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Archean crustal structure of the Kaapvaal craton, South Africa – evidence from the Vredefort dome
Earth and Planetary Science Letters 206 (2003) 133^144 www.elsevier.com/locate/epsl
Archean crustal structure of the Kaapvaal craton, South Africa ^ evidence from the Vredefort dome Cristiano Lana a , Roger L. Gibson a; , Alexander F.M. Kisters b , W. Uwe Reimold a a
Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa b Department of Geology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa Received 2 August 2002; received in revised form 13 November 2002; accepted 19 November 2002
Abstract Crystalline Archean basement rocks in the core of the Vredefort dome present a profile through a substantial part of the middle and lower crust of the Kaapvaal craton. Previously, this profile has been subdivided into two terranes with allegedly distinct lithologies and tectonometamorphic histories that were juxtaposed along a crustal-scale Late Archean brittle^ductile thrust zone. Lithological and structural mapping across the dome indicates, however, that the basement lithologies share a common polyphase tectonic history culminating in high-grade metamorphism and melting at V3.1 Ga. No evidence was found of the postulated tectonic terrane boundary, but the alleged boundary does coincide with a 1^2 km wide transition zone between upper amphibolite facies migmatitic gneisses and more restitic granulite facies gneisses. The implications of these results for Archean regional tectonic models for the Kaapvaal craton are discussed. 3 2002 Elsevier Science B.V. All rights reserved. Keywords: Archean; ‘Conrad discontinuity’; granulite^amphibolite transition; mid-crust; Kaapvaal craton; tectonics; Vredefort dome
1. Introduction The Kaapvaal craton of southern Africa (Fig. 1) represents one of only two extensive and largely pristine mid-Archean crustal fragments
* Corresponding author. Tel.: +27-11-7176553 (Sec. 7176547); Fax: +27-11-3391697. E-mail addresses: [email protected] (R.L. Gibson), [email protected] (A.F.M. Kisters), [email protected] (W.U. Reimold).
on Earth (the other being the Pilbara craton, Australia). It is, thus, an important natural laboratory for the investigation of the processes and events responsible for the formation of the earliest continental lithosphere and, in the speci¢c case of the craton, also for the formation of some of the world’s most spectacular ore deposits. Recent syntheses by De Wit et al. [1] and Moser et al. [2] have attempted to reconcile the large ¢eld-based mapping, geochemical, geochronological and geophysical databases available for the craton into crustal- and lithospheric-scale evolutionary mod-
0012-821X / 02 / $ ^ see front matter 3 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 1 0 8 6 - 5
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els. De Wit et al. [1] envisaged a two-stage process in which the cratonic nucleus formed by initial tectonic imbrication of slabs of hydrated oceanic lithosphere which melted between V3.2 and 3.1 Ga; and was then modi¢ed and enlarged by Cordilleran-type subduction^accretion processes between V3.1 and 2.6 Ga. Based largely on geophysics, they identi¢ed numerous sub-domains within the craton which they interpreted as lithospheric-scale features that were juxtaposed along thrusts during this latter period. Moser et al. [2], on the other hand, focused on events leading up to 3.1 Ga and proposed that events between 3.2 and 3.1 Ga led to assembly of the crust and formation of a thick mantle root, thereby allowing the subsequent development of the large Late Archean sedimentary and volcanic basins that characterize the craton. An integral part of their model is the development of a 10 km thick mid-crustal layer of granitoid melts derived from tectonically thickened lower crust at 3.1 Ga concomitant with this crust^root coupling. Both De Wit et al. [1] and Moser et al. [2] utilized in their models evidence from a unique pro¢le through the Kaapvaal crust exposed in the Vredefort dome in the central parts of the craton (Fig. 1). In particular, they focused on a speci¢c zone within the high-grade gneiss complex that has been correlated in various studies with regional geophysical features elsewhere in the craton. Named the ‘Vredefort discontinuity’ by Hart et al. [3], this zone has been interpreted as a crustal-scale thrust of post-3.1 Ga age by Hart et al. [3] and De Wit et al. [1] and, more recently, as the intrusive contact between the 3.1 Ga old molten mid-crust and older lower crust by Moser et al. [2]. In this paper, we present new lithological and structural mapping in the Archean gneiss complex exposed in the Vredefort dome, from which we reassess the nature of the ‘Vredefort discontinuity’ and its implications for these models for the evolution of the Kaapvaal craton.
2. Regional geology The Vredefort dome is an V80 km wide structural feature located some 120 km southwest of
Johannesburg (Fig. 1). It comprises a 40 km wide core of high-grade Archean gneisses that is rimmed by an V20 km wide collar of subvertically dipping Late Archean to Paleoproterozoic supracrustal strata (Fig. 2). The dome is largely obscured in the south and southeast by Phanerozoic sedimentary strata and dolerite sills. Estimates of the total thickness of crust exposed in the dome range from V25 km [4,5] to V36 km [3,6], with the latter suggesting that the paleoMoho is exposed at surface in the center of the dome. Evidence of shock metamorphism, impactmelt breccias and unusually voluminous pseudotachylitic breccias in the dome indicate that doming occurred as a result of a large meteorite impact event 2.02 Ga ago (see reviews in Reimold and Gibson [7] ; Gibson and Reimold [8,9]). The crystalline Archean basement rocks in the core of the Vredefort dome comprise a complex high-grade metamorphic terrane that is dominated by medium- to coarse-grained tonalitic and trondhjemitic gneisses and migmatites, granitic to granodioritic syntectonic intrusions, and volumetrically minor xenoliths of pelitic and ironstone paragneisses and ma¢c gneisses (Figs. 2 and 3). In the southeast of the dome, a sequence of amphibolite-facies meta-komatiites and komatiitic metabasalts with subsidiary ironstones is exposed in an inlier beneath the Phanerozoic strata [10]. The metamorphic and structural features in the gneiss complex clearly predate the deposition of the supracrustal rocks exposed in the collar of the dome, the oldest of which comprise the 3.074 N 0.006 Ga [11] Dominion Group lavas. Most rocks in the core of the dome are migmatized, although, locally, remnants of non-migmatized gneisses are preserved (Fig. 2). The migmatites are predominantly stromatic but grade locally into nebulitic to schlieric types that, in turn, grade into weakly deformed to undeformed granites and granodiorites. The gradational relationships between the smaller granitoid bodies and the migmatite leucosomes indicate that the former are largely locally derived. In the outer parts of the core of the dome, biotite N hornblende are the ferromagnesian minerals in the migmatite mesosomes; however, these are largely replaced by orthopyroxene N clinopyroxene in the central
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parts, with the result that the rocks are more appropriately described as charnockites or enderbites. This transition from upper amphibolite-facies to granulite-facies towards the center of the dome is consistent with the increasing levels of exhumation expected in a structural dome. The transition itself is complex, with pyroxene-bearing charnockitic to enderbitic lithologies interleaved with biotite- and hornblende-bearing lithologies in a zone 1^2 km wide that is best exposed between the towns of Vredefort and Parys (Fig. 3). The granulites in the central parts of the dome are more restitic than the amphibolite-facies rocks further from the center, suggesting signi¢cant melt loss from the higher-grade rocks. Within a radius of 5 km of the center, however, the Archean assemblages are overprinted and partially to totally destroyed by a distinctive static high-temperature recrystallization caused by the combined e¡ects of shock and thermal metamorphism triggered by the 2.02 Ga impact event [12,13] that led to Stepto [14,15] classifying these rocks as granofelses rather than gneisses (Fig. 2). Peak metamorphic temperatures during the Archean event within the granulite zone appear to have reached 800^ 850‡C at a pressure of 0.5^0.55 GPa [12,16].
3. Previous structural-tectonic interpretations of the Archean gneiss complex Previous studies of the crystalline Archean basement rocks in the core of the Vredefort dome have been largely con¢ned to geochemical, petrographic and geochronological investigations of selected areas and traverses (e.g. [2,3,4,6,17, 18]). Apart from limited structural mapping in the central parts of the core by Stepto [15] and C. Simpson (unpublished), no coherent structural study of the crystalline rocks has previously been attempted. Despite this, a key feature of the interpretation of the gneiss complex has been its subdivision into two terranes separated by a major tectonic contact believed to be of regional signi¢cance in the craton [1,3,19]. Hart et al. [4] ¢rst suggested that rocks in the outer parts of the core (‘Outer Granite Gneiss’, or OGG) were distinct from those in the inner core
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Greenstone belt Archean granitoids and gneisses Major late Archean sedimentary basins
Colesberg Lineament
N
Johannesburg
Edge of the Kaapvaal craton Vredefort dome
Cape Town
500 km
Fig. 1. Schematic map of the Kaapvaal craton, based on geological, aeromagnetic and gravity data (after De Wit et al. [1]), showing the location of the Vredefort dome and Colesberg Lineament. Note the varying structural trends of greenstone belts on either side of this lineament.
(‘Inlandsee Leucogranofels’, or ILG) based on geochemical analyses obtained along three traverses across the dome. Furthermore, initial Rb/Sr, Th/Pb and Pb/Pb whole-rock geochronological results [3,4] suggested that the metamorphic climax in the OGG rocks was signi¢cantly older (V3.1 Ga) than that in the ILG rocks (V2.8 Ga). The juxtaposition of these two terranes was attributed by Hart et al. [3] to a post-metamorphic regional thrusting event, evidence for which was best preserved in a narrow northeast-trending zone extending for V8 km between Parys and Vredefort, which they named the ‘Vredefort discontinuity’ (Fig. 2). Hart et al. [3] described this contact between the OGG and ILG as a 40^100 m wide brittle^ductile shear zone which truncates the main ductile shear fabric and associated folds in the ILG. They maintained that the OGG along the contact displays a gneissic fabric roughly parallel to this shear zone and that the presence of thin slivers of ILG lithologies in the OGG rocks up to 200 m from the contact is related to tectonic inter¢ngering caused by this thrusting. The age of the shear zone was estimated at between 2.8 Ga (that is, after the attainment of the metamorphic peak in the ILG) and 2.56 Ga (U/Pb single zircon age for an undeformed and unmetamorphosed
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C. Lana et al. / Earth and Planetary Science Letters 206 (2003) 133^144 27 O10’ 27 10’EE
27 O30’ E
Phanerozoic sediments Impact-related granofels 30
30
26 O45’ S
O
26 45’ 26 45’SS 30
r ve Ri al a V
Archean supracrustal strata
45
Granite Foliated porphyritic granodiorite
nT 0 nT 40 00 8
80
0
Amphibolite-facies trondhjemitic migmatitic gneiss (OGG)
nT T 0n
40
Charnockitic to enderbitic migmatitic gneiss (ILG) Tonalitic and trondhjemitic gneisses
VREDEFORT
Greenstones (amphibolite-facies) Subvertical to vertical fabric Fabric orientation S2 trend S3 trend
N
Magnetic anomaly (- 400 and - 800 nT contours) O
O
0
10 km
Trace of the alleged Vredefort discontinuity (after Hart et al. [3])
27 O30’ E
Fig. 2. Simpli¢ed geological map of the Vredefort dome showing the main lithologies and structures of the Archean gneiss complex in the core of the dome and the negative aeromagnetic anomaly [21]. The position of the structural discontinuity inferred by Hart et al. [3] is also indicated.
dolerite dike in the vicinity of the contact; Fig. 3) [3,18]. More recently, Moser et al. [2] suggested that the OGG^ILG contact is intrusive ; however, they also stated that it was deformed, although they failed to indicate if this deformation was similar to that described by Hart et al. [3]. Signi¢cantly, they, as well as Kamo et al. [20] and Hart et al. [18], obtained U/Pb single zircon metamorphic ages of V3.1 Ga from ILG rocks, contradicting Hart et al.’s [3,4] proposal that metamorphism in the ILG was up to 300 million years younger than in the OGG. The exposures between Parys and Vredefort correspond roughly to a broad negative magnetic anomaly that displays an arcuate pattern in the core of the dome (Fig. 2) [3,21]. In the absence of signi¢cant outcrop in large parts of the core of the
dome, Hart et al. [3,21] used this pattern to extend the discontinuity southeast of Parys and south of Vredefort (Fig. 2), where it disappears beneath the Phanerozoic sedimentary cover.
4. Structural geology The gneisses and migmatites in the core of the dome record a polyphase deformational history that preceded the deposition of the 3.07 Ga Dominion Group volcanics. Structures related to at least three main phases of ductile deformation (D1, D2, D3) are visible, although the D1 features have been largely transposed by D2 and D3 deformation. Field relations indicate that the D2 and D3 events overlapped the metamorphic
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Phanerozoic sediments
137
Amphibolite-facies trondhjemitic migmatitic gneiss (OGG)
supracrustal strata strata Archean supracrustal Granulite-amphibolite transition zone
Fabric orientation Vertical fabric
Pt Dol g
S2 trend S3 trend Pseudotachylite Dolerite dike
Charnockitic to enderbitic migmatitic gneiss (ILG)
85
75 85
75 75
85
88
Late-D3 granitoids
85
g
75
75 75
80 80 80 70
80 85
85
50
Parys
75
80
64 75 80
80 50 50
85
55
g
85
70
45
g
70
80 g
A)
Ironstone and pelitic granulites
78
85
75
85
78
80 75
85
N
75
75
85
85 75
75
85
80
75
g
80 85
80
85
70 85 70 85 80 80 70 75 80 g 70
g
70
75
75
g
70
70
85
85
Pt + Dol 85
80 80 75 75 85
85
85
70 80 77 75
75
80
82 85
80 77
80
77 80
82
85 75
75
85
75
80
N
75 80 g
85 88
N
58 85 75
88
75
87
80 88
76 85
Vredefort
82 80
75 75
0
75
75
86
75
85 80 75
77
C)
75 80 75 80
2 km
Poles to S2 N = 24 Poles to S3 N = 41
Poles to S2 N = 21 Poles to S3 N = 47
Fig. 3. (A) Structural and lithological map of the NW sector of the core of the dome, showing the dominant S3 fabric, the amphibolite^granulite transition zone and the locations of late-D3 granitoids and major pseudotachylite occurrences in the vicinity of this zone. (B) Lower hemisphere equal-area stereographic projection of poles to S2 and S3 in the ILG, showing the strong similarity in fabric orientations. (C) Lower hemisphere equal-area stereographic projection of poles to S2 and S3 in the OGG. The steeper dip of S2 in the OGG is related, at least in part, to rotation associated with formation of the dome.
peak and accompanying anatexis. The D2 and D3 structures are cut by pseudotachylite veins and dikes that are found throughout the wider environs of the dome [22] and that are related to the 2.02 Ga Vredefort impact event [20]. 4.1. Structures in the Outer Granite Gneiss Structures related to the earliest recognizable
deformation event (D1) are best preserved in the northeastern sector of the dome. S1 is a gneissic foliation locally preserved in centimeter- to meterscale isoclinal folds within the subhorizontal S1/ S2 transposition fabric, or in trondhjemitic xenoliths in the migmatites. In places, quartz^plagioclase leucosomes occur parallel to S1 but, more typically, they cut S1. The dominant fabric in the migmatites and gneisses in the outer parts of the
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A)
B)
10 cm
C) 20 cm
D)
Fig. 4. (A) Banded trondhjemitic gneiss with stromatic plagioclase^quartz leucosome (bottom) parallel to subhorizontal S2 foliation. Otavi quarry, northeast of Parys. (B) OGG migmatite with subvertical S3 foliation a¡ected by asymmetric extensional shear bands. Boudins of trondhjemitic gneiss (dark gray) lie in a banded granitic^trondjhemitic gneiss matrix. Locality V1 km NW of Vredefort. (C) Folded S2 foliation in subvertical S3 high-strain zone in trondhjemitic gneiss. The fold plunges shallowly to the NW and is crosscut by granitic leucosome that displays a weak S3 fabric. Locality 4 km SW of Parys. (D) Disaggregated ma¢c granulite in charnockite (left) cut by granite (right) in the granulite^amphibolite transition zone V1 km north of Vredefort. Both the charnockite and granite display a weak S3 foliation.
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gneiss complex is S2 (Figs. 2 and 4A). S2 has a subhorizontal to moderately dipping orientation where it is not a¡ected by D3 or by doming-related rotation close to the contact with the Dominion Group (Fig. 2) [23]. In the northwestern sector of the dome, the gneisses and migmatites are dominated by a northwest-trending subvertical S3 fabric (Fig. 4B) that is de¢ned by centimeter- to decimeterscale stromatic migmatitic layering that occurs in zones up to 500 m wide (Figs. 2 and 3). Chocolate-tablet boudinage of amphibolite sheets, and centimeter- to meter-scale conjugate extensional shear bands developed both in plan-view as well as in cross-section (Fig. 4B), indicate a large component of £attening strain within these zones. No consistent shear sense asymmetry was detected along the S3 fabric. In the western sector of the dome (Fig. 2), a porphyritic granodiorite shows alignment of euhedral K-feldspar megacrysts parallel to the S2 fabric in the adjacent migmatites, suggesting crystallization during D2. However, in the northwest, the migmatites in the OGG commonly grade into schlieric and homogeneous granites that display a variably developed S3 foliation de¢ned by aligned K-feldspar megacrysts in an undeformed matrix, consistent with crystallization having straddled the termination of D3 deformation. 4.2. Structures in the Inlandsee Leucogranofels zone The S1/S2 transposition fabric is also preserved locally in the granulite-facies rocks in the center of the dome, but this zone is dominated by the subvertical, NW-trending, S3 fabric (Figs. 2 and 3). Two large outcrops of ironstone east of Vredefort town preserve S2 in kilometer-scale, open to tight, D3 folds (Fig. 3). The D3 deformation is expressed either as discrete, centimeter- to decimeter-wide high-strain zones, between which the S1/S2 fabric is deformed into open to tight, upright, shallow to moderately NW- or SE-plunging folds (Fig. 4C), or as up to 100 m wide zones of highly strained gneisses in which S1/S2 is completely transposed. The S3 high-strain zones are characterized by stromatic
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trondhjemitic to granitic leucosomes and tonalitic to trondhjemitic mesosomes. The S3 stromatic fabric is sinuous, and locally displays more northerly and westerly trends (Fig. 3). It is locally cut by decimeter- to meter-wide, high-temperature retrograde mylonitic zones, which are parallel to sub-parallel to S3, and by conjugate extensional shear bands along which granitic leucosomes are commonly localized. Macroscopic mineral stretching lineations are conspicuously absent in the stromatic migmatitic bands, but oriented thin sections of samples from the mylonitic shear zones indicate that some pyroxene and biotite grains de¢ne a shallow (10^20‡) NW-plunging mineral lineation. The parallelism between S3 and the mylonitic shear zones, together with the high-grade mineral assemblages in the shear zones, indicates that the mylonitization probably occurred during the waning stages of D3, shortly after the metamorphic peak. Stringer- or pod-like granitoid bodies intrude predominantly parallel to S3, but they may also locally cut the S3 fabric (Fig. 4C). The granitoid stringers typically contain a weak planar S3 fabric de¢ned by aligned K-feldspar megacrysts and £attened quartz aggregates, indicating late syn-D3 crystallization. 4.3. Doming-related deformation Deformation associated with the formation of the Vredefort dome generated the ubiquitous pseudotachylites throughout the dome, and led to rotation of the Archean gneissic fabrics in a zone within a few kilometers of the core^collar contact in conjunction with upturning of the supracrustal strata in the collar of the dome [23]. This rotation is best seen in the northeast where the otherwise moderately dipping S2 fabric displays subvertical orientations tangential to the dome (Fig. 2). The pseudotachylites range in width from millimeters to V100 m, and the largest dike has a length in excess of 1 km. Reimold and Colliston [22] noted a preferred orientation of pseudotachylite veins parallel to lithological contacts and the Archean fabrics and proposed a dominantly subvertical attitude for the veins. Close analysis of three-dimensional outcrops indicates, however, that the complex deformation of
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the migmatitic and gneissic layering and foliations in the gneisses ensures that the veins, whilst largely parallel to these features, are more randomly oriented than they suggested. Signi¢cantly, no major o¡set or rotation of the Archean features has been noted along the veins.
5. Lithologies and structures in the vicinity of the ‘Vredefort discontinuity’ Hart et al. [3] described the contact between the OGG and ILG as a major retrograde shear zone that has been intruded by younger dikes of dolerite and pseudotachylite. Between Parys and Vredefort, this contact apparently trends NE^SW. Moser et al. [2], on the other hand, suggested a predominantly ESE^WNW trending intrusive contact cut by several faults with apparent kilometer-scale o¡sets in this area. Our mapping shows that the dominant rocks in the area grade from charnockitic and enderbitic migmatitic to restitic granulites in the south to trondhjemitic migmatitic and granitic gneisses farther from the center of the dome. Contact relationships are complex, with pyroxene-bearing lithologies interleaved with the biotite- and hornblende-bearing migmatitic gneisses and granites (Fig. 4D). Evidence supporting both intrusive and replacement origins for the charnockites appears to exist [9]. All the gneisses are dominated by the subvertical NW- to WNW-trending highstrain S3 foliation that anastomoses around lower-strain lenses characterized by moderately to shallowly plunging meter-scale upright folds of S2 (Figs. 3, 4C). Several meter-wide bodies of a magnetite-rich porphyritic granodiorite intrude both the charnockites and migmatites (Fig. 3). These contain a weak NW-trending fabric de¢ned by biotite and K-feldspar aggregates, indicating late-D3 crystallization. No evidence was found of the NE-trending retrograde brittle^ductile shear zone suggested by Hart et al. [3]. The discontinuous outcrop makes detailed investigation of the orientation of the post-metamorphic dolerite intrusion di⁄cult, but it appears to trend broadly NE^SW. A pseudotachylite-hosted breccia up to several meters wide is
well developed along the margins of, and within, the dolerite intrusion, but several other pseudotachylite occurrences of equal, or greater, size are also present elsewhere in the area (Fig. 3). These are generally oriented subparallel to the S3 fabric. As observed elsewhere in the dome, these pseudotachylites are not associated with broader zones of brittle-ductile deformation, and no discernable o¡set is apparent along their margins. Thinner pseudotachylite veins are found parallel to both S3 and the folded S2 fabric; however, there is no evidence that these are truncated by NE-trending structures as postulated by Hart et al. [3]. No evidence was found of the northerly-trending socalled ‘impactogenic’ faults identi¢ed by Moser et al. [2] from their aerial photograph interpretation and ¢eld mapping. Although they do not provide any further details of these faults, we assume that the ‘impactogenic’ designation means that they are associated with pseudotachylite; however, none of the large pseudotachylite dikes that we found shows the kilometer-scale o¡sets suggested by Moser et al. [2].
6. Discussion Lithological and structural mapping of the Archean gneiss complex and, in particular, the putative ‘Vredefort discontinuity’ outcrops in the Vredefort dome has failed to ¢nd any evidence for the large-scale Late Archean shear zone proposed by Hart et al. [3] and De Wit et al. [1]. Structural mapping results indicate a similar tectonic history for both the OGG and ILG, and a similar timing for these events relative to the metamorphic climax, with anatexis commencing during D2 and melt crystallization ceasing during the latter stages, or shortly after, the cessation of D3. The absence of a major terrane boundary is further supported by new geochemical data for the gneiss complex that suggest a strong compositional similarity between the lithologies in the OGG and ILG [24]. We conclude that the OGG and ILG rocks have evolved together since the initial consolidation of the central parts of the craton and that, apart from the heterogeneous distribution of lithologies characteristic of such gneiss complexes,
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the only signi¢cant di¡erence between them is the slightly higher grade of metamorphism experienced by the ILG rocks as be¢ts their deeper structural burial at 3.1 Ga. Moser et al. [2] suggested that the OGG represents part of a 10 km thick mid-crustal layer that was formed at 3.1 Ga by melts derived from granulite-facies metamorphism of the lower crust, represented here by the ILG. According to them, thus, the OGG^ILG contact would represent the lower contact of this intrusive mid-crust against the lower crust. Our results suggest, however, that this interpretation is somewhat simplistic. Most of the granitoid intrusive features in the vicinity of the contact are related to largely in situ or parautochthonous partial melts in the older trondhjemitic gneisses that occur on both sides of the contact. Furthermore, although some poorly foliated granitoid intrusions exist in the OGG, the predominant lithologies in the OGG are migmatitic gneisses in which a large part of the leucosome component appears to have remained essentially in situ. Thus, although the higher proportion of anatectic melt in the OGG relative to the highergrade ILG is consistent with more e⁄cient extraction of hotter, water-undersaturated, melts from the ILG and their emplacement into the overlying amphibolite-facies migmatites of the OGG, the scale of this process does not appear to have approached that envisaged by Moser et al. [2]. We conclude that the OGG^ILG contact is essentially the transition zone between partially melt-depleted granulites and melt-rich migmatitic amphibolite-facies gneisses formed during regional metamorphism of tonalite^trondhjemite^greenstone crust at 3.1 Ga. This zone became exposed as a result of tilting of the crust during the formation of the Vredefort dome at 2.02 Ga, and subsequent erosion. The mid-crustal nature of the Vredefort pro¢le has been con¢rmed by geobarometry on pelitic granulites which showed that the metamorphism occurred at ca. 0.5^0.55 GPa, corresponding to a depth of burial of V17^20 km [16]. These pelitic granulites lie only 2 km southeast of the ILG^OGG boundary (Fig. 3) and, thus, suggest a 15^18 km depth for the amphibolite^granulite transition. The coincidence of the OGG^ILG boundary
141
with a strong magnetic anomaly in the dome (Fig. 3) was originally interpreted by Corner et al. [19] as evidence for a magnetite-rich layer in the gneisses. Such enrichment might be expected where dehydration melting of hydrous ferromagnesian silicates at the amphibolite^granulite transition produces residual magnetite [34]. However, subsequent work has shown that the anomaly does not re£ect enhanced magnetic susceptibility but, rather, unusually high remanent magnetism [21,25,26]. Given that single-domain magnetite is the main carrier phase of remanence, Cloete et al. [27] and Hart et al. [25] proposed that the anomalously high Q-values (the measure of remanence versus susceptibility) in these rocks re£ect growth of extremely ¢ne-grained magnetite crystallites resulting from shock dissociation of ma¢c minerals during the 2.02 Ga impact event. The 2.0 Ga, post-doming, paleopole determined for these rocks [21] con¢rms that the magnetic characteristics are an artifact of the impact event. Elevated post-shock temperatures close to the magnetite Curie point temperature in these rocks [12,26,28] could also have facilitated setting of the remanence in ¢ne-grained shock-induced assemblages. 6.1. Regional implications Several workers have attempted to use the unique pro¢le through the Archean gneiss complex in the Vredefort dome to explain regional geophysical features in the Kaapvaal craton. Corner et al. [19] suggested that the magnetic anomaly in the dome might be linked to the Colesberg Lineament some 300 km west of the dome (Fig. 1). Corner et al. [19] attributed the lineament to a magnetite-rich zone in the Archean gneisses, but it is also a signi¢cant tectonic boundary that separates the older core of the craton from a N^S trending belt of younger greenstones and gneisses (Fig. 1) [1]. However, as discussed in the preceding section, not only is the magnetic anomaly in the Vredefort rocks unrelated to magnetite enrichment, but it also appears to be related to the 2.02 Ga impact event rather than being of Archean origin [21,25]. The tectonism along the Colesberg Lineament, in turn, appears to have been related to lateral accretion of Late Archean volcanic arcs
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onto the main cratonic mass up to several hundred million years after the events recorded in the Archean gneiss complex in the Vredefort dome [1]. De Wit et al. [1] correlated the OGG^ILG contact with a subhorizontal mid-crustal seismic re£ector identi¢ed by Durrheim [29] in the crystalline basement V100 km southwest of the dome. Durrheim et al. [30] proposed that the re£ector could represent a ma¢c sill complex related to one of the several volcanic events that have occurred on the Kaapvaal craton, but De Wit et al. [1] suggested that it represented one of the major Late Archean thrusts that transect the craton. Durrheim’s [29] original proposal, however, was that the re£ector could mark the so-called ‘Conrad discontinuity’ between more dense, restitic, granulite facies rocks in the lower crust and a more granitic middle crust. The exposure of the amphibolite^granulite transition in the Vredefort dome and evidence for vertical strati¢cation of the mid-crust could provide some support for the latter interpretation ; however, it remains to be seen whether this boundary is su⁄ciently sharp, and seismic velocities su⁄ciently di¡erent across this zone, to generate a strong re£ection. Despite the lack of evidence of a major Late Archean thrust zone in the Vredefort dome, a large body of evidence exists to support modi¢cation and growth of the Kaapvaal craton by subduction^accretion processes between 3.1 and 2.6 Ga [1]. The mid-Archean evolution of the Archean gneiss complex in the Vredefort dome is also consistent with De Wit et al.’s model of voluminous ca. 3.2 Ga tonalite^tondhjemite magmatism generated by thickening of precursor oceanic crust, followed by renewed deformation, highgrade metamorphism and partial melting at 3.1 Ga [10,24]. Voluminous sheet-like, 3.1 Ga, granitoid plutons intruding mid- to upper amphibolite-facies gneisses and greenstones in the Johannesburg dome and the eastern areas of the craton [31^33] may be analogous to the ‘granitic middle crust’ envisaged by Moser et al. [1], and suggest that the OGG section in the Vredefort dome may represent crustal levels below this zone. Such voluminous magmatism could also explain the crustal thickening necessary to generate the anticlock-
wise P-T path obtained for the pelitic granulites in the dome [16,34].
7. Conclusions Archean crystalline basement exposures in the core of the Vredefort dome preserve a mid-crustal upper amphibolite- to granulite-facies transition zone. No structural evidence was found to support previous interpretations of this zone as a major thrust ^ instead of two distinct terranes, the rocks demonstrate a shared tectonometamorphic history, culminating at 3.1 Ga in polyphase deformation, high-grade regional metamorphism and anatexis. These processes re£ect the ¢nal stages of consolidation of the Kaapvaal craton and stabilization of its lithospheric root prior to the development of major volcanic and sedimentary basins during the Late Archean.
Acknowledgements Funding for this study was provided by grants from the National Research Foundation of South Africa and the Research Council of the University of the Witwatersrand (to R.L.G. and W.U.R.). C. Simpson is thanked for her permission to use structural data from an unpublished map in Figure 3, and S. McCourt, D. van Reenen and A. Smit for their reviews. University of the Witwatersrand Impact Cratering Research Group Contribution No. 36.[AC]
References [1] M.J. De Wit, C. Roering, R.J. Hart, R.A. Armstrong, C.E.J. de Ronde, R.W.E. Green, M. Tredoux, E. Peberdy, R.A. Hart, Formation of an Archaean continent, Nature 357 (1992) 553^562. [2] D.E. Moser, R.M. Flowers, R.J. Hart, Birth of the Kaapvaal tectosphere 3.08 billion years ago, Science 291 (2001) 465^468. [3] R.J. Hart, M.A.G. Andreoli, M. Tredoux, M.J. De Wit, Geochemistry across an exposed section of Archean crust at Vredefort: with implications for mid-crustal discontinuities, Chem. Geol. 82 (1990) 21^50. [4] R.J. Hart, L.O. Nicolaysen, N.H. Gale, Radioelement
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concentrations in the deep pro¢le through Precambrian basement of the Vredefort structure, J. Geophys. Res. 86 (1981) 10639^10652. H. Henkel, W.U. Reimold, Integrated geophysical modeling of a giant, complex impact structure: anatomy of the Vredefort structure, South Africa, Tectonophysics 287 (1998) 1^20. M. Tredoux, R.J. Hart, R.W. Carlson, S.B. Shirey, Ultrama¢c rocks at the center of the Vredefort structure: further evidence for the crust on edge model, Geology 27 (1999) 923^926. W.U. Reimold, R.L. Gibson, Geology and evolution of the Vredefort impact structure, South Africa, J. Afr. Earth Sci. 23 (1996) 125^162. R.L. Gibson, W.U. Reimold, Deeply exhumed impact structures: A case study of the Vredefort Structure, South Africa, in: I. Gilmour, C. Koeberl (Eds.), Lecture Notes in Earth Sciences 91: Impacts and the Early Earth, Springer, Berlin, 2000, pp. 249^277. R.L. Gibson, W.U. Reimold, The Vredefort Impact Structure, South Africa (the scienti¢c evidence and a two-day excursion guide), Council for Geoscience Memoir 92 (2001) 111 pp. C. Lana, R.L. Gibson, W.U. Reimold, R.C.A. Minnitt, Geology of the Greenlands Greenstone Complex, Vredefort Dome, South Africa, S. Afr. J. Geol. (submitted). R.A. Armstrong, W. Compston, E.A. Retief, L.S. Williams, H.J. Welke, Zircon ion microprobe studies bearing on the age and evolution of the Witwatersrand Basin, Precambr. Res. 53 (1991) 243^266. R.L. Gibson, Impact-induced melting in Archaean granulites in the Vredefort dome, South Africa. I: Anatexis of metapelitic granulites, J. Metam. Geol. 20 (2002) 57^70. R.L. Gibson, U.W. Reimold, A.J. Ashley, C. Koeberl, Metamorphism on the Moon: A terrestrial analogue in the Vredefort Dome, South Africa, Geology 30 (2002) 475^478. D. Stepto, The geology and gravity ¢eld in the central core of the Vredefort structure, Tectonophysics 171 (1990) 75^104. D. Stepto, A geological and geophysical study of the central portion of the Vredefort Dome Structure, Ph.D. Thesis, University of Witwatersrand, Johannesburg, South Africa (1979) (unpublished). G. Stevens, R.L. Gibson, G.T.R. Droop, Mid-crustal granulite-facies metamorphism in the central Kaapvaal Craton: The Bushveld Complex connection, Precambr. Res. 82 (1997) 113^132. R.J. Hart, M.A.G. Andreoli, W.U. Reimold, M. Tredoux, Aspects of the dynamic and thermal metamorphic history of the Vredefort cryptoexplosion structure: implications for its origin, Tectonophysics 192 (1991) 313^331. R.J. Hart, D. Moser, M.A.G. Andreoli, Archean age for the granulite facies metamorphism near the center of the Vredefort structure, South Africa, Geology 27 (1999) 1091^1094. B. Corner, R.J. Durrheim, L.O. Nicolaysen, Relationships
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between the Vredefort structure and the Witwatersrand basin within the tectonic framework of the Kaapvaal craton as interpreted from regional gravity and aeromagnetic data, Tectonophysics 171 (1990) 49^61. S.L. Kamo, W.U. Reimold, T.E. Krogh, W.P. Colliston, A 2.023 Ga age for the Vredefort impact event and a ¢rst report of shock metamorphosed zircons in pseudotachylitic breccia and granophyre, Earth Planet. Sci. Lett. 144 (1996) 369^388. R.J. Hart, R.B. Hargraves, M.A.G. Andreoli, M. Tredoux, C.M. Doucoure¤, Magnetic anomaly near the center of the Vredefort structure: Implications for impact-related magnetic signatures, Geology 23 (1995) 277^280. W.U. Reimold, W.P. Colliston, Pseudotachylites of the Vredefort dome and the surrounding Witwatersrand Basin, South Africa, in: B.O. Dressler, R.A.F. Grieve, V.L. Sharpton (Eds.), Large Meteorite Impacts and Planetary Evolution, Geological Society of America Special Paper 293 (1994) 177^196. C. Lana, R.L. Gibson, W.U. Reimold, Impact tectonics in the core of the Vredefort dome, South Africa: Implications for central uplift formation in large impact craters, in: H. Henkel, C. Koeberl (Eds.), Meteoritics Plant. Sci. (submitted). C. Lana, R.L. Gibson, W.U. Reimold, Archaean crustal evolution of the central parts of the Kaapvaal craton: evidence from the Vredefort Dome, South Africa, Geocongress 2002, Windhoek, Bienn. Congr., Geol. Soc. S. Afr., extended Abstract (2002) CD-Rom, 3 pp. R.J. Hart, S.H. Connel, M. Cloete, L. Mare, M. Drury, M. Tredoux, ‘Super-magnetic’ rocks generated by shock metamorphism from the centre of the Vredefort impact structure, South Africa, S. Afr. J. Geol. 103 (2000) 151^ 155. H. Henkel, W.U. Reimold, Magnetic model of the central uplift of the Vredefort Impact Structure, South Africa, J. Appl. Geophys. 51 (2002) 43^62. M. Cloete, R.J. Hart, H.K. Schmid, M. Drury, C.M. Demanet, K.V. Sankar, Characterization of magnetite particles in shocked quartz by means of electron- and magnetic- force microscopy, Vredefort, South Africa, Contrib. Mineral. Petrol. 137 (1999) 232^245. R.L. Gibson, W.U. Reimold, G. Stevens, Thermal-metamorphic signature of an impact event in the Vredefort dome, South Africa, Geology 26 (1998) 787^790. R.J. Durrheim, Recent re£ection seismic developments in the Witwatersrand basin, in: M. Barazangi, L. Brown (Eds.), Re£ection Seismology, Am. Geophys. Union Geodyn. Series 13, 1986, pp. 77^83. R.B. Durrheim, L.O. Nicolaysen, B. Corner, A deep seismic re£ection pro¢le across the Archean-Proterozoic Witwatersrand Basin, South Africa, Am. Geophys. Union Geodyn. Series 22, 1991, pp. 213^224. C.R. Anhaeusser, The geology and geochemistry of the Archaean granites and gneisses of the Johannesburg-Pretoria dome, Spec. Publ. Geol. Soc. S. Afr. 3 (1973) 361^ 385.
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[32] C.R. Anhaeusser, Archean crustal evolution of the central parts of the Kaapvaal Craton, South Africa; Evidence from the Johannesburg dome, S. Afr. J. Geol. 95 (1999) 111^116. [33] M. Poujol, C.R. Anhaeusser, The Johannesburg Dome,
South Africa: new single U-Pb isotopic evidence for early Archaean granite-greenstone development within the central Kaapvaal craton, Precambr. Res. 108 (2000) 139^157. [34] S.L. Harley, The origin of granulites: a metamorphic perspective, Geol. Mag. 126 (1989) 215^247.
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Archean crustal structure of the Kaapvaal craton, South Africa ^ evidence from the Vredefort dome Cristiano Lana a , Roger L. Gibson a; , Alexander F.M. Kisters b , W. Uwe Reimold a a
Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa b Department of Geology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa Received 2 August 2002; received in revised form 13 November 2002; accepted 19 November 2002
Abstract Crystalline Archean basement rocks in the core of the Vredefort dome present a profile through a substantial part of the middle and lower crust of the Kaapvaal craton. Previously, this profile has been subdivided into two terranes with allegedly distinct lithologies and tectonometamorphic histories that were juxtaposed along a crustal-scale Late Archean brittle^ductile thrust zone. Lithological and structural mapping across the dome indicates, however, that the basement lithologies share a common polyphase tectonic history culminating in high-grade metamorphism and melting at V3.1 Ga. No evidence was found of the postulated tectonic terrane boundary, but the alleged boundary does coincide with a 1^2 km wide transition zone between upper amphibolite facies migmatitic gneisses and more restitic granulite facies gneisses. The implications of these results for Archean regional tectonic models for the Kaapvaal craton are discussed. 3 2002 Elsevier Science B.V. All rights reserved. Keywords: Archean; ‘Conrad discontinuity’; granulite^amphibolite transition; mid-crust; Kaapvaal craton; tectonics; Vredefort dome
1. Introduction The Kaapvaal craton of southern Africa (Fig. 1) represents one of only two extensive and largely pristine mid-Archean crustal fragments
* Corresponding author. Tel.: +27-11-7176553 (Sec. 7176547); Fax: +27-11-3391697. E-mail addresses: [email protected] (R.L. Gibson), [email protected] (A.F.M. Kisters), [email protected] (W.U. Reimold).
on Earth (the other being the Pilbara craton, Australia). It is, thus, an important natural laboratory for the investigation of the processes and events responsible for the formation of the earliest continental lithosphere and, in the speci¢c case of the craton, also for the formation of some of the world’s most spectacular ore deposits. Recent syntheses by De Wit et al. [1] and Moser et al. [2] have attempted to reconcile the large ¢eld-based mapping, geochemical, geochronological and geophysical databases available for the craton into crustal- and lithospheric-scale evolutionary mod-
0012-821X / 02 / $ ^ see front matter 3 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 1 0 8 6 - 5
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els. De Wit et al. [1] envisaged a two-stage process in which the cratonic nucleus formed by initial tectonic imbrication of slabs of hydrated oceanic lithosphere which melted between V3.2 and 3.1 Ga; and was then modi¢ed and enlarged by Cordilleran-type subduction^accretion processes between V3.1 and 2.6 Ga. Based largely on geophysics, they identi¢ed numerous sub-domains within the craton which they interpreted as lithospheric-scale features that were juxtaposed along thrusts during this latter period. Moser et al. [2], on the other hand, focused on events leading up to 3.1 Ga and proposed that events between 3.2 and 3.1 Ga led to assembly of the crust and formation of a thick mantle root, thereby allowing the subsequent development of the large Late Archean sedimentary and volcanic basins that characterize the craton. An integral part of their model is the development of a 10 km thick mid-crustal layer of granitoid melts derived from tectonically thickened lower crust at 3.1 Ga concomitant with this crust^root coupling. Both De Wit et al. [1] and Moser et al. [2] utilized in their models evidence from a unique pro¢le through the Kaapvaal crust exposed in the Vredefort dome in the central parts of the craton (Fig. 1). In particular, they focused on a speci¢c zone within the high-grade gneiss complex that has been correlated in various studies with regional geophysical features elsewhere in the craton. Named the ‘Vredefort discontinuity’ by Hart et al. [3], this zone has been interpreted as a crustal-scale thrust of post-3.1 Ga age by Hart et al. [3] and De Wit et al. [1] and, more recently, as the intrusive contact between the 3.1 Ga old molten mid-crust and older lower crust by Moser et al. [2]. In this paper, we present new lithological and structural mapping in the Archean gneiss complex exposed in the Vredefort dome, from which we reassess the nature of the ‘Vredefort discontinuity’ and its implications for these models for the evolution of the Kaapvaal craton.
2. Regional geology The Vredefort dome is an V80 km wide structural feature located some 120 km southwest of
Johannesburg (Fig. 1). It comprises a 40 km wide core of high-grade Archean gneisses that is rimmed by an V20 km wide collar of subvertically dipping Late Archean to Paleoproterozoic supracrustal strata (Fig. 2). The dome is largely obscured in the south and southeast by Phanerozoic sedimentary strata and dolerite sills. Estimates of the total thickness of crust exposed in the dome range from V25 km [4,5] to V36 km [3,6], with the latter suggesting that the paleoMoho is exposed at surface in the center of the dome. Evidence of shock metamorphism, impactmelt breccias and unusually voluminous pseudotachylitic breccias in the dome indicate that doming occurred as a result of a large meteorite impact event 2.02 Ga ago (see reviews in Reimold and Gibson [7] ; Gibson and Reimold [8,9]). The crystalline Archean basement rocks in the core of the Vredefort dome comprise a complex high-grade metamorphic terrane that is dominated by medium- to coarse-grained tonalitic and trondhjemitic gneisses and migmatites, granitic to granodioritic syntectonic intrusions, and volumetrically minor xenoliths of pelitic and ironstone paragneisses and ma¢c gneisses (Figs. 2 and 3). In the southeast of the dome, a sequence of amphibolite-facies meta-komatiites and komatiitic metabasalts with subsidiary ironstones is exposed in an inlier beneath the Phanerozoic strata [10]. The metamorphic and structural features in the gneiss complex clearly predate the deposition of the supracrustal rocks exposed in the collar of the dome, the oldest of which comprise the 3.074 N 0.006 Ga [11] Dominion Group lavas. Most rocks in the core of the dome are migmatized, although, locally, remnants of non-migmatized gneisses are preserved (Fig. 2). The migmatites are predominantly stromatic but grade locally into nebulitic to schlieric types that, in turn, grade into weakly deformed to undeformed granites and granodiorites. The gradational relationships between the smaller granitoid bodies and the migmatite leucosomes indicate that the former are largely locally derived. In the outer parts of the core of the dome, biotite N hornblende are the ferromagnesian minerals in the migmatite mesosomes; however, these are largely replaced by orthopyroxene N clinopyroxene in the central
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parts, with the result that the rocks are more appropriately described as charnockites or enderbites. This transition from upper amphibolite-facies to granulite-facies towards the center of the dome is consistent with the increasing levels of exhumation expected in a structural dome. The transition itself is complex, with pyroxene-bearing charnockitic to enderbitic lithologies interleaved with biotite- and hornblende-bearing lithologies in a zone 1^2 km wide that is best exposed between the towns of Vredefort and Parys (Fig. 3). The granulites in the central parts of the dome are more restitic than the amphibolite-facies rocks further from the center, suggesting signi¢cant melt loss from the higher-grade rocks. Within a radius of 5 km of the center, however, the Archean assemblages are overprinted and partially to totally destroyed by a distinctive static high-temperature recrystallization caused by the combined e¡ects of shock and thermal metamorphism triggered by the 2.02 Ga impact event [12,13] that led to Stepto [14,15] classifying these rocks as granofelses rather than gneisses (Fig. 2). Peak metamorphic temperatures during the Archean event within the granulite zone appear to have reached 800^ 850‡C at a pressure of 0.5^0.55 GPa [12,16].
3. Previous structural-tectonic interpretations of the Archean gneiss complex Previous studies of the crystalline Archean basement rocks in the core of the Vredefort dome have been largely con¢ned to geochemical, petrographic and geochronological investigations of selected areas and traverses (e.g. [2,3,4,6,17, 18]). Apart from limited structural mapping in the central parts of the core by Stepto [15] and C. Simpson (unpublished), no coherent structural study of the crystalline rocks has previously been attempted. Despite this, a key feature of the interpretation of the gneiss complex has been its subdivision into two terranes separated by a major tectonic contact believed to be of regional signi¢cance in the craton [1,3,19]. Hart et al. [4] ¢rst suggested that rocks in the outer parts of the core (‘Outer Granite Gneiss’, or OGG) were distinct from those in the inner core
135
Greenstone belt Archean granitoids and gneisses Major late Archean sedimentary basins
Colesberg Lineament
N
Johannesburg
Edge of the Kaapvaal craton Vredefort dome
Cape Town
500 km
Fig. 1. Schematic map of the Kaapvaal craton, based on geological, aeromagnetic and gravity data (after De Wit et al. [1]), showing the location of the Vredefort dome and Colesberg Lineament. Note the varying structural trends of greenstone belts on either side of this lineament.
(‘Inlandsee Leucogranofels’, or ILG) based on geochemical analyses obtained along three traverses across the dome. Furthermore, initial Rb/Sr, Th/Pb and Pb/Pb whole-rock geochronological results [3,4] suggested that the metamorphic climax in the OGG rocks was signi¢cantly older (V3.1 Ga) than that in the ILG rocks (V2.8 Ga). The juxtaposition of these two terranes was attributed by Hart et al. [3] to a post-metamorphic regional thrusting event, evidence for which was best preserved in a narrow northeast-trending zone extending for V8 km between Parys and Vredefort, which they named the ‘Vredefort discontinuity’ (Fig. 2). Hart et al. [3] described this contact between the OGG and ILG as a 40^100 m wide brittle^ductile shear zone which truncates the main ductile shear fabric and associated folds in the ILG. They maintained that the OGG along the contact displays a gneissic fabric roughly parallel to this shear zone and that the presence of thin slivers of ILG lithologies in the OGG rocks up to 200 m from the contact is related to tectonic inter¢ngering caused by this thrusting. The age of the shear zone was estimated at between 2.8 Ga (that is, after the attainment of the metamorphic peak in the ILG) and 2.56 Ga (U/Pb single zircon age for an undeformed and unmetamorphosed
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27 O30’ E
Phanerozoic sediments Impact-related granofels 30
30
26 O45’ S
O
26 45’ 26 45’SS 30
r ve Ri al a V
Archean supracrustal strata
45
Granite Foliated porphyritic granodiorite
nT 0 nT 40 00 8
80
0
Amphibolite-facies trondhjemitic migmatitic gneiss (OGG)
nT T 0n
40
Charnockitic to enderbitic migmatitic gneiss (ILG) Tonalitic and trondhjemitic gneisses
VREDEFORT
Greenstones (amphibolite-facies) Subvertical to vertical fabric Fabric orientation S2 trend S3 trend
N
Magnetic anomaly (- 400 and - 800 nT contours) O
O
0
10 km
Trace of the alleged Vredefort discontinuity (after Hart et al. [3])
27 O30’ E
Fig. 2. Simpli¢ed geological map of the Vredefort dome showing the main lithologies and structures of the Archean gneiss complex in the core of the dome and the negative aeromagnetic anomaly [21]. The position of the structural discontinuity inferred by Hart et al. [3] is also indicated.
dolerite dike in the vicinity of the contact; Fig. 3) [3,18]. More recently, Moser et al. [2] suggested that the OGG^ILG contact is intrusive ; however, they also stated that it was deformed, although they failed to indicate if this deformation was similar to that described by Hart et al. [3]. Signi¢cantly, they, as well as Kamo et al. [20] and Hart et al. [18], obtained U/Pb single zircon metamorphic ages of V3.1 Ga from ILG rocks, contradicting Hart et al.’s [3,4] proposal that metamorphism in the ILG was up to 300 million years younger than in the OGG. The exposures between Parys and Vredefort correspond roughly to a broad negative magnetic anomaly that displays an arcuate pattern in the core of the dome (Fig. 2) [3,21]. In the absence of signi¢cant outcrop in large parts of the core of the
dome, Hart et al. [3,21] used this pattern to extend the discontinuity southeast of Parys and south of Vredefort (Fig. 2), where it disappears beneath the Phanerozoic sedimentary cover.
4. Structural geology The gneisses and migmatites in the core of the dome record a polyphase deformational history that preceded the deposition of the 3.07 Ga Dominion Group volcanics. Structures related to at least three main phases of ductile deformation (D1, D2, D3) are visible, although the D1 features have been largely transposed by D2 and D3 deformation. Field relations indicate that the D2 and D3 events overlapped the metamorphic
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Phanerozoic sediments
137
Amphibolite-facies trondhjemitic migmatitic gneiss (OGG)
supracrustal strata strata Archean supracrustal Granulite-amphibolite transition zone
Fabric orientation Vertical fabric
Pt Dol g
S2 trend S3 trend Pseudotachylite Dolerite dike
Charnockitic to enderbitic migmatitic gneiss (ILG)
85
75 85
75 75
85
88
Late-D3 granitoids
85
g
75
75 75
80 80 80 70
80 85
85
50
Parys
75
80
64 75 80
80 50 50
85
55
g
85
70
45
g
70
80 g
A)
Ironstone and pelitic granulites
78
85
75
85
78
80 75
85
N
75
75
85
85 75
75
85
80
75
g
80 85
80
85
70 85 70 85 80 80 70 75 80 g 70
g
70
75
75
g
70
70
85
85
Pt + Dol 85
80 80 75 75 85
85
85
70 80 77 75
75
80
82 85
80 77
80
77 80
82
85 75
75
85
75
80
N
75 80 g
85 88
N
58 85 75
88
75
87
80 88
76 85
Vredefort
82 80
75 75
0
75
75
86
75
85 80 75
77
C)
75 80 75 80
2 km
Poles to S2 N = 24 Poles to S3 N = 41
Poles to S2 N = 21 Poles to S3 N = 47
Fig. 3. (A) Structural and lithological map of the NW sector of the core of the dome, showing the dominant S3 fabric, the amphibolite^granulite transition zone and the locations of late-D3 granitoids and major pseudotachylite occurrences in the vicinity of this zone. (B) Lower hemisphere equal-area stereographic projection of poles to S2 and S3 in the ILG, showing the strong similarity in fabric orientations. (C) Lower hemisphere equal-area stereographic projection of poles to S2 and S3 in the OGG. The steeper dip of S2 in the OGG is related, at least in part, to rotation associated with formation of the dome.
peak and accompanying anatexis. The D2 and D3 structures are cut by pseudotachylite veins and dikes that are found throughout the wider environs of the dome [22] and that are related to the 2.02 Ga Vredefort impact event [20]. 4.1. Structures in the Outer Granite Gneiss Structures related to the earliest recognizable
deformation event (D1) are best preserved in the northeastern sector of the dome. S1 is a gneissic foliation locally preserved in centimeter- to meterscale isoclinal folds within the subhorizontal S1/ S2 transposition fabric, or in trondhjemitic xenoliths in the migmatites. In places, quartz^plagioclase leucosomes occur parallel to S1 but, more typically, they cut S1. The dominant fabric in the migmatites and gneisses in the outer parts of the
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A)
B)
10 cm
C) 20 cm
D)
Fig. 4. (A) Banded trondhjemitic gneiss with stromatic plagioclase^quartz leucosome (bottom) parallel to subhorizontal S2 foliation. Otavi quarry, northeast of Parys. (B) OGG migmatite with subvertical S3 foliation a¡ected by asymmetric extensional shear bands. Boudins of trondhjemitic gneiss (dark gray) lie in a banded granitic^trondjhemitic gneiss matrix. Locality V1 km NW of Vredefort. (C) Folded S2 foliation in subvertical S3 high-strain zone in trondhjemitic gneiss. The fold plunges shallowly to the NW and is crosscut by granitic leucosome that displays a weak S3 fabric. Locality 4 km SW of Parys. (D) Disaggregated ma¢c granulite in charnockite (left) cut by granite (right) in the granulite^amphibolite transition zone V1 km north of Vredefort. Both the charnockite and granite display a weak S3 foliation.
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gneiss complex is S2 (Figs. 2 and 4A). S2 has a subhorizontal to moderately dipping orientation where it is not a¡ected by D3 or by doming-related rotation close to the contact with the Dominion Group (Fig. 2) [23]. In the northwestern sector of the dome, the gneisses and migmatites are dominated by a northwest-trending subvertical S3 fabric (Fig. 4B) that is de¢ned by centimeter- to decimeterscale stromatic migmatitic layering that occurs in zones up to 500 m wide (Figs. 2 and 3). Chocolate-tablet boudinage of amphibolite sheets, and centimeter- to meter-scale conjugate extensional shear bands developed both in plan-view as well as in cross-section (Fig. 4B), indicate a large component of £attening strain within these zones. No consistent shear sense asymmetry was detected along the S3 fabric. In the western sector of the dome (Fig. 2), a porphyritic granodiorite shows alignment of euhedral K-feldspar megacrysts parallel to the S2 fabric in the adjacent migmatites, suggesting crystallization during D2. However, in the northwest, the migmatites in the OGG commonly grade into schlieric and homogeneous granites that display a variably developed S3 foliation de¢ned by aligned K-feldspar megacrysts in an undeformed matrix, consistent with crystallization having straddled the termination of D3 deformation. 4.2. Structures in the Inlandsee Leucogranofels zone The S1/S2 transposition fabric is also preserved locally in the granulite-facies rocks in the center of the dome, but this zone is dominated by the subvertical, NW-trending, S3 fabric (Figs. 2 and 3). Two large outcrops of ironstone east of Vredefort town preserve S2 in kilometer-scale, open to tight, D3 folds (Fig. 3). The D3 deformation is expressed either as discrete, centimeter- to decimeter-wide high-strain zones, between which the S1/S2 fabric is deformed into open to tight, upright, shallow to moderately NW- or SE-plunging folds (Fig. 4C), or as up to 100 m wide zones of highly strained gneisses in which S1/S2 is completely transposed. The S3 high-strain zones are characterized by stromatic
139
trondhjemitic to granitic leucosomes and tonalitic to trondhjemitic mesosomes. The S3 stromatic fabric is sinuous, and locally displays more northerly and westerly trends (Fig. 3). It is locally cut by decimeter- to meter-wide, high-temperature retrograde mylonitic zones, which are parallel to sub-parallel to S3, and by conjugate extensional shear bands along which granitic leucosomes are commonly localized. Macroscopic mineral stretching lineations are conspicuously absent in the stromatic migmatitic bands, but oriented thin sections of samples from the mylonitic shear zones indicate that some pyroxene and biotite grains de¢ne a shallow (10^20‡) NW-plunging mineral lineation. The parallelism between S3 and the mylonitic shear zones, together with the high-grade mineral assemblages in the shear zones, indicates that the mylonitization probably occurred during the waning stages of D3, shortly after the metamorphic peak. Stringer- or pod-like granitoid bodies intrude predominantly parallel to S3, but they may also locally cut the S3 fabric (Fig. 4C). The granitoid stringers typically contain a weak planar S3 fabric de¢ned by aligned K-feldspar megacrysts and £attened quartz aggregates, indicating late syn-D3 crystallization. 4.3. Doming-related deformation Deformation associated with the formation of the Vredefort dome generated the ubiquitous pseudotachylites throughout the dome, and led to rotation of the Archean gneissic fabrics in a zone within a few kilometers of the core^collar contact in conjunction with upturning of the supracrustal strata in the collar of the dome [23]. This rotation is best seen in the northeast where the otherwise moderately dipping S2 fabric displays subvertical orientations tangential to the dome (Fig. 2). The pseudotachylites range in width from millimeters to V100 m, and the largest dike has a length in excess of 1 km. Reimold and Colliston [22] noted a preferred orientation of pseudotachylite veins parallel to lithological contacts and the Archean fabrics and proposed a dominantly subvertical attitude for the veins. Close analysis of three-dimensional outcrops indicates, however, that the complex deformation of
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the migmatitic and gneissic layering and foliations in the gneisses ensures that the veins, whilst largely parallel to these features, are more randomly oriented than they suggested. Signi¢cantly, no major o¡set or rotation of the Archean features has been noted along the veins.
5. Lithologies and structures in the vicinity of the ‘Vredefort discontinuity’ Hart et al. [3] described the contact between the OGG and ILG as a major retrograde shear zone that has been intruded by younger dikes of dolerite and pseudotachylite. Between Parys and Vredefort, this contact apparently trends NE^SW. Moser et al. [2], on the other hand, suggested a predominantly ESE^WNW trending intrusive contact cut by several faults with apparent kilometer-scale o¡sets in this area. Our mapping shows that the dominant rocks in the area grade from charnockitic and enderbitic migmatitic to restitic granulites in the south to trondhjemitic migmatitic and granitic gneisses farther from the center of the dome. Contact relationships are complex, with pyroxene-bearing lithologies interleaved with the biotite- and hornblende-bearing migmatitic gneisses and granites (Fig. 4D). Evidence supporting both intrusive and replacement origins for the charnockites appears to exist [9]. All the gneisses are dominated by the subvertical NW- to WNW-trending highstrain S3 foliation that anastomoses around lower-strain lenses characterized by moderately to shallowly plunging meter-scale upright folds of S2 (Figs. 3, 4C). Several meter-wide bodies of a magnetite-rich porphyritic granodiorite intrude both the charnockites and migmatites (Fig. 3). These contain a weak NW-trending fabric de¢ned by biotite and K-feldspar aggregates, indicating late-D3 crystallization. No evidence was found of the NE-trending retrograde brittle^ductile shear zone suggested by Hart et al. [3]. The discontinuous outcrop makes detailed investigation of the orientation of the post-metamorphic dolerite intrusion di⁄cult, but it appears to trend broadly NE^SW. A pseudotachylite-hosted breccia up to several meters wide is
well developed along the margins of, and within, the dolerite intrusion, but several other pseudotachylite occurrences of equal, or greater, size are also present elsewhere in the area (Fig. 3). These are generally oriented subparallel to the S3 fabric. As observed elsewhere in the dome, these pseudotachylites are not associated with broader zones of brittle-ductile deformation, and no discernable o¡set is apparent along their margins. Thinner pseudotachylite veins are found parallel to both S3 and the folded S2 fabric; however, there is no evidence that these are truncated by NE-trending structures as postulated by Hart et al. [3]. No evidence was found of the northerly-trending socalled ‘impactogenic’ faults identi¢ed by Moser et al. [2] from their aerial photograph interpretation and ¢eld mapping. Although they do not provide any further details of these faults, we assume that the ‘impactogenic’ designation means that they are associated with pseudotachylite; however, none of the large pseudotachylite dikes that we found shows the kilometer-scale o¡sets suggested by Moser et al. [2].
6. Discussion Lithological and structural mapping of the Archean gneiss complex and, in particular, the putative ‘Vredefort discontinuity’ outcrops in the Vredefort dome has failed to ¢nd any evidence for the large-scale Late Archean shear zone proposed by Hart et al. [3] and De Wit et al. [1]. Structural mapping results indicate a similar tectonic history for both the OGG and ILG, and a similar timing for these events relative to the metamorphic climax, with anatexis commencing during D2 and melt crystallization ceasing during the latter stages, or shortly after, the cessation of D3. The absence of a major terrane boundary is further supported by new geochemical data for the gneiss complex that suggest a strong compositional similarity between the lithologies in the OGG and ILG [24]. We conclude that the OGG and ILG rocks have evolved together since the initial consolidation of the central parts of the craton and that, apart from the heterogeneous distribution of lithologies characteristic of such gneiss complexes,
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the only signi¢cant di¡erence between them is the slightly higher grade of metamorphism experienced by the ILG rocks as be¢ts their deeper structural burial at 3.1 Ga. Moser et al. [2] suggested that the OGG represents part of a 10 km thick mid-crustal layer that was formed at 3.1 Ga by melts derived from granulite-facies metamorphism of the lower crust, represented here by the ILG. According to them, thus, the OGG^ILG contact would represent the lower contact of this intrusive mid-crust against the lower crust. Our results suggest, however, that this interpretation is somewhat simplistic. Most of the granitoid intrusive features in the vicinity of the contact are related to largely in situ or parautochthonous partial melts in the older trondhjemitic gneisses that occur on both sides of the contact. Furthermore, although some poorly foliated granitoid intrusions exist in the OGG, the predominant lithologies in the OGG are migmatitic gneisses in which a large part of the leucosome component appears to have remained essentially in situ. Thus, although the higher proportion of anatectic melt in the OGG relative to the highergrade ILG is consistent with more e⁄cient extraction of hotter, water-undersaturated, melts from the ILG and their emplacement into the overlying amphibolite-facies migmatites of the OGG, the scale of this process does not appear to have approached that envisaged by Moser et al. [2]. We conclude that the OGG^ILG contact is essentially the transition zone between partially melt-depleted granulites and melt-rich migmatitic amphibolite-facies gneisses formed during regional metamorphism of tonalite^trondhjemite^greenstone crust at 3.1 Ga. This zone became exposed as a result of tilting of the crust during the formation of the Vredefort dome at 2.02 Ga, and subsequent erosion. The mid-crustal nature of the Vredefort pro¢le has been con¢rmed by geobarometry on pelitic granulites which showed that the metamorphism occurred at ca. 0.5^0.55 GPa, corresponding to a depth of burial of V17^20 km [16]. These pelitic granulites lie only 2 km southeast of the ILG^OGG boundary (Fig. 3) and, thus, suggest a 15^18 km depth for the amphibolite^granulite transition. The coincidence of the OGG^ILG boundary
141
with a strong magnetic anomaly in the dome (Fig. 3) was originally interpreted by Corner et al. [19] as evidence for a magnetite-rich layer in the gneisses. Such enrichment might be expected where dehydration melting of hydrous ferromagnesian silicates at the amphibolite^granulite transition produces residual magnetite [34]. However, subsequent work has shown that the anomaly does not re£ect enhanced magnetic susceptibility but, rather, unusually high remanent magnetism [21,25,26]. Given that single-domain magnetite is the main carrier phase of remanence, Cloete et al. [27] and Hart et al. [25] proposed that the anomalously high Q-values (the measure of remanence versus susceptibility) in these rocks re£ect growth of extremely ¢ne-grained magnetite crystallites resulting from shock dissociation of ma¢c minerals during the 2.02 Ga impact event. The 2.0 Ga, post-doming, paleopole determined for these rocks [21] con¢rms that the magnetic characteristics are an artifact of the impact event. Elevated post-shock temperatures close to the magnetite Curie point temperature in these rocks [12,26,28] could also have facilitated setting of the remanence in ¢ne-grained shock-induced assemblages. 6.1. Regional implications Several workers have attempted to use the unique pro¢le through the Archean gneiss complex in the Vredefort dome to explain regional geophysical features in the Kaapvaal craton. Corner et al. [19] suggested that the magnetic anomaly in the dome might be linked to the Colesberg Lineament some 300 km west of the dome (Fig. 1). Corner et al. [19] attributed the lineament to a magnetite-rich zone in the Archean gneisses, but it is also a signi¢cant tectonic boundary that separates the older core of the craton from a N^S trending belt of younger greenstones and gneisses (Fig. 1) [1]. However, as discussed in the preceding section, not only is the magnetic anomaly in the Vredefort rocks unrelated to magnetite enrichment, but it also appears to be related to the 2.02 Ga impact event rather than being of Archean origin [21,25]. The tectonism along the Colesberg Lineament, in turn, appears to have been related to lateral accretion of Late Archean volcanic arcs
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onto the main cratonic mass up to several hundred million years after the events recorded in the Archean gneiss complex in the Vredefort dome [1]. De Wit et al. [1] correlated the OGG^ILG contact with a subhorizontal mid-crustal seismic re£ector identi¢ed by Durrheim [29] in the crystalline basement V100 km southwest of the dome. Durrheim et al. [30] proposed that the re£ector could represent a ma¢c sill complex related to one of the several volcanic events that have occurred on the Kaapvaal craton, but De Wit et al. [1] suggested that it represented one of the major Late Archean thrusts that transect the craton. Durrheim’s [29] original proposal, however, was that the re£ector could mark the so-called ‘Conrad discontinuity’ between more dense, restitic, granulite facies rocks in the lower crust and a more granitic middle crust. The exposure of the amphibolite^granulite transition in the Vredefort dome and evidence for vertical strati¢cation of the mid-crust could provide some support for the latter interpretation ; however, it remains to be seen whether this boundary is su⁄ciently sharp, and seismic velocities su⁄ciently di¡erent across this zone, to generate a strong re£ection. Despite the lack of evidence of a major Late Archean thrust zone in the Vredefort dome, a large body of evidence exists to support modi¢cation and growth of the Kaapvaal craton by subduction^accretion processes between 3.1 and 2.6 Ga [1]. The mid-Archean evolution of the Archean gneiss complex in the Vredefort dome is also consistent with De Wit et al.’s model of voluminous ca. 3.2 Ga tonalite^tondhjemite magmatism generated by thickening of precursor oceanic crust, followed by renewed deformation, highgrade metamorphism and partial melting at 3.1 Ga [10,24]. Voluminous sheet-like, 3.1 Ga, granitoid plutons intruding mid- to upper amphibolite-facies gneisses and greenstones in the Johannesburg dome and the eastern areas of the craton [31^33] may be analogous to the ‘granitic middle crust’ envisaged by Moser et al. [1], and suggest that the OGG section in the Vredefort dome may represent crustal levels below this zone. Such voluminous magmatism could also explain the crustal thickening necessary to generate the anticlock-
wise P-T path obtained for the pelitic granulites in the dome [16,34].
7. Conclusions Archean crystalline basement exposures in the core of the Vredefort dome preserve a mid-crustal upper amphibolite- to granulite-facies transition zone. No structural evidence was found to support previous interpretations of this zone as a major thrust ^ instead of two distinct terranes, the rocks demonstrate a shared tectonometamorphic history, culminating at 3.1 Ga in polyphase deformation, high-grade regional metamorphism and anatexis. These processes re£ect the ¢nal stages of consolidation of the Kaapvaal craton and stabilization of its lithospheric root prior to the development of major volcanic and sedimentary basins during the Late Archean.
Acknowledgements Funding for this study was provided by grants from the National Research Foundation of South Africa and the Research Council of the University of the Witwatersrand (to R.L.G. and W.U.R.). C. Simpson is thanked for her permission to use structural data from an unpublished map in Figure 3, and S. McCourt, D. van Reenen and A. Smit for their reviews. University of the Witwatersrand Impact Cratering Research Group Contribution No. 36.[AC]
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