This week’s VS-PG presentation by Dr. Olivier Vanderhaeghe who studies the tectonics of crustal roots through structural analysis of migmatites.

Speaker: Dr. Olivier Vanderhaeghe

Date & Time: Wednesday, June 2nd at 8:00 AM (Pacific Daylight Time)

You can replay the video below.

Dynamics of partially molten crustal roots: from the Ma to the Ga timescale

Most continents are cored by Archaean cratonic nuclei that comprise greenstone belts delineating crustal-scale domes cored by magmatic/migmatitic rocks of felsic composition (Bouhallier et al., 1995; Condie, 1981; de Wit, 1998; Van Kranendonk et al., 2004). The widespread occurrence of high-temperature metamorphic mineral assemblages and of migmatites attests to a high crustal geothermal gradient during the Archaean (Brown, 2008). The magmatic record of the oldest cratonic nuclei, exemplified by the Pilbara carton and the Barberton terrane of the Kaapvaal craton, respectively in Australia and South Africa, typically spans over one billion years (Nd model ages ranging from 4.0 to 3.0 Ga and U-Pb ages on zircon ranging from 3.7 to 2.7 Ga (Hickman and Kranendonk, 2012; Zeh et al., 2011). This widespread prolonged record of high-T metamorphism and magmatism for Archean cratonic nuclei has been typically attributed either to a succession of magmatic-tectonic events associated with recurrent mantle plumes or to repeated/prolonged subduction of slabs with a shallow dip (e.g. Bédard, 2006; Bleeker and Hall, 2007; Percival et al., 2001).

In this presentation, I will go through younger examples of crustal roots characterized by migmatites and granitoids, namely the Naxos dome in the Alpine belt exposed in Greece (Vanderhaeghe et al., 2018), the Variscan mid to lower crust exposed in the Ivrea Zone (Guergouz et al., 2018), and the Grenvillian belt (Turlin et al., 2018) and use these observations to document and model the thermal-dynamical evolution of partially molten crustal roots (Louis-Napoléon et al., 2020; Vanderhaeghe et al., 2019).

These investigations lead to a new model for the prolonged and widespread high-T and magmatic activity of Archean cratons. Indeed, the main outcome of these studies is a tautology, namely that crustal roots might remain partially molten as long as the heat source is active. As a consequence, these partially molten crustal roots might be animated by convective gravitational instabilities that account for their structural, metamorphic and geochronological records. Accordingly, Archean cratonic nuclei might remain partially molten and stirred by convection for hundreds of Myrs, which corresponds to the time required to reduce their content in the main heat-producing radioactive elements 40K, 232Th, 238U, 235U.

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