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1.
J.C. Bailey   《Lithos》2006,91(1-4):319-330
The distribution of boron has been studied in rocks and minerals of the Ilímaussaq complex, South Greenland, using optical emission spectrometry. In the silica-undersaturated rocks of intrusive phases 1 and 3, average B contents increased from 5.6 ppm in augite syenite to a maximum of 8.9 ppm in sodalite-rich agpaitic nepheline syenite (naujaite roof cumulate) and then decreased to 5.4 ppm in the final lujavrites. Boron only behaved as an incompatible element during certain stages of the fractionation history. Starting at the naujaite stage, sodalite crystals (60–45 ppm B) were fractionated by flotation and were also trapped among the heavy cumulus phases of the bottom cumulates. This prevented the significant build-up of B in late derivatives as seen in other nepheline syenites. Nevertheless, in late pegmatites and veins associated with the agpaitic rocks, B was locally concentrated in certain Be minerals and metamict/reworked minerals. In the silica-oversaturated rocks of intrusive phase 2, average B contents increased from 8.6 ppm in quartz syenite to 13 ppm in alkali granite.  相似文献   

2.
J.C. Bailey  R. Gwozdz   《Lithos》1994,31(3-4):207-225
Li was analysed by activation analysis in 363 whole-rock samples and 53 minerals from a 200 m drill core through aegirine lujavrite I, Ilímaussaq intrusion. The rocks are interpreted as cumulates and Li is dominantly located in the intercumulus material which is thought to represent the coexisting magma with 540 ppm: whole-rock contents generally range from 80–260 ppm Li and reflect variations in the amount of intercumulus material from 25–48 wt. %. The proportion of intercumulus material is lowest in the deeper parts of the drill core, in layers with a moderate density stratification resulting from showers of near-liquidus crystals and in compressed cumulates beneath a large naujaite xenolith from the roof. Intercumulus arfvedsonite with an average of 2200 ppm Li contains the bulk of Li in most samples, but nearly every sample contains Li (0–176 ppm) outside the rock-forming minerals (Li-ORFM) which was unlocated by conventional mineral separation. Li-ORFM formed during late-magmatic crystallization (a) from the final residues of intercumulus magma which crystallised as accessory Li-mica and alteration products and (b) during in situ zeolitization of arvedsonite which released Li. A later, more intense zeolitization released Li from the rocks of the drill core. Naujaite xenoliths contain 42–130 ppm Li, except where Li-metasomatised, and are associated with low Li values in adjacent lujavrites. Shearing of consolidated lujavrite allowed mobilization of the Li and its reprecipitation along narrow channels (< 10 cm). Li is not concentrated in late hydrothermal coatings.  相似文献   

3.
Microrhythmic layering is locally developed in agpaitic arfvedsonite lujavrite from the Ilímaussaq alkaline complex, South Greenland. Three–15-cm-thick laminated dark layers alternate with 1–10-cm-thick, light-coloured granular urtitic layers. Dark layers are uniform (isomodal) but the urtitic layers are enriched in early nepheline and eudialyte in their lower parts and in late analcime and REE phosphate minerals in the upper parts. The layers are separated by sharp contacts; they are draped around rafts from the overlying roof zone and lack structures indicative of current processes or post-cumulus deformation. Compared with the background arfvedsonite lujavrite of the complex, the dark layers are richer in sodalite, microcline and arfvedsonite and poorer in analcime and eudialyte. They have higher K2O, Cl, FeO and S but lower Na2O, H2O+, Zr and P contents, the opposite of the light-coloured layers. The complementary chemistry of the two types of layers oscillates about the composition of the background arfvedsonite lujavrite. Layers probably formed in a stagnant bottom layer of the lujavrite magma chamber. Each layer started as a liquid layer which exchanged components with the underlying crystallization front. On cooling, it crystallized primocrysts and exchanged components with the overlying magma which became a new, complementary liquid layer and, during further cooling and burial within the sequence of layers, it underwent largely closed-system interstitial crystallization. Exhaustion of Cl and a sharp decrease in aNaCl relative to aH2O terminated the crystallization of a sodalite-rich dark layer and initiated abundant crystallization of nepheline in the overlying liquid layer (urtitic layer). The layered sequence represents a local K2O-, Cl-rich but Na2O-, H2O-poor facies of arfvedsonite lujavrite and may have formed by exchanging components with sodalite-bearing rafts from the roof zone.  相似文献   

4.
The Ilímaussaq intrusion consists of an augite syeniteshell and a central cumulitic series of layered peralkaline(agpaitic) nepheline syenites with both roof and bottom cumulates.Microprobe analyses of the strongly zoned mafics show that thepyroxenes range from ferro-salite through hedenbergite to aegirine,the amphiboles from hastingsite through katophorite to arfvedsonite,and the olivines from Fo16.8 to FO0.2. Aenigmatite analysesare also presented. The crystal chemistry of the pyroxenes isdiscussed in detail. The behaviour of the various elements isdiscussed in relation to the conditions in the coexisting magma.The relation between oxygen fugacity and silica activity andthe stability of fayalite and Na-poor pyroxenes in over-andundersaturated magmas is considered. The persistent stabilityof these minerals in the strongly undersaturated Ilimaussaqmagma shows that the oxygen fugacities in Ilímaussaqwere lower than in any other known alkaline suites. Based onthe compositional development in the mafic minerals it is inferredthat the intrusion formed from at least three separate pulsesof successively more differentiated magma: (1) an augite syenitemagma congealed inwards from the sides, (2) a peralkaline undersaturatedmagma gave rise to the roof cumulates which congealed successivelydownwards. The differentiating magma was water-undersaturatedand underwent depletion in Mg and enrichment in Zr and Na. Thecontemporaneous bottom cumulates are hidden beneath the nowvisible bottom cumulates which possess a still more differentiatedliquidus mineral assemblage and are ascribed to a third separatemagma pulse.  相似文献   

5.
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   


6.
Partition coefficients for iron-rich olivine and pyroxene, sanidine, nepheline and apatite are reported from peralkaline trachytic to phonolitic dyke rocks and the agpaitic Ilímaussaq intrusion. Partition coefficients for many elements in olivine and pyroxene decrease with increasing peralkalinity and undersaturation of the magma, i.e. with decreasing polymerisation. The REE partition coefficients for olivine and pyroxene also show dependence on the mineral chemistry, i.e. the iron content. Probably due to the larger lattice sites in the iron end-members the heavy REEs enter the small six-coordinated lattice sites with increasing ease as the iron content of the mineral increases. La and Ce partition coefficients for apatite increase with increasing peralkalinity; this condition seems to stabilise a Na-REE-phosphate component in the mineral.  相似文献   

7.
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   

8.
Within the 1.16 Ga old Ilímaussaq intrusion, up to 700 m large autoliths occur in one stratigraphic unit of the layered floor series of agpaitic nepheline syenites (kakortokites). These autoliths consist of two different rock types: augite syenite and naujaite (agpaitic nepheline syenite). All three rock types show a number of alteration features related to the entrapment of the autoliths in the kakortokite magma caused by the interaction with a fluid phase.

In the kakortokites, the oxidation of primary arfvedsonite to aegirine and fluorite is restricted to the close proximity to the autoliths. Close to the surrounding kakortokite, the primary mafic phases of the augite syenites (augite, fayalite, Fe–Ti oxides) are completely replaced by arfvedsonite, aenigmatite, biotite, aegirine and fluorite. The decomposition of primary hastingsite to spectacular aegirine–augite–nepheline–aenigmatite symplectites can be observed up to several meters inside the autoliths. Additionally, fluorite formed at grain boundaries of primary nepheline. In the naujaite autoliths, primary arfvedsonite is replaced by aegirine–biotite intergrowths and abundant aenigmatite is occasionally replaced by Ti-rich aegirine and Fe–Ti oxides.

The mineral reactions in the autoliths are used to decipher details of the late to post-magmatic processes in a peralkaline syenitic intrusion. Mineral equilibria record an evolution governed by falling temperature (620 to ca. 500 °C) and increasing relative oxygen fugacity from FMQ + 1 to above FMQ + 4. Quantification of the observed mineral reactions reveals the infiltration of the autoliths with an oxidizing fluid phase rich in Na and F and minor addition of K. Volatiles (H and F) and in some cases also Fe, Ti and Ca (± Mg) released from primary autolith phases were mainly just relocated within the autoliths.  相似文献   


9.
Composition and zoning of amphibole in agpaitic pegmatites of the 1.16 Ga Ilímaussaq complex, South Greenland record the chemical evolution of the final stages of an already extremely fractionated melt. Our results show that the general differentiation trends found in the earlier rocks of the complex are continued in the pegmatites, albeit with some significant modifications: the dominating exchange mechanism of Na + Si Ca + Al in the amphiboles of the magmatic stage changes to K + Si Ca + Al and K Na in some pegmatitic samples. Na/K ratios in amphiboles, which generally increase in the course of the Ilímaussaq fractionation, partly display a reversal during the crystallization of the most differentiated amphiboles.

The alkali trends are probably related to the buffering of Na+and K+activity by the co-crystallization of albite and microcline. This buffering favors Na+in cooling fluids. This mechanism is lost when analcime replaces feldspar as a stable phase in the late stages of crystallization, e.g. due to locally elevated H2O activity. Analcime does not incorporate significant amounts of K and accordingly, amphibole incorporates more K in analcime-bearing assemblages. The Na–K variation in amphiboles in the Ilímaussaq pegmatites allow a detailed view into the late-stage evolutionary trends of a textbook agpaitic complex. The transition from silicate melt to aqueous fluid is recorded by the change of the dominant alkali ion in the pegmatitic amphiboles from Na to K.

Only in the very latest stage, virtually K-free mineral assemblages in analcime–aegirine veins support the existence of a Na-dominated aqueous fluid.  相似文献   


10.
Sm-Nd isotopic compositions were determined for the peralkaline Ilímaussaq Complex of the Gardar Province of southern Greenland. The majority of the samples in the agpaitic and augite syenitic units have near chondritic initial Nd(≈ 0), whereas a few samples trend towards Nd values as low as − 6 at the time of intrusion (1143 Ma). This latter value, from a sample taken from the margin of the complex, lying on the evolutionary trend for Ketilidian country-rock granitoids, suggests that large-scale contamination took place only at the margins of the complex. The similarity of the Nd isotopic compositions of the augite syenite and agpaitic units suggests that their parental magmas were derived from the same reservoir. A comparison of the Nd with existing Sr and Hf isotopic data for the complex suggests an origin by combined assimilation fractionation processes. Assimilation-fractional crystallization modeling of the isotopic compositions indicates that the Ilímaussaq magmas could have formed through fractional crystallization of a basaltic melt while assimilating granitic crust. The model requires initially higher assimilation rates from basalt to augite syenite composition with subsequent decreasing assimilation rates from augite syenite to agpaitic compositions. Alkali granites, which formed after the intrusion of the augite syenites, have isotopic compositions intermediate between those of the augite syenites and the surrounding Ketilidian basement. This implies even greater amounts of assimilation and is interpreted as evidence for an origin through fractionation of a basaltic or augite syenite magma with concurrent assimilation of Ketilidian crust.  相似文献   

11.
In this paper we report the results of the analysis of rare earth (REE), large-ion lithophile (LILE), and high field strength (HFSE) elements in minerals from the alkaline lamprophyre dikes of the Kola region and the Kaiserstuhl province by the local method of laser ablation inductively coupled plasma mass spectrometry. The contents of Y, Li, Rb, Ba, Th, U, Ta, Nb, Sr, Hf, Zr, Pb, Be, Sc, V, Cr, Ni, Co, Cu, Zn, Ga, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu were measured in olivine, melilite, clinopyroxene, amphibole, phlogopite, nepheline, apatite, perovskite, and the host fine-grained groundmass. The obtained data on trace element partitioning among the mineral phases of the alkaline ultrabasic rocks of the dike series indicate that the main mineral hosts for the HFSEs and REEs in alkaline picrites, olivine melanephelinites, and melilitites are perovskite and apatite comprising more than 90% of these elements. Among major rock-forming minerals, melilite, clinopyroxene, and highly magnesian amphibole make a significant contribution to the balance of REEs during the evolution of melanephelinite melts. The partition coefficients of Ni, Co, Cu, Zn, Sc, V, Cr, Ga, Y, Li, Rb, Ba, Th, U, Ta, Nb, Sr, Hf, Zr, Pb, Be, and all of the REEs were calculated for olivine, clinopyroxene, amphibole, phlogopite, nepheline, perovskite, and apatite on the basis of mineral/groundmass ratios. Variations in the composition of complex zoned clinopyroxene phenocrysts reflect the conditions of polybaric crystallization of melanephelinite melt, which began when the magmas arrived at the base of the lower crust and continued during the whole period of their ascent to the surface. The formation of green cores in clinopyroxene is an indicator of mixing between primary melanephelinite melts and phonolite magmas under upper mantle conditions. The estimation of the composition of primary melts for the rocks of the alkaline ultrabasic series of the Kola province indicated a single primary magma for the whole series. This magma produced pyroxene cumulates and complementary melilitolites, foidolites, and nepheline syenites.  相似文献   

12.
For the first time Pb isotope composition was established in Lovozero rocks and raremetal ores, which is important for identifying their sources. The world’s largest layered intrusion of agpaitic nepheline syenite-the Lovozero alkaline massif—is located near the center of the Kola Peninsula in Russia. This superlarge complex plutonic body hosts the economically important loparite and eudiallyte deposits [1]. These deposits contain immense resources of REE, Nb, Ta, Zr, and constitute a world class mineral district. The Lovozero massif belongs to the Kola ultramafic alkaline and carbonatitic province (KACP) of Devonian age. Previous bulk rock studies have shown that the initial Sr and Nd isotope ratios of Lovozero rocks plot in the depleted mantle quadrant of Sr-Nd diagrams [2]. More recently, Hf isotope data obtained by Kogarko et al. (3) confirm that the Lovozero and Khibina massifs with ?Hf between 6 and 8 are derived predominantly from a depleted mantle source. It was shown that Sr, Nd, and Hf abundances are significantly elevated in the Kola alkaline rocks, and thus their isotopic compositions are relatively insensitive to minor contamination by the overlying crustal rocks. By contrast, Pb in the KACP rocks is a much more sensitive indicator of a crustal component. In this paper we investigate the lead isotopic signature of all resentative types of Lovozero rocks (Table 1) in order to further characterize their mantle sources. The Lovozero massif consists of four intrusive phases. Rocks of phase I (mostly nepheline syenites) comprise about 5% of the total volume, phase II (urtites, foyaite, lujavrites) forms the main portion of the massif comprising 77% in volume, and phase III (eudialyte lujavrites) contributes about 18%. Country rocks are represented by Devonian effusive rocks and Archean gneisses.  相似文献   

13.
The eudialyte-group of minerals (EGM) is one of the most important index minerals of the peralkaline (agpaitic) nepheline syenites. They crystallize in varied physico-chemical conditions ranging from the early-magmatic (orthomagmatic) to late-magmatic and even in the post-magmatic (hydrothermal) stage. In India, the only agpaitic nepheline syenite gneisses of the Sushina Hill region contain both late-magmatic as well as hydrothermal eudialytes. Compositionally these are Mn-Nb-Ca rich eudialytes and are comparable to the other EGM occurrences such as Ilímaussaq (Greenland), Tamazeght (Morocco), Mont-Saint Hilaire (Canada) and Pilansberg (South Africa). High Mn content (>6.5 wt.%) for both varieties of the Sushina EGM indicates that they are highly evolved in nature. In terms of the calculated site occupancy, particularly the [M(3)] and [M(2)], the Sushina eudialytes mimic some Pilansberg eudialytes. In addition to the eudialyte, the host nepheline syenite gneiss also contains an unknown Na-Zr-silicate (NZS) which is often found to be replacing both types of eudialytes. Compositionally these NZS can be tentatively represented as Na2Zr2S6O17. These NZS are characterized by much higher Zr, but lower Mn and Nb concentrations compared to the associated eudialytes. Two distinct varieties of eudialyte and NZS indicate subtle changes in the alkalinity during their formations. The formation of the late-magmatic as well as hydrothermal eudialyte essentially took place at somewhat elevated pH conditions. The replacement or alteration of eudialytes by NZS indicates a decreasing pH condition. In terms of the chemical composition the late-magmatic eudialytes can be represented as a solid-solution series between the kentbrooksite-taseqite-aqualite while the hydrothermal eudialyte represents solid-solution between kentbrooksitetaseqite -Ce-zirsilite.  相似文献   

14.
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   

15.
The 1·13 Ga Ilímaussaq intrusive complex, SouthGreenland, is composed of various types of alkali granite andsilica-undersaturated alkaline to agpaitic nepheline syenitesrelated to three subsequently intruded magma batches. Mineralchemistry indicates continuous fractionation trends within eachrock type, but with distinct differences among them. The last,peralkaline magma batch is the most fractionated in terms ofXFemafic mineral, feldspar composition and mineral assemblage.This indicates that an evolving magma chamber at depth discontinuouslyreleased more highly fractionated alkaline melts. Fluid inclusionsin some sodalites record a pressure drop from 3·5 to1 kbar indicating that crystallization started during magmaascent and continued in the high-level magma chamber. On thebasis of phase equilibria and preliminary fluid inclusion data,crystallization temperature drops from >1000°C (augitesyenite liquidus) to <500°C (lujavrite solidus) and silicaactivity decreases from  相似文献   

16.
The Burpala alkaline massif is a unique geological object. More than 50 Zr, Nb, Ti, Th, Be, and REE minerals have been identified in rare-metal syenite of this massif. Their contents often reach tens of percent, and concentrations of rare elements in rocks are as high as 3.6% REE, 4% Zr, 0.5% Y, 0.5% Nb, 0.5% Th, and 0.1% U. Geological and geochemical data show that all rocks in the Burpala massif are derivatives of alkaline magma initially enriched in rare elements. These rocks vary in composition from shonkinite, melanocratic syenite, nepheline and alkali syenites to alaskite and alkali granite. The extreme products of magma fractionation are rare-metal pegmatites, apatite-fluorite rocks, and carbonatites. The primary melts were related to the enriched EM-2 mantle source. The U-Pb zircon ages of pulaskite (main intrusive phase) and rare-metal syenite (vein phase) are estimated at 294 ± 1 and 283 ± 8 Ma, respectively. The massif was formed as a result of impact of the mantle plume on the active continental margin of the Siberian paleocontinent.  相似文献   

17.
B. Bühn  R. B. Trumbull 《Lithos》2003,66(3-4):201-221
We compare the petrogenetic and chemical signatures of two alkali silicate suites from the Cretaceous Damaraland igneous province (Namibia), one with and one without associated carbonatite, in order to explore their differences in terms of magma source and evolution. The Etaneno complex occurs in close spatial proximity to the Kalkfeld bimodal carbonatite–alkali silicate complex, and is dominated by nepheline (ne)-monzosyenites and ne-bearing alkali feldspar syenites. The Etaneno samples have isotopic compositions of 87Sr/86Sr(i)=0.70462–0.70508 and Nd=−0.5 to −1.5, with the highest 87Sr/86Sr(i) and lowest Nd values observed in evolved samples. The magma differentiated via olivine, feldspar, clinopyroxene, and nepheline (ne) fractionation in a F-rich system, which fractionated Zr from Hf, and Y from Ho. Partly glassy, recrystallized inclusions in some samples are less evolved than their host rocks and contain a cumulate component (nepheline, plagioclase). The Kalkfeld ne-foidites (ijolites) and ne-syenites have 87Sr/86Sr(i)=0.70285–0.70592 and Nd=0.5 to 1.1. The isotope ratios show no consistent variation with rock composition, and they are in the same range as the associated carbonatites. The Kalkfeld silicate magma fractionated nepheline and alkali-feldspar in a CO2-dominated, F- and Ca-poor system. As a result, the rocks display some major and trace element trends distinctly different from those of the Etaneno samples.

We suggest that the Etaneno and the Kalkfeld magmas represent different melt fractions of a heterogeneous mantle source, resulting in different compositions especially with respect to CO2 contents of the primitive, parental magmas. In this scenario, the carbonated alkali silicate Kalkfeld parental melt contained a critical CO2 concentration and underwent liquid separation of carbonate and silicate melt fractions at crustal depths. The resulting silicate melt fraction experienced a very different mode of differentiation than the carbonate-poor Etaneno parental magma. Thus, the Kalkfeld rocks are depleted in Ca and other divalent cations, as well as F, rare-earth elements (REE), Ba, and P relative to the Etaneno syenites. We interpret these differences to reflect the partitioning of these elements into the carbonate melt fraction during immiscible separation.  相似文献   


18.
This work describes rare accessory minerals in volcanic and subvolcanic silica-undersaturated peralkaline and agpaitic rocks from the Permo-Triassic Cerro Boggiani complex (Eastern Paraguay) in the Alto Paraguay Alkaline Province. These accessory phases consist of various minerals including Th-U oxides/silicates, Nb-oxide, REE-Sr-Ba bearing carbonates-fluorcarbonates-phosphates-silicates and Zr-Na rich silicates. They form a late-stage magmatic to deuteric/metasomatic assemblage in agpaitic nepheline syenites and phonolite dykes/lava flows made of sodalite, analcime, albite, fluorite, calcite, ilmenite-pyrophanite, titanite and zircon. It is inferred that carbonatitic fluids rich in F, Na and REE percolated into the subvolcanic system and metasomatically interacted with the Cerro Boggiani peralkaline and agpaitic silicate melts at the thermal boundary layers of the magma chamber, during and shortly after their late-stage magmatic crystallization and hydrothermal deuteric alteration.  相似文献   

19.
At Gordon Butte (Crazy Mountains, Montana), agpaitic nepheline-syenite pegmatites intrude potassic alkaline rocks (principally, malignites and nepheline microsyenites). All pegmatite veins are composed predominantly of potassium feldspar, nepheline, prismatic aegirine, barytolamprophyllite, wadeite, eudialyte, loparite-(Ce) and altered rinkite ("vudyavrite") embedded in spherulitic and fibrous aegirine. Well-differentiated veins contain "pockets" filled with calcite, fluorapatite, mangan-neptunite, Mn-Ti-enriched prismatic aegirine, calcium catapleiite, and an unidentified Ca-Ti silicate. The potassium feldspar corresponds to Ba-rich sanidine with relatively low Na contents. The nepheline contains low levels of SiO2 and elevated Fe contents. The compositions of nepheline cluster in the lower portion of the Morozewicz-Buerger convergence field, indicating low-temperature crystallization and/or chemical re-equilibration of this mineral. The association of sanidine with nearly stoichiometric nepheline is unusual for agpaitic rocks and probably reflects inhibition of Al/Si ordering in the feldspar by Ba. At least four types of clinopyroxene can be distinguished on the basis of their morphology and composition. All these types correspond to Al- and Ca-poor aegirine (typically <0.6 and 2.6 wt% Al2O3 and CaO, respectively). The overall evolutionary trend of clinopyroxene in the Gordon Butte rocks is from Fe-poor diopside to aegirine-augite in the malignites and nepheline microsyenites, and culminates with the pegmatitic aegirine. This trend is characteristic for potassic alkaline complexes and results from preferential partitioning of Fe2+ into biotite during the magmatic crystallization. Barytolamprophyllite in the pegmatites is primary (as opposed to deuteric); only a few crystals contain a core composed of lamprophyllite. The evolutionary history of the Gordon Butte pegmatites can be subdivided into primary, agpaitic, and deuteric stages. The earliest paragenesis to crystallize included accessory zircon and thorite. Sr-rich loparite also precipitated relatively early serving as a major repository for Sr, REE, and Nb. During the agpaitic stage, diverse titano- and zircono-silicates (barytolamprophyllite, eudialyte, wadeite, and rinkite, among others) consumed most of the Ba, Sr, Ti, Zr, and Nb still remaining in the melt. The final stage in the evolution of the pegmatites involved interaction of the earlier-formed mineral assemblages with deuteric fluids. In common with the Rocky Boy pegmatites, Sr-REE-Na-rich fluorapatite, Ba-Fe titanates and REE-bearing carbonates (ancylite, calcio-ancylite, and bastnäsite-parisite series) are chief products of the deuteric stage. The alteration of the primary mineral assemblages by deuteric fluids also produced muscovite-zeolite pseudomorphs after nepheline, replacement of wadeite and eudialyte by catapleiite-group minerals, re-deposition of Ba in the form of hyalophane, baotite, and benitoite, and cation leaching from rinkite, eudialyte, and loparite. The mineralogy of the pegmatites from Gordon Butte, other potassic complexes, and sodic agpaitic occurrences is compared in detail.  相似文献   

20.
The three layered intrusions studied in the Laouni area have been emplaced within syn-kinematic Pan-African granites and older metamorphic rocks. They have crystallized at the end of the regional high-temperature metamorphism, but are free from metamorphic recrystallization, revealing a post-collisional character. The cumulate piles can be interpreted in terms of two magmatic liquid lines of descent: one is tholeiitic and marked by plagioclase–olivine–clinopyroxene cumulates (troctolites or olivine bearing gabbros), while the other is calc-alkaline and produced orthopyroxene–plagioclase rich cumulates (norites). One intrusion (WL (West Laouni)-troctolitic massif), shows a Lower Banded Zone where olivine-chromite orthocumulates are interlayered with orthopyroxene-rich and olivine–plagioclase–clinopyroxene cumulates, whereas the Upper Massive Zone consists mainly of troctolitic and gabbroic cumulates. The other two massifs are more homogeneous: the WL-noritic massif has a calc-alkaline differentiation trend whereas the EL (East Laouni)–troctolitic massif has a tholeiitic one. Separated pyroxene and plagioclase display similar incompatible trace element patterns, regardless of the cumulate type. Calculated liquids in equilibrium with the two pyroxenes for both noritic and troctolitic cumulates are characterized by negative Nb, Ta, Zr and Hf anomalies and light REE enrichment inherited from the parental magmas. Troctolitic cumulates have mantle-derived δ18O (+5 to +6‰), initial 87Sr/86Sr (Sri=0.7030 to 0.7054), Nd (+5 to −1) values whereas noritic cumulates are variably enriched in δ18O (+7 to +9‰), show negative Nd (−7 to −12) and slightly higher Sri (0.7040–0.7065). Based on field, isotopic ratios are interpreted as resulting from a depleted mantle source (Sri=0.7030; Nd=+5.1; δ18O=+5.1‰) having experience short term incompatible element enrichment and variable crustal contamination. The mantle magma was slightly contaminated by an Archaean lower crust in troctolitic cumulates, more strongly and with an additional contamination by an Eburnian upper crust in noritic cumulates. Lower crust input is recorded mainly by Sr and Nd isotopes and upper crust input by O isotopes. This is probably due to the different water/rock ratios of these two crust types. Assimilation of low amounts (<10%) of quartz-bearing felsic rocks, coming from both lower and upper crust, can explain the rise of SiO2 activity, the enrichment in 18O and 87Sr and the lowering of Nd in the noritic cumulates compared to troctolitic ones. The geodynamic model proposed to account for the Laouni tholeiitic magmatism involves a late Pan-African asthenospheric rise due to a rapid lithospheric thinning associated with functioning of shear zones, which allowed tholeiitic magmas to reach high crustal levels while experiencing decreasing degrees of crustal contamination with time.  相似文献   

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