Aspects of the dynamic and thermal metamorphic history of the Vredefort cryptoexplosion structure: implications for its origin

313

~~c~o~~~~~~~c~, 192 (1991) 313-331 Elsevier Science Publishers B.V., Amsterdam

Aspects of the dynamic and thermal metamorphic history of the Vredefort c~toexplosion structure: implications for its origin R.J. Hart a, M.A.G. Andreoli b, W .U. Reimold ’ and M. Tredoux d uSchonland Research Centre, University of the Witwatersrand, P.O. Wits 2MO>Johannesburg, South Africa

*

b Atomic Energy Corporation of South Africa Ltd., P.O. Box 582, Pretoria OGOI,South Africa

’ Economic Geology Research Unit, Department of Geology, University of the Witwatersrand, P. 0. Wits 2050, Johannesburg, South Africa d Department of Geology, Uniuersiiy of Cape Town, Rondebosch 7700, Sou:h Africa

(Received May 4,199O; accepted December X1990)

ABSTRACT Hart, R.J., Andreoli, M.A.G., Reimold, W.U. and Tredoux, M., 1991. Aspects of the dynamic and thermal metamorphic history of the Vredefort cryptoexplosion structure: implications for its origin. Tectonophysics, 192: 313-331. The Vredefort structure is the oldest and the largest known ~~t~xplosion structure on earth. An appro~mately 36 km deep section through the Archean sialic crust and the overlying Precambrian strata of the Kaapvaal craton is exposed in the core of the structure. The geology presented in the exposed section includes all the principal metamorphic facies in the crust and records a long and complex thermo-tectonic history which dates back to at least 3.5 Ga. The petrographic and geological observations in the basement rocks indicate that there is a complex interrelationship between the Archean geology and the 2.0 Ga dynamic and thermal metamorphic overprint (some of which are postulated to be indicative of impact processes). The dynamic and thermal metamorphic effects do not increase progressively towards the centre of the structure as found at known impact structures. In particular, dynamic deformation phenomena such as pseudotachylite and planar features in quartz reach maximum intensity in the rocks close to the Vredefort discontinuity, a brittle-ductile shear zone which separates upper crustal amp~bo~te facies rocks from lower crustal gram&es. In certain other litholo~cal zones, deformation phenomena are noticeably absent or diminished. We suggest that changes in the physical and chemical properties of the rocks from margin to centre of the basement may account for the variation in the intensity of the 2.0 Ga thermal and dynamic metamorphic effects observed at Vredefort. In conclusion, our overall impression of the Vredefort structure is that it is a relic of an ancient meteorite impact crater, but that there were thermo-tectonic events which occurred both prior to and after the postulated impact event, which complicates the interpretation of its origin.

Inanition

The origin of the circa 2.0 Ga Vredefort cryptoexplosion structure is controversial and has been the subject of debate for several decades. The morphology of the structure resembles that of a large meteorite impact structure (Daly, 1947; Die&, 1961), and consists of a central core of uplifted Archean crystalline rocks surrounded by a prominent collar of Precambrian strata (Fig. 1). Dynamic metamorphic effects, which include the formation of shatter cones, pseudotachylite, high-

* Seconded from the Geological Survey of South Africa, Silverton, Pretoria, South Africa. 0040-1951/91/$03.50

0 1991 - Elsevier Science Publishers B.V.

pressure silica polymorphs (coesite and stishovite), and planar ~crodefo~ation features in quartz, are evident over wide areas of the basement and the collar strata (e.g., Shand, 1916; Hargraves, 1961; Carter, 1965, 1968; Manton, 1965; Martini, 1978; Lilly, 1981; Schreyer 1983; Reimold et al., 1985; Albat, 1988; Carter et al., 1990; Grieve et al., 1990). Proponents of an impact origin for cryptoexplosive structures see no feasible mechanism to create these dynamic metamorphic features other than by an extraterrestrial projectile striking the earth’s surface. Bisschoff (1982) relates the intense thermal metamorphism that has affected both the collar strata and the crystalline basement rocks to the occurrence of several alkali granite intrusives and

R.J. HART

314

basic igneous complexes within and around the structure. These alkali granite plutons and associated mafic rocks are believed to have intruded < 70 Ma before the formation of the Vredefort structure (Nicolaysen et al., 1963; Walraven et al., 1990). Bisschoff (1982) suggests that these intrusive bodies are genetically related to the Bushveld Igneous Complex. The strong thermal metamo~hism in the central core region of the structure (see Fig. 2) has been ascribed to a cryptic mafic pluton thought to exist beneath the surface in this region (Schreyer, 1983); this author also provides evidence that the thermal metamo~~sm in the core of the structure started before, but outlasted the shock metamorphism. These findings imply that the shock metamorphism was superimposed on an “ongoing”

ET AL.

thermal metamorphic event. The coincidence in time and space of the localized magmatism, and the shock metamo~hism was interpreted by Schreyer (1983) as indicative of an endogenous origin for the Vredefort structure. The one point of agreement amongst all investigators to date (e.g., Bisschoff, 1982; Schreyer, 1983; Grieve et al., 1990) has been that the intensity of both the shock and the thermal metamorphism increase progressively towards the centre of the structure. These conclusions were reached despite the fact that in the central region the evidence for shock metamo~hism has been largely destroyed by the thermal metamo~~sm. In this study we present a re-examination of the relative intensities and distribution of (a) the dynamic deformation phenomena and (b) the ther-

I --

VREDEFORT DISCONTINUITY

AND MAFIC SUPRACRUSTALS

- GREENSTONES

Fig. 1. Geological

map of the Vredefort

structure

and N-S

section

across

boundary

fault.

the structure

(after

Hart et al., 1990a).

SEBF=

southeast

METAMORPHIC

HISTORY

OF VREDEFORT

CRYPTOEXPLOSION

STRUCTURE

ma1 metamorphic effects, characteristic of the 2.0 Ga event, with depth across the Archean crystalline crust. Evidence is presented that the intensity of the thermal and dynamic metamorphic effects do not follow a simple progressive increase towards the centre of the st~cture, but that there is a complex inte~elations~p between the 2.0 Ga metamorphic overprint and the Archean geology. Regional geology The Vredefort structure appears to be roughly circular in shape (Fig. 1) but the southeast part is hidden beneath a cover of Paleozoic sediments, and the total geometry is still unresolved. Geophysical evidence indicates that the structure is pear shaped (Antoine et al,, 1990), tapering off to the southeast beneath the cover rock. Hart et al. (1990a) proposed that the Vredefort structure is asymmetrical and that it is intersected by a NESW trending, vertically dipping shear zone (see Fig. 1) termed the southeast boundary fault (SEBF). In the exposed north-west sector of the structure, the collar strata are overturned and dip at high angles (70” to 80”) inwards towards the core. Hart et al. (19~a) suggest that the dips in the basement in the northwest of the structure are consistent with those of the overlying strata, that is they are near-vertical, and there is no change in the dip from the northwest basement margin inwards towards the core, where the section is truncated by the SEBF (see cross section, Fig. 1). Hart et al. (1990a) postulate that a section through the entire crust and possibly into the upper mantle of the Kaapvaal craton is exposed in the northwest sector of the Vredefort structure. The geology of the structure to the southeast of the SEBF is different from the overturned section in the northwest. The limited exposure of basement in the southeast of the structure is granitegreenstones (see Fig. l), and is both chemically and mineralogically different from all the lithologies in the northwest. The attitude of the collar strata in the southeast is also different from the northwest in that they are right way up and dip at shallow angles (< 45 o ) away from the core of the structure (Roering et al., 1990).

315

In the overturned section of crystalline basement, which is approximately 25 km thick, two distinct geological terrains have been identified (Hart et al., 1981a, 1990a; Stepto, 1990): the upper part of the Vredefort basement {in contact with the overlying Witwatersrand strata) consists of a 3.0 Ga differentiated sequence of felsic rocks in amphibolite facies, termed the Outer Granite Gneiss (OGG). Just beneath the basement-sediment unconformity, the OGG is a coarse-grained, relatively homogeneous granite (Hart et al., 1990a). With depth, the OGG changes gradually to strongly layered and foliated trondhjemites and granodiorites. Beneath the OGG, a complex and heterogeneous high grade granulite terrain consisting of charnockites, granulites (mafic and felsic) and supracrustal rocks is exposed. Collectively this sequence is known as the Inlandsee Leucogranofels (ILG) terrain. The upper half of the ILG terrain, known as the charnockite zone (Hart et al., 1990a), extends in a broad belt roughly 2 to 4 km wide around the whole of the structure (Fig. 1). The charnockite zone consists of about equal proportions of leucogranofels and charnockitic gneisses. The rocks in the central core region of the structure (see Fig. 2), which represent the lower half of the ILG terrain, and the deepest rocks exposed at Vredefort consist of leucogranofels with discrete xenoliths of mafic and ultramafic granulites. Beneath the charnockite zone, mafic and ultramafic rocks constitute about 10% of the outcrop, and are mainly metasediments (Stepto, 1990). Borehole and gravity data indicate that the proportion of mafic and ultramafic rocks probably increases relative to the leucogranofels towards the centre of the structure. At locality OKD-1 (see Fig 2), the - 100 m core recovered from a borehole indicates that mafic and ultramafic rocks which are mainly metagabbro, metanorite and metapyroxenite, constitute up to 50% of the rock in this region. The whole of the ILG terrain exhibits a complex thermo-tectonic history dating back to at least 3.5 Ga (Stepto, 1979; Hart et al., 1981b) and shows evidence of several generations of folding and shearing. Available evidence indicates that the contact between the charnockite zone and the lower ILG terrain is gradational.

R.J. HART

316

ET AL.

-

Fig. 2. Locality map. The positions of traverse A and traverse B are indicated by a dot dashed line. The inset shows the localities of samples taken in the vicinity of the Vredefort discontinuity along traverse A. The position (distance from ba~ment-s~iment contact) of the remainder of the samples along traverse A and traverse B are given in Tables 1 and 2. The VNT traverse of Reimold et al. (1987, 1990) is shown as a dashed line. The cordierite gneisses studied by Schreyer (1983) are indicated by Locality C. The localities of the boreholes OKD-1, and Beta-2 are indicated by stars. The shaded area represents the charnockite zone.

Outer GraniteGneis Leucogranofels and

=

Main Road

Fig. 3. Schematic geological map of the Vredefort dis~ontin~ty, from the type area along traverse A (see Fig. 2).

317

METAMORPHIC HISTORY OF VREDEFORT CRYFTOEXPLOSION STRUCTURE

The OGG and the ILG terrains are separated by a brittle-ductile shear zone known as the Vredefort discontinuity, which is interpreted as a mid-crustal tectonic discontinuity (Hart et al., 1990a). Available evidence (Hart et al., 199Oa), indicates that the OGG was thrust on top of the ILG between 2.5 and 2.8 Ga. The geology of the discontinuity in the type area (see Fig. 2) is represented schematically in Fig. 3, and consists of a - 30 m wide zone of dolerite, pseudotachylite with clasts of dolerite, OGG and ILG rocks. The Vredefort discontinuity can be traced over a distance of about 8 km. Although it cannot be seen around the whole of the basement, its further extension has been inferred (see Fig. 1) because the same litholo~cal contrast (OGG-ILG) occurs throu~out the exposed part of the basement (Hart et al., 1990a). Seismic reflection data (Durrheim, 1986) indicates that the Vredefort discontinuity is a regional feature which occurs at a depth of about 8 km beneath the Witwatersrand basin. At Vredefort the discontinuity was uplifted and overturned into its present position near the centre of the structure during the 2.0 Ga Vredefort event. Near the centre of the structure there is a small (+ 4 m2) outcrop of a massive, relatively homogeneous biotite-rich granite, termed the Central Intrusive Granite (Stepto, 1979). A Rb-Sr whole rock age of 1950 i 60 Ma was obtained for these rocks by Hart et al. (1981b). A vertical borehole (locality BTA-2, Fig. 2) located on this outcrop indicates that this biotite-granite grades into the leucogranofels with depth (- 10 m). Such findings suggest that this body may represent a local “pocket” of anatectic melt rather than an intrusive rock (Stepto, 1979), and we propose hereafter to call it the Central Anatectic Granite (CAG). Sampling the basement

A petrographic investigation has been carried out on about 60 samples taken from two radial traverses, which extend from the uppermost parts of the OGG to the centre of the structure ( f 25 km beneath the basement-sediment contact; see Fig. 2). Samples were taken at about 0.5 km intervals along both traverses, except across the Vredefort continuity where the sample spacing

was about 50 m (Al3 to A40, see inset, Fig. 2). Most of our petrographic data pertain to traverse A, because it has better and more continuous exposure. Major and trace element data for the same samples have been reported by Hart et al. (1990a). Brief descriptions of the samples taken &long traverses A and B are given in Tables 1 and 2. The density of planar and non-planar microdeformation features in quartz and feldspar have been determined in the samples taken along traverse A and B following the technique outlined by Short (1969). These data as well as the degree of recrystallization of quartz and feldspar are presented as a function of distance from the granitesediment unconformity towards the centre of the structure in Fig. 4. Metamorphic

profiles across the basement

Thermal and dynamic metamorphic features from the Vredefort basement have been described in detail by numerous investigators. The thermal metamorphism manifests itself in the form of mineralogical and textural changes (mainly recrystallization of quartz) and the local formation of anatectic melt rocks (Bisschoff, 1982; Schreyer, 1983). Deformation phenomena related to dynamic metamo~~sm include pseudotachylite, planar features and fractures in quartz and feldspar, and kink-banding in biotite and amphibole (Carter, 1965; 1968; Schwarzman et al., 1983; Schreyer, 1983; Reimold et al., 1985, 1990a; Carter et al., 1990; Grieve et al., 1990). The nature of the deformation features in Vredefort quartz, either as bona fida planar deformation features (also referred to as planar elements) diagnostic of shock metamorphism (Alexopoulos et al., 1988) or as planar fractures of no diagnostic value, is still controversial. This controversy is discussed in detail by Carter et al. (1990), Grieve et al. (1990) and Reimold (1990). The nature and orientation of quartz microdeformations from Vredefort have also been the subject of several earlier publications, in particular work by Carter (1965, 1968) and Lilly (1981). The latter author concludes that the Vredefort record comprises two “shock” events that are separated in time by a thermal (annealing) event. Carter (1965,

TABLE

1

Petrography Sample

ET AL.

R.J. HART

318

of samples Lithology

Locality

along traverse

A *

Distance

Archean

from collar

mineral

Level of recrystallization

Comments

assemblage non-dir.

(km)

direct.

X-cut by late PT vein;

Al

granite-gneiss

0.8

Q+Pl+Kf+Bt

3

A2

granite-gneiss

1.4

Q+Pl+Kf+Ms+Bt

2

A3

granite-gneiss

2.0

Q+Pl+Kf+Bt

2

As above;

A4

granite-gneiss

2.3

Q+Pl+Kf+Bt

2

Saussur.

A5

granite-gneiss

2.45

Q+Pl+Kf+Bt

A6

granite-gneiss

3.1

Q+Pl+Kf+Bt

Al

granite-gneiss

3.5

Q+Pl+Kf+Bt

1

A8

leucogranite

3.8

Kf+Q+Pl+Bt

1

A9

granodiorite

4.5

Pl+Q+Kf+Bt

2-3

A10

granodiorite

4.9

Pl+Q+Kf+Bt

2-3

All

granodiorite

5.15

Pl+Q+Kf+Bt

2-3

Al2

granodiorite

6.0

Pl+Q+Kf+Bt

2-3

saussuritized 1

PI.

X-cut by late PT vein; rare Pl. feat. in Q. Bt chloritized. PI; Bt chloritized.

As A3. As A4.

Prominent

Opaque

kinking

of Bt.

phase (Mt?) expelled

from kinked biotite. Al3

granodiorite

6.5

PI + Q + Bt + Kf

Q almost completely

recrystalliz.;

Bt chloritiz. Al4

biotite-tonalite

6.6

Pl+Q+Bt>Kf

Al5

biotite-grano-

6.65

Pl + Q + Kf + Bt

2

As A13.

6.75

Pl+Q+Bt+Kf

2-3

Mt decorates

As A12; Bt expels opaques.

diorite Al6

biotite-granodiorite

some

PI. feat. in Q.

A17

leucogranofels

6.8

Kf+Q

Prominent

A18

(fLG) biotite-grano-

6.9

PI + Q + Bt + Kf

Some Pl. feat. in Q

7.0

Pl+Q+Bt+Kf+Opx

Deuteric

diorite Al9

chamockitic

PI. feat. in Q.

decorated

granodiorite

by Mt.

alteration

prominent

of Opx;

recrystallization

A20

granodiorite

7.1

Pl+Q+Bt+Kf

Vredefort

A21

leucogranofels

7.25

Kf+Q+Opx+Bt

Bt chloritized.

A22

(fLG) charnockitic

7.4

PI + Q + Opx + Bt

Monazite,

of Pl.

discontinuity

- 7.15 km.

granulite A23

charnockitic

zircons-rich

Mt decorates 1.55

Pl+Q+Opx+Kf>Bt

As A21. Mt decorations feat. of Q.

rock;

some Pl. feat.

granulite A24

granulite

7.7

Pl+Q+Kf+Opx>Bt

A25

charnockitic

7.8

Pl+Q+Kf+Opx

8.05

Q+Pl>Bt+Kf+Opx

2

in PI.

PT vein.

enderbite A26

biotitegranulite

A21

biotite-

Opx, Bt deuterically

altered;

PI is albite-oligoclase. 8.2

Q+Pl>Bt+Kf+Opx

As above;

8.4

Kf+Pl+Q+Cpx+Bt

Kf is mesoperthitic,

thin PT vein.

granuhte A28

salitegranulite

-oligoclase;

PI is albite

rich in accessories

(Mt, Zirc, Ap).

METAMORPHIC

HISTORY

TABLE

1 (~ntinu~)

Sample

Lithology

Locality

OF VREDEFORT

CRYFTOEXPLOStON

Distance

Archean

from collar

mineral

Level of recrystallization assemblage non-dir.

(km) A29

biotite-

319

STRUCTURE

direct.

Very narrow

2

Q+Kf+Pl+Opx+Bt

8.6

Comments

PT vein;

Mt decorations

granuhte

in Q and Pl;

Opx chloritized. A30

leucogranofels

Pt vein with spherulitic

2

Q+Kf+Pl>Bt+Opx(?)

8.75

recrys-

talliz.; blocky extinct. A31

Ieucogranofels

8.95

Q+Pl>Kf+Bt

2

BIocky extinction

A32

l~ucogranulite

9.1

Q+Kf+Pl>Opx+Bt

2

PT vein; occasional

in Q.

in Q;

some Mt in PI. feat. feat.; deuteric A33

leucogranofels

Mt decorates

2

Q+Kf+Pl>Bt

9.25

Mt in PI.

alter.

Pi. feat, very narrow

PT/microfractures. A34

leucogranulite

Retrogressed

2

Q+Pl+OpxtKf+Bt

9.5

Archaean

granu-

lite; very thin PT vein and microfractures. A35

leucogranofels

9.8

2

Kf+Q+Pl

Microfractures recrystall.

A36

pyroxene

gran-

9.9

Q+PL+Opx+Kf+Bt

2

Two PT veins; deuteric

10.1

Qi-Kf+PI>Opx+Bt

2

Retrograded

alteration

uhte A37 A38

Ieucogranofeis leucogranofels

initiate of Q.

10.4

2

Q+Kf+Pl>Opx+Bt

of Opx. granulite

with prom-

inent Mt(?)-d~orated

fractures.

Mt(?) decorates

inter-/intragran-

ular fractures;

Opx deuterically

altered. A39

leucogranulite

10.65

2

Q+Kf+Pl>Opx

Intragranular

fractures

Q recrystalliz.;

initiated

Opx deuterically

altered. A40

leucogranulite

10.7

2

Kf+Q+PizOpx+Bt

Incipient

fractures

a cryptic A41

mafic granuhte

10.9

2

HbI+Pl+Cpx+Opx+Bt+Op

Extensive

development

intragranular decorated A42

intermediate

11.1

2

Pl+~px+Cpx+Bt+Op

Fractures

granulite

by Op. decorated

Apatite

contains

leucogranofels

11.3

Kf+Q>Pl

2

Irregular/subparallel,

A44

leucogranulite

11.5

Q+Kf+Pl>Bt+Opx

2

Intragranular

A45

felsic granulite

11.7

Q+Pl+Opx+Kf+Bt

2

Retrogressed

intragranular

fractures.

11.9

PI + Opx + Q + Bt

2

Prominent

* Direct = directional; chnopyroxene; indicates

of Q.

Archean

with Op-decorated granuhte

by Op; U/Th.

fractures

cause recrystall.

intermediate

of inter/

fractures

A43

A46

in PI suggest

PT/microfractures.

fractures

granulite fractures. decorated

by OP. Q = quartz;

Mt = magnetite;

very minor mineral

PI = plagioclase; Zirc = zircon;

constituent;

Kf = K-feldspar;

Ap = apatite;

Pl. feat = planar

features;

1968) describes planar features and fractures from Vredefort quartz and stresses a predominance of basal {OOOl}orientations for shock lamellae. This is also found by Grieve et al. (1990) who report

Bt = biotite;

Op = Opaque

Opx = orthopyroxene.

minerals;

MS = muscovite;

hypersthene;

Cpx =

Hbl = hornblende;

>

PT = pseudotachylite.

that 90% of their analysed microdeformation features exhibit basal orientations. These authors conclude that Vredefort planar features are anomalous compared to those at other known

R.J. HART

320

impact structures, with respect to their relative distribution and preservation. Recorded shock pressures, calculated on the basis of planar feature orientation, are shown not to regularly decrease outwards as observed at other impact structures. However, Grieve et al. (1990) interpret this result as the consequence of post-shock recrystallization. They conclude that “although anomalous, the evidence from planar features in quartz is still consistent with the Vredefort structure being an eroded remnant of a large, complex impact structure”.

ET AL.

Reimold (1990) reviews the available evidence from microdeformations in Vredefort quartz and finds that Vredefort “features” are in their overwhelming majority different from bonafidu planar deformation features with respect to the criteria defined by Alexopoulos et al. (1988) that is sharpness, straightness, continuity, length and spacing. Reimold (1990) suggests that most Vredefort features are only subplanar and frequently can be identified as subplanar and planar fractures. These fractures are frequently annealed and only fluid inclusion trails remain. Clearly there is still a lack

TABLE 2 Petrography of samples along traverse E * Sample

Lithology

Distance from collar

Archean assemblage

(km)

Level of recrystalfization Non-dir.

Bl

granite-gneiss

0.1

Q+Pl+Kf

3

B2

Ieucogranite

0.5

QiKl+Bl+Bt

2

B3

granite-gneiss

1.2

Q+PI+Kf+Bt

3

B4 B5

biotite-granite biotite-granitegneiss

1.3 1.5

Q+Kf+Pl+Bt Q+Pl+Kf+Bt

2 2

l-2 1-2

B6

2.5

Q+Pl+Kf+Bt

1

2-3

2.7

Q+Kf+Pl+Bt+Ms

2-3

2.9

Q+Kf+Pl+Bt

2-3

3.1

Q+Kf+Pl+Bt

2

BlO

porphyritic granitegneiss porphyritic granitegneiss granitegneiss biotite-rich granitegneiss granite-gneiss

3.4

Q+Kf+Pl+Bt+Ms

1

2

Bll B12 B13

leucogranites granite-gneiss leucogranofels

3.9 4.7 5.1

Q+Kf>Pl+Bt+Ms Q+Kf+Pl+Bt Q+Pl-tKf

1

2 3 2

5.4

Q+Pl+Kf+Opx+Bt

B7 B8 B9

l-2

(I=) B14

chamockitic grant&e

Comments

Direct

2

Plagioclase is saussuritized; late Pl. feat. in secondary Q. Feldspar records an (Archean?) penetrative fabric. As above; late PI. feat in secondary Q grains; Bt chloritized. Scattered fractures (incipient PT veins?) cause recrystalhz of Kf.

Bt kinked and rimmed by opaques. Some hematite in PI feat. Bt kinked and rimmed by opaques. Several thin PT veins; Bt chloritized. Vredefort discontinuity Ubiquitous granophyric rims between Q and feldspars; deuteric alteration. Deuteric alteration; Mt decorates secondary Q; inter/intragranular fractures.

* Q = quartz; KF = K-feldspar; PI = plagioclase; Bt = biotite; Ms = muscovite; Opx = orthopyroxen; indicates very minor mineral constituents; PT = pseudotachylite; PI feat. = planar features in quartz.

Cpx = clinopyroxene;

z

METAMORPHIC

HISTORY

OF VREDEFORT

CRYPTOEXPLOSION

321

STRUCTURE

II

Quart2 (only planar

3-

I’

deformations)

Dislance from basement-sediment

E z

6

Quartz(onlyplanar

2

unconfoimay

(km)

deformations)

4

6

6

10

12

Fig. 4. Planar microdeformation counts in quartz and feldspar (corrected for recrystallization) in the samples taken along traverse A and traverse B plotted as a function of distance from the granite-sediment unconformity inwards towards the centre of the structure. The geological symbols are the same as Fig. 1. Note the double dashed line indicates a change in the horizontal (distance) scale.

R.J. HART

322

of consensus

on

the

microdeformation however, features

nature

features

there is a general are indicative

tion processes.

Vredefort

to

origin that

these

various

lithologies

in

across

indicates

that

the

seen

predates About

unconformity phic effects. (Reimold

et al., 1985)

and

there

are

only

few

subplanar or planar features in the quartz (Fig. 5a from locality A3, Fig. 2). However, metamorphic effects which could be ascribed to either or hydrothermal alteration are pronounced quartz and feldspar A. The

biotite

pervasively

from the outer parts of traverse

and

altered

thermal in both

plagioclase

(oligoclase)

to sericite, chlorite

are

and epidote

there

the

the quartz

rarely shows any dynamic metamorPseudotachylite is relatively rare

the dynamic

the thermal along

metamor-

traverse

A also

metamorphism. is a sharp

of the thermal

plagioclase the basement-sediment

cones, and

This relationship

2 to 3 km from the sediment-basement

acter&es

The OGG just beneath

shatter

predate

the rim

the dynamic

effects Outer Granite Gneiss

rocks

therefore, near

unconformity

basement.

these

metamorphism; phism

the distribution metamorphism

rocks in the collar strata exhibit are both cut by pseudotachylite.

of high strain rate deforma-

and the dynamic the

of the

at Vredefort;

agreement

We now describe

of the thermal relation

and

in quartz

ET AL.

decrease

metamorphism

upper

parts

of

lar recrystallization, has the coarse-grained

the

char-

OGG:

the

sericitized

and

the

is now only marginally has not suffered

in

that

significant

intragranu-

that is the quartz generally fabric typical of the primary

OGG. The occurrence of subplanar and planar features and fractures in quartz in this zone is also extremely

rare (Fig. 5b, from locality

This microphotograph zation

is restricted

planar

deformation.

A7, Fig. 2).

also shows that recrystallito a narrow These bands

band

along

are generally

a less

than 200 pm wide. Further

inwards

along

traverse

A, there

is a

(Fig. 5a, left). Large quartz grains (up to several millimetres in width) have been recrystallized to

gradual increase in the frequency of planar features and fractures in quartz grains towards the

aggregates of smaller grains of different sizes (up to 500 pm) which form a complex mosaic texture

more frequent

(Fig.

5a, right).

tallized

both

straight

The new quartz and

curviplanar

aggregates grain

have

boundaries

base of the OGG

(Fig.

(see Fig. 4). We also note

occurrence

quartz 5~). The

of aggregates

extending degree

along

planar

of recrystallization

the

of recrysfeatures (mea-

and do not show any preferred orientation. These effects are attributed to thermal metamorphism (contact metamorphism) caused by the alkali

sured as volume% of fine grained quartz along planar features) also increases towards the base of

granite intrusions shown in Fig. 1. Contact metamorphism is also clearly evident in the overlying

crystallized quartz observed near the base of the OGG differ from those observed in the upper 2 to

Witwatersrand

3 km of the OGG

sediments

as a high-temperature

(T = 500-600 o C) and intermediate pressure (P = 4 kbar) aureole around the alkali-granite intrusive (Bisschoff, 1982). While the alteration described above is clearly evident along traverse A, the samples close to the basement-sediment unconformity along traverse

the OGG.

However,

the characteristics

described

above;

of the re-

the former are

localized along planar features in quartz, whereas the latter are pervasive and extend across entire quartz grains. The recrystallization textures also differ between the upper and the lower parts of the OGG: the grain size of the recrystallized quartz along

the planar

features

in the lower

OGG

is

B, which occurs some distance away from the alkali granite (see Fig. 2), do not show any of

much smaller (rarely in excess of 80 pm) and the grain shape is generally more angular. The tex-

these alteration effects (see Fig. 4b). This observation favours the interpretation that the metamorphic effects recorded along the outer part of traverse A (as discussed above) are due to the effect of the alkali granite. It is important to note that both the alkali granite and the metamorphic

tures in the lower OGG are either well developed mosaics of equigranular grains, or more heterogeneous aggregates of fine-grained clusters and more elongated prismatic crystals. These crystals are often orientated parallel to the planar features. These textures are similar to those produced in

Fig. 5. Microphotographs

above the Vredefort

discontinuity.

formation

(d) Locality

A, showing

that here recrystallization

development of the quartz

quartz of planar along planar

along

features;

is extremely

rare.

in the ILG terrain

note

The scale-bars

the Vredefort

in discontinuity;

note

in the OGG

textures

crystals. in quartz

recrystallization

to rounded

on the left and

in Figs. Sa and 5c are 0.5 plagioclase

these features beneath

along

the different

subangular

note sericitized

inwards. OGG,

and recrystallization

planar in quartz

features features

features

margin in the upper

(up to 500 pm in diameter)

effects

from the granite to smaller of planar

80 pm) recrystallized development

effects 1): thermal has been recrystallized 1): extensive

(less than

grain

A26 (Fig. 2, Table 1): extensive

A15 (Fig. 2, Table

of fine-grained

quartz

of metamorphic A3 (Fig. 2, Table

the variation

is 1 mm. (a) Locality

the large primary

in Fig. 5a. (c) Locality

with that shown

from traverse

the scale-bar

on the right;

1): limited

quartz

comparison

(annealed)

A7 (Fig. 2, Table

altered

of selected samples

of the photographs

(b) Locality

thermally

mm, for the remainder

R.J. HART

324

dynamic recrystallization (stress annealing and syntectonic recrystallization) experiments (Hobbs 1968; e.g., plates IE and IIIC). They also frequently resemble the experimentally produced textures shown by Urai et al., (1986) and interpreted as an indication of grain boundary migration. About 500 m above the Vredefort discontinuity, there is a sudden further increase in the effects of dynamic metamorphism. Planar features in quartz and ~crodefo~ation in feldspar are ubiquitous in the gneisses above the discontinuity and the degree of rec~stal~ation along planar features reaches a maximum (Fig. 5c, locality A15, Fig. 2).

The Vredefort discontinuity consists almost entirely of pseudotachylite and brecciated country rock. The highest proportion of pseudotachy~te observed in the structure occurs in a zone approximately 3 km wide which straddles the discontinuity (Reimold et al., 1985). Note that the dolerite, which is the dominant rock type along the discontinuity in the type area along traverse A, does not contain any characteristic shock deformation features, but is extensively brecciated and cut by pseudotachylite. I_tG terrain

The highest frequency of planar features are observed in the quartz from chamockitic rocks just beneath the Vredefort discontinuity (Fig. 4). In contrast to the rocks above the discontinuity the degree. of r~~st~li~tion along microdeformations in the rocks from this zone is notably diminished and extremely localized (Fig. 5d from locality A29, Fig. 2). Thus most of the rocks from the charnockite zone are characterized by relatively high dynamic metamorphism, but a low degree of recrystallization. Towards the base of the charnockite zone, in the region where we first encounter the mafic granulites, there is a sharp decrease in the frequency of planar features in quartz and feldspar. Thus towards the base of the chamockite zone

ET AL.

there is a narrow zone of rocks that is characterized by both diminished dynamic metamorphic effects and a low degree of recrystallization. Similar observations to those made in this study about the distribution of planar features for rocks from the VNT traverse (see Fig. 2) are reported by Reimold (1987) and Reimold et al., (1990a): these authors also noticed a pronounced increase in the frequency of occurrence of planar features across the chamockite zone followed by a sharp decrease towards the base of this zone. Grieve et al. (1990) have examined the o~entation and dist~bution of planar features in the same samples as those investigated by Reimold (1987), but do not report any significant changes over the chamockite zone. We suggest that the reason for this apparent difference in observation and interpretation is that we are reporting the overall metamorphic textures in the rock, whereas Grieve et al. (1990) base their calculations on the crystallographic orientations of planar features in individual grains only. Note also that the sample density along the VNT traverse is much more sparse than along traverse A. The metasediment and metabasite xenoliths also exhibit only minor dynamic deformation features and are thus like the gneisses at the base of the charnockite zone. Schreyer (1983) notes that “at first sight the dynamic metamo~~sm is less evident in the mafic metamorphic suite”, although he claims that there are “drastic mineralogical changes” due to the 2.0 Ga thermal event in these rocks. The replacement of primary garnet (in a garnet paragneiss) by a symplectitic pseudomorph consisting of cordierite, orthopyroxene and secondary garnet (locality C, Fig. 2) provides evidence of a thermal metamorphic event with estimated conditions of 700 o C and 5 kbar (Schreyer 1983). However, our observations in the adjacent gneisses do not support Schreyer’s (1983) claim that this section of the crust (which includes the base of the chamockite zone) was subjected to a uniform and intense thermal event at - 2.0 Ga. Rather our data suggests that the thermal metamorphism (as characterized by the recrystallization of quartz) is extremely localized in this region and only occurs in a few samples along traverse A (see Fig. 4).

METAMORPHIC

HISTORY

OF VREDEFORT

CRYPTOEXPLOSION

325

STRUCTURE

Several sections of the core recovered from the borehole at locality OKD-1 (see Fig. 2), which is about 7 km from the Vredefort discontinuity, and about 3 km from locality C discussed above, consist of mafic or ultramafic granulites. These rocks are cut by a network of narrow (2-10 mm) granophyric veins, consisting of a fine-grained intergrowth of quartz, feldspar, biotite, and euhedral clinopyroxene and orthopyroxene. The granophyric veins do not exhibit any deformation textures, indicating that they post-date the 2.0 Ga dynamic metamorphic event. In the central core region of the structure, where the surface exposures consist mainly of leucogranofels, our observations are essentially the same as those made by Schreyer (1983). In thin section the large lenticular quartz crystals of the leucogranofels appear to have been recrystallized into aggregates of smaller quartz grains. Granophyric veins, similar to those described from the OKD-1 borehole, but consisting of quartz-alkali feldspar intergrowths, often occur marginal to the quartz aggregates. Relic quartz grains with planar features are observed in some of the samples; however, the density of the planar features in unannealed ILG is substantially less than that observed in the rocks from the charnockite zone (Fig. 4). Exposure of pseudotachylite in the core region of the structure is rare, but its recognition is hampered by the fact that the pseudotachylite in this region has been recrystallized and has the same pale colour as the host leuco~~ofels. Elsewhere in the basement, the pseudotachylite is invariably dark. We disagree with Schreyer’s (1983) statement that pseudotachylite is “never” found in the core region, as we have found scattered occurrences. The Centrat Anatectic Granite

Petrographic observation of the CAG recovered from the Beta-2 borehole (see Fig. 2) show no evidence of thermal metamorphism and only minor evidence of dynamic metamorphism. With depth, the borehole core grades into leucogranofels containing recrystallized quartz grains with occasional planar features in their unannealed cores. The leucogranofels is extensively cut by granophyric

veins with a similar mineralogy to the CAG (i.e. quartz, feldspar and biotite). The granophyric veins grade into a quartz-feldspar paragenesis away from the CAG. This observation tends to support the suggestion made by Schreyer (1983), that the granophyric melts are related to the CAG, and that they are probably - 1950 Ma old, which is the Rb-Sr age obtained for the CAG. The initial 87Sr/86Sr ratio of the CAG is consistent with a rock formed as a result of melting of leucogranofels at - 1950 Ma (Hart et al., 1981b). Relationship between the thermal and the dynamic metamorphic

events

We concur with Schreyer (1983) that widespread high temperature effects (characterized by the rec~stalli~tion of quartz and the occurrence of granophyric veins) are more apparent near the centre of the structure, but unlike Schreyer (1983) we find very little evidence to suggest that the rocks in the central core region have been subjected to a long and uniform period of intense heating that preceded and outlasted the shock event. The cordierite hornfelses at locality C are of particular importance to this argument, as pseudotachylite veins are reported to cut the garnet pseudomorphs (Scbreyer, 1983). This relationship provides the prima facies evidence that the thermal metamorphic event preceded the shock metamorphism. The observations made in this study on the other hand indicate that the gneisses adjacent to the cordierite homfelses have not been affected by a pervasive thermal metamorphic event; this suggests that the metamo~~sm at this locality is either extremely localized, or alternatively that the cordierite gneisses are in fact Archean rocks, rather than metamorphic rocks formed as a result of a - 2.0 Ga thermal rnet~o~~c event. Th-Pb and Rb-Sr isotopic data obtained by Hart et al. (1981b) on mafic granulite xenoliths, including cordierite garnet granulites from locality C (Fig. 2), give ages of - 3.5 Ga. It is most improbable that these rocks could have been intensely baked at 2.0 Ga without effect on the Rb-Sr and Th-Pb isotopic systematics. Therefore we conclude that the cordierite homfelses could well be of Archean age and their formation is not related to a 2.0 Ga

326

thermal event. Cordierite gneisses are not that uncommon in Archean high-grade metamorphic terrains and have been reported from other Archean granulite terrains in southern Africa, e.g., the Limpopo Mobile Belt (Van Reenen, 1983). If a thermal metamo~~c event preceded the shock event in the centre of the Vredefort structure, it presupposes the presence of a local heat source (Bisschoff, 1982; Schreyer, 1983). Although there is no direct evidence of this, Bisschoff (1982) suggested that the 40 mGa1 positive gravity anomaly in the centre of the structure (Maree, 1944; Stepto, 1990) provides evidence for a dense (mafic) intrusive body beneath the surface near the centre of the structure. A borehole located close to the peak of the gravity anomaly indicates that the central region of the structure is underlain by Archean ultramafic rocks, interpreted as upper mantle material by Hart et al. (1990b). These lithologies could adequately account for the gravity anomaly near the centre of the structure (Hart et al., 1990b), but would not supply a circa 2.0 Ga heat source required for a thermal metamo~~c event in this region. The only evidence of a thermal event in the core region of the structure is the recrystallized quartz and feldspar, and the local development of granophyric veins. Both these phenomena clearly post-date the shock event. The available evidence indicates that the CAG, which is interpreted as a small pocket of anatectic melt and also post-dates the shock event, is genetically related to the granophyric veins. The Rb-Sr age of 1950 f 60 Ma obtained for the CAG overlaps with the age of 2002 & 52 Ma (Walraven et al., 1990) quoted for the granophyre dykes. These latter rocks are thought by some (Dence, 1971; French et al., 1989; French and Nielsen, 1990) to be impact melts. It may be significant that the mineralogy of the granophyre dykes (quartz, plagioclase, Kfeldspar, orthopyroxene and biotite}, is similar to the mineralogy of the granophyric veins from the OKD-1 borehole. These observations suggest that the CAG and the granophyric veins may be related to the granophyre dykes. If these analogs are correct, and if we accept the interpretation of French et al. (1989), then it is possible that all the thermal effects that we observe in the core of the

R.J. HART

ET AL.

basement may be related to a single event such as an impact, and that the Rb-Sr age of - 1950 Ma obtained for the CAG may represent the time of crystallization of an impact melt. In contrast to the melt rocks and the thermal metamo~hism near the centre of the structure, the alkali granites and the related metamorphic aureoles in the collar strata clearly predate the shock event (p. 322). We suggest that the thermal metamorphism in the collar strata and in the upper parts of the OGG is related to the Bushveld Igneous Complex and its satellites, which predates the Vredefort event. Distribution of metamorphic phenomena in relation to Archean geology

The results of our observations show that the distribution patterns of the 2.0 Ga metamorphic features across the basement are more complex than previously described, and that there is a strong correlation of metamorphic effects with litholo~cal variation, in particular at the Vredefort discontinuity. This relationship is represented schematically in Fig. 6. The highest frequency of micro-deformation phenomena, in particular planar features in quartz, occur in the rocks close to the Vredefort discontinuity (see Fig. 6). However, the planar features above the discontinuity appear distinctly different to those below the discontinuity, as they are invariably decorated by a narrow band of recrystallized quartz. Below the discontinuity, this phenomenon is rarely observed. Thus there is a marked difference in the way the rocks above the discontinuity and below the discontinuity have been met~o~hosed at circa 2.0 Ga. The rocks beneath the discontinuity are anomalous, as they are the only rocks in the basement of Vredefort that have been dynamically metamorphosed, but are not recrystallized. This zone of anomalous rocks is about 2 km wide and roughly corresponds with the charnockite zone, although towards the base of the chamockite zone there is a narrow band of rocks with little evidence for either thermal or dynamic metamorphism. These rocks have the appearance of pristine Archean gneisses from any high-grade granulite terrain. From about

METAMORPHIC

HISTORY

OF VREDEFORT

CRYPTOEXPLOSION

STRUCTURE

327

Fig. 6. Schematic presentation of thermal and dynamic metamorphic features across the Vredefort structure. The star indicates the approximate locality of coesite and stishovite.

this point on towards the centre, the outcrop is extremely poor and all inte~retation is based on sporadic outcrop and some borehole cores. Both Schreyer (1983) and Grieve et al. (1990) postulate that the shock metamo~~sm peaks near the centre of the structure. In actual fact there is very little evidence for these claims, as most of the evidence has been obliterated by the thermal recrystallization, which is most apparent near the centre. In rocks where it is still possible to identify planar features in relic quartz grains, our observations are that the frequency of occurrence of planar features appear less than those observed close to the discontinuity. Above the Vredefort dis~ntinuity the frequency of planar features (and annealed planar features) falls off gradually towards the upper regions of the OGG and there is little evidence of dynamic deformation in the granite-gneisses just beneath the basement-sediment unconformity. Above this unconformity, deformation effects are once again clearly evident in the collar strata in

the form of coesite and stishovite, shattercones and planar features in quartz. The latter two features are ubiquitous throughout the collar strata (Hargraves, 1961; Manton, 1965; Lilly, 1981; Albat, 1988). Like the microdeformation features, the highest proportion of pseudotachylite in the basement rocks also occurs close to and along the Vredefort discontinuity (see Fig. 6). Pseudotachylite also has been reported from throu~out the Witwatersrand basin, generally located along faults, bedding planes and shears (Reimold et al., 1986; Fletcher and Reimold, 1989). Until recently all of the pseudotachylite was thought to have been formed 2.0 Ga ago. This assumption was largely based on the observation that the alkali granite intrusives are extensively cut by pseudotachylite veins, whereas the slightly younger granophyre dykes are pseudotachylite free (Nicolaysen et al., 1963). Recent evidence presented by Reimold et al. (1990b) indicates that the granophyre is also cut by pseudotachylite. The latter observation suggests that at

ET AL.

R.J. HART

least at some localities dates

the Vredefort

observations

the pseudotachylite

event.

Fletcher

Based

and

Reimold

clude that the pseudotachylite lated

to tectonic

tersrand

basin

processes

post-

on the above (1989)

at Vredefort active

conis re-

in the Witwa-

both prior to and after the forma-

tion of the Vredefort

the structure. uration

result of impact researchers Hart

Reimold,

Any model tions described occurrence

or referred

of shatter

polymorphs

and

of the structure

the following

to in this study:

cones,

planar

high-pressure

features,

which

thought to be indicative of impact the fact that the density of planar not increase but rather

towards reaches

to the Vredefort

key observa-

the centre

a maximum

(1) the silica are

all

processes; (2) features does

of the structure, in the rocks close

discontinuity,

interpreted

as a

brittle-ductile shear zone; (3) the general asymmetry of the structure and the fact that a considerable vertical section (possibly

36 km) is exposed in

Roering

which

it today is a alone. Several

1954; Pretorius, 1987;

that

is related shaped

the

origin

Kaapvaal

and

and

Colliston

to regional

the

3.0 Ga

1986;

Fletcher

et al., 1990;

postulated

at circa

that the config-

by erosion

Winter,

structure

processes

on the formation

must take into account

1987;

have

starting

followed

1989;

Vredefort Origin of the Vredefort structure

envisage

as we perceive

(e.g., Du Toit,

et al.,

1990)

structure.

We cannot

of the structure

ending

of

the

tectonic craton, with

the

Vredefort

event 2.0 Ga ago. Some of these authors

suggested

that

was caused and

that

the central

uplift

of the structure

as a result of compressional the thrusting

was northwards

Fletcher

and Reimold

(1989) suggest

tensive

development

of

shears and thrusts there

directed. that the ex-

pseudotachylite

zones in the Vredefort

and in the greater fact that

tectonics,

Witwatersrand

are more

than

along structure

basin,

and the

one generation

of

pseudotachylite, are consistent with a tectonic model for the evolution of the structure. The asymmetry the northwest

of the Vredefort

structure

tilt of the structure)

(i.e.

is consistent

the northwest sector of the structure; (4) the age relationships and the regional distribution of pseu-

with a northerly directed post-Vredefort tectonic event, and we suggest that the configuration of the

dotachylite;

structure

(5) the local occurrence

of melt rocks

and recrystallized rocks in the centre of the structure, which postdate the dynamic metamorphism, and (6) contact

metamorphic

rocks

in the collar

that we observe

combination Structural pressional

of impact evidence tectonics

today and

is the result tectonic

of a

processes.

for northward directed comin the form of faults and folds

strata which predate the dynamic metamorphism. The morphology of the structure together with

in the collar strata of the structure which postdate the Vredefort event was reported by Simpson

the occurrence

(1977). The distribution

(see 1 above) pact

origin

interpretations between

of the characteristic provide for the

strong Vredefort

are correct,

the thermal

shock features

evidence then

for an im-

structure.

If our

the relationship

and the shock metamorphism

the pseudotachylite the

evidence

postdates

that

and age relationships

as discussed some

of the

the shock event (Fletcher

1989) provide

strong

support

above

of

including

pseudotachylite and Reimold,

for this model.

in the core of the structure are also consistent with a single event such as impact. The occurrence of igneous and metamorphic rocks in the collar strata which predate the shock event is also not surprising considering the close proximity of some of the

Another problem we encounter in trying to justify an impact model for the Vredefort structure is that the distribution of the dynamic and thermal metamorphic phenomena, and in particu-

satellites of the Bushveld Igneous Complex, and therefore a localized heat source specifically related to the Vredefort structure is not required. The major problem for an impact model for the Vredefort structure is the configuration of the structure and the fact that a - 36 km section of crust is turned on edge in the northern sector of

from that observed at other known impact structures, e.g., Charlevoix (Robertson, 1975; Grieve et al., 1990). However, at Charlevoix the shock features are largely confined to one lithology (Robertson 1975) and if we accept the interpretation that the Vredefort structure has been tilted on edge and that a - 36 km section through the

lar the distribution

of planar

features,

is different

METAMORPHIC

HtSTORY

OF VREDti-VRT

CRYFTOEXPLOSION

STRUCTURE

Archean sialic crust and the overlying Precambrian strata is exposed (Hart et al., 1990a, 199Ob), then the anomalous distribution of planar features at Vredefort is perhaps not surprising considering the variation in lithology. The geology presented in the exposed section includes all the principal metamorphic facies in the crust, and there is a marked variation in mineralogy, chemistry (including volatile content), texture and density of the rocks from the margin to the centre of the structure and in particular across the Vredefort discontinuity (Stepto, 1979; Hart et al., 1981a; Schreyer, 1983; Hart et al., 1990a). Several workers (e.g., Kieffer et al., 1976; Lambert, 1977; Reimold and Stoffler, 1978; Schaal et al., 1979) have investigated the effect that changes in the physical and chemical properties of a rock may have on the relative intensities and types of shock deformation features that develop in a rock under high strain. Factors such as porosity, texture, grain size, mineralogy, temperature and volatile content have all been found to significantly affect the elastic properties of a rock and consequently the development of shock metamorphic effects. If the planar features found at Vredefort formed as a result of impact, then it would appear that a shock wave propagating through the earth’s crust could have passed through certain lithological layers leaving only minor evidence of its passage, while severely disrupting the rocks above and below. We suggest that the correlation of changes in the type and intensities of the rnet~o~~c phenomena with changes in lithologies observed at Vredefort indicate that the variations in the physical and chemical properties of the lithology may have had a strong control on the distribution of the metamorphic effects at Vredefort. Therefore the difference between the distribution of shock features at Vredefort compared to other impact sites cannot be used to discount an impact origin for the Vredefort structure. Concluding remarks

From the data presented in this study we cannot identify a point source for the propagation of shock, and models which propose centrally gener-

329

ated shock propagation either by impact (Die& 1961) or by hot spot (Nicolayson, 1985), must take into account that the distribution of the shock deformation features do not reach maximum intensity towards the centre of the structure, and therefore this criteria cannot be used to identify the origin of the Vredefort structure. Nonetheless, on the basis of the presence of characteristic shock phenomena we conclude that Vredefort does represents the eroded remnants of the oldest and largest known impact structure on earth. However we postulate that thermo-tectonic events both prior to and after the impact event have had a profound effect on the structure as we observe it today, which further complicates the interpretation of it’s Origin.

Acknowledgements The authors would like to thank Maarten de Wit and Rod Green for there helpful discussion on this paper. We are indebted to the Anglo American Corporation, of South Africa Limited for generously providing us with the boreholes in the Vredefort basement. One of the authors (W.U.R.) does not believe that ch~a~te~stic shock effects are confirmed at Vredefort, and therefore does not share all our conclusions. References Albat, H.M., 1988. Shatter cone/bedding inte~elations~p in the Vredefort structure: evidence for meteorite impact? S. Afr. J. Geol., 91(l): 106-113. Alexopoulos, J.S., Grieve, R.A.F. and Robertson, P.B., 1988. Microscopic lamellar deformation features in quartz: discriminative characteristics of shock generated varieties. Geology, 16: 796-799. Antoine, L.A., Nicolaysen, L.O. and Nicol, S.L., 1990. Processed and enhanced magnetic images over the Vredefort Structure and their inte~retation. T~tonophysi~, 171: 63-74. Bisschoff, A.A., 1982. Thermal metamorphism is the Vredefort dome. Trans. Geol. SK S. Afr., 85: 43-57. Carter, N.L., 1965. Basal quartz deformation lamellae-a criterion for recognition of impactites. Am. J. Sci., 263: 786-806. Carter, N.L., 1968. Dynamic deformation of quartz. In: B.M. French and N.M. Short, (Mitors), Shock Metamorphism of Natural Materials. Mono Book, Baltimore, Md, pp. 453474.

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