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The Tectonic and Metamorphic History of UHP

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The Tectonic and Metamorphic History of UHP 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.
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