Magmatism in Extensional Structural Settings: The Phanerozoic African Plate

660 Pages·1991·17.96 MB·English

by  A. B. Kampunzu

A. B. Kampunzu . R. T. Lubala (Eds.)
Magmatism in
Extensional Structural
Settings
The Phanerozoic African Plate
With 215 Figures and 79 Tables
Springer-Verlag Berlin Heidelberg New York
London Paris Tokyo HongKong Barcelona Budapest
Professor Dr. A. B. KAMPUNZU
Professor Dr. R. T. LUBALA
Laboratoire de Petrologie
Universite de Lubumbashi, B. P. 1825
Lubumbashi - Zaire
Prepared under the aegis of the I.G.C.P. 227
Cover: Distribution of the main Phanerozoic volcanic provinces in Africa (after
Choubert and Muret, 1975 and others)
ISBN-13 :978-3-642-73968-2 e-ISBN-13:978-3-642-73966-8
DOl: 10.1007/978-3-642-73966-8
Library of Congres Cataloging.in·Publication Data
Magmatism in Extensional Structural Settings: the Phanerozoic African Plate / A. B. Kampunzu, R. T.
Lubala, eds.; [prepared under the aegis of the I.G.c.P. 227). Includes bibliographical references and
index.
ISBN·13 :978·3·642·73968·2
1. Magmatism - Africa. 2. Geology, Structural - Africa. 3. Petrology - Africa. I. Kampunzu, A. B.,
1942- . II. Lubala, R. T. III. I.G.c.P. 227 (Project) QE461.E89 1991 552'.1 - dc20
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Softcover reprint of the hardcover 1st edition 1991
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This book is dedicated
to the authors' indulgent wives
Pacy Kampunzu and Mary Lubala
Introduction
Since the end of the Pan-African orogenesis (ca. 600-500 Ma ago), the
history of the major part of the African Plate has mostly been characterized
by the development of domal uplift and rifting (e.g. the East African Rift)
leading sometimes to continental drift (e.g. Atlantic Ocean). Therefore, the
African continent provides the ideal model to improve our knowledge on the
magmatism produced during the different evolutionary stages of extensional
tectonics.
Our knowledge concerning the Phanerozoic magmatism of the African
Plate has greatly evolved in the last 20 years. Up to 1975, it was generally
agreed that, except in Afar and along the Atlantic Ocean margin, African
magmatism was alkaline to ultra-alkaline 1. Despite the nomenclature used
to confirm the above unquestionable assumption concerning the nature of
African anorogenic magmatism, many reported petrological and geochemi-
cal features of individual igneous provinces are not always compatible with
alkaline or ultra-alkaline affinities.
Alkaline (s. 1.) magmatism is not, as generally assumed, the sole typical
hallmark of an intracontinental extensional tectonic setting. It is therefore
necessary to redefine the relationships between each extensionally related
magmatic type (tholeiitic, transitional, alkaline, ultra-alkaline), on the one
hand, and the structural setting and the mineralizations, on the other hand.
To define these relationships, it was indispensable to update and exchange
data on specific African igneous provinces with several scientific teams of
different nationalities concerned with African geology. Consequently, the
idea of launching an International Geological Correlation Programme
(IGCP) was proposed to some participants at the last International Geologi-
cal Congress in Moscow in 1984. This project was agreed upon in 1985 by
UNESCO and lUGS as an IGCP Project 227 (Magmatism and Evolution
of Extensional Regions of the African Plate). Its inaugural meeting was held
at St. Andrews (Scotland), during the 13th Colloquium on African Geology.
The second meeting of this project took place in Lubumbashi (Zaire), in Au-
gust 1986.
The latter was the 1st. International Congress to ever specifically deal
with the various aspects of African extensional magmatism. During this
meeting, it was agreed that the organizers of the congress would edit a book,
1 In this book, ultra-alkaline refers to Le Bas's definition (e.g. Le Bas 1987). It is more ex-
plicit than its equivalent "strongly alkaline".
VIII Introduction
reviewing all the available data on the main Phanerozoic igneous provinces
of the African Plate.
The chapters of this book survey the geology (including the tectonic
aspects), the petrology and the geochemistry of igneous rock associations in
regions where:
i) the knowledge has evolved significantly since the publication of the latest
review books devoted to extensional magmatism in general (e.g. TUTTLE
and GITTINS 1966; SORENSEN 1974) or specifically to African magmatism
(CLIFFORD and GASS 1970);
ii) the available data could be integrated in reliable geodynamic models.
Some major igneous provinces like the carbonatitic-alkaline magmatism of
the eastern margin of the Atlantic (Namibia, Angola) or the western African
kimberlites do not fulfil the above criteria, so they are not examined in
specific chapters of this book.
In addition, three major igneous provinces of the African continent are not
given individual chapters:
i) the carbonatitic-alkaline province outcropping on the Kenya-Uganda
border which have recently received significant consideration in Le Bas's
(1977) book intitled Carbonatite-Nephelinite volcanism;
ii) the southern Africa Karoo magmatism which is extensively dealt with in
the collective book Petrogenesis of the volcanic rocks of the Karoo Province
edited by ERLANK (1984). In addition, a review paper related to this volca-
nic province appeared in the book devoted to the Flood Basalts Provinces
and edited by MACDOUGALL (1988).
iii) the northeastern African magmatism, extending mostly between Sudan,
Egypt and W. Ethiopia. The potential contributors approached (J. R. VAIL
and D. C. ALMOND) were not in a position to prepare a review chapter on
this topic. The same applies to the Oceanic volcanism (except the Canary
Is.).
The book is subdivided into four parts centring around geological topics
(parts I, III and IV) and geographical criteria (Part II).
In the first part of the book, contributors describe the main features (dis-
tribution, tectonic setting, petrology, geochemistry) of the East-African Rift
volcanism, both in its eastern and western branches. The abundance of
available data varies; some volcanic regions (e.g. Rungwe in Southwestern
Tanzania) are poorly known, hence the need to increase research on these
regions. This part of the book also studies the widespread occurrence of
transitional basalts and sometimes tholeiites in both branches of the in-
tracontinental East-African rift. The genesis of the K- and LILE- enriched
ultra-alkaline volcanism of the Western rift is also examined in the light of
trace element, isotopic and experimental data. The results of the different
chapters within the first part of this work call in question the generally ac-
cepted hypothesis that strongly alkaline magmas are only produced by low
amounts of partial melting.
Introduction IX
This part examines mantle heterogeneities at various scales and also
reveals the importance of enrichment processes including the metasomatism
in the genesis of rift magmatism, especially the ultra-alkaline one.
Finally, a short discussion of the controversial problem of genetic models
of rifting (i.e. active vs. passive rifting) is included in both parts, one and
two.
The second part discusses West-African magmatism associated with the
early opening of the Atlantic Ocean, during the fragmentation of Gond-
wanaland. A short chapter, devoted to the meagre available data on the
Cretaceous and the Cainozoic volcanism of the Senegal basin, examines the
magmatic growth of the continental margin alongside the ocean, during the
different spreading stages. This interesting topic is worldwide poorly covered
and invites further study.
The multiplicity of geological parameters, involved in magma emplace-
ment and genesis, in some oceanic islands is typified by the Canary Islands
case. It is important to note that the only oceanic carbonatites known in the
world are located in the Canary and Cape Verde Is., at this northwestern
African Plate. The complexity of the processes operating in the mantle
beneath West Africa is also illustrated in this second part and in a previous
but complementary paper devoted to the African Plate-related mantle xen-
oliths (DAUTRIA and GIROD 1987).
The link between the second and third parts of the book is assured by the
two complementary papers related to the AIr ring complexes and the chapter
devoted to the Cameroon line.
A detailed review of African ring complexes and related mineralization,
with special reference to the Phanerozoic examples, is given in the third part.
Something not clearly developed in this volume (except in the introduction
to the third part) but which will receive certainly more consideration in the
near future is:
i) the discrimination between tholeiitic, transitional and alkaline ring com-
plexes. Based on the available data (including our unpublished ones), several
so-called alkaline intrusions which occur in African ring complexes are asso-
ciated with tholeiitic or transitional basic rocks. The review chapter devoted
to the East-African rift stresses a similar biassed classification rectified only
recently for the comendites and pantellerites, mostly associated with transi-
tional rather than undersatured alkaline basalts.
ii) the petrological and geochemical criteria discriminating the anorogenic
ring complexes (including A - granites s.s.) from the post-orogenic alkaline
complexes. In fact, the papers given in the third part of this book review our
present knowledge of the main African ring complexes and emphasize the
need for a new classification of the so-called "alkaline" ring complexes.
The last part of this book is devoted to kimberlites, especially those supply-
ing the largest worldwide diamond production in South Africa and Zaire-
Angola. Their emplacement context, mostly intra-cratonic (Archean shields)
but in places along mobile belts, is clearly established by the contributors.
X Introduction
The mantle layers "sampled" by the kimberlites during their ascent to the
surface is also reviewed in the last two papers. The detailed data given in
these two chapters resulted in the reviews of the magma source features us-
ing petrological, geochemical and isotopic analysis, estimation of pressures
and temperatures of equilibration, and the structure and the composition of
the mantle.
Where important, cross-referencing between chapters emphasizes their
complementary character. Contributors have been encouraged to provide
the maximum of references related to the studied areas; this will help the
readers to thoroughly master the topic. Finally, the book is produced for all
those who are interested in various aspects of extensional magmatism, i.e.
geology, petrology, geochemistry, mantle metasomatism, relationships be-
tween progressive evolution of rifting and magmatism or between minerali-
zation and magmatism, etc.
Obviously, this book, essentially devoted to African Geology, offers data
and models which can be integrated in the plate tectonics scenarios in exten-
sional settings.
Lubumbashi, April 1991 A. B. KAMPUNZU and R. T. LUBALA
Acknowledgements
This volume has been prepared in the framework of the IGCP (International
Geological Correlation Programme), Project 227, sponsored by UNESCO
and the International Union of Geological Sciences (lUGS). Some of the
papers contained in this book were presented at the First International Con-
gress on Magmatism of Extensional Regions, held in Lubumbashi-Zaire,
10-15 August 1986.
Besides the staff of the sponsoring organizations, the compilation of this
book has depended on the dedicated work of many individuals. We greatly
thank the following persons who acted as referees: V. ARANA (Spain), F.
BARBERI (Italy), H. BERTRAND (France), R. BLACK (France), B. BONIN
(France), P. BOWDEN (0. K.), W. L. BROWN (France), C. COULON (France),
K. G. Cox (0. K.), A. CUNDARI (Australia), 1. M. DAUTRIA (France), D.
DEMAIFFE (Belgium), J. B. DAWSON (0. K.), J. DOSTAL (Canada), R. F.
EMSLIE (Canada), J. G. FITTON (0. K.), M. GIROD (France), 1. KINNAIRD
(0. K.), 1. KORNPROBST (France), 1. D. KRAMERS (Zimbabwe), M. J. LE BAS
(0. K.), F. E. LLOYD (0. K.), R. MACDONALD (0. K.), R. F. MARTIN
(Canada), M. A. MENZIES (0. K.), C. MOREAU (Senegal), G. MORTEANI (w.
Germany), P. H. NIXON (0. K.), W. POHL (w. Germany), G. ROCCI
(France), R. N. THOMPSON (0. K.), M. TREUIL (France), 1. R. VAIL (0. K.),
P. VIDAL (France), R. A. WIEBE (USA), L. A. 1. WILLIAMS (0. K.), A. R.
WOOLLEY (UK.) and B. ZANETTIN (Italy).
We are greatly indebted to the staff of the Laboratory of Petrology, both
at Lubumbashi (Zaire) and Marseille (France), in particular to L. AGUIRRE,
J. P. CARON, J. J. COCHEME, C. COULON, A. DEMANT, D. HERMITTE, M.
KABENGELE, D. KAPENDA, M. KANIKA, M. MAKUTU, M. POPOFF, B.
RUMVEGERI, K. TSHIMANGA and P.1. VELLUTINI for their friendly col-
laboration, especially during the congress.
We are deeply grateful to various international agencies and particularly
to those of their members without whom this project would not have been
possible. May Drs. SIBRAVA, VON BRAUN, MISSOTTEN, DUDICH and DRIES-
SLE (UNESCO), SINDING-LARSEN and REINEMUND (lUGS), LELEU and
BIDET (CIFEG-Paris) find in these lines our gratitude for their encourage-
ment and tremendous assistance.
We are grateful to the Zairian officials from the Office of the President
of the Republic, Mining and Energy Department, Home Affairs Department
and from the Presidents General of the following companies: Air-ZaIre,
Gecamines, I. Z. C. N., Sodimiza, Somikivu and Sominki for their great ma-
XII Acknowledgements
terial contribution to the realisation of this project. The following persons
of the above companies are thanked for their assistance: NTUMBA, LWANGO
(Mining Department), NGONGO, KATEKESHA and MBUYI (GCM), LEFEB-
VRE and TSHIAUKA (Sodimiza), FIOCCHI (Sominki), DIJKHUIZEN
(Somikivu), the Thchnical Director and Mrs. BASHIGE (I.Z.C.N.), BAMBI
(Air-Zaire) and MUKONKI (ERTS-Zaire).
The officials from the Lubumbashi University, especially the past Rector
BINGaI'O, are acknowledged for the administrative facilities provided.
Efficient collaboration received from Mrs. RUBANGO and Mr. DURIEUX
during the Congress is highly acknowledged.
N. ISSA, J. BIRINDWA, K. KAUMBA, M. MBUYI and M. W. MWAMBA are
thanked for their secretarial and drafting work.
We also thank Springer-Verlag, the publishers, especially Dr. W. ENGEL
(Geosciences Editorial) for the prompt preparation of this volume.
Finally, all the contributors are thanked for their active participation in
the activities of IGCP 227. We apologize to those who promptly sent their
manuscripts and had to wait patiently for the publication of this book. Last,
but not least, we express our gratitude to our wives Pacifique and Marie for
their patience and assistance.
A. B. KAMpUNZU and R. T. LUBALA