Overview

Project Highlights:

  • Exploring a modern analogue for early continental evolution and crustal construction
  • Geochemical and rock magnetic analyses
  • Fieldwork in SE Iceland
  • Granite emplacement and petrogenesis

Overview:

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.

 

Figure 1 – Intracrustal melting

Methodology

  • Icelandic fieldwork and sample collecting
  • Major and trace element analyses
  • Sr-Pb-Nd-Hf radiogenic isotope study
  • Thin-section petrographic analysis
  • Texture and fabric analyses achieved through AMS measurements of oriented outcrop samples.
  • 3D modelling of the geometry and structure of the pluton as well as the deformation and strain in the country rocks using geological modelling software MoveTM

Training and Skills

CENTA students are required to complete 45 days training throughout their PhD including a 10 day placement. In the first year, students will be trained as a single cohort on environmental science, research methods and core skills. Throughout the PhD, training will progress from core skills sets to master classes specific to the student's projects and themes. Project specific training will include major and trace element analyses through XRF and/or ICP-MS and radiogenic isotope analyses at NIGL. AMS analyses will be undertaken in the palaeomagnetic laboratory in Birmingham. Key rock magnetic analysis will be susceptibility and AMS used for fabric analysis. As well as laboratory rock magnetic analysis, the project will involve field sampling of oriented samples using block sampling and field drilling.

Timeline

Year 1:

  • Literature review and compilation of published data
  • Planning and development of field programme
  • Fieldwork
  • Major and trace element preparation and analysis

Year 2:

  • Substantive rock magnetic analyses
  • Petrological and geochemical analyses/interpretation
  • Second field season
  • Radiogenic isotope analyses/interpretations

Year 3:

  • Integration of dataset
  • Writing up main outputs and thesis
  • Presentation of major findings at international conference/publication

Partners and collaboration (including CASE)

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.

Further Details

Please contact the project supervisors for further details.

Alan Hastie: a.r.hastie@bham.ac.uk

Carl Stevenson: c.t.stevenson@bham.ac.uk