The melt rocks of the Vredefort impact structure – Vredefort Granophyre and pseudotachylitic breccias: Implications for impact cratering and the evolution of the Witwatersrand Basin

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Chemie der Erde 66 (2006) 1–35 www.elsevier.de/chemer

INVITED REVIEW

The melt rocks of the Vredefort impact structure – Vredefort Granophyre and pseudotachylitic breccias: Implications for impact cratering and the evolution of the Witwatersrand Basin Wolf Uwe Reimold,1, Roger L. Gibson Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa Received 22 February 2005; accepted 26 July 2005

Abstract The unique combination of its large size (250–300 km diameter), deep levels of erosion (47 km), and widespread regional mining activity make the Vredefort impact structure in South Africa an exceptional laboratory for the study of impact-related deformation phenomena in the rocks beneath giant, complex impact craters. Two types of impactgenerated melt rock occur in the Vredefort Structure: the Vredefort Granophyre – impact melt rock – and pseudotachylitic breccias. Along the margins of the structure, mining and exploration drilling in the Witwatersrand goldfields has revealed widespread fault-related pseudotachylitic breccias linked to the impact event. There, volumetrically limited melt breccia occurs in close association with cataclasite or mylonitic zones associated with bedding-parallel normal dip-slip faults that formed during inward slumping of the crater walls, and in rare subvertical faults oriented radially to the center of the structure. This association is consistent with formation of pseudotachylites by frictional melting. On the other hand, rocks in the Vredefort Dome – the central uplift of the impact structure – contain ubiquitous melt breccias that range in size from sub-millimeter pods and veinlets to dikes up to tens of meters wide and hundreds of meters long. Like fault-related pseudotachylites in the goldfields and elsewhere in the world, they display a close geochemical relationship to their wallrocks, indicating local derivation. However, although mm/cm- to, rarely, dm-scale offsets are commonly found along their margins, they do not appear to be associated with broader fault zones, are commonly considerably more voluminous than most known fault-related pseudotachylites, and show no consistent relationship between melt volumes and slip magnitude. Recent petrographic observations indicate that at least some of these melt breccias formed by shock melting, with or without frictional melting. Consequently, the nongenetic term ‘‘pseudotachylitic breccia’’ has been adopted for these Vredefort occurrences. These breccias formed during the impact in rocks at temperatures ranging from greenschist to granulite facies, and were subsequently annealed to varying degrees during cooling of the central uplift. In addition to the pseudotachylitic breccias, nine clast-laden impact melt dikes (Vredefort Granophyre), each up to several kilometers long, occur in vertical radial and tangential fractures in the Vredefort Dome. Unlike the pseudotachylitic breccias, they display a remarkably uniform bulk composition and clast populations that are largerly independent of their wallrocks, and they contain geochemical traces of the impactor. They represent intrusive offshoots of the homogenized impact melt body that originally lay within the crater. U–Pb single zircon and Ar–Ar dating indicates that the Vredefort Granophyre and pseudotachylitic breccias, and the Witwatersrand pseudotachylites Corresponding author. Tel.: +27 11 717 6565; fax: +27 11 717 6579. 1

E-mail addresses: [email protected] (W.U. Reimold), [email protected] (R.L. Gibson). Note that as of 1 January 2006, W.U. Reimold’s e-mail address will be: [email protected].

0009-2819/$ - see front matter r 2005 Elsevier GmbH. All rights reserved. doi:10.1016/j.chemer.2005.07.003

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all formed at 202075 Ma – the age of the impact event, making the breccias a convenient time marker in the evolution of the structurally complex Witwatersrand basin with its unique gold deposits. r 2005 Elsevier GmbH. All rights reserved. Keywords: Vredefort impact structure; Impact melt rock; Granophyre; Pseudotachylite; Pseudotachylitic breccia; Breccia formation; Impact cratering; Witwatersrand gold

1. Introduction Largely as a direct result of Space exploration, impact cratering has been widely accepted as one of the most important processes in the Solar System (e.g., Taylor, 2001). The surfaces of all solid bodies in our planetary system are densely covered with impact structures. On Earth, some 170 impact structures have been identified to date (www.unb.ca/passc/ImpactDatabase/). Whereas first-order indications of the presence of an impact structure in the form of circular morphology or a centered geophysical anomaly may lead to the initial recognition of an impact structure, positive evidence in the form of direct (meteoritic debris) or indirect (chemical signature in melt rock) evidence of the extraterrestrial projectile, or of bona fide shock metamorphism, is required as final proof (e.g., Sto¨ffler and Langenhorst, 1994; Grieve et al., 1996; French, 1998). The physical processes related to impact cratering are reasonably well understood and can be modeled in considerable detail (Pierazzo and Herrick, 2004), but the geology of impact structures and the nature of impactite formation and distribution in complex crustal targets are still the subjects of intense analysis. The impact cratering process (e.g., Melosh, 1989) is characterized by rapid transfer of enormous amounts of energy from the projectile to the target rock volume. Following the explosion of the projectile after its penetration into the target volume, a shock wave travels into the target, causing vaporization near the explosion site, surrounded by successive zones characterized

respectively by melting of bulk target rock, melting of individual minerals, and, further away, plastic and elastic deformation (e.g., French, 1998). The developing (transient) crater is excavated until its walls become unstable and collapse. In the case of relatively small events (craters of o2–4 km diameter), simple bowlshaped crater structures develop. In larger events, socalled complex impact structures result, as the strongest compressed part of the target below the crater floor rebounds. This leads to the formation of a central uplift, and the unstable, steep crater walls collapse to form terraced rim sections. Following rebound, the central uplift collapses onto itself and results in either central peak or central peak ring structures. The central complex is usually covered with fallback breccia and impact melt, and various impactites (impact breccias) accumulate in the ring syncline around the central uplift (see Fig. 1; and French, 1998 – also for impact breccia classification and definitions). Traditionally, the impact process is divided into several stages: (1) the initial ‘‘contact phase’’ when the projectile interacts with the target is followed by the ‘‘compression phase’’, during which the shock wave propagates through the target; (2) the ‘‘excavation phase’’, during which the transient cavity develops; and (3) the ‘‘collapse and modification phase’’, which involves the rebound-driven rise and collapse of the central uplift, and the collapse of the transient crater wall by inward sliding of large blocks from the crater rim, along listric faults (compare Fig. 1). Whilst these stages last only seconds to minutes, even for the largest

Fig. 1. Schematic cross section through a large, complex impact structure, illustrating the central uplift, distribution of various types of impact breccia, and the slumping of large blocks along listric faults around the edge of the structure. Impact-generated melt rocks can be expected as impact melt on top of the crater fill (including the upper part of the central uplift where gravitational settling into opening fractures can occur, as in Vredefort) and as pseudotachylitic breccia, development of which is likely concentrated in the upper part of the central uplift and along fault zones in the outer parts of the structure. It cannot be excluded that block movement below the crater moat can also result in formation of pseudotachylitic breccia.

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impact events, recent studies have also emphasized the importance of the subsequent stage of post-impact hydrothermal activity (e.g., review by Naumov, 2002), which takes place over tens to hundreds of thousands of years after an impact (Osinski et al., 2001; Abramov and Kring, 2004). Impact melting during the highly dynamic, turbulent cratering process is complex. Melts may represent bulk melting of rock types or mixtures of such bulk melts. Or they can represent melts derived from individual minerals or combinations of minerals. In this regard, impact melting is atypical of most crustal melting, which involves eutectic melting. Consequently, impact melting can result in a wide variety of unusual and even unique compositions, and typically creates clast-bearing melt breccias. Impact-melted material may be deposited as a coherent impact melt body within the crater (on top of the central uplift as well as in the rim syncline, Fig. 1) or as disseminated particles of sub-millimeter to meter size within the impact breccia deposits of the crater fill or circum-crater ejecta blanket. In addition, impact melt rock and impact melt-bearing breccia (impact melt breccia, as well as suevite that is dominated by clastic mass but still contains at least a small melt component) can become injected into the crater floor, below the central parts of large impact structures, as can purely clastic impact breccias (known as lithic or fragmental impact breccia). Finally, local melt breccias can develop in the subcrater basement, either due to shock melting or friction melting along fault surfaces, or both. Terminology around these latter melt breccias is somewhat loose (see review in Reimold and Gibson, 2005). They have generally been described as pseudotachylite; however, we will argue here for the use of a more general term – pseudotachylitic breccia – unless the cause of the melting is clear. Understanding impact melt rocks can provide important constraints on the energy thresholds of impact events of different magnitude, with scaling relationships between crater geometry and crater scale vs. impact melt volume having been evaluated for terrestrial and extraterrestrial impacts (Melosh, 1989). Furthermore, impact-generated melt rocks allow dating of impact events. Impact melt rocks contain clast populations that comprise material derived from all parts of the crater structure, thus characteristically including diagnostically shock metamorphosed clasts. The melts may also contain a projectile component, which is important for the identification of an impact structure. The emplacement mechanism of impact melt is important, as it provides information regarding the material flow during the impact process – vital information, for example, for constraining numerical models of the impact physics. The clast content of melt rocks provides a means of determining the composition of the target volume, which has largely been obliterated in the process and,

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in the case of our dynamic planet, may have been subject to later geological overprints.

1.1. The Vredefort impact structure Since the earliest detailed studies of the Vredefort Dome (or Ring), it was recognized that its origin involved catastrophic forces (e.g., Shand, 1916; Hall and Molengraaff, 1925). Boon and Albritton (1936) suggested first that this
90 km wide and roughly circular structure might be of impact origin, based on the recognition by Shand (1916), Hall and Molengraaff (1925) and Nel (1927) of extensive volumes of melt rock – the so-called ‘‘pseudotachylyte’’ (Shand, 1916; modern spelling ‘‘pseudotachylite’’) – and the general observation of intense rock deformation. The idea of meteorite impact was later debated by Daly (1947), who concluded that it was a more likely explanation than any other then-available endogenic hypothesis. Dietz (1947, 1959) promoted shatter cones as a recognition criterion for impact structures, and his prediction that the Vredefort Ring would contain this feature (Dietz, 1960, 1961) was confirmed by Hargraves (1961). Microdeformation features in quartz from Vredefort rocks (Carter, 1968), of pseudotachylite (Wilshire, 1971; Schwarzman et al., 1983), and Martini’s (1978) identification of coesite and stishovite in pseudotachylite (new spelling) strongly suggested an impact origin for the structure. Nevertheless, in South Africa the majority of geoscientists continued to adhere to internal processes as a cause of dome formation – by either tectonic processes (Du Toit, 1954; Colliston, 1990; Coward et al., 1995), diapirism (Brock and Pretorius, 1964; Ramberg, 1967), or tectonism plus cryptoexplosion (Nicolaysen and Ferguson, 1990; for a review, see Reimold and Gibson, 1996). Near-general acceptance of the impact origin for the dome came through the confirmation by Leroux et al. (1994) that the basal planar microdeformation features in quartz from the Vredefort Dome are a shock deformation effect. Kamo et al. (1996) identifed shocked zircon in Vredefort melt rocks, and Koeberl et al. (1996) determined a meteoritic component in Vredefort impact melt rock. Kamo et al. (1996) also dated the impact by U–Pb dating of single zircons in the impact melt rock and pseudotachylitic breccia to 202374 Ma, within error limits of the results by Gibson et al. (1997a), Moser (1997), and Spray et al. (1995). Until the beginning of the 1990s, the limits of the Vredefort Dome were considered to define the extent of the deformation related to the catastrophic doming event. However, when the limit of ‘‘pseudotachylite’’ development was extended far into the surrounding Witwatersrand basin (Fletcher and Reimold, 1989; Killick and Reimold, 1990) and the breccias in the Witwatersrand goldfields (see Fig. 2) yielded similar ages

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Fig. 2. Distribution of major lithologies and goldfields, and the outer limits of planar deformation feature (PDF) and shatter cone development, in and around the Vredefort Dome near the geographic center of the Witwatersrand Basin.

to that of the Vredefort breccias (Trieloff et al., 1994), it was recognized that the Vredefort impact structure had to have been originally much larger than the size of the Vredefort Dome. Scaling of the distribution of shatter cones, pseudotachylite, and shock deformation features in quartz (Therriault et al., 1997) suggested a ca. 300 km crater diameter (Fig. 3), which was confirmed by the comprehensive geophysical modeling of the VredefortWitwatersrand system by Henkel and Reimold (1998). This makes Vredefort a very large, complex impact structure, possibly the largest known impact structure on Earth; thus, the Vredefort Dome represents only the central uplift of this structure. The absence of both the craterform and extensive crater fill impact breccia volumes in the region suggested that the structure is deeply eroded. This was confirmed by detailed metamorphic studies in the dome (Gibson and Wallmach, 1995; Stevens et al., 1997; Gibson et al., 1998; Gibson and Reimold, 2000) that showed that the currently exposed level has been exhumed by 7–10 km.The recent developments have placed Vredefort into the elite triad of very large terrestrial impact structures, together with the Sudbury (
250 km, 1850 Ma) and Chicxulub (180–200 km, 65 Ma) structures. Grieve and Therriault (2000) compared these three structures and concluded that they represent the only multi-ring basins (Spudis, 1993) presently known on Earth. The Vredefort impact structure, thus, involves the Vredefort Dome (central uplift) and the surrounding Witwatersrand basin (ring basin) (Fig. 3). The Witwa-

Fig. 3. Large syn- and antiforms around the Vredefort Dome (after McCarthy et al., 1990; Therriault et al., 1997), and the 300 km limit that could represent the original diameter of the Vredefort impact structure.

tersrand basin hosts the world’s largest gold resource (e.g., Frimmel, 2004; Frimmel and Minter, 2002), which is now believed by some workers to have been preserved from erosion primarily as a consequence of the downfaulting and folding of the gold-bearing

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strata due to the Vredefort impact (McCarthy et al., 1986, 1990; Grieve and Masaitis, 1994; Reimold et al., 2005). Here, we synthesize the extensive knowledge about the impact-generated melt rocks in the Vredefort impact structure, the Vredefort Granophyre (impact melt rock), and the various breccias that, in the past, have been collectively referred to as pseudotachylite. This is then evaluated with regard to the involvement of such breccias in the impact cratering process and with regard to the relationship between the Vredefort impact and the Witwatersrand gold resource.

2. Geological background 2.1. Regional geological setting The Kaapvaal craton of north-central South Africa comprises a series of granite-greenstone fragments of 3.6–2.7 Ga age (Schmitz et al., 2004). Tectono-magmatic analysis in the last decade has led to the recognition of a core (shield) that formed through amalgamation of individual micro-continental blocks between 3.6 and 3.1 Ga, and that culminated in widespread granitoid magmatism at ca. 3.1 Ga (e.g., De Wit et al., 1992; Poujol et al., 2003; Schmitz et al., 2004). Subsequently, a series of rifting and basin-forming events took place, with associated deposition of a thick pile of supracrustal sedimentary and volcanic strata. The oldest event is the 3.07470.009 Ga (Armstrong et al., 1991) rifting stage that generated a bimodal sequence of basaltic andesite and felsic lavas with subsidiary rift-related clastic sediments known as the Dominion Group (Jackson, 1994). Between ca. 2.97 and 2.71 Ga (Robb and Robb, 1998), up to 7 km of clastic sediments, with minor banded ironstones, were deposited to form the Witwatersrand Supergroup. The basal West Rand Group comprises predominantly shallow-marine to subtidal argillaceous-arenaceous sediments, whereas the overlying Central Rand Group is dominated by relatively coarser-grained fluviatile to subtidal arenaceous-rudaceous sediments. Gold mineralization occurs mostly in the conglomerates of the Central Rand Group. At 2.714 Ga (Armstrong et al., 1991), Witwatersrand sedimentation was terminated by the extrusion of up to 3 km of tholeiitic flood basalts of the Klipriviersberg Group (Ventersdorp Supergroup), followed by deposition of up to 2 km of localized rift sediments and felsic volcanics (Platberg Group). A shallow craton-wide basin developed from
2.6 Ga, into which some 2 km of carbonates and subsidiary iron formation of the Chuniespoort Group of the Transvaal Supergroup were deposited. From about 2.35 to 2.15 Ga, sedimentation of the 3 km thick argillaceous-arenaceous Pretoria

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Group occured (Walraven and Martini, 1995). Following this, the central Kaapvaal craton experienced a major volcanic and intrusive magmatic event at 2.05–2.06 Ga (Bushveld Event; e.g., Buchanan et al., 2004). 2.1.1. Large-scale geology of the Witwatersrand Basin Along the N and NE margin of the basin the Witwatersrand Supergroup strata dip at shallow angles into the basin. The northern margin of the basin is formed by a crustal warp, the Rand Anticline. South of a line diagonally across the Vredefort Dome, the basin is entirely covered by Phanerozoic (300–180 Ma) Karoo strata (see also Fig. 4). The basin extends in a northeast-southwest direction for
250 km, but is narrower (
140 km) in the perpendicular direction. The 90 km wide Vredefort Dome is roughly located in the center of the basin, with the 50 km wide Potchefstroom Synclinorium along the northern margin of the dome. In this syncline, the Pretoria Group strata are folded into a series of concentrically arranged, kilometer-scale anticlines and synclines (Fig. 3). Smaller-scale thrusts and associated

Fig. 4. Schematic geological map of the Vredefort Dome.

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folding occur as far as around Johannesburg –
150 km from the center of the dome, and have been temporally associated with the Vredefort impact event (e.g., Simpson, 1978; McCarthy et al., 1986, 1990; Gibson et al., 1999). McCarthy et al. (1990) were the first to postulate that the preservation of these strata in what is today known as the ‘‘structural remnant of the Witwatersrand basin’’ is the result of downfaulting of Witwatersrand strata as a consequence of the impact event and the ensuing cover with thick impact breccia deposits.

2.2. Geology of the Vredefort Dome and immediate environs The geology of the Vredefort Dome (Gibson and Reimold, 2001) is depicted on the 1:250 000 West Rand and Kroonstad geological maps of the Council for Geoscience (1986, 2000) and a 1:50 000 geological map of the dome by Bisschoff (1999). Extensive lithological and structural mapping of the Archean basement rocks in the dome was subsequently done by Lana (Lana, 2004; Lana et al., 2003b, c). The dome is well exposed in its northern and western sectors (Fig. 4). It comprises a 40–45 km wide central core of Archean basement gneisses and migmatites, surrounded by a
20 km wide collar of subvertical to overturned supracrustal strata of the Dominion Group and the Witwatersrand, Ventersdorp, and Transvaal supergroups. The Archean basement core consists of trondhjemitic and granodioritic gneisses and granites, with subsidiary mafic, ultramafic, meta-pelitic and meta-ironstone xenoliths. The Greenlands Greenstone Complex occuring in the SE sector has strong similarities to parts of the Barberton Greenstone Belt, and could be as old as 3.3 Ga (Lana et al., 2003a). The core gneisses and migmatites have traditionally been divided into amphibolite-grade Outer Granite Gneiss and granulite-grade Inlandsee Leucogranofels (Stepto, 1990; Bisschoff, 1999), but recent mapping (Lana, 2004; Lana et al., 2003b, c) has shown that a similar, diverse sequence of trondhjemitic-tonaliticgranodioritic lithologies occurs throughout the core, although metamorphic grade varies with radial distance from the crater. The collar rocks are intruded by several alkali granitic and basic igneous bodies (Schurwedraai, Baviaanskrans, Roodekraal, Rietfontein, Lindequesdrift – Fig. 4). These complexes show Vredefort impact-related shatter coning and pseudotachylitic breccia occurrences and, thus, must predate the impact event. Various ages between 2.2 and 2.05 Ga have been obtained for these rocks, with most recent work suggesting a syn-Bushveld age (Walraven and Elsenbroek, 1991; I. Graham, Pretoria, pers. comm., 2003). In addition, the mafic intrusive Losberg Complex, geochronologically and geochemi-

cally linked to the Bushveld event (Coetzee and Kruger, 1994), occurs in the Potchefstroom Synclinorium, north of the dome. This body deserves further study to confirm its pre-impact origin. Numerous mafic sills, now metamorphosed to epidiorite, have intruded the collar strata and have been correlated with Ventersdorp magmatism (Pybus, 1995). Several mafic intrusions into the collar and adjacent terrane in the southwestern core, have been related by Bisschoff (1982) to the 2.06 Ga Bushveld event. A series of post-impact tholeiitic intrusions into the dome, including the 1.05 Ga Anna’s Rust Sheet (Pybus, 1995; Reimold et al., 2000) and drilling intersections from the central part of the dome, are all thought to represent a
100 m thick diorite sheet.

2.3. Regional metamorphism The Vredefort Dome exposes rocks that range in metamorphic grade from greenschist to granulite facies (Bisschoff, 1982; Gibson and Stevens, 1998). The metamorphic pattern is broadly concentric, with grade decreasing away from the center of the dome. The predominant upper amphibolite to granulite facies metamorphic pattern in the core, together with the mid-greenschist facies metamorphism in the Greenlands Complex in the southeastern sector, dates to Archean times (3.1 Ga, Hart et al., 1999; Lana et al., 2003a; Lana, 2004) and, thus, predates deposition of the collar rocks and their metamorphism. The current facies distribution reflects progressively deeper exhumation towards the dome center. The supracrustal rocks in much of the collar and surrounding Witwatersrand basin attained greenschist facies, whereas the Witwatersrand rocks of the innermost collar and the Dominion Group meta-lavas in the dome experienced mid-amphibolite grade (Gibson and Wallmach, 1995). The metamorphic assemblages are affected and cut by impact-related structures, indicating a pre-impact age of metamorphism. Gibson and Wallmach (1995) and Gibson et al. (2000) attributed this regional metamorphism to the Bushveld event. The impact itself generated a highly variable thermal overprint (granulite facies in the center of the dome to lower amphibolite to greenschist facies in the collar; Gibson et al., 1997b). This shock heating partially replaced the pre-impact metamorphic parageneses in the host rocks and produced recrystallization, or even slow crystallization, of impact-related breccias (see below).

2.4. Melt rocks and their occurrence The Vredefort Dome is famous for two specific types of melt rock that are related to the impact event: pseudotachylitic breccia and the Vredefort Granophyre.

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2.4.1. Pseudotachylitic breccias Nearly every lithology in the Vredefort dome affected by the impact event carries an abundance of ‘‘pseudo-

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tachylyte’’ (Shand, 1916; historical spelling). This material (Fig. 5) forms dark-grey to black veins and pods, and locally massive networks, comprising a

Fig. 5. (a) Small portion of the massive crystalline breccia at Leeukop quarry, N of Parys. Note well-rounded mega-clasts and apparent settling towards the bottom. This exposure was sampled for the dating of Kamo et al. (1996). (b) Several veins of pseudotachylitic breccia (quarry exposure N of Parys). The vein on the left (NE–SW trending) has some displacement, probably oblique to this quarry face. The main vein is a so-called ‘‘paired shear’’ (in analogy to such a development of pseudotachylite in tectonic setting – e.g., Reimold and Colliston, 1994), where country rock has been brecciated between two parallel shear surfaces. Hammer ca. 70 cm long. (c) A view into part of the Salvamento quarry north of Parys. This breccia face provides an impression of the size variation and shapes of large granite gneiss xenoliths, within the fine-grained crystalline matrix of this pseudotachylitic breccia (compare Fig. 19 of Dressler and Reimold, 2004). (d) Meter-scale fold structure in Hospital Hill quartzite (lower Witwatersrand Supergroup): dm-thick pseudotachylitic breccia (arrows) concentrated in hinge zone (to lower left of fold). Width of field of view: ca. 2.5 m.

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glassy-looking to microcrystalline groundmass supporting highly variable amounts of host rock clasts. Shand (1916) proposed that the ‘‘pseudotachylyte’’ was generated not by shearing but by shock or, alternatively, by gas-fluxing. Today, the recognition of similar-looking breccias in numerous crustal settings apparently unrelated to impacts has led to the term ‘‘pseudotachylite’’ becoming synonymous with ‘‘friction melt’’ (e.g., Magloughlin and Spray, 1992a, b; Reimold, 1998; Gibson et al., 1997b; Reimold and Gibson, 2005; pseudotachylite occurrences in high-T settings: Passchier, 1982; Clarke and Norman, 1993). They generally are directly related to fault and shear zones and, in favorable circumstances, these ‘‘pseudo-igneous’’ materials may allow dating of fault movements. Spray (1995) determined that pseudotachylite can be derived by initial cataclasis of precursor rock followed by frictional melting at very high (4101 s1) strain rates. Most frequently, such conditions occur in the brittle-ductile regime at relatively shallow crustal levels, with low ambient temperatures favoring elastic deformation. Besides tectonic occurrences, ‘‘pseudotachylites’’ have been described from a number of impact structures. The Vredefort Dome is the type locality (Shand, 1916); its abundance of such breccia is rivalled only by that of the Sudbury Structure (Dressler and Reimold, 2004). Similar breccias in other impact structures are generally not abundant and are usually restricted to a handful of occurrences of centimeter or even millimeter thickness and very limited extent. Given the exceptional strain rates that accompany impact cratering (X104 s1; Spray, 1998), impact structures might be considered favored sites for the development of friction melts. The challenge, however, is how to distinguish such ‘‘pseudotachylites’’ (sensu stricto) from other impact-related breccias and melts. It is, thus, perhaps not surprising that a range of terms for such materials (such as E( ¼ endogenic)- or S( ¼ shock)-pseudotachylite, A- or B-pseudotachylite, A1/A2 or B breccias, etc; e.g., Lambert, 1981; Martini, 1991; Reimold et al., 1992a; Spray 1998) has been proposed. Reimold (1998) and Reimold and Gibson (2005) have discussed this nomenclature and its pitfalls and proposed that the term ‘‘pseudotachylitic breccia’’ be applied, in a non-genetic sense, whenever the true genetic process is not known. Several impact workers have expressed the view that ‘‘pseudotachylite presence’’ is a diagnostic criterion for impact (e.g., Bland, 2003), but this has been solidly refuted (Reimold and Gibson, 2005). Breccias of pseudotachylite-appearance at and around Vredefort may well be the result of more than one genetic process, and certainly a range of different breccia types of pseudotachylitic appearance occurs in wide parts of the Witwatersrand basin. In Sudbury, as well, different types of Sudbury Breccia have been identified (Mu¨llerMohr, 1992).

2.4.2. Vredefort Granophyre A highly unusual, clast-laden melt rock (Fig. 6), with a micropegmatitic or granophyric matrix texture (compare, e.g., Fig. 7b) and unusual, regionally homogeneous chemical composition was known as ‘‘enstatitegranophyre’’ (Hall and Molengraaff, 1925), ‘‘basic granophyre’’ (Willemse, 1937), or ‘‘bronzite-granophyre’’ (Bisschoff, 1972), although the matrix pyroxene is in fact hypersthene (Reimold et al., 1990a). For the last 15 years the term ‘‘Vredefort Granophyre’’ has been applied to this lithology. This material was initially variably interpreted as ‘‘a flinty crush-rock formed by ultratrituration and fusion’’ or as ‘‘a glorified form of pseudotachylyte’’ (Hall and Molengraaff, 1925), based on their common dark-grey, fine-grained but clast-laden appearances. Willemse (1937) and Bisschoff (1988) suggested that this lithology could represent an igneous intrusion (diorite or lamprophyre) that had stoped out upper crustal rocks during its emplacement. Dietz (1960, 1961) proposed a possible origin as impact melt rock, largely because of the chemical homogeneity of the Granophyre. As these dikes regionally occur only in the Vredefort Dome, and as this lithology could not predate the Vredefort event due to a lack of the other Vredefort-related deformation features, this rock type was generally believed to be genetically linked to the updoming event. Accordingly, when K–Ar and then first 40Ar–39Ar dating of Granophyre samples yielded an
2 Ga age (Walraven et al., 1990; Allsopp et al., 1991), this age was widely regarded as a good approximation for that of the Vredefort event. Kamo et al. (1996) confirmed the 2020 Ma age for the Vredefort event by dating single zircon crystals of igneous morphology from both pseudotachylitic breccia

Fig. 6. Typical block of Vredefort Granophyre with some exceptionally large clasts. The angular, mottled clasts are derived from granitoid precursor rock; the much smoother inclusions are mostly quartzite-derived. The large, elongated and rectangular clast (center) is roughly 30 cm long.

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Fig. 7. (a) Fine-grained spherulitic groundmass of Vredefort Granophyre. Note recrystallization to finest-grained mosaics in quartz clasts (white). Long laths of hypersthene nucleated on one of the quartz clasts; some smaller and stubby, granular orthopyroxene crystals are visible, too. Plane polarized light, width of field of view 3.5 mm. (b) Micropegmatitic (granophyric) groundmass of Vredefort Granophyre. Intergrowth of quartz, plagioclase and alkali feldspar, with some small euhedral crystals of magnetite and rare granules of orthopyroxene. Crossed polarizers. Width of field of view 2.2 mm. (c) Ternary diagrams of the modal compositions of the different types of Vredefort Granophyre (after Therriault et al., 1996).

and Granophyre. Several groups (Schreyer, 1983; French et al., 1989; Reimold et al., 1990a) investigated the impact melt option by attempting to find chemical traces of a meteoritic component, but failed. Re–Os isotopic analysis finally allowed Koeberl et al. (1996) to confirm that the Vredefort Granophyre represents impact melt rock.

2.5. Shock metamorphism in the Vredefort Dome Boon and Albritton’s (1936) suggestion that the Vredefort Dome could represent an impact structure was based on its somewhat circular shape rather than currently acceptable criteria for impact. First tentative evidence came from Dietz’s (1960, 1961) recognition of shatter cones, however Simpson (1981) and Reimold and Colliston (1994) produced some evidence of temporal relationships between shatter cones and Vredefort-related fault gouge and pseudotachylitic breccia, respectively, that generated some circumspection with regard to shatter cones as impact-diagnostic

evidence. Following the identification of planar microdeformation features and other shock metamorphic effects in quartz and other rock-forming minerals in a number of impact structures, and their experimental investigation (e.g., French and Short, 1968), the detection of such features in quartz from Vredefort rocks (Carter, 1968) generated much interest in Vredefort as a possible impact structure. The enormous volume of pseudotachylitic breccia in the dome – similar to Sudbury, where shatter cones had also been known since the early 1960s (Dietz, 1964) and where planar deformation features (PDFs) in quartz had been identified (e.g., French, 1972) – also contributed to this view. As the PDFs in Vredefort quartz (Fig. 8a) are quite strongly annealed by the post-impact metamorphism and do not have the normal appearance of pristine PDFs (Reimold, 1990), their recognition as an impactdiagnostic feature remained unconfirmed until TEM studies (Leroux et al., 1994) showed that the quartz contained bona fide shock deformation effects in the form of basal Brazil twins. Planar fluid inclusion trails aligned along higher-order orientations are regarded as

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annealed planar deformation features (Grieve et al., 1990; Leroux et al., 1994). Martini’s (1978) discovery of coesite and stishovite in thin pseudotachylitic breccia veinlets from the northeastern collar of the dome convinced most of the international community of the validity of a Vredefort impact. The next milestone development was the recognition of shocked zircon in clasts of both Vredefort Granophyre and pseudotachylitic breccia by Kamo et al. (1996) and, shortly thereafter, in other rocks by Gibson et al. (1997a). Several hundred thin sections of Granophyre failed to provide any evidence of shock metamorphism in quartzite- and granite-derived inclusions (French et al., 1989; Reimold et al., 1990a; Therriault, 1992). The vast majority of mineral and lithic clasts is extensively melted and forms part of the melt matrix. Remaining clasts are generally completely or mostly recrystallized (annealed) to fine-grained mineral mosaics. Buchanan and Reimold (2003) eventually identified shocked clasts in the Granophyre, with PDFs as well as crystallographically controlled lamellar melting in quartz (Fig. 8). It was long considered strange that, besides the largely annealed anomalous deformation features in Vredefort quartz, no further shock-related deformation had been found in other minerals in rocks from the Vredefort Dome (Reimold, 1992). However, Gibson and Reimold (2005) described shock and post-shock thermal metamorphic textures from various minerals (Fig. 9) and concluded that the centralmost part of the dome experienced shock pressures of up to 40–50 GPa and shock temperatures of at least 800–1000 1C, and possibly as high as 1350 1C. The outermost part of the core is characterized by strong development of PDFs in quartz, characteristic of shock pressures between 10 and 30 GPa (e.g., Sto¨ffler and Langenhorst, 1994; French, 1998). Gibson and Reimold (2005) observed strong shock heterogeneity at even the mm-scale throughout the core, which is important with regard to the discussion of formation of pseudotachylitic breccia (Gibson and Reimold, 2005). Fig. 8. (a) Two sets of decorated planar deformation features (PDFs) in a partially annealed quartzite clast in Vredefort Granophyre. PDFs along the left margin of this grain seemingly trend into the adjacent quartz crystal; this is interpreted that this quartz grain is partially annealed and originally covered the area right up to the left margin of this image. Width of field of view 0.45 mm. (b) Part of a vermicular quartz grain illustrating the crystallographic control on melting. Width of field of view 1.1 mm. (c) Checkerboard feldspar (left) in contact to finest-grained Granophyre ‘‘matrix’’. This matrix material represents part of an original granitic clast. Width of field of view: 2 mm. All images taken with crossed polarizers.

3. Vredefort Granophyre 3.1. Occurrence and general appearance Nine dikes of Vredefort Granophyre occur in the Vredefort Dome (Fig. 4). Their total exposed length is 50 km, and widths vary from 10 to 50 m (Therriault, 1992; Therriault et al., 1996). Four Granophyre dikes occur in the inner core up to 9 km from the core-collar boundary, hosted entirely by granitoids. They strike in either NE–SW or NW–SE directions, which correspond to the main fabric orientations in the Archean basement

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Fig. 9. Shock zonation in the Vredefort Dome, after Gibson and Reimold (2005).

(e.g., Reimold and Colliston, 1994; Lana et al., 2003b). The other five dikes straddle the core-collar boundary, with exposures in granite-gneiss, quartzite, shale, and epidiorite. The core dikes that are radial to the structure are relatively small (up to 4.5 km long and 20 m wide) in comparison to the tangential core-collar boundary dikes (up to 9 km long and 65 m wide). The more or less vertical attitude of dikes is evident in a number of corecollar boundary exposures, and in shallow outcrop against charnockitic gneiss on farm Lesutaskraal 72. Dike edges are locally slickensided. Increased structural deformation and thermal alteration may occur in a narrow zone of host rock directly adjacent to a dike, and dikes may have up to 1.5 cm wide chilled margins (Therriault, 1992; authors’ observations). Reimold et al. (1990a) showed that dike margins may also have been exploited for the formation of very thin (up to several mm wide) pseudotachylitic breccia. These authors also cited a 1 cm wide pseudotachylitic breccia crosscutting a Granophyre dike on farm Rensburgsdrift at the northwestern core-collar boundary. Bisschoff (1972) described a Granophyre dike on farm Spitskop 1060 as cutting across and being chilled against pseudotachylitic breccia, but Therriault et al. (1996) held the opinion that the pseudotachylitic breccia at this locality could enclose blocks of Granophyre. Locally, dikes are exposed as discontinuous, sinuous or kinked arrays of boulder blocks on surface. Therriault et al. (1996) noted that the core/collar boundary dikes are topographically elevated by 40–100 m relative to the core dikes, and attributed differences in textural character in the dikes to this. Offshoots are rare and are generally not more than a few

meters long and up to a few meters wide. These authors discussed duplication of dike portions or possible offshoots represented by parallel dikes, but this could also represent displacement of a Granophyre dike due to faulting. Indeed, in several places, displacement by up to a few tens of meters is observed. The dikes terminate either by pinching out or ending abruptly in mid-slope or adjacent to a depression. Jointing and faulting is prominent but is typically rather widely spaced, at several decimeters. Displacements up to several decimeters are known from some narrow faults. Macroscopically, the Granophyre may be mistaken for pseudotachylitic breccia, in places. However, with the exception of very fine-grained selvages and quench margins, the typical spherulitic or granular textures of granophyre are distinctive (Fig. 7a and b). In contrast, the matrix textures of pseudotachylitic breccia are always extremely fine-grained, even in the strongly thermally overprinted area in the center of the dome. Locally, however, the pseudotachylitic breccia has a micro-spherulitic texture, very similar to spherulitic Granophyre. Melt rock types may show a pitted surface, with up to 20 cm sized open pits. These are devoid of any lining, but reaction aureoles may be visible in the adjacent groundmass. These void structures are interpreted to represent loci of weathered-out clasts. Clasts are ubiquitous in the Granophyre; they constitute about 20 vol%, on average (Therriault et al., 1996). Macroscopic clasts range from 80 cm to a few millimeters, and the microscopic evidence extends this range to o0.5 mm. Clasts larger than 10 cm (Fig. 6) are, however, rare. Most macroscopic clasts are either angular to subangular, but rounded inclusions can also

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be observed. Strong plastic deformation in the form of bending and folding of clasts is widespread. Inclusions commonly have irregularly shaped edges, as a result of resorption by, or reaction with, the matrix. The macroscopically visible clast population can be highly varied in clast size and abundance. Where the population is dominated by small (o1.5 cm) clasts, the Granophyre may appear quite clast-poor, whereas a clast population with a majority of inclusions larger than 1 cm may give the impression of clast-laden Granophyre. Contrary to the impression of earlier workers that the Granophyre clast population was dominated by quartzite clasts (99%, Manton, 1962), it has now been established (Reimold and Reid, 1989; Reimold et al., 1990a; Buchanan and Reimold, 2003; unpublished data of the authors) that 20–60% of all clasts are granitoid-derived, and 420% of the microclasts are granitoid-derived. Quartzite is the second most important component, followed by much rarer metapelite (shale) and mafic intrusion (epidiorite) clasts. These latter lithologies may be important only where a corecollar boundary dike cuts across shale or epidiorite strata and has sampled local material. Therriault et al. (1996) noted that, as matrix grain size increases towards the center of a dike, the proportion of large clasts decreases. Dikes in the core of the dome seem to have more large clasts than core/collar boundary dikes, and large clasts typically appear to be concentrated along one side of a dike. This was considered evidence for overturning of the dikes and their host rocks (Bisschoff, 1972). However, even adjacent dikes may exhibit this concentration on opposite sides, suggesting that this one-sided concentration is likely a result of flow dynamics (Therriault, 1992). In the field, distinct differences in matrix texture can be discerned at many sites: it may be granular, of varied mineral size (up to several mm), or spherulitic (Fig. 7a), with needle-shaped hypersthene forming distinct rosettes and spherulites that may reach 7 cm diameter locally. In some places, narrow (o6 cm) bands of apparently finer-grained groundmass are observed between less fine-grained granular material. Bisschoff (1972) regarded this as evidence of magmatic composite dikes, but Therriault (1992) explained these zonations by crystallization and flow mechanisms. Therriault et al. (1996) also reported that the spherulitic texture is more important in core dikes than at the core/collar boundary. Reimold et al. (1990a) noted that micro-clasts are particularly abundant where spherulitic texture is developed, and granular Granophyre contains comparatively more large clasts. Locally on farms Daskop 1103 and Zandfontein 194, Granophyre occurences have been used by San people for the production of petroglyphs (see, for example, the cover of this issue; also Reimold and Wallmach, 1991, Fig. 1).

3.2. Petrography 3.2.1. Groundmass texture and mode Therriault et al. (1996) distinguished four microtextural types (Table 1a) of Granophyre groundmass: spherulitic A (very fine-grained spherulites), B (fine- to coarse-grained spherulites), and C (micro-ophitic texture), and granular (hypidiomorphic texture). French et al. (1989) referred to a vitrophyric type, which, however, has not been sampled by us; Reimold et al. (1990a) noted an intersertal-spherulitic transition type. Table 1b gives mineralogical compositions of the four matrix types and Table 1c the compositions of the respective groundmasses. The Granophyre displays relatively uniform mineral composition, but there is significant variation in abundances of the major modal components (Fig. 7c). Important matrix minerals (including phenocrysts) are hypersthene, plagioclase, orthoclase, quartz, biotite, magnetite and ilmenite, besides traces of pyrite and chromite. Augite and pigeonite are accessories in granular Granophyre, with clinopyroxene at least partially overgrowing orthopyroxene crystals. Pyroxene is partially replaced by biotite, amphibole, and chlorite, indicative of either deuteric alteration of the primary assemblage or metamorphism under amphibolite to upper greenschist grades. The principal difference between the granular and spherulitic matrix types is the habit of pyroxene that occurs as laths and long blades, or as stubby, prismatic crystals. The resulting textures range from subophitic to ophitic. In-between this network of pyroxene crystals, the other phases (mostly quartz and feldspar minerals) form a micropegmatitic (‘‘granophyric’’), graphic intergrowth. The majority of clasts are extensively or completely recrystallized, but it is still possible to detect continuation between partially recrystallized/melted clasts and interstitial matrix. Reimold and Reid (1989) found micropegmatitic patches within granitic and quartzitic inclusions (Buchanan and Reimold, 2003). The proportion of groundmass in the Vredefort Granophyre varies from 31 to 50 vol% (Therriault et al., 1996). Mineral chemistry in the groundmass is complex: groundmass plagioclase varies from An36and orthoclase is Or8396Ab716 55Ab4561Or230 An03.5, whereas microphenocrysts have compositions of An5362Ab3747Or03 in cores and An3655Ab4561Or02 in rims. Reimold et al. (1990a) reported plagioclase of An2365, with Ab452 occuring in the groundmass. Both normal and reverse zoning occurs in orthopyroxene microphenocrysts, with compositions of Wo2En4278Fs2056. Pigeonite overgrowths are Wo515 En3970Fs2454 and augite is Wo1545En3157Fs1348. Buchanan and Reimold (2003) compared the compositions of various minerals in Granophyre groundmass with those of microlitic phases grown within melt pockets of granitic clasts. They found that plagioclase

Fine-to-coarsegrained spherulites

Micro-ophitic texture

Hypidiomorphic texture

Spherulitic B

Spherulitic C

Granular

b

Occurs as intergrowths in Matrix. Occurs as microphenocrysts and laths.

Very fine-grained spherulites

Spherulitic A

a

Textural description

Hypersthene Opaques Quartz Plagioclasea Orthoclase Biotite Hypersthene Biotite Plagioclasea Quartz Orthoclase Opaques

Hypersthene Plagioclaseb Biotite Quartz Orthoclase Plagioclasea Hypersthene Plagioclaseb Pigeonite Augite Quartz Plagioclasea Orthoclase

o4

o3

o2

Major

Mineralogy

o5

Grain Size (mm)

Biotite Opaques Amphibole Chlorite Sericite Zircon Hematite Apatite Limonite

Opaques Amphibole Zircon Apatite Limonite Hematite

Plagioclaseb Amphibole Chlorite Zircon Apatite Limonite Hematite

Plagioclaseb Amphibole Zircon Hematite

Minor

The Granophyre types from the Vredefort structure, after Therriault et al. (1996)

Textural type

Table 1a.

Glomerophyric pigeonite and augite grains. Ophitic intergrowth of hypersthene and plagioclase. Interstitial quartz and feldspars in micropegmatitic matrix.

Ophitic intergrowths of hypersthene and plagioclase. Fine-grained matrix of plagioclase +orthoclase+quartz. Pockets of quartz+orthoclase+biotite+amphibole+chlorite+ needles of opaques and apatite, with an overall granular texture.

Microphenocrysts of hypersthene commonly as acicular aggregates with domino-like effect or chains of stubby prisms. Acicular or plumose fibres of opaques aligned or hair-like cluster. Pockets of quartz+orthoclase+biotite+opaques and apatite needles, with a granular texture. Microcrystalline matrix of plagioclase+orthoclase+quartz showing fibrous domains.

Acicular hypersthene aggregates with ‘plumose’ texture. Chains of stubby hypersthene prisms. Small7 fibrous domains in microcrystalline matrix of plagioclase+ orthoclase+ quartz. Hair-like clusters of opaques.

Textural features

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Table 1b.

Modal composition of Vredefort Granophyre matrices and bulk rock

Textural type

Sample number

Spherulitic A

VAT-62a

Spherulitic B

Plagioclase matrix (%)

Orthoclase matrix (%)

Matrix bulk (%)

Plagioclase bulk (%)

Biotite bulk (%)

38

14

31

4

10

VAT-1 VAT-42 VAT-47 VAT-88

49 47 440 42

18 16 o20 8

40 50 49 42

2 3 n.I. 2

20 14 18 11

Spherulitic C

VAT-8 VAT-34

22 25

35 35

33 44

26 19

15 11

Granular

VAT-78 VAT-122 VAT-165

37 23 25

19 29 27

44 43 43

19 20 18

13 5 5

Note: Samples VAT-88, VAT-122, and VAT-165 are from core-collar boundary Granophyre dykes. Selected Vredefort Granophyre samples, after Therriault et al. (1996).

Table 1c.

Composition of micropegmatitic matrix of Vredefort Granophyre (data in vol%); after Therriault et al. (1996)

Textural type

Sample number

Plagioclase

Orthoclase

Quartz

Grain-size (mm)

Spherulitic A

VAT-72a

38

14

47

o100 (10)

Spherulitic B

VAT-1 VAT-42 VAT-88

49 47 42

18 16 8

33 37 49

o20 o100 o50(10)

Spherulitic C

VAT-8 VAT-34

22 25

35 35

44 40

o100(20) o50(20)

Granular

VAT-37 VAT-23 VAT-25

37 23 25

19 29 27

44 48 47

o100(10) o200 o100

 Values in parentheses are grain-size average of matrix in micrometers.

microlites in two granitic clasts have compositions of Or1Ab4079An2059, and orthoclase is Or7191Ab828 An0.21, very similar to the results of Therriault et al. (1996) and Reimold et al. (1990a). Comparison of pyroxene microlites within clasts and groundmass pyroxene gave orthopyroxene (clast) Wo0.8En4963 Fs3350 and orthopyroxene (groundmass) Wo1.2En5075 Fs2247, and for pigeonite (clast) Wo58.5En3944Fs4955 and clinopyroxene (groundmass) Wo1037En3759 Fs2544, indicating a close correlation with groundmass mineral compositions. Thus, much of the groundmass seems to be directly derived from melting of precursor rock constituents, and it is not impossible that some of the fine-grained minerals in the groundmass actually represent remnants of parent rock. 3.2.2. Microclasts Most microclasts are annealed and do not preserve shock deformation effects. Quartz cores that are not recrystallized may still show irregular fractures. Even

the remaining
1–2% unannealed clasts do not generally display characteristic deformation features (e.g., French et al., 1989; Reimold et al., 1990a). Reimold and Reid (1989) reported some granitic clasts with patches of possible in situ melt. Buchanan and Reimold (2003) conducted a detailed micropetrographic study of a suite of 0.5–1 cm sized granitic and arkosic inclusions from samples of dike 8 (Therriault et al., 1996; compare Fig. 4) and found that quartz-rich clasts of likely meta-sedimentary origin had experienced two phases of annealing, with the more pervasive later stage being related to the Vredefort event. Non-recrystallized quartz commonly displays undulatory extinction and mosaic texture that Buchanan and Reimold (2003) related to shock mosaicism. In only two clasts could they observe up to four sets of planar deformation features (PDFs) (Fig. 8a) in a small number of quartz grains. In addition, a new shock effect in quartz was identified, similar to the well-known checkerboard feldspar (compare Fig. 8c) known from clasts in impact

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melt rock from many impact structures. Buchanan and Reimold (2003) named this effect ‘‘vermicular quartz texture’’ (Fig. 8b). It is manifest as 10–30 mm spaced, near-planar melt veinlets surrounding blocky remnants of crystalline quartz. The melt veinlets are orthogonal to each other and commonly can be indexed with crystallographic orientations. With more prograde development of melt, these (sub)planar melt veinlets grade into curved surfaces dividing quartz crystals into aggregates of rounded and polygonal grains. Feldspars in these inclusions generally have lath-like to blocky shapes and abundant twinning. They may display mild undulatory extinction. Where thermal overprint has been more severe, a range of textures including rare feldspars with spherulitic texture and decomposition of individual crystals to fine-grained aggregates with vague outlines are noted. Rare checkerboard feldspar crystals were also observed. Several patches of originally mafic minerals are replaced by aggregates containing tiny Fe–Al-rich and Mg- and Crpoor spinel crystallites in devitrified or metamorphosed glass, as well as microlites of K-feldspar, quartz, orthoand clinopyroxene, and magnetite. While these melt pockets have average compositions similar to that of bulk Granophyre, compositions of individual melt pockets are quite variable (66–77 wt% SiO2). This is interpreted as variable mixtures of the main granitoidor arkose-forming minerals.

3.3. Geochemistry Fifty-six major element analyses of Vredefort Granophyre were generated by Hall and Molengraaff (1925), Willemse (1937), Wilshire (1971), Reimold et al. (1990a), Koeberl et al. (1996), and Therriault et al. (1997). Analyses of up to 34 trace elements in up to 34 samples by XRF and INAA techniques were reported by Therriault et al. (1996), Reimold et al. (1990a), and Koeberl et al. (1996). French et al. (1989) published INAA data with emphasis on Ir and other siderophile elements. 3.3.1. Major elements The Vredefort Granophyre is an extremely homogeneous lithology (Table 2). The Granophyre homogeneity is further enhanced when the most extraneous outliers are disregarded (Table 2, n ¼ 51). The earliest analyses reported were wet chemical analyses, the quality of which is not comparable to the subsequent XRF results. Further variability may have arisen from contamination from micro-clasts. Attempts to minimize this effect were made, for example, by Reimold et al. (1990a). An additional effect on a number of analyses is the local incorporation of country rock material where a Granophyre dike cross-cuts an epidiorite sill (e.g.,

15

Table 2. Major and trace element compositions of Granophyre, for the full data set (n ¼ 56) and with obvious outliers removed (n ¼ 51) n ¼ 56 Mean SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2 O P2O5 LoI CO2 Total

Sc Cr Co Ni Zn Ga As Se Br Rb Sr Y Zr Nb Sb Cs Ba La Ce Nd Sm Eu Dy Tm Yb Lu Hf Ta W Ir Au Th U Cu V Tb

n ¼ 51 Mean

1s

0.96 0.08 0.18 1.32 0.03 0.26 0.43 0.38 0.17 0.05 0.29 0 1.42

66.97 0.48 12.63 7.09 0.14 3.54 3.87 2.7 2.26 0.12 0.14 0.1 99.89

0.64 0.08 0.18 0.98 0.03 0.23 0.27 0.39 0.15 0.05 0.24 0 1.04

Mean

1s

n

12.9 350.2 23.53 104.4 63.25 10.4 2.13 0.06 0.19 75.76 231.7 16.89 146.7 7.16 0.33 3.1 438.8 32.66 51.86 23.2 4.17 0.88 2.78 0.22 1.18 0.19 3.7 0.55 0.48 0.52 1.04 6.39 1.42 42.11 97.41 0.51

0.79 130.6 2.61 13.18 10.77 4.1 1.96 0.03 0.06 6.02 13.83 1.29 12.77 0.83 0.05 0.32 61.73 2.5 8.01 2.89 0.46 0.12 0.19 0.01 0.31 0.04 0.47 0.21 0.21 0.16 0.89 0.4 0.25 3.49 7.71 0.05

10 32 34 33 24 5 10 5 5 34 24 19 34 19 9 10 34 10 10 9 10 10 5 5 9 10 10 10 5 5 9 10 10 19 17 5

66.77 0.49 12.63 7.03 0.14 3.58 3.95 2.7 2.23 0.12 0.17 0.1 99.76

1s

Mean values and 1s standard deviations; n ¼ number of samples analysed; LoI - loss on ignition.

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Therriault et al., 1996) or shale horizon. These authors also found that the elements Mg and Ca are marginally enriched in the core/collar boundary dikes, when compared to the composition of core dikes, which can be explained by local contamination by epidiorite. The chemical composition of Granophyre is unique, in comparison to the composition of all other known igneous rocks. At a SiO2 content of 767 wt% (i.e., granodiorite equivalent), the Granophyre values of Fe2O3, MgO and CaO of 77.3, 3.6, and 3.9 wt%, respectively, are abnormally high. Consequently, some workers assumed that the Granophyre represented a mafic and Ca-rich igneous magma (of dioritic or lamprophyric composition – Willemse, 1937; Bisschoff, 1972) that had assimilated significant amounts of crustal material while stoping upwards. This model demanded that the current geological configuration involve supracrustal rocks underlying at least part of the basement gneisses in the NW sector of the core of the dome, as a result of thrusting or intrusion of granitoids over the supracrustal material. As unique as the major element composition of the Vredefort Granophyre is, its composition is astonishingly similar to that of the Morokweng impact melt rock (Reimold et al., 1999b). This is understandable, as the target rock stratigraphies for both impact structures are remarkably similar. 3.3.2. Trace elements The variation in trace element abundances in the Granophyre is (in ppm): Co: 19–38; Ni: 87–134; Cr: 87–469; Cu: 34–46; Zn: 53–89; Rb: 63–85; Sr: 199–262; Y: 15–19; Zr: 117–165; Nb: 6–8; V: 76–139; Ba: 283–512; Cs: 2.6–3.5; U: 1.1–1.8; Th: 5.8–6.8; Sc: 12–22; La: 29–36; Ce: 41–62; Nd: 18.5–29; Sm: 3.6–5.1; Eu: 0.6–1.0; Gd: 2.6–4.0; Tb: 0.43–0.6; Dy: 2.6–3.1; Tm: 0.2; Yb: 0.8–1.7; Lu: 0.12–0.26; Hf: 2.7–4.6. Ta: 0.5–1.1; W: 0.6–0.7; As: o0.01–3.9; Sb: o0.4; Ga: 10–17; Au (ppb): 0.2–7.5. These ranges indicate that the Granophyre is reasonably homogeneous, certainly with regard to many (i.e., 15) trace elements, for which the variation, expressed as 1s standard deviation of the mean (Table 2), is o12%. Other trace elements have much higher variation, up to 50%, and in two cases (Au, As) up to 80% and 92%, respectively. For these elements, only a limited number of samples was analysed (Table 2), and alteration and ‘‘nugget’’ formation – the latter in the form of small sulfide crystals – could also play a role. There is clearly some variability in the modal compositions (ratio of felsic to mafic minerals). Individual samples are certainly ‘‘contaminated’’ with mafic country rock, and the overall variation in clast content must be considered as well. Average core/collar boundary and core sample compositions (Therriault et al., 1996) are in many cases similar (within standard deviation limits), but are significantly different for Sr,

Ba, and Cr (with the latter element apparently strongly enriched in core/collar boundary samples. The significant Ba variability is not readily explained, but is likely linked to differences in feldspar modal proportions between samples. The REE patterns of Granophyre match those of typical core granitoids, and Reimold et al. (1990a) took this as evidence supporting their mixing calculation results indicating that granitoids represented an essential precursor component for Granophyre generation. Their Co and Ni ranges match those of French et al. (1989) very well, but their Cr and Au values are significantly lower. Iridium contents were determined in 7 Granophyre samples by French et al. (1989) at 57–130 ppt. They also analysed 21 country rock samples, representing all major lithological components that could have contributed to an impact melt mixture. They found that some Witwatersrand sediments, especially meta-pelites, have Ir concentrations up to 330 ppt, indicating a likely important contribution to the indigenous component. Granitic country rocks were analysed by both French et al. (1989) and Reimold et al. (1990a). The latter group concluded that there was good correspondence between concentrations of many trace elements in the Granophyre and granitoids, with the exception of Sc, which is distinctly depleted in the basement granitoids. 3.3.3. Mixing models To test the impact hypothesis for the origin of the Vredefort Granophyre as a mixture of various target rock components and perhaps obtain constraints on the indigenous component of siderophile elements, mixing calculations with various target rock components were conducted (French and Nielsen, 1990; Reimold et al., 1990a; Therriault et al., 1997). These authors applied different mathematical tools and target component groups. The pre-Vredefort impact stratigraphy involves granitoid basement, clastic (quartzite, pelite) and chemical (banded ironstone) Witwatersrand metasediments, mafic Ventersdorp lava, and Transvaal metasediments that contain a significant carbonate proportion. French and Nielsen (1990) modeled the Granophyre as mixtures of Ventersdorp lava, Witwatersrand quartzite and shale components, and Archean basement granite. They concluded that a simple igneous assimilation mechanism could not duplicate the Granophyre composition unless unreasonable amounts of wallrock (4100% of the parent magma) were assimilated. They also determined that 30–80% of the clasts would need to have been basalt. A granite component, at 5–45%, varied inversely with basalt proportion, and 5–20% shale and
20% quartzite would have been required, too. Reimold et al. (1990a) used mixtures of Archean granitoid basement, Witwatersrand shale, and Witwatersrand quartzite, as these materials constitute the only important lithologies of the Granophyre clast

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population. They determined that
25% shale, 3.5% quartzite, and 75% granitoid could achieve a matching mixture, but that 9–14% shale, 9% quartzite, 60% granite and 20% lamprophyric magma (proposed by Bisschoff, 1972) would also result in a good match. French and Nielsen (1990) and Reimold et al. (1990a) pointed out that assimilation of at least 30%, and perhaps as much as 80%, of upper crustal material, as required by their respective mixing models, presents enormous kinetic problems for the small igneous intrusive component. Reimold et al. (1990a) did not model a Ventersdorp lava component, as clasts derived from this target lithology had not been observed in Granophyre. Therriault (1992) reported a very small and localized occurrence of mafic clasts in a few Granophyre samples. Reimold et al. (1990a) also did not involve a carbonate component (Transvaal dolomite equivalent), as no carbonate clasts had been observed in the Granophyre. They believed that the CaO component in the Granophyre (3.770.25 wt%) was adequately modeled by the 1.972.2 wt% CaO in granite. Therriault et al. (1997) obtained a best-fit mixture of ca. 40% lava, 30% quartzite, 25% granite gneiss, 3% shale, and 2% carbonate. They emphasized that these results were geologically reasonable but did not conform to the clast population in the dikes. This was, however, not necessary, as the clast populations in other impact melt sheets did not necessarily correspond to the proportions of melted components either. It is important to note that melt production took place at considerable distance from the current sampling level. The observed clast load is characterized by a very small proportion of shock deformed material, whereas the melt must have originated in the high-shock zone. Considering the diverse results of these mixing calculation studies, the Granophyre mixture can be produced, at varied proportions, with varied suites of components that would be texturally (clast population) or geologically reasonable. There is still large uncertainty whether Granophyre melt comprises a significant component of Ventersdorp lava, which is not validated by the actual clast population. However, the importance of Archean granitic basement at 25–75% was found unanimously. Comparing Ni and Co concentrations in the various target rocks (Reimold et al., 1990a; French et al., 1989), a 20–30% shale contribution to melt could easily explain the abundances of these elements found in the Granophyre, with an even lower proportion required by an indigenous contribution from quartzite and granitoid. A contribution of Ventersdorp lava would reduce the required shale component. 3.3.4. Extraterrestrial component Given the radially decreasing energy content (shock intensity) within an impact crater, impact melt is

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generated relatively close to the impact explosion site during the compression stage of cratering early on in the process. This allows a relatively small part of meteoritic projectile to become incorporated into impact melt, with extraterrestrial components (ETCs) generally not larger than 1%, and often much smaller. Only in exceptional cases have values up to 10% been measured (e.g., 2–5% in Morokweng impact melt rock; Koeberl et al., 2002). There are currently three techniques available to investigate the presence of an ETC in impact melt rock: (1) comparison of siderophile element abundances in target rocks (the so-called ‘‘indigenous component’’) with those in impact melt rock, (2) the Re–Os isotope method, and (3) investigation of Cr isotopic characteristics of the impact melt rock. Against the already discussed general lack of good constraints on the mixture of target rocks from which the Vredefort Granophyre melt was formed, nobody, to date, has attempted to calculate a reasonable indigenous component. It is essentially impossible to use the siderophile element abundances to determine a meteoritic component in Granophyre. The Granophyre does have elevated Ir, Ni, Co, and Cr contents in comparison with most possible target rocks analysed so far (if one assumes that the maximum shale input would have been 10%) but, as the actual contribution of the siderophile elementrich Witwatersrand shales is not known, a well-defined extraterrestrial component cannot be determined. Koeberl et al. (1996, 2002) analysed the range of likely target rock compositions (including Ventersdorp lava) for comparison with Vredefort Granophyre Os isotopic values. They found Os abundances of 0.11–1.11 ppb in Granophyre, and between 0.024 and 0.0525 ppb in country rocks, with one shale sample yielding 0.162 ppb. 187Os/188Os ratios of Granophyre range from 0.196 to 0.558, whereas the analysed country rocks range from 1.013 to 6.272. In a 187Re/188Os vs. 187 Os/188Os isochron plot the Granophyre data straddle a 2 Ga reference line and extend just into the range of chondritic and iron meteorites. The country rock data plot far off the isochron and most have very high 187 Re/188Os ratios. Thus, a definite presence of a small meteoritic component is indicated and, assuming a chondritic meteorite Os abundance of
500 ppb, Koeberl et al. (1996, 2002) calculated the ETC in Vredefort Granophyre as o0.2%. The variable 187Re/188Os ratios of Granophyre indicate that this component is not homogeneously distributed among the analysed samples, which is in agreement with the equally variable Ir, Co, Ni, and Cr abundances in Granophyre. The Cr isotopic method (Shukolyukov and Lugmair, 1998) is not only useful for the identification of meteoritic components in terrestrial impact melt rocks, but it also may allow constraints to be placed on projectile type. Koeberl et al. (2002) determined the 53 Cr/52Cr ratios in two Granophyre samples at

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0.0170.06 and +0.0370.03e (errors are 2s), which is indistinguishable from the terrestrial mean value (e ¼ 0). Cr contents in the various target rocks range from 6 to 750 ppm, with a likely average value around 300 ppm, whereas the average Cr content in Granophyre is
420 ppm. According to the detection limits for chondritic ETC using the Cr isotopic system (Koeberl et al., 2002), this translates to a detection limit of a possible chondritic component in Granophyre of 2% – much higher than the 0.2% suggested by the Re–Os isotopic results. Clearly, abundant terrestrial Cr camouflages the small extraterrestrial Cr component.

4. Pseudotachylitic breccias in the Vredefort impact structure 4.1. Occurrences in the Vredefort Dome Pseudotachylitic breccias with a wide variety (planar to irregular) of geometries (Fig. 5), often stretching for hundreds of meters, occur in most lithologies of the Vredefort Dome. Geometries range from orthogonal pairs of veins, well developed and relatively straight veins that could be compared to the generation veins of Sibson (1975) with or without offshoots/injection veins, en echelon sets, irregular pods, massive dikes up to many meters in thickness, to widespread network breccias of either elongated shape or entirely devoid of any preferred orientations. In contrast to fault- or mylonite-related occurrences, and even the occurrences in the Witwatersrand goldfields, the veins are not associated with broad zones of shear deformation or faulting. Breccias consist of a generally dark-grey to black, crypto- to microcrystalline (in massive occurrences) groundmass (Fig. 5), with a variable clast content. Where weathered, the groundmass is usually light grey. Clasts are typically from the immediate host rock, but may locally include exotic fragments, likely derived not far from the place of occurrence (Bisschoff, 1962; Reimold, 1991). Clast size distribution is variable (even along a single occurrence). Blocks of meter-size may be found in a groundmass laden with cm- to mmsized clasts. At other occurrences, maximum clast size may be in the dm to cm range. Fracture strength of precursor lithologies clearly plays a role regarding the degree of comminution. The abundance of pseudotachylitic breccia is much higher in the Archean basement rocks of the core than in the supracrustal collar strata. All of the largest occurrences – the largest being a dike of irregular geometry on farm Abel in the NE core that can be traced for 2.6 km at an apparent maximum width of 100 m (Dressler and Reimold, 2004) – occur in basement gneisses. It was long thought that pseudotachylitic

breccias were virtually absent in the centralmost, granulite facies, parts of the core. However, Gibson et al. (2002) identified light to dark grey breccias with flattened and locally melted clasts in a granulitic matrix comprising mainly feldspar, biotite, pyroxene and opaque minerals (Fig. 10). The grain size of the groundmass minerals is identical to that in aggregates replacing shock-damaged minerals in the host rocks, and the breccia margins are typically diffuse. Gibson et al. (2002) concluded that these ‘‘granulitic breccias’’ formed during the impact event under granulite-facies conditions, which enabled both crystallization and subsequent annealing of the molten matrix. Only one large occurrence of breccia has so far been recognized in the central part of the dome (on farm Helpmekaar 750, 4 km WSW of the Inlandsee pan). Dressler and Reimold (2004) reported detailed field studies of pseudotachylitic breccias throughout the core of the dome, including a number of 3D quarry exposures. They referred to exceptionally large ‘‘mother lode’’ dikes of irregular geometry that are the foci within larger areas with randomly distributed networks of thinner veins and veinlets. This is in contrast to observations by Reimold and Colliston (1994) who referred to instances of preferential breccia development on lithological contacts and where pre-existing zones of weakness, such as brittle shear zones or pre-impact mylonite zones, were exploited. They also noted that many vein occurrences follow the main Archean fabric orientations. Where thin pseudotachylitic breccia is formed along contacts between felsic and mafic bands in basement gneiss, the breccia is often developed on the mafic side of the contact. Reimold (1991) suggested that this could be due to the greater presence of hydrous ferromagnesian minerals on the mafic side of such a boundary, for example in an amphibolitic gneiss band. However, there are instances where the breccia transgresses the contact, sometimes obviously related to the existence of a pre-brecciation structural or lithological weakness (a small shear zone or a pegmatite vein). Where a relationship to faulting is evident at a pseudotachylitic breccia vein or veinlet, maximum observed displacement is o50 cm and mostly o5 cm. Unlike for tectonic pseudotachylite (Sibson, 1975), no consistent correlation appears to exist between Vredefort pseudotachylitic breccia thickness and displacement length. In summary, both structural and fabric controls, as well as large- to small-scale structural irregularities, are features of the development of pseudotachylitic breccias at this deep level of the Vredefort central uplift. In general in the collar strata, pseudotachylitic breccias are not as abundant or as voluminous as in the core of the dome (Reimold and Gibson, 2005). Veins are generally only a few cm wide, although several occurrences up to 25 cm width have been observed. They normally follow bedding contacts, where some veins

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Fig. 10. (a) Fine-grained crystalline breccia in granitic host rock on Leeukop hill – with plagioclase microlites that locally suggest a flow pattern. Only quartz forms relic clasts, all of which are recrystallized indicating that the melt must have been very hot. Crossed polarizers, width of field of view 2.2 mm. (b, c) Thin pseudotachylitic breccia veinlet in mafic amphibolite (NW part of the core of the Vredefort Dome, (b) plane polarized light; (c) with crossed polarizers). The veinlet occurs along a fracture that displaces the plagioclase grain (center) but is flanked by a 1–2 mm wide zone of damaged or recrystallized plagioclase, hornblende and biotite that locally display textures consistent with local elevation of shock pressure, such as partial amorphization and diaplectic glass (now recrystallized to bladed aggregates) formation against the veinlet, and unusual ripple-like features in the feldspar. Width ca. 4 mm. (d, e) Backscattered electron images of pseudotachylitic breccia veinlets from the center of the Dome: (d) Breccia veinlet of tonalitic composition similar to that of its host rock. The matrix is dominated by plagioclase, with subsidiary biotite needles and polygonal clinopyroxene grains. A relict quartz aggregate occurs as scattered grains throughout the matrix. The biotite laths and the quartz clasts show a weak NE–SW alignment parallel to the breccia margins, consistent with high-temperature flow. Scale bar: 200 mm. (e) Breccia veinlet in granulite from the Inlandsee borehole, showing a comparatively coarse-grained texture in the groundmass of a pseudotachylitic breccia veinlet, dominated by plagioclase (dark gray) with subsidiary, irregular K-feldspar grains (light gray) and interstitial quartz (black), large, irregular, poikilitic biotite (lightest gray), clinopyroxene and opaques (white). The breccia is chemically similar to the trondhjemitic granulite host rock, parts of which are visible at the top right and bottom left corners (note the considerably coarser grain size). Scale bar: 500 mm.

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have been followed over hundreds of meters. At places along such veins, they become discordant to bedding, which locally can be related to an existing plane of weakness (a joint or shear plane). Entirely beddingcrosscutting occurrences are seen as well and, in some cases, have been interpreted to represent offshoots from a locally covered bedding-parallel vein, but in others they can be followed over longer distances and seemingly do not represent offshoots. Very thin veinlets of pseudotachylitic breccia with the intensity of spacing of a fracture cleavage (several mm up to 10 cm) in the lower Witwatersrand Supergroup were described by Gibson et al. (1997b). The fractures show mm- to sub-mm-scale displacements and range from planar to irregular networks. No consistent geometric or slip sense could be determined on a scale larger than a few meters, suggesting that the pseudotachylitic breccias formed during brecciation of the target in a manner similar to the ‘‘megabreccia’’ model by Ivanov and Kostuchenko (1997). Geometries of peudotachylitic breccia developments may be far more complex than the mostly twodimensional outcrops suggest. Some of the breccia occurrences in the collar also comprise sets of en echelon veins. No general pattern of orientation of such extension veins could be established during detailed regional mapping of the inner collar of the dome. Our recent fieldwork has shown a notable increase in abundance of pseudotachylitic breccia within large-scale radial or transverse fault zones cutting the collar strata. Massive occurrences have also been noted in the hinge zones of hundreds of meters- to kilometer-scale folds. A meter-scale example of a fold structure in quartzite is shown in Fig. 5d. In addition to these folding-related occurrences of abundant breccia in the collar, there are several large (meter-scale) occurrences of irregular geometry that cannot be linked to a specific structural feature but to a lithological boundary, such as that between Coronation shale and an epidiorite sill northwest of the Smilin Thru Resort (northern collar), or at a contact between Witwatersrand quartzite and epidiorite immediately northeast of the Anna’s Rust gabbro, where irregular pods of melt breccia 41 m wide were noted. They contain rounded clasts of up to several decimeter size. Reimold and Colliston (1994) reported several occurrences of pseudotachylitic breccia that seem to pre-date the impact event. One occurrence in the western collar is overprinted by shatter cones, and another in the southwestern core exhibits a veinlet that has been folded together with the Archean gneiss – in pre-Vredefort times, and that is crosscut by a younger, possibly impact-related breccia veinlet. At Broodkop in the southeastern sector of the dome, a vein with ovoid inclusions of similar breccia occurs that was investigated by Spray et al. (1995), who showed by laser Ar dating that both phases are likely Vredefort impact-related.

Clast shapes generally vary from sub- to wellrounded, irrespective of whether the clast sizes are at the meter or smaller scales. This could be the result of mechanical abrasion within the highly dynamic crush zone and turbulence during the subsequent mixing and injection phases of melt formation and emplacement. Further abrasion could have taken place due to milling of clasts against each other. Alternatively, it could be due to thermal rounding by marginal melting of clasts within superheated melt.

4.2. Petrography of Vredefort pseudotachylitic breccia The groundmass of Vredefort pseudotachylitic breccia generally falls into two categories: (1) aphanitic matrix, loaded with magnetite microlites and devitrification phases including feldspar, pyroxene, and amphibole; and (2) micro-crystalline matrix with finest-grained subophitic, intersertal or spherulitic textures. The main groundmass minerals are plagioclase, pyroxene, amphibole, biotite and magnetite. Some alteration (chlorite, epidote, sericite, secondary biotite) is usually present. Statistical clast size analysis of breccia from the Vredefort Dome, at the outcrop and thin section scale, and comparison with tectonic pseudotachylites by Hisada (2004) showed that clast size distributions are fractal at both scales. On the thin section scale, Vredefort pseudotachylitic breccia has a higher proportion of clasts than fault-related pseudotachylite, and on the outcrop scale impact-generated breccia has a very high proportion of clasts. We feel, however, that these results do not have statistical validity. We have observed a large variation of clast content at both the outcrop and thin section scale for Vredefort pseudotachylitic breccias. Microscopic clasts are usually sub- to well-rounded and at least partially annealed. Where annealing has progressed, only relic quartz may be recognizable in granitoid-derived clasts. Large clasts have been rotated in a turbulent matrix, as frequently indicated by flow folds at their margins. They are also commonly marginally disrupted, with slivers of a large clast separated by melt matrix, indicating significant shear stress at the clast-matrix contact. Thin sections of pseudotachylitic breccia formed in quartzite (host rock) often show only annealed quartz/quartzite clasts, indicating the high level of thermal overprint resulting from the formation of these melt breccias. Reimold and Colliston (1994) reported the following deformation features in clasts: partial to complete cataclasis, buckling and microfaulting of plagioclase twin lamellae, and partially annealed planar microdeformation features (Reimold, 1990) in quartz. The latter features are somewhat reminiscent of features in quartz

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known from a few quartz clasts in Witwatersrand breccias. Martini (1978, 1991) identified coesite and stishovite in thin pseudotachylitic breccia veinlets, as well as in narrow zones within the wall rock immediately adjacent to them, at various sites in the Vredefort collar. The presence of these minerals in pseudotachylitic breccia veinlets was confirmed by transmission electron microscopy by White (1993), who also showed that such veinlets still contain glass.

4.3. Geochemistry of Vredefort pseudotachylitic breccias Reimold (1991) compared the chemical compositions of Vredefort pseudotachylitic breccias, sampled in veins and in massive network breccias, and their granitic and gabbroic host rocks, with results for tectonic pseudotachylites. Generally, Vredefort pseudotachylitic breccia samples in cm- to dm-wide veins have compositions that closely resemble those of the specific host rocks, indicating bulk melting and lack of extensive mixing between different lithologies. However, where massive breccia occurrences were investigated, mixing of several parent rock types was demonstrated, generally in accordance with the observed clast populations. Although not always present, some seemingly systematic differences between the compositions of breccia veins and host rocks were indicated, and these are different for different types of parent material. Vredefort pseudotachylitic breccia in granitic host rock typically shows depletion of SiO2 and enrichment of TiO2, Fe2O3, CaO, and, to a slightly lesser degree, of MgO. Vredefort breccia formed in mafic host rock is generally very similar in composition to the host, but may show slight enrichment in K2O and depletion in CaO. Reimold (1991) also reported that elements normally associated with feldspar, as well as Sc, are commonly enriched in the breccia. He concluded that the chemical systematics of Vredefort pseudotachylitic breccia were similar to those described from tectonic pseudotachylites, and that the formation involved selective melting of hydrous ferromagnesian minerals and, to a lesser degree, of feldspar minerals (also Spray, 1992). Coney (2002) compared the chemical composition of an
10-cm-wide vein in granite with that of much thinner (o 5 mm) offshoots and found that all samples had the same composition, suggesting effective homogenization within this vein system or that the offshoot fills were emplaced from main vein material after thorough homogenization. In contrast, Reimold (1991) reported a 2–3 mm wide veinlet, in which electron microprobe spot analyses revealed spatially variable matrix compositions consistent with those of quartz, feldspar, and biotite. This has been confirmed by P. Ogilvie (Univ. Witwatersrand, Johannesburg, pers.

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comm., 2004) recently through modal analysis of veins from the central part of the dome, using optical and scanning electron microscopy. Dressler and Reimold (2004) investigated compositions of breccia and host rock pairs for several places along cm- to dm-thick veins that cut across different host rock types. They noted that a single breccia vein has a different chemical composition when sampled opposite a different host rock type. In contrast to Coney’s (2002) findings, these results favor local melting and lack of (or, at least, limited) lateral mixing.

4.4. Geochronology of Vredefort and Witwatersrand breccias Argon chronology (bulk stepheating and laser argon dating), on pseudotachylite has become a widely used tool for the age determination of seismic and tectonic events (e.g., Di Vincenzo et al., 2004). First argon stepheating dating of Vredefort pseudotachylitic breccia (Reimold et al., 1990b) gave apparent plateau ages between 1.0 and 2.2 Ga. Allsopp et al. (1991) obtained a roughly 2.0 Ga age for Vredefort Granophyre – which at that time was adopted as the age of the formation of the dome (e.g., Walraven et al., 1990). However, this early work (also Reimold et al., 1992b) suggested that significant thermal overprint had disturbed these argon systematics later than 2 Ga. Argon dating on Witwatersrand pseudotachylitic breccias by Reimold et al. (1992c) and Trieloff et al. (1994) also yielded several reasonable plateau ages at
2 Ga, as well as significant information regarding post-2 Ga thermal and/or hydrothermal overprints. Laser argon chronology of Vredefort pseudotachylitic breccia by Spray et al. (1995), shortly followed by the definitive U–Pb single zircon dating study by Kamo et al. (1996), revealed that all breccias dated were of
2 Ga age. The single zircon study by Kamo et al. (1996) determined the impact age at 2.02370.004 Ga. Further evidence of thermal and hydrothermal overprint on both Vredefort and Witwatersrand breccias and minerals was provided by Reimold et al. (1995) and Zhao et al. (1999). Gibson et al. (2000) obtained argon ages on a range of metamorphic minerals from Vredefort collar rocks, which showed partial resetting at
2 Ga, sometimes only partial resetting to geologically unreasonable 2.5–2.3 Ga ages, as well as post-2 Ga resetting. Friese et al. (2003) addressed the argon chronology of various breccias – cataclasite, mylonite, and melt-bearing pseudotachylitic breccia – from the Welkom gold field in the southwestern part of the Witwatersrand basin and obtained very diverse results. Some samples were extensively reset at times later than 2 Ga, others gave good Vredefort (2.02 Ga) ages (for ultracataclasite and pseudotachylitic breccia), and also some Bushveld ages of ca. 2.06 Ga.

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In summary, the ages of Vredefort Granophyre and pseudotachylitic breccias are well-constrained, and a significant number of 2 Ga ages for pseudotachylitic breccias from the wider Witwatersrand Basin have demonstrated that not only the breccias from the Vredefort Dome, but also the majority of those of the wider basin have to be related to the impact event at 2.02 Ga ago.

4.5. Pseudotachylitic breccias in the wider Witwatersrand Basin 4.5.1. Occurrences and appearance Numerous occurrences of pseudotachylitic breccia have been described in the wider Witwatersrand basin, generally in conjunction with gold-mining operations. They include many occurrences of cm to dm thickness along and crosscutting contacts between lithologies of contrasting rheological character, such as quartz pebble conglomerate/quartzite, shale/quartzite, and conglomerate/Ventersdorp lava (Fig. 11). Many of these are linked to bedding-parallel faults. They may be relatively straight, continuous or discontinuous, and sometimes display offshoots orthogonal or oblique to the fault plane. In some places, veining becomes complex, with matrix engulfing cm to dm sized clasts, sometimes in the fashion of rip-off clasts known from tectonic pseudotachylites, and locally forming small, complex, clastic breccia zones. When breccia development occurs at the hanging-wall contact at a reef (a gold-bearing quartz pebble conglomerate), it is not uncommon for it to have exploited structural features, such as fractures, shear zones, or quartz veins, to cut across the conglomerate, and to then continue along the contact with the footwall lithology (generally a quartz arenite). There are several large-scale (tens of meters wide) fault zones that contain massive formations of pseudotachylitic breccia, such as the Master Bedding Fault (Zone) (near the top of the West Rand Group) and the Black Reef De´collement Zone at the base of the Transvaal Supergroup (Fletcher and Reimold, 1989) in the West Rand (Klerksdorp) and West Wits Line (Carletonville) goldfields to the NW and N of the Vredefort Dome. Killick et al. (1988) observed similar breccias along a fault zone at the base of the Ventersdorp Contact Reef (VCR, above the top of the Witwatersrand Supergroup). Reimold et al. (1992c) and Reimold and Boer (1993) investigated the petrography and geochemistry across this fault zone and adjacent lithologies (from hanging-wall Ventersdorp lava, across VCR conglomerate, into the footwall quartzite). In the VCR Fault Zone, breccia may be developed at either the hanging-wall or footwall contacts, or may change abruptly, via a o601 inclined fault, from one contact to the other.

Fig. 11. (a) Flow-banded breccia vein at contact between VCR conglomerate and footwall quartzite. Note the thin quartz vein parallel to the bottom contact of the vein. It is not disrupted tectonically but extends locally into the breccia vein. Thus, breccia and quartz vein likely are coeval. Also note the offshoot of breccia into the conglomerate (top part) – resembling a generation/injection vein case, as well known from tectonic pseudotachylite developments. Pen for scale ca. 12 cm. Detailed examination reveals that this is a composite vein composed of a thin, dark band of ultracataclasite or pseudotachylite and a somewhat wider band of mylonitic character. Such occurences of multiple breccia are well known from Elandsrand, and in some cases definitive cross-cutting relationships between different breccias have been recorded. (b) Specimen of pseudotachylitic breccia and VCR reef conglomerate from Elandsrand gold mine. The broad, flowbanded breccia at left represents an older generation of mylonitic breccia that is cut by a thin, dark band (arrows) of later breccia (likely an ultracataclasite or pseudotachylite) formed directly at the contact with the conglomerate.

Occurrences of such breccias have also been observed along several normal faults that trend radially towards the Vredefort Dome (e.g., the West Rand and Bank faults that traverse the Vredefort collar in the north, or the Mooi River Fault near Potchefstroom; Fletcher and Reimold, 1989), or which are tangential or concentric with respect to the dome (e.g., Friese et al., 2003). In addition to obvious fault-control, unfaulted contacts between lithologically (and presumably rheologically) different parent rocks may also form loci of breccia development.

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The bedding-parallel, breccia-bearing fault zones of the Klerksdorp and Carletonville goldfields generally dip at low angles (15–251) towards the Vredefort Dome. Exposures are limited to underground developments in mines, so that it is not possible to continuously follow the extent of these zones. However, the fact that they occur in the same stratigraphic position in several adjacent mines strongly suggests that they could be basin-wide structures. The distribution of major occurrences in the Witwatersrand Basin is shown schematically in Fig. 12. Several breccia specimens from Witwatersrand gold mines, especially Elandsrand Mine in the West Wits Line goldfield, contain more than one generation of pseudotachylitic breccia (Killick and Reimold, 1990; Fig. 11b). Berlenbach and Roering (1992) also suggested that some of the pseudotachylitic breccias in the Witwatersrand goldfields are as old as 2.7 Ga (Ventersdorp age). Killick and Roering (1998) estimated that the Witwatersrand ‘‘pseudotachylites’’ probably formed over a 1.9–6.6 km depth range under relatively low differential stresses and near-hydrostatic pore-fluid pressure. They concluded that this required high slip velocity and displacements in excess of 20 cm on very narrow faults. This latter requirement clearly is not applicable for the very wide, major bedding-parallel fault zones of regional extent. Witwatersrand breccias show a range of colors, which are a reflection of the parent materials involved in their formation (Reimold et al., 1999a). The breccias are generally formed at faults and in fault zones that occur at or across lithological contacts. Green and yellow breccias are generally derived from quartzite and quartz pebble conglomerate, but the lighter shades of these colors may also reflect alteration. In addition, several breccias, or inclusions of breccia in breccia, composed of mostly carbonate may also have a yellow or beige color. A black

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breccia that has a large carbonate component has also been noted. Where a significant mafic volcanic (e.g., Ventersdorp Lava) component is involved, the breccia may have a maroon or dark-grey to black appearance. 4.5.2. The De Pan breccia Recently, one of us (WUR) discovered a massive (10 m wide and more than 100 m long) breccia in granitic basement some 80 km north of the center of the Vredefort Dome at De Pan on the Rand Anticline, the northern margin of the structural remnant of the Witwatersrand basin. This breccia (Fig. 13) superficially resembles Vredefort pseudotachylitic breccia, although clast shapes are generally more angular. Petrographic analysis shows that it is a cataclasite with a thoroughly altered groundmass in which the mafic precursor rock minerals (biotite and amphibole) have been completely chloritized and uralitized, feldspar partially sericitized and saussuritized, and pore space infilled with later hydrothermal minerals. On this evidence, age dating seems unlikely to prove successful, but the massive nature of this ocurrence strongly suggests a direct link with the Vredefort event. 4.5.3. Petrography of Witwatersrand breccias A general problem with Witwatersrand breccias is that, despite the fact that there is a whole range of different breccia types developed in this region, all have been termed ‘‘pseudotachylite’’ by mining geologists. Cataclasites, ultracataclasites, mylonites, ultramylonites (many of which are also locally known as phyllonites due to their comprehensive alteration to phyllosilicate asemblages), bona fide friction melt, and even fault gouge and quartz veining have been collectively reported as ‘‘pseudotachylite’’. Melt development in these faultparallel occurrences is seemingly limited. Small pockets,

Fig. 12. Schematic distribution of pseudotachylitic breccia in the wider Witwatersrand Basin. Major concentrations of breccia are only found in the Vredefort Dome and along major fault zones in the northern and northwestern part of the basin.

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Fig. 13. A meter-wide portion of the extensive breccia zone at De Pan, north of Carletonville at the margin of the Witwatersrand Basin. The host rock is leucogranite. Note the angular shapes of the variably sized clasts. Knife for scale, 8.5 cm long.

generally of less than a few cm in size, and narrow, up to 1 cm wide but generally even thinner, schlieren contain local melts between cataclastic zones (Reimold et al., 1999a). Killick et al. (1988) described the breccias as very fine-grained with a dirty appearance due to presence of fine-grained iron oxide. Quartz clasts are commonly highly strained and show undulatory extinction and subgrain boundaries. Matrix proportions are between 30 and 60 vol%, with higher proportions in the clast-bearing, matrix-dominated breccias that most resemble pseudotachylite. Clasts in cataclasites were described as lacking high-strain deformation such as serrated boundaries and polygonization, which are seen in mylonites from the Witwatersrand goldfields. Killick et al. (1988) suggested that the intimate relationship between pseudotachylite and cataclasite is the result of pseudotachylite formation after cataclasite formation and that it reflects exploitation of pre-existing cataclasite zones for pseudotachylite development. We can only partially support this interpretation, because such a temporal relationship is not regularly indicated. In contrast, we cannot exclude the possibility of coeval formation of pseudotachylite and cataclasite, as the evidence for intrusive relationships is often poor or entirely absent. Schlieren of melt-bearing material in cataclasite or the converse relationship are commonly encountered without any hint that might suggest that one breccia type was formed after the other. Killick et al. (1988) reported that mylonite associated with quartz vein emplacement occurred after deposition of the upper Klipriviersberg Group (Ventersdorp Supergroup) but predated the cataclasites that show mylonite clasts and which, in turn, are cut by pseudotachylite. The example shown in Fig. 6d of Killick and Reimold (1990) suggests (1) mylonite formation, followed by (2) formation of a

second mylonite that contains clasts of the older one, and (3) formation of pseudotachylite that cuts across the two older phases. Reimold et al. (1999a) presented a detailed study of two sections through the Master Bedding Fault, one through the Bank Fault at a site close to the Master Bedding Fault, and one from the VCR Fault Zone. They also observed local evidence for melting, for example, as pockets with microlites in altered groundmass (they used various melt recognition features as discussed by Reimold and Colliston, 1994; Magloughlin and Spray, 1992b). Reimold et al. (1999a) concluded that 498% of breccia materials were clastic breccia, in contrast to the massive melt breccias found in the Vredefort Dome. However, their sections also contained dm- to m-wide zones of melt-dominated or melt-bearing breccia, with melts in the form of flow-banded veins or irregular schlieren. Clast lithologies include all possible host lithologies for these sections, including quartzite, shale, dolomite, and lava. Except for the very refractory quartz and orthoquartzite clasts, other clast types were partially or completely annealed and usually only recognizable as ghosts. This is in stark contrast to the cataclasite sampled at De Pan, where clasts are hardly annealed at all, as noted in Reimold et al. (1999a). The ratio of angular to rounded clasts is much higher in cataclasite than in melt-bearing pseudotachylite. Vesicles are abundant in some melt-bearing breccias. These authors also noted that such fault zones are particularly affected by hydrothermal alteration, commonly having cm-wide quartz veins intercalated with breccia or a high density of amygdales with quartz filling, indicating that these fault zones were important fluid channels. The petrographic study of the VCR Fault Zone on Elandsrand and Vaal Reefs No. 10 Shaft gold mines by Reimold et al. (1992c) and Reimold and Boer (1993) was focused on economic aspects. They discussed the general opinion that where several fault rock types (e.g., ultramylonite and pseudotachylite) occur intimately associated (e.g., Passchier, 1982), they are believed to have formed under different crustal conditions. This opinion cannot be reconciled with the observations on Witwatersrand breccias that all formed under upper crustal conditions (T ¼ 350  300 1C and P ¼ 0:2  0:3 GPa; e.g., Phillips, 1988). They placed a lower temperature limit for the formation of melt at 650–740 1C and pointed out that variation in chemical and mineralogical composition of precursor rocks, mixing of several parent lithologies, nature of contrasting lithologies in contact with the fault, as well as the local presence, composition and volume of fluids must be considered when attempting these estimations. 4.5.4. Geochemistry of Witwatersrand breccias Reimold et al. (1999a), Killick et al. (1988) and Killick (1994) reported major and trace element systematics for

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Witwatersrand fault zones. They found: (1) The pseudotachylitic breccia on the Bank Fault, East Driefontein gold mine, was mainly formed by local melting of the specific host rocks at the sides of the melt zone, but that mixing of different melts was noted in its inner part. (2) A drill core section sampled through the Master Bedding Fault on Doornfontein Gold Mine proved lithologically and chemically complex. Country rocks and breccias are chemically different, and mixing between different melts and at different proportions is required to produce this complex chemical profile. (3) A number of breccia-host rock pairs from Western Areas Gold Mine showed different effects for each case. Generally, the breccias were found to be enriched in trace elements compared to their host rock. Hydrothermal activity that was either directly related to the breccia-forming event or postdated it contributed to the chemical differences noted. Mixing between mafic and felsic precursor rocks was reported by Killick et al. (1988) and Killick (1994). (4) A second section through the Master Bedding Fault (Reimold et al., 1999a) had been affected by strong alteration, according to the authors ‘‘notably at times of breccia formation’’. Reimold et al. (1999a) further concluded that quartzite-hosted breccias had lower SiO2 and higher TiO2, Al2O3, and sometimes Fe2O3 and K2O contents than the respective host rocks. Breccias hosted in Ventersdorp lava are enriched in SiO2 and moderately enriched in TiO2, Fe2O3, MgO, and Al2O3, with variable CaO and alkali element behavior. Killick (1994) and Reimold et al. (1999a) determined that Witwatersrand pseudotachylitic breccias are consistently enriched in Au, irrespective of host rock composition. They concluded that their findings indicated hydrothermal activity along the fault zones. The timing of this could have either been prior to (Au would have been present in the host rock that became entrained in the breccia), during, or after breccia formation. However, the fact that this preferential enrichment is seen in the fault rocks and not affecting the adjacent lithologies strongly suggested (Reimold et al., 1999a) autometasomatism during fault breccia development, likely related to the thermal/hydrothermal effects that accompanied the Vredefort impact event (Reimold et al., 2005).

5. Summary and Discussion 5.1. Vredefort Granophyre 5.1.1. Homogeneity In terms of major elements, the Granophyre is extremely well homogenized (Table 2). This homogeneity of all Granophyre dikes within a 2000 km2 area implies that the melt was well homogenized in a

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common reservoir – the now completely eroded main impact melt body – prior to emplacement of the dikes. The regional homogeneity is also observed for many, especially the lithophile, trace elements. It is, however, also indicated that a number of trace elements are characterized by larger variability (10–20% relative, or even higher – Table 2), and a number of reasons for this can be discussed: heterogeneous distribution of a meteoritic component in the case of siderophile elements, scavenging of siderophile elements by mafic clasts or groundmass patches, accidental analysis of sizable clasts, and local alteration that could have resulted in leaching out or incorporation of mobile elements. 5.1.2. Mixing of target rock components Efforts to model the proportions of the various target rocks from which the Granophyre melt was formed are limited by the deep erosion level of the Vredefort Structure that inhibits investigation of the original stratigraphy and stratigraphic thicknesses of the target geology, as well as by the impact-generated structural disruptions which make it impossible to fix accurate thicknesses of stratigraphic intervals for the pre-impact dome region. It is known from other parts of the Witwatersrand – for example, the Rand Anticline (Fletcher and Reimold, 1989) – that thinning of strata and, presumably, warping occured in pre-Vredefort times. Thus, it is impossible to ascertain whether Ventersdorp lava, for instance, constituted an important contributor to Granophyre melt (although Ventersdorp strata occur west of the dome at
3.5 km thickness). It has been established that Archean granite gneiss was an important component – as confirmed by both the actual clast proportions and matrix mineralogy (Reimold et al., 1990a; Buchanan and Reimold, 2003) and the chemical modeling. We do not believe that the question of whether the clast population is a true reflection of the geological target composition has been sufficiently addressed with the limited information available from other impact structures. This issue deserves further detailed investigation. In particular, whether Transvaal carbonate was involved in Vredefort Granophyre formation remains an intriguing issue. The presence of substantial dolomite in the environs of the Vredefort Dome strongly suggests that the target region was carbonate-covered. The absence of an obvious carbonate clast component could indicate that either this component of the target stratigraphy was essentially impact-dissociated and not mixed into the impact melt, or that a small component has to date escaped observation – possibly due to its complete assimilation into Granophyre groundmass. 5.1.3. Meteoritic component The trace element data are consistent with a slight enrichment of siderophile element abundances in

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Granophyre in comparison to the target rocks. However, not having good constraints on the target lithologies and their respective proportions, it has not been possible to reliably delimit the indigenous component. Nevertheless, PGE data (Koeberl et al., 1996, 2002) support the presence of a chondritic component, and the Re–Os isotopic method has convincingly demonstrated that an
0.2% chondritic component was mixed into the Granophyre melt. It has been demonstrated already with the existing limited data that the extraterrestrial component in the Vredefort Granophyre is not homogeneously distributed. This is evident from the siderophile element data reviewed above, and also from the Re–Os isotopic results. The Granophyre in this regard is similar to the Morokweng impact melt rock. Maximum ETC abundance in Morokweng impact melt reaches 2–5% (Koeberl et al., 2002). In this case, heterogeneous distribution of PGEs and other siderophile elements is, at least partially, linked to scavenging of these elements by mafic and ultramafic inclusions, some rims of which, and reaction rims around which, contain very high Ir abundances. 5.1.4. Shock metamorphosed components Buchanan and Reimold’s (2003) analysis of granitoid and arkosic inclusions in Granophyre led to a number of conclusions: (1) Formation of the Granophyre involved thorough mixing of melts and inclusions from very different parts of the evolving crater structure. The different shock and thermal overprint-related textures in clasts in Granophyre relate to shock pressures from o5 GPa (i.e., essentially unshocked) to 430 GPa (including PDF development and mineral as well as bulk melting). (2) Micro-analysis of some feldspathic melts formed at quartz/feldspar contacts in granitic inclusions implies that at least some melting was eutectic and occured after incorporation of a clast into the hot impact melt. (3) Melt pockets formed at ambient temperatures between
650 and 1610–1727 1C (Weast, 1976; www.isis.rl.ac.uk/ISIS98/). 5.1.5. Granophyre formation The distinctive clast population in the Vredefort impact melt rock, with a strong metasedimentary and igneous component, can only be explained by emplacement of the Granophyre from above, where the melt was capable of assembling a clast population that involves all major supracrustal rock types. However, it also indicates that much of the melt was generated from the Archean basement complex, as the granitoid-dominated composition would otherwise not be achieved by those dikes along the core-collar boundary. One can estimate, by comparison with the Sudbury Structure of similar size and target stratigraphy, that the total impact melt volume must have been of the order of 5000–8000 km3 (e.g., Grieve et al., 1991; Dressler and Reimold, 2001,

2004). The bulk of this melt would have covered the collapsed central uplift or would have been located in the environs of the uplift structure. Obviously, it has been largely eroded, with only remnants preserved as Granophyre dikes. All these dikes are characterized by chemical homogeneity, as demonstrated above, despite the fact that they must have intruded downward by significant distances and in a highly dynamic environment. This scenario implies that the sinking of injected melt would have picked up additional clast load during descent, as seen in the collar dikes cutting mafic intrusions and ironstones. The confinement of Granophyre melt to dikes that are concentric and radial to the dome, and where the radial dikes follow the main fabric orientations in the Archean basement complex (Reimold and Colliston, 1994; Lana et al., 2003b) strongly suggests that opening of these fractures is related to a late stage of the formation of the central uplift, likely the collapse phase. This was an extensional event (also: Lilly, 1981; Simpson, 1978, 1981; Therriault et al., 1996) that allowed large fractures to open, likely with pre-existing weaknesses being exploited. The Granophyre dikes have not remained undeformed. Evidence of micro- and, rarely, macrofaulting has been observed by us. However, displacements are limited and rare. Joint spacing is wide, and fault or shear zones are entirely absent. This could imply that the emplacement of the dikes ensued towards the end of the uplift collapse and crater modification stages, late in the impact cratering process – after sufficient time for melt homogenization had passed. Therriault et al. (1996, 1997) emphasized that the dikes in the core of the dome have a dominant – but not exclusive – spherulitic texture, whereas those along the core/collar boundary have a granular texture, which could be the effect of different emplacement depth and dike width. Gibson et al. (1998) and Gibson and Reimold (2000), in contrast, argued that this could be the result of the strong lateral thermal gradient that occured across the dome in response to differential uplift and radially different degrees of shock heating (Gibson and Reimold, 2005). An important observation is that some dikes display clast concentration on one side, and different dikes show this on opposite sides. This is in obvious contrast to the crust-on-edge model for the basement of the Vredefort Dome (e.g., Slawson, 1976; Hart et al., 1981; Stepto, 1990), which has been challenged recently) following comprehensive structural analysis of the core of the dome (Lana et al., 2003d). Rapid cooling of the impact melt is indicated by the generally fine-grained nature of the Granophyre, the existence of quench features such as isolated spherulites and radial pyroxene growths in matrix-rich samples, the presence of chill-margins along dike edges, and the coarsening of spherulites toward the central parts of dikes. The melt reached solidus temperature rapidly,

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despite the 4500 1C temperature of the host rock (Dressler and Reimold, 2004). Therriault et al. (1996) interpreted mineral and rock textures to indicate a moderate to high degree of supercooling. The fact that the clast population is generally homogeneous throughout all sampled dikes was taken by Therriault et al. (1996) as supporting evidence that homogenized impact melt infiltrated opening fractures from above – in contrast to an ascending magma from below that would have stoped into geologically different sectors of the dome region and sampled varied country rock compositions. The temporal relationship between formation of Granophyre and pseudotachylitic breccia is not resolved. The field evidence is ambiguous, and only one narrow veinlet of pseudotachylitic breccia crosscutting a Granophyre block has ever been reported (Reimold et al., 1990a; Gibson and Reimold, 2001), besides a very small pocket of melt in the margin of one of the core dikes. However, whatever the interpretation of the Granophyre/pseudotachylitic breccia situation on farm Spitskop, it must be considered that both melt rocks were formed within a very short time, in a complex and rapidly evolving crater.

5.2. Pseudotachylitic breccias 5.2.1. Appearance and occurrence of pseudotachylitic breccias In general appearance, the pseudotachylitic breccias of the Vredefort Dome closely resemble pseudotachylites in tectonic settings. All geometries observed by Sibson (1975) in tectonic environments have been observed at Vredefort. Nevertheless, an exclusive link between faulting/shearing and breccia development cannot be established (Dressler and Reimold, 2004). While some pseudotachylitic breccias occur in veins along which limited displacement is observed, displacement is not the norm. No relationship between length of displacement and width of fault breccia has been established for Vredefort breccias. Pseudotachylitic breccias in the Vredefort Structure occur in two regimes: (1) In the Witwatersrand goldfields 480 km from the center of the structure, well outside the zone of notable shock metamorphism (even outside of the several GPa limit indicated by the occurrence of shatter cones (Fig. 2); and (2) in the Vredefort Dome, where shock pressures between 10 and 435 GPa (Gibson and Reimold, 2005) are typically indicated. Limited brecciation is found in the middle collar (upper Witwatersrand and Ventersdorp strata), where the rocks would not have experienced background shock pressures in excess of 5–10 GPa. No PDFs are known from these strata, although Martini (1991) reported an occurrence of coesite in a quartz vein

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intersected in a borehole in Ventersdorp lava of the southeastern collar. Most if not all breccias in the dome have a melt matrix. A direct association to faulting/shearing can only be made in a small sample of all known occurrences. The abundant, and sometimes truly massive, breccia occurences in the wider Witwatersrand region include bona fide pseudotachylite (friction melt), cataclasites and mylonitic breccias. In contrast to the majority of breccia occurrences in the Vredefort Dome, all known Witwatersrand occurrences are associated with fault systems. Further important observations include: (1) The breccias in the dome have melt volumes higher than those in the Witwatersrand breccias, by orders of magnitude. (2) There are also no major flanking breccia or mylonite zones evident in dome exposures (with the exception of those places where pseudotachylitic breccia formation exploited already existing mylonite occurrences – obvious planes of weakness), in contrast to the occurrences in the Witwatersrand goldfields. (3) Some slip is noted associated with vein systems on the dome, but no correlation between vein size/pseudotachylitic breccia volume and slip magnitude could be confirmed. A matter of much debate has been the mode of emplacement of the breccias in the rocks of the Vredefort Dome. There is a distinct relationship between immediate host rock and clast population and also chemical composition of the breccia matrix (generally veins). However, a similar relationship is noted between breccia in offshoots/injection veins and their immediate host rock. Consequently, it must be resolved whether the fillings of these ‘‘apophyses’’ and offshoots represent injections from a larger melt ‘‘pool’’, or whether they also formed in situ. A related issue is whether the concept of tectonic generation planes/ injection veining can be applied to the Vredefort breccias. An alternative is that the pseudotachylitic breccias in the dome formed during the shock compression phase of early impact cratering, either with or without a frictional heating component (Gibson et al., 2002; Dressler and Reimold, 2004; Gibson and Reimold, 2005; Melosh, 2005; Reimold and Gibson, 2005). The presence of voluminous melt breccias in dilational sites formed during central uplift development (e.g., in the hinges of large folds related to tangential shortening) suggests that melt formed in a vein network throughout an extended volume (under compressional condition) was pooled. There are two processes that could be responsible for such melting within large, geometrically unconstrained volumes: (1) melting under shock compression, with or without a frictional component (compare Kenkmann et al., 2000; Langenhorst et al., 2002), or (2) local excursions to ultra-high strain rate either during shock compression or during decompression (i.e., central uplift formation). While these

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processes would be active in extended rock volumes, melting by shock and/or friction would obviously be favored locally by the presence of older defects (deformation features, lithological contacts, local crush zones). The fact that individual and total pseudotachylitic breccia volumes are lower in the collar strata than in the basement gneisses may reflect the lower shock pressures in the collar rocks, relative to those determined for the core of the dome (Gibson and Reimold, 2005), or the anisotropic nature of the stratified collar rocks, which ensured that the breccias were distributed through a network of less voluminous veins, rather than accumulating into large individual volumes. Target rock experiences a number of different ‘‘tectonic’’ processes during the impact cratering event. The shock compression phase is compressional, whereas crater collapse is generally extensional, but locally compression can also occur at this stage (F. Wieland, pers. commun., 2005). The final modification stage may result in locally different ‘‘tectonic’’ regimes, with either compressional or extensional forces being active at different places in the crater structure. Strain rate, in particular, is enhanced by orders of magnitude over the rates related to tectonic deformation processes (Spray, 1998). If Henkel and Reimold’s (1998) estimate that the entire cratering process (projectile contact to end of crater collapse) at Vredefort essentially lasted some 15 min is correct, then the exact timing or emplacement relationship for Granophyre melt and pseudotachylitic breccia must be considered within a context very different from ‘‘normal’’ geological timeframes. In addition, the energy transfered to the target volume by the impacting bolide is incomparable to the magnitudes of tectonic forces released in the upper crust. Based on new shock pressure determinations, Gibson and Reimold (2005) suggested that although much of the pseudotachylitic breccia in the dome formed in rocks that experienced shock pressures well below those capable of inducing shock melting, the heterogeneous nature of the target rocks likely enhanced shock pressures locally through refraction and/or reflection of the shock waves. Baratoux and Melosh (2003) have shown this through numerical modeling of a shock wave interacting with a fracture. Whilst such modeling cannot rule out frictional heating due to differential acceleration of blocks along discontinuities, it is possible that both shock and friction heating contributed to melt development. An alternative explanation is that the development of melt facilitated slip along the fractures for a short time until quenching commenced. A shock origin for much of the pseudotachylitic breccia in the Vredefort dome might provide an explanation for its variable distribution. In the outer parts of the dome, the background shock pressure was too low to trigger melting except where pressures could

be significantly enhanced by void collapse or interference of secondary shock fronts caused by refraction or reflection off the myriad discontinuities in the strongly layered target rocks. Closer to the center, the increased background shock pressure ensured an increased frequency of breccia and larger volumes of individual breccias, as less enhancement of the shock pressure was needed to reach melting conditions. The distinct relationship between the high-pressure polymorphs coesite and stishovite and pseudotachylitic breccia veinlets in the inner collar of the dome (Martini, 1978, 1991), as well as the fact that these phases otherwise have only been observed in narrow zones of wall rock immediately adjacent to such veinlets, provides strong evidence for a shock (plus/minus friction) origin of, at least, these narrow breccia veinlets. Only a local, high (shock) pressure gradient from outside of these zones into the veins can account for the limited development of these high-pressure polymorphs. Gibson and Reimold (2005) interpret the voluminous breccia dikes as shock melt ponded in extensional sites as a result of the post-shock stresses associated with formation of the central uplift. In the central parts of the dome, the threshold shock pressure required to effect substantial shock modification of feldspar was exceeded throughout the rock mass, possibly aided by the lowering of this threshold due to the higher pre-impact temperatures of mid-crustal rocks relative to those from shallower crustal levels. This led to a fundamental change in the way the shock wave interacted with the rocks, which smoothed out, to a large extent, the shock pressure perturbations seen at lower background shock pressures, thereby restricting the development of the pseudotachylitic breccias. Nonetheless, the fact that the background pressure in these rocks was closer to the pressure necessary to generate shock melts ensured that some melts did form. Dressler and Reimold’s (2004) ‘‘flash replacement process’’ means nothing else but the explosive transfer of thermal shock energy during the compression to decompression phase of cratering; areas of widely differing size may be subjected to enormous thermal overprint, triggering melting of minerals according to their respective solidus temperatures. The structural setting of breccias in the wider Witwatersrand basin, as well as most of the related chronological results reviewed above, strongly suggest that these breccias in their majority are related to the Vredefort impact. The fact that multiple occurrences of pseudotachylitic breccia (of different breccia types) occur locally in these fault zones could mean either (1) fault-rock bearing fault zones were re-activated by the Vredefort impact, or (2) fault-rock was formed at several stages during the impact event (e.g., during the early compression phase when general outward movement of target rock was initiated, and later during the

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modification phase, when inward-directed collapse of the outer parts of the impact structure took place. The likely movement planes would have been important lithological contrasts and zones of weakness, such as bedding planes and fault zones. The observation of more than one generation of pseudotachylitic breccia in the Vredefort Dome and environs can be reconciled with a number of scenarios. Evidence for pre-impact formation of breccia, both in the dome and the wider Witwatersrand basin, has been presented (Reimold and Colliston, 1994; Berlenbach and Roering, 1992). Also, Spray et al. (1995) showed that two breccia generations (the vein itself and clasts in this vein) were formed during impact, which shows that the highly dynamic impact cratering process may lead to formation of pseudotachylitic breccia within intervals of minutes, hours or days – that cannot be resolved with current chronological techniques. Within the violent context of impact, melt breccia can form in a variety of ways: (1) by bulk shock melting in the regime relatively close to the point of impact, resulting in a large impact melt volume; (2) due to a combination of rapid change of pressure and temperature, under enormous strain rate, during both compression and decompression, within the volume of the central uplift; and (3) due to friction only associated with large displacements along faults that are developed either as a consequence of seismic activity triggered by the impact or may have been present before impact but are reactivated by the event. Massive breccias along bedding-parallel and normal faults in the outer impact stucture are likely the product of modification, the crater collapse, late in the cratering scheme. The fact that several types of breccias are found in these fault zones could mean that (1) these fault zones have been active at different times in the history of the basin – and the multiple breccias from Elandsrand gold mine do indeed favor this possibility; one or more of the several generations of mylonites would then have been caused by pre-impact tectonic events; or (2) that melting was limited to local rock volumes while the bulk of the fault zone was only subjected to brittle deformation and development of cataclasite. It has been suggested that massive occurrences of pseudotachylitic breccias in the outer parts of large impact structures (such as the Sudbury Breccias) signify a relationship between ring fault (so-called ‘‘superfaults’’) development and enhanced friction melting (Spray, 1997). Certainly in the case of Vredefort this idea is not supported by the existing, still limited – especially with regard to spatial distribution of impactrelated breccia – data from the environs of the Vredefort Dome. Information is restricted to the goldfields of the northern parts of the Witwatersrand basin – no evidence is available from the southern and eastern parts, and very limited breccia occurrence is known from the

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Welkom goldfield (Fletcher and Reimold, 1989; this Fig. 12). In addition, it is clear that the large beddingparallel fault zones in the northern environs of the Vredefort Dome were in existence prior to the impact event, and they do not compare with the stratigraphic position and physico-chemical system of the occurrences of massive Sudbury Breccia in basement gneiss around the Sudbury Structure. In fact, the Sudbury Structure is distinctly less eroded than the Vredefort Structure. Consequently, the direct comparison of breccia distribution is simply not warranted by the geology and structural context. 5.2.2. Geochemical aspects of Vredefort breccias The detailed comparison of the compositions of host rock and breccia pairs from the Vredefort Dome indicates the same systematics for tectonic pseudotachylites and impact-generated melt breccias. Cataclasis and preferential melting of hydrous ferromagnesian minerals, followed by melting of feldspars, seems to be the dominant process (Reimold, 1991) involved in the formation of these breccias and is governed by the physical properties that determine the fracture strength of these minerals (Spray, 1992). Lateral mixing of melts of different composition is involved in the formation/ emplacement of massive breccia pods. Geochemical analysis of Witwatersrand breccias has shown a close relationship of melt breccia composition to the compositions of adjacent rock type(s). Again, where massive occurrences of breccia have been analysed, mixing between two or more precursor rock types was indicated. It is obviously problematic to use chemical evidence alone to investigate the respective melt-forming processes in impact structures, as a close relationship between compositions of melt rock and host rock would be expected for the cases of both friction origin and shock origin of the melt breccia. 5.2.3. Meteoritic component in pseudotachylitic breccia? Several workers have recently discussed the results of searches for a meteoritic component in the chemical systems of what they called ‘‘pseudotachylite’’ (e.g., Mory et al., 2000, about breccia in the Woodleigh impact structure, Western Australia – commented on by Reimold et al., 2003). This is a puzzling issue, when one considers ‘‘pseudotachylite’’ the manifestation of ‘‘friction melt’’ in rocks of the floor of a meteorite impact structure. How should a meteoritic component be mixed into the in situ generated friction melt below the impact crater? One suggestion made was that meteoritic siderophile elements could have been transported through ‘‘volatilization’’ into fracture networks of the crater floor or deeper levels of a central uplift. This is an interesting idea, if these breccia veinlets were formed under initial high-shock pressure – which remains to be proven. Even where the presence of shock melts has

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been confirmed – such as Martini’s (1978, 1991) coesite and stishovite bearing veinlets at Vredefort – no evidence of a meteoritic component has to date been observed. In this context the ongoing discussion of the origin of socalled ‘‘shock veins’’ in meteorites is important, where veining with and without high-pressure (shock-produced) minerals has been contrasted. It has also been debated (e.g., Reimold, 1998; Reimold and Gibson, 2005) that in the past the term ‘‘pseudotachylite’’ has been applied to a range of very different breccia types, including locally formed friction melt or cataclastic, and even mylonitic rocks, as well as injections of impact breccia – and impact melt – into the crater floor. For example, multiple types of impactgenerated breccia have been investigated by Reimold et al. (1999b) in the crater floor of the Morokweng impact structure. In such cases, it is certainly possible that a meteoritic component could be present in impact melt rock occuring as dike breccia in a crater floor or upper central uplift setting.

6. Conclusions Much has been learnt in the last 20 years about the Vredefort impact structure and its associated melt rocks. In fact, the study of these melt rocks has fertilized the understanding of the long enigmatic Vredefort Structure. Study of Witwatersrand breccias has been a major factor in the recognition of the actual size of the Vredefort impact structure, the largest known impact structure on Earth. Since then, it has been an important part of the study of Witwatersrand gold ore resources, a field that still requires further work. However, there are still several important issues concerning the melt rocks of the Vredefort impact structure that must be further pursued, including:



 5.2.4. Importance of Witwatersrand pseudotachylitic breccias to the evolution of the Witwatersrand gold resources The Witwatersrand breccias are important as a convenient time marker, for both structural deformation as well as mineralization processes. The Witwatersrand basin experienced several major structural events in the 800–900 Ma prior to the Vredefort impact event (e.g., Jolley et al., 2004). In contrast to these workers, we emphasize that pre-Vredefort faults would have provided logical loci of Vredefort-triggered reactivation, as well as fluid flow. Fluid flow and associated leaching and subsequent mineralization of paragenetically late gold – which is ubiquitously present in the gold-bearing conglomerates of the Witwatersrand basin and is frequently observed in footwall rocks as well (e.g., Foya, 2001; Foya et al., 1999; Reimold et al., 1999a, 2005) – must be investigated in the framework of the specific structural and thermal regional situation around the central uplift feature of the Vredefort impact structure, as discussed by Reimold et al. (2005). That Witwatersrand breccias show extensive autometasomatism as well as enrichment in gold (Reimold et al., 1999a), together with the general late appearance of this hydrothermally deposited gold and as the Vredefort event coincides with the last interval in the evolution of the Witwatersrand basin when hydrothermal temperatures of the order of 300–350 1C were reached in the region, forces the conclusion that the Vredefort impact event is responsible for widespread activation of fluids and dissolution and redeposition of gold. Hayward et al. (2005) have shown that this fluid activation was likely of small-scale spatial significance but that it affected all important gold reefs and all goldfields of the basin.



 



the need to distinguish true pseudotachylite (friction melt) from other ‘‘pseudotachylitic breccias’’, to provide a better understanding of the emplacement and formation processes of such breccias in impact structure settings; further analytical work regarding the various friction, shock, and other breccia types in the various zones of impact structures – especially with regard to their geological and structural settings; and the question raised by Spray and Thompson (1995) whether the ocurrence of massive pseudotachylitic breccias in the outer parts of large impact structures can be related to large ring-fault structures (in analogy to the Lunar multi-ring impact structures); When do pseudotachylitic breccias form during the impact cratering process? What is the distribution of different breccia types throughout the crater floor to large impact structures? To date, only Sudbury and Vredefort provide information in this regard. Finally, in the context of the Vredefort breccias, there is still ample scope for further detailed field and laboratory studies of the breccias in the dome and outside; the relationship between Witwatersrand breccias and gold mineralization is now known but requires further refinement.

Acknowledgements This paper represents an invited review solicited by the editor, Klaus Keil. We are grateful for the extensive support from landowners in the Vredefort Dome area, who supported our field work, and information and support from a number of Witwatersrand mininghouses is gratefully acknowledged. Our Vredefort research has been mainly supported by the National Research Foundation of South Africa, and through funding from the Barringer Family Fund, the Geological Society of South Africa, the University of the

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Witwatersrand, and the National Geographic Society. Drafting and photography support was provided by Lyn Whitfield, Di Du Toit, and Henja Czekanowska. Some images were kindly provided by Cristiano Lana, Martin Tuchscherer, and Frank Wieland. Reviews by Bevan French, Klaus Keil, and Christian Koeberl greatly improved the manuscript. This is University of the Witwatersrand Impact Cratering Research Group Contribution No. 88.

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