Constraining the timing and migration of collisional ... - CiteSeerX

SHANNON D. JOHNSON, MARC POUJOL AND ALEXANDER F.M. KISTERS

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Constraining the timing and migration of collisional tectonics in the Damara Belt, Namibia: U-Pb zircon ages for the syntectonic Salem-type Stinkbank granite Shannon D. Johnson Department of Geology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa Present address: Martha Mine, Newmont Waihi Gold Ltd., PO Box 190, Waihi, New Zealand e-mail: [email protected]

Marc Poujol Facility for Isotopic and Elemental Determination – Memorial University of Newfoundland, 300 Prince Philip Drive, St John’s, NL, CANADA A1B 3X5. e-mail: [email protected]

Alexander F.M. Kisters Department of Geology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa corresponding author: e-mail: [email protected] © 2006 December Geological Society of South Africa

ABSTRACT A U-Pb zircon LAM-ICP-MS age of 549 ± 11 Ma from the Stinkbank granite in the south Central Zone of the Damara belt represents the first robust age for the crystallization of a member of the regionally widespread suite of syntectonic Salem-type granites. This age confirms previous assertions that suggest that the main phase of crustal shortening related to the collision between the Congo and Kalahari Cratrons occurred between 550 and ~540 Ma in the south Central Zone, the leading edge of the Congo Craton. Ages from syntectonic granites reported in the literature from the north Central and the Northern Zone record progressively younger ages of ~530 to ~515 Ma and late-stage folding in the Northern Zone is suggested to have occurred between ~510 to ~500 Ma. The younging of ages that date the main collisional phase towards the north is interpreted to track the northward propagation of the deformation and metamorphic front from the main suture zone to the northern foreland of the Damara belt. Geochronological data are sparse, and the lack of detailed structural data precludes the retrodeformation of the fold-and-thrust belt along this 250 km long section. However, the age brackets suggest a migration of the orogenic front at a rate of at least 5 to 7 mm/a, but more likely in the order of several centimetre per year, if the internal strain is removed.

Introduction The northeasterly trending Damara Belt is interpreted to represent the collisional suture between the Congo and Kalahari Cratons. At present, most studies place the timing of collisional tectonics in the central, high-grade metamorphic parts of the belt, the south Central Zone (sCZ), between ~570 to ~530 Ma (e.g. Kröner, 1982; Miller, 1983; Jacob et al., 2000; Nex et al., 2001; Seth et al., 2000; Tack and Bowden, 1999; Tack et al., 2002; Kisters et al., 2004). This timing is tentatively constrained by a suite of regionally widespread and variably deformed granitoids, locally referred to as the “Salem Granitic Suite” or “Salem-type granites”, that are generally considered to be syntectonic with respect to the main collisional event in the sCZ (Jacob, 1974; Miller, 1983; Miller and Burger, 1983; Allsopp et al., 1983). Reported ages for Salem-type granites include U-Pb zircon ages of 580 ± 30 Ma (Allsopp et al., 1983) from Goas, south of Karibib, in the southern Central Zone and 589 ± 40 Ma and 546 ±30 Ma from Salem-type granites in the Northern Zone and northern Central Zone (Miller and Burger, 1983). In addition, there are numerous RbSr whole rock ages of Salem-type granites documented

in the literature, but these show invariably large errors of 20-60 Ma, clustering around apparent ages of ~570 to ~550 Ma (see summary in Haack and Martin, 1983; Kröner, 1982; Miller 1983). Hence, there are, to date, no precise and robust age determinations of the Salem-type granites in the sCZ and the timing of the main phase of collisional tectonics remains only poorly defined. The prolonged and complex thermal evolution of the Damara Belt is reflected by the wide spread of mineral ages between ~550 and ~460 Ma (e.g. Kröner, 1982; Miller, 1983; Haack et al., 1983; Hawkesworth et al., 1983; Horstmann et al., 1990; Jacob et al., 2000; Jung et al. 1998, Jung and Mezger, 2003, to name but a few), which has somewhat complicated the identification of the exact timing of collisional tectonics in the belt. For example, Jung and Mezger (2003) applied a number of different isotope systems on a variety of rock-forming minerals (Sm-Nd garnet, U-Pb monazite, Rb-Sr biotite) taken from leucosomes in migmatites, identifying multiple phases of high-temperature events in rocks of the north Central Zone around Omaruru. Peak thermal conditions were attained between ~540 to ~500 Ma and as late as ~480Ma (e.g. Jung, 2005). Moreover, Pb-Pb

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Figure 1. Schematic geological map of the Damara belt in central Namibia, emphasizing the distribution of tectonostratigraphic zones (after Miller, 1983). Insert shows the location of the Damara belt between the Congo and Kalahari Cratons.

and U-Pb zircon ages from syntectonic granites in the northern Central and Northern Zones yield ages of 530 ±3 Ma, and ~526 to ~516 Ma (e.g. Seth et al., 2000; Jung et al., 1998). Goscombe et al. (2004; 2005) suggested the formation of late-stage, east-west trending folds in the Northern Zone, related to the collision between the Congo and Kalahari Cratons, to have occurred at ~510 to ~500 Ma. In summary, these recent studies suggest an either protracted collisional phase and associated granite plutonism between ~540 to ~490 Ma or an early Phanerozoic timing of the main collisional event in the Damara belt at ~500 Ma (e.g. Jung and Mezger, 2003; Kröner et al., 2004; Goscombe et al., 2004; 2005). In contrast, Jacob et al. (2000) concluded that ubiquitous U/Pb titanite ages of ~500 Ma from the Navachab Mine and surroundings in the sCZ record the recrystallization and isotopic resetting of the minerals in response to a high-temperature, but post-collisional event at that time.

Similarly, Nex et al. (2001) suggested a two-stage metamorphic evolution for the Central Zone of the Damara Belt. An early medium-P medium-T metamorphic event (M1) related to the main collisional event and tentatively constrained between 571 ± 64 and 534 ± 7 Ma was followed by a high-T low-P regional thermal event (M2) interpreted to have occurred between 534 ± 7Ma and 508 ± 2 Ma. Miller (2002) categorically states that all granites with ages up to ~540 Ma in the sCZ are convergence- and collisionrelated whereas younger granites are post-tectonic in the sCZ, but syn-tectonic in the north Central Zone (Figure 1). This highlights the existing controversy about the actual timing of collisional tectonics in the Damara belt. We document intrusive relationships between the Stinkbank granite (Marlow, 1983), a composite, variably deformed Salem-type granite and wall rocks of the

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Damara Sequence in the sCZ, southwest of the town of Usakos (Figures 1 and 2). We focus on the fabric development within the granites and the wall rocks as well as intrusive contact relationships, in order to illustrate the syn-kinematic timing of granite emplacement during regional-scale folding related to the main collisional phase in the sCZ (D2 after Jacob, 1974; Kisters et al., 2004; D3 after Miller, 1983). U-Pb zircon ages from one of the phases of the Stinkbank granite provide the first robust ages that constrain the absolute timing of the syntectonic Salem-type granite and, thus, the timing of collisional tectonics in the sCZ. These results will be compared and discussed with other age data that constrain the main phase of crustal shortening in the Damara belt. Geological setting The Damara belt in central Namibia is a east-northeasttrending collisional orogen that records the rifting, subsequent convergence and final collision between the Congo and Kalahari Cratons in the late Neoproterozoic and earliest Paleozoic (e.g. Kröner, 1982; Miller, 1983) (Figure 1). The belt shows a well-preserved bivergent symmetry, typical of collisional belts. The internal parts of the orogen consist of a high-grade metamorphic zone intruded by abundant syn- and post-collisional granites, which are situated on the leading edge of the overriding plate of the Congo Craton, the Central Zone. The Central Zone is bordered in the south by the medium- to highpressure metasedimentary belt of the Southern Zone, commonly interpreted to form the accretionary complex above the downgoing plate of the Kalahari Craton (Kasch, 1983; Stanistreet et al., 1991). The internides are bounded on both sides by foreland-verging fold-andthrust belts that are, in turn, succeeded by foreland basins on the Congo and Kalahari Cratons, respectively. The supracrustal rocks of the Damara Sequence preserve the late-Neoproterozoic lithological record of this evolution. Basal coarse-clastic, continental sediments and volcanic rocks of the Nosib Group unconformably overlie Paleo- to Mesoproterozoic basement gneisses. The rift succession is overlain by carbonate and turbidite successions of the Swakop Group, that represent shallow and deep marine sediments of an evolving shelf, followed by the thick and widespread flysch-type sediments of the Kuiseb Formation. Marine sedimentation lasted from ~740 Ma to probably ~580 to ~580 Ma and northwesterly-directed subduction of the Kalahari plate below the Congo Craton culminated in the collision of the two continental blocks (e.g. Miller, 1983; 2002). Syn- and post-collisional foreland molassetype deposits of the Mulden and Nama Groups are developed along the northern and southern margins of the orogen. U-Pb zircon ages from ash beds in the lower sections of the molasse-type sediments of the Nama foreland basin, south of the Damara belt, provide tight age constraints for the timing of foreland sedimentation and unroofing of the orogen between ~550 and ~540 Ma (Grotzinger et al., 1995; Prave, 1996).

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The structural inventory of the sCZ records a polyphase deformational history. Early generations of regional-scale recumbent folds and associated beddingparallel to –subparallel foliations are related to crustal convergence and thrust- and nappe-tectonics (D1 and/or D2 after Miller, 1983; Kasch, 1983). The main structural grain is dominated by northeasterly-trending, km-scale periclinal and/or sheath-fold like domal structures and intervening, tightly infolded synforms. Irrespective of the controversially discussed formation of the dome structures, regional-scale folding and crustal shortening are generally accepted to be related to the final head-on or slightly oblique collision between the Congo and Kalahari Cratons. Later tectonism is not penetrative and deformation is largely confined to shear zones and lineaments that are variably reported to show strike-slip or transpressional kinematics (e.g. Stanistreet et al., 1991; Poli and Oliver, 2001). Throughout much of this evolution, large parts of the Damara belt were intruded by voluminous syn-, late-, and post-tectonic granite plutons. The Central Zone, in particular, represents the central magmatic axis of the orgen. Granitic plutonism is recorded over a period of at least 150 Ma between ~620 and ~470 Ma (e.g. Kröner, 1982; Hawkesworth et al., 1983; Haack et al., 1983) Convergence-related calc-alkaline intrusives are rare and much of the voluminous granite plutonism is related to the collisional and, particularly, post-collisional stages of the orogen (e.g. Miller, 1983; 2002; Hawkesworth et al., 1983; Jacob et al., 2000; Tack and Bowden, 1995; Jung et al., 1998; Jung and Mezger, 2003). The internides of the Damara belt are characterized by high-grade metamorphic, amphibolite- to granulitefacies conditions. The Central Zone is commonly perceived to represent a high-T low-P collisional terrain, but recent metamorphic and geochronological work has hinted at a complex and long-lived history from early medium-P, medium-T metamorphism (M1, after Nex et al., 2001) to one or probably several phases of highT, low-P overprints (e.g. Jung et al., 1998; Jung and Mezger, 2003). Field relations and setting Intrusive contact relationships between Salem-type granitoids and rocks of the Damara Sequence are well exposed on the farm Gross Aukas 68, in tributary rivers that cut into the eastern banks of the Khan River, some 10 km southwest of the town of Usakos (Figure 2). These outcrops form the eastern margin of a larger, kmscale pluton largely that is covered by alluvial and eluvial deposits, but whose full extent is delineated by scattered outcrops to the west and southwest (Figure 2). Marlow (1983) referred to the southwestern extent of this pluton as the Stinkbank granite, and we adopt this name for the granites described in the following. Marlow (op cit.) reported a 10 point Rb-Sr whole-rock age of 601 ± 79 Ma for the Stinkbank granite, but also emphasized that the calculated age was likely to be of little significance due to the large scatter of data points

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Figure 2. Schematic geological map of the Usakos dome and Kransberg syncline, south of Usakos. The Stinkbank granite intrudes into the steep northwestern limb of the Usakos dome, along the contact between the Karibib and Kuiseb Formation. Section A-B refers to the location of the river section referred to in the text.

about the isochron. We describe cross-cutting and fabric relationships from a river section between coordinates 15° 32’ 35’’ E, 22°04’32’’ S and 15° 32’ 44’’ E, 22° 04’ 43’’S (Figure 3) that povides excellent 3-D and almost continuous outcrop through the contact zone between the various phases of the Stinkbank granitoids and the folded Damara Sequence. Figure 3a shows a simplified cross-section through the central parts of this traverse,

illustrating the sheeted nature of the marginal zones of the Stinkbank granite and the both concordant and sharply cross-cutting nature of the granites. Lithologies and wall-rock structures of the Damara Sequence The Stinkbank granite intrudes close to the steeplydipping contact between the marble-dominated Karibib

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Figure 3. (a) Simplified cross-section showing the intrusive relationships between the Stinkbank granite and wall-rocks of the Damara Sequence exposed along a tributary of the Khan River, the location of which is shown in the insert (see also Figure 2). Note the sheeted and largely concordant nature of the Stinkbank granite with the country rocks in the eastern parts of the cross-section while the central parts show a subhorizontal, high-angle contact between the granite and the folded Damara Sequence. The eastern part of the section is simplified and granite sheets are more abundant than can be shown. (b) Lower hemisphere, equal area projection of poles to S2 in rocks of the Damara Sequence and L2 lineations (intersection and stretching) in wall rocks. (c) Lower hemisphere, equal area projection of poles to magmatic (dots) and solid-state (triangles) foliations and magmatic lineations (squares) in granite phases of the Stinkbank granite.

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Figure 4. (a) Oblique plan view of a tight F2 fold developed in meta-turbidites of the Kusieb Formation, showing folded bedding (S0) and a well-developed, slightly fanning axial planar foliation (S2). (b) Cross-section of a relic tight fold developed in marble and interlayered calc-silicate horizons of the Karibib Formation (center) bounded by steeply dipping, highly strained domains of banded marbles (upright). The high-strain domains are parallel to the northeasterly-trending S2 foliation, resulting in the transposition of bedding into the S2 fabric.

Formation and the overlying, predominantly siliciclastic metaturbiditic sequence of the Kuiseb Formation. The Karibib Formation comprises white- to mediumgrey, finely laminated and banded marbles, marble breccias and massive calcitic marble beds. Bright white to creamish-coloured dolomitic marbles also occur. In places, the marbles are intercalated with 5 to 15 m thick packages of well-bedded, reddish-brown weathering calc-silicate felses made up of quartz, plagioclase, tremolite, biotite, clinopyroxene and garnet in various amounts. The Karibib Formation reaches a structural thickness of approximately 500 m at this locality. The overlying rocks of the Kuiseb Formation are mainly biotite and biotite-cordierite schists, minor metapsammites, calc-silicate felses and isolated marble horizons. Primary sedimentary structures are generally well preserved, including graded bedding, laminated and/or cross-bedding, load-structures and flute casts, all pointing to the turbiditic origin of much of the sequence (Badenhorst, 1992). The top of the Kuiseb Formation is

not exposed, but the sequence has a thickness of at least 1000 m. The rocks form the subvertical to slightly overturned nothwest limb of a km-scale northeasterly-trending fold structure, the Usakos dome, bordered by the large, firstorder Kransberg syncline to the immediate northwest (Figure 2; Johnson, 2005). The regional-scale folds are part of the D2 dome and syncline pattern that characterizes much of the Central Zone (e.g. Smith, 1965; Miller, 1983; Jacob, 1974, Oliver, 1994; Poli and Oliver, 2001; Johnson, 2005). Second- and lower-order parasitic folds are the most conspicuous structures in outcrop, forming tight, upright to northwesterly-verging, gently doubly plunging folds with wavelengths of between ~100 and ~300 m (Figure 3a). A steep southeasterlydipping axial planar foliation (S2) is well developed in schistose units of the Kuiseb Formation (Figure 4a) where it is mainly defined by the preferred alignment of biotite. Marble of the Karibib Formation commonly lacks evidence of the S2 foliation, due to the absence of mica or a grain-shape-preferred orientation of minerals in the recrystallized marble units. However, tight- to isoclinal, rootless, metre- to dekametre-scale fold hinges bounded by steeply-dipping, northeasterly-trending, highlystrained, banded marbles testify to the transposition of bedding into the S2 foliation on the steep northwestern limb of the Usakos dome (Figure 4b) (Kisters et al., 2004; Johnson, 2005). Hence, large parts of what appears to be bedding in the Karibib Formation represents, in fact, transposed bedding (S0/S2). Dolomitic marble horizons and calc-silicate felses are commonly boudinaged within this transposition fabric. Boudinage occurs in both horizontal and vertical sections, defining a chocolate-tablet boudinage of the more competent layers (X ≈ Y > Z; with X ≥ Y ≥ Z) in the plane of the S0/S2 transposition fabric. Mineral stretching lineations, defined by stretched biotite and/or quartz-feldspar aggregates in schists of the Kuiseb Formation and intersection lineations (S0/S2) plunge predominantly at shallow to moderate angles to the northeast (Figure 3b). Evidence of an earlier deformation phase is manifest by a weakly developed beddingparallel schistosity (S1), particularly in schists of the Kuiseb Formation. The schistosity is defined by the preferred alignment of biotite that wraps around cordierite porphyroblasts and the grain-shape preferred orientation of quartz and quartz-feldspar aggregates. The widespread biotite-cordierite- and, locally, garnetbearing mineral parageneses and the ductile deformation of all units testify to the amphibolite-facies conditions during deformation in this part of the sCZ (e.g. Steven, 1993; Masberg 2000). The Stinkbank granite The bulk of the Stinkbank granite is located in the core of the Kransberg syncline, corresponding to Miller’s (1983) general assertion that Salem-type granitoids are preferentially emplaced in synformal structures in rocks of the Kuiseb Formation and above the marble units of

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Figure 5. (a) Megacrystic, biotite-rich Stinkbank granite, typical of the rather homogeneous granites in the western parts of the river section in Figure 3a; (b) oblique plan view of an intrusive granite sheet containing abundant elongate enclaves of different plutonic rocks of mainly intermediate to mafic composition (from the eastern parts of the section in Figure 3a). (c) Mutliple sheet intrusions into a relatively homogeneous megacrystic variety of the Stinkbank granite; (d) Shallowly-dipping and sharp intrusive contact between the megacrystic Stinkbank granite (lower half of photo) and southeasterly-dipping bedding (S0) of banded marbles and calc-silicate felses of the Karbib Formation.

the Karibib Formation. The following chapters focus on intrusive and cross-cutting relationships as well as the fabric development and fabric relations between the granites and country rocks of the Damara Sequence. Detailed petrographic descriptions of Salem-type granites can be found in, inter alia, Miller, (1973, 1983), Jacob (1974), Blaxland et al. (1979), Kröner (1982), Hawkesworth et al. (1983), Marlow (1983), Allsopp et al. (1983), and Miller and Burger (1983). Granite varieties and intrusive contact relationships Much of the compositional heterogeneity shown by the Stinkbank granite can be attributed to the sheeted nature of this marginal zone (Figure 3a). Granite sheeting is particularly prominent within 150 to 200 m of the granite- wall-rock contact, where dozens of dm- to m- wide distinct granite sheets are intrusive into each other and into wall rocks. The granites become more homogeneous towards the west and central parts of

the Stinkbank granite. Broadly speaking, however, the granitoids correspond to the end-member types of Salem granite described by Jacob (1974) from the Swakop River. A biotite-rich, equigranular granite and a biotiterich megacrystic variety are the most common granite types. The two varieties dominate the western parts of the traverse towards the centre of the Stinkbank granite. Both consist of quartz, plagioclase, biotite (up to 25 %) and K-feldspar (mainly microcline). K-feldspar occurs in the matrix as well as tabular, up to 5 cm long xenocrysts typical for the megacrystic Salem-type granite (Figure 5a). Sphene, apatite, zircon, hornblende and garnet are present in accessory amounts, whereas sericite, calcite, and chlorite are secondary minerals. Leucocratic varieties are mainly composed of quartz, K-feldspar and plagioclase, with minor muscovite, biotite, zircon, sphene and garnet. Tourmaline-bearing quartz-feldspar-muscovite pegmatites are common. Numerous granite sheets contain abundant enclaves of

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Figure 6. (a) Plan-view of a northeast to southwest trending (northeast is to the right of the photo) boudinaged apophysis of the Stinkbank granite in marble of the Karibib Formation, showing the internal layering of the granite, preferred orientation of feldspar megacrysts and sheet margins that are all parallel to the surrounding S0/S2 fabric in the wall rocks. The boudin neck is filled by coarse-grained quartz. (b) Pervasive development of a solid-state gneissosity in megacrystic Stinkbank granite; (c) Magmatic fabric defined by the preferred alignment of closely packed K-feldspar megacrysts in pocket-like pods of megacrystic granite intrusive into dark, biotite-rich variety of the Stinkbank granite; (d) sketch of biotite tails developed in strain-shadow position around k-feldspar megacrysts, suggesting an origin as a submagmatic feature during the advanced crystallization of the Stinkbank granite.

other plutonic rocks, including mainly elongated enclaves of granitic and dioritic composition, but also more mafic material (Figure 5b). Wall-rock xenoliths are also present, but are rare compared to plutonic enclaves. Intrusive contact relationships of the granites with the Damara Sequence may be concordant as well as highly discordant. Most sheets intrude as bedding- and foliation (S2)- parallel or subparallel sills, showing steep, southeasterly dips, parallel to the steep northwestern limb of the Usakos dome (Figure 5c). Individual sheets range in width from merely centimetre to tens of metres. Strike extents are in excess of hundred metre for the wider sheets, but the full extent cannot be traced due to limited outcrop outside the river section. Sheet-in-sheet contacts are common, testifying to the multiple sheeting and progressive assembly of these marginal zones of the pluton. The sheer number of intrusive sheets precludes an identification of the detailed sequence of intrusion. In general, however, dark, biotite-rich granites appear to be the earliest phases. These are intruded by medium-

grey, porphyritic granites, followed by leucogranites, commonly rich in enclaves, succeeded by the final phase of pegmatites and aplites. The more homogeneous biotite-rich and commonly megacrystic granite varieties in the central and western parts of the traverse show predominantly high-angle cross-cutting intrusive relationships with the Damara Sequence, truncating the folded layering developed in marbles and calc-silicate felses of the Karibib Formation (Figures 3a and 5d). The subhorizontal, gently undulating contact can be traced for approximately 200 m along the river banks. The intrusive contact is sharp and there is no evidence for e.g. emplacementrelated strains in the rocks of the Damara Sequence. Contact metamorphic effects are limited to a 10 to 20 cm wide aureole characterized by the coarse recrystallization and bleaching of marbles and the growth of mainly vesuvianite and minor garnet. Deformation and fabrics in the Stinkbank Granite The Stinkbank granite contains a wide variety of

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fabrics, including magmatic, submagmatic and almost ubiquitous solid-state fabrics. In addition, numerous of the granitic sheets have undergone boudinage, testifying to the deformation of the Stinkbank granite and its marginal zones, in particular. Boudinage of granite sheets is particularly well developed where the sills have intruded marbles of the Karibib Formation (Figure 6a). The boudins are mainly symmetrical and rhomb- or barrel-shaped, showing separation widths of up to three metre. The boudin necks are commonly filled by either coarse milky quartz, intergrown with feldspar and, in places, by quartz-feldspar-muscovite pegmatites (Figure 6a). Boudinage of the granite sills is observed in both plan view and parallel to the northeasterly trend of the sills as well as in vertical sections, thus describing a similar type of chocolate-tablet type boudinage recorded for the more competent country-rock lithologies in the Karibib Formation. Virtually all phases of the Stinkbank granite contain planar and, locally, linear fabrics. The distinction of granite fabrics used here follows the criteria devised by e.g. Paterson et al. (1989), Tobisch et al. (1997) and Vernon (2000; 2004) who distinguish magmatic, from submagmatic and solid-state fabrics. Solid-state fabrics are almost penetratively developed in the 500 m wide contact zone where the granites

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intrude the Karibib and Kuiseb Formations, so that the granites appear almost invariably gneissose (Figure 6b). Solid state fabrics are defined by the preferred orientation of the basal planes of biotite, imparting a schistosity on biotite-rich varieties. A gneissic banding in more leucocratic granites is defined by alternating quartz-feldspar and biotite domains. Quartz is dynamically recrystallized and it is not uncommon to find quartz ribbons with aspect ratios of, in plan view, 10 to15 to 1. Feldspar megacrysts in the megacrystic granites preserve a macroscopically visible magmatic zonation, but are marginally recrystallized, forming ovoid augen wrapped around by anastomosing biotite foliae and elongated quartz-feldspar aggregates. Solid-state foliations in the granite trend northeast to southwest, showing subvertical to steep southeasterly dips parallel to the S2 foliation in the Damara Sequence (Figure 3c). The dynamic recrystallization of feldspar and the largely unretrogressed nature of biotite that defines the foliation in the granites point to at least upper greenschist- to lower amphibolite-facies conditions during the formation of the solid-state fabrics at temperatures in excess of ~450°C (Tullis and Yund, 1987). These conditions are indistinguishable from the regional metamorphic grades during the D2 deformation (e.g. Miller, 1983; Steven, 1993; Masberg, 2000).

Figure 7. SEM backscattered images of selected zircons from the Stinkbank granite showing well-developed magmatic zoning. Zr 9 preserves an inherited core giving a Mesoproterozoic age surrounded by a darker rim. SOUTH AFRICAN JOURNAL OF GEOLOGY

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Figure 8. Concordia plot of U-Pb zircon analyses for the Stinkbank granite.

Magmatic fabrics are evidenced by the preferred alignment of the long axis of tabular feldspar megacrysts in areas of lower fabric intensity, mainly towards the centre of the pluton (Figure 6c). Enclaves are commonly elongate showing a strong preferred orientation parallel to the walls of the mainly northeasterly-trending, steeply-dipping sills (Figure 5b). In plan view, the enclaves show aspect ratios of up to 10 to 1. Given that the medium- to fine-grained matrix of the granitic sheets appears largely unstrained, the elongation and alignment of enclaves has most likely occurred during magmatic flow and the emplacement of the sheets. Magmatic lineations, defined by the preferred orientation of the long axis of feldspar megacrysts (Figure 5a), are relatively weak, but show consistently shallow northeast plunges, parallel to L2 lineations in the wall rocks. Significantly, the Stinkbank granite contains evidence for submagmatic fabrics. The common occurrence of massive milky quartz or pegmatites in boudin necks of deformed granite sills, rather than calcite, as might be expected in the marble units, indicates that the sills were sufficiently strong to behave more competently than the surrounding marbles, but that melt was still present, probably as late-stage interstitial melt pockets (Figure 6a). The location of the quartz-feldspar aggregates in boudin necks suggests that the late-stage melts followed the hydraulic gradient set up in the low-stress sites of the boudin necks created during

boudinage. Biotite commonly forms tapering aggregates located at the short crystal faces of aligned tabular feldspar megacrysts (Figure 6d). This location corresponds to strain-shadow sites around the megacrysts during fabric formation and boudinage of the sheets, again suggesting the crystallization of latestage crystallization products in low-stress sites (e.g. Vernon, 2004). Geochronology For geochronological work, a biotite-rich, megacrystic granite variety from the main central granite in Figure 3 was chosen. The granite contains a moderately developed magmatic foliation, defined by the alignment of K-feldspar megacrysts and a weak solid state foliation, defined by the preferred orientation of biotite laths in the matrix and grain-shape preferred orienation of dynamically recrystallized quartz-feldspar aggregates. Analytical technique Mineral separates were prepared from a 5 kg rock sample. The sample was pulverized using a heavy-duty hydraulic rock splitter, jaw crusher and swing mill. Mineral separation involved the use of a Wilfley Table, heavy liquids (bromoform and methylene iodide) and a Frantz Isodynamic Separator. All the analyses were performed at Memorial University of Newfoundland, Canada.

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Table 1. LAM-ICPMS data for the Salem-type Stinkbank granite. LAM-ICPMS data: All errors are quoted at 1 sigma. Sample SALEM Radiogenic Ratios Grain. spot Zr15 Zr18 Zr19 Zr20 Zr21 Zr47 Zr41 Zr1 Zr7 Zr5 Zr9 Zr11 Zr14 Zr16 Zr4

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Pb/ 238 U 0.0912 0.0907 0.0944 0.0905 0.0829 0.0891 0.0881 0.0899 0.0889 0.0790 0.3058 0.0860 0.0855 0.0866 0.0966

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± 0.0066 0.0034 0.0052 0.0031 0.0050 0.0031 0.0048 0.0100 0.0035 0.0055 0.0256 0.0052 0.0021 0.0044 0.0127

Pb/ 235 U 0.850 0.700 0.791 0.749 0.639 0.713 0.711 0.799 0.741 0.697 5.055 0.771 0.700 0.799 0.867

207

± 0.080 0.047 0.071 0.033 0.091 0.055 0.048 0.118 0.043 0.063 0.369 0.040 0.037 0.054 0.147

Zircons were abraded following the technique described by Krogh (1982) and then mounted in epoxy, polished and photographed using SEM backscattered imagery. This explains the rounded shape of the grains in Figure 7. Zircon grains are generally euhedral and yellow in colour. Backscattered imaging of the grains reveals both fine-scale euhedral oscillatory zoning and sector zoning, with the exception of grain 9 with the presence of a bright core surrounded by a relatively homogeneous grey rim (Figure 7). The white rimmed square on Figure 7 represents the location of the analysis. LAM-ICP-MS The U-Pb method followed is that described by Kosler et al. (2002). Laser Ablation Microprobe-Inductively Coupled Plasma-Mass Spectrometry (LAM-ICP-MS) analyses were performed in the Microanalysis Facility at Memorial University, using an ELEMENT XR coupled to a GEOLAS 193 nm excimer laser system. Zircons were ablated using a laser repetition rate of 10Hz and laser energy of 3 mJ/pulse and a laser beam of 20 m in diameter. The sample cell was mounted on a computerdriven motorized stage. The computer driven stage was moved beneath the stationary laser to produce a square pit of 40 m. Although the technique does not require the use of external standards, we periodically acquired data for the 1065-Ma old zircon reference material 91500 (Wiedenbeck et al., 1995) and the 295 Ma old zircon reference material 02123 (Ketchum et al. 2001) to monitor the accuracy and reproducibility of our measurement. Final ages and concordia diagrams were produced using the Isoplot/Ex macro (Ludwig, 2000) in conjunction with the LAMdate Excel spreadsheet program (Kosler et al., 2002). The calculated isotopic ratios were corrected for gas blank and the small

Pb/ 206 Pb 0.0670 0.0574 0.0619 0.0600 0.0568 0.0583 0.0584 0.0639 0.0584 0.0602 0.1173 0.0649 0.0591 0.0663 0.0605

± 0.0038 0.0021 0.0022 0.0019 0.0060 0.0027 0.0015 0.0033 0.0026 0.0046 0.0029 0.0038 0.0023 0.0046 0.0050

Ages (in Ma) 206 Pb/ 238 U 562 560 582 558 514 550 544 555 549 490 1720 532 529 535 595

207

Pb/ U 625 539 592 568 502 547 546 596 563 537 1829 580 539 596 634 235

207

Pb/ Pb 838 508 671 604 483 541 543 740 543 610 1915 772 571 816 621

206

± 1 118 80 78 67 232 99 55 111 96 166 45 124 83 144 177

contribution of Pb and U from the tracer solution. This was typically less than 100 and 300 cps on masses 206 and 238. The amount of common Pb in the zircons was insignificant (typically less than 10 cps). Consequently no common Pb correction was applied to the data. Results Fifteen analyses were performed and the data are reported in Table 1 and plotted on Figure 8. Fourteen analyses plot in a concordant to sub-concordant position and define a Concordia age (Ludwig, 1998) of 549 ± 11 Ma (2 sigma, MSWD of concordance = 1.7). One point (Zr 9, Figures 7 and 8 and Table 1) plots in a discordant position with an apparent 207Pb/206Pb age of 1915 ± 45 Ma. The weighted average mean of the 206Pb/238U dates is identical “within error at 543 ± 12 Ma (2_).” The date of 549 ± 11 Ma Ma is interpreted as the emplacement age of this granite. The presence of an inherited core of ~1915 Ma reveals the presence of older crust in the region. Discussion Syntectonic timing of the Stinkbank granite Planar and linear fabrics in the marginal zones of the Stinkbank granite are virtually identical in orientation to the D2 –related fabrics in the Damara Sequence that are associated with the formation of regional-scale fold structures, such as the Usakos dome and Kranzberg syncline. The fabrics and the boudinage of intrusive granite sheets record the same northwest to southeast directed, subhorizontal shortening strain compared to the wall rocks on the northwestern limb of the Usakos Dome. One may interpret the mainly solidstate fabrics as a result of post-emplacement strains that have affected both the Damara Sequence and

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the granites. However, the continuity of granite fabrics from magmatic, via submagmatic to solid-state fabrics provides compelling evidence that fabric development not only occurred after, but also during the crystallization of the Stinkbank granite. Similarly, the sharp truncation of second-order D2 folds (Figure 5d) by the granites, which, in turn, contain planar and linear fabrics that are coaxial with D2 and fold-related fabrics in wall rocks is further evidence for the intrusion of the Stinkbank granite during D2 shortening. Given that the regional D2 dome-and-basin pattern in the sCZ is the result of the main collisional event in the Damara Belt, we interpret the U-Pb zircon age of 549 ± 11 Ma to indicate the timing of the main collisional phase in the sCZ of the Damara Belt. The presence of one zircon core with a Palaeo- to Mesoproterozoic age of ~1915 Ma is interpreted to be inherited from the underlying basement of the Congo Craton. Tectonic implications The results of this study correspond to those of earlier workers and e.g. Miller’s (1983) general assertion that all granites younger than ~540 Ma in the sCZ should be considered as being post-tectonic. It also agrees with previously reported U-Pb ages from granitoids in the sCZ, south of Karibib, namely the pre- to syntectonic Mon Repos diorite and granodiorite (564 ± 5 and 546 ± 6 Ma) and the post-tectonic Rote Kuppe Granite (539 ± 6 Ma) (Jacob et al., 2000) that bracket the main collisional event to between ~550 to ~540 Ma. Similarly, U-Pb zircon ages from early post-collisonal granites in the southwestern parts of the sCZ yield ages of 542 ± 6 Ma (Tack et al. 2002). In summary, the existing age data suggest that the main phase of crustal shortening in the sCZ, the central magmatic axis of the Damara belt, was largely concluded by ~540 Ma. Younger radiometric ages, including U-Pb ages from titanite and/or monazite are interpreted to reflect a posttectonic high-temperature thermal event in the sCZ, since there is no evidence for penetrative strains postdating the ~550 to ~540 Ma D2 fabrics and structures (Miller, 1983; 2002; Jacob et al., 2000). However, recent studies have described syn- to latetectonic granitoids and regional deformation considerably younger than ~540 Ma, mainly from lowergrade rocks to the immediate north of the sCZ in the northern Central Zone and Northern Zone of the Damara Belt. Seth et al. (2000) reported Pb-Pb a single zircon evaporation age of 530 ± 3 Ma from the Voetspoor intrusion, a syenitic complex in the Northern Zone of the Damara Belt, that they consider to be late tectonic. Jung et al. (2000) recorded U-Pb ages of ~526 and ~516 Ma from syn- to late-collisional, S-type granites from the Oetmoed Granite-Migmatite Complex in the northern Central Zone, some 100 km north of the Stinkbank granite. Based on detailed structural, petrographic and geochronological work in the Kaoko Belt, the north to south trending coastal branch of the Damara orogen, Goscombe et al. (2004; 2005) described

east to west trending, late-stage folds related to the main collisional event in the Damara Belt to have formed as late as ~500 Ma in the southeasternmost parts of the Kaoko Belt and Northern Zone of the Damara Belt. In conjunction, these younger ages have recently been suggested to indicate the timing of the main collision between the Congo and Kalahari Cratons. We suggest that the younging of syncollisonal granites from the leading edge of the Congo Craton in the sCZ to the northern foreland of the Damara Belt records the migration of the deformation and metamorphic front across the orogen with time. The existing age data are sparse, and a detailed structural framework and metamorphic evolution have still to be established in order to deduce orogenic rates in the northern parts of the Damara belt. However, considering that the effects of crustal shortening are over by ~540 Ma in the sCZ, but still recorded at ~500 Ma in the Northern Zone (e.g. Goscombe et al., 2005), some ~300 to ~250 km to the north, the propagation rate of the deformation front must have been in the order of 5-7 mm/a. This must be taken as a minimum value, since the internal shortening of the folded and thrusted sequence, nor the geometry of the thrust sheets have been accounted for. Retrodeformation and removal of internal strain will result in significantly higher displacement rates, likely in the order of centimetre per year. These rates are well within the range of convergence rates recorded for recent collisional orogens. Conclusion Deformation and cross-cutting relationships of the Salem-type Stinkbank granite and the Pan-African Damara Sequence illustrate the syntectonic timing of granite emplacement during the main phase of crustal shortening in the sCZ of the Damara Belt. A U-Pb (LAMICP-MS) zircon age of 549 ± 11 Ma is interpreted to date the crystallization of the granite and, thereby, the timing of the main collisonal phase in the sCZ, along the leading edge of the Congo Craton. Younger ages recorded from the northern Central and Northern Zone in the Damara Belt and related to the main collisional phase are interpreted to record the foreland-vergent propagation of the deformation front. Given the only sporadic age data and lack of detailed structural data that would enable a retrodeformation of the belt, the quantification of the propagation rate is tentative at best. However, existing age data suggest a propagation rate of several centimetre per year. The results may explain the wide scatter of syncollisional ages commonly cited for the main phase of collisional tectonics between the Congo and Kalahari Cratons. Acknowledgements We gratefully acknowledge the financial and logistic support of the Navachab Gold Mine (AngloGold Ashanti) and their permission to publish the results of this study. We particularly thank Frik and Juanita

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Badenhorst for their hospitality and Berti Roesener and Nick Steven for logistic support and discussions. The hospitality of farmers of the area and access to their lands is greatly appreciated. Helpful reviews by Alfred Kröner and Udo Zimmermann are greatly appreciated. References Allsopp, H.L., Barton, E.S., Kröner, A., Welke, H.J. and Burger, A.J. (1983). Emplacement versus inherited isotopic age patterns: a Rb-Sr and U-Pb study of Salem-type granites in the central Damara belt. Geological Society of South Africa Special Publication, 11, 281-287. Badenhorst, F.P. (1992). The Lithostratigraphy of area 2115B and D in the Central Zone of the Damara Orogen in Namibia: with emphasis on facies changes and correlation. Unpublished MSc thesis, University of Port Elizabeth, South Africa, 124pp. Goscombe, B., Gray, D., Hand, M. (2004). Variation in metamorphic style along the northern margin of the Damara orogen, Namibia. Journal of Petrology, 45, 1261-1295. Goscombe, B., Gray, D., Armstrong, R., Foster, D.A., and Vogl, J. (2005). Event geochronology of the Pan-African Kaoko Belt, Namibia. Precambrian Research, 140, 1-41. Grotzinger, J.P., Bowring, B.Z., Saylor, B. and Kaufman, A.J. (1995). Biostratigraphic and geochronologic constraints on early animal evolution. Science, 270, 598-604. Haack, U. and Martin, H. (1983). Geochronology of the Damara orogen – a review. In: H. Martin and F. Eder (Editors), Intracontinental fold belts. Springer, Berlin, Germany, 839-845. Haack, U., Hoefs, J., and Gohn, E. (1983). Genesis of Damaran granites in the light of Rb-Sr and _18O data. In: H. Martin and F. Eder (Editors), Intracontinental fold belts. Springer, Berlin, Germany, 847-872. Hawkesworth, C.J., Gledhill, A.R., Roddick, J.C., Miller, R.McG. and Kröner, A. (1983). Rb-Sr and K-Ar studies bearing on models for the thermal evolution of the Damara belt, Namibia. Geological Society of South Africa Special Publication, 11, 323-338. Horstmann, U.E., Ahrendt, H., Clauer, N. and Porada, H. (1990). The metamorphic history of the Damara orogen based on K/Ar data of detrital white micas from the Nama Group, Namibia. Precambrian Research, 48, 41-61. Jacob, R.E. (1974). Geology and metamorphic petrology of part of the Damara orogen along the lower Swakop River, South West Africa. Precambrian Research Unit Bulletin, University of Cape Town, South Africa, 17, 185pp. Jacob, R.E., Snowden, P.A. and Bunting, F.J.L. (1983). Geology and structural development of the Tumas basement dome and its cover rocks. Geological Society of South Africa Special Publication, 11, 157-172. Jacob, R.E., Moore, J.M. and Armstrong, R.A. (2000). Zircon and titanite age determinations from igneous rocks in the Karibib District, Namibia: implications for Navachab vein-style gold mineralization. Communications of the Geological Survey Namibia, 12, 157-166. Johnson, S.D. (2005). Structural geology of the Usakos dome, Damara Belt, central Namibia. Unpublished MSc thesis, University of Stellenbosch, South Africa, 159pp. Jung, S. (2005). Isotopic equilibrium/disequilibrium in granites, metasedimentary rocks and migmatites (Damara orogen, Namibia) – a consequence of polymetamorphism and melting. Lithos, 84, 168-184. Jung, S., Mezger, K. and Hoernes, S. (1998). Petrology and geochemistry of post-collisional metaluminous A-type granites – A major and trace element and Nd-Sr-Pb-O-isotope study from the Proterozoic Damara Belt, Namibia. Lithos, 45, 147-175. Jung, S., Hoernes, S. and Mezger, K. (2000). Geochronology and petrogenesis of Pan-African, syn-tectonic, S-type and post-tectonic A-type granite (Namibia): products of melting of crustal sources, fractional crystallization and wall-rock entrainment. Lithos, 50, 259-287. Jung, S. and Mezger, K. (2003). Petrology of basement-dominated terranes: I Regional metamorphic T-t path from U-Pb monazite and Sm-Nd garnet geochronology (Central Damara Orogen, Namibia). Chemical Geology, 198, 223-247. Kasch, K.W. (1983). Tectonothermal evolution of the southern Damara Orogen. Geological Society of South Africa Special Publication,

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Editorial handling: J. M. Barton

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