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Subduction or Intraplate? Granitoids in the Virgin Islands and implications for the growth of early-Earth-like continental crust

Alastair G. E. Hodgetts 1, Lindsay A. Young1, Alan R. Hastie1, J. Godfrey Fitton1, Richard Palin2, Marc-Alban Millet3, Sally Law1, Jeroen van Hunen4, Craig Storey5, Andrew Kerr3

  • Affiliations:  1 School of GeoSciences, University of Edinburgh, Edinburgh, United Kingdom; 2 Department of Earth Sciences, University of Oxford, Oxford, United Kingdom; 3 School of Earth and Environmental Sciences, Cardiff University, Cardiff, United Kingdom; 4 Department of Earth Sciences, Durham University, Durham, United Kingdom; 5 School of the Environment, Geography and Geosciences, University of Portsmouth, Portsmouth, United Kingdom 

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 30

  • Programme No: 1.7.17

  • Theme 1 > Session 7

Abstract

A compositionally heterogenous suite of tonalite-trondhjemite-granodiorite (TTG) rocks comprise the oldest preserved continental crust. This material is considered to be generated through partial melting of metamorphosed mafic source rocks. However, the geological mechanisms in the formation of TTG remains poorly understood. Both subduction and intraplate settings are proposed environments of formation. Petrological studies indicate early-earth basaltic surfaces share similar compositions to both Mesozoic Ocean plateaus and island arc basalts (IAB). This suggests that Archaean TTGs may have been formed from partial melting plateau and/or IAB compositions via subduction or intraplate processes. The Virgin Islands (VI) are a collection of over 100 islands located in the northeastern corner of the Caribbean plate. Since ~60 Ma, the 15-20 km thick Caribbean oceanic plateau has been underthrusting and subducting beneath the North American plate. This has formed two distinct tectonic environments in the VI: (1) an intraplate environment in the southern islands, with ~30 km of stacked ocean plateau and IAB material; and (2) deeper subduction towards the north. Through geological mapping and sampling, granitoids with TTG compositions have been identified throughout the VI, therefore providing a unique opportunity to determine whether these early-Earth-like TTGs have been derived from metamorphosed IAB and oceanic plateau material from the southerly intraplate setting and/or the northerly subduction environment. Here, we present field observations, initial whole rock major and trace element analyses and preliminary mass balance partial melting and fractional crystallization modelling for chosen starting compositions to determine the pressure-temperature conditions required to generate VI TTGs.