FRANCESCO STOPPA* - Consensus and open questions about Italian CO₂ - driven magma from the mantle

ABSTRACT - The significance of the heavy Carbon moves the frontier of interpretation for the Italian carbonatite magmatism and carbonatites worldwide toward progressively deeper mantle sources. Italian kamafugites and carbonatites are co-magmatic and erupt explosively in continental grabens. This system has some peculiarities which allow a direct link with the mantle. Spinel lherzolite xenoliths and other mantle debris are carried to the surface by carbonatites and kamafugites in the Italian Province. The extreme chemical nature of these rocks challenges conventional petrogenetic thinking. The gurus of the Italian magmatism cannot convincingly persist in preaching a geodynamic model based on Recent subduction.

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ALAN R. WOOLLEY* - Igneous silicate rocks associated with carbonatites: their diversity, relative abundances and implications for carbonatite genesis

ABSTRACT - Data on the diversity and relative abundance of igneous rock types associated with carbonatite have been compiled for 377 occurrences, which represents 82% of the ~450 carbonatite occurrences known to the author. Extrusive carbonatite is found at 40 localities and for the purposes of analysis these are treated separately. The extrusive carbonatite association is characterised by the high proportion of occurrences that contain mantle debris (xenoliths and/or xenocrysts) and the occurrence of melilite-bearing rocks in nearly half of them. 22% of intrusive carbonatite localities have no associated igneous silicate rocks while a diverse range of rock types are associated with the rest. These rocks are listed, quantified and described and lead to the conclusion that six series of rocks can be distinguished as being associated with carbonatites. These are: 1. melilitite-nephelinite-phonolite-trachyte (melilitolite), 2. nephelinite-phonolite trachyte (ijolite), 3. basanite-trachyte (alkali gabbro), 4. phonolite-trachyte (foid syenite), 5. trachyte (syenite), and 6. kimberlite. A high proportion of occurrences with representatives of series 1 to 3 also contain ultramafic rocks, which may be cumulates. The presence of mantle debris in some carbonatite occurrences and the absence of associated silicate rocks from others are taken as evidence of derivation of these carbonatites directly from the mantle. The remaining carbonatites are considered to have been generated by differentiation (separation) from magmas represented by the associated silicate rocks that are the result of partial melting in a metasomatised lithosphere. This model indicates that carbonatites can be generated in a number of ways, and the close association with a broad spectrum of silicate liquids implies that there is probably some variation in carbonatite liquid compositions.

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JOHN GITTINS* and ROBIN E.HARMER - Myth and reality in the carbonatite-silicate rock "association"

ABSTRACT - The very common "association" of carbonatite with alkalic silicate rocks is generally assumed to be a genetic relationship in which carbonatite was derived from silicate magma, or that both groups of rocks shared a common parent magma. It has become a dominant theory of carbonatite genesis on which much experimental petrology study has been based. We suggest that this assumption is unwarranted and misleading. The "association" is spatial rather than genetic and is caused by two separately generated magmas having used the same system of conduits to reach the crust from their generation sites in the mantle. Experimental petrology, field relations and isotope studies do not definitively confirm any mode of origin to the exclusion of others. Experimental petrology allows the development of carbonatites (largely as cumulates) by fractional crystallization of silicate magma and, under some conditions, of carbonatite magma derived by liquid immiscibility from a silicate magma, but it also allows the development of carbonatite magma by partial melting of carbonate-bearing mantle peridotite, unaccompanied by silicate magma generation. Most isotope data give no definitive proof that silicate magma is parental to carbonatite magma, - nor are they inconsistent with such a relationship. Furthermore, where the data are consistent with a derivative relationship they also show, quite definitively, that not all of the silicate rocks could be parental to carbonatite. However, there are South African and Zimbabwean carbonatite complexes in which isotopic data show that at least some carbonatites were generated at considerably greater depths than the silicate rocks that accompany them in the same intrusive complex, thus ruling out a directly derivative relationship between carbonatites and silicate rocks, either by fractionation or by liquid immiscibility. We believe that, in general, there is no liquid line of descent relationship between silicate magma and carbonatite magma, except in very minor amounts. It would be more meaningful to refer to carbonatites and their "accompanying", rather than to their "associated" alkalic silicate rocks.

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D. KEN BAILEY* and STUART KEARNS - Carbonatite magmas: natural examples and the phase relations they define

ABSTRACT - As the picture of carbonatite magmatism continues to improve, so the bimodal distribution of calcio-and magnesio-carbonatites becomes more apparent. Calcite carbonatites are much more abundant: some of these may be differentiates from silicate parent magmas. But carbonatite volcanism requires that calciocarbonatite can be sourced in the mantle, without involvement of silicate melts. Phase relations in the Ca-Mg carbonate system impose the requirement for separate sources for calcitic and dolomitic carbonatites. Dolomite is the expected product of incipient mantle melting. Calciocarbonatite may arise from melting of a higher level CaCO3 rich stockwork metasome, in which higher pressure polymorphs, possibly including aragonite, might be the controlling influence on the melt composition.

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GIUSY LAVECCHIA* and NICOLA CREATI - Lithosphere tectonic context of the carbonatite-melilitite rocks of Italy

ABSTRACT - The occurrence in Italy of Quaternary carbonatite-melilitite rocks, belonging to the Intra-mountain Ultra-alkaline Province (IUP), is considered in order to discuss a likely geodynamic environment for the Tyrrhenian-Apennine system. The IUP tectonic setting is described at the crustal and lithospheric scale and compared with the HK-series of the Roman Co-magmatic Province. It is concluded that a suitable mantle source for the IUP and HKS melts products (e.g. a radiogenic source with a phlogopite-bearing carbonate peridotite composition) does not need either the westward subduction of the Adriatic continental lithosphere or a mantle plume. The IUP and HKS geochemistry, as well as the deformation history of the Tyrrhenian-Apennine system, is explained in the frame of a substantially passive intra-continental rift context. The peculiar metasomatism and high radiogenic content of the HKS and IUP mantle source is attributed to fluids directly deriving from the lower mantle.

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PATRICIA B. DE ALBUQUERQUE SGARBI* and GERALDO NORBERTO C. SGARBI - Kamafugite volcanism in Brazil

ABSTRACT - In the Upper Cretaceous, a tectonic reactivation associated with extensive alkaline-ultramafic volcanism took place in the central-western plateau of Brazil. As a consequence of this diastrophism, the Brasilia Belt, an approximately N-S elongated structure, and two marginal depressions in both sides of the Belt, the southern Sanfranciscana Basin and the NNE border of the Paraná Basin, were formed. This magmatism constitutes the Minas Goiás Alkaline Province, located in Western Minas Gerais and Southern Goiás states. This paper describes the kamafugitic volcanic and volcaniclastic rocks of the province that occur in Mata da Corda and Santo Antônio da Barra areas.
The Cretaceous alkaline rocks of the province occur as lavas, pyroclastic rocks, volcaniclastic deposits, pipes, and plutonic complexes. Pipes and volcanic rocks from both areas are mostly kamafugites. Sr, Nd, Os, and Pb isotopic results indicate a lithospheric mantle derivation and the age data suggest that the magmatism is related to the Trindade plume activity.
The Mata da Corda and Santo Antônio da Barra kamafugites are mafurites and ugandites. The rocks are all feldspar-free, with abundant clinopyroxene and Ti-magnetite in very fine- to medium-grained porphyritic to seriated textures. These Brazilian kamafugites are all undersaturated in SiO₂, with high contents of CaO, FeOt and TiO₂, and relatively low MgO. Regarding the chemical features, a pattern of evolution promoted by fractional crystallization from mafurites toward ugandites is not totally apparent. However, if the possibility of this evolutionary connection is considered, it is most likely that olivine and clinopyroxene would be the major fractionated phases.
The volcaniclastic units occur discontinuously over an extensive area of the Minas Goiás Alkaline Province and are genetically related to a magmatic-tectono-sedimentary cycle that occurred in the region during the Upper Cretaceous.
The pyroclastic deposits of the province can be considered as being of hawaiian/strombolian type, forming small bodies around volcanic vents or diatremes. They are generally chemically altered to a large extent and consist of agglomerates, lapillites and tuffs.
The largest volume of volcaniclastic rocks present in the province consists of intensely altered epiclastic units that reflect continuous severe tropical weathering of the magmatic and pyroclastic units of the Minas Goiás Alkaline Province during the Tertiary and Quaternary periods. The detrital materials were transported and deposited by alluvial fans and torrential streams around the slopes of the region, forming sandstones and conglomerates, with diverse lithological contributions.

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SEBASTIAN TAPPE*, S.F. FOLEY and D.G. PEARSON - The Kamafugites of Uganda: a mineralogical and geochemical comparison with their Italian and Brazilian analogues

ABSTRACT - New mineral chemistry and whole-rock geochemistry of type kamafugites from the East African Toro-Ankole province are compared with Italian and Brazilian kamafugitic rocks showing the diversity of kamafugite compositions. Whereas Brazilian kamafugites are more akin to the Ugandan type rocks in showing a smooth convex-upward trace element abundance curve and similarities in the mineral chemistry, significant chemical differences to the Italian rocks are apparent. The latter are characterized by higher SiO₂, Al₂O₃ and K₂O contents and show negative anomalies for Eu and the high-field strength elements in normalized abundance patterns.
This could be due to slightly distinct source compositions as a result of the different nature of mantle metasomatism preceding kamafugite magmatism. Whereas the Ugandan and Brazilian subcontinental lithosphere was affected by metasomatizing agents derived from the asthenosphere the lithospheric mantle beneath Italy was probably affected by enrichment processes linked with subducted crust.

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STÉPHANE BRASSINNES*, DANIEL DEMAIFFE, ELENA BALAGANSKAYA and HILARY DOWNES - New mineralogical and geochemical data on the Vuorijarvi ultramafic, alkaline and carbonatitic complex (Kola Region, NW Russia)

ABSTRACT - The Vuorijarvi massif belongs to the famous Devonian (380-360 Ma) alkaline and carbonatitic province of Kola (NW Russia). It is a complex polyphase intrusion made of ultramafic rocks, ijolites-melteigites, carbonatites and related phoscorites. Ultramafic rocks (mainly clinopyroxenites) have been interpreted as cumulates, with variable amounts of trapped interstitial liquid. Ijolites are quite complex : some are aegirine-augite and nepheline cumulates, others are partly recrystallised rocks. Carbonatites and phoscorites (apatite-forsterite-magnetite rocks) are genetically linked; the latter correspond to cumulates of silicate and oxide minerals crystallised from a carbonatitic magma. Two generations of carbonatite have been recognized, calcitic varieties and dolomitic varieties. The three groups of rocks (ultramafic rocks, alkaline silicate rocks and carbonatites) have similar initial Nd-Sr isotopic compositions which suggest their cogenetic character. Apatite is a liquidus phase in the three main lithologies; it presents a continuous chemical evolution trend interpreted in terms of isomorphous substitution 2Ca²⁺ = REE³⁺ + Na⁺, which is compatible with a fractional crystallisation model.

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IRINA P. SOLOVOVA*, ANDREI V. GIRNIS, IRINA T. RASS, JOERG KELLER and NATALIA N. KONONKOVA - Different styles of evolution of CO2-rich alkaline magmas: the role of melt composition in carbonate-silicate liquid immiscibility

ABSTRACT - Melt and fluid inclusions were studied in olivine from the olivine melilitite of the Mahlberg complex, Germany. It was shown that the magmas were saturated in CO₂ at a pressure higher than 5 kbar and 1220°C and showed low SiO₂ and high CaO and alkali contents. The magma composition is similar to that of the melilitolites of the Gardiner massif in Greenland (Nielsen et al., 1997), which associate with carbonatite bodies and where evidence for carbonatite-silicate liquid immiscibility was detected in melt inclusions. The absence of carbonatite liquids in Mahlberg is probably related to a relatively high Al₂O₃ content, which shifted the composition of magma away from the carbonate-silicate liquid immiscibility field.

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IRINA A. ANDREEVA* and VYACHESLAV I. KOVALENKO - Magma compositions and genesis of the rocks of the Mushugai-Khuduk carbonatite-bearing alkaline complex (southern Mongolia): evidence from melt inclusions

ABSTRACT - Inclusions in mineral-forming media were studied using various techniques including X-ray spectral and ion microanalysis. Based on this study, the compositions, formation conditions, and magma evolution were evaluated in the silicate rocks and ores of the Mushugai-Khuduk carbonatite-bearing alkaline complex in southern Mongolia. It was found that the rock-forming minerals of melanephelinite, leucite phonolite, shonkinite, theralite, quartz syenite, rhyolite, magnetite-apatite and celestite-fluorite rocks crystallized from silicate, salt-silicate, and salt melts. The silicate melts form a continuous series from basic to rhyolitic compositions with SiO₂ contents from 47 to 77 wt %. The salt-silicate melts are silicate-phosphate in composition containing up to 10-20 wt % of P₂O₅. The salt melts show phosphate-carbonate, phosphate-sulphate, fluoride-sulphate, and chloride-sulphate compositions. The analysis of changes in melt composition during the crystallization of the complex allowed us to distinguish the processes that were responsible for the formation of specific rocks and ores and their evolution. It was established that the process of crystal fractionation played a leading role in the genesis of the volcanic and plutonic rocks. In addition, liquid immiscibility contribute to the formation of the rocks. The processes of liquid immiscibility were probably crucial for the genesis of the ore-bearing rocks.

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ANTON R. CHAKHMOURADIAN*, EKATERINA P. REGUIR and ROGER H. MITCHELL - Titanite in carbonatitic rocks: Genetic dualism and geochemical significance

ABSTRACT - Titanite is a relatively rare Ti silicate in carbonatitic rocks. It is a primary phase in alkali-rich carbonatites, and may also occur in silicocarbonatites whose composition was modified by assimilation of wallrock silicate material. More typical is late-stage titanite that forms by reaction of a precursor Ti mineral with deuteric fluids. Both genetic types show significant variations in chemical composition arising mostly from the substitution of Ti with Al, Fe, Nb and Zr. Cationic substitutions at the Ca site are limited to several atomic per cent. Zoning in primary titanite typically involves a decrease in the proportion of Nb and Zr toward the rim, whereas deuteric crystals show the reverse zoning pattern.

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NATALY KRASNOVA*, TOMAS PETROV and JACQUES MOUTTE - A new rock classification system applied to ultrabasic-alkaline and phoscorite-carbonatite rocks

ABSTRACT - The logically strict and simple universal systems to display the composition of any rock composition is suggested, which facilitates creation of databanks, search and grouping of materials in them. The ultrabasic-alkaline and carbonatite-phoscorite chemical rock composition is classified and ordered according to the suggested and described RHA method. R - the rank formula - is the succession of component's symbols ranged according to the diminishing of their content in the analysis. If components are chemical elements, the rank formula (Rchem) will correspond to a set of element symbols ranked in order of decreasing atomic %. Collections of more than 900 rank formulas (Rchem) with corresponding values of complexity measure (Hn) and purity measure (An) and about 500 RHA indexes for typical rock types are available on the web-sites (see files: RHA_collect.txt, RHA_typical.xls and the file RHACollectHelp). If components are minerals, the rank formula (Rmin) of a rock composition will correspond to a set of mineral symbols ranked in order of decreasing molecular %. The simple nomenclature of phoscorites and carbonatites based on use of rank formulas Rmin is worked out.

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JÖRG KELLER* and ANATOLY N. ZAITSEV - Natrocarbonatite Dykes at Oldoinyo Lengai transformed to Calcite Carbonatites

ABSTRACT - For the first time carbonatite dykes are described from the active natrocarbonatite volcano Oldoinyo Lengai, Tanzania. The dykes have a microporphyritic texture with polycrystalline, granular calcite microphenocrysts. The porous texture is indicative of leaching and alteration. The dykes are interpreted as the typical transformation products of a Lengai-type natrocarbonatite. Besides the major component calcite, groundmass phases are fluorite with 2.1-3.9 wt.% SrO, Mn-rich magnetite (up to 10.2 wt.% MnO), REE and silica-rich apatite (up to 20.6 wt.% REE2O3 and 9.9 wt.% SiO₂) and a Mn-Ba oxide/hydroxide. The chemical composition is Ca-carbonatitic with high concentrations of Sr, F, and Mn. Alkalis are <1%. Compared with fresh natrocarbonatites of Oldoinyo Lengai enrichment of trace elements include the REE, Sr, Y, Nb, Pb, Th , and depletion is shown for Rb, Cs, Ba, U.
Stable isotope ratios are d13C -1,95‰ (PDB), d18O +24,12‰ (SMOW), showing a great distance from the primary Lengai mantle box (Keller & Hoefs 1995), and indicating extensive atmospheric exchange and re-equilibration, in particular of the oxygene isotopes. All calcite is of secondary origin. Noteworthy for secondary calcite are the high Sr contents (up to 3.5 wt.% SrO), and varying Ba contents (0.1-2.2 wt.% BaO). Distinction from primarily calciocarbonatitic extrusives is based on textural evidence mimicking the porphyritic texture of natrocarbonatites, and on the polycrystalline nature of calcite pseudomorphs after nyerereite. The distinctive sheaf-like intergrowth texture of fluorite in natrocarbonatites is preserved as the only primary feature in the dyke calciocarbonatite.
Oldoinyo Lengai is the only active carbonatite volcano and is known for the unusually alkali-rich composition of its natrocarbonatites, not found elsewhere (e.g. Dawson 1989, Keller & Krafft 1990). From an actualistic point of view the questions are obvious: (1) Is the present carbonatite activity of Oldoinyo Lengai a key for the interpretation of past carbonatite magmatism? and (2) Where are the natrocarbonatites of the geological past? And how would we recognize these possible past equivalents of the present activity?

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LIA N. KOGARKO - Two-stage model of carbonatite origin: evidence from metasomatised mantle xenoliths

ABSTRACT - The geochemical study of mantle xenoliths from Montana Clara island (Canary archipelago), Fernando de Naronha island (Brazil) and East Antarctic revealed that mantle of these regions has been affected by intense carbonate metasomatism.
Primary generation olivine, orthopyroxene and clinopyroxene in mantle xenoliths of these regions are crossed by veinlets of fine-grained assemblage, which replaces minerals of the first generation, especially orthopyroxene. These finegrained zones contain second generation clinopyroxene, olivine, carbonate, glass, sulfides, sometimes apatite, Ba-rich mica, henrymeyerite, armalcolite, krishtonite, perovskite.
The carbonate metasomatism led to wehrlitization of the primary mineral assemblage according to the reactions:

4MgSiO₃ + CaMg(CO₃)₂ =
2Mg₂SiO₄ + CaMgSi₂O₆ + 2CO₂
3CaMg(CO₃)₂ + CaMgSi₂O₆ =
4CaCO₃ +2Mg₂SiO₄ + 2CO₂.

The interrelationships between the glass, sulfide and carbonate permit speculation that silicate, sulfide and Ca-rich carbonatite melts were in equilibrium with each other and originated from partial melting of metasomatized and wehrlitized peridotite.
On the basis of these data we developed the two-stage model of Ca-rich carbonatite formation: first stage-metasomatic wehrlitization and carbonatization of mantle substrate; second stage- partial melting of the carbonatised wehrlite, resulting in the formation of calciocarbonatites.

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VALENTIN NIVIN*, RUSLAN LIFEROVICH, SERAFIM IKORSKY, ELENA BALAGANSKAYA and VICTOR SUBBOTIN - Noble gas isotopes in minerals from phoscorites and carbonatites in Kovdor and Seblyavr ultramafic-alkaline complexes (Kola alkaline province, NWRussia)

ABSTRACT - Abundances of He and Ar isotopes in minerals from rocks composing the Paleozoic carbonatite-bearing Kovdor and Seblyavr alkaline-ultramafic complexes (the Kola Alkaline Province NW Russia) have been preliminary studied. The data obtained are to estimate their use as geochemical indicators, informative on conditions of the mineral-forming alkaline and carbonatite systems and sequence of mineral formation in these media. Noble gas isotope compositions show the extractable fluid phase in the most minerals studied to have varying input of the mantle, crustal and atmospheric components as well. All of these are found in both magmatic and postmagmatic minerals. Mixing of the mantle and crustal fluid phases might begin during the pre-crystallization and continue during differentiation and fractional crystallization of the mantle-derived magma. The most part of the mantle fluid (considered as primary fluid) is lost during magmatic evolution of alkaline and carbonatite complexes. Abundance patterns of the noble gas isotopes suggest predominantly autometasomatic character of higher-temperature postmagmatic processes and increasing input of exogenous fluid phase (meteoric water) at later postmagmatic stages. Distribution of the He and Ar isotopes and variations of 3He/4He and 3He/36Ar ratios, can be employed as an informative tool for recognizing of fluid sources cognate to the ore-forming and barren magma(s), and serve as a basis for reconstruction of the physical-chemical conditions of valuable mineralization typical of alkaline-ultramafic complexes (apatite, magnetite, phlogopite, minerals of high-field strength element like to Nb, Zr, Sc, LREE).

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IRINA RASS - Carbonatites derived from primary magmas with different Ca-contents: geochemical evidence

ABSTRACT - Rock geochemistry in two alkaline-ultrabasic-carbonatite complexes, the Odikhincha (Polar Siberia) and the Kaiserstuhl massifs, was investigated. In both massifs, trace element and P distributions in the sequential derivatives of melilite-bearing and melilite-free series are in accordance with the Rayleigh model of crystal fractionation of different parental magmas. The analysis of trace-element logarithmic relations and of triangular diagrams shows that nor nepheline syenites (phonolites), neither carbonatites could not be derived by fractional crystallization of both Ca-rich and Ca-poor parental magmas. They may be formed by liquid immiscibility of some primary magma, parental for the melilite-bearing series, at a certain degree of differentiation.

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VASYL M. ZAGNITKO*, STEPAN G. KRYVDIK AND MYKOLA O. DONSKIY - Isotopic geochemistry of carbonatites of Ukraine

ABSTRACT - In Ukraine there are carbonatites and carbonatite-like rocks of Precambrian (1.8-3.0 Ma) and Palaeozoic (0.4 Ma) age. Overall, these carbonatites have isotopic and geochemical characteristics similar to carbonatites in other world localities, but sometimes they are peculiar. Infact the oldest (2.09 Ma) carbonatites of Chernigovka complex show some peculiarities of isotope composition. In this rocks the d18O varies from 5.9 to 17.5‰ accordingly with the reduction of FeO contents in carbonatites. Unusual low ratio of 87Sr/86Sr (0.7005) has been measured in carbonates of "calciphyre" inclusions in Archean granites (3.0 Ma - granites and 2.9 Ma - "calciphyres")
Isotope composition of Palaeozoic carbonatites testifies about exchange between these rocks and surface material.

KEY WORDS: carbonatites, isotope composition, Ukraine, Chernigovka complex.

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YU XUEHU*, ZHAO ZHIDAN, MO XUANXUE, SU SHANGGUO, ZHU DEQIN, WANG YONGLEI - The petrological and mineralogical characteristics of Cenozoic kamafugite and carbonatite association in West Qinling, Gansu province: China

ABSTRACT - Kamafugites are ultra-potassic volcanic rocks which occur in continental rifts. Kamafugites are associated to carbonatites and this occurrence, to our knowledge, is only known from a very few places all over the world.
The Cenozoic kamafugite and carbonatite association in West Qinling, Gansu province was not know in the world so far. This paper provide a description of the petrological and mineralogical characteristics of the West Qinling kamafugite and carbonatite rocks in some detail. Representative, whole rock and essential minerals compositions from both kamafugite and carbonatite are shown, as well as Ar/Ar and K/Ar dating of 18 extrusive pyroclastic centers and dykes. The dating indicate that the West Qinling kamafugites and carbonatites were emplaced during the Miocene. The volcanic rocks are poor in SiO₂ and Al₂O₃, but rich in MgO, CaO, TiO₂ and alkalies (K₂O+Na₂O). As a whole, bulk rock composition is very similar to that one from Toro-Ankole kamafugites of Uganda.
The carbonatites are Ca-carbonatite. They occur as lapilli-tuff in surges deposits and high temperature pyroclastic flows. They are composed of essential calcite with minor amounts of nepheline, apatite, phlogopite and rare clinopyroxene. Carbonatite composition show characteristic high content of CaO, relatively high content of SiO2 and are very poor in alkalies. In general, the carbonatites in the West Qinling are very similar to other carbonatites associated to kamafugites elsewhere such as Fort Portal, (Uganda) and Polino (Italy).

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FRANCESCO STOPPA*, GIANLUIGI ROSATELLI, FRANCES WALL and MICHAEL J. LE BAS - Texture and mineralogy of tuffs and tuffisites at Ruri Volcano in western Kenya: a carbonatite, melilitite, mantle-debris trio

ABSTRACT - Ruri is located in the Kavirondo rift, near Homa in western Kenya It is a twin volcano with an ijolite-sövite core in its northern half, coupled with a carbonatite-melilitite pyroclastic centre in the southern half. The petrography and mineral chemistry of pyroclastic rocks from the collection of Ruri material at The Natural History Museum, London, have been studied in order to learn more of the nature of the volcanic activity and the erupted magmas
Four types of tuffs have been recognised at South Ruri, on the basis of rock structure and texture, the nature of fragmental materials and matrix relationships: 1- heterolithic tuff: a mixture of accidental fragments with no discrete physical juvenile component; 2- pelletal tuff: heterolithic tuff with a carbonatitic matrix, with a minor juvenile component of rounded melilititic lapilli cored by mafic HP xenocrysts; 3-lapilli tuff: formed mainly of juvenile lapilli, with concentric melilitite layers around a kernel of clinopyroxene or Cr-spinel, immersed in a carbonatite ash-matrix; 4-lapilli-ash tuff: mostly microporphyritic carbonatite lapilli in a micritic carbonatite matrix (ie. extrusive carbonatite). Types 1 to 3 are interpreted as conduit and/or vent facies and type 4 is a surge deposit.
The juvenile components of the deposits are of melilititic-carbonatitic type. Their proportion increases with eruptive sequence, indicating a progressively shallower level of magma fragmentation in the conduit. The juvenile silicate material is a feldspar-free association of melilite (now altered) and olivine, with foids, although these are also highly altered. Clinopyroxene, phlogopite and amphibole are slightly fresher. Some Cr-rich clinopyroxene, phlogopite, olivine and chromite have strain features and compositions indicating they are from disaggregated mantle xenoliths
The melilititic melts were co-eruptive with igneous carbonate at Ruri They may represent a small volume, near-primary magma that erupted directly from the mantle, whereas the subvolcanic rocks at Ruri are more differentiated, as are the carbonatites and nephelinites of the nearby Kisingiri stratovolcano. Ruri adds another example to the general pattern that the most primitive magma compositions are erupted at small centres adjacent to the large alkaline/carbonatitic volcanoes. Other examples are Deeti, close to Kerimasi and the Monticchio Lakes at Vulture.
At the liquidus temperature and low pressure of melilitite stability, carbonatite is not a miscible phase and may have erupted as mechanically separated, fine sprays of droplets and ash fragments that now form the matrix of the tuffs, as well as discrete porphyritic carbonatite lapilli.

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FRANCESCO STOPPA*, FELICITY E. LLOYD and GIANLUIGI ROSATELLI- CO₂ as the propellant of carbonatite-kamafugite cognate pairs and the eruption of diatremic tuffisite

ABSTRACT - Tuffisite pipes are characteristic of ultramafic rocks among which the rare association of kamafugite and carbonatite is prominent. Four main areas are known at present where this association is found: Katwe-Kikorongo and Bunyaruguru in Uganda, IUP localities in Central Italy, Mata da Corda in Brazil and Qinling in Gansu, China. In all areas dominant volcanic forms are pipes (diatremes) with maar-tuff rings. Tuffisite lapilli-bearing breccia fills the diatremes and tuffisite lapilli tuff surges form pyroclastic aprons around the vents. Tuffisite lapilli are so named to reflect their specific structure, which has probably been generated during a deep magmatic tuffisitisation. Mantle xenoliths are typical of these rocks and the only realistic agent of acceleration to the surface is a deep-seated concentration of volatiles with a high proportion of CO₂. The genesis of the kamafugite-carbonatite association can be assessed both in terms of field occurrence and geochemistry. In three cases out of the four the kamafugite-carbonatite association occurs in intracratonic grabens, only Italy being controversial. Specific magma composition is germane to a specific magma propagation mechanism producing a distinctive style of intrusive and extrusive pyroclastics that argues cogently for a fundamentally similar tectonic environment.

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FRANCESCO STOPPA*, ALDO CUNDARI, GIANLUIGI ROSATELLI and ALAN R.WOOLLEY - Leucite melilitolites in Italy: genetic aspects and relationships with associated alkaline rocks and carbonatites

ABSTRACT - New bulk-rock and mineral data on leucite melilitolite from Italy are presented, compared and discussed in terms of their parageneses, petrological significance and petrogenesis. Melilitolite is an intrusive assemblage with more than 10% modal melilite. Leucite-bearing melilitolite (italite?) is so far only known from Italy, contains about 30vol.% melilite and up to 25vol.% leucite. Other felsic constituents are kalsilite, nepheline and haüyne. It occurs as dykes, sills and a plug in the kamafugite -carbonatite suite forming the Pleistocene Intra-mountain Ultra-alkaline Province (IUP). In addition, ejecta of melilite-bearing, leucite and/or kalsilite clinopyroxenite as well as foid-free ultramelilitolite occur in alkaline, high-K volcanics from the Roman Comagmatic Region (RR). Essential mineral chemistry shows that the ubiquitous clinopyroxene signals crystallization from peralkaline liquids in its T site configuration but also, notably in RR ejecta, crystallization from metaluminous liquids.
Melilite is characterised by a high gehlenite composition, similar to the melilite from a Ugandan (Fort Portal) calcite carbonatite lava. All IUP leucite melilitolites yielded lower Mg# and Cr+Ni, relative to the associated melilitites, and their parental liquid is residual with respect to the initial melilititic melt. The melilitolite liquid was highly enriched in CaO and alkalis and depleted in Al₂O₃ (agpaitic index > 0.9). High CaO and association with carbonatites have been proved to be unrelated to sedimentary limestones, but are linked to CaCO₃ decoupling and reaction with the silicate fraction to form melilitolites and/or, by early CaCO3 immiscibility at high temperature, to form carbonatites. The occurrence of carbonate in globules, ocelli, and patches in melilitolite groundmass, is interpreted to have resulted from limited, late-stage immiscibility at relatively low temperature (670-800°C) and low pressure (<1 kb), favoured by residual fluid concentrations. Based on stratigraphic and structural observations, IUP melilitolites represent a final event in the related volcanic activity, inferred to have occurred as a slow, sub-volcanic intrusion which mechanically deformed the pre-existing rocks (brecciation, dragging and warping). IUP melilitolites and RR ejecta yielded a distinct mineral chemistry and modal abundance which reflect their initial peralkaline and metaluminous nature, respectively. This distinction is sharp for IUP melilitolites, but is blurred for RR ejecta. This may be due to the absence at the surface of a carbonatite component, non-essential modal melilite and essential feldspars in the RR assemblages. It is inferred that kamafugites may have originated from a deeper source, under a thicker lithosphere and lower heat flow, reflecting their close association with carbonatite, in contrast with conditions that prevailed for the generation of the much more abundant RR plagio-leucitite melts.

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