Journal of Petrology | Volume 39 | Number 11-12 | Pages 2043-2059 | 1998
© Oxford University Press 1998
Magmatic Evolution of the Melilitite–Carbonatite–Nephelinite Dyke Series of the Turiy Peninsula (Kandalaksha Bay, White Sea, Russia)
1 Department of Petrology, Geological Faculty, St Petersburg University St Petersburg, 199031, RUSSIA
2 Ottawa–Carleton Geoscience Centre, Department of Earth Sciences, Carleton University Ottawa, Ont., CANADA, K1S 5B6
Received September 30, 1997; Revised typescript accepted June 16, 1998
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ABSTRACT |
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 TOP ABSTRACT IntroductionGeological SettingPetrographyMineralogyGeochemistryDiscussionConclusionsReferences |
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Major and trace element data are presented from a suite of melilitite–carbonatite–nephelinite dykes, the youngest of three dykes swarms in the Turiy peninsula, Russia. The most primitive dykes consist of olivine melanephelinites and olivine–melilite melanephelinites that contain high-pressure phases (Cr-diopside and low-Ca forsteritic olivine). These dykes approximate the composition of the parental melt, which probably originated by low-degree partial melting of metasomatized peridotite. Least-squares mass-balance calculations and geochemical modelling indicate that differentiation was controlled by fractional crystallization involving olivine, clinopyroxene, melilite, Ti-magnetite, apatite, and perovskite. The calculated modal proportions of the cumulate minerals correspond to some of the rocks seen in alkaline ultramafic plutons elsewhere in the Kola peninsula. Calciocarbonatite dykes, with quenched primary magmatic fabrics, were probably continuously separated by liquid immiscibility from an evolved carbonated nepheline melilitite parent. The conjugate silicate liquid to the carbonatitic melt is a melilite nephelinite. The distribution of LREE, Zr, Hf, Ta, W, Pb, and Cu between the carbonatite and melilite nephelinite is in reasonable agreement with experimental data on element partitioning between alkaline silicate and carbonate melts.
KEY WORDS: carbonatite; fractional crystallization; Turiy peninsula; liquid immiscibility
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Introduction |
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TOPABSTRACTIntroductionGeological SettingPetrographyMineralogyGeochemistryDiscussionConclusionsReferences |
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During the last decade, several volumes have outlined the progress that has been made in carbonatite research (Bell, 1989
; Egorov, 1991; Bell & Keller, 1995). However, certain problems still remain unresolved. For example, what are the petrogenetic relationships between intrusive and volcanic carbonatites? Do carbonatite melts represent primary mantle liquids or secondary melts? Are carbonatites the products of liquid immiscibility fro alkaline silicate melts or do they represent the end products of fractional crystallization? If carbonatite melts are derived by differentiation processes, what were the parental silicate melts?
Some of these questions can be investigated from the study of the melilitite–carbonatite–nephelinite dyke series associated with the Turiy complex, Kola (Fig. 1), one of several alkaline ultramafic and carbonatite complexes of the Kola peninsula and northern Kareli (Kukharenko et al., 1965). One of the main geological features of the Turiy peninsula is the evidence for three distinct episodes of alkaline magmatism.
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In this paper, the mineralogy and geochemistry of the melilitite–carbonatite–nephelinite dyke series, the youngest of the three groups of dykes at Turiy, are discussed and a quantitative petrogenetic model is proposed for its origin.
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Geological Setting |
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TOPABSTRACTIntroductionGeological SettingPetrographyMineralogyGeochemistryDiscussionConclusionsReferences |
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Dykes of the Turiy peninsula are associated with the Kandalaksha Deep Fracture zone, which follows older structures associated with the Riphean Onega-Kandalaksha graben (Konstantinovskiy, 1977
). Reactivation during the Palaeozoic resulted in the emplacement of carbonatite massifs as well as numerous alkaline dykes and pipes. The largest dyke swarms, consisting of hundreds of dykes, are concentrated near the town of Kandalaksha, and on the Turiy peninsula (Ivanikov & Rukhlov, 1996). Geological relationships are shown in Fig. 1.
More than 300 dykes are exposed along the southern coast at Turiy and many were intersected during drilling of the carbonatite massifs in the central part of the Turiy peninsula (Bulakh & Ivanikov, 1984). On the basis of cross-cutting relationships, the dykes can be divided into three distinct age groups.
The dykes of the earliest group, made up of alkaline picrites, olivine melilitites and alkaline ultramafic lamprophyres, were formed before the emplacement of the main Turiy body, and are strongly deformed and altered. Abundant xenoliths, both of sandstones and deep-seated ultramafic rocks (Shurkin, 1959; Bulakh, 1962; Ivanikov, 1977), characterize this group. The second group is coeval with the emplacement of the main massifs and consists of olivine melteigite, micro-ijolite, ijolite–porphyry as well as turjite. Most of these dykes are subhorizontal.
Intrusion of the youngest group of dykes followed the emplacement of the main massifs. Most of these dykes trend north–south, have vertical or steeply dipping contacts, and have variable thicknesses, from a few centimetres up to 2m, similar to those shown by dykes of the other two groups. The greatest chemical variation is shown by the younger group of dykes. The dyke rocks of this group are, in order of emplacement: olivine melanephelinite, olivine–melilite melanephelinite, olivine–nepheline melilitite, nepheline melilitite, melilite nephelinite, carbonatite, nephelinite and feldspar nephelinite (Ivanikov, 1977; Bulakh & Ivanikov, 1984, 1996).
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Petrography |
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TOPABSTRACTIntroductionGeological SettingPetrographyMineralogyGeochemistryDiscussionConclusionsReferences |
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The youngest group of dykes has been described by Shinkarev & Ivanikov (1973)
, Ivanikov (1977), and Bulakh & Ivanikov (1984). The most primitive dykes are porphyritic, with large (up to 1.5–cm) phenocrysts of olivine (8 modal %), clinopyroxene (30%) and magnetite (1–3%). Olivine melanephelinite contains trace amounts of platy melilite restricted to the groundmass. The olivine–melilite melanephelinite contains 
20% melilite, mostly in the groundmass, but melilite can also occur as microphenocrysts. In the olivine-nepheline melilitite, melilite is in excess of 30%, equally distributed between phenocrysts and groundmass microlites. The groundmass consists of prismatic clinopyroxene and kaersutite, tabular melilite, and nepheline microphenocrysts set in a matrix of analcite or devitrified glass, with smaller crystals of phlogopite, perovskite, magnetite, sulphides, carbonate, cancrinite, pectolite, zeolite, chlorite, and serpentine. Nepheline phenocrysts occur in some of these dykes, and are accompanied by a reduction in the amount of olivine phenocrysts. Some olivine crystals are surrounded by rims of diopside.
The nepheline melilitite and melilite nephelinite grade into one another. Most are aphyric, but phenocrysts of melilite (up to 15%), clinopyroxene, nepheline, nosean and titanomagnetite occur in the central parts of the thicker dykes. These rocks contain rare, resorbed phenocrysts of olivine surrounded by clinopyroxene as well as phlogopite and magnetite. Rims of pyroxene, phlogopite and perovskite around the melilite indicate reaction with the enclosing melt. The groundmass contains euhedral nepheline, clinopyroxene, cancrinite, melilite, phlogopite, magnetite and perovskite. Ocelli of primary calcite (5%) have been documented in these rocks (Bulakh & Ivanikov, 1984).
Melilite is the most abundant mineral in the nepheline melilitites, whereas in the melilite nephelinites, nepheline dominates. In both rock types, clinopyroxene makes up between 15 and 30 modal % of the total minerals. Some rocks have been almost completely recrystallized, forming fine-grained aggregates of phlogopite, aegirine, Ti-garnet, pectolite, wollastonite, cancrinite, analcite, carbonate, zeolite, sericite, apatite, iron oxides and hydroxides, and rare albite and fluorite.
The nephelinite dykes are both porphyritic and aphyric. Porphyritic varieties contain phenocrysts of nepheline, sanidine, clinopyroxene, garnet and nosean, whereas the aphyric type and groundmass of the porphyritic varieties typically consist of nepheline, orthoclase, albite, aegirine, melilite, phlogopite, Ti-garnet, magnetite, perovskite, pectolite, wollastonite, cancrinite, analcite, zeolite, and carbonate. Accessory minerals include zircon, apatite, fluorite, titanite and sulphides.
The carbonatite dykes consist of tabular calcite grains set in a matrix consisting of anhedral ankerite–dolomite (up to 5–10%). The minor and accessory phases include chlorite, phlogopite, quartz, zeolites, analcite, K-feldspar, magnetite, sulphides, apatite, iron hydroxides, and fluorite. A magmatic origin is indicated by quench textures developed at the margins of several carbonatite dykes, including one similar to the comb-layering described from the Kaiserstuhl volcanic complex, Germany (Katz & Keller, 1981).
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Mineralogy |
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TOPABSTRACTIntroductionGeological SettingPetrographyMineralogyGeochemistryDiscussionConclusionsReferences |
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Presented in this paper are electron microprobe mineral analyses obtained using a CAMSCAN LINK AN-10000 energy-dispersive spectrometer operating at 20 kV, with a beam current of 4 nA at the Mehanobr-Analyt Laboratory (St Petersburg, Russia). Standards included magnesium oxide, diopside, sanidine, rutile, haematite, and pure metals. The ZAF 4/FLS program was used for a treatment of results of the measurements. Selected representative mineral analyses are listed in Table 1.
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Olivine
All olivine phenocrysts have relatively uniform Mg:Fe ratios (83.3–85.4% Fo), with increase in Mn and Ca contents at the outer margins of the crystals. The high Ca contents (0.46–0.50 wt % CaO) of the rims are consistent with low-pressure crystallization (Simkin & Smith, 1970
). Anhedral, kink-banded olivine phenocrysts, found in many of the primitive dykes from Turiy, may represent mantle xenocrysts, a feature also supported by the presence of high-pressure Cr-diopside. Using the method of Roeder & Emslie (1970), crystallization temperatures estimated for the olivine phenocrysts from the olivine melanephelinites range from 1140 to 1170°C. However, the geothermometers of Leeman & Scheidegger (1977) and Helz & Thornber (1987) indicate higher crystallization temperatures of 1285–1332°C and of 1231°C, respectively.
Clinopyroxene
The data are shown in Fig. 2. Three genetically different types of clinopyroxene are found in the dyke rocks: (1) colourless xenocrysts; (2) colourless, unzoned phenocrysts; (3) euhedral red–brown and bright green zoned phenocrysts and groundmass pyroxenes. The xenocrysts can be mantled by the other types of pyroxene.
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The anhedral colourless cores of the complexly zoned phenocrysts and xenocrysts, found only in the olivine melanephelinites and olivine–melilite melanephelinites, consist of Cr-diopside and are characterized by the highest MgO (mg-number = 82–92) and Cr2O3 contents (up to 1.0 wt %; average 0.8 wt %). Their SiO2 contents are high (an average of 52.4 wt %), and both Al2O3 (average <1.0 wt %) and TiO2 (average of 0.7 wt %) are low (Fig. 2c). Aluminum preferentially occupies the octahedrally coordinated site (an average of AlVI/AlIV = 1.1) indicating crystallization at pressures of 20–25 kbar (Thompson, 1974
; Velde & Kushiro, 1978). High-pressure jadeite and Al-tschermakite solid solutions are incorporated in the Cr-diopside, as illustrated by a plot of Na–Ti–AlIV (Fig. 2b). The Cr-diopside compositions of the Turiy dykes are similar to those of clinopyroxene from xenoliths in alkaline basalts, interpreted as mantle xenocrysts (Wilshire & Shervais, 1975). These are unlike the Cr-diopsides from the nephelinites of Napak, which have lower AlVI/AlIV value (0–0.65), typical of low-pressure (<10 kbar) igneous clinopyroxene (Simonetti et al., 1996).
Unzoned phenocrysts and some of the rims surrounding xenocrystal cores consist of augite, diopside, and Ti-diopside. Compared with the Cr-diopside, these pyroxenes have higher Al2O3 (2.5–5.1 wt %) and slightly lower SiO2 (50.0–52.7 wt %), Cr2O3 (0.1–0.5 wt %) and MgO (mg-number = 80–83). The AlVI/AlIV values (0.3–1.5) are lower than those from the Cr-diopside, but still higher than the limit of 0.25 dividing high-pressure from low-pressure clinopyroxenes (Aoki & Shiba, 1973). These pyroxenes are similar in composition to those of the Al-augite series that crystallized in weakly differentiated magmas within the upper mantle and/or lower crust (Wilshire & Shervais, 1975; Wass, 1979).
The clinopyroxene that forms the outer zones of phenocrysts and that occurs in the groundmass varies in composition (titaniferous diopside–hedenbergite, titaniferous fassaite, sub-silica titanian fassaite, aegirine–augite, aegirine) as the melt composition changes from mafic to more felsic compositions. Such compositional changes can be seen in individual phenocrysts from core to rim (Figs 2a and c). It should be noted that the term ‘fassaite’ is used in this paper to distinguish the strong enrichment in TiO2 (up to 6.2 wt %) and Al2O3 (up to 9.1 wt %), and depletion in SiO2 (41.6–49.1 wt %) relative to augite found in these pyroxenes, especially those from the melilitite dykes (Fig.–2 c). Such features indicate high-temperature crystallization in a strongly silica-undersaturated melt. Temperatures in the range 1289–1330°C have been estimated using the method of Nielsen & Drake (1979). Fully tetrahedrally coordinated Al and Ti form Ti- and Fe-tschermakite solid solutions, with low AlVI / AlIV values (<0.25) typical of low-pressure clinopyroxene (Yagi & Onuma, 1967).
Melilite
Melilite forms euhedral, tabular phenocrysts and occurs in the groundmass. Fresh melilite is rare and is usually pseudomorphed by aggregates of silicates and carbonate. The composition of fresh melilite from the nosean-bearing melilite nephelinite (W-281) is similar to that from the turjaites of the Turiy peninsula's central massif (Bell et al., 1996). It consists of about 70% akermanite (Ca2[Mg, Fe2+]Si2O7) and 30% sodium melilite (CaNaAlSi2O7) components.
Phlogopite
Phlogopite is found only in the groundmass and is attributed to subsolidus, autometasomatic crystallization. Phlogopite contains 5.5 wt % TiO2 and a low [IV]Al content. The latter is compensated by the accommodation of Fe3+ in the tetrahedrally coordinated sites reflecting the peralkaline nature of the melt.
Nepheline, nosean and analcite
Nepheline from melanephelinites contains up to 2.0 wt % CaO. Nepheline from the nephelinites is characterized by a partial substitution of Al by Fe3+ (up to 1.5 wt % Fe2O3). The content of the kalsilite end-member shows insignificant variation (5.5–7.1 wt % K2O).
Nosean, found in several melilite nephelinite dykes, forms abundant and rather large (up to 0.8 cm)phenocrysts. The analysed nosean from melilite-bearingnephelinite (Table 1) is characterized by a deficiency of Na, Al and Si. Nosean can crystallize only from a strongly silica-undersaturated melt enriched in CaO and alkalis at high temperatures (800–1000°C) and low pressures (Romanchev & Kuznetzova, 1982).
Analcite is widespread in the groundmass of most dykes and is also found in amygdales. It has a deficiency in Na2O (16.3 wt %) and Al2O3 (23.7 wt %) and contains little CaO. According to Henderson & Gibb (1983), a Ca-free analcite forms at low temperatures during the late stages of magmatic evolution.
Spinellide, ilmenite and perovskite
Spinellide occurs as a groundmass phase or as phenocrysts, and can also occur as small inclusions in olivine and clinopyroxene phenocrysts. Spinellide from theolivine melanephelinites and nepheline melilitites corresponds to titaniferrous (12.7–15.1 wt % TiO2) aluminous (4.6–7.5 wt % Al2O3) magnesian (4.3–5.2 wt % MgO) magnetite. The small grains in the nephelinite groundmass correspond in composition to magnetite with low TiO2 contents (Table 1, analysis W-50). Ilmenite occurs only in the nephelinite dykes, and its composition is similar to that from carbonatites and basalts (Mitchell & Bergman, 1991).
Perovskite commonly forms very small, euhedral, grains in the matrix of the melilitites and has also been found in melilite phenocrysts, accompanied by inclusions of olivine and Ti-magnetite. The perovskite (Table 1) contains Na2O (1.5 wt %) and iron (up to 1.0 wt % FeOt), typical of perovskites from alkaline rocks (Mitchell & Bergman, 1991).
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Geochemistry |
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TOPABSTRACTIntroductionGeological SettingPetrographyMineralogyGeochemistryDiscussionConclusionsReferences |
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Trace elements were determined using instrumental neutron activation analysis (INAA) (Ni, Co, Sc, Cr, Ba, Hf, Ta, La, Ce, Nd, Sm, Eu, Tb, Yb, Lu) at the Institute of Precambrian Geology and Geochronology, St Petersburg, X-ray fluorescence (XRF) (Rb, Sr, Y, Zr, Pb, Nb, U, Th), and flame photometry (Li, Cs) both at the Laboratory of Spectral Analysis of the Geological Survey and Exploration Company ‘NEVSKGEOLOGIYA’, St Petersburg. W, Pr, Gd, Er, Ho, Dy, Tm and Cu in six samples were analysed by inductively coupled plasma mass spectrometry (ICP-MS) at the Institute of Analytical Instruments, St Petersburg. The abundances of the major elements were analysed by classical wet chemical methods and XRF. Selected representative whole-rock compositions, along with some element ratios, are listed in Table 2. The estimated analytical uncertainties for the ICP analyses are considered to be ±10% of the quoted values, and the INAA data vary from ±3% to ±20% of the quoted values.
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Major Elements
The major element chemistry for the youngest group of dykes at Turiy has been described in detail by Bulakh & Ivanikov (1984)
. The most important chemical features of the silicate rocks are their undersaturation in SiO2, high alkali contents, Na2O/K2O >1, high CaO/MgO ratio, and high abundance of TiO2 (Table 2).
The more mafic rocks of the dyke series show a continuous trend towards nephelinite, with an enrichment in Na2O and Al2O3 and a decrease in MgO, FeOt and TiO2 (Bulakh & Ivanikov, 1984). The CaO and P2O5 contents show a slight increase from the olivine melanephelinite to nepheline melilitite dykes but decrease in the younger nephelinitic dykes. It is interesting that Na2O increases significantly (from 2.2 to 12 wt %), whereas K2O remains almost constant at 2.0 wt %.
One of the remarkable features of the dykes, including the youngest nephelinite dykes, is their high CaO contents. Normative larnite is present not only in the melilitites, but also in the more mafic olivine melanephelinites and in some of the younger peralkaline nephelinites. All dyke rocks are characterized by high CO2 and H2O abundances.
All of the carbonatites are calciocarbonatites that contain SiO2, FeO, MgO <10 wt % and K2O, Na2O, Al2O3 <1 wt %. The abundances of P2O5 in four of the six carbonatite dykes are significantly greater than those in the silicate dykes.
Trace elements
The Solidification Index (SI = 100xMgO/[MgO+FeO+Fe2O3+Na2O+K2O] in wt %) is used to reflect changes in the trace element variation diagrams (Fig. 3) with melt differentiation. For some trace elements, their abundances show regular trends from the mafic to the nephelinitic dykes. Ni, Cr, Co, and Sc decrease one or two orders of magnitude, whereas the Ta, Th, Rb, and U contents gradually increase. During the evolution of the dykes, Sr increases slightly, but then decreases in the later nephelinite dykes. Hf, Zr, Y and, to a lesser extent, Nb increase in the early stages of magmatic evolution and decrease in the younger dykes. Variations in Hf, Zr and Nb are less than one order of magnitude, whereas the range of Y is much more restricted (20–46 ppm). Although the Ba contents are fairly variable, they show no correlation with the differentiation index (Fig. 3). The rare earth elements (REEs) (Fig. 4) are strongly fractionated in the silicate dyke rocks [(Ce/Yb)N = 24.7–l39.9], and REEs abundances are high (
REEN = 817–1597).
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The carbonatite dykes contrast with the silicate rocks in terms of their trace element composition and are similar to the average calciocarbonatite given by Woolley & Kempe (1989)
. Compositional data are given in Fig. 5. Zr, Hf, Ta, Cs, Rb, and Cu are depleted in the carbonatites, whereas W, Pb and the light REEs (LREEs), are all enriched relative to the silicate dyke rocks of similar age (melilite nephelinites) (Table 2). With the exception of one sample, with a value of 304 ppm, Ce ranges from 773 to 1520 ppm in five of six carbonatite dykes. Although the Ce contents exceed those in the silicate rocks, the HREE contents are approximately equal in both. This results in a higher (Ce/Yb)N ratio in the carbonatites. The abundances of the remaining trace elements from the carbonatite dykes are similar to those seen in the melilitites and nephelinites, with the exception of Nb and Sr, which are enriched in most of the carbonatite dykes.
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Discussion |
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TOPABSTRACTIntroductionGeological SettingPetrographyMineralogyGeochemistryDiscussionConclusionsReferences |
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Parental magma and its source
According to recent experimental studies, melts with 100Mg/(Mg + Fe) ratios >68 could represent primary mantle melts (Eggler, 1989
). The larnite-normative olivine melanephelinites and olivine–melilite melanephelinites, the earliest dykes in the Turiy youngest dyke series, have mg-numbers that fall within this range and contain the highest abundances of Ni (up to 200 ppm) and Cr (176–304 ppm), features consistent with a primary mantle origin. The occurrence of high-pressure minerals (Cr-diopside and Al-augite as well as low-Ca forsteritic olivine) in these dyke rocks, also, provides mineralogical evidence to support their primary nature. The olivine melanephelinite and olivine–melilite melanephelinite dykes have similar compositions to lavas and subvolcanic intrusions that occur in other parts of the world (e.g. Homa Bay, Kenya; Kaiserstuhl, Germany; Gardiner, Greenland) and that are favoured as possible parental melts (Wimmenauer, 1963; Le Bas, 1977; Nielsen, 1980).
Using data from experiments on mantle peridotite compositions, Eggler (1989) derived a ‘mantle norm’ to determine the pressure of melt generation. Based on this method, the pressure of generation of the primary Turiy olivine–melilite melanephelinite is 25 kbar (75–80 km depth).
Fractional crystallization
Major element modelling
The new mineralogical and geochemical data can be used to evaluate fractional crystallization as a possible model for producing the melilitite–carbonatite–nephelinite dyke series of the Turiy peninsula (Ivanikov, 1977; Bulakh & Ivanikov, 1984). The evolutionary sequence of the dykes, along with the modal and compositional changes of major liquidus phases fractionating from the melt, corresponds to experimental data on crystallization in the ‘extended basalt tetrahedron’(Schairer & Yoder, 1964; Onuma & Yagi, 1967; Yoder, 1979; Pan & Longhi, 1989). Changes in residual liquid composition by crystal fractionation can be illustrated using several variation diagrams (Fig. 6). From these diagrams it can be seen that the chemical variations could be explained by a simultaneous extraction of clinopyroxene, olivine, titanomagnetite, and melilite from a mafic parent. Clinopyroxene and olivine play a more important role at the beginning of the fractionation process. Removal of olivine results in depletion in MgO and enrichment in CaO, leading to the appearance of melilite on the liquidus. Melilite and clinopyroxene then become the two major phases fractionating from the melt at lower temperatures, after olivine disappears from the liquidus. Clinopyroxene compositions change from diopside towards subsilica titanian fassaite and aegirine–augite.
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Fractional crystallization was quantitatively calculated using a least-squares mass-balance program (Wright & Doherty, 1970
). We have chosen to model the evolution of the liquid in three stages, each of which is based on petrographic and geochemical observations. The ‘parental liquid’ for each stage represents the residual melt from the previous stage. The proportion of minerals extracted, and degree of melt fractionation (F), i.e. the percentage of the parental melt that has solidified, for each of these three stages is presented in Table 3. The composition of the initial melt of the Turiy dyke series was based on an average of six olivine melanephelinite and olivine–melilite melanephelinite dykes. In the first stage, olivine–melilite melanephelinite 
nepheline melilitite (F = 42.3%), the fractionating mineral assemblage consists of olivine (8%), diopside (80%) and titanomagnetite (12%), corresponding to an olivine clinopyroxenite. Rocks with similar modal mineralogy occur in the alkaline ultramafic plutons of the Karelia and Kola peninsula regions and are thought to represent cumulateproducts (Orlova, 1983). In the second stage, nepheline melilitite melilite nephelinite (F = 56.5%), two variations are calculated. Small amounts of olivine occur in one of the two calculated liquidus assemblages and the percentages of melilite, clinopyroxene and titanomagnetite differ (see Table 3). Petrographic evidence favours the mineral assemblage that is olivine absent and consists of clinopyroxene (48.5%), melilite (38.9%) and titanomagnetite (12.6%). Finally, for the third stage, melilite nephelinite nephelinite (F = 76.7%), the major phases extracted from the melt were subsilica titanian fassaite (66.8%), Na-akermanitic melilite (16.2%), titanomagnetite (13.7%) and apatite (3.3%). The products of the two last stages are similar to plutonic uncompahgrites and melilite pyroxenites of the Turiy peninsula, supporting earlier suggestions that these rocks are cumulates (Bulakh & Ivanikov, 1984).
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Trace element modelling
The degrees of melt fractionation (F) and percentages of minerals precipitated for the three-stage model of fractional crystallization have been verified by trace element modelling, using published crystal–liquid distribution coefficients (Baker et al., 1977
; Nagasawa & Schreiber, 1980; Villemant et al., 1981; Furnes & Stillman, 1987). The similarity between calculated trace element abundances and concentration levels in the sample suite lends additional support for the fractional crystallization model (Table 3).
The compatible trace elements, including Ni, Cr, Co and Sc, show significant correlations (Figs 7a, c and e) implying precipitation of Fe–Mg minerals. From these diagrams it appears that bulk crystal–melt partition coefficients (D) values for Sc, Co, Cr and Ti had to be less than that for Ni during the first stage of magmatic evolution, indicating olivine fractionation during this stage. The sharp increase in D values for Cr, Co and Sc during the later stages of differentiation reflects the olivine + liquid clinopyroxene + liquid peritectic reaction. At this time, the main co-precipitating phases are clinopyroxene accompanied by melilite and, probably, by Fe–Ti oxides. Melilite fractionation should be reflected in the behaviour of Sr. According to Bell et al. (1996), melilite from turjaites contains up to 1 wt % SrO, which is much greater than the Sr contents of the Turiy dyke rocks. Although some Sr could be removed by apatite, the role of this mineral is probably insignificant given the absence of any correlation between Sr and P2O5.
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The removal of olivine, clinopyroxene, melilite, and magnetite is not reflected for other trace elements in the dyke rocks, suggesting that such elements were highly incompatible (D << 1). The contents of some of such elements do, however, decrease in the more evolved dyke rocks, suggesting fractional crystallization of accessory minerals. One example is shown by the discontinuities in the Hf, Zr and perhaps Y trends (Fig. 3), which might be attributed to the precipitation of zircon in the evolved melts. Zr and Hf might also be accommodated into alkaline clinopyroxenes.
Perovskite is an important accessory mineral in the Turiy dykes, but the incompatible nature of Ta (Fig. 3), along with its strong correlation with Nb (Fig. 7b) and with Th, implies that the role played by perovskite was insignificant. Slight decrease in Nb in the late nephelinitic dykes, however, may suggest some precipitation of perovskite or minerals of the pyrochlore group.
Attempts to model the melilite nephelinite–nephelinite stage by adding perovskite to th mixture of the calculated minerals hardly affect the LREE contents. LREE might be concentrated by apatite. However, using the highest known apatite–melt distribution coefficients for the LREE, removal of 10% of apatite from the melt is needed to obtain the low LREE contents seen i the nephelinites. This is inconsistent with the modal abundance of this mineral (2–3 vol. %) and the actual P2O5 contents (0.80–1.30 wt %) in the dykes. Therefore, removal of the essential and accessory minerals cannot explain all of the chemical features shown by the dyke rocks, particularly the late-stage nephelinites.
The origin of the Turiy carbonatites
The carbonatites in the Turiy dykes cannot be explained by fractional crystallization of the silicate magmas (Ivanikov, 1977; Bulakh & Ivanikov, 1984). At Turiy, all of the carbonatites are older than the late-stage nephelinites (Bulakh & Ivanikov, 1996), and hence it is unlikely that they could represent residual liquids produced by fractional crystallization of a silicate melt. The presence of calcite ocelli (5 vol. %) in the nepheline melilitite and melilite nephelinite dykes, coupled with the same ratio, volumetrically, of carbonatite to silicate dykes, are features consistent with the origin of the carbonatite dykes by liquid immiscibility (e.g. Kjarsgaard & Peterson, 1991).
The new geochemical data for the Turiy carbonatite dykes are consistent with liquid immiscibility. On the basis of the petrographic data, the carbonatites and melilite nephelinites are probably conjugate liquids. The partitioning of LREE, Zr, Hf, Ta, W, and Cu between the melilite nephelinite and carbonatite dykes is similar to the experimental partition coefficients (KD) of these elements determined for the alkaline silicate melt-carbonate melt pairs (Wendlandt & Harrison, 1979; Hamilton et al., 1989). The KD values for some elements vary significantly and are strongly dependent on the compositions of silicate and carbonate melts (degree of melt polymerization), and on temperature and pressure (Wendlandt & Harrison, 1979; Hamilton et al., 1989). If the experimental data are applied to evaluate pressure and temperature conditions of liquid immiscibility at Turiy, then separation of the two liquids took place at depths of 7–10 km. However, the presence of carbonatite dykes with relatively low LREE contents may also imply the continuous process of liquid immiscibility during magma ascent to higher levels. The variations in Ta content observed in the carbonatite dykes support this suggestion.
Partitioning and enrichment of the LREE in the carbonate melt and also the strongly decreased Ce/Yb ratios in the late-stage nephelinite dykes can be attributed to separation of a carbonatite and a melilite nephelinite from a parental nepheline melilitite. The Nb contents in the late-stage nephelinites (see Table 2) might also be explained by partitioning of Nb into a carbonatitic liquid. A much better agreement between the calculated and actual trace elements contents in the late-stage nephelinites (Table 3) occurs when a 5% carbonate melt fraction is added to the mixture of minerals extracted at the final stage of fractional crystallization. The scheme shown in Fig. 8 summarizes our preferred model for the evolution of the melilitite–carbonatite–nephelinite dyke series of the Turiy peninsula.
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None of the models proposed so far explains all of the data from the dyke series. For example, the behaviour of K cannot be explained by fractional crystallization. The K2O contents calculated using the fractional crystallization model appear to be 2–3 times higher than those from the youngest of the dykes, and this was attributed to partitioning of K into a late-stage magmatic fluid (Ivanikov, 1977
; Bulakh & Ivanikov, 1984). Similar mechanisms might also explain the behaviour of Cs, Li, Rb and Ba contents in the melilitite–carbonatite–nephelinite dyke series of the Turiy peninsula. Abundant amygdales, subsolidus recrystallization, and the development of phlogopite in the groundmass are consistent with such a process. |
Conclusions |
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TOPABSTRACTIntroductionGeological SettingPetrographyMineralogyGeochemistryDiscussionConclusionsReferences |
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The field relationships, mineralogy, and compositional diversification shown by the melilitite–carbonatite–nephelinite dyke series of the Turiy peninsula are consistent with fractional crystallization of a parental, olivine–melilite melanephelinitic melt. Mineralogical and geochemical modelling reveal fractional crystallization of olivine, clinopyroxene, melilite, titanomagnetite, apatite and accessory perovskite. The calculated assemblages of the precipitated minerals are similar to those for ultramafic rocks, including some melilitolites, associated with the carbonatite-bearing plutons of the Kola peninsula.
The origin of the carbonatites is best explained by immiscible separation from a carbonated nepheline melilitite. The melilite nephelinite melt is considered to be conjugate to the carbonatitic liquid. Volumetrically, the carbonatite forms 5% of the youngest dykes observed at Turiy, which is in keeping with experimental studies.
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Acknowledgements |
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Thanks are extended to Professor A. Bulakh (Department of Mineralogy, the University of St Petersburg), and to R. Ernst, B. Kjarsgaard, A. Lalonde, and A. Simonetti for their interest and their constructive reviews of an earlier version of this paper. This research was funded by the State Committee on Science and Higher Education of Russian Federation (Grant MGGA 11-2.1-5), by the International Soros Science Education Program (Grants s96-2297 and a97-353 awarded to A.R.), and by NSERC Grant A7813 (K.B.).
* Corresponding author.
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References |
|---|
|
TOPABSTRACTIntroductionGeological SettingPetrographyMineralogyGeochemistryDiscussionConclusionsReferences |
|---|
Aoki K., Shiba I. Pyroxenes from lherzolite inclusions of Itinomegata, Japan. Lithos (1973) 6:45–51.
Baker B. H., Goles G. G., Leeman W. P., Lindsrom M. M. Geochemistry and petrogenesis of a basalt–benmoreite–78trachyte suite from the southern part of the Gregory Rift, Kenya. Contributions to Mineralogy and Petrology (1977) 64:303–332.[Web of Science]
Bell K. Carbonatites: Genesis and Evolution (1989) London: Unwin Hyman.
Bell K., Keller J. Carbonatite Volcanism: Oldoinyo Lengai and the Petrogenesis of Natrocarbonatites. IAVCEI Proceedings in Volcanology 4 (1995) Berlin: Springer-Verlag.
Bell K., Dunworth E. A., Bulakh A. G., Ivanikov V. V. Alkaline rocks of the Turiy peninsula, Russia, including type-locality turjaite and turjite: a review. Canadian Mineralogist (1996) 34:265–280.[Web of Science]
Bulakh A. G. Explosive breccias of the Turiy peninsula and age of the Terskaya suite sandstones. Izvestiya Vishikh Uchebnikh Zavedeniy (1962) 3:44–52. (in Russian).
Bulakh A. G., Ivanikov V. V. The Problems of Mineralogy and Petrology of Carbonatites (1984) Leningrad: Leningrad University Publishing House (in Russian).
Bulakh A. G., Ivanikov V. V. Carbonatites of the Turja peninsula, Kola: role of magmatism and metasomatism. Canadian Mineralogist (1996) 34:403–409.[Web of Science]
Droop G. T. R. A general equation for estimating Fe3+concentrations in ferromagnesian silicates and oxides from microprobe analyses using stoichiometric data. Mineralogical Magazine (1987) 51:431–435.[Web of Science]
Eggler D. H. Carbonatites primary melts and mantle dynamics. In: Carbonatites: Genesis and Evolution—Bell K., ed. (1989) London: Unwin Hyman. 561–579.
Egorov L. S. Ijolite–Carbonatite Plutonism (1991) Leningrad: Nedra (in Russian).
Furnes H., Stillman G. J. The geochemistry and petrology of the alkaline lamprophyre sheet intrusion complex on Maio, Cape Verde Republic. Journal of the Geological Society, London (1987) 144:227–241.
Hamilton D. L., Bedson P., Esson J. The behavior of trace elements in the evolution of carbonatites. In: Carbonatites: Genesis and Evolution—Bell K., ed. (1989) London: Unwin Hyman. 405–427.
Helz R. T., Thornber C. R. Geothermometry of Kilauea Iki lava lake, Hawaii. Bulletin of Volcanology (1987) 49:651–668.
Henderson C. B., Gibb F. G. Felsic mineral crystallization trends in differentiating alkaline basic magmas. Contributions to Mineralogy and Petrology (1983) 84:355–364.[Web of Science]
Ivanikov V. V. Geology and petrology of the complex of alkaline–ultrabasic dykes of the Kandalaksha Graben. (1977) Leningrad University (in Russian). Ph.D. Thesis.
Ivanikov V. V., Rukhlov A. S. Dyke series of the Kandalaksha graben: petrographical nomenclature and genetic systematics. Vestnik St Petersburg University, 7th Section (1996) 2:128–137.
Katz K., Keller J. Comb-layering in carbonatite dykes. Nature (1981) 294:350–352.
Kjarsgaard B. A., Peterson T. D. Nephelinite–carbonatite liquid immiscibility at Shombole volcano, East Africa: petrographic and experimental evidence. Mineralogy and Petrology (1991) 43:293–314.[Web of Science]
Konstantinovskiy A. A. Onejsko-Kandalakshskii Riphean graben of the East-European Plate. Geotectonics (1977) 2:38–45.
Kukharenko A. A., Orlova M. P., Bulakh A. G., Bagdasarov E. A., Rimskaya-Korsakova O. M., Nefedov E. I., Ilyinskiy G. A., Sergeev A. B., Abakumova N. B. The Caledonian Ultramafic Alkaline and Carbonatite Complexes of the Kola Peninsula and Northern Karelia (1965) Moscow: Nedra (in Russian).
Le Bas M. J. Carbonatite–Nephelinite Volcanism (1977) Chichester, UK: John Wiley.
Leeman W. P., Scheidegger K. F. Olivine/liquid distribution coefficients and a test for crystal–liquid equilibrium. Earth and Planetary Science Letters (1977) 35:247–257.[Web of Science]
Mal'kov B. A. Differentiation in monchiquite dykes. Doklady Akademii Nauk SSSR, Earth Science Section (1970) 194(2):1239–1243.
Mitchell R. H., Bergman S. C. Petrology of Lamproites (1991) New York: Plenum.
Morimoto N. Nomenclature of pyroxenes. Canadian Mineralogist (1989) 27:143–156.[Web of Science]
Nagasawa H., Schreiber H. D. Experimental mineral/liquid partition coefficients of REE, Sc and Sr for perovskite, spinel and melilite. Earth and Planetary Science Letters (1980) 46:431.[Web of Science]
Nakamura N. Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites. Geochimica et Cosmochimica Acta (1974) 38:757–775.[Web of Science]
Nielsen T. F. The petrology of a melilitolite, melteigite, carbonatite and syenite ring dike system, in the Gardiner complex, East Greenland. Lithos (1980) 13:181–197.[Web of Science]
Nielsen R. L., Drake M. J. Pyroxene–melt equilibria. Geochimica et Cosmochimica Acta (1979) 43:1259–1272.[Web of Science]
Onuma K., Yagi K. The system diopside–akermanite–nepheline. American Mineralogist (1967) 52:227–243.[Web of Science]
Orlova M. P. Some problems on chemistry and petrology of the Caledonian complex of alkaline–ultrabasic rocks of the Kola peninsula. Mineralogical Magazine VSEGEI (1983) 3:288–294. (in Russian).
Pan V., Longhi J. Low pressure liquidus relations in the system Mg2SiO4–Ca2SiO4–NaAlSiO4–SiO2. American Journal of Science (1989) 289:1–16.
Roeder F. L., Emslie B. F. Olivine–liquid equilibrium. Contributions to Mineralogy and Petrology (1970) 29:275–289.[Web of Science]
Romanchev B. P., Kuznetzova S. L. Conditions of crystallization of nosean in alkaline melts. Newsletter of the Academy of Sciences of the USSR (1982) 1:134–137.
Schairer J. F., Yoder H. S. Jr. Crystal and liquid trends in simplified alkali basalts. Carnegie Institution of Washington, Yearbook (1964) 63:65–74.
Shinkarev N. F., Ivanikov V. V. Alkaline dyke series of the Turiy peninsula. In: Problems in Igneous Petrology—Sobolev V. S., ed. (1973) Novosibirsk: Nauka. 129–142. (in Russian).
Shurkin K. A. On Palaeozoic pseudoconglomerates of NorthernKarelia and the Kola peninsula. Doklady Akademii Nauk SSSR, Earth Science Section (1959) 125(6):1329–1332.
Simkin T., Smith J. V. Minor element distribution in olivine. Journal of Geology (1970) 78:304–325.[Web of Science]
Simonetti A., Shore M., Bell K. Diopside phenocrysts from nephelinite lavas, Napak volcano, eastern Uganda: evidence for magma mixing. Canadian Mineralogist (1996) 34:411–421.[Web of Science]
Thompson R. N. Some high-pressure pyroxenes. Mineralogical Magazine (1974) 39:768–787.[Web of Science]
Velde B., Kushiro I. Structure of sodium alumino-silica melts quenched at high pressure: infrared and aluminium K-radiation data. Earth and Planetary Science Letters (1978) 4:137–140.
Villemant B., Jaffrezic H., Joron J. L., Treuil M. Distribution coefficients of major and trace elements: fractional crystallization in the alkali basalt series of Chaîne des Puys (Massif Central, France). Geochimica et Cosmochimica Acta (1981) 45:1997–2016.[Web of Science]
Wass S. Y. Multiple origin of clinopyroxenes in alkali basaltic rocks. Lithos (1979) 12:115–132.[Web of Science]
Wendlandt R. F., Harrison W. J. Rare earth partitioning between immiscible carbonate and silicate liquids and CO2vapour: results and implications for the formation of light rare earth-enriched rocks. Contributions to Mineralogy and Petrology (1979) 69:409–419.[Web of Science]
Wilshire H. G., Shervais J. W. Al-augite and Cr-diopside ultramafic xenoliths in basaltic rocks from western United States. Physics and Chemistry of the Earth (1975) 9:257–272.
Wimmenauer E. Contribution to the petrography of the Kaiserstuhl. Neues Jahrbuch für Mineralogie, Abhandlungen (1963) 99:231–276.
Wood D. A., Tarney J., Varet J., Saunders A. D., Bougault H., Joron J. L., Treuil M., Cann J. R. Geochemistry of basalts drilled in the North Atlantic by IPOD Leg 49: implications for mantle heterogeneity. Earth and Planetary Science Letters (1979) 42:77–97.[Web of Science]
Woolley A. R., Kempe D. R. C. Carbonatites: nomenclature, average chemical compositions and element distribution. In: Carbonatites: Genesis and Evolution—Bell K., ed. (1989) London: Unwin Hyman. 1–14.
Wright T. L., Doherty P. C. A linear programming and least-squares computer method for solving petrologic mixing problems. Geological Society of America Bulletin (1970) 81:1995–2008.
Yagi K., Onuma K. The join CaMgSi2O6–CaTiAl2O6and its bearing on the titanaugites. Journal of the Faculty of Science, Hokkaido University, Series IV (1967) 13:117–138.
Yoder H. S. Jr. Melilite bearing rocks and related lamprophyres. In: The Evolution of the Igneous Rocks—Yoder H. S., ed. (1979) Princeton, NJ: Princeton University Press. 391–412.
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