Cryogeodynamics: testing potential feedback mechanisms between subduction, magmatism and glaciation

The Earth underwent a series of extensive glaciations during the Neoproterozoic. Feedbacks between glacial-deglacial events and tectonic activity have been proposed on a global scale due to enhanced erosion and lubrication of plates leading to increased rates of plate movement via subduction (Sobolev and Brown, 2019). These models require rigorous testing using the geological record of magmatism and sedimentation across this critical time period to assess the potential involvement of glacial-derived melts and eroded material in subduction zones and their derived mantle melts. 
The Cadomian orogeny in NW France involved arc magmatism, deformation and metamorphism from c.650 to 540 Ma, which was accompanied by development of intra- and back-arc sedimentary basins and deposition of the Brioverian Supergroup, which contains glaciogenic sediments. At this time, the region was located around the periphery of Gondwana and close to the South Pole. This PhD project aims to analyse the U-Pb, Lu-Hf and O isotopes of zircons from plutonic igneous rocks spanning the range of magmatic activity and also to analyse zircon rims from potentially glacial-influenced sediments of the Brioverian Supergroup. This will allow testing of whether the subduction zone magmas were contaminated by crustal-derived materials (sediments) formed under ice-house conditions either by subduction of sediments or by crustal contamination during emplacement. In addition, the extent to which the Brioverian Supergroup itself was formed in a glacial environment will be tested by measuring U-Pb-Hf-O isotopes of zircon rims where they have been partially melted in contact metamorphic aureoles to form new zircon overgrowths.