Fluid inclusions, planar elements and pseudotachylites in the basement rocks of the Vredefort structure, South Africa

Tectonophysics, 171 (1990) 169-183 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

169

Fluid inclusions, planar elements and pseudotachylites in the basement rocks of the Vredefort structure, South Africa A. FRICKE, 0, MEDENBACH and W. SCHREYER Institiit

ftirMineralogie, Ruhr-Universitdt Bochum, Universitiitstrasse 150, 4630 Bochum I (F.R.G.) (Revised

version accepted November 30,1988)

Abstract Fricke, A., Medenbach, 0. and Schreyer, W., 1990. Pluid inclusions, Planar elements and pseudotacbylites in the basement rocks of the Vredefort structure, South Africa. In: L.O. Nicolaysen and W.I_J. Reimold (Editors), Cryptoexplosions and Catastrophes in the Geological Record, with a Special Focus on the Vredefort Structure. Tectonophysics,

171: 169A83.

The basement rocks of the Vredefort structure are characterized by high-temperature static metamorphism, in the course of which a violent shock event occurred. Quartz in these rocks contains numerous fluid inclusions that consist of ahnost pure COa. Commonly, such fluid inclusions are aligned along planar elements in quartz that were formed during the shock event. Crystallographic orientations of the planar elements wefe used to estimate minimum shock pressures of 75-100 + 30 kbar. The fact that fluid inclusions are situated on planar elements indicates that the CO2 gas must have been available immediately after the shack event. Measurements of the homogenization temperatures of the fluid inclusions in samples from the Vredefort basement allow subdivision of the CO* inclusions into three groups. The main group (Type 1) representing nearly 99% of the COa inclusions homogenizes to liquid in the range 26 ‘-30°C and therefore has densities of between 0.6 and 0+7Jcm3. Inclusions of this most important gr5up are generally situated on planar elements. High-density (Type 2) and Iow&nsity (Type 3) inclusions occur isolated in coarse unsbocked quartz grains (without planar elements), in post-shock recrystallized quartz grains and in mafic minerals apparently unaffected by the shock event. Type-2 inclusions with densities of about 0.9 g/cm3 are believed to belong to an early generation which wns formed during the gram&e-facies metamorphism predating the static high-temperature Vredefort metamorphism. Low-density inclusions of Type 3 homogenizing to the gaseous state could be explained as a very late generation of inclusions formed under near-surface conditions. On the basis of the observation that Type-l in&sions are located on shock-induced planar elements, the time of entrapment must have been immediately after the shock event, and a minimum pressure for Co, between 2 and 5 kbar can be determined for this time within the entire basement based on independent temperature estimations. Theobservations that a multitude of Type-l inclusions has been found in a minute, millimetre-sized pseudotachylite vein produced by the shock event is explained by recrystallization of diaplectic glass. With this knowledge of microstructures in the Vredefort basement rocks it is suggested that the ubiquity of CO* and the shock event are related to each other.

Introduction The Vredefort Dome in South Africa, a nearly circular structure about 100 km in diameter, is one of the greatest cryptoexplosion structures on Earth (Bucher, 1963). A core area, consisting mainly of updomed Archaean basement, is rimmed by a ~-~~~~~~/~3.~~

Q 1930 Ebwier

S~~IIC~ Publishers B.V.

collar of unconformably overlying younger metasediments and volcanic rocks and an outer concentric synclinorium (I-&B and Molengra~f, 1925). According to Sawson (1976) the rocks of the core present a “vertical section through the crust”. The various basement rocks are dated at 2.8 Ma (Nicolaysen et al., 1963). The southeastern

170

A. FRICKE

part of the structure sediments.

cal observations undergone mum

is covered by younger

According

structure

a late static metamorphism

structure.

attained

This static the

(Nicolaysen

shock

of the

started

dated

has

with maxi-

in the centre

metamorphism event,

Karroo

and petrologi-

the entire Vredefort

temperatures

before

to geological

at

- 2.0

well

Inlandsee

Leucogranofels

zone of Outer

Granite

(ILG) Gneiss

followed (OGG).

ET AL.

by the

Lenses

separating

the ILG and OGG

are

Stepto

belonging

described

Steynskraal the

by

Metamorphic

samples

suitable

for

as Zone

(SMZ).

fluid

inclusion

to Most

the of

studies

Ma

et al., 1963) and, at least in the centre

of the structure,

continued

namic

metamorphism

event.

These

temporal

are discussed

in detail

and outlasted

associated and

with

spatial

by Schreyer

the dythe shock

relationships and Abraham

(1978) Schreyer et al. (1978), Schreyer Medenbach (1981) and Schreyer (1983a,b).

and

Features diagnostic of high shock pressures in the Vredefort structure include kink bands in biotite, extraordinary pseudotachylites, shatter cones,

TABLE

1

List of rock samples used for fluid inclusion- and planar element-studies. Nomenclature of zones after Stepto (1979): OGG = Outer

Granite

and

Gneiss;

ILG = Inlandsee

Leucogranofels; SMZ = Steynskraal Metamorphic Zone (small units within the ILG area). Localities are shown in Fig. 1 Sample

Rock type

Locality

Zone

VT 159

Granite

9

OGG

VT 508

Byroxenite

5

SMZ

planar elements in quartz crystals, and the presence of coesite and stishovite (Martini, 1978).

VT 709

Amphibole-pyroxenite

2

SMZ

VT 574

Hornblende-pyroxene

2

SMZ

There is a vigorous debate as to whether all these features strictly require an impact origin, or

VT 618

whether they might be related to shock released from within the Earth (e.g. during volcanic erup-

VT 593

tions). The present

paper focuses on investigations

of CO,-rich fluid inclusions sociated with shock-related

which are closely asfeatures in quartz, as

well as with rocks apparently unaffected by the shock event. Careful studies of these inclusions provide useful information on the metamorphic and fluid history of the Vredefort structure, and provide additional insight regarding the possible origin of the shock features. Microthermometrical methods

of

mafic bodies locally

have been

applied

to nearly

70 samples

granuhte VT 704 VT 606 VT 579 VT 608 VT 609 VT 620 VT 680 VT 586

Byroxene grant&e

1

Pyroxene amphibolite

1

Hypersthene-gneiss

Eyroxene gneiss Biotite gneiss

2

SMZ

4

SMZ

2

SMZ

2

SMZ

2

SMZ

2

SMZ

2

SMZ

2

SMZ

4

SMZ

2

SMZ

4

SMZ

4

SMZ

VT 524

4

SMZ

VT 530

4

SMZ

VT 595

2

SMZ

2

SMZ SMZ

VT 446 VT 523

VT 596

(

( Garnet paragneiss

from the core of the Vredefort structure. The phase changes induced in the fluid inclusions by cooling and heating were used to identify the components of the fluids and to determine their density, assuming that the inclusions have re-

VT 600

2

VT 605

2

SMZ

VT 617

2

SMZ

VT 642

2

SMZ

VT 700

5

SMZ

VT 121

1

ILG

mained closed systems with constant their formation.

VT 211

1

ILG

VT 142

7

ILG

VT 175

8

OGG

volumes

since

Samples studied Nearly 70 samples of the Vredefort basement were investigated, beginning below the unconformity against the overlying metasediments and extending towards the centre of the structure (Fig. 1). According to Stepto (1979) the crystalline core can be broadly divided into a central zone of

VT 186

3

ILG

4

ILG

VT 118

3

ILG

12945

1

ILG

Leek 24,OO

6

ILG

6

ILG

1

ILG

VT 679

Leek 25,7 VT 346B 8765

( Granitic gneiss

\ Eulite rock Biotite granite

10

OGG

FLUID

INCLUSIONS.

PLANAR

Fig. 1. Simplified

ELEMENTS

geological

sampling locations.

AND

map of the northwestern

The Steynskraal

Metamorphic

area is not shown on this map. I-quarry and Klipkraat; Sijverfontein

2-several

farms;

PSEIJDOTACHYLITES,

outcrops

I-Jagers d-Khprag

171

VREDEFOM

and central part of the Vredefort

structure (after Schreyer, 1983b) showing

Zone (Stepto, 1979) occurring as small units within the Inlandsee

Leucogranofels

on Beta Farm, outcrop near the northern end of the Inlandsee Pan, on the farms Inlandsee, on the farms Pretoriuskraal,

Vrede

Farm;

5-Steynskraal

Farm; P-Welbekend

Aankom,

Farm;

Farm; IO-Pedretti

(Table 1 and Fig. 1) are felsic plutonic and metamorphic rocks from the ILG and the SMZ, rich in quartz and with varying amounts of feldspar, mica, pyroxene, amphibole and garnet. Methods of investigation In addition to microthermometric measurements with a Chaixmeca heating and freezing stage (Poty et al., 1976) Raman spectrometry (Dhamelincourt et al., 1979) was used to investigate the fluid inclusions. The Raman spectral analyses were performed at the Free University of Amsterdam (Burke and Lustenhower, 1987). This Micro-Raman Laser Probe (Microdil 28) works with an Ar ion continous laser with a wavelength of 514 nm. The Raman effect is recorded by a multichannel (512) detector. For determination of planar element orientation in quartz a universal stage was used.

Goedverwacht

6-drillhole

and Helpmekaar;

on Leeuwkuil

quarry on Kopjeskraal

Farm;

3-Lincoln

7-Winddam

and Farm;

Farm.

Fluid inclusion studies In an unusu~ly large number of the investigated samples from the Vredefort basement CO, fluid inclusions are extremely abundant in quartz and they frequently decorate shock-induced planar elements (Schreyer and Medenbach, 1981). They also occur in coarse unshocked, relic quartz grains as well as in post-shock recrystallized quartz. Inclusions in other minerals are less abundant. The inclusions in quartz occur in various shapes and sizes as a function of the distance of the samples from the centre of the Vredefort structure. In the centre (Fig. 1, 1) they reach 10 pm in diameter and form excellent negative crystals or round cavities. Towards the rim (Fig. 1, 10) their average size decreases to 2 pm and the shape tends to become gradually more irregular. In the outermost part of the basement rocks they are mostly submicroscopic in size and form dust-like

A. FRICKE

172

aebll

a-1

Fig. 2. Characteristic

Raman

spectra

of two different

29so

2&a

29iaa

fluid inclusions

in pyroxene.

Eulite rock, locality

ET AL.

27Sa

1. a. CO,. b. CH,.

Analysis

by

E.A.J. Burke, Amsterdam.

decorations of the lamellae. At room temperature most of the inclusions from the various localities are two-phase liquid/gas inclusions, with a rapidly moving gas bubble. This gas bubble disappears during gentle heating. The two-phase inclusions in quartz melted in the very narrow range of - 56.6 k 0.5 o C, which indicates that they consist of pure CO, with negligible amounts of other components. This was confirmed by Raman microprobe analyses (Fig. 2a). No other components were found in the inclusions in quartz. In mafic minerals (e.g. in sample VT 524) the rare inclusions are not associated with microcracks or crystal planes and thus cannot be unam-

biguously related to the shock event. These inclusions have more variable compositions: pure CO, (Fig. 2a), pure CH, (Fig. 2b), and CO, with very small amounts of CH,. No N,-bearing inclusions were found in any of the samples studied. Fluid inclusions in the mafic minerals may also contain solid carbonates which were identified by Raman microprobe as magnesite in host pyroxenes and as dolomite in host amphiboles. In addition, isolated single-phase solid inclusions of carbonates occur in these same host minerals, with sizes up to 10 pm in diameter. This observation supports the suggestion that these carbonate-bearing inclusions are primary. Although so far unproven, these inclusions may represent one possible source of the

FLUID

INCLUSIONS,

PLANAR

ELEMENTS

AND

PSEUDOTACHYLITES,

CO, fluid-that of decomposition of carbonates during the shock event. In some samples from the outermost part of the Vredefort basement (Fig. 1, OGG, 10) three-phase water-CO, fluid inclusions were found located on planar features in quartz. The melting point of the H,O phase in these inclusions is 0 o C. The water is squeezed into sharp edges and corners of the irregularly shaped cavities, whereas the central part of the inclusion is occupied by the two-phase CO* assemblage. The regularly shaped inclusions in the rocks from the central part of the structure may contain only small amounts of such water which would remain undetected along the inclusion walls. Microthermometric measurements also determine the temperature of homogenization (Tu). This is the temperature at which the fluid in the inclusion leaves the univariant liquid/gas curve to form a homogeneous gas or liquid. Their further path in P-T space is then defined by the isochore representing the range of P-T conditions along which a fluid of that particular density could have been trapped (Schreyer and Medenbach, 1981, Figs. 16 and 17; Roedder, 1984, figs. 8.7 and 8.8). Most of the CO&h inclusions in our samples homogenize to liquid in the range 26 “-30 ’ C, i.e. close to the critical point of CO, (31’ C). A very distinct and sharp maximum is shown in Fig. 3a (sample 12945). This histogram of the inclusions in a granitic gneiss from locality 1, just at the centre of the structure, is also representative of nearly 80% of the samples taken from the basement rocks. It is obvious that inclusions situated on planar elements and those not related to microcracks have similar homogenization temperatures. The 25 histograms in Fig. 3 need little further explanation. However, some of the exceptional diagrams will be discussed in detail. The granitic gneiss VT 118 from locality 3, about 6 km east of the town of Vredefort, contains CO, inclusions in coarse, unshocked, relic quartz grains. Some inclusions in this sample also contain small amounts of additional H,O. The homogenization temperature of the two-phase CO, inclusions (Fig. 3 n) ranges from ll” to 29’ C (0.65-0.85 g/cm3). One inclusion in an unshocked relic quartz grain homogenized at - 2” C (0.93 g/cm3). Such obviously dif-

VREDEFORT

173

ferent but very rare Type-2 inclusions have been found in four samples of hypersthene gneisses, VT 620, VT 609 and VT 579 from locality 2 and VT 679 from locality 4 (Figs. 3 o-r). Some fluid inclusions (Type 3) in samples from these localities homogenize to vapour rather than to liquid. In sample VT 595 (Lot. 2) two isolated inclusions only 50 pm apart in the same garnet crystal homogenized to a liquid phase at 28” C and to a gaseous phase at 17’C respectively (Fig. 3 t), without any visible sign of leakage. Inclusions in quartz from this sample have homogenization temperatures of nearly 29 o C. The densities vary from 0.2 to 0.75 g/cm3 (Shepherd et al., 1985, fig. 6.17). We therefore conclude that samples from the core of the Vredefort structure contain three groups of CO, inclusions. Type 1 is the most abundant and most important group, with a density of about 0.6-0.8 g/cm3. The frequent occurrence of Type l-inclusions along planar elements is taken as evidence that they are related to the shock event. Type-2 inclusions are high-density inclusions (- 0.9 g/cm3); we suspect that the dense inclusions were formed during the grant&e-facies metamorphism (Touret, 1974) predating the static high-temperature Vredefort metamorphism. The low-density inclusions homogenizing to the gaseous state (Type 3) do not show any sign of leakage. A possible explanation for these inclusions is that they belong to a very late generation of inclusions formed under near-surface conditions, perhaps under the Karroo cover. Type-2 and Type-3 inclusions were never found on planar features. All types of CC& inclusions occur in coarse, unshocked relic quartz grains as well as in post-shock recrystallized quartz. Low-density CO, inclusions with additional CH, and pure CH, inclusions (demonstrated by Raman microprobe analysis) remain without phase transition during the freezing experiments. For this reason, it is not possible to give any P-T estimates for these types of inclusions. As mentioned above, the Type-l inclusions often decorate planar elements (see below) suggesting that they are secondary inclusions formed during the shattering event which opened fissures and allowed fluids to enter the broken crystals.

A. FRICKE

174

ET AL

feldspar microstructures in samples from the Vredefort core indicate that annealing and recrystallization of shocked quartz crystals was more rapid, so that the trapping of a fluid phase along fractures generated by shock is more a matter of minutes than of hours. This suggests that the fluid phase trapped in the cavities is the very fluid which percolated through the rock during the shock event. If one can obtain independent estimates on either pressure or temperature at the time of the entrapment of the fluid, the second variable prevailing at that time can be determined using the isochores. On the basis of the data of Bisschoff

The fracturing was caused by the shock event which developed the planar fractures. The fluid was trapped by healing and annealing of these deformation structures and then presumably remained unaffected throughout geological time. There is no evidence for subsequent necking or leakage. The shock event took place in a geological environment where at the same time a static metamorphism had reached peak temperatures of about 900 * C in the centre of the dome. Annealing experiments with shocked feldspars carried out by Ostertag and Stoffler (1982) showed that recrystallization of diaplectic plagiodase starts at 900 o C after 5 h and at 1000°C after 30 min. Quartz/

e

n

cl

VT 159

n 70

20

10

30 i.._n 10

15

20

25

30

TH [“Cl T, I”C1

n

bnt

A

120-

Leek

257

110 _ 100 -

10

15

10

15

20

25

20

25

T, [“Cl

90 30 80

TH [“Cl

h 70 -

nt

60 10

15

20 T,

25

[“Cl

30

50 10

10 -

15

20

25

30

T, i”Cl

d 30 20 lo-

pi 10

15

20

25

TH [“Cl Fig. 3. Frequency distribution of the homogenization temperatures (T,.,) measured on pure CO, fluid inclusions. n = number of measurements. See p. 176 for legend.

FLUID

INCLUSIONS,

PLANAR

ELEMENTS

AND

PSEUDOTACHYLITES,

175

VREDEFORT

tribution of garnet and newly formed cordierite. Pseudotachylites cross-cutting these rocks were formed while the static metamorphism was’operating. This observation shows that at this locality 5 kbar/700°C were the static conditions that predated and outlasted the shock event. For rocks from the centre of the structure Schenk (pers. commun., 1983) using pyroxene thermometry, found higher temperatures of between 875” and 900 o C. In the light of the Fe/(Fe + Mn + Mg)

(1969) temperatures between 500 o and 550 o C for the rocks near the unconformity of the collar have been deduced. Schreyer and Abraham (1978) estimated the conditions of static metamorphism in a homfelsed garnet granulite from the central part of the Vredefort structure (Fig. 1, 5) where former garnet-orthopyroxene granulites are changed into cordierite-hypersthene-garnet pseudomorphs, Pressures of - 5 kbar and temperatures of 700°C are estimated by the Mg/Fe dis-

m nk

10

15

20

25

20

25

30

20

25

30

20

25

30

30

T, [“Cl k

A

n

So Leek 24.0 LO-

10

3020loll,P1”m-wI 10

15

20

25

T, ["Cl

bi

0

15

T, ["Cl

n 90

VT620

80 70 60

1

L"o.VT121 3020 10 r 20

25 T,,

30

["Cl Fig. 3 (continued).

A. FRICKE

176

ratios

of eulites

from

(1978) calculated peratures

locality

pressures

of about

1, Schreyer

et al.

of 3-4 kbars and tem-

800°C.

of CO, inclusions lower

than

which

occur

The combination of the Type-l fluid inclusion densities with the P-T values of earlier workers

elements. Canada,

gives a fairly uniform of about 2-5 kbar

inclusions

lithostatic (Schreyer

1981, fig. 17; Roedder, In summary, fluid inclusions the rocks during

occurring

that the Type-l

along planar

of the Vredefort

Dome

CO,

features

in

were trapped

after a shock event

under

as secondary

those of the Vredefort

gations

somewhat

of granulite-facies

rocks

inclusions

and

on planar

Rocks of the Sudbury Complex which in some respects are similar with

quantities

1984, figs. 8.7 and 8.8).

we conclude

or immediately

trapping pressure and Medenbach,

which have densities

those

ET AL.

Dome, contain

varying

of CO,

salinity

inclusions.

and

mainly

H,O

only

small

Detailed

on rocks from the Sudbury

in to

investi-

complex

are in

progress. Planar elements

static metamorphic conditions of about 2-5 kbar (at a depth of 7-17 km in the crust). Compared to

The term “planar elements” is a collective for the parallel sets of deformation structures

basement rocks investigated worldwide, those of the Vredefort structure are unique in their richness

are closely spaced and crystallographically oriented in shocked quartz grains (Carter, 1965; Von

t n Lo 5 30 3 .o‘ P

I

VT595

n

=20-

n’ VT609

Lo

10-

30

i

\ q

T,l"Cl

n 1 "

VT679

50

30 20

I

10

T,[“CI

0 Fig. 3 (continued)

5

10

TJW

20

25

term that

FLUID

INCLUSIONS,

PLANAR

ELEMENTS

AND

F’SEUDOTACHYLITES,

Engelhardt and Bertsch, 1969; Stoffler, 1972). Subsequently, various authors have attempted to define and use more specific terms such as “planar features” (Carter, 1965), “shock lamellae” (Chao, 1968) and, “intragranular fractures” (Horn, 1978). For our description of the findings in the Vredefort structure we prefer to use the more neutral term “planar elements” because there is a complete transition between still-open “planar fractures” (Schreyer and Medenbach, 1981, fig. 9), “healed planar fractures” (Schreyer and Medenbath, 1981, fig. 10) and “fluid-decorated planes” within completely recrystallized new quartz fabrics (Schreyer and Medenbach, 1981, fig. 12). The orientations of planar elements may yield information on the pressures of shock metamorphism (Hbrz, 1968; Mtiller and Defourneau, 1968). The classification of Robertson (1975) provides

177

VREbEFORT

the following relationships between planar element orientations and minimum shock pressures. Type Type Type Type

A: B: C: D:

Planes Planes Planes Planes

parallel parallel parallel parallel

to to to to

(0001); 75 + 30 (1013); 100 f 30 (2241); 140 f 30 (1072); 160 f 30

As described above, planar elements occur frequently in all Vredefort basement rocks. In the outer part of the basement rocks (Fig. l., 10) only planes parallel to (0001) have been observed. Towards the centre, textures were modified by the post-shock static metamorphism. This caused recrystallization which started along planar elements (Wilshire, 1971) or at grain boundaries and finally lead to a totally recrystallized fabric. The planar elements may then cross-cut grain boundaries re-

yn 50

I

1

VT 175

LO

VT 680

30

80

20 10

60

t/-+A

3 I

2oTH2&CI FLUID

INCLUSIONS

homogenlzlng

m

10

20

laquld

in postshock

m

25

to the

on

planar

in

lss4

IVT

in garnet u!Jmu

Or opatite

5

10

20

T,'f°CI

25

quartz

groins

quartz or

I Leek

pseudotochyltte inclusions

pyroxene

(VT

5951.

amphibole

IVT

I VT

6801.

gas

phase

25.71

in garnet

5791 iVTS79.

INCLUSIONS

homogenizing

0

groins

elements

lenticular 5951

quartz

recrystallized

in 0 CO, -rich

FLUID

phase

in non-recrystallized

0 5

kbar kbar kbar kbar

to the

30 ra

Fig. 3 (continued).

in

quartz

or amphibole

IVT 606, VT 5791.

VT6061

178

A. FRICKE

ET AL.

Fig. 4. a. Completely recrystallized former single quartz crystal with planar elements that now intersect the grain boundaries of the newly formed fine-grained quartz fabric. The planar elements are mainly oriented parallel to {1073). Plane polarized light. Granitic gneiss 12945, locality 1 (Fig. 1). b. Same sample under crossed nicols.

gardless of the c~stallo~ap~~ o~entation of these newly formed grains (Figs. 4a and b). This was described in detail by Schreyer and Medenbach (1981). Although different inclusion trails are visible in the recrystallized fabric, in most cases their orientation relative to the former single quartz crystal is difficult to reconstruct. However, in some aggregates from the most intensely metamorphosed central part of the dome, incomplete recrystallization started from grain boundaries

while the central part remained unchanged. Such relics are characterized by undulatory extinction within the matrix of stress-free neoblasts (Fig. 5). In these cases, the orientation of planar elements could be measured relative to the c-axes of the old crystal with the universal stage. In other, fully recrystallized aggregates such measurements were also successful, assuming that the most prominent trails of inclusions represent the generally most abundant (0001) planes. Other attempts of orient-

FLUID

INCLUSIONS,

Fig. 5. Partly preserved

PLANAR

recrystallized

ELEMENTS

quartz

AND

fabric

of a former

in the core. Here the orientation

4, the new fabric

consisting

of many

PSEUDOTACHYLITES,

single crystal.

of the quartz lamellae

individual

grains

Granitic

An unusual granofels

pseudotachylite

veinlet

Relics of the old grain

with respect

is intersected

to the old crystal

by planar

gneiss 12945, locality

ing the planar sets were not in agreement with possible crystallographic directions in quartz. Using the same technique, planes parallel to {1073) and, rarely, parallel to {1072) in the central part of the Vredefort structure could be reconstructed (Figs. 6a and b). Thus, we conclude that dynamic pressures of 75-100 kbar were attained in the centre of the structure.

179

VREDEFORT

elements

exhibiting

undulatory

can be measured

of different

orientations.

Fig. 6. Reconstruction measurements. various

The

orientations

of the orientation aggregates histograms

show

frequency

of the planar the normal

c-axis of the relic quartz

grains

the granitic

lamellae

of universal

sent the angles between of the aggregates.

of the quartz the

elements. to planar

stage of the

The values reprefeatures

which are preserved

and the

in the core

a. Sample

12945, also shown in Fig. 5. b. In

gneiss

from

elements

{ lOi3)

are by far the most dominant.

site 1, planar

parallel

to

Crossed

nicols.

to be a thin pseudotachylite veinlet which dissects a leucogranofels in a sample from a shallow drillhole (27.4 m) on Leeuwkuil Farm (Fig. 1, 6). The rock consists of coarse crystals of perthitic K-

in a leuco-

by means

are

As in Fig.

1 (Fig. 1)

Abundant veins and dykes of extraordinary pseudotachylite ranging in width from 1 mm to > 20 m are present in the Vredefort terrain. Here we describe what appears macroscopically

in two recrystallized

extinction

directly.

LO 5p 60;70 -~. (1012J (oiizl

IlOill (IlflJ (5 .-.

(0111)

A. FRICKE

180

Fig. 7. a. Old quartz

gram located

on the boundary

of the CO,-rich

veinlet (lower part of the figure) is easily distinguished particularly

concentrated

b. Same area under

at the boundary

crossed

because

pseudotachylite

of the extreme

of the veinlet. Plane polarized

nicols. The coarse

recrystallization

of quartz

veinlet against

abundance

light. Leucogranofels, in the normal

the unaffected

of fluid inclusions Leek 25.7, locality

ET AL.

rock. Part of the

(5 vol%?), which are 4 (Fig. 1). See text.

rock and the very fine grain

size inside the

veinlet are remarkable.

feldspar (+ 2.5 mm) and quartz, forming a granular fabric, which is cross-cut by a veinlet approximately 1 cm wide consisting of the same minerals but with much smaller (@ 80 pm) grain sizes (Figs. 7 and 8). The contacts of the veinlet are curved, forming embayments into the neighbouring large grains which are obviously older. Most importantly, however, the grain

boundaries between quartz and K-feldspar of the old, coarse fabric extend fully undisturbed into the veinlet, where the grain sizes of both minerals decrease. Quartz is coarsely recrystallized (400 pm) outside the veinlet, but recrystallized to a much finer grain size (80 ,um) inside the veinlet. The fine-grained quartz within the veinlet contains a multitude of CO,-fluid inclusions (estimated at

FLUID

INCLUSIONS,

Fig. 8. Close-up

PLANAR

view of CO,

ELEMENTS

AND

fluid inclusions

PSEUDOTACHYLITES,

in fine-grained

quartz of the pseudotachylite polarized light.

to 5 vol%), whereas the coarser quartz outside contains only a few CO, inclusions. All fluid inclusions inside and outside the veinlet have the same homogenization temperature (28”-30 o C, Fig. 3f). Those outside are aligned, as usual, along former planar elements formed prior to recrystallization. Those inside are irregularly distributed (Fig. 3f). Compared to most other samples there is no difference in the homogenization temperatures of this particular sample. We interpret the above observations by assuming that the rock volume within the veinlet was transformed into a structureless, glassy, diaplectic state by the shock event. Because the grain boundaries of the earlier fabric are largely preserved, no movement or flow seems to have occured within the veinlet. Thus, one might use the term “in-situ pseudotachylite” for this feature. Subsequent heating of the diaplectic material led to the crystallization of the fine-grained, new fabric of both quartz and K-feldspar within the boundaries of the old grains. The abundance of fluid inclusions inside the veinlet seems to be linked to the high magnitude of shock pressure which must have been focused locally on this particular part of the rock, leading to the diaplectic nature of the quartz and feldspar. The occurrence of planar elements in quartz outside up

181

VREDEFORT

veinlet (Fig. 7) described

in the text. Plane

the veinlet shows that this quartz was able to retain its crystal structure thus indicating lower shock pressures outside the veinlet. The concentration of CO, fluid in the quartz parts of the veinlet might be caused by a preferential solution of CO, in the transient, diaplectic, glassy state formed by the shock. Whatever the exact process was, it is clear from these observations that the introduction of the CO, must have been closely linked, both in time and space, to the shock event. It should be clear that with the above attempt at a genetic discussion of this veinlet that we do not imp3 similar origins for the pseudotachylite at Vredefort and that at other occurrences elsewhere. Discussion

The controversy over the origin of the Vredefort structure has been reviewed, among others, by Schreyer (1983a, b). He pointed out that there is considerable geological evidence against the meteorite impact hypothesis - a hypothesis which would demand a rather unlikely time and space coincidence between the endogenic long-lasting and localized static metamorphism and an impact at precisely this site. In addition, a meteorite body would have struck the centre of the circular structure precisely after the updoming and while the

A. FRICKE

182

static metamorphism fluid inclusion ments

in support

studies

have

trapped

during

event.

The

still persisted.

studies

of an endogenic

shown

that

the

or immediately

filling

fluid

is dense

CO,,

to have

contrast,

Page1 and Poty (1975) described inclusions

generated on planar

These

inclusions after

of

argu-

origin.

unlikely

sity aqueous

been

The results

may yield additional

were

the

shock

(1975),

this points

those

observed

structure.

In

of 10 GPa (pressures

from

coesite

to stishovite.

in the

bath

(1981)

has

explosion capable nitude.

structure, data indithe subse-

quent trapping of CO, fluids took place at a depth of at least 7-15 km in the crust. Unresolved, ing the origin

centre

however, is the question concernof the CO, fluids. One explanation

pressures

of the

mini-

100 kbar), far above

from quartz

to coesite and

Schreyer

suggested

that

at the crust-mantle of generating

than

Vredefort

et al. (1986) have derived

the transition

enclosed

petrological event and

the

and {lOiO}.

scale of Robertson

to even higher

in

Carter

mum pressures

Charlevoix structure in Canada as clear evidence for an impact origin. The uniform density of the together with independent cates that the catastrophic

{1122}, {O&l}

low-den-

by impact.

fluids over the entire Vredefort

{lOi2},

to the barometric

is

which

elements

lel to {lOi3}, According

ET AL.

and

Meden-

a violent

boundary

shock pressures

CO,

might be of this mag-

Acknowledgements The authors are grateful to Dr. D. Stepto for providing many samples of the Vredefort basement.

Furthermore,

we thank

Professor

J. Touret

might be the breakdown of carbonates such as those included in some of the investigated miner-

and his team for the help with the Raman analyses and their interest in our studies. M. Crawford,

als. This, however, seems to be most unlikely, because of the scarcity of carbonates throughout the Vredefort structure. Values for CO, of S”C =

A.E. Shoch, U. Reimold.

- 21.1% (Hoefs, Unpubl.) are in reasonable ment with 613C values of CO, in granulites

agreeof the

deeper crust. This is supported by the model of Slawson (1976) in which the ILG forming the central core is regarded as deeper continental crust. It seems to be probable, therefore, that the fluids were remobilized by the shock event from existing inclusions in the Archaean granulite-type rocks. However, the questions about the unique richness and about the primary origin of the CO, remain unsolved. The model of a violent CO, explosion -perhaps as an additional source of the CO, fluid-could

also be taken into account.

If the Vredefort Structure was indeed formed by an endogenic process, high-magnitude shock waves must have been generated to deform minerals and to transform quartz to a diaplectic state and even to the high-pressure phases coesite and stishovite. For a long time there was no reliable evidence for the existence of an endogenic process which is capable of producing such deformation features. However, evidence for shock deformation in volcanic ejecta has been discussed by Carter et al. (1986) who described the presence of planar elements in quartz crystals with orientations paral-

Drs. Komor

as well as two anonymous reviewed and constructively script.

and Valley,

persons criticised

thoroughly the manu-

References Bisschoff,

A.A., 1969. The petrology

morphic parts

rocks

of the Potchefstroom

Pretoria, Bticher,

Dome

syncline.

and meta-

and the adjoining

D. SC. Thesis,

Univ.

243 pp.

W.H.,

within

of the igneous

in the Vredefort

1963. Cryptoexplosion

the earth?

structures

(“Astroblemes”

caused

or “Geoblemes”.)

from Am. J.

Sci., 261: 591-649. Burke, E.A.J. and Lustenhower, a multichannel the analysis Carter,

laser

N.L., 1965. Basal quartz

Carter,

N.L.,

caldera:

boundary

C.B.,

Possible

phenomena.

E.C.T.,

1968.

servations.

In:

B.M.

Metamorphism

Baltimore,

lamellae-a

C.A.

and

of volcanic

relevance Geology,

crite-

Rose,

ejecta

W.I.,

from

the

to Cretaceous/Tertiary 14: 380-383.

and

temperature

rocks-based French

to

Am. J. Sci., 263: 786-806.

Chesner,

Pressure

impact-metamorphosed Shock

deformation

deformation

of

(Microdil-28)

Chem. Gel., 61: 11-17.

of impactites.

Officer,

1986. Dynamic Toba

microprobe

of fluid inclusions.

rion for recognition

Chao,

W.J., 1987. The application

Raman

and

of Natural

histories

on petrographic N.M.

Short

Materials.

of ob-

(Editors),

Mono

Book,

pp. 5-138.

Dhamehncourt,

P., Beny, J.M., Dubessy,

Analyse

d’inclusions

Raman.

Bull. Mineral.,

fluides

J. and Poty, B., 1979.

ii la microsonde

102: 600-610.

Mole a effect

FLUID

INCLUSIONS,

Hall,

A.J.

and

mountain Orange Horn,

PLANAR

ELEMENTS

Molengraaff,

AND

G.A.F.,

land in the southern

1925.

Free State. Verh. Akad. Ries Bohrkem.

Heidelberg,

Vredefort

and the northern

Wet. Sect. 2, 24: 183 pp. und Ejektasequenz

measurements indices

In: B.M. French

of deformation

in experimentally

struc-

shock loaded

and N.M. Short (Editors),

of Natural

Materials.

Mono

Shock

Book,

Bal-

J.E.J.,

1978. Coesite

Dome, South Africa. Miiller,

W.F.

strukturen

and

and stishovite

Nature,

in the Vredefort

W.,

1968.

als Indikatoren

Untersuchung

Deformations-

ftir Stosswellen:

an Quarzkristallen.

Eine

Z. Geo-

L.O., Burger,

The origin isotopic Ostertag,

A.J. and Van Niekerk,

of the Vredefort

structure

C.B., 1963.

in the light of new

data. In: Gen. Assem. Int. Union Geod. Geophys.,

13th (Berkeley

shocked

D., 1982. Thermal feldspar

crystal.

annealing

of experi-

J. Geophys.

Res.,

87:

A457-463. of the Charlevoix Poty,

B., Leroy, pour

J. and

Mineral.,

(Quebec,

Jachimowicz,

studies

Canada). L., 1976.

des temperatures

de microthermom6trie

in rocks Forts&r.

Un nouvel

sous le microChaixmeca.

Bull.

99, 182-186. impact

of shock

structure.

Quebec.

metamorphism

at the

Bull. Geol. Sot. Am.,

86: 1630-1638. Roedder,

Inclusions

W., 1983a. Metamorphism

basement

(Reviews

Sot. Am., Washington,

of the Vredefort

in Mineralogy, DC.,

and fluid inclusions

Dome, South Africa:

to the origin of the structure.

644 pp.

J. Petrol.,

in the

Guidelines

24 (1): 26-47.

their

Petrol.,

of quartz

South

Africa.

in the

relations

to

68: 53-62.

O., 1981. CO,-rich

W., Stepto, D., Abraham,

Clinoeulite

(Magnesian

metamorphosed

fluid inclu-

in basement Contrib.

rocks of

Mineral.

Pet-

Contrib.

T.J., Rankin,

Practical

K. and Miiller, W.F., 1978.

clinoferrosilite)

iron formation

South Africa.

Guide

in eulysite

in the Vredefort

Mineral.

Petrol., 65: 351-361.

A.H. and Alderton,

to Fluid

of a

structure,

Inclusion

D.H.M.,

Studies.

1985. A

Blackie,

Glas-

gow, 239 pp. Slawson,

W.F.,

Stepto,

1976. Vredefort Geochim.

portion

core. A cross

Cosmochim.

D., 1979. A geological

central Stoffler,

minerals

I. Behavior

sion. Forts&r. Touret,

and geophysical Johannesburg, and

natural

Mineral.,

deformation Germany.

of the Ph.D.

642 pp. of rock-

experimental under

et fluides

Cristalline.

shock

shock compres-

structures Contrib.

carboniques.

de la Belgique,

In:

Geolo-

Liege, pp. 267-287.

Bertsch,

W., 1969. Shock in quartz

Mineral.

H.G., 1971. Pseudotachylite

South Africa.

study

49: 50-113.

granulite

W. and

of the

Structure.

transformation and

de la SociCtt Geologique

Von Engelhardt,

Wilshire,

Dome

of minerals

J., 1974. Facib

Centenaire

planar

by

section

Acta, 40: 117-121.

of the Vredefort

D., 1972. Deformation

forming

crater,

E., 1984. Fluid

Vol. 12.) Mineral. Schreyer,

Dome,

and

Mineral.

elements

gie des Domaines

P.B., 1975. Zones

Charlevoix

Africa,

of contact

granulites

rol., 77: 93-100.

processes.

la mesure

scope: l’installation Robertson,

structure

52: 479-486.

appareil

Contrib.

cordierite-

as products

basement

Thesis, Univ. Witwatersrand,

Pagel, M. and Poty, B., 1975. Fluid inclusion Mineral.,

South

W. and Medenbach,

upper crust?

Calif.). (Abstr.)

R., Stoffler,

mentally

structure,

sions along planar

Shepherd,

phys., 34: 483-504. Nicolaysen,

Schreyer,

K., 1978. Symplectitic assemblages

of pre-existing

pseudotachylite.

Krater

Gasexplosion?

70: 388-395.

W. and Abraham,

Vredefort

in Stidafrika:

oder einer irdischen

Naturwissenschaften, Schreyer,

Schreyer,

272: 715-717.

Defoumeaux,

in Quarz

experimentelle

W., 1983b. Der Vredefort-Dom

eines Riesenmeteoriten

the Vredefort

timore, pp. 243-263. Martini,

Schreyer,

metamorphism

tures and refractive Metamorphism

183

VREDEFORT

orthopyroxene-garnet

Thesis, Ruprecht-Karl-Univ.,

87 pp.

H&z, F., 1968. Statistical quartz.

The

Transvaal

W., 1978. Stosswellenmetamorphose

im Nordlinger

PSEUDOTACHYLITES,

J. Geol. 79: 195-206.

Petrol.,

from

induced the

Ries

20: 203-234.

from the Vredefort

Ring,