Geothermobarometry and Monazite Geochronology of the
Supracrustal Rocks on South Midøy, Western Gneiss Region, Norway
Doctorandus / MSc Thesis
H.J. Kooijman
June 2006
Structural Geology and Tectonics Department
Faculty of Geosciences
Abstract
The islands Otrøy and Midøy (Moldefjord area, Western Gneiss Region, Norway) consist of (U)HP grt-peridotite and eclogite bearing basement (Basal Gneiss Complex (BGC)), overlain by a tectonic stack of supracrustal rocks (e.g. Carswell and Harvey, 1985). Detailed structural, metamorphic and geochronologic studies are required to investigate the evolution and nature of this nappe complex and to identify its role in the tectonostratigraphy of the Scandinavian Caledonides.
The main objective of this research was to determine the metamorphic grade and metamorphic age of the rocks of the Supracrustal Nappe Complex (SNC) on southern Midøy. Combination of PT estimates and age data resulted in the construction of a metamorphic PTt path. In combination with similar data on the northern basement rocks, the metamorphic character of the SNC rocks provided constraints on the role of the SNC in the Caledonian orogenic evolution. The secondary objective was to determine the metamorphic grade of a coronitic gabbro body within the SNC. Uncertainty existed about whether the body is in-situ or a tectonic lense. Comparison of the PT estimates of the gabbro with estimates from the surrounding SNC rocks provided constraints.
The research required a multidisciplinary approach including: (1) field work for recognition of tectonostratigraphy and rock units, construction of a detailed geological map and sample collection for lab research, (2) light-microscopic analysis for determination of mineralogy and microstructures, (3) quantitative Electron Microprobe (EMP) analysis to determine major element mineral chemistry for classification, zoning studies and geothermobarometry and (4) EMP U-Pb geochronology of metamorphic monazite.
Field study indicated that the SNC on southern Midøy consists of a sequence of metapelitic and metabasic (amphibolitic) gneisses, intruded by numerous basic and felsic melts of host-rock and allochthonous source. Main structures include a compositional banding (S0), a regional general flattening foliation (Sreg), large scale synforms (Sreg+1) and mylonite zones cross-cutting all other structures (Sreg+2). The SNC rocks are equivalent to the Norwegian Blåhø-Surna Nappe (Seve Nappe in Sweden).
A combined PTt path was constructed for the SNC. The path starts at ~600 ˚C and ~8 kbar, increasing through prograde metamorphism to peak conditions in the upper amphibolite facies (~750 ˚C and ~12 kbar). The peak metamorphic mineral assemblage is associated with a monazite age of 450 ± 19 Ma. The start of the retrograde metamorphic path involved the growth of coarse-grained amphibole under upper amphibolite facies conditions. A subsequent deformation event (Freg) caused the formation of the regional foliation (Sreg), which consists of elongated biotite and fine-grained amphibole under middle amphibolite facies conditions. Aligned matrix monazites date the formation of Sreg at 405 Ma ± 18 Ma.
PT conditions of the rock unit at the base of the SNC (the MGA unit) deviate from other lithologies with peak metamorphic conditions in the high-pressure granulite facies. This indicates a different metamorphic history, which could point to a relation of the MGA unit to the BGC. The PT path of the MGA unit overlaps with other SNC paths in the lower amphibolite facies, when juxtaposition to the other SNC rocks occurred.
The PT path of the coronitic gabbro coincides with trajectories of surrounding SNC rocks (excluding the MGA unit). This is in strong support of an in-situ character of the body. Beside the typical SNC metamorphic path, the gabbro yields PT conditions that reflect initial granulite facies intrusion conditions.
The metamorphic event of ~450 Ma can be correlated to the Jämtlandian Orogeny. The metamorphic conditions of the SNC in the Jämtlandian phase correspond to a supracrustal environment in an accretionary system similar to the Seve Nappe terrains in Sweden. The peak at ~405 Ma relates to the Scandian Orogeny, which occurred from 425-390 Ma. The imprint of this orogeny on the SNC mainly concerns retrogression and the origin of middle amphibolite facies compressive structures (Sreg), which are related to the exhumation of the BGC.
1. Introduction
1.1 Research setting
The present research focuses on the Scandinavian Caledonides, which are part of the Caledonian Orogen that exposes in Greenland, Newfoundland, the United Kingdom, Ireland, and the East coast of the United States of America. The Scandinavian Caledonides were formed during the closure of the Iapetus and Ægir oceans, which resulted in the final collision between Baltica and Laurentia. Baltica subducted deeply below Laurentia resulting in (U)HP metamorphism in the Baltic crust, subsequently exposed due to exhumation. The highest (U)HP metamorphic conditions are recorded in the Western Gneiss Region, which represents an excellent region to study (U)HP metamorphism and exhumation processes with numerous garnet-peridotite and eclogite bodies. In the past decades many projects have focused on the Proterozoic basement rocks resulting in various complex geodynamic models (further explained in section 2.5).
Apart from the Proterozoic basement gneisses, a series of heterogeneous cover nappes is present in the Western Gneiss region. The nappes consist of material from the pre-collisional basins of the Iapetus and Ægir oceans that has been stacked towards the east in allochthonous sequences on top of basement rocks and autochthonous sedimentary cover rocks. Metamorphic and geochronologic studies indicate that the nappes have recorded different (peak) metamorphic conditions and that metamorphic and tectonic events often vary in age. The structural and metamorphic relations between the (U)HP basement and the superimposed cover nappes are not completely understood. Many nappes have not yet been placed in the Scandinavian tectonostratigraphy and determination of their place and role in the geodynamic models can provide necessary constraints to the Caledonian orogenic evolution.
Figure 1.1: (a) Geographic map of Norway with location of Western Gneiss Region (WGR) and islands Otrøy and Midøy. (b) Close-up of islands Otrøy and Midøy. (Geological map after Carswell and Harvey, 1985).
Otrøy and Midøy
The research area is located on the islands Otrøy and Midøy in the northern part of the Western Gneiss Region (Moldefjord area) (figure 1.1a). On the northern part of Otrøy UHP minerals were discovered in garnet peridotites (figure 1.1b). Studies of the surrounding mid-proterozoic basement gneisses resulted in recognition of pre-Caledonian granulite facies and Caledonian UHP metamorphic conditions. In contrast, the southern parts of Otrøy and Midøy consist of lower grade supracrustal nappe rocks of presumed late Proterozoic to early Palaeozoic age (figure 1.1b). The exact age and metamorphic grade of the supracrustals, and their relation with the basement rocks remains to be determined.
1.2 Objectives of the research
The research comprises different MSc projects concerning the tectonic contact between basement and supra-crustal rocks, age of the supracrustal and basement rocks, nature of the metamorphism, and structural evolution and deformation conditions of the supracrustal and basement rocks. A PhD project was dedicated to the garnet peridotites. These projects are conducted by various workers. The main objective is to complete the picture of the Caledonian orogenic evolution in this area.
The present study focuses on the island Midøy. The main objectives are:
• To determine the metamorphic age of the supracrustal rocks of southern Midøy using chemical age dating of monazite. This age is important in correlating the nappe with the tectonostratigraphy of the Scandinavian Caledonides and exhumation models.
• To obtain pressure and temperature estimates of the supracrustal rocks. In combination with the determined ages, a metamorphic PTt path can be constructed. This, in combination with similar data from the northern basement rocks provides constraints on the Caledonian orogenic evolution and exhumation, in particular on the role of the nappe complexes.
• To determine the metamorphic grade of a coronitic gabbro body present within the supracrustal nappe complex on eastern Midøy. Uncertainty exists about whether the body is ‘in-situ’ or a tectonic lense. Comparison of PT estimates with the metamorphic nature of surrounding rocks and basement rocks should provide constraints.
1.3 Research approach
In July 2004 a geological mapping project was carried out by a team of five workers on the islands Midøy and Otrøy, Moldefjord area, Norway. The project was supervised by H.L.M. van Roermund, Utrecht University, the Netherlands. Detailed (1:5.000) geological maps were constructed of the supracrustal nappe complex on the southern sides of both islands including the contact with the northern Basement Complex. In addition, cross-sections through the study area were constructed and samples were taken for lab analysis. Results of the field research are presented in chapter 3.
From Midøy island, 25 samples are selected for further research, based on lithology and location in the field. Thin sections (30μ thick) are made of the samples and studied by light microscopy techniques to obtain information on deformation microstructures and metamorphic fabric and mineral assemblages.
The faculty of Earth Sciences in Utrecht is equipped with a JEOL JXA-8600 Superprobe, which was subsequently used for detailed mineral analysis. Mineral chemistries are measured and geothermobarometry applied to obtain (peak) metamorphic conditions. Internal variations in minerals provided information on prograde or retrograde formation. In addition, the electron microprobe is used for U-Th-P geochronology of metamorphic monazite. Results of geothermobarometry and geochronology are presented in chapters 4 and 5, respectively.
2. Regional Geology
2.1 Scandinavian Caledonides
The Scandinavian Caledonides were formed during the closure of the Iapetus and Aegir oceans, resulting in the collision between Baltica in the east and Laurentia in the west. Final closure of the Iapetus ocean during the Scandian Orogeny (420-400 Ma) resulted in various nappe complexes that were thrusted towards the east over the Fennoscandian basement and autochthonous sedimentary cover rocks, illustrated in the terrane map of figure 2.1. The total tectonostratigraphy of the nappe complexes and basement rocks comprises five major units (Roberts and Gee, 1985), which are from base to top, the Basement Autochthon, the Lower Allochthon, Middle Allochthon, Upper Allochthon and Uppermost Allochthon (figure 2.1). Each nappe by itself is also a composite nappe complex.
2.1.1 The Basement Allochthon
The basement primarily consists of gneiss complexes of igneous origin. Protolith compositions range from fully basic to granitic and crystallisation ages yield Archean in the northern Scandinavian Caledonides and Baltic Shield to Sveconorwegian in the south (Gorbatchev, 1985). These age ranges are considered periods of widespread thermal instability. Orogenic components and tectonics of these phases are badly constrained. Basal gneisses expose as tectonic windows along the axis of the Scandinavian orogen. Basement gneisses to the west of the axis are highly deformed and metamorphosed to (U)HP conditions (Griffin et al., 1985). The character of the (U)HP metamorphic imprint will be discussed in a latter section of this chapter.
The Archean-Sveconorwegian basement contains a thin orogen-parallel sequence of sedimentary rocks along the frontal décollement of the orogenic axis. These sedimentary rocks are weakly deformed and consist of sand- and limestones deposited in Ordovician and Cambrian times (Roberts and Gee, 1985).
2.1.2 The Lower Allochthon
The Lower Allochthon nappe complex is a composite terrain exposed along the orogenic axis. The composite consists of Precambrian crystalline rocks and sedimentary rocks of mostly Ordovician and Cambrian age. The separate lithologies of the Lower Allochthon are deformed and displaced into tectonic lenses and duplex structures. Lithological similarities indicate that the Lower Allochthon sequences are tectonically processed basement lithologies (Roberts and Gee, 1985).
2.1.3 The Middle Allochthon
The Middle Allochthon is a tectonic composite of disrupted Vendian and Cambrian sedimentary sequences and Precambrian crystalline rocks. The composite terrane is heavily intruded by Proterozoic plutons and doleritic dykes. The Middle Allochthon successions are sheared and thinned into discontinuous and highly deformed beds in regions of intense strain. Most strain is accommodated in the Caledonian and surfaces in Middle Allochthon exposures west or along the orogenic axis.
2.1.4 The Upper Allochthon
Seve nappe (Roberts and Gee, 1985). The Köli nappe consists purely of marine sediments and ophiolitic fragments (intruded sediments, dolerites, ultramafics). The Köli rocks are typical arc-volcanic complexes.
2.2 Orogenic Events
The complicated structure of the Scandinavian Caledonides is built when convergent movements caused the Iapetus and Ægir oceans to close. These closures resulted in ocean-continent and continent-continent collisions and subsequent subduction and eduction of crustal terranes. The presence of (U)HP metamorphic terranes on the surface indicates that the subducted terranes have educted again and were placed onto the Baltoscandian margin. At least three events are recognised that resulted in HP and UHP metamorphism and the introduction of peridotite bodies into the crust: the Finnmarkian orogeny at ~500 Ma, the Taconian / Jämtlandian orogeny at ~450 Ma and the Scandian orogeny from 420-400 Ma (Brueckner and Van Roermund, 2004). It is important to note that geological processes are diachronous. As nappe complexes may be traced over hundreds of kilometres, a specific metamorphic event may vary in age laterally (Roberts, 2003).
The Pre-Finnmarkian (>500 Ma) situation is presented in figure 2.2. Brueckner and Van Roermund (2004) propose that prior to the Finnmarkian orogeny, Iapetus/Ægir was a contracting, complex ocean system with, from east to west, the components: A, Baltica; B, an ocean of unknown width similar to that between Australia and New Zealand today; C, a microcontinent similar to the South Island of New Zealand that had previously rifted off from the western edge of Baltica; D, eastern Iapetus (and/or the Ægir Sea); E, an island arc system; F, western Iapetus; and G, Laurentia (figure 2.2). Fossil evidence indicates components A–C have Baltoscandian affinities, whereas components E–G have Laurentian affinities (Bruton and Harper, 1988; Pedersen et al., 1992; Roberts and Stephens, 2000). Component D is of uncertain affinity (Brueckner and Van Roermund, 2004).
Figure 2.2: Pre-Finnmarkian (>500 Ma) configuration of the Iapetus domain (Brueckner and Van Roermund, 2004).
2.2.1 The Finnmarkian Orogeny
Figure 2.3: The Finnmarkian Orogeny (at circa 500 Ma) according to Brueckner and Van Roermund (2004).
2.2.2 The Jämtlandian Orogeny
After the Finnmarkian orogeny, the ocean between the VNCT and Baltica (component B in figure 2.2) closed as the result of subduction along a west dipping subduction zone beneath the VNCT (figure 2.3). Melts resulting from the subduction intruded within the VNCT, causing igneous rocks to occur within the lower Köli Nappe complex. As the ocean (component B, figure 2.2) closed, the VNCT collided with Baltica, a collision called the Jämtlandian orogeny (Brueckner and Van Roermund, 2004). The thin western edge of Baltica was pulled into the mantle by the oceanic crust at ~450 Ma (figure 2.4a). The Baltic host rocks were metamorphosed under eclogite facies conditions and mantle peridotite bodies were emplaced. During the eductive phase of the Jämtlandian orogeny, the subducted Baltic margin delaminated from the underlying mantle and was buoyantly exhumed and thrusted westward over the inland part of Baltica. The Jämtlandian orogeny appears to have occurred largely along central Scandinavia.
Figure 2.4: Jämtlandian (a) and Scandian (b) orogenies according to Brueckner and Van Roermund (2004).
a)
2.2.3 The Scandian Orogeny
The Scandian Orogeny (425-390 Ma) is classically identified as the final closure of Iapetus resulting in the continental collision between Baltica and Laurentia. This orogenic phase is preceded and accompanied by renewed hinterland thrusting and tectonic block reorganisation. During the Scandian, parts of the Baltic crust were subducted underneath Laurentia resulting in (U)HPM at depth (figure 2.4b). During subduction, the Baltic terrain was enriched with fragments from the hanging wall of the subduction system. Most of these peridotite bodies display secondary garnet-bearing assemblages that indicate subduction after introduction. This concept is elaborated by Brueckner and Van Roermund (2004).
Subduction stagnated at the delamination of negatively buoyant oceanic crust. The largely metastable Baltic terrain educted via detachment zones and underwent retrogression through the lower eclogite, granulite and upper amphibolite facies. Under the driving forces of the educting terrain and orogenic stack instability, the collision zone ultimately collapsed resulting in slip on orogen-scale detachment zones (e.g. Nordfjord-Sogn detachment, Møre-Trøndelag Fault, etc.). Most of these zones acted as listric fault systems accommodating Old Red Sandstone sediment accumulation.
The Scandian affected the outward tips of the Baltic and Laurentic protoplates and the earlier Caledonian rocks in between. Metamorphic and deformational imprints of this phase are recorded in the complete allochthon stack, in remobilised Baltic and Laurentic crust, and in basement exposures in the vicinity of the orogenic front.
2.3 Western Gneiss Region
2.3.1 GeologyThe Western Gneiss Region (WGR) (figure 1.1a) occupies an area of about 50.000 km2. The terrain is composed of a variety of nappes of heterogeneous metamorphic grades lying on top of the Proterozoic basement rocks, subsequently folded into (U)HP remobilised Proterozoic basement rocks of the WGR. The (U)HP basement rocks of the WGR are the high grade metamorphic equivalents of crystalline basement rocks defining the Baltic Shield. Metamorphism of these rocks occurred when they were forced downward during the Scandian Orogeny.
The (U)HP basement terrain displays a discontinuous increase in deformational and metamorphic grade from the southeast to the northwest. Deformation grades vary from weakly foliated to highly folded, overturned and highly foliated. Metamorphic temperatures vary from 600
º
C in the southeast of the WGR to 800º
C in the northwest (figure 2.5b). Metamorphic pressures range from high pressure in the Sognefjord (in the south, figure 2.5a) towards coesite / microdiamond stability conditions in the UHP zone at Stadlandet (Nordfjord) / Fjørtoft (Nordøyane), both north-western WGR (figure 2.5a).(U)HP metamorphism in the north-western WGR is expressed by relicts of (U)HP minerals in eclogite and garnet-peridotite bodies, scattered across the basement exposure. Most basement gneisses are fully recrystallised under amphibolite facies conditions, but locally (U)HP minerals are also found in basement gneisses. Most eclogite and peridotite bodies have acted more resistant to regrogression.
2.3.2 Eclogites
Eclogites in the north-western WGR occur as country-rock and internal eclogites. These terms refer to eclogite field relation with respect to corresponding garnet-peridotites. The eclogites show complex multigenerational fold structures, lineations and foliations, hinting to a complex tectonic history. Many eclogites of the north-western WGR have remained remarkably fresh. Retrogression is highly localised, presumably because of low fluid mobility.
Figure 2.5: (a) Generalised geological map of the Western Gneiss Region between the Sognefjord and Moldefjord areas with occurrences of UHP eclogites, diamond gneiss and peridotite bodies (Carswell and Cuthbert, 2003). (b) Regional temperature gradient across the WGR based on Fe2+/Mg2+ partitioning
between garnet and omphacite in eclogites (after Griffin et al., 1985) plus indications of a pressure gradient based on PT estimates for eclogite samples from various localities (Cuthbert et al., 2000; Carswell et al., 2003a; Terry et al., 2000b).
a)
2.3.3 Garnet-peridotites
Peridotites in the WGR are garnet-peridotites or retrograde chlorite-bearing dunitic equivalents. The bodies occur as discontinuous bodies, supposedly included into the subducting host gneiss complex through “sinking intrusion” mechanisms as posed by Brueckner (1998) and Brueckner and Van Roermund (2004). Observations on prograde rims in secondary garnet assemblages indicate that some peridotites have experienced reburial, which is as expected from the above introduction mechanism.
Some garnet-peridotites contain microdiamonds (e.g., at Bardane on Fjørtoft (Van Roermund et al., 2002) (figure 2.5a). These minerals are presumably precipitates from fluxes of metasomatic fluids percolating through the host gneisses. These minerals are in equilibrium with secondary garnet assemblages ascribed to the Scandian phase (Carswell and Van Roermund, 2005).
Age dating has shown that the garnet peridotites are mantle fragments that underwent different stages of melting, refertilisation and depletion from as early as the Archean (Brueckner et al., 2002; Spengler et al., 2006). Majoritic components of garnet, reconstructed from excess silica, now exsolved as clino- and orthopyroxene needles in garnet, hints to derivation from great depth (up to 375 km (Van Roermund and Drury, 1998; Spengler et al., 2004)) before incorporation into the Caledonian orogenic system.
2.4 Metamorphic and structural history of Moldefjord area
In the past decades many research groups have worked in the Moldefjord area, Western Gneiss Region, Norway. These studies were focussed on the geological characteristics of the UHP basement, its relation to bounding supracrustal complexes and the garnet peridotites included in basement rocks (Carswell, 1968, 1973). Prime studies are listed and discussed below.
2.4.1 Carswell and Harvey (1985)
Carswell and Harvey (1985) classified entire Midøy (and Otrøy) as a basement terrain. Lithologies consist of augen gneiss, heterogeneous paragneiss, migmatic amphibolites and metapelites, peridotites, eclogites and metadoleritic amphibolites.
The study of Carswell and Harvey includes a tectonometamorphic history of the basement terrain. This history includes four prime phases. Phase (1) is the creation of felsic igneous protoliths. This plutonic event was dated at 1478 ± 42 Ma. Phase (2) occurred a few hundred Ma later (in the Sveconorwegian Phase (1200-1000 Ma) and included basic intrusion and associated wall-rock melting of the igneous province. Phase (3) includes Caledonian metamorphism under conditions of T = 750 ºC / P = 17-21 kbar. Late Caledonian flattening, foliation, shearing and pegmatite intrusion defines phase (4). Now UHP metamorphism should be correlated with phase (3) metamorphism. (U)HP peak conditions are most likely much more extreme than metamorphic conditions estimated by Carswell and Harvey (1985) (Carswell et al., in press), although the possibility exists that metastability inhibited phase transitions and recording of UHP metamorphism.
2.4.2 Robinson (1995)
The study of Robinson (1995) has a structural and tectonostratigraphic approach to the research area. Robinson (1995) subdivided the islands Midøy and Otrøy into two prime tectonostratigraphic units: the (U)HP basement and a stack of undifferentiated supracrustal rocks. The overlying supracrustals on Midøy are correlated to the Blåhø-Surna nappe complex (affirmed in (Robinson et al., 2003)), a thinned equivalent of the Essandsjø nappe complex (Kollung, 1990), which is part of the Seve Nappe. Metamorphic grades of these rocks reach into the upper amphibolite facies.
Mylonite at Nordøyane. The high strain zone on the south of Midøy is an identical feature and corresponds to this generation of shear zones.
2.4.3 Terry and Robinson (2003; 2004)
These studies are based upon research on the Nordøyane district (islands Haramsøy, Fjørtoft, Flemsøy, etc., all islands in Nordøyane which is shown in figure 2.5). The studies recognise deformation structures of different metamorphic grades. High-grade deformation structures in the basement, such as eclogite facies shear zones and UHP mineral lineations are correlated with structures within peridotites and eclogites. Shape lineations, asymmetric fabrics and foliations from the upper staurolite zone of the amphibolite facies overprinted these high-grade structures and obliterated much of their record. These upper amphibolite facies structures are also found in cover sequences. All upper amphibolite structures are overprinted by late stage open folding, mylonitisation and the formation of sheath and tabular folds and greenschist facies shape lineations.
Terry and Robinson (2003; 2004) identified a period of deep subduction of Baltic crust, constrained in the Scandian by age determinations in (Terry et al., 2000; Carswell et al., 2003). Deep subduction was accompanied by the activity of eclogite facies shear zones (i.e. on Haramsøy) and is reflected by high-grade mineral lineations. Syncollisional exhumation of the (U)HP terrain has lead to the juxtaposition of tectonostratigraphic units and the formation of amphibolite facies deformation structures. The exhumation of the (U)HPM terrain is thought to have been accompanied by the rupture of the (U)HPM crust into separate nappe units (most contacts not found in the field and for instance discredited by subsequent PT analysis (Carswell et al., in press)). The formation of these basement slices progressed plate inward from the suture zone and culminated in the formation of a massive basal thrust. This contact is envisioned to have deflected into the mantle. Sections of the mantle were transferred through this contact zone into the basal gneiss terrain. During the exhumation and rupture of the UHP basement, the overthickened orogenic stack collapsed. This lead to the low-grade deformation record as found in supracrustal nappes and basement.
2.4.4 Wiggers-de Vries (2004), Van Straaten (2004) and Smit (2006)
These studies included detailed multidisciplinary research on the islands of Midøy and Otrøy. The (2004) studies were focussed on the tectonometamorphic evolution of the basement gneiss complex. Wiggers-de vries provided Scandian (U)HP conditions from external bi-mineralic and opx-eclogites. The opx-eclogites yielded UHPM conditions at around 45 kbar. Progressive retrogression was accompanied by the formation of foliations and lineations. The geodynamic significance of these structures is not (yet) retrieved.
Van Straaten (2004) also found a peculiar garnet granulite facies metamorphic path. This grt-granulite facies metamorphic path is ascribed to the initial melt crystallisation of the igneous protolith, although this not microstructurally constrained.
The study of Smit (2006) is focussed upon establishing metamorphic PTtD paths of the supracrustal complex on Otrøy. Subsequently his results are contrasted to the basement in the north. The multidisciplinary study on the supracrustals affirms earlier diagnoses that these rocks on Midøy and Otrøy are part of the Blåhø-Surna nappe complex. These rocks show petrochemical, microstructural and geochronologic similarities to amphibolite facies Seve terrains elsewhere in Norway and in Sweden. The study also indicates that there is a third (and perhaps a fourth) tectonostratigraphic unit on the islands, which is (are) part of the basement or basement-related allochthons.
2.5 Geodynamic models for the evolution of Iapetus
The studied supracrustal nappe complex is believed to have recorded parts of the evolution of the Iapetus (related) domains. Three recent models are relevant to this evolution: (Roberts, 2003; Brueckner and Van Roermund, 2004 and Hacker and Gans, 2005). These models are briefly discussed in this section.
2.5.1 Roberts (2003)
Roberts compiled stratigraphic, metamorphic and geochronologic data into a geodynamic model that incorporates four major Caledonian tectonothermal / tectonometamorphic phases: (1) The Finnmarkian (~500 Ma), (2) The Trondheim Event (~475 Ma), (3) The Taconian (~450 Ma) and (4) The Scandian (~420-400 Ma). The phases are clarified in figure 2.4a to d.
Metamorphism dated at ~500 Ma (Finnmarkian, northern Seve nappes, figure 2.4a) is envisioned to mark a subductional phase in the northern Iapetus oceanic domain (Ægir Ocean). The phase included deep westward subduction and rapid exhumation of marine terrains. The composite terrain contains metapelitic and meta-ophiolitic lithologies of the Seve and lower Köli nappes.
The Trondheim event or Trondheim disturbance is seen as a plutonic period, which largely affected tectonostratigraphic units and stacks in the Trondheim area (Seve and Köli terrains, figure 2.4b). The Taconian orogenic phase as seen by Roberts (2003) affected the Laurentia-affinitive Uppermost Allochthon by HP metamorphism and deformation (figure 2.4c). Subduction in an easterly direction is thought to have buried large fractions of granulitic material of this nappe. The Taconian imprint is prominent in Laurentic terrains (with Laurentic fossil records), for instance in the Greenland Caledonides, Newfoundland and northern Norway.
Subsequently, the complete orogenic stack is processed during the main tectonometamorphic phase in the Scandinavian Caledonides: the Scandian (figure 2.4d). Large amounts of Baltic crust were subducted under the influence of propagating Laurentia. Nappe stacks are reshuffled, deformed and ramped onto the Baltic margin. After delamination of the oceanic slab, the metastable positively buoyant Baltic crust is educted into the orogenic wedge, which collapsed instantaneously.
2.5.2 Brueckner and Van Roermund (2004)
A very feasible reconstruction of the pre-Scandian Iapetus/Ægir evolution is provided by the Dunk Tectonics paper of Brueckner and Van Roermund (2004). This paper acknowledges at least four tectonometamorphic (subductive) phases throughout the Caledonian period: (1) The Finnmarkian (~500 Ma, figure 2.2b), (2) the Jämtlandian (~455 Ma, figure 2.3a), (3) The Taconian (~450 Ma) and (4) The Scandian (410 Ma, figure 2.3b). The Finnmarkian phase is recognised in a similar fashion as described by Roberts (2003). Metapelites, oceanic volcanites and related basement units of the Seve Nappe are subducted beneath an island arc with present-day Köli greenschists. After rapid exhumation (eduction), the composite terrain functions as an island arc system.
Eastern Iapetus terrains (component B in figure 2.2b) are subducted until the island arc composite is ramped onto the Baltic margin. Westward tips of the Baltic margin are subducted to HP depth to form the eclogites and related HP rocks of the Jämtland Seve nappe. This phase is marked as the “Jämtlandian” phase; a newly discovered HP metamorphic event, recognisable throughout the central and southern Seve nappe complex. Contemporary with this phase, Uppermost Allochthon granulites are subducted in an oppositely vergent subduction zone at the other side of the western Iapetus. Brueckner and Van Roermund (2004) emphasise that synchronicity of the Taconian and Jämtlandian phases holds no significance. The events occurred in different parts of the Iapetus Ocean.
Finale closure of the Iapetus Ocean during the Scandian (figure 2.3b) is accompanied by deep westward subduction of large masses of Baltic crust underneath Laurentia. Parts of the Baltic terrain educted rapidly after the delamination of the oceanic slab. Exhumation is achieved in a two-way-street fashion where the subducted plate is wrenched between a bounding normal and thrust fault and educted approximately the same way as it went down. This mechanism (“Dunk Tectonics”) is proposed to explain the massive exhumation rates as deduced from geochronologic and petrochemical studies.
As a useful addition to the highly credible geodynamic reconstruction of Brueckner and Van Roermund (2004), Smit (2006) proposed a mechanism to explain the geochronologic information deduced from the Bergen Arcs (Lindås nappe) anothosites and basement terrains near Solund, Råna and Vestranden. These basement-related granulites reflect HP metamorphism prior to the Scandian (in the 435-425 Ma period). It is proposed that these are fragments of the subducting Baltic terrain that decoupled from the subducting terrain under the influence of deformation in the upper levels of the slab. The fragments rapidly exhumed under the influence of their own positive buoyancy, while the parental slab subducted further into the UHP metamorphic field.
2.5.3 Hacker and Gans (2005)
The geodynamic evolution of Hacker and Gans (2005) for the Trondheim area proposes four major tectonometamorphic phases: (1) A phase of early subduction of Seve and Köli terrains (480-470 Ma, figure 2.5a), (2) subduction of central (HP) and southern (upper amphibolite facies) Seve terrains (~450 Ma, figure 2.5b), (3) pre-Scandian basement subduction and HP metamorphism (435-415 Ma, figure 2.5d) and (4) The Scandian phase (410-400 Ma, figure 2.5e). The phase between 445 and 432 Ma (figure 2.5c0 is seen a magmatic phase represented by large scale volcanism and the formation of ophiolites.
3. Description of the study area
3.1 Introduction
In July 2004 field work was carried out on the islands of Midøy and Otrøy, Moldefjord area, Norway. The research was focussed on detailed mapping of the unidentified nappe complex on the southern sides of both islands including the contact with the northern Basement Complex. In addition, cross-sections through the study area were constructed and samples were taken for lab analysis.
The islands cover approximately 150 m2 and have maximum altitudes of ~800 m (Otrøy) and ~500 m (Midøy). The mountains, which have steep cliffs (figure 3.1), are predominantly present on the southern parts of both islands. The origin of the island morphology relates to the last ice age, which created the typical fjord landscape. Most parts of the islands are reasonably well accessible, despite the absence of paths in some high parts of the area. The parts along the sea are flat, but covered in swamps and vegetation, which make outcrops scarce (figure 3.1).
Several geological maps of the islands were made in the past, but the scale of the maps is large and differences exist between the maps. The peridotites and surrounding mid-Proterozoic ultrahigh pressure gneisses of the Basement Complex have recently been studied and mapped by the Structural Geology Group, Utrecht University, supervised by dr. H.L.M. van Roermund and dr. M.R. Drury. An overview of past mapping studies on Otrøy and Midøy is provided in section 3.2.
In sections 3.4 to 3.6 results of the current field research are presented including presentation of the geological maps (3.4), descriptions of the lithological units (3.5) found in the area (lithostratigraphic column), and description of the main structures (cross-sections) (3.6).
3.2 Past mapping studies
Figure 3.2 shows an overview of different geological maps of the Otrøy/Midøy area. In 1985 a mapping study by Carswell and Harvey was presented (figure 3.2a). Five lithologies were distinguished in the research area: the Augen Orthogneiss Complex, the Heterogeneous Paragneiss Complex, peridotites, eclogites and metadolerites. They assumed that all lithologies were part of the Western Gneiss Region basal gneiss complex and did not distinguish between the gneisses in the north and those in the south.
Mørk (1989) constructed a geological map (scale 1:50,000) of Midøy and Western Otrøy, which was reproduced by Wiggers - de Vries (2004) (figure 3.2b). In contrast to Carswell and Harvey, Mørk made a distinction between the tectonic stack of paragneisses in the south and the basal gneiss complex in the north, but did not define a tectonic contact. The map contains more detail on the northern basal gneiss complex, which contains eclogites and ultramafic rocks. The southern paragneiss complex is described as a sequence of micaschists, mylonitic gneiss, garnet amphibolite gneiss and banded amphibolitic and quartzofeldspathic gneiss, crosscut by pegmatites.
Tveten et al. (1998) created a geological map (scale 1:250,000) of the Ålesund area, which is provided by the Norwegian Geological Survey (NGU). The part of the map including Otrøy and Midøy is shown in figure 3.2d. The geological map indicates that both islands predominantly consist of basal gneisses, except
Figure 3.2: Geological maps of (a) Carswell and Harvey, 1985; (b) Mørk, 1989 (modified by Wiggers-deVries, 2004); (c) Wiggers-de Vries,, Van Straaten, 2004; (d) Tveten et al., 1998.
a)
b) c)
for the blue and brown parts, which indicate supracrustal sequences. In contrast to the other maps, extensive fault structures are present that cross-cut both islands.
The geological map of Van Straaten and Wiggers – de Vries (2004) shows the western part of Otrøy (figure 3.2c). The northern basement lithologies are classified as a sequence of quartzofeldspathic and amphibolitic gneiss units with bodies of eclogite and garnet peridotite. This largely coincides with the maps of Carswell and Harvey (1985) and Mørk (1989). The southern supracrustal rocks are described as amphibolitic and coarse amphibolitic gneiss units, which display a circular pattern. The contact between northern and southern units is classified as a broad mylonite zone. More detailed information on the mapping studies can be found in referred articles.
3.3 Field methods
The field mapping project was done in cooperation with M.A. Smit, J. Linckens, W.L. van Mierlo, en H.W. de Goede. The first collective field days were used to investigate the various lithologies in the area and to make agreements on names and colour coding. After that, separate teams were made to speed up the mapping process. Teams as well as mapping locations were changed daily to make sure everyone got a clear image of the entire study area.
Silva compasses were used to measure planar and linear structures in the field. Mapping was done on 1:5,000 topographic maps provided by the Norwegian Statens Kartverk. On these maps a local Norwegian grid was present, which had a 2 degree west deviation from the magnetic north as was confirmed by field measurements. This declination was used when putting foliation orientations on the map. For global reference a UTM WGS 84 coordinate system was added to the maps in a later stage. The field area is situated in UTM zone 32N, which required a seven degree clockwise rotation (subtraction of 7 degrees) of measured orientations.
Various field trajectories were studied perpendicular to the main foliation, which enabled construction of north-south profiles. Observations and descriptions were supplemented with photographs of striking structures and lithologies. In addition to the mapping, samples were taken from different lithologies for geothermobarometry and dating.
3.4 Geological maps
Lithological and structural variations occurred on cm-scale, which made it impossible to make a detailed geological map of the entire area. Therefore, it was decided to make a 1:20,000 overview map of both islands, and 1:5,000 detailed maps of areas that showed various lithologies on small (10 m-) scale and/or remarkable structures. This way the overview map could be completed with detailed information. The 1:20,000 overview map was constructed by enlargement and combination of the 1:50,000 maps of Bratvag and Vestnes. Detailed mapping was performed on 1:5,000 maps provided by the Norwegian Geological Survey (NGU).
3.4.1 Overview map
The overview map (figure 3.3 and appendix A1) shows the position of the Supracrustal Nappe Complex (SNC) with respect to the Basal Gneiss Complex (BGC). BGC lithologies were constructed using data of Carswell and Harvey (1985), Carswell et al. (2006) and Spengler (2006). The northern high strain zone (NHSZ) that defines the contact between the BGC and the SNC is striped on the map. The thickness of this zone is variable throughout the area.
Midøy (Drynjasund) (figure 3.3). Scattered over the study area the largest pegmatite dykes have been indicated. Two striking kyanite-bearing metapelitic gneiss banks, which could be traced in most parts of the area, were used as marker units. These banks are probably continuous, but were only mapped where observed in the field. The southern high strain zone (SHSZ) is also indicated by a striped pattern. On the easternmost part of the studied area a marked different lithology is found south of the SNC, which could indicate the start of a different nappe complex. Note that the SHSZ continues in this lithology. The locality of the main rock units and above mentioned villages are illustrated in figure 3.3 and appendix A1.
Figure 3.3: Geological overview map of Midøy and Otrøy including position of Basal Gneiss Complex (BGC) and Supracrustal Nappe Complex (SNC), the Northern High Strain Zone (NHSZ) and Southern High Strain Zone (SHSZ), locations of Midsundet, Magerøy and Raknestangen eclogite bodies, Breivik and Lomtjern retro-eclogite pods, granite sheets at Orset, granite body at Bløvatnet, coronitic gabbro at Drynjasund, and Rauthaugane, Ugelvik and Midsundvatnet peridotite bodies (geological map by Smit et al., 2006). Legend and details can be viewed in appendix A1.
Representative orientations of the regional foliation are indicated by strike-dip signs. Local dip variations are common, but dips on the map are considered representative of the average dip. Characteristics and variations in strike and dip will be further discussed in section 3.6.
3.4.2 Detailed maps
Seven different detailed (scale 1:5,000) geological maps were constructed to enlighten local lithological and structural variations. Three locations were chosen on Otrøy, and four on Midøy. Only the Midøy maps are presented in this study (appendix A2a-d). Details from Otrøy can be obtained from the Master thesis of M.A. Smit (2006).
The areas on Midøy selected for detailed mapping are the west coast near Drynjasund (appendix A2a), the southwest area near Rossfjellet (appendix A2b), the elevated area of Høgfjellet (appendix A2c), and the Ramsberget-Høgset area including the island Midsundholmen (appendix A2d). Exact locations of
these areas are shown in appendix A1. The maps show smaller scale lithological variations taking into account specific characteristics including the quartz content of a micagneiss and the mica content of amphibolites. Note on the Høgfjellet map that the distinct exposure pattern of the lithologies is mainly caused by variations in elevation combined with layer orientations. On the Rossfjellet map, the SHSZ seems to be confined to a single lithology. This is not per definition the case, but severe mylonitisation changed the main characteristics of the rocks, which were therefore given another colour on the map.
3.5 Lithostratigraphy
The BGC on northern Otrøy and Midøy is well studied by various workers (e.g. Carswell and Harvey, 1985; Mørk, 1989; Van Straaten, 2004). Therefore, in this field research the complex was not studied in great detail. For reference a short overview of characteristics is provided in this section. In contrast, the supracrustal nappe complex in the south was studied in great detail, because little was known about the characteristics of these rocks and their place in the tectonostratigraphy of the Scandinavian Caledonides. Regional correlation with the tectonostratigraphy present in the Scandinavian Nappes will be made in section 4.7 after petrographic study of the rocks.
A lithostratigraphic column was constructed to illustrate the sequence of lithologies (figure 3.4). It is important to note that laterally the lithologies may vary in thickness. Some are discontinuous and wedge out entirely. In addition, the presence or absence of intrusive bodies varies throughout the area. The units of this column will be described in detail below.
3.5.1 Basal Gneiss Complex
The BGC mainly consists of orthogneisses with a characteristic Augen gneiss texture as defined by Carswell and Harvey (1985). The gneisses predominantly consist of K-feldspar, (which gives them a reddish appearance), quartz, plagioclase and some amphibole ± biotite. Garnet is only present in highly deformed parts of the gneisses (Van Straaten, 2004).
In addition to the felsic gneisses, amphibolitic gneisses are present, which consist of amphibole, brown mica, garnet and some K-feldspar. Locally migmatites and leucosomes are present with increased quartz and feldspar content. The amphibolitic gneisses and orthogneisses are both intruded by pegmatites and small-scale granitic bodies that crosscut the lithological layering.
On the island of Otrøy three garnet peridotite bodies are present within the gneiss sequences (Rauthaugane, Ugelvik and Midsundvatnet). Their locations can be viewed in figure 3.3. In the field they can be easily recognised by their caramel-coloured appearance, caused by weathering of olivine. The garnet peridotites mainly consist of olivine, ortho- and clinopyroxene, garnet, and spinel. Furthermore, they contain garnet aggregates, megacrysts, and pyroxenites (Carswell, 1973).
In the Otrøy basement gneisses many (retro-)eclogites are present as dark green pods enclosed by the deformed amphibolite facies gneisses. Most of them are elongated in the foliation direction. Well-known eclogites are the bi-mineralic Midsundet eclogite body, the Magerøy and Raknestangen opx-eclogite bodies and the retro-eclogite pods at Breivik and Lomtjern (figure 3.3). The latter two are recently studied by Smit (2006). Small eclogite pods are often fully retrograded to garnet amphibolites.
3.5.2 Supracrustal Nappe Complex
High strain zone
Quartzitic gneiss with minor mica and amphibolitic gneiss Amphibolitic gneiss with felsic melt contamination Quartzitic gneiss with mica content
Amphibolitic gneiss with quartzitig gneiss intercallation Pegmatite body
Coronitic gabbro body Kyanite micagneiss marker unit Micaschist and –gneiss (Grt) Amphibolite gneiss (Retro-)eclogite
Granitic Augen gneiss with (retro-)eclogite boudins Migmatic amphibolitic gneiss with (retro-)eclogite boudins
Figure 3.4: Lithostratigraphic column based on the lithological sequence of western Midøy. White numbers indicate where samples were taken.
Metapelitic gneisses
The most common rocks on the islands Otrøy and Midøy are metapelitic schists and gneisses (appendix A2). Their modal abundance increases towards the south of the area. The unit occurs as bands of different size up to several meters thickness. The rocks are strongly foliated and show a characteristic reddish-brown weathering colour. In the southernmost area strong mylonitisation has taken place.
The mineral content includes brown mica, quartz, plagioclase, garnet, and some minor amphibole. Quartz occurs in strings surrounded by elongated biotite. Garnet porphyroclasts occur throughout the unit, sizes ranging up to 1 cm. On the island Midøy white mica has been found in some places. Locally high concentrations of kyanite are found with crystal sizes up to 1 cm (figure 3.5). These banks are traceable throughout the study area and are used as a marker unit during mapping.
Locally metapelitic gneiss bands alternate with quartzofeldspathic gneiss layers. These are especially well exposed at the southern coastal areas, where some bands classify as quartzites with only traces of mica. The quartz-rich bands are more massive than mica-quartz-rich bands and show less distinct weathering.
Locally metapelitic gneisses are enriched in amphibole, which gives them a green appearance. These amphibole enriched gneisses mainly consist of brown mica, amphibole, and plagioclase (dioritic composition). The rock type is predominantly found at outer margins of metapelites in contact with amphibolitic rocks and is the result of metasomatism. Samples were taken from all types of metapelitic gneisses for geochemical analysis and monazite dating.
Amphibolites
In addition to metapelitic gneisses, amphibolitic gneisses are present in the supracrustal nappe complex. Main constituents of these gneisses are amphibole and plagioclase. Locally, garnet is present and some minor brown mica may occur. Gneisses contain > 50% amphibole. The amphibolitic gneisses show very strong alternation with metapelitic gneisses, on m- to dm-scale. This made separate mapping of the two lithologies on scale 1:20,000 impossible. On the detailed (scale 1:5,000) maps, a distinction was made based on the dominant lithology (section 3.4). The amphibolite gneisses are predominantly orientated parallel to the banding in the metapelitic gneisses (figure 3.6a). A weak foliation is developed. However, irregularly shaped contacts that cross-cut the layering are also observed (figure 3.6b). Along these contacts, the metapelitic gneisses are often enriched in quartz. These quartz-rich contact zones are probably the result of melt extraction caused by temperature increase and fluids that refer to the intrusion of the amphibolites.
Figure 3.6: (a) Foliated amphibolitic gneisses. (b) Igneous contact between amphibolite and quartzofeldspathic gneisses. A quartz-rich margin is visible in the contact zone.
Scattered over the study area amphibolitic dolerite pods are found, embedded in metapelitic gneisses (figure 3.7a). These are especially abundant close to the southern mylonite zone and the northern basement contact zone, where the pods occur as mafic boudins (‘sausage rock’) (figure 3.7b). The doleritic texture is preserved as coarse amphibole grains and randomly orientated feldspars. On south-western Otrøy a dolerite dyke swarm is present. The dolerite dykes experienced severe mylonitisation. Samples were taken from all types of amphibolitic rocks for lab-analysis. The dolerite dyke swarm was studied by M.A. Smit (2006).
Figure 3.7: (a) Amphibolitic dolerite pod embedded in foliated gneisses on eastern Midøy. (b) Boudinaged amphibolitic and quartzofeldspathic gneisses.
MGA gneisses
On the north side of the SNC, adjacent to the NHSZ, a continuous sub-unit of (garnet-) amphibolites is present. This unit corresponds to the Massive Garnet Amphibolites as recognised by Mørk (1989) and is therefore referred to as MGA. The unit has an average thickness of approximately 20 m and could therefore not be mapped on the overview map (appendix A1). Part of the unit can be viewed on the detail map of Drynjasund (appendix A2a). The rocks are more resistant to weathering than SNC gneisses and
10 cm
a) b)
can be traced along the entire basement-nappe contact. Mineral content includes coarse grained amphibole and feldspar. Locally, particularly large garnets (cm-scale) are abundant. Differences with SNC gneisses include larger grain size and the absence of a clear foliation. Samples were taken from different locations in the unit (sample numbers and locations are shown in appendix A3).
Coronitic gabbro body
At Drynjasund on the west side of Midøy a mafic coronitic gabbroic body is present (figure 3.3, appendix A1, appendix A2a). The body is very badly exposed, except for the coastline and a small road outcrop land inwards (appendix A2a). The field evidence is limited, but the body is assumed to have a lens shape. The contact with surrounding metapelitic gneisses is not exposed, which makes it difficult to determine whether the body is in situ or tectonically bounded. Comparison of metamorphic PT estimates with the metamorphic nature of surrounding SNC gneisses could provide constraints.
The main mineral content is plagioclase, clinopyroxene, garnet, amphibole, and biotite. Grain sizes are relatively large and garnet coronas around clinopyroxene and amphibole can be observed with the naked eye. The body is characterised by evident gabbroic textures defined by course grained igneous cumulates. Although the elongated body is orientated parallel to the regional foliation, no foliation is present within the body.
Throughout the massive body mm-thick fractures occur, which are filled with a black coloured glassy material. These fractures may represent pseudotachylites, but further research is necessary to confirm this. The coronitic gabbro body is the only body present on Otrøy and Midøy with these characteristics.
Felsic intrusives
In the supracrustal nappe complex, pegmatite bodies and veins are present of sizes varying from centimetres to tens of meters. A high concentration of large elongated bodies is found on south-western Otrøy. Smaller pegmatites occur throughout the area. The various pegmatites appear as marked white bodies that are resistant to weathering compared to surrounding gneisses. The primary mineral content includes plagioclase, quartz, and k-feldspar. Mica contamination occurs along the edges of most bodies. The average grain size is relatively large (~1 cm) and locally high concentrations of garnet are found. The large elongated bodies often stretch along the regional foliation, but smaller bodies and veins cross-cut the foliation and are often strongly boudinaged as well as folded (figure 3.8a and b). Small veins protruding from larger bodies often follow the present foliation (figure 3.9a).
Figure 3.8: (a) Strongly boudinaged and folded pegmatite. (b) Folded pegmatite that cross-cuts the foliation on south Otrøy.
In addition to pegmatites, granite bodies (10 to 100m scale) are found in the study area. Two distinct granite sheets are present on central Otrøy (near Orset; figure 3.3, appendix A1) and an elongated granite body is found on central Midøy (figure 3.3, appendix A2c). On Otrøy the sheets are orientated parallel to the regional foliation and appear concordant with the surrounding gneisses. The Midøy body is also elongated in this direction, but it does show igneous contacts with surrounding gneisses (figure 3.9b). The composition of the granites is quartzofeldspathic with minor amphibole and the average grain size is coarse (cm-scale).
Figure 3.9: (a) Pegmatitic vein follows the foliation where the granite body cross-cuts the foliation. (b) Granitic melt intrusion in amphibolitic host rocks on Midøy.
3.5.3 Origin of the lithologies
The amphibolitic gneisses and orthogneisses of the BGC are generally assumed to be of the same tectonostratigraphic unit. The orthogneisses originated as crystalline cratonic crust that was intruded by granitic and pegmatitic melt in various stages (Wiggers – de Vries, 2004, Van Straaten, 2004). The amphibolite and mica gneisses are probably remnants of crust injected with gabbroic and dioritic bodies. The (retro-) eclogites are the product of (U)HP metamorphism of basic melt, associated with deep crustal subduction.
Garnet peridotites are derived from the mantle at depths of at least 60 km. Relics of majoritic garnet indicate that the Otrøy peridotites were derived from depths of at least 185 km (Van Roermund and Drury, 1998). Other estimates indicate that depths may exceed 246 km (Van Roermund et al., 2001).
The metapelitic gneisses of the SNC are probably metamorphic products of sediments (protoliths clays and sandstones). The variation in mica and quartz bands may represent original sedimentary layering. The mica-rich rocks then refer to clays and the quartz-rich rocks to sands from higher energetic environments. The presence of amphibole in the metapelites is probably the result of metasomatism, caused by adjacent mafic bodies.
The amphibolitic gneisses are probably metamorphic products of mafic melt intrusion into the metapelitic and quartzofeldspathic sequence. Doleritic textures and igneous contacts with host gneisses confirm this theory. The intrusion of mafic melts may be initiated in a rifting environment. Doleritic textures indicate that intrusion occurred at medium depth.
The pegmatites probably have a leucosome origin, indicated by melanosome characteristics of surrounding gneisses like melt veining and felsic depletion. In cases where pegmatites are orientated parallel to the foliation, leucosome melt probably migrated along the layering. On weak spots in the
sequence melt may have accumulated or cross-cut the layering. The granites on Otrøy and Midøy lack melanosome and melt veining characteristics, which indicates they probably had an external source.
3.6 Structural geology
The most important structures as observed and measured in the field will be described shortly in this section. A short recap on structural geology of the Basal Gneiss Complex will be made to enable comparison with the supracrustal nappe complex.
3.6.1 Foliations
The dominant foliation in BGC is the regional amphibolite facies foliation (Sreg) (studied on Otrøy by Wiggers-de Vries (2004) and Van Straaten (2004)). In Otrøy basement rocks Sreg overprints a diagenetic relict (S0) and a series of deformation structures (S(-6 to –1)) including eclogite facies folds (Terry and Robinson, 2004), N-S lineations, and various foliations. Some pegmatites and granite intrusions are affected by Sreg, others cross-cut Sreg. The regional foliation is recorded throughout the Western Gneiss Region.
In the SNC of Otrøy and Midøy, the regional foliation Sreg represents the dominant structure. Sreg is the only foliation recognised in the field. In the SNC, Sreg is orientated (sub-)parallel to the compositional banding, defined as S0 (or relatively speaking Sreg-1), described in section 3.5. On small scale Sreg is defined by elongated minerals and planar alignment of amphibole, mica, quartz, and feldspar. Porphyroclasts (mainly garnet and feldspar) show cracks perpendicular to Sreg. The lack of a significant rotational component indicates that general flattening occurred. Srge might be a transpositional foliation that overprints older deformation structures. No relicts of such structures are found but the possibility of their former presence is not excluded.
Compositional heterogeneity throughout the nappe complex causes variation in characteristics of Sreg depending on rock type. In metapelitic rocks the foliation is developed best and can be described as an anastomosing zonal gneissosity. In amphibolitic gneisses Sreg is less developed and classifies as a parallel spaced foliation. Most pegmatite and granite bodies in the research area are also affected by Freg and bodies are orientated parallel to the foliation. Some smaller pegmatitic veins cross-cut the foliation.
Some amphibolitic pods as well as amphibolitic gneisses along the basement contact have not recorded any foliation and lack Sreg. In contrast, the metapelitic gneisses in direct contact with these rocks show a well developed Sreg.
The orientations and variations of the regional foliation on scale of the study area can be viewed on the geological maps (appendix A). Stereo plot calculations of the orientations of Sreg are performed by M.A. Smit (2006). The orientations of the Sreg-planes in the SNC is similar to that of the BGC. Three areas show significant deviations from the regional Sreg-plane, including the NHSZ, the SHSZ and the area around the island Midsundholmen (see figure 3.3 for locality). The last deviatory area comprises a variation in strike, which is discussed in detail by M.A. Smit (2006). The two high strain zones will be described in section 3.6.2.
3.6.2 High strain zones
The nature of the NHSZ can be classified as mylonitic over the studied domain (appendix A). The zone is characterised by a very strong decrease in grain size and alternation of compositional banding on dm to cm-scale. Characteristics of the zone vary laterally throughout the area. On east Midøy the NHSZ is ultramylonitic with a homogeneous dark coloured and very fine-grained matrix containing rounded clasts of feldspar. On western Midøy the zone can be classified as cataclastic with mylonitic blocks.
The SHSZ is mylonitic to ultra-mylonitic with distinct compositional alternation on a very small scale (cm). Lateral variations in this zone are not significant. An important characteristic of the zone is the presence of tight and isoclinal folds that fold Sreg and Sreg-1 (figure 3.x). All lithological units in the area are affected by these folds. Similar folds have been observed in the SNC on Otrøy (Smit, 2006) and in the BGC (Wiggers – de Vries, 2004, Van Straaten, 2004). A second important characteristic of the SHSZ is the presence of asymmetric boudinage patterns (figure 3.x). These ‘tilted bookshelf’ structures indicate that boudinage as well as shear occurred. The boudins show Sreg-1 and Sreg tilted with respect to the surrounding rocks (figure 3.10), indicating that mylonitisation (Fmyl) occurred after Freg. In other words, Smyl is Sreg+x.
In the vertical plane, shear sense indicators are predominantly symmetrical. In the horizontal plane the indicated shear sense is predominantly sinistral, but dextral movement senses are also found. This observation indicates that the dominant shear sense was sinistral, i.e. that there was a relative movement of the northern block to the WSW.
The orientation of the NHSZ is sub-parallel to Sreg. The average strike of the SHSZ is 5 to 10° higher (~80º in contrast to the general strike of ~65-70º). Sreg strike smoothly bends into the SHSZ and undergoes a gradual strike increase. The dip of Sreg in high strain zones is significantly larger than the average dip of Sreg elsewhere in the SNC. The variation in Sreg dip is enlightened by five cross-sections from Midøy (appendix B). Locations are indicated on the detailed maps of appendix A. Sreg throughout the nappe has an average dip of 30 to 50° S. Towards the high strain zones the dip of Sreg increases gradually up to a vertical position. Locally a slight north dip is found in the southern high strain zone of Midøy. All the above structural aspects can be retraced in the detailed geological maps in appendix A2.
The similarity in nature and orientation of the high strain zones indicates that these are probably related. Comparable high strain zones in the Moldefjord area are also interpreted to be synchronous (Terry and Robinson, 2003).
3.6.3 Structural inferences
The dominant structure in the SNC is the regional foliation Sreg, which is also present in the BGC under approximately the same orientation. Presence of Sreg in both units indicates that Sreg arose after juxtaposition of the two complexes. Sreg is overprinted by mylonite zones that altered the orientations of this structure. The NHSZ defines the contact between the BGC and SNC.
Structural maps of Robinson (1995) indicate that the regional foliation is folded into deep synclines in the Moldefjord area. This folding postdates tectonic juxtaposition of the complexes and foliation formation. Terry and Robinson (2003) report that the synclines are crosscut by a series of steep east-west trending mylonites (e.g. Brattvåg, Åkre). In (micro)texture and orientation the mylonites are identical to the high strain zones on Otrøy and Midøy. These implications suggest that the NHSZ was reactivated. Initially the NHSZ was active when the nappe was imposed on the BGC. Subsequent cross-cutting of
4. Geothermobarometry
4.1 Introduction
The main scope of geothermobarometry is to reconstruct the metamorphic evolution of each different rock unit. Important questions are whether the units experienced multiple metamorphic events, and whether different units underwent metamorphic events of different grade and at different places in time. In order to find this out, mineral assemblages were analysed and peak metamorphic conditions determined.
To determine the conditions of peak metamorphism, the chemical compositions of different mineral assemblages were analysed. The heterogeneity in the SNC (rock units: metapelites, amphibolitised metabasites and coronitic gabbros) enabled the use of many different geothermo- and geobarometers. This was done to gain most perspective into the metamorphic conditions for the SNC rock units. Results per rock unit are discussed in separate paragraphs, later to be combined and correlated.
4.2 Methods
4.2.1 ApproachDifferent samples were studied from each of the three different rock types found in the area: metapelites, amphibolitised metabasites and coronitic gabbros. A total of 25 samples were hand-sawn and polished to thin sections (~30μm thick), 18 of which were used for geothermobarometry. Sample locations are shown in appendix A3. First the main mineral content of each sample was determined by light microscopic study. In order to calculate the pressure and temperature conditions at which the coexisting minerals equilibrated, the exact mineral compositions were measured using the electron microprobe. These compositions were used to calculate the equilibrium constants. Using this constant in combination with experimental results on thermodynamic quantities entropy (∆S), enthalpy (∆H), specific heat (∆Cp) and volume change (∆V), a line of constant equilibrium can be drawn in P-T space. It is assumed that the rock has equilibrated somewhere along this line and by using two or more different thermo- and barometers several lines of equilibrium can be drawn in a P-T diagram. The intersection between lines of equal mineral paragenesis then gives a unique temperature and pressure at which the paragenesis has equilibrated (Spear, 1993).
Many different geothermometers and geobarometers were used to estimate the metamorphic conditions depending on the mineral assemblages found in the samples. The best geothermometers are based on reactions that show considerable temperature sensitivity and small pressure sensitivity. For the best geobarometers, the opposite holds. For each sample, as many different thermometers and barometers were used to get a good grip on PT conditions of various mineral parageneses. For processing of large amounts of data, Microsoft Excel sheets were used to program the formulas needed for calculation. To get a direct value for pressure or temperature (instead of a line in a P-T plot), the possibility was created to fill in known temperatures or pressures, which indicate a point on the line and give direct values.
4.2.2 The Electron Microprobe
The JEOL JXA-8600 Electron Microprobe (EMP) at the University of Utrecht, equipped with an energy dispersive detection system (EDS) and five wavelength dispersive spectrometers (WDS), was used to determine the chemical composition of minerals in the samples. This apparatus was used for both geothermobarometry and monazite geochronology.
difference of 15 kV (beam current 20 nA). This set-up is the standard for major element analysis on silicates, oxides, most phosphates. When dealing with heavy elements in trace element concentration the set-up includes a 50 nA current and 20 kV acceleration voltage. These electrons are then focused through a series of electromagnetic lenses into a beam with a diameter of 2- 3 μm at the analysis spot.
When the beam interacts with the sample, part of the electrons that hit the sample are scattered back. These backscattered electrons are detected to create a backscatter electron (BSE) image. The measured backscatter electron signal intensity, depending on the mean atomic number of a mineral creates a grey-scale image of the sample, where the brightness depends on the mean atomic number of the detected elements. The remaining high-energy electrons in the beam cause the sample electrons to get excited to higher energy atomic shells (or perhaps outside of the particle). If possible, they immediately fall back into their normal energy state, thereby emitting X-rays. These X-rays have wavelengths and energies, which are element specific, therefore enabling the elements in the measured mineral to be quantified. With EDS a spectrum of all collected X-ray energies can be made, which is characteristic for that particular mineral. However, X-ray signals with a certain energy characteristic can interfere with X-rays of the same energy emitted from relaxation in another electron shell in a different element, which causes distortion. WDS measurements are based upon dispersion on the basis of X-ray wavelength and only measure one element per run. These analyses have a far higher accuracy, since the signal has a narrower peak and suffers far less distortion. Therefore, for accurate chemical analysis, WDS are be used.
In WDS analysis, the X-rays are diffracted by crystals which are capable of diffracting X-rays with specified wavelength intervals by Bragg reflection. By using various crystals, the complete wavelength spectrum is covered. The diffraction crystals used for this study are: Thallium Acid Phthalate (TAP), Pentaerythritol (PET) and Lithium Fluoride (LIF) (table 4.1). The X-ray signal is diffracted by the crystal and lead through a detector gas, which in this analysis was Ar-gas. The gas is ionised in the counter by the X-rays and the ionised Ar-particles are attracted by the change of the wire in the centre of the counting tube. The number of particles detected per second per nA (counts/s/nA) is a direct measure for the concentration of incident X-rays. Analysis of the peak signal as well as the background X-ray radiation determines the actual electron relaxation. Comparison of the WDS signal with calibrated homogeneous standards enables calculation of the element concentrations in the analysed sample.
4.2.3 Error correction and calculation
The element concentrations obtained by WDS measurement may show deviation as peak and background signals often interfere. Therefore, corrections have to be made for inter-element effects (Bence-Albee correction). Furthermore, a ZAF correction is applied by the electron microprobe, comprising: (i) the Z factor; for backscatter and retardation effects, (ii) the A factor; for absorption effects, and (iii) the F factor; to compensate secondary fluorescence of X-rays. The corrections are applied using PROZA iterations (up to 6 iterations) incorporating all ZAF components.
The relative standard deviation of EMP measurements on major elements is calculated from formula (I) below. In the present study, peak X-ray signals are measured for 60 seconds. Background measurements are done over a time lapse of 10 seconds. A standard peak - background count rate ratio of 36 over 1 is acknowledged for the current set-up (Wittke, 2003). From this information, a relation between standard deviation and peak count rate can be constructed. The result is graphically presented in figure 4.2.
