Extrusive Carbonatites

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Lithos 85 (200

Extrusive carbonatites: A brief review

A.R. Woolleya,*, A.A. Churchb

aDepartment of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UKbLe Grand Crolier, Rue de la Trigale, Torteval, Guernsey, GY8 0PX, Channel Islands, UK

Received 27 February 2004; accepted 11 March 2005

Available online 14 June 2005

Abstract

49 known extrusive carbonatite occurrences are listed with brief details of their tectonic setting, structure, lithologies,

associated silicate rocks, chemistry and presence or absence of included mantle materials. Half the occurrences appear to be

related to tephra cones, tuff rings, diatremes and maars and the rest occur within strato-volcanoes. Pyroclastic carbonatitic rocks

are present at all the localities, with carbonatite lava flows occurring at only 14 of them. The pyroclastic rocks, which include

fallout tephra and deposits from pyroclastic surges and flows and products of phreatomagmatic eruptions, vary from rocks

composed principally of carbonate to varieties with as little as 20% igneous carbonate. The most abundant silicate rocks

associated with extrusive carbonatites are melilite-bearing rocks, nephelinite and/or ijolite, and phonolite and/or nepheline

syenite; seven occurrences have no associated silicate rocks. 16 occurrences, most of them associated with small extrusive

centres, contain mantle xenoliths or megacrysts, details of which are tabulated, with spinel lherzolite the most abundant rock

type, but amphibole, phlogopite and garnet are also recorded. The lack of such materials in intrusive carbonatites may reflect

their less energetic environment of emplacement. It is proposed that carbonatites are essentially of two types: (a) those rising

energetically and rapidly from the mantle, which form small explosion craters, ash or tuff cones, or diatremes, have only low-

volume associated silicate rocks, and entrain mantle debris, and (b) those which occur in strato-volcanoes, are associated with

large volumes of silicate rocks and follow a more complex genesis, probably involving ponding and differentiation (separation

from carbonate-bearing silicate magma) at higher levels in the mantle and/or crust. Most of the classic intrusive carbonatite

complexes probably fall into the second category.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Carbonatite review; Extrusive carbonatite; Pyroclastic; Lava

1. Introduction

It has long been appreciated that most chemical

analyses of carbonatite do not represent the composi-

0024-4937/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.lithos.2005.03.018

* Corresponding author.

E-mail address: [email protected] (A.R. Woolley).

tions of the primary liquids from which they are

derived. For instance, most intrusive carbonatites

have fenite aureoles that indicate loss of alkalis

(e.g., Woolley, 1969), and probably other elements,

while it is probable that the carbonate textures dis-

played by most large intrusive bodies of carbonatite

are the result of extensive re-crystallization (Barker,

5) 1–14

Table 1

World occurrences of extrusive carbonatite with brief details of tectonic setting, structure, nature of extrusive carbonatite, associated silicate rocks and presence or absence of mantle

debris

Locality Tectonic

setting

Structure Nature of

extrusive

carbonatite

Nature of

carbonate

Extrusive

silicate

rocks

Intrusive

silicate

rocks

Intrusive

carbonatite

Mantle

debris

Selected references

1 Castignon Lake, Canada C SVE P cc,ank Me Me C – Dimroth, 1970

2 Mount Grace, Canada T ? P cc – NS C – Hoy and Pell, 1986

3 Qagssiarsuk, Greenland R? SVE P,L cc,(dol) – Px,M C – Stewart, 1970

4 Santo Antonio da Barra, Brazil ? V L cc A,M,N P,A – – Gaspar and Danni, 1981

5 Cerro Manomo, Bolivia F V P,L dol,ank? – – D,Ank? – Fletcher et al., 1981

6 Kaiserstuhl, Germany R V P(Lp, Ash) cc N,Te,P M,E,T C,Al Keller, 1989

7 Hegau, Germany R SVE P cc M P,M – X Keller et al., 1990

8 Auf Dickel, Germany nR SVE P(Lp, Ash) cc N,M,T – – – Riley et al., 1996

9 San Venanzo, Italy R SVE P cc M,P M – X Stoppa and Woolley, 1997

10 Cupaello, Italy R SVE P cc M – – X Stoppa and Woolley, 1997

11 Monticchio, Italy T SVE P cc M – – X Stoppa and Woolley, 1997

12 Oricola, Italy nR SVE P cc Lc – – X Stoppa et al., pers. comm., 2003

13 Chabrieres, France C V P dol B,P – – – Chazot et al., 2003

14 Calatrava, Spain C SVE P cc? M,N,Lc,B – – X Bailey et al., 2003

15 Khan Neshin, Afghanistan R V P,L cc,ank P Lc,P C – Vikhter et al., 1976

16 Dasht-I-Nawar, Afghanistan ? V P cc? TB,D – – – Vikhter et al., 1976

17 Lixian, China ? V P,L cc Lc – – – Yu et al., 2003

18 Kontozero, Russia R V P,L M,N Px,Mlt,NS,Sy C – Pyatenko and Saprykina, 1976

19 Khaluta and Arschan, Russia nR SVE? P cc ? Sy C – Yarmolyuk et al., 1997

20 Uyaynah, U.A.E. O? V P cc – – – X Woolley et al., 1991

21 Hatta Zone, U.A.E. O? V P cc B – – – Nasir and Klemd, 1998

22 Tamazert, Morocco T SVE Lp Dol,ank (cc) N,TB,A Px,NS C,Ank X Mourtada et al., 1997

23 Fort Portal, Uganda R SVE P,L cc M – C X Nixon and Hornung, 1973

24 Katwe-Kikorongo, Uganda R SVE P cc M,N – – X von Knorring, 1967

25 Bunyaraguru, Uganda R SVE P cc M,Lc,K – – X Stoppa et al., 2003

26 Tinderet, Kenya R V P cc M,N,P,Ba,T C,Al – Deans and Roberts, 1984

A.R.Woolley,

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27 Homa Mountain, Kenya R V P cc N,P,M I C,Al,F – Clarke and Roberts, 1986

28 Nyamaji, Kenya R V P cc N,P I C – Le Bas, 1977

29 Ruri, Kenya R V P cc,ank P,N NS,P C,Al,F – Le Bas, 1977

30 Rangwa, Kenya R V P cc N,M I,NS,M C,Al,F – Le Bas, 1977

31 Shombole, Kenya R V Ash cc N,P,F – C – Peterson, 1989

32 Oldoinyo Lengai, Tanzania R V P,L Alk N,P – – – Bell and Keller, 1995

33 Kerimasi, Tanzania R V P,L cc (dol) N,P,M I,Mlt,M,Px, C,Al,D X Church, 1996

34 Sadiman, Tanzania R V P cc M,N,P – – – Hay, 1978

35 Arusha-Monduli, Tanzania R SVE P cc M,B – – X Rudnick et al., 1993

36 Hanang, Tanzania R V P,L cc N M,I C – Thomas et al., 1966

37 Kwahera, Tanzania R V P,L cc N,M – C – Mudd and Orridge, 1966

38 Basotu tuff cones, Tanzania R SVE P cc,dol – – – – Downie and Wilkinson, 1962

39 Kaluwe, Zambia R ? P,L cc,(dol) – – – – Turner and Rex, 1991

40 Mwambuto, Zambia R SVE,V P dol,ank – L D,Ank X Bailey, 1989

41 Chasweta, Zambia R SVE,V P dol,ank – – C X Bailey, 1989

42 Gross Brukkaros, Namibia C SVE P dol,cc – – D(C ) – Stachel et al., 1995

43 Goudini, South Africa C V P cc N Px,NS C – Verwoerd, 1993

44 Kruidfontein, South Africa C V P cc P,T,R Sy C – Verwoerd, 1993

45 Melkfontein, South Africa C SVE P cc – – – – Boctor et al., 1984.

46 Catanda, Angola C? V P,L cc? – Ti,P – X Silva, 1973

47 Santiago, Cape Verde Islands O V P dol N N D – Silva et al., 1984

48 Brava, Cape Verde Islands O V L cc P,N I,Sy,NS C – Hoernle et al., 2002

49 Mont Auber de la Rue, Kerguelen O V P dol H – – – B.N. Moine, pers. comm., 2003

A Chagatai, Uzbekistan nR SVE P cc – T C X Djuraev and Divaev, 1999

B Polino, Italy nR SVE P cc – – C X Stoppa and Lupini, 1993

C Natron-Engaruka, Tanzania R SVE P cc N,M – – X Dawson and Powell, 1970

D Laacher See, Germany nR V P cc P – – – Taylor et al., 1967

For significance of bottom four localities (A–D) see text. Tectonic setting column: R rift; nR near rift; O oceanic; C cratonic; T thrust belt; F fold mountains. ? Not clear. Nature of

extrusive carbonatite column: P pyroclastics; L lava; Lp lapilli tuff; Ash ash. Extrusive silicate rocks column: M melilitite or melilite nephelinite; N nephelinite; Te tephrite; P

phonolite; T trachyte; B basalt; Ba basanite; TB trachybasalt; Me meimechite; A analcimitite; Lc leucitite; D dacite; H hawaiite; R rhyolite. Intrusive silicate rocks column: Px

pyroxenite; Mlt melteigite; I ijolite; E essexite; Sy syenite; M melilitolite; L lamprophyre; P phonolite; NS nepheline syenite; Ti tinguaite; T trachyte; A analcime-bearing phonolite.

Intrusive carbonatite column: C calciocarbonatite; Al alvikite; Ank ankerite carbonatite; D dolomite carbonatite; F ferrocarbonatite. Mantle debris column: X mantle debris identified.

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A.R. Woolley, A.A. Church / Lithos 85 (2005) 1–144

1989). Many carbonatite intrusions appear to have

undergone some degree of alteration resulting in the

development of, amongst others, secondary minerals

of the RE and other rare elements, while ‘dolomi-

tizationT can be seen at, for instance, Jacupiranga.

Because carbonate glasses are unknown in nature,

an approach to primary compositions via quenched

melt products is difficult, although some dykes may

possibly be close to primary in composition. We

believe, however, that considerable information can

be gleaned about primary carbonatite magma compo-

sitions from a study of the extrusive products, a point

already stressed by Bailey (1993) who noted that

bEvidence from effusive carbonatite is essential for

understanding the wider aspects of carbonatite mag-

matism, and for identifying the most relevant applica-

tions of the results from experimental petrologyQ. We

find that considerable petrogenetic information is in-

deed provided by the extrusives that is not revealed by

a study of intrusive carbonatites.

Although the Oldoinyo Lengai volcano and its

pristine natrocarbonatite lavas is well known, few

other occurrences of extrusive carbonatite have been

investigated in such detail, and it does not seem to be

widely appreciated how many such occurrences have

in fact been recognised. Of the N500 carbonatites that

have been described (Woolley and Kjarsgaard, in

press.) only some 10% are solely extrusive or include

an extrusive component. It is probable that some of

the carbonatites considered at present to consist only

of intrusive rocks will prove, on further work, to

include some extrusive components. Further, the na-

ture of the carbonate mineral phases in some alkaline

volcanic rocks, particularly tuffs, will prove to be of a

primary carbonatitic nature. We are aware of 49 lo-

calities (Table 1) that include extrusive carbonatite

and feel confident that this number will increase

substantially once it is more widely appreciated that

the carbonate in some pyroclastic sequences is of

primary magmatic origin.

Outside the Russian literature modern investiga-

tions of extrusive carbonatites can be considered to

begin with papers by Keller (1981) and Deans and

Roberts (1984). The work of the latter does not seem

to have been fully appreciated partly, we suggest,

because of the general disagreement with their iden-

tification of certain rectangular crystals or crystal

aggregates of calcite as being after nyerereite, a ubiq-

uitous phase in the recent Oldoinyo Lengai natro-

carbonatite lavas. This calcite is now generally con-

sidered to be primary (e.g., Bailey, 1993), but the

disagreement seems to have precluded the widespread

appreciation of the fact that Deans and Roberts had

correctly recognised several carbonatitic tuff

sequences in East Africa.

A most useful and well illustrated review of extru-

sive carbonatites is that of Keller (1989) who pointed

out that carbonatitic lavas, ashes, tuffs, tuff breccias

and phreatomagmatic deposits occur and that tear-

drop lapilli, juvenile carbonatite bombs, agglutinated

lapilli tuffs, welded spatter and agglomerates are com-

mon and characteristic of carbonatite volcanic activity.

He also noted that quenched juvenile fragments in

carbonatite pyroclastic deposits are important in con-

straining the physical properties and original chemical

composition of carbonatite melts. Keller (1989) refers

to some of the principal occurrences of extrusive

carbonatite then known and is thus a source of

many references on these occurrences. Bailey

(1990), in discussing the extrusive carbonatites of

southeast Zambia, noted the importance of consider-

ing those carbonatites that are apparently generated

directly in the mantle and in that paper, and a later

review (Bailey, 1993), discussed at some length the

importance of the extrusive carbonatites when consid-

ering a whole range of carbonatite petrogenetic pro-

blems. Many of Bailey’s conclusions, such as the

importance of the distinction of dprimaryT and

dderivedT (high level) carbonatites, possible relation-

ships to kimberlites, and the role of rapid eruption are,

we feel, substantiated by consideration of the extru-

sive carbonatites as a whole.

The present paper lists 49 occurrences of extrusive

carbonatite (Table 1), all that we have been able to

trace from the literature. The principal objective is to

categorise these occurrences, with a view to trying to

discern patterns that might promote a further under-

standing of their nature and genesis, while also inves-

tigating any differences from intrusive carbonatites

that might also have genetic significance.

Special mention must be made of four occurrences,

which are given at the foot of Table 1, the classifica-

tion of which is not certain. The Chagatai (Uzbeki-

stan) and Polino (Italy) occurrences are certainly

carbonatites but essentially intrusive. However, the

fascinating Chagatai centre (it contains diamonds

A.R. Woolley, A.A. Church / Lithos 85 (2005) 1–14 5

and melilite) comprises a series of dykes and two

ddiatremesT which Djuraev and Divaev (1999) indi-

cate are sub-volcanic, while the Polino occurrence

consists of a dvery shallow-level diatremeT, accordingto Stoppa and Lupini (1993). Although there is no

evidence at present for a truly extrusive carbonatitic

facies at either of these localities, it appears very

likely that there was an extrusive component, that

has not been preserved, which probably had a com-

position close to, if not identical with, that of the

diatreme rock. (F. Stoppa, pers. comm., 2004).

The Natron-Engaruka explosion craters in Tanza-

nia include a suite of tuff rings, tuff cones and maars

(Dawson and Powell, 1970) and in some cones there

are extensive carbonate-rich tuffs. It is probable that

these tuffs will prove to be carbonatitic, but at

present a definitive description is lacking. Carbona-

tite has been described from the pyroclastic rocks of

the Laacher See volcano in Germany (Taylor et al.,

1967) but this comprises xenoliths of sovite, so there

is no direct evidence that a carbonatitic melt reached

the surface (J. Keller, pers. comm., 2004). For these

various reasons these four localities have not been

included in the main list of extrusive carbonatite

occurrences, but they are shown at the bottom of

Tables 1 and 4.

It should also be noted that the Igwisi Hills in

Tanzania have been omitted. This locality has been

described as including carbonatite but it is now gen-

erally thought to be kimberlitic (Dawson, 1994). The

Jabal El Arab locality in Syria has been identified as

including extrusive carbonatite (Mahfoud and Beck,

1995), but we consider that it is probably not so; it

has, therefore, also been omitted.

2. Distribution, form and lithology

The setting and distribution of extrusive carbona-

tite occurrences does not appear to differ significantly

from those of intrusive carbonatites. They are almost

invariably intra-plate and on the African continent,

where 35% of all known carbonatite occurrences are

located, appear to be concentrated in zones marginal

to the ancient craton cores. There is a very clear

relationship to continental rifts, about two thirds

lying within or closely adjacent to them. Carbonatites

are found in the Cape Verdes, Canary Islands and

Kerguelen, but these are the only occurrences

known in an oceanic setting.

For the purposes of this paper, occurrences of

extrusive carbonatite have been divided into two

types: those associated with large volcanoes, generally

strato-volcanoes, and those forming, or associated

with, smaller volcanic edifices such as tephra cones,

tuff rings, diatremes and maars. There appears to be

no general volcanic term for the latter so these are

referred to in Table 1 simply as bSVEQ - smaller

volcanic edifices.

Extrusive carbonatites generally form only a small

part of the volcanic succession in either large volca-

noes or SVEs. An exception is the Fort Portal oc-

currence which comprises numerous carbonatitic tuff

cones and volcanic craters, a single carbonatite lava

flow, and 142 km2 of carbonatitic tuffs, apparently

with no contemporaneous silicate lavas or tuffs. In

the large volcanoes (30 are listed in Table 1) the

carbonatitic facies is predominantly pyroclastic, with

lava flows present in only 11 occurrences, although

the identification of some of the carbonatite as lava

is not always conclusive. Even in Oldoinyo Lengai,

which is well known for its recent carbonatite lavas,

carbonatitic tuffs and breccias are probably the more

voluminous. The Kontozero complex in Russia is

instructive in so far as it consists of a 55 km2 caldera

containing a 2 km thick succession of which some

10% comprises carbonatite tuffs and lavas. In carbo-

natite-bearing SVEs also pyroclastic rocks are pre-

dominant, with carbonatite flows being found at only

two of the 18 such localities listed in Table 1. Thus

the field evidence for the SVEs, in particular, is for a

generally explosive regime, which is supported by

the widespread occurrence of debris of mantle origin.

The identification of some of the Fort Portal carbo-

natitic tuffs as ignimbrites (Barker and Nixon, 1989)

also points to a high energy, gas-charged extrusive

environment.

The observed carbonatitic ashes and tuffs vary

from varieties in which the carbonate is minor to

types, almost invariably lapilli tuffs, consisting of a

high proportion of carbonate. Primary carbonates in

the ashes is commonly in the form of broken crystals

and fragments as well as lapilli and fragments of

carbonatite rock. There is generally also a secondary

carbonate cement. The rest of the rock tends to consist

of a wide range of crystal and lithic fragments of

Table 3

Comparison of abundances of silicate rock types present in occur-

rences of extrusive and intrusive carbonatites

Combined number of intrusive and

extrusive silicate rock types

associated with the 49 occurrences

that include extrusive carbonatite

Number of occurrences

of silicate rocks

associated with 337

intrusive carbonatites*

Melilitite and melilitolite 21 43% 28 8%

Nephelinite and

ijolite/melteigite

21 43% 116 34%

Phonolite, tinguaite,

tephrite and

nepheline syenite

21 43% 147 44%

Trachyte and syenite 6 12% 109 32%

No associated silicate rocks 7 14% 68 20%

*Data taken from Woolley (2003, Table 4), which based on 337

carbonatite occurrences lacking extrusive carbonatite (see Woolley,

2003, Table 1).


primary igneous origin, as well as abundant accidental

material, presumably from the walls of the feeder

conduit. Some detailed petrographic descriptions are

given by Deans and Roberts (1984) and there are

descriptions and some excellent photomicrographs in

Keller (1989).

A feature of many of the lavas is the presence of

euhedral to subhedral calcite phenocrysts which ap-

pear to be of undoubted primary origin, and not

pseudomorphs after nyerereite, as originally suggested

by Deans and Roberts (1984). The presence of nyer-

ereite and gregoryite phenocrysts in the recent lavas of

Oldoinyo Lengai appear to be unique to that volcano,

whilst the Fort Portal lava and lava bombs are appar-

ently also unique their mineral assemblages including

primary spurrite and periclase.

3. Associated silicate rocks

Table 2 indicates the nature and abundance of the

extrusive and intrusive silicate rocks found in associ-

ation with the 49 extrusive carbonatite occurrences.

The relative abundances of the silicate rocks associ-

ated with the extrusive carbonatite occurrences appear

to differ little, at least statistically, from the silicate

rocks associated with intrusive carbonatites (i.e., car-

bonatite occurrences with no extrusive carbonatite

component). In Table 3 the data for both the extrusive

and intrusive silicate rocks associated with the extru-

Table 2

Nature of silicate rocks associated with occurrences of extrusive

carbonatite

Associated extrusive

silicate rocks

Associated intrusive

silicate rocks

Melilite-bearing rocks 18 Melilite-bearing rocks 7

Nephelinite 21 Ijolite and melteigite 6

Phonolite and tephrite 14 Phonolite/tinguaite/

nepheline syenite

11

Trachyte 3 Trachyte and syenite 4

Basalt, basanite and

trachybasalt

7 Essexite 1

Meimechite 1 Meimechite 1

Analcimitite 1 Analcime-bearing

phonolite

1

Leucitite 4 Lamprophyre 1

Other rock types 3 Pyroxenite 4

No silicate rocks 11 No silicate rocks 27

Numbers indicate number of occurrences at which rock type occurs
.
sive carbonatites (Tables 1 and 2) have been combined

and are shown in columns 2 and 3 in terms of number

of localities and percentages of the total number of

localities. Table 3 indicates that 21 extrusuve carbo-

natite occurrences, representing 43% of the total,

include melilite-bearing rocks (melilitite, melilite

nephelinite and melilitolite). In contrast, only 8% of

occurrences of intrusive carbonatite are associated

with melilite-bearing rocks. Of the 18 centres cate-

gorised as SVEs (Table 1) 10 include melilitite, whilst

four lack associated silicate rocks. One third of the

carbonatite-bearing centres classified as strato-volca-

noes contain melilitite-bearing rocks, although nephe-

linites are generally the most voluminous rock type.

The possible significance of the apparent relative

abundances of melilite-bearing rocks in the extrusive

and intrusive carbonatite associations will be dis-

cussed later, but it is stressed that, in the opinion of

the authors, the clear association of extrusive carbo-

natites with melilite-bearing rocks is of considerable

significance when the petrogenesis of the carbonatites

comes to be considered.

Of the 29 extrusive carbonatite localities which do

not include melilite-bearing silicate rocks eight are

associated with nephelinites, 14 contain phonolite,

generally in combination with nephelinite, melilite

rocks or both, and eight occurrences include neither

extrusive nor intrusive silicate rocks. The relative

abundances of nephelinite (ijolite/melteigite) and

phonolite, tephrite and tinguaite (nepheline syenite)

occurring in the intrusive and extrusive carbonatite


associations are closely similar (Table 3), as are the

proportions of occurrences with which no silicate

rocks are associated. In contrast, there is a difference

in the abundance of trachyte and syenite in the

extrusive and intrusive carbonatite associations,

only 12% of the former but 32% of the latter in-

cluding these rock types, and this is the second

noteworthy difference which this examination of

the data brings to light.

4. Mantle debris

There has not, as far as we aware, been any

general investigation of mantle xenoliths and mega-

crysts in carbonatitic rocks, so that they are not

distinguished as a particular category in, for instance,

the comprehensive volume on mantle xenoliths edi-

ted by Nixon (1987), although there have been de-

Table 4

Extrusive carbonatite occurrences with brief details of the mantle materia

Locality Structure

7 Hegau, Germany SVE

9 San Venanzo, Italy SVE

10 Cupaello, Italy SVE

11 Monticchio, Italy SVE

12 Oricola, Italy SVE

14 Calatrava, Spain SVE

20 Uyaynah, U.A.E. V

22 Tamazert, Morocco SVE

23 Fort Portal, Uganda SVE

24 Katwe-Kikorongo, Uganda SVE

25 Bunyaraguru, Uganda SVE

33 Kerimasi, Tanzania V

35 Arusha-Monduli, Tanzania SVE

40 Mwambuto, Zambia SVE,V

41 Chasweta, Zambia SVE,V

46 Catanda, Angola V

A Chagatai, Uzbekistan SVE

B Polino, Italy SVE

C Natron-Engaruka, Tanzania SVE

V volcano; SVE small volcanic edifice.

tailed studies of mantle assemblages from particular

localities e.g., Monticchio, Italy (Jones et al., 2000) .

Carbonatites, however, clearly hold considerable po-

tential for mantle petrogenetic studies. We are un-

aware of any identifiable mantle debris, in the form

of xenoliths or xenocrysts, having been described

from intrusive carbonatites, with the following

exceptions. Some material is found in the small,

dtypeT, damkjernite intrusion near the Fen carbonatite

complex, Norway and in dykes and intrusions of

alnoite near the Alno complex, Sweden (Griffin

and Kresten, 1987). The Chagatai and Polino dia-

tremes have already been mentioned, the former

remarkable for containing diamonds, and it is con-

sidered probable that true extrusive facies were gen-

erated at these localities. Sixteen of the extrusive

carbonatite occurrences listed in this paper do con-

tain mantle-derived material. Table 4 lists these lo-

calities and includes brief mineralogical details.

ls they contain

Nature of mantle debris

Spinel lherzolite xenoliths and cores to lapilli and grains of

Cr-spinel, Cr-diopside and orthopyroxene

Ultramafic nodules and crystal fragments of olivine (FoN92),

Cr-diopside and phlogopite

Rare Cr-diopside and Cr-spinel grains

Xenoliths of Cr-diopside, orthopyroxene, olivine, Cr-spinel.

Also Cr-diopside, olivine and Cr-spinel cores to lapilli

Xenocrysts of Cr-diopside

Xenoliths of wehrlite and lherzolite; xenocrysts of Cr-spinel

Grains of Cr-spinel

Grains of Cr-spinel

Spinel lherzolite xenoliths

Xenoliths of clinopyroxenite with phlogopite and minor apatite,

Ti-magnetite and titanite

Xenoliths of clinopyroxenite with phlogopite, wehrlite and

phlogopite dunite

Pargasite and clinopyroxene xenocrysts

Xenoliths of harzburgite, lherzolite, wehrlite, olivine

orthopyroxenite, clinopyroxene dunite

Xenocrysts of Cr-spinel

Xenocrysts of Cr-spinel

Xenocrysts of Cr-spinel and Cr-diopside

Diamond

Xenocrysts of olivine and phlogopite

Xenoliths of clinopyroxenite with amphibole and phlogopite;

amphibole peridotite; dunite

Fig. 1. Plot of SiO2 against CaO (wt.%) for published analyses of

extrusive carbonatites. Carbonatite lavas and pyroclastic rocks are

distinguished and the abundant data from Fort Portal (Nixon and

Hornung, 1973 and unpublished data) are also differentiated. A

clear trend is apparent but the analysis of Oldoinyo Lengai natro-

carbonatite lava and four other analyses, which are named individ-

ually, lie off this trend. Santiago (S), Goudini (G) and Mwambuto

(M) are dolomitic carbonatites. The main rock type at Castignon

Lake (C) is dmeimechiteT, the categorisation of which is problem-

atical. For further discussion see text. References: lava from Cas-

tignon Lake, Canada (Dimroth, 1970); dolomitic tuff from

Mwambuto (Bailey, 1990); dolomitic tuff from Goudini, South

Africa (Verwoerd, 1967); dolomitic carbonatite lava from Santiago,

Cape Verde Islands (Kogarko, 1993).


Information on the mantle materials found at the

Chagatai, Polino and Natron-Engaruka localities

(mentioned earlier) have been appended to Table 4.

Spinel lherzolite xenoliths, and xenocrysts derived

from them, are the most abundant mantle rock type

(Table 4), but amphibole and phlogopite have been

found in some occurrences. The presence of pyrope

in a xenolith from Monticchio, Italy (Jones et al.,

2000), suggests sampling of the mantle at greater

depths at these localities, as does the occurrence of

diamond at Chagatai. Pargasite xenocrysts at Keri-

masi and Monticchio all suggest that metasomatised

mantle is being sampled, as do the phlogopite-bear-

ing xenoliths at Katwe-Kikorongo and the phlogopite

xenocrysts at San Venanzo. The minor apatite, Ti-

magnetite and titanite in the Katwe-Kikorongo xeno-

liths probably have a similar metasomatic origin

(Lloyd et al., 1987).

The presence of mantle phases in these carbonatitic

rocks is clear evidence of direct eruption from a

mantle source and suggests that these carbonatites

have not undergone dhigh levelT (crustal?) fraction-

ation processes. It is noteworthy that most of these

occurrences take the form of SVEs (Table 4) and

hence the entrained mantle debris probably survived

because of rapid transport to the surface. The occur-

rences with mantle inclusions are also characterised

by an association with melilite-bearing silicate rocks,

11 of the 16 listed in Tables 1 and 4 revealing such an

association.

The apparent lack of mantle xenoliths and xeno-

crysts in intrusive carbonatites is probably a reflection

of the physical processes of magma transport and

emplacement. Intrusive carbonatite complexes are

likely to have been emplaced relatively passively, in

comparison with SVEs, and probably involve one or

more stages of ponding and differentiation of the

magma within the mantle or crust, or both, before

final emplacement. Any solid mantle fragments

would thus almost invariably be altered or, if not,

such high density material would rapidly settle grav-

itationally in low density and allegedly ultra-low vis-

cosity carbonatite magma. If differentiation of a

carbonated silicate magma is involved, whether by

crystal fractionation or a process of liquid immiscibil-

ity, then the mantle materials would probably remain

in the silicate fractionation and so be efficiently sep-

arated from the carbonate portion.

5. Chemistry

Chemical analyses of 64 extrusive carbonatite

lavas and fragmental rocks (tuffs, ashes, breccias)

have been found in the literature, of which 26 are

from Fort Portal (Nixon and Hornung, 1973). We also

have access to a further 29 analyses of Fort Portal

lavas and tuffs (collected by N. Eby, F.E. Lloyd, F.

Stoppa and A.R. Woolley) which are as yet unpub-

lished. Because tuffs, in particular, generally contain a

proportion of entrained debris, they probably do not

represent liquid compositions and it is clearly difficult

to manipulate the data in a meaningful way. An idea

of the range of compositions represented is illustrated

by a plot of SiO2 against CaO (Fig. 1). The plot

demonstrates the variation from relatively dpureT car-bonatites with N50 wt.% CaO, and hence N90%

Fig. 2. Plot of SiO2 against MgO (wt.%) for published analyses of

extrusive carbonatites. The majority of the data define a positive

trend (dot-dash arrow) passing through the carbonatitic lavas of Fort

Portal, and this trend corresponds to increasing contamination with

debris of mantle origin (olivine, pyroxene etc.). However, the

carbonatitic tuffs from Fort Portal form a distinct trend from the

lavas towards higher SiO2values, which is explicable in terms of

contamination with crustal materials. The carbonatite tuffs at

Mwambuto are dolomites and heavily contaminated with K-feld-

spar, and thus lie on a dolomite–K-feldspar trend (dashed arrow).

Localities indicated by letters as on Fig. 1.


calcite, to carbonatitic rocks with only some 20 wt.%

of CaO. The former rocks cluster, in terms of Fig. 1,

around an average calcite carbonatite composition for

intrusive carbonatites (Woolley and Kempe, 1989). It

should be noted, however, that Woolley and Kempe

(1989) excluded carbonatites with N10% SiO2 for

purposes of calculating their average and it could be

that some of the silica-rich carbonatites do in fact

represent primary liquids and not simply carbonatite

liquids plus entrained silicate debris.

The fragmental rocks define a very clear trend from

rocks with negligible silica to those with silica values

approaching 40%, and it is the data for the Fort Portal

tuffs that dominate the values from about 15% to 38%

SiO2. The other very high silica tuffs are from San

Venanzo and Monticchio in Italy (Stoppa and Wool-

ley, 1997) and Gross Brukkaros, Namibia (Stachel et

al., 1995). Although data on these high silica carbo-

natitic tuffs are scarce, apart from Fort Portal, this

scarcity does not reflect the abundance of these rocks,

merely the fact that they have been widely ignored,

probably because the abundance of entrained debris

would seem to preclude any chemical data as being

meaningful. The well-defined trend on Fig. 1 from

zero SiO2 and about 55% CaO to around 40% SiO2

and about 18% CaO probably reflects mixing of

carbonatite liquid and crustal debris and, for Fort

Portal rocks, a lesser amount of mantle material.

The tuffs from Mwambuto and Goudini that plot on

Fig. 1 well below the overall trend are dolomitic, and

hence the CaO is partly replaced by MgO.

Excluding the data for the Fort Portal lavas, and the

lavas from Santiago and Castignon Lake (Fig. 1)

which are dolomitic, it is apparent that the analysed

lavas do not generally extend to such high values of

silica as the tuffs, although the data are scarce. The

Fort Portal analyses (Nixon and Hornung, 1973, and

unpublished data) also demonstrate that generally the

lavas are lower in SiO2 than the tuffs, the Fort Portal

lavas having SiO2 values that average 15.54%, where-

as the tuffs average 24.9% SiO2. The principal reason

for this difference is undoubtedly that the more ener-

getically erupted tuffs contain a significantly higher

proportion of entrained crustal and mantle material.

Although the lavas are probably closer in composition

to primary carbonatitic liquid than the tuffs, it is

difficult to decide what proportion of the silica repre-

sents secondary material. Intrusive calcite carbonatites

generally have much lower silica values than the Fort

Portal extrusives, 116 analyses giving an average of

2.72% SiO2 (Woolley and Kempe, 1989). Although

this average was reached after excluding carbonatites

with N10% SiO2, in fact there are not very many such

analyses in the literature, the majority being repre-

sented by dykes associated with the Alno complex

(von Eckermann, 1958), so that the average is not

increased substantially if such rocks are included. We

surmise that the silica value of primary intrusive

carbonatite liquids is rather higher than the calculated

average carbonatite, but is reduced through gravita-

tional settling of silicate phases while, conversely, the

value for the extrusives was enhanced by incorpora-

tion of crustal material.

On a plot of SiO2 against MgO (wt.%) (Fig. 2) a

clear positive correlation is indicated for the low silica

extrusive carbonatites, which are essentially calciocar-

bonatites. This trend extends to the Fortal Portal lavas,


and is explicable in terms of increasing amounts of

debris of mantle, and possibly cognate, origin (oliv-

ine, pyroxene, phlogopite etc.). The Fort Portal tuffs,

however, show a decrease of MgO with increasing

silica, probably reflecting an increase in contamina-

tion by crustal materials, particularly quartz and feld-

spar. The only strongly dolomitic carbonatite

represented is that from Mwambuto, Zambia (SiO2

17.2%, MgO 12.9%; Bailey, 1990), and this appears

to lie on a trend from dolomite to K-feldspar, the two

principal phases of this rock.

The Fort Portal lavas appear to show evidence of a

trend of increasing MgO with increasing SiO2. This

trend may reflect the greater importance of mantle

debris (high in MgO) in the lava in comparison with

the tuffs (low in MgO high in SiO2). Both Figs 1 and

2 make clear the distinct chemistry of the Fort Portal

carbonatites and the difference between the lava and

tuffs. It is unfortunate that this is the only suite of

extrusive carbonatites in the world that have been

investigated chemically in any depth.

Fig. 1 underlines the uniqueness of the chemistry

of the Oldoinyo Lengai natrocarbonatite lavas, this

property, or otherwise, having been widely discussed

in the literature. We are of the opinion that the Lengai

lavas are not unique and that similar lavas have been

erupted elsewhere in the past. For instance, the lime-

stones that form a carapace on the adjacent volcano of

Kerimasi are considered to have been originally natro-

carbonatitic. The ubiquity of fenite aureoles indicates

that most intrusive carbonatite magmas contain alka-

lis, but there is probably a spectrum of abundances

from a few percent to values in the 30–40 wt.% range

as represented by the Lengai natrocarbonatites.

Although the mixed nature of many carbonatite

and carbonatitic tuffs makes them difficult to interpret,

while the fact that most have also been subjected to

some degree of weathering exacerbates the difficulties

further, the relatively abundant data from Fort Portal

suggest that much more information could be gleaned

from these rocks. The lavas, in particular, should

certainly afford much more insight while Keller

(1981) has demonstrated that some idea of melt com-

position can be obtained by analysis of carbonatite

lapilli, which are to be found at a number of localities.

Dolomitic carbonatites are relatively rare amongst

intrusive carbonatites and, apart from Mwambuto,

hardly feature amongst the extrusives. There is clearly

a need for a thorough chemical study of the Zambian

extrusive carbonatite occurrences, which appear to be

the only extensive dolomitic ones in the world.

6. Discussion

This review was undertaken for a number of

reasons including the belief that few, if any, intrusive

carbonatites represent liquid compositions but that

extrusive carbonatites might do so, that the mantle

xenoliths and xenocrysts being discovered in extru-

sive carbonatites were of fundamental importance to

understanding the genesis of these rocks and finally,

that by considering these rocks as a whole some

differences from intrusive carbonatites would come

to light which might further the understanding of

carbonatite genesis as a whole. It is believed that

these objectives were met in so far as it can be

demonstrated that there are differences between ex-

trusive and intrusive carbonatites in terms of (a)

general geology and mechanism of emplacement,

(b) the nature of the associated silicate rocks and

(c) the presence or absence of materials derived

from the mantle.

It is considered that some of the differences of the

extrusive carbonatites from the intrusive is caused by

the physical differences of their emplacement, but that

these differences may also reflect fundamental differ-

ences of genesis. For instance, the widespread pres-

ence of mantle materials in some extrusives,

particularly those around diatremes, is probably the

result of their energetic emplacement, the very rapid

rise of a turbulent mixture of solid, liquid and gas

components enabling dense mantle material to be

transported to the surface, while the mere presence

of the mantle phases demonstrates that these particular

carbonatites are generated at mantle depths and are

not high level products of crystal fractionation or

separation by immiscibility processes. Conversely,

the carbonatites found in large volcanoes may repre-

sent ponding at high levels of carbonate-rich melt in

the uppermost mantle, or even crust, with subsequent

differentiation, and loss of volatiles to the surrounding

rocks to produce fenitization. Whether the second

type of carbonatite is simply the product of a more

quiescent physical environment at high level mantle/

crustal levels working on the same primary precursor


is not immediately apparent, but some evidence may

be provided by the associated silicate rocks.

It was pointed out earlier that a high proportion of

all the occurrences of extrusive carbonatite are asso-

ciated with melilite-bearing rocks, but that the ratio

is much lower for the intrusive carbonatites. What

does this mean? Although some carbonatite intru-

sions may represent magmatic activity that was

never expressed at the surface, it is thought probable

that a high proportion of such intrusions did indeed

have a surface manifestation, from which it is con-

cluded that there is unlikely to be any fundamental

difference between the magmas represented by the

extrusive and the intrusive carbonatites and their

associated silicate rocks. It is therefore suggested

that the relative paucity of melilite-bearing rocks in

the intrusive carbonatite association is a high level

effect and both physical and chemical mechanisms

suggest themselves.

One such mechanism involves the physical re-

placement of early melilitite-bearing rocks by later

melilite-free rocks, notably carbonatite. Many centres

are dominated by large carbonatite intrusions which

are emplaced at the very end of the igneous events

and which probably destroyed and replaced earlier

intrusions. Chilwa Island, in the Chilwa Province of

Malawi, is an example of a complex consisting

overwhelmingly of carbonatite which forms an ap-

proximately circular intrusion that probably occupies

an earlier vent through which a suite of silicate

magmas passed.

Another mechanism may involve the instability of

melilite in high level intrusions. Wollastonite is a

common constituent of ijolite in many carbonatite

centres and it seems likely that it, and perhaps garnet,

are produced by the breakdown of melilite.

That some melilitites are very intimately associated

with carbonatite are nicely demonstrated by the ac-

cretionary lapilli from Italy in which inner melilitite

zones, around cores of mantle minerals, are rimmed

by carbonatite (Stoppa and Woolley, 1997), and by the

similar, but even more spectacular lapilli from the

Deeti tuff cones, Tanzania (Johnson et al., 1997). A

mechanism for the generation of the extrusive carbo-

natites by separation of a carbonate-rich liquid from

carbonate-bearing melilitite or nephelinite seems to be

supported by both the chemistry and the zoned meli-

litite-carbonatite lapilli.

It was noted earlier that generally the extrusive

carbonatites are higher in silica than the intrusive

ones, and that the higher values of many extrusives

are undoubtedly in part caused by entrained crystals

and fragments of silicate country rocks picked up

during the emplacement process, whereas, it is sug-

gested, the more passive conditions pertaining in

intrusions would allow settling of silicate phases

and particularly any dense mantle xenoliths or xeno-

crysts that may be present. However, it seems pos-

sible that a relatively high silica content may be a

primary feature of many carbonatite magmas. An

impression has been gained that, apart from the

entrained dforeignT materials of high level origin,

the extrusive carbonatites contain an abundance of

cognate silicate phases and that such relatively sili-

cate-rich carbonate rocks may reflect more closely

the primary composition of typical carbonatite mag-

mas than the relatively silicate-free carbonate rocks

typical of most carbonatite intrusions. Such a con-

clusion does, of course, have important implications

for genetic modelling and for experimentalists mod-

elling carbonatite systems.

In an earlier paper (Woolley, 1969) the role of

alkalis in carbonatites was discussed and it was as-

sumed that most carbonatites are endowed with alkalis

that are lost to the surrounding rocks to produce

fenites, or as brines at the surface. Earlier in this

paper it was suggested that the Oldoinyo Lengai

natrocarbonatite is probably not unique but that at

the adjacent Kerimasi volcano, for instance, alkalis

were certainly lost from alkali-rich carbonatites during

weathering. The alkali problem may be related to the

two general types of carbonatite that have been dis-

tinguished here, namely those associated with small

volcanic edifices, involving carbonatitic magmas that

moved rapidly to the surface directly from their source

deep in the mantle, and those in large volcanoes, in

which the carbonatite may have been generated by

high level fractionation/immiscibility processes. It is

proposed that in the latter type high level fractionation

of carbonate-bearing peralkaline magmas generates

carbonatites with a high alkali content, like those of

Oldoinyo Lengai, but that the deeply sourced carbo-

natites contain rather less alkalis, as a result of which

there is little fenitization associated with them. Fur-

ther, the rapid ascent involved in the SVEs is probably

not so conducive to fluid loss to the walls of the


conduit. In the case of the high level generation of

carbonatite it would be expected that a range of

carbonatite magmas containing varying amounts of

alkalis (and other incompatible elements) would be

produced, depending on the chemistry of the parental

magmas, and particularly their degree of peralkalinity.

Further, the ratio of Na :K would differ, although at

crustal levels this might also be controlled by differ-

ential loss of the alkalis during the fenitization pro-

cess, as discussed earlier (Woolley, 1969).

Acknowledgements

We are most grateful to Prof. D.K. Bailey for his

interest in this work and for many pertinent comments

and useful suggestions for improving the paper. Dr

Chris Stanley also kindly commented on the MS.

Prof. J. Keller and Dr B.A. Kjarsgaard gave detailed

and useful reviews, which are much appreciated. We

are grateful to N. Eby, F.E. Lloyd and F. Stoppa for

allowing us to use unpublished data of Fort Portal

tuffs and lavas.

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Extrusive Carbonatites