Nyerereite from calcite carbonatite at the Kerimasi Volcano,Northern Tanzania

The extinct Quaternary Kerimasi volcano located in the southern part of the Gregory Rift, northern Tanzania, contains both intrusive and extrusive calciocarbonatites. One carbonate mineral with a high content of Na and Ca has been found in a sample of volcanic carbonatite, which is probably a cumulate rock. On the basis of Raman spectroscopy and SEM/EDS, this mineral was identified as nyerereite, ideally Na₂Ca(CO₃)₂. It occurs as solid inclusions up to 300 × 200 μm in size in magnetite and contains (wt. %) 25.4–27.4 Na₂O, 26.0–26.8 CaO, 1.6–1.9 K₂O, 0.6–1.8 FeO, 0.3–0.6 SrO, <0.4 BaO, 1.4–2.3 SO₃, and 0.6–0.9 P₂O₅. The average mineral formula is (Na_1.84K_0.08)_Σ1.92(Ca_1.00Fe_0.03Sr_0.01)_Σ1.04[(CO₃)_1.91(SO₄)_0.05(PO₄)_0.02]_Σ1.98. A few inclusions in magnetite also contain calcite, which is considered here to be a late-stage, subsolidus mineral. The occurrence of nyerereite in carbonatite supports Hay’s (1983) idea that some of the extrusive carbonatites at the Kerimasi volcano were originally alkaline rich and contained both calcite and nyerereite as primary minerals.     

Petrology and genesis of natrocarbonatite

Microprobe analyses of phenocrysts and groundmass, and crystal-size distributions of phenocrysts of pahoehoe natrocarbonatite lavas of the 1963 eruption of Oldoinyo Lengai have been determined. Nyerereite phenocrysts are homogeneous, with average composition Nc₄₁Kc₉Cc₅₀ (neglecting F, Cl, P₂O₅, and SO₃) where Nc=Na₂CO₃, Kc=K₂CO₃, and Cc= (Ca,Sr)CO₃. Gregoryite phenocrysts have turbid, pale brown, oscillatorily zoned cores (average composition Nc₇₇Kc₅Cc₁₈) with 0–30% oriented inclusions of exsolved nyerereite. Overgrowths on gregoryites (30 m wide) are relatively sodic (Nc₈₁Kc₄Cc₁₅) and are free of inclusions. Cores and rims are rich in SO₃ (4%) and P₂O₅ (2%). Blebs of pyrite-alabandite mixtures (100 m) occur in the groundmass. The groundmass has the simplified composition Nc₆₅Kc₁₅Cc₂₀, less calcic than the composition of the 1-kbar nyerereite+gregoryite +liquid cotectic in the ternary system Nc-Kc-Cc. Groundmass quench growth of alkali halides + carbonate was followed by slower growth of coarse-grained and irregular gregoryite +KCl+BaCO₃. Crystal size distributions of gregoryite and nyerereite in one sample are linear, implying little loss or gain of phenocrysts by crystal settling. AverageG is 0.15 mm, compared toG=0.03 mm for combeite phenocrysts from consanguineous nephelinite. Assuming an equal residence time () for both lavas, the apparent crystal growth rate (G) in carbonate melt is 5 times greater than in peralkaline undersaturated silicate melt. Data from experiments with natrocarbonatite and related synthetic systems indicate that Na–K–Ca carbonatite magmas which crystallize calcite cannot fractionate to nyerereite+gregoryite +liquid assemblages. Natrocarbonatites plot in the liquidus field of nyerereite, and minor fractionation of nyerereite to produce the erupted lavas is indicated. The term natrocarbonatite has been inappropriately applied to other eruptive rocks with calcite phenocrysts, and the only known occurrence of gregoryite-bearing natrocarbonatite is Oldoinyo Lengai. Natrocarbonatite probably originates by liquid immiscibility from strongly peralkaline nephelinites, which have also been erupted at Oldoinyo Lengai.     

Altered rocks of the Onguren carbonatite complex in the Western Tansbaikal Region: Geochemistry and composition of accessory minerals

The paper discusses the mineralogy and geochemistry of altered rocks associated with calcite and dolomite–ankerite carbonatites of the Onguren dyke–vein complex in the Western Transbaikal Region. The alteration processes in the Early Proterozoic metamorphic complex and synmetamorphic granite hosting carbonatite are areal microclinization and riebeckitization; carbonates, phlogopite, apatite, and aegirine occur in the near-contact zones of the dolomite–ankerite carbonatite veins; and silicification is displayed within separated zones adjacent to the veins. In aluminosilicate rocks, microclinization was accompanied by an increasing content of K, Fe³⁺, Ti, Nb (up to 460 ppm), Th, Cu, and REE; Na, Ti, Fe³⁺, Mg, Nb (up to 1500 ppm), Zr (up to 2800 ppm), Ta, Th, Hf, and REE accumulated in the inner zone of the riebeckitization column. High contents of Ln _Ce (up to 11200 ppm), U (23 ppm), Sr (up to 7000 ppm), Li (up to 400 ppm), Zn (up to 600 ppm), and Th (up to 700 ppm) are typical of apatite–phlogopite–riebeckite altered rock; silicified rock contains up to (ppm): 2000 Th, 20 U, 13000 Ln _Ce, and 5000 Ва. Ilmenite and later rutile are the major Nb carriers in alkali altered rocks. These minerals contain up to 2 and 7 wt % Nb₂O₅, respectively. In addition, ferrocolumbite and aeschynite-(Ce) occur in microcline and riebeckite altered rocks. Fluorapatite containing up to 2.7 wt % (Ln _Ce)₂O₃, monazite-(Ce), cerite-(Ce), ferriallanite-(Ce), and aeschynite-(Ce) are the REE carriers in riebeckite altered rock. Bastnäsite-(Ce), rhabdophane-group minerals, and xenotime-(Y) are typical of silicified rock. Thorite, monazite-(Ce), and rhabdophane-group minerals are the Th carriers.     

Primary, agpaitic and deuteric stages in the evolution of accessory Sr, REE, Ba and Nb-mineralization in nepheline-syenite pegmatites at Pegmatite Peak, Bearpaw Mts, Montana

Summary The pegmatites at Pegmatite Peak (Bearpaw Mts., Montana) crystallized from an evolved fraction of nepheline-syenitic melt enriched in Sr, Ba, light REE and Nb. These rocks are composed essentially of microcline (up to 1.1 wt.% Na₂O and 1.0 wt.% BaO), altered nepheline (replaced by analcime, zeolites, muscovite and gibbsite), and prismatic aegirine set in an aggregate of fibrous and radial aegirine. The early accessory assemblage includes Mg-Fe mica, rutile, zircon, titaniferous magnetite and thorite. Precipitation of these phases was followed by crystallization of a plethora of rare minerals enriched in Sr, Ba, light REE and Nb. Three major stages are distinguished in the evolution of this mineralization: primary, agpaitic and deuteric. Primary repositories for Sr, REE and Nb included betafite, loparite-(Ce), crichtonite and ilmenite-group minerals. Betafite (Ta-poor, REE- and Th-rich) is present in very minor amounts and did not contribute significantly to the sequestration of incompatible elements from the nepheline-syenite melt. Loparite-(Ce) evolved predominantly by depletion in Sr and Ca and enrichment in Nb, Na and REE, i.e. from strontian niobian loparite (up to 22.0 wt.% SrO) to niobian loparite (up to 17.6 wt.% Nb₂O₅). Crichtonite contains minor Na, Ca and K, lacks detectable Ba and REE, and is unusually enriched in Mn (7.0–13.6 wt.% MnO). The ilmenite-group minerals evolved from manganoan ilmenite to ferroan pyrophanite, and have relatively low Nb contents ( 0.9 wt.% Nb₂O₅). During the agpaitic stage, the major repositories for incompatible elements were silicates, including lamprophyllite, titanite and chevkinite-group minerals. Lamprophyllite is generally poor in Ba, and contains relatively minor Ca and K; only few small crystals exhibit rims of barytolamprophyllite with up to 26.3 wt.% BaO. Titanite is devoid of Al and depleted in Fe, but significantly enriched in Nb, Sr, REE and Na: up to 6.4, 4.5, 4.4. and 2.9 wt.% oxides, respectively. The chemical complexity of titanite suggests involvement of several substitution mechanisms: Ca²⁺+Ti⁴⁺Na¹⁺+Nb⁵⁺, Ca² Sr²⁺, 2Ca²⁺Na¹⁺+REE³⁺, and Ca t++OZ-~--Nal+ + (OH)^1–. Chevkinite group minerals evolved from Sr-rich (strontiochevkinite) to REE-rich compositions [chevkinite-(Ce)]. Strontiochevkinite from Pegmatite Peak is compositionally similar to the type material from Sarambi, and has high ZrO₂ (up to 7.8 wt.%) and low FeOT ( 2.5 wt.%) contents. During the final stages of formation of the pegmatites, a deuteric F-bearing fluid enriched in Sr and REE precipitated carbonates and minor phosphates confined to fractures and cavities in the rock. In this youngest assemblage of minerals, ancylite-(Ce) is the most common Sr-REE host. Some discrete crystals of ancylite show significant enrichment in Th (up to 6.0 wt.% ThO₂). Ancylite-(Ce) and bastnaesite associated with metaloparite and TiO₂ (anatase?) comprise a replacement assemblage after primary loparite. The typical replacement pattern includes a loparite core with locally developed metaloparite, surrounded by a bastnaesite-anatase intermediate zone and an ancylite rim. Fluorapatite is rare, and has very high Sr, Na and REE contents, up to 21.4, 2.6 and 12.9 wt.% oxides, respectively. Compositionally, this mineral corresponds to the solid solution series between fluorapatite and belovite-(Ce). At this stage, hollandite-group minerals became a minor host for Ba; they demonstrate the evolutionary trend from priderite (5.2 wt. % K₂O, 7.4 wt. % BaO) to Ba-Fe hollandite (19.2–21.4 wt. % BaO). Thus, the evolution of Sr, REE, Ba and Nb mineralization was a complex, multi-stage process, and involved primary crystallization, re-equilibration phenomena and late-stage deuteric alteration.

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Die primäre, agpaitische und deuterische Hauptphase in der Entwicklung der akzessorischen Sr, REE, Ba und Nb-Mineralisation in den nephelinsyenitischen Pegmatiten von Pegmatite Peak, Bearpaw Mts., Montana

Zusammenfassung Die Pegmatite von Pegmatite Peak (Bearpaw Mts., Montana) sind aus dem Restdifferentiat einer nephelinsyenitischen Schmelze, die an Sr, Ba, leichten SEE und Nb angereichert war, auskristallisiert. Diese Gesteine bestehen hauptsächlich aus Mikroklin (max. 1.1 Gew.% Na₂O und max. 1.0 Gew.% BaO), alteriertem Nephelin (verdrängt durch Analcim, Zeolithe, Muscovit und Gibbsit) und prismatischem Agirin, welcher von einem Aggregat aus fasrigem und strahligem Ägirin umgeben ist. Als frühe akzessorische Mineralien sind Mg-Fe Glimmer, Rutil, Zirkon, titanführender Magnetit und Thorit auskristallisiert. Anschließend bildete sich eine Vielzahl seltener, Sr-, Ba, leichter SEE- und Nb-reicher Mineralien aus. In den Proben von Pegmatite Peak sind drei Hauptphasen in der Entwicklung der akzessorischen Sr-, Ba-, SEE- und Nb-Mineralisation zu unterscheiden: eine primäre, eine agpaitische und eine deuterische. Primär wurden Sr, SEE und Nb in Betafit, Loparit-(Ce), Crichtonit und Mineralien der Ilmenitgruppe eingebaut. Betafit (Ta-arm, SEE- und Th-reich) ist ein sehr seltenes Mineral in den Pegmatiten, und hat die inkompatiblen Elemente nur unbedeutend konzentriert. Loparit-(Ce) entsteht im wesentlichen durch den Austausch von Sr und Ca durch Nb, Na und SEE; d.h. durch Umwandlung von strontium- und niobhältigem Loparit ( 22.0 Gew.% SrO) zu niobhältigem Loparit ( 17.6 Gew.% Nb₂O₅). Crichtonit enthält eine geringe Menge Na, Ca und K, ist ohne feststellbare SEE und Ba und ist gewönlich Mn-reich (7.0-13.6 Gew.% MnO). Mineralien der Ilmenitgruppe entwickeln sich von manganfiihrendem Ilmenit hin zu eisenführendem Pyrophanit und haben relativ niedrige Nb-Gehalte ( 0.9 Gew.% Nb₂O₅). Während der agpaitischen Phase waren Silikate wie Lamprophyllit, Titanit und Mineralien der Tscheffkinitgruppe die wichtigsten Träger von inkompatiblen Elementen. Lamprophyllit ist generell Ba-arm und ist durch relativ niedrige Ca- und K-Gehalte charakterisiert. Nur wenige kleine Kristalle zeigen barytolamprophyllitische Ränder (< 26.3 Gew.% BaO). Fe ist im Titanit (Al-frei) abgereichert während Nb, Sr, SEE und Na (jeweils max. 6.4, 4.5, 4.4 und 2.9 Gew.% Oxid) angereichert wurden. Die chemische Zusammensetzung des Titanits kann durch mehrere Substituierungen erklärt werden: Ca l++Ti4+~Nal+-I-Nbs+, Ca²⁺ Sr²⁺, 2Ca²⁺ Na¹⁺+REE³⁺, und Ca²⁺ +O2^– Na¹⁺ +(OH)^1–. Mineralien der Tscheffkinitgruppe entwickeln sich aus Sr-reichen (Strontiotscheffkinit) hin zu SEE-reichen Gliedern [Tscheffkinit-(Ce)]. Strontiotscheffkinit von Pegmatite Peak mit hohem ZrO₂-(< 7.8 Gew.%) und niedrigem FeOT-Gehalt (< 2.5 Gew.%) hat eine ähnliche Zusammensetzung wie der Holotyp von Sarambi. Während der letzten Phasen der Bildung der Pegmatite brachte ein deuterisches, F-haltiges, Sr- und SEE-reiches Fluid Karbonate und in geringer Mengen Phosphate in Spalten und Hohlräumen im Gestein zur Ausfällung. Ankylit-(Ce) ist das häufigste Sr- und SEE-führende Mineral dieser jüngsten Mineralassoziation. Manche einzelne Ankylitkristalle zeigen eine bedeutende Anreicherung von Th (< 6.0 Gew.% ThO₂). Ankylit, Bastnäsit, Metaloparit und TiO₂ (Anatas?) ersetzten den ursprünglichen Loparit. Typische Verdrängungen zeigen sich als Körner mit loparitischen Kernen, welche örtlich mit Metaloparit verwachsen sind, weiters einer Bastnäsit-Anatas Zwischenzone und einem ankylitischen Rand. Fluorapatit ist hier ein seltenes Mineral und hat sehr hohe Sr-, Na- und SEE-Gehalte (jeweils 21.4, 2.6 und 12.9 Gew.% Oxid). Von der chemischen Zusammensetzung aus gesehen gehört dieses Mineral zur Fluoapatit-Belovit-(Ce)-Mischkristallreiche. Während der deuterischen Phase dienten die Mineralien der Hollanditgruppe untergeordnet als Träger für Ba; sie legen die Entwicklung von Priderit (5.2 Gew.% K20, 7.4 Gew.% BaO) zu Ba-Fe-Hollandit (19.2–21.4 Gew.% BaO). Somit ist die Entwicklung der Sr-, SEE-, Ba- und Nb-Mineralisation ein komplexer mehrphasiger Prozeß und umfaßt die primäre Kristallisation, Reäquilibrierungsphänomene und eine späte deuterische Alteration.

     

Rare earth elements in phoscorites and carbonatites of the Devonian Kola Alkaline Province,Russia: Examples from Kovdor,Khibina, Vuoriyarvi and Turiy Mys complexes

The Devonian (ca. 385–360 Ma) Kola Alkaline Province includes 22 plutonic ultrabasic–alkaline complexes, some of which also contain carbonatites and rarely phoscorites. The latter are composite silicate–oxide–phosphate–carbonate rocks, occurring in close space-time genetic relations with various carbonatites. Several carbonatites types are recognized at Kola, including abundant calcite carbonatites (early- and late-stage), with subordinate amounts of late-stage dolomite carbonatites, and rarely magnesite, siderite and rhodochrosite carbonatites. In phoscorites and early-stage carbonatites the rare earth elements (REE) are distributed among the major minerals including calcite (up to 490 ppm), apatite (up to 4400 ppm in Kovdor and 3.5 wt.% REE₂O₃ in Khibina), and dolomite (up to 77 ppm), as well as accessory pyrochlore (up to 9.1 wt.% REE₂O₃) and zirconolite (up to 17.8 wt.% REE₂O₃). Late-stage carbonatites, at some localities, are strongly enriched in REE (up to 5.2 wt.% REE₂O₃ in Khibina) and the REE are major components in diverse major and minor minerals such as burbankite, carbocernaite, Ca- and Ba-fluocarbonates, ancylite and others. The rare earth minerals form two distinct mineral assemblages: primary (crystallized from a melt or carbohydrothermal fluid) and secondary (formed during metasomatic replacement). Stable (C–O) and radiogenic (Sr–Nd) isotopes data indicate that the REE minerals and their host calcite and/or dolomite have crystallized from a melt derived from the same mantle source and are co-genetic.     

Silicate and salt melts in the genesis of the industrial’noe tin deposit: Evidence from inclusions in minerals

The data obtained on melt and fluid inclusions in minerals of granites, metasomatic rocks, and veins with tin ore mineralization at the Industrial’noe deposit in the southern part of the Omsukchan trough, northeastern Russia, indicate that the melt from which the quartz of the granites crystallized contained globules of salt melts. Silicate melt inclusions were used to determine the principal parameters of the magmatic melts that formed the granites, which had temperatures at 760–1020°C, were under pressures of 0.3–3.6 kbar, and had densities of 2.11–2.60 g/cm³ and water concentrations of 1.7–7.0 wt %. The results obtained on the fluid inclusions testify that the parameters of the mineral-forming fluids broadly varied and corresponded to temperatures at 920–275°C, pressures 0.1–3.1 kbar, densities of 0.70–1.90 g/cm³, and salinities of 4.0–75.0 wt % equiv. NaCl. Electron microprobe analyses of the glasses of twelve homogenized inclusions show concentrations of major components typical of an acid magmatic melt (wt %, average): 73.2% SiO₂, 15.3% Al₂O₃, 1.3% FeO, 0.6% CaO, 3.1% Na₂O, and 4.5% K₂O at elevated concentrations of Cl (up to 0.51 wt %, average 0.31 wt %). The concentrations and distribution of some elements (Cl, K, Ca, Mn, Fe, Cu, Zn, Pb, As, Br, Rb, Sr, and Sn) in polyphase salt globules in quartz from both the granites and a mineralized miarolitic cavity in granite were assayed by micro-PIXE (proton-induced X-ray emission). Analyses of eight salt globules in quartz from the granites point to high concentrations (average, wt %) of Cl (27.5), Fe (9.7), Cu (7.2), Mn (1.1), Zn (0.66), Pb (0.37) and (average, ppm) As (2020), Rb (1850), Sr (1090), and Br (990). The salt globules in the miarolitic quartz are rich in (average of 29 globules, wt %) Cl (25.0), Fe (5.4), Mn (1.0), Zn (0.50), Pb (0.24) and (ppm) Rb (810), Sn (540), and Br (470). The synthesis of all data obtained on melt and fluid inclusions in minerals from the Industrial’noe deposit suggest that the genesis of the tin ore mineralization was related to the crystallization of acid magmatic melts. Original Russian Text@ V.B. Naumov, V.S. Kamenetsky, 2006, published in Geokhimiya, 2006, No. 12, pp. 1279–1289.     

Evolution of the trachydacite and pantellerite magmas of the bimodal volcanic association of Dzarta-Khuduk, Central Mongolia: Investigation of inclusions in minerals

Using various methods of melt inclusion investigation, including electron and ion microprobe techniques, we estimated the composition, evolution, and formation conditions of melts producing the trachydacites and pantellerites of the Late Paleozoic bimodal volcanic association of Dzarta-Khuduk, Central Mongolia. Primary crystalline and melt inclusions were detected in anorthoclase from trachydacites and quartz from pantellerites and pantelleritic tuffs. Among the crystalline inclusions, we identified hedenbergite, fluorapatite, and pyrrhotite in the trachydacites and F-arfvedsonite, fluorite, ilmenite, and the rare REE diorthosilicate chevkinite in the pantellerites. Melt inclusions in anorthoclase from the trachydacites are composed of glass, a gas phase, and daughter minerals (F-arfvedsonite, fluorite, villiaumite, and anorthoclase rim on the inclusion wall). Melt inclusions in quartz from the pantellerites are composed of glass, a gas phase, and a fine-grained salt aggregate consisting of Li, Na, and Ca fluorides (griceite, villiaumite, and fluorite). Melt inclusions in quartz crystalloclasts from the pantelleritic tuffs are composed of homogeneous silicate glasses. The phenocrysts of the trachydacites and pantellerites crystallized at temperatures of 1060–1000°C. During thermometric experiments with quartz-hosted melt inclusions from the pantellerites, the formation of immiscible silicate and salt (fluoride) melts was observed at a temperature of 800°C. Homogeneous melt inclusions in anorthoclase from the trachydacites have both trachydacite and rhyolite compositions (wt %): 68–70 SiO₂, 12–13 Al₂O₃, 0.34–0.74 TiO₂, 5–7 FeO, 0.4–0.9 CaO, and 9–12 Na₂O + K₂O. The agpaitic index ranges from 0.92 to 1.24. The glasses of homogenized melt inclusions in quartz from the pantellerites and pantelleritic tuffs have rhyolitic compositions. Compared with the homogeneous glasses trapped in anorthoclase of the trachydacites, quartz-hosted inclusions from the pantellerites show higher SiO₂ (72–78 wt %) and lower Al₂O₃ contents (7.8–10.0 wt %). They also contain 0.14–0.26 wt % TiO₂, 2.5–4.9 wt % FeO, 9–11 wt % Na₂O + K₂O, and 0.9–0.15 wt % CaO and show an agpaitic index of 1.2–2.05. Homogeneous melt inclusions in quartz from the pantelleritic tuffs contain 69–72 wt % SiO₂. The contents of other major components, including TiO₂, Al₂O₃, FeO, and CaO, are close to those in the homogeneous glasses of quartzhosted melt inclusions in the pantellerites. The contents of Na₂O + K₂O are 4–10 wt %, and the agpaitic index is 1.0–1.6. The glasses of melt inclusions from each rock group show distinctive volatile compositions. The H₂O content is up to 0.08 wt % in anorthoclase of the trachydacites, 0.4–1.4 wt % in quartz of the pantellerites, and up to 5 wt % in quartz of the pantelleritic tuffs. The content of F in the glasses of melt inclusions in the phenocrysts of the trachydacites is no higher than 0.67 wt %, and up to 1.4–2.8 wt % in quartz from the pantellerites. The Cl content is up to 0.2 wt % in the glasses of melt inclusions in the minerals of the trachydacites and up to 0.5 wt % in the glasses of quartz-hosted melt inclusions from the pantellerites. The investigation of trace elements in the homogenized glasses of melt inclusions in minerals showed that the trachydacites and pantellerites were formed from strongly evolved rare-metal alkaline silicate melts with high contents of Li, Zr, Rb, Y, Hf, Th, U, and REE. The analysis of the composition of homogeneous melt inclusions in the minerals of the above rocks allowed us to distinguish magmatic processes resulting in the enrichment of these rocks in trace and rare earth elements. The most important processes are the crystallization differentiation and immiscible separation of silicate and fluoride salt melts. It was also shown that all the melts studied evolved in spatially separated magma chambers. This caused the differences in the character of melt evolution between the trachydacites and pantellerites. During the final stages of differentiation, when the magmatic system was saturated with respect to ore elements, Na-Ca fluoride melts were separated and extracted considerable amounts of Li.     

Major Element Compositions and Rare—earth Element Abundaces of Cenozoic Basalts in Eastern China：Implications for a Pressure Control over LREE／HREE Fractionation in Continental Basalt

Major element compositions and rare-earth element (REE) and transition element(Ni,Cr and V) abundances have been determined on 44 basalt samples from eastern China.These basalts have SiO2 contents ranging from 38.63 to 55.24(wt.%),and Na2O K2O from 3.1 to 9.4(wt.%).Ni and Cr abundances are largely variable,respectively falling in ranges 60-605 and 78-1150 ppm.REE abundances,especially light rare-earth elements(LREE), are highly variable.La/Sm and La/Yb ratios vary 2.8 to 7.6 and 1.8 to 8.1. Although the segregation mainly of olivine and clinopyroxene is requested to account for the vari-able and low MgO,CaO/Al2O3,Cr and Ni characteristic of these basalts studied here,the differ-ences in REE composition of the basalts are still related mainly to the partial melting process.Obvious varations in REE abundances could be principally attributed to the partial melting process.Obvious variations in REE abundances could be principally attributed to the partial melting processes that took place at different depths,in spite of some variations caused by the fractional crystallization processes.REE abundances and La/Sm and La/Yb ratios systematically decrease with increasing SiO2,which probably indicated that the basaltic magma derived from a deeper level has higher LREE and LREE/HREE ratios than that from a shallower level.As viewed from the fact that the D^Yb/D^La ratios of clinopyroxenes in the basaltic system increase with increasing pressure,the increase of LREE/HUEE ratios with increasing melting depth can be interpreted as the pressure dependence of bulk D^HREE/D^LREE ratios of silicate minerals,in addition to the pressure control over the melting degree.     

Ore geochemistry,zircon mineralogy,and genesis of the Sakharjok Y-Zr deposit,Kola Peninsula,Russia

The Sakharjok Y-Zr deposit in Kola Peninsula is related to the fissure alkaline intrusion of the same name. The intrusion ∼7 km in extent and 4–5 km² in area of its exposed part is composed of Neoarchean (2.68–2.61 Ma) alkali and nepheline syenites, which cut through the Archean alkali granite and gneissic granodiorite. Mineralization is localized in the nepheline syenite body as linear zones 200–1350 m in extent and 3–30 m in thickness, which strike conformably to primary magmatic banding and trachytoid texture of nepheline syenite. The ore is similar to the host rocks in petrography and chemistry and only differs from them in enrichment in zircon, britholite-(Y), and pyrochlore. Judging from geochemical attributes (high HSFE and some incompatible element contents (1000–5000 ppm Zr, 200–600 ppm Nb, 100–500 ppm Y, 0.1–0.3 wt % REE, 400–900 ppm Rb), REE pattern, Th/U, Y/Nb, and Yb/Ta ratios), nepheline syenite was derived from an enriched mantle source similar to that of contemporary OIB and was formed as an evolved product of long-term fractional crystallization of primary alkali basaltic melt. The ore concentrations are caused by unique composition of nepheline syenite magma (high Zr, Y, REE, Nb contents), which underwent subsequent intrachamber fractionation. Mineralogical features of zircon-the main ore mineral—demonstrate its long multistage crystallization. The inner zones of prismatic crystals with high ZrO₂/HfO₂ ratio (90, on average) grew during early magmatic stage at a temperature of 900–850°C. The inner zones of dipyramidal crystals with average ZrO₂/HfO₂ = 63 formed during late magmatic stage at a temperature of ∼500°C. The zircon pertaining to the postmagmatic hydrothermal stage is distinguished by the lowest ZrO₂/HfO₂ ratio (29, on average), porous fabric, abundant inclusions, and crystallization temperature below 500°C. The progressive decrease in ZrO₂/HfO₂ ratio was caused by evolution of melt and postmagmatic solution. The metamorphic zircon rims relics of earlier crystals and occurs as individual rhythmically zoned grains with an averaged ZrO₂/HfO₂ ratio (45, on average) similar to that of the bulk ore composition. The metamorphic zircon is depleted in uranium in comparison with magmatic zircon, owing to selective removal of U by aqueous metamorphic solutions. Zircon from the Sakharjok deposit is characterized by low concentrations of detrimental impurities, in particular, contains only 10–90 ppm U and 10–80 ppm Th, and thus can be used in various fields of application.     

Abundances of F,Cl S and P in Volcanic Magmas and Their Evolution,Wudalianchi

F, Cl, S and P were determined, using electron microprobe, in magmatic inclusions trapped within minerals and glass mesostasis from Wudalianchi volcanic rocks. The initial volcanic magma from Wudalianchi corresponds to the basanitic magma crystallized near the surface ( pressure < 91 Mpa ). The potential H₂O content of this magma is in the range 2 — 4 wt. %. The initial composition of volcanic magmas varies regularly from early to late volcanic events. From the Middle Pleistocene to the recent eruptions (1719 – 1721 yr.), the basicity of volcanic magma tends to increase, as reflected by an increase in MgO and CaO contents and by a progressive decrease in SiO₂ and K₂O contents. Meanwhile. from early (Q₂ ) to late (Q₃) episodic eruptions of the Middle Pleistocene, the initial concentrations of chlorine in volcanic magmas range from 1430 – 1930 ppm to 1700 ppm and decrease to 700 — 970 ppm for the first episodic eruption during the Holocene (Q ₄ ¹ ). The chlorine concentrations of volcanic magmas of recent eruption (Q ₄ ² ) are increased again to 2600 – 2870 ppm. A parallel evolution trend for phosphorus and chlorine concentrations in magmas has been certified: 1500 – 5970 ppm (Q₂)→ 3500 – 4210 ppm (Q₃)→ 1100– 3500 ppm (Q ₄ ¹ )→ 6800– 7900 ppm (Q ₄ ² ). The fluorine contents of volcanic magmas, from early to late volcanic events, show the same trend: 770 – 2470 ppm → 200–700 ppm → 700 – 800 ppm. During the crystallization-evolution of volcanic magmas, fluorine and phosphorus tend to be enriched in residual magmas as a result of crystal-melt differentiation. for example. the fluorine contents reach 5000– 6800 ppm and the phosphorus contents, 2.93wt.% in residual magmas. An appreciable amount of chlorine may be lost from water rich volcanic magmas prior to eruption as a result of degassing. Apparently, water serves as a gas carrier for the chlorine. The chlorine contents of residual magmas may decrease to 100 – 300 ppm. The volcanic magmas from Wudalianchi are poor in sulfur, normally ranging from 200 to 400ppm. On account of the behavior of sulfur in magmas and the strontium and oxygen isotopic analyses ((⁸⁷Sr /⁸⁶Sr)_i=0.70503– 0.70589; δ¹⁸O = + 5.50 – + 6.89 ‰ ), it can be considered that the basanitic magmas in the Wudalianchi volcanic area came from the upper mantle and have not yet been contaminated probably by continental crust materials.     

Unique accessory Ti-Ba-P mineralization in the Kvalöya ultrapotassic dike,Northern Norway

Unusual ultrapotassic dikes were recently found on the Kvalöya Island in Northern Norway. The dikes crosscutting granites 1.8 Ga in age are 0.1–1.0 m thick and consist of phlogopite phenocrysts in a fine-grained groundmass of K-magnesioarfvedsonite, orthoclase, apatite, and secondary chlorite. According to the composition of the rock-forming minerals (4.5–6.0 wt % K₂O and 0.7–3.5 wt % TiO₂ in magnesioarfved-sonite, 1.6–3.6 wt % FeO in orthoclase, 9.2–10.7 wt % Al₂O₃ and 2.1–2.6 wt % TiO₂ in phlogopite) and its bulk chemical composition (K/Na = 2.3–2.9, K/Al = 1.0–1.2, (Na + K)/Al = 1.4–1.7, Mg# V = 65–73, (La/Yb) _n = 100–140, 3.2–4.0 wt % TiO₂, 0.55–1.47 wt % BaO, 2.5–3.0 wt % P₂O₅, 2650–3000 ppm Zr, 900–1260 ppm REE total, 2300–2500 ppm Sr), the rock corresponds to lamproite of the transitional type. The unique chemical composition of the rock resulted in uncommon Ti-Ba-P accessory mineralization, including baotite Ba₄(Ti,Nb)₈Si₄O₂₈Cl (up to 5 vol %), Sr-apatite (5–7 vol %), and previously unknown Na-Mg-Ba phosphate. Baotite forms anhedral elongated and isometric grains 10–500 μm in size. It is characterized by low Nb (0.03–0.05 f.c.); admixtures of K (0.04–0.12 f.c.) and Sr (0.04–0.07) replacing Ba and Fe (0.01–0.03 f.c.); and Al (0.03–0.04 f.c.) substituting Ti. Euhedral elongated zonal apatite crystals are extremely enriched in SrO (8–12 wt %) and REE₂O₃ + Y₂O₃ (6–9 wt %) in the marginal zone. Na-Mg-Ba phosphate occurs as prismatic grains 10–100 μm in size. The atomic ratio of its major cations Na: Mg: Ba: P ～ 2: 1: 1: 2 corresponds to the conventional formula Na₂MgBa(PO4)2; the mineral contains Sr, Mn, Fe, Ca, Si, and Al admixtures.     

A new finding of Cu-Au alloy in association with rodingite minerals in the Kaa-Khem ophiolitic belt,Tuva

A new Cu-Au alloy occurrence is located at the southeastern flank of the Malye Kopty massif of ultramafic rocks in the Vendian-Early Cambrian Kaa-Khem ophiolitic belt. Lithic clasts with Cu-Au alloy segregations (up to 15 mm in size) intergrown with other minerals were found in alluvium of the Kara-Oss Creek valley, which extends along the fault zone crosscutting ultramafic rocks. Cu-Au alloy occupies the main volume of clasts and fills the network of veinlets in grained aggregates consisting of andradite (2–18% grossular component) and diopside (X _Fe = 0.01–0.05). Cu-Au alloy contains small ingrowths of andradite (up to 43% grossular component), diopside (X _Fe = 0.14–0.19), chlorite (penninite), chalcocite that contains up to 1.5 wt % Au, Cu-bearing greenockite (6.07–13.67 wt % Cu, 0.48–1.56 wt % Zn, and 0.76–1.06 wt % Au), and magnetite. The chemical composition of Cu-Au alloy is nonuniform. The central parts of large Cu-Au alloy segregations consist of Ag-bearing tetraauricupride (AuCu) blocks (3.2–6.4 wt % Ag). They contain veinlet-shaped AuCu zones with 13.3–14.5 wt % Ag. The AuCu blocks are cemented by late Cu-Au alloy, whose composition is close to auricupride (AuCu₃). Taking into account the limits of component miscibility in the Au-Ag-Cu system, the temperature of the Cu-Au alloy formation was estimated at 350–600°C. This temperature corresponds to the formation conditions of garnet-pyroxene rodingite mineral assemblage (Plyusnina et al., 1993). The studied Cu-Au alloy samples from the Malye Kopty massif are very similar to Cu-Au alloy minerals hosted in the Alpine-type ultramafic rocks of the Karabash massif in the southern Urals. This similarity is confirmed by identical chemical compositions of pyroxene, garnet, and chlorite, and similar PT conditions of their formation. The data show that primary ore mineralization of gold-rodingite type occurs in the Kaa-Khem ophiolitic belt.     

Pingüino In-bearing polymetallic vein deposit,Deseado Massif,Patagonia, Argentina: characteristics of mineralization and ore-forming fluids

The Pingüino deposit, located in the low sulfidation epithermal metallogenetical province of the Deseado Massif, Patagonia, Argentina, represents a distinct deposit type in the region. It evolved through two different mineralization events: an early In-bearing polymetallic event that introduced In, Zn, Pb, Ag, Cd, Au, As, Cu, Sn, W and Bi represented by complex sulfide mineralogy, and a late Ag–Au quartz-rich vein type that crosscut and overprints the early polymetallic mineralization. The indium-bearing polymetallic mineralization developed in three stages: an early Cu–Au–In–As–Sn–W–Bi stage (Ps₁), a Zn–Pb–Ag–In–Cd–Sb stage (Ps₂) and a late Zn–In–Cd (Ps₃). Indium concentrations in the polymetallic veins show a wide range (3.4 to 1,184 ppm In). The highest indium values (up to 1,184 ppm) relate to the Ps₂ mineralization stage, and are associated with Fe-rich sphalerites, although significant In enrichment (up to 159 ppm) is also present in the Ps₁ paragenesis associated with Sn-minerals (ferrokesterite and cassiterite). The hydrothermal alteration associated with the polymetallic mineralization is characterized by advanced argillic alteration within the immediate vein zone, and sericitic alteration enveloping the vein zone. Fluid inclusion studies indicate homogenisation temperatures of 308.2–327°C for Ps₁ and 255–312.4°C for Ps₂, and low to moderate salinities (2 to 5 eq.wt.% NaCl and 4 to 9 eq.wt.% NaCl, respectively). δ³⁴S values of sulfide minerals (+0.76‰ to +3.61‰) indicate a possible magmatic source for the sulfur in the polymetallic mineralization while Pb isotope ratios for the sulfides and magmatic rocks (²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of 17.379 to 18.502; 15.588 to 15.730 and 38.234 to 38.756, respectively) are consistent with the possibility that the Pb reservoirs for both had the same crustal source. Spatial relationships, hydrothermal alteration styles, S and Pb isotopic data suggest a probable genetic relation between the polymetallic mineralization and dioritic intrusions that could have been the source of metals and hydrothermal fluids. Mineralization paragenesis, alteration mineralogy, geochemical signatures, fluid inclusion data and isotopic data, confirm that the In-bearing polymetallic mineralization from Pingüino deposit is a distinct type, in comparison with the well-known epithermal low sulfidation mineralization from the Deseado Massif.     

Geochemistry and Petrology of Emeishan Basalts and Subcontinental Mantle Evolution in Southwestern China

Three major volcanic rock sequences in the P2β formation(Emeishan basalts)were sampled dur-ing a comprehensive study of the Late Permian volcanics associated with the Panxi paleorift in southwestern China .Two of the three sections-Emei and Tangfang are composed of continental flood basalts(CFB) while the third-Ertan is an alkalic center.Multi-element chemical analyses indi-cate a predominance of low MgO transitional quartz tholeiites at Emei and Tangfang,whereas the Ertan suite ranges from high-MgO alkaline olivine basalts to rhombic porphyry trachytes and quartz-bearing aegerine-augite syenites.Consanguineity of the rocks from the three sections is sug-gested by consistently high TiO2 ,K2O,incompatible trace elements and uniformly fractionated REE patterns typical of alkalic compositions,but antypical of CFB.Sr isotope data for ten Emei basalt samples(^87Sr/^86Sr=0.7066-0.7082)which show no correla-tion with Rb/Sr ratios (0.02-0.12) and Nd isotopes for two of the samples(^143Nd/^144Nd=0.51171-0.51174)are interpreted as being related to the mantle evolution.The primary magmas re-sponsible for all the three sequences have been modeled in terms of a uniformly metasomatized man-tle source.Trace element models support the derivation of the Emei and Tangfang primary magmas from 10-15 percent partial melting of spinel lherzolite,followed by fractional crystallization of olivive and clinopyroxene.The primary alkaline olivine basalts at Ertan are generated by 7-10 percent par-tial melting of a chemically equivalent source in the garnet-peridodite stability region.The assumed mantle composition is characterixzed by Rb=3.8-5.5 ppm,Sr=62-83ppm,Ba=45-64 ppm,La=3.8-5.6ppm,and Yb=0.46-0.57ppm.The proposed mechanism of regional mantle enrichment requires metasomatic stabilization of phlogopite which becomes depleted later during par-tial melting.Such enrichment is consistent with the models proposed for alkalic systems in which a large mantle diaper acts as the agent for upward enrichment as well as uplift and extension of the crust.     

Mineralogy and petrography of a kimberlite-picrite dike in the northwestern Urik-Iya Graben,the eastern Sayan region

Mica kimberlite and alkali picrite were identified in the northwestern Urik-Iya Graben of the eastern Sayan region. Typomorphism of Cr-diopside and high-Cr (up to 55.22 wt % Cr₂O₃) spinel from kimberlite of the Bushkanai dike indicate that the melt was generated in the mantle, composed of spinel peridotite. The high content of Cr-spinel (45–55 wt % Cr₂O₃) microlites in the groundmass of kimberlite and small amounts of ulvospinel and titanomagnetite in the absence of perovskite testifies to the diamond potential of this kimberlite. Picroilmenite, manganoilmenite with an anomalously high MnO content (11.37–17.78 wt %), and barium titanate with (wt %) 62.21 TiO₂, 0.61 Cr₂O₃, 15.89 FeO, 4.05 MnO, 1.71 CaO, and 11.13 BaO close in composition to a new mineral species from the Murun pluton were identified in the groundmass for the first time. Kimberlite from the Bushkanai dike belongs to the Zolotitsa low-Ti geochemical type of kimberlites derived from the slightly enriched lithospheric mantle EM1. The distribution of trace elements, including REE, in picrite from the same dike corresponds to the slightly depleted asthenospheric mantle. Different mantle sources of kimberlite and picrite from the same dike indicate that these rocks are related to independent melts rather than to products of fractionation of a common parental alkaline ultramafic magma.     

Composition,sources, and genesis of granitoids in the Irtysh Complex,Eastern Kazakhstan

Intrusions of the Irtysh Complex are spatially restricted to the regional Irtysh Shear Zone (ISZ) and are hosted in blocks of high-grade metamorphic rocks (Kurchum, Predgornenskii, Sogra, and others) in the greenschist matrix of the ISZ. The massifs consist of contrasting rock series from gabbro to plagiogranite and granite at strongly subordinate amounts of diorite and the practical absence of rocks of intermediate composition (tonalite and granodiorite). The complex was produced in the Early Carboniferous, simultaneously with the onset of the origin of the ISZ itself. The granitoids composing the complex affiliate with diverse petrochemical series (from subaluminous plagiogranite of the andesite series to granite of the calc-alkaline series) and contain similar REE and HFSE concentrations [total REE = 103–163 ppm (La/Yb) _n = 3.59–5.44, Zr (200–273 ppm), Nb (7.6–10.6 ppm), Hf (6.1–7.6 ppm), and Ta (0.68–1.19 ppm)] but are different in concentrations in LILE [Rb (3–9 and 121–221 ppm), Sr (213–375 and 77–148 ppm), and Ba (67–140 and 240–369 ppm)] and isotopic composition of Nd (ɛ_Nd(T) from +5.3 in the plagiogranite to −1.2 in the granite) and O (δ¹⁸O from +9.4 in the plagiogranite to +14.5 in the granite). Data on the geochemistry and isotopic composition of metamorphic rocks of the Kurchum block and numerical geochemical simulations indicate that the granitoids were generated via the melting of a heterogeneous crustal source, which consisted of upper crustal metapelites and metabasites of the oceanic basement of the blocks of high-grade metamorphic rocks. The differences in the chemical and isotopic compositions of the granitoids were predetermined by the mixing of variable proportions of granitoid magmas derived from metapelite and metabasite sources.     

Mechanisms of formation of barium-rich phlogopite and strontium-rich apatite during the final stages of alkaline magma evolution

Carbonatite veinlets in fergusite from the Dunkeldyk potassium-rich basaltoid complex (southeastern Pamirs) are composed of clinopyroxene, phlogopite, and apatite phenocrysts embedded in a crystallized calcite-bearing groundmass. The examination of back-scattered electron images revealed areas of significantly different compositions in fluorapatite and fluorphlogopite. The content of BaO in the phlogopite ranges from 0.68 to 10.9 wt %. There are also variations in MgO and F contents. The maximum BaO content corresponds to high mole fractions of the Ba end member kinoshitalite (up to 0.24) in the phlogopite. The zoned fluorapatite phenocrysts are rich in SrO (0.77–25.4 wt %). An increase in SrO content is accompanied by an increase in Ce₂O₃, La₂O₃, and BaO and a distinct decrease in CaO. Most of the apatite grains are rimmed by elongated colorless crystals showing the maximum SrO contents. Based on the experimentally determined Ba and Sr partition coefficients between these minerals, silicate and carbonate melts, and fluid, a model was proposed for the enrichment of phases in these trace elements. It was shown that the mineral-forming media of the Ba-rich phlogopites was a residual melt enriched in volatiles (including F) and fluid-mobile elements. During that stage, the decomposition reactions of early Ba-bearing feldspars with subsequent incorporation of BaO in Ba-rich phlogopites played an important role. The mechanism of formation of Sr-rich apatites is fundamentally different: early apatite grains with moderate Sr contents recrystallized under the influence of Sr-rich fluids released during the late magmatic stage. Thus, despite their close association in a single rock, the Ba-bearing phlogopite and Sr-rich apatite were formed by significantly different mechanisms. Our previous investigations of melt and fluid inclusions in minerals from the rocks of the Dunkeldyk complex and the results obtained in this study allowed us to suggest that the barium, fluorite-carbonatite, and rare metal mineralization occurring in the region developed owing to the prolonged evolution of primary magmas, resulting in the formation of melt-solutions (brines) and hydrothermal systems.     

Gold potential of the volcanoplutonic zones in the southeastern Siberian Platform and physicochemical conditions of the deposit formation

Numerous gold deposits and occurrences were recognized in the regions of tectonomagmatic activation in the southeastern Siberian Platform. They are located in four metallogenic zones: the Ket-Kap (skarns, quartz veins, and stockworks; gold-bearing lodes in silicitolites; and argillisite-sericite metasomatites), Ulkan (clayey-micaceous metasomatites, quartz veins), Preddzhugdzhur (quartz veins, skarns, and sericite-hydromicaceous metasomatites), and Uda (sericite-hydromicaceous metasomatites). The skarn mineralization is of Meosozoic age, while the mineralization in the quartz veins, quartz-hydromicaceous metasomatites, and quartz-sulfide veins may have a Meosozoic, Paleozoic, or Late Paleozoic age. The highest temperatures were determined for the ore formation in the Preddzhugdzhur skarns (500–715 °C) and the hydrothermal-metasomatic rocks of the Ket-Kap zone (510–530 °C). The composition of gas-liquid inclusions in the minerals of these rocks is dominated by aqueous Na, K, and Ca chloride solutions with salinity up to 40 wt % NaCl equiv; fluid contains CO₂. Quartz veins and stockworks of the Ket-Kap zone were formed under high (up to 465°C) and moderate temperatures and salinity up to 32 wt % NaCl equiv. Sometimes, the minerals in these rocks contain inclusions of low-density CO₂. The gold-bearing veins of the Preddzhugdzhur zone formed at 225–230°C and salinity of 1–2 wt % NaCl equiv. The ore-bearing solutions in the gold-bearing veins of the Ulkan zone are characterized by a potassium-sodium-chlorine composition and salinity of 2–10 wt % NaCl equiv., and the temperature of their formation was 220–280 °C.     

Origin of the Maoduan Pb–Zn–Mo deposit, eastern Cathaysia Block, China: geological, geochronological, geochemical, and Sr–Nd–Pb–S isotopic constraints

The Maoduan Pb–Zn–Mo deposit is in hydrothermal veins with a pyrrhotite stage followed by a molybdenite and base metal stage. The Re–Os model ages of five molybdenite samples range from 138.6 ± 2.0 to 140.0 ± 1.9 Ma. Their isochron age is 137.7 ± 2.7 Ma. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating of the nearby exposed Linggen granite porphyry gave a ²⁰⁶Pb/²³⁸U age of 152.2 ± 2.2 Ma and the hidden Maoduan monzogranite yielded a mean of 140.0 ± 1.6 Ma. These results suggest that the intrusion of the Maoduan monzogranite and Pb–Zn–Mo mineralization are contemporaneous. δ ³⁴S values of sulfide minerals range from 3.4‰ to 4.8‰, similar to magmatic sulfur. Four sulfide samples have ²⁰⁶Pb/²⁰⁴Pb = 18.252–18.432, ²⁰⁷Pb/²⁰⁴Pb = 15.609–15.779, and ²⁰⁸Pb/²⁰⁴Pb = 38.640–39.431, similar to the age-corrected data of the Maoduan monzogranite. These isotope data support a genetic relationship between the Pb–Zn–Mo mineralization and the Maoduan monzogranite and probably indicate a common deep source. The Maoduan monzogranite has geochemical features similar to highly fractionated I-type granites, such as high SiO₂ (73.7–75.2 wt.%) and alkalis (K₂O + Na₂O = 7.8–8.9 wt.%) and low FeO_t (0.8–1.3 wt.%), MgO (~0.3 wt.%), P₂O₅ (~0.03 wt.%), and TiO₂ (~0.2 wt.%). The granitic rocks are enriched in Rb, Th, and U but depleted in Ba, Sr, Nb, Ta, P, and Ti. REE patterns are characterized by marked negative Eu anomalies (Eu/Eu^* = 0.2–0.4). The Maoduan monzogranite, having (⁸⁷Sr/⁸⁶Sr) _t  = 0.7169 to 0.7170 and ε_Nd(t) = −13.8 to −13.7, was probably derived from mixing of partial melts from enriched mantle and the Paleoproterozoic Badu group in an extensional tectonic setting.     

The geochemistry and petrogenesis of the lavas of the Vulsinian District,Roman province,Central Italy

The Vulsinian lavas are dominated by a suite of undersaturated leucite-bearing basic to intermediate compositions. The remaining lavas are mainly oversaturated and have shoshonitic affinities. One hundred and thirty-five samples have been analysed for major elements and most for 20 trace elements. Twenty-seven lavas have been analysed for REE. They are all perpotassic (for the undersaturated lavas: K₂O/Na₂O=2–8) and have very high LIL element concentrations, (e.g. Rb=400–800 ppm, Th=25–150 ppm, REE/REE_cho=c.200, (LREE/HREE)_cho=c.20) even in the most basic rocks.The undersaturated lavas appear to be interrelated by fractional crystallization of cpx±olivine (from 14 to 11 wt.% CaO), cpx+leu±plg±mica (from 11 to 8 wt.% CaO), cpx+leu+plg+apa+magnetite±mica (from 8 to 5 wt.% CaO), and additional sanidine (or hyalophane)±haüyne (from 5 to 3 wt.% CaO). The saturated lavas and the few slightly undersaturated shoshonite basalts are thought to be evolved from the undersaturated magma(s) by crustal contamination or mixing with silica-rich magmas. The parental Vulsinian magma having: Mg-value=c.73, Cr=300–700 ppm, Ni=100–125 ppm, Sc= 40–50 ppm, Fo_89–92, Di_77–97 approximates a primary, mantle-derived liquid. Enrichment in LIL elements (incl. REE) and LREE/HREE suggest a small degree of partial melting from fertile mantle; whereas the low concentrations of Na, Ti and P suggest larger degrees of partial melting. This indicates that either the primary magma or the parental mantle was metasomatized by a fluid, which previously equilibrated with subducted continental material. This model agrees with published high ¹⁸O, high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd.