Akademia Górniczo-Hutnicza
im. Stanisława Staszica
Wydział Geologii, Geofizyki i Ochrony Środowiska
Katedra Mineralogii, Petrografii i Geochemii
Rozprawa doktorska
Exhumation dynamics of high pressure rocks during
Caledonian orogenesis: A geochronological perspective
Tempo ekshumacji skał wysokociśnieniowych podczas
orogenezy kaledońskiej: Perspektywa geochronologiczna
Christopher J. Barnes
Promotorzy:
Dr inż. Jarosław Majka, prof. (AGH University of Science and Technology) Dr. David A. Schneider, prof. (University of Ottawa)
Acknowledgements
I’m always amazed that I can pinpoint the start of my scientific career to a grade 12 biology class that I attended at A.B. Lucas Secondary School. This class was taught by Mr. McLean, whose innovative, energetic and engaging teaching style instilled in me an incredible interest in science and an understanding of what it means to be great teacher. This class alone persuaded me to pursue a BSc degree in biochemistry at the University of Ottawa, and although I did not proceed in this subject, it directly led to my discovery and love of geology. The next significant moment in my education was the completion of my Honours BSc in geology at the University of Ottawa. For this, I give sincere thanks to David Schneider who guided me through my honours project and put up with me when I was finishing my BSc thesis while traveling across Europe. The BSc thesis helped develop my love for academic research, and through Dave’s help, I gained invaluable experience in the trials and tribulations of fieldwork, laboratory work (calcite dating, anyone?), scientific writing, etc., that has been crucial for my career. More thanks to Dave for bringing me back to the University of Ottawa to my MSc degree after I had left to move to western Canada, and for continuing to be instrumental for the development of my career. The MSc project not only furthered my abilities as a research science and provided me ample opportunities to attend conferences, workshops and to build my scientific network, but it allowed me to participate in my first fieldwork in the Arctic, a region which has now captivated me. The fieldwork on Svalbard also led me to meet a particular group of geoscientists working in Poland and Sweden... I give fist-big thanks to Jarosław Majka who provided me with the opportunity to come to AGH University of Science and Technology to pursue my PhD studies. With his immense help throughout these studies, I have been able to work with an excellent research team, engage in international collaboration, and produce top-notch work that has been presented at a multitude of conferences. All-in-all, I am forever grateful to Jarek for providing me this opportunity to choose Poland (or did Poland choose me?). That decision was a large undertaking. To move from Canada to Poland, and almost immediately begin my PhD degree, could have been an overwhelming and difficult experience. However, the people who I soon met at AGH and Uppsala University, who have all since become my close friends, helped to make this adjustment rather smooth. I could go on writing paragraphs about you all, but I am running out of space. So quickly, I give thanks to: Michał, my number one partner in fieldwork and research over these past four years (and I wouldn’t have it any other way); Grzesiek (Potato) my cribbage partner and audiologist who checked to see if my left ear was still working every day in the office; Bartek, whose positive energy and smile never fails to brighten a dull day, and for allowing me to participate in his hydrological experiments; Kasia, who welcomed me to her family when I couldn’t be with mine, and who never complained when I dragged her through Canada’s “finest” cities; Karolina K., with a great big heart to always to lend help and support for whatever issues may arise; Maciek, who has taken great care of me during my time in Poland, thanks for teaching me the ways of śmigus-dyngus. Pauline J., crêpe extraordinaire, whose been a great mentor and greater friend both in and out of the field; Iwa, whose infectious laugh and attitude can be bested by no other person; Marysia, whose quirkiness amazes me, I still haven’t figured you out after four years; and to Ania and Kunek (sorry I missed your wedding), Karolina R., Pauline M., and all of the other people who have helped me in countless ways over these past four years. Above everyone else, I thank my mother Susan. Without her unconditional love and support over the past ~27 years as a single mother raising two kids through numerous hardships, I would never be in the position to obtain my PhD. All of the accomplishments and events that have been briefly outlined here, and everything else that I have been able to do in my life is because of you. Thank you for everything you have done for Jason and I, and that you continue to do. This thesis is dedicated to you. I still haven’t forgotten about your dog house that I promised to buy for you all those years ago.
Table of Contents
1. List of Articles included in the PhD thesis and
statements of co-author contributions ... 1
2. Abstract ... 5
3. Streszczenie... 7
4. List of Abbreviations ... 9
5. Introduction ... 10
5.1 Scope of the thesis ... 10
5.2 Subduction-exhumation processes ... 13
5.3 Zircon U-Pb geochronology and trace element geochemistry depth-profiling ... 14
5.4 In-situ monazite Th-U-total Pb geochronology ... 15
5.5 In-situ white mica 40Ar/39Ar geochronology ... 16
6. Geological Background ... 17
6.1 Vaimok and Tsäkkok lenses ... 17
6.2 Vestgӧtabreen Complex ... 19
7. Methods... 21
7.1 Fieldwork ... 21
7.2 Transmitted light and electron microscopy ... 22
7.3 Zircon U-Pb geochronology and trace element geochemistry depth profiling ... 23
7.4 In-situ monazite Th-U-total Pb geochronology ... 24
7.5 In-situ white mica 40Ar/39Ar geochronology ... 24
8. Conclusions ... 26
8.1 Summary of PhD thesis work ... 26
8.2 Implications for the Caledonian orogeny and comparisons with global tectonics ... 30
8.3 Future Directions ... 34
9. Verification of the PhD thesis ... 35
Funding ... 37
1
1. List of Articles included in the PhD thesis and statements of co-author contributions
[A1] Barnes, C. J., Majka, J., Schneider, D., Walczak, K., Bukała, M., Kośmińska, K., Tokarski, T., Karlsson, A. (2019). High-spatial resolution dating of monazite and zircon reveals the timing of subduction–exhumation of the Vaimok Lens in the Seve Nappe Complex (Scandinavian Caledonides). Contributions to Mineralogy and Petrology, 174:5. doi: 10.1007/s00410-018-1539-1
[A2] Barnes, C. J., Walczak, K., Janots, E., Schneider, D., & Majka, J. (2020). Timing of paleozoic exhumation and deformation of the high-pressure Vestgӧtabreen Complex at the Motalafjella Nunatak, Svalbard. Minerals, 10(2), 1–23. doi: 10.3390/min10020125
[A3] Barnes, C.J., Jeanneret, P., Kullerud, K., Majka, J., Schneider, D., Bukała, M., & Klonowska, I. (2020). Exhumation of the high-pressure Tsäkkok Lens, Swedish Caledonides: Insights from the structural and white mica 40Ar/39Ar geochronological record. Tectonics. doi:
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High-spatial resolution dating of monazite and zircon reveals the timing of subduction– exhumation of the Vaimok Lens in the Seve Nappe Complex (Scandinavian Caledonides).
Contributions to Mineralogy and Petrology 174:5 (2019), 21.12.2018 (date of publication) Authorship Number Name of the Author Percent of Contribution Scope of Contribution Signature 1 Christopher Barnes 55% Formulation of main scientific idea; collection of all data;
interpretation of all data; creation of the
manuscript 2 Jarosław Majka 10% Funding of research; interpretation of data; preparation of manuscript 3 David Schneider 10% Funding of research; interpretation of data; preparation of manuscript 4 Katarzyna Walczak 5% Collection of zircon depth-profiling data; preparation of the manuscript 5 Michał Bukała 5% Sample collection; preparation of the manuscript 6 Kośmińska Karolina 5% Collection of monazite Th-U-total Pb geochronology; preparation of the manuscript 7 Tomasz Tokarski 5% Assistance with Argon ion milling of
zircon; preparation of manuscript 8 Andreas Karlsson 5% Assistance with imaging of milled zircon grains; preparation of manuscript
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Timing of Paleozoic Exhumation and Deformation of the High-Pressure Vestgӧtabreen Complex at the Motalafjella Nunatak, Svalbard. Minerals 10 (2020), 125
31.01.2020 (date of publication) Authorship Number Name of the Author Percent of Contribution Scope of Contribution Signature 1 Christopher Barnes 55% Formulation of main scientific idea; funding of research; collection of all data;
interpretation of all data; creation of the
manuscript 2 Katarzyna Walczak 15% Collection of samples and data; interpretation of the data; preparation of the manuscript 3 Emilie Janots 10% Collection and interpretation of monazite Th-U-total Pb geochronological data; preparation of the manuscript 4 David Schneider 10% Interpretation of white mica 40Ar/39Ar
geochronological data; preparation of the manuscript 5 Jarosław Majka 10% Interpretation of monazite Th-U-total Pb data; preparation of the manuscript
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Exhumation of the high-pressure Tsäkkok Lens, Swedish Caledonides: Insights from the structural and white mica 40Ar/39Ar geochronological record. Tectonics (2020)
12.06.2020 (date of publication) Authorship Number Name of the Author Percent of Contribution Scope of Contribution Signature 1 Christopher Barnes 50% Formulation of main scientific idea; collection of all data; interpretation of all data; creation of the
manuscript 2 Pauline Jeanneret 15% Collection and interpretation of structural data; creation of the manuscript 3 Kåre Kullerud 10% Collection of structural data; preparation of the manuscript 4 Jarosław Majka 7.5% Funding of research; preparation of the manuscript 5 David Schneider 7.5% Interpretation of white mica 40Ar/39Ar geochronological data; preparation of the manuscript 6 Michał Bukała 5% Collection of samples; preparation of the manuscript 7 Iwona Klonowska 5% Collection of samples; preparation of the manuscript
5 2. Abstract
The evolution of the Caledonian orogeny constituted Cambrian to early Silurian closure of the Iapetus ocean, followed by Silurian to Devonian collision between the Laurentia and Baltica. Within the tectonostratigraphy of the Scandinavian Caledonides, the Seve Nappe Complex includes predominantly continental high pressure and ultra-high pressure rocks. The unique rock record provides direct evidence for the subduction and exhumation of the passive margin of Baltica in the late Cambrian to Late Ordovician periods beneath Iapetus ocean volcanic arc(s). More specifically, both the Vaimok and Tsäkkok lenses of the Seve Nappe Complex in Norrbotten, Sweden, possess evidence of (ultra-)high pressure metamorphism in the Early Ordovician period. In contrast, other (ultra-)high pressure exposures of the Seve Nappe Complex farther southwest in Jämtland, Sweden, are thought to have attained peak metamorphism in the Middle to Late Ordovician period. In the Southwestern Caledonian Basement Province of Svalbard, high pressure metamorphism of the comparatively more oceanic Vestgӧtabreen Complex is synchronous with the Norrbotten localities. However, despite the similar timing of (ultra-)high pressure metamorphism between Norrbotten and Svalbard, the Silurian to Devonian tectonic histories of these regions is markedly different. In Norrbotten, and the Scandinavian Caledonides in general, this period is marked by dominant southeast-vergent thrusting during collision, whereas the Caledonian rocks on Svalbard were transported along major sinistral strike-slip faults. Altogether, the (ultra-)high pressure localities exposed in Norrbotten and Svalbard provide a direct geodynamic record for the mechanisms of the Iapetus ocean closure. Yet, the dynamics and possible spatial correlations for subduction of these localities preceding peak pressure metamorphism, and subsequent exhumation from mantle depths, are not well resolved. As a result, the current understanding of the evolution of the Caledonian orogen is limited. Without a detailed examinations of these processes, a more comprehensive and realistic model for the Caledonian orogeny cannot be realized. The aim of the thesis is to resolve the timing of subduction and exhumation of the (ultra-)high pressure Vaimok and Tsäkkok lenses in Norrbotten, as well as the Vestgӧtabreen Complex on Svalbard. The work presented here ultimately improves the understanding of the evolution of the Arctic Caledonides, as well as the subduction and exhumation mechanisms of continental and oceanic crust in general. To achieve this, metasedimentary rocks that host high pressure lithologies (i.e., blueschists and eclogites) were targeted for application of the following modern geochronological techniques: 1) zircon U-Pb geochronology and trace element geochemistry depth-profiling; 2) in-situ monazite Th-U-total Pb geochronology; and 3) in-situ white mica 40Ar/39Ar geochronology. The results of these analyses were combined
with structural observations and mineral chemistry studies to resolve the temporal evolution of deformation and metamorphism that ultimately record subduction to the peak (ultra-)high pressure conditions and subsequent synorogenic exhumation. Altogether, the thesis provides a prime example of applications of small spatial resolution geochronology in resolving large scale tectonic processes. In the Vaimok lens, monazite Th-U-total Pb geochronology revealed that these rocks were subjected to high pressure in the late Cambrian period, consistent with previous studies that indicate that ultra-high pressure metamorphism was achieved in the Early Ordovician period. Depth-profiling analyses of zircon from the Vaimok lens metasedimentary rocks indicate exhumation in the Early to Middle Ordovician period. In-situ 40Ar/39Ar
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geochronology of white mica from the Tsäkkok lens metasedimentary rocks provides a robust record of Early Ordovician exhumation directly following high pressure metamorphism. Monazite and white mica ages also illustrate that exhumation of the Vestgӧtabreen Complex on Svalbard was coeval with the Tsäkkok lens from similar pressure and temperature conditions, a record which was later overprinted during Silurian to Devonian strike-slip tectonism. Altogether these results indicate that subduction of the Baltican passive margin was protracted for parts of the Seve Nappe Complex in Norrbotten (i.e., the Vaimok lens). Peak pressure metamorphism was followed by rapid Early Ordovician exhumation of both Norrbotten and Svalbard localities, suggesting that they belonged to the same subduction system that was dismembered due to the later-stage difference in compressional vs. strike-slip geodynamics along strike of the orogen. Regardless, the Early Ordovician exhumation of these localities is in direct contrast with the presumed timing of subduction leading up to the documented Middle to Late Ordovician peak pressure metamorphism for the localities to the southwest in Jämtland. If these localities in the Seve Nappe Complex also represent the same subduction system, it would suggest that either a late Cambrian to Early Ordovician record of subduction and exhumation has gone undocumented for Jämtland, or different tectonic regimes governed the history of the Jämtland and Norrbotten due to complex subduction geometries. Therefore, the knowledge gained from the PhD thesis helps elucidate the evolution of the Arctic Caledonides, which in turn provides a foundation for tectonic models of Caledonian orogenesis.
7 3. Streszczenie
Przebieg orogenezy kaledońskiej złożony jest z kambryjskiego do wczesnosylurskiego zamknięcia oceanu Iapetus oraz sylursko-dewońskiej kolizji pomiędzy Laurencją i Baltiką. W tektonostratygrafii kaledonidów skandynawskich grupa płaszczowin Seve zawiera głównie skały skorupy kontynentalnej zmetamorfizowane w warunkach wysokich i ultrawysokich ciśnień. Ta unikatowa jednostka dostarcza bezpośrednich dowodów na subdukcję i ekshumację pasywnej krawędzi Baltiki pod łuk wulkaniczny (lub łuki wulkaniczne) oceanu Iapetus w okresie od późnego kambru do późnego ordowiku. W szczególności łuski tektoniczne Vaimok i Tsäkkok z grupy płaszczowin Seve w Norrbotten w Szwecji zawierają dowody na metamorfizm (ultra-) wysokociśnieniowy we wczesnym ordowiku. Dla kontrastu, inne lokalizacje (ultra-) wysokociśnieniowego metamorfizmu w grupie płaszczowin Seve położone są na południowy zachód, w Jämtland w Szwecji, osiągnęły pikowe warunki metamorfizmu w okresie od środkowego do późnego ordowiku. W południowo-zachodniej prowincji kaledońskiego podłoża krystalicznego Svalbardu metamorfizm wysokociśnieniowy stosunkowo bardziej oceanicznego kompleksu Vestgӧtabreen jest synchroniczny z lokalizacjami w Norrbotten. Jednak pomimo podobnego czasu zajścia metamorfizmu (ultra-) wysokociśnieniowego w Norrbotten i na Svalbardzie, ewolucja tektoniczna w tych rejonach, w sylurze i dewonie, jest wyraźnie różna. W Norrbotten, jak i reszcie kaledonidów skandynawskich okres ten charakteryzuje się głównie nasuwaniem płaszczowin o południowo-wschodniej wergencji, podczas gdy skały podłoża kaledońskiego na Svalbardzie zostały przetransportowane wzdłuż głównie lewoskrętnych uskoków przesuwczych. Sumarycznie lokalizacje (ultra-) wysokociśnieniowego metamorfizmu w Norrbotten i na Svalbardzie zawierają fragmenty bezpośredniego zapisu geodynamicznego mechanizmów zamykania oceanu Iapetus. Dynamika i możliwe korelacje przestrzenne dla warunków subdukcji poprzedzających warunki maksymalnego ciśnienia metamorfizmu, a następnie ekshumacji z głębokości płaszczowych w obu tych regionach, nie zostały jeszcze rozwikłane. W rezultacie obecne rozumienie ewolucji kaledońskiego orogenu jest ograniczone. Bez szczegółowych badań tych procesów nie można zbudować bardziej kompleksowego i realistycznego modelu orogenezy kaledońskiej. Celem pracy jest określenie czasu subdukcji i ekshumacji (ultra-) wysokociśnieniowych skał z łusek Vaimok i Tsäkkok w Norrbotten, a także kompleksu Vestgӧtabreen na Svalbardzie. Przedstawione tutaj wyniki poszerzają zrozumienie ewolucji arktycznych kaledonidów, a także ogólnie mechanizmów subdukcji i ekshumacji skorupy kontynentalnej i oceanicznej. Aby to osiągnąć, minerały skał metaosadowych, goszczących litologie charakterystyczne dla wysokociśnieniowego metamorfizmu (tj. łupki glaukofanowe i eklogity), zostały poddane analizie przy użyciu następujących nowoczesnych technik badań geochronologicznych: 1) cyrkon metodą U-Pb i profilowania wgłębnego (depth profilling) wraz z analizą pierwiastków śladowych; 2) monacyt metodą Th-U-total Pb in situ; oraz 3) biała mika metodą 40Ar/39Ar in situ. Rezultaty analiz połączono z obserwacjami
strukturalnymi i badaniami składu chemicznego minerałów skał metaosadowych, aby wyjaśnić ewolucję epizodów deformacji i metamorfizmu, które rejestrują subdukcję do warunków pikowych metamorfizmu (ultra-) wysokich ciśnień, a następnie ekshumację synorogeniczną. Podsumowując, praca ta stanowi doskonały przykład zastosowań geochronologii o wysokiej małej rozdzielczości w rozwiązywaniu procesów tektonicznych na dużą skalę. W łusce tektonicznej Vaimok, należącej do grupy płaszczowin Seve, badania geochronologiczne
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monacytu metodą Th-U-total Pb ujawniły, że skały te zostały pogrążone do głębokości odpowiadających warunkom metamorfizmu wysokich ciśnień w późnym kambrze, co jest zgodne z wcześniejszymi badaniami, które wskazują, iż pik metamorfizmu (ultra-) wysokich ciśnień został osiągnięty we wczesnym ordowiku. Analizy profilowania wgłębnego cyrkonu ze skał metaosadowych łuski Vaimok wskazują na początkową dekompresję związaną z ekshumacją w czasie od wczesnego do środkowego ordowiku. Badania geochronologiczne białej miki ze skał metaosadowych z soczewki Tsäkkok metodą in situ 40Ar/39Ar przedstawiają
dokładny zapis wczesnoordowickiej ekshumacji bezpośrednio po metamorfizmie wysokociśnieniowym. Wieki monacytu i białej miki ilustrują również, że ekshumacja kompleksu Vestgӧtabreen na Svalbardzie i soczewki Tsäkkok miały miejsce w tym samym czasie, rozpoczynając się w podobnych warunkach ciśnienia i temperatury, jednakże zapis ten został później częściowo zatarty podczas sylursko-dewońskiego nasuwania i aktywności uskoków przesuwczych. Przedstawione wyniki wskazują, że subdukcja pasywnej krawędzi Baltiki ma swoje przedłużenie w części grupy płaszczowin Seve w Norrbotten (tj. w łusce Vaimok). Po osiągnieciu maksymalnych warunków ciśnienia metamorfizmu nastąpiła szybka ekshumacja skał wysokociśnieniowych zarówno w regionie Norrbotten, jak i na Svalbardzie, co sugeruje, że należały one do tego samego systemu subdukcji we wczesnym ordowiku. System ten został rozczłonkowany z powodu późniejszej przestrzennej zmienności geodynamiki procesów kompresyjnych i ścinających wzdłuż rozciągłości orogenu. Niezależnie od późniejszych procesów, tak wczesna ekshumacja w Norrbotten i na Svalbardzie jest niezgodna z przypuszczalnym okresem trwania subdukcji skał w Jämtland, a udokumentowanym dla warunków wysokociśnieniowego metamorfizmu od środkowego do późnego ordowiku. Jeśli te lokalizacje w grupie płaszczowin Seve reprezentują ten sam system subdukcji, sugerowałoby to, że zapis procesów subdukcji i ekshumacji od późnego kambru do wczesnego ordowiku został całkowicie zatarty dla skał w Jämtland lub różne reżimy tektoniczne, wynikające ze złożonej geometrii procesów subdukcji, decydowały o ich przebiegu w Jämtland i Norrbotten. Rezultaty przedstawione w tej pracy doktorskiej poprawiają zrozumienie ewolucji kaledonidów arktycznych, co z kolei stanowi podstawę dla szerszych modeli tektonicznych orogenezy kaledońskiej.
9 4. List of Abbreviations
BSE Back-scattered electron
EDS Energy-dispersive spectrometry
EMP Electron microprobe
HP High pressure
ID-TIMS Isotope dilution thermal ionization mass spectrometry
KNC Köli Nappe Complex
LA-ICP-MS Laser ablation inductively coupled mass spectrometry
OIIL Oscar II Land
REE Rare earth elements
SCBP Southwestern Caledonian Basement Province
SE Secondary electron
SIMS Secondary ion mass spectrometry
SNC Seve Nappe Complex
UHP Ultra-high pressure
10 5. Introduction
5.1 Scope of the thesis
The tectonostratigraphy of the Caledonian orogen provides a unique, telescoped record of the typical Wilson-cycle style of tectonics through opening of the Iapetus ocean, followed by ocean closure and eventual continental collision between Baltica and Laurentia (e.g., Gee et al., 2008). The Caledonides are exposed in the north Atlantic/Arctic region in Scandinavia, eastern Greenland, Svalbard (Fig. 1) and the northernmost Canadian Arctic Islands. The different tectonic stages can be extracted from the individual tectonostratigraphic levels (from the Parautochthon to the Lower, Middle, Upper and Uppermost Allochthons; Fig. 2a) and from variations of the metamorphic and structural records along strike of the orogen within the same tectonostratigraphic level (e.g., Gee et al., 1985, 2020; Stephens et al., 1985). The thesis herein focuses on closure of the Iapetus ocean that is recorded in the high pressure (HP) and ultra-high pressure (UHP) rocks of the Seve Nappe Complex (SNC) in the Scandinavian Caledonides (Fig. 2a) and in the Southwestern Caledonian Basement Province (SCBP) of the Svalbardian Caledonides (Fig. 2b). A multitude of geological investigations have focussed on these regions over the preceding decades, laying the foundation for a general understanding of the tectonics and geodynamics of the orogen. In the time since these pioneering studies were conducted, however, significant advances in the spatial and mass resolution of many geochronological techniques have been made, as well as an increased understanding of the isotopic systems in different minerals have been realized. Although the original studies do offer significant insight into the tectonic history of the Caledonides, the theoretical and technical progress provides tremendous potential for more detailed, comprehensive investigations.
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Figure 1 – Overview map of the distribution of the Caledonides in Scandinavia, eastern Greenland, and on Svalbard.
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Figure 2 – A) The tectonostratigraphic map of the Scandinavian Caledonides (modified after Gee et al., 2013). High pressure localities in the Seve Nappe Complex exposed in southern Norrbotten are subjects
of this PhD thesis. The locations of other (ultra-)high pressure localities in the Seve Nappe Complex in southern Västerbotten/northern Jämtland and west-central Jämtland are indicated. B) Bedrock map of Svalbard (modified after Gee and Teben’kov, 2004) showing the location of the Caledonian Basement
Provinces. High pressure rocks located in the Vestgӧtabreen Complex exposed at the Motalafjella nunatak in Oscar II Land are subjects of this PhD thesis. Abbreviation: OIIL, Oscar II Land
Most of the recent studies in the Scandinavian and Svalbardian Caledonides have focused on the lithologies that still retain a (U)HP mineral paragenesis to extract the pressure-temperature conditions of (U)HP metamorphism (e.g., Agard et al., 2005; Bukała et al., 2018; Janák et al., 2013; Klonowska et al., 2014; 2016, 2017; Kośmińska et al., 2014; Majka et al., 2014), and the timing of the peak metamorphic conditions (e.g., Brueckner and van Roermund, 2007; Fassmer et al., 2017; Grimmer et al., 2015; Majka et al., 2012; Petrík et al., 2019; Root and Corfu, 2012). The work presented here employs modern high-spatial resolution geochronological techniques integrated in a structural and mineral chemistry framework to extract tectonic histories from retrogressed and deformed metasedimentary rocks. The techniques utilized include: 1) zircon U-Pb geochronology and trace element geochemistry depth-profiling; 2) in-situ monazite Th-U-total Pb geochronology; and 3) in-situ white mica 40Ar/39Ar geochronology.
Zircon, monazite and white mica are ubiquitous minerals in metasedimentary rocks that readily record metamorphism or deformation making them especially useful mineral for understanding dynamics of subduction and exhumation. Zircon and monazite are both relatively stable minerals that can (re)crystallize and endure over a wide range of pressure and temperature conditions in response to metamorphic and fluid processes (e.g., Engi, 2017; Rubatto, 2017; Rubatto et al., 2001). They contain a wide array of trace elements, including U and Th, which can be used to directly identify and resolve metamorphic processes in time (Foster et al., 2000; Hermann and Rubatto, 2003; Kohn et al., 2005; Majka et al., 2012; Rubatto, 2002; Rubatto et al., 2001, 2013). Furthermore, the contents of Ti in zircon (e.g., Ferry and Watson, 2007; Watson et al. 2006) and Y in monazite (e.g., Pyle et al., 2001, Seydoux-Guillaume, 2002) can be directly linked to temperature, providing a means to quantify the conditions of metamorphism recorded by the minerals. Like zircon and monazite, white mica is also quite stable in metasedimentary rocks over a wide array of metamorphic conditions, but its geochronological systematics are much more sensitive. Diffusion of 40Ar can be thermally activated, driven by recrystallization, or facilitated by fluid interaction with the mica grains (Cosca et al., 2011; Cossette et al., 2015; Harrison et al., 2009; Kellett et al., 2016; Kramar et al., 2001; Mulch & Cosca, 2004; Schneider et al., 2019; Uunk et al., 2018; Warren et al., 2012). White mica is also commonly
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(re)crystallized along major structural features of rocks, including foliations, lineations, shear bands, etc. Therefore, white mica is an extremely advantageous mineral that is uniquely suited to date tectonic processes resulting in rock cooling, deformation events, or fluid activity. Together, geochronological analysis of zircon, monazite and white mica are incredibly powerful tools for resolving metamorphic and tectonic processes. In this regard, these minerals offer great potential to provide crucial information for the subduction and exhumation dynamics of key (U)HP localities in the Arctic Caledonides, providing a significant contribution for understanding the large-scale dynamics of subduction and exhumation along strike of the orogen.
5.2 Subduction-exhumation processes
Subduction of continental crust to UHP conditions was first recognized for the Dora Maira whiteschists in the western Alps in Italy (Chopin, 1984) and was quickly followed in the Western Gneiss Region of the Caledonides in Norway (Smith 1984). Since these ground-breaking discoveries, many more localities of continental rocks metamorphosed under (U)HP conditions have been globally recognized (e.g., Chopin, 2003; Ernst et al., 1997; Froitzheim et al., 2003; Gilotti et al., 2013; Hacker & Gerya, 2013; Liou et al., 2004, 2009; Majka et al., 2014; O’Brien et al., 2019; Platt, 1993; Warren, 2013). The foundering of continental crust, however, is not favoured in most subduction models due to the generally thicker, more buoyant characteristics of the material compared to oceanic crust (e.g., Zheng and Chen, 2016). Furthermore, the exhumation of such rocks from mantle depths continues to generate significant debate (e.g., Gerya et al., 2002; Guillot et al., 2009; Hacker and Gerya, 2013; Platt, 1993; Rubatto and Hermann, 2001; Warren, 2013), although the positive buoyancy of continental rocks relative to the mantle is thought to be a key driver of exhumation. Evidence for buoyancy-driven exhumation is supported by the preservation of metamorphic microdiamonds (>3.0 GPa) in (U)HP continental crust (e.g., Klonowska et al., 2017; Majka et al., 2014; Petrik et al., 2019; Sobolev and Shatsky, 1990), whereas subducted oceanic rocks typically do not exhume from pressure conditions exceeding ~2.3 GPa (Agard et al., 2009). The fact the such rocks can be subducted to achieve such great pressures, and an increase in the density with the conversion of quartz (2.65 g/cm3) to coesite (2.92 g/cm3) in UHP conditions, requires changes in plate kinematics that would allow for buoyancy forces to drive the upward movement of deep-seated rocks toward the surface. Multiple mechanisms have been proposed (e.g., Hacker and Gerya, 2013; Platt, 1993; Warren, 2013), yet a common theme that emerges from these models is the change of plate kinematics of either the lower (subducting) plate or the upper (overriding) plate.
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The former case includes mechanisms such as eduction, slab-rollback and (micro)plate rotation (e.g. Hacker and Gerya, 2013; Warren, 2013), whereas the latter can be due to upper plate retreat (e.g. Warren, 2013) from near-field tectonics (e.g., Froitzheim et al., 2003; Majka et al., 2014) or far-field plate processes (e.g., Faccenna et al., 2018). The style of exhumation is also reflected in the size and rate of the exhumed (U)HP terrane (Kylander-Clark et al., 2012; Hacker and Gerya, 2013; Warren, 2013). In the Scandinavian Caledonides, for example, the Western Gneiss Region can be classified as a large UHP terrane that likely exhumed due to slow, full-scale reversal (eduction) of the subducting slab (e.g., Andersen et al., 1991; Kylander-Clark et al., 2008). In contrast, the (U)HP localities in the Seve Nappe Complex constitute volumetrically small terranes, which typically exhume rapidly during subduction (Warren 2013). Synorogenic exhumation of (U)HP suggests creation of transient divergent forces within the subduction channel, creating an opportunity for buoyancy forces of the continental materials to rise along the channel during subduction (Liao et al., 2018). Therefore, to better understand the mechanisms that govern subduction and exhumation of (U)HP rocks that are subject of this thesis, and for the Caledonian orogen in general, resolving the timing and duration of subduction and exhumation are critical. Comparing the temporal evolution of the Caledonian orogen along strike, and across the tectonostratigraphic levels is also crucial to understand whether buoyancy-assisted exhumation was driven by changes in lower or upper plate motions.
5.3 Zircon U-Pb geochronology and trace element geochemistry depth-profiling
Preferred zircon U-Pb geochronological methods include isotope dilution thermal ionization mass spectrometry (ID-TIMS), laser ablation inductively coupled mass spectrometry (LA-ICP-MS) or secondary ion mass spectrometry (SIMS). ID-TIMS involves dissolution of entire zircon grains and high precision analyses, whereas the latter two provide for better spatial precision as the micrometer-scale zircon volume that is ablated can be strategically chosen. For conventional use of LA-ICP-MS and SIMS, the zircon are typically separated from the rock sample and mounted in an epoxy to be polished, which reveals a cross section of each zircon grains for analysis. However, in metasedimentary rocks that experienced (U)HP metamorphism, metamorphic zircon is often found as neoblastic or recrystallized mantles on xenocrystic cores. The existence of multiple zircon generations present in a single grain is not well-suited for ID-TIMS analysis. Furthermore, the neoblastic zircon are often thinner than the diameter of the incident beam of the conventional ablation systems, precluding the unequivocal extraction of the metamorphic record without contamination through mixing with the xenocrystic core.
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To overcome the challenge, the depth-profiling technique ablates an unpolished zircon (or other mineral phase) to create profiles of U, Th, and Pb isotopic species and trace elements from the exterior of the crystal into the xenocrystic core. The resolution of the depth-profiling method allows for thin (<10 μm) overgrowths to be analyzed without contamination of xenocrystic zircon volumes (e.g., Breeding et al., 2004; Kelly et al., 2014; Marsh and Stockli, 2015; Schneider et al., 2011, 2012; Skipton et al., 2016). Trace elements can also be profiled through the zircon that can be used to fingerprint metamorphic reactions recorded by the zircon. Therefore, combining the geochronological profiles with certain trace elements (e.g., REE, Ti, P, Y) provides a direct link between the numerical dates and metamorphic reactions that occur in response to tectonic processes. A significant amount of time is required for sample preparation and analysis and quality control of the data. The amount of time required is significantly greater than conventional zircon dating via LA-ICP-MS or SIMS approaches. Other problems may also arise, such as the fractionation of isotopes with increasing pit depth during the ablation profiling, but the advantages of analysing very small volumes of neoblastic zircon in metasedimentary and polymetamorphic rocks far outweigh the disadvantages.
5.4 In-situ monazite Th-U-total Pb geochronology
Isotopic analysis of monazite using the LA-ICP-MS or SIMS methods are the often preferred approaches for geochronological analyses, but the size of the grains and of subcrystalline domains may be too small compared to the laser or ion beam diameter, which may lead to mixing of distinct metamorphic domains during analysis or preventing analysis altogether. Particularly problematic is monazite that have been partially retrogressed to apatite, allanite and clinozoisite (Budzyń et al., 2011; Spear et al., 2010) or monazite that grew in greenschist facies conditions (Rasmussen et al., 2001; Seydoux-Guillaume et al., 2012). Instead of isotopic analysis, in-situ monazite Th-U-total Pb geochronology is an analytical method that utilizes an electron microprobe (EMP), which can achieve an electron beam diameter of 1-3 μm to keep excitation volumes minimal (Montel et al., 1996; Williams et al., 2017; Konečný et al., 2018). Furthermore, major and trace element concentrations of monazite can be obtained simultaneous with Th, U and Pb (Williams and Jercinovic, 2002; Williams et al., 2002, 2007, 2017). Consequently, every Th-U-total Pb date is coupled with trace element chemistry from the same volume, which is extremely important for making an informed interpretation about the processes responsible for development of the monazite domain (Engi et al., 2017; Williams et al., 2007). Therefore, careful examination of monazite chemistry can reveal the ages of distinct metamorphic events representing large-scale tectonic processes. Another advantage of this
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technique is that the small volumes of monazite are dated directly in standard 30 μm-thick thin sections, thereby retaining the petrological and structural context of the monazite, allowing monazite in different textural positions to be targeted (Williams et al., 2002). For example, monazite found as inclusions in garnet or in the matrix in association with foliation-defining minerals can be dated separately. However, there are a few major shortcomings of the technique. The precision of the technique is much larger than LA-ICP-MS or SIMS that are derived from limitations of the electron microprobe (Spear et al., 2009). Furthermore, the dates are calculated from the total elemental concentrations of Th, U and Pb, and therefore the isotopic systems cannot resolve problems arising from Pb-loss, nor can common-Pb contamination be evaluated (Williams et al., 2017). Although monazite is generally perceived to contain insignificant common-Pb (Parrish, 1990), some cases of monazite generated during UHP metamorphism have been found to contain significant common-Pb (Holder et al., 2015). These factors cannot be ignored, but attempts can be made to mitigate them by correlating monazite dates and chemistry, and by applying sophisticated statistical methods to calculate the ages of metamorphic events recorded by the phosphate phases. (Williams et al., 2006).
5.5 In-situ white mica 40Ar/39Ar geochronology
Employment of 40Ar/39Ar geochronology can be done by step-heating, total fusion, or in-situ techniques applied to potassium-bearing minerals. Potassic white mica is especially useful for dating as it is very common in metasedimentary and felsic rocks. Both step-heating and total fusion analyses have advantageous aspects for dating white mica. The former can be useful for detecting extraneous 40Ar in the micas, (e.g., Dallmeyer and Stephens, 1991), whereas the latter is useful for quickly obtaining large data sets across several samples (e.g., Uunk et al., 2018). However, both of these methods are conducted on white mica that are separated from the rock sample, thereby removing the target grains from their structural context. In metasedimentary samples, several generations of structures that may be linked with white mica (re)crystallization can be present (e.g., Cossette et al., 2015; Schneider et al., 2019). Analyzing mineral separates without structural context can therefore mix the different mica generations into a single dataset. Therefore, in-situ white mica 40Ar/39Ar is an extremely advantageous technique (e.g., Kramar et al., 2001; Mulch and Cosca, 2004, Mulch et al., 2002) as the white mica are directly dated from a 200-500 μm-thick thin section. The ability to target and date distinct generations of structures and structural types (e.g., foliations, folds, shear bands, etc.) allows for resolving different generations of tectonic events. Acquisition of white mica chemistry is also crucial for interpretation of the 40Ar/39Ar as major element chemistry of the mica grains is also strongly
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controlled by metamorphic conditions and can be (partially) re-equilibrated during recrystallization (Cossette et al., 2015; Dempster, 1992; Giorgis et al. 2000; Schneider et al., 2019), providing evidence for deformation that is independent of the 40Ar/39Ar datasets. However, in-situ 40Ar/39Ar geochronology is not able to detect extraneous 40Ar and is a very time intensive process compared to total fusion. Nevertheless, the ability to preserve the structural context of grains and to investigate the mineral chemistry to help interpret the 40Ar/39Ar is an unmatched tool for dating deformation events.
6. Geological Background
Several (U)HP localities are exposed in the Arctic regions of the Scandinavian and Svalbard Caledonides, and the thesis focuses on three key localities: 1) the Vaimok lens, 2) the Tsäkkok lens (Figs. 2a, 3), and 3) the Vestgӧtabreen Complex (Figs. 2b, 4). The UHP Vaimok lens, the HP Tsäkkok lens, and the amphibolite-facies Sarek lens together constitute the Seve Nappe Complex (SNC) in southern Norrbotten, Sweden, whereas the Vestgӧtabreen Complex is part of the Southwestern Caledonian Basement Province (SCBP) of Svalbard. Previous studies have built a solid geological framework from lithological mapping, structural analysis, metamorphic petrology, and geochronology, which are summarized below.
6.1 Vaimok and Tsäkkok Lenses
The Vaimok and Tsäkkok lenses both comprise Neoproterozoic siliciclastic and carbonaceous metasedimentary rocks that host eclogitic bodies (Fig. 3; Albrecht, 2000; Andréasson et al., 1985; Kullerud, 1987; Santallier, 1988; Albrecht, 2000). The bulk of the Vaimok lens metasediments consist of metapsammitic and metapelitic rocks with subordinate marble layers, whereas the Tsäkkok lens has a much more significant carbonaceous component in the form of thick marble layers and calc-silicate sequences, in addition to metapelitic and metapsammitic rocks. The eclogite protoliths of the Vaimok lens are typically regarded to be dolerite dykes associated with the breakup of Rodinia and development of the passive margin of Baltica. Protoliths of the Tsäkkok lens eclogites can be similarly interpreted, however, undeformed and eclogitized pillow basalts have been found in the lens, indicating a component of subaqueous basaltic volcanism (Kullerud et al., 1990). Altogether, these two lenses represent the rifted passive margin of Baltica during the breakup of Rodinia and subsequent opening of the Iapetus ocean (Albrecht, 2000; Jakob et al., 2019; Kjøll et al., 2019; Kullerud et al., 1990).
In both the lenses, the predominant structures in the metasedimentary rocks formed during exhumation and are associated with retrogressive metamorphism (Albrecht, 2000;
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Kullerud, 1987; Snilsberg, 1987). The prograde-to-peak metamorphic record was nearly obliterated as a result. The relic foliation (S1) is only locally preserved within the more dominant exhumation-related foliation (S2). In the Vaimok lens, S2 is associated with intense shearing associated with development of lineation-parallel isoclinal folds, sheath folds, anastomosing shear zones and meter-scale shear bands (Albrecht, 2000).
The exhumation-related structures are predominantly confined to the metasedimentary units, but retrogression during exhumation affected the majority of eclogite bodies. The evidence of subduction and (U)HP metamorphic conditions for both lenses is provided by a few remaining, pristine eclogite bodies. For the Vaimok lens, peak pressure conditions were initially reported to be 2.0-2.7 GPa at 690-730°C (Albrecht, 2000). Recently, Bukała et al. (2018) re-calculated the metamorphic conditions, indicating initial burial to 2.8-3.1 GPa at ~700°C, which was followed by nearly isothermal decompression at ~2.1 GPa and ~735°C. In comparison, the peak pressure conditions for the Tsäkkok lens are considerable lower, with original calculations of >1.5 GPa at 610 ± 90°C (Stephens and van Roermund, 1984), and more recent estimates of ~2.2 GPa at ~590°C (Bukała et al., 2020a). Nearly isothermal decompression has also been interpreted for the Tsäkkok lens following peak pressure metamorphism (Kullerud, 1987). The conditions are broadly constrained by the metasedimentary rocks hosting the eclogites, providing P-T estimates of ~1.0-0.6 GPa between ~500-400°C (Snilsberg, 1987).
Pioneering work to resolve the timing of eclogite-facies metamorphism was done by Mørk et al. (1988) who reported a garnet-omphacite Sm-Nd isochron dates of 503 ± 14 Ma and 505 ± 18 Ma for eclogites of the Vaimok lens and Tsäkkok lens, respectively. For the Vaimok lens, the interpreted timing of eclogite-facies metamorphism was in agreement with previous work of Dallmeyer and Gee (1986) who investigated the timing of exhumation of the Vaimok lens with 40Ar/39Ar step-heating performed on hornblende from retrogressed eclogites and on white mica from the surrounding metasedimentary rocks. They obtained plateau dates of 491 ± 8 Ma (hornblende), 447 ± 7 Ma and 436 ± 7 Ma (white mica), taken to represent cooling during exhumation. However, as more work revealed the complexity in the Vaimok lens, the oldest dates were called into question. Essex et al. (1997) reported prograde metamorphism of the Vaimok lens from c. 500-475 Ma, dates that were obtained from titanite U-Pb geochronology on the metasedimentary rocks. The most recent study on the timing of eclogite-facies metamorphism provided identical dates of 482 ± 1 Ma for both lenses using (ID-TIMS) U-Pb geochronology on eclogitic zircon (Root and Corfu, 2012). The consistency of the latest work result supports Essex et al. (1997) and suggests that extraneous 40Ar in hornblende and disequilibrium of Sm and Nd systematics in garnet were responsible for the older age estimates
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(Root and Corfu, 2012). For the Tsäkkok lens, no age of prograde metamorphism has been reported. The timing of exhumation has been broadly dated using 40Ar/39Ar step-heating on
hornblende from retrogressed eclogite and white mica from the metasedimentary rocks (Dallmeyer and Stephens, 1991). The dates produced were 465 ± 1 and 463 ± 6 Ma (hornblende), and 468 ± 1 Ma and 448 ± 2 Ma (white mica).These results, and the results of 40Ar/39Ar geochronology of the Vaimok lens, were all interpreted to represent protracted cooling of the rocks. However, the structural history of these metasedimentary rocks suggests that the spread of dates could be the result of multiple tectonic events and recrystallization.
Figure 3 – Tectonostratigraphic map of the Seve Nappe Complex in southern Norrbotten (modified after Thelander, 2009). Pristine and retrogressed eclogite bodies are hosted within the siliciclastic and
carbonaceous metasedimentary rocks in the Tsäkkok and Vaimok lenses.
6.2 Vestgӧtabreen Complex
The Vestgӧtabreen Complex is best exposed at the Motalafjella Nunatak in Oscar II Land on Svalbard (Fig. 4). At this locality, the complex is divided into the Upper and Lower units that are separated by a southwest-dipping thrust (Ohta et al., 1986). The Lower Unit is unconformably overlain by Late Ordovician to early Silurian sedimentary rocks of the Bullbreen
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Group (Armstrong et al., 1986; Ohta et al., 1983; Scrutton et al., 1976). The Upper Unit is composed of siliciclastic and carbonaceous metasedimentary rocks that host bodies of blueschist and eclogite, whereas the Lower Unit is dominated by green and black phyllites, calc-silicates and blueschists without eclogites (Agard et al., 2005; Horsfield, 1972; Hirajima et al., 1984, Kanat, 1984; Kanat and Morris, 1988). The P-T conditions of peak metamorphism differ between the units. Eclogites of the Upper Unit have yielded conditions of metamorphism at1.8-2.4 GPa at 575-645°C (Hirajima et al., 1988) in contrast to the Lower Unit where carpholite-bearing rocks provide 1.5-1.6 GPa at 380-400°C (Agard et al., 2005).
Geochronological studies of the Vestgӧtabreen Complex have only focused on lithologies from the Upper Unit. The timing of eclogite-facies metamorphism for the Upper Unit has been broadly dated to a Pb-loss event at 476 ± 30 Ma based U-Pb geochronology of zircon from eclogite (Bernard-Griffiths et al. 1993). The timing of exhumation for the Upper Unit is even more convoluted. Horsfield (1972) provided whole-rock K-Ar dates of 428 ± 24 Ma and 402 ± 12 Ma from metapelites. A subsequent study used 40Ar/39Ar step-heating of white mica also from metapelitic rocks, which yielded variably discordant 40Ar/39Ar spectra and plateau dates from 475 ± 2 Ma to 453 ± 2 Ma (Dallmeyer et al., 1990). These dates were interpreted as cooling of the Upper Unit during exhumation. A c. 425-400 Ma low-grade thermal overprint was also noted. The large variability of these dates obtained from the metasediments reflects the complex structural history of the Vestgӧtabreen Complex that was reproduced by Labrousse et al. (2008). In both units, the schistosity of the rocks are parallel to the southwest-dipping bounding thrust with dominant top-to-S shear sense in the rocks. However, proximal to the bounding thrust, the shear sense is directed top-to-N. The unconformable contact of the Lower Unit with the Bullbreen Group is also folded into a recumbent syncline around a northeast-southwest axis (Labrousse et al., 2008). Altogether, it suggests at least two deformation events affected the Vestgӧtabreen Complex after peak metamorphism. Therefore, the spread and discordance of the
40Ar/39Ar dates could be a result of recrystallization of the white mica during these deformation
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Figure 4 – Tectonostratigraphic bedrock map of the Vestgӧtabreen Complex exposed at the Motalafjella nunatak on Svalbard (modified after Ohta et al., 1986). The complex is divided into Upper
and Lower units that are separated by a southwest-dipping thrust.
7. Methods 7.1 Fieldwork
Fieldwork was conducted in all three localities (Figs. 2 and 3) over the past several years to obtain samples, field observation and photographs, and to acquire structural measurements. The field strategy was to acquire metasedimentary rocks samples of different lithologies (i.e., metapsammites, metapelites, calc-silicates, marbles) that recorded various generations of structures and strain intensities. The sampled metasedimentary rocks were targeted from regions where they hosted either pristine or retrogressed (U)HP lithologies (i.e., eclogites or blueschists). Structural data was also collected for the samples and throughout the localities as a whole to place the samples in the context of previous works investigating the structural histories of the localities, i.e., for articles [A1, 2], or to reconstruct and present the structural history, i.e., for article [A3].
22 7.2 Transmitted light and electron microscopy
Standard 30 μm polished thin sections were prepared from the samples obtained during fieldwork for purpose of transmitted light microscopy. Mineral paragenesis, microtextures, and microstructures were investigated using the thin sections to determine the samples that would be suitable for in-situ dating of monazite or white. The thin sections were also used to search for zircon grains to select samples for crushing and heavy mineral separation for purpose of zircon depth profiling. Electron microscopy was conducted on samples that were determined to be of interest after investigation with transmitted light microscopy and for preparation of geochronological analysis. Back-scattered electron (BSE) and secondary electron (SE) imaging was performed on the zircon mount that was prepared for depth profiling analysis for article [A1]. The mount was imaged at AGH University of Science and Technology (Kraków, Poland) with a FEI Quanta 200 scanning electron microprobe (SEM) operating under low-vacuum conditions with a 20 KeV accelerating voltage. For qualitative and quantitative analyses of monazite and white mica in articles [A1, 2, 3], energy-dispersive spectrometry (EDS) and wavelength-dispersive spectrometry (WDS) were used on an electron microprobe (EMP). The analyses were done with the JEOL JXA8230 EMP at AGH University of Science and Technology (Kraków, Poland) or the JEOL JXA8530F EMP at Uppsala University (Uppsala, Sweden).
Characterization of the monazite presented in article [A1] was done with the EMP at AGH University of Science and Technology. BSE images were taken for all monazite grains for article [A1]. The grains were then mapped for Al, Ca, Th, U, and Y using WDS, and for Ca, Fe, La, Nd, P and S using EDS. The operating conditions were set at 15 KeV and 100 nA. Identification of monazite grains that were dated in article [A2] was conducted with the EMP at Uppsala University. BSE images were obtained for these grains, but no mapping was conducted due to the very small grain size (typically <<50 μm).
Thick sections (~500 μm) were prepared and examined with the EMP to select and characterize white mica of interest for in-situ 40Ar/39Ar geochronology. For white mica in article [A2], BSE images and WDS chemical mapping was performed with the EMP at Uppsala University. The chemical mapping was done for the major elements Si, Al, Mg, Fe, and K. The analytical conditions for the mapping were 15 KeV and 200 nA. For white mica presented in article [A3], BSE imaging and chemical mapping was done with the EMP at AGH University of Science and Technology. The major elements of Si, Al, Mg, Fe and K were obtained using analytical conditions of 15 KeV and 40 nA. Furthermore, quantitative WDS spot analyses were conducted on standard 30 μm polished thin sections that mirrored the thick sections of the
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respective samples. The spot analyses targeted white mica in the same structural positions as those chosen for dating in the thick sections. The spot analyses were conducted on the thin sections to avoid volatilization of elements (e.g., K) in white mica that was targeted for dating. The electron microprobe was operated using an accelerating voltage of 15 KeV and a beam current of 50 nA, with a 5 μm spot size for WDS analyses.
7.3 Zircon U-Pb geochronology and trace element geochemistry depth profiling
The zircon depth profiling utilized for article [A1] was conducted at the University of New Brunswick’s LA-ICP-MS Laboratory (Fredericton, Canada). Preparation for depth profiling involved crushing of metasedimentary rock samples from the Vaimok lens and standard heavy mineral separation techniques. The zircon crystals from three samples were hand-picked from the 250-63 μm size fraction. The zircon were subsequently embedded into indium within an aluminum disc. Crystal faces of the zircon were parallel to the mount surface and the zircon grain were not polished. Depth profiling of the zircon grains was conducted using a Resonetics S-155-LR 193 nm Excimer laser ablation system coupled to an Agilent 7700x quadrupole ICP-MS. Plešovice zircon (U-Pb: 337.13 ± 0.37 Ma; Sláma et al., 2008) and zircon FC-1 (U-Pb: 1099 ± 1 Ma; Paces and Miller, 1993) were used as the primary and secondary reference materials, respectively. NIST 610 was used as the trace element reference material. Both the geochronological and trace element references were analyzed at the beginning and end of each sample, as well as intermittently between every 3-6 unknowns.
The laboratory protocols outline by McFarlane and Luo (2012) for U-Pb dating of zircon via LA-ICP-MS were used. Two depth profiles were ablated on every zircon using a 33 um lateral spot size to acquire both trace element and geochronological isotopic data. The locations of the ablation pits were selected according to the BSE and SE images of the zircon crystals to avoid cracks, overgrowths, and irregularities of the zircon. The ablation pits were placed adjacent to each other on the same crystal face. The first ablation pit used a 3 Hz repetition rate and was pulsed for a total of 206 s with a total quadrupole sweep time of 0.543 s, analyzing elements P, Ti, V, Sr, Y, Rare Earth Elements (REE), Hf, as well as U, Th and Pb isotopes. The second ablation pit utilized a 2.5 Hz repetition rate and was pulsed for a total of ~15 s with a total quadrupole sweep time of 0.316 s, and only analyzed U, Th and Pb isotopes. The raw data from both ablation pits was reduced with Iolite v.2.5 software (Paton et al., 2011) using VisualAge data reduction scheme for geochronology (Petrus and Kamber, 2012) and Trace_Element_IS for all trace elements. Further statistical analysis of the datasets derived from the Iolite software were conducted using Isoplot v.4.15 plug-in (Ludwig, 2012) for Microsoft Excel.
24 7.4 In-situ monazite Th-U-total Pb geochronology
For article [A1], the monazite Th-U-total Pb geochronology was conducted at Slovakian State Geological Institute (Bratislava, Slovakia), whereas for article [A2], dating was done at the Institut des Sciences de la Terre (ISTerre; Grenoble, France). For both laboratories, standard 30 μm polished thin sections were prepared. The analytical targets for the monazite dated in both articles were chosen according to the BSE images, and EMP chemical maps for article [A1]. The results from both laboratories were investigated using statistical methods offered in the Isoplot v.4.15 plug-in (Ludwig, 2012) for Microsoft Excel.
A Cameca SX 100 electron microprobe was used to date the monazite at the Slovakian State Geological Institute. The dating procedure followed the methodology detailed in Konečný et al. (2018). For the analyses, a 3 μm spot was utilized with operating conditions of 15 KeV and 180 nA. The contents of Th, U, Pb, P, Y, REE, Fe, S, Ca, Sr, Al, Si, Al, and K were obtained for each analysis.
At ISTerre, a JEOL JXA-8230 electron microprobe was used to date the monazite. The procedure followed a similar protocol to the one described in Grand’Homme et al. (2018). For the analyses, a 1 μm spot size was used with operating conditions set to 15 KeV and 200 nA. The elements analyzed were Th, U, Pb, P, Y, La, Ce, Pr, Sm, Gd, Dy, Ca and Si. To provide accurate date calculations, the counting times for U, Th and Pb of 160, 300 and 480 s were used. Manangotry monazite reference material (Montel et al., 2018) was run as an unknown regularly during the analyses.
The different laboratories were chosen according to the analytical spot size that could be implemented. The monazite dated in article [A1] were much larger (up to 50 μm in diameter) relative to the monazite dated in article [A2] (typically <<50 μm in diameter, but with domains suitable for analysis on the order of only a few μm). Therefore, the 3 μm spot size used at the Slovakian State Geological Institute could be applied to the monazite of article [A1]. The 1 μm spot size used at ISTerre was more suitable for the smaller monazite of article [A2] to avoid mixing with other mineral phases. However, by choosing to use the smaller spot size, the analysis of Eu, Tb, Ho, Er, Tm, Yb, Lu, Fe, S, Sr, Al, and K were sacrificed.
7.5 In-situ white mica 40Ar/39Ar geochronology
For articles [A2] and [A3], in-situ 40Ar/39Ar analytical work was performed at the University of Manitoba (Winnipeg, Canada) using a multi-collector Thermo Fisher Scientific ARGUS VI mass spectrometer, linked to a stainless steel Thermo Fisher Scientific extraction/purification line, a Photon Machines (55 W) Fusions 10.6 CO2 laser, and a Photon
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Machines (Analyte Excite) 193 nm laser. The 193 nm ultra-violet (UV) laser was operated at a frequency of 25 Hz, ablating for 80 s over an area of 100 x 100 μm or 200 x 50 μm, depending on the size and shape of the white micas. Argon isotopes (masses 40 to 37) were measured using Faraday detectors with low noise 1012 Ω resistors and mass 36 was measured using a compact discrete dynode (CDD) detector. The sensitivity for argon measurements is ~7.392 x 1017 moles/fA as determined from measured aliquots of Fish Canyon sanidine age standard (Dazé et al., 2003; Kuiper et al., 2008).
The 40Ar/39Ar UV laser thick sections used for dating were produced by cutting ~1 cm2 areas from ~500 μm-thick polished thin sections that had previously been investigated by optical microscopy and scanning electron microprobe. These unknowns were re-polished prior to irradiation to remove the carbon coating used for the scanning electron microprobe. Age standards and unknowns were placed in 2 mm deep wells in 18 mm diameter aluminium irradiation disks, with the age standards (28.201 Ma Fish Canyon sanidine; Kuiper et al., 2008) placed strategically so that the lateral neutron flux gradients across the disks could be monitored. Planar regressions were fit to the age standard data, and the 40Ar/39Ar neutron fluence parameters, J values, were interpolated for the unknowns. All the specimens were irradiated for 20 hours in the cadmium-lined inner-core irradiation tube (CLICIT) facility of the Oregon State University TRIGA reactor. After irradiation, standards were loaded into a Cu laser sample tray, covered with a KBr coverslip, and placed in a stainless-steel chamber with a differentially pumped ZnS viewport attached to a Thermo Fisher Scientific extraction/purification line, and baked with an infrared lamp for 24 hours. Single crystals of the age standards were fused using the CO2 laser. The unknowns were affixed to a quartz slide using a ceramic adhesive (PELCO),
and were placed into a stainless-steel laser chamber fitted with a sapphire viewport that was attached to the same stainless-steel high vacuum extraction system as the CO2 laser, and baked
with an infrared lamp for 20 hours. Reactive gases were removed for both the age standard and unknown analyses for 3 minutes, using three NP-10 SAES getters (two at room temperature and one at 450 °C) prior to being admitted by expansion to an ARGUS VI mass spectrometer. Five argon isotopes were measured simultaneously over a period of 6 minutes. Measured isotope abundances were corrected for extraction-line blanks, which were determined before every sample analysis. Blanks in both the Excimer and CO2 laser lines averaged ~5.63 fA for mass 40,
~0.01 fA for mass 39, ~0.02 fA for mass 38, ~0.02 fA for mass 37, and ~0.02 fA for mass 36. Detector intercalibration (IC) between the different Faraday cups was monitored (using the Thermo Qtegra software) every four days by peak hopping on the 40Ar peak using an air aliquot. Calculated values are ICH1: 1.0000 (40Ar), ICAX: 1.0745 (39Ar), ICL1: 1.0637 (38Ar),
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and ICL2: 1.0534 (37Ar), with an error of ~0.2% (1 σ). The intercalibration factor between H1
and the CDD was measured using 20 air aliquots that were evenly interspersed with the unknowns resulting in an ICCDD value of 1.0058 ± 0.0002 per amu (36Ar; 2σ). An atmospheric
40Ar/36Ar ratio value of 295.5 was used for the purposes of routine measurement of mass
spectrometer discrimination using air aliquots, and correction for atmospheric argon in the
40Ar/39Ar age calculation (Steiger and Jaëger, 1977). Corrections are made for neutron-induced 40Ar from potassium, 39Ar and 36Ar from calcium, and 36Ar from chlorine (Renne and Norman,
2001; Renne et al., 1998; Roddick, 1983). The isotope values were also corrected for the blanks, mass discrimination, and radioactive decay. Data collection was performed using Pychron (Ross, 2017) and data reduction, error propagation, date calculation and plotting were performed using MassSpec software (version 8.091; Deino, 2013). The decay constants used were those recommended by Steiger and Jäger (1977). The 40Ar/39Ar dates were examine by statistical methods of the Isoplot v.4.15 plug-in (Ludwig, 2012) for Microsoft Excel.
8. Conclusions
8.1 Summary of PhD thesis work
In-situ monazite Th-U-total Pb geochronology conducted on the Vaimok lens metasedimentary rocks revealed high pressure prograde metamorphism at 498 ± 10 Ma (Fig. 5). The monazite domains that yielded the age of metamorphism were produced by dissolution-reprecipitation of a pre-existing monazite volume (603 ± 16 Ma) and are characterized by a depletion of Y content with respect to the Neoproterozoic monazite (Fig. 5). The late Cambrian monazite was also found as inclusions in garnet, which together with the Y depletion indicates monazite formation during garnet crystallization. A third, volumetrically subordinate monazite generation that was characterized by the highest Y content and is interpreted to represent new monazite growth at 479 ± 29 Ma due to partial dissolution of garnet during decompression. All monazite in the investigated metasedimentary rocks were subsequently partially-to-fully retrogressed to coronas of apatite, allanite and clinozoisite due to external Ca-rich fluid influx destabilizing the monazite. Retrogression of monazite coincided with dissolution-reprecipitation of zircon forming metamorphic rims that were detected using LA-ICP-MS depth profiling. The zircon rims are characterized by increased LREE, U, Th, and P, reflecting the simultaneous breakdown of monazite. The age of dissolution-reprecipitation was calculated from three samples to 480 ± 22 Ma, 475 ± 26 Ma, and 479 ± 38 Ma, that are interpreted to represent the timing for exhumation of the Vaimok lens (Fig. 5).
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Figure 5 – Graphical illustration summarizing the important mineral reactions and trace element distributions amongst major and accessory minerals that record subduction and exhumation of the Vaimok lens. Abbreviations: Aln, allanite; Ap, apatite; Czo, clinozoisite; Mnz, monazite; Rt, rutile. Monazite and zircon were dated using the Th-U-total Pb and depth profiling techniques, respectively.
A structural investigation of the Tsäkkok lens was performed in conjunction with in-situ white mica 40Ar/39Ar geochronology applied to the metasedimentary rocks in order to resolve
the timing and style of exhumation. Three main phases of deformation were described (Fig. 6): foliation development (S1) during prograde-to-peak metamorphism (D1), closed to isoclinal folding (F2) and foliation development (S2) during exhumation (D2), and late stage, broad scale folding (F3) post-dating continental collision and nappe stacking (D3). The in-situ white mica
40Ar/39Ar geochronology targeted a multitude of structures of S1, F2 and S2 to elucidate the
temporal evolution for deformation of the lens. The dating results were also interpreted in light of EMP major element mapping and quantitative chemistry of the white mica that defined the structures that were dated. Results of the work demonstrate that following eclogite-facies metamorphism (c. 482 Ma; Root and Corfu, 2012), the Tsäkkok lens underwent decompression then cooled in middle-to-shallow crustal levels at 477 ± 4 Ma to 475 ± 4 Ma (Fig. 6). The white mica that provided the record of cooling are associated with an array of structures (S1, F2, S2) and are characterized by homogeneous chemical compositions, indicating that deformation likely occurred at temperatures >500°C. Some F2 samples provided dates that coincided with the cooling record, but also considerably younger dates as well, as young as 448 ± 7 Ma, suggesting that vertical shortening of the lens continued after cooling was recorded by white mica (Fig. 6). Furthermore, samples of S2 that contain non-coaxial structures provided the largest dispersion with 40Ar/39Ar dates as young to 424 ± 6 Ma (Fig. 6). The scatter in dates
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represents partial resetting of the cooling record due to recrystallization of the white mica crystal. Recrystallization is also reflected in the heterogeneous white mica chemistry of these samples, demonstrating incomplete re-equilibration of white mica chemistry. Altogether, these partially reset samples indicate that folding and vertical shortening of the Tsäkkok lens persisted in temperature conditions cooler than the white mica closure temperature, and subsequent penetration of deformation in the Tsäkkok lens associated with continental collision and nappe stacking.
Figure 6 – Graphical illustration summarizing the evolution of the Tsäkkok lens during exhumation. A) Vertical shortening and development of F2 and S2 prior to/during cooling at 477 ± 4 Ma to 475 ± 4 Ma. B) Localized, late-stage F2 during continued shortening in temperature after cooling of the Tsäkkok lens is recorded by white. The late-stage F2 recrystallized white mica and partially reset the cooling record to produce 40Ar/39Ar dates as young as 448 ± 7 Ma. C) Nappe stacking of the Sarek and Tsäkkok
lenses, and the overlying Köli Nappe Complex during continental collision. Non-coaxial deformation penetrated the Tsäkkok lens during this event and recrystallized white mica grains, producing 40Ar/39Ar
dates as young as 424 ± 6 Ma. D) Development of F3 after nappe stacking. The timing of all events are resolved by in-situ white mica 40Ar/39Ar geochronology.
