Exploring the tectono-magnetic evolution of Mesoproterozoic giant sill and dyke complexes

Overview

Igneous complexes of sills and dykes commonly represent globally significant tectono-magmatic rift events (Ernst, 2014). Some of the most exciting examples of fossilised rift zones are the Mesoproterozoic giant dyke and sill complexes of Sweden, Canada, Zimbabwe and Greenland that formed between the Nuna and Rodinian supercontinents (Gorbatschev et al., 1979; LeCheminant and Heaman, 1989; B. G. J. Upton, 2013; Ripa and Stephens, 2020a). The eroded cores of these failed rift zones reveal economically significant critical metal resources within mafic to ultra-mafic sheet intrusions that range from a few 100 m to > 1 km thick (e.g. Rosa et al. (2020), Koopmans et al. (2021)). Giant dykes and sills appear to be unique to the period between 1.6 – 1.0 Ga, an observation that infers rifting and magmatic process operated differently during the Mesoproterozoic (B. G. J. Upton, 2013; Ripa and Stephens, 2020a). Whilst the mechanism of giant dyke formation remains poorly understood, their occurrence suggests that a specific geodynamic criterion was met during the Mesoproterozoic that favoured their emplacement in several cratons over a ~ 600Ma period. This study asks What is the tectonic significance of Giant Dyke complexes?
The study will focus on the Central Scandinavian Dolerite Group (CSDG), a suite of Mesoproterozoic giant sills and dykes in the Baltica craton in east Sweden. In the Baltica craton, three widespread anorogenic magmatic events have been identified, (1) rapakivi anorogenic complexes emplaced ~ 1.45 Ga simultaneously with dolerite dyke swarms (Ripa and Stephens, 2020b), (2) the Central Scandinavian Dolerite Group (CSDG) that was emplaced at 1.27 – 1.25 Ga, which consists of up to 1 km thick giant dolerite sills and dyke swarms (Gorbatschev et al., 1979) and (3) the 1.1 Ga ultramafic lamprophyre to carbonatitic sheeted intrusions of north-eastern Sweden (Kresten et al., 1981; Ripa and Stephens, 2020a). Provisional regional AMS studies on the CSDG reveal a NW-SE magnetic lineation in the giant sills of the CSDG which have been interpreted to represent a magma flow direction however additional data are needed before a holistic interpretation can be developed (Elming and Mattsson, 2001; Mattsson and Elming, 2003). By comparing isotope compositions from different areas of the CSDG, the project will investigate differences in source composition and together with magnetic fabric studies explore structural relationships observed to ask the question Are giant dykes the result of large melt production/fast rifting or caused by external factors such as crustal thickness and rheology?.
The CSDG was emplaced at the same time as the 1.3 to 1.1 Ga Gardar province in SE Greenland and the 1.27 Ga Mackenzie dyke swarm in Canada (Upton and Emeleus, 1987; Upton et al., 2003; Brian G. J. Upton, 2013). Geodynamic studies reveal that Baltica was rotated 90 degrees during the Mesoproterozoic and infer the existence of either a volcanic arc or a mantle plume associated with the rifting events (LeCheminant and Heaman, 1989; Söderlund et al., 2006; Cawood et al., 2010; Ripa and Stephens, 2020a, 2020b). The successful candidate will collaborate with ongoing research at the University of St Andrews on the Giant Dykes of SE Greenland with a view to testing the hypothesis that the complexes are tectonically related.
The PhD is designed to be flexible, the successful candidate will be encouraged to pursue their own exciting ideas using the wide range of state-of-the-art analytical facilities and expertise made available by the advisory team and the IAPETUS network.

Methodology

Fieldwork, palaeomagnetic and geochronological analysis are central to this study.
The project consists of two field campaigns to Sweden (Year 1 and Year 2) to map and sample the intrusions associated with Mesoproterozoic rifting. Fieldwork includes state-of-the art UAV based structural mapping of internal features (e.g. ophitic textured layers and pegmatites) in the sills. Fieldwork will be supported by Mattsson who has more than ten years of experience in geological fieldwork in Sweden focussing on magma dynamics and structural geology.
Rock magnetic analysis, including Anisotropy of Magnetic Susceptibility and palaeomagnetic investigations, will investigate the interrelationship between tectonic deformation and magma flow dynamics during the formation of giant dykes and sills. Magma flow patterns are instrumental in understanding the interdependency of rift development and large-scale magma transport in continental rift zones. Palaeomagnetic investigations will investigate movement of the Baltica craton during Mesoproterozoic tectono-magnetic events and the arising data will be compared to existing data compiled form the Giant Dykes of SE Greenland.
High precision isotope geochronology will be used to place robust constrains on the timing of tectono-magmatic events and evaluate the rate at which volumes of magma were moved through the CSDG. Isotopic analysis will be undertaken at the BGS facility at Keyworth under the supervision of Dr Dan Condon.

Project Timeline

Year 1

Introduction to project & literature review.
Conduct new, high-resolution mapping and structural analysis of internal units, using drone, satellite and digital field mapping techniques, to efficiently record structural data from the sills and to identify structures related to magma flow at Höga Kusten, Sweden.
Collect first rock magnetic and petrographic data to interpret magma flow patterns in context of regional structural data.

Year 2

Second field campaign to Central and northern Sweden to collect samples from various Mesoproterozoic intrusions and follow up rock magnetic and geochemical data analysis.
Palaeomagnetic investigation on Mesoproterozoic rocks from across the Baltica craton, as well as other contemporary intrusions in the Laurentian craton to reveal the tectonic evolution in the Mesoproterozoic.
Selection of samples for geochronological study to place constraints on the timing and rate of magma supply.
Write up first paper with tutelage from supervisors

Year 3

Acquire new, high-resolution geochemical data for the dykes to constrain potential magma evolution differences between contemporary dyke swarms and giant sills in the Baltica craton.
Integrate existing palaeomagnetic and isotopic data from other giant dyke and sill complexes to produce a conceptual model describing dyke emplacement and evolution of Mesoproterozoic rifting in the Baltica and Laurentian cratons.
Write up second/third paper.

Year 3.5

Write-up, submission of papers and thesis

Training
& Skills

The successful candidate will receive bespoke training in: (1) digital and traditional mapping techniques (Mattsson & McCarthy); (2) palaeomagnetic and rock fabric analysis techniques (McCarthy and Mattsson); (3) transmitted light petrography (all supervisors); and (4) major and trace element, isotopes and radiometric dating (Condon). Training will be largely one-to-one. Over the course of the PhD you will gain many transferable skills such as scientific writing, statistical analysis, time management and independent research planning skills. Formal training courses, as part of the fulfilment of DTP transfer requirements, will also be undertaken. At the end of the PhD, you will become a confident and independent researcher with transferable skills applicable to both academic and non-academic jobs.

References & further reading

Cawood, P. a., Strachan, R., Cutts, K., Kinny, P.D., Hand, M., Pisarevsky, S., 2010. Neoproterozoic orogeny along the margin of Rodinia: Valhalla orogen, North Atlantic. Geology 38, 99–102. https://doi.org/10.1130/G30450.1
Elming, S., Mattsson, H.J., 2001. Post Jotnian basic Intrusions in the Fennoscandian Shield, and the break up of Baltica from Laurentia: a palaeomagnetic and AMS study. Precambrian Research 108, 215–236. https://doi.org/10.1016/S0301-9268(01)00131-0
Gorbatschev, R., Solyom, Z., Johansson, I., 1979. The Central Scandinavian Dolerite Group in Jämtland, central Sweden. Geologiska Föreningen i Stockholm Förhandlingar 101, 177–190. https://doi.org/10.1080/11035897909448572
Koopmans, L., Webster, R.A., Changleng, R., Mathieson, L., Murphy, A.J., Finch, A.A., McCarthy, W., 2021. New insights from field observations of the Younger giant dyke complex and mafic lamprophyres of the Gardar Province on Tuttutooq island, South Greenland. GEUS Bulletin 47. https://doi.org/10.34194/geusb.v47.6526
Kresten, P., Åhman, E., Brunfelt, A.O., 1981. Alkaline ultramafic lamprophyres and associated carbonatite dykes from the Kalix area, northern Sweden. Geologische Rundschau 70, 1215–1231. https://doi.org/10.1007/BF01820189
LeCheminant, A.N., Heaman, L.M., 1989. Mackenzie igneous events, Canada: Middle Proterozoic hotspot magmatism associated with ocean opening. Earth and Planetary Science Letters 96, 38–48. https://doi.org/10.1016/0012-821X(89)90122-2
Mattsson, H.J., Elming, S.-Å., 2003. Magma flow directions of post Jotnian dolerite sills in central-east Sweden: a magnetic fabric and paleomagnetic survey. GFF 125, 7–16. https://doi.org/10.1080/11035890301251007
Ripa, M., Stephens, M.B., 2020a. Chapter 12 Dolerites (1.27–1.25 Ga) and alkaline ultrabasic dykes ( c. 1.14 Ga) related to intracratonic rifting. In: Stephens, M.B. (Ed.), Sweden: Lithotectonic Framework, Tectonic Evolution and Mineral Resources. Memoir. The Geological Society of London, London, 315–323. https://doi.org/10.1144/M50-2017-5
Ripa, M., Stephens, M.B., 2020b. Chapter 10 Magmatism (1.6–1.4 Ga) and Mesoproterozoic sedimentation related to intracratonic rifting coeval with distal accretionary orogenesis. Geological Society, London, Memoirs 50, 269–288. https://doi.org/10.1144/M50-2017-4
Rosa, D., Sandrin, A., Nielsen, T.F.D., Vesturklett, H., 2020. Petrography, geochemistry and magnetic susceptibility of the Isortoq Fe-Ti-V deposit, Isortoq Giant Dykes, South Greenland. GEUS Bulletin 44. https://doi.org/10.34194/geusb.v44.4626
Söderlund, U., Elming, S.-Å., Ernst, R.E., Schissel, D., 2006. The Central Scandinavian Dolerite Group—Protracted hotspot activity or back-arc magmatism? Precambrian Research 150, 136–152. https://doi.org/10.1016/j.precamres.2006.07.004
Upton, Brian G. J., 2013. Tectono-magmatic evolution of the younger Gardar southern rift, South Greenland. Geological Survey of Denmark and Greenland (GEUS) Bulletin 29, 1–24. https://doi.org/10.34194/geusb.v29.4692
Upton, B.G.J., Emeleus, C.H., 1987. Mid-Proterozoic alkaline magmatism in southern Greenland: The Gardar province. Geological Society Special Publication 30, 449–471. https://doi.org/10.1144/GSL.SP.1987.030.01.22
Upton, B.G.J., Emeleus, C.H., Heaman, L.M., Goodenough, K.M., Finch, A.A., 2003. Magmatism of the mid-Proterozoic Gardar Province, South Greenland: Chronology, petrogenesis and geological setting. Lithos 68, 43–65. https://doi.org/10.1016/S0024-4937(03)00030-6

Further Information

If you are interested in applying for this project please contact Will McCarthy at wm37@st-andrews.ac.uk

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