A continental back-arc setting for the Namaqua belt: Evidence from the Kakamas Domain
P.H. MaceyRobert J. ThomasAlex KistersJohann F.A. DienerM. AngombeS. DoggartC.A. GroenewaldChristopher W. LambertJodie MillerH. MinnaarHarold P. SmithH. F. G. MoenEwereth MuvanguaAnna NgunoG. ShifotokaJ. IndongoDirk FreiChristopher J. SpencerPetrus le RouxRichard A. ArmstrongChristel Tinguely
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The evolution of Earth's biosphere, atmosphere and hydrosphere is tied to the formation of continental crust and its subsequent movements on tectonic plates. The supercontinent cycle posits that the continental crust is periodically amalgamated into a single landmass, subsequently breaking up and dispersing into various continental fragments. Columbia is possibly the first true supercontinent, it amalgamated during the 2.0–1.7 Ga period, and collisional orogenesis resulting from its formation peaked at 1.95–1.85 Ga. Geological and palaeomagnetic evidence indicate that Columbia remained as a quasi-integral continental lid until at least 1.3 Ga. Numerous break-up attempts are evidenced by dyke swarms with a large temporal and spatial range; however, palaeomagnetic and geologic evidence suggest these attempts remained unsuccessful. Rather than dispersing into continental fragments, the Columbia supercontinent underwent only minor modifications to form the next supercontinent (Rodinia) at 1.1–0.9 Ga; these included the transformation of external accretionary belts into the internal Grenville and equivalent collisional belts. Although Columbia provides evidence for a form of 'lid tectonics', modern style plate tectonics occurred on its periphery in the form of accretionary orogens. The detrital zircon and preserved geological record are compatible with an increase in the volume of continental crust during Columbia's lifespan; this is a consequence of the continuous accretionary processes along its margins. The quiescence in plate tectonic movements during Columbia's lifespan is correlative with a long period of stability in Earth's atmospheric and oceanic chemistry. Increased variability starting at 1.3 Ga in the environmental record coincides with the transformation of Columbia to Rodinia; thus, the link between plate tectonics and environmental change is strengthened with this interpretation of supercontinent history.
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The regional ultrahigh–temperature (UHT) metamorphism of the Highland Complex, Sri Lanka is well established and has an important role in our understanding of the tectonic history of the Gondwana supercontinent. U–Pb zircon dating of sapphirine–bearing Mg–Al granulites yielded two major metamorphic age populations at approximately 620–590 and 563–525 Ma with no evidence of older zircon cores. Pelitic granulite samples with a Grt–Sil–Spl–Crd assemblage have similar metamorphic ages with concordant data clusters at ~ 602, 563, and 526 Ma and inherited zircon cores aged from 2040 to 1600 Ma. The pelitic granulites also underwent two stages of metamorphism (565–520 and 622–580 Ma). Some of these pelitic granulite samples have inherited zircon cores ranging from 3060 to 760 Ma. Zircons in mafic granulite samples have age ranges of 566–533 and 620–578 Ma. A calc–silicate granulite sample also has similar age populations at 591, 541, and 524 Ma. Combining these new results with previously published ages from Sri Lanka and formerly adjacent continental fragments of Gondwana, we propose that the terranes in southern Madagascar (south of Ranotsara Shear Zone), Northern and Southern Madurai and the Trivandrum Blocks of southern India, the Highland Complex of Sri Lanka, and the Skallen Group in the Lützow–Holm Complex of East Antarctica represent a unique metamorphic belt that regionally experienced the Ediacaran–Cambrian UHT event during the amalgamation of the Gondwana supercontinent.
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On the basis of analysis and generalization of modern data the features of the structure and tectonic evolution of granulite-gneiss (high-grade) belts of the Earth are considered. Their continental collisional tectonic nature, polycyclic and inherited character of development, expressed in repeated manifestations in the same belt of several stages of granulite metamorphism, separated by intervals of several hundred million years, are confirmed. Granulite-gneiss belts are permanent mobility structures that maintain endogenous activity in all stages of their existence, including intraplate environments. The relationship between high-grade belts and supercontinental cyclicity is revealed, which is expressed in the spatial coincidence of the majority of them to the outskirts of the young oceans that arose during the breakup of Pangea; in the control of assembly and breakup of ancient supercontinents along granulite belts; in correlation of manifestations of different types of granulite metamorphism in these belts with the stages of the supercontinent cycle. In the evolution of these belts there is a complex interaction of plate-tectonic and mantle-plume mechanisms, which is expressed in the combination of continental collision and underplating processes. The possibility of using granulite-gneiss belts in paleotectonic analysis along with other indicators of geodynamic settings is shown.
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Because magmatism associated with subduction is thought to be the principal source for continental crust generation, assessing the relative contribution of pre-existing (subducted and assimilated) continental material to arc magmatism in accreted arcs is important to understanding the origin of continental crust. We present a detailed Nd isotopic stratigraphy for volcanic and volcaniclastic formations from the South Mayo Trough, an accreted oceanic arc exposed in the western Irish Caledonides. These units span an arc–continent collision event, the Grampian (Taconic) Orogeny, in which an intra-oceanic island arc was accreted onto the passive continental margin of Laurentia starting at ∼ 475 Ma (Arenig). The stratigraphy corresponding to pre-, syn- and post-collisional volcanism reveals a progression of ε Nd(t) from strongly positive values, consistent with melt derivation almost exclusively from oceanic mantle beneath the arc, to strongly negative values, indicating incorporation of continental material into the melt. Using ε Nd(t) values of meta-sediments that represent the Laurentian passive margin and accretionary prism, we are able to quantify the relative proportions of continent-derived melt at various stages of arc formation and accretion. Mass balance calculations show that mantle-derived magmatism contributes substantially to melt production during all stages of arc–continent collision, never accounting for less than 21% of the total. This implies that a significant addition of new, rather than recycled, continental crust can accompany arc–continent collision and continental arc magmatism.
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