JOURNAL OF PETROLOGY
VOLUME 39
NUMBER 6
PAGES 1117–1146
1998
Isotope and Trace Element Geochemistry of
Cretaceous Damaraland Lamprophyres and
Carbonatites, Northwestern Namibia:
Evidence for Plume–Lithosphere Interactions
ANTON P. LE ROEX∗ AND RUTH LANYON†
DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CAPE TOWN, RONDEBOSCH 7700, SOUTH AFRICA
RECEIVED JULY 7, 1997; REVISED TYPESCRIPT ACCEPTED DECEMBER 12, 1997
Major and trace element and radiogenic and stable isotope data are
reported for Cretaceous lamprophyres and carbonatites associated
with alkaline igneous complexes in Damaraland, northwestern
Namibia. Lamprophyres, emplaced as late-stage dykes, stocks and
diatremes, include both alkaline and ultramafic varieties. Many of
the latter contain abundant carbonate-rich ocelli. The carbonatites
are primarily sövites and Fe-rich beforsites emplaced as plugs and
dykes. Both lamprophyres and carbonatites are characterized by
strong, but variable, enrichment in incompatible trace elements (e.g.
La 45–7232 ppm; Nb 89–1961 ppm; Sr 700–43 000 ppm),
and variable but coherent initial 87Sr/86Sr (0·70351–0·70468),
143
Nd/144Nd (0·51244–0·51266) and Pb isotope ratios (e.g.
206
Pb/204Pb = 17·77–19·99). Individual lamprophyre types
show distinct, but restricted, ranges in isotopes and incompatible
element ratios (Zr/Nb = 0·9–2·0; Y/Nb = 0·15–0·34; La/
Ybn = 20–58), indicating derivation from a trace element enriched,
heterogeneous mantle source. Lamprophyres and carbonatites found
within the same complexes have similar isotope ratios, indicating
derivation from a common source. Isotope and trace element variations, coupled with the temporal and spatial association with the
palaeoposition of the Tristan plume, are thought to indicate that
the Damaraland lamprophyre and carbonatite magmatism formed
as a consequence of melting of metasomatic vein material introduced
into an isotopically depleted subcontinental lithospheric mantle by
alkaline melts or fluids derived from the upwelling Tristan mantle
plume at the time of continental break-up.
KEY WORDS:
geochemistry; lamprophyre carbonatite; Namibia; Tristan plume
∗Corresponding author. Telephone:+27-21-650-2921. Fax:+27-21650-3783. e-mail: [email protected]
Present address: Port Pirie Environmental Health Centre, 117 Gertrude
Street, Port Pirie, S.A., Australia 5540.
INTRODUCTION
The close association of alkaline silicate rocks (ijolites,
nephelinites and phonolites) and carbonatites within certain intra-plate intrusive complexes is a well-recognized
and geographically widespread phenomenon (Le Bas,
1987; Barker, 1989). However, the genetic relationship
between them and the processes involved in their generation are still not fully understood. Considerable debate
exists as to whether carbonatite magmas are derived via
the direct partial melting of mantle peridotite (e.g. Gittins,
1988; Green & Wallace, 1988; Wallace & Green, 1988;
Yaxley et al., 1991), or whether they represent mantlederived magmas which have undergone modification at
lower pressures by a process of silicate–carbonate liquid
immiscibility from, or extensive fractionation of, a carbonated silicate parental magma (e.g. Middlemost, 1974;
Koster van Groos, 1975; Le Bas, 1987, 1989; Kjarsgaard
& Hamilton, 1989; Lee & Wyllie, 1994).
A linear series of anorogenic igneous complexes extend
through Damaraland in northwestern Namibia (Fig. 1)
and provide an example of the contiguous alkaline silicate
and carbonatite lithologies. The composition of these
complexes shows a general northeasterly spatial trend
from granitic associations close to the Atlantic coast,
through differentiated basic, to peralkaline and carbonatitic compositions located ~250–350 km inland
(Martin et al., 1960). In addition to the silicate rock–
carbonatite associations within three of the Damaraland
complexes, carbonate-rich globular structures (ocelli)
have also been described within the late-stage nephelinitic
 Oxford University Press 1998
JOURNAL OF PETROLOGY
VOLUME 39
NUMBER 6
JUNE 1998
Fig. 1. Simplified geological map of northwestern Namibia, showing the main outcrop regions of the Etendeka Group volcanic rocks, Cretaceous
Damaraland igneous complexes, and basement ages (from Milner & le Roex, 1996).
and/or lamprophyric intrusions associated with the
Okenyenya, Okorusu and Ondurakorume complexes
(Prins, 1981; Lanyon & le Roex, 1995). The Okenyenya
igneous complex, the most northeasterly located of the
differentiated basic complexes, marks the position northeast of which carbonatites occur as discrete intrusive
phases. In addition, Okenyenya also contains the most
extensive and chemically varied set of lamprophyre intrusions.
This study is aimed at geochemically characterizing the
Damaraland lamprophyres, nephelinites and associated
carbonatites, so as to place constraints on their petrogenesis and location of their source regions, and evaluating the possible role of the Tristan mantle plume
in initiating this Cretaceous alkaline magmatism. An
investigation into the relationship between the carbonatites and the carbonate ocelli-bearing lamprophyres
and nephelinites associated with the Okenyenya, Okorusu
and Ondurakorume complexes was also undertaken
using trace elements as well as stable and radiogenic
isotopes.
GEOLOGICAL SETTING
Regional geology
The Mesozoic Damaraland complexes of northwestern
Namibia intrude a predominantly Late Proterozoic
basement that consists of Pan-African (930–470 Ma)
Damara Sequence metamorphic rocks and syntectonic
bodies of Salem granite (Martin et al., 1960; Miller, 1983).
Although they define a broad northeasterly-trending linear feature which extends from Cape Cross on the
Atlantic coast to Okorusu ~350 km inland (Fig. 1), no
temporal significance can be attached to their linear
arrangement (Milner et al., 1995). However, their emplacement between ~137 and 123 Ma (Milner et al., 1995,
and references therein), coincident with the eruption of
the voluminous Paraná–Etendeka flood basalts of Brazil
and northwest Namibia and the opening of the South
Atlantic Ocean (Siedner & Mitchell, 1976; Renne et al.,
1992; Turner et al., 1994), suggests that they are a product
of thermal activity associated with the break-up of western
Gondwana (Milner et al., 1995; Milner & le Roex, 1996).
1118
LE ROEX AND LANYON
GEOCHEMISTRY OF LAMPROPHYRES AND CARBONATITES
Initial upwelling of the Tristan mantle plume head beneath the previously contiguous regions of South America
and southern Africa resulted in both extensive flood
volcanism (e.g. Duncan, 1984; White & McKenzie, 1989;
Thompson & Gibson, 1991; Hawkesworth et al., 1992)
and the emplacement of a series of high-level subvolcanic
intrusions, the Damaraland igneous complexes, along an
ancient crustal suture (Milner et al., 1995).
The Damaraland complexes
Okenyenya igneous complex
The Okenyenya igneous complex, located midway between Cape Cross and Okorusu, comprises a series of
intrusive phases emplaced between ~129 and 123 Ma
(Milner et al., 1993; Watkins et al., 1994) and is thought to
represent the sub-volcanic remnants of a central volcano
complex (Simpson, 1954; Watkins & le Roex, 1991,
1994). Tholeiitic rocks dominate the southern two-thirds
of the complex and comprise a suite of olivine gabbro
to quartz monzodiorite rock types (~129 Ma) which are
both rimmed and cross-cut by later picritic gabbro,
syenite and quartz syenite dykes (~127 Ma). Tholeiitic
magmatism at Okenyenya alternated with the emplacement of plugs of alkaline gabbro (~128 Ma) within
the central and northwestern regions, and a series of
concentric andesine essexite, oligoclase essexite (~126
Ma), and nepheline syenite (~123 Ma) intrusions in the
northeast. The final highly alkaline phase of magmatic
activity at Okenyenya, of particular interest to this study,
consists of lamprophyric rock types which form a series
of plugs, dykes and diatremes and intrude both the
tholeiitic and alkaline gabbros in the southern half of the
complex. Their geochemistry has been described in detail
by le Roex et al. (1996).
Carbonatite complexes
The Damaraland carbonatite complexes are all located
further inland and to the east of Okenyenya. The closest,
and smallest, is the Osongombo complex, which outcrops
~70 km east-northeast of Okenyenya (Fig. 1). It is dominated by volcanic breccia with a central plug of Fe-rich
beforsite and scattered outcrops of iron ore, and lacks
associated silicate rock types (Martin et al., 1960; Verwoerd, 1966; Prins, 1981). The Kalkfeld igneous complex
lies ~15 km to the northeast of Osongombo (Fig. 1) and
consists of a central carbonatite plug of equigranular grey
sövite with micaceous- and apatite-rich portions and
ankeritic veins (Prins, 1981), and is surrounded by fenitized intrusions of nepheline syenite and syenite.
About 15 km northeast of the Kalkfeld igneous complex, the Ondurakorume complex is made up of a central
carbonatite plug which penetrates volcanic breccia and
which is surrounded by scattered and fenitized outcrops
of syenite and nepheline syenite, and cross-cut by olivine
dolerite dykes and oxidized iron ore dyke-like bodies
(Verwoerd, 1966; Prins, 1981). The Ondurakorume carbonatite consists of micaceous sövite, sövite, apatite-rich
beforsite and amphibole- or rare earth-beforsite (Prins,
1981).
Okorusu, the most inland of the Damaraland complexes, is located ~100 km north-northeast of the Ondurakorume complex. Okorusu is characterized by an
extensive fluorspar deposit and comprises a series of
confocal alkaline intrusions, including hortonolite monzonite, syenite, foyaite and urtite, which are intruded by
dykes and plugs of foyaite, tinguaite, bostonite, melanephelinite, nephelinite (Van Zijl, 1962; Prins, 1981)
and rare lamprophyre. Homogeneous sövitic carbonatite
occurs only as minor plugs (Verwoerd, 1966) associated
with pyroxene fenite (Van Zijl, 1962).
ANALYTICAL TECHNIQUES
Whole-rock major and trace element abundances were
determined by X-ray fluorescence (XRF) spectrometry
following the procedures routinely applied in the Department of Geological Sciences at the University of
Cape Town, with errors and detection limits similar
to those quoted by le Roex (1985). Whole-rock CO2
abundances were calculated following duplicate determinations of CaCO3 using the Karbonat-bombe
method of Birch (1981). Whereas the precision for samples
with >5% CaCO3 is 2% relative, precision decreases to
~4% relative for samples with <5% CaCO3.
Whole-rock REE abundances were determined using
high-pressure ion chromatography (HPIC) following the
procedures outlined by le Roex & Watkins (1990); errors
are typically <5% relative. Stable isotope (C and O)
analyses of whole-rock and ocellar carbonates were obtained following the procedures outlined by Martinez
et al. (1996). Carbonatite and whole-rock lamprophyre
analyses were performed on the same powder splits as
used for all other analytical procedures, whereas chips
([180 lm) of lamprophyre ocellar and groundmass
phases were hand separated before analysis. An internal
standard calibrated against NBS-19 was used to correct
raw data to the SMOW and PDB scales assuming d18O
and d13C values for NBS-19 to be 28·64‰ and 1·95‰,
respectively. Typical reproducibility for analyses of the
in-house standard (Namaqualand marble) are 0·05‰
(d13C) and 0·1‰ (d18O).
Sr, Nd and Pb were separated using conventional ionexchange techniques and all radiogenic isotope analyses
were performed on a VG Sector multi-collector mass
spectrometer operated in either static (Pb) or dynamic
(Sr and Nd) multi-collector mode. To correct for mass
fractionation effects, measured 87Sr/86Sr and 143Nd/144Nd
1119
JOURNAL OF PETROLOGY
VOLUME 39
values were normalized to 86Sr/88Sr = 0·1194 and 146Nd/
144
Nd = 0·7219, respectively. Lead isotopes were corrected for fractionation using the values of Catanzaro et
al. (1968) for international standard NBS 981: 206Pb/
204
Pb = 16·937, 207Pb/204Pb = 15·491, and 208Pb/204Pb =
36·721; average fractionation factors were 0·14% per
a.m.u. Mean and 2rpop errors for repeated analyses of
standards performed during the course of this study are
as follows: NBS 987 87Sr/86Sr = 0·710222 ± 24 (n =
9); La Jolla 143Nd/144Nd = 0·511820 ± 12 (n = 8); and
NBS 981 206Pb/204Pb = 16·902 ± 6, 207Pb/204Pb =
15·453 ± 9, 208Pb/204Pb = 36·575 ± 30 (n = 11),
where the errors relate to the least significant digit/s.
Where possible, the trace element and REE concentrations used to calculate the initial isotopic ratios
were obtained by XRF and HPIC, respectively; trace
element and REE concentrations in the remaining
samples were obtained by isotope dilution using mixed
87
Rb–84Sr and 149Sm–150Nd spikes.
RESULTS
Okenyenya lamprophyres
Both alkaline and ultramafic lamprophyres have been
identified at Okenyenya (Lanyon & le Roex, 1995, and
references therein). Detailed petrographic descriptions,
accompanied by representative electron microprobe mineral analyses, have been presented by Lanyon & le
Roex (1995), and a petrographic summary is provided
in Table 1.
The alkaline lamprophyres have been further classified
as camptonites in that they possess groundmass plagioclase and feldspathic segregations (Rock, 1991; Lanyon
& le Roex, 1995). They occur as thin dykes cross-cutting
gabbros in the southeast of the complex and as a stocklike body near the centre of the complex, comprising both
camptonite and differentiated tinguaite. Four distinctive
petrographic varieties of ultramafic lamprophyre dykes
occur within the southern half of the complex, and as
two brecciated and xenolith-rich diatremes which outcrop
in the centre of the complex. The classification of the
southeastern diatreme as an alnöite is based primarily
on the presence of groundmass melilite and perovskite
(Baumgartner, 1994). Most of the ultramafic lamprophyre
dykes, dominated by phenocrystic phlogopite and abundant groundmass carbonate, are classified as damkjernites
(Lanyon & le Roex, 1995). The remainder, as well as the
more centrally located diatreme, have been subdivided on
the basis of their petrographic textures into either ‘seriatetextured’ or ‘ocelli-rich’ varieties (Lanyon & le Roex,
1995). Although Lanyon & le Roex (1995) originally
considered the seriate-textured, ultramafic lamprophyres
to lack distinctive enough modal mineralogy to allow
classification beyond ultramafic, it is possible that, with
NUMBER 6
JUNE 1998
their lack of groundmass feldspar and melilite, they might
represent ouachitites, a term that will be used here.
Major elements
Distinction between the alkaline and ultramafic lamprophyres was originally made primarily on the basis of
mineralogy (Lanyon & le Roex, 1995), but differences
are also evident in terms of their whole-rock major and
trace element chemistry. In particular, the ultramafic
lamprophyres are readily distinguished from the alkaline
varieties by their lower SiO2 and Al2O3 abundances, their
higher TiO2 and CaO contents, and their higher CaO/
Al2O3 ratios (Table 2; Fig. 2). The alkaline lamprophyres
(camptonites) show a wide range of mg-number (68–39;
assumed Fe2O3/FeO = 0·20), with the most primitive
having mg-number close to expected values for primary
magmas. The most evolved in terms of mg-number is
classified as tinguaite. The considerable internal variation
shown by the camptonite plug (Fig. 2) is qualitatively
consistent with fractionation of observed phenocryst
phases, namely, olivine, clinopyroxene, amphibole and
Fe–Ti-oxide.
With the exception of a single evolved sample (OKJ94-3: mg-number ~50), the ultramafic lamprophyres have
relatively primitive compositions (mg-number = 60–74),
and the different sub-types can be readily distinguished
from one another on the basis of major elements (Fig. 2).
The significantly higher LOI values (6·42–15·91 wt %)
and CO2 contents (5·35–14·22 wt %) of the damkjernites,
as compared with the other ultramafic lamprophyres
(LOI = 2·80–6·45 wt %; CO2 <1·00 to 4·69 wt %),
is reflected in the greater abundance of groundmass
carbonate.
Trace elements
Compatible trace element abundances (e.g. Ni, Cr, Sc)
show a regular decrease with decreasing mg-number; the
more primitive ultramafic and alkaline lamprophyres
have >200 ppm Ni, and >500 ppm Cr (Fig. 3; Table 2).
In terms of incompatible trace element abundance variations, the individual lamprophyre types have generally
distinct and coherent characteristics (Fig. 3). All of the
Okenyenya lamprophyres have REE patterns characterized by strong chondrite-normalized light rare earth
element (LREE) enrichment (Fig. 4). The camptonites
are less enriched in the LREE (La/Ybn = 20–30) than
the ultramafic lamprophyres (La/Ybn = 27–58). The
camptonites also have a more concave-upwards chondrite-normalized REE pattern than the other varieties
(Fig. 4).
The different lamprophyre types present at Okenyenya
are readily distinguished from one another in terms of
their incompatible element ratios, which show remarkably restricted ranges for each type. The damkjernites are distinguished by their low Zr/Nb and Y/Nb
1120
1121
amph (64–77) + cpx (61–77; 50–58) +
Ti-mag ± ol (85–86)
amph (53–70) + biot (53–59) + cpx
(61–89) + ol (74–91) + rare phlog
(~74)
amph (45–67) + ol (77–87) + cpx
(70–88; green hedenbergite: 45–46) +
rare biot + Ti-mag
Phenocrysts and microphenocrysts
plag (An33–59) + amph + cpx (67–73) +
nepheline + Ti-mag + apatite +
carbonate
plag (~An37) + sanidine (An1Ab42Or57–
An2Ab28Or70) + analcime + sodalite +
cancrinite + carbonate + rare
muscovite
plag (An30–53) + amph (63–68) + cpx
(69–70) + biot + Ti-mag ± sodalite ±
analcime ± minor sanidine (Ab16Or84)
Groundmass
ol + cpx + sanidine
OKU-94-5
ol (85–86) + phlog (74–79) + cpx
(66–82; green cores: 51–57) + rare biot
(~51) + ilmenite + apatite
ol (85–89) + cpx (66–83; green cores:
54–66) + Ti-mag
cpx (~80) + biot (48–59) + nepheline
+ sodalite + calcite + Ti-mag +
apatite
ol (~89) + cpx (76–83) + nepheline +
sanidine (An3Ab30Or67–An6Ab43Or51) +
amph (38–50) + calcite + analcime +
Ti-mag + apatite + spinel
ocelli: calcite ± analcite ± sodalite ±
albite
ocelli: calcite ± analcime ± mica ±
?cancrinite
ocelli: sodalite + calcite
segregations: zeolites ± orthoclase ±
calcite/magnesian calcite ± phlog
phlog (72–82) + cpx (71–79) +
nepheline + analcime + sodalite +
calcite + Ti-mag + apatite
phlog (74–81; biot cores: 39–63) + ol
(83–90) + cpx (46–48; 72–85; green
cores: 58–60; ~88) + Ti-mag + calcite
1. ocelli: analcime cores; calcite ±
orthoclase rims
2. ocelli: ankerite + Ti-mag (OKJ-94-5)
3. ocelli: calcite + Ti-mag (OKJ–94-3)
phlog (72–80) + cpx (67–83) +
ocelli: calcite + Ti-mag ± zeolites
analcime + Ti-mag + apatite ± calcite segregations: calcite + analcime (OKJ± perovskite
94-9)
phlog (67–83) and/or biot (63–66) +
cpx (72–75) + nepheline + orthoclase
(An1Ab4Or95–An1Ab2Or97) + calcite +
ilmenite + apatite ± Ti-mag (89–211:
nepheline + sodalite) (OKJ-94-5:
nepheline + analcime + albite)
phlog (76–81; biot cores: 39–40) + ol
(86–91) + cpx (65–87; green cores:
51–87) + Ti-mag ± ilmenite
phlog (76–85) + ilmenite ± biot
(60–63) ± cpx (71–86; green cores:
42–72) ± ol (84–85) ± calcite
phlog (~75) + ol (87–90) + cpx (66–83; phlog (75–78) + cpx (57–60; 73–83) + ocelli: calcite + analcime
green cores: 58–64) + Ti-mag +
analcime + calcite + perovskite + Tiapatite
mag + Cr-pleonaste + apatite
ocelli: analcime cores; ankerite +
albite rims
segregations: plag + ankerite + Timag
ocelli: plag + calcite + zeolites
segregations: plag + biot + amph +
Ti-mag ± cpx
Leucocratic structures
Unless otherwise stated, all numbers in parentheses represent mg-number = 100 Mg/(Mg + Fe); abbreviations for the more common minerals are: ol, olivine;
cpx, clinopyroxene; plag, plagioclase; amph, amphibole; biot, biotite; phlog, phlogopite; Ti-mag, titanomagnetite.
∗Minerals as described by Baumgartner (1994).
cpx
cpx + ol
phlog ± cpx
cpx + ol
amph + cpx + ilmenite + amph (67–81) + phlog (74–76; biot
phlog (~88) + cpx (~78) + calcite +
apatite + phlog
cores: ~59) + ol (63–67; 80–90) + cpx sodalite + monticellite + melilite∗ +
(72–84; green cores: 60–68) + ilmenite perovskite∗ + Ti-mag + apatite
+ Ti-mag + apatite
ol + cpx + plag
OKORUSU
OKU-94-1
Ouachitite
Seriate-textured
OKJ-162
OKJ-284
OKJ-94-9
Ocelli-rich
OKJ-94-7
Ultramafic dykes
Damkjernite
OKJ-135
89-211
OKJ-94-3
OKJ-94-5
Ouachitite diatreme
OKJ-94-10
Alnöite diatreme
OKJ-12
OKJ-60
OKJ-94-1 (dyke)
OKJ-94-2 (dyke)
OKJ-24 (tinguaite plug)
Sample no.
OKENYENYA
Camptonite
OKJ-11 (plug) OKJ-55 (plug)
Megacrysts
Table 1: Mineralogy of lamprophyre and nephelinite intrusions associated with the Okenyenya and Okorusu igneous complexes
LE ROEX AND LANYON
GEOCHEMISTRY OF LAMPROPHYRES AND CARBONATITES
1122
55·6
80·3
7·55
20·5
3·56
1·11
2·58
0·44
2·90
1·70
1·88
1176
206
152
704
266
16
11
47
49
2·5
55
7·9
9·4
39·1
90·9
142
15·3
46·5
8·24
2·28
5·60
0·83
4·73
2·35
2·17
1465
119
169
1469
222
26
12
31
139
5·4
78
21
18
3·3
12·2
5·6
50·0
1·82
45·75
1·34
19·03
7·50
0·22
3·06
7·55
5·31
3·78
0·67
4·87
0·06
99·14
OKJ-94-1
Camptonite
Dyke
88·1
145
14·7
46·8
8·89
2·15
5·62
0·86
4·83
2·34
2·35
1618
124
174
1446
224
26
5·8
14
135
3·2
80
16
18
3·1
16·1
8·9
43·5
1·65
46·25
1·28
19·42
7·32
0·23
2·68
7·03
6·48
4·05
0·65
4·22
0·16
99·77
OKJ-94-2
Camptonite
Dyke
105
170
16·4
55·2
10·4
4·56
8·39
1·15
6·00
2·95
1·52
1029
43
89
2125
145
30
116
126
311
25
84
82
53
2·4
15·0
12·0
60·9
1·98
36·20
2·62
12·49
11·87
0·23
8·80
14·78
4·95
2·14
1·22
4·12
0·08
99·50
OKJ-12
Alnöite
Diatreme
118
180
22·5
64·0
11·3
3·55
8·43
1·20
6·04
2·38
1·89
800
38
94
2356
144
29
168
169
294
23
86
85
52
3·0
16·0
40
63·6
2·19
35·12
2·71
11·47
11·81
0·23
9·79
14·57
5·66
2·31
1·38
4·66
0·10
99·80
OKJ-60
Alnöite
Diatreme
107
199
21·9
77·8
13·3
3·49
8·73
1·09
5·26
2·01
1·31
1485
112
136
1498
120
23
32
16
230
20
81
125
51
62·1
8·86
31·33
2·20
10·76
9·12
0·19
7·11
18·33
4·03
3·64
0·96
10·51
0·14
98·32
OKJ-135
Damkjernite
Dyke
71·0
150
17·2
62·7
11·8
2·46
6·54
0·87
4·35
1·73
1·07
1668
138
132
1000
128
22
69
119
244
23
67
129
46
2·5
9·4
4·6
64·8
5·35
36·16
2·23
12·39
9·04
0·18
7·91
15·26
3·58
4·19
0·67
7·43
0·05
99·09
89-211
Damkjernite
Dyke
114
195
21·0
72·0
12·8
3·28
8·06
0·99
5·26
2·16
1·83
1553
155
169
1805
149
26
32
10
264
8·9
92
97
36
3·0
13·0
9·0
11·10
0·21
53·2
32·02
1·77
12·02
8·75
0·21
4·73
14·81
3·85
5·24
1·04
12·47
0·10
97·20
OKJ-94-3
Damkjernite
Dyke
NUMBER 6
44·6
85·2
7·15
29·8
5·70
1·59
4·57
0·67
4·03
2·04
1·46
947
94
114
818
189
21
220
747
192
18
66
67
46
68·3
2·39
51·24
0·65
20·91
3·81
0·12
1·16
2·91
9·18
5·80
0·17
4·89
0·18
101·02
OKJ-24
Tinguaite
Plug
VOLUME 39
Rare earth elements (ppm)
La
49·2
Ce
81·8
Pr
8·71
Nd
27·2
Sm
5·50
Eu
1·82
Gd
4·64
Tb
0·64
Dy
3·77
Er
1·99
Yb
1·75
Trace elements (ppm)
Ba
916
Rb
99
Nb
112
Sr
777
Zr
191
Y
20
Ni
196
Cr
620
V
188
Sc
19
Zn
65
Cu
57
Co
41
U
2·5
Th
9·5
Pb
6·4
67·8
<1·00
CO2
F
mg-no.
1·61
OKJ-55
Camptonite
Plug
46·08
1·60
15·51
8·52
0·20
8·72
9·07
5·60
2·83
0·43
2·70
0·24
100·67
OKJ-11
Camptonite
Plug
Major elements (wt %)
SiO2
45·96
1·54
TiO2
15·74
Al2O3
FeO∗
8·15
MnO
0·18
MgO
8·18
CaO
8·77
4·89
Na2O
2·95
K 2O
0·40
P2O5
LOI
2·90
0·18
H2O–
Total
99·84
Sample no.:
Rock type:
Intrusion:
Okenyenya
Table 2: Whole-rock major, trace and REE chemistry of the lamprophyre and nephelinite intrusions associated with the Okenyenya, Okorusu
and Ondurakorume igneous complexes
JOURNAL OF PETROLOGY
JUNE 1998
14·22
0·15
65·4
1123
68·0
130
13·5
51·2
9·07
2·59
7·15
0·97
5·31
2·23
1·58
1377
90
125
1016
253
27
292
737
262
25
74
70
55
2·7
10·0
5·2
73·5
1·97
39·99
2·76
11·66
9·21
0·19
12·12
13·07
2·80
2·23
0·68
4·94
0·16
99·81
Dyke
OKJ-162
Ouachitite
82·1
150
15·2
57·2
10·4
2·95
7·85
1·08
5·95
2·63
1·54
1527
93
136
1201
257
29
214
527
275
23
76
77
51
3·3
9·6
5·4
69·9
1·71
39·34
3·08
12·71
9·55
0·20
10·56
13·14
5·01
1·98
0·81
4·06
0·10
100·54
Dyke
OKJ-284
Ouachitite
1515
85
119
1023
227
26
247
531
269
26
70
71
53
<2·6
10·4
<4·2
72·2
1·81
39·48
2·95
12·07
9·19
0·18
11·35
13·08
5·23
1·84
0·69
3·43
0·05
99·54
Dyke
OKJ-94-9
Ouachitite
73
129
15·3
51·6
10·3
2·86
7·37
1·01
5·42
2·27
1·96
1985
100
121
1414
236
27
257
784
382
34
73
77
77
2·2
9·9
4·7
72·1
<1·00
39·63
3·02
12·28
9·52
0·18
11·68
14·09
2·19
1·47
0·75
5·01
0·32
100·13
Diatreme
OKJ-94-10
Ouachitite
104
180
20·3
69·8
13·6
3·64
9·18
1·17
6·16
2·42
1·81
884
95
137
1217
267
28
49
37
260
23
79
68
45
2·1
16·0
6·9
60·3
4·69
34·54
3·10
12·03
10·86
0·21
7·83
13·81
5·60
2·25
1·17
7·66
0·19
99·23
OKJ-94-7
Ocelli-rich
lamp
Dyke
Okorusu
710
867
80·0
205
27·3
7·42
17·3
2·64
14·5
7·56
6·04
2897
121
334
4801
176
91
39
24
167
9·0
98
92
33
6·5
52
12·3
5·64
0·36
56·4
34·28
1·52
13·77
8·67
0·36
5·34
14·90
6·75
2·58
2·20
7·89
0·18
99·06
OKU-94-1
Ultramafic
lamp
Dyke
74·9
131
14·1
44·4
7·51
2·14
5·38
0·77
4·21
2·04
1·89
1034
67
115
965
156
24
296
515
160
23
73
87
53
3·2
16·3
7·1
72·4
1·59
42·90
1·12
13·53
8·79
0·19
10·95
11·20
5·34
2·39
0·47
2·97
0·02
99·84
Dyke
OKU-94-5
Ol nephelinite
90·6
133
13·4
44·6
8·24
2·44
6·27
0·85
4·83
2·03
1·42
<2·9
6·5
4·1
26
14222
46
140
1452
130
25
68·4
4·80
38·83
1·41
11·63
10·39
2·51
10·67
10·73
0·81
1·49
0·34
7·43
2·39
98·63
ON-6
Ultramafic
lamp
Dyke
Ondurakorume
mg-number =100 atomic Mg/(Mg + Fe2+) assuming Fe2O3/FeO = 0·20. LOI, loss on ignition. Major and trace element data for samples OKJ-11, OKJ-24 and OKJ135 from le Roex et al. (1996); major and trace element data for ON-6 from Prins (1981) except Sc, U, Th and Pb; all REE data from this study.
Rare earth elements (ppm)
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Er
Yb
Trace elements (ppm)
Ba
1960
Rb
105
Nb
135
Sr
1417
Zr
133
Y
20
Ni
41
Cr
107
V
233
Sc
26
Zn
62
Cu
121
Co
35
U
2·7
Th
12·2
Pb
5·8
CO2
F
mg-no.
Major elements (wt %)
SiO2
31·29
1·75
TiO2
10·44
Al2O3
FeO∗
7·36
MnO
0·17
MgO
6·61
CaO
16·70
2·87
Na2O
3·55
K 2O
0·68
P 2O5
LOI
16·73
0·19
H2O–
Total
98·48
Dyke
Intrusion:
Okenyenya
OKJ-94-5
Damkjernite
Sample no.:
Rock type:
LE ROEX AND LANYON
GEOCHEMISTRY OF LAMPROPHYRES AND CARBONATITES
JOURNAL OF PETROLOGY
VOLUME 39
NUMBER 6
JUNE 1998
N
Fig. 2. Selected major element variations of the Okenyenya alkaline and ultramafic lamprophyre intrusions: Χ, ouachitite dykes; ,ouachitite
diatreme; Μ, ocelli-rich ultramafic dyke; Α, alnöite diatreme; Β, damkjernite dykes; Φ, camptonite dykes; ×, differentiated camptonite plug.
ratios and their high Ce/Yb, K/Nb, K/Th, Rb/Nb
and K/U ratios compared with the other lamprophyres
(Table 3; Fig. 5). The ouachitite and ocelli-rich ultramafic
varieties have the lowest K/U and K/Nb ratios and the
highest Zr/Nb ratios, whereas the alnöites have the
highest La/Nb and Y/Nb at intermediate Zr/Nb ratio
(Table 3; Fig. 5). The alkaline lamprophyres have lower
Ce/Yb, La/Nb and Ti/Zr ratios than the ultramafic
varieties.
The Okenyenya lamprophyres have primitive mantle
normalized trace element patterns that are characterized
by an increase in abundance with increasing
incompatibility (Fig. 6). Superimposed on this general
increase are a number of variably pronounced negative
anomalies (relative to adjacent elements). The alkaline
lamprophyres have slightly negative Ti and Sm anomalies
but no significant K anomalies. The damkjernites display
negative Zr and P anomalies, strong negative Ti anomalies, and have no significant K anomalies. In contrast,
the remaining varieties of ultramafic lamprophyre have
strong negative K and Rb anomalies. Whereas the alnöite
diatreme samples also possess negative Zr and minor
negative Ti anomalies, the ocelli-rich ultramafic lamprophyre lacks a significant negative Zr anomaly and
the ouachitites show no significant negative Ti or Zr
anomalies.
1124
LE ROEX AND LANYON
GEOCHEMISTRY OF LAMPROPHYRES AND CARBONATITES
Fig. 3. Selected trace element variations of Okenyenya alkaline and ultramafic lamprophyre intrusions. Symbols as for Fig. 2.
Fig. 4. Chondrite-normalized rare earth element variations of Okenyenya and Ondurakorume lamprophyre intrusions and Okorusu lamprophyre
and nephelinite dykes. Chondrite normalizing values from Sun & McDonough (1989).
1125
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NUMBER 6
JUNE 1998
Table 3: Selected incompatible element ratios for primitive lamprophyres (mg-number >0·60) from
the Okenyenya igneous complex
Ultramafic
Damkjernite
Alkaline
Alnöite
Ouachitite
Ocelli-rich
Zr/Nb
0·9–1·0
1·5–1·6
1·9–2·0
1·9
Zr/Y
5·2–6·7
4·8–5·0
8·7–9·4
9·5
Y/Nb
0·15–0·17
0·31–0·34
0·21–0·22
0·20
Ce/Yb
140–152
95–112
66–97
Rb/Nb
0·78–1·04
0·40–0·48
0·68–0·83
99
0·69
Camptonite
1·7
9·0–9·6
0·18
47–58
0·82–0·88
K/Nb
218–263
200–204
101–148
136
K/Th
2420–3700
1180–1200
1230–1850
1170
2580
K/U
11120–13910
6390–7400
4980–6860
8895
9800
0·54–0·78
1·2–1·3
0·54–0·60
Ti/Zr
79–110
108–113
65–78
70
48–51
Nb/U
51–53
31–37
41–55
65
45
26
La/Nb
Ce/Pb
33
4–14
25–28
(La/Sm)n
3·9–5·2
6·5–6·7
4·6–5·1
(La/Yb)n
48–58
45–50
27–38
0·76
4·9
41
206–219
0·39–0·44
13
5·1–5·8
20–22
Chondrite-normalizing values from Sun & McDonough (1989).
Radiogenic and stable isotopes
The initial radiogenic isotope compositions (calculated
at 124 Ma, Table 4; Fig. 7) of the alkaline lamprophyres
are virtually indistinguishable. They have high eNd and
low 87Sr/86Sri ratios, and Pb isotope ratios intermediate
to the range displayed by the ultramafic lamprophyres
(Table 5; Fig. 8). The ultramafic lamprophyres define a
relatively narrow range of initial eNd values (+1·6 to
+3·1), signifying time-integrated LREE depletion of their
source region relative to Bulk Earth, and a moderate
range of initial 87Sr/86Sr ratios (0·70385–0·70461). With
the exception of the ouachitite diatreme sample (OKJ94-10), the ultramafic lamprophyres define a narrow
range of initial 207Pb/204Pb (Table 5), but with significant
206
Pb/204Pb variation (17·769–19·070), and plot well
above the Northern Hemisphere Reference Line (NHRL)
in 207Pb/204Pb vs 206Pb/204Pb space (Fig. 8). However,
they form an approximately linear array along the NHRL
on a plot of 208Pb/204Pb vs 206Pb/204Pb (Fig. 8).
Small, yet coherent and distinct, differences amongst
the initial radiogenic isotope signatures of the different
lamprophyre types found at Okenyenya are evident in
Figs 7 and 8. Whereas the ocelli-rich ultramafic lamprophyre dyke has a similar Sr, Nd and Pb isotopic
signature to the camptonites, the ouachitite dykes have
slightly lower eNd and higher 87Sr/86Sr values (Table 4).
The ouachitite diatreme (OKJ-94-10) has the highest
initial ratios of 87Sr/86Sr (0·70461) and 207Pb/204Pb
(15·705) of all the Okenyenya lamprophyres (Fig. 8), and
the latter in particular is considered to reflect crustal
contamination. The damkjernite dykes have a limited
range of initial 87Sr/86Sr and eNd values (Table 4) which
plot within the depleted mantle quadrant at 124 Ma
(Fig. 7). They also have the least radiogenic initial 206Pb/
204
Pb values (Table 5; Fig. 8). The alnöite diatreme has
similar initial Sr and Nd isotope ratio to the damkjernites,
but is more similar to the camptonites and ocelli-rich
ultramafic dyke with respect to Pb isotopes (Fig. 8).
Groundmass carbonates within the Okenyenya
damkjernite dykes have a narrow range of d13C values
(–5·7 to –5·0‰) and a virtually constant d18O signature
(~9·2‰), and lie within the field defined by Deines (1989)
for most primary carbonatites (Table 6; Fig. 9). This
groundmass carbonate can therefore be considered to
have a primary magmatic origin.
Okorusu and Ondurakorume lamprophyres
and nephelinites
Late-stage ocelli-bearing ultramafic lamprophyre and olivine nephelinite dykes also occur at Okorusu and Ondurakorume. The mineralogy of these dykes is
summarized in Table 1 and their whole-rock geochemistry is presented in Table 2. Ondurakorume sample
ON-6 was originally described as a carbonatite by Prins
(1981) but its low CO2 (4·80 wt %) content and generally
similar major and trace element composition to some of
1126
LE ROEX AND LANYON
GEOCHEMISTRY OF LAMPROPHYRES AND CARBONATITES
Fig. 5. Covariations of selected incompatible trace element ratios in the Okenyenya lamprophyre intrusions. Approximate 2r errors are shown.
Symbols as for Fig. 2.
the Okenyenya ultramafic lamprophyres and the Okorusu olivine nephelinite (Table 2) suggests that it is
more correctly classified as an olivine nephelinite, a term
adopted here. The pisolitic structures in this sample are
similar to the carbonate-bearing ocelli found in ultramafic
lamprophyres from Okenyenya and Okorusu. The Okorusu and Ondurakorume nephelinite dykes have similarly
high mg-number to the primitive ultramafic lamprophyres
at Okenyenya (Table 2), but have lower TiO2 and P2O5,
and slightly lower LOI and CaO contents. In contrast,
the ultramafic lamprophyre (OKU-94-1) from Okorusu
has a more evolved composition than the nephelinite
dyke (Table 2), and is highly enriched in incompatible
trace elements.
The Okorusu and Ondurakorume dyke samples are
strongly LREE enriched (La/Ybn = 28–84) and have
concave-up chondrite-normalized REE patterns similar
to the Okenyenya camptonites (Fig. 4). With the exception
of the unusual enrichment in Ba in the Ondurakorume
dyke, the incompatible trace element patterns of the
Okorusu and Ondurakorume nephelinites are similar
when normalized to primitive mantle abundances (Fig. 6),
with both samples showing negative Ti, P, K and Rb
anomalies.
The Okorusu lamprophyre and nephelinite dykes have
similar initial 87Sr/86Sr but lower initial eNd values than
the Okenyenya lamprophyres (Table 4; Fig. 7), whereas
the Ondurakorume nephelinite has significantly lower
initial 87Sr/86Sr but intermediate eNd (Table 4; Fig. 7). Pb
isotope values for the Okorusu ultramafic lamprophyre
dyke are similar to those obtained for the Okenyenya
damkjernites (Table 5; Fig. 8). Groundmass carbonate
within the Okorusu ultramafic lamprophyre dyke has
slightly lower d13C (–6·8‰) and d18O (7·4‰) values than
those within the Okenyenya ultramafic lamprophyres
(Table 6; Fig. 9). However, it too lies within the primary
carbonatite field of Deines (1989), and therefore also
appears to be mantle-derived with no significant crustal
influence.
Damaraland carbonatites
The Damaraland carbonatites analysed in this study
were originally studied in detail by Prins (1981). Specific
1127
JOURNAL OF PETROLOGY
VOLUME 39
NUMBER 6
JUNE 1998
Fig. 6. Primitive mantle normalized incompatible trace element abundance patterns of primitive (mg-number >60) Okenyenya and Ondurakorume
lamprophyre intrusions and the Okorusu nephelinite dyke; primitive mantle normalizing values from Sun & McDonough (1989). Shaded fields
correspond to primitive basanites from Tristan da Cunha (le Roex et al., 1990), representing the present-day composition of Tristan plume
eruptives.
sampling localities, general petrographic and mineralogical descriptions, and the classifications of the various types of carbonatites have been given by Prins (1981,
and references therein).
The two samples analysed from Osongombo are classified as Fe-rich beforsites; sample OS-4 was collected
from the main intrusive body, whereas sample OS-5 was
collected from a cross-cutting beforsite vein. Kalkfeld
carbonatite samples K-2 and K-22 were classified by
Prins (1981) as micaceous sövite and ankeritic sövite,
respectively, although sample K-22 is hydrothermally
altered. The four samples from Ondurakorume are
thought to represent the four main stages of carbonatite
intrusion, and include sövite (ON-8 and ON-10), apatiterich beforsite (ON-9) and rare earth-beforsite (ON-4).
Okorusu carbonatite samples (OKU-6 and OKU-18)
were collected by Prins (1981) from two sövite plugs.
Major and trace element analyses of the Damaraland
carbonatites are reported in Table 7. All are greatly
enriched in REE and incompatible elements and
generally display steep chondrite-normalized patterns of
LREE/HREE enrichment with (La/Er)n ratios ranging
from 34·4 to 649 (Fig. 10). The primitive mantle
normalized trace element patterns reflect the strong
incompatible element enrichment and are generally
similar (Fig. 11), with variable but marked depletions
in Ti, Zr, P, K and Rb. These depletions are similar
to, although considerably more enhanced than, those
observed in the Damaraland lamprophyres and nephelinites.
Considered as a whole, the Damaraland carbonatites
define a limited range of initial eNd values which cluster
around Bulk Earth, overlap with values for the associated
lamprophyre and nephelinite dykes at Okorusu and
1128
LE ROEX AND LANYON
GEOCHEMISTRY OF LAMPROPHYRES AND CARBONATITES
Table 4: Measured and initial whole-rock Sr and Nd isotope ratios for the Okenyenya,
Okorusu and Ondurakorume lamprophyre and nephelinite intrusions and selected Damaraland
carbonatite samples
Sample no.
Okenyenya lamprophyres
Camptonite
OKJ-11 (plug)
87
Sr/86Sr
( 87Sr/86Sr)i
(eSr)i
143
Nd/144Nd
( 143Nd/144Nd)i
(eNd)i
0·512755(7)
0·512656
+3·4
0·704540(11)
0·703890
−9·2
OKJ-55 (plug)
0·704470(13)
0·704480(13)
0·703884
0·703894
−9·2
−9·1
OKJ-24 (plug—tinguaite)
0·705533(13)
0·704315(14)
0·704041
0·703902
−7·0
−9·0
0·512730(7)
0·512643
+3·2
Diatremes
OKJ-12 (alnöite)
0·704342(13)
0·704239
−4·2
0·512698(5)
0·512606
+2·4
OKJ-60 (alnöite)
0·704363(13)
0·704277(14)
0·704260
0·704195
−3·9
−4·9
OKJ-94-10 (ouachitite)
0·704299(14)
0·704969(14)
0·704217
0·704608
−4·5
+1·0
0·512686(6)
0·512588
+2·1
0·704573(14)
0·704578(14)
0·705154(13)
0·705146(17)
0·704829(11)
0·704396(11)
0·704312(18)
0·704245(15)
0·704192
0·704197
0·704450
0·704442
0·704391
0·703944
0·703917
0·703848
−4·9
−4·8
−1·2
−1·3
−2·1
−8·4
−8·8
−9·8
0·512658(6)
0·512574
+1·8
0·512655(7)
0·512563
+1·6
0·512652(7)
0·512698(9)
0·512709(11)
0·512735(6)
0·512565
0·512611
0·512620
0·512639
+1·6
+2·6
+2·7
+3·1
OKJ-94-1 (dyke)
Ultramafic dykes
OKJ-135 (damkjernite)
89-211 (damkjernite)
OKJ-94-3 (damkjernite)
OKJ-162 (ouachitite)
OKJ-284 (ouachitite)
OKJ-94-7 (ocelli-rich)
Okorusu and Ondurakorume lamprophyre and nephelinite dykes
OKU-94-1
0·704487(14)
0·704358
−2·5
0·512518(16)
0·512453
−0·5
OKU-94-5
ON-6
0·704892(13)
0·703716(14)
0·704538
0·703547
0·0
−13·9
0·512521(7)
0·512605(6)
0·512438
0·512514
−0·8
+0·7
Damaraland carbonatites
Osongombo
OS-4
OS-5
0·704360(14)
0·705363(13)
0·704359
0·704683
−2·5
+2·1
0·512571(6)
0·512563(7)
0·512492
0·512487
+0·2
+0·1
Kalkfeld
K-2
K-22
0·703761(21)
0·703716(13)
0·703731
0·703716∗
−11·4
−11·7
0·512528(8)
0·512518(7)
0·512470
0·512439
−0·2
−0·8
Ondurakorume
ON-4
ON-8
ON-9
0·703512(13)
0·703530(13)
0·703553(14)
0·703512∗
0·703530∗
0·703553∗
−14·6
−14·3
−14·0
0·512547(7)
0·512578(16)
0·512565(8)
0·512475
0·512510
0·512497
−0·1
+0·6
+0·3
ON-10
0·703547(11)
0·703537
−14·2
Okorusu
OKU-6
OKU-18
0·704379(15)
0·704418(13)
0·704348
0·704417
−2·7
−1·7
0·512521(5)
0·512528(7)
0·512472
0·512474
−0·2
−0·1
Numbers in parentheses represent the errors (2rmean) associated with individual Sr and Nd measurements and indicate
within-run precision only. Initial ratios were calculated using an age of 124 Ma and data from Tables 2 and 7. Initial epsilon
values were calculated using ( 87Sr/86Sr)UR = 0·7047, ( 87Rb/86Sr)UR = 0·0926, ( 143Nd/144Nd)CHUR = 0·51264 and ( 147Sm/144Nd)CHUR =
0·1967.
∗Initial 87Sr/86Sr values were calculated using Rb and Sr concentrations (ppm) obtained using isotope dilution: K-22: Rb =
1·50, Sr = 41 020; ON-4: Rb = 0·46, Sr = 24 243; ON-8: Rb = 2·20, Sr = 35 570; ON-9: Rb = 0·47, Sr = 24 813.
1129
3·25
2·21
1130
3·30
2·10
OKJ-284 (ouachitite)
OKJ-94-7 (ocelli-rich)
44
41
38
K-2
ON-9
OKU-18
212
31
70
1030
52
421
55
61
70
12·3
6·88
5·40
9·03
4·60
4·71
12·0
5·55
6·40
Pb (ppm)
Pb/204Pb
18·180
18·427
18·544
21·052
18·685
18·866
19·726
18·179
18·518
19·668
18·751
19·325
18·988
206
Pb/204Pb
15·539
15·611
15·614
15·725
15·589
15·571
15·625
15·531
15·580
15·734
15·567
15·607
15·582
207
Pb/204Pb
38·652
38·697
39·402
52·819
39·135
39·139
39·338
38·048
38·722
39·559
38·392
39·057
38·746
208
18·069
17·504
17·641
19·987
18·028
18·484
18·950
17·769
17·845
19·070
18·504
18·588
18·501
( 206Pb/204Pb)i
15·534
15·566
15·570
15·673
15·557
15·552
15·587
15·511
15·547
15·705
15·555
15·571
15·558
( 207Pb/204Pb)i
38·449
38·469
38·932
45·490
37·406
38·186
38·599
37·477
37·893
38·683
37·886
38·151
38·140
( 208Pb/204Pb)i
NUMBER 6
Initial ratios were calculated using an age of 124 Ma. Although the 2r errors associated with the XRF elemental abundances used to calculate the initial ratios
(U = 0·8 ppm, Th = 1·2 ppm and Pb = 1·4 ppm) will have little effect on samples with high U, Th and Pb concentrations, such as the Okorusu ultramafic
lamprophyre and the Damaraland carbonatites, the Okenyenya lamprophyres have much lower abundances of these elements and their calculated initial Pb
values should therefore be considered as approximations only.
49
OS-4
Carbonatite
16·0
9·60
12·9
9·40
9·88
15·0
12·2
9·50
Th (ppm)
VOLUME 39
OKU-94-1
6·47
3·03
OKJ-94-3 (damkjernite)
Okorusu lamprophyre
2·50
89-211 (damkjernite)
Ultramafic dykes
2·40
OKJ-12 (alnöite)
OKJ-94-10 (ouachitite)
Diatremes
2·50
OKJ-94-1 (dyke)
U (ppm)
OKJ-11 (plug)
Camptonites
Okenyenya lamprophyre
Sample no.
Table 5: Whole-rock Pb isotope data for the Okenyenya and Okorusu lamprophyre intrusions and selected Damaraland carbonatite
samples
JOURNAL OF PETROLOGY
JUNE 1998
LE ROEX AND LANYON
GEOCHEMISTRY OF LAMPROPHYRES AND CARBONATITES
Fig. 7. Initial Sr and Nd isotope ratios of Damaraland lamprophyre and carbonatite intrusions, as calculated for 124 Ma. Osongombo (×
+) and
Kalkfeld (Μ) are carbonatites; Ondurakorume samples include carbonatites (Ε) and lamprophyre (Φ); Okorusu samples include carbonatites
(Ο) and lamprophyre–nephelinite dykes (Η); Okenyenya lamprophyre symbols as for Fig. 2. Small crosses indicate carbonate ocelli compositions,
with associated tie lines indicating host rock composition. BE124, Bulk Earth at 124 Ma. The 2r errors are less than symbol size.
Ondurakorume, but plot well below the Okenyenya
lamprophyres over a similar range of initial 87Sr/86Sr
(Table 4; Fig. 7). Whereas the Osongombo and Okorusu
carbonatites have similar initial Sr isotope signatures, the
initial 87Sr/86Sr ratios of the Kalkfeld and Ondurakorume
carbonatites are distinctly lower (Table 4). The well below
Bulk Earth initial eSr values (–14·8 to –11·4) of the
Kalkfeld and Ondurakorume carbonatites imply a timeintegrated source depletion in Rb/Sr, without associated
depletion in Nd/Sm. In this respect the Kalkfeld and
Ondurakorume carbonatites are similar to East African
carbonatites in plotting below the mantle array (Fig. 7;
Bell & Blenkinsop, 1989), and are unlike comparable
aged Brazilian carbonatites, which have higher 87Sr/86Sr
at similar 143Nd/144Nd ratios (Roden et al., 1985).
Although only one Pb isotope dataset is available for
each of the Damaraland carbonatite complexes (Table 5),
it is obvious that samples from the Kalkfeld and Ondurakorume complexes, which are the least radiogenic
in terms of initial 87Sr/86Sr, are similar in having the lowest
initial 206Pb/204Pb ratios of all the analysed Damaraland
samples (Fig. 8). Okorusu carbonatite sample OKU-18
has initial Pb isotope ratios which are similar to the
Okenyenya and Okorusu lamprophyres. Osongombo
carbonatite sample OS-4 has anomalously high initial
Pb isotope values, which are particularly evident in terms
of 206Pb/204Pb and 208Pb/204Pb. Brazilian carbonatites of
comparable age have significantly lower 206Pb/204Pb ratios
(Toyoda et al., 1994) than those found in the Damaraland
carbonatites (Fig. 8).
Whole-rock stable isotope values for the Damaraland
carbonatites are variable (Table 6; Fig. 9), with all but
two samples plotting within the field encompassed by
most primary carbonatites (Deines, 1989). Although carbonatite samples K-22 and OS-5 have d13C values consistent with a mantle origin they have significantly higher
d18O signatures. The description of Kalkfeld sample K22 as hydrothermally altered sövite (Prins, 1981) suggests
that its high d18O value (23·1‰) is probably a product
of equilibration with low-temperature fluids (e.g. Deines,
1989). The high d18O value (24·8‰) of Osongombo
sample OS-5 may result from the sample being a carbonatite dyke. According to Deines (1989), although d13C
values do not vary significantly between different intrusive
carbonatite types, carbonatite dykes and veins are less
likely to retain their original oxygen isotope signature
because they are less massive than carbonatite plutons.
Although sample OS-5 has the highest d18O and initial
87
Sr/86Sr values of any of the analysed carbonatites, the
latter is still representative of a mantle value and within
the range of initial Sr isotope ratios shown by the Damaraland lamprophyres. Thus it seems unlikely that
crustal contamination has played a significant role in the
genesis of the Osongombo carbonatites. It is noteworthy
that the Damaraland carbonatites have similar d18O
values but slightly higher d13C values than the noncontaminated Brazilian carbonatites of equivalent age,
but significantly lower d13C values than the limestonecontaminated, Brazilian Mata Peto carbonatites (Fig. 9;
Santos & Clayton, 1995).
1131
JOURNAL OF PETROLOGY
VOLUME 39
NUMBER 6
JUNE 1998
Fig. 8. Initial Pb isotope ratios of Damaraland lamprophyre and carbonatite intrusions, as calculated for 124 Ma. Field for the Tristan plume
encompasses data from Tristan da Cunha, Inaccessible Island, Gough Island and DSDP holes 527 and 528 on the Walvis Ridge (Richardson
et al., 1982; le Roex, 1985; le Roex et al., 1990; Cliff et al., 1991). l, 238U/204Pb; j, 232Th/204Pb. Data for contemporaneous Brazilian carbonatites
from Toyoda et al. (1994). NHRL, Northern Hemisphere Reference Line (Hart, 1984). The 2r errors are less than symbol size. Symbols as for
Fig. 7.
Carbonate-bearing ocelli within the
Okenyenya and Okorusu dykes
Leucocratic globular structures within all of the Okenyenya lamprophyres (Lanyon & le Roex, 1995), as well
as the Okorusu and Ondurakorume nephelinite and
ultramafic lamprophyre dykes, consist of spherical ocelli
or more irregularly shaped segregations, and their mineralogy varies according to the host rock (Table 1).
Of the three mineralogical types of globular structures
defined by Rock (1991), the feldspathic variety occurs
within the Okenyenya camptonites, whereas the ultramafic lamprophyre and nephelinite dykes at Okenyenya,
Okorusu and Ondurakorume are dominated by carbonate–analcime ocelli.
Ocellar carbonates within the Okenyenya lamprophyres have stable isotope signatures within the prescribed range of Deines (1989) for primary carbonatites
(Table 6; Fig. 9). They have d13C values (–3·9 to –3·0‰)
higher than those of the groundmass carbonates (–5·7
to –5·0‰), and more similar to the generally higher
Damaraland whole-rock carbonatite values (–5·0 to
–2·4‰); the slight shift is attributed to fractionation
during formation of the ocelli. Ocelli from Okorusu have
d13C values (–6·6 to –5·7‰) which are similar to, or even
1132
LE ROEX AND LANYON
GEOCHEMISTRY OF LAMPROPHYRES AND CARBONATITES
Sr and Nd isotope ratios than their corresponding groundmass components (Fig. 7). However, they lie well within
the field defined by the Okenyenya lamprophyres and
can be considered to derive from the same source as
their host rocks. The lower initial eNd values (–0·7 to +0·3)
of ocelli within the Okorusu dykes, as compared with
the Okenyenya lamprophyre ocelli (+2·0 to +3·1), are
consistent with the lower host-rock initial eNd values and
those obtained for the associated carbonatite, suggesting
derivation from a similar source.
Table 6: Carbon and oxygen itotope
data for the Damaraland carbonatites
and groundmass and ocellar carbonate
phases within the Okenyenya and
Okorusu lamprophyre and nephelinite
intrusions
Sample
d13C (‰)
OS-4
whole-rock
−2·44
12·02
OS-5
whole-rock
−3·61
24·77
K-2
whole-rock
−4·90
7·99
K-22
whole-rock
−2·65
23·07
ON-8
whole-rock
−4·97
11·12
ON-9
whole-rock
−4·31
6·47
OKU-6
whole-rock
−3·45
11·84
OKU-18
whole-rock
−4·77
8·11
OKJ-135
groundmass
−5·25
9·26
OKJ-94-3
groundmass
−5·02
9·22
89-211
groundmass
−5·69
9·21
OKJ-94-5
ocelli
−3·92
8·67
OKJ-94-9
ocelli
−3·04
11·49
OKU-94-1
groundmass
−6·76
7·36
OKU-94-1
ocelli
−6·64
5·03
ocelli
−5·67
8·98
Sample no.
d18O (‰)
Carbonatites
Osongombo
DISCUSSION
Petrogenesis of Damaraland lamprophyres
and nephelinites
Kalkfeld
Ondurakorume
Okorusu
Lamprophyre
Okenyenya
Okorusu
Nephelinite
Okorusu
OKU-94-5
All d values are given as per mil units relative to V-SMOW
(d18O) and V-PDB (d13C).
lower than, the Okenyenya groundmass values. The
groundmass and ocellar carbonates within Okorusu ultramafic lamprophyre sample OKU-94-1 have similarly
low d13C values (–6·8 to –6·6‰), although the ocellar
carbonates have a significantly lower d18O signature
(+5·0‰) compared with groundmass carbonate
(+7·4‰).
The initial radiogenic Sr and Nd isotope signatures of
ocelli within the lamprophyre and nephelinite intrusions
at Okenyenya and Okorusu have also been determined
(Table 8). Ocelli separates from two of the Okenyenya
ultramafic lamprophyre dykes have slightly higher initial
Lamprophyres are highly alkaline, H2O- and/or CO2rich magmas, generated by very small degrees of partial
melting of a hydrous mantle (e.g. Rock, 1991). The
mantle-like low 87Sr/86Sr and high 143Nd/144Nd ratios
obtained for all the Damaraland lamprophyre intrusions,
coupled with the presence of hydrous mafic and ultramafic mantle xenoliths (amphibole-bearing lherzolite,
wehrlite and clinopyroxenite) within the Okenyenya alnöite diatreme, are consistent with derivation from hydrous mantle. Together with their primitive nature (mgnumber >60), these features make the Okenyenya lamprophyres ideal for placing constraints on the petrogenesis
of lamprophyric magmas in general. Although reference
will be made to the Okorusu and Ondurakorume lamprophyres and nephelinites, the following discussion will
emphasize the Okenyenya lamprophyres in view of their
greater abundance, wider variety, and close spatial proximity.
Of the lamprophyre types recognized at Okenyenya,
the camptonites and ouachitites have representatives close
in composition to primary magmas (i.e. mg-number >67).
The most primitive damkjernites and alnöites have mgnumber ~65 and, although they may have experienced
as little as 10% olivine fractionation, their compositions
closely reflect those of primary magmas.
The most noticeable features of the Okenyenya lamprophyres are the distinct and restricted range in incompatible trace element ratios of each of the different
varieties (Fig. 5), their unique primitive mantle normalized trace element abundance patterns (Fig. 6), and
the associated distinct radiogenic isotope signatures
(Fig. 7). These geochemical features are inconsistent with
derivation of the different Okenyenya lamprophyres from
a homogeneous mantle source. The damkjernites must
have derived from a mantle source with lower 206Pb/
204
Pb and 143Nd/144Nd and higher 87Sr/86Sr ratios than
the other ultramafic lamprophyre dyke varieties. The
high 87Sr/86Sr, 206Pb/204Pb and particularly 207Pb/204Pb
ratios of the ouachitite diatreme sample OKJ-94-10 are
1133
JOURNAL OF PETROLOGY
VOLUME 39
NUMBER 6
JUNE 1998
Fig. 9. Plot of d13C vs d18O compositions of Damaraland lamprophyre and carbonatite intrusions. Data for Brazilian carbonatites from Santos
& Clayton (1995); field for primary carbonatites from Deines (1989). The 2r errors are less than symbol size.
attributed to shallow-level crustal contamination. The
alnöite diatreme has similar 143Nd/144Nd and Pb isotope
ratios to the camptonites, ouachitites and ocelli-rich ultramafic lamprophyre dyke, and its slightly elevated 87Sr/
86
Sr ratio could also reflect minor crustal contamination
or late-stage alteration.
The only two lamprophyre varieties with virtually
identical initial isotope ratios, and thus potentially derived
from the same source, are the camptonites and the ocellirich ultramafic dyke. This would require the former to
be derived by higher degrees of melting to account for
its lower absolute abundances of Zr, Ce and P (at
equivalent mg-number; Table 2) and lower (La/Yb)n ratio
(Fig. 5; Table 3). The lower K/Nb (Fig. 5), K/Th and
K/U and slightly lower Rb/Nb ratios (Table 3) of the
ocelli-rich ultramafic dyke, as compared with the
camptonites, are consistent with derivation of the former
by lower degrees of partial melting in the presence
of residual phlogopite or amphibole. Furthermore, the
considerably higher volatile content (H2O and CO2) of
the ocelli-rich dyke compared with the camptonite would
also be consistent with derivation by lower degrees of
melting of a hydrous/carbonated mantle. The absence
of a negative K-anomaly in the camptonites (Fig. 5)
requires derivation from an amphibole free source, or
by a degree of melting that fully consumes any K-bearing
phase (e.g. Späth et al., 1996). The slightly higher Zr/
Nb and La/Nb of the ocelli-rich ultramafic lamprophyre
would require minor residual ilmenite, which is consistent
with the negative Ti anomaly evident in Fig. 6, but not
obviously consistent with the lower Ti/Zr ratio of the
camptonite.
Although there is ample evidence for the individual
lamprophyre varieties found at Okenyenya to have derived from a locally heterogeneous mantle source (where
scale of heterogeneity is comparable with melting volume
for a particular magma), there are many similarities in
their overall geochemistry, which allow some general
comment to be made on their petrogenesis. Of particular
note is their uniformly strong chondrite-normalized
LREE enrichment (La/Ybn = 20–58) and substantial
enrichment in incompatible elements, as well as H2O
and CO2 (Tables 2 and 3). In terms of their REE
abundances, La ranges from 200 to 600 times chondrite,
whereas Yb abundances are fairly constant at ~10 times
chondrite (Fig. 4). Quantitative modelling of REE abundance variations [using partition coefficients from McKenzie & O’Nions (1991) and modal mineralogy from
Späth et al. (1996)] suggest that if derived by 1–2% melting
of amphibole-bearing lherzolite, the mantle source of
the ultramafic lamprophyres would need to be enriched
by a factor of 7–10 over chondritic values for the LREE
and by a factor of 1·4–1·5 for the HREE, if melting
occurred in the garnet stability field. Choice of lower F
(e.g. 0·1–0·2%) would scale these values accordingly. If
melting occurred as a one-stage process in the spinel
stability field then a similar enrichment of the LREE is
required, but HREE concentrations would be lower than
chondritic by a factor of 2·5–3·3, which seems unrealistic.
Our preferred model is therefore one in which the
1134
1135
74
1879
1
868
3629
165
961
2·55
4771
7284
519
1566
184
45·3
98·0
12·6
58·0
15·3
440
18
1200
145
975
24686
242
223
5·94
92·24
15·85
0·99
4·83
1·54
9·62
1·91
5·02
23·16
1·23
3·24
1·04
22·13
0·34
1·34
tr
K-2
Micaceous
Sövite
Kalkfeld
3771
5858
468
1074
97·2
16·3
34·8
3·16
14·9
4·06
706
42685
800
83
23
36940
5
110
7232
9835
826
2424
391
95·1
161
13·2
45·3
8·49
2804
6200
8·10
9·57
0·03
0·24
16·74
16·24
5·41
5·73
10·57
1·01
0·01
1·87
23·18
0·74
1·10
0·62
0·26
93·32
ON-4
Rare earth
Beforsite
3763
1200
8·44
0·44
27·31
1·86
2·37
0·11
0·05
89·94
4·42
4·84
24·54
1·79
0·18
1·39
20·64
K-22
Ankeritic
Sövite
Ondurakorume
3944
6049
597
1788
260
60·9
132
14·3
67·9
16·2
1005
33874
785
276
3801
7·56
90·80
2·94
0·04
0·59
4·67
4·75
3·63
8·09
28·20
0·00
0·03
4·78
30·76
0·71
1·61
ON-8
White
Sövite
1760
3894
487
1533
211
51·7
112
14·2
70·2
19·5
8·28
768
23967
573
285
2089
4·08
95·43
1·72
0·02
0·23
1·95
5·53
2·16
4·96
35·81
0·00
0·00
7·37
34·68
0·57
0·43
ON-9
Apatite
Beforsite
All major and trace element data from Prins (1981); REE data obtained by HPIC in this study on the same rock powders.
6·51
1·31
765
1054
82·2
277
42·7
9·96
20·8
2
12317
114
225
855
27
19
2·04
97·03
7·15
0·06
15·85
1·52
1·32
7·08
27·04
0·73
1·52
95·52
1·55
0·77
18·11
32·18
0·54
8·34
9·64
OS-5
Beforsite
7·28
0·22
0·21
6·94
0·85
1·28
1·70
40·67
OS-4
Beforsite
Rare earth elements (ppm)
La
2793
Ce
4015
Pr
372
Nd
1141
Sm
183
Eu
55·3
Gd
156
Tb
25·4
Dy
159
Er
56·7
Yb
31·2
Trace elements (ppm)
F
Cl
Ba
Rb
Nb
Sr
Zr
Y
Traces (wt %)
Major elements (wt %)
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K 2O
P2O5
CO2
H2O–
H2O
F
O(F,Cl)
Total
Sample no.:
Rock type:
Osongombo
1225
2296
291
1007
161
46·2
98·9
12·0
58·7
17·4
7·92
2235
33
1440
16738
479
275
3·83
96·03
4·68
0·03
0·00
1·30
5·16
1·21
2·06
42·29
0·00
0·01
4·36
33·01
1·35
0·57
ON-10
White
Sövite
1013
1739
152
368
36·6
8·61
13·6
1·64
8·73
4·70
4·75
94
2514
60
1961
9866
50
64
2·77
97·61
16·40
1·49
3·87
14·43
2·53
0·44
2·62
26·41
0·38
2·12
2·66
23·10
0·45
0·71
OKU-6
Sövite
Okorusu
1488
2857
202
614
67·8
21·2
53·0
7·39
38·3
13·4
5·57
8274
2
355
16299
55
194
4·24
94·12
0·04
1·34
36·36
0·49
1·21
1·22
0·01
0·49
3·73
0·44
0·58
0·68
47·53
OKU-18
Sövite
Table 7: Whole-rock major, trace element and REE chemistry of carbonatites associated with the Damaraland intrusive complexes
LE ROEX AND LANYON
GEOCHEMISTRY OF LAMPROPHYRES AND CARBONATITES
JOURNAL OF PETROLOGY
VOLUME 39
NUMBER 6
JUNE 1998
Fig. 10. Chondrite-normalized rare earth element variations of Damaraland carbonatites. Chondrite normalizing values from Sun & McDonough
(1989).
Fig. 11. Primitive mantle normalized incompatible trace element abundance patterns of Damaraland carbonatites; primitive mantle normalizing
values from Sun & McDonough (1989).
magmas are either derived by direct melting within the
garnet stability field, or are derived by melting within
the spinel stability field, but inherit their residual garnet
REE pattern by predominantly melting vein material
introduced into the spinel lherzolite as a consequence of
metasomatism by low volume alkali melts or fluids derived
1136
1137
ocelli
groundmass
ocelli
OKU-94-1
OKU-94-5
OKU-94-5
3·87
72·0
5·27
97·2
12·7
1·44
101
56·9
117
Rb (ppm)
5980
1013
134858
4540
6451
2752
1080
1340
1577
Sr (ppm)
Sr/86Sr
0·704597(13)
0·704889(13)
0·704368(10)
0·704528(14)
0·703989(14)
0·703984(14)
0·704338(11)
0·704722(14)
0·704780(14)
87
0·704594
0·704527
0·704368
0·704419
0·703979
0·703981
0·703861
0·704505
0·704402
( 87Sr/86Sr)i
7·68
1·00
−0·8
51·8
−0·2
24·4
−2·4
8·31
−1·7
−7·9
−7·9
9·73
11·4
−9·6
10·6
−0·5
Sm (ppm)
−1·9
(eSr)i
8·63
48·1
380
160
29·6
60·0
69·5
66·4
Nd (ppm)
Nd/144Nd
0·512555(18)
0·512516(6)
0·512513(6)
0·512540(6)
0·512775(6)
0·512694(8)
0·512663(6)
0·512651(7)
143
0·512498
0·512437
0·512446
0·512465
0·512637
0·512614
0·512582
0·512572
( 143Nd/144Nd)i
+0·3
−0·8
−0·7
−0·3
+3·1
+2·6
+2·0
+1·8
(eNd)i
Numbers in parentheses represent the errors (2rmean) associated with individual Sr and Nd measurements and indicate within-run precision only. Initial ratios
were calculated using an age of 124 Ma. Initial epsilon values were calculated using ( 87Sr/86Sr)UR = 0·7047, ( 87Rb/86Sr)UR = 0·0926, ( 143Nd/144Nd)CHUR = 0·51264 and
( 147Sm/144Nd)CHUR = 0·1967. All elemental abundances were measured by isotope dilution.
groundmass
OKU-94-1
Okorusu
ocelli
OKJ-94-9
groundmass
OKJ-94-9
ocelli
ocelli
OKJ-94-9
groundmass
OKJ-94-5
Sample
OKJ-94-5
Okenyenya
Sample no.
Table 8: Sr and Nd isotope data for the groundmass and ocellar components of selected Okenyenya and Okorusu lamprophyre and nephelinite
intrusions
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from depths within the garnet stability field (see below
for further discussion).
The alkaline lamprophyres and ultramafic damkjernites are the only lamprophyre varieties that do not show
evidence for a significant negative K anomaly (Fig. 6),
indicating the absence of a residual K-bearing phase
such as phlogopite or amphibole in their source region.
The damkjernites, however, have marked negative Zr,
Ti and to a lesser extent P anomalies, indicating either
relative depletion of these elements in the source, or
fractionation against residual accessory phases that host
these elements (e.g. zircon, ilmenite, and apatite). The
higher volatile and CO2 content of this ultramafic lamprophyre variety (Table 2) suggests, furthermore, that
their source was more volatile rich, with a higher CO2/
H2O ratio. In contrast, the remaining ultramafic lamprophyre varieties (including those at Ondurakorume
and Okorusu) have higher H2O/CO2 ratios and all show
strong negative K anomalies (Fig. 6; Table 3), suggesting
a residual K-bearing phase in their source regions. The
broadly constant Ba/Nb and Rb/Nb ratios with varying
Nb content of these magmas suggest that amphibole
rather than phlogopite is the most likely residual Kbearing phase, as these ratios are strongly fractionated
during low degrees of melting in the presence of residual
phlogopite (Späth & le Roex, in preparation). The different varieties also variably show negative Ti, Zr and/or
P anomalies, which, like the damkjernites, can be taken
to indicate either relative depletion in their source regions
(as a consequence of a metasomatic overprint; e.g. Hauri
et al., 1993), or the presence of residual accessory phases
hosting these elements.
In summary, qualitative considerations combined with
semi-quantitative modelling suggest that the Damaraland
lamprophyres were formed as a consequence of melting
of a metasomatically enriched, hydrous or carbonated
mantle with heterogeneously distributed accessory phases
such as amphibole, ilmenite, zircon and apatite. The
alkaline lamprophyres appear to have derived from a
less volatile and incompatible element enriched mantle
source by greater degrees of melting than the ultramafic
lamprophyres. The different ultramafic lamprophyre
varieties owe their origin to minor differences in the
degree of source enrichment (including volatile composition), degree of partial melting and residual accessory
phases. The damkjernites seem to have derived from a
more volatile and CO2-rich source than the other ultramafic lamprophyre varieties.
Petrogenesis of Damaraland carbonatites
The most remarkable features of carbonatite magmas
the world over are their low SiO2 contents, exceptional
enrichment in highly incompatible elements, and their
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very steep chondrite-normalized REE patterns (Woolley
& Kempe, 1989). The Damaraland carbonatites are no
exception. They are characterized by enrichment of
highly incompatible elements to ~10 000 times primitive
mantle values (e.g. Fig. 11), LREE abundances
4000–40 000 times chondrite values, and La/Ybn ratios
of >400 (Fig. 10). Comparison of primitive mantle normalized incompatible trace element patterns for the Damaraland carbonatites and the associated ultramafic
lamprophyres and nephelinites shows that the carbonate
and silicate magmas have remarkably similar overall
patterns. However, the carbonatite patterns are considerably more accentuated, displaying greater absolute
enrichment in the highly incompatible elements and
greater negative K, (Rb), P, Zr and Ti anomalies (Figs
6 and 11). Interpretation of trace element abundance
variations in carbonatites is subject to considerable uncertainty; the degree of fractional crystallization is hard
to evaluate, carbonatites are notoriously rich in exotic
accessory phases, many of which host the otherwise highly
incompatible elements, partition coefficients are not as
well constrained as for silicate magmas, and, perhaps most
importantly, recognition of what constitutes a ‘primary’
magma is difficult.
Considerable debate exists around the origin of carbonatite magmas, and the alternative petrogenetic models
have been highlighted in the book edited by Bell (1989),
and more recently reviewed by Lee & Wyllie (1994,
1997) from an experimental perspective. The debates are
concerned with: (1) whether the composition of primary
carbonatite magma is calcic, dolomitic or sodic, and (2)
whether carbonatites are derived from parental silicate
magmas by extensive fractional crystallization or immiscibility. An underlying cause of much of this debate
is the difficulty in distinguishing unequivocally between
a ‘liquid’ and a cumulate-enriched magma, and the
extent of volatile and alkali loss (to fenitizing fluids) from
the original magma composition. In this regard, the
Damaraland carbonatites are no exception.
Phase relationships indicate that carbonatites formed
either through extensive fractional crystallization or as
an immiscible phase from a carbonated alkaline silicate
magma should be rich in alkalis (e.g. Lee & Wyllie, 1994).
If the Damaraland carbonatites formed at crustal or
mantle depths as immiscible carbonate magmas from an
alkaline silicate magma similar to the spatially associated
ultramafic lamprophyres (which show evidence for the
localized development of an immiscible carbonate phase
in the form of ocelli) then, from a consideration of phase
relationships (e.g. fig. 7 of Lee & Wyllie, 1994), alkalis
should form at least 40% of the cation component of the
carbonatite magma. The Damaraland carbonatites are
notably bereft of alkalis, and either did not form as
a consequence of immiscibility, or such a significant
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GEOCHEMISTRY OF LAMPROPHYRES AND CARBONATITES
proportion of the original alkalis has been lost that a
rigorous evaluation of their petrogenesis is negated.
Primary carbonatite magma derived directly by partial
melting of carbonated mantle peridotite will be dolomitic,
with a maximum calcite content of 50–80% (Lee &
Wyllie, 1994, 1997), and low ca-number [Ca/
(Ca + Mg) = 0·51–0·53; Sweeney, 1994]. As the Damaraland carbonatites have high ca-number (0·71–0·94)
and are primarily sövites or differentiated Fe-rich beforsites (Prins, 1981) they are unlikely to represent primary
magmas. Furthermore, following the arguments of Lee
& Wyllie (1994, 1997) that all sövitic carbonatites are
cumulates, they are also unlikely to represent liquid
compositions.
In summary, although there are many uncertainties
regarding the petrogenesis of the Damaraland carbonatites, it is unlikely that they represent primary
magmas, based on their low MgO contents, nor are they
likely to represent liquid compositions formed through a
process of immiscibility as they are too calcic and too
poor in alkalis. Based on the limited available evidence,
and without a more in-depth study of individual complexes to establish a possible fractionation history, the
most likely explanation is that the Damaraland carbonatites represent calcite-rich cumulates. There is therefore little value in speculating on detailed compositional
relationships amongst and between the Damaraland carbonatites and ultramafic lamprophyres. However, the
similar Sr and Nd radiogenic isotope compositions of the
carbonatites, their associated ultramafic lamprophyres
and the carbonate ocelli found within the latter (e.g.
Fig. 7) provide compelling evidence that all are intimately
related and derive by petrogenetic processes as yet undefined from a common trace element enriched mantle
source.
Source of Damaraland lamprophyre and
carbonatite magmatism
On the basis of spatial, temporal and geochemical relationships, the magmatism that gave rise to the Cretaceous Damaraland alkaline complexes is believed to be
related to the Etendeka volcanism and to mantle melting
associated with the upwelling Tristan plume when it was
located beneath this region ~130–123 my ago (Milner
et al., 1995; Milner & le Roex, 1996). As field and
geochronological evidence (Prins, 1981; Milner et al.,
1993; Watkins & le Roex, 1994; Watkins et al., 1994)
indicates that the lamprophyre intrusions represent the
final stage of magmatic activity associated with the Damaraland alkaline complexes, they are therefore also
likely to be related to the influence of the Tristan plume
(Milner & le Roex, 1996).
The present-day Tristan plume composition is characterized by elevated 87Sr/86Sr (greater than Bulk Earth),
low 143Nd/144Nd (eNd <0) and Pb isotope compositions
which plot above the NHRL (Figs 8 and 12). Milner &
le Roex (1996) have shown that the Okenyenya alkaline
gabbros and Etendeka Tafelkop basalts have similar
isotope and trace element compositions to those of the
present-day Tristan plume, leading them to the conclusion that the plume composition has remained broadly
constant for the past 130 my. It is evident from Fig. 12
that the Okorusu and Osongombo lamprophyres and
carbonatite magmas also have Sr and Nd isotope compositions similar to the calculated Tristan plume composition at 124 Ma (based on the age of the Okenyenya
lamprophyres and assuming a source Rb/Sr of 0·05 and
Sm/Nd of 0·20; values calculated for the source of
the Tristan da Cunha basanites), and are thus likewise
consistent with derivation by direct melting of Tristan
plume material. In contrast, the Okenyenya lamprophyres and the Kalkfeld and Ondurakorume carbonatites and lamprophyre dyke have isotope ratios
displaced towards less radiogenic Sr and more radiogenic
Nd compositions than the Tristan plume (Fig. 12;
Table 4), suggesting involvement of depleted mid-ocean
ridge basalt (MORB) mantle (DMM) or a HIMU (high
238
U/204Pb) component in their genesis. It is noteworthy,
in this regard, that a small subset of Inaccessible Island
lavas, similarly displaced towards low 87Sr/86Sr values
(Fig. 12), have also been interpreted as having a depleted
asthenosphere or HIMU mantle component involved in
their genesis (Cliff et al., 1991), although this is not as
evident with respect to Pb isotopes. Also significant is the
fact that, unlike the Okenyenya tholeiitic gabbros, the
bulk of the Etendeka lavas (Milner & le Roex, 1996) and
the Cretaceous potassic and carbonatitic rocks of southern
Brazil (Toyoda et al., 1994; Carlson et al., 1996), the
Sr–Nd isotope data obtained during this study provide
no compelling evidence for a significant contribution
from ancient enriched continental lithospheric mantle (i.e.
eSr q0) in the genesis of the Damaraland lamprophyres or
carbonatites.
Variations in Pb isotopes are less systematic. The
Okorusu lamprophyre and carbonatite magmas and the
Okenyenya lamprophyres all have Pb isotope compositions broadly similar to the calculated composition
of the Tristan plume at 124 Ma (assuming a plume l =
15 and j = 4; Fig. 8). Their Pb isotope compositions are
therefore consistent with derivation from the upwelling
plume, although all have slightly lower 208Pb/204Pb ratios.
As a group the Okenyenya lamprophyres define an
isotopic trend across the field of the Tristan plume which
extends towards HIMU (Fig. 8); a feature also seen in a
plot of 87Sr/86Sr vs 206Pb/204Pb (Fig. 13). The single
analysis for Osongombo is similarly displaced to high
207
Pb and 206Pb. However, its extremely high 208Pb/204Pb
ratio is anomalous (Fig. 8), in part perhaps reflecting
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NUMBER 6
JUNE 1998
Fig. 12. Initial Sr and Nd isotope ratios of Damaraland lamprophyre and carbonatite intrusions compared with the inferred composition of
the Tristan plume at 124 Ma. Hatched extension to Tristan field is for a subset of Inaccessible Island lavas (data sources as for Fig. 8).
problems inherent in age corrections given the exceptionally high Th abundance (relative to Pb) in this
sample (Table 5). Alternatively, the magma must have
derived from a source with unusually high Th/Pb (and
U/Pb) ratio. In contrast, the Ondurakorume and Kalkfeld
carbonatites are both displaced to low 206Pb relative to
the Tristan plume. These carbonatites also have Sr–Nd
isotope compositions that plot well below the mantle
array (Fig. 12) and, like the East African carbonatites
(Bell & Blenkinsop, 1989), correspond more closely to
the LoNd array of Hart et al. (1986) extending between
EM-I and HIMU in Sr–Nd space. In this regard, Tilton
& Bell (1994) and Bell & Simonetti (1996) have pointed
out that most young carbonatites appear to be mixtures
between HIMU and EM-I components. However, the
very low 206Pb/204Pb ratios of the Ondurakorume and
Kalkfeld carbonatites negate any involvement of HIMU
source material in their formation, and their low 87Sr/86Sr
negates significant involvement of an EM-I component
(Fig. 13).
The extreme concentrations of incompatible elements
(including the LREE) in the Damaraland lamprophyres
and carbonatites require a source that is highly enriched
in incompatible trace elements, despite evidence for a
contribution from a source component with low Sr and
high Nd isotope ratios relative to Bulk Earth. Numerous
recent studies have emphasized the importance of metasomatism as a precursor to highly alkaline magmatism.
Experimental and petrological evidence suggests that
such metasomatic fluids could be silicate or carbonatitic
in composition (e.g. Wallace & Green, 1988; Yaxley et
al., 1991; Hauri et al., 1993; Lee et al., 1996), and
would carry the necessary inventory of incompatible trace
elements. Invasion of peridotitic mantle by low-viscosity
carbonatite or hydrous alkaline melts leads to cryptic or
modal metasomatism in which a range of accessory
phases such as amphibole, phlogopite, apatite, zircon,
ilmenite, titanates, monazite and whitlockite may host
many otherwise highly incompatible elements (Haggerty,
1989; Meen et al., 1989; Yaxley et al., 1991; Rudnick et
al., 1993). These accessory phases not only result in an
absolute increase in certain incompatible trace element
abundances, but can also lead to fractionation of certain
incompatible trace element ratios. Most notably, partition
coefficients for elements such as Ti, Zr, P and K (e.g.
Sweeney et al., 1995) indicate that incipient carbonatite
melts will be depleted in these elements relative to a
silicate melt, and will impose this characteristic on metasomatized peridotite. Characteristic features of carbonatite metasomatism are believed to include a decrease
in the Ti/Eu ratio of the host peridotite with increasing
La/Yb ratio (Rudnick et al., 1993) and a high Ca/Al
ratio as a consequence of increased modal clinopyroxene
(Yaxley et al., 1991). The high (>1·1) Ca/Al ratio of
Damaraland ultramafic lamprophyres, and their low and
decreasing Ti/Eu ratio with increasing (La/Er)n ratio
(Fig. 14; Er is used here in place of Yb as many of the
carbonatites have Yb values below detection), coupled
with the negative Ti, Zr, P and K anomalies superimposed
on their otherwise smooth primitive mantle normalized
1140
LE ROEX AND LANYON
GEOCHEMISTRY OF LAMPROPHYRES AND CARBONATITES
Fig. 13. Covariation of 87Sr/86Sr, 143Nd/144Nd and 206Pb/204Pb isotope ratios in Damaraland lamprophyres and carbonatites. Shaded field marks
composition of Tristan plume at 124 Ma.
patterns (Fig. 6), is consistent with derivation from a
mantle source that has experienced metasomatism by an
incipient carbonatitic melt. Of importance is that the
strong metasomatic enrichment is evident in the trace
elements, but not the isotopes, indicating that this
metasomatism is not an ancient feature but must have
occurred shortly before the magmas were emplaced at
~125 Ma.
Lee et al. (1996) have shown that major and trace
element and radiogenic isotope compositions of mantle-
derived spinel lherzolite xenoliths from the Cameroon
line provide evidence for metasomatic enrichment of
previously depleted sub-continental lithospheric mantle.
Their isotope compositions indicate that portions of the
sub-continental lithosphere beneath this region of West
Africa are comparable with sub-oceanic lithosphere, i.e.
are isotopically depleted. Petrographic evidence for
metasomatism of sub-continental lithosphere beneath
northwestern Namibia is similarly found in xenoliths
from the Okenyenya complex in the form of abundant
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JUNE 1998
Fig. 14. Variation in (La/Er)n with respect to Ti/Eu ratio in Damaraland lamprophyre and carbonatite intrusions. Schematic trend representing
the influence of carbonatite metasomatism on source peridotite from Rudnick et al. (1993).
amphibole present in peridotitic xenoliths (Baumgartner,
1994). Calculated temperatures and pressures of equilibration of these xenoliths (~950–1050°C; 18–20 kbar)
indicate that this metasomatism is likely to have occurred
within the sub-continental lithospheric plate, at a depth
immediately above the H2O–CO2–peridotite solidus
ledge (i.e. ~70 km; Wyllie, 1989).
Although no isotope data are available for the Okenyenya xenoliths, by analogy to the Cameroon line
xenoliths, it is proposed that at the time of continental
break-up, similar material existed beneath northwestern
Namibia. The evidence for a HIMU component in some
samples is limited, but if indeed present it could reflect a
dispersed component within the depleted sub-continental
lithosphere, formed by a previous metasomatic event
unrelated to the Tristan plume [e.g. Hart et al., 1986;
also, Burke (1996) has noted the near ubiquitous presence
of a HIMU signature in most African alkaline magmatism], or it is present as a minor heterogeneity within
the upwelling Tristan plume itself, as perhaps suggested
by the Inaccessible Island data (Cliff et al., 1991).
Following Meen et al. (1989), the isotopic variations
in the Damaraland lamprophyres and carbonatites are
attributed to melting of metasomatized (depleted)
lithosphere produced by invasion of carbonated alkalic
melt, or incipient carbonatite (Wallace & Green, 1988),
derived from the Tristan plume immediately before the
opening of the South Atlantic at ~130 Ma. Water- and
CO2-rich magmas from depths in excess of ~70 km (i.e.
within the garnet stability field) reacted with depleted
peridotite as they crossed the solidus ledge to produce a
variety of mineral assemblages at temperatures <1100°C
and pressures of between 22 and 17 kbar (Wyllie, 1989).
The metasomatized mantle produced was a carbonated
amphibole-bearing lherzolite which acted as a potential
lamprophyre (and carbonatite?) source rock. The products of this high-temperature metasomatism are envisaged
as veins enriched in clinopyroxene (wehrlites are common
in the xenolith assemblages at Okenyenya), and this
metasomatized mantle had Rb/Sr, U/Pb and Th/Pb
ratios controlled by partitioning between mantle phases
such as clinopyroxene, accessory phases such as apatite,
calcite and whitlockite, and melt or fluid (Meen et al.,
1989). Later melting of this metasomatized mantle (with
dominant contribution from the metasomatic vein material, and variable contributions from more refractory,
depleted, host sub-continental lithosphere) yielded CO2rich ultramafic lamprophyres, carbonated nephelinites
and perhaps primary carbonatite magmas (e.g. Sweeney,
1994) with strong enrichment in incompatible elements,
but with relatively low 143Nd/144Nd, moderate 87Sr/86Sr
and variable Pb isotope ratios.
The strong plume signature in these late-stage alkaline
melts contrasts with a similar situation in Hawaii, where
post-erosional, highly alkaline lavas show evidence for
dominant melting of lithospheric mantle (e.g. Chen &
Frey, 1983). The reasons for this difference are not clear,
but our preferred model for magma generation and
plume–lithosphere interaction beneath northwestern
Namibia is illustrated in Fig. 15, and can be summarized
as follows:
1142
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GEOCHEMISTRY OF LAMPROPHYRES AND CARBONATITES
Fig. 15. Schematic diagram (not to scale) showing evolution of plume–lithosphere interaction beneath northwestern Namibia. (1) Eruption or
emplacement of early (mildly alkaline) magmas uncontaminated by cold sub-continental lithosphere (o Etendeka Tafelkop basalts and Okenyenya
alkaline gabbros; Milner & le Roex, 1996). (2) Conductive heating of lithosphere and metasomatism by the Tristan plume. Large volume melts
contaminated by, or generated within, sub-continental lithosphere as a result of raised geothermal gradient (o Etendeka LTZ- and HTZ-basalts
and Okenyenya tholeiitic gabbros; Milner & le Roex, 1996). (3) Because of waning heat flow, melting is largely restricted to metasomatic veins
(originally derived from the plume), giving rise to lamprophyre and carbonatite magmas. (See text for further discussion.)
Stage 1. The Tristan plume head arrives and spreads out
beneath the sub-continental lithosphere. Decompression
melting within the plume gives rise to magmas that rise
through the cold continental lithosphere, and if suitable
pathways exist they erupt as basaltic magmas uncontaminated by sub-continental lithospheric mantle; i.e.
carry the plume signature (equivalent to the mildly
alkaline Etendeka Tafelkop basalts and Okenyenya
alkaline gabbros; Milner & le Roex, 1996).
Stage 2. Conductive heating of the sub-continental lithosphere by the plume, coupled with rise of melts into the
sub-continental lithosphere, leads to a raised geothermal
gradient within the lithosphere. Volatiles or low-volume
carbonate-rich alkaline melts escape from rising plume
and infiltrate or metasomatize the base of the lithosphere.
Magmas derived from the plume are contaminated en
route to the surface by sub-continental lithospheric mantle
and erupted as flood basalts; contamination is aided by
the raised geothermal gradient and development of lowvolume melts derived from the sub-continental lithospheric mantle which mix with plume magmas, and/or
magmas are derived by direct melting of (hydrous
or metasomatized) sub-continental lithosphere (e.g.
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Gallagher & Hawkesworth, 1992; equivalent to the majority of Etendeka LTZ- and HTZ-basalt types).
Stage 3. With continued drift of the African plate to the
northeast heat flow wanes and leads to a falling geothermal gradient. Melting of the sub-continental lithosphere is restricted to easily fusible components
(metasomatic veins—originally derived from the plume),
with perhaps a limited contribution from the host peridotite, leading to the formation of lamprophyric and
carbonatitic magmas that carry the Tristan plume signature.
ACKNOWLEDGEMENTS
Logistic support provided by the Geological Survey of
Namibia and financial support provided by the Foundation for Research Development and the University of
Cape Town are gratefully acknowledged. Petrie Prins
is thanked for allowing us access to his Damaraland
carbonatite collection housed in the Geology Department
at the University of Stellenbosch. Steve Richardson,
Andreas Späth, and particularly Richard Armstrong,
provided invaluable help with radiogenic isotope analyses;
Chris Harris and Kevin Faure provided equivalent assistance with stable isotope analyses; Simon and Debbie
Milner are thanked for their hospitality in Windhoek.
Informal reviews by Chris Harris and Phil Janney, valuable insights from Conny Class, and formal reviews by
Fred Frey, Godfrey Fitton and Keith Bell greatly improved earlier drafts of the manuscript, and for their
conscientious efforts we are greatly indebted.
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