Damara Orogen Projects

Damara Orogen Metamorphism: Latest findings 2017. [1]

The Damara Orogen is a well exposed, Pan-African collisional orogen between the Congo and Kalahari Cratons, in Namibia. A project to document the metamorphic evolution and thermo-barometric patterns in the Damara Orogen was initiated in 1994. The project has grown and now involves structural, thermochronology and provinence studies in collaboration with Prof. David Gray [2] (Melbourne University) and Prof. David Foster [3] (University of Florida). Structural studies in the Ugab Terrane at the junction with the Kaoko Belt, have also been undertaken in collaboration with Prof's Cees Passchier and Rudolph Trouw. Metamorphic and chronological studies are continuing in collaboration with Prof. David Foster and his students, Ben Wade (Adelaide Microscopy) and Prof. David Gray. From 2013 a new detailed mapping project was established to investigate the geometries across the triple junction from the Kaoko Belt through the Ugab Terrane and into the Damara Belt.

Below are links to summaries of different aspects of the Damara Orogen project:

• Northern margin metamorphism [4]

• Paired metamorphic collisional orogen [5]

• Granite intrusion mechanisms [6]

• Ar-Ar thermochronology [7]

• Accretion of turbidites [8]

• Comparison with Lachlan [9]

• West Gondwana amalgamation [10]

• Detrital zircon provenance [11]

• Logitudinal variation [12]

(1) VARIATION IN METAMORPHIC STYLE ALONG THE NORTHERN MARGIN OF THE DAMARA OROGEN, NAMIBIA.

Ben Goscombe¹, David Gray², Martin Hand¹
¹School of Earth and Environmental Sciences, University of Adelaide, 5005, S.A.
²School of Earth Sciences, University of Melbourne, Parkville, 3010, Victoria.
See Journal of Petrology 45, 1261-1295 (2004)

The northern margin of the Inland Branch of the Pan-African Damara Orogen in Namibia, shows dramatic along strike variation in metamorphic character during convergence between the Congo and Kalahari Cratons (M3). Low-P contact metamorphism with anticlockwise P-T paths dominate in the west and high-P Barrovian metamorphism with a clockwise P-T path occurs in the east.

The sequence of M3 mineral growth in contact aureoles show early growth of cordierite porphyroblasts that were pseudomorphed to biotite-chlorite-muscovite at the same time that a andalusite-biotite-muscovite transposed foliation is developed in the matrix. The peak metamorphic assemblages were overprinted by crenulations and retrograde chlorite-muscovite. The KFMASH PT pseudosection for metapelites in the Ugab Zone and western Northern Zone contact aureoles, indicate tight anticlockwise P-T loops through peak metamorphic conditions of 540-570 ºC and 2.5-3.2 kb. These semi-quantitative P-T loops are consistent with average PT calculations using THERMOCALC, which give a pooled mean of 556±26 ºC and 3.2±0.6 kb, indicating a high average thermal gradient of 50 ºC/km.

In contrast, the eastern Northern Zone experienced deep burial, high-P/moderate-T Barrovian metamorphism with an average thermal gradient of 21 ºC/km and peak metamorphic conditions of approximately 635 ºC and 8.7 kb. The calculated PT pseudosection and garnet compositional isopleths in KFMASH, appropriate for the metapelite sample from this region, document a clockwise P-T path. Early plagioclase1-kyanite-biotite parageneses evolve by plagioclase consumption and the growth of garnet to increasing XFe, XMg and XCa and decreasing XMn compositions, indicating steep burial with heating. The developed kyanite-garnet-biotite peak metamorphic parageneses were followed by the resorption of garnet and formation of plagioclase2 moats, indicating decompression, which was followed by retrogressive cooling and chlorite-muscovite growth. The clockwise P-T loop is consistent with the foreland vergent fold-thrust belt geometry in this part of the northern margin.

The Ugab Zone and western Northern Zone show evidence for an earlier M2 metamorphism that produced regionally pervasive foliations with low-P biotite-muscovite-chlorite±andalusite parageneses formed on high average thermal gradients, possibly due to a period of high heat flow and associated granite emplacement at 580-570 Ma. Pervasive matrix foliations (M2) were overprinted by contact metamorphic parageneses (M3) in the aureoles of 530±3 Ma granite in the Ugab Zone and 553-514 Ma granite in the western Northern Zone. Available geochronological data suggests that convergence between the Congo and Kalahari Cratons was essentially coeval in all parts of the northern margin, with similar ages of 535-530 Ma for the main phase of deformation in the eastern Northern Zone and Northern Platform. NNE-SSW directed convergence across the Inland Branch of the Damara Orogen was also broadly contemporaneous with the 538-505 Ma deformation and high-grade metamorphism of the Central Zone immediately to the south.

It is often assumed that there is no change in metamorphic style along the length of linear to arcuate convergent orogens. The northern margin of the Inland Branch clearly illustrates the potential for contrasting styles of metamororphism developing at the same time in different sectors along the length of a single orogenic margin. In this case, two processes were important in this disparate thermal histories; (1) initial and evolving differences in crustal architecture, possibly including thickness of the passive margin lithosphere and thickness of the overlying sedimentary succession, and (2) variation in input of heat into different sectors by granite emplacement. Variation in metamorphic style in apparently simple linear/arcuate margins of high-angle convergent orogens, may be more wide spread than is currently acknowledged.

KEY WORDS: Pan-African Orogeny; P-T paths; pseudosections: low-P metamorphism; contact metamorphism; Barrovian metamorphism

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(2) Metamorphic model for the Inland Branch of the Damara Orogen: A Paired Metamorphic Mountain Belt (PMMB).

Ben Goscombe, Martin Hand
Department of Geology and Geophysics, Adelaide University, Adelaide, S.A. 5005, Australia.
See SGTSG conference abstract, Ulverstone, 2001.

The Damara Orogen is a Neoproterozoic fold and thrust belt with a triple junction geometry, consisting of two coastal arms and a ENE-trending Inland Branch which is the focus of this study. The Damara Orogen experienced a single orogenic cycle of Pan-African age (550-520 Ma) and can be divided into some 16 zones that are lithologically, structurally and metamorphically distinct, although a coherent Neoproterozoic stratigraphy can be recognised throughout. This study, for the first time, rationalizes a large body of new and existing metamorphic data in order to characterise the P-T evolution and average thermal gradient in each tectono-metamorphic zone and map out the average metamorphic field gradient across the entire Inland Branch of the orogen. Metamorphic conditions have been estimated from P-T calculations on 74 samples using the average P-T approach of Powell & Holland, (1994). Because only one thermobarometric method and thermodynamic dataset have been used, the P-T results are probably directly comparable throughout the sampled region, and give some useful insights into variation in metamorphic style in different parts of the Damara orogenic system.

The Inland Branch of the Damara Orogen consists of two terranes with contrasting metamorphic "style" (a composite of the average thermal gradient, P-T path and metamorphic grade), that are confined to either the upper, or lower-plate domains of a crustal-scale over-thrust system. Goscombe and Hand (2000) have referred to this type of orogenic architecture as a Paired Metamorphic Mountain Belt (PMMB). The Inland Branch displays some degree of symmetry in structural and metamorphic style; a high-grade, granite-rich Central Zone is flanked by the Northern and Southern Zones which are both high-P, Barrovian metamorphic sequences dominated by outward thrusting onto the cratonic basement in their forelands. At both margins, the Central Zone occupies the upper-plate position of a crustal-scale over-thrust system, and thus gives the appearance of being an extruded orogenic core. The Central Zone experienced high-T/low-P (35-50 ºC/km) metamorphism (700-800°C, 3.5-6 kbar) characterised by isobaric cooling paths. Both the Northern and Southern Zones occupy lower-plate positions and experienced moderate-T/high-P (15-25°C/km) Barrovian metamorphism and clockwise P-T paths. Both zones contain an inverted metamorphic sequence with the highest metamorphic grade occupying the highest structural levels immediately below the contacts with the Central Zone.

Aside from contrasts in the style of metamorphism between the core and margins of the Inland Branch of the Damara Orogen, the Central Zone is characterised by large volumes of granite (30-60% of the current erosion surface). An important question that needs to be addressed is whether the granites are the cause, or the consequence, of the high thermal gradient regime in the Central Zone. The granites are derived from melting of the Damara Sequence (McDermott et al., 1996), suggesting they are a consequence of the high thermal gradient conditions. There is no obvious evidence of mafic magmatism, or a significant volume of highly radigenic heat producing rocks, and the ultimate source of the high temperature conditions is still unresolved. However we speculate that the formation of low-P/high-T conditions in the Central Zone of the Inland Branch is a consequence heat advection during rapid mass flow within the orogen in which a relatively hot lower crust was extruded upwards and outwards, and across the Northern and Southern zones. In such a scenario it is possible that all earlier high-P parageneses and the record of isothermal decompression would be obliterated during pervasive ductile flow of the hot terrain as it decompressed (giving rise to decompression melting and pervassive granitoids), and then underwent rapid (isobaric) cooling.

The spatial metamorphic patterns with respect to gross structural architecture of the Inland Branch, are very similar to those developed in the Eastern Himalayas (Goscombe & Hand, 2000). Both of these examples are PMMB’s that record high-T/low-P metamorphism in the upper-plate and Barrovian metamorphism in the lower-plate. This suggests that development of PMMB patterns may be a common feature of collisional orogens with crustal-scale over-thrust architectures.

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(3) Intrusion mechanisms in a turbidite sequence: the Voetspoor and Doros Plutons in NW Namibia

CEES W. PASSCHIER^a, RUDOLPH A. J. TROUW^b, BEN GOSCOMBE^c, DAVID GRAY^d, ALFRED KRÖNER^a
^aInstitut für Geowissenschaften, Johannes Gutenberg University, 55099 Mainz, Germany
^bInstituto de Geociencias, Universidade Federal do Rio de Janeiro, 21949-900 Rio de Janeiro, Brazil
^cContinental Evolution Research Group, Department of Geology and Geophysics, Adelaide University, Adelaide, S.A. 5005, Australia
^dSchool of Earth Sciences, The University of Melbourne, Vic. 3010, Australia
See Journal of Structural Geology 29, 481-496 (2007)

Two syntectonic plutons of Cambrian age intruded Neoproterozoic metaturbidites in Namibia at the junction of the NS trending Kaoko- and EW trending Damara belts. Sinistral transpression in the Kaoko Belt produced km-scale upright D1 folds overprinted by minor D2 folds. D3 is associated with N-S shortening in the Damara Belt. The plutons show two main pulses of intrusion: hornblende syenite intruded late during D1 or during D2 and biotite granite during D3. Each tectonic event produced a strain shadow defined by the shape of folds and the foliation trend around the plutons. The internal igneous fabric and the arrangement of wall rock xenoliths that locally make up 50% of the intrusion mass suggest that the plutons have a disk or wedge shape. A marginal shear zone indicates that one of the syenite intrusions descended during emplacement with respect to the wall rock. Emplacement is therefore inferred to be at least partly accommodated by descend of the intrusion floor. The biotite granite intruded into mega-strain shadows and tension gashes alongside and in the syenite during D3. The plutons show evidence of sinistral solid state rotation with respect to the wall rocks in response to D1-D2 transpression.

Keywords: granite emplacement, Namibia, Lower Ugab Domain, Neoproterozoic
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(4)⁴⁰Ar/³⁹Ar thermochronology of the Damara Orogen, Namibia, South West Africa, with implications for tectono-thermal evolution

David R. Gray^a, David A. Foster^b, Ben Goscombe^a, Cees Passchier^d, Rudolph Trouw^e
^aSchool of Earth Sciences, University of Melbourne, Melbourne, 3010, Vic. Australia
^bSchool of Geological Sciences, University of Florida, Gainesville, FL, 32611-2120, USA^dDeInstitut für Geoswissenschaften, JohannesGutenburg University, 55099 Mainz, Germany^eInstituto de Geociências, Universidade Federal do Rio de Janeiro, 21910-900 Rio de Janeiro, Brazil
See Precambrian Research 150, 49-72 (2006)

Forty three new Ar-Ar step-heating experiments on micas, hornblende and whole rocks from the Damara Orogen, Namibia provides a regional perspective on cooling across the orogen, as well as documenting continued activity on major shear zones and the thermal effects of late-syn to post-tectonic granite intrusions. These data show regional cooling through mica blocking temperatures from 545-520 Ma in the Central Kaoko Zone, 530-510 Ma in the Eastern Kaoko Zone, and 495- 480 Ma in the Southern Zone of the Damara Belt. Discordant Ar-Ar age spectra relate to 1) resetting of Ar in micas by continued activity in the major, Kaoko Belt shear zones such as the Purros and Three Palms Mylonite Zones giving apparent ages of 467 ± 6 Ma, 481 ± 3 Ma and ~492 Ma; and 2) thermal effects from granites particularly in the Ugab Domain of the southern Kaoko Belt (475- 500 Ma) and in the granite-dominated Central Zone of the Damara Belt (460- 490 Ma). Phyllites of the lower grade flank regions record apparent Ar-Ar crystallization ages of ~517 Ma (Eastern Kaoko Zone), ~526 Ma (Northern Zone, Damara Belt), and ~553-568 Ma (Southern Foreland Zone, Damara Belt), but show some discordance due to influences of detrital mica. Rates of cooling vary between zones within the Kaoko Belt. In the Central Kaoko Belt hornblende, biotite and white mica apparent Ar-Ar ages are similar, suggestive of rapid cooling through the 350°C-550°C temperature range, whereas in the Orogen Core or Western Kaoko Belt hornblende and biotite apparent ages are older than the white mica ages. These data support models of diachronous deformation and metamorphism of the component parts of the orogen, and suggest that Pan African orogenesis is broadly bracketed between 580 and 500 Ma with final movements through 480 Ma.

Keywords: Pan African orogenesis; Damara Orogen; Ar-Ar thermochronology; Namibia

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(5) Continental growth in accretionary orogens involving large turbidite fans

David R. Gray, School of Earth Sciences, University of Melbourne, Melbourne, 3010, Vic. Australia
David A. Foster, School of Geological Sciences, University of Florida, Gainesville, FL, USA.
Catherine V. Spaggiari, Geological Survey of Western Australia, Perth, 6004, W.A., Australia.
Robert T. Gregory, Department of Geology, Southern Methodist University, Dallas, TX, 75275, USA
Ben Goscombe, Geology Dept. Adelaide University, Australia.
K.H. (Charlie) Hoffmann, Namibian Geological Survey, Windhoek, Namibia.
See GSA special publication, in press (2007).

Convergent margin tectonic settings involving large accumulations of turbidites represent important sites of growth and regeneration of new continental crust. The newly accreted crust consists of a supra-crustal recycled layer (turbidites) underlain by mafic oceanic crust as well as underplated magmatic materials. Examples from the Neoproterozoic Pan African Damaran Orogen of SW Africa, the Palaeozoic Tasman Orogen of eastern Australia, and the Mesozoic Rangitatan Orogen or Rakaia wedge of New Zealand provide a disconnected history of tectonic process through time and illustrate continental growth from marine successions involving subduction-accretion. Lithologies include basalt, chert and/or turbidite in the low-grade belts (e.g. Lachlan Orogen) or monotonous quartzo-feldspathic schist, with varying amounts of micaceous schist, greenschist and metachert (e.g. Southern Zone, Damara Orogen and Otago Schist belt of the Rangitatan Orogen). The spatial and temporal variations in deformation, metamorphism and magmatism across these orogens illustrate how large volumes of monotonous turbidites eventually become part of crust of continental thickness and character. The timing of deformation and metamorphism reflect the crustal thickening phase whereas the post-tectonic granitoids give the timing of cratonisation. The conditions of metamorphism reflect the tectonic setting, with 1) the low-T/high-P metamorphism defining the subduction channel, 2) the moderate-T/moderate-high-P defining structural thickening above the subduction interface, and 3) high-T/low-P defining the magmatic arc. Deformation and structural style of the turbidite package depends on the tectonic setting (forearc, backarc, continental margin), the thickness of the turbidite fan, the residence time of the turbidites on the seafloor prior to deformation, and the preserved level within the subduction-accretion system. The Rangitatan Orogen, the Lachlan Orogen and present day Makran have large volumes of turbidite sediments that have undergone internal deformation and subsequent structural interleaving with slices of oceanic lithosphere. Very large sediment volumes tend to clog up the associated subduction zone resulting in marked deformation within the thrust-wedge (accretionary prism). The turbidite package may show stratal disruption and classic mélange/broken formation of accretionary prisms (e.g. Howqua accretiionary complex of the Lachlan Orogen), chevron folding (e.g. western Lachlan Orogen) and/or fold nappes and schistosity (e.g. Southern Zone, Damara Orogen and Otago Schist belt, Rangitatan Orogen). The preservation of, as well as the nature of the preserved oceanic lithosphere (Tethyan or Oman type versus Cordilleran, Turkic or Lachlan Orogen type) to a first order approximation depends on tectonic setting (forearc, backarc, continental margin), the age of oceanic lithosphere (young or old), and the thickness of the turbidite fan. Cordilleran or Lachlan type involve thick fans sitting on old, cold oceanic lithosphere whereas Tethyan ophiolites form in sediment-starved settings with pelagic sequences, and involve overthrusting of young, hot oceanic lithosphere (e.g. Oman).

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(6) Parallels between the Palaeozoic Lachlan and Neoproterozoic Damaran Orogens: structural insights and tectonic evolution

David Gray, Ben Goscombe and David Foster

Conference abstract (2004)

Turbidites are a major lithologic component of both the Neoproterozoic Damara and Palaeozoic Lachlan orogens, but despite this there are major structural and metamorphic differences between the two. The Damara orogen is part of the Pan African orogenic system and within Namibia consists of inland and coastal branches that define a triple junction. Turbidites constitute a major component of the central and southern zones of the inland or intra-continental branch. Structural and metamorphic relationships within the metamorphosed Kuiseb Schist provide information about closure of the Khomas trough depositional region between the Congo and Kalahari cratons. The flanks of the former basin now consist of craton-verging thrust systems within shelf carbonates (e.g. Hakos and Naukluft nappes in the southern margin zone). The basinal Khomas turbidite sequence is now broken into a high T-low P, granite- dominated central zone (metamorphosed between 538-505Ma) floored by attenuated Congo-like craton, with dome and basin fold interference and possible extensional structures. The thicker basinal sequence has undergone Barrovian series metamorphism and is now part of a ~100km wide southern zone consisting of homoclinally, N-dipping, transposed foliation and schistosity. The southern zone is a major shear zone interface transitional into basement-cored fold- and thrust-nappes of the southern margin zone. Although geometrically distinct in terms of crustal architecture both the Lachlan and Damara orogens have had a similar history of hypotheses of geodynamic evolution. Debate has involved ensimatic models with turbidites sitting on attenuated craton to those ensialic models involving closure of an ocean basin floored by oceanic lithosphere between the Congo and Kalahari cratonic segments. Relict oceanic lithosphere is now preserved as a thin (200-300m thick), but laterally extensive (over 250 km in length), shear zone-hosted Matchless Amphibolite that consists of basalt, pillowed basalt, chert and gabbro, with tholeiitic geochemistry. The Damara structural and metamorphic examples provide instructive fingerprints to some of the processes inferred to have taken place in the closure of the Lachlan back-arc basin. The similarities and differences between the two orogens will be used to assess the particular models of Lachlan orogenesis.

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(7) A Damara orogen perspective on the assembly of southwestern Gondwana

David R. Gray, David Foster, Joe Meert, Ben Goscombe, Richard A. Armstrong, Rudolph Trouw, Cees Passchier.

School of Earth Sciences, University of Melbourne, Melbourne, 3010, Vic. Australia;
Geology Dept. Adelaide University, SA, Australia;
PRISE, School of Earth Sciences, Australian National University, Canberra, ACT. 0200 Australia;
School of Geological Sciences, University of Florida, Gainesville, FL, 32611-2120, USA;
Institut für Geoswissenschaften, JohannesGutenburg University, 55099 Mainz, Germany;
Instituto de Geociências, Universidade Federal do Rio de Janeiro, 21910-900 Rio de Janeiro, Brazil.

See: PANKHURST, R. J., TROUW, R. A. J., DE BRITO NEVES, B. B. & DE WIT, M. J. (eds) West Gondwana: Pre-Cenozoic Correlations Across the South Atlantic Region. Geological Society, London, Special Publications, 294, 257–278. The Geological Society of London 2008.

New kinematic, geochronological and thermochronological data from the Damara Orogen of Namibia SW Africa supports previously published temporally distinct suturing and amalgamation of South America (Rio del Plata craton) with the Congo and Kalahari cratonic nuclei of southern Africa. This three-pronged orogenic system is essentially a collisonal triple junction (after Hoffman et al., 1994) made up of a coastal arm, with north and south expressions as the Kaoko and Gariep Belts respectively, and an inland Damara Belt extending through the Lufilian Arc and Zambezi Belt into the Mozambique Belt of eastern Africa. Oblique convergence between the Rio del Plata craton (South America) and the Congo and Kalahari cratons (Africa) involved sequential closure of the Adamastor Ocean, where the main ocean basin and/or sub-basins were closed first in the north and subsequently southwards from 550 Ma.

Transpressive convergence in the Kaoko and Gariep coastal arms produced major west-dipping listric shear zones that caused oblique crustal overriding of external or more outboard parts over and towards the African cratonic nuclei. Transpressional orogenesis in the Kaoko Belt occurred from 580-550 Ma and had clearly ceased by ~535 Ma, with cratonisation marked by intrusion of post-kinematic granite and pegmatite between 535-505 Ma. The Kaoko Belt is a thermally softened margin in transpression, dominated by major, sinistral strike-slip shear zones defining regional scale shear lozenges within high-grade amphibolite facies Damara sequence turbidites, incorporating basement slivers and sheared Pan-African age granitoids. The Gariep Belt underwent transpressional convergence between ~550-540 Ma (Frimmel and Frank, 1998) with erosion into the Nama foreland basin commencing at ~540 Ma (Gresse and Germs, 1993; Gresse, 1994; Frimmel, 2000) and intrusion of post-tectonic granites at ~530-520 Ma (Allsopp and others, 1988). E-directed thrusting continued inboard within the Nama foreland basin through 496 Ma (Gresse and others, 1988). The Gariep Belt is mostly of low- metamorphic grade, consisting of an arcuate belt of SW-vergent, stacked oceanic thrust-sheets including mélange with blueschist blocks, metagreywacke turbidites and metabasalts, thrust over the passive continental margin of the Kalahari craton.

The Damara Belt (Inland Branch) is a classic doubly vergent orogen with craton-directed thrust systems on both margins (e.g. Hakos and Naukluft nappes in the Southern Margin Zone and Southern Foreland). It records closure of the Khomas Ocean basin by high angle convergence between the Congo and Kalahari cratons, with a marked tectono-thermal magmatic history extending from 570 Ma through ~490 Ma with the intrusion of A-type granites (McDermott et al., 2000; Jung & Mezger, 2003). Ar-Ar data suggests Naukluft thrust-nappe emplacement on the southern foreland was at ~570-550 Ma. Overall, the Damara and Gariep Belts appear to show both younger deformation and metamorphism than the Kaoko Belt, although deformation may have been occurring in the Damara Belt by ~570 Ma. Continued high-angle convergence through 530 Ma in the Damara Belt coincides with the Shortening Phase deformation in the Kaoko Belt, reflected by large-scale open E-W trending folds and by major NE-trending kink-like flexures, local subvertical NE-trending crenulation cleavage and penetrative foliation in contact aureoles in the Ugab Domain (Southern Kaoko Belt). Data from the Damara Orogen suggest that the assembly of this part of West Gonwana had occurred by 480 Ma.

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(8) The accretion of north and south Gondwana: evidence from U-Pb ages and Hf-isotopic compositions of detrital zircons from the Damara Orogen, Namibia.

Foster, Goscombe, Mueller, Gray, Meert
See Abstracts for AESC, Perth, 2008.

The breakup of Rodinia and accretion of Gondwana define a one of the most critical periods in Earth history both tectonically and biologically. In recent years, it has become particularly apparent that proper identification of ancient suture zones is critical to accurately defining supercontinent cycles and identifying the nature of juxtaposition of cratons in the context of global geodynamics. The configurations of continental fragments within Rodinia, the dispersal of these fragments, and their subsequent amalgamation to form Gondwana remain some of the more profound questions in continental evolution, geodynamics, and paleogeography. There are two very different hypotheses for the relationship between the Congo and Kalahari cratons, which now make up southern Africa and comprised the core of Gondwana. These cratons are presently separated by the Neoproterozoic-Cambrian DamaraLufilian-Zambezi orogenic system. The Congo and Kalahari cratons may have been located in different parts of Rodinia (or not in Rodinia at all) and then collided when Gondwana formed. Alternatively, Congo and Kalahari may have been together from the time that Rodinia formed and in roughly the same relative positions as they are today, separated by a small Red Sea-like basin at Rodinia breakup, and reconnected upon amalgamation of Gondwana. Unfortunately, there are no reliable paleomagnetic data for Kalaharia and only limited data from Congo for late Neoproterozoic time. The results of preliminary analysis of U-Pb ages and Hf-isotopic compositions of detrital zircon from Neoproterozoic metaseclimentary rocks within the Damara orogen show distinct sources from the Congo and Kalahari cratons and as well as other Gondwanan fragments, These data constrain the accretionary history of north and south Gondwana and suggest the presence of a significant ocean between these cratons in Neoproterozoic time.

U‐Pb Age and Lu‐Hf Isotopic Data of Detrital Zircons from Neoproterozoic Damara Sequence: Implications for pre‐Gondwana proximity of Congo and Kalahari

David A. Foster, Ben D. Goscombe, Brittany Newstead, Ben Mapani, Paul A. Mueller, Laura C. Gregory and Ewereth Muvangua

Submitted to Gondwana Reserch, in review

The proximity of the Congo and Kalahari cratons during the Neoproterozoic breakup of the supercontinent Rodinia and during subsequent assembly of Gondwana is unclear. Neoproterozoic metasedimentary rocks from the rifted margins of Congo and Kalahari in the Damara Orogen yield distinctive detrital zircon U-Pb age distributions that correspond to the ages of prominent crustal components within the respective cratons. The most abundant zircons from Neoproterozoic strata deposited on the Congo margin give ages of 1150-1000 and 800-600 Ma, whereas, the most abundant zircons from the Kalahari margin strata range from 1350-1100 Ma. A 1350-1200 Ma detrital zircon population in the Kalahari margin strata is absent in the Damara-Congo strata. A prominent c. 1050-1000 Ma detrital zircon age population from Damara-Congo strata is nearly absent from the Damara-Kalahari strata, even though orogenic events of this age are found on both cratons. Damara strata on the Kalahari margin also lack detrital zircons with U-Pb ages of 900-600 Ma. The differences in detrital zircon age distributions are robust when comparing strata of the same age on both cratons, and remains so, even when younger, deeper water facies are excluded, which could have been biased by other sediment sources. These data suggest that the Congo and Kalahari cratons were not proximal in Rodinia, and did not establish their current relative positions until the end of the Neoproterozoic when they were sutured together during the collisional orogenies that formed Gondwana.

[9] Longitudinal Variation in Metamorphic Response

Goscombe, B., Gray, D., Foster, D. and Wade, B., 2014.

Metamorphic evolution of Gondwana 2. The Damara Orogenic System: amalgamation of central Gondwana and evolution of orogen architecture. Geoscience Australia Record 2014/XX (in review).

The northern margin of the Damara Orogen shows dramatic along-strike variation in metamorphic character during convergence between the Congo and Kalahari Cratons. M3 metamorphism in the Ugab Zone and western Northern Zone is low-P anticlockwise path contact metamorphism around late-kinematic granites. M3 metamorphism in the eastern Northern Zone (Outwedge III described above) is high-P Barrovian metamorphism with clockwise P-T paths. The sequence of mineral growth in M3 contact aureoles shows early growth of cordierite porphyroblasts that were pseudomorphed to biotite-chlorite-muscovite at the same time that an andalusite-biotite-muscovite transposed foliation was developed in the matrix. The peak-T metamorphic assemblages and fabrics were overprinted by crenulations and retrograde chlorite-muscovite. KFMASH P-T pseudosection for metapelites indicates tight anticlockwise P-T loops through peak metamorphic conditions of 540-570 ºC and 2.5-3.2 kb. These semi-quantitative P-T loops are consistent with average P-T calculations using THERMOCALC, which give a pooled mean of 556±26 ºC and 3.2±0.6 kb, indicating a high average thermal gradient of 50 ºC/km.

In contrast, the eastern Northern Zone experienced deep burial, high-P/moderate-T Barrovian metamorphism with an average thermal gradient of 17 ºC/km and peak metamorphic conditions of approximately 630 ºC and 10.5 kb. Prograde garnet compositional isopleths and phase relations in KFMASH pseudosection document a clockwise P-T path. Early plagioclase-kyanite-biotite parageneses evolved by plagioclase consumption and the growth of garnet to increasing XFe, XMg and XCa and decreasing XMn compositions, indicating steep burial with heating. The developed kyanite-garnet-biotite peak metamorphic parageneses were followed by the resorption of garnet and formation of plagioclase moats, indicating decompression, which was followed by retrogressive cooling and chlorite-muscovite growth. The clockwise P-T loop is consistent with the foreland vergent fold-thrust belt geometry in this part of the northern margin.

Main phase deformation occurred at the same time in all parts of the Northern Foreland, Ugab Zone, Northern Zone and Central Zone. Syn-kinematic granites accompany low-strain shortening in the Ugab Zone have 535-530 Ma age, main foliations in the Northern Platform have 530-535 Ma age and main phase deformation and metamorphism in the Central Zone occurred at 538-510 Ma. Post-kinematic granites in the Ugab Zone have 511 Ma age and the peak of metamorphism after deformation and burial in the eastern Northern Zone occurred at 510±4 Ma. Consequently, main phase orogenesis during NNE-SSW convergence in the Damara Orogen gave rise to contrasting styles of metamorphism in different parts of the northern margin during the same M3 metamorphic cycle. In the west, heat transfer was dominated by conduction and externally driven by granites, whereas in the east, heat transfer was dominated by advection and internally driven radiogenic heat production. The ultimate cause was along-orogen variation in crustal architecture, including thickness of the passive margin lithosphere and thickness of the overlying sedimentary succession, with thick rift sequences in the east and a thin 1.7 km package on a basement high in the west.

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