B. CLARKE, R. UKEN AND J. REINHARDT
THE GEOMETRY AND EMPLACEMENT MECHANICS OF A BUSHVELD COMPLEX PERIDOTITE BODY, SOUTH AFRICA B. CLARKE Council for Geoscience, PO Box 900, Pietermaritzburg 3200, KwaZulu-Natal, South Africa Current affiliation: The MSA Group email: [email protected]
R. UKEN AND J. REINHARDT School of Geological Sciences, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000, South Africa email: [email protected]
; [email protected]
© 2009 June Geological Society of South Africa
ABSTRACT The Apiesdooringdraai peridotite is a syn-Bushveld Complex, dominantly harzburgitic intrusion of Lower Zone parentage. Detailed mapping and three-dimensional modeling shows the intrusion to comprise several discordant segments linked by concordant, silllike segments. Deformation in the contact aureole is strongly partitioned adjacent to the discordant segments of the intrusion and is detectable at distances from the contact on the order of the intrusion width. In contrast, areas above and below the intrusion, and at greater distances from the discordant intrusive segments, are effectively undeformed. The geometry of structural features suggests that the peridotite is a variably structurally-evolved stepped-sill system that was emplaced as kilometre-scale magma fingers from the northwest to the southeast, a trend heavily utilised by the mafic-ultramafic magmas of the Bushveld Complex. Deformation in the country rock was affected by the lateral expansion of the magma fingers at high postulated strain rates, and linkage of offset fingers formed prominent steps in the intrusion. Emplacement-related country rock deformation is postdated by a shallow dipping foliation that records compaction and crustal loading during contact metamorphism.
Introduction Metasediments of the Palaeoproterozoic Pretoria Group (Transvaal Supergroup) in the Burgersfort area of the Mpumulanga Province, South Africa, form the floor to the ~2.06 to ~2.05 Ga (Buick et al., 2001; Scoates and Friedman, 2007) mafic-ultramafic Rustenburg Layered Suite (RLS) of the Bushveld Complex, the world’s largest layered intrusion. Associated with the RLS in this area, and intrusive into the floor rocks, is an extensive peridotite body, known as the Apiesdooringdraai peridotite (ADP), which has an outcrop areal extent of ~26 km2 (Figure 1), forming part of a feature known as the Burgersfort Bulge. The form of the ADP is enigmatic, and has been previously described as a “discordant cushion” (Sharpe, 1982), separating the RLS from the floor rocks. The focus of this study is the gross form of the intrusion, the mechanics of its emplacement and the implications of these findings for intrusive trends in the RLS. Regional geological setting and previous investigations The subdued topography immediately around the town of Burgersfort is markedly different from the rugged and locally mountainous terrain that characterises the region (Figure 2). This flat area owes its existence to two main features, namely the confluence of the Steelpoort and Spekboom Rivers, and the erosive susceptibility of the underlying bedrock. The soft substrate is due entirely to
the presence of the ADP, which is extensively weathered to magnesite to depths up to 25 m (Figure 3). Combined with the extensive alluvial terraces developed in the area, the deeply weathered bedrock has resulted in a region of ~30 km2 of very flat topography around Burgersfort. West of the town, mountainous terrain, underlain by erosively more resistant Marginal and Lower Zone rocks of the RLS, rises ~650 m above the adjacent Spekboom River floodplain, whereas to the east metamorphosed floor rocks of the Pretoria Group define prominent westward-dipping cuestas (cf. Figure 2). The marginal rocks of the eastern RLS have been studied in great detail by Sharpe (1981), Sharpe (1982), Harmer and Sharpe (1985) and Sharpe and Hulbert (1985). Sharpe (1981) recognised four main marginal border groups, of which the B1 laminated Marginal Zone is representative of the marginal rocks in the study area. It comprises alternating sheets of norite (including conediabases and quench textured micronoritic sills), xenolithic norite, pyroxenite and harzburgite. From field relations, Sharpe (1981) noted that this group was deformed prior to the intrusion of the ultramafic Lower Zone. The Lower Zone of the RLS is not associated with its own border facies as it is always in contact with the laminated Marginal Zone which separates it from the Pretoria Group floor rocks (Figure 4). Two types of ultramafic bodies associated with the Lower Zone were, however, considered by Sharpe (1981) to also occur as intrusions in the floor rocks. These are discordant,
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GEOMETRY AND EMPLACEMENT OF A PERIDOTITE BODY
Figure 1. Geological map of the eastern Bushveld Complex and its contact aureole, showing the location of the Apiesdooringdraai peridotite (ADP). The prominent eastwards bulge of the Bushveld Complex – Pretoria Group contact is the Burgersfort Bulge. Modified after Clarke et al. (2005).
harrisitic pods and the more extensive suite of peridotites exposed around Burgersfort. The peridotites, of which the ADP is one of the largest, are envisaged by Sharpe (1981) and Sharpe and Hulbert (1985) to be very early fractionates of the Lower Zone magmas that were periodically expelled from the magma chamber. Sharpe and Hulbert (1985) envisaged tectonism in the magma chamber as a likely driving force for magma expulsion. Liebenberg (1964) also noted the presence of ultramafic bodies around Burgersfort that intruded the Marginal Zone. The presence of these easily and preferentially weathered ultramafic rocks is also responsible for the accentuation in the appearance of the Burgersfort Bulge, an apparent downward transgression of the RLS into the Pretoria Group floor. Recent examination of the “Bulge”
(Clarke et al., 2005) indicates that, rather than being a downward transgression of the RLS, it represents a deep trough of RLS rocks that ponded between two upwarps of the metasedimentary floor, the Steelpoort and Derde Gelid periclines. These two structures originated as interfinger deformation zones between intruding Lower Zone magma fingers (Uken and Watkeys, 1997; Clarke et al., 2005). Regardless of the true origin of the “Bulge”, the ADP results in a prominent eastward shift of the outcrop position of the Pretoria Group-RLS contact (cf. Figure 1). Aside from the recognition of its discordance (Sharpe, 1981), little detailed work has been carried out to resolve the structure and gross geometry of the intrusion, other than a gravity and field mapping survey
SOUTH AFRICAN JOURNAL OF GEOLOGY
B. CLARKE, R. UKEN AND J. REINHARDT
Figure 2. Oblique, southeasterly-facing view of the Burgersfort area. The image comprises the 15 m resolution panchromatic band of the Landsat 7 image draped on the 20 m digital elevation model of the area. Note the extensive area devoid of relief around Burgersfort, which is largely due to the deeply weathered (to magnesite) nature of the ADP (shown in black). Vertical exaggeration ~4 x.
Figure 3. Example of the abandoned magnesite workings west of Burgersfort (tyres in foreground for scale). The ADP (here deeply weathered to magnesite) at this locality is intrusive into the noritic Marginal Zone of the RLS, which forms the hills in the background.
done by Liebenberg (1964), who noted the locally wedge-shaped nature of the body. Limited drilling of the body was also carried out intermittently by Gold Fields South Africa during the 1980s, primarily in the hope of intersecting nickel mineralisation (Picken, 1990). This exploration programme partially elucidated the geometry of the intrusion, with borehole intersections revealing continuity of the body to depths of 400 m in places. Stratigraphically, the quartzitic Magaliesberg Formation of the Pretoria Group forms the floor to the RLS in the immediate vicinity of Burgersfort (Figure 4). To the north and west, however, the updomed Derde Gelid and Steelpoort periclines result in extremely discordant contacts, with the RLS cutting across many hundred metres of floor rock stratigraphy. Recent investigations (e.g. Cawthorn et al., 1998; Kruger, 2005; Clarke et al., 2009) support a sill-like model for the RLS, which intruded the floor concordantly to slightly discordantly (Cawthorn, 1998). Importantly, palaeomagnetic evidence indicates the RLS was emplaced horizontally (Hattingh, 1986; Letts et al., 2009) and later subsided isostatically to form the current, basin-like geometry of the RLS.
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GEOMETRY AND EMPLACEMENT OF A PERIDOTITE BODY
Figure 4. Simplified geometric stratigraphic column showing the relationship between the RLS, ADP and Pretoria Group floor. Modified after Sharpe (1981).
The Pretoria Group in the Burgersfort area was not subjected to pre- or post-Bushveld penetrative deformation and there is no evidence of the diapirism that affected much of northeastern contact aureole (Uken and Watkeys, 1997; Gerya et al., 2003). Deformation related to diapiric deformation of the Steelpoort pericline is restricted to the RLS sequence within ~5 km of the pericline (Clarke et al., 2005), whereas the Derde Gelid structure is not of diapric origin (Clarke et al., 2009). However, local zones of intense deformation are present in the country rock adjacent to the ADP and these must be attributed to a local causal factor. Local geological and stratigraphic setting The dominantly volcanoclastic Machadodorp Member of the Silverton Formation is the oldest stratigraphic unit exposed in the area (Figure 5) and is one of only a few volcanogenic horizons in the Pretoria Group. This member, whose base is not exposed in the study area, comprises predominantly volcanoclastic agglomerates with subordinate tuffaceous, pelitic and calcareous rocks. The agglomeratic rocks, in particularly,
are spectacularly exposed in the bed of the Monaps River in the eastern part of the study area, where volcanic bombs up to 20 cm in diameter are embedded in a fine-grained, grey matrix. The agglomeratic rocks show little evidence of deformation other than brittle, post-Bushveld high strain zones and associated dolerite dykes that transect them. Minor shaly interbeds are present, and tuffaceous rocks are also exposed along railway cuttings in the area. Thin (