ABSTRACT. Karroo basaltic lavas of southern Malawi are divided into two groups on the basis of stratigraphy, petrography, and the data from 20 newly analysed ...
M I N E R A L O G I C A L MAGAZINE, SEPTEMBER 1983, VOL. 47, PP. 281-9
Karroo basalts of southern Malawi and their regional petrogenetic significance R. MACDONALD
Department of Environmental Sciences, University of Lancaster, Lancaster LA1 4YQ R. CROSSLEY
Department of Earth Sciences, Chancellor College, PO Box 280, Zomba, Malawi AND K. S. WATERHOUSE
Department of Environmental Sciences,University of Lancaster, Lancaster LA1 4YQ
ABSTRACT. Karroo basaltic lavas of southern Malawi are divided into two groups on the basis of stratigraphy, petrography, and the data from 20 newly analysed rocks. A lower group of porphyritic lavas is notably rich in plagioclase phenocrysts and has contents of K, Ti, P, Ba, and Zr unusually high for tholeiitic basalts. They are overlain by an upper group of aphyric or sparsely microphyric lavas whose chemistry is more normal for tholeiites and is comparable with that of the Karroo dolerite dykes in Malawi. Though they outcrop in the northern part of the Karroo province, the lavas and dykes have chemical characteristics more similar to southern province rocks, adding some complexityto the concept of geochemical provinces established by earlier workers. Inter-suite variations in incompatible trace element contents and ratios indicate that the basalts of each group were derived from heterogeneous mantle sources. THE concept of geochemical provinces within the volcanic rocks of the Karroo system was first clearly established by Cox et al. (1967). They distinguished a southern province comprising the basalts of Lesotho (see Cox et al. (1967) and Woolley et al. (1979) for Karroo locality maps), Swaziland and the dolerites of S. Africa, and having a typical tholeiitic chemistry, from a northern province comprising mainly the lavas of Rhodesia and containing in the mafic members, values of K, Ti, P, Ba, Sr, and Zr abnormally high for tholeiitic rocks. A more complicated pattern of chemical variation was described by Cox (1972). In the Nuanetsi area of Rhodesia, part of the northern province of Cox et al. (1967), rocks from the uppermost part of the sequence, in particular the Inter-bedded Basalts, were recognized as having southern province character, thus negating any simple relationship ~) Copyright the Mineralogical Society
between magma chemistry and geographical or tectonic situation. This was confirmed by studies of Woolley et al. (1979) on the dolerite dykes of southern Malawi in the northern part of the Karroo outcrop. They showed that these rocks apparently have southern province chemistry. Woolley et al. (1979) suggested that regional geochemical variations reflect heterogeneity in the mantle sources. Erlank et al. (1980) also invoked mantle heterogeneity, in the horizontal and perhaps vertical senses, as the explanation for compositional variability in the Karroo basalts. On trace element and isotopic evidence they proposed that such heterogeneity resulted from complex pre-Karroo metasomatic events which enriched some areas of mantle in incompatible dements and depleted others. Continuing studies (Erlank et al., 1980) are attempting to document fully the range of chemical variation in the Karroo rocks as functions of geographical location and stratigraphical position within sequences. Such studies will eventually place some constraints on the mechanisms generating northern and southern province chemistries. As a contribution, this paper presents chemical and petrographic data for a hitherto poorly known northern part of the Karroo sequences, the lavas of southern Malawi. The fact that both northern and southern province chemistries can be found in one area makes an explanation of the use of the provincial terminology necessary. Cox (1972) preferred such terms as potassic and potassium-poor (or sodic) types while Erlank et al. (1980) informally used enriched and
282
R. M A C D O N A L D
ET AL.
I 16%
INDIAN
Basalts of S. Malowi p
Lupata Gorge on the Zambezi
LAKE CHtLWA
~
\
OO
"~" b
"" . ~'r~
OCEAN
--
\
"
..\.'\\\.
s \
. . . . J~' 0
~::.:S:.
30 • 0 00
0
0
0
o
0
0
0
0 0
0
0
0 0
o OO~ o 0 ~"
0 0 0
0
c
0 0 0 0
~ -'---~
0
9
-34~
I
I
"
Alluvium in the Shire Trough l a n d Urema G r a b e n
.~s FSEZ~Marine andcoastal J
~ -33~
..... Pt ~
o
I
Post-Karroo continental sediments
33%--
Lupata alkaline [avas and tuffaceous sediments
o
0
Jsediments
0
and minor ~
Karroo continental sediments
--~ Pre-Karroo basement rocks
-32~
O0 100
Kitometres
I
I
32~
-
Igneous centres
----"-- Basaltic dykes .......:..... Alkaline dykes
17%
F
18%
I
FIG. 1. Simplified geological map of southern Malawi and adjacent areas of Mozambique, to show the relationships between the Karroo lavas studied here (Triangle 1) and related lavas and the dolerite dyke swarm.
283
K A R R O O BASALT P R O V I N C E S normal to describe Karroo basalts, with particular reference to incompatible trace element contents. Such terminology is, of course, imprecise unless some reference datum is specified. The terms 'enriched' and 'normal' are used here to describe the potassic and less-potassic (sodic) varieties of Karroo tholeiite, except where reference to earlier papers is being made and the terms northern and southern provinces are also given to avoid ambiguity.
Geology Southern Malawi lies on the SW margin of the Zambezi trough and the stratigraphic units are in a general way tilted away from Malawi towards the Zambezi (fig. 1). Erosion has stripped away many of the younger units from the uplifted margin so that in southern Malawi the older units are found whereas the younger sequences are incomplete or missing. A generalized Mesozoic stratigraphy for southern Malawi is given in Table I; for completeness, the stratigraphy at Sinjal-Doa, some 20 km SW of the Malawi basalt outcrops, and at Lupata gorge, some 70 km to the west, are also shown in Table I. The great thickness of Karroo lavas at Sinjal-Doa suggests that no more than the lower third of the full Karroo volcanic sequence is preserved in southern Malawi. TABLE I .
The lowermost 130 m of the basalt sequence, the
porphyritic group of this study, is notably plagioclase-phyric and crops out in poorly exposed terrain along with purple-red basaltic pyroclastic rocks. The indifferent exposures make it almost impossible to define stratigraphical relationships within the group and accordingly as wide an area as possible, some 30 km 2, was sampled in the hope of collecting the full petrographic range. Among the analysed samples (Table II), the only relative age known is that M4-2 overlies M4-3. The fragments in M4-2 belong petrographically to the porphyritic group but the rock is chemically akin to the overlying aphyric group. It is taken to mark the transition between the two, thus establishing M4-3 as the highest member locally of the porphyritic group. The overlying basalts of the aphyric group form a 300 m thick sequence, with individual flows ranging between 3 m and 25 m. The local vent agglomerate noted by Dixey (1930) near the top of this sequence is somewhat exceptional since pyroclastic rocks are much less common amongst the aphyric lavas than the porphyritic group. The aphyric rocks form higher ground and are rather better exposed than the underlying group. Nine of the analysed specimens were collected in two stratigraphical sequences on Murukanyama Hill and a hill 3 km away; from bottom to top in each case, they are
Generalized Mesozoic stratigraphy of S. Malawi, with comparative sequences.
SOUTH MALAWI
SINJAL-DOA
sandstone
LUPATA GORGE
Sena sandstone
Sena sandstone
unconformity
CRETACEOUS
PHONOLITE LAVAS
sandstone with
300 m+ 300 m l l 5 • 10 m.y.
TUFFACEOUS SANDSTONE
some ASH
150 m
50 m
conglomeratic sandsto~ne
lO0 m
unconformity
unconformi t~/
unconformity BASALT LAVAS
c~
BASALT LAVAS (aphyric)
300 m
BASALT LAVAS (porphyritic) and PYROCLASTICS
130 m
sandstones (upper)
800 m
< v
BASALT LAVAS
80 m
sandstone
90 m
unconformity 200 m
RHYOLITE LAVAS and TIIFFS c~
RHYOLITE LAVA
BASALT LAVAS
600 m •
130 m
llO0 m
unconformity red beds
300 m
Mwanza g r i t s and shales
lO00 m
sandstones (lower)
1300 m
coal shales PERMIAN
Karroo sandstones
Karroo sandstones
600 m
unconformity basement complex
Sources of information:
Dixey and Campbell Smith (1929);
Dixey (Ig30);
Flores (1964);
Hal)good (1963).
166 • lO m.y.
284
R. M A C D O N A L D E T A L .
M l l , M12, M14, M15, M16, and M27, M28, M29, M30 (Table II). M18 is probably from near the base of the aphyric sequence, while M16 is a dyke intruding the porphyritic group, which presumably once fed aphyric extrusives subsequently stripped off. The only leucocratic rock found is a rhyolite dyke (M31) cutting the porphyritic group. While not directly relevant to the main aims of this study, the dyke is of some interest for its likely magmatic affinity. While the only ferromagnesian phase in the rock is magnetite (in various stages of oxidation to hematite) and the dyke itself is corundum-normative (c = 1.4 ~o), the low A120 3 and high Fe203, Ce, Nb, Y, Zn, and Zr contents are more consistent with peralkaline chemistry. It seems probable that during emplacement and crystallization of this body, loss of Na20 occurred, which caused precipitation of magnetite rather than Na-Fe silicates. This effect has been described from peralkaline intrusions in SW Greenland by Macdonald (1969) and Larsen and Steenfelt (1974). Peralkaline Karroo rhyolites have been recorded in the upper part of the sequence in the southern Lebombo (Assun~ao et al., 1962). Relationships with the dolerite dyke swarm. The NE-trending dolerite dyke swarm can be seen on fig. 1 on the shoulder of the alluvium-filled trough opposite the basalt lava outcrops. The representation of the dykes on this map is necessarily diagrammatic; for a better illustration of the intensity of the swarm see Woolley et al. (1979). There are no radiometric dates for the dykes but they are considered to be of Jurassic age because dykes and sills of dolerite intrude sediments of late Triassic to early Jurassic (Stormberg) age in the Chikwawa area and the swarm is cut by intrusions belonging to the early Cretaceous Chilwa alkaline province. It would seem likely therefore, on the basis of age, geographic proximity and rock type, that the dolerite dyke swarm and basalt lavas are representatives of the same phase of igneous activity. Petrography Lavas of the porphyritic group. Two types may be distinguished on the basis of phenocryst assemblages (specimen numbers in brackets): olivineaugite-plagioclase-phyric basalts (5, 4-3, 23, 25, 3) and olivine-plagioclase-phyric basalts (9, 22). Chemical data (Table II) indicate that the latter type is more evolved, less magnesian, than the former. All the rocks are characterized by a relative abundance, 8 % to 25 % modally, of plagloclase phenocrysts up to 8 mm long and commonly occurring in glomeroporphyritic aggregates or stellate clusters. Olivine phenocrysts are fresh in
specimen 23 but in other rocks are pseudomorphed by iddingsite or serpentine. Their maximum dimension is 1 mm and abundance varies from < 1 ~o to 5~. Colourless augite phenocrysts (up to 8~o) occur as discrete, subhedral crystals up to 2 mm across but more often are found in glomeroporphyritic aggregates. In some specimens, the phenocryst phases grade down in size to a holocrystalline matrix dominated by laths of plagioclase, intergranular augite and pigeonite and Fe-Ti oxides ranging in shape from rod-like to equant. More commonly, the matrix contains variable, up to 30~, amounts of glass, normally heavily charged with reticulate ore and occasionally devitrified. Lavas and dyke of the aphyric group. While the 'aphyric' designation is convenient in the field, many rocks in this group are actually sparsely microphyric. The following microphenocryst assemblages, listed approximately in order of decreasing MgO contents of the host rocks, have been recognized: olivine (18, 6); olivine-augite (12); augite-plagioclase (27); augite-plagioclase-Fe-Ti oxides (11). The aphyric rocks (14, 15, 16, 28, 29, 30) are the most evolved rocks in the group; textural relationships indicate that oxides were the earliest crystallizing phase. The aphyric rocks and the matrices of the microphyric types consist of plagioclase, augite, pigeonite, Fe-Ti oxides and small amounts of glass. The most common texture is ophitic or subophitic, which has not been seen in the lavas of the porphyritic group. This compares with the general relationship noted by Cox et al. (1967, p. 1471), that subophitic relationship had not been observed in northern province rocks but was common in southern province lavas. Some Malawi specimens (11, 12, 14, 27) contain coarser patches of doleritic texture which may contain carbonate ocelli. P etr oc hemistr y
Seven specimens from the porphyritic group, 12 from the aphyric group and a rhyolite dyke were analysed for major and 15 trace elements (Table II). All the marie specimens are hypersthene + quartz normative, even after adjustment of the oxidation ratio 100Fe3+/(Fe 2 + + F e a+) = 10 (Cox and Hornung, 1966, p. 1425), indicating that on a chemical basis, the Malawi basalts are quartz tholeiites. None of the rocks is notably magnesian, the highest MgO content being 8.44 ~. No extrusive equivalent of the picritic Karroo dykes of Malawi noted by Woolley et al. (1979) has thus been recorded. The relatively low MgO mafic Karroo volcanic rocks have traditionally been termed basalts yet, as Wilkinson and Binns (1977)
KARROO TABLE I I .
BASALT PROVINCES
Analyses of Malawi Karroo lavas and dykes
Porphyritic group 5 SiO2 TiO2 Al203
4.3
23
25
3
Aphyric group 9
51.9 5 1 . 3 5 0 . 5 5 0 . 3 50.fi
285
22
4-2
50.3 49.8
57.8
1.40 1 . 6 2 1 . 8 9 1 . 9 3 1.81 2 . 0 3 2.02 15.20 14,33 14.51 15.07 16.65 15.99 16.59
18
II
12
14
15
16
Rhyolite 27
28
29
30
6
31
5 1 . 2 5 2 . 8 5 0 . 8 5 0 . 4 5 2 . 3 5 2 . 4 5 2 . 4 5 2 . 8 5 2 . 5 5 2 . 7 4 9 . 0 72.7
0.63 I1.08
0.54 0 , 7 2 0.68 0.90
0.98
0 . 9 7 0.71 0 . 7 7 0.88
1 . 0 0 l.O0
Fe203
5.12
6.14
6.98 ]0.22
8.17 10.40
9.75
6.66
3 . 9 2 2 . 8 0 3 . 8 9 2.31 4.03
FeO
5.94
5.94
6.54
3.36
3.02
2.57
3.01
0.40
5 . 5 2 7,40 6 . 1 6 8 . 8 7 8.41 8 . 9 6 7.26 8.17
MnO
0.13
0.17
0.13
0.11
0.II
0.I0
0.13
0.17
0.13 0 . 1 9 0.12
MgO
5.34
5.22
4.57
3.91
3.22
2.72
2.48
3.00
8.44 6.35
CaO
8.72
8,97
8.35
8.64
7.98
7.25
8.58
6.13 II.54 I0.80 I0.76 I0.79
Na20
2.30
2.29
2,44
2.49
3.03
3.08
2.99
1.66
1 . 8 4 2.08
2 . 9 6 2 . 4 4 2 . 7 0 3 . 1 4 3 . 4 8 5.82 8.28
0 . 1 6 0 . 1 7 0.16
6 . 3 9 6 . 0 8 6.13
0.12
7 . 8 6 8.88
0.18 0.18
5.58 0.00
0 . 1 5 0 . 1 4 0.24
5.74 6 . 8 2 6 . 2 9 6 . 0 7 5 . 7 7 7 . 1 0 0.16
9.88 9.60 l l . l l
10.29 9 . 8 8 9,94 lO.21 0.17
2 . 2 9 1.94 2 . 2 2 2 . 4 0 2.01 2 . 2 2 2 . 1 3 2.19 2 . 5 3 2.80
K20
1.84
1,68
1.82
1.89
2.16
2.94
2.00
5.56
0 . 7 4 0.98
1.66 0.48 0.98
0 . 9 9 0 . 4 9 0.88
1.04 0.93 0.84
P205
0.28
0.31
0.36
0.38
0.36
0.42
0.43
0.06
0.08 0.12
0.12
0.14 0.15
0.14
O.ll
0.13
H20+
0.85
0.67
1.06
1.02
0.94
1.50
1.13
1.39
0 . 7 0 0 . 2 0 1 . 3 3 3.05 0.48
0.89
1.02 0.30
CO2
0.17
0,68
n.d.
n.d.
1.13
n.d.
n.d.
4.76
Total
9 9 . 1 999.32 99.15 99.32 99.38 99.30 98.91
0.44
14.87 14.41 14.13 14.42 13.98 14.05 14.96 14.38 14.06 13.89 16.28 I0.05
0.16 0.87
0.12
n_d. n.d.
3.49
0 . 1 4 0.21 0.01
1.30 0.31 1 . 0 5 0.99 n.d,
n.d.
n.d.
99.30 99.52 99,01 99.20 99.54 99.71 99.26 99.45 99.10 99.17 99.38 99.76 99.33
Trace elements (ppm) Ba
60?
659
702
718
752
820
794
295
168
249
291
183
311
287
230
265
204
240
330
358
Ce
69
71
78
71
70
72
70
18
22
30
13
22
31
22
23
14
20
24
29
526
Co
50
43
39
46
31
40
42
24
43
46
39
37
48
44
43
45
36
44
44
18
Cr
217
179
127
109
70
34
16
169
397
148
168
45
26
31
156
64
83
22
132
