Table Of ContentAlkaline Igneous Rocks
Geological Society Special Publications
Series Editor K. C 0 E
GEOLOGICAL SOCIETY SPECIAL PUBLICATION NO 30
Alkaline Igneous Rocks
EDITED BY
J. G. FITTON & B. G. J. UPTON
Grant Institute of Geology
University of Edinburgh
Edinburgh EH9 3JW
1987
Published for
The Geological Society by
Blackwell Scientific Publications
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First published 1987 ISBN0-632-01616-7
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Alkaline igneous rocks.
(Geological Society special publication; no. 30)
Bibliography: p.
Includes index.
1. Alkalic igneous rocks. I. Fitton, J. G.
II. Upton, B. G.J. III. Geological Society of
London. IV. Series.
QE462.A4A43 1987 552'. 1 86-26364
ISBN0-632-01616-7
Contents
Preface vii
Introduction: FITTON, J. G. & UPTON, B. G.J. ix
BAILEY, D. K. Mantle metasomatism--perspective and prospect 1
MENZIES, M. Alkaline rocks and their inclusions: a window on the Earth's interior 15
EDGAR, A. D. The genesis of alkaline magmas with emphasis on their source regions: inferences 29
from experimental studies
LE Bhs, M. J. Nephelinites and carbonatites 53
TWYMAN, J. D. & GITTINS, J. Alkalic carbonatite magmas: parental or derivative ? 85
DAWSON, J. B. The kimberlite clan: relationship with olivine and leucite lamproites, and 95
inferences for upper-mantle metasomatism
BERGMAN, S. C. Lamproites and other potassium-rich igneous rocks: a review of their 103
occurrence, mineralogy and geochemistry
ROCK, N. M. S. The nature and origin of lamprophyres: an overview 191
CLAGUE, D. A. Hawaiian alkaline volcanism 227
WEAVER, B. L., WOOD, D. A., TARNEY, J. & JORON, J. L. Geochemistry of ocean island basalts 253
from the South Atlantic: Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha
HARRIS, C. & SHEPPARD, S. M. F. Magma and fluid evolution in the lavas and associated 269
granite xenoliths of Ascension Island
FITTON, J. G. The Cameroon line, West Africa: a comparison between oceanic and continental 273
alkaline volcanism
BAKER, B. H. Outline of the petroIogy of the Kenya rift alkaline province 293
MACDONALD, R. Quaternary peralkaline silicic rocks and caldera volcanoes of Kenya 313
WOOLLEY, A. R. & JONES, G. C. The petrochemistry of the northern part of the Chilwa alkaline 335
province, Malawi
BOWDEN, P., BLACK, R., MARTIN, R. F., IKEI E. C. KINNAIRD, J. A. & BATCHELOR, R.A. 357
Niger-Nigerian alkaline ring complexes: a classic example of African Phanerozoic anorogenic
mid-plate magmatism
LI/~GEOIS, J. P. & BLACK, R. Alkaline magmatism subsequent to collision in the Pan-African 381
belt of the Adrar des Iforas (Mali)
FLETCHER, C. J. N. & BEDDOE-STEPHENS, B. The petrology, chemistry and crystallization 403
history of the Velasco alkaline province, eastern Bolivia
BARKER, D. S. Tertiary alkaline magmatism in Trans-Pecos Texas 415
EBY, G. N. The Monteregian Hills and White Mountain alkaline igneous provinces, eastern 433
North America
vi Contents
UPTON, B. G. J. & EMELEUS, C. H. Mid-Proterozoic alkaline magmatism in southern 449
Greenland: the Gardar province
LARSEN, L. M. & SORENSEN, H. The Ilimaussaq intrusion--progressive crystallization and 473
formation of layering in an agpaitic magma
NIELSEN, T. F. D. Tertiary alkaline magmatism in East Greenland: a review 489
DOWNES, H. Tertiary and Quaternary volcanism in the Massif Central, France 517
KOGARKO, L. N. Alkaline rocks of the eastern part of the Baltic Shield (Kola Peninsula) 531
Index 545
Preface
The papers contained in this volume were presented at a symposium held
in Edinburgh in September 1984, which marked the passage of ten years
since the publication of The Alkaline Rocks edited by Henning Sorensen.
In organizing the symposium and compiling this volume we aimed to
review recent developments in the petrology and geochemistry of alkaline
igneous rocks. We have, for example, paid particular attention to work on
lamprophyres and carbonatites which are rock associations of current
interest not covered in Sorensen's book. Reviews of recent work on some
of the classic alkaline provinces, such as East Africa, southern Greenland
and the Kola Peninsula, are included together with reviews of less well-
known areas. Other papers discuss the impact of experimental, geochem-
ical and isotopic studies on our understanding of the generation and
evolution of alkaline magmas.
We are indebted to the contributors for their collaboration in producing
this volume and it is with sadness that we note the death, on 14 February
1986, of Brian Baker, whose pioneering field studies formed the basis for
much of our knowledge of the tectonic and volcanic evolution of the East
African Rift. An obituary and appreciation of his work is published in the
Journal of Volcanology and Geothermal Research (28, v-vii). We are grateful
to The Geological Society and The Royal Society of Edinburgh for their
generous assistance with the symposium costs, to Lucian Begg and Dodie
James for their help with organizing the symposium and producing this
volume, and to our colleagues for the care and enthusiasm with which
they reviewed the manuscripts. The efforts of Edward Wates and his staff
at Blackwell Scientific Publications are also gratefully acknowledged. To
all these we offer our sincere thanks.
J.G.F.
B.G.J.U.
vii
Introduction
J. G. Fitton & B. G. J. Upton
Alkaline igneous rocks may be defined as those which have higher concentrations of alkalis
than can be accommodated in feldspars alone, the excess appearing as feldspathoids, sodic
pyroxenes, sodic amphiboles and other alkali-rich phases. These rocks are, therefore,
deficient in silica and/or alumina with respect to alkalis and will have nepheline and/or
acmite in their norms. In practice the term 'alkaline' is used to encompass a wide range of
igneous rocks, not all of which conform to this rigid definition. Carbonatites, for example,
are certainly silica-deficient but are rarely alkali-rich. True (nepheline-normative) alkali
basalts grade into hypersthene-normative transitional basalts without any obvious change
in mineralogy. Since transitional basalts are often closely associated with alkali basalts in
the field, they are traditionally regarded as alkaline. It is now usual practice to define
alkaline igneous rocks simply in terms of their alkali (Na20+ K20) and silica contents
(see, for example, Le Bas et al. 1986). We have not attempted to review the classification of
alkaline igneous rocks in this volume as this has been dealt with elsewhere (e.g. Sorensen
1974; Streckeisen 1967, 1980). The only alkaline rocks not covered in previous reviews are
those hydrous mafic to ultramafic hypabyssal rocks known as the lamprophyres. The
present volume includes three papers on this group. A comprehensive overview of
lamprophyres is given by Rock and of the sub-group of lamproites by Bergman. The
relationship between lamproites and kimberlites (which arguably belong to the lampro-
phyres) is discussed by Dawson.
Volumetrically, alkaline rocks account for less than one per cent of all igneous rocks.
Despite this, their remarkable mineralogical diversity has brought them repeatedly to the
attention of petrologists and mineralogists, with the result that alkaline rocks account for
about half of all igneous rock names. Sorensen (1974) lists no fewer than 400 alkaline rock
types. This diversity springs largely from an abundance of alkalis and deficiency in silica
which together generate a large number of mineral species not stable in more silica-rich,
alkali-poor magmas. However, a large part of the attention given to alkaline rocks is due to
their characteristic high concentrations of incompatible or large-ion lithophile elements
(LILE). These are often of more than academic interest as most of the world's resources of
niobium, tantalum and the rare-earth elements are found in or around alkaline igneous
rock bodies. The economic importance of alkaline igneous rocks is further enhanced by
their association with economic deposits of apatite (Kogarko) and with diamonds (Dawson;
Bergman).
Evidence for continental alkaline magmatism can be found as far back as the late
Archaean. For example, biotites from the Poohbah Lake syenite in north-western Ontario
have been dated at around 2.7 Ga (Mitchell 1976) and a similar age has been reported by
Larsen et al. (1983) for the Tupertalik carbonatite in western Greenland. At the present
time, alkaline magmas are erupted in all tectonic environments with the possible exception
of mid-ocean ridges. Even here, though, mildly alkaline lavas are sometimes erupted from
off-axis volcanoes, as in the Vestmann Islands of Iceland. Alkaline igneous rocks are found
on all the continents and on islands in all the ocean basins. Their occurrences may be
classified on the basis of tectonic setting into three categories; continental rift valley
From: FITTON, J. G. & UPTON, B. G. J. (eds), 1987, Alkaline Igneous Rocks, ix
Geological Society Special Publication No. 30, pp. ix-xiv.
x Introduction
magmatism, oceanic and continental intraplate magmatism without clear tectonic control,
and alkaline magmatism related to subduction processes. In practice, however, this
classification is not always easy to apply.
Continental rift valleys provide, volumetrically, the most important occurrences of
alkaline igneous rocks although continental rifting is not always accompanied by
magmatism. The best known example (arguably the type example) is the East African Rift
which, in the course of its long history, has yielded almost the entire spectrum of alkaline
magmas. Three papers in this volume are devoted to various aspects of magmatism in the
East African Rift. Baker reviews its magmatic associations with respect to tectonic
development and discusses the origin of the magmas, particularly in relation to the
problems presented by the strongly bimodal distribution of basic and salic lava compositions.
He concludes that the salic magmas evolved from basic parental magmas by processes of
crystal fractionation (cf. Bailey). Macdonald focusses attention on the peralkaline silicic
central volcanoes of Kenya and also favours an origin by crystal fractionation for most of
the evolved magmas. There is little evidence for contamination of the evolving magmas
with ancient continental crust except in the case of the Naivasha comendites. It is not
always possible to demonstrate a genetic link between basic and evolved magmas in the
East African Rift, however. The Chilwa alkaline province in Malawi, at the southern end
of the rift, is an essentially intrusive province in which salic rocks predominate. The
scarcity of basic rocks in this part of the rift has led Woolley & Jones to suggest that the
evolved magmas were produced directly by melting of metasomatised mantle and lower
crust.
Insight into the processes occurring at depth beneath rift valleys may be gained by
studying ancient and deeply eroded examples. The Proterozoic Gardar province in South-
West Greenland is probably the best studied of these and is reviewed by Upton & Emeleus.
One of the most striking features of the province is the presence of giant dykes, up to 800
metres wide. These are dominantly basic but in places show in situ differentiation into more
salic rocks. Salic magma generated in the wider portions of these dykes migrated upwards
and may ultimately have accumulated to produce central complexes in which basic magma
was subordinate or absent (e.g. the Ilimaussaq intrusion, Larsen & Serensen). The giant
dykes, therefore, play a crucial role in understanding the relationship between basic and
salic magmas in this and possibly other rift systems.
The separation of continents to form ocean basins must always be preceded by a phase
of continental rifting, leaving volcanic and intrusive complexes stranded along passive
continental margins. The vigorous magmatism which accompanies continental separation
is generally tholeiitic in character as, for example, in the Karoo and Deccan flood basalt
provinces. Alkaline magmas, however, may be emplaced along the trailing continental
margin during the waning phase of magmatism, long after the spreading centre is
established off-shore. The Tertiary volcanic rocks exposed along the east coast of Greenland
(described by Nielsen) provide an excellent example of such an alkaline province.
A second major occurrence of alkaline igneous rocks is provided by intraplate magmatic
provinces whose activity and siting are not subject to any obvious tectonic control. In the
ocean basins such magmatism manifests itself as ocean islands which are sometimes
aligned in chains with ages increasing away from the active centres, as in the Hawaiian
islands. In these cases it is possible to relate the magmatism to convective plumes within
the asthenosphere. The Hawaiian islands (reviewed by Clague) show clearly defined
