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Tectonomagmatic formation of Earth’s first continents, the Slaufrudalur Pluton, SE Iceland


Project Description

Earth is the only planet in our solar system that has distinctive oceanic and continental crust. Importantly, the growth of Earth’s first stable and preserved continental crust about 4 billion years ago modified the early mantle, hydrosphere and atmosphere and enabled terrestrial life to evolve. However, the mechanism responsible for early continental development remains disputed, although there is growing geological evidence suggesting continental genesis was initiated by intracrustal melting of 25–45 km thick basaltic crust (Fig. 1). Further evidence is required to support the intracrustal theory as subduction processes have not been completely ruled out. Up to 90% of continental crust from 4.0–3.6 billion years ago (the Eoarchaean Era) is composed of mineralogically and geochemically unique granitic rocks that are called tonalites, trondjhemites and granodiorites (TTG). Therefore, in order to study ancient continental crust forming environments it would be ideal if a modern analogue that has granitic rocks intruded into thick predominantly mafic crust could be investigated. Such an analogue exists with Iceland, which has an average mafic crustal thickness of 30 km, is compositionally similar to possible Eoarchaean mafic crust and is the site of granitic plutonic intrusions. We want to know: (1) how was granitoid magma generated (fractional crystallisation from mantle plume partial melts or basaltic intracrustal partial melt); and (2) is the accommodation and emplacement of magma consistent with its genesis (is magma added to the crust from the mantle or passively transferred in the case of intracrustal magma genesis).

The Slaufrudalur pluton in SE Iceland is the largest granitic pluton exposed on Iceland. It is a roughly 8 x 3 km elongate granite body with a gently dipping roof and steep exposed sides. Glacial valleys provide good access to the 3D geometry of the body, which displays clear relationship between faulting and magma emplacement by subsidence of the floor of the pluton.
This project has two main objectives. The first is to undertake comprehensive major and trace element and Sr-Pb-Nd-Hf radiogenic isotope analyses to determine if the pluton has TTG compositions and determine if it has been derived from an Icelandic basaltic protolith or fractionated mantle partial melt. Thin section petrographic analysis of the collected samples will also be undertaken to help constrain whether silicic crustal material was formed through crustal fusion. The second objective is to test the floor down drop emplacement model against our petrogenetic analysis. palaeomagnetic and magnetic fabric analysis will be used to measure emplacement related deformation of the country rocks and examine internal fabrics that record magma flow. This will provide structural data that will allow the emplacement related kinematics of the magma and country rocks to be detected.

In summary, we will determine if the thick mafic crust and granitic pluton relationship in Iceland is analogous to the Eoarchaean geodynamic make-up of the early Earth. If so, the geochemical and structural analyses will enable us to better understand how the first silicic continental nuclei were generated and progressively constructed.

The principal supervisor will be Dr Alan Hastie (Geochemistry and Petrology) and co-supervision by Carl Stevenson (Structure and Rock Magnetism). External supervision panel will include Professor Thor Thordarsen (Reykjavik) and Professor Godfrey Fitton (Edinburgh), both experts in Icelandic geology and geochemistry, and by Dr Steffi Burchardt (Uppsala) who is an expert in magma emplacement mechanisms and developed the currently accepted model for the Slaufrudalur Pluton.

Please contact the project supervisors for further details: Alan Hastie - , Carl Stevenson -

Funding Notes

CENTA studentships are for 3.5 years and are funded by NERC. In addition to the full payment of their tuition fees, successful candidates will receive the following financial support:

Annual stipend, set at £14,777 for 2018/19
Research training support grant (RTSG) of £8,000

References

Johnson, T. E., Brown, M., Gardiner, N.J., Kirkland, C.L. & Smithies, H. Earth's first stable continents did not form by subduction. Nature 543, 239-242 (2017).
Dhuime, B., Hawkesworth, C. & Cawood, P., Storey, C.D. A change in the Geodynamics of Continental Growth 3 Billion Years Ago. Science 335, 1334-1336 (2012).
Moyen, J.-F. M., H. . Forty years of TTG research. Lithos 148, 312-336 (2012).
Burchardt, S., Tanner, D. and Krumbholz, M., The Slaufrudalur pluton, southeast Iceland—an example of shallow magma emplacement by coupled cauldron subsidence and magmatic
stoping. Geological Society of America Bulletin, 124, pp.213-227 (2012).
Stevenson, C.T., Owens, W.H., Hutton, D.H., Hood, D.N. and Meighan, I.G. Laccolithic, as opposed to cauldron subsidence, emplacement of the Eastern Mourne pluton, N. Ireland: evidence from anisotropy of magnetic susceptibility. Journal of the Geological Society, 164(1), 99-110 (2007).

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