Metamorphic Response in the Zambezi Belt and Assembly of Gondwana
- Tectonic evolution of the Zambezi Belt
- Archaean to Stenian basement
- Tonian-Cryogenian-Ediacaran
- Early Pan-African Orogenic Cycle
- Pan-African suture
- Late Pan-African Orogenic Cycle
- Tectonic vergence switching
- Timing of collisions between Kuunga and Mozambique Belts
- Thermal regimes and supercontinent growth
[9] Metamorphic Evolution of the central Kuunga Orogen
Assembly of central Gondwana along the Zambezi Belt: Metamorphic response and basement reactivation during the Kuunga Orogeny
Ben Goscombe, David A. Foster, David Gray, Ben Wade
Focus Review accepted for publication in Gondwana Research, December 2019.
Central Gondwana was assembled by three continental collisions in relatively quick succession: late Cryogenian East Africa Orogen, early Ediacaran West Antarctica Orogen and late Ediacaran Kuunga Orogen. The Kuunga Orogen involved diachronous closure of the South Adamastor–Khomas–Mozambique Oceans and accretion of Kalahari Craton and cratonic elements in Antarctica, with a previously assembled North Gondwana. The two older orogens were still hot and deforming at the time of final assembly by the Kuunga Orogen, and were therefore reworked and re-metamorphosed. The Central Kuunga Orogen is comprised of the Lufilian Arc, Zambezi Belt, Malawi–Unango Complex and the Lurio Belt. This region was the site of earliest collision in the Kuunga Orogen at ~575 Ma, and involved collision of two buoyant, previously metamorphosed rigid basement promontories. Pivoting on the Zambezi Belt indenters led to clockwise rotation of the Kalahari Craton and oblique collision within the Damara Belt ~20–30 m.y. later. The Central Kuunga Orogen is a relatively cold collisional belt dominated by eclogite, whiteschist and Barrovian series metamorphic parageneses, and contrasts with the paired metamorphic response in the Damara Belt to the west, and low-P/high-T metamorphism in the East Kuunga Orogen. Metamorphic parageneses are preserved from each stage of the full Wilson Cycle: from initiation of continental lithosphere thinning at ~940 Ma, widespread rifting between 725 and 805 Ma, and passive margin sedimentation until ~580 Ma. Eclogite-facies subduction parageneses indicate consumption of ocean lithosphere was underway by ~630–660 Ma. Collision at ~575 Ma involved deep burial of continental crust and formation of very high-P, low T/depth metamorphic parageneses, followed by Barrovian series thermal peaks at ~545 and ~525 Ma. Isostatic compensation and stress switches associated with plate reconfigurations once Gondwana was assembled, resulted in exhumation and local extension in an intra-continental setting from ~518 Ma.
[8] Tectonic Evolution of the Zambezi Belt
Goscombe, B., Gray, D. and Foster, D., 2015.
Metamorphic evolution of Gondwana 3. The Pan-African Orogenic System: Thermo-mechanical response to orogenesis during Gondwana assembly. Geoscience Australia Record 2015/XX (in review).
The Lufilian-Zambezi Belt is the central sector of the Pan-African Orogenic System (Kuunga Orogen) that stretches across southern Africa from Damara Orogen in the west to Malawi Mosaic and Lurio Belt in the east. Metamorphic events in the Pan-African Orogenic System have been labelled by an extension of the scheme used for the Damara Orogen (Goscombe et al., 2014). The Early Pan-African Orogeny or "Mozambiquean Phase" consists of two distinct metamorphic events: M1a at 640-660 Ma and M1b at 610-630 Ma. The Late Pan-African Orogeny has two phases: the "Adamastor Phase" or M2 at 575-585 Ma and the "Kuungan Phase" or M3 at 500-555 Ma. The protracted M3 metamorphic cycle is sub-divided into three moderately distinct metamorphic events that have very similar ages throughout the Pan-African Orogenic System. M3a ranges 530-555 Ma, M3b ranges 515-530 Ma and M3c ranges 500-515 Ma. An overlapping and younger M4 metamorphic event ranges 517-470 Ma and is characterized by extensional telescoping, exhumation and cooling events in both the Damara Orogen and Zambezi Belt. The Chewore inliers in Zimbabwe are centrally located, well exposed and contain many of the crustal elements involved in the evolution of the Zambezi Belt. Consequently, structural, metamorphic and geochronological research from this region (Goscombe et al., 1994, 1998, 2000; Johnson et al., 1996, 2005, 2007; Johnson and Oliver, 1997, 1998, 2004; this record) forms a framework for the tectonic evolution of the Zambezi Belt. This framework has been expanded to include the entire belt by incorporating research from southern Zambia (e.g. Hansen et al., 1994a,b; this record), northeast Zimbabwe (e.g. Vinyu et al., 1997; Vinyu and Jelsma 1997; Dirks et al., 1998; this record) and northwest Zimbabwe (e.g. Dirks et al., 1997, 1999; Dirks and Sithole 1997, 1999; this record).
Archaean to Stenian Basement Terranes
Like the Damara Orogen to the west and Lurio Belt in the east, the ancient basement terranes of Archaean to Tonian age on either side of the Zambezi Belt cannot be correlated with each other. There is no evidence, such as orogens in common or dyke swarms, that would suggest the Congo and Kalahari Cratons were in contact prior to collision in the Late Pan-African Orogenic Cycle at ~585 Ma. As a result, previous models for the Zambezi Belt (and Damara Orogen) as a closed intra-continental rift-geosyncline (Kroner, 1977; Hanson et al., 1988a, 1994a; Hanson, 1993) need to be discounted. The southern and northern limits of different basement terranes and orogenic elements all end within the Zambezi Belt, indicating that it is the site of a true suture between different continental plates. Like the Damara Orogen to the west (Goscombe et al., 2014), we suggest that an ocean basin separated the Congo and Kalahari Cratons and the sedimentary sequences on the two passive margins developed independent and unconnected from each other. Cryogenian to Cambrian sequences developed on both margins show distinctly different provenances akin with their respective craton hinterlands, suggesting they were widely separated by an ocean basin (Foster et al., 2013).
The southern margin of the Zambezi Belt is Kalahari Craton crust consisting of a complex mosaic of different age tectono-metamorphic terranes and orogens. The northern nucleus of the wider Kalahari Craton is the Zimbabwe Craton. The Zimbabwe Craton has a small central terrane of Palaeoarchaean age, is otherwise predominantly typical granite-greenstone of Neoarchaean age that was cratonized and intruded by mafic dykes from 2574-2577 Ma. The northern 50-80 km margin of the craton, along the edge of the Zambezi Belt, consists of a number of Archaean gneissic terranes of different ages. The Migmatitic Gneiss Terrane in northeast Zimbabwe has U-Pb zircon ages of 2552-2625 Ma (Vinyu et al., 1997) and the Escarpment and Chipisa-Kariba Gneisses in the north and northwest have very poorly constrained Rb-Sr and Pb/Pb zircon evaporation ages of 2406-2947 Ma (Vail and Snelling, 1971) and 2290-2704 Ma (Loney, 1969; Dirks et al., 1997) respectively. The E-W trend of these Archaean orogenic terranes at the margin of the much younger Zambezi Belt, suggests this zone is a long-lived zone of lithospheric weakness developed in the Archaean and was reactivated by rifting in the Cryogenian. Indeed the presence of Mesoproterozoic putative oceanic crust of 1393 Ma age (Johnson et al., 1996), preserved in the Ophiolite Terrane at the southeast margin of the Chewore Inliers, suggests the Zambezi Belt zone may have experienced reactivation a number of times. Archaean crust is overlain by Palaeoproterozoic sequences such as the Lomagundi and Piriwiri Groups in the Magondi Belt in northwest Zimbabwe. The Magondi Belt was deformed and metamorphosed at 1963 Ma and intruded by granites and enderbite such as the Kariba orthogneiss at approximately 1910-1960 Ma (Dirks et al., 1997; Master, 1991).
In contrast, Congo Craton crust at the northern margin has Archaean nuclei very distant from the southern margin, as the internal Tanzania and Kasai Cratonic nuclei. Similarly, Palaeoproterozoic sequences such as the Bangweulu Block and orogenic belts such as the Ubendian and Usagaran Belts are very distant and disconnected from the southern margin. Furthermore, these Palaeoproterozoic elements are older and do not correlate in age with the Magondi Belt in Kalahari crust. The Ubendian and Usagaran Belts experienced eclogite and main phase metamorphism in the range 1970-2002 Ma and possibly older events at 2026-2084 Ma and younger high-grade events at 1946-1950 Ma. Crucially, the southern margin of Congo crust is dominated by a broad Irumide Orogenic System of Stenian age, with no correlatives south of the Zambezi Belt in the Kalahari Craton. The Stenian aged Choma-Chobe-Ghanzi-Sinclair Belt on the northwest margin of the Kalahari Craton experienced events that do not directly correlate with the Irumide Orogenic System in the Congo Craton. Choma-Chobe-Ghanzi-Sinclair orogenic system contains felsic volcanic protoliths of 1290-1335 Ma age was intruded by granites at 1090-1110 and 1160-1210 Ma and metamorphosed at 1145 and 1045-1055 Ma.
The Irumide Orogenic System consists of the Barrovian Irumide Belt thrust NW onto the southwest margin of the Bangweulu Block (Daly 1986) and the high-grade gneissic Southern Irumide Province, which extends into the Zambezi Belt and is reworked by the Late-Pan-African Orogenic Cycle. The Irumide Belt contains numerous granites that range in age 1000-1050 Ma (e.g. Daly 1986; N'gambi et al., 1986; Hanson et al., 1988b; Ring et al., 1993). Metamorphic age determinations indicate Barrovian series metamorphism at 1004-1021 Ma. The Southern Irumide Province has granitic orthogneisses of 1035-1120 Ma age and the host supracrustal gneisses are equally deformed and metamorphosed and interpreted to be of similar age, deposited between approximately 1000 and 1200 Ma. High-T/low-P granulite facies metamorphism of 1015-1018 and 1030-1047 Ma age, accompanied recumbent isoclinal folding, south over north transport and granitic orthogneisses. Age determinations indicating a third granulite facies metamorphism averaging 943±20 Ma (n=15), are widely spread across the Southern Irumide Province as far east as the Malawi Mosaic and Unango Complex in Mozambique (Barr et al., 1978; Jourde and Wolff, 1974; Haslam et al., 1983; Costa et al., 1983; Bingen et al., 2009). Consequently, there was either high-grade metamorphic condition over a very protracted period or multiple events in the 945-1047 Ma period. There is no evidence for tectono-metamorphic events of 945, 1004-1021, 1015-1018 and 1030-1047 Ma age in the Zambezi Belt to the south of the Chewore Granulite Domain.
Stenian high-grade metamorphism of the Southern Irumide Province was of high-T/low-P type with high T/depth ratios of 35-38 ºC/km indicating high heat flow settings. The Chewore Granulite Domain and Champira Dome experienced anticlockwise P-T paths and isobaric cooling and the Chipata Granulites are interpreted to have followed tight clockwise paths followed by isobaric cooling (Goscombe et al., 1998; this record). These P-T evolutions and high T/depth metamorphism, suggest tectonic settings involving lithospheric extension, magmatic accretion or lower crust delamination. These settings are incompatible with the Barrovian metamorphism and collisional tectonics in the Irumide Belt to the north, indicating a paired orogenic system. The apparently widespread and disconnected thermal anomalies of ~945 Ma age may indicate isolated zones of crustal extension, corresponding to widely distributed rift basins with mantle lithosphere thinning and high-T/low-P metamorphism at depth. Rifting and deposition of the Fombwe Group at the base of the Muva Supergroup began at 940 Ma in the Lufilian Arc and Zambezi Belt, and may have been initiated by lithospheric extension associated with the 945 Ma thermal anomalies.
Tonian-Cryogenian-Ediacaran Magmatism and Sedimentation
The Tonian and Cryogenian period in the Lufilian Arc and Zambezi Belt is marked by rifting, eruption of felsic and mafic volcanics and intrusives, and deposition on the Congo and Kalahari Craton passive margins. Rift and drift sequences were deposited on these passive margins until the end of the Ediacaran, between 940 and 580 Ma. These sequences are of similar age to those in the Damara Orogen, with which they are correlated (Goscombe et al., 2014). Different sedimentary sequences of similar age were deposited on passive margins either side of the Zambezi Belt suture, on the Congo Craton to the north and Kalahari Craton to the south. Metamorphosed sequences of equivalent age to the Muva and Katanga Supergroups are widespread throughout the Zambezi Belt, and were deposited on Southern Irumide Province basement rocks of the Congo Craton. Sequences correlated with the rift stage Muva Supergroup are the Quartzite Domain in the Chewore Inliers and the Fingoe and Mchinje Groups in the Tete region. Tonian and Cryogenian sequences deposited on Kalahari crust to the south are the Rushinga Complex and equivalents in northeast Zimbabwe, Zambezi Domain sequences in the Chewore Inlier, Makuti Group meta-sediments of ~700-800 Ma age and Tengwe Group in northwest Zimbabwe.
The Zambezi Belt appears to show similar rift stage relationships as the Damara Orogen. Rift stage felsic volcanics are preserved on both passive margins of the Damara Orogen, though these volcanics are not the same age, not correlated and thought to have formed independently (Goscombe et al., 2014). Intensely sheared felsic volcanics and granitic orthogneiss of Cryogenian to upper Tonian age are not uncommon within both north and south margins of the Zambezi Belt. In the north on Congo crust, the Muva Supergroup contains the Kafue rhyolites of 867-880 Ma age, Kafue mafics of 845-855 Ma age and younger rhyolites of 735, 760, 765 and 820 Ma age. Syn-tectonic granites in the west Zambezi Belt, such as the Lusaka Granite and Ngoma orthogneiss in Zambia are of 820-880 Ma age and others of 730-760 Ma age (Barr et al., 1978; Hanson et al., 1988a; this record). The Lufilian Arc has 718 Ma granite and 735-765 Ma rhyolites. Otherwise Cryogenian granites and felsic volcanics are uncommon in the Lufilian Arc, north Zambezi Belt in Zambia, Irumide Belt and South Irumide Province. Rare boudinaged and metamorphosed dolerite dykes in the Chewore Granulite and Zambezi Domains pre-date Pan-African orogenesis, indicating N-S crustal extension that may coincide with the Tonian rift stage.
To the south on Kalahari crust, the Makuti Terrane in northeast Zimbabwe contains large volumes of Cryogenian felsic volcanics and granites. Age determinations from granites of the Basal Rushinga Igneous Complex have a large range in ages from 721-870 Ma (Barton et al., 1991; Vinyu et al., 1997; this record). Strongly sheared and metamorphosed felsic volcanics within the Makuti Terrane range in age 720-805 Ma and possibly as old as 870 Ma, and sheared granitic orthogneiss of 725-855 Ma age (Loney, 1969; Dirks et al., 1997, 1999). The Mavuradonha and Masoso Complexes in the Zambezi Allochthonous Terrane in northeast Zimbabwe are bimodal magmatic terranes thrust onto the Kalahari Craton margin. The Masoso Complex contains mafic volcanics or intrusives of 845-855 Ma age and numerous granitic orthogneisses of 725-855 Ma age. The Cryogenian episode of felsic magmatism is widespread throughout the southern Zambezi Belt on the margin of the Zimbabwe Craton, and was originally named the Zambezi Orogeny (Barton et al., 1991) and interpreted to be extensional (Dirks and Sithole, 1997, 1999).
Early Pan-African Orogenic Cycle
The Zambezi Belt preserves metamorphic and chronologic evidence for at least four Pan-African tectono-metamorphic events. The two oldest events are constrained by very few metamorphic age determinations, but these are widely scattered on both margins of the belt. These oldest metamorphic events constitute the Early Pan-African orogenic cycle with M1a metamorphic event at 640-660 Ma and M1b in the 600-630 Ma age range. M1a metamorphic age determinations range 629-659 Ma in Zambia and in northeast Zimbabwe are 649 and 691 Ma in the Masoso Terrane and 646-652 Ma in the Rushinga Complex. M1b metamorphic age determinations from Zambia are 602-614 Ma and 600 Ma in the Masoso Terrane and 608-616 Ma in the Rushinga Complex. The significance of these metamorphic age determinations is not known and they are not directly related to known metamorphic parageneses and P-T evolutions. Molasse shed to the south of the Zambezi Belt and onto the Kalahari Craton in northwest Zimbabwe, is called the Sijarira Group and was deposited between 570 and 500 Ma. Consequently, the M1a and M1b metamorphic events are not associated with widespread main phase collisional orogenesis and development of topography in the Zambezi Belt. The M1 metamorphic events may represent early extensional events, or subduction or other oceanic settings prior to collision.
Pan-African Suture Between Congo and Kalahari Crust
The core of the Lufilian Arc - Zambezi Belt has a trace of widely separated high-P parageneses such as eclogite, whiteschist and high-P granulites. These are contained within both Congo Craton passive margin stratigraphy and units on the Kalahari Craton margin and so do not necessarily indicate the suture line (see below). These trace a line of "pop-ups", out-wedging and upthrusting of deep crustal rocks in the core of this collisional orogen. These high-P parageneses formed in the M2 period at ~570-580 Ma and interpreted to represent deep burial during initial collision. These include eclogites and whiteschists in the central Dome Belt of the Lufilian Arc and eclogites at Chombwa and Lusaka in the central part of the West Zambezi Belt. Chombwa eclogites formed at maximum pressure conditions of 21.0-23.0 kb at 520-550 ºC, and peak metamorphic conditions are 650-725 ºC and 16.0-20.0 kb. Maximum pressure conditions in the Lusaka eclogites are 18.0-23.0 kb at 550-600 ºC, and peak metamorphic conditions are 680-740 ºC and 10.0-16.0 kb. These eclogites and host parageneses have been dated at approximately 568-595 Ma (John et al., 2003, 2004; this record). Eclogites in the Makuti Terrane at the southern margin of the West Zambezi Belt formed at maximum pressure conditions of 17.0-22.0 kb at 600-750 ºC, and peak metamorphic conditions are 700-820 ºC and 12.0-19.0 kb. Upper-amphibolite facies reworking of the Makuti Terrane occurred at conditions of 700-770 ºC and 10.5-12.0 kb, at 536-557 Ma (John et al., 2004; Dirks et al., 1999; this Record), indicating eclogite metamorphism at >557 Ma.
The East Zambezi Belt has a number of moderately high-P thrust sheets within the core of the belt and southern margin. A major thrust slice adjacent to the Southern Irumide Province contains the Chewore Quartzite Domain and lateral equivalent Fingoe and Mchinje Groups in the Tete region. Peak metamorphic conditions in the Quartzite Domain are 675-720 ºC and 8.5-9.5 kb and metamorphic age determinations correlate with the M2 period at 568-580 Ma (John et al., 2004; this record). Whiteschists in the Chewore Ophiolite Terrane formed at peak metamorphic conditions of approximately 580-680 ºC and 14.0-19.5 kb, with a single metamorphic age determination of ~580 Ma. High-P upper amphibolite facies rocks in the Zambezi Allochthonous Terrane were thrust onto the southern margin in northeast Zimbabwe. Peak metamorphic conditions were 810 ºC and 13.3 kb, with a single metamorphic age determination of 573±17 Ma, correlating with the M2 period. Both of these high-pressure thrust slices imply an extensional structure in the hanging wall to accommodate exhumation. These hypothetical extensional structures are not exposed and sited within the Zambezi graben, which may in part have been located by reactivation of these and similar crustal structures.
The suture between Congo and Kalahari crust is south of the median line of high-P parageneses exhumed in the core of the Zambezi Belt. The suture is defined by the southern limit of Katanga and Muva Supergroups, and southern limit of Southern Irumide Province basement terranes, both of which do not occur in Kalahari crust. Metamorphosed sequences of equivalent age to the Muva and Katanga Supergroups are widespread throughout the Zambezi Belt, and were deposited on Southern Irumide Province rocks of the Congo Craton. Metamorphosed sequences correlated with the Muva Supergroup are the Quartzite Domain in the Chewore Inliers and the Fingoe and Mchinje Groups in the Tete region. These criteria for locating the suture are clear in most parts of the Zambezi Belt, though obscured below Karoo aged rift sequences. Only where exposed in the centrally located Chewore Inliers is the exact location of the suture ambiguous, with two possible alternatives; either within high-P schists south of the Ophiolite Terrane or the shear zone with dismembered ultramafics between the Quartzite and Zambezi Domains.
The Quartzite Domain is correlated with the Mpanshya Group in the Muva Supergroup and thus part of unambiguous Congo crust. Further south the Zambezi Domain cannot be correlated confidently and may be either Southern Irumide Province or underlain by Palaeoproterozoic basement similar to the Magondi Belt in the Kalahari Craton (Goscombe et al., 2000). Orthogneiss within the Zambezi Domain contain inherited zircons with Palaeoproterozoic age spectrum similar to the Magondi Belt. Consequently, the Zambezi Domain and closely associated Ophiolite Terrane is interpreted to be part of the Kalahari Craton (Goscombe et al., 2000). The northwest margin of the Zambezi Domain is marked by a major thrust that contains a dismembered 25 m thick unit of sheared olivine websterite ultramafic, serpentinite-talc rock and hornblende-chlorite schists and is interpreted to be the Pan-African suture between Congo and Kalahari crust.
Alternatively, if the Zambezi Domain is correlated with the Southern Irumide Province, the Pan-African suture will be further south and possibly at the whiteschists in the Ophiolite Terrane. Supracrustals in the Zambezi Domain are intimately interlayered with granitic orthogneisses of 1066-1082 Ma age, and there is no obvious basement versus cover relationship between the two. The Zambezi Domain probably experienced a complex metamorphic evolution with at least two metamorphic cycles. Early high-T/low-P sillimanite-spinel inclusions within peak metamorphic garnets are incompatible with clockwise P-T paths experienced during the Late Pan-African metamorphic cycle. It is possible that the Quartzite and Zambezi Domains may have also experienced the Irumide metamorphic cycle, but were almost totally reworked and recrystallized during the Late Pan-African metamorphic cycle.
Late Pan-African Orogenic Cycle
Main phase orogenesis and development of the Zambezi Belt occurred in the Late Pan-African orogenic cycle, consisting of: M2 metamorphic event at 575-585 Ma, M3a at 530-555 Ma and M3b at 510-530 Ma. The first widely distributed main phase metamorphic event with recognised metamorphic parageneses, in the Zambezi Belt and most parts of the wider Kuunga Orogen, was middle Ediacaran M2 metamorphism between 565-585 Ma. These widely distributed age determinations are associated with eclogite and other high-P metamorphic parageneses and are interpreted to represent the initial collisional and deep burial event in the Kuunga Orogen. This middle Ediacaran event is recognised in all parts of the Zambezi Belt, Malawi Mosaic, Lurio Belt, Madagascar, Sri Lanka, Southern India, Dronning Maud Land and Prydz Bay in Antarctica. This event is also probable, but not confirmed, in the Damara Orogen to the west. The Zambezi Belt in Zambia contains eclogites ranging in age 560-595 Ma. The oldest metamorphic age determinations in the Chewore Inliers are between 568-580 Ma. M2 metamorphic age determinations in northeast Zimbabwe are 573 Ma from the Masoso Terrane and 568-596 Ma from the Rushinga Complex. Eclogite within the Makuti Group are constrained in age to be younger than the youngest felsic volcanic unit of 720 Ma age, and older than the regional amphibolite facies metamorphism of 557 Ma age. Collision between the Congo and Kalahari Cratons involved SW-directed transport, crustal over-thickening, pervasive deformation and in the latter stages, juxtaposition of thrust-bound terranes (Goscombe et al., 1994). Collisional orogenesis resulted in pervasive regional Barrovian series metamorphism and clockwise P-T paths in all parts of the Zambezi Belt. Reworking was heterogeneous, with pervasive recrystallization of basement in most terranes, ranging to merely thermal re-equilibration and minor recrystallization in some basement terranes such as the Chewore Granulite Domain.
Like the Damara Orogen to the west, regional metamorphism of the Zambezi Belt was a protracted Late Pan-African M3 metamorphic cycle ranging 555-510 Ma. This M3 regional metamorphic cycle affected all parts of the Zambezi Belt as well as variably reworking basement terranes. Regional metamorphic Barrovian series conditions and clockwise P-T paths indicate burial and convergent orogenesis. Peak metamorphic conditions in the Chewore Zambezi Domain are 650-670 ºC and 8.5-9.5 kb, and in the Rushinga Metamorphic Complex are 629 ºC and 9.3 kb (Vinyu and Jelsma, 1997; Hargrove et al., 2003; this record). M3 metamorphic age determinations cluster in two probable metamorphic events: M3a ranging 530-555 Ma and M3b ranging 500-530 Ma. The Zambezi Belt in Zambia experienced regional Barrovian series M3a metamorphism at 539-542 Ma and the Makuti Terrane was pervasively sheared at upper amphibolite facies conditions at 536-557 Ma. Regional metamorphism in northeast Zimbabwe occurred at 532-550 Ma in the Masoso Terrane and 532-554 Ma in the Rushinga Complex. Most parts of the Zambezi Belt experienced the M3b youngest metamorphic event, except the Makuti Terrane. The Zambezi Belt in Zambia experienced its youngest metamorphic event at 524-531 Ma and contains Ar-Ar muscovite, cooling ages of 524 Ma. The Masoso Terrane in northeast Zimbabwe experienced both lower and middle Cambrian metamorphism at 511-518 and 525-526 Ma. The youngest metamorphic event of 511-518 Ma in the Masoso Terrane has been interpreted as accompanying extensional tectonics and anticlockwise P-T paths through 650 °C and 5 kb (Vinyu and Jelsma, 1997; Dirks et al., 1997). The Rushinga Complex in northeast Zimbabwe experienced similar aged metamorphic events of 500-509 and 518-521 Ma. Barrovian series metamorphism accompanied reworking of older terranes in the Chewore Inliers at 517-526 Ma, producing regional metamorphism in the Zambezi Domain and re-equilibrated minerals, reaction textures and shear zones in the Granulite Domain. To the east, the Malawi Mosaic did not develop Cambrian M3b metamorphic parageneses, except for a single metamorphic age determination of 523 Ma in the Blantyre Domain. Almost all terranes in Madagascar, Southern India and Sri Lanka experienced both lower and middle Cambrian metamorphic events. Many of the Antarctic terranes experience one or both of the lower and middle Cambrian metamorphic events.
[9] Tectonic Vergence Switching in the Pan-African Orgenic System
Goscombe, B., Gray, D. and Foster, D., 2015. Metamorphic evolution of Gondwana 3. The Pan-African Orogenic System: Thermo-mechanical response to orogenesis during Gondwana assembly. Geoscience Australia Record 2015/XX (in review).
First-order tectonic vergence is indicated by the direction of transport of high-P thrusts, "pop-ups" and out-wedges within the Zambezi Belt, Damara Orogen to the west and Lurio Belt to the east. The entire length of the Pan-African Orogenic System shows vergence switching along its length. From southeast vergent in the Gariep Belt, predominantly south vergent in Damara Orogen (minor north vergent), north vergent in Lufilian Arc, symmetric up-wedge in West Zambezi Belt in Zambia, south vergent in East Zambezi Belt in Zimbabwe, north and west vergent in south Malawi Mosaic and south vergent nappes emanated from the Lurio Belt in the east. The change in polarity of metamorphic zonation and site of exhumation of deeply buried very high-pressure parageneses changes along the length of the Lufilian-Zambezi-Malawi-Lurio orogenic system. The site of up-thrusting or out-wedging and rapid exhumation of deeply buried rocks was within the core of the orogen in almost all sectors; including the Lufilian Arc, most of the West Zambezi Belt, East Zambezi Belt, Malawi Mosaic and Lurio Belt. The site of exhumation of deeply buried rocks shifted to the margin of the orogen only in the Makuti Terrane.
The first-order metamorphic pattern of the Lufilian-Zambezi-Malawi-Lurio sector differs significantly from the Damara Orogen portion to the west. The Lufilian-Zambezi-Malawi-Lurio sector has a symmetric to asymmetric bivergent metamorphic pattern and in this sense is superficially similar to the Damara Orogen. However, unlike the Damara Orogen, the most deeply exhumed, high pressure, domains are in the core of the orogen and not exhumed crustal wedges on the margins. Furthermore, the Lufilian-Zambezi-Malawi-Lurio sector does not have a paired metamorphic pattern with zones of contrasting metamorphic style, such as zones of Barrovian (low T/depth) and Buchan (high T/depth) series metamorphism. The Lufilian-Zambezi-Malawi-Lurio sector has low T/depth Barrovian to eclogite series main phase parageneses throughout, and typically metamorphic grade decreases to both the north and south from a high-grade and high- to very high-P orogen core. There is no high T/depth Buchan series main phase regional parageneses anywhere in the Lufilian-Zambezi-Malawi-Lurio sector. Though the Damara Orogen and Lufilian-Zambezi-Malawi-Lurio Orogen are part of the same collisional orogenic system, there is a marked difference in metamorphic response between these two sectors. Polarity of metamorphic zonation from orogen core towards foreland margins, switches radically along the length of the orogenic system. There is symmetric decrease in grade out from the core of the Lufilian Arc. The West Zambezi Belt has opposite polarity of symmetric increase in grade towards margins. There is apparent asymmetric zonation of decreasing grade towards the southern margin in the East Zambezi Belt, Malawi Mosaic and Lurio Belt. However, the East Zambezi Belt may have an obscured symmetric zonation, but Pan-African parageneses are hard to recognize in basement rocks that predominate in the north. This variation in metamorphic polarity is also reflected by tectonic vergence reversals along the length of the orogen.
[10] Timing of Collisions at Mozambique-Kuunga-Umkondo Junction
Goscombe, B., Gray, D. and Foster, D., 2015.
Metamorphic evolution of Gondwana 3. The Pan-African Orogenic System: Thermo-mechanical response to orogenesis during Gondwana assembly. Geoscience Australia Record 2015/XX (in review).
Previous literature has assumed that the Mozambique Orogen was a single orogenic system stretching from the Arabian Platform in the north to Dronning Maud Land in Antarctica to the south (e.g. Grantham, 2001; Collins and Pisarevsky, 2005). The now well-established timing of collisional stitching of the Congo and Kalahari Cratons at 520-580 Ma by the Pan-African Orogenic System (Kuunga Orogen) shows that this entrenched paradigm is not possible on simple over-printing criteria. The young age of collision at the Kuunga Orogen indicates that the older orogenic systems along the eastern margin of Africa and west margin of Antarctica cannot be a single orogen, because north and south components were brought together after main phase orogenesis within them. The Mozambique Belt to the north of the Lurio Belt, formed at ~640 Ma during collision of the Azania strip continent with the Congo Craton. The Umkondo Belt and Dronning Maud Land are a separate orogen (West Antarctica Orogen) that brought the Kalahari and Antarctica Cratons together at ~615 Ma. The two older orogenic systems were brought together during closure and collision of north and south Gondwana in the Kuunga Orogen at ~580 Ma, and their current near alignment is merely coincidental.
The southern Malawi Mosaic is a complex of granulite grade metamorphic domains that experienced different metamorphic conditions. The age of protoliths in this sector are poorly constrained but are correlated with the Unango Complex in Mozambique of Stenian age. Almost all metamorphic age determinations from the Malawi Mosaic and Unango Complex are of 525-575 Ma age, similar to the Late Pan-African metamorphic cycle in the Lufilian Arc and Zambezi Belt to the west. Most domains experienced high-pressure (10-13 kb) granulite facies conditions with T/depth ratios of 19-24 ºC/km, which are typical of the Barrovian series. Only the Dedza and northern Blantyre domain experienced lower pressures of 8.0-9.0 kb and higher T/depth ratios of 28-29 ºC/km. The highest pressures and temperatures are 13 kb and 900 ºC in the southern Blantyre domain, which represents the core of the orogen, and lateral equivalent of the Lurio Belt. Further south the Nsanje domain is of upper amphibolite grade at 10 kb pressure and typical T/depth ratios of 22 ºC/km. Consequently, the broad metamorphic pattern for the Malawi Mosaic is from highest temperatures and pressures and lowest T/depth ratio within the lateral equivalent of the Lurio Belt and an irregular decrease in temperature and pressure to both the north and south.
West of the Malawi Mosaic is a lower grade margin within the Southern Irumide Province. This region has been reworked at amphibolite facies conditions and shows Barrovian series zonation to lower grades towards the southwest. Metamorphic conditions range from the garnet-kyanite/sillimanite field closest to the Malawi granulites, progressing through the garnet-staurolite field to garnet-biotite field 60 km from the Malawi granulites. Similar amphibolite facies Barrovian metamorphics immediately north of the Chipata Granulites may also be part of the low-grade Barrovian margin to the Malawi Mosaic. This Barrovian reworked margin of the Southern Irumide Province, suggests it was the foreland margin of an orogen cored by the Malawi-Unango granulites. Though the Barrovian margin and granulite core share similar T/depth ratios, the large difference in metamorphic grades and sharp boundary give the appearance of a paired metamorphic system with strongly exhumed high-pressure, high-grade orogen core and low-grade Barrovian margin that reworked adjacent older crust. Metamorphic polarity indicates that tectonic vergence was towards the WSW.
Map geometries suggest a triple junction in the Pan-African Orogenic System centred on the southern Malawi region. The Zambezi Belt to the west, Malawi-Unango Belt to the NNW and Lurio Belt to the ENE, all intersect in this region. The Malawi Mosaic and Unango Complex together, appear to form a broad embayment to the NNW, at high-angle to the Kuunga collisional suture. This NNW orogenic branch appears to terminate in the north within central Malawi, though probably propagates further to the NNE by reactivation and reworking within the Mozambique Orogen. For example, forming the Pan-African eclogites of 533±5 Ma age in the Lulomo Domain (Ring et al., 2002). The Malawi-Unango orogenic branch possibly represents an orogenic front against Irumide crust to the west. This branch was active after collision in the Mozambique Orogen during the Early Pan-African orogenic cycle (630-650 Ma). The Malawi-Unango branch has no Early Pan-African age determinations, suggesting it did not experience the 630-650 Ma or 613-615 Ma events. The Unango-Malawi orogenic branch did not experience orogenesis until the Late Pan-African orogenic cycle at 525-575 Ma. This indicates that collision of the Mozambique Orogen crustal block with the Irumide crustal block did not occur until this time. Further north, Mozambique Belt crust had collided with Ubendian-Usagaran-Tanzania Craton crust much earlier, during the Early Pan-African orogeny at 630-650 Ma. As a result, the Malawi-Unango branch indicates inboard stepping of the orogenic front, during the younger Late Pan-African (525-575 Ma) amalgamation of north and south Gondwana at the Kuunga suture.
Similar triple junction geometry exists immediately to the west where the N-S Umkondo Belt links with the Zambezi-Lurio Belt. The Umkondo Belt represents a similar inboard orogenic front that was active during the Late Pan-African orogenic cycle at 520-556 Ma. Together these two slightly offset triple junctions linking four orogens of similar age, suggesting that four crustal blocks came together at close to the same time. These blocks are Zimbabwe crust in the Kalahari Craton, Irumide and Ubendian crust in the Congo Craton, Mozambique Orogen crust in the Azania strip continent and the Nampula-Barue Complex in the Antarctic Craton. There are insufficient robust age determinations to confidently resolve absolute timing of these collisional events. However, there is weak evidence for older 613-615 Ma parageneses in the Umkondo Belt and along strike in the Dronning Maud Land Belt in Antarctica, suggesting collision between the Kalahari and Antarctica Cratons was during M1b in the Early Pan-African orogenic cycle. Collision of Mozambique Belt crust with Ubendian-Usagaran-Tanzania Craton crust in the north occurred as early as 630-650 Ma (M1a) in the Early Pan-African orogenic cycle. However, the southern part of Mozambique Belt crust did not collide with Irumide crust at the inboard Malawi-Unango Belt until 575 Ma during final convergence between the Mozambique Orogen and the Congo Craton. This final stage of collision coincides exactly with collision of north and south Gondwana at ~575 Ma along the Zambezi-Lurio Belt sector of the Kuunga Orogen. Activation of orogenesis at the inboard Malawi-Unango Belt may be in response to rotation of the stress field in response to anticlockwise bending of the Zambezi-Lurio Belt during ongoing convergence at this major collisional belt.
[11] Evolution of Crustal Thermal Regime with Supercontinent Growth
Goscombe, B., Gray, D. and Foster, D., 2015.
Metamorphic evolution of Gondwana 3. The Pan-African Orogenic System: Thermo-mechanical response to orogenesis during Gondwana assembly. Geoscience Australia Record 2015/XX (in review).
Cycles of assembly and dispersal of supercontinents have driven geological change since the Neoarchaean and been the major influence on a range of Earth systems. Recent growing interest in secular change of thermal regimes in continental crust (Brown 2006, 2007) has highlighted our immature understanding of the evolving mantle thermal structure resulting from supercontinent growth, and its' effect on crustal thermal regimes. Gurnis (1988) and Coltice et al. (2007) modelled changes in mantle thermal structure consequent to different aerial extents of continental crustal lithosphere. This modelling predicts profound increases in sub-crustal temperatures below supercontinents due to the thermal blanketing effect of continental crust. Sub-crustal temperatures below supercontinents the size of Gondwana and Pangea would be 135 to 180 ºC higher than that below typical continental fragments the size of the Kalahari Craton. It follows that supercontinental assembly will induce marked changes in thermal structure of underlying mantle and thus profoundly influence the thermal structure of the overlying continental crust. This should be reflected in a secular change in the thermo-barometric and mechanical response to orogenesis in continental lithosphere at different stages in the growth of supercontinents. It follows that elevated sub-crustal temperatures would self-seed the destruction of supercontinents. Crustal lithosphere that is sufficiently thermally weakened will enable rifting and supercontinent break up, as indicated by the fast break away of Laurentia soon after formation of the Pangea mega-supercontinent, in contrast to longer-lived (and smaller) Gondwana and Laurentia supercontinents.
Gondwana is an exceptional experimental laboratory for investigating this process because it has a three-stage collisional assembly history of ~630-660, ~570-580 and ~550-510 Ma, followed by growth in a protracted accretionary phase. Investigation of collisional and accretionary episodes will determine the influence on crustal lithosphere, of successively hotter mantle thermal regimes during progressive supercontinent growth. Utilizing a large (n=4112) metamorphic database from Gondwana and other orogenic belts, thermo-barometric arrays (e.g. P versus T/depth) have been used to characterize metamorphic conditions in different periods of Gondwana and Earth history. There is no significant difference in thermal regime between the Palaeoproterozoic, Mesoproterozoic and two oldest periods of Gondwana assembly in the Neoproterozoic. Only the final period of Gondwana assembly at ~550-510 Ma is different to all other periods, including Phanerozoic orogens. The Kuunga Orogen that brought north and south Gondwana together in the latest period of assembly has on average significantly higher thermal regimes than all other periods of Earth history. The "hot orogen" conditions are heterogeneously distributed throughout this orogenic system; centred on the Damara Orogen in the west and Madagascar - Southern India - Sri Lanka sectors in the east (Goscombe et al., 2014). This concentration of high T/depth metamorphic conditions late in Gondwana assembly is consistent with progressive thermal blanketing of the mantle during supercontinent growth as modelled.
Characterizing the evolution of thermal regimes in this way, of plotting the full distribution of metamorphic conditions as arrays, is difficult to compare between different populations. Consequently, an alternative gross simplification is to compare the average metamorphic conditions, based on the total population of PT determinations available from specific time periods in Earth history. These averages are highly influenced by sampling preferences. For example, most metamorphic studies in literature target particularly high-T and high-P parageneses and lower grade rocks are neglected, resulting in skewed datasets. Nevertheless, if sample populations are sufficiently large, it can be assumed skewing effects will be similar in each sample set, and the gross averages will indicate relative differences in metamorphic conditions between different times in Earth history. Average T and P are strongly dependent on sampling effects; though usefully compare near-maximum conditions experienced. Whereas, T/depth ratios (thermal regime) effectively normalize sampling effects by being independent of the depth or grade sampled. Consequently, average T/depth ratios are reliably representative of thermal regimes in general for each period.
The pooled average metamorphic conditions for different periods indicate a near systematic decrease in thermal regime throughout Earth history. Average T/depth ratios decrease from 33-43 ºC/km (n=1664) in the Archaean and Palaeoproterozoic to 17-26 ºC/km (n=1205) in the Phanerozoic. Barrovian series metamorphic conditions typical of continent-continent collision (21-29 ºC/km) dominate on average from the Mesoproterozoic to present (n=2223). The systematic "cooling" trend is interrupted by only two significant anomalous "hot" periods. The first averaging 31 ºC/km (n=225) is the Grenvillian Phase associated with major crustal growth in the Stenian period. The second averaging 30-35 ºC/km (n=622) is located in the east (Madagascar-India) and west (Damara) ends of the Kuunga Orogen during the Late Pan-African orogenic cycle (500-550 Ma), associated with the latest stage of Gondwana assembly. These highest thermal regimes out of the Archaean, is consistent with extreme sub-crustal temperatures arising from thermal blanketing of the mantle in the latest stage of supercontinent growth, and this being expressed by steep thermal gradients in the overlying crust.
