Archaean geodynamics of the Zimbabwe craton

St Andrews, School of Earth and Environmental Sciences In partnership with Durham University and the BGS Supervisory Team • Nick Gardiner, University of St Andrews

• Jeroen van Hunen, Durham University • Laurence Robb, University of the Witwatersrand • Kathryn Goodenough, British Geological Survey

Key Words Archaean TTG granitoids, Geodynamics, Fieldwork; Isotope

crustal evolution; Numerical modelling Overview How Eo- to Paleoarchaean (4.0–3.2 Ga) granite-greenstone terranes were built, and the geodynamic settings in which this occurred, are fundamental questions in early Earth geology. The Archaean Earth was likely hotter thus the prevailing geodynamic regime, and the nature of crust formation, were distinct to that on the modern Earth. Constraining the origin of Archaean tonalite-trondjhemite-granodiorite (TTG) crustal rocks and their derivatives, poses fundamental questions in Earth Sciences such as: the onset of plate tectonic processes; the emergence and stabilization of the continents; the origin of life; the nature of mineralization on Earth and other rocky planets.

Figure 1. Different models for early Archaean crust formation. From Van Kranendonk et al. (2014). Models for the formation of early Archaean granite-greenstone terranes include horizontal subduction-accretion type processes (e.g., de Wit et al., 1992), but also infracrustal melting within a volcanic plateau (e.g., Smithies et al., 2009) (Fig. 1). In the latter scenario, a vertically-accreting volcanic substrate reaches a critical thickness where hydrated basalt partially melts to form TTG. One new approach to constrain crust production and evolution on the Archaean Earth, is to integrate fieldwork and petrological studies of appropriate

rocks with geodynamic modelling. Modern geochemical and isotopic techniques (e.g., Hf and O isotopes and trace elements in zircon) applied to TTG help constrain both the nature of magmatic source and the processes of melting. These results can then provide a framework for numerical modelling to understand the nature, and crucially timescales, of mantle and crust melting.

Figure 2: Numerical models for modern and Archaean subduction dynamics (Moyen & van Hunen, 2012). The candidate will undertake targeted fieldwork in the Zimbabwe Craton using modern analytical techniques. The craton comprises an early Archaean (3.5 Ga) gneissic core surrounded by major granite-greenstone belts hosting 2.9—2.7 Ga TTG domes (Horstwood et al., 1999; Rollinson & Whitehouse, 2011), giving the opportunity to interrogate secular changes in magmatic style. Isotopic and geochemical analyses of appropriate samples will be undertaken and then integrated with new geodynamic models to yield a holistic view of the setting and timescales of early Archaean granite-greenstone terrane development. The project builds on previous work by the supervisors on Archaean terrane growth (Johnson et al. 2017; Gardiner et al., 2019) in collaboration with geodynamic modelling expertise at Durham (e.g. Van Hunen & Moyen, 2012). The project will benefit from

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summarizes the typical model results, in which the subduction process for a Phanerozoic setting is compared to an Archaean setting (with an assumed 200 K hotter mantle). The Phanerozoic model illustrates continu-ous subduction with a typical 5−10 cm/yr convergence rate, representative for today’s subduction. The Archaean setting displays a different behavior, as slabs frequently break off from the trailing plate and sink down into the transition zone. This change in subduction behavior is a consequence of several effects: (1) the thicker oceanic crust creates a larger tensile stress between the buoyant crust near the surface and the dense (eclogitic) crust at depth; (2) due to the larger average subduction velocity, oceanic plates are younger and therefore thinner and weaker when arriving at the trench; (3) a weaker mantle leads to more vigorous sublithospheric small-scale convection and subsequent lithospheric thinning (van Hunen et al., 2005); (4) the thick, intrinsically weaker oceanic crust leads to a reduced inte-grated strength of the subducting plate; and (5) the weaker mantle pro-vides less support for the sinking slab. This combination of effects leads to weaker slabs that cannot maintain the encountered tensile stresses during subduction, and therefore frequently yield in the form of slab breakoff. Such breakoff would lead to a temporal loss of slab pull, and a period in which subduction would be absent or very slow, with no or very little vola-tile input in the mantle, and subsequent magmatic quiescence.

These results suggest a subduction episodicity of a few million years, but the exact duration of break-off events is somewhat uncertain. The dura-tion of modern, continental collision-triggered events is a topic of signifi-cant debate, with estimates ranging from a few million years up to 20 m.y. (Andrews and Billen, 2009; Duretz et al., 2011; van Hunen and Allen, 2011). This is controlled by a range of geodynamical parameters, most of which are unconstrained for the generic Archaean scenario studied here.

DISCUSSION AND CONCLUSIONThe use of a geochemical “arc” signature as a marker of subduc-

tion is debatable. Firstly, a range of petrogenetic processes can yield “arc” signatures (Bédard et al., 2010; van Hunen and Moyen, 2012), and burial of mafic rocks in the mantle is not the only option; however, the close

temporal association of distinct types of rocks, all with some kind of arc affinity, suggests that this petrogenetic scenario is still the most likely on a regional scale (Smithies et al., 2005). Secondly, the burial of mafic rocks in the mantle can occur in nonsubduction environments (e.g., by delami-nation of the mafic crust; Bédard, 2006), but in these models, it is unclear how hydrous rocks are transported down. We acknowledge, however, that geochemistry puts only weak constraints on the size and shape of the buried parcels of mafic rocks. Thirdly, there may be preservation issues such that some time slices could be missing, therefore creating an appar-ent episodicity. However, in the Abitibi case (as in most other examples cited), we do have a continuous or nearly continuous stratigraphic record, without long breaks (Fig. 1; see the Data Repository). Therefore, we can confidently state that no known event of “arc” magmatism in the Archaean lasted longer than a few tens of millions of years.

Cessation of arc magmatism can occur for reasons other than slab breakoff. (1) During modern subduction of oceanic buoyant plateaus, magmatism ceases temporarily due to flattening of the subducting slab. Such a mechanism, however, is not likely to be viable in an Archaean, hotter mantle (van Hunen et al., 2004). (2) Modern subduction stops with continental collision, and the same probably occurred in the Archaean as well. Depending on the intermediate ocean size, Phanerozoic collision intervals varied from a few to several hundred million years (i.e., a much larger range than observed in the Archaean rock record), and applying this mechanism to the observed short-lived “arc” signature would suggest a dramatic change of the size of ocean basins throughout Earth history. The mechanism of spontaneous, frequent slab breakoff of weak slabs in a hotter Earth is more appealing, because it only requires a (well-accepted) change in mantle temperature to explain the difference between Archaean and modern subduction dynamics. It does provide an elegant explanation to the classical interleaving of both “arc” and “plume” rocks in Archaean rock sequences (i.e., plume-arc interaction; Wyman et al., 2002).

We do not know when the plate tectonics–style of convection started on Earth, with estimates ranging from nearly 4.5 Ga. down to 2 Ga (or even later according to some authors; Stern, 2005). Our work suggests a way to evolve a single-plate, stagnant lid style of tectonics into mod-ern plate tectonics. Early subduction events in a hot mantle were small and short. As the mantle cooled down, the size and duration of the down-wellings increased, and they evolved into proper, permanent subduction. This short-term episodicity may have been superimposed to a longer-term periodicity (100 m.y.), related to episodic mantle overturns (Davies, 1995), supercontinent formation and breakup (Silver and Behn 2008), or to intermittent plate locking (O’Neill et al., 2007). Therefore, plate tec-tonics may not have “appeared,” but rather matured, over a long period of time (perhaps of several gigayears), during which subduction became progressively more widespread, efficient, and stable.

ACKNOWLEDGMENTSThis work was supported by the European Research Council (ERC StG 279828).

This is a contribution to the IGCP-SIDA project #599 “The Changing Earth” spon-sored by UNESCO/SIDA.

REFERENCES CITEDAbbott, D., Burgess, L., Longhi, J., and Smith, W.H.F., 1994, An empirical ther-

mal history of the Earth’s upper-mantle: Journal of Geophysical Research, Solid Earth, v. 99, p. 13835–13850, doi:10.1029/94JB00112.

Andrews, E.R., and Billen, M.I., 2009, Rheologic controls on the dynamics of slab detachment: Tectonophysics, v. 464, p. 60–69, doi:10.1016/j.tecto.2007.09.004.

Ayer, J., Thurston, P., Dube, B., Gibson, H.L., Hudak, G., Lafrance, B., Lesher, C.M., Piercey, S.J., Reed, L.E., and Thompson, P.H., 2004, Discover Abitibi Greenstone Architecture Project: Overview of results and belt-scale impli-cations: Ontario Geological Survey Open File Report 2004, p. 37-1–37-15.

Bédard, J., 2006, A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle: Geochimica et Cosmochimica Acta, v. 70, p. 1188–1214, doi:10.1016/j.gca.2005.11.008.

Bédard, J., Leclerc, F., Harris, L.B., and Roy, P., 2010, Calc-alkaline interrup-tions of a tholeitic sequence, Chibougamau, Abitibi belt: Remelting of a

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Figure 2. Numerical model calculations for subduction dynamics through time. Left: Subduction velocity (top) and subduction dy-namics (bottom) for the present-day mantle potential temperature and 7-km-thick oceanic crust, illustrating a coherent, continuously subducting slab. Right: Subduction in a 200 K hotter mantle with a 15-km-thick oceanic crust. A weaker slab results in intermittent sub-duction due to frequent slab break-off events. Further details about the model calculations are provided in the Data Repository (see foot-note 1) and in van Hunen and van den Berg (2008).

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project partners at the University of the Witwatersrand (Laurence Robb & Carl Anhaeusser).

Methodology This explorative project will undertake fieldwork and sampling in the craton and apply modern isotopic and geochemical techniques to constrain the age and nature of TTG genesis. It will then build geodynamic models for the formation of Archaean granite-greenstone terranes. At least one field season in Zimbabwe will be undertaken in collaboration with the BGS and the University of the Witwatersrand. On-the-ground work in Zimbabwe has recently been undertaken by members of the supervisory team Analytical work will be carried out at the University of St Andrews Isotope Geochemistry Laboratory (StAIG), which is equipped with laser ablation ICP-MS facilities to undertake in-situ trace element and isotopic analyses of mineral phases, and to date magmatic rocks through zircon U-Pb. Combined geodynamical and petrological modelling in this project will be undertaken at Durham, with the state-of-the-art community-supported code ASPECT (https://aspect.geodynamics.org) coupled to the widely used software Perple-X (http://www.perplex.ethz.ch/).

Timeline Year 1) Familiarity of current controversies in Archaean geodynamics. Field season to Zimbabwe. Year 2) Geochemical and isotopic analyses; 21-month progress report; preparation for publication of first key results in a peer-reviewed journal. Year 3) 6 month academic secondment to Durham University to build numerical models - results by full integration of geodynamical models with observables, such as seismic tomography, kimberlite xenolith data, tectonic activity, and magmatism; 33-month progress report; first publication and preparation for publications of further research. Participation in international conference. Final 6 months) Finalizing further publications of research outcomes; thesis completion and submission.

Training & Skills The PhD student will join the Solid Earth and Planetary Science Research Group at the University of St Andrews and become part of a vibrant research culture. Full training on the appropriate field, geochemical, isotopic, and geodynamic modelling will be provided

by the project supervisors, in the field, at St Andrews, and at the University of Durham, where the candidate is expected to spend a 6 month secondment. The student is expected to attend national and international conferences to disseminate research results and to spend time away from St Andrews to integrate project partners at the partner institutes.

References & Further Reading de Wit, M.J., de Ronde, C.E.J., Tredoux, M., Roering,

C., Hart, R.J., Armstrong, R.A., Green, R.W.E., Peberdy, E., Hart, R.A. 1992. Formation of an Archaean continent. Nature 357 553-562.

Gardiner, N.J., Hickman, A.H., Kirkland, C.L., Lu, Y.J., Johnson, T.E., Zhao, J.X., 2017. Processes of Crust Formation in the Early Earth Imaged through Hf isotopes from the East Pilbara Terrane. Precambrian Research 297 56-76.

Horstwood, M.S.A., Nesbitt, R.W., Noble, S.R., Wilson J.F. 1999. U-Pb zircon evidence for extensive early Archaean craton in Zimbabwe: A reassessment of the timing of craton formation, stabilization, and growth. Geology 27 707-710.

Johnson, T.E., Brown, M., Gardiner, N.J., Kirkland, C.L., Smithies, R.H., 2017. Earth's first stable continents did not form by subduction. Nature 543 239-242.

Moyen, J-.F., van Hunen, J. 2012. Short-term episodicty of Archean plate tectonics. Geology 40, 451-454.

Rollinson, H.R., Whitehouse, M.J. 2011. The growth of the Zimbabwe Craton during the late Archaean: an ion microprobe U–Pb zircon study. Journal of the Geological Society 168 941-952

Smithies, R.H., Champion, D.C., Van Kranendonk, M.J., 2009. Formation of Paleoarchean continental crust through infracrustal melting of enriched basalt. Earth and Planetary Science Letters 281 298-306.

van Hunen, J., Moyen, J-.F. 2012. Archean subduction: Fact or fiction? Annual Review of Earth and Planetary Sciences. 40, 195-219.

Van Kranendonk, M.J., Smithies, R.H., Griffin, W.L., Huston, D.L., Hickman, A.H., Champion, D.C., Anhaeusser, C.R., Pirajno, F., 2015. Making it thick: a volcanic plateau origin of Palaeoarchean continental lithosphere of the Pilbara and Kaapvaal cratons. Geological Society, London, Special Publications 389, 83-111.

Further Information For any information on the project please contact Nick Gardiner (nick.gardiner@st-andrews.ac.uk).