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Alkaline plumes of continents and oceans
Russian Geology and Geophysics 51 (2010) 965–971 www.elsevier.com/locate/rgg
Alkaline plumes of continents and oceans V.G. Lazarenkov * G.V. Plekhanov State Mining Institute (Technical University), Dvadtsat’ Pervaya Liniya 2, St. Petersburg, 199106, Russia Received 15 April 2009; received in revised form 27 January 2010
Abstract Series of continental and oceanic alkaline associations have been compared. Comparison confirms that alkaline plumes originated from the Earth’s liquid core under the continents and, less often, under the oceans. The spatial distribution of alkaline complexes has been analyzed in terms of the plume magmatism theory. Analysis suggests that the zoning and lateral migration of alkaline magmatic centers in alkaline provinces were determined by the migration of an alkaline plume (multiplume) and its alkaline basaltic, alkaline ultramafic, carbonatitic, kimberlitic, and other derivates. Two components are well pronounced in the chemical history of alkaline plume magmatism. The first is the foidaphile component, which persists in all igneous and metasomatic rocks of various alkaline complexes. It includes elements associated with Na and K: rare alkali metals, alkaline earth metals, radioactive elements, rare earths, and others. They make up the important part of the plume that might have separated from the liquid core. The second component is rock-forming mantle–lithospheric, which formed in the asthenosphere during the mixing of mantle and lithospheric sources while the plume ascended to the Earth’s surface. © 2010, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: alkaline plume; superplume; multiplume; series of alkaline complexes and associations
Introduction The theory by Yu.A. Kuznetsov on igneous complexes and series of igneous complexes and associations (Kuznetsov, 1964; Polyakov and Izokh, 2003) is especially important now, in the context of the plume magmatism theory. The idea of mantle plumes manifested on the Earth’s surface as hotspots originated in plate tectonics. It won support due to achievements in the study of the Earth’s thermal field and satellite data on Jupiter’s Great Red Spot, Io, Jupiter’s satellite, and Ariel and Miranda, moons of Uranus. In modern view (Brandon and Walker, 2005; Dobretsov, 2008; Dobretsov and Kirdyashkin, 2000; Letnikov, 2001), a plume is a fluid, probably essentially hydrogen, heat flow generated in the D2 layer. Plume magmatism originates in the asthenosphere, from a depth of 600–700 km upward. According to seismic data, potassic volcanic rocks on the island of Java form at approximately this depth (Whitford and Nicholls, 1976). From the D2 layer (about 2900 km) to the lower asthenospheric boundary, the plume moves in fluid form (Letnikov, 2001).
* Corresponding author. E-mail address: [email protected] (V.G. Lazarenkov)
Comparison of alkaline plume magmatism in the continents and oceans To locate the roots of alkaline plumes, we have to consider how series of alkaline associations differ in the continents and oceans. Alkaline magmatism takes place predominantly in the continents. Here it is better pronounced in platforms, especially shields, than in mobile zones. Within the latter, it mainly occurs in rigid blocks of median masses. Alkaline magmatism also occurs at passive continental margins, in pericontinental zones of the continent–ocean boundary (Table 1). In the oceans it is poorly developed: the series of alkaline associations is dramatically shorter than in the continents. Here highly alkaline plumes are observed primarily on islands at the ocean–continent boundary, near passive continental margins, for example, on the Cape Verde Islands, the Canary Islands in the Atlantic, the Comoros in the Indian Ocean, and others. For example, the Canary volcanic complexes form the following time series: tholeiite (Cretaceous–Paleogene), alkali rhyolite–trachyte–alkali basalt (13.7–13.0 Ma), and phonolite– trachyte–alkali basalt (4.4–3.4 Ma); the Comoros complexes form the following series: alkali basalt (Miocene?), phonolite– trachyte–alkali basalt (Miocene–Pleistocene), and nephelinite– phonolite–alkali basalt (Emerick and Duncan, 1982). Complexes in the insular alkaline provinces of perioceanic rifts
1068-7971/$ - see front matter D 2010, V . S. Sabolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2010.08.006
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Table 1. Time series of volcanic associations in rift zones (Lazarenkov, 1988) Continental
Oceanic
Inland
Pericontinental
Intraplate uplifts
Mid-Ocean
Leucitite
Leucitite
–
–
Alkali rhyolite
Alkali rhyolite
–
–
Alkali trachyte–trachyte
Alkali trachyte–trachyte
–
–
Phonolite
Phonolite
–
–
Kimberlite
Kimberlite (?)
–
–
Phonolite–nephelinite–carbonatite
Phonolite–nephelinite–carbonatite
Basanite–phonolite–nephelinite
–
Phonolite–trachyte–alkali basalt
Phonolite–trachyte–alkali basalt
Phonolite–trachyte–alkali basalt
–
Rhyolite (alkali rhyolite)–trachyte– alkali basalt
Rhyolite (alkali rhyolite)–trachyte– alkali basalt
Rhyolite (alkali rhyolite)–trachyte– alkali basalt
Rhyolite (alkali rhyolite)–trachyte– alkali basalt
Alkali basalt
Alkali basalt
Alkali basalt
Alkali basalt
Basalt
Basalt
Basalt
Basalt
Note. The series are arranged from early to late ones (from the bottom upward). Line separates possible derivates of basaltic magma.
(Canary, Comoro, Cape Verde, Gulf of Guinea Islands, and others) correspond to early continental complexes of alkaline volcanic rocks. In the oceans highly alkaline plumes are observed on islands located on intraplate uplifts: the Seychelles, Kerguelen Island in the Indian Ocean, the Hawaiian Islands in the Pacific, and others. The crust on oceanic rises is 20–40 km thick, 2–5 times thicker than common oceanic crust, its section sometimes resembling that of continental crust, for example, under the Seychelles (Ben-Avraham et al., 1981). Such thick crust results from long evolution, and its alkalic rocks comprise alkali-basalt and phonolite–trachyte–alkali basalt complexes. Finally, some islands are located on mid-ocean ridges. For example, Iceland lies on the Mid-Atlantic Ridge. It is considered a hotspot, underlain by oceanic crust 10–20 km thick, and marked by rhyolite (alkali rhyolite)–alkali basalt volcanism. Iceland as a region of rhyolite and alkali-rhyolite magmatism is not unique, because rhyolites (a rhyolite–basalt province in the Atlantic Ocean) are also present on islands in the South Atlantic strung out along the Mid-Atlantic Ridge (Ascension and Tristan da Cunha Islands, Gough, Bouvet). All of them are interpreted as mantle plumes anchored to the center of mid-ocean ridges (Dobretsov, 2008). Apart from series of alkaline associations, an important indicator of alkaline magmatism in the continents and oceans is the abundance of alkalic rocks. The latter account for 0.2% of continental area, and in the oceans they occupy an area smaller by 2–3 orders, that is, a tiny one (Lazarenkov, 1988). Petrographic provinces with a wide areal extent of alkalic rocks are unknown in the oceans. Consequently, the continents experienced active alkaline magmatism, whereas the oceans, that is, vaster areas, did not. Under the plume magmatism theory, this evidences that alkaline plumes have been very active in the continents for a long time, since between the Archean and Proterozoic (2.7– 2.5 Ga (Kogarko, 2004; Vladykin, 2009)), and poorly devel-
oped in the oceans, mainly in the Cretaceous and Cenozoic, only for 0.15 Ga. However, it should be borne in mind that the present-day oceans are younger than 0.2 Ga, with fragments of ancient oceanic crust accounting for no more than 0.1%. Why has alkaline plume magmatism been strong and long-lived in the continents, but weak and short-lived in the oceans? To answer this question, the geochemistry of the largest oceanic ultramafic massifs, for example, Massif du Sud in New Caledonia (4950 km2), the largest in the world, was studied, along with the composition of mantle inclusions in oceanic alkali basalts, for example, on the Hawaiian Islands. Studies showed that ultramafic oceanic mantle, at least lithospheric, was poor in alkalis and incompatible elements and, apparently, only slightly influenced plume alkalinity and the content of incompatible elements. The Earth’s geological history suggests that the primary mantle under the future oceans and continents was of the same origin and roughly the same ultramafic composition. If so, neither continental nor oceanic mantle could generate alkaline plumes or accompanying foidaphile elements. Therefore, most likely the liquid core or D2 layer generated them. Conclusion 1. Series of continental and oceanic alkaline associations have been compared. Comparison confirms that alkaline plumes originated from the Earth’s liquid core under the continents and, less often, under the oceans.
A fixed plume vs. a mobile plate, or the other way round? Hotspot–plate interaction was used in plate tectonics for determining the plate-motion distance and vectors. The fundamental idea of plate tectonics was that plumes—hot mantle currents generating a hotspot—were fixed for a long time and “burned through” the slab sliding over them to create volcanic chains, with volcanic edifices aging with distance from the
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recent eruption center (Khain, 2002). Take, for example, the Hawaiian Islands, which extend about 3000 km NW and EW. Here the age of volcanic edifices decreases from Late Cretaceous at the northwest extremity of the chain to recent on Hawaii Island. Plate tectonists interpret this as a result of the West Pacific Plate drifting northwest. Within this plate, north of the Hawaiian Islands, there is the Emperor Seamounts province; south of the Hawaiian Islands, there is the Line Islands alkaline province. Southeast of the Line Islands, there are the Society, Tuamotu, and Marquesas Islands, which make up the southern extension of the Polynesian zone of alkaline provinces. Like on the Hawaiian Islands, the volcanic edifices on all of them seem to migrate along island chains, from the northwest southeastward. In terms of plate tectonics, this evidences that the entire West Pacific Plate drifted northwest. Now let us imagine that the plate was fixed and the plume mobile. Physically, this seems quite natural if the plume is superfluid up to the lower asthenospheric boundary (Letnikov, 2001). There are examples when magmatic eruption centers shifted rapidly: the 1974–1976 eruption of the Tolbachik Volcano in Kamchatka (The Great..., 1984) or the formation of the Khibiny massif (Galakhov, 1975). These examples evidence for the development of large crustal fissures rather than for gas flow in the lower mantle. However, let us consider oceanic basaltic magmatism in terms of the plume magmatism theory. Trap magmatism in India, manifested in the Late Cretaceous–Early Paleogene (65–40 Ma) Deccan Traps, continues in the Indian Ocean southward on the Maldives, Chagos Archipelago, Seychelles, and Mascarene Islands, with volcanic edifices becoming younger. The island of Réunion (Mascarene Islands) is marked by recent volcanism (Tiwari et al., 2007) (Fig. 1). Like in the Pacific, in this part of the Indian Ocean, hotspots corresponding to an active basalt plume shifted southward. Now let us turn to alkaline plume magmatism in continents. The world’s most remarkable alkaline province is the East African one, contiguous to the East African Rift Zone (Fig. 2). The province extends from the north southward, from the Afar Triangle (Ethiopia) to Uganda and Kenya, spans the West African Rift, and continues southward to Tanzania. This province belongs to the world system of Cretaceous–Cenozoic alkaline provinces and is dominated by volcanic complexes, mainly alkaline ones. The composition of these complexes is extremely diverse. Early plume magmatism is manifested in the Eocene–Oligocene Yemeni–Ethiopian tholeiitic complex—a wide strip extending roughly from the north southward, through Egypt and the Sudan to Kenya (Ebinger and Sleep, 1998). Afterward the plume shifted southward, to Uganda and Kenya, to produce the Ugandan carbonatite– nephelinite complex (23–16 Ma) in the Miocene. Later on, it may have migrated farther south, to Tanzania, to produce the Tanzanian kimberlitic complex and Comoros alkaline complexes (10–0 Ma) (Emerick and Duncan, 1982). After that the plume produced the remarkable Miocene–Pliocene Kenyan plateau-phonolite complex (14–10 Ma) and the Pliocene–Quaternary Ethiopian pantellerite–comendite complex farther north. According to analysis, East African alkaline magmatism
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Fig. 1. Hotspot distribution in the India–Réunion Island zone, the western sector of the Indian Ocean. Numbers indicate the seafloor age (Ma); circles and diamonds are drill holes (Tiwari et al., 2007).
has migrated since the Cenozoic (45–0 Ma) from Yemen to Mozambique, for several thousand kilometers. This seemingly chaotic migration, nevertheless, follows a reciprocating pattern. Lateral zonation is conspicuous in the arrangement of alkaline-ultramafic/carbonatite massifs in the Maimecha–Kotui province (P2–T1). Magmatism changes from the southern boundary of the Khatanga trough deep into the Siberian Platform. For example, variation is well-pronounced in the dunitic (Guli massif, 250.2 ± 0.3 Ma (Kamo et al., 2003)), clinopyroxenite–ijolitic (Magan massif), and carbonatitic (Guli, Yraas, Essei massifs) components of this complex. Note also the nearest kimberlite fields of the Kuonamka complex (T2–J1). Alkalic rocks in the Maimecha–Kotui province are coeval with 251–248 Ma basalts near Noril’sk (Wooden et al., 1993) and in the other fields of the Siberian superplume (Dobretsov, 1997, 2008). The fact that this complex is larger (about 4000 m) and older than the other trap complexes of the Siberian Platform suggests the existence of a powerful P2–T1 superplume (Dobretsov, 2008). Let us consider an example of kimberlitic platform magmatism. According to (Heaman and Kjarsgaard, 2000), in eastern North America, there is a WNW-trending zone about 4000 km long, where kimberlite fields become younger southeastward, from 214 to 40 Ma. The hotspot migration is
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Fig. 2. Cretaceous–Cenozoic East African alkaline province. 1, Eocene–Oligocene Yemeni–Ethiopian tholeiitic complex (P2–P3); 2, Ugandan carbonatite–nephelinite complex (23–16 Ma N1); 3, Tanzanian kimberlitic complex (K–N); 4, Kenyan complexes: a, plateau phonolite (14–10 Ma), b, trachyte (2.4–0.6 Ma) (N2–N1); 5, Ethiopian pantellerite–comendite complex (N2–Q); 6, Bufumbira leucititic complex (N2–Q).
traced here. Along with kimberlites, this zone hosts the Monteregian alkaline-ultramafic/carbonatite complex. This complex formed at 141–117 Ma, the massifs consecutively shifting from the west eastward. Its 17 intrusions form a chain about 120 km long. Its composition exhibits dramatic lateral variations. Mont Oka in the west is mainly carbonatitic (56%) and ijolitic (29%), whereas Monts Saint Bruno and Rougemont are mainly peridotitic and pyroxenitic (70% in the former, 65% in the latter). On the other hand, in eastern massifs, ultramafic and mafic rocks are not so abundant, but the proportion of pulaskites and nordmarkites increases: Mont Shefford (48%), Mont Brome (57%), Mont Mégantic (100%) (Eby, 1985). In accordance with the general migration trend
in this zone, the hotspot in the Monteregian complex shifted eastward, to the continent–ocean boundary. There are many other examples of lateral zonation in alkaline provinces. Conclusion 2. The spatial distribution of alkaline complexes has been analyzed in terms of the plume magmatism theory. Analysis suggests that the zoning and lateral migration of alkaline magmatic centers in alkaline provinces were determined by the migration of an alkaline plume (multiplume) and its derivates. The latter include alkali basalts, alkaline ultramafic rocks, carbonatites, kimberlites, phonolites, alkali rhyolites, leucitites and other superplume or multiplume components. Here multiplume igneous complexes are derivates and a multiplume is a series of igneous complexes
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(Kuznetsov, 1964). In other words, the term superplume emphasizes power, whereas the term multiplume emphasizes variable structure.
Series of alkaline complexes as a clue to the composition and structure of alkaline multiplumes
Table 2. Chemical composition (ppm) of oceanic and continental basalts (Hess, 1989) Element
Basalts Oceanic
Continental
1–6 2
11 0–30
120 43
410 200
4 3.5 21–32
7.7 4.1 24
86 1.5–5.0
100 8–11
290 318 50 110
250 160 46 85
Rare alkali metals Li Rb Alkaline earth metals
This section discusses time series of continental alkaline complexes in alkaline provinces of different ages. Take, for example, a representative time series of alkaline complexes in the Brazilian province. It comprises the Paraná tholeiitic complex (160–140 Ma), the Jacupiranga alkaline-ultramafic/carbonatite complex (138–81 Ma), the Paranaíba kimberlitic complex (87–79 Ma), and the Posos de Caldas phonolite–nepheline syenite complex (77–51 Ma). This series resembles that in the East African province. In other alkaline provinces, this series is incomplete or shortened. Note that, in different provinces and periods, its members form a fairly clear time sequence, which can be considered an important regularity in alkaline magmatism (Lazarenkov, 1981, 1984). In terms of the plume magmatism hypothesis, all this is due to an alkaline plume (multiplume) which evolved more or less similarly in different parts of the continents. Conclusion 3. Series of alkaline complexes have a roughly similar composition in different continents and areas regardless of age. This suggests that the original alkaline plumes (multiplumes) had a more or less the same composition, which evolved similarly.
Geochemical differences between alkaline complexes as a clue to the composition of alkaline plumes Series of alkaline complexes and associations display their foidaphile character already when we compare the chemical composition of continental and oceanic basalts (Table 2). Continental and oceanic asthenospheric basalts have a similar content of major elements, except K (Hess, 1989). As regards their geochemical composition, oceanic basalts are obviously poor in incompatible elements. So, continental and oceanic basalts have a similar content of major elements, but their geochemical composition differs substantially in the content of some incompatible, essentially foidaphile, elements. However, as suggested above, if plumes originated not in the upper mantle, but in the D2 layer, these differences also originated there. This idea agrees well with the important fact that the alkaline complexes—carbonatites, phonolites, and alkali rhyolites—which differ greatly in their petrography and chemistry of major elements have a relatively similar content of foidaphile elements (rare alkali metals, alkaline earth metals, radioactive elements, rare earths, HSFE, and others) (Lazarenkov, 1988). In other words, the petrography and chemistry of alkaline complexes (carbonatites, phonolites, alkali rhyolites, and others), like that of basalt ones, formed in the
Sr Ba Rare earths La Sm Y Incompatible elements Zr Nb Iron-group elements V Cr Co Ni
asthenosphere, whereas their geochemistry was influenced by the lower mantle and core. Significantly, the constancy of geochemical composition shows that it was essentially independent of the composition of the asthenosphere or the entire mantle. If we bear in mind that the foidaphile character of alkaline complexes determines their metallogeny, we should admit that the relationship between alkaline magmatism and metallogeny is rooted in the origin and initial composition of the alkaline plume. Conclusion 4. Two components are well-pronounced in the chemical history of alkaline plume magmatism. The first is the foidaphile component, which persists in all igneous and metasomatic rocks of various alkaline complexes. It includes elements associated with Na and K: rare alkali metals, alkaline earth metals, radioactive elements, rare earths, and others. They make up the important part of the plume which originated at the core–mantle boundary. The second component is rock-forming mantle–lithospheric, which formed in the asthenosphere by the mixing of mantle and lithospheric sources while the plume ascended to the Earth’s surface.
Chemical composition of alkaline plumes (multiplumes) This section discusses the composition of alkaline plumes on the basis of the continental alkaline associations which are foidaphile-richest. These are primarily Na complexes of agpaitic nepheline syenites, alkali granites, and carbonatites, along with K complexes of leucitites and pseudoleucitic syenites. The agpaitic coefficient is a criterion for their alkalinity. The Ilimaussaq, Lovozero, and Khibiny massifs are the most agpaitic in the world. The Ilimaussaq massif has the
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highest alkalinity, but a small area (~100 km2). The Lovozero one is less agpaitic, but larger (~650 km2), and the Khibiny one is still larger (1327 km2). Kenyan plateau phonolites are the world’s largest nepheline syenite occurrences. The general geochemistry of these rocks gives insight into the chemistry of powerful alkaline plumes. It is especially well studied in many papers on the Ilimaussaq, Lovozero, and Khibiny massifs. Particularly interesting are the sodalite–nepheline syenites of the Ilimaussaq massif (kakortokites, naujaites, kuanites). Their remarkable coarse texture is strong evidence for crystallization from volatile-rich magma. Note that khibinites in the Khibiny massif have the same texture. Ilimaussaq alkalic rocks, like Lovozero naujaites and lujavrites, are enriched in trace elements. Ilimaussaq kakortokites, naujaites, and kuanites are commercial ores not only of U, Th, Be, Zr, Hf, Nb, Ta, Tr, Ti, but also of P, Zn, V, Ga, Tl, and other elements. The Lovozero massif is geochemically similar (Geochemistry..., 1966): its alkalic rocks are considerably enriched in many trace elements (Nb, Ta, Zr, Hf, REE, U, Th) and volatiles (F, Cl, S). Loparite and eudialyte lujavrites in this massif are ores of Nb, Ta, Zr, REE, and other elements. Of great interest for the compositional analysis of alkaline plumes is the composition of alkaline pegmatites and hydrothermal rocks in alkaline massifs influenced by fluid alkaline magmas. Ilimaussaq kuanites, which form pegmatite veins, are foidaphile-richest (Semenov, 1969) and rich in F, Cl, S, and P. Like kuanites, the pegmatites and hydrothermal rocks of the Lovozero massif are rich in foidaphile elements. Their mineral composition is extremely diverse, about 300 minerals (Pekov, 2001). They contain much more mineral species than the other rocks, including granite pegmatites. Although the fluid phase of the alkaline pegmatites was by no means chemically equivalent to alkaline plumes, it can be assumed that the chemical composition of the alkaline plume was reflected in these rocks more than in the other alkalic rocks. Host-rock xenoliths in the agpaitic nepheline syenites of the Khibiny and Lovozero massifs also carry useful information about the composition of the alkaline mantle fluid (Korchak, 2008). They were metasomatized to nepheline syenites by the fluid in three stages: the high-temperature “hornfelsic,” medium-temperature fenitic, and low-temperature ones. The xenoliths consist of 169 mineral species. Particularly interesting for our purposes are the high-temperature minerals—anorthoclase, sekaninaite, cordierite, freudenbergite, andalusite, annite, and fayalite—influenced by a K–Al fluid at the early magmatic distillation stages of agpaitic nepheline syenites. This is because they reflect the chemistry of exactly the early fluid. Twenty-six minerals in these xenoliths contain V, W, Sn, Sb, and Te—elements observed neither in the Khibiny or Lovozero massifs nor in the host rocks. However, they were part of the original alkaline melt, which, as we can see, differed from the pegmatitic and hydrothermal fluid. Finally, interstitial glasses in ultramafic mantle inclusions in alkali basalts have been attracting considerable researchers’ attention in the last few years. These glasses are of unusual calc-silicate composition, not typical of
igneous rocks (Lazarenkov et al., 2000; Litasov et al., 2003). We think that they originated from the melt/fluid which penetrated the inclusions from alkali basalt magma and may reflect the composition of the plume which caused alkali basalts to melt. Conclusion 5. The most alkaline massifs are richest in foidaphile elements. Evidently, their chemical composition was most influenced by an alkaline plume and least by the asthenosphere. In terms of the plume magmatism theory, alkalinity and the content of foidaphile elements depend on the interaction between the alkaline plume and the asthenosphere. On the whole, lateral and time series of alkaline associations give insight into the roots, initial composition, and chemical differentiation of alkaline plumes.
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V.G. Lazarenkov / Russian Geology and Geophysics 51 (2010) 965–971 Kuznetsov, Yu.A., 1964. Fundamental Types of Igneous Rocks [in Russian]. Nedra, Moscow. Lazarenkov, V.G., 1981. Time and lateral series of alkaline platform formations. Geologiya i Geofizika (Soviet Geology and Geophysics) 22 (11), 61–69 (51–57). Lazarenkov, V.G., 1984. Tectonic position and formation series of young alkaline provinces of continents and oceans. Izv. AN SSSR, Ser. Geol., No. 7, 84–92. Lazarenkov, V.G., 1988. Formational Analysis of Alkaline Rocks of Continents and Oceans [in Russian]. Nedra, Leningrad. Lazarenkov, V.G., Maslov, V.A., Talovina, I.V., 2000. Anatectic glasses in the mantle xenoliths of Sverre Volcano, Spitsbergen, in: Abstracts of Papers of the Seminar on Igneous Rocks [in Russian]. GEOKhI RAN, Moscow, p. 7. Letnikov, F.A., 2001. Ultradeep fluid systems of the Earth and problems of ore formation. Geologiya Rudnykh Mestorozhdenii 43 (4), 291–307. Litasov, K.D., Simonov, V.A., Kovyazin, S.V., Litasov, Yu.D., Sharygin, V.V., 2003. Interaction between mantle xenoliths and deep-seated melts: results of study of melt inclusions and interstitial glasses in peridotites from basanites of the Vitim volcanic field. Geologiya i Geofizika (Russian Geology and Geophysics) 44 (5), 436–450 (417–431). Pekov, I.V., 2001. Lovozero Massif: History of Research, Pegmatites, and Minerals [in Russian]. Zemlya, Moscow.
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Polyakov, G.V., Izokh, A.E., 2003. Scientific legacy of Academician Yu.A. Kuznetsov and topical problems of formational analysis of igneous complexes. Geologiya i Geofizika (Russian Geology and Geophysics) 44 (12), 1235–1242 (1192–1198). Semenov, E.I., 1969. Mineralogy of the Ilimaussaq Alkaline Massif [in Russian]. Nauka, Moscow. The Great Tolbachik Fissure Eruption [in Russian], 1984. Nauka, Moscow. Tiwari, V.M., Grevemeyer, I., Singh, B., Phipps Morgan, J., 2007. Variation of effective elastic thickness and melt production along the Deccan–Reunion hotspot track. Earth Planet. Sci. Lett. 264 (1–2), 9–21. Vladykin, N.V., 2009. Potassium alkaline lamproite-carbonatite complexes: petrology, genesis, and ore reserves. Russian Geology and Geophysics (Geologiya i Geofizika) 50 (12), 1119–1128 (1443–1455). Whitford, D.F., Nicholls, I.A., 1976. A potassium variation in lavas across the Sunda arc, Java and Bali, in: Johnson, R.W. (Ed.), Volcanism in Australia. Elsevier, Amsterdam, pp. 63–75. Wooden, J.L., Czamanske, G.K., Fedorenko, V.A., Arndt, N.T., Chauvel, C., Bouse, R.M., King, B.-S.W., Knight, R.J., Siems, D.F., 1993. Isotopic and trace-element constraints on mantle and crustal contributions to Siberian continental flood basalts, Noril’sk area, Siberia. Geochim. Cosmochim. Acta 57, 3677–3704.
Alkaline plumes of continents and oceans V.G. Lazarenkov * G.V. Plekhanov State Mining Institute (Technical University), Dvadtsat’ Pervaya Liniya 2, St. Petersburg, 199106, Russia Received 15 April 2009; received in revised form 27 January 2010
Abstract Series of continental and oceanic alkaline associations have been compared. Comparison confirms that alkaline plumes originated from the Earth’s liquid core under the continents and, less often, under the oceans. The spatial distribution of alkaline complexes has been analyzed in terms of the plume magmatism theory. Analysis suggests that the zoning and lateral migration of alkaline magmatic centers in alkaline provinces were determined by the migration of an alkaline plume (multiplume) and its alkaline basaltic, alkaline ultramafic, carbonatitic, kimberlitic, and other derivates. Two components are well pronounced in the chemical history of alkaline plume magmatism. The first is the foidaphile component, which persists in all igneous and metasomatic rocks of various alkaline complexes. It includes elements associated with Na and K: rare alkali metals, alkaline earth metals, radioactive elements, rare earths, and others. They make up the important part of the plume that might have separated from the liquid core. The second component is rock-forming mantle–lithospheric, which formed in the asthenosphere during the mixing of mantle and lithospheric sources while the plume ascended to the Earth’s surface. © 2010, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: alkaline plume; superplume; multiplume; series of alkaline complexes and associations
Introduction The theory by Yu.A. Kuznetsov on igneous complexes and series of igneous complexes and associations (Kuznetsov, 1964; Polyakov and Izokh, 2003) is especially important now, in the context of the plume magmatism theory. The idea of mantle plumes manifested on the Earth’s surface as hotspots originated in plate tectonics. It won support due to achievements in the study of the Earth’s thermal field and satellite data on Jupiter’s Great Red Spot, Io, Jupiter’s satellite, and Ariel and Miranda, moons of Uranus. In modern view (Brandon and Walker, 2005; Dobretsov, 2008; Dobretsov and Kirdyashkin, 2000; Letnikov, 2001), a plume is a fluid, probably essentially hydrogen, heat flow generated in the D2 layer. Plume magmatism originates in the asthenosphere, from a depth of 600–700 km upward. According to seismic data, potassic volcanic rocks on the island of Java form at approximately this depth (Whitford and Nicholls, 1976). From the D2 layer (about 2900 km) to the lower asthenospheric boundary, the plume moves in fluid form (Letnikov, 2001).
* Corresponding author. E-mail address: [email protected] (V.G. Lazarenkov)
Comparison of alkaline plume magmatism in the continents and oceans To locate the roots of alkaline plumes, we have to consider how series of alkaline associations differ in the continents and oceans. Alkaline magmatism takes place predominantly in the continents. Here it is better pronounced in platforms, especially shields, than in mobile zones. Within the latter, it mainly occurs in rigid blocks of median masses. Alkaline magmatism also occurs at passive continental margins, in pericontinental zones of the continent–ocean boundary (Table 1). In the oceans it is poorly developed: the series of alkaline associations is dramatically shorter than in the continents. Here highly alkaline plumes are observed primarily on islands at the ocean–continent boundary, near passive continental margins, for example, on the Cape Verde Islands, the Canary Islands in the Atlantic, the Comoros in the Indian Ocean, and others. For example, the Canary volcanic complexes form the following time series: tholeiite (Cretaceous–Paleogene), alkali rhyolite–trachyte–alkali basalt (13.7–13.0 Ma), and phonolite– trachyte–alkali basalt (4.4–3.4 Ma); the Comoros complexes form the following series: alkali basalt (Miocene?), phonolite– trachyte–alkali basalt (Miocene–Pleistocene), and nephelinite– phonolite–alkali basalt (Emerick and Duncan, 1982). Complexes in the insular alkaline provinces of perioceanic rifts
1068-7971/$ - see front matter D 2010, V . S. Sabolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2010.08.006
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Table 1. Time series of volcanic associations in rift zones (Lazarenkov, 1988) Continental
Oceanic
Inland
Pericontinental
Intraplate uplifts
Mid-Ocean
Leucitite
Leucitite
–
–
Alkali rhyolite
Alkali rhyolite
–
–
Alkali trachyte–trachyte
Alkali trachyte–trachyte
–
–
Phonolite
Phonolite
–
–
Kimberlite
Kimberlite (?)
–
–
Phonolite–nephelinite–carbonatite
Phonolite–nephelinite–carbonatite
Basanite–phonolite–nephelinite
–
Phonolite–trachyte–alkali basalt
Phonolite–trachyte–alkali basalt
Phonolite–trachyte–alkali basalt
–
Rhyolite (alkali rhyolite)–trachyte– alkali basalt
Rhyolite (alkali rhyolite)–trachyte– alkali basalt
Rhyolite (alkali rhyolite)–trachyte– alkali basalt
Rhyolite (alkali rhyolite)–trachyte– alkali basalt
Alkali basalt
Alkali basalt
Alkali basalt
Alkali basalt
Basalt
Basalt
Basalt
Basalt
Note. The series are arranged from early to late ones (from the bottom upward). Line separates possible derivates of basaltic magma.
(Canary, Comoro, Cape Verde, Gulf of Guinea Islands, and others) correspond to early continental complexes of alkaline volcanic rocks. In the oceans highly alkaline plumes are observed on islands located on intraplate uplifts: the Seychelles, Kerguelen Island in the Indian Ocean, the Hawaiian Islands in the Pacific, and others. The crust on oceanic rises is 20–40 km thick, 2–5 times thicker than common oceanic crust, its section sometimes resembling that of continental crust, for example, under the Seychelles (Ben-Avraham et al., 1981). Such thick crust results from long evolution, and its alkalic rocks comprise alkali-basalt and phonolite–trachyte–alkali basalt complexes. Finally, some islands are located on mid-ocean ridges. For example, Iceland lies on the Mid-Atlantic Ridge. It is considered a hotspot, underlain by oceanic crust 10–20 km thick, and marked by rhyolite (alkali rhyolite)–alkali basalt volcanism. Iceland as a region of rhyolite and alkali-rhyolite magmatism is not unique, because rhyolites (a rhyolite–basalt province in the Atlantic Ocean) are also present on islands in the South Atlantic strung out along the Mid-Atlantic Ridge (Ascension and Tristan da Cunha Islands, Gough, Bouvet). All of them are interpreted as mantle plumes anchored to the center of mid-ocean ridges (Dobretsov, 2008). Apart from series of alkaline associations, an important indicator of alkaline magmatism in the continents and oceans is the abundance of alkalic rocks. The latter account for 0.2% of continental area, and in the oceans they occupy an area smaller by 2–3 orders, that is, a tiny one (Lazarenkov, 1988). Petrographic provinces with a wide areal extent of alkalic rocks are unknown in the oceans. Consequently, the continents experienced active alkaline magmatism, whereas the oceans, that is, vaster areas, did not. Under the plume magmatism theory, this evidences that alkaline plumes have been very active in the continents for a long time, since between the Archean and Proterozoic (2.7– 2.5 Ga (Kogarko, 2004; Vladykin, 2009)), and poorly devel-
oped in the oceans, mainly in the Cretaceous and Cenozoic, only for 0.15 Ga. However, it should be borne in mind that the present-day oceans are younger than 0.2 Ga, with fragments of ancient oceanic crust accounting for no more than 0.1%. Why has alkaline plume magmatism been strong and long-lived in the continents, but weak and short-lived in the oceans? To answer this question, the geochemistry of the largest oceanic ultramafic massifs, for example, Massif du Sud in New Caledonia (4950 km2), the largest in the world, was studied, along with the composition of mantle inclusions in oceanic alkali basalts, for example, on the Hawaiian Islands. Studies showed that ultramafic oceanic mantle, at least lithospheric, was poor in alkalis and incompatible elements and, apparently, only slightly influenced plume alkalinity and the content of incompatible elements. The Earth’s geological history suggests that the primary mantle under the future oceans and continents was of the same origin and roughly the same ultramafic composition. If so, neither continental nor oceanic mantle could generate alkaline plumes or accompanying foidaphile elements. Therefore, most likely the liquid core or D2 layer generated them. Conclusion 1. Series of continental and oceanic alkaline associations have been compared. Comparison confirms that alkaline plumes originated from the Earth’s liquid core under the continents and, less often, under the oceans.
A fixed plume vs. a mobile plate, or the other way round? Hotspot–plate interaction was used in plate tectonics for determining the plate-motion distance and vectors. The fundamental idea of plate tectonics was that plumes—hot mantle currents generating a hotspot—were fixed for a long time and “burned through” the slab sliding over them to create volcanic chains, with volcanic edifices aging with distance from the
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recent eruption center (Khain, 2002). Take, for example, the Hawaiian Islands, which extend about 3000 km NW and EW. Here the age of volcanic edifices decreases from Late Cretaceous at the northwest extremity of the chain to recent on Hawaii Island. Plate tectonists interpret this as a result of the West Pacific Plate drifting northwest. Within this plate, north of the Hawaiian Islands, there is the Emperor Seamounts province; south of the Hawaiian Islands, there is the Line Islands alkaline province. Southeast of the Line Islands, there are the Society, Tuamotu, and Marquesas Islands, which make up the southern extension of the Polynesian zone of alkaline provinces. Like on the Hawaiian Islands, the volcanic edifices on all of them seem to migrate along island chains, from the northwest southeastward. In terms of plate tectonics, this evidences that the entire West Pacific Plate drifted northwest. Now let us imagine that the plate was fixed and the plume mobile. Physically, this seems quite natural if the plume is superfluid up to the lower asthenospheric boundary (Letnikov, 2001). There are examples when magmatic eruption centers shifted rapidly: the 1974–1976 eruption of the Tolbachik Volcano in Kamchatka (The Great..., 1984) or the formation of the Khibiny massif (Galakhov, 1975). These examples evidence for the development of large crustal fissures rather than for gas flow in the lower mantle. However, let us consider oceanic basaltic magmatism in terms of the plume magmatism theory. Trap magmatism in India, manifested in the Late Cretaceous–Early Paleogene (65–40 Ma) Deccan Traps, continues in the Indian Ocean southward on the Maldives, Chagos Archipelago, Seychelles, and Mascarene Islands, with volcanic edifices becoming younger. The island of Réunion (Mascarene Islands) is marked by recent volcanism (Tiwari et al., 2007) (Fig. 1). Like in the Pacific, in this part of the Indian Ocean, hotspots corresponding to an active basalt plume shifted southward. Now let us turn to alkaline plume magmatism in continents. The world’s most remarkable alkaline province is the East African one, contiguous to the East African Rift Zone (Fig. 2). The province extends from the north southward, from the Afar Triangle (Ethiopia) to Uganda and Kenya, spans the West African Rift, and continues southward to Tanzania. This province belongs to the world system of Cretaceous–Cenozoic alkaline provinces and is dominated by volcanic complexes, mainly alkaline ones. The composition of these complexes is extremely diverse. Early plume magmatism is manifested in the Eocene–Oligocene Yemeni–Ethiopian tholeiitic complex—a wide strip extending roughly from the north southward, through Egypt and the Sudan to Kenya (Ebinger and Sleep, 1998). Afterward the plume shifted southward, to Uganda and Kenya, to produce the Ugandan carbonatite– nephelinite complex (23–16 Ma) in the Miocene. Later on, it may have migrated farther south, to Tanzania, to produce the Tanzanian kimberlitic complex and Comoros alkaline complexes (10–0 Ma) (Emerick and Duncan, 1982). After that the plume produced the remarkable Miocene–Pliocene Kenyan plateau-phonolite complex (14–10 Ma) and the Pliocene–Quaternary Ethiopian pantellerite–comendite complex farther north. According to analysis, East African alkaline magmatism
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Fig. 1. Hotspot distribution in the India–Réunion Island zone, the western sector of the Indian Ocean. Numbers indicate the seafloor age (Ma); circles and diamonds are drill holes (Tiwari et al., 2007).
has migrated since the Cenozoic (45–0 Ma) from Yemen to Mozambique, for several thousand kilometers. This seemingly chaotic migration, nevertheless, follows a reciprocating pattern. Lateral zonation is conspicuous in the arrangement of alkaline-ultramafic/carbonatite massifs in the Maimecha–Kotui province (P2–T1). Magmatism changes from the southern boundary of the Khatanga trough deep into the Siberian Platform. For example, variation is well-pronounced in the dunitic (Guli massif, 250.2 ± 0.3 Ma (Kamo et al., 2003)), clinopyroxenite–ijolitic (Magan massif), and carbonatitic (Guli, Yraas, Essei massifs) components of this complex. Note also the nearest kimberlite fields of the Kuonamka complex (T2–J1). Alkalic rocks in the Maimecha–Kotui province are coeval with 251–248 Ma basalts near Noril’sk (Wooden et al., 1993) and in the other fields of the Siberian superplume (Dobretsov, 1997, 2008). The fact that this complex is larger (about 4000 m) and older than the other trap complexes of the Siberian Platform suggests the existence of a powerful P2–T1 superplume (Dobretsov, 2008). Let us consider an example of kimberlitic platform magmatism. According to (Heaman and Kjarsgaard, 2000), in eastern North America, there is a WNW-trending zone about 4000 km long, where kimberlite fields become younger southeastward, from 214 to 40 Ma. The hotspot migration is
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Fig. 2. Cretaceous–Cenozoic East African alkaline province. 1, Eocene–Oligocene Yemeni–Ethiopian tholeiitic complex (P2–P3); 2, Ugandan carbonatite–nephelinite complex (23–16 Ma N1); 3, Tanzanian kimberlitic complex (K–N); 4, Kenyan complexes: a, plateau phonolite (14–10 Ma), b, trachyte (2.4–0.6 Ma) (N2–N1); 5, Ethiopian pantellerite–comendite complex (N2–Q); 6, Bufumbira leucititic complex (N2–Q).
traced here. Along with kimberlites, this zone hosts the Monteregian alkaline-ultramafic/carbonatite complex. This complex formed at 141–117 Ma, the massifs consecutively shifting from the west eastward. Its 17 intrusions form a chain about 120 km long. Its composition exhibits dramatic lateral variations. Mont Oka in the west is mainly carbonatitic (56%) and ijolitic (29%), whereas Monts Saint Bruno and Rougemont are mainly peridotitic and pyroxenitic (70% in the former, 65% in the latter). On the other hand, in eastern massifs, ultramafic and mafic rocks are not so abundant, but the proportion of pulaskites and nordmarkites increases: Mont Shefford (48%), Mont Brome (57%), Mont Mégantic (100%) (Eby, 1985). In accordance with the general migration trend
in this zone, the hotspot in the Monteregian complex shifted eastward, to the continent–ocean boundary. There are many other examples of lateral zonation in alkaline provinces. Conclusion 2. The spatial distribution of alkaline complexes has been analyzed in terms of the plume magmatism theory. Analysis suggests that the zoning and lateral migration of alkaline magmatic centers in alkaline provinces were determined by the migration of an alkaline plume (multiplume) and its derivates. The latter include alkali basalts, alkaline ultramafic rocks, carbonatites, kimberlites, phonolites, alkali rhyolites, leucitites and other superplume or multiplume components. Here multiplume igneous complexes are derivates and a multiplume is a series of igneous complexes
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(Kuznetsov, 1964). In other words, the term superplume emphasizes power, whereas the term multiplume emphasizes variable structure.
Series of alkaline complexes as a clue to the composition and structure of alkaline multiplumes
Table 2. Chemical composition (ppm) of oceanic and continental basalts (Hess, 1989) Element
Basalts Oceanic
Continental
1–6 2
11 0–30
120 43
410 200
4 3.5 21–32
7.7 4.1 24
86 1.5–5.0
100 8–11
290 318 50 110
250 160 46 85
Rare alkali metals Li Rb Alkaline earth metals
This section discusses time series of continental alkaline complexes in alkaline provinces of different ages. Take, for example, a representative time series of alkaline complexes in the Brazilian province. It comprises the Paraná tholeiitic complex (160–140 Ma), the Jacupiranga alkaline-ultramafic/carbonatite complex (138–81 Ma), the Paranaíba kimberlitic complex (87–79 Ma), and the Posos de Caldas phonolite–nepheline syenite complex (77–51 Ma). This series resembles that in the East African province. In other alkaline provinces, this series is incomplete or shortened. Note that, in different provinces and periods, its members form a fairly clear time sequence, which can be considered an important regularity in alkaline magmatism (Lazarenkov, 1981, 1984). In terms of the plume magmatism hypothesis, all this is due to an alkaline plume (multiplume) which evolved more or less similarly in different parts of the continents. Conclusion 3. Series of alkaline complexes have a roughly similar composition in different continents and areas regardless of age. This suggests that the original alkaline plumes (multiplumes) had a more or less the same composition, which evolved similarly.
Geochemical differences between alkaline complexes as a clue to the composition of alkaline plumes Series of alkaline complexes and associations display their foidaphile character already when we compare the chemical composition of continental and oceanic basalts (Table 2). Continental and oceanic asthenospheric basalts have a similar content of major elements, except K (Hess, 1989). As regards their geochemical composition, oceanic basalts are obviously poor in incompatible elements. So, continental and oceanic basalts have a similar content of major elements, but their geochemical composition differs substantially in the content of some incompatible, essentially foidaphile, elements. However, as suggested above, if plumes originated not in the upper mantle, but in the D2 layer, these differences also originated there. This idea agrees well with the important fact that the alkaline complexes—carbonatites, phonolites, and alkali rhyolites—which differ greatly in their petrography and chemistry of major elements have a relatively similar content of foidaphile elements (rare alkali metals, alkaline earth metals, radioactive elements, rare earths, HSFE, and others) (Lazarenkov, 1988). In other words, the petrography and chemistry of alkaline complexes (carbonatites, phonolites, alkali rhyolites, and others), like that of basalt ones, formed in the
Sr Ba Rare earths La Sm Y Incompatible elements Zr Nb Iron-group elements V Cr Co Ni
asthenosphere, whereas their geochemistry was influenced by the lower mantle and core. Significantly, the constancy of geochemical composition shows that it was essentially independent of the composition of the asthenosphere or the entire mantle. If we bear in mind that the foidaphile character of alkaline complexes determines their metallogeny, we should admit that the relationship between alkaline magmatism and metallogeny is rooted in the origin and initial composition of the alkaline plume. Conclusion 4. Two components are well-pronounced in the chemical history of alkaline plume magmatism. The first is the foidaphile component, which persists in all igneous and metasomatic rocks of various alkaline complexes. It includes elements associated with Na and K: rare alkali metals, alkaline earth metals, radioactive elements, rare earths, and others. They make up the important part of the plume which originated at the core–mantle boundary. The second component is rock-forming mantle–lithospheric, which formed in the asthenosphere by the mixing of mantle and lithospheric sources while the plume ascended to the Earth’s surface.
Chemical composition of alkaline plumes (multiplumes) This section discusses the composition of alkaline plumes on the basis of the continental alkaline associations which are foidaphile-richest. These are primarily Na complexes of agpaitic nepheline syenites, alkali granites, and carbonatites, along with K complexes of leucitites and pseudoleucitic syenites. The agpaitic coefficient is a criterion for their alkalinity. The Ilimaussaq, Lovozero, and Khibiny massifs are the most agpaitic in the world. The Ilimaussaq massif has the
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highest alkalinity, but a small area (~100 km2). The Lovozero one is less agpaitic, but larger (~650 km2), and the Khibiny one is still larger (1327 km2). Kenyan plateau phonolites are the world’s largest nepheline syenite occurrences. The general geochemistry of these rocks gives insight into the chemistry of powerful alkaline plumes. It is especially well studied in many papers on the Ilimaussaq, Lovozero, and Khibiny massifs. Particularly interesting are the sodalite–nepheline syenites of the Ilimaussaq massif (kakortokites, naujaites, kuanites). Their remarkable coarse texture is strong evidence for crystallization from volatile-rich magma. Note that khibinites in the Khibiny massif have the same texture. Ilimaussaq alkalic rocks, like Lovozero naujaites and lujavrites, are enriched in trace elements. Ilimaussaq kakortokites, naujaites, and kuanites are commercial ores not only of U, Th, Be, Zr, Hf, Nb, Ta, Tr, Ti, but also of P, Zn, V, Ga, Tl, and other elements. The Lovozero massif is geochemically similar (Geochemistry..., 1966): its alkalic rocks are considerably enriched in many trace elements (Nb, Ta, Zr, Hf, REE, U, Th) and volatiles (F, Cl, S). Loparite and eudialyte lujavrites in this massif are ores of Nb, Ta, Zr, REE, and other elements. Of great interest for the compositional analysis of alkaline plumes is the composition of alkaline pegmatites and hydrothermal rocks in alkaline massifs influenced by fluid alkaline magmas. Ilimaussaq kuanites, which form pegmatite veins, are foidaphile-richest (Semenov, 1969) and rich in F, Cl, S, and P. Like kuanites, the pegmatites and hydrothermal rocks of the Lovozero massif are rich in foidaphile elements. Their mineral composition is extremely diverse, about 300 minerals (Pekov, 2001). They contain much more mineral species than the other rocks, including granite pegmatites. Although the fluid phase of the alkaline pegmatites was by no means chemically equivalent to alkaline plumes, it can be assumed that the chemical composition of the alkaline plume was reflected in these rocks more than in the other alkalic rocks. Host-rock xenoliths in the agpaitic nepheline syenites of the Khibiny and Lovozero massifs also carry useful information about the composition of the alkaline mantle fluid (Korchak, 2008). They were metasomatized to nepheline syenites by the fluid in three stages: the high-temperature “hornfelsic,” medium-temperature fenitic, and low-temperature ones. The xenoliths consist of 169 mineral species. Particularly interesting for our purposes are the high-temperature minerals—anorthoclase, sekaninaite, cordierite, freudenbergite, andalusite, annite, and fayalite—influenced by a K–Al fluid at the early magmatic distillation stages of agpaitic nepheline syenites. This is because they reflect the chemistry of exactly the early fluid. Twenty-six minerals in these xenoliths contain V, W, Sn, Sb, and Te—elements observed neither in the Khibiny or Lovozero massifs nor in the host rocks. However, they were part of the original alkaline melt, which, as we can see, differed from the pegmatitic and hydrothermal fluid. Finally, interstitial glasses in ultramafic mantle inclusions in alkali basalts have been attracting considerable researchers’ attention in the last few years. These glasses are of unusual calc-silicate composition, not typical of
igneous rocks (Lazarenkov et al., 2000; Litasov et al., 2003). We think that they originated from the melt/fluid which penetrated the inclusions from alkali basalt magma and may reflect the composition of the plume which caused alkali basalts to melt. Conclusion 5. The most alkaline massifs are richest in foidaphile elements. Evidently, their chemical composition was most influenced by an alkaline plume and least by the asthenosphere. In terms of the plume magmatism theory, alkalinity and the content of foidaphile elements depend on the interaction between the alkaline plume and the asthenosphere. On the whole, lateral and time series of alkaline associations give insight into the roots, initial composition, and chemical differentiation of alkaline plumes.
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