet phyllite country rock.  相似文献   

7.
Aegirines with almost 7.0 wt.% ZrO2 have been discovered in nepheline syenites from the Motzfeldt Centre, South Greenland. The analyses require the postulate of a new endmember pyroxene composition with the formula NaFM0.5Zr0.5Si2O6.A possible acronym is FM-NAZ. Aegirines rich in this component occur in rocks in which there is no other zirconium-bearing phase such as eudialyte.These zirconian pyroxenes have crystallised from magmas which were peralkaline, low in lime and silica and relatively low in oxygen fugacity compared with other nepheline syenites. These factors have combined to prevent the usual crystallisation of such Zr-phases as eudialyte, zircon or baddeleyite.  相似文献   

8.
Aegirines with almost 7.0 wt.% ZrO2 have been discovered in nepheline syenites from the Motzfeldt Centre, South Greenland. The analyses require the postulate of a new endmember pyroxene composition with the formula NaFM0.5Zr0.5Si2O6.A possible acronym is FM-NAZ. Aegirines rich in this component occur in rocks in which there is no other zirconium-bearing phase such as eudialyte.These zirconian pyroxenes have crystallised from magmas which were peralkaline, low in lime and silica and relatively low in oxygen fugacity compared with other nepheline syenites. These factors have combined to prevent the usual crystallisation of such Zr-phases as eudialyte, zircon or baddeleyite.  相似文献   

9.
B. A. Worley  A. F. Cooper  C. E. Hall 《Lithos》1995,35(3-4):183-199
Chemically-evolved carbonate-bearing nepheline syenites are intruded into basement metasediments of the Koettlitz Group on Dismal and Radian Ridges in the Pipecleaner Glacier region of Southern Victoria Land, Antarctica. Whole rock XRF data from the Dismal Nepheline Syenite defines a broad trend which is consistent with the removal of a cumulate fraction of approximate composition 70% hedenbergite, 15% nepheline, 10% titanite and 5% apatite. Stable isotope, major and trace element and mineralogical characteristics of the syenites are very similar to those of cross-cutting calcite-rich dykes indicating derivation from closely-related source magmas. The general association of carbonatites and nepheline syenites with extensional environments, suggests that the Dismal and Radian Ridge nepheline syenites and carbonatite dykes indicate a period of early Paleozoic (531 Ma) rifting or intrusion into localised tensional structures in an overall compressional regime. Assimilation of marble by the syenite magmas is evidenced by abundant rafts and xenoliths within the Dismal Nepheline Syenite, however, carbon and oxygen isotopic ratios from syenite and carbonatite calcites are distinctly lighter than values from the marble country rock indicating a magmatic source. Graphite and calcite commonly occur as aggregates in the Dismal Nepheline Syenite suggesting equilibrium between these two carbon-bearing phases. Isotopic fractionation between calcite and graphite, via the equilibrium: C + O2 = CO2 has resulted in enrichment of the 13C isotope in calcites from the Dismal Nepheline Syenite.  相似文献   

10.
The igneous alkaline rocks at Elchuru start from a parent ijolite-melteigite association to basic malignite, melalusitanite and shonkinite followed by nepheline syenites and then biotite lamprophyres (as dykes) at the waning phase of the evolutionary course of the complex. The distinct alkalinity of the rocks is manifested by the development of modal nepheline and calcic amphibole (kaersutite). For both the basic rocks,i.e. alkali gabbro and biotite lamprophyre, the percentages of normative nepheline are always higher than modal nepheline, indicating silica deficiency and alkali enrichment of the mafics. It is evident from detailed petrological and geochemical studies that the two basic members are very much akin to each other and there is no major deviation in their bulk chemistry.  相似文献   

11.
The Sivamalai alkaline complex lies at the southern margin of the Cauvery Shear System that separates the Archaean and Proterozoic domains of the Southern Granulite Terrain in India. U–Pb TIMS dating of zircon from a pegmatitic syenite sample in the complex yields a concordant age of 590.2 ± 1.3 (2σ) Ma which is interpreted to date the intrusion of the alkaline rocks. A lower concordia intercept at 168 ± 210 Ma defined by two grains with high common lead may indicate post-magmatic disturbances due to recrystallisation which is also evident in the CL images of the zircons. EPMA dating of monazite from a post-kinematic pegmatite which intrudes the crystalline basement hosting the alkaline rocks yields an age of 478 ± 29 (2σ) Ma and provides a lower bracket for the main phase of tectonism in this part of the Southern Granulite Terrain. The Pan-African high-grade metamorphism and ductile deformation has thus most likely affected the alkaline rocks. This is supported by the presence of a metamorphic foliation and extensive recrystallisation textures seen in the rocks. The major and trace element concentrations measured on selected samples reveals the presence of both enriched and depleted rock types. The enriched group includes ferrosyenite and nepheline syenite while the depleted group has only nepheline syenites. The trace element depletion of some nepheline syenites is interpreted to be a result of fractional crystallization involving the removal of accessory phases like zircon, titanite, apatite and allanite.  相似文献   

12.
To the north of the Qinling-Dabie orogen there exists a Mesozoic (K1) granite-syenite belt called the Beihuaiyang granite-syenite belt, in which the Xianghongdian alkaline complex outcrops in the Devonian Foziling schist and also intruded into the slightly earlier volcanic rocks. This alkaline complex is composed of syenite, alkaline syenite and nepheline syenite, but the nepheline syenite occurs inside the alkaline syenite in the pod-like, irregular, lenticular or layered shape. Researches on the petrogenesis of the alkaline complex and the relationship between the alkaline syenite and the nepheline syenite must rely on precise isotopic ages. Previous researches have reported iso-topic dating results merely by the 40Ar/39Ar method on hornblende and biotite separated from the alkaline syenite and nepheline syenite. In view of this, the authors have made fairly detailed isotopic geochronological studies of the alkaline syenite and nepheline syenite by using Rb-Sr isochron for the whole rock and minerals,  相似文献   

13.
The unaltered magmatic rocks of the Zadoi Massif were analyzed for Sr isotopic composition and concentrations of major oxides and trace elements by ICP MS. The evolution of the massif involved four phases: (i) perovskite and ilmenite clinopyroxenites, (ii) ijolites, (iii) nepheline syenites, and (iv) carbonatites. The perovskite clinopyroxenites have anomalously high Ce/Pb (223–1132) and Pr/Sr × 1000 (70–360) ratios at a low initial Sr isotopic ratio (87Sr/86Sr)0 = 0.70247–0.70285. The ilmenite clinopyroxenites have Ce/Pb and Pr/Sr × 1000 ratios approaching those in basalts of oceanic islands (OIB) (decreasing to 39 and 30, respectively) at a simultaneous increase in the (87Sr/86Sr)0 ratios (0.7030–0.7036). The ijolites and nepheline syenites have patterns of incompatible trace elements similar to those in OIB and the highest (87Sr/86Sr)0 ratios (0.70346–0.70414). The carbonatites are complementarily enriched in incompatible elements of the nepheline syenites and have (87Sr/86Sr)0 = 0.7029–0.7034, which is comparable with the range of analogous ratios for the ilmenite clinopyroxenites. Our geochemical data indicate that the carbonatites were formed as an immiscible liquid or fluid, which separated from the ijolite-nepheline syenite melt during its interaction with the source material of the perovskite and ilmenite clinopyroxenites.  相似文献   

14.
We present new geochemical data on alkali and nepheline syenites from various complexes of different age within the Ukrainian Shield. The results reveal a correlation between the content of trace elements in the syenites, their assignment to a particular rock complex, the chemistry of primary melts, and the degree of their differentiation. The data also suggest regional geochemical heterogeneity in the ultramafic-alkaline complexes of the Ukrainian Shield. The alkali and nepheline syenites in the ultramafic-alkaline massifs from the eastern and western parts of the region exhibit similar REE contents and Eu/Eu* ratios but are markedly different in Nb, Ta, Zr, and Hf content and are of the miaskitic type. These rocks have lower REE, Nb, and Zr and higher Sr and Ba compared with early foidolites. The rocks of the gabbro-syenite complexes define a distinct Fe-enrichment fractionation trend from early syenitic intrusions to more differentiated varieties; they are also characterized by lower Sr, Ba, and Eu/Eu* and significantly lower contents of some major elements, e.g., Ti, Mg, and P. The agpaitic index and concentrations of Zr, Nb, Y, and REE increase in the same direction. A similar geochemical feature is observed in the alkali syenites genetically associated with anorthositerapakivi-granite plutons, which show incompatible-element enrichment and strong depletion in Sr and Ba. The distinctive evolutionary trends of alkali and nepheline syenites from different rock complexes of the Ukrainian Shield can be explained by different mechanisms of their formation. The main petrogenetic mechanism controlling the distribution of trace elements in the rocks of ultramafic-alkaline complexes was the separation of parent melts of melanephelinite and melilitite types into immiscible phonolite and carbonatite liquids. The gabbro-syenite complexes and alkali syenites from anorthosite-rapakivi granite plutons evolved via crystallization differentiation, which involved extensive feldspar fractionation.  相似文献   

15.
The Sivamalai alkaline complex comprises ferro-, pyroxene-hornblende- and nepheline-syenites. Field relations show that the nepheline syenites followed the emplacement of non-feldspathoidal syenites. Mineralogical data on the syenite suite have been reviewed. The Sivamalai alkaline rocks are not strongly enriched in rare-earth elements like most miaskites. Rb-Sr isotopic analyses of a suite of six samples from the various members of the complex define an isochron corresponding to an age of 623 ±21 Ma (2σ) and initial Sr ratio of 0.70376 ±14 (2σ). This is consistent with a model of fractional crystallization of a parent magma derived from an upper mantle source with apparently no isotopic evidence for more than one magma source for the complex. The Sivamalai alkaline complex represents a Pan-African alkaline magmatic event in the southern granulite terrane of Peninsular India.  相似文献   

16.
LA-ICP MS data are presented for ilmenites from different rocks of the alkaline complexes of the Ukrainian Shield (Chernigovka carbonatite, Oktyabr’skii, Malaya Tersa, and Southern Kal’chinskii gabbrosyenite massifs). Ilmenites from the early intrusive phases (alkaline pyroxenites, gabbroids, and ultramafic rocks) have the elevated contents of Cr, Co, Ni, and V, while ilmenites from later alkaline and nepheline syenites, monzonites, and carbonatites are significantly enriched in Nb and Ta, which is caused by change in the alkalinity of the mineral-forming medium. Zr shows the more intrinsic behavior: its content is higher in the ilmenites from basic and ultrabasic rocks than in those from the nepheline syenites and carbonatites. This is mainly caused by temperature conditions of the formation of differentiated alkaline complexes. The carbonatites contain magnesian ilmenite (up to 22 mol % MgTiO3). Variations of Mg contents in ilmentes are correlated with Mg number of mafic minerals and depend also on the iron oxidation state (amount of magnetite) in the carbonatites. In the alkaline massifs of the Ukrainian Shield, ilmenites usually have the low contents of hematite end member (3–7 mol %). Ilmenite serves as a sensitive indicator of temperature, oxygen fugacity, and alkalinity of the mineral-forming medium during crystallization.  相似文献   

17.
《International Geology Review》2012,54(11):1296-1308
Geochronological, geochemical, and whole-rock Sr–Nd isotopic analyses were performed on a suite of Palaeozoic nepheline syenites from Zijinshan to characterize their ages and petrogenesis. Laser ablation inductively coupled plasma-mass spectrometry U–Pb zircon analyses yield consistent ages of 525.7 ± 2.8 million years for a sample (HYK01). These intrusive rocks belong to the foid syenite magma series in terms of K2O?+?Na2O contents (14.3–15.2 wt.%) and to the shoshonitic series based on their high K2O contents (5.42–5.61 wt.%). The nepheline syenites are further characterized by high light rare earth element contents [(La/Yb) N ?=?29.1–36.1]; show modest negative Eu anomalies (δEu?=?0.5–0.6) and positive anomalies in Rb, Th, U, Pb, Zr, and Hf; are depleted in Ba and high field strength elements (P and Ti). In addition, all the nepheline syenites in this study display relatively low radiogenic Sr (87Sr/86Sr) i (0.7042–0.7043) and positive ?Nd (t) (0.7–0.8). These results suggest that the nepheline syenites were derived from depleted continental crust. The parent magmas likely experienced fractional crystallization of plagioclase, Ti-bearing oxides (e.g. rutile, ilmenite, and titanite), apatite, and zircon during ascent, with negligible crustal contamination before final emplacement at a high crustal level.  相似文献   

18.
The U-Pb geochronological study (by the classic technique and on an ion microprobe) of syenites from central Karelia has established their Archean age. The age values obtained for individual massifs are 2735 ± 15 Ma for syenites from the Sjargozero Massif and 2745 ± 10 Ma for syenite from the Khizhjarvi Massif. The syenites are demonstrated to have been emplaced nearly synchronously with sanukitoid massifs in central Karelia, whose average age is 2743 ± 3 Ma (Bibikova et al., 2005). The syenites of the Sjargozero Massif and granodiorites of the Ust-Volomsky Massif were determined to have practically identical ages of 2735 and 2738 Ma, respectively, a fact also corroborating the coeval character of the syenites and granodiorites. Some zircon grains from the Sjargozero syenites contain cores with an age of about 2755 Ma, which suggests that the syenites could have been contaminated with the material of the host volcanic rocks of basaltic and andesitic composition that were metamorphosed at 2750–2760 Ma. The results of the isotopic geochronologic research indicate that the different rock groups composing the Archean postorogenic association of sanukitoids, syenites, and granitoids in central Karelia have been generated in a single stage at approximately 2740 Ma, i.e., 60–70 m.y. after the origin of the syntectonic tonalites. The zircons have elevated Th/U ratios, which is consistent with the mantle genesis of the rocks. Significant crustal contamination was identified in the most acid members of the sanukitoid series: syenites and granitoids. Our data obtained for zircons from the sanukitoids and syenites of the Karelian craton in the Baltic Shield are in good agreement with the results obtained on the sanukitoids of the Canadian Shield.  相似文献   

19.
Seven pyroxene varieties were identified in nepheline syenites and foidolites of the Khibiny pluton: enstatite, ferrosilite, diopside, hedenbergite, augite, aegirine-augite, and aegirine. Enstatite and augite are typical of alkaline and ultramafic rocks of dike series. Ferrosilite was found in country quartzitic hornfels. Diopside is a rock-forming mineral in alkaline and ultramafic rocks, alkali gabbroids, hornfels in xenoliths of volcanic and sedimentary rocks and foyaite, melteigite-urtite that assimilate them, and certain hydrothermal pegmatite veins. Hedenbergite was noted in hornfels from xenoliths of volcanic and sedimentary rocks and in a hydrothermal pegmatite vein at Mount Eveslogchorr. Aegirine-augite is the predominant pyroxene in all types of nepheline syenites, phonolites and tinguaites, foidolites, alkaline and ultramafic rocks of dike series, fenitized wall rocks surrounding the pluton, and xenoliths of Devonian volcanic and sedimentary rocks. Aegirine is an abundant primary or, more often, secondary mineral in nepheline syenites, foidolites, and hydrothermal pegmatite veins. It occurs as separate crystals, outer zones of diopside and aegirine-augite crystals, and homoaxial pseudomorphs after Na-Ca amphiboles. Microprobe analyses of 265 pyroxenes samples allowed us to distinguish ten principal trends of isomorphic replacement and corresponding typomorphic features of pyroxenes. Compositional variations in clinopyroxenes along the sampled 35-km profile from the margin of the Khibiny pluton to its center confirm the symmetric zoning of the foyaite pluton relative to semicircular faults of the Minor Arc and the Main (Central) Ring marked by Devonian volcanic and sedimentary rocks, foidolites, and related metasomatic rocks (rischorrite, albitite, and aegirinite). Changes in the composition of pyroxenes are explained mainly by the redistribution of elements between coexisting minerals of foyaites in the process of their intense differentiation under the effect of foidolite melts that have intruded into the circular fault zones.  相似文献   

20.
The Ditrău Alkaline Massif is an intrusion into the Bucovina nappe system that is part of the Mesozoic crystalline zone located in Transylvania, Romania, in the Eastern Carpathians. Nepheline syenites are the most abundant rocks in the central and eastern part of the Massif, and represent the last major intrusion of the complex. Fluid inclusions in nepheline, aegirine and albite were trapped at magmatic conditions on or below the H2O-saturated nepheline syenite solidus at about 400–600 °C and 2.5–5 kbars. Early nepheline, and to a lesser extent albite, were altered by highly saline fluids to produce cancrinite, sodalite and analcime, during this process cancrinite also trapped fluid inclusions. The fluids, in most cases, can be modeled by the H2O–NaCl system with varying salinity; however inclusions with more complex fluid composition (containing K, Ca, CO3, etc., in addition to NaCl) are common. Raman spectroscopic analyses of daughter minerals confirm the presence of alkali-carbonate fluids in some of the earliest inclusions in nepheline, aegirine and albite.

During crystallization, the melts exsolved a high salinity, carbonate-rich magmatic fluid that evolved to lower salinity as crystallization progressed. Phases that occur early in the paragenesis contain high-salinity inclusions while late phases contain low-salinity inclusions. The salinity trend is consistent with experimental data for the partitioning of chlorine between silicic melt and exsolved aqueous fluid at about 2.0 kbars. The activity of water (aH2O) in the melt increases during crystallization, resulting in the formation of hydrous phases during late-stage crystallization of the nepheline syenites.  相似文献   


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