Journal of Petrology | Volume 44 | Number 4 | Pages 679-711 | 2003
© Oxford University Press 2003
The Metamorphic Architecture of a Transpressional Orogen: the Kaoko Belt, Namibia
1 CONTINENTAL EVOLUTION RESEARCH GROUP, DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF ADELAIDE, ADELAIDE, S.A. 5005, AUSTRALIA
2 SCHOOL OF EARTH SCIENCES, UNIVERSITY OF MELBOURNE, MELBOURNE, VIC. 3010, AUSTRALIA
E-mail: ben.goscombe{at}adelaide.edu.au
RECEIVED MARCH 16, 2002; ACCEPTED OCTOBER 28, 2002
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ABSTRACT |
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The Kaoko Belt in Namibia represents the deeply eroded core of a classic sinistral transpressional orogen with a half flower structure centred on the crustal-scale Purros Mylonite Zone. The Kaoko Belt consists of three NW-trending structural zones each with distinct kinemetamorphic style. The Eastern Kaoko Zone contains upright-folded, Neoproterozoic Damara Sequence shelf carbonates. The Central Kaoko Zone comprises an inverted Barrovian metamorphic series within large-scale east-vergent nappes, whereas the Western Kaoko Zone is predominantly of high metamorphic grade and intruded by numerous granitoids. The Western Kaoko Zone has orogen-parallel panels of distinctly different metamorphic grade separated by strike-slip ductile shear zones with overall isograd pattern being indicative of extrusional tectonics in the orogen core. The Kaoko Belt evolved through three distinct phases of a protracted Pan-African Orogeny in the late Neoproterozoic to Cambrian: (1) an early Thermal Phase (early M2) was responsible for pervasive partial melting, high-grade parageneses and granite emplacement between 580 and 570 Ma; (2) the main deformation Transpressional Phase (580–550 Ma) reworked early M2 parageneses in the pervasive orogenic fabric producing M2 assemblages that formed as a result of progressive sinistral transpression that evolved from wrench-style to high-angle convergence accompanying foreland-vergent thrusts and nappes; (3) the post-transpression Shortening Phase generated upright, open folds during north–south shortening (530–510 Ma). In the Western Kaoko Zone, peak metamorphic conditions were attained during early M2 at moderate to high average thermal gradients (29–40°C/km) and were intensely reworked by lower-grade pervasive fabrics during M2. (Average thermal gradient is simply the calculated metamorphic temperature divided by the calculated depth assuming a density of 2·8 g/cm3. It should not be confused with the instantaneous thermal gradients in the vicinity that an assemblage formed, or imply that the thermal gradients are time equivalent.) In the northern part of the Western Kaoko Zone, immediately adjacent to the Purros Mylonite Zone, the amphibolite-grade Khumib Terrane experienced peak M2 metamorphism at 573°C and 5·4 kbar. Along strike to the south the granulite-grade Hoarusib Terrane experienced peak early M2 conditions at 843°C and 8·1 kbar and M2 reworking at approximately 560–580°C and 4·8 kbar. In the western margin of the orogen, the Coastal Terrane experienced early M2 metamorphism at sillimanite–K-feldspar–melt grades and was reworked during M2 at muscovite–biotite grade. In the Central Kaoko Zone, metamorphic grade increases towards the west to higher structural levels. Peak metamorphic matrix assemblages formed during pervasive deformation in the Transpressional Phase (M2) at conditions in the range of 530–690°C and 8·5–9·0 kbar with consistently low average thermal gradients (17–23°C/km). Clockwise P–T paths were experienced in both the Central Kaoko Zone and Western Kaoko Zone. Garnet Sm–Nd geochronology indicates that matrix parageneses, early M2 in the Western Kaoko Zone and M2 in the Central Kaoko Zone, formed at the same time within uncertainties (576 ± 15 Ma). This indicates that the thermal peak was contemporaneous across the belt, even though deformational phases of equivalent structural style were diachronous across the Kaoko Belt.
KEY WORDS: Pan-African Orogeny; transpression; metamorphism; geochronology; metamorphic field gradients; orogen architecture
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INTRODUCTION |
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Deeply eroded transpressional orogens show strike-slip shearing in the internal part, often with a major crustal-scale median shear zone, through progressively more oblique convergence to high-angle overthrusting onto the foreland (e.g. Shackleton & Ries, 1984
; Holdsworth & Strachan, 1991; Vassallo & Wilson, 2002). Transpressional orogens are an important style of collisional orogenesis (Jones et al., 1977; Tapponnier et al., 1982; Woodcock, 1986; Holdsworth & Strachan, 1991; Jones & Strachan, 2000), the structural aspects of which have been well studied in the upper crust (e.g. Harland, 1971; Lowell, 1972; Wilcox et al., 1973; Sylvester & Smith, 1976; Kirkwood et al., 1995). Only more recently has the deeply eroded ductile middle to lower crust of transpressional orogens been investigated in detail (e.g. Johnson & Kattan, 2001; Little et al., 2002; Vassallo & Wilson, 2002). Apart from the studies by Jones & Strachan (2000) and Whitney et al. (2001), metamorphic studies of mid-crustal exposures of transpressional orogens are almost non-existent. Thermal modelling of transpressional orogens by Thompson et al. (1997) indicated that exhumation rate is proportional, and heating is inversely proportional, to the angle of convergence obliquity. Consequently, granulites should be expected in the orogen cores and Barrovian metamorphic conditions in the margins of the orogens. This modelling also considers extrusional exhumation to be an important process in transpressional orogens, which typically have flower-structure architectures dominated by crustal-scale shear zones.
In this paper we document the P–T–t evolution of the mid-crust across all metamorphic terranes within a typical transpressional orogen to compare with the predicted thermobarometric evolution of transpressional systems, and develop tectonometamorphic models for transpressional orogens in general. We selected the Kaoko Belt for chronometamorphic analysis because it is a classic example of a transpressional orogen presenting typical structural styles, orogen architecture and progressive deformation both across the orogen and through time (Goscombe et al., 2002). Furthermore, the Kaoko Belt is continuously exposed throughout, from the orogen core to the foreland, and contains rock-types well suited to documenting the thermobarometric evolution across the orogen. Until very recently the Kaoko Belt was poorly understood and largely unmapped (Guj, 1970) because of its remoteness and the security problems in northern Namibia throughout the 1970 s and 1980 s. A number of geological studies have presented details of the stratigraphy (Miller, 1983; Hoffman et al., 1998), structural evolution (Guj, 1970; Dingeldey et al., 1994; Dürr & Dingeldey, 1996; Dingeldey, 1997), metamorphism (Dingeldey, 1997; Franz et al., 1999) and geochronology (Seth et al., 1998; Franz et al., 1999).
Our analysis of the Kaoko Belt is based on five field seasons (1997–2002), involving reconnaissance observations throughout the entire Kaoko Belt and detailed mapping of a 30 km wide swath across the belt (Fig. 1; Goscombe, 1999a, 1999b, 1999c, 1999d). This detailed mapping and structural analysis established a tectonic framework that has been presented by Goscombe (1998), Goscombe & Hand (2001a) and Goscombe et al. (2002). The chronometamorphic analysis presented in this paper is integrated with this established tectonic framework and utilizes extensive datasets (sourced from our work and published work) of all available Damara Orogeny geochronological data (Electronic Appendix A) and petrological data (1600 samples) covering the entire Kaoko Belt. Published geochronological data are entirely restricted to U–Pb zircon ages from basement and Neoproterozoic and Palaeozoic granitoids (Miller & Burger, 1983; Seth et al., 1998; Franz et al., 1999), U–Pb zircon and monazite ages from a reworked garnet gneiss (Franz et al., 1999), K–Ar and Ar–Ar cooling ages from micaceous schists (Clauer & Kröner, 1979; Ahrendt et al., 1983; D. Gray & D. Foster, unpublished data, 2001) and apatite fission-track ages (Brown et al., 1990). In this paper we also present Sm–Nd ages from peak metamorphic garnet-bearing assemblages from the four metamorphic terranes that make up the Kaoko Belt, and for the first time directly date peak metamorphic parageneses, formed during transpressional orogenesis in the Kaoko Belt.
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REGIONAL GEOLOGY OF THE KAOKO BELT |
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The Kaoko Belt is the NNW-trending northern arm of the Neoproterozoic Damara Orogen extending some 700 km from the Ugab Zone in the south to Angola in the north (Fig. 1). The western margin equivalents of the Kaoko Belt in Brazil are the sinistral transpressional Dom Feliciano and Ribeira Belts flanking the Rio de la Plata Craton (Porada, 1989
; Trompette & Carozzi, 1994; Chemale et al., 1994). In the Kaoko Belt a mosaic of Archaean, Palaeoproterozoic and Mesoproterozoic basement metamorphic and igneous complexes, which form the SW margin of the otherwise predominantly Archaean Congo Craton, is unconformably overlain by the Neoproterozoic Damara Sequence. Deposition of the Damara Sequence was terminated by collision in the late Neoproterozoic and was followed by a protracted tectonothermal evolution collectively called the Damara Orogeny, which occurred from the late Neoproterozoic to Cambrian (Miller, 1983; Prave, 1996).
The Kaoko Belt is subdivided into five zones of characteristic tectonic and metamorphic style (Miller, 1983), three NNW-trending parallel zones in the central Kaoko Belt region (the region of interest in this study) and two at each end of the belt (Figs 1 and 2).
- The Eastern Kaoko Zone (EKZ) is the foreland and consists of sub-greenschist platform carbonates deformed by east–west shortening and upright folds. Its western margin is marked by the shallow west-dipping Sesfontein Thrust, which formed under brittle conditions late in the Damara orogenic cycle (Dürr & Dingeldey, 1996).
- The Central Kaoko Zone (CKZ) ranges from lower-greenschist in the east to upper-amphibolite grade in the west, and experienced intense fabric development during sinistral transpression, culminating in the formation of large-scale east-vergent nappes. The western margin of the CKZ is delineated by the Purros Mylonite Zone (PMZ) (Goscombe, 1998; Goscombe et al., 2002), which is a crustal-scale, upper-amphibolite-grade mylonite and ultramylonite zone. The PMZ is the sub-vertical median shear zone running the entire length of the Kaoko Belt and extends north into Angola and south to the Ogden mylonites (Miller, 1983) on the western margin of the Ugab Zone (Fig. 1).
- The Western Kaoko Zone (WKZ) is the wrench-style orogen core composed of shear zone bounded panels of amphibolite- to granulite-grade Damara Sequence with a high proportion of partial melt and Neoproterozoic granitoids and a single lower-amphibolite-grade panel.
- The Kunene Zone in the NE Kaoko Belt is dominated by basement with low-grade Damara Sequence and characterized by north–south shortening without transpression.
- The Ugab Zone in the south consists of greenschist-grade, thin turbiditic Damara Sequence that has been pervasively deformed by very tight folding without involvement of the basement (Freyer & Hälbich, 1994).
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CHRONOSTRATIGRAPHIC ROCK UNITS |
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Archaean to Mesoproterozoic basement units (>1500 Ma)
The Damara Sequence is underlain by a mosaic of basement terranes that were intensely reworked in the CKZ and WKZ during the Damara Orogeny. Archaean Andib Terrane granitic and dioritic orthogneisses of 2645–2585 Ma age (Seth et al., 1998
) are contained in a single CKZ antiformal nappe in the southern CKZ (Fig. 1). All other basement in the EKZ and CKZ is Palaeoproterozoic in age and dominated by granitoid orthogneisses. The Epupa Metamorphic Complex in the northern and central regions (Figs 1 and 2) is >2100 Ma in age (Miller, 1983) and contains orthogneisses of 1985–1961 Ma age (Seth et al., 1998) and a younger Mesoproterozoic sequence. The Kamanjab Inlier in the SE (Fig. 1) consists of metamorphic sequences of 1811 ± 35 Ma (Tegtmeyer & Kröner, 1985) and 1987 ± 4 Ma age (A. Kröner, personal communication, 1997) and granitoids of approximately 1800 ± 80 Ma and 1580–1547 Ma age (Burger et al., 1976). The WKZ contains Mesoproterozoic meta-igneous gneisses of 
1507 Ma age (Seth et al., 1998). Mesoproterozoic orthogneisses have not been reported from the CKZ and no Palaeoproterozoic ages have been reported from the WKZ.
Multiple pre-Damara tectonometamorphic cycles are inferred in basement terranes by relic high-grade mineral assemblages, strain-hardened textures, gneissic layering, migmatites, granitoids and the geochronological record (Goscombe et al., 2002). Palaeoproterozoic fabrics and mineral assemblages are preserved almost entirely un-reworked in basement exposures in the EKZ and Kunene Zone (Fig. 1). The Epupa Metamorphic Complex in the Kunene Zone preserves granulite gneisses with two-pyroxene and garnet–sillimanite quartzo-feldspathic assemblages that have been variably overprinted by lower-grade Damaran-aged foliations. Basement in the CKZ and WKZ is almost entirely reworked and recrystallized during the Damara Orogeny. Relic pre-Damara porphyroclasts indicate garnet–biotite–quartz–K-feldspar–plagioclase and clinopyroxene–hornblende–quartz–plagioclase ± garnet upper-amphibolite assemblages with no apparent variation in grade across the CKZ.
Neoproterozoic Damara Sequence (770–600 Ma)
The Damara Sequence is a marine sequence deposited on a passive margin, progressing from shelf carbonates in the EKZ (Hoffman et al., 1998) to slope and deep basin facies in the CKZ and WKZ. The basal Damara Sequence is represented by rift-related siliciclastics of the Nosib Group in the EKZ, which contains quartzites, conglomerates and arenites and has upper limiting ages of 750 Ma (Hoffmann et al., 1994, 1998; Prave, 1996). Deposition of the Damara Sequence is interpreted to have spanned the late Neoproterozoic, between 770 and 600 Ma (Miller, 1983; Hoffman, 1994; Frimmel, 1996; Prave, 1996). Overlying Mulden Group siliciclastic molasse of 620–600 Ma age (Miller, 1983) is preserved only east of the Sesfontain Thrust in the EKZ (Fig. 2; Guj, 1970). West of the Sesfontain Thrust, the Damara Sequence is dominated by meta-turbidite and meta-greywacke schists with minor carbonate, mafic schists, meta-quartzite, meta-arkose and two diamictite units interpreted to be of 750–735 Ma and 700 Ma age (Fig. 2; Hoffman, 1994; Frimmel, 1996; Folling et al., 1998; Hoffman et al., 1998; Goscombe et al., 2002). The westernmost Coastal Terrane (Fig. 2) is composed of meta-greywackes and meta-arenites and is largely devoid of mafics and carbonates.
Neoproterozoic and Palaeozoic granitoids (660–550 Ma)
Neoproterozoic and Palaeozoic granitoids are entirely absent from the EKZ, are rare granitic veins (centimetre-scale) and granitic orthogneiss sills (metre-scale) in the westernmost CKZ, and pervade the WKZ as stromatic partial melt segregations (centimetre-scale) and kilometre-scale lenticular-shaped granitoid bodies. Granitic segregations and bodies are strongly sheared, concordant and have sharp to gradational contacts, and are interpreted to have formed by in situ melting. There are at least three generations of WKZ Neoproterozoic and Palaeozoic granitoids: (1) rare 656–645 Ma dioritic orthogneiss in the westernmost WKZ (Seth et al., 1998; Franz et al., 1999); (2) stromatic migmatitic segregations and a variety of coeval S-type granitic orthogneiss bodies of 580–552 Ma age (Seth et al., 1998), which were emplaced before and possibly also during the pervasive Transpressional Phase of deformation in the Koako Belt (Goscombe et al., 2002); (3) minor volumes of cross-cutting pegmatitic and granitic veins (centimetre-scale), which were emplaced axial planar to upright folds late in the Transpressional Phase (Goscombe et al., 2002). In the south, the Ugab Zone contains 570–573 Ma granites that post-date first-generation folds (Kröner, 1982; Miller & Burger, 1983) and a 530 Ma composite syenite pluton (Seth et al., 2000) that post-dates pervasive deformation associated with transpression in the Kaoko Belt and is syntectonic with late-stage north–south shortening.
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Tectonic framework
The temporal and spatial framework of deformational events recognized in the Kaoko Belt has been described in detail by Goscombe et al. (2002)
. The Damara Orogeny was a protracted tectonothermal cycle spanning 660–510 Ma and contains three distinct phases in a progressive continuum that can be recognized throughout the Kaoko Belt.
- The Thermal Phase produced quartz veins in the CKZ and high-grade parageneses, partial melting and granitoid formation in the WKZ at 580–570 Ma (Seth et al., 1998; Franz et al., 1999), with no apparent deformation structures preserved with the exception of rare inclusion trails in garnet porphyroclasts.
- The architecture of the Kaoko Belt, including almost all structures and fabrics, formed in the intense and pervasive Transpressional Phase. This involved progressive deformation, both through time and spatially out towards the orogen margin, from a Wrench Stage to Convergence Stage. The Transpressional Phase occurred synchronously with, or subsequent to, emplacement of the 580–552 Ma granitic orthogneisses in the WKZ and before the late-stage 530 Ma granites in the Ugab Zone.
- The Kaoko Belt was moderately reworked in a Shortening Phase that buckled the belt at the time that the Congo and Kalahari Cratons underwent high-angle convergence that resulted in intense pervasive deformation within the Inland Branch of the Damara Orogen (Fig. 1). The Shortening Phase accompanied 530 Ma granite emplaced in the Ugab Zone (Seth et al., 2000) and 530 ± 11 Ma mineralization (Kamona et al., 1999) and 535 ± 13 Ma K–Ar ages (Clauer & Kröner, 1979) in the Northern Platform. The main tectonothermal phase in the Inland Branch (Fig. 1) is widely regarded to have occurred over the interval 530–510 Ma (Miller, 1983; Jung et al., 2000). Maximum K–Ar whole-rock ages of 499 ± 11 and 490 ± 11 Ma from the Kaoko Belt and Ugab Zone respectively (Ahrendt et al., 1983) indicate that these regions had cooled through 350°C at this time.
Transpressional Phase
Wrench Stage
The first regionally preserved deformation in the Kaoko Belt produced the pervasive and intense bedding-parallel L–S fabric and associated small-scale isoclinal folds. This stage of the Damara Orogeny reworked basement resulting in geometrically identical fabrics in both basement and cover. At lower grades the dominant foliation is defined by a schistosic alignment of micas and quartz ± feldspar aggregate ribbons. At higher grades in the WKZ it is a grain-refinement foliation developed either as fine-grained shearbands and foliation seams that overprint the coarse-grained polygonal granoblastic matrix, or as mylonitic to ultramylonitic fabrics in high-strain zones. The dominant stretching lineation is defined by mineral aggregate ribbons of quartz and feldspar sub-grains, trains of fine micas and aligned laths of mica, sillimanite and amphibole. This lineation is regionally pervasive, typically sub-horizontal to shallow NNW-plunging and interpreted to be the vector of local tectonic transport throughout the Wrench Stage. There is an almost 90° swing in stretching lineation orientation from NNW-plunging in the strike-slip core of the orogen (WKZ) to down-dip west-plunging in the overfolding and nappe margin (CKZ) (Figs 1 and 2). Everywhere shear sense indicators such as shearband cleavages, asymmetric mantled porphyroclasts, flanking folds and asymmetric boudins (Simpson, 1984; Passchier & Simpson, 1986; Passchier, 2001; Goscombe & Passchier, 2002) are consistently sinistral along shallow lineations throughout the WKZ and CKZ (Figs 1 and 2).
The dominant L–S fabric is also expressed in the WKZ as steep, crustal-scale mylonite to ultramylonite sinistral strike-slip shear zones that dissect the WKZ into panels of different metamorphic grade (Fig. 3). The orogen median Purros Mylonite Zone (PMZ) (Goscombe, 1998; Goscombe & Hand, 2001a) is 4·5 km wide, forms the eastern margin of the WKZ, controls the architecture of the whole Kaoko Belt and remained active throughout the Transpressional Phase, from early in the Wrench Stage to being the root zone of outward-vergent overfolds and nappes during the Convengence Stage. The westernmost Three Palms Mylonite Zone experienced sinistral extensional movements along shallow SSE-plunging lineations and forms the eastern margin of the Coastal Terrane that experienced Transpressional Phase reworking (M2) at lower metamorphic grades than most of the WKZ.
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Convergence Stage
Map-scale, shallow NNW- and SSE-plunging, east-vergent, tight to isoclinal nappes and asymmetric folds dominate the structure of the Kaoko Belt (Fig. 2) and are characteristic of the Convergence Stage of deformation late in the Transpressional Phase. Fold inclination varies systematically across the Kaoko Belt, defining the belt's gross architecture of a steep divergent flower structure in the core of the orogen, centred on the PMZ, outwards to progressively shallower east-vergent nappes in the eastern CKZ (Figs 2 and 4). In the WKZ, Convergence Stage folds in the high-grade Hoarusib Terrane (Fig. 3) have thin (0·5–3 cm) axial planar migmatitic segregations and pegmatites and develop a weak axial planar mica foliation. Crenulation cleavages are developed at lower grades such as in the Khumib Terrane and CKZ. Convergence Stage mineral lineations defined by coarse-grained hornblende and biotite laths that plunge down-dip to the west to WNW are sparsely developed. These overlap in orientation with Wrench Stage mineral aggregate lineations, indicating progressive deformation from sub-horizontal transport to higher angles of oblique transport in the Convergence Stage (Goscombe et al., 2002
).
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Convergence Stage mineral lath lineations and boudin trains were formed by an approximately east–west-trending maximum extension axis, which represents the transport vector during west over east Convergence Stage nappe folding (Goscombe & Passchier, 2002
). Convergence Stage folds in the EKZ are kilometre-scale upright, symmetric cylindrical folds with horizontal north–south axes, that formed by pure shear east–west shortening (Goscombe et al., 2002). Latest formed Convergence Stage structures include progressive tightening of folds, late-stage crenulations and the semi-brittle Sesfontain Thrust. The Wrench Stage and Convergence Stage lineation and boudin train populations and fold geometries indicate that two temporally sequential and geometrically distinct, end-member strain states bracketed the progressive Transpressional Phase of orogenesis (Goscombe et al., 2002).
Shortening Phase
A later phase of deformation, entirely distinct from the Transpressional Phase, involved minor north–south shortening strain resulting in large-scale (5–50 km) and mesoscopic upright buckling and kinkbands in the central Kaoko Belt region (Fig. 2). Shortening Phase strain was greatest at both ends of the Kaoko Belt within the Kunene and Ugab Zones (Fig. 1). Upright, open folds and steep retrograde shear zones of ESE trend developed in the Kunene Zone and Northern Platform. Shortening Phase folds in the Ugab Zone have an east to ENE trend, steep axial surfaces and axes and are invariably asymmetric with sinistral vergence (Freyer & Hälbich, 1994). Shortening Phase deformation involved pure shear north–south shortening with little non-coaxial shear component. Change in the deformation regime was due to a reorganization of the Damara Orogeny on crustal plate scale, with the deformation front being within the Inland Branch during high-angle convergence of the Congo and Kalahari Cratons (Fig. 1; Coward, 1983; Freyer & Hälbich, 1994).
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Metamorphic events
M1 metamorphic cycle
Evidence for an early metamophic cycle (M1) before transpressional deformation of the Kaoko Belt is very limited. Evidence for M1 is restricted to a U–Pb zircon age of 656 ± 8 Ma from a monzogranitic orthogneiss (Seth et al., 1998
) and a Pb–Pb metamorphic zircon and monazite age of 645 ± 3·5 Ma from a reworked high-grade garnet gneiss on the Atlantic coast (Franz et al., 1999). Both samples are from a restricted region in the western Coastal Terrane (Fig. 5). Similar ages are yet to be found from elsewhere in the Coastal Terrane or other terranes forming the Kaoko Belt, which is dominated by 580–570 Ma peak metamorphic ages and 580–550 Ma granite ages (Electronic Appendix A). Thus M1 metamorphism is interpreted to have been restricted to the westernmost part of the Coastal Terrane. The high-grade garnet–sillimanite–K-feldspar–plagioclase–biotite assemblage in the garnet gneiss dated by Franz et al. (1999) has a close association with a granitic orthogneiss of similar age. This suggests that M1 high-T/low-P metamorphism may be driven by magmatism, conceivably resulting in a restricted areal extent for the metamorphism (Hand & Buick, 2001). Alternatively, M1 metamorphism is restricted to an outboard terrane separated from the Coastal Terrane by an as yet undocumented shear zone in this comparatively poorly known portion of the Kaoko Belt. Mineral parageneses that can be attributed to M1 have not been recognized in the areas investigated by this study. All mineral parageneses discussed in this paper formed in the M2 metamorphic cycle (580–550 Ma) associated with Pan-African transpressional orogenesis in the Kaoko Belt.
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M2 metamorphic cycle
Thermal Phase (early M2). There is evidence for an early metamorphic event (early M2) in the Damara Sequence, before the development of the pervasive L–S fabric and matrix mineral assemblages formed during the Transpressional Phase (M2). Stromatic quartz ± K-feldspar ± muscovite veins are widespread in the CKZ and are strongly deformed during development of the pervasive L–S fabric, suggesting that they pre-date deformation in the Transpressional Phase. These veins record a progression in parageneses from quartz ± calcite in the east to quartz ± muscovite and quartz ± K-feldspar in the western CKZ and ultimately quartz–plagioclase–K-feldspar ± biotite. The last are interpreted to be partial melt segregations in the PMZ and WKZ. Rare relic early M2 porphyroclasts of garnet, biotite, muscovite and hornblende are deformed, enveloped and grain-refined by the pervasive L–S fabric (Goscombe et al., 2002
), indicating coarse mineral growth before the Transpressional Phase (M2). Biotite and muscovite porphyroclasts are preserved in the eastern CKZ and garnet and hornblende porphyroclasts in the western CKZ, implying an east to west increase in grade in concert with M2 metamorphic conditions. Restricted to the WKZ are coarse polygonal granoblastic textures, K-feldspar and garnet porphyroclasts, stromatic partial melt segregations and a variety of granitic veins, sills and orthogneiss bodies (Figs 3 and 5). All are strongly deformed, and show high degrees of grain-refinement, boudinage and reworking by the regionally pervasive L–S fabric, indicating that high-grade metamorphism, partial melting and granite generation occurred before Transpressional Phase deformation. Relic early M2 porphyroclasts and mineral parageneses are most evident in the WKZ and western CKZ, indicating that matrix assemblages are dominantly early M2 in the WKZ and M2 in the CKZ (Fig. 5). Transpressional Phase (M2). Early M2 mineral parageneses, segregations and granitoids were reworked and overprinted by the regionally pervasive L–S fabric that constitutes the matrix mineral assemblage (M2) formed during the Transpressional Phase. In the CKZ and low-grade Khumib Terrane in the WKZ (Fig. 3), the foliated matrix assemblage in metapelites is defined by aligned micas, kyanite, sillimanite or staurolite laths, syntectonic garnet porphyroblasts and annealed quartz and feldspar aggregate ribbons with polygonal granoblastic textures and curved to lobate grain margins. In mafic schists, hornblende and micas are well aligned in a polygonal matrix. Early generations of the progressively evolving pervasive fabric are preserved as linear and sigmoidal inclusion trains within garnet, staurolite and hornblende porphyroblasts. Garnet porphyroblasts record prograde growth, which continued throughout the Wrench Stage and to idioblastic overgrowth rims that overprint the pervasive L–S fabric. Coarse-grained biotites in metapelite and hornblende laths in mafic and calc-silicate rocks define lineations formed in both the Wrench Stage and Convergence Stage, implying growth throughout a protracted period. Coarse mica laths are axial planar to Convergence Stage crenulation cleavages and overgrow the main foliation (Fig. 4).
M2 metamorphism is expressed differently in the WKZ. M2 assemblages also formed in the pervasive L–S fabric that formed during the Wrench Stage, but differ from the CKZ because this fabric overprints a coarse polygonal granoblastic early M2 matrix assemblage that formed during peak conditions in the WKZ. M2 assemblages are recorded in thin, fine-grained sillimanite–biotite foliation seams and in diffuse grain-refinement fabrics ranging from protomylonitic to ultramylonitic. In the low-grade Khumib Terrane of the WKZ, the M2 assemblage associated with the pervasive Wrench Stage fabric consists of aligned micas with syntectonic garnet porphyroblasts. In this region there is little record of early M2 porphyroclasts. The schistosic matrix is overprinted by micaceous shearbands of the same metamorphic grade that formed during the Convergence Stage. In the high-grade Hoarusib Terrane, Convergence Stage fabrics are low-angle discordant foliation seams and cross-cutting spaced aligned micas.
M3 metamorphic cycle
The M3 metamorphic cycle accompanied the Shortening Phase of deformation. Low-strain buckling and crenulation of M2 mineral assemblages and foliations occurred with no new mineral parageneses developed in the Kaoko Belt proper. M3 metamorphic assemblages developed only in the Ugab Zone within the contact aureole of syn-Shortening Phase granites (Goscombe et al., 2002).
Petrography
Damara Sequence in Central Kaoko Zone
There is a continuous and gradual increase in metamorphic grade of the M2 matrix assemblage across the CKZ towards the west, from greenschist to upper-amphibolite grade (Fig. 3). The Sesfontein Thrust marks a sharp increase in grade from sub-greenschist in the EKZ, which contains essentially unmetamorphosed arenites and carbonates and phyllitic schists and shales composed of smectite group minerals (Fig. 5). The biotite-in isograd occurs in the easternmost CKZ (Fig. 3), which is entirely of lower-greenschist grade. Aluminous rocks are quartz–sericite ± chlorite ± albite ± biotite schists in which biotite is typically fine grained and in low modal proportions (Electronic Appendix B). Mafic schists have quartz–plagioclase–actinolite ± chlorite ± epidote assemblages (Electronic Appendix C) and carbonates have quartz–muscovite–calcite–dolomite assemblages.
The garnet-in isograd occurs in the Gomatum Region and greenschist to lower-amphibolite grades persist in this region (Fig. 3). Aluminous schists have quartz–muscovite ± biotite ± garnet ± albite ± ilmenite assemblages and rarely also chlorite, epidote and/or calcite (Electronic Appendix B). Matrix chlorite is aligned with the pervasive foliation and often overgrown by sub-parallel biotite. Large aligned biotite and muscovite laths are both enveloped by and overgrow the finer-grained biotite–muscovite pervasive foliation, indicating progressive reworking of the main foliation over a period with little change in metamorphic grade. Biotite is occasionally overgrown by late unaligned chlorite, and chlorite also forms as a retrograde phase on the margin of garnet. Garnet typically has cores with sigmoidal, straight or crenulated inclusion trails and idioblastic, inclusion-free rims that post-date the main foliation (Figs 4 and 6b). Less commonly, garnet forms as masses that are elongate and skeletal with inclusion trails continuous with the main foliation. Inclusions are typically quartz and rarely biotite or muscovite. Albite forms as irregular-shaped porphyroblasts that often have straight inclusion trails of biotite and muscovite and are enveloped by the main foliation.
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In the eastern Gomatum Region mafic schists have quartz–plagioclase–hornblende assemblages with rare late-stage idioblastic garnet and either biotite or chlorite and epidote or titanite (Electronic Appendix C). Carbonates and calcareous schists contain quartz, biotite or muscovite, plagioclase and rare actinolite laths. Throughout the central Gomatum Region aluminous schists have quartz–albite–biotite–muscovite–garnet ± chlorite assemblages, also with porphyroblastic albite and garnet (Figs 4 and 6b). Garnet may also be rimmed by coronas of albite (Electronic Appendix B). Mafic schists have quartz–hornblende ± plagioclase ± chlorite ± biotite ± epidote ± titanite assemblages and are devoid of garnet. Rare calc-silicate horizons contain quartz–plagioclase–epidote–biotite assemblages.
West of the Gomatum Region metapelitic schists have quartz–oligoclase–biotite–muscovite–garnet–ilmenite assemblages that are devoid of matrix chlorite (Electronic Appendix B). Both oligoclase and garnet are irregular-shaped porphyroblasts that both overgrow and are enveloped by the main foliation. In contrast to lower-grade metapelites to the east, garnets do not develop idioblastic overgrowths (Fig. 4). Inclusion-free coarse biotite laths are enveloped by the pervasive biotite–muscovite foliation and in part overgrown by late-stage muscovite plates. Mafic schists have quartz–hornblende–garnet ± oligoclase ± ilmenite ± clinopyroxene or quartz–oligoclase–hornblende– biotite–epidote–ilmenite–titanite assemblages (Electronic Appendix C; Fig. 4). Oligoclase forms as coarse clots, garnet has hornblende inclusion trails oblique to the enveloping main foliation, clinopyroxene is overgrown by blue–green hornblende coronas and chlorite is a late-stage retrogressive phase. Carbonates have annealed textures and contain quartz, muscovite and rare tremolite that is partially replaced by fine-grained muscovite coronas. Calc-silicates contain polygonal granoblastic assemblages of quartz–plagioclase–epidote–carbonate ± hornblende.
The staurolite-in isograd coincides with the central Omapungwe Region (Fig. 3). Metapelite matrix assemblages west of the staurolite-isograd are quartz–oligoclase–biotite–muscovite–garnet–ilmenite ± stauro- lite ± kyanite (Electronic Appendix B; Fig. 4). Both garnet and staurolite are poikiloblastic, synkinematic and occur in textural equilibrium in the matrix assemblage. Staurolite growth continued after cessation of garnet growth and staurolite is most typically part of the pervasive foliation that envelops garnet porphyroblasts. Garnets may be inclusions within staurolite porphyroblasts, partially enclosed by staurolite coronas, and garnets are resorbed by moats of staurolite–biotite–quartz with retrogressive chlorite (Fig. 6c; Electronic Appendix B). Garnet porphyroblasts are poikiloblastic, synkinematic and either rounded and showing moderate resorption (Fig. 6c) or elongate parallel to the main foliation (Figs 4 and 6a). Chlorite rarely occurs as an inclusion phase in garnet (Electronic Appendix B) and is a common retrogressive phase corroding both garnet and staurolite margins (Fig. 6c). Muscovite also occurs as a late-stage mineral across biotite, and fibrolite occurs as a late-stage mineral on the margins of garnet. Primary Fe–Ti oxides are invariably ilmenite ± rutile and retrograde reaction textures contain haematite.
Mafic schists in the central Omapungwe Region have quartz–andesine–hornblende–garnet–titanite–ilmenite ± epidote assemblages and garnet-free samples contain clinopyroxene or biotite (Electronic Appendix C). Hornblende occurs as large poikiloblastic porphyroblasts and finer laths in the matrix and both are well aligned and in textural equilibrium. Epidote is both a matrix phase and a common coronal phase. Garnet occurs as relic grains that are invariably enclosed by andesine ± quartz ± hornblende moats (Electronic Appendix C; Figs 4 and 6d). Carbonates and calc-silicates have well-annealed polygonal textures with straight grain margins. Typical carbonate assemblages are calcite–quartz–plagioclase–phlogopite ± muscovite ± tremolite and calc-silicates have quartz–plagioclase–biotite–epidote–titanite ± garnet ± actinolite ± calcite assemblages. In the western margin of the CKZ, immediately east of the PMZ (Fig. 3), matrix assemblages and reaction textures in general differ little from those in similar rock-types in the central Omapungwe Region. However, some metapelite samples contain minor K-feldspar and the modal proportion of muscovite is low, with most being late formed. Mafic schists rarely have biotite but otherwise have similar garnet- or clinopyroxene-bearing assemblages and andesine coronas on garnet. Carbonates have identical assemblages, except that muscovite also occurs as a retrograde phase after plagioclase and few samples have calcite–wollastonite–titanite assemblages.
Damara Sequence in Western Kaoko Zone
Immediately west of the first ultramylonite defining the boundary to the PMZ is a marked increase in metamorphic grade. Modal matrix muscovite drops sharply and is entirely absent in the high-grade core of the WKZ, and K-feldspar porphyroblasts are common (Electronic Appendix B; Fig. 4). Interpreted partial melt segregations, biotite-pegmatite and granite sills and veins are absent in the CKZ and present in the PMZ, and increase in proportion towards the core of the WKZ (Fig. 3). Typical matrix assemblages in the PMZ are quartz–plagioclase–K-feldspar–biotite–hornblende ± epidote in intermediate gneisses, quartz–plagioclase–K-feldspar–biotite ± garnet ± sillimanite ± kyanite in metapelites, tremolite– plagioclase–calcite in carbonates, and plagioclase ± quartz–clinopyroxene–hornblende ± biotite and plagioclase–hornblende–meionite in mafic gneisses (Electronic Appendices B and C). Coarse early M2 matrix assemblages are polygonal granoblastic and invariably the same as those developed in the mylonitic M2 sub-grain foliation. Mylonitic quartz and feldspar aggregate ribbons are typically strained with serate to lobate sub-grain margins and less commonly annealed and polygonal granoblastic. Prograde reaction textures are rarely preserved. Early muscovite is replaced internally by sillimanite aggregates and kyanite is replaced by coronal and fibrous sillimanite. Garnets contain inclusion trails continuous with the pervasive foliation. Retrograde muscovite forms within and on the margin of matrix hornblende, plagioclase and K-feldspar. In mafic and intermediate gneisses plagioclase is replaced by epidote ± chlorite or actinolite aggregates and coronas. Biotite laths rarely grow across matrix hornblende laths but are typically coplanar and in textural equilibrium.
West of the PMZ, the high-grade Hoarusib Terrane constitutes the core of the WKZ. All rock-types are coarse-grained early M2 granulite-grade gneisses with cross-cutting M2 sub-grain aggregate ribbons that are also annealed and polygonal granoblastic. Kyanite and muscovite are absent, garnet, sillimanite, cordierite and K-feldspar porphyroblasts occur, and all quartzo-feldspathic and metapelitic gneisses contain stromatic early M2 and cross-cutting M2 partial melt segregations (Fig. 4). Matrix assemblages in mafic gneiss are plagioclase–orthopyroxene–clinopyroxene–garnet–hornblende and quartz–plagioclase–clinopyroxene–hornblende ± biotite ± garnet (Electronic Appendix C). Clinopyroxene is partially replaced by actinolite–calcite–hornblende and garnet in orthopyroxene-free samples is corroded by hornblende– plagioclase–ilmenite symplectites (Fig. 7a). Carbonates contain calcite–quartz–plagioclase–epidote–tremolite or calcite–quartz–plagioclase–muscovite–hornblende. Quartzo-feldspathic gneisses in the east have matrix assemblages of quartz–plagioclase–K-feldspar– biotite–garnet ± sillimanite. Garnet was partially corroded by silica-undersaturated symplectites of corundum–plagioclase–biotite–ilmenite–hercynite (Fig. 7c) and feldspar is corroded by retrograde muscovite and biotite (Electronic Appendix B). Metapelitic gneisses in the west are migmatitic and have coarse polygonal granoblastic early M2 matrix assemblages of cordierite–garnet–sillimanite–K-feldspar ± biotite (Electronic Appendix B). Rare kyanite inclusions in cordierite are overgrown by fine sillimanite, which also occurs within inclusion trails defining an early foliation (Fig. 7b). Cordierite is corroded by sillimanite–biotite aggregates on grain margins and garnet is corroded by sillimanite–biotite–plagioclase coronas. The early M2 matrix assemblage is cross cut by fine M2 sillimanite–biotite foliation seams (Fig. 7d).
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Two lower-grade metamorphic terranes exist in the WKZ: the Khumib Terrane, centred on the Khumib and Nadas Regions between the PMZ and Khumib Mylonite Zone and directly along strike from the high-grade Hoarusib Terrane, and the Coastal Terrane west of the Three Palms Mylonite Zone (Figs 3 and 5). The low-grade Khumib Terane has M2 matrix mineral parageneses similar to those developed in the central Omapungwe Region of the CKZ. Metapelitic schists have matrix assemblages of quartz–biotite–muscovite–oligoclase ± garnet, the garnet porphyroblasts are typically synkinematic with the main Wrench Stage foliation and overprinted and enveloped by the latest formed Convergence Stage foliations. Metapelites on the western margin of the Khumib Terrane are in transition to the high-grade Hoarusib Terrane, with developed garnet–biotite–K-feldspar–oligoclase ± fibrolite matrix assemblages and pegmatitic sills and segregations. Mafic schists in the low-grade zone have garnet-free, hornblende–quartz–plagioclase–titanite–epidote matrix assemblages and secondary epidote overgrowths. Calc-silicates have polygonal granoblastic assemblages of garnet–hornblende–quartz– plagioclase–ilmenite–biotite (Electronic Appendix C).
In contrast, the Coastal Terrane (Fig. 3) experienced high-grade early M2 metamorphism and a low-grade M2 overprint during Transpressional Phase deformation. Relic porphyroclasts and boudin lenses of coarse-grained polygonal granoblastic early M2 parageneses include quartz, garnet, plagioclase, sillimanite, K-feldspar, biotite and migmatitic segregations, all of which have been extensively grain-refined, recrystallized and overprinted by proto-mylonitic to mylonitic Wrench Stage fabrics. These relic parageneses indicate extensive K-feldspar–sillimanite–melt grade early M2 metamorphism throughout the Coastal Terrane, similar to much of the Hoarusib Terrane (Figs 3 and 5). The Wrench Stage grain-refined foliation contains biotite–muscovite–oligoclase–quartz assemblages and K-feldspar, whereas sillimanite and garnet are not developed (Electronic Appendix B). Thus the Coastal Terrane differs from the remainder of the WKZ by recording overprinting M2 parageneses that are of considerably lower metamorphic grade than the matrix early M2 assemblages.
Mineral chemistry
Methods
Mineral analyses from the Kaoko Belt were performed on a Cameca SX51 electron microprobe at Adelaide University and a Camebax microbeam at the University of Cape Town. An operating voltage of 15 kV and 20 nA was used for all phases except micas (10 nA) and feldspar (15 nA), and a beam radius of 2 µm for most phases and 4 µm for micas and feldspars. Natural silicates were used as standards and checked periodically. Mineral analyses from metapelite samples are described in detail below and representative analyses are contained in Electronic Appendix D. A summary of the full compositional range displayed by all metapelite and metabasic samples, in each metamorphic domain, is tabulated in Fig. 4.
Garnet. Representative garnet compositional maps from typical metapelites from each of the four metamorphic domains are presented in Fig. 8. Garnets from the CKZ and Khumib Terrane show typical growth zoning of increasing Fe2+ and Mg and decreasing Ca and Mn from core to rim. Garnets in the CKZ are typically large (1–15 mm) with poikiloblastic syntectonic cores and idioblastic overgrowths showing no significant resorption. Compositional variation is smooth in the synkinematic cores and most of the compositional range is within the idioblastic overgrowths, which rarely also illustrate stepped compositional zoning patterns (Fig. 8a). Mole fractions in the octahedral site have the following core-to-rim range in a typical sample (K211): XFe2+ 0·57–0·69, XMg 0·02–0·12, XCa 0·26–0·18, XMn 0·14–0·008 and Fe/(Fe + Mg) ratios 0·96–0·88. In contrast, garnets in the Khumib Terrane are smaller (0·5–1 mm), show smooth zoning of a smaller compositional range and have very thin Mn-enriched rims, which probably record garnet resorption (Fig. 8b). Core-to-rim growth zoning in a typical example (KK108c) has the following compositional range, not including the thin rims affected by resorption: XFe2+ 0·51–0·62, XMg 0·07–0·09, XCa 0·16–0·08, XMn 0·25–0·21 and Fe/(Fe + Mg) ratios 0·89–0·87. Garnets from the Hoarusib and Coastal Terranes in the WKZ have distinct flat compositional patterns with thin Mn-enriched and Ca- and Mg-poor resorption rims (Fig. 8c and d). Garnets in the high-grade Hoarusib Terrane are coarse grained (up to 30 mm) with irregular shapes and coarse-grained inclusions of the matrix assemblage. These garnets have cores that are rich in Fe2+ and Mg and poor in Ca and Mn, and the resorption rim is further enriched in Fe2+. For all these samples the range in core compositions are: XFe2+ 0·67–0·79, XMg 0·15–0·29, XCa 0·02–0·03, XMn 0·02–0·04 and Fe/(Fe + Mg) ratios 0·70–0·84. In contrast, relic garnets from the lower-grade Coastal Terrane have lower Fe and Mg and higher Ca and Mn core compositions, and the thin resorption rims are lower in Fe2+. The total range in core compositions in Coastal Terrane garnets is: XFe2+ 0·67–0·70, XMg 0·08–0·11, XCa 0·03–0·04, XMn 0·17–0·20 and Fe/(Fe + Mg) ratios 0·87–0·92.
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Cordierite. Cordierites in the high-grade Hoarusib Terrane are coarse grained (up to 20 mm) and part of the matrix assemblage. These cordierites have flat compositional profiles with slightly Mg-enriched rims. Fe/(Fe + Mg) ratios range from 0·30 to 0·36 in all samples. Na cations range from 0·04 to 0·12 in all samples.
Biotite. Biotite in the Gomatum Region metapelite is unzoned, Fe/(Fe + Mg) ratios range from 0·39 to 0·49 and Ti cations from 0·08 to 0·10 in all samples. At higher grades in the Omapungwe Region biotites are also unzoned and Fe/(Fe + Mg) ratios range from 0·39 to 0·69 and Ti cations from 0·08 to 0·16. Khumib Terrane biotite is unzoned, Fe/(Fe + Mg) ratios range from 0·43 to 0·60 and Ti cations from 0·08 to 0·15 in all samples. Biotite in the low-grade foliation in the Coastal Terrane has 0·18 Ti cations and Fe/(Fe + Mg) ratios range from 0·52 to 0·64. Biotites from the high-grade Hoarusib terrane are unzoned, have 0·09–0·35 Ti cations and Fe/(Fe + Mg) ratios of 0·43–0·58.
Muscovite. Muscovite in Gomatum Region metapelite is weakly zoned, Si cations in all samples range from 2·98 to 3·29 and K/(K + Na + Ca) ratio from 0·62 to 0.81. Omapungwe Region muscovites are unzoned, and Si cations range from 2·97 to 3·04 and K/(K + Na + Ca) ratio from 0·61 to 0·79. Muscovite in the low-grade foliation in the Coastal Terrane has 3·2 Si cations and K/(K + Na + Ca) ratio of 0·92. Khumib Terrane muscovites have 3·14–3·31 Si cations and K/(K + Na + Ca) ratios of 0·79–0·96.
Chlorite. Matrix chlorite in Gomatum Region metapelite is unzoned, Fe/(Fe + Mg) ratios range from 0·43 to 0·45 and Al/(Al + Si) ratios from 0·50 to 0·52 in all samples. In the Omapungwe Region, inclusion chlorites in garnet porphyroblasts have Fe/(Fe + Mg) ratio of 0·66 and Al/(Al + Si) ratio of 0·53, and retrogressive chlorites have Fe/(Fe + Mg) ratios of 0·45–0·48 and Al/(Al + Si) ratios of 0·51–0·53. Matrix chlorite in Khumib Terrane metapelite is unzoned, Fe/(Fe + Mg) ratios range from 0·42 to 0·44 and Al/(Al + Si) ratios from 0·50 to 0·52 in all samples.
Feldspar. Matrix plagioclase in metapelites from the CKZ shows weak zoning with increasing anorthite component towards the rims. Xan in plagioclase ranges from 0·01 to 0·28 in the Gomatum Region and from 0·14 to 0·34 in the Omapungwe Region. Khumib Terrane plagioclase displays weak zoning to lower anorthite rims, Xan ranges from 0·17 to 0·24 in the east and from 0·25 to 0·35 in higher-grade metapelites in the west. Feldspars in the high-grade Hoarusib Terrane are homogeneous, Xan ranges from 0·15 to 0·21 in plagioclase in the west and from 0·29 to 0·33 in the east. Xab ranges from 0·07 to 0·51 in K-feldspar from Hoarusib Terrane samples. Plagioclase in the Coastal Terrane is weakly zoned with Xan ranging from 0·27 in cores to 0·25 in rims.
Staurolite. Staurolite in metapelites from the Omapungwe Region is essentially unzoned with only slightly Fe-enriched rims. The compositional range in all samples is Fe/(Fe + Mg) of 0·78–0·85, Al/(Al + Si) of 0·70–0·71 and ZnO of 0·10–0·36%.
Pressure–temperature calculations
Methods
Samples with typical metapelite bulk compositions representing all metamorphic terranes in the Kaoko Belt were selected for PT calculations. A number of metapelite samples have been analysed by whole-rock X-ray fluorescence (XRF) analysis and bulk composition is also estimated from the modal mineralogy and average mineral chemistry of the most intensely investigated samples (Electronic Appendix E). The metapelite samples used in this study have a restricted range of compositions, meaning that bulk compositional variations should not exert systematic bias in the calculated PT results. A small number of metabasic and calc-silicate rocks were also investigated for comparison with the results from metapelites to test for bulk composition effects on PT calculations. Garnet compositional maps show that CKZ and Khumib Terrane garnets have typical, prograde growth zoning (Spear, 1993; Fig. 8a and b) and rarely develop thin Mn-enriched rims as a result of garnet resorption. Consequently, average PT calculations using the outermost rim of prograde garnet growth, careful to avoid the thin resorption rim, in conjunction with matrix mineral cores, are interpreted to represent peak M2 metamorphic conditions. Garnets from the high-grade Hoarusib Terrane and Coastal Terrane are compositionally flat (Fig. 8c and d) and typical of garnets that have been homogenized at high grades (e.g. Tuccillo et al., 1990; Spear, 1993). We assume that adjacent grains approach equilibration during this homogenization stage. This homogenization may or may not correspond to the peak of early M2 metamorphism, or alternatively some post-peak equilibration. However, the PT results from mineral cores match well with assemblage stability fields in pseudosections. Thin Mn-enriched garnet margins are due to resorption of garnet during M2 reworking (Fig. 8c and d).
Mineral end-member activities were calculated by the method of Holland & Powell (1990) using the program AX (Powell et al., 1998). PT calculations on peak metamorphic assemblages used the average PT approach of Powell & Holland (1994) and were done using THERMOCALC v3.0 (Powell & Holland, 1988) using the 1998 thermodynamic dataset (Powell et al., 1998). Partial melt segregations in granulite metapelite samples are devoid of garnet and aH2O is assumed to range between 0·5 and 0·7 (Electronic Appendix F). All sub-granulite metapelites were assumed to have XH2O = 1·0 (Electronic Appendix F). All results satisfy the 
2 test and errors from THERMOCALC, incorporating activity uncertainties for each mineral end-member and errors in the thermodynamic dataset, average ±50°C and ±1·3 kbar for all calculations (Electronic Appendix F). PT calculations of peak-metamorphic and retrograde conditions preserved in early M2 and M2 mineral parageneses (Electronic Appendix F), are summarized in Table 1 and represented in Figs 8 and 9.
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Because only one thermobarometric method and thermodynamic dataset have been employed, using predominantly typical metapelite bulk compositions, the PT results should be comparable across the Kaoko Belt. Twenty-eight samples from a continuous profile across the central Kaoko Belt region (Fig. 3) were analysed and the PT calculations define the metamorphic field gradient for this profile (Fig. 9). An additional sample from the CKZ in the Orumpembe Region and three samples from the WKZ in the Hoanib Region (Fig. 3) were analysed in the same manner to test the regional applicability of the findings from the central detailed profile. These results do not differ significantly from those in the central detailed profile (Table 1), suggesting very similar metamorphic conditions laterally along the length of each metamorphic zone defined by matrix assemblages (Fig. 5). Three samples from the low-grade Khumib Terrane in the WKZ were analysed to constrain the PT conditions of formation in this distinct metamorphic terrane (Fig. 3). Within the errors of the THERMOCALC calculations, all results are consistent with the phase stability field of the samples' matrix assemblage (Figs 10
–12) and thus considered plausible estimates of the equilibration conditions.
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Results
Peak M2 conditions average 534 ± 47°C and 9·0 ± 1·1 kbar (n = 5) in the Gomatum Region, 658 ± 45°C and 8·5 ± 1·6 kbar (n = 5) in the Omapungwe Region, and 689 ± 49°C and 8·5 ± 1·3 kbar (n = 4) in the western margin of the CKZ (Figs 9 and 10; Table 1). These results are similar to those documented by conventional geothermobarometry in a profile across the southern part of the Kaoko Belt (Dingeldey, 1997
; Franz et al., 1999). These peak metamorphic conditions define average thermal gradients of 17 ± 1·6, 22 ± 2 and 23 ± 2°C/km in the Gomatum Region, Omapungwe Region and western margin of the CKZ, respectively. (Average thermal gradient is simply the calculated metamorphic temperature divided by the calculated depth assuming a density of 2·8 g/cm3. It should not be confused with the instantaneous thermal gradients in the vicinity that an assemblage formed, or imply that the thermal gradients are time equivalent.) These average PT results (Table 1; Electronic Appendix F) are entirely consistent with the phase stability fields of the respective matrix assemblages (Figs 9 and 10; Electronic Appendices B and C). In the Gomatum Region, garnet–biotite–muscovite schists form at T > 510°C (Powell & Holland, 1990) and actinolite mafic schists from below the metapelite garnet isograd form at T < 490°C (Moody et al., 1983). In the western CKZ, garnet + staurolite ± kyanite schists and epidote- and garnet-bearing amphibolites encompass conditions of 550–660°C and >5·0 kbar (Fig. 10; Ghent et al., 1979; Powell & Holland, 1990). In two samples from the western Omapungwe Region, average PT calculations using garnet analyses from the Mn-enriched rim and rims of matrix minerals are interpreted to represent post-peak re-equilibration conditions. The re-equilibrated rim assemblage formed at lower pressures, averaging 640 ± 33°C and 6·5 ± 1·2 kbar (Table 1), suggesting decompression through the peak of M2 metamorphism.
Average PT calculations from mineral cores in the Hoarusib Terrane indicate peak early M2 conditions averaging 843 ± 64°C and 8·1 ± 1·6 kbar (n = 4) in the east and 811 ± 58°C and 6·2 ± 0·7 kbar (n = 2) in the west, corresponding to an average thermal gradient of 30 ± 3 and 38 ± 3°C/km respectively (Fig. 9; Table 1). Phase stability fields of diagnostic early M2 metamorphic assemblages in Hoarusib Terrane rocks (Electronic Appendices B and C) are coincident with PT calculations from the same rock (Figs 10 and 12). Diagnostic matrix assemblages in mafic gneisses are orthopyroxene–clinopyroxene–hornblende–garnet–plagioclase, limiting conditions to >6 kbar and >800°C (Spear, 1981; Kohn & Spear, 1990). Garnet–sillimanite–K-feldspar–melt ± biotite assemblages in the eastern Hoarusib Terrane formed at 750–900°C and 7–10 kbar, consistent with PT calculations (Figs 10 and 12; Powell & Holland, 1990; White et al., 2001). Garnet–cordierite–sillimanite–K-feldspar–melt ± biotite matrix assemblages in metapelites from the western Hoarusib Terrane are constrained to form at 780–880°C and 6–7 kbar, consistent with the slightly lower PT calculations from these samples (Figs 10 and 12; Powell & Holland, 1990; White et al., 2001).
Late-stage silica-undersaturated spinel–corundum–plagioclase–biotite symplectites that corrode peak metamorphic garnet in the eastern Hoarusib Terrane give 583 ± 85°C and 4·4 ± 1·2 kbar (Table 1). Similar moderate-T/low-P conditions are calculated from elsewhere in the WKZ, such as matrix M2 parageneses of the Khumib Terrane and M2 parageneses formed during reworking of the Coastal Terrane (see below; Fig. 10; Table 1). This group of PT results are broadly representative of conditions during M2 reworking in all the terranes of the WKZ and collectively average 561 ± 37°C and 4·9 ± 1·0 kbar (n = 7). PT calculations from mineral cores in an M2 garnet–sillimanite–biotite pegmatite in the high-grade Hoarusib Terrane give an inaccurate PT result of 698 ± 54°C and 5·3 ± 3·6 kbar (Table 1). This is consistent with typical M2 assemblages of biotite–sillimanite–plagioclase ± K-feldspar in the overprinting foliation seams in the Hoarusib Terrane, with a phase stability field indicating conditions of 660–750°C and 4·0–7·0 kbar (Powell & Holland, 1990; White et al., 2001; Figs 10 and 12).
Average PT results from peak metamorphic assemblages in the Khumib Terrane average 573 ± 19°C and 5·4 ± 1·0 kbar (n = 3) and define an average thermal gradient of 30 ± 1°C/km (Table 1). A single average PT calculation using a garnet analysis from the outer Mn-enriched rim and rims of matrix minerals is interpreted to represent post-peak re-equilibration conditions. The rim compositions give lower pressures, at 576 ± 29°C and 3·7 ± 1·3 kbar (Table 1), suggesting decompression through the peak of M2 metamorphism in the Khumib Terrane. A garnet– plagioclase–biotite–muscovite metapelite from the higher-grade western margin of the Khumib Terrane gives peak metamorphic conditions of 704 ± 47°C and 4·2 ± 1·4 kbar, calculated using growth zoning rims in garnet and matrix cores (Table 1). PT conditions calculated from the compositionally homogeneous relic garnet grains and adjacent micas in a low-strain domain within Coastal Terrane metapelite sample (KK87c) are 689 ± 43°C and 4·9 ± 1·3 kbar (Table 1). M2 metamorphic conditions during pervasive reworking in the Coastal Terrane cannot be accurately estimated from the grain-refined foliation assemblage in metapelite samples. The M2 foliation assemblage in metapelites is typically quartz– plagioclase–muscovite–biotite, which can form across a wide range from 400 to 650°C (Fig. 10). A Coastal Terrane metabasic schist with a garnet–hornblende–plagioclase–quartz matrix foliation assemblage and homogeneous mineral interior compositions gives 547 ± 51°C and 4·3 ± 1·0 kbar (Table 1), which may represent M2 conditions in this terrane (Fig. 10).
Pressure–temperature paths
Garnet zoning. CKZ and Khumib Terrane garnet porphyroblasts preserve typical growth zoning compositional patterns, with smooth or stepped progressive increase in Fe and Mg and decrease in Ca and Mn towards rims (Fig. 8a and b; Tracy, 1982; Loomis, 1983; Tuccillo et al., 1990; Spear, 1993). Consequently, these garnet compositions document a portion of the prograde P–T trajectory during garnet growth (Spear et al., 1984; St-Onge, 1987). Variation in the composition across select garnets displaying growth zoning in the CKZ (K211) and Khumib Terrane (KK108c) (Fig. 8a and b) is used as a semi-quantitative constraint on the prograde P–T path in these Barrovian parts of the belt (Fig. 13). Variation in garnet composition is compared with the XFe, XCa and Xmn isopleths in PT pseudosections calculated by Vance & Mahar (1998) (Fig. 13), for metapelite samples of similar compositions to those in the Kaoko Belt (Electronic Appendix E). Decreasing XMn and XCa with increasing XFe in sample (K211) from the CKZ document a prograde P–T path that must have involved increasing P (Fig. 13a). Garnet growth during heating with burial, at mid-amphibolite grade, is consistent with the modal garnet calculations of Spear (1993) and Vance & Mahar (1998), for broadly similar metapelite bulk compositions used in this study. Decreasing XMn and XCa, with increasing XFe, from core to rim in sample KK108c from the Khumib Terrane preserves a decompressive portion of the prograde P–T path (Fig. 13b).
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Some garnets in the CKZ and Khumib Terrane are partially resorbed and develop thin (0·01–0·02 mm) Mn-enriched rims (Fig. 8b). PT calculations from these re-equilibrated rim assemblages are invariably of lower pressures (
P = 1·5–2·7 kbar) than conditions obtained using mineral interiors (Table 1), suggesting a broadly decompressive evolution. Compositional zoning in garnet porphyroblasts from the Hoarusib Terrane of the WKZ (Fig. 8c) differs markedly from those in the CKZ. Garnet cores are compositionally flat, suggesting homogenization at high temperatures (Tuccillo et al., 1990; Spear, 1993). Thin (0·02–0·06 mm) rims as a result of garnet resorption are developed that typically have slightly lower Ca and Mg and higher Fe and Mn, although lower Fe and higher Mg rims occur in some samples. Rim compositions formed by retrograde diffusion, during which the cations may have decoupled (Tracy, 1982; Tuccillo et al., 1990). Most samples show minor Ca decrease, suggesting post-peak decompression. Average PT calculations from rim compositions in one sample (K1335) give pressures 1·5 kbar lower than the peak of metamorphism (Electronic Appendix F).
CKZ phase relationships. Mineral reaction textures strongly suggest that retrograde paths in the CKZ involved decompression. Decompressive P–T paths are implied by plagioclase coronas on garnet in metapelites (e.g. Harley, 1992) and plagioclase ± quartz ± hornblende moats around resorbed peak metamorphic garnet in metabasic rocks (Fig. 6d; Electronic Appendix C) (e.g. Kohn & Spear, 1990). In metapelites the peak metamorphic alumino-silicate is invariably kyanite, with sillimanite occurring only as late-stage growth corroding garnet margins (Electronic Appendix B). Decompression through peak metamorphic conditions is indicated by the sequence of mineral growth in staurolite-bearing metapelites (Electronic Appendix B). Early chlorite–ilmenite–quartz inclusion assemblages were overgrown by garnet–biotite–plagioclase prograde parageneses, which were replaced by garnet–biotite–staurolite matrix assemblages. Staurolite–biotite growth continued in the main foliation, which envelops the earlier formed garnet (Fig. 6c). Staurolite is typically later formed than garnet, as indicated by garnet inclusions in staurolite, staurolite–biotite–quartz moats around resorbed garnet porphyroblasts (Fig. 6c), partial staurolite coronas and idioblastic staurolite overgrowing the foliation (Electronic Appendix B). Matrix assemblages are in turn overprinted by retrograde chlorite, muscovite, haematite, plagioclase and fibrolite assemblages that formed as a result of garnet and staurolite breakdown (Fig. 6c).
The general topology of published KFMASH P–T pseudosections for moderately aluminous amphibolite-grade metapelites (above the garnet–chlorite tieline) is similar over a range of typical metapelite compositions (Powell et al., 1998; Vance & Mahar, 1998; Spiess et al., 2000; White et al., 2000). The bulk compositions of four Barrovian-grade metapelite samples from the Kaoko Belt have been determined directly by whole-rock XRF analysis and three have also been estimated using average mineral compositions and modal proportions (Electronic Appendix E). The bulk composition of CKZ metapelites (Electronic Appendix E) and their mineral chemistry (Fig. 4) are typical of metapelites in general and comparable with the bulk composition used in Fig. 11. Furthermore, T–XFe relations (Powell et al. 1998) show that the general topology of metapelitic phase relations does not change over the compositional range displayed by Kaoko Belt amphibolite-grade metapelite samples (Electronic Appendix E). Consequently, the sequence of mineral growth recorded in Kaoko Belt metapelite samples can be used to define a semi-quantitative P–T path (Fig. 11).
The sequence of mineral growth documented in a suite of garnet–staurolite metapelites from the Omapungwe Region of the CKZ (K332, K931b, K940 and K978) constrains the general form of a clockwise P–T evolution in that part of the orogen (Fig. 11). Prograde trajectories are from chlorite-bearing parageneses to matrix assemblages within the staurolite–garnet–biotite field, which is also restricted to the kyanite field. Decompression from the garnet–staurolite–biotite field into the staurolite–biotite field is evident by continued staurolite growth after cessation of garnet growth, garnet resorption textures and also sillimanite growth. The retrogressive cooling sector of the clockwise P–T path is documented by late-stage chlorite and muscovite growth by resorption of garnet and staurolite (Fig. 11). Clockwise P–T paths, constrained by conventional geothermobarometry, have also been reported from the southern CKZ (Dingeldey, 1997; Franz et al., 1999).
WKZ phase relationships. WKZ matrix parageneses are overprinted by plagioclase-bearing coronas on garnet in mafic gneisses (Fig. 7a), kyanite in the PMZ is overgrown by sillimanite and corundum–spinel– biotite–plagioclase symplectic intergrowths corrode garnet (Fig. 7c). These coronitic and symplectic overgrowths suggest decompression (Hensen, 1987; Kohn & Spear, 1990; Harley, 1992). Late-stage symplectites formed at moderate-T/low-P conditions of 583 ± 85°C and 4·4 ± 1·2 kbar (Table 1; Fig. 10), and M2 pegmatites formed at approximately 698 ± 54°C and 5·3 ± 3·6 kbar (Table 1), both suggesting decompressive cooling from peak metamorphic conditions. Overall, M2 mineral parageneses from all terranes in the WKZ formed at around 561 ± 37°C and 4·9 ± 1·0 kbar, indicating decompressive cooling during evolution from early M2 to M2 (Fig. 10).
The diagnostic sequence of developed mineral parageneses in Hoarusib Terrane metapelites constrains the P–T loops in that part of the orogen (Fig. 12). The bulk compositions of samples KK67a and KK68d were estimated using average mineral compositions and modal proportions, and determined directly by XRF whole-rock analysis (Electronic Appendix E). The bulk compositions of these samples are comparable with those used in Fig. 12. In these Hoarusib Terrane rocks, kyanite inclusions are overgrown by sillimanite, which is also a common inclusion phase in matrix cordierite, and matrix assemblages are migmatitic cordierite–garnet–sillimanite–K-feldspar ± biotite. This sequence of mineral growth suggests prograde heating accompanying decompression. Cordierite is corroded by sillimanite–biotite aggregates, suggesting decompression with cooling (Fig. 12). Inclusion, matrix and late-stage parageneses document a clockwise P–T loop (Fig. 12) and core-to-rim PT calculations preserve the retrograde portion involving decompressive cooling from the peak of metamorphism (Fig. 10).
Clockwise P–T loops were apparently experienced in all parts of the Kaoko Belt. However, specific to particular zones in the orogen, different portions of these P–T loops are recorded by the matrix mineral parageneses in each zone. The Kaoko Belt is an asymmetric orogenic system in which the granulitic WKZ represents the hot orogen core and the Barrovian CKZ is a diffuse mobile shear zone in a crustal-scale overriding system. Samples in the Barrovian CKZ preserve the prograde heating and burial portion of the P–T loop. Compositional zoning of garnets indicates growth during prograde P–T trajectories culminating in peak metamorphic conditions represented by rim analyses (Fig. 10; Table 1). In the western margin of the CKZ, adjacent to the PMZ, and also in the high-grade Hoarusib Terrane of the WKZ, retrograde trajectories with decompression are preserved. These decompressive P–T vectors are documented by both mineral reaction textures and low-P/moderate-T conditions recorded by PT calculations in re-equilibrated garnet rim assemblages and retrograde symplectites (Fig. 10; Table 1). Thus matrix mineral growth occurred at different points on the clockwise P–T loops that the different zones of this asymmetric transpressional orogen evolved through.
Metamorphic field gradients
Matrix assemblages indicate the existence of an inverted metamorphic sequence in the CKZ, increasing in grade to higher structural levels towards the west. Metamorphic isograds are sub-parallel to the pervasive west-dipping fabric, are unfolded and cut across map-scale Convergence Stage folds, indicating that the peak of metamorphism was attained during or subsequent to these folds (Fig. 5). The CKZ has a typical high-P/moderate-T Barrovian field gradient from sub-greenschist in the foreland (EKZ), across biotite-, garnet-, staurolite- and kyanite-in isograds to a maximum of kyanite–sillimanite grade in the westernmost CKZ. This smooth and continuous metamorphic field gradient changes abruptly at the PMZ and the Kaoko Belt can be divided into two metamorphic terranes of contrasting metamorphic style, the WKZ and CKZ, separated by the orogen-median PMZ (Fig. 5).
Remarkably, peak metamorphic P does not vary significantly from east to west across the belt. Calculated conditions document an insignificant decrease from 9·0 kbar in the Gomatum Region, to 8·5 kbar in the Omapungwe Region, 8·5 kbar in the western margin of the CKZ and 8·1 kbar in the eastern Hoarusib Terrane of the WKZ (Fig. 9; Table 1). Peak metamorphic temperatures rise steeply from east to west; from <400°C in the EKZ, to 534°C in the Gomatum Region, 658°C in the Omapungwe Region, 689°C in the western margin of the CKZ and 843°C in the eastern Hoarusib Terrane in the WKZ (Fig. 9; Table 1). Furthermore, there is a consistently higher average thermal gradient in all terranes of the WKZ (29–40°C/km) in contrast to low average thermal gradients between 17 and 23°C/km in the CKZ (Table 1).
Matrix assemblages in the WKZ, both early M2 in the Hoarusib Terrane and M2 in the low-grade Khumib Terrane, are characterized by extreme variation in metamorphic grade both along and across the belt, ranging from biotite grade to granulite grade (Fig. 5). In the south, metamorphic grade increases steeply towards the west across the PMZ, which is coincident with the muscovite- and kyanite-out isograds and incoming of K-feldspar, sillimanite and a second generation of partial melt (Figs 4, 5 and 9). Highest metamorphic grades were experienced in the south, peaking in the Haorusib Terrane, bound by the PMZ and Three Palms Mylonite Zone. These rocks experienced granulite-grade early M2 metamorphism with sillimanite-, cordierite-, K-feldspar- and garnet-bearing assemblages in metapelites (Figs 4 and 5) and two-pyroxene mafic granulites, and were reworked by M2 assemblages characterized by sillimanite–biotite seams (Fig. 7d) and a second generation of partial melting associated with Convergence Stage folds (Fig. 4). Metamorphic grade varies across the Hoarusib Terrane from 843°C and 8·1 kbar in the east to 811°C and 6·2 kbar in the west (Table 1).
Polarity of the metamorphic field gradient across the PMZ is reversed in the north where the low-grade Khumib Terrane containing biotite–muscovite ± garnet grade metapelites occurs immediately west of the PMZ. As elsewhere in the WKZ, metamorphic grade increases across the Khumib Terrane towards the west and reaches a maximum K-feldspar–sillimanite–melt grade in the adjacent Hoarusib Terrane towards the west (Fig. 5). There is a continuous variation in metamorphic grade from the central part of the Khumib Terrane, which equilibrated at 573 ± 19°C and 5·4 ± 1·0 kbar, to 704 ± 47°C and 4·2 ± 1·4 kbar in the western margin of the Khumib Terrane, and to 811 ± 58°C and 6·2 ± 0·7 kbar in the western Hoarusib Terrane. Despite the marked variation in metamorphic grade, all terranes in the WKZ formed at similarly high average thermal gradients, between 29 and 40°C/km (Table 1). Consequently, a common thermal regime appears to have existed across the WKZ, and this differed from that experienced by the CKZ, which had an average thermal gradient of 16–23°C/km. There is a sharp decrease in metamorphic grade of both early M2 and M2 parageneses to the west of the Three Palms Mylonite Zone, that is, in the Coastal Terrane (Figs 3, 5 and 10). Like the Hoarusib Terrane, the Coastal Terrane also contains earlier formed early M2 migmatitic segregations, garnet and K-feldspar porphyroclasts and granitoids (Fig. 3), but early M2 matrix assemblages are devoid of sillimanite and equilibrated at relatively lower-grade conditions of 689 ± 43°C and 4·9 ± 1·3 kbar (Table 1). Early M2 mineral parageneses are extensively reworked by the pervasive L–S fabric giving M2 mineral assemblages in this fabric that are typically oligoclase–muscovite– biotite and are devoid of garnet, sillimanite, K-feldspar and second-generation partial melt in metapelites. M2 metamorphic conditions in the Coastal Terrane are constrained to be 547 ± 51°C and 4·3 ± 1·0 kbar by average PT calculations from the foliated matrix assemblage in a garnet amphibolite schist (Table 1).
Lower-temperature rocks in the WKZ, such as the Coastal Terrane and Khumib Terrane, also formed at lower pressures of 4·2–5·7 kbar, indicating that these terranes represent shallower crustal levels juxtaposed against the high-grade Hoarusib Terrane that was metamorphosed at deeper crustal levels of 6·2–8·1 kbar. Stretching lineation arrays in the WKZ suggest low-angle downward trajectories of the Khumib and Coastal Terranes and low-angle upward and southward extrusion of the Hoarusib Terrane between these two low-grade terranes (Goscombe et al., 2002). The low-grade Khumib ‘trough’ coincides with a convex arc in the PMZ and south-plunging stretching lineations, giving low-angle extensional movements and downward trajectories in this region (Goscombe et al., 2002). Decrease in grade across the Three Palms Mylonite Zone into the Coastal Terrane is due to a component of west-down movement along shallow south-plunging lineations (Goscombe et al., 2002). The gross metamorphic isograd pattern in the WKZ (Fig. 5) implies that the high-grade Hoarusib Terrane was extruded upwards and southward along shallow NNW-plunging lineations during the Transpressional Phase.
The metamorphic isograd pattern of the Kaoko Belt is only in part typical of those in high-angle convergent orogens, and also shows marked differences because of the low-angle convergence (transpressional) nature of the orogen. The inverted Barrovian metamorphic sequence in the CKZ is within what can be considered a foreland-vergent, orogen-scale diffuse shear zone (escape zone). This is typical of high-angle convergence, crustal overriding orogenic systems, such as the Himalayas (Goscombe & Hand, 2000), Appalachians (Armstrong et al., 1992) and Inland Branch of the Damara Orogen (Hoernes & Hoffer, 1979; Goscombe & Hand, 2001b), and is consistent with thermal models of the foreland block in transpressional orogens (Thompson et al., 1997). In contrast, the WKZ has developed a more complex spatial pattern of metamorphic isograds (Fig. 5), as a result of metamorphism accompanying lateral shearing along sub-horizontal stretching lineations at very low angles to the orogen. Metamorphic grade varies discontinuously between orogen-parallel panels separated by crustal-scale, sinistral strike-slip shear zones. Shearing along low-angle lineations, both within panels by pervasive deformation and in bounding shear zones, gave rise to alternating domains of shallow upward or downward trajectories and the resulting metamorphic pattern (Fig. 4; Goscombe et al., 2002).
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GARNET Sm–Nd GEOCHRONOLOGY |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGY OF THE...CHRONOSTRATIGRAPHIC ROCK UNITSSTRUCTURAL...METAMORPHIC...GARNET Sm-Nd GEOCHRONOLOGYDISCUSSIONSUPPLEMENTARY DATAREFERENCES |
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Samples
There are few direct constraints on the timing of metamorphism in different parts of the Kaoko Belt. To constrain the age of peak metamorphism, four Damara Sequence metapelite samples with single-phase garnet growth were selected from each of the four metamorphic terranes: the Barrovian CKZ (K209), granulite-grade Hoarusib Terrane (K1336b), low-grade Khumib Terrane (KK105f) and the high-grade Coastal Terrane that experienced low-grade reworking (KK87c). Samples K209 and KK105f are garnet– biotite–muscovite–oligoclase–quartz schists with all minerals syntectonic with the pervasive Wrench Stage foliation, defining the M2 assemblage. Garnet porphyroblasts have synkinematic cores with sigmoidal inclusion trails and idioblastic inclusion-free rims. Garnet compositional zoning in similar closely associated samples reveals a single phase of progressive garnet growth (Fig. 8a and b). Sample K1336b is texturally equilibrated coarse-grained polygonal granoblastic semi-pelitic garnet–biotite–K-feldspar–plagioclase–quartz gneiss with minimal overprinting by M2 biotite– sillimanite foliation seams. Sample KK87c contains rounded relic early M2 porphyroclasts and domains of garnet–biotite–quartz–K-feldspar–plagioclase that are enveloped by a protomylonitic biotite–muscovite–quartz–oligoclase M2 fabric. In similar closely associated samples from both high-grade terranes in the WKZ, garnet is compositionally homogeneous (Fig. 8c and d) and formed during high-grade early M2 metamorphism before Wrench Stage reworking and garnet resorption during M2. An additional basement garnet–biotite–muscovite–oligoclase–quartz metapelite from the CKZ was analysed in an attempt to constrain the age of the Proterozoic metamorphic cycle (K983b).
Analytical methods
Individual garnets were initially seperated from the matrix and rims were ground off to minimize the effects of the compositional modification during the retrograde history of the terrane. The garnet fractions and matrix whole-rock fractions were then crushed, milled and sieved, and mineral fractions were obtained via magnetic and heavy liquid separation. Surface contamination on the mineral separates was removed by an ultrasonic cleaner in 1N HCl solution. For mineral separates, between 50 and 300 mg of sample was used. The mineral separates were milled under ethanol in an agate mortar to a grain size < 2 µm. To minimize contamination of mineral fractions by rare earth element (REE)-rich inclusions such as monazite, the milled fractions were leached in cold 6N HCl for 15 h. The leachate was separated from the residual solid material. The solid material was washed and centrifuged in doubly distilled water a number of times to remove any trace of the leachate fraction. For the whole-rock component, around 150 mg of milled whole rock was dissolved in HNO3–HF acid mixtures for periods between 1 and 10 days. All samples were spiked with a mixed 147Sm–150Nd spike before dissolution. Nd isotopic compositions were measured by thermal ionization mass spectrometry (TIMS) on a Finnigan MAT 262 system in static mode, taking 10 blocks of 10 scans each. The isotopic ratios were corrected for fractionation to 146Nd/144Nd = 0·7120903. Sm concentrations were measured by isotope dilution TIMS on a Finnigan MAT 261 system in dynamic mode, taking four blocks of 10 scans each. Fractionation corrections were made to a 152Sm/149Sm ratio of 1·9347.
Results
Results of Sm–Nd isotopic analysis are contained in Table 2. Apart from the basement sample, all Damara Sequence metapelites give reasonably well-constrained ages of 574 ± 10 Ma in the CKZ (K209), 573 ± 8 Ma in the Hoarusib Terrane (K1336b), 579 ± 16 Ma in the Khumib Terrane (KK105f) and 595 ± 13 Ma in the Coastal Terrane (KK87c). The majority of the analysed garnet fractions have Nd/Sm ratios >1, indicating that they are relatively light REE (LREE) enriched. However, the majority of existing data (e.g. Getty et al., 1993; Baxter et al., 2002) suggest that garnets should be comparatively enriched in heavy REE (HREE). The relatively LREE-rich compositions of most of the garnets in Table 2 reflect either unusual garnet compositions or contamination by LREE-rich micro-inclusions. Given the degree to which LREE-rich inclusions can mask the isotopic compositions of garnet (e.g. Zhou & Hensen 1995), it is likely that some of the garnets analysed in this study have been affected by inclusions despite being finely ground and acid leached.
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In medium- and high-grade metapelitic rocks, monazite is the most common LREE accessory mineral, and is an accessory in most metapelites in the Kaoko Belt. Whereas it is a comparatively abundant detrital mineral, monazite is uncommon in greenschist-grade metapelites (Kingsbury et al., 1993
), suggesting that it undergoes partial dissolution under hydrous low-grade conditions. Kingsbury et al. (1993) showed that metamorphic monazite forms via dehydration reactions that occur at around the staurolite-in isograd. Therefore it seems likely that REE-rich inclusions in garnets in the Kaoko Belt would have formed during the prograde metamorphic evolution of the terrane (e.g. Rubatto et al., 2001). Because the behaviour of a garnet micro-inclusion system will be largely controlled by the diffusional characteristics of the garnet host, one interpretation of the ages obtained from the potentially contaminated garnet compositions in Table 2 is that they represent average prograde ages derived from grain-scale domains. On this basis we interpret the Sm/Nd ages to be indicative of the timing of prograde or peak conditions in at least the CKZ, where the garnets preserve prograde zoning. The similarity of the ages across the terrane lends support to this interpretation. If the isotopic data reflected averages involving detrital REE-bearing minerals, it is unlikely that the ages would be consistent across the Kaoko Belt.
Samples KK87c, K209 and KK105f equilibrated at approximately 690, 534 and 566°C, respectively (Table 1), all of which are near or below most estimates of garnet Sm–Nd closure temperatures for moderate grain size and cooling rate (e.g. Becker, 1997; Ganguly et al., 1998). However, garnet in sample KK1336b from the Hoarusib Terrane grew at temperatures of >800°C. These temperatures are above the typically considered range of closure temperatures for garnet Sm–Nd systems (e.g. Burton et al., 1995), raising questions about the interpretation of the age. However, the garnet in sample KK1336b has a diameter of >10 mm, and for cooling rates of around 10°C/Myr, Sm–Nd closure is likely to be in the vicinity of 800°C (Ganguly et al., 1998). If this is the case, the age obtained from KK1336b is probably close to that of the metamorphic peak. This is supported by many studies of granulite terranes (T >800–850°C) that report garnet Sm–Nd ages near that of the peak of metamorphism as determined by U–Pb zircon and monazite ages (Friend & Kinny, 1995; Zhou & Hensen, 1995; Miller et al., 1997; Mawby et al., 1999).
The three Damara Orogeny garnet Sm–Nd ages from the CKZ and high-grade and low-grade panels of the WKZ are the same within uncertainties, spanning 576 ± 15 Ma. These samples represent all the metamorphic terranes that make up the Kaoko Belt apart from the outboard Coastal Terrane, suggesting that peak metamorphism was contemporaneous across the core and foreland margin of the orogen. That is, the amphibolite-grade M2 schistosic matrix assemblages associated with pervasive deformation of the Transpressional Phase in the CKZ and Khumib Terrane formed at the same time as the high-grade coarse-grained, polygonal granoblastic early M2 assemblages developed during the Thermal Phase in the Hoarusib Terrane. The Coastal Terrane garnet Sm–Nd age of 595 ± 13 Ma is slightly older, but nevertheless the same within uncertainty. Coastal Terrane garnets are relic early M2 mineral parageneses that formed at moderate metamorphic grades with migmatitic assemblages, before extensive reworking at low metamorphic grades during the Transpressional Phase in M2. The age determined from the relic, partially resorbed garnets (Fig. 8d), possibly represents a mixed early M2 and M2 age (Fig. 14). Basement sample K983b from the Omapungwe Region of the CKZ gives a poorly constrained two-point isochron age of 1239 ± 69 Ma. Compositional maps from that sample show two phases of garnet growth, the younger of which is interpreted to be Daraman in age. No significant tectonic interpretation can be attributed to the 1239 ± 69 Ma age, apart from suggesting that the Proterozoic metamorphic cycle in this portion of the basement mosaic was older than 1240 Ma.
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, age derived from a matrix metamorphic mineral; 
, age derived from a granitoid. Frequency distribution of age data illustrates the M1 (656–645 Ma) and M2 (580–570 Ma) metamorphic peaks and spread in late synkinematic granite ages to 552 Ma and syn-Shortening Phase granite (M3) in the Ugab Zone at 530 Ma. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGY OF THE...CHRONOSTRATIGRAPHIC ROCK UNITSSTRUCTURAL...METAMORPHIC...GARNET Sm-Nd GEOCHRONOLOGYDISCUSSIONSUPPLEMENTARY DATAREFERENCES |
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Tectonometamorphic evolution ofthe Kaoko Belt
Structural evolution and architecture of the Kaoko Belt (Goscombe et al., 2002
) is typical of transpressional orogens elsewhere (e.g. Shackleton & Ries, 1984; Holdsworth & Strachan, 1991; Vassallo & Wilson, 2002). This is exemplified by wrench-style tectonics dominated by crustal-scale strike-slip shear zones in the orogen core (WKZ), and temporal and spatial progression outwards to the foreland margin (CKZ) with accompanying large-scale overfolds, nappes and thrusts. These different parts of the orogen display contrasting metamorphic style and isograd patterns. Metamorphic grade in the CKZ increases toward higher structural levels, constituting an inverted Barrovian field gradient. This is typical of foreland-vergent orogen-scale overriding systems with high-angle convergence, such as the Himalayas (e.g. Goscombe & Hand, 2000), Acadian metamorphism in the Appalachians (Armstrong et al., 1992) and both margins of the Inland Branch of the Damara Orogen (Goscombe & Hand, 2001b). In contrast, the wrench-style dominated orogen core in the WKZ has a very complex isograd pattern with pronounced variation in metamorphic temperatures between shear zone bound terranes (Fig. 5). Metamorphic grade varies both across the grain of the WKZ and along strike (Fig. 5). The pattern in metamorphic grade is suggestive of the high-grade Hoarusib Terrane having been extruded from deeper crustal levels (Fig. 10) along shallow NNW-plunging stretching lineations (Goscombe et al., 2002). Extrusional tectonics were possible in the WKZ because transpressional deformation continued throughout metamorphism with high-grade conditions generating rheologies that were conducive to extrusion between panel-like crustal-scale shear zones (e.g. Thompson et al., 1997). The average thermal gradient is consistently high across all WKZ terranes, ranging from 29 to 40°C/km, in contrast to 17–23°C/km across the CKZ (Table 1). The crustal level exposed is approximately the same across most of the Kaoko Belt, ranging from 9 kbar in the CKZ to 8·1 kbar in the Hoarusib Terrane of the WKZ, and is significantly lower only in the Khumib (4·2–5·4 kbar) and Coastal (4·4–4·9 kbar) Terranes of the WKZ. Thus the metamorphic architecture of the Kaoko Belt is characterized by two contrasting halves separated by the orogen-median PMZ: (1) a wrench-style dominated orogen core (WKZ) with consistently high average thermal gradient and mosaic of shear zone bounded panels of different metamorphic grade and crustal depth; (2) oblique convergence with overfolding and thusting at the orogen margin (CKZ) with a smoothly varying inverted Barrovian metamorphic sequence of moderate average thermal gradient.
Absolute horizontal length-scale, metamorphic field gradient across the Kaoko Belt (not to be confused with the average thermal gradient at each locality), contrasts strongly with all high-angle collisional orogens documented by Sonder & Chamberlain (1992). Over a distance of 32 km from the Gomatum Region in the east to the Hoarusib Terrane in the west, estimates of peak metamorphic conditions (Table 1) document a very low pressure gradient of –28 bar/km and moderate temperature gradient of +9·6°C/km (Fig. 9). These do not overlap with typical high-angle convergence collisional orogens, which plot in a broad field with much higher pressure gradients ranging from +13 to +375 bar/km and temperature gradients ranging from –2 to +33°C/km but typically lower than in the Kaoko Belt (Sonder & Chamberlain, 1992).
Thermal modelling of crustal-scale transpressional systems, with varying obliquity of convergence (Thompson et al., 1997), generates the same metamophic architecture as developed in the Kaoko Belt. High temperatures and high average thermal gradients are generated in the orogen core where the lowest degrees of obliquity (low pure/simple shear ratios) are experienced and long residence time of sub-horizontal transport in the lower crust generates granulites (Thompson et al., 1997). Indeed, metamorphism appears longest lived in the core of the Kaoko Belt, where high-grade early M2 metamorphism and partial melting produced matrix parageneses before extensive reworking during M2, which is when matrix parageneses were produced in the CKZ. Coincident high strain (Goscombe et al., 2002) and high grade (Fig. 5) in the PMZ and WKZ is due to a high early M2 average thermal gradient resulting in a thermally weakened (‘soft’) zone conducive to partitioning strain during the subsequent Transpressional Phase (M2). In contrast, high obliquity of convergence (high pure/simple shear ratios) at the orogen margin (CKZ) implies faster rates of exhumation and erosion, leading to moderate average thermal gradients and Barrovian-style metamorphism (Thompson et al., 1997).
Clockwise P–T paths were experienced in both the CKZ and all terranes in the WKZ (Figs 10
–12). However, the portion of the P–T loop that is preserved by matrix mineral parageneses varies across the orogen. The prograde phase is preserved by garnet growth zoning in the CKZ and the exhumation phase from peak metamorphic matrix assemblages to overprinting reaction textures and fabrics is preferentially preserved in WKZ rocks (Fig. 10). This is also represented by the distribution of mineral textures across the Kaoko Belt (Fig. 4). Synkinematic garnets with typical growth zoning and idioblastic post-tectonic overgrowths are typical in the CKZ and compositionally flat garnets without idioblastic overgrowths are typical in the WKZ (Fig. 8c). These relationships are typical of convergent orogens such as the Himalayas (Goscombe & Hand, 2000), where the high-grade extruded orogen core has sufficient thermal energy to re-equilibrate and homogenize matrix mineral parageneses, obliterating evidence of prograde mineral growth. Consequently, matrix mineral growth occurred on different sectors of the P–T loops that the different metamorphic terranes evolved through.
In high-angle convergent orogens such as the Himalayas, the crustal-scale shear zone separating the two metamorphically distinct halves of the orogen is shallowly inclined and sub-parallel to isograds (Goscombe & Hand, 2000). Transpressional orogens such as the Kaoko Belt are very different and though the orogen median shear zone and metamorphic isograds are similarly sub-parallel, they are steeply inclined and at high angles to the isobars. In both orogenic systems the orogen median shear zone controls the architecture of the orogen and coincides approximately with the muscovite-out, sillimanite-in, K-feldspar-in and partial melt-in isograds (Goscombe & Hand, 2000). The observation by Sonder & Chamberlain (1992) that metamorphic grade is controlled by thermal parameters and erosion rates, and not solely by burial depth, is particularly applicable to transpressional orogens such as the Kaoko Belt, where metamorphism of the currently exposed rocks took place at constant pressures of 8·5 kbar. The isograds in the Kaoko Belt cannot remain sub-vertical throughout the crustal column, and presumably the crustal-scale shear zones are discordant to the isograds at depth and in the stripped-off shallow crust.
Timing of metamorphism in the Kaoko Belt
The high-grade portions of the WKZ, the Coastal and Hoarusib Terranes, experienced two texturally distinct metamorphic episodes (early M2 and M2). Coarse-grained, polygonal granoblastic textures and migmatitic segregations formed during early M2 were reworked to varying degrees by fine-grained foliation seams and proto-mylonitic to mylonitic fabrics and shear zones during M2. The earliest thermal event (early M2) is less apparent at metamorphic grades that did not exceed the solidus, such as the Khumib Terrane and CKZ. In these lower-grade parts of the belt, early M2 is recognized as the earliest stage of porphyroblast growth that continued progressively into synkinematic M2 matrix assemblages associated with Transpressional Phase fabrics. This suggests that early M2 and M2 were texturally distinct episodes in a single metamorphic cycle, early M2 occurring before pervasive reworking in the Transpressional Phase, which accompanied M2. In the CKZ early M2 mineral core parageneses occurred during prograde metamorphism (Fig. 10) and were followed by M2 matrix rim parageneses developed at the peak of metamorphism (Fig. 10; Table 1). In contrast, in the WKZ early M2 accompanied the peak of metamorphism and M2 accompanied reworking at lower grades during retrogression.
Sm–Nd dating of matrix garnet in all metamorphic terranes forming the Kaoko Belt shows the peak of metamorphism to be approximately contemporaneous at 576 ± 15 Ma. Thus peak metamorphism in the two halves of the orogen, early M2 in the WKZ and M2 in the CKZ, are approximately time equivalent. This implies that reworking of the WKZ by Wrench Stage shear zones and overprinting sillimanite–biotite foliation seams continued to develop in the WKZ after generation of the pervasive Wrench Stage foliation and matrix assemblages developed in the CKZ. Thus Wrench Stage foliations and shear zones in the two halves of the orogen are not time equivalent. This implies that strike-slip shear zones and overprinting foliations that are characteristic of the Wrench Stage continued to form in the core of the orogen, at the same time as Convergence Stage overfolding and nappes operated in the CKZ.
Early M2 mineral parageneses in the Hoarusib Terrane have coarse polygonal granoblastic textures with only weak mineral alignments and rare aligned inclusion trails. Thus either early M2 metamorphism occurred in a low-strain environment or earlier deformational fabrics were annealed at the peak of metamorphism. At present, only the peak of early M2 metamorphism has been directly dated and the age of M2 reworking in the Hoarusib Terrane is unknown. Peak metamorphic garnet in the high-grade core of the WKZ is dated at 571 ± 6 Ma, which is coeval with the oldest U–Pb igneous zircon ages of 580 ± 3 and 576 ± 5 Ma (Electronic Appendix A), determined from granitic orthogneisses in the WKZ (Seth et al., 1998; Franz et al., 1999). Late-tectonic granites in the Ugab Zone (Fig. 1) have similar, but imprecise, ages of 570 ± 20 Ma (Miller & Burger, 1983) and 573 ± 33 Ma (Kröner, 1982), suggesting that the main Transpressional Phase of deformation occurred at the same time (580–570 Ma) throughout the length of the Kaoko Belt (Fig. 14).
WKZ granitic orthogneisses are synkinematic and partitioned variable degrees of strain, suggesting emplacement over a protracted period during transpressional orogenesis (Goscombe et al., 2002). This is consistent with the spectrum of U–Pb zircon and monazite ages (Fig. 14), ranging from 580–576 Ma granites coeval with the peak of metamorphism to younger granites of 552 Ma (Seth et al., 1998; Franz et al., 1999). However, four of the six younger ages from granites (i.e. 567–552 Ma) cannot be used to infer even the age of emplacement of the granite. Three are minimum ages derived by the single zircon evaporation technique (Seth et al., 1998) (Electronic Appendix A). Furthermore, a concordant U–Pb monazite age of 553·6 ± 1·4 Ma is reported to be from a post-kinematic granite (Franz et al., 1999), and therefore does not date the peak of metamorphism but does, nevertheless, give a minimum age constraint on the termination of the Transpressional Phase in the Kaoko Belt.
M2 reworking in the Hoarusib Terrane has not been directly dated, but must be older than 553·6 ± 1·4 Ma post-kinematic granite (Franz et al., 1999). The interval of time between formation of the early M2 (i.e. 580–570 Ma) and M2 mineral parageneses must therefore be <16–26 Myr. M2 metamorphic garnet growth was syntectonic with development of the pervasive matrix foliation in both the CKZ and Khumib Terrane, and has been directly dated at 574 ± 10 Ma and 579 ± 15 Ma, respectively. M2 reworking in the Hoarusib Terrane is not necessarily time equivalent to M2 parageneses in the CKZ and Khumib Terrane. However, the similar age of early M2 in the Hoarusib Terrane and M2 in the CKZ and Khumib Terrane suggests that M2 reworking in the Hoarusib Terrane cannot be significantly younger than the peak of early M2 metamorphism. Early M2 and M2 mineral parageneses are of similar sillimanite grade in the Hoarusib Terrane and reworking at sillimanite grade did not occur in the adjacent low-grade Coastal and Khumib Terranes. Thus the grade of M2 reworking was intimately linked with the grade of early M2 metamorphism in the respective terranes. Stated another way, early M2 and M2 share similar metamorphic field gradients decreasing towards the west across the WKZ, indicating that both formed in one progressive metamorphic cycle. Thus, early M2 and M2 constitute successive episodes in a single progressive Damara metamorphic cycle (M2) and cannot be interpreted as separate metamorphic events separated by a significant period of time.
Contemporaneous formation of peak metamorphic mineral parageneses in all metamorphic terranes forming the Kaoko Belt, with the exception of the outboard Coastal Terrane, is a significant finding and may be a universal aspect of transpressional belts. First, this finding suggests that metamorphism of disparate style and average thermal gradient operated at the same time across the belt. Peak metamorphism is correlated across the belt, and this is regardless of temporal relationships between deformation fabrics and peak metamorphic parageneses in different parts of the orogen, suggesting that apparently correlatable deformational fabrics are in fact diachronous. Pervasive fabrics associated with Wrench Stage tectonics persisted after the peak of metamorphism in the orogen core (WKZ) and Wrench Stage fabrics are associated with prograde and peak metamorphism in the crustal overriding margin (CKZ). Second, contemporaneous metamorphism across a transpressional orogen is in stark contrast to documented diachronous metamorphism across high-angle convergent orogens. For example, metamorphism in the overriding upper plate of the central Himalayan orogen occurred at 22 Ma (Hubbard & Harrison, 1989; Vannay & Hodges, 1996) and in the lower plate at 6–8 Ma (Harrison et al., 1997), possibly as a result of progressive foreland-vergent propagation of both the deformation and thermal front. Opposite vergence is illustrated by the Delamerian fold–thrust belt in South Australia with foreland-vergent thrusts, where the deformation and thermal front propagated progressively towards the hinterland (Webb et al., 2002). The findings from the Kaoko Belt suggest that peak metamorphism occurred at approximately the same time across all parts of this transpressional belt, without a consistent foreland or hinterland propagation of the deformation and metamorphic front being apparent. Furthermore, deformation has been shown to be diachronous across the Kaoko Belt, because Wrench Stage deformation is coeval with peak metamorphism in the CKZ but this style of deformation persists in shear zones subsequent to the peak of metamorphism in the WKZ.
K–Ar ages from mica schists in the CKZ and EKZ give cooling ages ranging from 442 ± 11 to 499 ± 11 Ma (Fig. 14; Ahrendt et al., 1983). K–Ar whole-rock ages from the Northern Platform (Fig. 1) are in the range of 560 ± 14, 535 ± 13 and 455 ± 13 Ma (Fig. 14; Clauer & Kröner, 1979). The Ugab Zone has K–Ar whole-rock ages of 490 ± 11 Ma and 418–430 Ma (Ahrendt et al., 1983) and Ar–Ar plateau ages in micas ranging from 512 ± 2 to 486 ± 3 Ma (D. Gray & D. Foster, unpublished data, 2001). Apatite fission-track ages from the Ugab Zone range from 81·0 ± 2·9 Ma to 109·8 ± 3·5 Ma (Brown et al., 1990). These cooling ages and peak metamorphic ages by direct dating of peak metamorphic garnet by Sm–Nd, as discussed above, document the cooling curves for different parts of the Kaoko Belt (Fig. 14). Initially moderate cooling rates of 5·4–2·1°C/Myr were experienced in the WKZ and CKZ, cooling from peak metamorphic conditions ranging from 843–534°C at 575 ± 15 Ma to 350°C (Dodson & McClelland-Brown, 1985) closure of K–Ar isotopic systems at 480 Ma (Ahrendt et al., 1983). There are currently no K–Ar or Ar–Ar isotopic data from anywhere in the WKZ, thus cooling rates are unconstrained. Nevertheless, cooling rates are predicted to be highest in the high-grade orogen core, where extrusional tectonics was responsible for exhumation of high-grade rocks along shallow trajectories from 8·1 kbar pressures to juxtaposition of all WKZ terranes at 4·4 kbar during the Transpressional Phase (M2).
Regional implications
The Sm–Nd garnet dates presented here are the first direct dating of the peak of metamorphism and associated Transpressional Phase of deformation in the Kaoko Belt. The 575 ± 15 Ma peak metamorphic age is the same in the core of the WKZ and CKZ and is also consistent with the oldest synkinematic granitoid ages of 580–576 Ma in the WKZ and 573–570 Ma in the Ugab Zone. This constraint on the age of metamorphism and Transpressional Phase deformation in the Kaoko Belt has important regional implications for the Damara Orogen as a whole. The main phase of sinistral transpressional deformation is similar in both coastal arms of the Damara Orogen, the Kaoko Belt in the north and Gariep Belt in the south. The Gariep Belt is interpreted to have experienced an early subduction and closure phase at 576–573 Ma (Frimmel & Frank, 1998) immediately followed by transpression and metamorphism, which culminated at 547–543 Ma (Frimmel, 1998; Frimmel & Frank, 1998).
Transpressional deformation on the western margin of the southern African continent occurred 55 Myr before the main phase of deformation by high-angle convergence in the Inland Branch of the Damara Orogen, as argued on structural grounds by Goscombe et al. (2002). The Shortening Phase of the Damara Orogeny occurred during collision and NNE–SSW convergence between the Congo and Kalahari Cratons (Coward, 1983; Miller, 1983), giving foreland-directed thrusting and overfolding on both margins of the Inland Branch (Coward, 1983; Miller, 1983). Shortening Phase deformation was minor in the Kaoko Belt and occurred subsequent to emplacement of the youngest granites (Goscombe et al., 2002), and thus must be less than 552 Ma in age (Seth et al., 1998; Franz et al., 1999). Shortening Phase deformation in the Ugab Zone is dated directly by a syenite pluton of 530 ± 3 Ma age (Seth et al., 2000) that was emplaced during a protracted period of north–south shortening (Goscombe et al., 2002). Peak metamorphism accompanying pervasive deformation in the Central Zone of the Inland Branch occurred between 535 and 510 Ma (Miller, 1983; Jung et al., 2000) and syntectonic Pb–Zn–Cu mineralization at Tsumeb in the Northern Platform gives Pb–Pb model ages from galena of 530 ± 11 Ma (Kamona et al., 1999). These age constraints and overprinting relationships in all zones of the Kaoko Belt (Goscombe et al., 2002) and elsewhere (Coward, 1981, 1983; Miller, 1983; Freyer & Hälbich, 1994; Frimmel, 1998) constrain north–south convergence in the Inland Branch to have occurred 55 Myr after transpressional orogenesis in the two coastal branches. Timing of north–south convergence between the Congo and Kalahari Cratons was remarkably consistent throughout the entire Pan-African Orogenic System (Fig. 1), from the Inland Branch of the Damara Orogen in the west through the southern Lufilian Arc and Zambezi Belts into the Mozambique Belt in the west (Goscombe et al., 1998, 2002).
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Supplementary data for this paper are available on Journal of Petrology online.
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Murray Hasseler and Bonza are sincerely thanked for their great company in the field. Cees Passchier, Rudolph Trouw, Paul Hoffman, Roy Miller and Charlie Hoffman are acknowledged for their encouragement and discussions in the field. This research resulted from work undertaken by Goscombe for the Namibian Geological Survey (1997–1999), private work (1997–2000) and a Monash University post-doctorate (2001). Funding for mineral analyses and fieldwork was sourced from the Namibian Geological Survey, private funding, ARC Discovery Grant awarded to D.G. (2001) and funds awarded to M.H. Adelaide University, Monash University and Peerce Hardware are sincerely thanked for their support during the writing up of this work. The work in Namibia was made possible by the administrative support of Mimi Duneski. The constructive reviews of Armin Zeh, Peter Crowley and Ed Ghent and encouraging editorial input by Kurt Bucher are gratefully acknowledged.
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REFERENCES |
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