MONAZITE GEOCHRONOLOGY AND GEOTHERMOBAROMETRY OF THE HØYVIK GROUP OF THE CALEDONIAN MIDDLE ALLOCHTHON AND ITS STRUCTURAL SIGNIFICANCE TO THE UNDERLYING HP BASEMENT, WGR, WESTERN NORWAY V. van Schijndel Structural Geology Group Institute of Earth Sciences, Utrecht University Budapestlaan 4, 3508 AT, Utrecht, The Netherlands e-mail: [email protected] ABSTRACT The Høyvik Group is part of the Middle Allochthon and its Precambrium sediments were unconformable deposited on the crystalline Dalsfjord Suite during the Late Proterozoic. The rocks of the Høyvik Group are deformed under upper greenschist facies conditions and lie now within a close range of the HP rocks of the Western Gneiss Region (WGR), western Norway. In this study geochronology by EMP monazite age dating and geothermobarometry using chlorite-white mica pairs were performed on several samples of the Høyvik Group. Monazite CHIME ages of 605 ± 91 Ma, 587 ± 71 Ma and 290 ± 48 Ma are found. The Precambrium ages are linked to dyke intrusion events dating the opening of the Ægir/ Iapetus Sea at 590-610 Ma. The rifting age is attained from several areas in both Norway and Sweden (e.g. the Seve Nappe, Central Sweden). These dykes are similar in composition to the dykes occurring in the Høyvik Group. Earlier 40Ar/39Ar dating on rocks of the Høyvik Group suggested a tectonic event at 445- 450 Ma. For this event no evidence was found by means of monazite age dating in this research. Geothermobarometry using a new program applied to chlorite-white mica solid solution phases show PT conditions of 400-475ºC and 8-11 kbar. These PT conditions may be linked to the deformation event at 445- 450 Ma. This event is also seen in eclogites of the Seve Nappe, Central Sweden. Due to several similarities between the Høyvik Group and the Seve Nappe, Sweden, both in terms of origin and tectonic evolution it is suggested that the latter may be a HP variant of the Høyvik Group. The monazite CHIME age of 290 ± 48 Ma is believed to be the result of resetting of older or the growth of new monazite minerals. This age is linked to reactivation of the Dalsfjord Fault and the simultaneously dyke intrusion during the Late Permian, indicating early rifting preluding the opening of the Atlantic Ocean. This study gave more insight into the tectonic evolution of parts of the Høyvik Group that are not highly affected by the Nordfjord Sogn Detachment Fault (NSDZ) during exhumation of the WGR. However, the role of the Høyvik Group in relation to the WGR during the Caledonian event and exhumation remains to be investigated. 1 LIST OF CONTENTS ABSTRACT 1 1. INTRODUCTION 4 1.1 Research setting 4-6 1.2 Research aim 6-7 1.3 Outline 7 2. PLATE TECTONICS 8-9 3. GEOLOGICAL SETTING 10 3.1 Regional Geology 10 3.1.1 Scandinavian Caledonides 10-12 3.1.2 Autochthon/Parauchthon 12 3.1.3 Lower Allochthon 12 3.1.4 Middle Allochthon 12 3.1.5 Upper Allochthon 12 3.1.6 Uppermost Allochthon 12 3.1.7 Devonian Basins 12-13 3.2 Geology of the research Area 13 3.2.1 WGR 13-16 3.2.2 Lower Allochthon 16 3.2.3 Middle Allochthon 17 3.2.3.1 Høyvik Group 18 3.2.3.2 Dalsfjord Suite 18 3.2.4 Upper Allochthon 18 3.2.5 Kvamhesten Basin 19 3.2.6 Geological Structures 19 3.2.6.1 Nordfjord-Sogn Detachment zone 19-20 3.2.6.2 Dalsfjord Fault 20-21 4. REGIONAL CORRELATIONS 22-23 4.1 Seve Nappe 23 4.2 Hyllestad Complex 23 4.3 Jotun Nappe & Valdres Sparagmite 23 4.4 Lindås Nappe 24 4.5 Correlations of the Middle Allochthon and the WGR 24 5. FIELD DATA 25 5.1 Sample collection 25 5.2 Structural observations 25 5.2.1 WGR 25 5.2.2 NSDZ 26 5.2.3 Høyvik Group 26-27 5.2.4 Dalsfjord Fault 27 5.3 Analysed Samples 27-28 5.4 Mineral Description of the analysed samples 28 5.4.1 Sample 6.8N06 micaschist 28 5.4.2 Sample 6.9N06 micaschist 28 5.4.3 Sample 9.2N06 psammite 28-29 2 6. ANALYTICAL PROCEDURES 31 6.1 Introduction to the EMPA 31 6.2 Data corrections 32 6.2.1 Peak Interferences 32 6.2.2 ZAF 32 6.2.3 Standard deviation 33 6.2.4 Valencies 33 6.3 Sample Preparation 33 6.4 Analytical Procedures for Monazite 33-36 6.5 Analytical Procedures for Chlorite-White Mica 37 7. INTRODUCTION to MONAZITE 38 7.1 Introduction 38 7.2 Monazite mineral 38-39 7.3 First appearance of monazite 39 7.4 Monazite growth and occurrence 39 7.5 Monazite resetting 40 8. GEOCHRONOLOGY 41 8.1 Chemical age dating-methodology 41 8.2 Apparent age dating 41 8.3 Isochron age dating 41-42 8.4 Error quantification 42-44 8.5 Results 45-47 9. GEOTHERMOBAROMETRY 48 9.1 Introduction 48 9.2 Thermobarometry 48-49 9.3 Fe2+/Fe3+ error 49-50 9.4 Results 51-52 10. DISCUSSION 53 10.1 Pre-Caledonian events 53-54 10.2 Caledonian events 54 10.3 Caledonian events 54-55 11. CONCLUSIONS 56 12. ACKOWLEDGEMENTS 57 13. REFERENCES 58-62 APPENDIX I 63-64 APPENDIX II 65 APPENDIX III 66-72 APPENDIX IV 73-74 APPENDIX V 75 APPENDIX VI 76-80 APPENDIX VII 81-84 3 1. INTRODUCTION 1.1 Research setting The research area is situated in Western Norway in the surrounding area of Askvoll and the island of Atløy (figures 1 and 2). The Scandian orogeny was the last stage of a series of events, together called the Caledonian orogeny, which were responsible for the tectonostratigraphic structure of the Scandinavian Caledonides. The Scandian orogeny included the collision of Laurentia and Baltica and ended with a post-orogenic extensional phase. The continental collision resulted in the formation of a nappe complex (Allochthons) that was thrusted to the east over the Baltoscandian basement and its autochthonous sedimentary cover. The nappes are subdivided into the Lower, Middle, Upper and Uppermost Allochthons (Roberts & Gee, 1985) (table 1). The Uppermost Allochthon consists of rocks from the Laurentian continent. The Upper Allochthon includes composites of oceanic terranes and marginal areas from Baltica or microcontinents. The Middle and Lower Allochthons consist of Baltica related sedimentary, igneous and crystalline rocks. The rocks of the Askvoll Area consist of Allochthoneous nappes and reworked basement rocks that are separated by the Nordfjord-Sogn Detachment Zone (NSDZ). The nappes in the research area are from top to bottom the Stavfjord (Høyvik and Herland Group) and Dalsfjord nappes (Corfu and Andersen, 2002), both nappes form parts of the Middle Allochthon (Tillung, 1999; Hacker et al., 2003) (figures 1 and 2). The probably Late Precambrium metasedimentary sequence of the Høyvik Group lies unconformable on the crystalline rocks of the Dalsfjord Suite. The Høyvik Group is in its turn unconformably overlain by the Silurian sediments of the Herland Group. The Dalsfjord Suite, Høyvik Group and Herland Group are part of the Middle Allochthon. The Solund-Stavfjord Ophiolite and cover of the Upper Allochthon lie structurally above the Herland Group and the units are separated by the Sunfjord Melange, formed during the obduction of the ophiolite (Andersen et al., 1990). Structurally under the Dalsfjord Suite lies the WGR, part of the continental basement which includes intrusions and high-grade orthogneisses (Andersen et al., 1990). Scandinavian Caledonides Research Area Devonian Basins Kvamshesten Basin Uppermost Allochthon Not present Upper Allochthon Stavenes Group Solund-Stavfjord Ophiolite Sunnfjord Melange Middle Allochthon Stavfjord Nappe Herland Group Høyvik Group Dalsfjord Suite Dalsfjord Fault Lower Allochthon “Aksvoll Group” Autochthon/Parautochthon Nordfjord-Sogn Detachtment Zone WGR Table 1: Tectonostratigraphy of the research area with respect to the Scandinavian Caledonides. 4 Figure 1: Simplified geological map of the research area and surroundings of the Sunnfjord area (after Andersen, 1998). The white mica, 40Ar/39Ar cooling ages are from Berry et al. (1993, 1995) and Andersen et al. (1998). The black box indicates the research area. The WGR is highly influenced by an U(HP) event during the last Scandian phase of the Caledonian orogeny. The post-orogenic extensional phase during early Devonian times caused the final exhumation of the WGR. The Western Gneiss Region (WGR) is an area with mainly retrograde amphibolite facies Pre-Caledonian basement migmatites and gneisses with local eclogite occurrences. The U(HP) rocks of the Western Gneiss Region form the root of the Caledonian orogen and represent the basement on which the Allochthonous rocks lie. 5 The Allochthonous segments and syn/post-orogenic Devonian basins are structurally detached from the deep crustal rocks of the WGR by the NSDZ, this structural element is a zone of extensional mylonites in the footwall on top of the WGR. The NSDZ is formed by top to the west extension during the exhumation of the WGR. The Devonian Dalsfjord fault (figure 2) has truncated the NSDZ and was reactivated during Permian, Jurassic and Cretaceous times (Eide et al., 1997; Torsvik et al., 1992). Figure 2: Geological map of the research area (after Andersen et al., 1998). The black outline around the Kvamhesten Basin and Dalsfjord Suite marks the Dalsfjord Fault. 1.2 Research Aim The main idea for this project was to construct a geological time frame through parts of the WGR, the NSDZ and the structurally overlying nappes to give more constraints about the origin of the nappes and relations with other parts of the Scandinavian Caledonides and the exhumation of the WGR.