The tectonic and metamorphic history of UHP basal gneisses and Blåhø- Surna cover complexes on Otrøy, Moldefjord, northern Western Gneiss Region, Norway

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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

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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 S

0

, (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.

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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 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. Smit

1

1_{m.a.smit1@students.uu.nl; m.a.smit@uni-muenster.de}

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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

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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

6.02.02. 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

6.02.03. 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

7.01.01. Compositional banding in the Basal Gneiss Complex (ScBGC) 70

7.01.02. Compositional banding in the GK nappe complex (ScGK) 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

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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

7.04.02b. Mylonites: the instigators of Fr+x and LrGK ? 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

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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

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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

13.02.02. Tectonometamorphism in the Scandinavian Caledonides 165 13.03. The mid-Caledonian period ( 460 Ma < T < 440 Ma ) 166

13.03.02. Tectonometamorphism in the Scandinavian Caledonides 168 13.04. Late Caledonian Pre-Scandian period ( 440 Ma < T < 420 Ma ) 170

13.04.02. Tectonometamorphism in the Scandinavian Caledonides 171

13.05. The Scandian Orogeny ( 420 Ma < T < 395 Ma ) 173

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

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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.

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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

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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

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(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 PT

D

-path?

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

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(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.

(14)

(15)