S . A f r . J . G e o i . , | 9 9 8 , I 0 l ( 2 ) , I I 9 - l 2 2
Geochemical variations in a long Karoo dyke, Eastern Cape
J.S. Marsh and M.J. Mndaweni
l)epartnrent of Geology, Rhodes University, Grahamstown, 6140 Republic of South Atiica c - m a i l : ' i s m @ r o c k . r u . a c . z a
A L t : e p t e d l0 D e c e m b e r 1 9 9 7
A prominent vertical dolerite dyke, up to 30 m wide and which can be traced fbr over 100 km, is compositionally homo- geneous along much of its length and shows no evidence of flow difl'erentiation. Associated parallcl dykes arc composition- ally distinct and are unrelatcd by closed-system processcs. The main dyke has geochemical characteristics of thc Maloti or Senqu Units in the upper part of the main Lesotho basalt sequence, whereas the parallcl dykes are geochemicirlly similar to thc lower Maflka Lisiu Unit. On the basis of size and composition the main dvke mav well reorcscnt a flssure which oncc l'ed ellusions of Karoo basalt.
'n Opvallende vertikale dolerietgang, tot 30 m wyd en wat vir meer as 100 km ver gevolg kan word, het 'n
homogene same- stclling langs 'n
groot gcdeelte van sy lcngtc en toon geen tekens van vloeidiff'erensiasie nie. Geassosieerdc cwcwydigc gangc verskil in samestclling cn is nie verwant a.g.v. prosesse in'n geslote stelsel nie. Die hoofgang het geochemiesc cicn- skappe van die Maloti of Senqu Eenhede in die boonste deel van die hoofopeenvolging van Lesothobasalt, tcrwyl die ewew- ydigc gange geochemies soortgelyk aan die laer Maflka Lisiu Eenheid. Op die grondslag van grootte en sarnestelling kan die hoofgang 'n spleet vcrtccnwoordig wat eens uitvloeiings van Karoobasalt gevoed het.
lntroduction
Most geochemical studies on Karoo intrusions have focused on differentiation in thick sills and intrusions (e.g. Eales, 1990; Lightfbot & Naldreu, 1984; Le Roex & Reid, 1978), involving multiple sampling of the bodies, or on regional studies of dykes and sills involving single samples from sepa- rate intrusions (e.g. Marsh & Eales, 1984). Detailed studies involving multiple samples of single Karoo dykes are not available. This is an important omission considering that dykes may well have fed the tlssures fiom which Karoo basalts were erupted. An underlying assumption of the regional geochemical studies involving single samples from numerous dykes over a wide area is that such dykes are com- positionally homogeneous. This has never been demonstrated fbr the Karoo Province.
F i e l d r e l a t i o n s h i p s a n d s a m p l i n g
This note reports results of a study of a c. 1O0-km-long Karoo dyke, which is well exposed between Middleburg and the Mountain Zebra National Park east ol'Cradock, Eastern Cape ( F i g u r e 1 ) . It i s d e p i c t e d o n t h e l : 1 0 0 0 0 0 0 G e o l o g i c a l M a p o f S o u t h A f r i c a ( 1 9 8 4 ) a n d in m o r e d e t a i l o n t h e 1 : 2 5 0 0 0 0 geological maps 3124 Middleburg (1996) and 3224 Graaff Reinet (1993). These sheets have been used to prepare Figure l. This dyke is vertical, up to 30 m wide, and particularly prominent along much of its outcrop, fbrming a 'wall' extending south-southeast some 40 km across the plains south of the Witkransnek road pass from which it is easily viewed. However, at its northern termination in the property of the Grootibntein Agricultural College, east of Middleburg, it has more subdued relief. North of the Mountain Zebra Park, geological maps show the dyke bifurcating and several other 'dykes
intersecting the main dyke at various orientations. In this area, problems were experienced in locating the position of the main dyke. The geological maps also indicate that the dyke cuts across the thick dolerite sheet forming the high mountains in the Mountain Zebra Park and extending to the south. Although it was possible to trace the dyke to the north- west border of the Park, it was not possible to confidently find
its extension to the south. The dyke was sampled at seven sep- arate localities along 85 km of its length (Figure l). At some of these localities, samples from both the chilled contacts, as well as the coarse-grained interior of the dyke, were collectcd.
Samples MJM-02 to MJM-12 are unambiguously fiom the main dyke. Sample MJM-01 is from a parallel dyke lying to the east of the main dyke near its northern termination. Sam- ple MJM-15 was collected lrom the apparent continuation of the main dyke south of its oblique intersection with another large dyke. This oblique dyke yielded sample MJM-14, and MJM- l3 is from a parallel dyke at the same locality. MJM- I 3 to MJM-15 are all from the area ol dyke complexity where the course of the main dyke is difllcult to define as mentioned above.
Petrography
On a petrographic basis the dykes subdivide into two groups.
The majority of samples (MJM-02 to MJM-12 and MJM-14) are olivine-free and are extremely similar in having sporadic aggregates of plagioclase phcnocrysts encloscd in a l'iner groundmass. The glomeroporphyritic texture is particularly obvious in the chilled contact samples where the phenocryst aggregates are up to 3 mm across and the groundmass is inter- granular. In the coarser-grained samples from the rnterior of the dyke, similar sized aggregates are enclosed in a medium- grained subophitic mass of pigeonite-cored augite and plagio- clase laths. Minor opaque minerals are also present. Samples MJM-13 and MJM-15 are identical in having sparse corroded olivine and plagioclase aggregates in a coarse subophitic groundmass. MJM-01 is hypocrystalline with altered pscudo- morphs after olivine and aggregates of plagioclase pheno- crysts in a medium-grained groundmass dominated by a crudc variolitic texture. In terms of petrography, it correlates closely with MJM-13 and MJM-15. In the sections that follow it is shown that these petrographic groupings corespond to gco-
? . -
ti
l z 0 3 , 0 4
l^"'"*k r,,
05 0 9 0 6 , 0 7 , 0 8
N
I
Figure I lv'lap showing the location ol'thc dyke sarnpled in this study. Solid lincs - dykes; dottcd lines - roads. Sample locations are indicatcd by nurnbered circlcs along the dyke length. Thc inset map shows outcrops o1' Karoo basalt (black) in southern Africa relative to the location of thc study area.
chemical groupings, indicating that to the south of the high- way R6 I . the main dyke swings to the west (Figure l).
Geochemistry
Major and tracc analyses iire presented in Table I and Figures 2 and 3. Thc results show that the data clusJer in two groups (Figurc 2). One group, the olivine-frce dolerites comprising the majority of samples collected fiom along the entire length o f t h e d y k e . h a s M g O - 6 . 5 o k , S i O 2 - 53%, Zr - 110 ppm, and Ni - 50 ppm. The other group of three olivine-bearing sam- p l e s ( M J M - 0 1 , M J M - 1 5 , a n d M J M - 1 3 ) i s d i s t i n c t l y m o r e primitive, with higher MgO and Ni and lowcr SiO2 and Zr concentrations, indicating that there is a pcrfect comelation betwecn petrography and geochemical character
Thc tight geochemical clustering ol'samples from along thc cntire length of thc main dyke, at least to .just north of the Mountain Zcbra Park, a distance of 75 km, indicates a remarkable compositional homogeneity. Furthcrmore, there is no difl'erentiation between chilled margin samples and those fiom the intcrior of the dyke. At its northern and southcrn ter- minations, the olivine-bcaring parallel dykes have a signifi- cantly difl'crent composition, suggesting that they are not
S . A f r . T y d s k r . C e o l . , | 9 9 8 , l 0 I ( 2 )
oll'shoots or an en echelon subset of the main dykc. There is no data indicating direction of magma propagation within the dyke, but the dyke shows no evidcncc of flow diffcrentiation.
In all these respects thc dykc diffcrs fiom the largc sheet-like Karoo intrusions, which usually show cvidence of modest intcrnal difl'erentiation by phenocryst accumulation some- times induccd by llow (Lc Roex & Reid, 1978; Richardson,
1979). This contrast is illustratcd by comparativc data ficlm sills also plotted in Figurc 2.
One of the important problems in the Karoo province is to detcrmine the mode of eruption of the basaltic lavas and the location of the conduits which I'cd surlace eruptions. It is widcly believed that the dykes represent the roots of cruptive flssures. If so, the occurrencc of'dykes throughout the Karoo basin suggests that the Karoo lava pile was built fiom a wide- spread and diffuse plexus o1'llssures; an eruptive charactcris- tic that stands in contrast to that of somc othcr flood basalt provinces. For examplc, in thc Columbia River Province most of the major basalt units wcre erupted from geographically constrained linear fissure zones located in the eastcrn part o1' the province, with the lavas f'lowing westwards firr l00s of km (Swanson et al., 1915).
The geochemical stratigraphy in the basalt remniints clf' Lesotho and the Springbok Flats established by Marsh c/ a/.
(1997) offers the potential of idcntifying intrusions which corrclatc with specific volcanic units, and which potentially fbrm part of the magma system f'eeding the volcanic scquence. These authors dcmonstratcd that the lowermost 50
200 1 5 0 2 100 50 lz0 100
Upper sill
Main Dyke O margin O interior y parallel dyke c..l
a
4 6 8 1 0
Mgo
Figure 2 Variation diagrams showing concentrations of somc clc- ments in the main dyke (dots and circles) and parallel dykes (stars).
The enclosed elongate llelds illustrate the composition variation in two moderately differcntiated sills on the Nico Malan Pass, near Seymour, Eastern Cape (Marsh. unpubl. data).
L<
N
60 40
< A
5 3 5 2 5 1
s0
S . A f i . J . G e o l . | 9 9 8 . l 0 l ( 2 )
Table 1 Major oxide and trace-element analyses of dyke rocks
M J N , I . O I M J M - O 2 M J M - O 3 M J M - 0 4 M J M - 0 5 M J M - 0 6 M J M _ 0 7 M J M - 0 8 M J M - 0 9 M J M . I O M J M - I I N , I J M . I 2 M J M - I 3 M J M - I 4 M J N , I . I 5
a/'
S i O r 5 l . l 7 5 1 . 9 9 5 2 . 1 8 5 2 . 1 8 5 2 . 4 1 5 1 . 9 9 5 2 . 9 9 T i O l 0 . 9 3 L 0 6 I . 0 5 | . 0 5 L 0 5 1 . l Z l . l 2 A l 2 O i 1 5 . 3 2 1 4 . 6 6 1 4 . 1 2 1 4 . 1 2 1 1 . 6 2 1 4 5 0 1 4 . 5 0 F e O * 1 0 . 0 5 1 0 . 8 1 1 0 . 8 8 1 0 . 8 8 1 0 . 7 1 1 1 . 0 6 1 1 . 0 6 M n O 0 . l l l 0 . 1 9 0 . 1 9 0 . 1 9 0 . 1 9 0 . 1 9 0 1 9
MgO 6.95 6.30 6.50 6..50 6.39 6.30 6.30
CaO 10.71 9.99 9 88 9.88 9.94 9 95 9.95
Na2o 2. 19 2.26 2.41 2.11 2.4ti 2.43 2.43 K : o 0 2 3 0 . 7 1 0 . 8 0 0 . 8 0 0 . 7 9 0 . 7 5 0 . 7 5 P : O : 0 . 1 4 0 . 1 9 0 . 1 9 0 | 9 0 . 2 0 0 . 2 0 0 . 2 0 L O I r . 9 4 1 . 4 8 I 1 3 r . 1 6 1 . 3 6 t . 3 3 1 . 3 3 H z o - 0 3 3 0 . 1 9 0 . 2 3 0 . z z 0 . 2 0 0 . 1 9 0 . 1 9 T O T A L 1 0 0 2 0 9 9 . 8 3 9 9 . 8 9 9 9 . 9 t t 0 0 . 4 6 1 0 0 . 0 1 t 0 l . 0 l ppm
Z n 1 0 0 9 8 1 0 6 l 0 l 1 0 6 I 0 5 l 0 l
C u 1 0 2 9 7 9 3 1 0 5 9 2 l l l
52.68 52.37 52.t4 1 . 0 5 1 . 0 4 I . l 0 14.95 14.94 t4.'32 1 0 . 8 3 1 0 . 7 8 I I . l 0
0 . 1 9 0 . 1 9 0 1 9 6.29 6.27 6 28 9 . 9 5 9 . 8 9 1 0 . 0 3 2 . 4 1 2 . . 3 6 2 . 3 9 0 7 5 0 . 1 4 0 7 6
0.20 0.20 0.20
| . 2 8 1 . 2 5 t . 6 4 0 . 2 5 A . t 4 0 . t 6 1 0 0 8 3 1 0 0 . 1 7 1 0 0 . 8 1
t 0 7 1 0 3 1 0 3 9 1 1 0 7 9 3
46 48 ,18
4 2 4 1 4 2 l 8 l t 7 5 1 8 0
248 258 253
9 2 9 . 3 U . 9 l l l t 0 7 1 0 8 3 0 . 6 3 0 . 9 3 0 . 6
205 209 199
l t 1 6 l 8
5 2 0 9 5 2 . t ' 7 5 1 3 9 l . l 1 1 . 0 6 t . 0 2 t 4 . 1 3 1 4 6 5 1 5 . 5 9 I t . 8 | 1 0 . 8 8 9 . 5 6
0 . 2 1 0 . 1 9 0 . I t t 6 6 0 6 . 3 1 7 . 0 5 9 . 8 8 9 . 9 5 l t . t 8 2 . 3 0 2 5 | 2 . 5 0 0 . 8 0 0 . 7 6 0 . 5 1 0 . 2 0 0 . 2 0 0 . 1 8 1 . 3 8 | . 2 1 1 .1 0 0 1 6 0 . 1 7 0 . 2 2 r 0 0 . 7 3 r 0 0 . 0 9 1 0 0 . 7 8
r 0 8 1 0 6
i l t 9 1
7 6 1 l
4 2 ' 1 1 l 8 7 t 1 9 2 8 0 2 5 3 t i 6 9 . 2 I t 3 t j l 3 3 . 1 2 9 . 8
I 9 6 2 0 3
n 2 0
8 9 t 0 5
| 0 9
1 a
329 26u 9 . 9 1 8 23.1 231 8 . 3
52.31 .5 1.24 r . 0 5 t . 0 4 1 4 . 6 5 1 5 . t 7 t 0 8 0 9 . 9 5
0 . r 9 0 . 1 8 6 . 3 3 6 9 0 9 . 8 9 r 0 . 9 4 2 . 5 3 2 . 4 3 0 . 1 9 0 . 5 3 0 . 2 1 0 1 8
1 1 6 t 3 4 0 . 1 7 0 . r 3
| 00. l4 | 00.03
| 0.5 r05 9 t . 7 r t t ) 1 1 t o l
.+l 44
lTfl 296 2-19 211,1
9 . 5 8 9 l t O 7 9 3 0 . 4 2 3 6
208 235
2 0 t 0 N i
C o Cr'
N b 1 . 1 8 . 4 9 . 1 7 . 8 t 0 . l ' t
6 9 . 2
99 49 48 41 49 47
'16 41 42 39 40 .10
il0 5 2 40
7 t
Y S r
30,1 | 85 t95 113 | 82 ll3 t75
164 259 258 256 251 263 258
t 5 1 0 7 1 0 5 1 0 8 1 0 8 I l 0 I I I 26.0 30.,1 30.1 30.7 30.0 3 | . I 32.4
194 203 200 197 203 205 207
R b : 1 . 6 1 5 1 8 1 8 l 8 l 1 1 7
Major and trace elements determined by XRF spectrometry at Rhodes University. FeO* all Fe as FeO
- 200 m of' the lava sequence is built of a number of basaltic magma types with quite diverse geochemical characteristics.
These flows have been grouped into the Barkly East Forma- tion of the Drakensberg Group. The main sequence of basalts is also built of several basalt magma types which are more
relationships between the two. These diagrams also indicatc that the main dyke has strong geochemical affinities to thc dis- tinctive Maloti Unit or thc overlying Senqu Unit in the Lesotho Formation. Thc three olivine-bearing parallel dykes are similar to basalts of the Mafika Lisiu Unit which underlies voluminous, are less variable in composition,
and form thick units which extend throughout the Lesotho basalt remnant. These units have been groupcd into the Lesotho Formation and fiom the base up are the Mafika Lisiu, Maloti, Senqu, and Mothae Units. Figure 3 compares the dyke data with those of the basalts on geo- chemical discrimination diagrams fiom Marsh e/ al. (1991). These diagrams utilize a number of immobile incompatible element ratio diagrams to discriminate between difTer- ent stratigraphic units, and can be applied to data fiom very fresh rocks, as well as those exhibiting modest degrees of alteration.
The diflerence in incompatible element ratios between the two groups of dykes in rela- tion to the slight differences in degree of dif- ferentiation preclude a closed-system genetic
PlZr
Mafika S e n q
M o t
Maloti
PlZr
Mafika Lisiu
Senqu & Mothae
Maloti
A/ g.oxbow
""\ Wonderkop 1l a-
wonderkop
: , . \ A
t c o J r K . ' , \ . ' R o t ui r
- .,,' i .., 'Moshesh,s Ford Lisiu
t 0
r 0 0 2
TilZr ZrN
Figure 3 Geochemical discrimination diagrams takcn fiom Marsh el (1t. (l()9i,) showing the dyke compositions in relation to the composition of stratigraphic units in the main Lesotho basalt remnant. Symbols as in Ficure 2.
t ) 2
the Maloti Unit. The main dyke lies some 200 km southwest of the ncarcst outcrops of these basalt units in the Lesotho rcmnant. and it is l'easible that it could have actcd as a f'eeder to both thc preserved flow packages and those since removed b y c r o s i o n .
C o n c l u s i o n s
A promincnt, north-northwest-trending, I l0-km-long, verti- cal Karoo dolcrite dyke, situatcd between Cradock and Mid- dleburg in Eastern Cape, is compositionally homogeneous along much of its length. On the basis of geochemistry, it cor- rclates with thc Maloti or Senqu basalt magma types which occur in thc upper portions of the Lesotho basalt remnant of Karoo basalts. The homogeneity of the dyke is consistent with it being a possible f'eeder to recognized units of basalt f l o w s w h i c h a l s o s h o w v e r y li m i t e d c o m p o s i t i o n a l l y v a r i a b i l - ity ovcr large areas.
Acknowledgements
This rescarch was supported by an Honours bursary to MJM tiom An-clo American Corporation, and an FRD research grant to JSM. We thank Michele Booysen and Andre Visagie for assistancc in the fleld and laboratory.
S . A f r T y d s k r . G e o l . , I 9 9 8 , | 0 I ( 2 )
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