Magnetic fabric in the Umvimeela Dyke, satellite of the Great Dyke, Zimbabwe

TECTONOPHYSICS ELSEVIER

Tectonophysics 242 (1995) 241-254

Magnetic fabric in the Umvimeela Dyke, satellite of the Great Dyke, Zimbabwe M.P. Bates *, M.F. Mushayandebvu Department of Physics, University of Zimbabwe, PO Box MPI6Z Mount Pleasant, Harare, Zimbabwe

Received 4 August 1993; revised version accepted 21 July 1994

Abstract

Anisotropy of magnetic susceptibility (AMS) has been determined for 22 locations from the 500-km-long gabbroic Umvimeela Dyke of Zimbabwe. AMS is controlled by the presence of multi-domain magnetite grains which appear to have preserved a variety of magnetic fabrics related to magma movement at the time of dike emplacement. Three locations along the Umvimeela Dyke have been identified as feeder points at which magma flowed into the dike from below. Lateral flow away from these points supplied the rest of the dike system. The presence of several loci of igneous activity along the Umvimeela Dyke parallels the development of the neighbouring contemporaneous Great Dyke layered complex, which consists of a line of contiguous subchambers. Individual feeder points to the Umvimeela Dyke may be directly related to activity in adjacent subchambers.

I. Introduction

The variation in magnetic susceptibility with direction in a rock sample is known as anisotropy of magnetic susceptibility (AMS). AMS is used to recognise subtle rock fabrics that are difficult to identify using optical or image analysis techniques. The AMS fabric of basic dikes and sills reflects the direction of magma emplacement (E1lwood, 1978; Knight and Walker, 1988; Park et al., 1988; Ernst, 1990; Puranen et al., 1992; Cadman et al., 1992). Other studies have linked the AMS of intrusive rocks to cooling-related or regional stresses during the late stages of crystal-

* Corresponding author.

lization (Ellwood and Fisk, 1977; Park et al., 1988). We have gathered AMS data on the Umvimeela Dyke, one of the satellite dikes of the Great Dyke of Zimbabwe, to see if the magnetic fabric of the dike provides information relating to its mode of emplacement and thereby an insight into the origin of the Great Dyke itself.

1.1. Geology

The Great Dyke (Fig. 1) is an elongate layered mafic/ultra-maflc complex, with an Y-shape cross section, 550 km long and up to 11 km wide (J.F. Wilson et al., 1987). It strikes north-northeast across the granite-greenstone terrain of the Archean Zimbabwe craton, which it intruded at 2461 _+ 16 Ma (Hamilton, 1977). For most of its

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242

M.P. Bates, M.F. M u s h a y a n d e b v u / Tectonophysics 242 (1995) 2 4 1 - 2 5 4

1011-

18

19

20

/J 5

110

- - u

O

o

22 ~

0

100

0

I

! km

Fig. 1. L o c a t i o n of s a m p l e sites from the U m v i m e e l a Dyke, and the m a j o r subdivisions of the G r e a t Dyke. U D = U m v i m e e l a Dyke; G D = G r e a t Dyke; E D = E a s t Dyke; M S = M a i n S a t e l l i t e Dyke; M U S = M u s e n g e z i S u b c h a m b e r ; D A R = Darwendale Subchamber; SEB = Sebakwe Subchamber; SEL = Selukwe Subchamber; WED = Wedza Subchamber; HA = Harare; L = Lalapanzi.

length the Great Dyke is only slightly deformed with the most significant brittle deformation related to three cross-cutting faults with displacements of up to 5 km. The Great Dyke is unaffected by the Limpopo Mobile Belt at its southern end, but Late Proterozoic Pan-African folding and thrusting in the Zambezi Belt has bent the northern end into a distinctive S-shape (Chimbodza, 1987; Chiyanike, 1987). Gravity data indicate that the Great Dyke consists of two major chambers (north and south)

which are separate at depth and only coalesce near the present surface at Lalapanzi (Fig. 1; Podmore and Wilson, 1987). The chambers were initially separated by floor topography, but became linked as the chambers grew. The chambers can be subdivided into five subchambers from north to south: Musengezi, Darwendale, Sebakwe, Selukwe and Wedza (Fig. 1; A.H. Wilson and Prendergast, 1988). The Great Dyke is flanked by the sub-parallel Umvimeela and East Satellite dykes (Fig. 1) composed of quartz gabbro and gabbro-norite (A.H. Wilson and Prendergast, 1988). The flanking dikes are variably altered and sheared in the vicinity of the S-bend at their northern ends (Wiles, 1972), and they extend 80 km further south than the Great Dyke into the Northern Marginal Zone of the Limpopo Belt where the Umvimeela Dyke has been remagnetized (Jones et al., 1975). Preliminary aeromagnetic interpretation (Podmore and Mushayandebvu, 1990) indicates that the flanking dikes are sub-vertical and approximately 300 m thick. Apart from the obvious spatial relationship, the correlation between the flanking dikes and the Great Dyke has been confirmed on the basis of petrology (Worst, 1960) and palaeomagnetism (McElhinny and Gough, 1963; Jones et al., 1975; Mushayandebvu, 1991). The Great Dyke and its associated satellites were intruded under conditions of limited extension of the pre-existing Popoteke fault set, driven by NNE-directed compression in the Limpopo Belt (J.F. Wilson, 1990).

1.2. Anisotropy of magnetic susceptibility In reporting the AMS data, the recommendations of Ellwood et al. (1988) have been followed, and S.I. (volume basis) units are used throughout. The initial magnetic susceptibility K is a dimensionless parameter defined as: J=UJ~

(1)

where J is the induced magnetization ( A / m ) and H is the applied magnetic field ( A / m ) of an intensity similar to the Earth's magnetic field (less than 70,000 nT). If K is anisotropic, the susceptibility of a given sample can be repre-

M.P. Bates, M.F. Mushayandebvu / Tectonophysics 242 (1995) 241-254

sented as a second-rank tensor which in a specific reference frame is described by three orthogonal eigenvectors K t, K 2 and g3, the directions of maximum, intermediate and minimum susceptibility which define the principal axes of the magnitude ellipsoid (Hrouda, 1982; Nye, 1987). The shape of the AMS magnitude ellipse is described by the following parameters (Jelinek, 1981): (1) Anisotropy degree: P' = e x p ~ / { 2 [ ( r l - "to)Z+ ( r 2 - "r0)2+ (73 - ~0)21}

(2)

(2) Shape factor: 2 r 2 -- 7-1 -- ~3

7" =

(3) ?I

--

7"3

where 7"1,7"2 and 7"3 are ln(K 0, ln(K 2) and In(K3), respectively, and 7"0 = (7"1 + 7"2 + 7-3)/3. T ranges from + 1 for an oblate ellipsoid to - 1 for a prolate ellipsoid.

243

anisotropy, and always produces orthogonal mean axes.

2.2. Results Site mean values of P', T and principal axes directions are given in Tables 1 and 2. While site level data are of varied quality (Fig. 2) the axes are not randomly distributed in orientation at a single site. Bulk susceptibilities vary from approximately 6 × 10 - 4 to 2 x 10 -2. The significance of this variation is discussed below and by Mushayandebvu et al. (1994). The degree of anisotropy for most sites is below 10%; only three sites have a P' value which exceeds 1.1 (Table 1; Fig. 3). Both prolate and oblate anisotropies are present in approximately equal numbers (Table 1; Fig. 3), but no particular relationship between axis orien-

Table 1 Site mean susceptibility and anisotropy data

2. AMS study

2.1. Data acquisition technique A suite of samples from the Umvimeela Dyke (Fig. 1) was collected at the University of Zimbabwe in the course of several palaeomagnetic investigations (see Appendix). The cores were all oriented by sun compass, drilled in the field and subsequently cut into 2.5-cm-long specimens with a diameter of 2.4 cm. The margins of the dike are not exposed and thus the location of each site relative to the dike's margins could not be ascertained. Twenty-two sites (U1-U22) have been analyzed. AMS and bulk susceptibility for specimens from each core were measured using Sapphire Instruments SI-2 susceptibility apparatus. Site mean directions were determined using the tensor-averaging technique described by Ernst and Pearce (1989), based on the work of Hext (1963) and Jelinek (1978). The technique calculates an average ellipse for each site which automatically weights each specimen according to its degree of

Site U1 U2 U3 U4 U5 U6 U7 U8 U9 U10 Ull U12 U13 U14 U15 U16 U17 U18 U19 U20 U21 U22

n 5 5 4 6 9 5 3 7 4 8 5 4 5 4 4 5 4 4 4 6 6 6

K (×10

s.d,

P'

T

0.403 0.216 0.266 0.378 5.608 11.221 4.174 83.501 4.480 5.926 31.753 11.330 65.816 2.613 7.208 9.621 2.852 51.165 36.144 12.897 16.545 3.301

1.0247 1.0140 1.0427 1.0231 1.1040 1.0648 1.0165 1.0823 1.0385 1.0152 1.0193 1.0214 1.0213 1.0068 1.0170 1.0131 1.0144 1.0385 1.0443 1.0362 1.2607 1.1414

0.235 -0.150 - 0.457 - 0.252 -0.565 -0.261 0.430 0.773 0.189 - 0.177 - 0.146 -0.680 -0.068 - 0.151 0.014 0.454 -0.159 0.607 0.142 0.212 0.825 0.573

4)

7.343 6.311 6.906 6.394 58.037 52.103 52.298 101.687 50.724 67.029 101.004 83.425 194.491 93.624 81.457 81.010 43.299 93.159 82.497 55.991 114.054 40.987

n = number of samples; K = bulk susceptibility; s.d. = standard deviation of K; P' = degree of anisotropy of the average AMS ellipse; T = shape parameter of the average AMS ellipse.

M.P. Bates, M.F. Mushayandebc, u / Tectonophysics 242 (19951 241-254

244

tation and anisotropy shape or degree can be established.

showed that there was no significant change in AMS. The shape of a specimen can influence the A M S measured. Ideally the ratio of length over diameter is 0.865 (Noltimier, 1971); the specimens used have the ratio 0.96 and are slightly long. The K 3 (minimum susceptibility) axes show a tendency to cluster around the axis of the specimen, with the other axes spread in a plane perpendicular to the specimen axis. This is not what might be expected from over-long specimens; if specimen shape is significant, clustering of K~ (maximum susceptibility) axes around the specimen axis would be observed. The observed correlation in fact arises from the fact that due to the flat nature of the outcrop and the particular drilling rig used by Jones et al. (1975), many cores were drilled at near-vertical angles. It is concluded that the A M S of the satellite dike specimens is real and of geological significance.

2.3. Systematic errors

Specimens which had been heated during palaeomagnetic analysis were avoided in case of alteration. In some cases it was necessary to use specimens that had been demagnetized in an alternating field (AF). This procedure does not generally alter the susceptibility of a rock; however, Potter and Stephenson (1990) found that AF demagnetization may change the A M S observed. This p h e n o m e n o n they ascribed to movement of magnetic domain walls in magnetite. To check whether this was a concern, some sites were analyzed twice using duplicate sets of specimens from the same cores with one set untreated and the other AF demagnetized. The results

Table 2 Site information and mean directions of the principal axes of anisotropy Site

Lat S / L o n g E

S

n

U1 U2 U3 U4 U5 U6 U7 U8 U9 UI0 Ull U12 U13 U14 U15 UI6 UI7 U18 U19 U20 U21 U22

16.67/30.88 16.71/30.89 16.73/30.88 16.76/30.87 16.86/30.78 16.88/30.78 16.97/30.73 17.08/30.67 17.48/30.53 17.81/30.43 17.95/30.38 18.74/30.19 18.95/30.15 19.70/29.90 19.83/29.87 20.11/29.70 20.48/29.58 20.88/29.48 20.98/29.48 21.03/29.48 21.33/29.43 21.47/29.40

344 344 31 29 13 13 11 15 22 23 17 19 18 13 32 14 15 3 2 2 348 355

5 5 4 6 9 5 3 7 4 8 5 4 5 4 4 5 4 4 4 6 6 6

Minimum

Intermediate

Maximum

Tr/P1

A

a

Tr/PI

A

a

Tr/PI

A

a

187/65 179/19 174/2 213/23 90/59 101/6 111/13 57/71 220/87 250/73 328/44 256/3 120/77 48/37 347/63 342/34 258/29 80/13 179/54 166/71 184/74 204/59

18 28 22 13 21 18 39 5 15 34 37 30 25 44 32 20 60 27 24 25 4 6

14 17 4 2 3 5 2 3 3 18 17 19 13 211 9 6 27 12 3 8 3 1

89/4 274/14 264/14 121/5 298/28 350/75 354/63 227/18 29/3 95/15 67/9 348/37 245/8 231/53 218/18 244/12 13/37 173/10 5/35 270/5 55/10 95/11

35 27 22 17 20 17 54 20 27 33 36 31 33 41 32 27 60 30 24 24 12 7

9 12 8 11 3 3 35 4 7 22 15 9 16 25 9 13 14 23 5 13 2 6

358/24 38/66 78/76 19/67 201/12 193/14 207/23 318/3 119/1 3/7 166/45 162/53 336/11 139/2 121/20 138/54 140/39 299/74 273/3 1/18 323/12 359/29

36 19 8 17 7 9 53 20 26 25 21 20 32 33 12 27 33 30 6 15 11 6

11 13 1 6 3 4 12 4 3 16 10 14 19 20 5 10 14 4 3 13 4 1

Lat S / L o n g E = latitude south and longitude east; S = strike of dike; n = number of samples; Tr/P1 = axis trend and plunge; A and a = long and short axes of cone of 95% confidence. All values except n are in degrees.

M.P. Bates, M.F. Mushayandebvu / Tectonophysics 242 (1995) 241-254 N

245

N

b. U6

a. U3

~

N

N

t

~

.

N

Fig. 2. E x a m p l e s of site level A M S axial d a t a p l o t t e d on e q u a l a r e a s t e r e o g r a p h i c projections. M e a n axes with c o n e s of 95% c o n f i d e n c e are shown. All symbols are on the l o w e r h e m i s p h e r e . • = m a x i m u m K; • = i n t e r m e d i a t e K; • = m i n i m u m K; o u t l i n e d symbols = site m e a n axes; a r r o w = dike strike.

1.0

oJ

'G JO O

00

T 0

••

111

112

p'~-

etll e

0J o

-1.0 Fig. 3. Plot of site m e a n d e g r e e of a n i s o t r o p y ( P ' ) a g a i n s t site m e a n s h a p e p a r a m e t e r (T). No s y s t e m a t i c p a t t e r n is a p p a r e n t .

246

M.P. Bates, M.F. Mushayandebt;u / Tectonophysics 242 (1995)241-254

2. 4. Magnetic mineralogy Progressive d e m a g n e t i z a t i o n analysis of Umvimeela Dyke specimens indicates that multidomain (MD) and single-domain (SD) magnetite and titano-magnetite are the major magnetic phases, with minor hematite and ilmeno-hematite (Mushayandebvu, 1991). All these mineral phases carry a palaeomagnetic remanence of G r e a t Dyke age which is probably primary. Although magnetite is only a minor phase, its high susceptibility ( K up to 3; Maher, 1988) means that it generally dominates AMS, although paramagnetic contributions from iron-bearing silicates (typically K = 1 x 10 4; Borradaile, 1988) can be significant if the total rock K value is less than 10 3 (Rochette, 1987). Hematite too has a much lower susceptibility than magnetite ( K = 1 x 10 2; Uyeda et al., 1963) and when present as a minor phase is unlikely to be important. Low field susceptibility of M D magnetite is principally controlled by grain shape such that the long axis of inequant magnetite grains has the highest susceptibility (Khan, 1962; Uyeda et al., 1963). Therefore, AMS in basic dikes will reflect the preferred orientation of the magnetite grains, although a complication may arise with SD magnetite which may have its maximum initial susceptibility along its shortest grain dimension (Potter and Stephenson, 1988). The interpretation of the magnetic fabric will depend on whether or not AMS is dominated by SD or M D magnetite, with or without a paramagnetic silicate contribution. Therefore it is important to determine the role of SD magnetite in controlling susceptibility values. This was investigated in three different ways.

The Lowrie-Fuller test The domain state of magnetite was examined using a modified Lowrie-Fuller test described by Dunlop (1983). Demagnetization curves were determined for induced high-field isothermal remanent magnetizations (IRM; B = 0.5 T) and lowfield anhysteretic r e m a n e n t m a g n e t i z a t i o n s (ARM; B = 34662 nT, peak AF = 100 mT) for a suite of samples from eleven of the sites. I R M curves decay rapidly either exponentially or sub-

1

J/Jo

i

BImT)

50

Fig. 4. Example of ARM ([3) and IRM ( o ) demagnetization curves from specimen U13-4-C. J/Jo = ratio of intensity to initial intensity of magnetization; B = AF demagnetization field. Both scales are linear.

exponentially, compared to more resistant A R M curves (Fig. 4). The large difference between I R M and A R M curves strongly suggests a bimodal grain size distribution between fine SD magnetite controlling A R M characteristics, and much larger MD magnetite which carries most of the I R M (Dunlop, 1983).

Susceptibility and intensity of remanence There is no correlation between intensity of natural remanent magnetization (NRM, usually dominated by SD magnetite in gabbros) or lowfield A R M and bulk susceptibility K. However, K has a positive correlation with saturation I R M and therefore with the total amount of M D magnetite present (Fig. 5). If the MD magnetite fraction has isotropic susceptibility, we would expect a negative correlation between I R M / A R M and the degree of anisotropy. No relationship is apparent, so MD magnetite grains dominate bulk susceptibility and contribute to the AMS. Anisotropy of magnetic remanence (AMR) A M R of single specimens from seventeen of the 22 sites were analyzed. The methodology

M.P. Bates, M.F. Mushayandebvu / Tectonophysics 242 (1995) 241-254

247

(RES) of the A M R ellipse were calculated as follows:

200

G O F = 100 x (root mean square residual) 150

•

"

/MRmean RES = 100 x (root means quare residual)

-f x

(4)

100

/ ( M R m a x - M R min)

5O

0

0

s'0

1;0

A 150

IRM (A/m) Fig. 5. P l o t o f specimen

intensity o f saturation

isothermal

r e m a n e n t magnetization (IRM) against specimen bulk susceptibility (K). T h e linear regression is marked by the sloping line; note the positive relationship between I R M and K (correlation coefficient 0.806).

employed is similar to that described by McCabe et al. (1985): (a) A F demagnetization at 100 mT; (b) m e a s u r e m e n t of the remaining base level remanence; (c) induction and m e a s u r e m e n t of lowfield A R M (ARM; B = 34662 nT, p e a k A F = 100 mT) in nine different orientations (after Girdler, 1961), with A F demagnetization at 100 m T between each treatment; (d) removal of the base level r e m a n e n c e from the measured A R M values to give the actual induced A R M in each orientation. Principal axes of the A M R tensor are calculated using the algorithms described by Girdler (1961) 1; a best fitting magnetic remanence (MR) ellipsoid is calculated from the nine A R M values, and new values for the nine orientations are derived from the best fitting ellipsoid. T h e difference between these calculated values and the observed values gives the residual values; the larger the residuals, the poorer the fit of the ellipse. Goodness of fit ( G O F ) and resolution

1 Note that element (1,9) of the 6 by 9 matrix on p. 40 of Girdler (1961) should in fact be - 2.

(5)

Both are expressed as percentages. G O F indicates m e a s u r e m e n t errors and noise, and should not be greater than a few percent (Jackson, 1991). Due to the low anisotropy, A M R from only one specimen (U3-4-1) failed this criterion ( G O F > 5%) but a second specimen (U3-5-2) provided a good fit. Also, a well-determined ellipsoid will have RES of less than 10% (Jackson, 1991). It has been found that M D magnetite gives rise to AMS and A M R with coincident principal axes (Jackson, 1991; Rochette et al., 1992). Conversely, maximum and minimum axes are reversed between AMS and A R M when SD magnetite controls the magnetic fabric. Of those A M R ellipsoids which are well resolved, four have all three axes coincident (from sites U1, -20, -21 and -22). A further four well-resolved A M R ellipsoids (from sites U6, -8, -16 and -19) have coincident or near coincident minimum axes only, with maximum and intermediate M R axes that are reversed with respect to the orientation of their equivalent AMS axes in two of the ellipsoids (from sites U8 and -19) and unrelated in any way in the other two. Of the less well resolved A M R ellipsoids, five have all three axes coincident (from sites U3, -4, -9, -13 and -14; RES = 23, 12, 11, 18 and 21%, respectively). Two more A M R ellipsoids (from sites U15 and -18; RES = 21 and 13%) have coincident or near coincident minimum axes only, with maximum and intermediate M R axes that are unrelated. The A M R ellipsoid from site U2 appears to have totally unrelated A M R and AMS axes, but the specimen has a G O F of 5% which is at the limit of acceptability. In all the above cases, either all three axes, or at least the minimum A M R and AMS axes are coincident, indicating that M D magnetite is the

248

M.P. Bates, M.F. Mushayandebt:u / Tectonophysics 242 (1995) 241-254

primary source of AMS in the Umvimeela Dyke. It is possible that the presence of significant SD magnetite is the cause of those instances where the maximum and intermediate axes are reversed, but the minimum axis always reflects the attitude of the M D grains. The only exception to this is site U17 (RES = 11%) where maximum, intermediate and minimum AMS axes coincide with intermediate, minimum and maximum A M R axes, respectively. As well as magnetite domain state, the question of paramagnetic minerals must be addressed. Such minerals may contribute significantly to AMS at sites U 1 - U 4 which have low bulk susceptibilities. The paramagnetic minerals present (pyroxene, phyllosilicates) have anisotropies and shapes both controlled by crystallography in such a way that their maximum susceptibilities will generally lie along the long axis of the grain (Rochette et al., 1992). The pyroxene is bronzite (A.H. Wilson and Prendergast, 1988), so the problems of AMS interpretation associated with orthoferrosilite (Weidenmann et al., 1986) are not encountered. Therefore, assuming that the paramagnetic minerals describe the same petrofabric as the magnetite, they will all contribute to the same AMS fabric. In other investigations, a positive correlation between bulk K and degree of anisotropy has been observed and ascribed to varying contributions to AMS from separate mineralogical sources (Rochette et al., 1992). No such correlation exists in this study. These observations lead us to conclude that K 1 axes reflect preferred directions of crystal alignment in M D magnetite, as has been assumed in previous studies of this kind, although the degree of anisotropy may be reduced by parallel aligned SD magnetite grains, and reinforced by a paramagnetic contribution in some sites.

3. Magnetic fabrics in dikes

The mechanism by which grain shape fabrics, and hence AMS, arises in an igneous intrusive is open to debate. Liquid m a g m a is generally assumed to be an incompressible Newtonian fluid

that moves by laminar flow (Shaw and Swanson, 1974; Delaney and Pollard, 1981). In a planar conduit such as a dike, flow direction will be in the plane of the conduit, laminae will usually be parallel to the conduit walls, and the maximum velocity gradient will be across the conduit and therefore normal to the laminae. However, as the m a g m a cools, the ratio of crystals to melt increases, the fluid is unlikely to remain Newtonian (Park et al., 1988) and may become a Bingham or pseudo-plastic material (Delaney and Pollard, 1981). Ultimately, the m a g m a becomes a solid. If magnetite crystals are formed when the m a g m a is still a Newtonian fluid, their orientation will be governed directly by the laminar flow. Alternatively, early forming inequant silicate crystals may become aligned in the fluid, and latestage magnetite fills interstices in this fabric when flow had ceased. Two contrasting models of particle behaviour in a fluid have been proposed. Jeffery (1922) suggests that prolate (cigar-shaped) ellipsoids become aligned such that their long axes are normal to the direction of flow and to the direction of maximum velocity gradient because they stop tumbling when their long axes become parallel to the direction of maximum vorticity. In addition, Happel and Brenner (1965) suggest that oblate (disc-shaped) ellipsoids align such that their short axes are normal to flow direction because in that orientation their tumbling rate is at a minimum. A combination of these two effects results in intermediate axes lying parallel to flow direction, a prediction that is consistent with the experimental work of Khan (1962) who found that AMS axes correspond to the physical shape axes of magnetite and that intermediate susceptibility and shape axes are parallel to flow. This led Ellwood (1978) to propose that any significant cluster of intermediate susceptibility axes should be used to define the direction of m a g m a flow. Owens (1974) predicts that in a deforming rock the fabric of inequant grains is determined by cumulative strain such that long axes align with the direction of maximum extension, and short axes align with the direction of maximum compression. This model may be applicable to the magma as it becomes more like a Bingham body.

M.P. Bates, M.F. Mushayandebvu / Tectonophysics 242 (1995) 241-254

Exact coincidence between grain and strain axes is not expected due to collisions between particles (Rees, 1968, 1979) and the tumbling motion of grains due to rotational (non-plane) strain (Willis, 1977). Knight and Walker (1988) inferred from Owen's work that "a mixture of prolate and oblate grains will develop a preferred orientation of long axes in the direction of flow and short axes normal to it in the plane of shear". Furthermore, they explain the tendency for maximum axes to become inclined to flow by up to 30 ° at dike margins due to a process of imbrication whereby grains become stacked up "like roof tiles" against the dike wall. Cadman et al. (1992) point out that

249

imbrication may occur across the entire dike width due to gradual accretion of grains onto the margins such that the conduit becomes progressively narrower. Modification of the fabric may occur as the magma becomes plastic, but continues to flow. For example, Duff (1975) suggests that long axes will realign normal to flow direction. Fabrics may be distorted or completely randomized by degassing, continued crystallization or oxidation (Sparks et al., 1977; Ellwood, 1978). High postemplacement strains due to later tectonic stress may lead to grain rotation, grain deformation or even complete recrystallization (Park et al., 1988).

D.S.

D.S. b.

•

i &

D.S.

°ll

~S.

d

A

Fig. 6. Equal area stereographic projections of site mean principal axes for groups of sites 1-4 (a-d), plotted in a dike strike reference frame such that dike strike (D.S.) is rotated to due north. Symbols as for Fig. 2.

250

M.P. Bates, M.F. Mushayandebvu / Tectonophysics 242 (1995) 241-254

In the particular case of columnar jointed basalts, Ellwood and Fisk (1977) show that thermal contraction resulting in horizontal strains may cause local rotation of grain short axes into the horizontal after crack initiation but before complete solidification, but in general short axes become vertical by grain rotation due to compaction by overburden (Park et al., 1988). Despite the problems of interpretation presented by the differing models proposed, there are important similarities between them. If a flow-related fabric is present in a dike, the direction of flow will be within the plane of the dike and normal to the alignment of grain short axes.

4. Interpretation of AMS Site mean AMS directions vary when viewed both in situ (Table 2) and in a dike strike reference frame (Fig. 6), and it is unlikely that the observed magnetic fabrics arise from a single large-scale post-solidification tectonic strain effect. This is compatible with the undisturbed nature of the G r e a t Dyke, with the notable exception of the northern S-bend. We conclude that the magnetic fabric is more likely to arise from processes operating at the time of dike emplacement. Adjacent sites often have similar K~ orientations (U2, -3 and -4; U5, -6 and -7; U l l and -12; U13 and -14; U16 and -17; U20, -21 and -22) and four groups of sites with specific characteristics can be identified. G r o u p 1, representing half the sites analyzed (e.g., Fig. 2c and e), have relatively steep K 3 axes (U1,-5,-8,-9,-10,-13,-15,-19,-20,-21 and-22, Fig. 6a). Their level of exposure along the length of the Umvimeela Dyke is more or less constant. If the sub-vertical K 3 axes of group 1 were due to compaction during cooling (Park et al., 1988) we might expect this fabric to be observed at all sites. The fact that it is not suggests that this pattern may arise during lateral movement. K~ and K 2 axes show a wide range of sub-horizontal directions, both parallel and perpendicular to the dike. U n d e r fluid conditions both K~ and K 2 axes align themselves parallel to flow; indeed Knight

and Walker (1988) observed both types of fabrics in coeval dikes in close proximity to each other. Add to this the fact that imbrication of the fabric during gradual accretion could occur across the whole width of a dike (Cadman et al., 1992) and the observed variation in K 1 and K 2 becomes readily explicable. Group 2 sites (e.g., Fig. 2b) have what are considered by many to be typical dike AMS patterns, with K 1 and K 2 defining a plane parallel to, and K 3 axes perpendicular to, the dike plane (U6, -7 and -18, Fig. 6b). However, the presence of both K1 and K 2 a x e s in the dike plane means that interpretation is ambiguous since both may be parallel to the azimuth of flow. The presence of imbrication would allow us to determine which process was operating, but this would require sampling from both sides of the dike which was not possible. Site U14 does not have any particular group affinity, but does have a relatively steep K 2 axis close to the plane of the dike like U6 and -7. K 1 and K 3 lie away from the dike plane at U14, so that the K 2 axis represents the likely azimuth of flow. Group 3 sites (e.g., Fig. 2d) have K I axes dipping steeply towards the southeast and can be split into two spatially related pairs ( U l l and -12, U16 and -17, Fig. 6c); although it should be noted that U l l and -12 are separated by an apparent break in the dike. There is no evidence for lateral flow at these sites, and as such they represent sectors of the dike where the flow trajectory was steep. As in group 2, K~ or K 2 may be the true indicator of flow azimuth. The dip of the K~ axes may reflect imbrication of the fabric relative to the dike walls, or a dip of the dike plane to the east. Gravity and magnetic profiling of the dike show that it has a vertical attitude for most of its length, but dips 80 ° east at its southern end (F. Podmore and M.F. Mushayandebvu, pers. commun.) which includes site U17. The group 4 sites (e.g., Fig. 2a) form a distinctive set at the northern end of the dike (U2, -3 and -4, Fig. 6d). K~ axes are near vertical coupled with near dike perpendicular K 2 axes. The obvious interpretation is that flow was vertical at this point in the dike. However, these sites occupy the S-bend where the Umvimeela Dyke has been

M.P. Bates, M.F. Mushayandebvu / Tectonophysics 242 (1995) 241-254

already steep, and the K 2 axes were rotated towards the plane of shear. A further complication at sites U1 to -4 is the possibility of a p a r a m a g n e t i c silicate contribution to A M S since susceptibility is of the o r d e r of 1 × 10 -3. Analysis of the p a r a m a g n e t i c minerals would require a detailed study of crystallographic orientation which is b e y o n d the scope of this study.

d e f o r m e d by lateral shearing along closely spaced E - W - s t r i k i n g vertical planes which resulted in a distinctive t o m b s t o n e surface morphology. Even t h o u g h sampling was carried out away f r o m shear zones, all four of the most northerly sites have susceptibilities an o r d e r of m a g n i t u d e less than the o t h e r sites, suggesting that alteration of the magnetic minerals has taken place. A l t h o u g h U1 has a g r o u p 1 pattern, it is also the only one of the four sites which has not b e e n totally remagnetized during the P a n - A f r i c a n O r o g e n y (Mushayandebvu, 1991) and has u n d e r g o n e a lower degree of alteration. F u r t h e r m o r e , the magnetic foliation containing K 1 and K 2 axes at each site is close to parallel to the shear planes, suggesting that rotation of magnetite grains has occurred. H o w e v e r , it should be n o t e d that d e e p - s e a t e d c h a m b e r s or lateral sill-like extensions have b e e n identified at the n o r t h e r n end of the G r e a t Dyke ( P o d m o r e and Wilson, 1987) which suggests a possible f e e d e r point to the U m v i m e e l a Dyke. Also, there is no obvious reason why g I axes would have b e e n aligned vertically by the shearing described above. T h e possibility therefore remains that d e f o r m a t i o n has only modified a preexisting flow p a t t e r n in which the K t axes were

S o u t h Chamber

WED I= I

J

I

~

I

_. --~-I

5. D i s c u s s i o n

A n overall interpretation of A M S results based on inferred m a g m a flow in the U m v i m e e l a Dyke is shown in Fig. 7. D u e to the inherent ambiguities of A M S interpretation, the solution is nonunique. Nevertheless, if flow was in the plane of the dike, and g 3 (minimum) axes of A M S are parallel to m i n i m u m grain shape axes of multid o m a i n magnetite and n o r m a l to flow directions, we can present a sensible geological scenario which can account for the A M S at all sites. T h e locations of steep flow are the likeliest points at which the dike was fed from below. T h e presence of several such points along the dike's length is not surprising given that the G r e a t Dyke

=

SEL l_

North

I I I I I I

Ii H

16

IF

0

MUS "~ ~

I I

:~

I I I I I

E

D

4rlEIN

DDI-- qI

DAR -;-

I

I I I I I I

Chamber

SEB

1

I I I I I

251

I

C

b--

B A

-ffq

100

I

I

km

Fig. 7. Schematic side profile of the Umvimeela Dyke. Individual segments of the dike are labelled A to J. Note that the gaps between sectors are only in the surface outcrop, and the dike may be continuous at depth. Flow azimuths are marked by lines in the plane of the dike determined from the AMS at 22 sites. Where the interpretation is ambiguous, two crossing lines are shown. The positions of the adjacent subchambers of the Great Dyke are indicated above the dike (abbreviations as for Fig. 1), and the likely points of magma injection and lateral flow in the Umvimeela Dyke are indicated below the dike. The vertical scale is arbitrary.

252

M.P. Bates, M.F. Mushayandebvu / Tectonophysics 242 (1995) 241-254

was developing contemporaneously close by to the east. It appears that different G r e a t Dyke subchambers may be associated with distinct feeder points to the Umvimeela Dyke. The interpretation of the most northerly sites is complicated by Pan-African deformation, but vertical emplacement is the preferred interpretation of the AMS at these sites (group 4 above); sectors A, B and C are fed from the northern end of the G r e a t Dyke from a point that may be associated either with the Darwendale Subchamber or possibly the Musengezi Subchamber which lies just to the north. Sector D is fed from a point related to the Sebakwe Subchamber, although the exact location of this feeder point is unclear due to a lack of data and ambiguity in the data from site U12. It is possible, but perhaps less likely, that this second feeder point also contributed m a g m a to the southern end of sector C. A third feeder point has fed sectors G to J; lateral flow in that part of the Umvimeela Dyke which extends beyond the southern tip of the G r e a t Dyke is consistent with the source of m a g m a lying within that part of the Umvimeela Dyke adjacent to the G r e a t Dyke. Whether or not the Selukwe and Wedza subchambers are in fact associated with separate sources for the Umvimeela Dyke is unclear due to a lack of data at their boundary. For the same reason it is not known if sectors F and E were fed from the north, south or below; lateral flow along the dike away from the source points means that different parts of the Umvimeela Dyke are not always adjacent to their related subchamber. Palaeomagnetic results from the Great Dyke and its satellites (McElhinny and Gough, 1963; Jones et al., 1975; Mushayandebvu, 1991) indicate that e m p l a c e m e n t of the different parts of the Umvimeela Dyke, and the adjacent subchambers of the G r e a t Dyke, were contemporaneous. This conclusion is tenuous in regard to the northern sector because only one site related to the Musengezi Subchamber (U1) has retained the primary remanence. It is important to note that although half the sites analyzed provided evidence for lateral m a g m a flow, the whole Umvimeela Dyke was not fed from one end and the distances of lateral

movement were up to a maximum of about 100 km. Given that feeder points can be localised relative to the length of a dike, care must be taken when postulating large distances of lateral m a g m a emplacement based on lateral flow fabrics in studies of this kind. Identification of the source area and the associated dike AMS fabrics (e.g., Ernst, 1990) is important when drawing such conclusions. If the separate feeder points of the Umvimeela Dyke are indeed associated with the separate subchambers of the G r e a t Dyke, geochemical and petrological analysis of the Umvimeela Dyke may provide evidence for the parental m a g m a of the different subchambers. Such information would be invaluable to further the understanding of the genesis of the G r e a t Dyke.

6. Conclusions M a g m a movement during emplacement of the Umvimeela Dyke is reflected in the magnetic fabric of the rock, with modification of the fabric during Pan-African deformation at the dikes' northern end. Flow azimuth is indicated by either K 1 o r K 2 axes at different locations along the dike. Three likely points of sub-vertical injection have been identified, each associated with an adjacent subchamber or chamber of the G r e a t Dyke. E m p l a c e m e n t from these points was contemporaneous at least in the central and southern portions of the Umvimeela Dyke. Much of the Umvimeela Dyke developed through lateral m a g m a motion away from the feeder points, with the actual distances of lateral flow limited to a maximum of 100 km.

Acknowledgments The authors would like to thank D.L. Jones, G. Djulov and D.J. Robertson for helpful comments and discussion. The reviews of G. Borradaile, J.K. Park, T. Engelder and G. Oertel are greatly appreciated. G.W. Pearce and R.E. Ernst kindly provided tensor averaging software.

M.P. Bates, M.F. Mushayandebvu / Tectonophysics 242 (1995) 241-254

Appendix The samples analyzed in this paper were originally collected for palaeomagnetic work in two separate studies. For clarity of presentation, the site numbers were changed from those used previously. The original site numbers and their source are listed below.

UI:US(1) U5:UR(1) U9:Q(2) U13:UI(1) U17:UN(1) U21:M(2)

U2:UU(1) U6:UQ(1) U10:UE(1) U14:O(2) U18:N(2) U22:L(2)

U3:UT(1) UT:UH(1) UII:UF(1) UI5:P(2) UI9:UP(1)

U4:UA(1) U8:UG(1) U12:UK(1) U16:UM(1) U20:UO(1)

(l) Mushayandebvu (1991); (2) Jones et al. (1975).

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