The Tectonic and Metamorphic History of UHP

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The tectonic and metamorphic history of UHP basal gneisses and Blåhø- Surna cover complexes on Otrøy, Moldefjord, northern Western Gneiss Region, Norway: New insights into the pre-Scandian evolution of Iapetus and exhumation of (U)HP metamorphic terranes

A comparative structural, metamorphic and geochronologic study

Doctorandus / MSc Thesis

Matthijs A. Smit

Utrecht University

May 2006 Abstract

This MSc research is focussed upon two aspects. The first focus is to identify and classify tectonometamorphic signals and to establish their significance with respect to the dynamics of the Iapetus Ocean before late-Caledonian (Scandian) closure and concurrent (ultra-) high pressure ((U)HP) metamorphism. The second focus is to monitor mechanisms and processes that account for the exhumation of the (U)HP gneisses of the Western Gneiss Region. The island of Otrøy, Moldefjord, in the northern (UHP part) of the Western Gneiss Region, was chosen as the research area. On this island (U)HP gneisses, grt-peridotites and eclogites are exposed, that are overlain by a stack of unidentified allochthonous metapelites and metabasites. Correlative field studies indicate that the latter rocks (addressed in this study as the GK-Nappe) are part of the Blåhø-Surna nappe complex, a Norwegian equivalent of the Seve Nappe Complex in Sweden. The multidisciplinary research includes: (1) a field study into lithologies and structures, (2) light-microscopic analysis into microstructures and mineralogy, (3) quantitative Electron Microprobe (EMP) analysis into major element chemistry and (4) EMP U-Pb monazite geochronology. These data provide relevant information on the evolution of the nappe before Scandian collision. Furthermore, Comparison of these data to information on the UHP basal gneisses provides insight into the physical processes in both foot- and hanging wall complexes as a consequence to exhumation From the field study it is concluded that the GK nappe complex consists of (1) a metapelitic and amphibolitic complex and (2) a massive garnet amphibolite gneiss. Structural field studies indicated the following sequence of deformational events: (1) the creation of an initial S0, (2) intrusion of basic and felsic melts, in this order, (3) formation of a flattening foliation with no rotational component, (4) folding into large synclinal structures and (5) crosscutting of the area by near-vertical WSW-ENE-striking mylonites and reactivation of older tectonic contacts. Points (1) and (2) are not constrained as contemporary in each tectonostratigraphic unit, while points (3) to (5) homogeneously affect all units. Morphological light microscopic studies indicate that the flattening foliation is spaced and is made up of aligned amphibole, quartz and brown mica. The foliation disrupts coarse grt-bearing assemblages. Matrices in samples from the massive garnet amphibolite and retro-eclogites, included in the basal gneiss unit, are symplectitic and unfoliated PT-analysis and mineral zoning studies yield different metamorphic paths for different units in the tectonostratigraphy. Most GK lithologies only reflect burial to the upper amphibolite facies. The massive garnet amphibolite at the base of the GK nappe complex has a grt-granulite fingerprint. Basal gneiss eclogites show retrogression out of the UHPM field. Microstructural and geochemical analyses indicate that the various PT paths unify in the upper amphibolite facies and share a common retrogressive path during exhumation. U-Th-Pb geochronology on monazites of the GK nappe complex (yields three particular distributions within the Caledonian cycle: early Caledonian (500-465 Ma), mid-Caledonian (460- 440 Ma) and late-Caledonian ((early to late) Scandian, 420-400). Monazites attached to coarse upper amphibolite facies porphyroclasts of the GK nappe complex gave mid-Caledonian apparent ages, while monazites defining foliations that anastomose around the porphyroclasts reveal Scandian ages. These textural and geochronologic signals are evident blueprints for the Seve nappe. Along with data from the rest of this study, geochronology provides a very strong indication that these rocks indeed belong to the Seve complex. One monazite is found as an inclusion in garnet. This monazite yields Paleoproterozoic apparent ages. Such ages are only rarely encountered in allochthon complexes. The monazite is diagnosed to harbour a basement-derived detrital fraction.

II In the light of research focus (1), the multidisciplinary study indicates that Seve-equivalent GK nappe rocks were buried to upper amphibolite facies conditions in the mid-Caledonian; a period that marks the age of HP metamorphism in the Seve terranes in Jämtland (SWE) and metamorphic overprint of older Seve HP terranes in Norrbotten (SWE). Combination of these data with background information on tectonometamorphic evolutions of other parts of the Scandinavian tectonostratigraphy enables the reconstruction of the pre-Scandian Caledonian (500-425 Ma) evolution of the Iapetus Ocean that once separated cratons Laurentia and Baltica. Note:elsewhere you defined Scandian as 425-390 Ma. In the light of focus (2), the combination of studies indicates that exhumation of Western Gneiss Region (U)HP rocks was accompanied by massive spatial problems at crustal levels. These spatial problems were most likely induced by deficiencies in mass removal rates. Compressive stresses controlled by positive plate-buoyancy induced general flattening and plate- parallel foliations in the suture zone under lower amphibolite facies conditions. Further striving to isostatic stability caused deep synclinal folding under more ample ((sub-)greenschist facies) conditions. Post-Scandian orogenic collapse caused massive listric growth fault systems throughout the Western Gneiss terrane and translated to the formation of mylonites and overprinting and more brittle deformation structures in the research area. The results of this MSc research forms a decent basis for future research. The current study advocates subsequent multidisciplinary studies into structural and metamorphic histories of basal crystalline terranes and hanging-wall composites with special emphasis on geochronology.

M.A. Smit1

1 [email protected]; [email protected]

Front-page photos of the high plain near Midsundvatnet on Otrøy and of Gangstad on Midøy. Thin section images of micaschist with peculiar biotite question-mark shape and garnet in a HP garnet-amphibole gneiss.

III 1. Table of Contents

2. Introduction 9 2.01 The Scandinavian Caledonides 9 2.02 Research focus and questions 10 2.03 Research Strategy 12

3. Geology of the Scandinavian Caledonides 13 3.01. Anatomy of the Scandinavian Caledonides 13 3.02. Geological characteristics of the main tectonostratigraphic units 16 3.02.01. The basement 16 3.02.01a. Geology and pre-Caledonian evolution of the (Para-) Autochthon 16 3.02.01b. Caledonian overprint in basement rocks 17 3.02.02 The Caledonian orogenic nappes – Geology and deformation 17 3.02.02a. The Lower Allochthon 17 3.02.02b. The Middle Allochthon 18 3.02.02c. The Upper Allochthon 19 3.02.02d. The Uppermost Allochthon 19 3.03. The Caledonian Orogeny and its timing 21 3.03.01. The basement 21 3.03.01a. Deformation 21 3.03.01b. (U)HP metamorphism 21 3.03.01c. Eclogites 22 3.03.01d. (Gt) peridotites 23 3.03.01e. Caledonian geochronology 23 3.03.02. The Lower Allochthon 25 3.03.03. The Middle Allochthon 25 3.03.04. The Upper Allochthon 27 3.03.04a. The lower section: Seve Nappes 27 3.03.04b. The upper section: Köli Nappes 30 3.03.05. The Uppermost Allochthon 30 3.03.06. The Old Red Sandstones, Detachments and orogenic extension 30

4. Geodynamic modelling 33 4.01. Recent geodynamic models for the Caledonian evolution 33 4.01.01. The origin and closure of Iapetus and Ægir 33 4.01.02. Dunk Tectonics: Multiple subduction / eduction stages in the Caledonides 34 4.01.03. The paradigm of multiple collisions 37 4.02. Scandian UHP metamorphism and exhumation 39 4.02.01. The WGR: A regional duplex? 39 4.02.02. Deep subduction and root delamination 40 4.02.03. Two partly synchronous extension modes 40 4.02.04. Sinistral crustal transtension in the WGR 41 4.02.05. Gravity tectonics and ductile rebound 42 4.02.06. Crustal imbrication and peridotite entrainment 44 4.02.07. Two-stage exhumation and HP / UHP mixing 44 4.02.08. Dunk tectonics: a two-way-street subduction-eduction model 45 4.02.09. Dual exhumation processes in the WGR 47

3 4.02.10. Shredding Baltica: Syncollisional exhumation and symmetric collapse 48 4.02.11. Earthquakes, eclogitisation and subduction channel flow 50 4.03. Conclusive remarks: research goals refined 52 4.03.01. Significance of this research regarding the evolution of Iapetus 52 4.03.02. Contributions to WGR exhumation studies 52

5. The research island Otrøy 53 5.01. Setting and accessibility 53 5.02. Past geological (mapping) studies on Otrøy 54 5.02.01. Carswell and Harvey [1985], Griffin and Carswell [1985] 54 5.02.02. Mørk [1989] 54 5.02.03. Tveten et al. [1998] 55 5.02.04. Robinson et al. [2003] 55 5.02.05. Wiggers-de Vries [2004] and Van Straaten [2004] 57

6. General tectonostratigraphy of Otrøy 58 6.01. The Basal Gneiss Complex 59 6.01.01. Basement gneisses 59 6.01.01a. Granitic gneisses 59 6.01.01b. Granodioritic gneisses 59 6.01.01c. Felsic intrusives 59 6.01.01d. Derivation 59 6.01.02. (U)HP rocks: Gt-peridotites and (external opx-) (retro-)eclogites in the BGC 60 6.01.02a. Garnet peridotites 60 6.01.02b. (Retro-) eclogites 61 6.01.02c. Derivation 61 6.02. The Gangstad – Klauset nappe complex 61 6.02.01. Metapelites 61 6.02.01a. (Garnet-bearing) Micaceous schists and gneisses 61 6.02.01b. Mica-bearing quartz-feldspar schists and gneisses 62 6.02.01c. Amphibole-bearing micaschists and –gneisses 62 6.02.01d. Derivation 63 6.02.02. Amphibolites 63 6.02.02a. (Garnet-)Amphibole gneisses 63 6.02.02b. Dolerite pods and bodies 64 6.02.02c. Basal garnet-amphibole gneisses (“MGA sub-unit”) 65 6.02.02d. Derivation 65 6.02.03. Felsic bodies 65 6.02.03a. Pegmatites 65 6.02.03b. Granites 66 6.02.03c. Derivation 66 6.03. The Upper Augen Gneiss Complex 67 6.04. Tectonostratigraphic Column 67 6.05. Linkage to tectonostratigraphy of the Scandinavian Caledonides 67 6.05.01. Basal Gneisses 67 6.05.02. GK nappe complex 67 6.05.03. The Upper Augen Gneiss Complex (UAGC) 68 6.06. Prologue to the analytical section 68

7. Structures on Central and southern Otrøy 70 7.01. Compositional banding structures (Sc) 70 BGC 7.01.01. Compositional banding in the Basal Gneiss Complex (Sc ) 70 GK 7.01.02. Compositional banding in the GK nappe complex (Sc ) 70 7.02. Dominant regional foliation (Sr) 71 7.02.01. General characteristics and trends of Sr 71 7.01.02. Deviatory areas 72 7.01.02a. High strain zones 72 7.01.02b. The Midsundholmen deviation 76 GK BGC 7.03. Lineations (deformation structure Lr and Lr ) 78

4 7.04. Discussing relative timing and structural settings 79 7.04.01. Sr: axial plains of the Moldefjord syncline? 79 7.04.02. Mylonites: Cause and consequence 79 7.04.02a. Regional linkage and relative timing 79 GK 7.04.02b. Mylonites: the instigators of Fr+x and Lr ? 80 7.04.02c. “Dr+2”: Otrøy before and after 81 7.05. Recap: deformational history of the GK nappe complex 82

8. Mineralogy and Microstructures 85 8.01. Basal Gneiss Complex retro-eclogites 85 8.01.01. The Breivik retro-eclogite 85 8.01.02. The Lomtjern retro-eclogite 86 8.02. The Gangstad-Klauset nappe complex 87 8.02.01. Metapelites 87 8.02.01a. Migmatic (garnet-bearing) micaschists and –gneisses 87 8.02.01b. Mica-bearing quartz schists and gneisses 89 8.02.01c. Amphibole-bearing micaschists and -gneisses 90 8.02.02. Amphibolites 91 8.02.02a. (Garnet-bearing) Amphibole gneisses 91 8.02.02b. Basal garnet-bearing amphibole gneisses (MGA sub-unit) 92 8.03. Recap on mineral content 93

9. EMP mineral chemical analysis and classification 96 9.01. The EMP technique 96 9.01.01. Electron beam and bombardment 96 9.01.02. Electron emission, refraction and detection 96 9.01.03. Calibration and correction 97 9.01.04. Errors corrections 98 9.01.04a. Analytical standard deviations 98 9.01.04b. Ferrous / ferric iron correction 99 9.02. Mineral Geochemistry 100 9.02.01. Garnet (X3Y2(TO4)3) 100 9.02.01a. Garnets in the GK nappe complex 100 9.02.01b. Garnets in the BGC retro-eclogites 102 9.02.02. Pyroxene (XYT2O6) 103 9.02.02a. Clinopyroxene from the GK nappe complex 103 9.02.02b. Clinopyroxene in the BGC retro-eclogites 103 9.02.02c. Orthopyroxene 106 - 9.02.03. Amphibole (XY2Z5T8O22(N )2) 106 9.02.03a. Amphibole in the GK nappe 107 9.02.03b. Amphibole in the BGC retro-eclogites 107 9.02.04. Feldspar (XYSi3O8) 110 9.02.04a. Feldspar in the GK nappe 110 9.02.04b. Feldspar in the BGC retro-eclogites 110 9.02.05. Mica (XY2-3 Z4 O10(OH)2) 112 9.02.05a. Brown mica in the GK nappe 112 9.02.05b. Brown mica in the BGC retro-eclogites 112

10. Preface to PT: Zoning and equilibration 114 10.01. Minerals tested for equilibration 114 10.02. Zoning characteristics 115 10.02.01. Garnet + Clinopyroxene ± Plagioclase assemblage 115 10.02.01a. The Grt + Cpx ± Pl in the GK nappe complex 115 10.02.01b. Grt + Cpx ± Pl in the BGC retro-eclogites 116 10.02.02. Garnet + Amphibole ± Plagioclase mineral assemblage 117 10.02.02a. Grt + Amph ± Pl in the GK nappe complex 117 10.02.02b. Grt + Amph ± Pl in the BGC 118 10.02.03. Garnet + Brown Mica mineral assemblage 118 10.02.04. Zoning in Ilmenite, Rutile and Titanite 119 10.02.05. A note on zoning in garnets 119

11.Geothermobarometry 121 11.01. Geothermo- and geobarometers 121 11.01.01. Garnet + Clinopyroxene ± (Plagioclase + Quartz) geothermobarometry 121 11.01.01a. Geobarometry (GADS) 121 11.01.01b. Geothermometry (GARC) 121 11.01.01c. PT-linkage 122 11.01.02. Garnet + Hornblende ± (Plagioclase + Quartz) geothermobarometry 122 11.01.02a. Geobarometry: net transfer equilibrium geobarometry (GHPQ) 122 11.01.02b. Geobarometry: Aluminium-in-Hornblende (Al-Hbl) 123 11.01.02c. Al-in-Hbl Geobarometry: constraining microstructures 123 11.01.02d. Geothermometry and PT-linkage (GARH) 123 11.01.03. Two-Pyroxene (Opx + Cpx) geothermobarometry 124 11.01.04. Garnet + Biotite geothermometry (GARB) 124 11.01.05. Chlorite geothermometry 125 11.02. PT Results 125 11.02.01. Garnet + Clinopyroxene ± (Plagioclase + Quartz) geothermobarometry 126 11.02.02. Garnet + Hornblende ± (Plagioclase + Quartz) geothermobarometry 126 11.02.02a. Al-in-Hbl Geobarometry: constraining microstructures 128 11.02.03. Two-Pyroxene (Opx + Cpx) geothermobarometry 130 11.02.04. Garnet + Biotite geothermometry (GARB) 130 11.02.05. Chlorite geothermometry 130 11.02.06. Compiling results 132 11.03. Discussing PT 133 11.03.01. Deduction of PT-paths for tectonostratigraphic units 133 11.03.02. Eclogites and their retrogression in the HP granulite and amphibolite facies 134 11.03.03. MGA sub-unit HP granulite path 134 11.03.04. Garnet zoning: a link between nappe and basement? 135 11.03.05. Amphibolite facies metamorphism and retrogression 135 11.03.06. Distinction and reclassification 135 11.03.07. PT and microstructure relations 136

12.Monazite geochronology 137 12.01. Introductory: Background on monazite 137 12.01.01. The mineral family Monazite 137 12.01.02. The formation and characteristics of monazite 138 12.01.02a. Melt-crystallised monazite 138 12.01.02b. Hydrothermal monazite 138 12.01.02c. Metamorphic monazite 139 12.01.02d. Closure temperatures (Tc) in monazite 140 12.01.03. Zoning in monazite 140 12.01.03a. Growth zoning 140 12.01.03b. Zoning by Th-U-Pb diffusion / resetting 141 12.01.03c. Implications for monazite geochronology 141 12.02. EMP analysis technique and errors 141 12.02.01. The EMP set-up 141 12.02.02. Standard deviations of measurement 142 12.02.03. Monazite selection 143 12.02.04. Subdividing the monazite population 143 12.03. Guidelines for EMP data processing 143 12.03.01. Part I: Constraining degree of substitution 143 12.03.02. Part II: Age Calculations / geochronology 144 12.03.02a. Population recognition 144 12.03.02b. Apparent age tapp: Calculation procedure 144 12.03.02c. Apparent age tapp: Error evaluation 145 12.03.02d. Apparent age tapp: statistical processing 145 12.04. Results part I: GK nappe monazite substitution geochemistry 146 12.04.01. Observations 146 12.04.02. Discussing monazite geochemistry 146

6 12.04.02a. Degree and character of substitution 146 12.04.02b. Error evaluation for geochemistry 148 12.05. Results part II: GK nappe monazite geochronology 150 12.05.01. Age populations 150 12.05.02. The 400-500 Ma isochron age cluster 151 12.05.02a. A distributional approach through age histograms 151 12.05.02b. Statistical approach through Isoplot 152 12.05.03. The 1705 Ma isochron age cluster 154 12.05.03a. Age derivation 154 12.05.03b. Mineral chemistry – chemical age linkage for A02M03 156 12.06. Discussion: Geochronology and its geological meaning 156 12.06.01. The Caledonian (400-500 Ma) age domain 156 12.06.01a. Character 156 12.06.01b. Microstructural ages 157 12.06.01c. Mid Caledonian metamorphic ages: A blueprint for the Seve Nappe 157 12.06.02. The paleoproterozoic (?) A02M03 monazite 158 12.06.02a. Processes and ages 158 12.06.02b. Substitution 159 12.06.03. Error evaluation for geochronology 159

13.Tectonometamorphic history 160 13.01. The pre-Caledonian era (T > 505 Ma ) 161 13.01.01. The Blåhø-Surna Complex on Otrøy 161 13.01.01a. Structural record 161 13.01.01b. Mineralogy and metamorphic record 161 13.01.01c. Geochronologic record 161 13.01.02. Tectonometamorphism in the Scandinavian Caledonides 162 13.02. The early Caledonian period ( 505 Ma < T < 460 Ma ) 163 13.02.01. The Blåhø-Surna Complex on Otrøy 163 13.02.01a. Structural record 163 13.02.01b. Mineralogy and metamorphic record 164 13.02.01c. Geochronologic record 164 13.02.02. Tectonometamorphism in the Scandinavian Caledonides 165 13.03. The mid-Caledonian period ( 460 Ma < T < 440 Ma ) 166 13.03.01. The Blåhø-Surna Complex on Otrøy 166 13.03.01a. Structural record 166 13.03.01b. Mineralogy and metamorphic record 166 13.03.01c. Geochronologic record 167 13.03.02. Tectonometamorphism in the Scandinavian Caledonides 168 13.04. Late Caledonian Pre-Scandian period ( 440 Ma < T < 420 Ma ) 170 13.04.01. The Blåhø-Surna Complex on Otrøy 170 13.04.01a. Structural record 170 13.04.01b. Mineralogy and metamorphic record 170 13.04.01c. Geochronologic record 171 13.04.02. Tectonometamorphism in the Scandinavian Caledonides 171 13.05. The Scandian Orogeny ( 420 Ma < T < 395 Ma ) 173 13.05.01. The Blåhø-Surna Complex on Otrøy 173 13.05.01a. Structural record 173 13.05.01b. Mineralogy and metamorphic record 174 13.05.01c. Geochronologic record 174 13.05.02. Tectonometamorphism in adjacent tectonostratigraphic units 174 13.05.02a. The Basal Gneiss Complex 174 13.05.02b. The MGA grt-granulites 175 13.05.03. Scandian geodynamics of the rocks on Otrøy 177 13.05.03a. Burial and initial exhumation 177 13.05.03b. Exhumation, constriction and foliation 177 13.05.03c. Self-stabilizing deformation accommodation 179 13.05.04. Tectonometamorphism elsewhere in the Scandinavian Caledonides 180 13.06. The Post-Caledonian / post-Scandian Period (T < 395 Ma) 181

7 13.06.01. The Blåhø-Surna Complex on Otrøy 181 13.06.01a. Structural record 181 13.06.01b. Mineralogy and metamorphic record 182 13.06.01c. Geochronologic record 183 13.06.02. Post-Scandian geodynamics of the rocks on Otrøy: Mylonitisation 183 13.06.02a. Mylonites: a shallow phenomenon 183 13.06.02b. Explaining the sinistral character 183 13.06.03. Tectonometamorphism in the Scandinavian Caledonides 184

14. Conclusive notes 185 14.01. Point-by-point conclusions 185 14.02. Answering research questions 186 14.02.01. On the tectonometamorphic history of nappes 186 14.02.02. On exhumation of UHP rocks in the northern Western Gneiss Region 187 14.03. Suggestions for future research 187 14.03.01. Future research on the island Otrøy 187 14.03.02. Future research in the Scandinavian Caledonides 188 14.03.03. Future research in a broad (global) context 189 14.04. Acknowledgements 189

15. References 190

Appendix I. Geological maps and cross sections 9pp. Appendix II. Studied thin sections 21pp. Appendix III. Mineral chemistry 35pp. Appendix IV. PT analysis 15pp. Appendix V. Background on monazites 14pp. Appendix IV. Geodynamics Compiled 3 pp.

8 2. Introduction

This introductory note aims to explain the main goals and research questions of this MSc thesis. Introductory information on the geology of the Scandinavian Caledonides, deduced tectonometamorphic histories and a summary of geodynamic models that may explain the formation and exhumation of Norwegian UHP rocks will be provided in chapters 3 and 4.

2.01 The Scandinavian Caledonides

The Scandinavian Caledonides have formed during Caledonian orogeny. This early-Paleozoic orogeny occurred not only in Scandinavia but also in Greenland, Newfoundland, The United Kingdom, Ireland. and along the east coast of the USA. The Scandinavian Caledonides extend well over 2000 km from southern Norway to its northern continental tip at North Cape. Beyond this stretch in Scandinavia, the orogen is also exposed on the Norwegian polar island of Svalbard. This orogenic belt in Scandinavia marks the collision zone between three proto-plates Baltica, Laurentia and (to a minor extent) Siberia. In the southwestern part of the Scandinavian Caledonides (figure 2.02), the Caledonian collision (~420 Ma, Scandian) involved only two plates: Laurentia and Baltica with the Iapetus ocean in between (review by [Roberts, 2003]). Following final closure of the Iapetus ocean, collision between the two proto-plates induced subduction and (U)HP metamorphism of sections of the Baltic crust below the Laurentian plate. Relict fractions of these deeply subducted terranes were exhumed and are now exposed in the (U)HPM terrane of the Western Gneiss Region (WGR) and HPM terranes along the west-coast of Nordland and the archipelago of Lofoten (highlighted in figure 2.01). The highest metamorphic-P conditions are sofar recorded in the WGR. The WGR provides a perfect field area to study UHP metamorphic processes and related exhumation mechanisms. Decades of study on this UHPM terrane yielded great new insights and numerous plausible geodynamic reconstructions for the behaviour of the WGR during continental subduction/collision . During Caledonian closure of the Iapetus (and Ægir) ocean, oceanic and Laurentian-related exotic terranes were stacked upon the Baltoscandian plate. These terranes form now heterogeneous allochthonous sequences/nappe complexes that lie above the Pre- Caledonian basement rocks in Sweden, Finland and Figure 2.01: UHPM and HPM terranes in the central-south Norway. In between these exotic nappe Scandinavian Caledonides (map after [Roberts and Gee, 1985].

9 complexes (Upper and Uppermost Allochthons) and the Baltic basement additional nappe complexes occur that have strong affinities to Baltica (Lower and Middle Allochthons). The allochthonous nappe complexes comprise variable lithologies that underwent often contrasting (peak) tectono-metamorphic histories. Additional geochronologic studies indicate these tectono-metamorphic signals, as recognised in the individual nappe complexes, often vary in age (e.g. contrasting data from [Mørk and Mearns, 1986; Williams and Claesson, 1987; Mørk et al., 1988; Essex et al., 1997; Terry et al., 2000b; Brueckner et al., 2004; etc, etc]). From this geological record in the Scandinavian Caledonides, it can be deduced that the (U)HP metamorphic event that caused extreme Scandian metamorphism in the Western Gneiss Region and related terranes was preceded by a series of tectonic and metamorphic phases that define orogenic Caledonian events that pre-date the Scandian. This new information opens up a new chapter of the geodynamic history of the Scandinavian Caledonides.

Figure 2.02: The reconstruction of the evolution of the protoplates Laurentia, Baltica and Siberia and separative oceanic domains. Reconstructions after Torsvik [1998], Torsvik et al. [1996] and Cocks and Torsvik [2002] with slight modifications by Roberts [2003].

2.02 Research focus and questions

The current research is focussed on contributing to: (1) insights into exhumation history of the UHP Western Gneiss Region and (2) insights into the pre-Scandian tectonometamorphic evolution. The research area was chosen on the island of Otrøy, in northernwestern part of the Western Gneiss Region (WGR), Moldefjord district (for location see figure 2.03). On Otrøy, basement gneisses of the WGR are overlain by allochtonous supracrustal rocks of weakly identified character and adherence. The island forms an excellent study ground for this research. General research questions on Otrøy in the context of the two goals are:

Tromsø

Bodø

Norrbotten (SWE)

Jämtland (SWE)

Trondheim

Molde

Otrøy

Ålesund

Bergen

Stavanger

Oslo

Figure 2.03: the geographical position of Otrøy and some larger cities / towns in Norway.

(1): o What drove exhumation of the UHP terrane and how is this reflected in possible deformation structures within the nappe? o Was exhumation a mono-phase mono-mechanic process or was it achieved through different steps of variable tectonic mode? o Under what circumstances?? What do you mean with that?? and when did the juxtaposition of supracrustals and high grade basement gneisses occur, i.e. from what point onward do PTt(D)- paths coincide? o Why is the UHP complex overlain by the allochthonous nappe stack? Is this a purely random feature? o When was the exhumation completed and what happened afterwards?

(2): o The allochthonous nappe: what rock types does it contain and where were the protoliths formed? o What tectonic and metamorphic signals are recorded in the nappe and how does this translate into a Caledonian PTD-path?

11 (2) continued: o What are the ages of metamorphism and deformation in the nappe? What is the tectonometamorphic history (PTtD-path) of the nappe ? o What does this means for the Pre-Scandian evolution of this metamorphic terrane. (How does this translate to the reconstruction of tectonics in the Iapetus oceanic domain before closure and UHP metamorphism?).

2.03 Research Strategy

In order to resolve answers to questions of this nature, a structural, metamorphic, geochemical, and geochronologic study is required. The following research strategy is applied:

I. Field work A careful study was made of the lithologies, tectonostratigraphy, deformation-structures and metamorphism of nappe exposures on Otrøy. The fieldwork includes sampling of representative rocks that are suitable for pressure (P) temperature (T) - and geochronologic analyses.

II. Structural analysis Research into the full structural record of the nappe. This is done in order to establish relative timing of deformation structures and thus to retrieve the deformation history of the nappe.

III. Microstructural analyses through light microscopy The thin section study was done in order to attain mineralogical contents of rock samples, establish metamorphic assemblages and retrieve microstructural characteristics of dominant deformation structures.

IV. Electron Microprobe (EMP) analysis to establish the major elements content of minerals. The data, obtained from sub-micron sized areas in minerals, will facilitate mineral classification, mineral zoning and establishment of equilibrated mineral assemblages. Subsequently these mineral assemblages will be subjected to PT analysis (using geothermobarometric techniques). A combination of calculated PT-data and microstructural information allow the construction of PTd paths

V. U-Th-Pb geochronology using EMP analysis on monazites Monazites from supracrustal rocks are analysed for their REE content and U-Th-Pb concentrations in order to attain their primary crystallisation- or re- equilibration/recrystallisation age. Submicron-sized geochronologic age data in combination with monazite microstructures could potentially constrain the age of the metamorphism and associated microstructures.

The results, including a “review” of comparative regional studies, will be presented in the analytical section of this thesis (Chapters 6-12). Chapter 13 deals with the integration of the analytical data into a final geodynamic model. However preceding chapters 6 to 13, two chapters (chapters 2 and 3) are presented that deal with the geological characteristics of the Scandinavian Caledonides and how they were formed. These two introductory chapters summarise the fundamental knowledge (orogen- scale geology, main tectonostratigraphy, geochronology, existing geodynamic models that exist in the literature about the Scandinavian Caledonides. This knowledge is required in order to understand the possible impact of a small geological study on the evolution of the entire mountain belt. Therefore chapters 2 and 3 will underline the research relevance of this thesis and provide complimentary geologic and geodynamic information on the Scandinavian Caledonides and its main tectonostratigraphic units.

Appendix I: Geological maps and cross sections

A note on measuring orientations on Otrøy

Deformation macrostructures are measured with application of 2 degrees West-declination. This has been tested in the field to be the correction from the map North (the north of the Statens Kartverk 1:5000 topographic maps used for mapping) to the magnetic North. The tests enabled projection of structural orientations on the field research topographic map set. Convention indicates a 7 degrees West rotation (addition of 7 degrees) for projection of orientations in the UTM WGS 84 coordinate system. The field area is situated in UTM zone 32N. For this zone there is a seven degrees clockwise rotation (subtraction of 7 degrees) of orientations with respect to the geographic North. So, in order to attain geographically projected orientations, two degrees are subtracted from the initial dataset. The orientation dataset for basement gneisses, as constructed by Wiggers-de Vries [2004] and v. Straaten [2004], is defined with respect to coordinate systems. Since this set is only used for global comparison, the possible error induced by declination differences has been ignored. The geological maps show the classical compass rose as given by the Norwegian Geological Survey. The relations of this rose are questioned, yet shown for classical reference.

Contents:

Geological maps distinguish between UHP rocks, basal gneisses and nappe complexes. Maps and cross sections with the annotation (detail) classify on a detailed lithological basis.

I. Geological map of Western Otrøy, Midøy and Dryna, Moldefjord, Norway

II. Geological map of Otrøy and sample locations (note sample 11-14 = 11-16)

III. Geological map and cross section of the Arneset lighthouse area (detail)

IV. Geological map and cross section of the Midsundhornet area (detail)

V. Cross section of the Storlihaugane – Klauset transect (UTM 386200, 6953250 to UTM 384800, 6971700)

VI. Geological map of the Lomtjern area (detail)

I Basal Gneiss Complex Sample (MSc)22-12, The Lomtjern retro-eclogite Garnet amphibolite gneiss, retro-eclogite

XXI Basal Gneiss Complex Sample (MSc)24-1, The Breivik retro-eclogite Coarse garnet amphibolite gneiss, retro-eclogite

XXII Appendix II. Studied thin sections

Contents: images of the following thin sections:

The Gangstad-Klauset / Blåhø-Surna nappe complex:

10-8 11-16* 12-1 12-5 15-5 21-6* 23-7 23-13-1 and –2 A2* D1-2, D2 and D3

Massive Garnet Amphibolite (MGA) stratum:

13-8 13-10 22-15 22-18 28-14

Basal Gneiss Complex:

22-12, The Lomtjern retro-eclogite 24-1, The Breivik retro-eclogite

* Samples studied in geochronology. Thin section images are enhanced with monazite locations.

I MGA (sub-) unit Sample (MSb)13-8 Garnet amphibolite gneiss

XVI MGA (sub-) unit Sample (MSa)13-10 Garnet amphibolite gneiss

XVII MGA (sub-) unit Sample (MSa)22-15 Garnet amphibolite gneiss (partly very coarse-grained)

XVIII MGA (sub-) unit Sample (MSa)22-18 Garnet amphibolite gneiss

XIX MGA (sub-) unit Sample (MSb)28-14 Garnet amphibolite gneiss