Event geochronology of the Pan-African Kaoko Belt, Namibia

Transcription

1 Precambrian Research 140 (2005) 103.e1 103.e41 Event geochronology of the Pan-African Kaoko Belt, Namibia Ben Goscombe a,, David Gray a, Richard Armstrong b, David A. Foster c, James Vogl c a School of Earth Sciences, The University of Melbourne, Vic. 3010, Australia b School of Earth Sciences, Australian National University, Canberra, Australia c Department of Geological Sciences, P.O. Box , University of Florida, Gainesville, FL, USA Received 13 April 2005; received in revised form 28 June 2005; accepted 5 July 2005 Abstract Zircon and monazite U Pb dates, garnet Sm Nd dates and hornblende 40 Ar/ 39 Ar data from the transpressional Kaoko Belt of the late Neoproterozoic Pan-African Orogenic system confirm three distinct tectono-metamorphic cycles: M1 ( Ma), M2 ( Ma) and M3 ( Ma). The high-grade M1 metamorphic cycle and associated intrusive complexes are evident only within the westernmost Coastal Terrane. The isotopic data record a progressive and protracted history for the M2 metamorphic cycle that is initiated by collision and terrane docking, but with three distinct tectono-thermal periods including (1) peak metamorphic parageneses and voluminous granitoid emplacement at Ma, (2) overlapping whole-scale transpressional orogenesis and reworking dominated by crustal-scale shear zones, throughout the period Ma, and (3) cessation of transpressional strain before Ma, the age of late-kinematic pegmatite dykes that cross cut the major shear zones. M1 metamorphism of the exotic Coastal Terrane at 650 Ma must have occurred out-board from the Kaoko Belt passive margin, where M1 intrusives and metamorphic mineral parageneses have not been recognised. Accretion of the Coastal Terrane to the Kaoko Belt proper must have occurred between 645 Ma (M1) and 580 Ma, prior to the peak of M2 metamorphism accompanying transpressional orogenesis. Low-grade buckling of the Kaoko Belt, minor post-kinematic granite and pegmatite intrusions and post-metamorphic cooling occurred between 535 and 505 Ma during the M3 metamorphic cycle accompanying NNE SSW directed, high-angle convergence between the Congo and Kalahari Cratons Elsevier B.V. All rights reserved. Keywords: Pan-African orogeny; Transpression; Suture; U Pb monazite geochronology; U Pb zircon chronology; SHRIMP geochronology; Exotic terrane; Zircon provenance; Sm Nd garnet chronology; Ar Ar hornblende chronology Supplementary data associated with this article (Electronic Appendices A M) can be found, in the online version, at doi: /j.precamres Corresponding author at: Northern Territory Geological Survey, P.O. Box 8760, Alice Springs, NT 0871, Australia. Tel.: address: ben.goscombe@nt.gov.au (B. Goscombe). 1. Introduction The Kaoko Belt, Namibia, SW Africa (Fig. 1) is a classic example of a Neoproterozoic transpressional orogen (e.g. Dürr and Dingeldey, 1996; Passchier et al., 2002; Goscombe et al., 2003a, 2005; Konopásek et al., 2005; Goscombe and Gray, in review), linked by a triple /$ see front matter 2005 Elsevier B.V. All rights reserved. doi: /j.precamres

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3 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e e3 junction with the high-angle convergent Inland Branch of the Damara Orogen between the Congo and Kalahari Cratons (Coward, 1981; Miller, 1983; Porada, 1989). Previous geochronological investigations using U Pb zircon and monazite dating of granites and granitic orthognesisses, have delineated crust forming events within the basement domains, placed broad limits of Ma on the main phase of Kaoko Belt transpressional orogenesis, and recognised a second earlier high-grade event at Ma in the western most parts of the belt (Seth et al., 1998, 2000; Franz et al., 1999; Kröner et al., 2004). Sm Nd dating of peak metamorphic garnet porphyroblasts has shown that highgrade and low-grade domains within the Orogen Core, as well as in the obliquely convergent Escape Zone (Fig. 1), were found to have experienced peak metamorphism at 576 ± 15 Ma (Goscombe et al., 2003b). In this paper we attempt to (1) further define the timing of recognised deformational, metamorphic and intrusive events in the Kaoko Belt by using U Pb SHRIMP zircon and monazite geochronology from a range of partial melt segregations, granitoids and pegmatite dykes, (2) constrain both provenance of the Damara Sequence in the Kaoko Belt and the minimum and maximum ages of deposition utilising U Pb SHRIMP geochronology on detrital zircons from a meta-sediment sample interpreted to be from high in the stratigraphic succession, and (3) investigate the post-kinematic thermal evolution in different parts of the belt using Ar Ar hornblende samples representing the Kaoko Belt as a whole and Sm Nd garnet samples in the vicinity of post-kinematic pegmatite swarms. The samples for U Pb SHRIMP zircon and monazite geochronology were selected on the basis of overprinting relationships with respect to deformation fabrics, mesoscopic structures and metamorphic assemblages, in the context of an integrated deformation-thermo-barometric framework (e.g. Goscombe et al., 2005). Most of the 10 analysed zircon samples give meaningful and accurate concordia ages for zircon populations and two samples with discordant age spectra also gave concordant single zircon ages that are meaningful. All of the five analysed monazite samples, present concordant robust age determinations for the crystallization of the analysed igneous rocks. The new geochronology presented here, in conjunction with previous geochronological investigations (Seth et al., 1998; Franz et al., 1999; Goscombe et al., 2003b; Kröner et al., 2004), tightly constrains the temporal evolution of structural, metamorphic and intrusive events during this protracted Pan-African evolution of the Kaoko Belt. Geochronology from the NW Namibian region (Table 1) has been integrated with the results of detailed mapping and structural and metamorphic investigations (Goscombe et al., 2003a, 2003b, 2004, 2005; Goscombe and Gray, in review), to fully document the protracted Pan-African evolution of the Kaoko Belt. This event geochronology has been linked with the thermal and barometric evolution of different parts of the Kaoko Belt based on Goscombe et al. (2003a, 2003b, 2005) as well as with the spatial distribution of strain, metamorphism and magmatism (granitoid intrusion) during the three orogenic phases (M1, M2 and M3) based on Goscombe et al. (2005) and Goscombe and Gray (in review). 2. Background Previous geochronological investigations (see above) quantified the age of basement gneisses and granitoids, the age range of Pan-African granitoids and recognised two metamorphic cycles. Goscombe et al. (2003a, 2003b, 2005) and Goscombe and Gray (2005) gave a framework for the deformation and metamorphic evolution of the Kaoko Belt, which is temporally compatible with the southern Kaoko Belt, or Ugab Zone in the triple junction region, and the Fig. 1. Geological map of the Kaoko Belt simplified after Goscombe et al. (2005). Crustal-scale shear zone abbreviations in boxes: ST Sesfontain Thrust, PMZ Purros Mylonite Zone, TPMZ Three Palms Mylonite Zone and OMZ Ogden Mylonite Zone. Tectono-stratigraphic zone abbreviations (Miller, 1983): EKZ Eastern Kaoko Zone, CKZ Central Kaoko Zone and WKZ Western Kaoko Zone. Indicated samples are the new U Pb, 40 Ar 39 Ar and Sm Nd geochronology presented in this paper. Earlier Sm Nd samples (Goscombe et al., 2003b) are presented for reference (Table 1). The inset outlines the location of the Kaoko Belt branch of the Damara Orogen, in the regional context of the shaded Neoproterozoic-Cambrian Pan-African Orogenic System (Goscombe et al., 2000). Dashed boxes out line detailed maps of post-kinematic pegmatite swarms in Fig. 11.

4 103.e4 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e41 Table 1 Summary of Pan-African (Damara Orogeny) geochronological data from the Kaoko Belt and adjacent domains Location a Sample Stratigraphic unit Rock-type Method n Age (Ma) Interpretation b Reference c Coastal Terrane West Hoarusib corridor Nd149 Early Pan-African granitoid Granodioritic orthogneiss U Pb, upper intercept (zrn) [10] 730 ± 15 Crystallization age Kroner et al. (2004) West Khumb corridor Na00/15 Early Pan-African granitoid Granitic orthogneiss Pb 207 /Pb 206 SHRIMP, weighted mean (zrn) [5] 694 ± 9/703 ± 1 Crystallization age Kroner et al. (2004) West Hoarusib corridor Nd137 Early Pan-African granitoid Granitic orthogneiss U Pb, upper intercept (zrn) [5] 661 ± 21 M1 crystallization, Kroner et al. (2004) approximate age West Hoarusib corridor Na115 Early Pan-African granitoid Granitic orthogneiss U Pb, upper intercept (zrn) [7] 655 ± 39 M1 crystallization, approximate age Kroner et al. (2004) West Hoanib corridor Early Pan-African granitoid M1 granitic orthogneiss U Pb SHRIMP, concordia age (zrn) 656 ± 8 M1 crystallization age Seth et al. (1998) Mowe Bay DK342 Coastal Terrane Sequence Meta-pelite gneiss Pb 207 /Pb 206 intercept (zrn, mnz) 645 ± 3.5 M1 metamorphic age Franz et al. (1999) Southern coast region U252a Pan-African granitoid M2 granite sill Pb 207 /Pb 206 SHRIMP, weighted mean (zrn) 640 M1 inheritance, approximate age West Hoarusib corridor NK184c Early Pan-African granitoid M1 granite sill U Pb SHRIMP, concordia age (mnz) [12] ± 5.8 M1 crystallization age This paper West Hoanib corridor Early Pan-African granitoid M1 granitic orthogneiss U Pb SHRIMP (zrn) 630 ± 8 M1 crystallization, Seth et al. (1998) minimum age West Hoarusib corridor NK184c Early Pan-African granitoid M1 granite sill Pb 207 /Pb 206 SHRIMP, weighted mean (zrn) [12] 625 ± 15 M1 crystallization, minimum age West Hoarusib corridor NK184c Early Pan-African granitoid M1 granite sill U Pb SHRIMP, upper intercept (zrn) [17] 620 ± 18 M1 crystallization, minimum age West Khumib corridor KK87c Coastal Terrane Sequence Meta-pelite gneiss Sm Nd isochron (grt, WR) [2] 595 ± 13 Mixed M1 and M2 metamorphic age This paper This paper This paper Goscombe et al. (2003b) Mowe Bay DK344 Pan-African granitoid Granitic orthogneiss Pb 207 /Pb 206 intercept (zrn) 576 ± 5.3 M2 crystallization age Franz et al. (1999) Southern coast region U252a Pan-African granitoid M2 granite sill Pb 207 /Pb 206 SHRIMP, weighted mean (zrn) [30] 576 ± 11 M2 crystallization age This paper Southern coast region U253a Pan-African granitoid M2 syenogranite Pb 206 /U 238 SHRIMP, weighted mean (zrn) [30] 568 ± 5 M2 crystallization age This paper West Hoanib corridor BK43 Pan-African granitoid Granitoid orthogneiss U Pb SHRIMP & evaporation (zrn) 565 ± 13 M2 crystallization age Seth et al. (1998) Mowe Bay DK344 Pan-African granitoid Granitic orthogneiss U Pb forced upper intercept (zrn) ± 7.3 Late-M2 crystallization, minimum age West Khumib corridor Na118/2 Pan-African granitoid Granitic orthogneiss Pb 207 /Pb 206 evaporation (zrn) [3] 549 ± 4 Late-M2 crystallization, minimum age Southern coast region U254 Pan-African granitoid Post-kinematic peg. Pb 206 /U 238 SHRIMP, single concordant (zrn) [1] ± 5.3 M3 crystallization age This paper Southern coast region U254 Pan-African granitoid Post-kinematic peg. Pb 207 /Pb 206 SHRIMP, single concordant (mnz) [1] 531 ± 6 M3 crystallization age This paper Southern coast region U252b Pan-African granitoid Post-kinematic peg. U Pb SHRIMP, concordia age (mnz) [14] ± 4.9 M3 crystallization age This paper Franz et al. (1999) Kroner et al. (2004) Orogen Core, Hoarusib Domain Middle Hoanib corridor BK425 Pan-African granitoid Granitic orthogneiss U Pb SHRIMP, concordia age (zrn cores) 580 ± 3 M2 crystallization age Seth et al. (1998) Middle Hoanib corridor Pan-African granitoid Granitic orthogneiss U Pb SHRIMP, lower intercept (zrn) 578 ± 57 M2 crystallization, Seth et al. (1998) approximate age Middle Hoarusib corridor K1336b Damara Sequence Meta-pelite gneiss Sm Nd isochron (grt, WR) [2] ± 6.1 M2 metamorphic age Goscombe et al. (2003b) Middle Hoanib corridor BK24 Pan-African granitoid Orthogneiss Pb 207 /Pb 206 evaporation (zrn) [1] ± 1.5 M2 crystallization, minimum age Middle Hoanib corridor BK43? Pan-African granitoid Orthogneiss Pb 207 /Pb 206 evaporation (zrn) ± 1.5 M2 crystallization, minimum age Middle Hoanib corridor DK363 Pan-African granitoid Granitic orthogneiss U Pb upper intercept (mnz, zrn) 554 Late-M2 crystallization, approx. age Seth et al. (1998) Seth et al. (1998) Franz et al. (1999) Middle Hoanib corridor DK363 Pan-African granitoid Granitic orthogneiss U Pb concordia age (mnz) ± 1.2 late-m2 crystallization age Franz et al. (1999) Middle Hoanib corridor DK365 Pan-African granitoid Post-kinematic granite Pb 207 /U 235 concordia age (mnz) ± 1.4 Late-M2 crystallization age Franz et al. (1999) Middle Hoanib corridor BK19 Pan-African granitoid Granitic orthogneiss Pb 207 /Pb 206 evaporation (zrn) [1] ± 1.5 Late-M2 crystallization, minimum age West Hoarusib corridor Na00/14 Pan-African granitoid Granitic orthogneiss Pb 207 /Pb 206 evaporation (zrn) [3] 550 ± 1 Late-M2 crystallization, minimum age Seth et al. (1998) Kroner et al. (2004) Middle Hoarusib corridor KK43b Damara Sequence Late-M2 granite vein Pb 206 /U 238 SHRIMP, weighted mean (zrn) [14] ± 1.9 Late-M2 crystallization age This paper Middle Hoarusib corridor Na173 Pan-African granitoid Inter-boudin pegmatite U Pb upper intercept (zrn) [4] 539 ± 6 Late-M2 crystallization age Kroner et al. (2004) Middle Hoanib corridor NK189b Damara Sequence meta-pelite gneiss Sm Nd isochron (grt, WR) [2] ± 8 M3 Nd isotopic disturbance This paper

5 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e e5 Orogen Core, Khumib Domain Khumib corridor KK105f Damara Sequence Meta-pelite gneiss Sm Nd isochron (grt, WR) [2] 579 ± 15 M2 metamorphic age Goscombe et al. (2003b) Nadas corridor NK62a Damara Sequence Partial melt segregation Pb 206 /U 238 SHRIMP, single concordant (zrn) [1] ± 4.0 M2 crystallization age This paper Nadas corridor NK58 Damara Sequence Meta-pelite schist Sm Nd isochron (grt, grt-l, WR) [3] ± 4.7 M3 Nd isotopic disturbance This paper Orogen Core, Hartmann Domain Hartmann corridor NK91 Damara Sequence Meta-quartz arenite Pb 206 /U 238 SHRIMP age (zrn overgrowth) [1] ± 5.1 M2 metamorphic age This paper Hartmann corridor NK108b Pan-African granitoid Late-M2, pre-sz peg. U Pb SHRIMP, concordia age (zrn) [6] ± 4.9 Late-M2 crystallization age This paper Hartmann corridor NK108b Pan-African granitoid Late-M2, pre-sz peg. Pb 207 /Pb 20 SHRIMP, weighted mean (mnz) ± 5.8 Late-M2 crystallization age This paper Nadas corridor NK79g Damara Sequence Amphibolite gneiss Ar Ar, plateau age (hbl) ± 6.6 Post-M2 cooling through 500 C Hartmann corridor NK89a Damara Sequence Amphibolite gneiss Ar Ar, plateau age (hbl) ± 6.5 Post-M2 cooling through 500 C Hartmann corridor NK85a Pan-African granitoid Post-kinematic peg. U Pb SHRIMP, concordia age (mnz) [19] ± 5.3 M3 crystallization age This paper Hartmann corridor NK84a Damara Sequence Quartz-pelite mylonite Sm Nd isochron (grt, grt-l, WR) [3] ± 5.4 M3 Nd isotopic disturbance This paper This paper This paper Escape Zone Gomatum corridor K209 Damara Sequence Meta-pelite schist Sm Nd isochron (grt, WR) [2] ± 9.7 M2 metamorphic age Goscombe et al. (2003b) Gomatum corridor KK130 Damara Sequence Amphibolite schist Ar Ar, plateau age (hbl) 526 ± 4 Post-M2 cooling through 500 C Ugab Zone/Southern Kaoko Belt b Huab River mouth Pan-African granitoid Granite U Pb concordia age (zrn) 570 ± 20 M2 crystallization, approximate age Footspore granite Go-1 Pan-African granitoid Syenogranite Pb 207 /Pb 206 evaporation (zrn) [5] ± 3.2 M3 crystallization, minimum age Footspore granite Go-5 Pan-African granitoid Syenogranite Pb 207 /Pb 206 evaporation (zrn) [3] ± 3.2 M3 crystallization, minimum age Goantagab region U355a Damara Sequence Amphibolite schist Ar Ar, plateau age (hbl) ± 7.5 Post-M3 cooling through 500 C This paper Miller and Burger (1983) Seth et al. (2000) Seth et al. (2000) Northern Central Zone, Inland Branch of Damara Orogen b Oetmoed Complex Damara Sequence Migmatite Pb 207 /U 235 concordant (mnz) [2] Early-M3 melting age Jung et al. (2000b) Oetmoed Complex Pan-African granitoid Garnet-cordierite granite Pb 207 /U 235 near concordant (mnz) [2] Early-M3 crystallization, Jung et al. (2000a) min. age Oetmoed Complex Pan-African granitoid Garnet-cordierite granite Pb 207 /Pb 206 near concordant (mnz) [2] M3 crystallization, Jung et al. (2000a) minimum age Oetmoed Complex Damara Sequence Metapelite and migmatite Pb 207 /U 235 concordant (mnz) [2] M3 peak metamorphic age Jung et al. (2000b) Oetmoed Complex Damara Sequence Migmatite Sm Nd isochron (grt, WR) [2] 511 ± 11 M3 peak metamorphic age Jung et al. (2000b) Oetmoed Complex Damara Sequence Migmatite Sm Nd isochron (grt, WR) [2] 508 ± 6 M3 peak metamorphic age Jung et al. (2000b) This paper Oetmoed Complex Pan-African granitoid Hornblende granite Pb 207 /U 235 concordant (ttn) [2] Late-M3 crystallization age Jung et al. (2000a) Oetmoed Complex Pan-African granitoid Hornblende granite Pb 207 /U 235 concordant (mnz) [2] Late-M3 crystallization age Jung et al. (2000a) Oetmoed Complex Damara Sequence Migmatite Pb 207 /U 235 near concordant (mnz) [4] Late-M3 metamorphic age Jung et al. (2000b) Oetmoed Complex Damara Sequence Leucosome Sm Nd isochron (grt, WR) [2] 473 ± 3 Late-M3 metamorphic age Jung et al. (2000b) Oetmoed Complex Damara Sequence Leucosome Sm Nd isochron (grt, WR) [2] 473 ± 5 Late-M3 metamorphic age Jung et al. (2000b) Age data in each domain grouped by metamorphic cycle. Details and methodologies used to generate the new data presented in this table are outlined in detail in the text and appendices. a Corridor locations outlined in Goscombe et al. (2005). b Our interpretation of age data may differ to the original published source. Crystallization age indicates the crystallization of an igneous rock, not metamorphic crystallization, possibly close to intrusion age. c Only hornblende Ar Ar data is presented here, additional cooling ages and Rb Sr ages are summarized in Goscombe et al. (2003b, 2004).

6 103.e6 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e41 northern margin of the Inland Branch of the Damara Orogen (Passchier et al., 2002; Goscombe et al., 2004). The Neoproterozoic Damara Sequence was deposited on Congo Craton passive margin basement between 770 and 600 Ma (Miller, 1983; Prave, 1996; Hoffman et al., 1998). Protracted late-neoproterozoic to Cambrian deformational, metamorphic and magmatic events, collectively called the Damara Orogeny (Miller, 1983; Prave, 1996), reworked the Damara Sequence and underlying basement units during different juxtapositions of the Congo, Kalahari and Rio De La Plata Cratons. Crustal-scale shear zones divide the Kaoko belt into distinct domains and terranes that experienced contrasting kinematic and metamorphic response to transpressional orogenesis (Goscombe et al., 2003a, 2003b, 2005; Goscombe and Gray, in review). Shelf carbonates of the eastern foreland margin experienced moderate E W shortening at sub-greenschist metamorphic grade (Hoffman et al., 1998; Goscombe et al., 2003a). The foreland is separated from the highgrade, strike-slip, Orogen Core by a broad Escape Zone containing an inverted Barrovian-style metamorphic sequence (Fig. 1). Both Damara Sequence and a complex basement mosaic (Miller, 1983; Seth et al., 1998; Kröner et al., 2004; Konopásek et al., 2005) were intensely reworked in large-scale, east-vergent nappes (Guj, 1970; Dürr and Dingeldey, 1996; Goscombe et al., 2003a, 2003b; Goscombe and Gray, in review). The Orogen Core is bounded by the oblique-reverse Purros Mylonite Zone on the east, and obliqueextensional Three Palms Mylonite Zone on the west (Goscombe et al., 2005). The Orogen Core contains high-grade Hartmann and Hoarusib Domains in the north and south, respectively and the lowergrade turbiditic Khumib Domain in the centre (Fig. 1; Goscombe et al., 2003b, 2005). The Three Palms Mylonite Zone marks a suture between the exotic Coastal Terrane and passive margin of the Congo Craton with cover Damara Sequences. Coastal Terrane Sequences of meta-greywacke were intruded and metamorphosed at upper-amphibolite grade, prior to being extensively reworked and down graded during docking and transpressional orogenesis (Goscombe et al., 2005; Goscombe and Gray, in review). The Neoproterozoic evolution of this region includes early histories within two separated terranes. (1) The exotic Coastal Terrane with possible arc affinities (Goscombe et al., 2005; Masberg et al., 2005), and (2) the remainder of the Kaoko Belt consisting of Damara Sequences deposited on a complex reworked passive margin, of Mesoproterozoic, Palaeproterozoic and Archaean basement (Seth et al., 1998; Kröner et al., 2004). Subsequent to suturing of these two terranes along the Tree Palms Mylonite Zone during oblique collision between the Rio De La Plata and Congo Cratons, both terranes experienced ongoing transpressional orogenesis that was terminated by high-angle convergence between the Congo and Kalahari Cratons. Three successive, but temporally unrelated, Pan-African tectono-metamorphic cycles that formed in entirely distinct crustal stress regimes, have been recognised and defined by Goscombe et al. (2003a, 2003b, 2005); M1 has been labelled the Thermal Phase ( Ma), M2 the Transpressional Phase ( Ma) and M3 the Shortening Phase ( Ma). Thermal Phase metamorphism and intrusions are only recognised in the once outboard Coastal Terrane. The Transpressional Phase was responsible for granite intrusion, the main phase deformation of the Kaoko Belt proper, reworking of the Coastal Terrane and the crustal-scale architecture of the entire belt. Transpressional orogenesis progressed in style from the Wrench Stage which produced the regionally pervasive S-L fabrics and long-lived shear zones, to Convergence Stage fold and nappe structures produced by the contractional component of transpression (Goscombe et al., 2003a; Goscombe and Gray, in review). The Shortening Phase in the Kaoko Belt involved buckling of earlier transpressional fabrics during convergence between the Congo and Kalahari Cratons (Goscombe et al., 2003a). 3. U Pb SHRIMP geochronology 3.1. Samples Basement in the Hoarusib Domain Sample KK51d ( E, S) is partial melt segregations of 1 2 cm width, separated from coarse-grained meta-pelitic gneiss of Palaeoproterozoic basement within the high-grade Hoarusib Domain. These partial melt segregations are stromatic and define the earliest planar fabric in the outcrop, parallel to

7 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e e7 compositional layering. These segregations are sheared by the regionally pervasive main foliation developed during the Wrench Stage of deformation in the Transpressional Phase Coastal Terrane sequence and intrusives Sample NK184c ( E, S) from the Coastal Terrane Sequence is a 25 cm wide granitic partial melt segregation within M1 upper-amphibolite facies meta-greywacke gneiss. This sample has been moderately reworked at mid-amphibolite facies grades during the Transpressional Phase. Samples U252a ( E, S), U252b ( E, S), U253a ( E, S) and U254 ( E, S) are all intrusives of the Torra Baai granitoids (Miller and Grote, 1988) from the southern most extremity of the Coastal Terrane in the Kaoko Belt, 90 km north of the Ogden Mylonite Zone (Fig. 1). Sample U252a is a 2 m thick granitic sill that post-dates early gneissic fabrics and is also sheared during development of the pervasive transpressional foliation. Sample U252b is a late-stage, discordant and undeformed pegmatite. Sample U253a is a medium-grained syeno-granite body that has been moderately sheared during transpressional reworking. Sample U254 is an undeformed, coarse-grained K- feldspar-rich pegmatitic sill that cross cuts all earlier partial melt segregations and granite phases and all deformation fabrics Damara sequence and intrusives in the Hartmann Domain Sample NK91 ( E, S) is a quartz-rich meta-arenite of the Hartmann Group in the Hartmann Domain (Goscombe and Gray, in review). Sample NK108b ( E, S) in the Hartmann Domain is a weakly deformed pegmatite sill (80 cm wide) within the Khomas Subgroup. This pegmatite sill cross cuts the regionally pervasive main foliation and pre-dates shearing in the steeper crustal-scale shear zones. Sample NK85a ( E, S) is the latest generation of biotite-muscovite pegmatite dykes in the Hartmann Domain and intrudes the Khomas Subgroup. This coarse-grained pegmatite is entirely undeformed and discordant across both the regionally pervasive main foliation and the steeper crustal-scale shear zones Damara sequence and intrusive in the Khumib Domain Sample NK62a ( E, S) is an early-m2, garnet-bearing leucocratic partial melt segregation of 3 5 cm width. This leucosome is from amphibolite facies schistose gneiss after a metapelite layer in the turbiditic Khomas Subgroup within the Khumib Domain. The partial melt segregations are stromatic, parallel to compositional layering and sheared and boudinaged by the regionally pervasive main foliation developed during the Wrench Stage of deformation. Sample NK62b ( E, S) is an undeformed coarse-grained, K- feldspar-rich pegmatite sill (30 50 cm wide) within the Khomas Subgroup. This pegmatite sill cross cuts all earlier partial melt segregations, pegmatite sill generations and all deformation fabrics. However, no zircons worth analysing were extracted from this rock Damara sequence in the Hoarusib Domain Sample KK43b ( E, S) is a late-stage Transpressional Phase coarse-grained pegmatitic partial melt segregation in meta-pelitic gneiss of the Khomas Subgroup within the high-grade Hoarusib Domain. This pegmatitic segregation cross cuts the pervasive Wrench Stage fabric and is axial planar to a tight Convergence Stage fold Results Analytical procedures are contained in Electronic Appendix A and all analytical results are contained in Electronic Appendices A K Hoarusib Domain The zircons from partial melt in Damara Sequence sample KK43b are large (up to 600 m in length), generally acicular and euhedral. Cathodoluminescence imaging shows well developed oscillatory-zoned margins surrounding either sector-zoned or unzoned interiors. Some highly acicular grains (L:B aspect ratios of 10:1) have strongly altered interiors and possible remnant quenched melt inclusions (Fig. 2a). Small cores of zoned zircon are present in some of these zircons, and a few were analysed in this study. These zircons produced some excellent results (Electronic Appendix A; Fig. 3a) with the general magmatic population yielding mainly concordant data. A weighted mean

8 103.e8 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e41 Fig. 2. (a) Cathodoluminescence image of a selection of zircons from sample KK43b. The numbers refer to the analyses in Electronic Appendix A, and the ellipses mark the spots analysed by SHRIMP. (b) Cathodoluminescence image of a random selection of detrital grains from the meta-quartzite NK91. The SHRIMP spot sites are marked by the ellipses and the numbering refers to the grains as listed in Electronic Appendix K. The rim #6.1 is the youngest measured spot at 572 Ma. This appears to be metamorphic and thus provides a minimum age of deposition. 206 Pb/ 238 U age of ± 1.9 Ma [MSWD = 1.10; probability = 0.44] was calculated from the fourteen analyses plotting as a group on concordia. One analysis (#14.1) was highly discordant. Two analyses of xenocrystic zircon cores give minimum 207 Pb/ 206 Pb ages of 1628 ± 62 Ma and 1416 ± 10 Ma. Basement sample KK51d has a very complex population of zircon with at least two, and possibly three generations of growth observable from the cathodoluminescence images. They comprise sectoror oscillatory-zoned (magmatic) cores with over- Fig. 3. (a) Conventional Wetherill U Pb concordia plot of SHRIMP data for zircons from the sample KK43b, showing all data. The analyses 3.2 and 5.1 are of inherited cores. Inset is an enlarged view of the data for the igneous population of zircons indicated by grey ellipses. (b) U Pb concordia plot of SHRIMP data for zircons from the basement sample KK51d. The black error ellipses represent analyses of high-u, low-th/u rims and the grey error ellipses represent the cores. Analysis #3.1 is a concordant rim age and may represent a basement metamorphic age. growths or embayments of dark-cl, high-u and low Th/U zircon. These overgrowths appear to of metamorphic origin. Rare examples of a possible second generation of overgrowth are also present. It is clear from the plotted data (Fig. 3b) that the complexity noted above are compounded by the effects of Pb-loss, mostly within the dark-cathodoluminescence, high U overgrowths. A zircon rim (spot #3.1) has a concordant Pb 207 /Pb 206 age of 1592 ± 4.6 (Electronic Appendix B) and this could represent a Neoproterozoic basement metamorphic age. An additional near concordant age of ± 3.8 Ma is also from a zircon rim. The cores tend to have lower uranium and thorium

9 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e e9 data show a large range in discordance directly related to the U and Th contents (Electronic Appendix C). Nevertheless, a number of analyses (n = 6) do plot as a group on concordia and yield a Concordia Age of ± 4.9 Ma. Good quality monazite grains were found in the heavy mineral separate and were also analysed. These monazites have exceptionally high Th and U contents and the data show some spread consistent with Pb-loss (Electronic Appendix C). Apart from one analysis (#23.1), however, a weighted mean 207 Pb/ 206 Pb age of ± 5.8 Ma can be calculated (Fig. 4b). This is just slightly younger than the age calculated from the zircons in the same rock. Late-stage pegmatite sample NK85a within the Khomas Subgroup of the Hartmann Domain, has few very poorly preserved, high uranium zircons. Good quality clear, light yellow monazites were however also separated and these were used for the SHRIMP analyses. Nineteen analyses on different grains were carried out and all data conform to a single age population with a Concordia Age of ± 5.3 Ma (95% confidence limits). The data are plotted on a conventional concordia diagram (Fig. 5a) and are listed in Electronic Appendix D. Fig. 4. (a) U Pb concordia plot of SHRIMP data for zircons from the sample NK108b. The grey error ellipses represent the analyses defining the concordia age with 95% confidence limits. (b) U Pb concordia plot of SHRIMP data for monazites from the sample NK108b. concentrations and are consequently less metamict and more concordant. A weighted mean 207 Pb/ 206 Pb age calculated from eleven of the more concordant data (i.e. those <10% discordant) from xenocrystic cores give 1768 ± 8 Ma [MSWD = 1.3; probability = 0.20] Hartmann Domain Zircons from pegmatite sill sample NK108b in the Khomas Subgroup, of the Hartmann Domain, are quite variable in shape and size. These range from rounded, equi-dimensional types with broad sector zoning to more elongate forms with well-developed oscillatory zoning. Many have a dark-cathodoluminescence rim or overgrowth with high uranium content. Unfortunately it is impossible to date these rims as they have extreme U contents and are metamict and too discordant. This is shown in a concordia plot of the data (Fig. 4a) and the Khumib Domain Sample NK62a of partial melt segregations within the Khomas Subgroup of the Khumib Domain, yielded many zircons that were unfortunately extremely metamict and poorly preserved, making them unsuitable for geochronology. Using cathodoluminescence and both transmitted and reflected microphotographs small least altered areas within the grains were identified and some analyses made. As suspected, most of these are highly discordant and contain unacceptably high levels of common Pb, with up to 63% of the 206 Pb measured in one spot is common 206 Pb (Electronic Appendix E). Even in these selected areas the zircons contain up to 9000 ppm uranium. Determining an age for this partial melt segregation is obviously complicated by the poor quality of the zircons, but a regression of all the data (apart form the two points 9.1 and 10.1 which had too much common Pb to calculate a meaningful 207 Pb/ 206 Pb composition) gives an upper-intercept date of 547 ± 25 Ma (Fig. 5b). This upper-intercept age is not considered reliable after the arguments of Mezger and Krogstad (1997). The best estimate of the age of this partial

10 103.e10 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e41 Fig. 5. (a) U Pb concordia plot of SHRIMP data for monazites from the sample NK85a. (b) U Pb concordia plot of SHRIMP data for zircons from the sample NK62a. The white error ellipses represent the analyses with extremely high common Pb contents. melt segregation is given by the single, near-concordant ( 4% discordance) 206 Pb/ 238 U age determination of ± 4.0 Ma (Fig. 5b; Table 1) Coastal Terrane Early granitic partial melt segregation sample NK184c in the Coastal Terrane contains euhedral zircons with sector-zoned central areas grading to oscillatory zoned margins where uranium concentrations are high (and spots located in these areas are severely discordant). Cores are common, but it is difficult to unequivocally identify all these cores as older, inherited components or if they are simply an earlier phase of magmatic growth. Analysis shows that the latter scenario to be more common. Most analyses are discordant, with some of the higher uranium areas showing up to 58% discordance. Many of these discordant analyses also have very high common Pb contents (Elec- Fig. 6. (a) Conventional Wetherill U Pb concordia plot of SHRIMP data for zircons from the sample NK184C, showing all the data. The least discordant analyses indicated by grey ellipses are used to calculate a weighted mean 207 Pb/ 206 Pb age. The analyses #4.2 and #6.1 represented by black error ellipses are from cores. The white error ellipses show data not included in the calculated mean 207 Pb/ 206 Pb age. (b) Monazite U Pb data for sample NK184C. tronic Appendix F), an uncertainty factor which when propagated through, result in the large error ellipses shown in Fig. 6a. Calculating an age from this set of data is limited to the use of the calculated upper intercept from regression analysis, or to the calculation of a weighted mean 207 Pb/ 206 Pb age from the least discordant analyses. The upper intercept age calculated from all the data (apart from the two identified cores) is 620 ± 18 Ma (MSWD = 0.47; probability of fit = 0.96). These data are shown in the concordia plot in Fig. 6a. A more reliable age is calculated from the weighted mean 207 Pb/ 206 Pb ages from the 12 least discordant analyses shown in Fig. 6a. This age is 625 ± 15 Ma (MSWD = 0.20; probability of fit = 0.998). Of the two cores analysed, one is highly discordant (#6.1) and the

11 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e e11 other (#4.2) gives an inaccurate 207 Pb/ 206 Pb age of 755 ± 89 Ma. Monazites from this sample were also analysed in this study. The data show much less discordance than the zircon data and are plotted in Fig. 6b and listed in Electronic Appendix F. Twelve of the 14 analyses plot as a group on concordia and give a Concordia Age of ± 5.8 Ma. Although within error of the zircon 207 Pb/ 206 Pb age for the same rock, the higher precision and the overall concordance of the data set suggests this is a more reliable age for this rock. The Th/Pb age calculated from this same data set gives a comparable age but with a large scatter (Electronic Appendix F) Southern Coastal Terrane granitoids Sample U252a is an early granitoid sill that predates transpressional reworking in the southernmost Coastal Terrane exposures. This sample produced many zircons that unfortunately are complicated by inheritance and possibly also alteration. The magmatic zircon is strongly zoned with generally high uranium contents and many of the spots located in these areas are highly discordant and high in common Pb (Electronic Appendix G). Cores are generally seen as bright cathodoluminescence areas within the zircons, but some discrete zircons with low uranium are also interpreted to be xenocrysts. Determining an age for this complex population of zircons is difficult and subject to the correct characterisation of the zircon. Over 30 analyses were done to try and unravel the complexities but due to the Pb-loss only a 207 Pb/ 206 Pb age is attempted. The best estimate of the age is given by the weighted mean 207 Pb/ 206 Pb age of 576 ± 11 Ma on the more concordant zircons as shown in Fig. 7a. Age of inheritance ranges between a single Archaean grain to a group of zircons that are apparently not much older than the magmatic population at about 640 Ma (Electronic Appendix G). Sample U252b is a late-stage and undeformed pegmatite that yielded both zircons and monazites. The zircons are dark, anhedral, pitted and show the effects of extreme alteration and metamictisation. This is a consequence of the very high uranium concentrations (up to a percent in one analysis) found in these minerals (Electronic Appendix H). Three reconnaissance analyses were done on selected areas within grains, which appeared to be the least affected. These data show unac- Fig. 7. (a) U Pb concordia plot of SHRIMP data for zircons from sample U252a. The least discordant analyses indicated by grey ellipses are used to calculate a weighted mean 207 Pb/ 206 Pb age. (b) A Tera-Wasserburg U Pb concordia plot of SHRIMP data for monazites from the sample U252b. The data are plotted after correction for common Pb. ceptably high discordance (Electronic Appendix H) and it was clear that even on small areas analysed the severe alteration could not be avoided. The monazite data show a more coherent picture with all fourteen points analysed plotting on concordia and giving an age of ± 4.9 Ma (Fig. 7b; Electronic Appendix H). This is interpreted to be the best estimate of the age of this pegmatite. Sample U253a is a sheared syeno-granite. Zircons from this sample are subhedral to euhedral and somewhat variable in shape from squat almost equidimensional forms to more acicular forms. Most grains show well-developed oscillatory zoning and rare sector zoning in the interiors. Rare cores are present. A total of thirty analyses were done on this sample, with the results presented in Electronic Appendix I

12 103.e12 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e41 Fig. 9. A U Pb concordia plot of all the SHRIMP analyses of detrital grains from sample NK91. The analysis of the young metamorphic overgrowth (#6.1) is shown. The inset is a cumulative probability plot of preferred age analyses, which are less than 10% discordant. Fig. 8. (a) U Pb concordia plot of SHRIMP data for zircons from the sample U253a. The white error ellipses are excluded from the age calculation shown. (b) U Pb concordia plot of SHRIMP data for magmatic zircons (shaded error ellipses) and older inherited zircons (black error ellipses) from the sample U254. An older analysis #13.1 plots off the diagram (Electronic Appendix J). and plotted on a concordia diagram in Fig. 8a. The majority of the data points plot as a concordant group yielding a weighted mean 206 Pb/ 238 U age of 568 ± 5 Ma (MSWD = 1.17; probability = 0.27). Apart from two analyses, all the identified outliers grains have suffered variable Pb-loss. The other two outliers have older apparent 206 Pb/ 238 U ages and could be inherited or xenocrystic components. As with the other post-kinematic pegmatite samples, the zircons separated from sample U254 have extreme U contents (and high Th in some cases), leading to advanced metamictisation and alteration. The occasional very high common Pb contents are a directly measurable consequence of this alteration (Electronic Appendix J). Some inheritance was found both as cores (e.g. grain #1) or as rare discrete grains of low-u zir- con (e.g. grain #4). For the analyses the best possible areas were chosen based on the cathodoluminescence and other images, hoping to avoid damaged or altered zones. Even with this approach it was impossible to avoid the highly discordant areas, making it difficult to determine a precise age for this rock (Fig. 8b). Regression analysis of all 16 data points from the magmatic, zoned zircons gives an inaccurate upperintercept age of 555 ± 23 Ma (MSWD = 0.58; probability = 0.88) that is not considered reliable (Mezger and Krogstad, 1997). The best estimate of the age of this post-kinematic pegmatite is given by the single, concordant (0% discordance) 206 Pb/ 238 U age determination of ± 5.3 Ma (Fig. 8b; Table 1). A single 207 Pb/ 206 Pb analysis of a monazite from this separate gives an identical age of 531 ± 6 Ma (1-sigma) (Table 1). The age of zircon inheritance is variable, with the oldest at about 1600 Ma, and the remainder giving apparent Neoproterozoic ages (Electronic Appendix J) Hartmann Domain detrital zircons The zircons from meta-sediment sample NK91 of the Hartmann Group, are typically rounded and preserve percussion and abrasion marks suffered during the erosion, transportation and deposition processes. Cathodoluminescence imaging (Fig. 2b) shows many of the zircons are structured with cores and overgrowths and all represent pre-erosion geological histories except for a single grain (grain #6). A total of 68 analyses were performed on 59 different zircons, with the data presented in the form of a concordia plot

13 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e e13 and a cumulative probability plot (Fig. 9). The data are listed in Electronic Appendix K. These detrital zircons show a wide range of ages, suggesting a diverse source. The oldest grain gives a minimum 207 Pb/ 206 Pb age of 2509 ± 11 Ma and the youngest (a rim) gives a 206 Pb/ 238 U (apparent) age of ± 5.1 Ma. Between these extremes there are several groupings of data, indicating the main provenance areas. The age groupings of 1950, 1230, 1760, 1440 and 1005 Ma are given in decreasing relative abundance and are based on analyses that are less than 10% discordant. Zircons giving Mesoproterozoic ages between 1000 and 1230 Ma, are definitely detrital and indicate a maximum age constraint on sedimentation. Further constraints on the age of deposition depend on the interpretation of the origin of the 572 Ma zircon rim found on grain #6. This grain has a 1000 Ma core overgrown by a dark cathodoluminescence, low Th and Th/U rim (Fig. 2b). This zircon is interpreted to be a post-depositional phase of metamorphic zircon growth and thus provides a minimum age of deposition of these sediments Ar/ 39 Ar thermochronology 4.1. Samples Four amphibolite samples were selected from along the length of the Kaoko Belt (Fig. 1) for 40 Ar/ 39 Ar analysis, to constrain timing of hightemperature ( C) cooling of the belt. Samples NK89a ( E, S) and NK79g ( E, S) are from the Hartmann Domain; NK89a from the Khomas Subgroup in the northern-most exposures and NK79g from the Hartmann Group to the south. Sample NK79g is mediumgrained amphibolite gneiss with moderate L-S fabric defined by aligned hornblende laths. The matrix is a polygonal granoblastic assemblage of hornblendeclinopyroxene-plagioclase-quartz-ilmenite in textural equilibrium with small garnet porphyroblasts (<2 mm diameter). Sample NK89a is a foliated amphibolite with a polygonal matrix of aligned green hornblende, plagioclase, ilmenite and titanite. Sample KK130 ( E, S) is from the Khomas Subgroup in the western part of the Barrovian-style Escape Zone in the middle of the belt (Fig. 1). Sample U355a ( E, S) is from the Khomas Subgroup in the eastern Ugab Zone (Fig. 1). This sample is in the outer margin of a contact aureole, 3 km distant from a post-transpressional Phase granite. This sample is fine-grained mafic schist with strongly aligned matrix of green hornblende, epidote, plagioclase and quartz, with scattered coarse hornblende laths Results Analytical procedures and all analytical results are contained in Electronic Appendix L. Hornblende from sample NK79g gives a plateau age of ± 6.6 Ma (2-sigma) for steps comprising 91% of the gas released (Fig. 10). Sample NK89a yields a relatively flat age spectra with four steps comprising 93% of the gas released giving a mean age of ± 6.5 Ma (2- sigma) (Fig. 10). These two steps, however, are separated by one small step giving an older age and different K/Ca ratio suggesting it is due to degassing of a K-rich inclusion. Hornblende from sample U355a gave a relatively discordant age spectra with a total fusion age of ± 8.1 Ma, which we take as the best estimate of the cooling age for this sample. The age spectra for hornblende from KK130 gives an age gradient from 0 to about 15% 39 Ar released. The age gradient is followed by eight steps, comprising 81% of the gas released, which give a plateau age of 526 ± 4 Ma. Inverse isochrons for the steps used to calculate the plateau or average ages give trapped 40 Ar/ 36 Ar ratios within error of atmosphere. These four hornblende ages are interpreted to record cooling of their respective samples below C, depending on cooling rate (McDougall and Harrison, 1999). 5. Sm Nd geochronology 5.1. Samples and previous results Three metapelite samples with 3 10 mm diameter garnet porphyroblasts, one each from the southern Khumib Domain, Escape Zone and northern Hoarusib Domain, all from the Khomas Subgroup of the Damara Sequence, have been previously dated using Sm Nd two and three point isochrons (Goscombe et al., 2003b). All three results were within error, centred on 576 ± 15 Ma and are the only direct dating of M2 metamorphic parageneses in the Kaoko Belt (Table 1).

14 103.e14 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e41 Fig Ar/ 39 Ar age spectra for hornblendes from the Kaoko Belt (Electronic Appendix L). For sample locations see Fig. 1. (a) Sample NK79g from the south Hartmann Domain. (b) Sample NK89a from the north Hartmann Domain. (c) Sample U355a from the east Ugab Domain. (d) Sample KK130 from the Central Kaoko Zone or Escape Zone. A garnet-bearing meta-greywacke from the Coastal Terrane gave a result of 595 ± 13 Ma. This sample preserves two stages of garnet growth; relict homogeneous M1 garnet cores and thin resorbed margins that re-equilibrated with the matrix foliation during transpressional reworking (Goscombe et al., 2003b, 2005). The thin, re-equilibration rims constitute 38 58% of the total garnet volume. Consequently, Sm Nd analysis of the whole garnet fraction is dating near equal portions of two periods of garnet growth, most plausibly M1 growth at 650 Ma and M2 growth during transpressional reworking at 570 Ma, resulting in the mixed age of 595 Ma (Table 1). Three additional samples have been dated in this study to extend the dataset to the northern and southernmost domains in the Orogen Core, and to test for the thermal effects associated with post-kinematic pegmatite swarms within these domains Sample NK84a Sample NK84a ( E, S) is a quartz-rich, migmatized metapelite of upperamphibolite grade from the central Hartmann valley in the Hartmann Domain (Fig. 1). This sample contains mm diameter garnets, and plagioclase and K-feldspar porphyroclasts within a strongly foliated matrix of biotite and aggregate ribbons of K-feldspar, plagioclase and quartz. The matrix assemblage is in textural equilibrium with garnet within pressure shadows and otherwise the Wrench-Stage matrix foliation envelops the rounded garnet porphyroclasts (Fig. 11e). Garnets show partially homogenized compositional patterns with flat cores and rims slightly enriched in Fe 2+ and are poor in Mg (Goscombe et al., 2005). The sample site is within a swarm of 2 4 m wide, NEtrending, post-kinematic muscovite-pegmatite dykes comprising 5% of exposure (Fig. 11b and d). The nearest pegmatite dyke is 10 m distant from the sample site Sample NK58 Sample NK58 ( E, S) is metamorphosed semi-pelite schist of mid-amphibolite facies grade from the Nadas River valley in the northern Khumib Domain (Fig. 1). The sample has mm

15 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e e15 Fig. 11. (a) New Sm Nd isotopic data from metapelite samples within three post-kinematic pegmatite swarms in the Kaoko Belt. Analytical data is contained in Electronic Appendix M. Samples are located in Fig. 1. (b and c) Foliation trace maps of the sampling regions with post-kinematic pegmatite and granite intrusions plotted in black. Post-kinematic intrusions are mapped by the authors from field observations, areal photographs and landsat images, as well as published maps (Guj, 1970; Schreiber, 2002). Stars Sm Nd sample sites. Ar Ar samples NK89a and NK79g are indicated for comparison. (d) Lower hemisphere, equal-area stereonets of post-kinematic pegmatite and microgranite dykes in the vicinity of the three sampling sites. (e) Microphotographs in plane polarized light, illustrating representative examples of the garnets analysed in each sample. diameter (typically <0.6 mm), sub-idioblastic garnet porphyroblasts that are in textural equilibrium with the schistose matrix (Fig. 11e). The matrix assemblage is quartz-oligoclase-biotite-garnet-ilmenite and the sample contains late-stage muscovite and retrograde chlorite. Garnets show typical growth zoning compositional patterns with very thin resorbed margins that are slightly enriched in Mn (Goscombe et al., 2005). These garnets are syn-kinematic, as they both overgrow and are enveloped by the penetrative Wrench-Stage foliation. This region contains numerous E- and N-trending, post-kinematic dykes of

16 103.e16 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e41 tourmaline-muscovite ± garnet pegmatite and microgranite (Fig. 11b and d). These range from 5 40 cm up to m in width, constitute 5 10% of exposure in the Nadas River valley region and the nearest is 4 m distant from the sample site (Fig. 11b and d). Random, coarse-grained, muscovite flakes that cross cut both the main foliation and all crenulation cleavages, are common in metapelites in this region Sample NK189b Sample NK189b ( E, S) is coarse-grained, high-grade migmatized metapelitic gneiss from the western Hoanib River in the southern Hoarusib Domain. The coarse, polygonal granoblastic matrix assemblage of quartz-biotite-plagioclasegarnet-k-feldspar is only weakly foliated and contains rare late-stage muscovite. Round garnet porphyroblasts are up to 12 mm in diameter and very poikiloblastic with large quartz, feldspar and biotite inclusions. These sieve-like garnet porphyroblasts are a spaced network of garnet sub-grains of mm diameter (Fig. 11e). The sampling site contains numerous E-trending, post-kinematic, muscovitetourmaline ± garnet-bearing pegmatite and microgranite veins and dykes of cm width (Fig. 11c and d). Two kilometres to the south is an extensive area (22 km 15 km) pervaded by the Uhima swarm of large, post-kinematic pegmatitic leucogranite bodies (Guj, 1970) that comprise approximately 20% of areal exposure (Fig. 11c) Results Results of Sm Nd isotopic analysis of these new samples are contained in Electronic Appendix L. All samples give well constrained, two or three points isochron ages (Fig. 11a) of ± 5.4 Ma in the Hartmann Domain (NK84a), ± 4.7 Ma in the north Khumib Domain (NK58) and ± 8Ma in the Hoarusib Domain (NK189b). All analysed garnet fractions have Nd/Sm ratios <1, indicating that they are relatively HREE-enriched, which is typical of garnet (Getty et al., 1993; Baxter-Ethan et al., 2002). Consequently, these garnets are not contaminated by LREE-rich micro-inclusions (Zhou and Hensen, 1995). All three results are remarkably similar and average 505 ± 8 Ma which is approximately 70 Ma younger than the 576 ± 15 Ma age of peak metamorphic garnet porphyroblasts previously dated from elsewhere in the Kaoko Belt (Goscombe et al., 2003b). All the new Sm Nd dated samples contain a single phase of peak metamorphic garnet growth associated with the development of the regionally pervasive and penetrative Wrench-Stage matrix foliation. All samples are distant from the crustal-scale shear zones and do not develop secondary foliations or structures. No secondary garnet growth is evident and the samples are not retrogressed, nor develop secondary mineral parageneses. Consequently, these age determinations have been derived from peak metamorphic garnets and cannot be correlated with any younger fabric or mineral parageneses. Thus these ages are anomalous, being incompatible with both the age of peak metamorphism in the Kaoko Belt ( Ma) and the age range of transpressional orogenesis in the Kaoko Belt between 580 and 550 Ma (Seth et al., 1998; Franz et al., 1999; Goscombe et al., 2003a, 2003b; Kröner et al., 2004). The age range of transpressional orogenesis and the peak of metamorphism is well constrained by other Sm Nd samples (Goscombe et al., 2003b) and many U Pb monazite and zircon ages from throughout the Kaoko Belt, including samples closely associated in the same domains (Table 1; Seth et al., 1998, Franz et al., 1999; Kröner et al., 2004). The anomalously young Sm Nd ages come from typical peak metamorphic mineral parageneses (Fig. 11e). Consequently, these ages must have been isotopically disturbed subsequent to the formation of the peak metamorphic mineral assemblages. Furthermore, this isotopic disturbance event was subsequent to cessation of all transpressional orogenesis in the Kaoko Belt, dated independently at >530 Ma by post-kinematic pegmatites (Table 1), which cross cut peak metamorphic mineral parageneses and all deformational fabrics. Garnet grain size is exceptionally small in all three isotopically disturbed samples, typically in the range mm. These small garnet grains have a high surface area to volume ratio of between 5 and 30. The diffusivity of Nd in garnet is dependent on temperature, cooling rate and grain size (Burton et al., 1995; Becker, 1997; Ganguly et al., 1998). The small absolute grain size and attendant high surface area to volume ratio of the analysed garnets, is conducive to Nd-isotopics being re-equilibrated in subsequent thermal events. All three isotopically disturbed samples were collected

17 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e e17 from within post-kinematic pegmatite swarms, indicating a local thermal perturbation subsequent to formation of the matrix assemblage. The local thermal perturbation, coupled with the very small garnet grain size, is interpreted to have resulted in re-equilibration of Nd-isotopics in these samples at temperatures lower than the C blocking temperature range for coarse garnets of a few mm diameter (Burton et al., 1995; Becker, 1997; Ganguly et al., 1998). In contrast, previous Sm Nd garnet dates (Goscombe et al., 2003b), were sampled at large distances (43 55 km) from the post-kinematic pegmatite swarms, have garnet porphyroblast diameters of 3 10 mm and surface area to volume ratios of only Therefore, by being both distant from the pegmatite swarms and with large garnet grainsize, these samples were significantly less likely to be isotopically disturbed and still preserve peak metamorphic (M2) Nd-isotopics. The anomalously young Sm Nd garnet ages are younger than hornblende 40 Ar/ 39 Ar age determinations of Ma, from widely separated localities in the Kaoko Belt (Table 1). Ar Ar hornblende samples in the Kaoko Belt come from regions 9, 20 and 35 km distant from the post-kinematic pegmatite swarms (Figs. 1 and 11; Table 1) and have apparently not been affected by these younger swarms. Consequently, the Kaoko Belt region as a whole is interpreted to have cooled through C approximately Ma prior to the localised Sm Nd isotopic disturbance event. As a result, Nd isotopic disturbance cannot be due to an orogen-scale thermal pulse affecting the whole Kaoko Belt, because the 40 Ar/ 39 Ar system remained apparently undisturbed. This implies that Nd isotopic disturbance must have been localised to within the vicinity of the post-kinematic pegmatite swarms where the disturbed samples are sited. There is no expression in the analysed samples that Sm Nd isotopic disturbance has been induced by strain. Late-stage, post transpressional deformation fabrics and new mineral growth have not recognised anywhere in the Kaoko Belt. The only late-stage structures are map-scale ( km wavelength) upright warps with steep axes (Goscombe et al., 2003a) and minor semi-ductile reactivation in parts of the Village Mylonite Zone (Konopásek et al., 2005) and Three Palms Mylonite Zone (Goscombe and Gray, in review). All anomalously young Sm Nd samples are closely associated with swarms of post-kinematic pegmatites (Fig. 11), of which there are only three in the north Kaoko Belt and two in the southern Hoarusib Domain (Figs. 1 and 11; Guj, 1970; Goscombe and Gray, in review). Consequently, isotopic disturbance in these three samples most plausibly occurred as a result of localized high-heat flow associated with the emplacement of networks of post-kinematic pegmatites and microgranites. Four U Pb monazite and zircon age determinations from post-kinematic pegmatites, spanning the length of the belt, have a wide range of ages between 530 and 508 Ma, the lower limit overlapping with the young garnet ages (Table 1). 6. Discussion 6.1. Protracted Pan-African evolution Preliminary tectonic frameworks for the NW Namibian region have been successively developed in Dürr and Dingeldey (1996), Passchier et al. (2002), Goscombe et al. (2003a, 2003b, 2004, 2005), Konopásek et al. (2005) and Goscombe and Gray (in review). These frameworks are based on the developed structural and intrusive elements and associated metamorphic parageneses (Fig. 12), with tentative temporal correlations made with the existing geochronology (Seth et al., 1998; Franz et al., 1999; Seth et al., 2000; Goscombe et al., 2003b). The tectonic framework has been modified and the temporal calibration refined by the new geochronological data presented in this paper and the recent publication of Kröner et al. (2004) and this framework is summarized below (Figs. 12 and 13). Currently available age determinations from each domain and terrane in the Kaoko Belt and adjacent Inland Branch are summarized in a time space diagram (Fig. 14) to illustrate the temporal evolution of NW Namibia during the Damara Orogeny. Evolution of the Kaoko Belt spanned almost continuous tectonic activity throughout a protracted and complex Pan-African orogenic cycle spanning 660 to 500 Ma. Pan-African orogenesis has been divided into three distinct tectono-metamorphic periods (Goscombe et al., 2003a, 2003b, 2005; Goscombe and Gray, in review). These are summarized as, M1 thermal phase ( Ma), M2 transpressional phase ( Ma) and M3 Shortening Phase ( Ma). M1 events are

18 103.e18 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e41 Fig. 12. Summary of age determinations integrated with the P-T evolution of all the tectono-metamorphic domains and terranes that constitute the Kaoko Belt. The P-T paths have been constrained in earlier investigations (Goscombe et al., 2003b, 2004, 2005) by the sequence of mineral growth in metapelite rocks, interpreted within P-T pseudosections calculated for average metapelite compositions (White et al., 2001; Tinkham et al., 2001; Johnson et al., 2003). Dashed P-T paths are inferred trajectories linking the documented (solid) portions of different metamorphic events. For each domain/terrane a single, representative P-T path is chosen, from a suite of nested similar P-T paths across a range of temperature. Superimposed on these representative P-T paths are granite and/or pegmatite emplacement events (cross-hatched circles), peak metamorphic events (M1, M2, M3 in circles), cooling through the closure temperature of low-temperature isotopic systems (black hexagons) and all available age determinations (in boxes). Peak metamorphic events that have been dated directly (in black boxes) are from by Sm Nd garnet isochrons (Goscombe et al., 2003b) and U Pb ages from metamorphic monazite and zircon in high-grade meta-paragneisses (Franz et al., 1999). Known granite and/or pegmatite emplacement events are placed with respect to their interpreted relationship to the peak of metamorphism and the P-T evolution (Goscombe et al., 2003a, 2003b, 2004, 2005; Goscombe and Gray, in review). Intrusive rocks have been dated by a variety of workers and methods using U Pb zircon and monazite geochronology (Miller and Burger, 1983; Seth et al., 1998, 2000; Franz et al., 1999; this paper). Cooling ages include Ar/Ar geochronology (Goscombe et al., 2003b, 2004) and apatite fission track dating (Brown et al., 1990). Sm Nd isotopic disturbance in some samples was most plausibly the result of localized thermal perturbations associated with M3 pegmatites and diagrammatically represented by the tight dashed P-T loops. evident only in the then outboard Coastal Terrane, with isotopic, intrusive, stratigraphic, geochemical and metamorphic relationships suggestive of an arc-like setting (see below; Goscombe et al., 2005; Masberg et al., 2005). M2 was associated with accretion of the exotic Coastal Terrane, SSE-directed oblique collision and ultimately progressive sinistral transpressional orogenesis between the Rio De La Plata and Congo Cratons. M3 resulted in weak deformation in a heterogeneously cooling Kaoko Belt, and deformation and metamorphism intensity increased southwards into the Inland Branch proper, which experienced NNE SSW-directed, high-angle collisional orogenesis between the Congo and Kalahari Cratons Sediment provenance The spectrum of age determinations from the detrital zircons and metamorphic overgrowth, in metasediment sample NK91 offer both constraints on the age of sedimentation and the provenance sources of these sediments. The youngest detrital core of with a concordant age of 1001 ± 9 (spot #4.1) is the best estimate of the maximum age of sedimentation. The metamorphic overgrowth age of ± 5.1 Ma with 4% discordance (spot #6.1) is entirely consistent with other estimates of the age of peak-m2 metamorphism and offers the minimum age limit for sedimentation. The least discordant ages (<10% discordance) from detrital

19 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e e19 Fig. 13. Distribution of deformation strain and expressions of metamorphism (at temperatures >400 C) during the different metamorphic events (M1, M2, M3) experienced during the protracted Pan-African Damara Orogeny in NW Namibia. The distribution of M1 and M2 granites are not presented for clarity, but M1 granites are restricted to the Coastal Terrane and M2 granites are restricted to the Coastal Terrane and Orogen Core only. The lower-strain and lower metamorphic grade (<400 C) orogenic margins at each stage are represented by wider-spaced hatching. Structural, metamorphic and geochronological constraints for this summary are based on a wide range of sources and primarily Goscombe et al. (2003a, 2003b, 2004, 2005) and Goscombe and Gray (in review). zircon cores give a spectrum of ages with significant peaks at 1950, 1760, 1440, 1230 and 1005 Ma and additional clusters at approximately 1160 and 1880 Ma and a single Archaean age of 2509 ± 11 (Figs. 9 and 15). Almost all of these detrital zircon age spectra can be explained by zircons sourced from a mixture of basement geological provinces proximal to the Kaoko Belt (Figs. 15 and 16). For example, some of these peaks are coincident with peaks of zircon ages from basement slivers within the Kaoko Belt (Fig. 15; Seth et al., 1998; Kröner et al., 2004, this study). Significantly, not all Kaoko Belt basement peaks are evident in the

20 103.e20 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e41 Fig. 14. Time-space diagram for the Kaoko belt and adjacent regions, presenting all new and published (Table 1), high-temperature thermochronometer ages (U Pb zircon and monazite and Sm Nd garnet) as well as the range of Ar Ar cooling ages in the Kaoko Belt and Ugab Zone. Black symbols are age determinations from metamorphic mineral phases such as zircon, garnet and monazite. Cross-hatched symbols are zircon and monazite age determinations from syn-kinematic granites. Diagonal stripped symbols are zircon and monazite age determinations from late-kinematic syenogranite (Ugab Zone) and post-kinematic pegmatite (Kaoko Belt). Open symbols are the pooled range of Ar Ar cooling age determinations. Single arrow indicates a minimum age and double arrows indicate in inaccurate age determination (Table 1). Shaded columns indicate the thermal peak of each of the three metamorphic cycles. Data from the Central Zone of the Inland Branch is presented for comparison at the bottom and for simplicity these are presented as ranges of pooled age determinations (Table 1).

21 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e e21 Fig. 15. Comparison of U Pb age determinations, from detrital zircons in Damara Sequence sample NK91 in the Orogen Core of the Kaoko Belt, with the U Pb zircon age determination distributions of broad geological provinces throughout Gondwanaland and Laurentia (Fig. 17. Data from sample NK91 are preferred 206 Pb/ 238 U and 207 Pb/ 206 Pb age determinations with less than 10% discordance, from detrital zircon cores. The frequency distribution of ages in all other geological provinces (n = 1973) are restricted to zircon U Pb age determinations from igneous and metamorphic rocks sourced from a large body of literature. The only exception is province #3 with U Pb age determination from volcanics within the Damara Sequence (i.e. Miller and Burger, 1983; Hoffman et al., 1994), giving constraints on the age of deposition of these sequences. Provinces are numbered for reference to their location in Fig. 17. The shaded vertical lines represent peaks in the NK91 detrital zircon population. The geological provinces presented in this figure are proximal to the Kaoko Belt, and these can account for almost all the detrital zircon peaks in sample NK91, with the exception of the Ma peak.

22 103.e22 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e41 Fig. 16. Same as for Fig. 15 with the geological provinces represented being more distal from the Kaoko Belt. All of these provinces have age distributions interpreted to not necessarily be reflected in sample NK91, with the possible exception of the Grenville Orogen and Central USA provinces (#23 and #24). detrital age spectrum, specifically the 2600, 1500 and 1600 Ma peaks (Fig. 1). The detrital zircon peaks at 2500, 1880, 1440, 1160 and 1005 Ma are not significantly represented by basement terranes within the Kaoko Belt. Most of these detrital zircon peaks can be found in basement terranes proximal to the Kaoko Belt in the southern African continent, such as the Kamanjab Inlier, Sinclair and Rehoboth terranes and other components of the Zambezi Belt and Kalahari Craton (Figs. 15 and 17). The detrital zircon spectrum from NK91 does not match those from the South American Rio De La Plata and Sao Francisco Cratons

23 B. Goscombe et al. / Precambrian Research 140 (2005) 103.e1 103.e e23 Fig. 17. Populations of U Pb zircon age determinations from geological provinces in Gondwanaland and Laurentia are compared with the detrital zircon population in sample NK91 (Figs. 15 and 16). Geological provinces are numbered the same as in Figs. 15 and 16. Specifically, #1 is the Kaoko region, #7 and #8 the Kalahari Craton and #11 and #12 the Congo Craton. The presented continental reconstruction is after Powell et al. (2001) for 1105 Ma and used only as an approximation for the time of sedimentation ( Ma). For example the Khomas Ocean was narrower and the southern African components were closer to the northern components during deposition of the Damara Sequences. Arrows indicate gross transport vectors of detrital zircons from numbered geological provinces containing age populations consistent with being provenance sources for sample NK91 (Figs. 9 and 15). (Fig. 17). The 1440 Ma detrital zircon peak has no obvious basement source proximal to the Kaoko Belt within the southern African region, or within South America or other components of Gondwana (Figs. 16 and 17). A plausible source for these 1440 Ma zircons are the Grenville Orogen and Central Plains Provinces in Laurentia or the Rondonian Province of South America (Figs. 16 and 17; Bickford et al., 1986; Gower and Tucker, 1994; Tassinari and Macambira, 1999; Steltenpohl et al., 2004). These sources would be consistent with the proposition that the Kalahari Craton was adjacent to southern Laurentia in the supercontinent of Rodinia (Powell et al., 2001; Hanson et al., 2004) M1 thermal phase and evolution of the exotic Coastal Terrane The oldest recognised Pan-African granitoid plutons and sills are from the Coastal Terrane and these have ages ranging Ma (Seth et al., 1998; Franz et al., 1999; Kröner et al., 2004; this paper). The most accurate and concordant of these ages are 656 ± 8Ma (Seth et al., 1998) and 638 ± 6 Ma (this paper). Minimum age determinations from Coastal Terrane M1 granitoids, such as slightly discordant ages and those by the Kober method, range Ma (Seth et al., 1998; this paper; Table 1) putting a minimum age constraint on the emplacement of these