j. .geol. vol. 142, conference...

J . .geol. Soc. London, Vol. 142, 1985, pp. 697-708. Printed in Northern Ireland

Conference Report

Alkaline igneous rocks: a review symposium

J. G. Fitton & B. G. J. Upton Department of Geology, University of Edinburgh, West Mains Road,

Edinburgh EH9 3JW. UK

Report of a meeting of the Volcanic Studies Group of the Geological Society held at Heriot-Watt University, Edinburgh, on 13-16 September 1984. The organizers were Dr J . G . Fitton and Professor B. G . J . Upton. The papers presented at the symposium will be published as a Special Publication of the Geological Society.

The symposium was held to mark the passage of 10 years since the publication of the review volume The Alkaline Rocks, edited by Henning S~rensen, who opened the meeting and presented a summary of the proceedings at the end. Two and a half days were devoted to the presentation of papers and one and a half days to local field excursions. The meeting was well attended with about 150 delegates, 60 of whom were from overseas. In addition to the 30 papers presented there were several poster displays.

Presentations included specific case histories of individual alkaline volcanoes and regional reviews of alkaline rock occurrences in both continental and oceanic settings. Reviews were presented of the occurrence of various alkaline associations, including kimberlite, lamproite, lamprophyre, nephelinite, and carbonatite, and the impact of isotopic and experimental petrological studies on the subject was emphasized.

Alkaline magmas are characterized by high concen- trations of large-ion lithophile elements (LILE). The causes of this have been the subject of debate over the past decade, with metasomatic enrichment of the mantle source before melting being a currently favoured hypothesis. Studies of mantle enrichment and melting processes are clearly essential to an understanding of alkaline magma genesis. The case for a metasomatically enriched mantle source for both mafic and felsic alkaline magmas was presented by D. K. Bailey in the first paper. Clear evidence for mantle metasomatism from geochemical and isotopic studies of mineral phases in mantle xenoliths was described by M. A. Menzies. There can be no doubt that samples of mantle brought to the surface as xenoliths in alkaline magmas have been metasomatically enricheboften

several hundred Ma before entrainment. Since basanite magma is the most likely metasomatizing agent in many cases (Menzies), the question of whether mantle metasomatism is the cause of alkaline magmatism or merely a consequence of it remains open. High- pressure experimental petrology can help here. A. D. Edgar reviewed experimental studies on alkaline rocks and showed the evidence in favour of heterogeneous sources and the importance of volatile phases in their generation.

Evidence for the development of vertically stratified magma bodies, particularly in continental settings (Kenya Rift; Eifel Province, Germany; Gardar Pro- vince, Greenland) was stressed by B. H. Baker; R. Macdonald; H.-U. Schmincke, H. Mertes & L. Viereck; and B. G. J. Upton & C. H. Emeleus. Macdonald reminded the conference of the relative neglect of studies of pyroclastic formations.

The problems associated with the genesis of carbonatites and fenites were addressed by M. J. Le Bas and by J. D. Twyman & J. Gittins. A marked divergence of views emerged over the genesis of alkaline carbonatites (natrocarbonatites). These were regarded by Le Bas as parental (through alkali loss) to sovite, alvikite and ferrocarbonatite. Twyman & Gittins challenged this view, claiming that alkali carbonatites are not parental to other carbonatites but are themselves late differentiates of more normal olivine sovites. Le Bas noted that carbonatites, while typically continental, were also known from oceanic islands. He reminded the conference that carbonatites are typically associated with olivine-poor nephelinites and that olivine-rich nephelinites tended to occur, in association with basanite and alkali basalt, in carbonatite-free associations.

The genetic association of salic and basic alkaline magmas in continental environments was stressed by several contributors, most of whom invoked crystal fractionation as the principal mechanism involved. The most basic parental magmas varied widely from hawaiite (Trans-Pecos Province, Texas; D. S. Barker) and high-AI transitional olivine basalt (Gardar Pro- vince, Greenland; Upton & Emeleus), on the one hand, and to highly silica-deficient magmas such as olivine

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698 J . G. Fitton &

melilitite (SW German Province; J. Keller), on the other. In the Monteregian Hills Province (Canada), G. N. Eby concluded that felsic, silica-saturated rocks dominate in zones with thicker crust, while thinner crust correlates with a dominance of ultramafic and mafic rocks. Three separate categories of primary magma were invoked by A. R. Woolley & G. C. Jones to explain relationships in the Chilwa Province, Malawi.

Whereas a mantle origin for salic alkaline rocks of the continents was supported by evidence presented by several speakers, a critical role for crustal involvement was claimed by others. Macdonald concluded that the Naivasha (Kenya) comendites arose from volatile-induced melting in crustal chambers followed by extreme fractionation. According to P. Bowden, some of the Nigerian alkaline salic magmas originated within the crust. The abundance of salic rocks in the Chilwa Province was thought by Woolley & Jones to preclude derivation from a basic parental magma. Eby noted that the more silicic Monteregian Hills magmas must be due either to genesis within the crust or to contamination of mantle-derived melts by crustal materials. Assimilation of lower crustal granulites by mantle-derived magmas was proposed by H. Downes to explain the petrogenesis of the Cantal (France) volcanic rocks.

Formation of highly peralkaline, silica- undersaturated salic magmas (agpaites) extremely enriched in LILE appears to be wholly confined to the continents. L. N. Kogarko reviewed the agpaites of the Kola peninsula (USSR) and concluded that they originated from LILE-enriched mantle sources. The Ilimaussaq agpaites (Greenland) were considered by L. M. Larsen to have originated by extreme crystal fractionation in a closed magma chamber although she noted that there had been partial loss of a volatile phase. This, with Bowden’s evidence for alkali metasomatism around the West African alkaline granites, was a reminder that while fenitization is typically associated with carbonatites it is by no means uniquely confined to such magmas.

The frequent association of alkaline magmatism with continental rifting was discussed by several speakers. Examples ranged over geological time from the Proterozoic Gardar Province (Upton & Emeleus; Larsen) to the alkaline volcanoes of the Kenya Rift reviewed by Baker. Other studies of rift-related magmatism were presented by J. A. Gamble (McMur- do Volcanic Group, Antarctica), Keller (Rhine Graben), Macdonald (Kenya) and Woolley & Jones (Chilwa Province, Malawi). Rift systems occasionally develop into ocean basins leaving alkaline provinces along passive continental margins. One such example was described from the Tertiary of East Greenland by T. F. D. Nielsen.

A review of the Cameroon line volcanoes, which extend from the African continent to the Atlantic

B. G. J . Upton

Ocean, was given by J. G. Fitton. He showed that the basalts of the oceanic and continental sectors are geochemically indistinguishable. If mantle metasomatism is a necessary precursor to alkaline magmatism, thisLmust .have been identical in both the oceanic and the continental mantle sources. Rejecting this proposi- tion he concluded that the magmas originated in LILE-depleted mantle and underwent enrichment in LILE by scavenging during slow ascent through overlying mantle. He suggested that this process takes place beneath both rift valleys and oceanic islands.

The subject of alkaline magmatism on oceanic islands was taken up by a number of speakers. Reviews of individual islands and island groups were given by D. A. Clague (Hawaiian islands), J. H. Natland and E. Wright (SW Pacific), S. Steinthorsson (Iceland), C. Harris (Ascension Island), and A. Giret (Kerguelen). B. L. Weaver, D. A. Wood, J. Tarney & J.-L. Joron presented a review of geochemical variation among the South Atlantic islands and R. J. Tracy & E. M. Stolper discussed the evolution of alkaline magmas on Tahiti based on experimental studies. Discussions largely concerned partial melting of heterogeneous mantle sources and subsequent crystal fractionation of the resulting magmas. Clague showed that the late-stage primitive Hawaiian melilitites, nephelinites, basanites, and alkali basalts developed as small-degree melt fractions from garnet lherzolite that had undergone light rare-earth element enrichment shortly before the onset of magmatism. Steinthorsson struck out against the general consensus in favour of a mantle origin for oceanic alkaline magmas by propos- ing that the Icelandic alkaline magmas were generated by partial melting within the crustal lava pile.

In a discussion of the remarkable development of alkaline ring complexes of Kerguelen, Giret concluded that whether salic residual magmas evolved towards silica over- or undersaturation depended on whether or not calciferous amphibole was involved. This reflected on earlier comments by Downes who had also remarked on the control of differentiation trends at Cantal by the presence or absence of amphibole in the fractionating assemblage.

In a review of the alkaline rocks of Ascension Island, where comenditic magmas were derived by crystal fractionation from olivine basalt magmas, Harris discussed the nature of the fluid inclusions. He concluded that the alkali granite (xenoliths) had crystallized from a H20-saturated magma and that the aqueous fluids exsolved during this crystallization were composed of H20, CO*, NaCI, and KCl.

A review of lamprophyres presented by N. M. S. Rock showed that these could be divided into four categories and that some of these might represent primary magmas. This presentation and that by S. C. Bergman helped to clarify the distinction between lamproites and ultramafic lamprophyres. As became clear, however, from the reviews of Bergman and J.



Conference Report 699

B. Dawson, there may be considerable overlap between the definitions of lamproite and Group 2 (highly micaceous, ilmenite-poor) kimberlites. These reviews made clear the necessity of further revision and, if possible, simplification of the terminology of these extremely silica-undersaturated rock types.

Since 1974 the major changes in outlook on alkali rock petrogenesis have arisen from the far greater geochemical and isotopic data base. Emphasis has also shifted from crustal to mantle processes and the topic of mantle metasomatism accompanying or preceding alkali magmatism is now much more widely accepted. Processes clearly critical to the generation of the great variety of alkaline rocks include metasomatism (both in the mantle and the crust), variable degrees of partial melting of source rocks at varying pressures and partial pressures of H20 and CO2, crystal fractionation, magma mixing and assimilation of wall rocks. Processes whereby magma bodies may become vertically stratified (by upward diffusion of volatile components and complexed ions) and of trace element enrichment of magmas percolating upwards through the mantle are also gaining wider acceptance.

Received 31 January 1985

Mantle metasomatism-perspective and prospect D. K. Bailey (Univ. Reading, UK)

Mantle replenishment in lithophile elements has been discerned in the patterns of trace elements and isotopes in lavas. One replenishment process is identified as metasomatic replacement, seen in ultramafic xenoliths brought up in high velocity alkaline eruptions. Thus alkaline magmatism provides the best prima facie evidence of metasomatism and open system conditions in the upper mantle. The list of added lithophile elements includes the following: K, AI, Ti, Fe, Mn, Ca, Na, H, C , S, P, Rb, Sr, Y , Zr, Nb, Ba, and REE. Some metasomatism may be due to wall-rock altera- tion near magma bodies, but there are plenty of cases for metasomatism prior to melting, and for its necessity as a precursor to alkaline magmatism. In some igneous provinces the metasomatism is widespread, intense and pervasive; in others it appears as veining of variable intensity. Metasomat- ism as a large-scale process is best indicated by the widespread distribution of alkaline magmatism in space and time.

Metasomatism thus provides the possibility of a wide range of mafic melts from the mantle, giving new freedom to petrogenesis. But the freedom does not end there. Some major problems of the alkaline associations (and, indeed, the calc-alkaline) still await solution. These include the diversity of magmas, the generation of large felsic volumes, and composition gaps in magma series. The old concepts of magma evolution remain inadequate, even allowing for an increased range of mafic parents. As yet, the potential of metasomatic processes acting in different physical regimes has been scarcely considered, but can provide a key to the outstanding problems. This is part of the new horizon offered by open system melting.

Volcanic and petrochemical associations in the Kenya Rift alkaline province and their tectonic setting B. H. Baker (Univ. Oregon, USA)

In Kenya and N Tanzania, 220,000 km3 of alkaline volcanic rocks were erupted over 30Ma, giving an average eruption rate of 7.3 X 10-3 km3 yr-l. Most of this volume was erupted during the last 15 Ma in the central sector, p g a more representative linear eruption rate of 3x10- km3 yr-' per km of length of the central rift. Taking into account unexposed plutonic rocks, this is about the same magma production rate as that of mid-ocean ridges. Volcanic structures and their petrological associations show some regularities according to their position and the stage of evolution of the rift from pre-rift basin, to half-graben, to complex graben. The associations are (1) medium to large nephelinite-phonolite (ijolite-carbonatite) central cones on the western flank and southern termination of the rift; (2) very large central cones with mixed associations, nephelinite- phonolite and alkali basalt-trachyte, with mafic rocks dominant (these are located on the floor and eastern edge of the rift); (3) basanite-flood phonolite shields, with phonolite dominant, located in the central and northern rift and formed during its second (half-graben) phase of development; (4) transitional basalt-trachyte-alkali rhyolite shields and caldera volcanoes, formed within the axial graben; and (5) basanitic multicentre uplift. The within-rift basanite-phonolite and basalt-trachyte associations (3 and 4 above) are characterized by a preponderance of salic eruptives. The igneous rocks became less alkaline and more concentrated within the rift as it evolved. The earlier strongly alkaline rocks show evidence of high pressure fractionation, whereas the less alkaline suites differentiated in shallow, stratified, and replenished reservoirs. The alkalinity and compositional ranges of suites was determined by their tectonic setting, the depth and size of the reservoir, and the relative importance of crystal and liquid fractionation. Bimodality was caused by the scarcity of intermediate magma and the fluid dynamics of stratified reservoirs. Geochemical and isotopic data show that mafic and silicic rocks were comagmatic and that there was trivial crustal contamination. The implied volumes of parental magma are so large as to require a mantle plume to supply source rock.

Tertiary alkaline magmatism in Trans-Pecos, Texas Daniel S. Barker (Univ. Texas, Austin, USA)

Alkaline magmatism in far west Texas extended from 43 to 16Ma. BP, peaking in volume between 35 and 30Ma. Primitive lavas are rare, if not entirely lacking, but ne- and 01 + hy-normative hawaiite and mugearite liquids (Ni c 120 pprn; 100 Mg/(Mg + 0.9 Fetota,) 66)) erupted through the entire time interval. These and q-normative trachybasalt liquid evolved by fractional crystallization through benmore- ite and trachyte to phonolite and rhyolite liquids, including peralkaline varieties of both q-normative and ne-normative suites. Phenocrysts are olivine. Ti-augite, kaersutite, plagioclase, apatite, and titanomagnetite in the mafic magmas, and fayalitic olivine, hedenbergitic to acmitic clinopyroxenes, edenitic to arfvedsonitic amphiboles, and alkali feldspars in the felsic.

Remnants of shield volcanoes and caldera complexes preserve lava flows and domes, radial dikes, and ash-flow and



700 J . G. Fitton &

air-fall tuffs. Sills, laccoliths, plugs and discordant sheets occur in discrete clusters of silica-oversaturated or undersaturated bodies reflecting subjacent plutons with a mean diameter of 40 km. These clusters differ in ratios among Si, Ti, K, Rb, Sr, Y, Zr, and Nb, suggesting that each cluster was fed from a unique reservoir. Trace element data favour fractional crystallization over fractional fusion as the most satisfactory explanation of compositional diversity within each cluster, but magma mixing was locally important.

Fifteen nepheline-bearing felsic plutons each exceed 1 km2 in outcrop area; these and many smaller bodies define a belt at least 400 km long at the northeastern limit of the Trans-Pecos magmatic province parallel to the southwestern margin of the North American plate. Major and trace element ratios show gradation from the Trans-Pecos south- westward into the Sierra Madre Occidental of Mexico, an enormous tract of subduction-related calcalkaline rocks coeval with the Trans-Pecos alkaline rocks. Trans-Pecos magmatism mostly predated Basin and Range normal faulting, and its relation to compressional or tensional regimes remains unresolved.

Lamproites: their occurrence, geochemistry and mineralogy S. C. Bergman (ARCO, Dallas, USA)

Lamproites are potassium- and magnesium-rich lamprophyres that possess a diagnostic mineralogy and geochemistry. Known lamproites occur in 20 major suites or localities in continental regions with a variety of geologic and tectonic environments; they range in age from the early Proterozoic dikes at Holsteinsborg, West Greenland and Chelima, India, to the middle Pleistocene flows and the recent volcano of the Leucite Hills, Wyoming, USA and Gaussberg, Antarctica, respectively. Intrusive and extrusive forms of lamproites include flows, pyroclastics, cinder cones, dikes, sills, and diatremes. Whereas kimberlites tend to be carrot-shaped, the shape of lamproite diatremes approxi- mates a champagne glass. The recent discovery of diamondiferous lamproites of large volumetric proportion in the East and West Kimberleys, NW Australia and the reclassification of the diamondiferous micaceous peridotite at Prairie Creek, Arkansas, USA as a lamproite, substantiate their economic importance.

The petrographic diversity of lamproites has historically hindered the development of a concise and universal classification and nomenclature. Lamproites are distinguished from kimberlites and alkali-basalts (and other lamprophyres) in terms of mineralogy, geochemistry and volcanic extrusive character. Relative to kimberlites, lamproites are enriched in K, Si, Ti, AI, Rb, Sr, Zr, and Ba and depleted in COz, Ca, Mg, Fe, Ni, CO, and Cr. Lamproites are characterized by the diagnostic presence of leucite, K-richterite, phlogopite, diopside, and occasional glass, olivine, sanidine, priderite, perovskite, apatite, chrome spinel, Fe-Ti oxides, and wade- ite, whereas kimberlites contain olivine and phlogopite with occasional monticellite, pyrope, carbonate, clinopyroxene, perovskite, apatite, chrome spinel, and picroilmenite. Lam- proite magmas are produced by the partial melting of a refractory mantle peridotite (approaching a dunite or harz- burgite in mineralogy) that was enriched in K-bearing phases such as phlogopite most probably as a result of some previous metasomatic event.

B. G. J . Upton

Niger-Nigerian alkaline ring complexes: West African repre- sentatives of African Phanerozoic anorogenic magmatism P. Bowden (Univ. St Andrews, Scotland, UK) & R. F. Martin (McGill Univ., Canada)

Following the close of the Pan African orogeny during late Precambrian to early Palaeozoic times, Phanerozoic anorogenic magmatism expressed in part as alkaline ring complexes developed in various uplifted domains of Pan-African mobile zones throughout most of the African continent. The Niger-Nigeria anorogenic province represents one of these regions in Africa where progressive uplift has been accompanied by periodic sequential development of chains of volcanoes now exposed as ring complexes with Palaeozoic and Mesozoic Rb-Sr ages. The magmatic lineage comparable to other alkaline provinces can be established from preserved volcanic sequences. Associated andesitic compositions are attributed to magma mixing.

In Niger an important petrogenetic parameter is the occurrence of leucogabbros, anorthosites, and monzo- anorthosites as part of the Palaeozoic subvolcanic association with syenites, peralkaline granites and biotite granites. The partially eroded volcanic cover of rhyolitic ignimbrites has provided the source for substantial uranium deposits. Three southern centres of Upper Silurian-Lower Devonian age are mineralized with columbite and cassiterite.

The general geological and geochemical features of the Nigerian Triassic-Jurassic anorogenic centres are well known. While their magmatic derivation from mantle and crustal sources can be conclusively demonstrated from isotopic studies, petrological and geochemical research has shown that many of the younger granite centres demonstrate substantial evidence for postmagmatic metasomatism linked to mineralization. There are certain parallels between alkali metasomatism in alkaline granite complexes and fenitization associated with carbonatites.

Hawaiian alkaline volcanism David A. Clague (US Geol. Surv., Menlo Park, USA)

Hawaiian volcanoes are constructed of lavas erupted during four sequential stages. Stage 1 is characterized by submarine eruption of a variety of lavas ranging from basanite to tholeiitic basalt-Loihi seamount off the SE coast of Hawaii is presently in this eruptive stage. Stage 2 is characterized by eruption of enormous volumes of fluid tholeiitic basalt that forms gently sloping shield volcanoes. Stage 3 follows collapse of the summit caldera; a capping of alkali basalt and related differentiates may erupt from vents generally situated along the rift zones developed during the preceding tholeiitic stage. Stage 4 follows a period of volcanic quiescence of up to 2.5Ma; flows of melilitite, nephelinite, basanite, and alkali basalt may fill erosional valleys cut into the preexisting tholeiitic shield and alkalic cap. These stage 4 post-erosional lavas erupt in small volume from vents unassociated with preexisting structures.

Stage 1 and 3 lavas are generally moderately differentiated, indicating substantial residence time in subcaldera magma chambers. Stage 4 lavas, on the other hand, are near-primary magmas. Initial lavas from stage 1 have never been sampled, but we speculate that they may be near-primary strongly alkalic lavas similar to stage 4 lavas. Stage 1 alkalic lavas from Loihi and stage 4 lavas contain common lherzolite




xenoliths; a few post-erosional vents even contain deep- seated garnet-bearing xenoliths. In contrast, stage 3 alkalic lavas contain dunite, wehrlite, a variety of cumulate gabbroic xenoliths crystallized from earlier tholeiitic and alkalic magmas, and cumulates from oceanic crustal layer 3.

Trace element and isotopic data constrain models for the origin of all these lavas. In general, stage 4 lavas can be derived by varying small degrees of melting of relatively homogeneous recently light rare earth element (REE) enriched garnet Iherzolite. Lavas from each volcano are chemically distinct, but tholeiitic lavas have the most radiogenic Sr isotopic ratios and the least light REE enriched patterns, whereas strongly alkalic lavas have the least radiogenic Sr isotopic ratios and the most light REE enriched patterns. These observations can be explained by models of magma genesis in which the volume of lava erupted and the degree of partial melting increase from stages 1 to 2 and decrease through stages 3 and 4. The early and late lavas are equilibriated with deeper, more depleted sources than stage 2 lavas. Complex mixing relations are required between partial melts and geochemically distinct source regions, including the underlying oceanic lithosphere and at least two other source compositions.

The kimberlite clan-relationship to olivine- and leucite- lamproites J. B. Dawson (Univ. Sheffield, UK)

Kimberlites are rare, low-volume melts that originate in the diamond stability field of the earth’s upper mantle. They are petrographically complex since in addition to the magmatic component (high-pressure phenocrysts plus igneous matrix) they contain a variety of upper mantle and lower crustal xenoliths and xenocrysts. Wide mineralogical and chemical variations in the kimberlite matrix suggest kimberlite should be regarded as a clan or group of rocks rather than a single, narrowly-defined rock-type. In the Mesozoic kimberlites of South Africa, the ilmenite-rich, mica-poor variety (group 1) were intruded at between 80 and 90Ma whereas the highly micaceous, ilmenite-poor kimberlites (group 2) were intruded at dates varying from 100 to 140Ma. Moreover, recent neodymium and samarium isotope work suggests that these two groups of kimberlite were derived from isotopically distinct areas within the upper mantle.

Recent finds of diamonds in olivine and leucite lamproites in Western Australia indicate that kimberlite is not the only potassic, low-volume magma to originate in the diamond stability field. Furthermore, recent detailed petrography of the diamondiferous so-called kimberlite of Prairie Creek, Arkansas, USA, indicates that it is an olivine lamproite.

Nd and Sr isotope studies on the W Australian lamproites indicate that although they have strong affinities with the group 2 kimberlites they are isotopically distinct from the group 1 kimberlites.

Tertiary and Quaternary volcanisrn of the Massif Central, France Hilary Downes (Univ. Edinburgh, UK)

The continental alkaline volcanics of the Massif Central range in age from 65 Ma to 3450 yr BP and can be subdivided by geography and age into 20 separate areas. There is no correlation between the age of an area and its geographical location.

Two magma series are widely represented: a silica- undersaturated basanite-tephrite-phonolite series and an alkali basalt-trachyandesite-trachyte-rhyolite series. Rare nephelinites are also present. The primitive magmas of both main series may be derived by 2-20% partial melting of a light rare earth element-enriched mantle with variable amounts of garnet in the residue; nephelinites require a source which is chemically and isotopically distinct.

Fractional crystallization of observed phases accounts for the diversity of evolved lavas. Amphibole fractionation in basalts at depth, indicated by megacrysts and cognate xenoliths, causes the trend to silica-saturation, while alkali feldspar dominates the final stages of fractional crystallization. Removal of small amounts of sphene and zircon is also required to explain trace element depletions in phonolites and rhyolites.

Crustal contamination has occurred in the evolved magmas but is generally absent from their basic parents, suggesting the presence of crustal magma chambers. Sr, Nd, and new Pb data have been used to estimate the extent of contamination; the most probable contaminant is undepleted granulite-facies lower crust. Variations in Sr and Nd isotope ratios also constrain petrogenetic hypotheses such as amphibole fractionation and magma mixing.

Evidence of magma mixing is found in lava flows and nuCes ardentes. Mineralogical disequilibrium, compositional band- ing and emulsification are present, and the injection of basic magma into a more evolved magma body may have been the eruptive trigger. Some flows are homogeneous hybrid magmas in which only the unusual composition and rare mineral disequilibria indicate the mode of formation.

The Monteregian HiUs and White Mountain alkaline igneous provinces, Eastern North America G . Nelson Eby (Univ. Lowell, USA)

Continental alkaline magmas were emplaced in New England (White Mountain) and Quebec (Monteregian Hills) between 90 and 220Ma. Major periods of magmatism, which have been related to events in the opening of the North Atlantic Ocean, occurred between 117 and 141Ma and 168 and 196Ma (White Mountain province). In regions of thick continental crust felsic and silica-saturated rocks predominate, whereas in regions of thinner continental crust ultramafic and mafic rocks predominate.

During the younger period of magmatism, slightly silica- undersaturated to critically saturated magmas were emplaced between 128 and 141 Ma and strongly undersaturated magmas were emplaced between 117 and 121 Ma. These magmas were derived from an isotopically depleted mantle which was enriched in incompatible elements before melting. Petrogenetic models indicate that the parental magmas for the slightly undersaturated to critically saturated rocks were produced by 5 1 0 % melting of a garnet or spinel lherzolite source, while the parental magmas for the strongly undersaturated rocks were produced by very small degrees of melting of a spinel lherzolite source. Felsic rocks containing significant amounts of quartz crystallized either from mantle melts contaminated by crustal material or from magmas which originated in the lower crust.



702 J . G. Fitton & B. G. J . Upton

Experimental studies pertinent to alkaline rocks: an overview of the last decade Alan D . Edgar (Univ. Western Ontario, Canada)

Experimental petrology has always been an important tool for petrologists interested in the genesis of alkaline rocks. In contrast to the majority of earlier studies which were mostly pertinent to the genesis of alkaline rocks of felsic compositions, particularly those of nepheline syenites and alkali granites, experimental studies during the last 1&15 years have concentrated on alkali basalts, especially those of extreme compositions, which are believed to be melts from heterogeneous mantle sources.

This review concentrates on high pressure experiments done under various pressure conditions corresponding to those of the upper mantle. Experiments with both synthetic and natural compositions are described and the importance of volatiles, particularly H 2 0 and COz, is emphasized. These studies indicate that many alkali magmas may be derived by partial melting of heterogeneous mantle sources, in some cases quite different from model pyrolite mantle compositions. Experiments on mantle metasomatism and melting of metasomatized mantle nodules are also considered. Recent advances in experimental petrology pertinent to the genesis of alkali magmas derived at shallower depths are briefly reviewed. The direction which future experiments on alkaline rock genesis might take is also considered.

The Cameroon line, West Africa: a comparison between oceanic and continental alkaline volcanism J. G. Fitton (Univ. Edinburgh, UK)

The Cameroon line is a unique example of a within-plate volcanic province which straddles a continental margin. It consists of a chain of Tertiary to Recent, generally alkaline volcanoes stretching from the Atlantic island of Pagalu to the interior of the African continent. It provides, therefore, a unique area in which to study the differences, if any, between the sub-oceanic and sub-continental mantle sources for alkali basalt. Although the Cameroon line does not have a graben structure, its origin is closely linked to the development of continental rifts and its volcanic rocks are closely similar in their composition and range to those found in the Kenya rift.

The geochemical and isotopic data available show no significant differences between basaltic rocks in the continental and oceanic sectors. However, the more evolved rocks in the two sectors are quite distinct. The continental magmas evolve towards peralkaline rhyolites, whereas those in the ocean evolve towards phonolite. Progressive crustal contamination of the continental sector magmas accompanied by crystal fraction is required to explain the distinction between the evolved magmas in the two sectors.

The striking geochemical similarity between basaltic rocks in the two sectors suggests that their parental magmas had very similar mantle sources. This source must lie in the asthenosphere. The young Atlantic Ocean lithosphere mantle will be isotopically and chemically different from the old lithosphere mantle beneath Africa and so involvement of lithosphere would be readily detected in the geochemistry of the basalts.

Recent models of rift valley evolution have stressed the importance of mantle metasomatism as a precursor to magmatism. If this is correct then the oceanic and continental

basalts in the Cameroon line must have been produced from mantle which has undergone idenrical metasomatic processes. Metasomatized mantle xenoliths are abundant in continental alkali basalt provinces (including the continental sector of the Cameroon line) but are very rare in oceanic islands. This may indicate that these xenoliths originate in the lithosphere. Continental lithosphere mantle is older and therefore more likely to have suffered metasomatism than oceanic lithosphere mantle. It follows that these xenoliths are not related to the genesis of their host basalts, at least in the Cameroon line.

If the Cameroon line data can be applied to rift-valley magmatism in general then it implies that the strong large-ion lithophile (L1L)-element enrichment observed in rift-valley basalts cannot be metasomatic in origin. An alternative explanation is proposed in which basaltic liquid generated at depth in a LIL-element depleted mantle rises to the surface along grain boundaries and become strongly enriched in LIL elements by a process akin to zone refining. This model implies a similar origin for rift-valley and ocean-island magmas and accounts for the similarity between the two.

The McMurdo volcanics of the Ross Dependency, East Antarctica J. A. Gamble (Victoria Univ. Wellington, New Zealand)

In the Ross Dependency of E Antarctica the McMurdo Volcanic Group comprises a rifted intraplate volcanic association varying from primitive basanite to evolved trachyte, phonolite, and peralkaline rhyolite. The available K-Ar age data indicate that volcanism commenced in Miocene times and ages range from 15.4Ma to a presently active volcano (Mt Erebus). A variety of volcanic forms and eruptive mechanisms can be recognized with large strato volcanoes of basanite and evolved lavas (e.g. Mts Discovery and Erebus), scoria cones of basanite (e.g. Foster Crater, Sulphur Cones) and peralkaline ignimbrites, possibly associated with caldera collapse structures (e.g. Mts Morning and Overlord). Subglacial eruptions have formed extensive areas of hyaloclastite.

In the Erebus Volcanic Province two fractionation lineages linking basanite and phonolite are distinguished by the presence or absence of kaersutite. Many, but not all, of the basanitoid scoria cones are hosts to a diverse suite of crustal and mantle nodules and megacrysts (kaersutite and clinopyroxene). The mantle sample is mineralogically and texturally complex and includes spine1 peridotite, dunite and pyroxenite with variable amounts of amphibole and mica. The mica (glimmerite) and amphibole-rich nodules are considered to represent disrupted vein systems from the subcontinental mantle, rare specimens consisting of both vein and wall rock. Petrogenetic modelling of the primitive basanites suggests derivation from a heterogeneous garnet-bearing source region.

Alkaline volcano-plutonic complexes in Kerguelen Islands; Southern Indian Ocean Andre Giret (Univ. Pierre et Marie Curie, Paris, France)

Kerguelen Islands, which cover a surface of 6500 km2, form the third largest oceanic archipelago after Iceland and Hawaii. Their geological history began about 50Ma ago,




when they were probably on the Eastern Indian Ocean Ridge, with abundant flood basalts of tholeiitic, transitional, and alkaline compositions. The islands are now in a typical intraplate location and they have been intruded since 26 Ma to Recent by alkaline ring-complexes displaying different structural horizons from volcanic to plutonic, according to the level of erosion.

The alkaline complexes are composed of a wide variety of rocks, from gabbros and intermediates to differentiated syenites and granites, and they constitute two provinces whose boundary can be related to the lithospheric structure of this oceanic area. The first is a series evolving to quartz-bearing rocks (syenites and granites) and it occupies a western position. The second which evolves to nephelinesyenites lies to the east.

The petrological studies suggest a common initial magma for all of these alkaline complexes. The evolution towards silica oversaturated or undersaturated liquids is mainly governed by the fractionation or the non-fractionation of the calciferous amphiboles.

Magma and fluid evolution in the volcanic rocks of Ascension Island Chris Harris (Univ. Oxford, UK)

The lavas of Ascension Island range in composition from olivine-basalt through hawaiite, trachybasalt, trachyandesite, and trachyte to comendite (SiOz = 74wt%). Major and minor element variation within the suite suggests that crystal fractionation is the major differentiation process and this is confirmed by the existence of a large number and variety of cumulate plutonic inclusions. The most evolved plutonic inclusions are alkali-granite in composition and are chemically very similar to the comendites. The presence of fluid inclusions in these granites enables direct study of the fluids associated with acid peralkaline magmas to be made. Mixed silicate meltkaline-fluid inclusions show that the granites crystallized from an H20 saturated magma and the initially exsolved fluid evidently belonged to the system H20-CO2- NaCI-KCI. The 6"O and 6D values for separated amphibole are consistent with a mantle origin for this fluid. The fluid inclusions show that unmixing of the fluid into C02-H20 rich 'vapour' and NaCI-KCI-HzO 'fluid' occurred. Dissipation of CO2-H,O vapour in this manner may be an important influence on trace element chemistry in other peralkaline volcanoes (although not apparently in this one).

Tertiary alkaline volcanism of SW Germany: Rhinegraben, Kaiserstuhl, Hegau, Uracb Jorg Keller (Univ. Freiburg, FRG)

Tertiary volcanism in SW Germany is exclusively alkaline and is dominated by highly silica-undersaturated magmas. Olivine melilitite is a common magma composition in all sub-provinces.

Volcanic activity is related to multiphase rifting of the Rhinegraben system, the more distant areas being considered as shoulder activity. Volcanism started about 80 Ma and peaked during the Miocene at about 2&10 Ma.

Geographically and compositionally three sub-provinces are distinguished:

Southern Rhinegraben has dominantly unfractionated olivine nephelinites and olivine melilitites occurring as dykes, necks, and diatreme-pipes along the graben faults. Starting at

80Ma, this activity precedes and alternates with the major rifting phases of the graben. Kaiserstuhl is the only larger volcanic complex within the graben and shows in a restricted time interval (18-13 Ma) a great complexity of highly evolved magmatic series (e.g. phonolites, leucite-tephrites, melilitites, carbonatites).

In the Hegau area, about 60 km east of the graben, primitive olivine melilitites are associated with obvious petrogenetic links with highly fractionated phonolites. Evolved melilitites and carbonatites have recently been recognized among the juvenile components of voluminous diatreme tuff-breccias. The age range of Hegau volcanism is 15-7 Ma.

The diatreme province of the Swabian A16 near Urach is formed by >300 tuff-pipes and associated olivine-melilitite intrusions. The age range is 2C-10 Ma.

"Sria6Sr for olivine nephelinites and olivine melilitites are 0.7034-0.7038. Values increase up to 0.7045 in evolved magmas. Sr and Nd isotope ratios indicate an originally depleted, oceanic-island type mantle source which became enriched in large-ion lithophile and rare-earth-element con- centrations with the evolution of the rifting system.

Geochemistry of the alkaline rocks of the eastern part of the Baltic Shield (Kola Peninsula) L. N. Kogarko (Vernadsky Inst. Geochemistry and Analytical Chemistry, Moscow, USSR)

Two large epochs of the development of alkaline magmatism on Kola peninsula are distinguished according to our isotopic and geochronological data: middle Proterozoic (alkali-gabbro massifs: Grem'yaka Vyrmes, Yelet' Ozero) and Palaeozoic (numerous massifs of alkaline ultrabasic rocks: Afrikanda, Lesnaya, Ozernaya Varaka, Kovdor, Turij Mis, etc., and gigantic complexes of agpaitic nepheline syenites: Khibina and Lovozero).

Isotopic investigations showed that ultrabasic alkaline massifs, agpaitic complexes and apatite and rare-metal deposits connected with them have mantle sources ("Sr/86Sr = 0.7038-0.7040). The surprising homogeneity of the Devo- nian mantle of Kola peninsula in respect of radiogenic isotopes is of great interest. Palaeozoic ultrabasic alkaline complexes develop under one and the same scheme: olivinites-pyroxenites-melteigites-ijolites-nephelinesyenites- carbonatites.

Two lines of differentiation of Khibina massif distinguished are ijolites-rischorrites (high-potassium nepheline syenites), and khibinites-foyaites-agpaitic nepheline syenites. A special trend of accumulation of dark-coloured minerals in the process of differentiation takes place in the Lovozero intrusion: miaskitic nepheline syenites-agpaitic foyaite- lujavrite. The geochemical evolution of Khibina and Lovozero is considered. Experimental investigations truly show that agpaitic magmas are dry (H20 content -4.5%) and, of those which equilibrated at 1000°C, redox conditions correspond to the Q-F-M buffer. Experimental studies of ApNe-Di-H,O and Eud-Ne-Px-H20 melting diagrams allowed us to construct models for the genesis of apatite and eudialyte deposits. Using data on geochemistry of ultrabasic effusives of the Kola peninsula, the composition of the upper mantle was determined. Mantle of the Palaeozoic complexes was anomalously enriched in rare lithophile elements and light rare earths.



704 J . G. Fitton &

The Iliaussaq intrusion revisited the evolution of an agpaitic magma Lotte Melchior Larsen (Geol. Surv. Green- land, Copenhagen, Denmark)

The strongly peralkaline undersaturated (agpaitic) magma of the Ilimaussaq intrusion differentiated in a closed magma chamber measuring around 17 X 8.5 X > 1.5 km. The bottom is not known. The exposed uppermost 1500m of agpaitic rocks comprise both roof and bottom cumulates and a ‘sandwich’ horizon. In the early stages heat loss through the roof was relatively high, and crystallization proceeded from the roof downwards. Concomitant bottom cumulates probably formed but are not exposed. The magma probably developed layering, with volatiles concentrating upwards; this would produce a stable situation with cooler and lighter magma towards the top of the chamber. Individual magma layers could still convect. The upwards concentrating volatiles eventually suppressed the crystallization under the roof. The exposed bottom cumulates are younger than the roof rocks; they are rhythmically density stratified and show signs of magma convection. The layers are horizontally persistent, and each layer is isochronous. The ‘sandwich’ horizon formed after a major roof collapse had divided the remaining 200- to 300-m magma column into isolated subchambers and brecci- ated the magma-exposed and altered lower surface of the roof cumulates.

Differentiation mechanisms in the agpaitic magma include volatile transfer (probably by double-diffusive convection), crystal fractionation (where fractionation of eudialyte led to characteristic changes in element ratios with differentiation), formation of metal complexes in the melt, formation of a fluid phase, and escape of part of this in the closing stage of consolidation. These processes have been documented geochemically.

Nepbeliiites and carbonatites M. J . Le Bas (Univ. Leices- ter, UK)

Carbonatites are found in strongly alkaline intraplate petrographic provinces associated with olivine-poor nephelinites and with phonolites. Olivine-rich nephelinites occur in basanitic and alkali basaltic provinces, normally without carbonatites. The alkalinity is largely controlled by the PH,o/Pco, ratio in the asthenosphere source. Alkaline igneous provinces are marked by epeirogenic uplift, some- times with rifting.

Nephelinites, ijolites and carbonatites form discrete intrusive events and correspond to the silicate-carbonate conju- gate immiscible relationships observed in the laboratory. Carbonate liquids of variable alkali content can separate from both nephelinitic and phonolitic liquids. The silicate liquids give rise to ijolites, nepheline-syenites and nephelinitic pyroclast-rich strato-volcanoes. The carbonate liquids lose alkalies and fractionate to sovite, alvikite and ferrocarbonatite each with increasing incompatible element content. Further fractionation of carbonatite magma can produce mineralizing fluids rich in rare earth elements, F, Ba, U and Th. Primary dolomite carbonatite forms only at depths in the crust greater than c. 2 km; secondary dolomitization also occurs. Explosive carbonatite volcanism produces widespread carbonate tuffs, rarely with lavas.

Fenitization characterizes nephelinite-carbonatite complexes. Syenitic fenites form as 500-m wide aureoles around

B. G. J . Upton

ijolites in granitic terrains, with nepheline-syenite commonly formed as a contact reaction rock. Chemically distinct feldspar-rich syenitic fenite aureoles develop around the early carbonatites.

Quaternary peralkaline silicic rocks and caldera volcanoes of Kenya R. Macdonald (Univ. Lancaster, UK)

The late Quaternary trachytic caldera volcanoes of the Kenya Rift provide an unrivalled opportunity to study the mechanisms of evolution of large peralkaline volcanic complexes and to assess the fundamental problems of magma genesis and chemical differentiation. In a northern set of centres (The Barrier, Emuruangogolak, Silali, and Paka) basalt is a major component, and mugearites, while scarce, may also be present. Caldera collapse was possibly of Kilauean type. Geochemical variations and the relationships between eruptive rocks and suites of plutonic nodules are consistent, with fractional crystallization being the dominant differentiation mechanism in these centres.

The southern, basalt-absent, centres (Menengai, Longo- not, and Suswa) show Krakatau-style collapse. The pyroclastic sequences, especially syncaldera ash-flow deposits, indicate the ubiquity of striking compositional zonations within the magma chambers. Chemical variations in these centres have been ascribed to complex interplays of sidewall crystallization, magma mixing and liquid-state differentiation processes, especially involving volatile complexing. Transi- tions from ne- to q- and from an- to ac-normative character have been recorded in the trachytes. The ultimate origin of the trachytes at these centres is still debatable; crystal fractionation and partial melting of basalt are both viable mechanisms. There is no evidence, however, of a substantial component of ancient continental crust in their genesis.

The comendites of the young Naivasha dome field are different: they apparently formed by volatile-induced melting of crustal sources, followed by extreme differentiation in high level chambers related to volatile complexing f thermo- diffusion.

Alkaline magmas: a window on the earth’s mantle Martin Menzies (Open Univ., Milton Keynes, UK)

Alkali basalts provide a valuable insight into mantle processes since the majority are derivatives of mantle liquids and basanitic varieties transport ultramafic xenoliths to the surface. Trace element characteristics of alkali basalts reveal that their mantle source is enriched in large ion lithophile (LIL) and light rare earth elements (REE) relative to the source of mid-ocean ridge basalts MORB. Furthermore, the extreme depletion in heavy REE ([Ce],/[Yb],) is consistent with (a) derivation from a garnet-bearing source region, or (b) derivation from spinel-bearing mantle modified by influx of a low degree of melt from the garnet stability field. Isotopic variation suggests that alkaline magmas are derived from source regions that have been depleted in light REE for a considerable period of time. One can conclude from this that the isotopic characteristics of alkaline magmas, in particular those that contain mantle nodules, do not represent a time-integrated response to their trace element ratios (high Rb/Sr and NdSm). This decoupling can best be explained by relatively recent enrichment events in depleted




mantle prior to melt formation. An insufficient period of time has elapsed (c0.2 billion years) to permit the isotopes to adjust to the recent addition of Rb and Nd to the mantle.

Alkaline basalts in themselves act as an important transport mechanism for LILE and light REE through the earth’s mantle. Interaction of high-temperature basanite and mantle wall rocks leads to chemical modification of wall rock adjacent to melt conduits and the production of mantle veined with pyroxenite segregations or more evolved pegma- tites (amphiboles f apatite f feldspar). This alkaline volcanism has a profound effect on the petrology and geochemistry of the earth’s mantle due to chemical transfer.

Alkaline volcanism in the SW Pacific James H. Natland & Elizabeth Wright (Scripps Inst. Oceanography, La Jolla, USA)

Alkaline lavas of the youthful linear island chains in the SW Pacific are particularly enriched in K, Rb, Ba, and s7Sr/86Sr. Thus alkaline basaltic lavas have low Na20/K20 (1.5-2.5), intermediate lavas commonly carry amphibole, and magmatic lineages with diverse trachytic and phonolitic residua are well developed. The lineages can be traced from distinctive basaltic parents, even where two lineages occur on the same volcano (e.g. Tahiti, and Ua Pu, Marquesas). The greatest known petrologic and isotopic diversity occurs in the Samoan chain, where three lineages occur on different volcanoes, and S7Sr/s6Sr ranges from 0.7042 to 0.7066, generally increasing during the history of the volcanoes as well as westward along the chain. In greatly incised, deep structural levels of Samoan shield volcanoes, tholeiitic and alkalic basalts are interbed- ded, whereas consistently alkalic lavas occur at higher structural levels. The least depleted basaltic lavas have erupted along a 300-km long post-erosional volcanic rift zone produced by plate bending into the nearby Tonga Trench. This demonstrates a laterally widespread rather than focused (plume-type) undepleted source for the youngest Samoan lavas. Similarities of these to lavas of the other SW Pacific chains suggest that this source extends over 2000 km to the east. Similarities to lavas dredged from Cretaceous seamounts in the central and western Pacific, and reconstruction of plate motions, indicate that this source has been active for c. 120 Ma. The Cretaceous seamounts are unusually shallow because plate motion carried them over a prominent plate mantle swell, the Darwin Rise. The subdued relic of this swell occurs today as a geoidal high in the SW Pacific, and we associate it with the mantle sources of lavas of SW Pacific volcanic islands.

Tertiary alkaline magmatism in East Greenland a review T. F. D. Nielsen (Geol. Surv. Greenland, Copenhagen, Denmark)

There are still large gaps in our knowledge of the Tertiary alkaline magmatism in East Greenland, related to the opening of the North Atlantic. The alkaline rocks are found along more than 1200km of the East Greenland coastline and up to 300 km inland. The chemical range is very large-from mildly alkaline to peralkaline and from ultramafic to strongly differentiated. Even carbonatites occur. Three general types of alkaline magmatism are identified: (a) nephelinitic-carbonatitic-basanitic-andesitic magmatism in

inland areas, (b) late, variable mildly alkaline to strongly alkaline magmatism in coastal regions, and (c) granitic to foyaitic plutonism and related dyke swarms.

The review focusses on the relationship between the alkaline magmatism and the continental break-up process and the restrictions imposed by this on the petrogenetic models. The chemical variation is believed to reflect heterogeneity of source, variation in depth, and duration of storage and contamination with upper mantle or crustal material.

The nature and origin of lamprophyres: an overview N. M. S. Rock (British Geol. Surv., Edinburgh, UK)

This paper is an attempt to expurgate many of the myths which have become attached to lamprophyres. Far from being rare rocks, lamprophyres are collectively among the most abundant of alkaline rocks. Far from being difficult to recognize, many are identifiable from even the most cursory field or thin-section examination. Far from being difficult to classify, lamprophyres can be divided into four petrologically and mineralogically quite distinct ‘branches’: Calc-alkaline lamprophyres (minettes etc.), alkaline lamprophyres (camp- tonites etc.), ultrarnafc lamprophyres (alnoites etc.), and lumproites (orendites, etc.). With few exceptions, members of different branches are nowhere associated together within coeval rock-suites. Far from being mere textural variants of ‘common’ igneous rocks, all lamprophyres are of distinctive and, in some cases, unique composition. Far from being late-stage residua of insignificant volume, some lamprophyres may represent primary mantle melts. Far from being petrogenetically insignificant, lamprophyres may tell us at least as much about lower crustal and mantle processes as perhaps even granites or kimberlites.

Mafic magmas from the Quaternary Eifel volcanic fields H.-U. Schmincke, H. Mertes & L. Viereck (Ruhr-Univ., Bochum, FRG)

Two adjacent Late Quaternary volcanic fields in the Eifel area (Germany) are made up of c. 240 (W Eifel) and c . 100 (E Eifel) volcanic centres erupted during the past c. 0.7 Ma. Magmas are K-rich (K,O/Na,O = c . 1) and comprise leuci- tites, nephelinites, olivine nephelinites, melilite nephelinites, and basanites.

In the W Eifel most magmas are primitive (as indicated by major- and compatible trace-element composition and higher abundance and larger size of lherzolite nodules) compared with those of the E Eifel. Volcano spacing and degree of differentiation increase towards the centre of the main field which reflects a NW-SE oriented magma detachment zone in the mantle about 50 km long. Age and chemical data allow the field to be subdivided into an older foiditic part, partly overlapping with a smaller, younger 01-nephelinite-basanite subfield in the southeast which developed during the past 0.1 Ma. These magmas possibly represent larger degrees of partial melting generated at shallower depth.

Two sharply bounded adjacent mantle domains are also distinguished in the E Eifel, both the older foiditic western and the basanitic eastern subfield showing increasing volcano spacing and larger volumes of intermediate and highly evolved magmas towards the centres of the fields. The magma volume of the youngest eruption, S km3 of highly



706 J . G. Fitton &

evolved phonolite from Laacher See volcano erupted 11,000 years BP, exceeds the combined volume of all (c. 300) mafic Eifel volcanoes.

The chemical character of the Eifel magmas will be contrasted with mafic potassic magmas from subduction- related tectonic settings.

Icelandic alkaline rocks Sigurdur Steinthorsson (Science Inst., Univ. Iceland)

Alkaline rocks appear in Iceland in two off-rift areas, the Vestmann Islands off S Iceland and the Snaefellsnes volcanic belt in W Iceland. The volcanics in both areas are shown to be largely crustal in origin, in sharp contrast to the generally accepted mode of origin for oceanic alkaline volcanism.

Resulting from the NW drift of the North Atlantic plate system relative to the Iceland hotspot, the rift system has been successively relocated to the southeast during the geologic history of Iceland. While developing, the new rifts propagate through older, chemically and mineralogically stratified crust, giving rise to alkaline volcanism at the leading tip, followed by FeTi basalts and later by tholeiitic rocks. This distinctive temporal and spatial spectrum of compositions results from the interaction of primary mantle-derived ocean tholeiite melt with the stratified crust.

Conversely, the Snaefellsnes belt constitutes a volcanic outlier in the Western plate, a dying remnant of the relocated rift system. The volcanism is of deep crustal origin: deep as shown by the lack of hydrothermal activity in the area, and crustal as shown by Sr isotopes.

The chemistry of the two alkaline areas is almost identical both in major and minor elements. However, the volcanic centres of Snaefellsnes have evolved towards slightly more potassic compositions owing to smaller influx of mantle- derived olivine tholeiite.

A reinterpretation along similar lines for Hawaii and the Canary Islands shows the initial alkaline phase of intraplate plume activity (Loihi, Lanzamte) to be tectonically and petrochemically closely similar to the tip of a propagating rift, whereas the final transitional to alkaline rock suites of these islands resemble the plate-trapped volcanic centres of Snaefellsnes. Oceanic alkaline volcanism in general is thus interpreted in terms of the interaction of a primary tholeiitic source and the oceanic crust.

A petrogenetic model for Tahitian volcanism Robert J. Tracy & Edward M. Stolper (Yale Univ., USA)

Volcanism in the Society Islands, and on Tahiti in particular, is dominantly alkaline. Of rock analyses known to the authors, 91 of 106 Society Islands rocks are Ne-normative (86%) and 70 of 73 Tahitian rocks (97%) are Ne-normative. The apparent reason for this behaviour is that the suite of Tahitian volcanics was generated through variable fractionation of an alkaline parent magma that was sampled at various times in a subvolcanic magma chamber.

To test this model, we performed a series of crystallization experiments on a Tahitian basanite (CIPW norm: Or 6.3, An 16.4, Ne 10.3, Lc 1.1, Di 28.4, 0133.1, Ilm4.0). The occurrence of abundant spinel lherzolite xenoliths in the basanite, its high Mg-number (0.70) and generally primitive character made the basanite a candidate as a possible

B. G. J . Upton

Tahitian parent magma. Experiments were done at 1 atmosphere at QFM using a CO-C02 atmosphere and Pt-Fe loops and at 5, 7.5 and 10 kbar in MO capsules in a solid media apparatus. Compositions of quenched glass and crystals from each experiment were determined by electron microprobe. Comparison of compositions of experimental liquidus olivines with basanite olivine phenocrysts (Fo 89) suggests that the natural rock is actually a quenched liquid and does not represent phenocryst-enriched material. The liquidus at 1 atm is 1350°C and increases slightly with increasing pressure. Onset of Cpx crystallization is at 1170" (1 atm), 1225" (5 kbar), 1250" (7.5 kbar), and 1275" (10 kbar). Compositions of fractionated liquids (represented by quenched glass) in the experiments are very similar to most of the compiled analyses of Tahitian extrusive rocks. The experimental results especially suggest an 01-Cpx control on fractionation at 5-8 kbar to explain the compositional trends of the natural samples. This may indicate the presence of a magma chamber beneath Tahiti in which a parent magma similar to our starting compositions (the basanite) was fractionating to produce successive batches of alkaline magma. The occurrence of spinel lherzolite xenoliths in the basanite, its high Mg-value (0.70) and the fact that the basanite composition can yield the great bulk of the Tahitian volcanic suite upon fractionation suggest that the basanite probably represents a primitive, mantle-derived magma which in this very unusual case bypassed the magma chamber(s) and came directly to the surface.

Alkalic carbonatite magmas: parental or derivative? James D. Twyman & John Gittins (Univ. Toronto, Canada)

During the 1950s and 1960s the Tanzanian volcano Oldoinyo Lengai erupted a highly alkalic lava composed almost exclusively of the Na-K-Ca carbonate minerals nyerereite and gregoryite and containing about 32% Na20 and 7% K20. For many years the lava was considered a petrological curiosity, but gradually the idea developed that alkalic carbonatite magmas might develop generally during the evolution of commoner carbonatite magmas. In 1981 Le Bas ascribed to the Oldoinyo Lengai magma a parental status and erected a scheme whereby other commoner carbonatite rock types are derived from it. The alkalic carbonatite magma is one of two derived by immiscible separation from a nephelinitic magma, the other being ijolitic. The carbonatite liquid has 500" of superheat and loses alkalis progressively in an aqueous fluid that is said to escape from the magma as it cools to its liquidus temperature of 40G600"C, by which stage it has become a calcitic-dolomitic liquid. This liquid then differentiates to produce the commoner types of carbonatite rocks.

We argue that such a scheme would require the magma to be water-saturated at the moment that immiscibility occurs in order for an aqueous fluid to develop and that progressive loss of water and alkalis would induce crystallization, thus preventing the formation of a calcite-dolomite magma. We argue that the 500" of superheat is a consequence of the design of the Freestone and Hamilton experiments and does not exist (the carbonatite magmas generally begin to crystallize at temperatures greater than 900"C), that rare earth element compositions do not permit of an immiscible derivation of alkalic carbonatite magma from nephelinite




magmas, and that the mafic silicate mineralogy of carbonatites demonstrates alkalic enrichment rather than alkali depletion.

We propose that the commonest parental magma of carbonatites is a mildly alkalic olivine sovite composition and that most carbonatite rock types are derived from it by fractional crystallization, allowing cumulates to develop that are well lubricated by inter-cumulus liquid and capable of much subsequent movement, deformation, and re-intrusion as a crystal mush.

Continued fractionation of alkali-deficient and anhydrous minerals increases both the alkali and water contents of the magma until water-saturation is reached. This in turn causes the development of an aqueous fluid which buffers the alkali content of the magma and limits the degree to which alkali enrichment can develop. The composition of the magma at which water-saturation develops is controlled by the rate of rise of the magma and so aqueous fluids of varied Na/K can develop and control the type of fenitization that occurs. Under plutonic conditions alkalies in excess of the amount that can be fixed as silicate minerals are carried away as fenitizing fluids.

In a dry magma, kept liquid by alkalies and fluorine, fractionation may produce an alkalic carbonatite liquid of Oldoinyo Lengai type because no aqueous fluid develops to remove alkalies and buffer the magma composition.

Alkalic carbonatite magmas are, therefore, late differentiates of a more normal mildly alkalic olivine sovite magma developed under low water fugacity. They are of very small volume and require long quiescence to develop. They are not to be considered parental to other carbonatite rock types.

The Gardar alkaline province, South Greenland B. G. J. Upton (Univ. Edinburgh, UK) & C. H . Emeleus (Univ. Durham, UK)

Intraplate basaltic magmatism related to extensional tectonics was widespread in South Greenland during the interval 135C1150Ma. Basic dyke swarms and fissure-fed lavas, mainly of mildly alkaline basalts, tend to have ‘troctolitic’ compositions, with high AI,O,/CaO ratios and low normative clinopyroxene contents. Anorthositic xenoliths are common in the basic and intermediate rocks. Alkaline salic rocks, mainly of trachyte, quartz trachyte, alkaline rhyolite or phonolitic parentage are principally represented by annular (sub-volcanic) intrusive complexes emplaced during tectonically quiescent (non-dilatational) interludes.

Field, mineralogical, geochemical, and isotopic studies point to an intimate genetic relationship between the salic and basic magmas. Generally low 87Sr/s6Sr ratios for the salic rocks (typically 0.7035-0.7045), and the occasional presence of rock-suites showing compositional continuity from basic to salic via Fe-rich intermediaries, suggest that crystal fractionation processes were of primary importance in the genesis of the alkaline salic magmas and that crustal contamination may have been relatively insignificant.

It is concluded that basic magmas, comparable to those erupted to high crustal levels, were largely retained at depth (mantle-crust boundary?) and underwent extensive fractionation, producing ultramafic (mainly dunitic) and coarse anorthositic cumulates. The latter acted as a crustal under- plating which was highly susceptible to stoping during intermittent ascent of relatively undifferentiated, denser,

basic magmas, thus producing the xenolith suites. Low- density salic magmas with high Fe/Mg, and often high (Na + K)/AI ratios produced as residues after the prolonged plagioclase separation involved in anorthosite genesis, rose through the crust largely by stoping and/or ring-faulting.

Geochemical nature of mantle sources of Atlantic Ocean island basalts B. L. Weaver, D. A. Wood, J. Tarney & J.-L. Joron (Univ. Leicester, UK)

Oceanic islands are commonly regarded as hot-spots that tap mantle reservoirs significantly different in composition from the asthenospheric source of mid-ocean ridge basalts, and which frequently erupt alkalic lavas. Radiogenic isotope ratios of ocean island lavas exhibit considerable diversity both between islands and within individual islands. This has been taken to indicate that several distinct chemical components are involved in the evolution of their mantle sources. Here we report the preliminary results of a comprehensive study of the major and trace element chemistry of basaltic lavas from 11 Atlantic Ocean islands and island groups, and attempt to integrate these with isotopic data to place constraints on the processes involved in the development of the mantle sources of alkali basalts.

All ocean island lavas are enriched in the incompatible trace elements, with high levels of the rare earth elements (REE) and large ion lithophile elements (LILE). There is, none the less, wide variation in their trace element composition, both in terms of element abundances and ratios (e.g. degree of light rare earth element (LREE) enrich- ments). Each island (or island group) exhibits a relatively consistent trace element character, although small differences are seen between different islands in the same group (for example in the Cape Verde Islands).

In assessing the chemical characteristics of the mantle sources for ocean island volcanics it is important that fractionation effects due to partial melting can be con- strained. Consideration of highly incompatible trace elements (e.g. Th, Ba, Rb, K, Nb, Ta, and the LREE) in rocks derived by moderate degrees of melting (alkali basalts) minimizes this problem. Ocean island basalts are characterized by a pronounced enrichment in Nb and Ta relative to the LREE and LILE. However, the magnitude of this enrichment (as expressed by La/Ta and ThRa ratios) is variable. Basalts from Gough and Tristan da Cunha (which among ocean islands have distinctive Nd and Sr isotopic compositions) have appreciably higher ThiTa and LaKa ratios than Ascension, St Helena, and Bouvet. High Th/U ratios for Gough and Tristan da Cunha accord with high 208Pb/Z04Pb ratios. These features (particularly the marked relative depletion in Nb and Ta) suggest a subducted sediment component in the mantle source for these two islands, as implied by He isotope data. Ocean islands such as Ascension, St Helena and Bouvet may be derived from depleted mantle sources which have been metasomatized by an undersaturated liquid which is strongly enriched in incompatible trace elements.

The Chilwa alkaline igneous province, Malawi: petrochemis- try and petrogenesis A. R. Woolley & G. C. Jones (British Museum (Natural History), London, UK)

The Chilwa Province of alkaline igneous rocks and carbonatites lies at the southern end of the East African Rift, and is



708 J . G. Fitton &

unique within the Rift for its essentially intrusive nature, extrusive rocks being minimal. The province comprises numerous carbonatite centres, some with nepheline syenite and nephelinite, large complexes of nepheline syenite and syenite and plutons of essentially peralkaline syenite, quartz syenite, and granite, together with dykes equivalent to all the major rock types. Over 200 rocks from the western half of the province have been chemically analysed and these seem to indicate that three rock series are present, namely (a) carbonatite-nephelinite (?basanite)-nepheline syenite, (b) nepheline syenite-syenite and (c) syenite-quartz syenite- granite, and it has not proved possible to reconcile these to a scheme of differentiation from a single parental magma. Furthermore, the overwhelming salic nature of the province seems to preclude the likelihood of differentiation from a basic parental magma or magmas, so it is concluded that three primary magmas of carbonatitic-nephelinitic, phonolitic, and quartz trachytic type were involved. A model for the production of these three magmas is suggested, involving partial melting at different levels of a zoned, metasomatized (fenitized) lithosphere wedge produced by the mechanism of lithosphere focusing proposed by D. K. Bailey. The metasomatism may have reached the base of the crust, and it was from this region that quartz trachyte magmas were produced by the melting of rocks similar to syenitic fenites, the degree of peralkalinity of the magma being determined crucially by the level at which melting took place. Phonolitic magmas

B. G. J . Upton

were generated directly at intermediate levels and nephelinites at the base of the metasomatized wedge. The relationship of nephelinite and carbonatite magma is discussed and the possibility suggested of a discrete carbonate-rich fraction existing in the source region rather than separating as an immiscible liquid just before emplacement. The beginning of the Chilwa igneous event is envisaged as being triggered by the fracturing (rifting) of the crust by the growing low density lithosphere wedge, the pressure release leading to more rapid uprising of volatiles with consequent partial melting. The energy involved would not come from a ‘mantle plume’ but be that stored during the long metasomatizing and heat focussing event of probably tens if not hundreds of millions of years.

The mechanism of the partial melting of zoned, metasomatized lithosphere can be applied not only to the Chilwa and other alkaline provinces in the East African Rift, but to intraplate alkaline provinces elsewhere, both continental and oceanic, as well as to plate margin situations, both construc- tive and destructive, where volcanism of alkaline type is perhaps more abundant than generally visualized. The degree of metasomatism, depth or depths of partial melting and differentiation, and the nature and volume of liquids contributed from deeper unmetasomatized mantle will deter- mine the relative abundance and type of any alkaline magmas produced.



j. .geol. vol. 142, conference...

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