Nephelinite Research Articles

Sanidine-olivine-diopside association in a metasomatized garnet lherzolite xenolith from southeastern China

A garnet lherzolite xenolith hosted in olivine nephelinite from Xilong, Zhejiang Province, southeastern China, shows clear evidence of modal metasomatism involving a special sanidine-olivine-diopside (SOD) association which was produced by replacement of primary orthopyroxene. The fluid responsible for the metasomatism was a silica-undersaturated vapour, rich in K, Ba, Sr and Ti. It is suggested that the SOD assemblage is the product of metasomatism of a depleted lherzolite precursor, and formed in the upper mantle prior to entrainment and eruption.

Pb isotopic systematics of alkaline volcanic rocks and carbonatites from the Kaiserstuhl, Upper Rhine rift valley, F.R.G.

Lead isotopic data of alkaline volcanic rocks and carbonatites from the Kaiserstuhl are presented in combination with Sr isotopic ratios. As a whole the initial lead ratios cover the range typical for other Tertiary to Recent alkaline rock provinces in Europe: 206 Pb 204 Pb = 18.9–19.7 , 207 Pb 204 Pb = 15.61–15.70 and 208 Pb 204 Pb = 38.7–39.7 . Especially in the 208 Pb 204 Pb vs. 206 Pb 204 Pb diagram the data reveal a linear array which is interpreted in terms of mixing of two mantle components. In detail the data comprise three distinct groups which are correlated with different magmatic rock types: unfractionated primary mantle melts (olivine nephelinites, olivine melilitites), fractionated alkaline rocks (tephrites, phonolites), and a group with carbonatites, bergalites and hauynophyres. These results are interpreted to reflect either derivation from different mantle sources, or the data trace a mixing line between two mantle reservoirs, one of them being more enriched. Contamination with crustal lead is unlikely: it may have affected the tephritic magma series, but not the carbonatitic rocks. The uppermost part of the updomed mantle beneath the Kaiserstuhl, which is known by petrographic evidence to be partially metasomatized, probably is the site of the enriched reservoir. A “plume”-like structure within the deeper mantle is suggested as the initial source of this mechanism.

Geochemistry of Cenozoic alkali basaltic lavas from Shandong Province, eastern China.

Cenozoic alkali basaltic rocks from the central and eastern parts of Shandong province, eastern China include alkali basalts, basanites and olivine nephelinites, some of which contain spinel peridotite xenoliths and megacrysts predominantly of clinopyroxene and anorthoclase. The abundances and ratios of incompatible elements in the basalts are similar to those of many continental and oceanic island alkali basalts. The magmas were derived from metasomatised garnet-clinopyroxene-bearing peridotites by variable but small degrees of melting. A metasomatism which produced an enrichment of strongly incompatible elements was superimposed on a depleted source shortly prior to a melting event. The basaltic rocks from the central and eastern regions have distinct major and trace element characteristics and Sr-Nd-Pb isotopic compositions. The Eastern Shandong lavas have higher abundances of Na, P and incompatible trace elements but are lower in Mg and compatible trace elements than those from the central part. The source of the Eastern Shandong basalts was relatively homogeneous in Sr and Pb isotopic compositions and was metasomatically enriched, at least in part, by carbonate-rich fluids. Phlogopite and probably apatite remained in the residue after the extraction of Eastern Shandong nephelinites. The basaltic rocks of Central Shandong were generated from a heterogeneous multicomponent source. δ18O values negate a significant role of upper crustal contamination in the genesis of the Shandong basalts. The compositional similarities between the Shandong continental alkali basaltic lavas and continental and oceanic island alkali basalts suggest that their mantle parents were metasomatically enriched by fluids derived from the same source—rising asthenospheric plumes.

Tertiary basalts and peridotite xenoliths from the Hessian Depression (NW Germany), reflecting mantle compositions low in radiogenic Nd and Sr

A total of 17 alkali basalts (alkali olivine basalt, limburgite, olivine nephelinite) and quartz tholeiites, and of 10 peridotite xenoliths (or their clinopyroxenes) were analyzed for Nd and Sr isotopes. 143Nd/144Nd ratios and 87Sr/86Sr ratios of all basalts and of the majority of ultramafic xenoliths plot below the mantle array with a large variation in Nd isotopes and a smaller variation in Sr isotopes. The tholeiites were less radiogenic in Nd than the alkali basalts. Volcanics from the Eifel and Massif Central regions contain Nd and Sr, which is more radiogenic than that of the basalts from the Hessian Depression. Nd and Sr isotopic compositions of all rocks from the latter area, with the exception of one tholeiite and one peridotite plot in the same field of isotope ratios as the Ronda ultramafic tectonite (SW Spain), which ranges in composition from garnet to plagioclase peridotite. The alkali basaltic rocks are products of smaller degrees of partial melting of depleted peridotite, which has undergone a larger metasomatic alteration compared with the source rock of tholeiitic magmas. For the peridotite xenoliths such metasomatic alteration is indicated by the correlation of their K contents and isotopic compositions. We assume that the upper mantle locally can acquire isotopic signatures low in radiogenic Nd and Sr from the introduction of delaminated crust. Such granulites low in radiogenic Nd and Sr are products of early REE fractionation and granite (Rb) separation.

Isotope studies on alkaline volcanics and carbonatites from the Kaiserstuhl, Federal Republic of Germany

Sr, Nd and Pb isotopic data on alkaline volcanic rocks and carbonatites from the Kaiserstuhl Miocene alkaline volcano in the southern part of the Rhinegraben rift valley are presented. Isotopically, three distinct groups of volcanic rocks can be distinguished which are correlated to different magmatic series: 1. (1) Unfractionated primary mantle melts (olivine nephelinites, olivine melilitites) and nepheline basanites: 87 Sr 86 Sr =0.7032-0.7040 ; ϵ Nd=4.18-5.03; 206 Pb 204 Pb =18.95-18.98 ). Geochemically, these rocks correspond to a Na-dominated magmatic series. 2. (2) Fractionated alkaline rocks (tephrites, phonolites: 87 Sr 86 Sr =0.7039-0.7051 ; ϵ Nd=2.07-3.96; 206 Pb 204 Pb =19.10−19.42 ), corresponding to a K-dominated magmatic series. 3. (3) A group with carbonatites, bergalites, hauynophyres ( 87 Sr 86 Sr =0.7036-0.7040 ; ϵ Nd=2.81-4.04; 206 Pb 204 Pb =19.24-19.66 ) These results are discussed in terms of derivation from different mantle sources and interaction of asthenospheric melts with subcontinental lithospheric mantle during their ascent. Contamination with crustal material is important in the tephritic magma series, also in some phonolites. The isotopic data point to a metasomatic overprinting of the upper mantle below the Kaiserstuhl. This metasomatism is thought to be directly connected with the updomed mantle structure in this region. For the carbonatitic rocks a genetic linkage to olivine nephelinites is most likely. This implies an asthenospheric origin of the carbonatitic melts.

The system Mg2SiO4-Ca2SiO4-CaAl2O4-NaAlSiO4-SiO2: one atmosphere liquidus equilibria of analogs of alkaline mafic lavas

One atmosphere liquidus relationships in the system Mg2SiO4 (Fo)−Ca2SiO4 (La)−NaAlSiO4 (Ne)−CaAl2O4 (CA)−SiO2 (Sil) are presented as analogs for alkaline mafic lavas. Liquidus diagrams are constructed from electron microprobe analyses of quenched liquids and coexisting mineral phases produced in melting experiments and they are depicted in terms of sub-projections within the pseudo-quaternary system Fo−La−[Ne,CA]−Sil. The Ne and CA components are combined to create a normative feldspathoid component defined as Ne#=Ne/[Ne+CA]. Liquidus relations at Ne#=0.5 from this study are compared to relations at Ne#=0.0 and 1.0 from previous studies. In general, liquidus temperatures decrease and positions of liquidus boundaries involving feldspathic phases shift toward the [Ne, CA] component as Ne# increases. The pseudoinvariant points fo+di+pl+mel+l and fo+pl+mel+sp+l exist at Ne#=0.5. These equilibria between forsterite-plagioclase-melilite-liquid are not present in the system when Ne#=1.0 because a boundary curve (fo+di+ne+l) separates the plagioclase and melilite liquidus fields. The liquidus diagrams provide useful analogs for the crystallization sequences of natural primary alkali olivine basalts, basanitoids, basanites, olivine nephelinites, olivine-melilite nephelinites and olivine melilitites.

The Mantle Source for Olivine Nephelinite, Basanite, and Alkaline Olivine Basalt at Fort Selkirk, Yukon, Canada

Olivine nephelinite, basanite, and transitional alkaline basalt lavas of the Quaternary Fort Selkirk volcanic complex in the central Yukon represent three distinct alkaline magma series which have evolved along diverging paths. They cannot be related by low-pressure crystal-liquid fractionation, and systematic isotopic differences make it difficult to derive them by variable degrees of melting of a common mantle source. Field evidence requires, however, that these three magma series are intimately related in time and space, and they share a number of anomalous chemical characteristics including low Ca/Na ratios with respect to the majority of terrestrial equivalents. When the effects of differential olivine fractionation are ignored, the compositional spectrum of the Fort Selkirk lavas approximates a binary mixing line between transitional alkaline basalt and olivine nephelinite. A population gap along this mixing line, located between the compositions of the nephelinite and basanite lavas, coincides with the compositions of amphibole and/or amphibole–garnet–clinopyroxene assemblages observed in mantle xenoliths. This compositional gap may represent a thermal divide separating two minimum-melt compositions in a mantle source consisting of a lherzolite host cut by amphibole–garnet–clinopyroxenite veins. The olivine nephelinite endmember may have been derived by early melting in the amphibole–garnet–clinopyroxenite veins, whereas the transitional alkaline basalt would represent more extensive melting of the host lherzolite.

Metasomatically altered peridotite xenoliths from the hessian depression (Northwest Germany)

Abstract A set of 36 spinel lherzolite and spinel harzburgite xenoliths from 7 occurrences of Tertiary alkali olivine basalt and olivine nephelinite have been investigated for modal composition, major elements, and 39 minor elements. The peridotites were depleted during former partial melting events. The proportion of clinopyroxene (7.3% cpx), elements indicating depletion (1.9% A12O3, 1.9% CaO), and compatible trace elements (Ni, Mn, Co, Sc, Cr, V, Zn) are on average close to the means of the worldwide subcontinental lithosphere. Accumulation of potassium and of correlated incompatible trace elements has been used as an index of metasomatic alteration. It divides the peridotites into an L-group ( 1000 ppm K). Immediately before the alkali-basaltic volcanism, mantle metasomatism produced phlogopite, amphibole, and secondary clinopyroxene (the latter is enriched in LREE). Some proportions of these minerals have formed an interstitial melt from heat of the transporting magma. The metasomatic alteration of the sampled mantle varies in a complicated three-dimensional pattern without vertical layering. Mobilization of elements in mantle fluids (H2O-CO2-C1) and accumulation in certain peridotites is a complex open-system process. Accumulation in H-group (relative to L-group) peridotites exceeds primitive mantle rocks and decreases in the following sequence: Ba, P, K, F, Nd, Sm, Tl, Eu, Gd, Rb, Sr, Ce, La, Ti, Dy, Ta, Nb, Zr, Y, Hf, Er, Li, Bi, Pb, Yb, Na. This order is almost conformable to increasing compatibilities in mantle minerals (or decreasing accumulation in the continental crust). The exceptional behaviour of high field-strength elements might be controlled by the formation of Ti minerals. The metasomatic transport can be explained by extraction of the incompatible elements either from mantle minerals or from subducted crustal sources into water-rich fluids. Chlorine in phlogopite has equilibrated with fluids containing chloride concentrations close to seawater. Values of δ18O in bulk peridotites and in phlogopite exceeding typical mantle compositions probably reflect a crustal contribution to the metasomatic fluids. Nd and Sr isotopic ratios of both peridotites and alkali basalts received a crustal signature from an environment with time-integrated LREE accumulation and Rb depletion.

Nephelinitic and Basanitic Rocks

Basanites (which mineralogically should be characterized by an abundance of plagioclase and only a little feldspathoid) and nephelinitic rocks (which normally lack modal plagioclase but should contain nepheline) are not easily distinguished in the field, and both are commonly confused with basalts. Since the mineralogical distinction sometimes cannot be applied, as in the case when no modal felsic minerals are apparent in the groundmass, a chemical distinction between nephelinitic and basanitic rocks becomes desirable. The best solution is provided by the use of the CIPW norm with basanites recognized as having > 5% normative ab and < 20% normative ne, melanephelinites as having < 5% normative ab and < 20% normative ne, and nephelinites as having > 20% normative ne. It is proposed that melanephelinites defined in this manner can be further divided and that rocks formerly termed olivine nephelinite should now be called olivine melanephelinite, and that the pyroxene-phyric, olivine-poor melanephelinites should now be termed pyroxene melanephelinite.

Spinel‐lherzolite‐bearing quaternary volcanic centers in San Luis Potosí, Mexico: 1. Geology, mineralology, and petrology

The central portion of San Luis Potosí State, Mexico, contains many isolated basic alkalic volcanic centers of Quaternary age, including seven large maar complexes. These vents are divided into two geographically distinct groups, Ventura to the southwest and Santo Domingo to the northeast. The maars and other volcanic centers contain xenoliths of mantle‐derived spinel lherzolite and lower‐crustal granulites. The Santo Domingo maars also contain kaersutite megacrysts derived from kaersutite‐rich veins in spinel lherzolite. Basic volcanic rocks form a well‐defined petrologic series from strongly silica‐undersaturated olivine nephelinites and basanites rich in spinel‐lherzolite xenoliths and xenocrysts which, are dominant at Ventura, through alkali olivine basalts and basalts having little to no lherzolitic debris, which are dominant at Santo Domingo. All rocks have olivine phenocrysts with Cr‐Al spinel inclusions, titanaugite‐augite phenocrysts or microphenocrysts, and groundmass crystals of one or both Fe‐Ti oxide series. Plagioclase decreases in abundance with increasing normative ne + lc and is absent or restricted to the groundmass in the most undersaturated rocks. Normative ne + lc is also strongly correlated with the abundances of incompatible elements, among which the degree of enrichment in the suite decreases in the sequence Nb, Ta, P, La, Th, Ce, Sm, Sr, Ba, Zr, Eu, Hf, K, Ti, Yb, and Na. Based on experimental analogs and major and trace element abundances, the entire volcanic suite from San Luis Potosí can be modeled by progressive degrees of partial melting of a garnet peridotite mantle source. In this scheme, the Santo Domingo magmas were produced by relatively larger degrees of partial melting. Equally plausible, however, is formation of the San Luis Potosí magmas as mixtures of melts from two different mantle sources. In this scenario, the Ventura‐type mantle is enriched in Nb, Ta, P, La, and related elements compared with the Santo Domingo‐type mantle. In Pier et al. [this issue] we describe Sr and Nd isotopic data for these samples that demand multisource models.

Transport and composition of mantle fluid into the lower crust

Alkaline lavas from southwestern Poland have been analysed for major and trace elements and Sr and Nd isotope composition. K-Ar age determinations indicate that the olivine nephelinites, basanites, hawaiites and trachytes were erupted during three episodes at about 30 Ma. A depleted mantle source is inferred for the mafic lavas from their 87Sr/86Sr (0.70342–0.70376) and 143Nd/144Nd (0.51278–0.51288) ratios, while the trachytes give higher 87Sr/86Sr ratios (0.7045) explained by some crustal assimilation. Chemical and isotopic differences between nephelinites and basanites suggest that each rock type was derived from a distinct mantle source.

Influence of Sr on an Established Petrological Incompatibility: The Association Melilite + K-Feldspar in a Nephelinite from Djebel Targou, Central Morocco

Influence of Sr on an Established Petrological Incompatibility: The Association Melilite + K-Feldspar in a Nephelinite from Djebel Targou, Central Morocco

Lherzolite inclusions in olivine nephelinite tuff from Salt Lake Crater, Hawaii

Spinel lherzolites are one of the most common rock-types in inclusions in melilite olivine nephelinites from the Salt Lake Crater. They are classified by textural criteria into two types: coarse-grained and fine-grained. The textural variation is correlated with compositions of olivine and spinel: forsterite-rich olivine and MgCr 2O 4-rich spinel in the coarse-grained type; in contrast to fayalite-rich olivine and FeAl 2O 4-rich spinel in the fine-grained type. Chemical characteristics of the inclusions and minerals suggest that the coarse-grained type has more refractory features than the fine-grained one. Equilibrium temperatures calculated by the Wells and Fabries thermometers are within a temperature range of 740°–1100°C and there is no systematic relation between the temperature and texture. Garnet- and plagioclase-bearing spinel lherzolites included in the coarse-grained type are found as rare rock-types. Based on phase stability experiments and thermodynamics, pressures of the plagioclase and garnet lherzolites are estimated at about 3 kbar at 740°C (Fabries' T) and 20 kbar at 1070°C, respectively. Temperature of the plagioclase lherzolite is much higher than that expected from a steady-state oceanic geotherm. The high-temperature equilibrium of the lherzolite is the result of local heating by tholeiitic magmatism.

Fluid activity in the lower crust and upper mantle: mineralogical evidence bearing on the origin of amphibole and scapolite in ultramafic and mafic granulite xenoliths

AbstractXenoliths in an olivine nephelinite from the McBride Province, North Queensland, include Cr-diopside lherzolites, spinel and garnet websterites, felsic, 2-pyroxene and garnet granulites, and hornblendites. The spinel and garnet websterites are interpreted as crystal segregations from olivine basalt or alkali olivine basalt magma at ∼ 12 kbar followed by isobaric cooling (to approximately 900–1000°C) and subsolidus reequilibration. Garnet and 2-pyroxene granulites are mineralogically and texturally distinct and are considered to represent relatively large degrees of crystallization of basaltic magmas at comparable or slightly lower pressures (8–12 kbar). Mafic and ultramafic xenoliths have been modified to varying degrees following the relatively recent influx of a H2O- and CO2-bearing fluid. Variable amounts of amphibole and mica developed in response to the introduced fluid and it is argued that some hornblendites are the end-products of this process acting on spinel websterites. Felsic and 2-pyroxene granulite xenoliths display only minor evidence of increasedPH2O. Mineralogical and textural evidence indicates high-sulphur Ca-rich scapolite in several garnet granulites did not form in response to the increased fluid activities. It is proposed the scapolite was a primary cumulate phase precipitated from alkali basaltic magma under elevatedfo2andfso2conditions.

Trace and isotope geochemistry of the alkaline Tertiary volcanism in southwestern Poland

Alkaline lavas from southwestern Poland have been analysed for major and trace elements and Sr and Nd isotope composition. K-Ar age determinations indicate that the olivine nephelinites, basanites, hawaiites and trachytes were erupted during three episodes at about 30 Ma. A depleted mantle source is inferred for the mafic lavas from their 87Sr/86Sr (0.70342–0.70376) and 143Nd/144Nd (0.51278–0.51288) ratios, while the trachytes give higher 87Sr/86Sr ratios (0.7045) explained by some crustal assimilation. Chemical and isotopic differences between nephelinites and basanites suggest that each rock type was derived from a distinct mantle source.

Stable isotope (O, H, S) relationships in Tertiary basalts and their mantle xenoliths from the Northern Hessian Depression, W.-Germany

18O/16O, 34S/32S, and D/H ratios as well as vacuum-fusion H2O+ contents were measured for late Tertiary volcanic basaltic rocks ranging in composition from quartz tholeiites and alkali olivine basalts to melilite-bearing olivine nephelinites and for peridotite xenoliths from the Northern Hessian Depression of W.-Germany. Measured Oisotope ratios in both basalts and peridotites were corrected for variable degree of post-eruption, secondary alteration. The ranges and means of corrected δ18O values (‰ SMOW) for the North Hessian lavas and peridotites are: (i) 8 tholeiites: ca. +6.1 to +7.3 (¯x=+6.6), (ii) 21 alkali olivine basalts: ca. +5.4 to +7.6 (¯x=+6.5), (iii) 19 nepheline basanites, limburgites, and olivine nephelinites: ca. +5.3 to +8.0 (¯x=+6.6), and (iv) 23 peridotites: +5.1 to 7.0 (¯x+6.0). The δ34S values (‰ CDT) for the tholeiites range from −0.6 to +1.4 (¯x=−0.03) and for the alkali basalts range from +0.9 to +8.6 (¯x=+2.5). The approximate δD value (‰ SMOW) of the pristine basalts and peridotites is estimated to have been ca. −90‰

Quelques precisions sur la composition mineralogique des laves tertiaires de la province volcanique de la Presqu'île du Cap-Vert (Senegal)

The late Tertiary volcanism from the volcanic province of Cap Verde Peninsula includes only silica-undersaturated basic lavas: olivine nephelinites, basanites and olivine alkaline basalts. This undersaturated alkaline series is sodic and indifferentiated. Lavas contain olivine Fo 88 to Fo 56, clinopyroxene Wo 54En 34Fs 12 (fassaïte) to Wo 40En 42Fs 18 (augite) present both in phenocrysts and in groundmass. Plagioclase An 67An 48 is only found in basanites and olivine alkaline basalts. The olivine nephelinite is characterized by a soda-rich nepheline (Ne 77Ks 16Q 7). Microphenocrysts of iron-titanium oxides may form within the more differentiated lavas of the series.

Petrochemistry of gem-bearing basalt in the Nong Bon area, Trat Province, Eastern Thailand

Basaltic rocks of Nong Bon area average 30m thick. They are generally massive with few vesicular layers. Megacrysts of clinopyroxene, garnet, ilmenite and magnetite, and xenoliths of ultramafic rocks are common in these rocks. Petrographic studies so far indicate that some of the rocks are olivine nephelinites, porphyritic with phenocrysts of olivine, clinopyroxene and spinel. Geochemi­ cal data are also available for the basanitoid basalt. On the basis of the data available, the origin of corundum found in the study area cannot be geneti­ cally related with the basanitoid basalts. It is suggested that pre-existing corundum was picked up from a depth just above the level of formation of the basanitoid basalts.

REE and selected trace elements in kimberlites from the Kimberley area (South Africa)

New data by INA methods on Sc, Cs, REE, Ta, Th and U in 11 kimberlites, 1 phlogopite garnet harzburgite xenolith and 1 eclogite xenolith from the larger Kimberley area are used to constrain models for kimberlite genesis. Kimberlite magmas are produced by melting of 61% clinopyroxene, 60% olivine, 22% phlogopite and 3% garnet of mantle rocks linked with precipitation of 46% orthopyroxene. The accumulation of the incompatible elements La, Ce, Sr, Rb in kimberlites relative to garnet harzburgite has been modelled with the distribution coefficients of major mantle minerals to melts. There is not much conformity between computed and observed abundances. The degree of partial melting is estimated as 2–3% based upon kimberlite—clinopyroxene balances. Complete melting of accessory mantle phases (phlogopite, phosphates, carbonates, etc.) and fractional melting of clinopyroxene (eventually plus phlogopite) of metasomatically altered garnet harzburgite can have contributed the major proportion of incompatible elements in kimberlites (and melilite-bearing olivine nephelinites). The major fraction of HREE in the latter rocks must have been supplied by mantle clinopyroxene and not by garnet. This precludes the general discrimination between garnet peridotite and spinel peridotite equilibrated melts by means of the HREE concentration of primary mantle magmas.

Origin of the Tertiary basaltic volcanism in the northern Hessian Depression

Volcanic activity started about 20 Ma before present with quartz tholeiites (QTh), had a climax with alkali olivine basalts (AOB) 13 to 14 Ma ago and ended 7 Ma ago with nepheline basanites (NB) and olivine nephelinites (ON). AOB covers 73% of the volcanic area. About 250 basalts and peridotite xenoliths were sampled for investigation. An upper mantle layer ranging from about 90 to 60 km depth has been conditioned for a preferential alkali basalt production by advection of H2O-CO2-fluids containing Si, Al, Ca, K, Na, P as major constituents beside numerous incompatible minor elements. At the onset of the geodynamically triggered mantle conditioning locally restricted diapirism into shallow depth has caused formation of olivine tholeiite magmas (OTh) at about 1,300° C by partial melting. All of these OTh primary melts intruded due to a favourable compressibility into granulites of the lower crust. The rare QTh basalts are their derivative magmas which have been slightly contaminated in the crust. Magmas of the subsequent alkali basaltic volcanism (AOB, bAOB, NB, ON, MON) formed by in-situ partial melting at about 75 to 90 km depth after depression of the peridotite solidi by fluids to temperatures ≦1,200° C. Except many AOB these magmas are primary melts as characterized by olivine/melt distribution coefficients of Mg/Fe2+ (KD=0.29 to 0.34), by Ni concentrations (260 to 330 ppm) and the occurrence of peridotite xenoliths. Rapid rise of gas charged melts due to saturation in CO2 prevented separation of olivine etc. and of xenoliths. The sequence of magmas from OTh to ON (or MON) is formed from decreasing proportions of orthopyroxene (opx) and increasing contributions of clinopyroxene (cpx) and phlogopite (ph) at almost equal proportions of spinel (sp). Incongruent melting of opx (and cpx) for OTh, AOB, NB and ON is correlated with precipitation of olivine. The average xenolith composition (73% ol, 18% opx, 7% cpx, 1.1% sp and 1.3/0.5% ph) was used to model the sources of the investigated melts by 9 incompatible elements and to calculate degrees of partial melting. The occurrence of garnet cannot be reliably excluded by modelling on the basis of HREE distribution coefficients. The average xenolith composition was used for modelling because of its resemblance with worldwide sampled depleted mantle inclusions. For avoiding to exhaust at least one mineral of the model mantle in the support of the norm composition of OTh, AOB, NB and MON magmas the degrees of partial melting cannot exceed 12.5%, 6%, 6% and 4% respectively. Mantle containing about 500 ppm K (and the correlated incompatible elements), like the average of 36 xenoliths, allows to explain the formation of OTh magmas. AOB, NB and ON melts require peridotite with slightly less than 1,500 ppm K, 670 ppm P and proportions of the correlated elements LREE, Sr, Ba, Zr, Rb, Cs, Ta, Th, Hf, U, which are higher than their abundance in primitive mantle rocks. About 20% of the xenoliths have this composition. Metasomatism of fluids with these elements must have been an immediate precursor of the alkali basaltic volcanism. Otherwise the preservation of a local disequilibrium in 87Sr/86Sr ratios between cpx cores and total rock at upper mantle temperatures cannot be explained.