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.
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.
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 project partners at the University of the Witwatersrand (Laurence Robb & Carl Anhaeusser).
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Figure 1. Different models for early Archaean crust formation. From Van Kranendonk et al. (2014).
Figure 2: Numerical models for modern and Archaean subduction dynamics (Moyen & van Hunen, 2012).
Figure 3: Outline geological map of the ZImbabwe Craton.
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/).
Familiarity of current controversies in Archaean geodynamics. Field season to Zimbabwe.
Geochemical and isotopic analyses; 21-month progress report; preparation for publication of first key results in a peer-reviewed journal.
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.
Finalizing further publications of research outcomes; thesis completion and submission.
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.
For any information on the project please contact Nick Gardiner (firstname.lastname@example.org).