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. Earlier geochronical research on the Høyvik Group was performed by Andersen et al. (1998) (figure 1). In this research the method of 40Ar/39Ar dating on white mica (phengite) was applied. The results were in the order of 445-450 Ma, while other areas of the Middle Allochthon in Norway contain ages of around 410 Ma and are interpreted as being overprinted by the Scandian event (Andersen and Austrheim, 2003). The white 40 39 mica Ar/ Ar ages are cooling ages, since the closure temperature (Tc) of white mica lies around 350°C. The closure temperature of monazite lies around 750°C. The question was if this early-Caledonian event which influenced the rocks of the Høyvik Group may also be found with monazite age dating or maybe even an older Pre- Scandian event. Locally the gneisses of the WGR were dated to be 395-399 Ma, with white mica 40Ar/39Ar dating by Andersen et al. (1998), indicating a cooling age during exhumation by the NSDZ. By monazite age dating of the rocks of the WGR and the NSDZ a possible difference in timing would become visible which will give more insight in the timing and mechanisms of exhumation of the WGR.
Monazite age dating was not done earlier in the Sunnfjord area and was chosen here to give a more decisive answer in terms of dating and origin of this area and the relationship with the exhumation of the WGR. The monazite age dating was performed by Chemical Th-U-total Pb Isochron Method (CHIME) with the use of EMP analyses, a relatively new age dating technique (Suzuki et al., 1991).
6 Sampling of felsic rocks for monazite dating took place along sections through the structural transition of low grade nappes (Høyvik Group), the mylonites of the NSDZ and the structurally underlying rocks of the WGR. The samples consist of mylonites, metasedimentary rocks and in addition gneisses from the WGR. The metamorphic PT path of the Høyvik Group is not fully known. During this study PT calculations were performed on rocks of the Høyvik Group for more insight in the metamorphic history in relation with other thrust nappes and the WGR. A new geothermobarometric technique (Vidal, 2005), based on EMP mineral analyses of chlorite-white mica pairs, is used to obtain information on the metamorphic grade. The new data from monazite age dating and chlorite-mica PT-calculations will be compared with geochronologic and geothermobaromtric results of other studies in the Scandinavian Caledonides. So by computing monazite ages of the WGR, NSDZ and the overlying nappes supported by additional PT estimates of the Høyvik Group, a better understanding of the role of the Høyvik Group during the Caledonian orogenesis and during exhumation of the WGR may be attained. Also the link between the Høyvik Group and other thrust nappes may become clearer.
1.3 Outline In this paper first in Chapter 2 an overview of plate tectonic motions leading to the development of the Scandinavian Caledonides will be given, starting with the break up of the supercontinent Rodinia. Chapter 3 contains an overview of the geology of the Scandinavian Caledonides and detailed description of the research setting. In Chapter 4 the similarities of the research area with other parts of the Scandinavian Caledonides will be high lighted. Chapter 5 contains information on the field research and sample collection. Also a more detailed description of the analysed samples is given in this chapter. In chapter 6 a general introduction of the laboratory techniques used will be given, followed by explaining the methods of measuring monazite for geochronologic calculations and chlorite-white mica pairs for geothermobarometric calculations. In Chapter 7 a general introduction of the monazite mineral and the occurrence in metamorphic rocks is given. In Chapter 8 the results of the geochronologic calculations of samples 6.9N06 and 9.2N06 of the Høyvik Group are presented. In chapter 9 the results of the geothermobarometric calculations of samples 6.8N06, 6.9N06 and 9.2N06 of the Høyvik Group are presented. Chapter 10 is the final discussion, in this part the analytical results are compared with other studies to give more insight in the geological history of the Høyvik Group with respect to other Allochthons and the WGR. In chapter 11 the overall conclusions of the paper will be given.
7 2. PLATE TECTONICS The movements of the continental plates result in subduction, mounting building, crustal thickening, extensional collapse and sea floor spreading. These large scale geodynamic processes can form and break up supercontinents. The supercontinent Rodinia (figure 3) existed from 1100 Ma to 800-700 Ma (Meert and Torsvik, 2003). The exact position of continental parts like Laurentia and Baltica are still under debate. For example Hartz and Torsvik (2002) suggested that Baltica must be rotated ~120° with respect to classic Rodinia reconstructions (figure 3b). This puts Norway on the eastern side of Rodinia instead of lying against the margin of Laurentia in the more classical reconstruction (figure 3a).
Figure 3: a) Classic reconstruction of Rodinia at 750 Ma, b) New reconstruction of Rodinia at 750 Ma (Torsvik, 2003).
The break up of Rodinia started on the western margin of Laurentia with an intercontinental rift to allow Australia, East Antarctica to drift away. Along the eastern margin of Laurentia a rift started to develop around 700-750 Ma, during this period the rift did not proceed to the oceanic stage (Torsvik et al., 1996). Laurentia and Baltica remained attached until the opening of the northern part of the Iapetus Sea when the rift between Laurenta and Baltica proceded at 570 Ma and continued up to 550 Ma (Hartz and Torsvik, 2002). The opening of the Ægir Sea appears synchronous with the early rifting events on the western margin of Laurentia (Hartz and Torsvik, 2002). The rifting stage was overlapped by a subduction along the continental margin. Subduction of Ægir crust along the continent boundaries overlaps in part with the continued seafloor spreading in the Ægir region as recorded in Caledonian and Siberian nappes (Hartz and Torsvik, 2002). The occurrence of several ~600 Ma MORB-type dykes in northern Sweden and Norway may be connected to the Iapetus or Ægir rifting. When these dykes are correlated to the Ægir Sea and not linked to the opening of the Iapetus, it would position Baltica geographically inverted with respect to classic interpretations prior to 580–590 Ma. Baltica moved steadily away from Laurentia (figure 4), the collision with an island arc and the outermost Baltoscandian margin is recognized as an early Caledonian event, the Finnmarkian (~500 Ma) (Andréasson et al., 1998). This migration of Baltica away from Laurentia made the Iapetus Sea wider and it was at its widest at the Cambro- Ordovician boundary (~490 Ma) (Cocks and Torsvik, 2005).
8 Figure 4: Projection of the continents at ~550 Ma. The beginnings of the Ægir and the southern Iapetus Sea are shown and the full break up of Laurentia and Baltica is starting (Cocks and Torsvik, 2005).
During Ordovician times (~480 Ma) Baltica started to rotate anticlockwise and drifted towards Laurentia, causing westward subduction of Baltica under Laurentia. The collision of Baltica against Laurentia is called the Scandian phase of the Caledonian orogenic events. The final closure of the Iapetus Sea began from the SW to the NW of Norway (~425 Ma) (Torsvik et al., 1996). This closure forms the beginning of the formation of another new supercontinent Pangea in late Permian times (figure 5). During the westward subduction several arc accretion related events are recognized, like the Trondheim (20 Ma younger than the Finnmarkian), and the Jämtlandian events (Roberts, 2003; Breuckner and van Roermund, 2007). During Carboniferous times Gondwana collided with Laurussia to form Pangea (~330 Ma), Hercynia orogeny. During the Late Permian (~250 Ma) the supercontinent Pangea was at its peak (figure 5). In some parts however Pangea was already starting to break up, caused by the creation of oceanic crust which formed the Neotethys Sea along the eastern margin of Pangea. Multi-phase rifting during the Late Paleozoic in the Baltoscandian shield is linked to these early stages of super-continental break up. Resulting in the final separation of Greenland and Norway during Tertiary times (~54 Ma) (Torsvik and Cocks, 2005).
Figure 5: Construction of Pangea Supercontinent during the Late Permian (~250 Ma).
For a more detailed palaeographic path see Appendix I
9 3. GEOLOGICAL SETTING
3.1 Regional Geology
3.1.1 Scandinavian Caledonides The formation of the Caledonides of Scandinavia is a combination of collision events that took place from ~500-400 Ma, separated by intervening extensional phases (Roberts, 2003; Hacker and Gans, 2005). The last event was the collision of Baltica against the eastern margin of Laurentia and final closure of the Iapetus Sea (figure 6) (Hacker, 2007). These orogenic events in the Caledonides of Scandinavia are also recognized in the four stages of (ultra)high-pressure (U)HP metamorphism and also the introduction of peridotite from the mantle into the crust (figure 7) (Brueckner and van Roermund, 2004). The first phase, called the Finnmarkian, is generally interpreted as a collision between an island arc and mainland Baltica at around 500 Ma. This event occurred in the Ægir Sea, Baltica was still moving away from Laurentia at this time, where the so called Virisen Arc (Dallmeyer and Gee, 1986) collided with the Scandian margin of Baltica. This event mainly affected the northern parts of the Northern Scandinavian Caledonides.
Figure 6: Collision of Baltica against Laurentia (~425 Ma) and formation of thrust nappes causing crustal thickening (Torsvik et al., 1996)
In the Seve Nappe Complex in Jämtland, central Sweden, evidence is found for another HP event at ~454 Ma, now called the Jämtlandian (figure 7; Brueckner et al., 2004). During this time the Scandian margin of Baltica was located on the eastern side of the Iapetus Sea and Laurentia on the western side. Similar 450 Ma ages are found in the Tromsø Nappe, Troms region, northern Norway, corresponding to subduction underneath Laurentia during Taconian times (Brueckner and van Roermund, 2007). The Taconian event occurred along the eastern Laurentian margin around 450 Ma, so there was still a wide ocean between Baltica and Laurentia.
The last stage of (U)HP metamorphism was during the Scandian event around 435-390 Ma (Torsvik et al., 1996; Terry et al., 2000; Roberts and Stephens, 2000; Carswell et al., 2003). During this phase Baltica underthrusts Laurentia which resulted in a final nappe stacking phase of earlier assembled Allochthons. The nappes were thrusted onto the Baltoscandian basement from east to west (Roberts and Gee, 1985). At that time these nappes were subdivided into two groups: Lower and Upper units (Stephens and Gee, 1985). The Lower and Middle Allochthons and probably the Seve nappes of the Upper Allochthon form here the Lower unit. These rocks are all derived from the Baltoscandian platform and miogeocline. Rocks of the Upper unit, Köli nappes of the Upper Allochthon and Uppermost Allochthon, were derived from terranes outboard of the Baltoscandian miogeocline.
10
Figure 7: Simplified tectonic map of the Scandinavian Caledonides in which the major allochthonous units are indicated. The (dated) HP/UHP terranes (eclogite) are indicated with a green asterisk. The occurence of mantle-derived peridotite/pyroxenite with an ellipse, when the peridotite contains garnet the ellipse has a star (Breuckner and van Roermund, 2004). The Baltic Shield is the Autochthon and the Remobilized Shield represents the Parautochthon. The location of the research area is indicated by the black box.
Post Scandian (U)HP decompression to amphibolite facies was around 400-390 Ma (Andersen and Jamtveit, 1990) and caused the exhumation of root zone of the Caledonian Orogen, Western Gneiss Region (WGR), to crustal levels. This was followed by post-orogenic extensional collapse which resulted in large scale movements of vertical shortening and horizontal stretching of the lower crust, accommodated by extensional detachment zones with top-W shearing (NSDZ) (Andersen and Jamtveit, 1990; Andersen et al., 1991, 1994). The WGR is a tectonic window of Baltican basement and is the lowest tectonostratigraphic unit of the Scandinavian Caledonides (figures 6 and 7).
11 3.1.2 Autochthon/Parautochthon The Autochthon (figure 7) includes Precambrian crystalline basement with a thin sedimentary cover (Roberts and Gee, 1985). The Autochthon represents the Baltic Shield to the east of the thrust front of the Allochthons. The Precambrian basement rocks west of the Allochthons occur predominantly in tectonic windows and are referred to as Parautochthon and give Caledonian recrystallization ages and represent the Baltic Shield overprinted by Caledonian events (Breuckner and van Roermund, 2004). The WGR is a U(HP) variant of these crystalline basement windows.
3.1.3 Lower Allochthon The Lower Allochthon (figure 7) contains mainly Late Precambrian and Early Paleozoic sediments deposited on top of the Baltican Autochthon and folded and thrusted to the east during nappe stacking. Towards the core of the orogen crystalline basement rocks get more profound (Roberts and Gee, 1985) and are influenced by the Caledonian orogeny (Hacker and Gans, 2005).
3.1.4 Middle Allochthon The Middle Allochthon (figure 7) is dominated by highly deformed Precambrian crystalline rocks and Late Precambrian unfossiliferous psammites, with a middle-upper greenschist facies metamorphic grade (Roberts and Gee, 1985). The main period of basin formation at the margin of Baltoscandia was at ~800-700 Ma (Kumpulainen and Nystuen, 1985). Like the Lower Allochthon, the Middle Allochthon consists of crystalline and sedimentary rocks that have been derived from Baltica, but formed farther outboard than the autochthon (Hacker and Gans, 2005).
3.1.5 Upper Allochthon The Upper Allochthon (figure 7) is very heterogeneous and consists predominantly of oceanic and continental nappes of the Baltica margin (Stephens and Gee, 1985) and microcontinents (Andersen and Andresen, 1994). The metamorphic grade of the Upper Allochthon ranges from greenschist facies to eclogite facies rocks (Roberts and Gee, 1985). Lower parts of the Upper Allochthon, like the Seve nappe, Sweden, consist of gneisses, psammites and schists, amphibolites and eclogitized mafic dykes and volcanics (van Roermund, 1985). Structurally above the Seve Nappes lies the Köli Nappes, which are mainly greenschist facies volcano- sedimentary rocks, ophiolite fragments are also present (Roberts and Gee, 1985). The Upper Allochthon forms the continent-ocean transition along the rifted Baltic margin (Andersen and Austrheim, 2003).
3.1.6 Uppermost Allochthon The Uppermost Allochthon (figure 7) is believed to be a composite of eastern margin Laurentia and outboard terranes (Roberts et al., 2002; Roberts, 2003). It is composed of units, including thick carbonate complexes and crystalline basement, as well as outboard terranes accreted to this margin on the western side of the Iapetus Sea (Breuckner and Van Roermund, 2007). The heterogeneous nappes of the Uppermost Allochthon are only found in Nordland and Troms in northern Norway, like the eclogite bearing Tromsø Nappe (Andersen and Austrheim, 2003).
3.1.7 Devonian Basins The Devonian basins of western Norway (figure 7) were formed during late- to post- orogenic large scale extensional tectonics associated with the collapse of the orogen. One of such extensional detachment zones is called the Nordfjord-Sogn Detachment Zone (NSDZ).
12 The basins are situated in the hanging wall of the mylonites of the NSDZ (Andersen and Jamtveit, 1990; Andersen, 1998; Osmundsen and Andersen, 2001) and are influenced by late brittle movements of low-dipping normal faults.
3.2 Geology of the Research Area
3.2.1 WGR In western Norway (figures 7; 8) is an area that consists crystalline basement rocks of Baltica of (Mid)Proterozoic age which became subducted to (U)HP eclogite conditions. This area is called the Western Gneiss Region (WGR) and is exhumed by post- Scandian extension. The rocks of the WGR are now exposed in a basement window in the Scandinavian Caledonian and includes often migmatite that became overprinted by the Scandian (U)HP event during the continental subduction of Baltica underneath Laurentia.
Figure 8: Geological NS- profile of Western Norway (Johnston et al., 2007). The presence of UHP rocks under HP rocks is interpretation.
In general the rocks of the WGR are reworked (Mid)Proterozoic orthogneiss, quartz- feldspatic gneiss of granitic to granodioritic compositions, K-feldspar augengneiss, minor amounts of anorthosite, mafic rock, ultramafic rock in addition to metasediment (Tucker et al., 1990), including different sized lenses of metagabbro and eclogite.
The rocks of the WGR show a Scandian metamorphic zoning pattern, from amphibolite facies in the eastern part to eclogite facies in the western part. The metamorphic pressure differs from 12-16 kbar in the south to 40-60 kbar in the north. This WNW- increasing pressure/temperature gradient reflects the westward subduction of Baltica (WGR) beneath Laurentia (Griffin et al., 1985). The WGR shows a WNW-increasing pressure/temperature gradient as is shown in figures 9 and 10. Next to the diagram of
13 the regional temperature gradient for the peak metamorphism of the WGR originally by Krogh (1977) a new diagram by Kylander-Clark et al. (2008) is shown.
Figure 9: Regional temperature gradient across the Western Gneiss Region of Norway (Griffin et al., 1985; Carswell and Cuthbert, 2003). The black box in the SW indicates the research area.
Early studies concerned with peak Scandinavian metamorphic conditions demonstrated a temperature/pressure gradient from ~500°C, 10-12 kbar in the Sunnfjord area (SE) to 800-900°C, 18-20 kbar in the Nordfjord area (NW) of the WGR (Griffin et al., 1985). Later peak metamorphic studies recorded that the WGR contained UHP areas and were calculated to have temperatures and pressures of 700-850°C, >30 kbar (Coleman and Wang, 1995; Wain et al., 2001). These estimates correspond to the presence of coesite and micro-diamond and indicate that the rocks reached during collision a maximum depth of 125 km and maybe even up to 150-200 km (Dewey, 1993; Scambelluri et al., 2008). Nowadays peak pressures of ~55 kbar in the Nordøyane UHP domain (figure 10), NW of the WGR, are found (Vrijmoed et al., 2006; Spengler et al., 2006). UHP metamorphic rocks occur dominantly in the NW parts of the WGR (figure 10).
The new diagram of Kylander-Clark et al. (2008) (figure 10) also shows an upgrade in the temperature gradient, which curves around the Nordøyane UHP domain. The Nordøyane area contains UHP eclogite with the highest measured paleo-pressure and also shows the highest paleo-temperature.
14 Figure 10: Regional temperature gradient across the Western Gneiss Region of Norway, including cor- responding eclogite ages (Kylander-Clark et al, 2008). The black box in the SW represents the research area.
An example of an HP eclogite in the southern part of the WGR, near the research area, is formed by the Drøsdal eclogite in the Sunnfjord area (figure 11), with metamorphic temperatures of 700-800°C and pressures of 19-21 kbar (Foreman et al., 2005). These estimates have temperatures up to 200°C higher than other estimates for this region as can be seen by figures 9 and 10) and it is uncertain if they represent Caledonian HP event temperatures. The Drøsdal eclogite is little affected by later stages of deformation and retrograde reactions during exhumation (Andersen and Austrheim, 2003). It might be a possibility that these paleo-temperatures of the Drøsdal eclogite are calculated with wrong Fe2+/Fe3+ estimations, which have a high influence on the temperature calculation. In general the PT conditions of the eclogites in the Sunnfjord area are around 580°C and 21 kbar (Cuthbert et al., 2000). Recalculations by Labrousse et al. (2004) using THERMOCALC gave peak metamorphism of the Vårdalsneset eclogite (figure 11) at 615±15ºC and 22.7±0.1kbar.
15
Figure 11: Simplified geological map southern part of the research area. D stands for the Drøsdall mafic body and V for Vårdalsneset eclogite body (Foreman et al., 2005).
There is a similarity between muscovite cooling ages in the east and eclogite ages in the west of the WGR (figures 13) (Kylander-Clark et al., 2008). This suggests that the WGR remained a coherent structure throughout its exhumation, beginning at lower to mid- crustal depths and continuing through mid-crustal levels. Exhumation started in the eastern and southern parts of the WGR, while the northwestern part was still at U(HP) depths. Exhumation of the UHP domains to mid-crustal levels was complete by ~390 Ma and further exhumation through crustal levels was controlled by tectonic unroofing by the NSDZ (Kylander-Clark et al., 2008). The final (brittle) exhumation gave a weak greenschist facies overprint that is stronger near the detachment fault, the structural transition towards the overlying nappes. As a response to this extensional phase during the Devonian low-angle detachment controlled basins formed (Chauvet and Dallmeyer, 1992).
3.2.2 Lower Allochthon This unit includes Precambrian crystalline rocks and upper Proterozioc and/or lower Palaeozoic platform and miogeocline sediments with Baltican affinity that were deposited and thrusted on top of the basement (Hossack et al., 1985). In the upper parts of the Lower Allochthon parts of basement rocks derived from Baltica can be present (Andersen & Austrheim, 2003). For example the Askvoll Group (see figure 7) on Atløy is the transition zone between the HP rocks of the WGR and the Middle Allochthon, but the geological significance of the Askvoll Group is however unclear. The following interpretation has been suggested by Swensson and Andersen (1991): - The Askvoll Group is the original cover to the underlying Precambrian basement, - it is a parautochthonous to highly allochthonous composite unit emplaced during the compressional phase of the Caledonian orogeny, or - it is a highly deformed, phyllonitic part of the basement.
16
Figure 12: Regional ages across the Western Gneiss Region of Norway, including corresponding eclogite ages (after Kylander-Clark et al., 2008). Geochronological data set after Hacker et al. (2007). Contours of muscovite 40Ar/39Ar ages in Ma are shown by heavy lines. The black box in the SW indicates the research area. The 40Ar/39Ar age of 395 Ma in the box indicates the age from the WGR and the age of 449 Ma is from the Høyvik Group.
3.2.3 Middle Allochthon The nappes of the Middle Allochthon that lie in the study area (mainly on Atløy) are the Stavfjord nappe, containing the Herland and Høyvik Group, and the Dalsfjord Suite (Corfu and Andersen, 2002). The Middle Allochthon consists mainly of crystalline and meta-sedimentary rocks that have been interpreted to be part of the Late Precambrian passive margin sequence of Baltica. The Høyvik Group includes the Laukeland formation; mica & quartz schists, marble and pillow lava and the Atløy formation; meta- sandstones locally cut by pre-tectonic metadolerites (Andersen et al., 1990). The pelitic Kvitanes formation also contains marble and the Granesund formation consists of massive bedded psammites (Andersen & Austrheim, 2003). The Høyvik Group is cut by rift related mafic dykes. The Dalsfjord magmatic suite is an allochthonous crystalline unit and lies structurally above the WGR (Corfu and Andersen, 2002). The massif thrust nappe is unconformably overlain by the Høyvik Group meta-sedimentary cover. The latter is unconformably overlain by the low-grade Silurian deposits of the Herland Group, which is on his turn discordantly succeeded by Solund-Stavfjord Ophiolite (Upper Allochthon) and the Sunnfjord Melange, which is formed during the obduction of the ophiolite (Andersen et al., 1990; Corfu and Andersen, 2002).
17 3.2.3.1 Høyvik Group The Høyvik Group (figure 13) is influenced by poly-phase pre-Scandian deformation, thought to be middle-upper greenschist facies, Tmax <450° C, constrained by quartz-rich, biotite-muscovite schists and psammites (Andersen, 1998). The main mineralogy of the metasediments of the Høyvik Group is characterized by quartz with undulatory extinction and sutured grain boundaries, alkali feldspar porphyroclasts and tabular phengites and chlorite that are gently kinked or undeformed with accessory titanite and pyrite (Eide et al., 1999). Andersen et al. (1998) suggest one episode of white mica (phengite) growth and deformation at temperatures from 400–450oC. In this paper also a cooling age is calculated for the Høyvik Group by 40Ar/39Ar white mica (phengite) dating giving a minimum deformation age of 445-450 Ma. The closing temperature (Tc) of white mica lies around the 350°C, the age of 445-450 Ma is a minimum deformation age timed when the rocks reached a cooling temperature of ~350°C (Andersen and Austrheim, 2003). Since the deformation age of 450 Ma age was not reset during the Scandinavian event it was thus interpreted to represent a deformation phase before the thrusting of the nappes. After that the nappes were only weakly influenced by the obduction of the Solund-Stavfjord Ophiolite and the Scandian continental collision between Baltica and Laurentia during the Silurian-Devonian time. The rocks of the Høyvik Group were exhumed before the deposition of the non-conformably overlying deposits of the Herland Group which were deposited during the Wenlock time 423-428 Ma (Andersen et al., 1998). A general overview of the metamorphic ages measured by different studies in the surroundings of the Askvoll area and the tectonostratigraphy is shown in figure 12. The age of ~443 Ma in the Solund-Stavfjord Ophiolite and Cover is dated by U-Pb on zircon. This is sedimentary zircon from the cover sediments and does not represent a metamorphic age of these deposits. The obduction of the ophiolite is reflected in the stratigraphy of the Silurian rocks of the Herland Group, the emplacement onto the continental margin can be relatively accurately timed to the Wenlock time 423-428 Ma (Andersen et al., 1998).
3.2.3.2 Dalsfjord Suite The Dalsfjord Suite (figure 13) has a U-Th zircon age of 1,634±3 Ma, and a cross-cutting gabbro was formed at 1,464±6 Ma. Both rocks were strongly overprinted during the Sveconorwegian orogeny (Corfu and Andersen, 2002). In the lowermost part of the nappe younger ages of ~383 Ma are calculated on K- feldspar. This difference in cooling age of the Dalsfjord Suite and the Høyvik Group is attributed to stronger Caledonian deformation and heating during the Scandian event since the Dalsfjord rocks directly overlie the NSDZ (Eide et al., 1999).
3.2.4 Upper Allochthon North of the research area lies the obducted Solund-Stavfjord Ophiolite Complex (figure 13) which contains a U-Pb date of ~443 Ma (Dunning and Pedersen, 1988). The Ophiolite Complex is tectonically separated from the rocks of the Middle Allochthon by the Sunnfjord Melange. The Melange is a zone of disrupted rocks with ophiolite lenses and was formed when the ophiolite was emplaced on the continental deposits of the Herland Group (Andersen et al., 1990).
18
Figure 13: Schematic cross-section showing simplified age relationships and tectonostratigraphy of the Sunnfjord Region (Andersen et al., 1998).
3.2.5 Kvamhesten Basin The Devonian Kvamshesten Basin (figure 13) in western Norway is one of the late- to post-orogenic basins situated in the hanging wall of the extensional NSDZ, located near the research area. The Kvamhesten basin is folded into a syncline by N-S shortening, during the E-W extension, with the axis subparallel to the ductile lineations in the detachment zone (Osmundsen et al, 1998). The Dalsfjord Fault outlines the eastern, northern and southern margins of Kvamhesten basin. The low-angle fault cross-cuts the basin and has juxtaposed the basin against the extensional mylonites of the NSDZ, much of this stage occurred after the sedimentation of the basin, since no clasts of the WGR rocks or from the mylonites of the NSDZ are identified in the basin (Osmundsen et al., 1998).
3.2.6 Geological Structures
3.2.6.1 Nordfjord-Sogn Detachment zone The Nordfjord-Sogn Detachment Zone (NSDZ) is the structural transition zone in the western part of Norway between the WGR and the overlying Allochthons. In the western part of the WGR underneath the Devonian basins the detachment zone consists of mylonite (high strain) zones of 2-3 kilometres thick. The major shear zones at the structural top that separate the allochthonous rocks, including the Devonian sediments, from the rocks of the WGR are characterised by both mylonitic and cataclastic rocks, whereas mylonites dominate in the footwall and the cataclasites in the hanging wall (Swensson and Andersen, 1991). There is a transition of symmetric extensional fabrics from the high strain and high temperature zones also high T to the amphibolite- greenschist facies asymmetric shearing closer to the contact with the overlying nappes (Andersen and Jamtveit, 1990; Dewey et al., 1993; Andersen et al., 1994; Johnston et al., 2007). This major tectonic boundary between the WGR and the overlying nappes is also found on Atløy. The extension was controlled by considerable top-W displacement of the NSDZ, which opened the Kvamshesten Basin (Osmundsen et al., 1998 & 2000). Kinematic indicators in the mylonites of the NSDZ suggest that during the ductile
19 deformation of the extension the Caledonian thrusting fabrics of top-E were overprinted (Chauvet and Dalmeyer, 1992).
Figure 14: Simplified structural N-S section of the tectono-stratigraphy of the research area with several age data for the area and a local PT estimation for the WGR (after Johnston et al., 2007).
In the research area the NSDZ follows the Askvoll Group into the footwall, resulting in a sequence of heterogeneous and highly deformed rocks structurally overlying the gneisses of the WGR (Swensson and Andersen, 1991). The name Askvoll Group is introduced by Skjerlie (1969) and described as a meta-sedimentary and meta-volcanic sequence forming part of the Baltic continental basement or deposited during the Late Precambrium. In this paper it is merely used to give constraints on an area, since the detailed tectonostratigraphy of the Askvoll Group is not yet known.
3.2.6.2 Dalsfjord Fault The mylonites of the NSDZ in this area are truncated by the Dalsfjord Fault (DF) (figures 14 and 15), this is a brecciated fault gauge zone which separates the mylonites of the footwall from the cataclastic rocks of the hanging wall (Swensson and Andersen, 1991). The Dalsfjord Fault is the result of the brittle stage of extension during the exhumation of the WGR. The Dalsfjord Fault is a long lived fault zone, the most important faulting was during the Devonian extension which also leads to the formation of the Devonian basins like the Kvamhesten Basin (figures 14 and 15). The Dalsfjord Fault underwent reactivation during the Permian (250-270 Ma) together with Mid-Late Permian dolerite dykes (Torsvik et al., 1997), fault rocks of Upper Jurassic/Lower Cretaceous (~150 Ma) (figure 15) and fault gauges younger than 96 (Torsvik et al., 1992; Eide et al., 1997). Of these reactivation events, the late Permian event was dominant (Torsvik et al., 1997). These ages are linked to major extension phases which finally resulted in the opening of the Atlantic Ocean (Andersen and Austrheim, 2003).
20
Figure 15: Structural location of the Molvær dykes, Dalsfjord dyke and the Dalsfjord Fault rocks on Atløy (Torsvik et al., 1997). Arrow 1 stands for the movement direction during the thrusting event of the Scandian phase and arrow 2 stands for the movement direction during Post-Scandian extension.
For a schematic overview of the tectonostratigraphy of the research area see Appendix II
21
3. REGIONAL CORRELATIONS
The Høyvik Group is in earlier studies connected to other terrains for different kinds of reasons (figure 16). In a tectonic sense and maybe sedimentary origin it may be linked to the Seve Nappe, Jämtland, Sweden. But in a sense of sedimentary origin the Høyvik Group could also be linked to the Hyllestad Complex (Tillung, 1999) and/or Valdres Sparagmite (Corfu and Andersen, 2002). The latter correlation is partly made by the existence of the underlying Jotun Nappe which is in turn correlated to the Dalsfjord Suite.
Figure 16: Areas in Norway and Sweden that are related to the research area.
22
The Høyvik Group was intruded by pre-Caledonian mafic dykes (Andersen et al., 1998). These dykes of the Høyvik Group may be correlated (figure 16) to ~608 Ma MORB-type dykes in the Seve Nappe Complex, Sarek Dyke Swarm, northern Sweden (Svenningsen, 2001). Same can be said about the Sm/Nd-dated (582 ± 30 Ma) dolerite dykes of MORB type character in the Corrovarre Nappe of the Middle Allochthon, Troms, northern Norway (Zwaan and van Roermund, 1990). Further the Egersund dykes, southern Norway, have an intrusion age of ~616 Ma (Bingen et al., 1998) and 612 ± 33 Ma from Seiland gabbros, Finnmark, northern Norway (Daly et al., 1991). The dykes are probably originated during rifting episodes in the Ægir Sea (Rehnström et al., 2002).
4.1 Seve Nappe There are several areas in the Scandian Caledonides that have evidence for dyke intrusion at ~600 Ma, but only one area also shows a tectonic event at ~450 Ma. The area of northern Jämtland, Seve Nappe, central Sweden, includes a HP metamorphic region represented by eclogites and peridotites with an age of ~450 Ma (Brueckner and van Roermund, 2004 and 2007). The Seve Nappe has another HP region; Norrbotten, northern Sweden, which contains also eclogites and peridotites but the age of this HP metamorphism is ~500 Ma (Mørk et al., 1988; Svenningsen 2001). These ages clearly indicate two different HP events, corresponding to the Jämtlandian (~450 Ma) and the Finnmarkian (~500 Ma). The Høyvik Group and Seve Nappe in Sweden are interpreted as sedimentary deposits on the western edge of Baltica, which was thinned into a passive margin during rifting and covered with shelf sediments (Breuckner and van Roermund, 2007). The Seve Nappe Complex in North Jämtland (Upper Allochthon), can thus be correlated to the Høyvik Group (Middle Allochthon) by the occurrence of similar tectonic events, and stratigraphical similarities. The Seve Nappe in Sweden may be a HP variant of the Høyvik Group (Dallmeyer, 1988). More evidence for this deformation event may be found in metamorphic clasts in the Devonian Basins. The Seve Nappe in the Norrbotten Region does not share the same tectono- metamorphic history, a reason for this may be due to a geographical factor. The northern part of Norway has undergone a local subduction phase during the Finnmarkian. Central Sweden and parts of SW Norway shared a subduction phase during the Jämtlandian.
4.2 Hyllestad Complex The Hyllestad Complex, which lies structurally direct above the WGR, has been correlated with the Høyvik Group (Tillung, 1999). This correlation is made by similarity in composition, which makes it possible that the areas have a similar origin in sedimentation. It is suggested by Tillung (1999) that this unit may have been deposited directly on the rocks of the WGR.
4.3 Jotun Nappe & Valdres Sparagmite The Dalsfjord Suite has correlations with rocks of the Jotun Nappe Complex, both in tectonic events (late Sveconorwegian metamorphism) and lithographical similarities. Both structures have a cover of comparable Late Precambrian psammitic rocks deposited on the continental margin of Baltica, Valdres Sparagmite on the Jotun Complex, and the Høyvik Group on the Dalsfjord Complex (Corfu and Andersen, 2002). This suggests a long lived history between the nappes until the deposition of the Høyvik Group and the Valdres Sparagmite. However the metasedimentary covers, Høyvik Group and Valdres Sparagmite, show no evidence of having undergone the same tectonic events (Corfu and Andersen, 2002).
23 It is also suggested that the Jotun Nappe may be a terrane outlined in the Iapetus Sea and this would link the Jotun Nappe to the Upper Allochthon (Andersen and Andresen, 1994). 4.4 Lindås Nappe Zircon ages of ~456 Ma and ~419 Ma for HP eclogites within the Lindås Nappe of the Bergen Arcs were found (Bingen et al., 2001), these ages are re-interpreted as an eclogite-forming age of ~425 Ma, during the early Scandian event by Bingen et al. (2004). The Lindås Nappe has a lithographical correlation and late Sveconorwegian metamorphism with the crystalline rocks of the Jotun nappe (Bingen et al., 2001; Corfu and Andersen, 2002), this may link the Lindås Nappe to the Middle Allochthon. Since the Lindås nappe shows lithographical and tectonic similarities with the Jotun Nappe, it may therefore also be linked to the Dalsfjord Suite.
4.5 Correlations of the Middle Allochthon and the WGR A reasonable fluid isograd from SE WGR to the NW suggests a transition between UHP and HP rocks without any significant metamorphic gaps and structural features within the complex itself. This implies that the rocks of the WGR stayed together as one region after the last (U)HP event, attached to lower pressure crust during exhumation (Young et al., 2007; Kylander-Clark et al., 2008). This is in contradiction with earlier local (U)HP events in the nappes, like the Finnmarkian and Jämtlandian events, which were more small scale events. Indicating that the latest (U)HP phase during the collision of Laurentia with Baltica had the biggest extent. General calculated eclogite conditions in Sunnfjord area lie around 500°C and 16 kbar (Cuthbert et al., 2000) and peak conditions calculated on the Vårdalsneset eclogite are 615±15ºC and 22.7±0.1kbar (Labrousse et al., 2004). These estimates have a difference of at least 30 km in burial and subduction conditions with the upper greenschist conditions of the Høyvik Group. Johnston et al. (2007) suggested that the WGR ascended buoyantly through the mantle until the lower crust by vertical pure shear thinning. This would account for most of the exhumation of the WGR up to the lower crust (Dewey et al., 1993; Young et al., 2007), but in UHP areas no evidence of post-eclogite flattening in the eclogite field is found, all deformation always started in granulite facies conditions or later. Top-W shear within the NSDZ did not initiated until lower crustal depths of 30-40 km and occurred between 410 Ma and 400 Ma during or immediately after UHP metamorphism (Johnston et al., 2007b) and ended at ~380 Ma marked by the Dalsfjord Suite near the NSDZ. Late brittle-ductile detachment faults, like the Dalsfjord fault, truncated the mylonites of the NSDZ and were responsible of the final stage of juxtaposing (U)HP rocks against lower crustal rocks and Devonian basins (Johnston et al., 2007a).
24 5. FIELD OBSERVATIONS
5.1 Sample collection One of the goals of this project was to create a geochronological EMP monazite age profile, across a HP area of the WGR, the transition to the overlying nappes (NSDZ) and the LP metasediments of the Høyvik Group (Middle Allochthon). Monazite occurs in felsic rocks which reached upper greenschist facies conditions and higher. Felsic gneisses, mylonites and schists, which reached at least the amphibolite facies, were taken as samples from the WGR and NSDZ. Schists, psammites and mylonites samples collected from the Høyvik Group are upper greenschist/low amphibolite facies rocks and towards the south the metamorphic grade apparently decreases. The Høyvik Group was chosen to collect samples for nappe dating since the rocks from the Dalsfjord Complex, which lies tectono-stratigraphically direct on top of the WGR, have high amphibole content and therefore no monazite. The Høyvik Group is exposed on Atløy and on a peninsula north of Askvoll (figures 17 and 20) and has a structural thickness of >1500 m on Atløy (Andersen & Austrheim, 2003). The rocks of the Høyvik Group are mainly micaschists and quartzites. The micaschists may contain garnet and are often rich in quartz veins (figure 18). The Høyvik Group has been deformed one time penetratively, resulting in a well developed cleavage/schistosity/S1 that dips generally towards to the north. Evidence for F2 a second phase of deformation is found in isolated folds deforming S1, but this was not a penetrative deformation and is not visible in all rocks. Local extensional shearzones form a third deformation phase, towards the basal contact with NSDZ this third deformation phase becomes more pronounced.
The locations of the samples that have been collected are indicated in table 2 and on figure 17. For an overview of all the thin sections of the samples listed in table 2 that were examined for this research see Appendix III.
Sample Tectonostratigraphic Unit 6.8N06, 6.9N06, 9.2N06 Middle Allochthon (Høyvik Group) 6.12N06, 7.9N06, 7.10N06, 13.5N06, 15.5N06 NSDZ 6.1N06, 7.5N06, 7.6N06, 13.2N06, 15.8N06 WGR Table 2: Sample numbers and their associated tectonostratigraphic units.
5.2 Structural observations
5.2.1 WGR The Vårdalsenet eclogite (east of Askvoll, location of samples 7.9N06 and 7.10N06) is an outcrop of in WGR at the base of the NSDZ. The eclogite contains in addition to omphacite and garnet, quartz, kyanite and phengite extension veins. The felsic gneisses present contain epidote which is evidence for retrograde reactions to epidote- amphibolite facies. The folding and lineations are not defined by eclogite facies minerals but instead by amphibolite facies minerals. The rocks reworked during exhumation and the Post-Caledonian extensional phase. The extensional shear indicators like garnet porphyroblasts and k-feldspar bands show predominantly symmetric shearing.
25
Figure 17: Locations of the samples and the tectonostratigraphic features of the research area. NSDZ: Nordfjord-Sogn Detachment Zone, DF: Dalsfjord Fault (location of figure 19).
5.2.2 NSDZ On Atløy, location of sample 6.12N06, the mylonites of the NSDZ are exposed structurally below the Dalsfjord Fault. These greenschist mylonites show good shear sense indicators with top-W shearing and near the detachment fault the mylonites are brecciated (Andersen and Austrheim, 2003). At Dorhella (SE of Askvoll, location of samples 7.5N06 and 7.6N06) the detachment mylonites of the NSDZ can be seen. Highly banded gneisses of the WGR with migmatite veins and gabbroic bands are present here. Shear sense indicators are well exposed in this outcrop. Asymmetric (horizontal Z-folds) of the migmatite veins and rotation of porphyroclasts show top-W shearing. Most folds are turning towards the E-W trending lineation and some folds still have an angle with the foliation (Appendix IV).
5.2.3 Høyvik Group The sampled rocks of the Høyvik Group on Atløy and the peninsula north of Askvoll (Figure 17 and 20) were micaschists, with locally garnet present, or quartzite. Within the schists many quartz veins were present, as can be seen at the outcrop picture of sample location 6.8N08 & 6.9N06 (figure 18). The Høyvik Group north of Askvoll also consists of pelitic schists with alternating bands of quartzite. At sample location of 9.2N06 pelitic schists and quartzite (old sandstone banks) alternate with each other, both are of greenschist grade judged from the high amount of chlorite in the rocks. In the outcrop there are folded quartz veins (~20 cm thick) present. Here no top the west shearing may be seen in the rocks so the lineation is not influenced by the high strain.
In most parts both bedding (S0) and foliation (S1) (sub-parallel to S0), are folded by large scale F2 folds and a S2 is formed by tight folding, also seen on thin section scale. The S2 foliation has a NNW-SSE strike and a NW dip in most of the area, with isolated-folding (F2).
26 Osmundsen and Andersen (1994) suggest that both small and large scale F3 folds in the Høyvik Group near the Sunnfjord-Melange are formed by normal/oblique-slip re- activation of the S2 foliation with top-W shear.
Figure 18: Outcrop on Atløy of a micaschist of the Høyvik Group with a quartz vein, samples 6.8N06 & 6.9N06
Figure 19: Dalsfjord Fault on the peninsula north of Askvoll. The hammer points to the fault-gauge zone, which is localized to <0.5 m, same as on Atløy.
5.2.4 Dalsfjord Fault The Dalsfjord Fault is a highly localized fault and may be found on Atløy or NE of Askvoll (Figure 19). This picture was taken along a road section NE of Askvoll (figure 17). The Dalsfjord Fault marks the top of the greenschist facies mylonites of the NSDZ (footwall) and the base of the Dalsfjord Complex (hanging wall).
5.3 Analysed samples EMP analyses for geochronologic calculations were performed on monazites of samples 6.9N06 and 9.2N06 (figure 20) of the Høyvik Group. No monazites were found in other samples of the Høyvik Group, the gneisses or metasediments of the WGR or mylonites of the NSDZ in the Sunnfjord area. EMP for geothermobarometric calculations were performed on chlorite and adjacent white mica’s of samples 6.8N06, 6.9N06 and 9.2N06 (figure 20) of the Høyvik Group
27
Figure 20: Field locations of samples of the Høyvik Group; 6.8N06, 6.9N06 and9.2N06. These samples were used for geothermobarometric calculations after microprobe measurements. Samples 6.9N06 and 9.2N06 contained monazites which were measured with the microprobe for the monazite age dating.
5.4 Mineral Description of the analysed samples
ampl e 5.4.1 Sample 6.8N06 micaschist Main mineralogy of the sample consists of white mica, chlorite, albite, quartz (irregular grains formed in bands) and apatite. Accessory minerals are REE-rich apatite, Fe- Sulphides and zircon. The albites in the sample are porphyroclasts and some blasts show twinning. In thin section a main foliation (S1) was visible and locally the beginning of a second foliation (S2) in the form of an (extensional) crenulation cleavage is visible.
5.4.2 Sample 6.9N06 micaschist The mineralogy of samples 6.9aN06 and 6.9bN06 consists of quartz minerals that are irregular in size and shape. The smaller minerals have formed subgrains. Small mica and chlorite bands are present in the thin section and are folded and little biotite is present. A few albite porphyroclasts are present and some show twinning. Accessory minerals are monazite, REE-apatite, zircon, titanite, and Fe-Sulphides.
5.4.3 Sample 9.2N06 psammite The dominant mineral content of this sample is quartz with small amounts of white mica and chlorite. This sample is a quartzite. The thin section includes metal-oxides, indicating that fluid transport has occurred through the rocks. This sample contains monazite and Fe-Sulphides as accessory minerals.
28
Figure 21: BSE image illustrating main mineralogy of sample 6.8N06. Dark grey: quartz, light grey: chlorite/white mica, white: apatite. Scale bar is 1 mm.
Figure 22: BSE image illustrating a close up of the main mineralogy of sample 6.9aN06. CHL=chlorite.
29
Figure 23: BSE image illustrating main mineralogy of sample 9.2N06. Dark grey: quartz, light grey: chlorite/white mica, white: Fe-Sulphides. Scale bar is 1 mm.
30 6. METHODS & TECHNIQUES
6.1 Introduction to the EMP The microprobe used is a JEOL JXL-8600 Superprobe at the Institute of Earth Sciences at the Utrecht University. It contains five Wavelength Dispersive X-ray spectrometers (WDS), and an Energy Dispersive X-ray Spectrometer (EDS). Figure 24 is showing a schematic drawing of an electron microscope.
Figure 24: Schematic drawing of the principles of an electron microprobe (from http://www4.nau.edu/microanalysis/Microprobe /Probe.html)
Electrons are generated from a tungsten filament (figure 24). These electrons are accelerated through a bias at a specific voltage (15-20 kV). The sample is then bombarded with the high energy beam of electrons, which are focused through a series of electromagnetic lenses into a narrow beam. The beam current used may be 20-50 nA. The spot size of the beam on the sample strongly depends on the height (Z-axis) of the sample in the EMP. The height must be kept constant, the spotsize is then set using the lenses. When the electron beam hits the sample it results in exited electrons and an emitted X-ray spectrum which is used for chemical analysis (figure 24). The generated X-rays have multiple wavelengths that become separated by the analysing crystals (TAP, PET, LIF). In this way all elements from atomic number 9 to 92 can be detected and analysed. Qualitative analysis of the individual X-ray lines in the spectrum and determination of the concentration of the elements present occurs by comparison with standards of know composition followed by correcting procedures for atomic number (Z, the effects of absorption (A) and fluorescence (F) in the sample, ZAF correction. Generated back scattered electrons (BSE) are also used to created a back scattered image, displayed as a grey scaled image. Figure 25: intensity of the BSE image (n) is dependent on the atomic number of the mineral (Z).
In a BSE image the contrast or difference in intensity (η) is strongly dependent on the mean atomic number of the mineral (Z) (figure 25). Heavy minerals appear white, while light minerals appear black on a screen with a grey-scale. The grey scale can be adjusted for the intensity needed.
31 6.2 Data corrections To use the quantitative EMP analyses for age dating calculations and geothermobarometry, a number of corrections have to be made. The initial intensity data which represent the counts made by the EMP must be corrected for peak overlap, background and factors caused by the instrument itself. All data is then referenced to standards of the EMP and presented as a ratio. This corrected ratio may be affected by X-ray absorption, secondary fluorescence, electron backscattering and the generation of heat in the sample. Due to these factors the ratio of a characteristic X-ray line is not directly linked to the concentration of the element. These factors are corrected by the following methods to make the analytical errors as small as possible.
6.2.1 Peak interferences The X-rays of the different elements that are reflected, after the bombardment with the electron beam, are generated at the same time. These X-rays are characteristic for the elements present in the sample and are used to measure the concentrations of the elements. Only heavy elements may have characteristic X-rays that cause peak interferences. Interference of K-lines, L-lines and M-lines during a measurement is an important factor in accurate quantitative analysis of REE-phosphates. For example Yttrium (Y) may be one of the REE’s in monazite. The amount of Y can vary off course but a Y amount of 1 wt% or higher is not thought to be a true value. This is then the consequence of overlap in measured peaks of Pb and Y. The X-ray peak of Y interferes with Pb. The use of β lines in monazite avoids the need for several element interference corrections. The most serious problem arises from the Y Lγ line which is superimposed upon the Pb Mα line.
For example for Th, U and Pb in monazite analyses the M-lines are usually used. The Mα of Th is free for typical monazite compositions. Mα and Mβ have both interferences with Th but the correction for the Mβ is smaller. For Pb the choice of best line is a bit disputable but it is suggested that that the Mα line is superior if analysis is performed with Xe detector gas (Pyle et al., 2002b). This is also the line used for chemical age dating on the electron microprobe in Utrecht.
During EMP analyses on monazite peak interference of X-ray lines for Y and Pb may cause an overestimation of the measured Pb content. Extrapolation down to the Y content of monazite (≤1 wt%) shows that this creates a maximum overestimate of the Pb content of about 30 ppm (Montel, 1996). The measurements containing a Y amount higher than 1 wt% must be disregarded.
The used lines for analyses are shown in table 3 and 4. However some level of peak interference may still occur, therefore a ZAF matrix correction is used to eliminate these errors.
6.2.2 ZAF During the bombardment of X-rays on minerals, especially small minerals, matrix elements can affect the measurement. Chemical composition of any mineral measured with the EMP is determined by comparing the intensities of characteristic X-rays from the sample with standard intensities. The correction used with the Superprobe is the ZAF correction.
Z differences in the average atomic number A absorption of X-rays by matrix elements of the sample F generation of secondary fluorescence of X-rays
32
6.2.3 Standard deviation The standard deviation is an empirically determined relationship between counts and concentration. The standard deviation (σ) of the mean of the EMP measurements is:
+ TRTR ))/()/(( σ = pp bb , where − RR bp )(
Rp = count rate of the peak measurement (count/s) Rb = count rate of the background measurement (count/s) Tp = duration of the peak measurement (s) Tb = duration of the background measurement (s)
Information about EMP and data corrections from http://www4.nau.edu/microanalysis/Microprobe/Probe.html
6.2.4 Valencies The microprobe is not able to measure the valence of an element, i.e. the ratio of Fe2+/Fe3+ in chlorite and white mica. This data is needed for calculating the paleo- temparatures and paleo-pressures. Therefore the geothermobarometric program of Vidal (2007) had several options for estimating the ratio of Fe2/Fe3+.
6.3 Sample Preparation In order to use the EMP for chemical analysis the samples had to be prepared. First were the samples cut into smaller pieces with a diamond saw and. Then these pieces were polished down to ~30 μm-thick uncovered thin sections. Before these thin sections were suitable for EMP they had to be coated with a thin carbon layer (in vacuum) to assure good conduction of electrons in the EMP. These thin sections were also used for light microscopy.
6.4 Analytical Procedures for Monazite Monazite geochronology was performed on felsic rocks samples from the Høyvik Group. These rocks underwent upper greenschist/ mid amphibolite metamorphose and the size of the monazite grains had a range of 5-15 μm. This grain size is the limit for EMP monazite age dating using a JEOL JXL-8600 Superprobe since the spotsize of the microprobe analyses needed for accurate chemical analyses of the monazites is 5 μm. The small grain size and a low Th content of the monazite minerals make the outcome of the chemical analyses disputable. On every monazite mineral only one measurement was done, mainly because most of the grains were too small in size for multiple measurements and a high possibility of an overlap with matrix minerals during measurements was possible. Monazite contains no SiO2 in the structure. So analyses containing a high SiO2 content (≥1.5 wt%) were excluded from the data set since it is clear that the electron beam was not solely directed on a monazite. Any kind of contamination can also be seen by the total wt% of the measurements. For monazite the total wt% must be around 100 wt%. Measurements lower than 96 wt% or higher than 106 wt% were in general also excluded from further age calculations. Some data from sample 6.9N06 showed an elevated level of Y. These minerals with elevated Y could also be xenotime instead of monazite and were excluded from the age calculations.
BSE images were used to find the monazite grains in the thin sections. Monazite appears bright white while other minerals are black or dark grey. Monazite is one of the
33 heaviest minerals in nature because they contain REE in addition to Th, U, Pb, and thus they will always appear white while other lighter minerals will appear grey or black. With this method also any chemical zoning in a single monazite grain can be spotted.
When a heavy mineral is analysed with the energy dispersive system (EDS) the main elements of this mineral are instantaneously shown in an energy dispersive spectrum (figure 26). In this way a mineral can be fully identified and a more accurate measurement can be performed by the WDS.
Figure 26: Energy dispersive spectrum of one of the analysed monazite minerals. The main elements of monazite are P, O, Th and the REE elements, La, Ce and Nb. The REE elements have higher energies and are thus concentrated at the right side of the figure.
34 Figure 27: Schematic drawing of the interaction volume and interaction effects. Next to the reflected energies also heat is created which stays in the sample.
The accurate quantitative analyses of minerals were performed using the wavelength dispersive spectrometers (WDS) of the EMP in combination with the computer facilities available. For heavy minerals like monazite, an accelerating voltage of 20 kV and a beam current of 50 nA is used. Due to the relative counting times (table 3) that are used it takes about 15 minutes to measure a monazite grain. A spot size of 5 μm was used during analyses. This defocused beam is used because a focused beam (spotsize 1 μm) will locate too much energy on the spot and burns away the mineral. Especially with the long counting times for monazites this is an important factor, since all the generated heat from the electron beam goes into the mineral. A defocused beam only changes the spotsize; the interaction volume with the sample stays the same (figure 27).
The measured monazites for geochronological calculations were too small for more than one measurement per single mineral, since the spot size used was 5 µm. The general size of the monazites was 5-10 μm (figures 28), so line scans or core-rim measurements were not possible. All the analysed points were measured manually, since the EMP in question is not precise enough to find the right spots that are indicated beforehand for this kind of accuracy. Before and after measuring the samples a standard monazite sample (m378-3-T2) with a known age was measured to check the accuracy of the EMP settings.
The data of the measured analyses used for geochronological calculations are shown in Appendix V.
35
Figure 28: BSE images of sample 9.2N06 showing on the left a monazite (mona) located in the middle of several muscovite (musc) minerals. On the right the same monazite is illustrated using another intensity on the black & white scale. The upper monazite mineral shows little zonation and thus little variation in the chemical content. This more homogeneous monazite mineral was used for measurements.
Element Line Crystal Spectrometer Calibration Standard Counting Time (s)
La Lα1 LIF 4 LaP5O14 50 Ce Lα1 LIF 4 CeP5O14 50 Pr Lβ1 LIF 4 PrP5O14 50 Nd Lβ1 LIF 4 NdP5O14 50 Sm Lβ1 LIF 4 SmP5O14 50 Gd Lβ1 LIF 4 GdP5O14 50 Dy Lβ1 LIF 4 DyP5O14 50 Ca Lβ1 PET 5 Diopside 20 U Lβ1 PET 3 UO2 200 Th Mα1 PET 3 ThF4 200 Pb Mα1 PET 5 PbS 300 Si Kα1 TAP 1 Diopside 50 Y Lα1 TAP 2 DyP5O14 50 P Kα1 TAP 2 Apatite 20 Tabel 3: Standard WDS analysis set up for elements of monazite minerals. Accurate analyses of Th, U, and Pb are important for geochronology, so these elements have longer counting times.
36 6.4 Analytical Procedures for Chlorite-White Mica With the lighter silicates the accelerating voltage used is 15 kV and a beam current of 20 nA. The counting times are also lower (table 4) and it takes only a few minutes to measurement one silicate mineral. The spot size is set to ~5 μm, because the lighter ions of white mica, K and Na, tend to move (diffuse) away when using a focused beam. This results in too low K and Na values and overestimated values of the other elements; Si, Fe and Al. So in order to gain more accurate and reliable estimates the defocused beam was used for all EMP measurements. EMP mineral analyses for geothermobarometry were performed by line tracks on chlorite and white mica (figures 29a and b). The chemical compounds that were measured are: SiO2-TiO2-Al2O3-FeO-MnO-MgO-CaO-Na2O-K2O. To assure results closest to equilibrium during solid solution mixing between the minerals, chlorite and white mica must be in direct contact with each other. Since equilibrium at grain scale is more likely then equilibrium on thin section scale and it is possible that there more generations of chlorite and mica present. The samples 6.8N06, 6.9aN06, 6.9bN06 and 9.2N06 (Høyvik Group) contained adjacent grains of chlorite and white mica. The data of the measured analyses used for geothermobarometrical calculations are shown in Appendix VI.
Figure 29: example of a line track of measurements on chlorite and white mica minerals which are adjacent, surrounded by quartz, sample 6.9bN06 (left mineral: chlorite, right mineral: white mica)
Element Line Crystal Spectrometer Calibration Standard Counting time (s) Fe Kα1 LIF 4 Hematite 50 Mn Kα1 LIF 4 Tephroite 50 Ti Kα1 PET 3 TiO 30 Ca Kα1 PET 5 Diopside 50 K Kα1 PET 5 KTiPO5 30 Cr Kα1 PET 3 Chromium 50 Si Kα1 TAP 1 Diopside 30 Al Kα1 TAP 1 Corundum 30 Mg Kα1 TAP 2 Forsterite 30 Na Kα1 TAP 2 Jadeite 30 Tabel 4: Standard WDS analysis set up for the elements of silicate minerals.
37 7. INTRODUCTION to MONAZITE
7.1 Introduction Monazite is an accessory mineral that is common in metapellitic rocks, felsic gneisses, igneous rocks, pegmatites and less abundant in mafic of calsic bulk compositions (Spear and Pyle, 2002). It is a light rare earth element (LREE) phosphate that may contain relative large amounts of radioactive uranium and thorium that decays into lead as time proceeds. Age dating, using microprobe analyses on monazite, is based on the Th, U and Pb content in the mineral. Three daughter Pb isotopes are the result from radioactive decay of thorium and uranium, 206Pb, 207Pb, and 208Pb. 204Pb is the only non- radiogenic lead isotope. Phosphate tends to take Th and U in its structure, no significance amount of 204Pb is incorporated into the structure during growth of the mineral. Since it is assumed there is negligible non-radiogenic lead (Pb204) in the mineral, all lead analysed can thus be correlated to the decay of uranium and thorium (Parrish, 1990). For the monazite age dating the total amounts of lead, uranium and thorium present in the mineral can be determined by electron microprobe analyses and there are no isotope measurements needed. The Th content of monazite may be up to 25 wt% and U content from ppm levels to 5 wt%. The accumulation of radiogenic Pb in monazite is only after ~100 Ma at a level that can reasonable be analyzed with the microprobe (~0,06 wt%) and therefore only rocks older than ~100 Ma can be used for age dating (Montel, 1996).
7.2 Monazite mineral
Monazite is a variant of one of three orthophosphates:
Monazite, (CePO4), monoclinic Apatite, (Ca5PO4), hexagonal Xenotime, (YPO4), tetragonal
In xenotime Y comprises 75-85% of the mineral with the HREE’s comprise the bulk of the remainder (Spear and Pyle, 2002). Just like Monazite it is possible for xenotime to occur as a minor accessory mineral, in felsic gneisses, igneous rocks, pegmatites and metasediments. Monazite and xenotime may break down during low- temperature hydrothermal alteration to form light REE-rich apatite or epidote. Monazite and xenotime show a similar alteration pattern, but the retrograde products contain a different REE distribution (Broska et al., 2005).
Figure 30: Endmember diagram for monazite
Monazite has the nominal composition (LREE)PO4 and the LREE’s generally comprise approximately 75% of the total cation proportions (exclusive P) of most metamorphic monazites. Monazites also contain additional Th, U, Ca, Si, HREE’s, Y, and Pb (Spear and Pyle, 2002). The two other endmembers of monazite are (figure 30):
38
Huttonite (REE3+P5+ = Th4+Si4+) Brabantite (2REE3+ = Ca2+Th4+)
The LREE sites in monazite may be filled with any rare earth element, but Ce is usually the most abundant followed by Nd and La (figure 26). The chemical distribution of the REE’s in a monazite is dependent on the bulk composition of the rock and other minerals present.
7.3 First appearance of monazite The first growth of monazite in a prograde reaction path is still not fully understood. The range of conditions suggested for the first appearance of monazite during metamorphism (~350-550°C) probably reflects differences in bulk composition and especially the bulk rock Ca (and possible Al) concentration. In rocks with a bulk composition that has a low amount of Ca but some phosphate and REE’s, monazite appears in the lower greenschist facies because the potential sinks for REE’s and Thorium (e.g. clays) become unstable at those grades (Overstreet, 1967; Spear and Pyle, 2002). So this process is important for metasediments.
A last possibility is that monazite grows as a response to hydrothermal fluids. Hydrothermal monazites are small and it is typical to have low Thorium contents, commonly less than 1 wt% (Meyer et al., 2006).
If a rock is more mafic in composition and contains amphibole, or epidote, monazite growth will not be in favor, because amphibole will be the sink for REE elements. This means that monazite appears to be stable in rocks of appropriate bulk composition at all metamorphic grades at the greenschist facies and above (Spear and Pyle, 2002). The crystal size of monazite tends to increase with increasing metamorphic grade (Rubatto et al., 2001). Monazite minerals can be present in greenschist facies rocks, but the individual grains will not be very big (5-15 μm).
7.4 Monazite growth and occurrence Monazite was only found in metasediments of the Høyvik Group Middle Allochthon (6.9N06 and 9.2N06). The monazites are believed to be metamorphic of origin and are around 5-15 μm in size. The Thorium, Uranium and Lead content of the minerals was in general low, and that makes the age calculation less accurate (8.4 Error quantification). Averages of Thorium lay around ~1.5 wt%, for Uranium ~0.1 wt% and for Lead ~0.05 wt%. Age and chemical zoning of the monazites was not seen on BSE images but the small size of the minerals made it hard to see if there was real chemical zoning.
Samples that were collected from a HP zone of the WGR, and the mylonites from the NSDZ in the Sunnfjord area did not contain monazite. The possibility exists that monazite was present in these rocks but that these minerals were dissolved during exhumation. The reason for the absence of monazite is not due to the metamorphic grade. The samples were felsic gneisses or metasediments, which were believed to have reached at least amphibolite facies conditions. However, the samples did contain (retrograde) minerals like epidote or amphiboles. The occurrence of epidote and amphibole even in more felsic samples seems to be a reason for the lack of monazite. Also most of the samples include metal-oxides, especially in the high strain zones, which marks the importance of the role fluids play during ductile or brittle deformation related to the exhumation of the HP rocks of the WGR. It is thought that these fluids caused existing monazites to be dissolved. The answer to why are there no monazites in the
39 collected samples of the WGR and NSDZ remains uncertain. Retrograde reactions can be an option, but then it is suspected that REE-carbonates are found. This was true for a few samples from this region.
7.5 Monazite resetting During deformation recrystallization of monazite can occur. These events may reset the TH-U-Pb ratios, if so the calculated age refers to the recrystallization event (Spear and Pyle, 2002). Temperature is an important factor in age dating. At high temperature some elements are more mobile than others. The temperature at which an element stops moving freely and sticks to the crystal lattice is called the closure temperature (Tc). This Tc depends on the kind of mineral and element in question and the size of the mineral. Loss of Pb by solid state diffusion is thought to be stopped at a closure temperature of TC ~750°C (Spear and Parrish, 1996). Minerals formed below the closure temperature of Pb will record the age of growth. However at lower temperatures the amount of Pb in a mineral can still be affected. The presence of (hydrothermal) fluids can change the amount of the parent and daughter products in monazite by dissolution.
40 8. GEOCHRONOLOGY
8.1 Chemical age dating-methodology With the Chemical Th-U-total Pb Isochron Method (CHIME) using EMP monazite analyses the main assumption is that there is no Pb initial in the monazite structure so all the Pb in monazites is radiogenic and proportional to Th, U (Montel et al., 1996, Suzuki, 1990). The relation between Th-U-Pb ratio is due to radioactive decay and thus proportional to the time elapsed. Ages can be calculated with different methods, but depend all on the same decay system (table 5).
Parent Decay constant λ (y-1) Half-life (y) Daughter 232Th 4.9475 x 10-11 1.4 x 10-10 208Pb 235Th 9.8485 x 10-10 7.07 x 10-10 207Pb 238U 1.5513 x 10-10 4.47 x 10-9 206Pb Table 5: radioactive decay system of Th and U
8.2 Apparent age dating (t) The apparent age (t) of single monazite grains is calculated using a computer program developed by H.L.M. van Roermund with the following formula.
204 208 207 206 Total PbPb initial +++= PbPbPb (1)
()λ ⋅t 208Pb = 232Th [e 232 −1] ()λ ⋅t 207Pb = 235U [e 235 −1] ()λ ⋅t 206Pb = 238U [e 238 −1]
204 Assuming no initial lead can be incorporated, so the term Pbinitial = 0
(λ ⋅t ) (λ ⋅t ) ThO ⎡ 235 238 ⎤ PbO 2 ()λ232 ⋅t UO2 e 88.137 ⋅+ e Total = + [e 1]+− ⋅ ⎢ −1⎥ (2) WPb WTh WU ⎣ 88.138 ⎦
Where WPb, WTh, WU are the molecular weights for respectively Pb, Th, and U. With this formula the apparent age is calculated of a single measurement of the elements PbO, ThO2, and UO2 from one monazite mineral. This equation is used for calculating the apparent age (t).
8.3 Isochron age dating The Chemical Th-U-total-Pb Isochron Method (CHIME) by Suzuki et al. (1991) uses a ThO2* value, which represents all ThO2 present. ThO2* is the sum of measured ThO2 and the equivalent amount of ThO2 that would replace the measured amount of UO2 during time (t).
* ThO2 ThO2 += 2 tfUO )( (3)
Filling this equation out using equation (2)
41 (λ ⋅t ) (λ ⋅t ) ⋅WUO ⎡e 235 88.137 ⋅+ e 238 ⎤ ThO ∗ = ThO + 2 Th ⋅ −1 (4) 2 2 ()λ232 ⋅t ⎢ ⎥ U ()eW −1 ⎣ 88.138 ⎦
When the ThO2* is plotted against PbO the data points should plot on a straight line. This line has slope a and intercept b:
PbO = a · ThO2* + b (5)
Since the assumption is made that there is no initial Pb in monazite, all best-fit lines go through the origin, which makes b in equation (5) zero. The PbO/ThO2 * ratio may then be substituted in equation (6), which yields the formula for the isochron age (T).
Isochron age (T) is given by; 1 ⎛ W PbO ⎞ T = ln⎜ Th +⋅⋅ 1⎟ (6) λ ⎜W ∗ ⎟ 232 ⎝ Pb ThO2 ⎠
The isochron age (T) is based on all the data, and is therefore more accurate than a single apparent age calculation (t).
Theoretically, in a ThO2* versus PbO diagram, all the age data would plot on a straight line, this is called the isochron. However, this is usually not the case, since other factors like Pb loss may play a role. So, a best-fit regression line is constructed, to determine the age, which is referred to as the isochron age. With a perfect fit all data are located on the line and R2 = 1. Normally the regression line with R2 closest to one gives the most accurate age (T). A high variability in ThO2 and UO2 spreads the data points in the diagram and helps to make the isochron more accurate.
So, assuming 1) no initial lead can be incorporated in the monazite structure and all radiogenic Pb is produced by the decay of Th 2) the amount of radiogenic lead in a monazite is proportional to the Th-U content the calculated monazite age is the time (t) elapsed since growth and/or cooling below the closing temperature (Tc= ~750˚ C).
8.4 Error quantification The EMP has a high detection limit for Th, U, Pb and other trace elements, compared to other techniques. This boils down to the fact that enough radiogenic lead has to be accumulated in the monazite, otherwise it cannot be detected above the background noise of the measurement and no accurate measurements can be taken. For example, monazites younger than ~100 Ma contain too little Pb and cannot be used for age dating with this method. The accuracy of a measured compound weight percentage (wt%) is highly dependent on the amount that is measured. The uncertainty becomes larger when the Th, U and Pb content in a mineral that is measured becomes smaller. This uncertainty factor is expressed as a standard deviation (σ). When the weight percentages are plotted against deviation measured by the microprobe, it shows that the concentration (in oxide wt%) and standard deviation have a power law relation (figures 29-31). The deviations may also be calculated manually with a program in excel, in that case the daily variations in for example beam current and temperature will not be taken into account.
42
Graphs 31-33 show that data of low wt% must be rejected for the analytical error becomes to high and the calculated ages inaccurate. When measurements with Pb < 0.075 wt%, Th < 1.0 wt% and/or U < 0.1 wt% will be disregarded the relative error would now be ≤10% for Pb and U, and for Th ≥1%. However, monazite was not abundant in the samples and the monazites that were present were small and low in Th, U and Pb content. Not many of the measurements were adequate enough for further age calculations and the inaccuracy is mainly caused by low amounts measured for Pb and sometimes even 0 wt% for U. However, a group of data can give more information, even with higher errors, than a single measurement. Therefore the relative error limit of the measured elements is lowered. The limitation used for the measured points is for Pb ≥0,01 wt% and for Th ≥0,7 wt%.
Compound wt% PbO - standard deviation y = 0.9387x-0.8684 50 45 40 35 30 25 20 15 error relative % 10 5 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 wt % PbO
Figure 31: calculated error PbO
Compound wt% ThO2 - standard deviation y = 0.9849x-0.333 1.6 1.4 1.2 1 0.8 0.6
error relative % 0.4 0.2 0 0123456 wt% ThO2
Figure 32: calculated error ThO2
43 Compound wt% UO2- standard deviation y = 1.4561x-0.8684 50 45 40 35 30 25 20 15 error relative % 10 5 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 wt% UO2
Figure 33: calculated error UO2
The relative standard deviation / error (ε) is equal to the standard deviation (σ) expressed as a relative fraction. This fraction is dependent on the peak counts, counting time and the beam current. The relative standard deviation / error can be calculated by the following formula, which is an uncertainly due solely to X-ray counting, peak and background, statistics:
⎛ N ⎞ ⎛ N ⎞ ⎜ P ⎟ + ⎜ B ⎟ ⎜ 22 ⎟ ⎜ 22 ⎟ ⎝ currt PP ⎠ ⎝ currt BB ⎠ ε −BP = ⎛ N P ⎞ ⎛ N B ⎞ ⎜ ⎟ − ⎜ ⎟ ⎝ currt PP ⎠ ⎝ currt BB ⎠
Where NP and NB are the number of peak and background counts, t is the counting time and curr stands for beam current.
An absolute error of the monazite ages is calculated by converting the deviation error of Pb, U, and Th into the corresponding error of weight percentages (figures 29-31). This is used to calculate a minimum and maximum in weight % of each measured element.
For example (table 6), for a measured amount of 2,4 wt% ThO2 the calculated deviation is 0,736 %, this results in a minimum of 2,22 wt% and a maximum 2,58 wt% ThO2. This is then also done for Pb and U. With these new compound weight percentages a minimum and a maximum age may be calculated. The difference between these ages, divided by two, gives the absolute error for that particular age.
(ε) calculation ThO2 (wt%) UO2 (wt%) PbO (wt%) Age (Ma) Measured value 2,4 0,032 0,67 626 Minimum value 2,22 0,023 0,60 613 Maximum value 2,58 0,041 0,74 639 Table 6: Example of absolute error calculation with monazite ages.
The absolute error of the example is then ~13 Ma and the final error on the age calculation would be t = 626 ± 13 Ma.
44 8.5 Results For the Høyvik samples (6.9N06 and 9.2N06) the selected data is plotted in a mean apparent age diagram and an isochron diagram (after Suzuki, 1991) (figures 34-37). The mean apparent age plot shows all the separate monazite ages of the single measurements. The isochron plot shows the data points following the ThO2*/PbO method. In the mean apparent plot and the isochron diagram of sample 6.9N06 two trends are visible. The oldest mean apparent age is 592 ± 42 Ma. The isochron age for this data set is 605 ± 91 Ma. The monazite grains with younger ages have a mean apparent age of around 266 ± 31 Ma and an isochron age of 290 ± 48 Ma. Here the isochron doesn’t fit the data as well as the older data set, as can be seen by the R2, indicated in the diagram.
An overview of the measured monazite data used for geochronological calculations is shown in Appendix V.
Figure 34: Distribution of the mean apparent age calculated for monazite in sample 6.9N06. The solid lines indicate the mean apparent ages; T=266 ± 31 and T=592 ± 42 Ma.
45 * Figure 35: ThO2 -PbO isochron diagram for sample 6.9N06 showing two trends
Figure 36: Distribution of the mean apparent age calculated for monazite in sample 9.2N06. The solid line indicates the mean apparent age; T= 571 ± 65 Ma.
46
* Figure 37: ThO2 -PbO isochron diagram for sample 9.2N06
Sample Isochron age Apparent age 6.9N06 605 ± 91 Ma 592 ± 42 Ma 9.2N06 587 ± 71 Ma 571 ± 65 Ma 6.9N06 290 ± 48 Ma 266 ± 31 Ma Table 7: Results of monazite dating
The data of sample 9.2N06 shows one trend in both the mean apparent age plot (figure 36) and the isochron diagram (figure 37). The regression line shows that not all the data points could be fitted on the line. This scatter may be due to a high inaccuracy of the measurements and not as a result due to loss of PbO. Loss of PbO only occurs during a temperature above the closing temperature (Tc= ~750°C for monazite). The samples of the Høyvik Group clearly didn’t undergo these high temperatures during metamorphism. The mean apparent age is 571 ± 65 Ma and the isochron age (after Suzuki, 1991) is 587 ± 71 Ma. The data set of sample 9.2N06 is more scattered. There were some loose data points, but they didn’t seem to fit any age lines determined for sample 6.9N06. The data set of sample 9.2N06 was too small to be more conclusive about possible other age lines.
The three different ages calculated by the mean apparent age method are all lower than the ages calculated by the isochron method of Suzuki et al (1991) but fit within the large error-range of the isochron ages (table 7). The absolute error, as can be seen in table 3, is for all three cases high but the isochron ages and the apparent ages are similar and mutually overlap. The high error in the ages is caused by the low amount of PbO, ThO2 and UO2 in the monazites. The wt% of the measured elements lies close to the accuracy limit of the microprobe. But since the isochron age and the apparent age of the two samples are in close range with each other, it seems an accurate estimation to give overall ages of ~590 Ma and ~280 Ma.
47 9. GEOTHERMOBAROMETRY
9.1 Introduction For this research study several samples were taken at different locations in the Sunnfjord Region from the WGR, NSDZ and the Høyvik Group for monazite age dating. The few rocks that did contain monazites were samples of the Høyvik Group. A new question came up; what were the PT conditions of these rocks during peak metamorphism. According to the little mineral content the metapelites from the Høyvik Group are greenschist facies rocks (Andersen et al, 1998). The growth of different kinds of minerals below 450°C is very limited under a wide pressure range. For an accurate PT estimation there must be minerals present for which the solid solution phases are an indicator for equilibrium PT conditions. However chemical equilibria of minerals are strongly subjected to pressure, temperature and mineral composition. This is because adjacent minerals interact with each other by exchanging chemical components and this is pressure and temperature dependent. Most geothermobarometrical methods use garnet, for example garnet-biotite relations, to determine paleo-pressures and temperatures. However, there is no garnet (or chloritoid) present in the samples from the Høyvik Group, this is probably caused by the little amount of MnO present in the system (see figure 38). In the samples of the Høyvik Group there was only quartz, albite, chlorite and white mica present (see Chapter 5).
Figure 38: FeO-MgO-MnO triangular diagram indicates that for forming Garnet and Chloritoid minerals there must be sufficient MnO present in the system.
For this reason the use of a new, but not yet fully developed geothermobarometric calculation technique (Vidal et al., 2001 & 2005), has been applied, and is based on chlorite-mica-quartz equilibria. The program can be found on:
http://lgca.obs.ujf-grenoble.fr/perso/ovidal/downloads/Downloads.htm
With the use of chemical EMP analyses of the mineral compositions of chlorite and white mica (phengite) the geothermobarometric program may determine the equilibrium PT conditions of the samples. These calculations were done to get more constraints on the temperature conditions but especially to get a better estimation of the lithostatic pressure during metamorphism of the Høyvik Group rocks.
9.2 Geothermobarometry Chlorite-mica-quartz multi-equilibrium is a principle brought up by Vidal & Parra (2000). From a thermodynamic point of view in a system with C independent chemical components, the number of independent reactions (IR) can be written for a given mineral assemblage described by end-members (EM) is given by: EM = IR – C
48 If minerals can coexist in equilibrium, then all the independent reactions intersect at a single point in PT space, this corresponds to the equilibrium conditions. This method is a good indicator for PT-conditions of rocks containing chlorite-quartz- mica (mainly meta-pelites). The mineral chemistry of chlorite and white mica can be used to determine equilibrium PT conditions. Chlorite and white mica have several substitutions that can be modelled by several end members to describe the PT positions of different compositions in the FeO-MgO-Al2O3-SiO2-H2O system. Chlorite is stable over a wide range of PT-conditions and occurs in a large variety of rocks. The chemical compositions of the different end members of chlorite are dependent on the conditions of mineral formation. Vidal et al. (2001) proposed a four-component model (Mg-amesite = Mg-Am, clinochlore = Clin, daphnite = Daph, and sudoite = Sud) to explain the three main substitutions observed in natural chlorite, i.e., Fe = Mg (FM), di/trioctahedral (DT): VI2AlVI = 3(Mg, Fe2+)VI and Tschermak (TK): SiIV(Mg, Fe2+)VI = AlIVAlVI substitutions. The equilibrium temperatures are computed by these four equilibria (2 independent) that are true for a Chl + Qrzt + H2O assemblage, with the use of the chlorite solid-solution model of Vidal et al. (2001, 2005). The equilibrium temperatures of the following reactions depend on the activities and therefore the proportions of the five end members in chlorite (clinochlore, daphnite, sudoite, Fe- and Mg-amesite).
2 Clin + 3 Sud = 4 MgAm + 7 Qrtz + 4 H2O (1) 4 Clin + 5 FeAm = 4 Daph + 5 MgAm (2) 16 Dph + 15 Sud = 20 FeAm + 6 Clin + 35 aQrtz + 20 H2O (3) 4 Dph + 6 Sud = 5 FeAm + 3 MgAm + 14 aQrtz + 8 H2O (4)
After this calculation an equilibrium pressure can be calculated for Chl + Phg + Qrzt + H2O mineralogy. The exchange of Fe and Mg between chlorite and white mica at a certain temperature is of high importance for calculating paleo-pressure. With the microprobe it is not possible to measure valencies, therefore the amount of Fe3+ and Fe2+ is estimated by calculations provided by the program. First a paleo-temperature is 3+ calculated by using the Chl-Qrtz-H2O assemblage, and then the amount of Fe in white mica is adjusted to come to a Fe-Mg exchange reaction between chlorite and white mica that occurs at the same temperature. The program then calculates a pressure with several reactions in the Chl-Phg-Qrtz-H2O system.
9.3 Fe2+/Fe3+ error The main limitation in accuracy with the calculations using the geothermobarometric model by Vidal (2007), results from the uncertainties with the [Fe3+] calculations. Since the microprobe is not able to measure valencies, the ratio of Fe2+/Fe3+ in chlorite and white mica must be attained with other techniques. This ratio is of importance in estimating paleo-temperatures and paleo-pressures because it takes part in different substitutions. Al3+ and Fe3+ substitution for example occurs in chlorite, like in most phyllosilicates. A minimum amount of Fe3+ in chlorite and white mica is estimated by the excel spreadsheet after the stoichiometric criteria given in Vidal et al. (2005) before further calculation. Because the amount of Fe3+ present will affect the out come of the activity of end members, this will also affect the calculation of the paleo-temperature and finally the paleo-pressure. Therefore the program has two options for estimating Fe3+; P-T Fe3+ min and P-T Fe3+ variable. One of the end member reactions is the exchange of Fe-Mg in chlorite and white mica, this is used to estimate the amount of Fe3+ (in phengite at the temperature obtained with the Chl-Qrtz-H2O assemblage. The temperature here is first constrained using the Chl- 3+ Qrtz-H2O assemblage, then the amount of Fe in phengite is adjusted to locate the Fe-
49 Mg exchange reaction between chlorite and phengite at a similar temperature, and finally the pressure is calculated with the reactions from the Chl-Phg-Qrtz-H2O reactions). With the total of five end members for chlorite and four for dioctahedral mica, 64 (of which five independent) reactions can be written in the system Si-Al-Fe-Mg-K for the different sequences of formation for chlorite, white mica and quartz in the presence of water. In this model only 11 of these reactions are used to attain approximations of paleo-temperature and paleo-pressure. This is partly due to the lack of understanding on the behaviour of certain end members like phlogopite and annite (end members of mica). So further investigation on this method and measurements on end members is still needed and therefore caution is required in the use of this method.
9.4 Results EMP analyses were performed on samples of the Høyvik Group which contained adjacent chlorite and white mica. These were thin sections from the rock samples 6.8N06, 6.9N06 and 9.2N06. The last two samples were also the only rock samples in which monazite minerals were present. The thin sections 6.9a and 6.9b are from the same rock sample. The measured EMP data of the chlorite and white mica are shown in Appendix VI. The differences in chemical compound, important for the PT-calculations, are shown in Appendix VII. In the graphs of Appendix VII it may be easily seen that sample 9.2N06 has the biggest differences in chemical compounds and that these differences exist both in chlorite and in white mica. In general the trend within a single mineral grain is the same for the chlorite and mica pairs. The biggest variation in chemical compounds occurs between the mineral pairs. The fact that in the mineral pairs the chemical variations are smaller may indicate that the mineral pairs established equilibrium. The EMP analyses may safely be used for geothermobarometry.
The differences between the “Fe3+ at minimum” and “Fe3+ variable” graphs (figures 39- 42) are mainly expressed in temperature. The calculations with “Fe3+ at minimum” give a higher temperature estimate of ~50-100°C than “Fe3+ variable”. The results of “Fe3+ variable” show less spreading in the results as is seen in the graphs (figure 39-42). That most iron is ferric is in agreeing with the white mica observations of Andersen et al. (1998); 1) the samples contain metal-oxides, which indicates a high oxidation state; 2) there is enough magnesium in the minerals to compensate the excess Si by a Tschermak substitution without ferrous iron needed (Si+Mg → Al+Al); 3) assuming ferric instead of ferrous iron the calculated number of octahedral cations is closer to four and the calculated weight percent totals closer to 100 wt%. In the geothermobarometric graphs (figure 38-42) of the analysed samples there are separate generations of chlorite and white mica growth visible. In the graphs of 6.8N06 and especially 9.2N06 (figures 38 and 40) there is a bigger spreading in the data points than in the graphs of 6.9N06. Samples also show the most variability in the analytical chemical measurements, so there is higher probability that these scattered points don’t represent true values. Data points indicating a temperature between 400-500°C in both the graphs of sample 6.9N06 (figures 39-40) may be seen as two stages of growth, one at ~400°C and one at ~475°C. In sample 6.8N06 (figure 38) only one phase of chlorite-white mica growth at a temperature of ~475°C is seen by the data points. The graphs of sample 9.2N06 (figure 42) indicate that there is a possible low temperature (~250°C) generation of chlorite-mica growth at a pressure range of 11- 14 kbar. These apparent high pressure values at low temperature are a common feature in the program, and are due to a problem in the thermodynamics of phengite at low temperature or a mixing of low temperature mica with clays (O. Vidal, pers. comm.). The
50 occurrence of monazite in sample 9.2N06 shows that the low temperature chlorite and white mica minerals are probably reset by recrystallization and therefore the occurence of clay minerals in sample 9.2N06 is not likely.
Mg and Fe end-members, Fe3+ in Mg and Fe end-members, Fe3+ in Chl Chl at min. variable 17 17
15 15
13 13
11 11
9 9 P (kbar) P (kbar) 7 7
5 5
3 3
1 1 200 300 400 500 600 700 200 300 400 500 600 700 T (°C) T (°C)
a b Figure 39: geothermobarometric graph of sample 6 .8N06 a; Fe3+ in chlorite at minimum and b; Fe3+ in chlorite variable
Mg and Fe end-members, Fe3+ in Chl Mg and Fe end-members, Fe3+ in Chl at min. variable 17 17 15 15 13 13 11 11
9 9 P (kbar) P (kbar) 7 7
5 5
3 3
1 1 200 300 400 500 600 200 300 400 500 600 T (°C) T (°C) a b Figure 40: geothermobarometric graph of sample 6 .9aN06 a; Fe3+ in chlorite at minimum and b; Fe3+ in chlorite variable
51
Mg and Fe end-members, Fe3+ in Mg and Fe end-members, Fe3+ in Chl Chl at min. variable
17 17
15 15
13 13
11 11
9 9 P (kbar) P (kbar)
7 7
5 5
3 3
1 1 200 300 400 500 600 700 200 300 400 500 600 T (°C) T (°C) a b Figure 41: geobarothermometric graphs of sample 6.9bN06 a; Fe3+ in chlorite at minimum and b; Fe3+ in chlorite variable
Mg and Fe end-members, Fe3+ in Mg and Fe end-members, Fe3+ in Chl Chl at min. variable 17 17
15 15
13 13
11 11
9 9 P (kbar) P (kbar) 7 7
5 5
3 3
1 1 100 200 300 400 500 600 700 100 300 500 700 T (°C) T (°C) a b Figure 42: geobarothermometric graphs of sample 9.2N06 a; Fe3+ in chlorite at minimum and b; Fe3+ in chlorite variable
As a summary of the PT calculations illustrated in figures 39-42, the rocks of the Høyvik Group have reached greenschist facies conditions with a general pressure estimate of 8- 11 kbar and a temperature range of 400-500°C These conditions are associated with depths of ~30 km and are mainly reached by burial and regional deformation during collision.
52 10. DISCUSSION
10.1 Pre-Caledonian events The rocks of the Høyvik Group are metasediments that are unconformable deposited on the magmatic rocks of the Dalsfjord Suite. The exact date of sedimentation is difficult to constrain but it is safely said that it took place ≥600 Ma ago, since that corresponds to the age obtained age by the monazite age dating. The break up of Rodinia began around 700-750 Ma with early rifting on the western margin of Laurentia (Torsvik et al., 1996). So in the classic reconstruction were Norway lies in the centre of Rodinia it is possible to form margin sediments (figure 3). The Dalsfjord Suite yields ages from 1,634 ± 3 Ma and overprinting ages from the Sveconorwegian orogeny of 882 ± 29 Ma (Corfu and Andersen, 2002). Parts of the Dalsfjord Suite and the Høyvik Group are intruded by dykes with a MORB- affinity and meta-basalts with a poorly preserved pillow structure (Andersen, 1990). The metasediments and mafic dykes of the Høyvik Group can be correlated with the Late Proterozoic rocks and dykes of the Seve Nappe, Sweden (Svenningsen 1993; Andersen et al., 1998). These dykes, also called the Sarek Dyke Swarm in Sweden, are dated to be ~608 Ma and are believed to date the initiation of the sea floor spreading of the Iapetus Sea (Svenningsen, 2001). If this is indeed the opening of the Iapetus Sea is not certain, since with the classic paleographic reconstruction Norway is located against Laurentia when Rodinia was forming (figure 3). Hartz and Torsvik (2002) discussed that this is a wrong reconstruction of Rodinia and that the position of Baltica should be rotated by 120˚C which makes Norway the eastern margin of the supercontinent. The rift related dykes would then indicate the opening of the Aegir Sea instead of the Iapetus Sea. Similar ages for rift related mafic dykes found in the Corrovarre Nappe of the Middle Allochthon, North Norway. The Corrovarre is the highest tectonic unit of the Kalak Nappe Complex. The dykes are associated with magmatism during continental thinning and seafloor spreading and have an age of ~580 Ma (Zwaan and van Roermund, 1990). The Precambrian monazite age of this study lies around ~590 Ma and the monazite growth is thought to be a consequence of the heating of the surrounding rocks of the dykes intruding the Høyvik Group. These dykes are not dated yet but the chemical and age similarities with other dyke swarms in the Allochthonous units make it reasonable to link them. Contact metamorphism in general can affect a large area of rocks but the impact of metamorphism lies in the heat capacity and conductivity of the country rock. Also heat transported by hydrothermal fluids circulating through the rocks may cause localized metamorphism. The average grain size of hydrothermal monazite is often small and has low Th contents, less than 1 wt% (Meyer et al., 2006). The monazite minerals from the samples 6.9N06 and 9.2N06 show these features. But the pressure and temperature conditions for this monazite growth are not really clear. In the figures of the chl-phg thermobarometric calculations some high-temperature and low-pressure points occur, but are more likely to be an inaccuracy of the geothermobarometric calculations. Most chlorite and white mica minerals are probably formed or re-equilibrated during later deformation processes so that an earlier metamorphic stage in which monazite was formed is not visible by the chl-mica geothermobarometric calculations. No monazite ages of ~450 Ma are found, this suggests that the existing monazites were stable at the deformation stage(s) at 400-500°C and 8-11 kbar and no new monazite were formed during this conditions. But why was there no monazite growth during the 445-450 Ma deformation event? Probably higher temperatures were needed to form new monazites with the existing mineralogy. In sample 6.8N06 a lot of phosphate in the form of apatite, and REE-rich apatite minerals were spotted. This may indicate that there were monazites present in this sample but that these were dissolved or re-crystallized during
53 later events. The remaining monazites of the Høyvik Group were formed ~590 Ma during a Pre-Scandian phase of metamorphism with higher temperatures than calculated by the geothermobarometrical calculations on chlorite-white mica analyses.
10.2 Caledonian events The Caledonian deformation phase recognized in the Høyvik Group had a polyphase deformational character and occurred at conditions ranging from 400-475°C and 8-11 kbar. The geothermobarometry study of the rocks of the Høyvik Group shows that the possibility of two generations of chl-mica growth exists, one at ~400°C and one at ~475°C with comparable pressure conditions. This is not in agreement with Andersen et al. (1998), they thought the variability in petrography and chemical analyses within their samples of the Høyvik Group to be too small to conclude more episodes of chl-mica growth. His temperature estimate was ≤450°C. The Høyvik Group underwent polyphase deformation at 445-450 Ma and exhumation was established before the deposition of the Wenlockian continental-margin deposits of the Herland Group (Andersen et al., 1998) and before the exhumation of the WGR. Sample 6.9N06 contained mica and chlorite pairs which defined the tight folds of the sample. This indicates that the minerals have grown syntectonic or recrystallized during deformation. The PT conditions calculated by the chlorite and mica minerals are inferred to represent the conditions of the polyphase deformation of 445-450 Ma. This deformation phase may be linked to the Jämtlandian (Breuckner et al., 2004; Breuckner and van Roermund, 2007) also occurring in the Seve Nappe Complex, Sweden. Eclogites and garnet pyroxenites of this area give Sm-Nd ages of ~458 Ma. The Høyvik Group forms probably part of this subduction phase given the cooling ages of 445-450 Ma but stayed at higher crustal levels during deformation. The thrusting sequence of the Caledonian orogeny (425-390 Ma) puts the Stavfjord Nappe and Dalsfjord Suite on the Baltic Shield. The Solund-Stavfjord Opiolite (~443 Ma) was thrusted on top of the Herland and Høyvik Group. The low grade Herland Group is recognised to be of Wennlockian (423-428 Ma) age, dated by fossils thus the obduction of the ophiolite is dated to be during this period (Andersen et al., 1998). The rocks of the Høyvik Group showed no evidence for internal deformation during this phase of Caledonian thrusting and/or the exhumation of the WGR. This is controlled by the fact that the Høyvik Group does not lie in direct contact with the mylonites of the NSDZ. The post-collision extensional event was responsible for exhumation of the HP rocks of the WGR in the Sunnfjord area (580°C and 21 kbar); (Cuthbert et al., 2000). The NSDZ was responsible for the last stage of exhumation of the WGR around 395-399 Ma (Andersen et al., 1998). In the rocks of the Dalsfjord Suite an age of ~380 Ma is found in the lowermost part of the nappe unit near the NSDZ (Eide at al., 1999), this is a much younger age than for the stratigraphically higher rocks and it shows that the Dalsfjord Suite is indeed affected by Post-Scandian exhumation.
10.3 Post-Caledonian events Monazite ages with an estimation of ~280 Ma are found in sample 6.9N06 of the Høyvik Group. Late events may be of influence in resetting earlier existing monazites or the formation of new minerals. The reactivation of the Dalsfjord Fault during the Late Permian and Mesozoic times are directly linked to hydrothermal fluids that could move freely through the breccia network (Torsvik et al., 1992 & Eide et al., 1997). Data from the Permian brittle fault breccia at Atløy (250-270 Ma) could be related to the Sunnfjord (e.g. Molvær) dykes in an E-W orientated extension (Torsvik et al., 1997). If the monazite minerals are grown due to the hydrothermal fluids or if they are reset by any fluids is not certain.
54 Sample 6.9N06, in which the young monazite ages were found, doesn’t give any information about another growth of chlorite and muscovite at low PT conditions or any influences on the solid solution equilibrium of the chl-phg due to (hydrothermal) fluids. Evidence for low temperature mineral growth is only spotted in sample 9.2N06, but from this sample there is not enough monazite data to conclude that this can be linked to Late Permian movements.
55 11. CONCLUSIONS
>625 Ma Sedimentation of the Høyvik Group, unconformable onto the Dalsfjord Suite, during rifting of the Ægir Sea or shelf sediments from an earlier sea. ~590 Ma Monazite age of the Høyvik Group (Pre-Caledonian). The age is interpreted to indicate contact metamorphism of rift related dykes and be related to the opening of the Ægir Sea. 40 39 445-450 Ma Ar/ Ar dating on mica (Tc= 350ºC) (Andersen et al., 1998). The Høyvik Group underwent polyphase deformation; Jämtlandian deformation event, at upper greenschist facies conditions of 400-475°C and 8-11 kbar ~443 Ma Ophiolite formation, U-Pb Zircon age (Dunning & Pedersen, 1988). The metasediments of the Høyvik Group show little evidence of nappe stacking during the Scandian event. ~425 Ma Høyvik Group is unroofed, dated by Wenlockian sediment deposits of the Herland Group. Full exhumation together with ophiolite emplacement after the deposition of the Herland Group sediments 395-399 Ma Local timing of exhumation of the WGR (Andersen, 1998). Høyvik Group shows no monazites ages timing the exhumation of the WGR. ~380 Ma Dalsfjord Suite age (Post-Caledonian) (Eide et al., 1999). Rocks of the Dalsfjord Suite near the NSDZ show much younger ages than structurally higher rocks. ~280 Ma Monazite age of the Høyvik Group (Post-Caledonian). (Hydrothermal) resetting by dissolution or new growth caused by reactivation of the Dalsfjord fault and dolerite dykes during the Late Permian.
56 12. ACKNOWLEDGEMENTS
My first thanks goes to the Molengraaff Fonds, who made this project possible by funding the fieldwork in Norway and the EMPA research at the Utrecht University. I would like to thank my field partner C.M. de Vries for the time in Norway. The help I got with the microprobe from T. Bouten is very much appreciated, especially the times during the holidays. Otto Stiekema helped with the sample preparation and made the thin sections. Also the correspondence with O. Vidal by email about his geothermobarometric program is highly appreciated. I like to give my special thanks to my supervisor H.L.M. van Roermund for his overall support and his enthusiasm for the geology of Norway, which made all the difference for bringing this project to a good end. And last, but off course not least, I’m thanking my friends and family for just merely being there. It helped!
57 13. REFERENCES
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59 Hacker, B.R., Andersen, T.B., Root, D.B., Mehl, L., Mattinson, J.M., and Wooden, J.L., Exhumation of high-pressure rocks beneath the Solund Basin, Western Gneiss Region of Norway, J. Metamorphic Geol., 21, 613-629, 2003a Hacker, B.R., and Gans, P.B., Continental collisions and the creation of ultrahigh- pressure terranes: Petrology and thermochronology of nappes in the central Scandinavian Caledonides, GSA Bulletin, 117, 1/2 117-134, 2005 Hacker, B.R., Ascent of the ultrahigh-pressure Western Gneiss Region, Norway, Geological Society of America, Special Paper 419, 2007 Hartz, E.H., Torsvik, T.H., Baltica upside down: A new plate tectonic model for Rodinia and the Iapetus Ocean, Geology, 30, 3, 255-258, 2002 Johnston, S.M., Hacker, B.R., and Duce, M.N., Exhumation of ultrahigh-pressure rocks beneath the Hornelen segment of the Nordfjord-Sogn Detachment Zone, western Norway, GSA Bulletin, 119, 9/10, 1232-1248, 2007a Johnston, S.M., Hacker, B.R., and Andersen, T.B., Exhuming Norwegian ultrahigh- pressure rocks: Overprinting extensional structures and the role of the Nordfjord- Sogn Detachment Zone, Tectonics, 26, 2007b Kylander-Clark, A.R.C., Hacker, B.R., Mattinson, J.M., Slow exhumation of UHP terranes: Titanite and rutile ages of the Western Gneiss Region, Norway, Earth and Planetary Science Letters, 2008 Krogh, E.J., Evidence for a Precambrian continent-continent collision in western Norway, Nature, 267, 17-19, 1977 Kumpulainen, R., and Nystuen, J.P., Late Proterozoic basin evolution and sedimentation in the westernmost part of Baltoscandia, In: Gee, D. G. & Sturt, B. A. (eds) The Caledonide Orogen-Scandinavian and Related Areas. Wiley, Chichester, 55-68, 1985 Labrousse, L., Jolivet, L., Andersen, T.B., Agard, P., Hébert, R., Maluski, H., and Schärer, U., Pressure-temperature-time deformation history of the exhumation of ultra-high pressure rocks in the Western Gneiss Region, Norway, Special Paper 380: Gneiss Domes in Orogeny, 380, 155-183, 2004 Meert, J.G., Torsvik, T.H., The making and unmaking of a supercontinent: Rodinia revisited, Tectonophysics, 375, 268-288, 2005 Meyer, F.M., Kolb, J., Sakellaris, G.A., and Gerdes, A., New ages from the Mauritanides Belt: recognition of Archean IOCG mineralization at Guelb Moghrein, Mauritania, Terra Nova, 18, 345–352, 2006 Osmundsen, P.T, Andersen, T.B., Caledonian compressional and late-orogenic extensional deformation in the Staveneset area, Sunnfjord, Western Norway, Journal of Structural Geology, 16, 10, 1385-1401, 1994 Osmundsen, P.T, Andersen, T.B., The middle Devonian basins of western Norway: sedimentary response to large-scale transtensional tectonics?, Tectonophysics, 332, 51-68, 2001 Powell, R., Holland, T., Calculated mineral equilibria in the pelite system, KFMASH (K2O-FeO-MgO-Al203-SiO2-H2O), American Mineralogist, 75, 367-380, 1990 Pyle, J.M., Spear, F.S., Wark, D.A., Daniel, C.G., and Storm, L.C., Contributions to precision and accuracy of monazite microprobe ages, American Mineralogist, 90, 547-577, 2005 Rehnström, E.F., Corfu, F., and Torsvik, T.H., Evidence of a Late Precambrian (637 Ma) deformational event in the Caledonides of northern Sweden: Journal of Geology, 110, 591-601, 2002 Roberts, D., The Scandinavian Caledonides: event chronology, palaeogeographic settings and likely modern analogues, Tectonophysics, 365, 283– 299, 2003
60 Roberts, D., and Gee, D.G., An introduction to the structure of the Scandinavian Caledonides In: Gee, D.G. & Sturt, B.A. (eds) The Caledonide Orogen- Scandinavian and Related Areas. Wiley, Chichester, 55-68, 1985 Roermund, H.L.M. van, Eclogites of the Seve Nappe, central Scandinavian Caledonides Caledonides In: Gee, D.G. & Sturt, B.A. (eds) The Caledonide Orogen- Scandinavian and Related Areas. Wiley, Chichester, 873-886, 1985 Rubatto, D., Williams, I.S., and I.S. Buick, Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia, Contrib. Mineral Petrol., 140, 458-468, 2001 Scambelluri, M., Pettke, T., and Roermund, van H.L.M, Majoritic garnets monitor deep subduction fluid flow and mantle dynamics, Geology, 36, 1, 59-62, 2008 Spear, F.S., Pyle, J.M., Apatite, monazite, and xenotime in metamorphic rocks, Reviews in Mineralogy and Geochemistry, 49, 293-335, 2002 Spengler, D., Roermund, H.L.M. van, Drury, M.R., Ottolini, L., Mason, P.R.D., Davies, G.R, Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway, Nature, 440, 913-917, 2006 Stephens, M., and Gee, D.G., A tectonic model for the evolution of the eugeoclinal terranes in the central Scandinavian Caledonides, In: Gee, D.G. & Sturt, B.A. (eds) The Caledonide Orogen-Scandinavia and Related Areas. Wiley, Chichester, 953-978, 1985 Suzuki, K., Adachi, M., The Chemical Th-U-total Pb Isochron Ages of Zircon and Monazite from the Gray Granite of the Hida Terrane, Japan, J. Earth Sci. Nagoya Univ., 38, 11-38, 1991 Svenningsen, O.M., Onset of seafloor spreading in the Iapetus Ocean at 608 Ma: precise age of the Sarek Dyke Swarm, northern Swedish Caledonides, Precambrian Research 110, 241-254, 2001 Swensson, E., Andersen, T.B., Contact Relationships between the Askvoll Group and the Basement Gneisses of the Western Gneiss Region (WGR), Sunnfjord, Western Norway, Norsk Geologisk Tidsskrift, 71, 1, 15-27, 1991 Tillung, M., Structural and Metamorphic Development of the Hyllestad–Lifjorden Area, Western Norway, Cand. Scient. Thesis, University of Bergen, 1999 Tillung, M., Andersen, T.B., Eide, E.A., and Walderhaug, H.J., The age and tectonic significance of dolerite dykes in western Norway, Journal of the Geological Society, 154, 961–973, 1997 Torsvik, T.H., The Rodinia Jigsaw Puzzle, Science, 200, 1379-1381, 2003 Torsvik, T.H., Cocks, L.R.M., Norway in space and time: A Centennial cavalcade, Norwegian Journal of Geology, 85, 73-86, 2005 Torsvik, T.H., Smethurst, M.A., Meert, J.G., Voo, van der R., McKerrow, W.S., Brasier, M.D., Sturt, B.A., and Walderhaug, H.J., Continental break-up and collision in the Neoproterozoic and Palaeozoic - A tale of Baltica and Laurentia, Earth-Science Reviews, 40, 229-258, 1996 Torsvik, T.H., Sturt, B.A., Swensson, E., Andersen, T.B., and Dewey, J.F., Palaeomagnetic dating of fault rocks: Evidence for Permian and Mesozoic movements along the Dalsfjord Fault, Western Norway. Geophysical Journal International, 109, 565-580, 1992 Vidal, O., Parra, T., Exhumation paths of high-pressure metapelites obtained from local equilibria for chlorite-phengite assemblages, Geol. J., 35, 139-161, 2000 Vidal, O., Parra, T., Trotet, F., A thermodynamic model for Fe-Mg aluminous chlorite using data from phase equilibrium experiments and natural pelitic assemblages in the 100° to 600°C, 1 to 25 kb range, American Journal of Science, 301, 557– 592, 2001
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62
APPENDIX I
PLATE TECTONICS
63
Figures by Cocks and Torsvik http://www.geodynamics.no/platemotions/500-400/
64 APPENDIX II
Tectonostratigraphy of the Sunnfjord area (Andersen & Austrheim, 2003, compiled from different studies).
65 APPENDIX III
THIN SECTION DESCRIPTIONS
a) Thin section 6.1N06 is from a mica-schist of the WGR on Atløy containing chlorite, biotite, quartz, little muscovite and deformed feldspar augen (2-3 mm). Main foliation is 18/305 and no visible lineation. b) Thin section 6.8N06 is from a mica-schist from the Høyvik Group on Atløy with larger matrix minerals than 6.1N06. Mineral content is chlorite, quartz and a little muscovite. The chlorite minerals form the foliation and there is a lot of oxidation visible indicating high iron content. Main foliation is 39/315.
66
Thin sections 6.9aN06 & 6.9bN06 are from the same mica-schist sample of the Høyvik Group on Atløy. The sample was collected near sample 6.8N06. Main mineralogy consists of chlorite, biotite, muscovite, quartz and a few albite porphyroblasts. The schist underwent polydeformation seen by the folding of the foliation. Main foliation is 39/315. This sample contained small monazites (~10 µm) used for age dating.
67 a) Thin section 6.12N06 is from a low grade greenschist facies mylonite of the NSDZ at Atløy, structurally located direct under the Dalsfjord Fault contact. Mineral content of the sample is comparable to samples 6.8N06 and 6.9N06 and consits mainly of quartz and muscovite, with little chlorite and (pre-tectonic) feldspar porphyroclasts. The difference lies in the structure of the sample. There is a very strong foliation and the quartz grains are recrystallized. The quartz minerals show a strong preferred orientation, have subgrains and form long thin lines separated by chlorite and mica minerals. The sample did not contain monazite minerals or good aligned chlorite-white mica minerals and is not used for EMP measurements. Main foliation is 41/358 with top-W shearing. b) Thin section 9.2N06 is from a quartzite sample of the Høyvik Group north from Askvoll. The mineral content is a lot of quartz with small bands of chlorite and muscovite (lineation /26/291). Next to the quartzite in the field lies folded pelitic schist. The main foliation is 36/293 and the folds have a foldaxis of 06/140, no top-W was visible in this outcrop but N-S related folding.
68
a) Thin section 7.5N06 is a gneiss of the WGR. The sample contains biotite, epidote, feldspar and quartz. Main foliation (27/003) is parallel to the lineation (08/84). b) Thin section 7.6N06 is a good foliated gneiss of the WGR with mainly feldspar, biotite and muscovite that probably is foliated during the amphibolite facies, but also has a lot of epidote from later retrograde reactions. The outcrop also contains k-feldspar bands of ~10 cm thick. Top to west shearing is not clearly visible.
Both samples are from the same location near the Vårdalsenet eclogite zone (Dørhella), east of Askvoll. This sample location is part of the transition of the NSDZ mylonite zone and the WGR.
69
Thin sections 7.9N06 and 7.10N06 of the Vårdalsenet eclogite area are platy, mylonitic gneisses of the NSDZ (east of Askvoll and NE of sample locatio of samples 7.5N06 and 7.6N06) with a lot of biotite and k-feldspar (augen and bands of <3cm) and with occasionally garnet. Main foliation 62/349 and a lineation of 12/75, top-W shearing is not clearly visible. The black dots seen on the thin section are Fe-Sulfides.
70 a) Thin section 13.2N06 is a folded gneiss of the WGR on Atløy with mainly muscovite, biotite, quartz, epidote and parts with (k-)feldspar. The main foliation is 26/341 with a lineation of 05/93 and a fold axis of 12/36 of subhorizontal folds. b) 13.5N06 is a felsic mylonite with biotite, chlorite and epidote of the WGR on Atløy (NSDZ). The black dots seen on the thin section are Fe-Sulfides. Main foliation of 28/335 and a lineation of 04/78.
71
a) Thin section 15.5N06 is a garnet mylonite with chlorite around the porphryoblasts from the NSDZ above the Kvamhesten Basin. Main foliation is 84/172, sub-vertical, with a lineation of 06/104 and symmetric shearing by the garnet pyphoroclasts can be seen in the thin section, but top the west indicators were also found in the outcrop. b) Thin section 15.8N06 is a gneiss containing hornblende with retrograde aerols of epidote. Sample is from the WGR (north of sample location 15.5) above the Kvamhesten Basin with vertical folds, foldaxis 13/258. Main foliation is 77/001 and a lineation of 12/81 and no indication of top to the west shearing. The black dots seen in the thin section are Fe-Sulfides.
72 APPENDIX ΙV
FIELD DESCRIPTIONS
Photographs of the Dalsfjord Fault located NE of Askvoll. The hammer points to the fault-gauge zone, which is localized to <0.5 cm. This contact may also be seen on Atløy. (July, 2006)
Dalsfjord Fault The brittle transition between the greenschist facies mylonites of the NSDZ (footwall) and the base of the Dalsfjord Suite (hanging wall) is marked by the Dalsfjord Fault. These pictures are taken NE of Askvoll.
NSDZ The potographs below show the mylonites of the high strain / high temperature zone of the NSDZ with folded migmatite veins and gabbroic bands at Dorhella SE of Askvoll. A lot of folding and rotation of porphyroblasts could be seen in this detachment mylonite. Samples of this sight that were taken didn’t contain monazite minerals for age dating.
73
Photographs of mylonitic gneisses of the NSDZ near Dorhella, SE of Askvoll (July, 2006).
74 APPENDIX V
MONAZITE MEASUREMENTS USED FOR AGE DATING
Total Age PbO ThO2 U2O (wt%) (Ma) 6.9N06 1 0,0230,759 0,069 97,895 547,1 2 0,0491,248 0,158 98,475 646,7 3 0,0391,509 0,02 107,173 581,2 4 0,0261,068 0,021 103,323 537,2
5 0,0371,091 0,08 105,131 638,7
6 0,0141,015 0,027 97,928 300,2 7 0,0483,505 0,037 96,482 312,9 8 0,0141,213 0 98,394 273,1 9 0,0151,377 0,034 108,805 238,9 10 0,0070,672 0,024 87,822 221,2 11 0,010,708 0,15 108,946 198,8 12 0,0191,417 0,037 98,416 292,3
13 0,0795,519 0,292 85,683 288,9
14 0,0210,808 0,339 75,641 260,5 9.2N06 1 0,0240,955 0,022 95,6 548,6 2 0,0562,574 0,008 98,523 506,6 3 0,0311,337 0 97,347 544,9 4 0,0331,767 0,03 97,058 544,9 5 0,0311,337 0,000 97,347 632,8 6 0,0612,137 0,037 98,588 745
7 0,071,631 0,168 98,05 537,5
8 0,0281,198 0,008 99,168 417,1
75 APPENDIX VI
Chlorite and white mica (phengite) measurements used for geothermobarometry
6.8N06 Chlorite SIO2 TIO2 AL2O3 FEO MNO MGO CAO NA2O K2O total 57 24.8246 0.071729 23.4323 30.052 0.461969 11.8945 0.084377 0.022192 0.47803 91.3217 58 24.1169 0.100713 21.2132 27.2104 0.427796 11.8979 0.02837 2.12E-09 2.48E-09 84.99528 59 23.9654 0.073072 20.8794 27.2871 0.377948 12.8038 0.008985 0.009296 2.48E-09 85.405 60 22.6782 0.06278 21.1544 31.726 0.382536 13.1772 0.037077 2.18E-09 2.46E-09 89.21819 61 24.2252 0.074912 21.2184 27.1816 0.380084 13.8995 0.00914 0.002351 2.49E-09 86.99119 62 24.4882 0.09211 21.3773 27.824 0.48458 13.4528 0.012513 0.028755 2.48E-09 87.76026 63 24.5548 0.088237 21.137 26.7042 0.370391 14.0707 2.46E-09 2.09E-09 2.49E-09 86.92533 64 24.4211 0.08045 21.602 27.0444 0.428539 13.6489 0.001003 2.1E-09 2.49E-09 87.22639 65 24.2325 0.05888 21.3697 26.8335 0.348601 13.9526 2.46E-09 2.09E-09 2.49E-09 86.79578 66 24.3782 0.068569 21.5784 26.7421 0.355299 13.5622 0.008505 0.003683 2.49E-09 86.69696 67 24.5299 0.07669 21.2076 27.4475 0.406625 13.5938 2.45E-09 0.003534 2.48E-09 87.26565 68 26.2021 0.094372 20.9229 26.8103 0.423979 13.4732 2.46E-09 0.017724 0.192954 88.13753 73 22.3566 0.057471 20.994 26.1364 0.361661 13.5624 0.000906 0.015602 2.48E-09 83.48504 74 24.0713 0.06001 21.3062 26.1945 0.42378 13.5358 2.46E-09 0.029141 2.49E-09 85.62073 75 23.8162 0.089449 21.9618 26.8117 0.448687 14.2215 0.002054 2.09E-09 2.49E-09 87.35139 76 24.3414 0.074635 21.0643 26.2071 0.34219 14.081 2.46E-09 0.003168 2.49E-09 86.11379 94 24.5848 0.06135 19.7792 26.0008 0.465527 13.9285 2.46E-09 2.09E-09 2.49E-09 84.82018 95 24.008 0.070194 21.0175 26.1355 0.368807 13.9855 2.46E-09 2.09E-09 2.49E-09 85.5855 96 24.1882 0.07484 21.1418 26.0309 0.290469 13.8511 2.46E-09 2.08E-09 2.49E-09 85.57731 97 23.4575 0.068505 20.6789 26.2803 0.355091 13.3212 0.016205 0.021486 2.49E-09 84.19919 98 23.2886 0.04002 21.0346 26.4635 0.431783 13.832 2.45E-09 2.1E-09 2.49E-09 85.0905
76 6.8N06 Phengite SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O total 49 44.9317 0.553996 31.6784 5.0034 0.051815 2.64888 0.023625 0.147154 8.62026 93.65923 50 43.6682 0.483156 29.2166 9.81557 0.082356 3.76245 0.022697 0.161424 7.17542 94.38787 51 45.3524 0.508134 31.9131 5.00228 0.017277 2.11784 0.013947 0.362247 8.17629 93.46351 52 44.4906 0.499728 31.3687 4.41186 7.75E-09 1.74909 0.004512 0.257941 8.56863 91.35106 53 49.0919 0.323237 27.1684 6.31495 0.043168 3.86086 0.029842 0.16875 6.92778 93.92889 54 48.887 0.30912 28.1879 4.7055 7.76E-09 3.40093 0.010896 0.197658 8.02123 93.72023 55 47.116 0.429607 33.6872 3.14739 7.77E-09 1.54677 2.64E-09 0.461867 7.8872 94.27603 56 46.8066 0.401758 33.0616 3.21516 0.038121 1.58202 2.64E-09 0.392199 8.324 93.82146 69 48.246 0.376614 31.2634 3.85554 0.003461 2.19178 0.00408 0.436834 7.83348 94.21119 70 47.7447 0.372903 32.6323 3.47124 0.008665 1.8045 0.012884 0.520815 7.71016 94.27817 71 48.0414 0.409942 32.5168 3.35214 0.025999 1.81927 0.007985 0.467824 7.71009 94.35145 72 47.3558 0.414986 33.3352 3.27429 0.024264 1.63271 0.003646 0.473397 7.77836 94.29265 77 47.372 0.477902 32.9481 4.03558 7.76E-09 1.93683 0.00487 0.348903 7.49794 94.62212 78 47.3255 0.413035 33.3468 3.40711 0.012129 1.7414 2.64E-09 0.426305 8.05693 94.72921 79 48.1774 0.384965 31.2778 3.23701 0.062382 1.94485 0.007845 0.317951 8.84658 94.25678 80 47.3424 0.403392 32.0198 3.1155 0.029468 1.78581 2.64E-09 0.434374 8.15799 93.28873 81 46.285 0.40488 31.789 4.00741 0.389186 1.7653 0.020754 0.482683 7.95508 93.09929 82 47.1541 0.412986 32.2537 3.07903 0.024266 1.79933 2.64E-09 0.497711 8.40813 93.62925 83 47.3764 0.430767 32.2358 3.11059 0.026 1.79272 0.007121 0.480409 8.29958 93.75939 84 47.5729 0.390795 32.3313 3.07957 7.77E-09 1.89359 2.64E-09 0.515344 8.4334 94.2169 85 46.9737 0.430902 31.8233 3.18293 0.025997 1.75498 2.64E-09 0.448231 8.0834 92.72344 87 46.9129 0.426095 32.5146 3.25279 0.001731 1.75765 2.64E-09 0.467921 8.35707 93.69076 88 46.8963 0.441663 32.601 3.14859 0.034658 1.69001 2.64E-09 0.529941 8.42809 93.77025 89 46.87 0.398449 32.6002 3.27854 0.017326 1.75792 2.64E-09 0.480115 8.4342 93.83675 90 46.9862 0.38396 32.4906 3.20225 0.010399 1.6857 2.64E-09 0.496553 8.03012 93.28578 91 46.4192 0.445972 33.2545 3.11252 0.005193 1.55748 2.64E-09 0.507532 8.36708 93.66948 92 45.9721 0.336124 33.2098 3.1277 0.038123 1.34218 2.64E-09 0.406604 8.16157 92.5942 93 49.6083 0.326532 29.2578 3.85834 0.017328 2.85377 2.64E-09 0.125603 7.87488 93.92255
6.9aN06 Chlorite SIO2 TIO2 AL2O3 FEO MNO MGO CAO NA2O K2O total 8 23.4965 0.040459 20.2381 24.4332 0.275714 13.5188 0.017435 0.037711 2.49E-09 82.05792 9 24.1447 0.077482 20.9366 24.6804 0.230829 14.5719 2.47E-09 0.030378 2.5E-09 84.67229 10 24.5079 0.058494 21.4095 24.7283 0.244395 15.4397 0.016276 0.021476 2.5E-09 86.42604 11 24.8229 0.06041 20.5398 24.4298 0.33309 15.0812 0.015315 0.038829 2.5E-09 85.32134 12 24.7109 0.082806 20.6188 24.5915 0.261105 15.2778 0.009552 0.025176 2.5E-09 85.57764 13 25.5704 0.060877 20.9036 24.0275 0.261467 15.4351 0.000962 0.043615 2.5E-09 86.30352 14 25.3182 0.049652 20.1895 25.7895 0.220635 14.9281 0.010445 0.05297 2.49E-09 86.559 15 25.3538 0.040779 20.6165 25.0138 0.242609 14.7861 0.011278 0.05282 2.5E-09 86.11769 16 24.8562 0.043904 20.864 24.8964 0.302823 14.9413 0.0095 2.05E-09 2.5E-09 85.91413 29 25.5007 0.083206 19.3256 26.0698 0.248887 14.7801 0.012589 2.08E-09 2.49E-09 86.02088 30 25.7724 0.078144 19.6528 26.0094 0.267321 14.6434 0.005004 0.000176 2.49E-09 86.42864 31 26.4802 0.056107 18.9315 25.4661 0.29274 15.4495 0.019385 0.021327 2.5E-09 86.71686 32 25.2006 0.073387 19.9056 25.4863 0.200646 15.1644 0.005153 2.07E-09 2.5E-09 86.03609 33 25.6918 0.060551 21.0266 26.2647 0.240655 14.6997 0.010228 2.07E-09 2.49E-09 87.99423 34 25.5101 0.038752 20.597 25.7408 0.262464 14.7074 0.006447 0.01573 2.5E-09 86.87869 35 25.2965 0.078082 19.9915 25.0238 0.185714 15.102 0.006069 0.019782 2.5E-09 85.70345 36 25.1134 0.064764 19.2613 25.2384 0.168854 14.821 0.003643 0.006959 2.49E-09 84.67832
77 6.9Na06 Phengite SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O total 6 46.4555 0.417914 32.4971 3.1501 7.77E-09 1.93899 0.012741 0.323966 8.13639 92.9327 18 47.3805 0.614314 31.2541 4.02381 0.152267 2.37921 0.033994 0.17663 8.40088 94.41571 19 48.0189 0.385668 33.0005 2.70654 0.031227 1.75121 0.028351 0.42239 7.84445 94.18924 20 47.8775 0.484146 32.261 3.15091 0.071073 1.96396 0.051127 0.231925 7.90477 93.99641 21 48.096 0.39782 30.1686 3.01031 0.682358 2.44211 0.08021 0.342239 8.49226 93.71191 22 48.7082 0.447915 30.3225 3.24377 0.034674 2.50901 0.027134 0.295303 7.55688 93.14539 23 49.1483 0.451238 30.6643 3.45329 0.055461 2.53437 0.031269 0.370663 7.40268 94.11157 37 49.1924 0.448666 29.3601 2.98799 0.001734 2.79112 0.006273 0.261762 7.52911 92.57915 38 49.0656 0.342692 29.1011 3.06137 0.057238 2.86051 0.006971 0.231161 8.06076 92.7874 39 49.4878 0.401073 28.9929 3.06434 0.003466 2.89857 2.64E-09 0.310735 8.02188 93.18076 40 46.7396 0.564492 31.741 3.30591 0.050249 2.21249 2.64E-09 0.292686 8.0182 92.92463 41 47.869 0.438393 31.1108 3.05315 0.015607 2.23605 0.015749 0.363895 7.78589 92.88853 42 47.1136 0.45808 32.2918 2.79371 0.019076 1.90562 2.64E-09 0.342284 8.3373 93.26147 44 44.6996 0.420253 32.0754 4.52732 0.025944 3.20834 0.006755 0.253679 7.17176 92.38905 46 45.8954 0.454396 34.0406 2.43453 7.77E-09 1.42329 0.013563 0.32633 8.92508 93.51319 47 47.6904 0.452533 31.3267 2.98313 0.001733 2.25679 2.64E-09 0.468294 7.77747 92.95705 48 45.9425 0.433947 31.8055 3.05222 0.020788 1.94031 0.007824 0.535797 9.33862 93.07751 49 46.7022 0.474968 32.7868 3.01702 0.039872 1.85375 2.64E-09 0.451234 7.85571 93.18155
6.9bN06 Chlorite SIO2 TIO2 AL2O3 FEO MNO MGO CAO NA2O K2O total 73 24.8754 0.052947 20.0248 25.7266 0.223921 14.9291 2.46E-09 0.009788 2.49E-09 85.84256 74 24.7707 0.058496 19.999 25.149 0.187301 15.1163 2.46E-09 0.002312 2.5E-09 85.28311 75 24.9208 0.087095 19.7411 24.7055 0.321276 15.5628 2.47E-09 0.018596 2.5E-09 85.35717 76 23.8096 0.077014 21.24 24.7204 0.260858 14.3383 2.46E-09 0.013856 2.49E-09 84.46003 77 24.2068 0.059381 21.1268 24.5041 0.239301 14.8616 2.47E-09 2.05E-09 2.5E-09 84.99798 78 24.2594 0.070258 21.2468 24.5779 0.262712 14.812 2.47E-09 2.05E-09 2.5E-09 85.22907 79 24.0993 0.077487 21.1934 24.7505 0.277679 14.8578 2.47E-09 2.06E-09 2.5E-09 85.25617 80 24.0928 0.089389 21.0887 24.5702 0.26432 14.6439 2.47E-09 2.05E-09 2.5E-09 84.74931 81 23.9494 0.075667 21.1824 24.5014 0.302789 14.6136 2.47E-09 0.014809 2.5E-09 84.64006 82 24.571 0.086725 21.098 24.6269 0.24263 14.6035 2.47E-09 0.016941 2.5E-09 85.2457 83 24.1897 0.092044 20.8009 24.681 0.299374 14.5806 2.46E-09 0.002968 2.5E-09 84.64659 48 24.0992 0.058906 21.4129 25.1427 0.28754 14.4329 0.042741 0.049824 2.49E-09 85.52671 49 24.6956 0.083041 21.8781 24.6536 0.331497 15.2556 0.0085 2.04E-09 2.5E-09 86.90594 58 25.1876 0.073703 20.9893 24.8688 0.276173 15.1732 2.47E-09 0.000867 2.5E-09 86.56964 59 24.2654 0.045795 20.9658 24.7683 0.322738 14.3332 2.46E-09 0.019473 2.5E-09 84.72071 60 24.7329 0.067654 21.6914 25.0161 0.291149 14.9651 2.47E-09 0.019561 2.5E-09 86.78386 61 24.4427 0.077656 21.2558 25.0502 0.304386 14.789 0.00048 0.017144 2.5E-09 85.93737 62 25.2349 0.06497 22.2045 24.9654 0.292989 15.0145 0.013377 0.015242 2.5E-09 87.80588
78
6.9bN06 Phengite SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O total 63 46.6678 0.476749 33.4624 2.9421 0.032941 1.77477 0.006346 0.38207 7.94654 93.69172 64 46.7216 0.421903 33.2655 2.84692 0.036414 1.80146 0.001157 0.441741 7.9328 93.4695 65 46.9815 0.403913 32.4342 3.22778 7.77E-09 2.01983 0.007321 0.39593 7.81821 93.28868 66 46.3567 0.44802 32.9913 2.91065 0.04161 1.81893 0.008223 0.456884 7.92891 92.96123 67 46.8354 0.419077 32.5127 3.09574 0.053732 1.97035 2.64E-09 0.42079 8.08155 93.38934 68 47.5274 0.472422 32.0373 3.1645 7.77E-09 2.14226 2.64E-09 0.412836 7.99817 93.75489 69 47.6793 0.428086 31.5875 3.32819 0.02773 2.3437 2.64E-09 0.397972 8.01225 93.80473 70 47.8347 0.441661 31.4266 3.17592 0.04334 2.32894 0.003443 0.353834 8.01368 93.62212 71 47.818 0.446603 31.5309 3.47498 7.77E-09 2.38609 2.64E-09 0.360115 7.94312 93.95981 72 47.5985 0.442694 31.2832 3.68587 0.02252 2.46004 2.64E-09 0.339298 7.87937 93.71149 50 46.5972 0.440014 32.4938 3.2764 0.003461 1.81751 0.01794 0.30439 8.89967 93.85039 51 47.6677 0.361517 32.1866 3.15412 0.013869 2.01974 0.014978 0.511096 7.98599 93.91561 52 47.6267 0.396077 32.4365 3.02668 0.041607 1.94201 0.006942 0.544395 8.28252 94.30343 53 48.9424 0.328366 30.9742 3.13609 7.78E-09 2.47555 2.64E-09 0.353 7.89552 94.10513 54 50.5401 0.264831 28.8997 3.28914 7.78E-09 3.0564 0.006504 0.34097 7.5701 93.96774 55 48.0005 0.48605 31.7959 3.40754 7.77E-09 2.21287 0.008938 0.176811 7.92447 94.01308 56 48.3763 0.429399 31.0803 3.31028 0.005196 2.40137 2.64E-09 0.407 7.81848 93.82832
9.2N06 Chlorite SIO2 TIO2 AL2O3 FEO MNO MGO CAO NA2O K2O total 9 26.2725 0.125647 18.024 22.6975 0.18302 16.8648 0.066999 0.015205 2.51E-09 84.24967 10 27.2586 0.19534 19.2433 22.6619 0.189975 17.3847 0.048283 0.008994 2.51E-09 86.99109 11 27.5431 0.630387 19.9273 22.7531 0.159801 17.9538 0.047571 1.99E-09 2.51E-09 89.01506 12 26.9946 0.215973 18.5227 22.499 0.26559 17.6848 0.032026 2E-09 2.51E-09 86.21469 13 26.9449 0.15898 18.8029 22.2394 0.190026 17.4609 0.041124 0.029388 2.51E-09 85.86762 21 24.1294 0.094004 17.8441 22.9871 0.135762 15.9423 0.059678 0.030777 2.5E-09 81.22312 22 25.5979 1.06526 17.4399 22.9569 0.157633 15.9169 0.065023 2.03E-09 2.5E-09 83.19952 23 27.2481 0.070189 19.2111 23.7758 0.157819 17.4403 0.053649 0.010677 2.51E-09 87.96763 24 28.1868 0.055093 19.6045 22.2714 0.166712 18.478 0.029976 1.98E-09 2.52E-09 88.79248 31 25.3626 0.24073 17.3394 23.1033 0.191178 16.8404 0.024933 0.02036 2.5E-09 83.1229 32 26.1482 0.082078 17.9441 23.3431 0.150984 16.6601 0.050026 0.001887 2.51E-09 84.38047 33 26.9758 0.09638 18.6017 24.0176 0.162799 17.6859 0.031438 0.001198 2.51E-09 87.57282 34 25.9933 0.064449 17.892 24.0756 0.184394 17.293 0.047569 2.04E-09 2.5E-09 85.55031 35 27.8613 2.18379 19.2676 23.4507 0.095789 17.5972 0.029822 2.01E-09 2.5E-09 90.4862 36 27.0989 0.082508 18.4676 24.0516 0.142616 16.9212 0.034544 0.008789 2.51E-09 86.80776 54 22.7245 0.122722 18.1291 22.2165 0.154241 16.4916 0.045844 0.016732 2.5E-09 79.90124 55 23.3254 0.100543 18.6931 22.2026 0.176216 17.1639 0.029486 2.02E-09 2.51E-09 81.69124 56 22.3432 0.133825 17.8062 22.3212 0.216079 15.9343 0.052644 2.04E-09 2.5E-09 78.80745 57 22.884 1.83469 17.7715 22.0334 0.211275 16.5894 0.020539 2.03E-09 2.49E-09 81.3448 58 24.4531 0.276125 19.5715 21.9486 0.210002 17.1246 0.042188 2E-09 2.51E-09 83.62612
79 9.2N06 Phengite SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O total 15 49.5879 1.03344 26.2914 5.29734 0.05876 3.56426 0.062915 0.037121 8.8562 94.78934 16 49.7427 1.00825 25.3299 5.23852 0.010373 3.50518 0.032493 0.054122 7.99943 92.92097 17 50.8554 1.05376 25.75 5.17537 0.019019 3.58995 0.049069 0.046958 8.36836 94.90789 18 48.9485 1.27099 27.5655 5.28419 0.025918 2.9825 0.062135 0.101888 8.87041 95.11203 19 50.1082 0.993947 25.0704 4.81059 0.031138 3.67801 0.022922 0.066133 8.14153 92.92287 26 48.3965 1.48196 29.3822 4.98333 0.029379 2.57533 0.053264 0.112259 8.24654 95.26076 27 49.795 0.874379 26.8214 4.74607 0.041524 3.54035 0.091579 0.055702 7.81042 93.77642 28 51.4881 0.844381 25.0943 4.86252 0.006923 3.92298 0.028791 0.026085 7.8385 94.11258 29 48.1578 1.37602 27.5314 5.06044 0.029373 2.9128 0.039388 0.052276 8.59476 93.75426 40 48.6905 1.36476 27.0286 5.3579 7.74E-09 3.10849 0.033242 0.044022 8.66736 94.29487 41 46.8985 1.07654 24.8888 5.32646 0.008632 3.20472 0.037283 0.068238 10.0093 91.51847 42 50.6494 0.939301 25.3035 5.60761 0.031102 3.43959 0.042232 0.055026 8.8591 94.92686 43 51.7936 0.979101 25.8467 5.31799 0.039762 3.57229 0.024441 0.047159 9.27667 96.89771 44 50.0634 1.03671 25.6089 5.11939 0.036301 3.40076 0.028569 0.024131 9.28795 94.60611 45 48.419 1.20666 27.4383 5.06476 0.015554 3.05308 0.021974 0.086596 8.35415 93.66007 61 40.4565 1.27339 25.137 5.2246 0.079338 2.89982 0.045804 0.061529 8.09373 83.27171 62 42.9157 1.2115 25.6274 5.25602 0.013808 2.95584 0.027085 0.089805 8.18115 86.27831 63 38.6849 1.34067 25.4617 4.9846 0.003444 2.7013 0.124545 0.104739 8.22368 81.62958 64 42.5408 1.18629 27.7508 4.77073 0.013816 2.75731 0.06278 0.100883 8.66824 87.85165 67 43.7217 0.499701 24.3476 3.73593 0.00173 3.81591 0.083899 0.020892 7.6716 83.89896
80 APPENDIX VII
Chemical variance of chlorite and mica in thin sections 6.8N06, 6.9aN06, 6.9bN06 and 9.2N06
Chlorite 6.8N06
35
30
25
FeO MgO Al2O3 SiO2 compound wt% compound 20
15
10 0 5 10 15 20 25 number
Phengite 6.8N06
50
45
40
35
30 FeO MgO 25 Al2O3 SiO2 K2O compound wt% compound 20
15
10
5
0 0 5 10 15 20 25 30 number
81 Chlorite 6.9aN06
30
28
26
24
22
FeO MgO 20 Al2O3 SiO2 compound wt% compound 18
16
14
12
10 0 2 4 6 8 1012141618 number
Phengite 6.9aN06
60
50
40
FeO MgO 30 Al2O3 SiO2 K2O compound wt%
20
10
0 0 2 4 6 8 101214161820 number
82 Chlorite 6.9bN06
30
28
26
24
22
FeO MgO 20 Al2O3 SiO2 compound wt% compound 18
16
14
12
10 0 2 4 6 8 101214161820 number
Phengite 6.9bN06
50
40
FeO 30 MgO Al2O3 SiO2 K2O compound wt% compound
20
10
0 0 2 4 6 8 1012141618 number
83 chlorite 9.2N06
30
28
26
24
22
FeO 20 MgO
compound wt% compound Al2O3 18 SiO2
16
14
12
10 0 5 10 15 20 number
Phengite 9.2N06
50
40
30 FeO
MgO
Al2O3 compound wt% SiO2
20 K2O
10
0 0 5 10 15 20 number
84