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PhD thesis defense
PhD thesis defense — Rikke Vestergaard 17 APRIL
Date & Time:
Aud. C, Department of Geosciences and Natural Resource Management, Øster Voldgade 10, 1350 Copenhagen K
Rikke Vestergaard defends her thesis,
Making and shaping of South Greenland: Assessing crustal growth and evolution of the Ketilidian Orogen through the lens of U-Pb-O-Hf isotopes in zircon
Professor Tod Waight, IGN
Associate Professor Anna Pietranik, University of Wroclaw – Poland
Professor Jochen Kolb, Karlsruhe Institute of Technology – Germany
Associate Professor Nicolas Thibault (chair), IGN
The granitic continental crust of Earth is unique, and elucidating the mechanisms and rates governing the formation and preservation of continental crust is essential to understanding the geochemical differentiation of Earth. The aim of this PhD project is to provide new insights into the geological evolution in a regional context of South Greenland by means of a geochemical and petrogenetic investigation. Zircon is a typical accessory mineral in granitic rocks and has many favourable attributes such as resilience to physiochemical alteration, diagenesis and metamorphism, thus it is an ideal mineral for geochronology studies given that it readily incorporates uranium yet little to no lead during crystallisation. Moreover, it includes up to several weight percent hafnium, and together with oxygen can be used to fingerprint the sources of the granitic magmas. This is why zircon is the chosen pivot for extraction of recorded information during the development of South Greenland. This project mainly aimed to carry out the first detailed U-Pb-Hf-O isotope analyses in zircons from igneous and metasedimentary rocks from the west parts of South Greenland. The thesis of this project provides three manuscripts that presents new zircon U-Pb geochronology from both the Julianehåb Igneous Complex (JIC) and the Southern Domain, thus expanding the regional age coverage and increases the understanding of the temporal evolution of the Ketildian Orogen that constitutes South Greenland. The JIC is the arc and locus of the Ketilidian Orogen and the Southern Domain represents a fore-arc adjacent to the JIC that later was intruded by plutonic bodies of the Ilua Suite. Furthermore, Hf-O isotope compositions in zircons from the JIC witness a more complex source than what was previously conceived. The interpretation that the sediments in the fore-arc are derived from erosional products of JIC still stands, however the oxygen isotope composition in zircons from the metasediments complicates the interpretation. This project suggest that oxygen in zircons in the metasediments were disturbed and equilibrated with surrounding sediments during high-grade metamorphism, whilst U-Pb and Hf isotopes remained unperturbed. The isotopic composition of the Ilua Suite mimic that of the surrounding country rocks.
A digital version of the PhD thesis can be obtained from the PhD secretary Mikala Heckscher at firstname.lastname@example.org up until the time of the defence.