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Metamorphic Evolution of Relict Eclogite-Facies Rocks in the Paleoproterozoic Nagssugtoqidian Orogen, South-East Greenland”

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Metamorphic Evolution of Relict Eclogite-Facies Rocks in the Paleoproterozoic Nagssugtoqidian Orogen, South-East Greenland” “Metamorphic evolution of relict eclogite-facies rocks in the Paleoproterozoic Nagssugtoqidian Orogen, South-East Greenland” Von der Fakultät für Georessourcen und Materialtechnik der Rheinisch-Westfälischen Technischen Hochschule Aachen zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften genehmigte Dissertation vorgelegt von M.Sc. Geowissenschaften Sascha Müller aus Münster Berichter: PD Annika Dziggel Ph.D. Prof. Dr. Jochen Kolb Tag der mündlichen Prüfung: 14. Dezember 2018 Diese Dissertation ist auf den Internetseiten der Universitätsbibliothek online verfügbar. Foreword and Acknowledgements The following thesis was written over the course of 5 years, starting in August 2013, in the framework of the joint GEUS-MMR “SEGMENT (South-East Greenland Mineral Endowment Task)”-project. During the course of this study, I had the opportunity to witness the beautiful scenery and outstanding geology of Greenland firsthand during a one-month fieldtrip to the area around Tasiilaq in June and August of 2014, for which I greatly appreciate funding by the Geological Survey of Denmark and Greenland (GEUS) and the Ministry of Mineral Resources of Greenland (MMR). Regular funding was provided by the Deutsche Forschungsgemeinschaft from 2013 to 2016, with additional funding until late 2017 by employment at the Institute of Applied Mineralogy and Economic Geology at RWTH Aachen. At first, I want to thank my supervisor Annika Dziggel for her great support and guidance, but also for initiating this interesting project in the first place. Thank you for your support during fieldwork, for encouraging me to keep a sharp mind during analysis and interpretation and for teaching me how to properly present my data. Many thanks also go out to my Co-Supervisor Sven Sindern for his support during the countless hours I spent in the laboratory, as well as with the whole-rock data and isotopic dating. Thanks to Jochen Kolb for stimulating discussions about the geology of South-East Greenland and his help during fieldwork. I further want to thank Thomas Find Kokfelt and Axel Gerdes for their help in deciphering the geochronology of the high-pressure rocks of the Kuummiut Terrane. Thomas is additionally thanked for his efficient editorial handling of the field report and his support during the trace element analysis of zircon. I am very grateful that I was given the opportunity to present the results of my work at international conferences and meetings in different countries, as well as during in-house seminars. Special thanks go out to GEUS and the members of the 2014 SEGMENT-Expedition for all their support before, during and after fieldwork. Further thanks also go out to the numerous roommates and all the friends I made, as well as the past and current staff members of the Institute. Thanks for the helpful discussions, analyses, preparations, assistance in any other form and overall for an enjoyable time: Katja Pesch, Ernowo, Tobias Schlegel, Paula Niinikoski-Fußwinkel, Tobias Fußwinkel, Ramon Reifenröther, Stefan Horn, Normann Schadock, Lars Gronen, Markus Schramm, Abdellatif Hddine, Irina Knisch, Roman Klinghardt, Martin Brand, Bettina Noll, Peter Zimmermann, Thomas Wagner, Gudrun Günther, André Hellmann, Franz Michael Meyer, Nicolaus Gussone, Annemarie Wiechowski. Last but not least I would like to thank my family, my friends and my girlfriend for their continuing support and encouragement. I would not have finished without you. I Abstract The Nagssugtoqidian Orogen in the Tasiilaq region of South-East Greenland, is a roughly southeast- northwest trending, ~200 km wide Paleoproterozoic collisional orogen. It predominantly consists of a variety of Archean and Paleoproterozoic rocks, which were variably affected by several stages of deformation and metamorphism. The orogen has been subdivided into three different terranes. From north to south, these are the medium-pressure granulite-facies Schweizerland Terrane, the mainly high-pressure amphibolite-facies Kuummiut Terrane and the low-pressure amphibolite-facies Isertoq Terrane with the calc-alkaline Ammassalik Intrusive Complex. In the Kuummiut Terrane, variably retrogressed high-pressure mineral assemblages are preserved within mafic dykes, as well as boudins and boudinaged layers of mafic to ultramafic supracrustal rock in TTG gneiss. These mineral assemblages had previously not been investigated in much detail, but may provide important insight into the conditions of subduction, metamorphism and exhumation in the Paleoproterozoic, an era from which the geothermal regimes and nature of plate-tectonic processes are not well understood and high-pressure rocks are scarce. Within the framework of a larger expedition program to South-East Greenland, this thesis investigated the mineral textural evolution, PT-path and geochronology of variably retrogressed eclogite-facies rocks of the Kuummiut Terrane, via detailed petrological and mineral textural analysis, bulk-rock and mineral chemistry, conventional geothermobarometry, pseudosection modelling, and U-Pb isotopic dating. Well-equilibrated high-pressure and amphibolite-facies mineral assemblages, with only minor replacement textures, were found in garnet-pyroxenite, garnet-amphibolite and garnet-kyanite schist. Retrogressed eclogite, in contrast, is characterized by complex mineral reaction textures and the formation of two chemically and mineralogically distinct domains, leading to domainal equilibration volumes. A clinopyroxene domain is dominated by a fine-grained, worm-like to globular, diopside- plagioclase symplectite, which is often intergrown with and partially replaced by a coarser-grained hornblende-plagioclase symplectite. The fine-grained symplectite is interpreted to have grown at the expense of omphacite, which is only preserved in a Na-rich retrogressed eclogite sample. In a garnet domain, coarse-grained garnet is surrounded and variably pseudomorphed by corona-textured plagioclase ± amphibole ± clinopyroxene ± orthopyroxene. Geothermobarometry and pseudosection modelling, in combination with the various mineral textures, reveal evidence for four metamorphic stages along a clockwise PT-path. Ca-rich cores of large garnet grains in retrogressed eclogite are interpreted as prograde in origin and yield PT- conditions of 14-19 kbar and 600-750 °C (I), followed by eclogite-facies metamorphism at 17-19 kbar and 740-810 °C (II). The retrograde PT-evolution is initially characterized by near-isothermal decompression to high-pressure granulite-facies conditions of 13.8-15.4 kbar and 760-880 °C (III), with subsequent decompression and minor cooling to high-pressure amphibolite-facies conditions of II 8.8-10.9 kbar and 660-840 °C (IV). The PT-path implies that the Kuummiut Terrane probably experienced an initially rapid, tectonically driven exhumation. Furthermore, a large degree in consistency in the PT-data for the variably retrogressed high-pressure rocks suggests that they underwent the same metamorphic history, with the degree of retrogression and type of replacement assemblage mainly being controlled by fluid activity. LA-SF-ICP-MS U-Pb dating was carried out on zircon, monazite, titanite and rutile from retrogressed eclogite and the garnet-kyanite schist. A large range in 207Pb/206Pb dates between 2634 ± 63 and 1617 ± 91 Ma has been obtained, of which zircon yields the oldest and rutile the youngest dates. Detrital zircon in garnet-kyanite schist gives Archean to Paleoproterozoic dates and confines the maximum deposition of the precursor to the metasediment at 2107 ± 21 Ma. In retrogressed eclogite, the oldest zircon dates at 2146 ± 63 to 2092 ± 22 Ma are proposed to reflect the age of dyke emplacement into the Archean TTG gneiss. Dyke emplacement and deposition of supracrustal rocks most likely occurred near-contemporaneously during Paleoproterozoic extension and basin formation, as indicated by overlapping dates for detrital and magmatic zircon. The remaining data characterize the metamorphic evolution of the Kuummiut Terrane, about 200 m.y. after dyke emplacement. Consistent metamorphic dates between 1891 ± 10 and 1882 ± 3 Ma are obtained from the majority of the zircon, monazite and titanite analyses. The REE and U-Pb systematics in zircon indicate decoupling during retrograde metamorphism, with the REE patterns typical of zircon growth at eclogite-facies conditions, whereas the U-Pb dates and microtextures indicate recrystallization at high-pressure amphibolite-facies conditions. Based on previous studies and a similar decoupling in the magmatic zircons, the dates are interpreted to reflect mineral growth and recrystallization during high-pressure amphibolite-facies metamorphism. Regional medium-pressure amphibolite-facies metamorphism, associated with the collision of the Rae and North Atlantic cratons, is reflected by the 1872 ± 70 to 1821 ± 31 Ma monazite and titanite dates. Afterwards, the Kuummiut Terrane experienced relatively slow erosion-controlled cooling, with only minor thermal perturbations, as identified via rutile cooling ages at 1793 ± 10 Ma, 1738 ± 14 to 1720 ± 12 Ma, 1645 ± 63 Ma and 1617 ± 91 Ma. This thesis shows that a combination of different datasets from several variably retrogressed lithologies, rather than just examining eclogite-facies mineral assemblages, provides an invaluable tool for the characterization of the tectonometamorphic evolution of a Paleoproterozoic collisional orogen. III Kurzfassung Das Nagssugtoqidian Orogen in der Tasiilaq Region von Südost-Grönland ist ein ungefähr Südost-
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