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Contribution of AMS measurements in understanding the migmatitic
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terrains of Pointe Géologie, Terre Adélie (East-Antarctica)
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Jérôme Bascou1, Bernard Henry2, René-Pierre Ménot1, Minoru Funaki3, Guilhem Barruol4
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Université de Lyon, Université Jean Monnet and UMR-CNRS 6524, Laboratoire Magmas et Volcans, 23
rue du Dr Paul Michelon, 42023 Saint Etienne, France
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Paléomagnétisme, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ ersité Paris Diderot
and UMR-CNRS 7154, 4 avenue de Neptune, 94107 Saint-Maur cedex, France
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National Institute of Polar Research, 10-3 Midori-cho Tachikawa Tokyo 190-8518, Japan
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Université de la Réunion, CNRS and Institut de Physique du Globe de Paris, Géosciences Réunion, 97715
Saint Denis cedex 9, La Réunion, France
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Abstract
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A detailed magnetic mapping using Anisotropy of Magnetic Susceptibility (AMS) technique
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was carried out in Pointe Géologie archipelago (Terre Adélie, East Antarctica) that represents
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a deep and hot crust that experienced a long-live anatectic event during Paleoproterozoic
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times, 1.69 Ga ago. AMS measurements allowed to better characterize the tectonic structure
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of the crystalline basement that is built up by rocks affected by various degrees of partial
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melting and then, devoid of clear markers of deformation. AMS sampling was performed
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from main rocks types of Pointe Géologie: migmatites including leucosomes and
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melanosomes, coarse-grained pink granites, anatexites and mylonitic gneisses. For
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melanosomes, the magnetic foliation is dominantly in agreement with the observed field
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foliation, i.e. dominantly sub-vertical N-S in shear zones and gently inclined in dome
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structures. AMS technique reveals a sub-horizontal magnetic lineation in migmatites from
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shear zones and a gently dipping one in dome structures. Magnetic properties of leucosomes
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and dykes of coarse-pink granites contrast with melanosomes. The bulk susceptibility and
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anisotropy degree are significantly lower in granitic magmas that in melanosomes. In
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addition, in well-defined leucosomes, granitic dykes and anatexites, the magnetic ellipsoid is
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characterized by a more dipping magnetic lineation, which tends to be vertical. This is
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associated to a rheological contrast between the solid-state deformation suffered by oxide
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grains in the melanosomes and their reorientation in a viscous flow during the transfer of
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felsic melt in the granitic dikes. Magnetic structure of leucosomes, granitic dykes and
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anatexites allows highlighting the role of the gravity-induced upwelling of a crust undergoing
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high degree of partial melting.
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Keywords: Migmatite, anatexite, AMS, Pointe Géologie, Antarctique
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1. Introduction
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Migmatites and anatexites result from progressive melting of rocks under high-grade
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metamorphism conditions (Wimmenauer and Bryhni, 2007` and references therein). These
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rocks are often the only key to determine the structural patterns in the deep crust. However,
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the structural analysis of migmatites is often not very easy because of their complex and
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multiphase evolution. Moreover, the products, leucosomes and melanosomes, from the
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melting of an initial highly deformed rock acquire a fabric that mostly cannot be determined
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by classical structural approaches. In partially melted gneisses, i.e. migmatites, this fabric can
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differ from that of the initial parent rock. In dominantly melted rocks, the only visible fabric is
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mostly related to the orientation of rafts of melanosomes or mesosomes, i.e. respectively
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restites or protoliths, and therefore this fabric only offers an incomplete signature of the
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tectonic events suffered by the rocks during melting. Melanosomes and mesosomes usually
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exhibit features of metamorphic rocks whereas the leucosomes display an igneous-like
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appearance. Therefore, one of the characteristics of the migmatite structures is their local
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variability and then their analysis imply by a statistical approach. To this aim, the field
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observations are limited to the few visible markers markers of the fabric and generally not
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well-adapted.
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The anisotropy of magnetic susceptibility (AMS) technique (Graham, 1954); see
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(Tarling and Hrouda, 1993; Borradaile and Henry, 1997; Borradaile and Jackson, 2004) and
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references therein) has been widely used for structural applications. It offers several
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advantages relatively to field observations: It gives not only foliation and lineation but also
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intensity and shape of the fabric and it can be applied to a set of samples representative of a
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significant area allowing simple statistical analysis. Ferré et al. (2003) and Kruckenberg et al.
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(2010) applied the magnetic tool for migmatites studies. They showed that despite rheological
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variations during deformation (solid-state deformation followed by viscous flow), AMS axes
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were consistent with observed regional field structures and thus highlighting the interest of
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the AMS data for the strain characterizations in high-temperature metamorphic rocks. In
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paramagnetic migmatites, the biotite fabric can evidenced these two types of deformation
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(Hasalovà et al., 2008). Others authors have shown that anatexites could have specific AMS
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signature, associated with changes towards weak solid-state rheology and that AMS
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characterizations could therefore help to better model the different stages of migmatitic
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terranes (Charles et al., 2009; Schulmann et al., 2009; Kratinová et al., 2012).
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The Pointe Géologie archipelago (Terre Adélie, East Antarctica) represents a deep and
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hot crust that experienced a long-live anatectic event during Paleoproterozoic times, 1.69 Ga
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ago (Ménot et al., 2007 and references therein). This archipelago is partially covered by the
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ice cap and limited outcrops are key zones to constrain the Terre Adélie Craton tectonics.
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Pelletier et al. (2002) and Gapais et al. (2008) proposed a detailed structural and kinematic
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analyses of the area based on foliation and lineation measurements of migmatites. But we
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believe these structural data may be supplemented by taking into account the magnetic fabric
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of the leucosomes and anatexites, which correspond to more than 50% of the outcrops and
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which are devoid of clear markers of deformation. This would greatly improve the knowledge
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of the structural pattern of this region.
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The first aims of this paper is thus to specify the deformation framework associated to
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the Paleoproterozoic event in the Terre Adélie Craton from a large sampling in the Pointe
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Géologie rocks. For this, a detailed AMS study was performed in the migmatites, the larger
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granitic dykes and the anatexites observed on the field. Finally, we focus on various
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migmatitic structures of centimetric to decametric size; they correspond to various phases of
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segregation, extraction and transfer of melts. We discuss the relationships between these
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different migmatitic habitus and their related AMS signature in order to constrain the
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rheological evolution of the molten crust.
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2. Geological setting
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The Terre Adélie Craton (TAC) is located on the easternmost part of the East
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Antarctic Shield. It crops out as islands and capes scattered along the coast between 135° and
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145°E but most of the outcrops are located in three main archipelagos, from West to East:
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around Pointe Géologie (66°50’S, 140°E), East of Port Martin (66°80’S, 142°25’E) and East
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of Cape Denison (67°S, 142°25’E) up to the Mertz glacier. This crystalline basement consists
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of Late Archean and Paleoproterozoic formations devoid of any significant reworking since
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1.5 Ga and then it is considered as part of the Mawson block or Mawson continent (Fanning
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et al., 1995; 2002). At the scale of the TAC, the Late Archean and Paleoproterozoic terrains
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are distributed westwards and eastwards from Port Martin, respectively (Monnier et al., 1996;
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Ménot et al., 2007), Figure 1.
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This paper deals only with the Paleoproterozoic formations of the Pointe Géologie area,
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i.e. close to the permanent French Polar Station Dumont d’Urville (DDU). As described by
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(Bellair, 1961b, a), the main rock types are migmatitic to anatectic gneisses that are cross cut
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by dykes of apparently isotropic coarse-grained pink granites. The detailed mapping, carried
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on during the GEOLETA project, improves this previous description and points out some new
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and significant features (Monnier et al., 1996; Peucat et al., 1999; Gapais et al., 2008).
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Migmatitic gneisses and anatexites derived from a sedimentary sequence dominated by
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aluminous pelites together with subordinate greywackes, quartzites and calc-silicates.
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Metagreywakes appear as intercalations ranging from tens of centimetres to hundreds of
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meters in thickness. Quartzites and calc-silicates form dismembered layers and enclaves of
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mesosomes within the migmatites and they mark out a poorly preserved sedimentary layering
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(Figure IIa). Amphibolitized dolerites and gabbros occur as dykes and sills and they are
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coeval with the anatectic event as shown by magma mingling features between the mafic
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intrusions and the granitic magmas. Isotopic data indicate Paleoproterozoic sources for both
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sediments and magmas (Peucat et al., 1999 and references therein). According to Monnier et
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al. (1996), mesosomes, neosomes and anatexites represent respectively 10%, 60% and 30% of
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the rock types in the Pointe Géologie archipelago.
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These migmatitic and anatectic formations underwent a polyphased tectonic and
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metamorphic evolution with three phases of deformation leading to a composite sub vertical
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regional S0-S1-S2 foliation (Monnier et al., 1996; Pelletier et al., 2002; Pelletier et al., 2005).
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Neosomes display two successive parageneses related to both the S1 and S2 metamorphic
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foliations and bearing comparable Sil+/-Kfs+/-Pl+/-Bt+/- Cd+/-Grt+/-Ilm+/-Qtz mineral
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assemblages. Moreover, mineral chemistry of the S1 and S2 parageneses is similar and argues
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for a single continuous LP-HT metamorphic event, the peak conditions of which reaching
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750-700°C at 0.6-0.4 GPa. Large amounts of anatectic melts have been produced all along the
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tectonic and metamorphic evolution as suggested by the various habitus of the leucosomes.
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The oldest leucosomes appear as mylonitic lenses isoclinally folded by the S1-S2
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transposition, but most of them are undeformed and concordant within the S2 foliation and the
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latest melts cross cut the S2 planes. A retrograde imprint with Bt+/-Ms+/-And assemblages
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marks the later evolution to about 550-450°C and 0.5-0.4 GPa (Monnier et al., 1996; Pelletier
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et al., 2002; Gapais et al., 2008).
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Bellair and Delbos (1962) assumed a 1.5 Ga age (Rb/Sr on micas) for the partial melting
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event. More recently, new radiometric data (Peucat et al., 1999, U-Pb on zircons and
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monazites, Sm-Nd and Rb-Sr on whole rocks and minerals) gave precise time constraints on
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the geological evolution of the meta-volcano-sedimentary formations of Pointe Géologie.
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Time of deposition is bracketed between the inherited zircon ages (1.72 -1.76 Ga) and the
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peak metamorphism and partial melting age at 1.69 Ga. The later cooling path was
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constrained by Nd garnet ages (650-600°C at 1.63-1.60 Ga) and Rb-Sr muscovite and biotite
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ages (450-350°C at 1.5 Ga).
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3. High temperature rocks types and regional structures
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While the lithology is very homogeneous in the scale of the archipelago, field
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relationships between different rock types are much more complex for the scale of outcrops.
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Four main rocks types have been sampled for AMS measurements: migmatites including
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leucosomes and melanosomes, coarse-grained pink granites, anatexites and strongly mylonitic
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gneisses.
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Migmatitic gneisses: they have been described previously as the K feldspar gneisses
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(Monnier et al., 1996; Pelletier et al., 2005) and they are the dominant facies of the area,
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especially on the Pétrels Island (Dumont d’Urville station, Figure I). We will discuss more
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specifically the space and time relationships between leucosomes and melanosomes in the
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migmatitic gneisses. As partial melting developed all along a polyphased tectonic and
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metamorphic event, leucosomes display various habitus relative to the composite foliation,
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which is marked on the field by melanosomes and septas of mesosomes. The melanosomes
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defined continuous layers in migmatites (Figure IIa):
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(i)
The earliest leucosomes occur as mylonitized and elongated lenses, sometimes
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affected by intrafolial folds within the foliation. They bear evidence of successive
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segregation and deformation processes (Figure IIa).
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(ii)
The most common leucosomes are undeformed and concordant within the
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migmatites foliation and they may be later folded together with the foliation. They
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represent syn to late-foliation melts (Figure IIb).
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(iii)
The latest leucosomes form dykelets cross-cutting, and post-dating, the S1-2
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foliation. They may be connected to the concordant leucosomes, tapping off and
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transferring melts upwards (Figure IIa, c).
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Coarse-grained pink granites: they appear as dykes of decimetric to decametric
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thickness which cross cut the regional foliation (Figure IId). They are devoid of any visible
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markers of orientation. Such dykes, collecting the dykelets, drain the more evolved melts
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during the late- to post-tectonic stages of the partial melting and metamorphic event.
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Anatexites: they outcrop widely on the W area of Pointe Géologie (e.g., Gouverneur,
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Balance, Taureau and la Vierge Islands, Figure IX). They derived from a higher degree of
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partial melting than migmatitic gneisses and show a mineral assemblage comparable to that of
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leucosomes but more enriched in Qtz and depleted in Crd. Anatexites display more isotropic
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textures but a flow fabric or a palimpsest structure of the parent gneisses is often marked by
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some oriented rafts of migmatitic gneisses or schlierens of biotite and oxides (Figure IIe).
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Mylonitic gneisses: they outcrop in a restricted and narrow zone on the western flank of
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the Pétrels Island (Rocher Jakobsen) and some islands of Pointe Géologie, i.e. along a major
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shear zone (sites II and V, Figure X). They consist of Bt + Grt gneisses with only tiny Grt-
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bearing leucosomes and are characterized by a vertical mylonitic foliation. They derive from
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the K-feldspar migmatitic gneisses and then may be considered as late- to post-migmatisation
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tectonites (Figure IIf).
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As carefully described by Pelletier et al. (2002) and Gapais et al. (2008), the Pointe
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Géologie archipelago show two types of structural domains: NS to N340° striking vertical
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shear zones and domes both of them developing during the metamorphic and anatectic event.
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Between the vertical zones, foliation attitudes point out dome-shaped structures marked by
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flat lying to gently dipping fabrics. Dome displays a mineral lineation outlined by a preferred
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orientation of sillimanite crystals while such a lineation is almost invisible in the vertical
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zones. The transition from flat lying to vertical foliation zones may be sharp, with
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development of vertical foliation planes cutting across the horizontal foliation of domes, or
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gradual when folds, with horizontal axes, induce vertical planes. Such a transition from dome
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to vertical shear zone never exceeds five hundred meters in thickness. In both cases, vertical
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structures seem to be later than flat lying ones. According Gapais et al. (2008), the kinematic
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markers (lineations, boudinage, ...) suggest that shear zones reflect a dextral transpressive
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movement while flat areas are the result of shortening combined with stretching parallel to the
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shear zones. Such patterns might be explained in the frame of a regional convergence of a hot
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and weak juvenile crust leading to horizontal crustal flow and strain partitioning with
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localized deformation along transpressional shear zones.
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4. Sampling and analyses procedures
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Because of outcrops restrictions (areas non accessible because of ice cover or of birds
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protection) and high variability of the visible deformation, sampling was often made in sites
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relatively close one to another in the main island (Pétrels Island). Other sites were scattered in
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different smaller islands (Figure IX). Most sampled sites correspond to a surface up to 50m2.
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For magnetic fabric study, cores were sampled with a gasoline-powered portable drill.
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All the cores were oriented only with a sun compass (Figure IIf) because of the immediate
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proximity of the southern magnetic pole. That introduced a strong limitation for sampling
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because of climatic conditions. The sampling was made during three different summer
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fieldworks, in 1992, 2009 and 2011. As a whole, 819 rock cores were obtained in 33 sites,
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including 20 on the Pétrels Island (Figure X). When possible, different facies (from gneiss to
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anatexites) were sampled on the same site. The cores were cut in the laboratory in specimen
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of standard paleomagnetic size.
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Magnetic susceptibility and its anisotropy have been measured using Kappabridges
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KLY3 and MKF1 (AGICO, Brno). Thermomagnetic experiments were performed on
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representative samples by heating in argon or in air using the CSL equipment and the CS2-3
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oven (AGICO, Brno) associated with the Kappabridges. Hysteresis loops were obtained for
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small cores (cylindrical samples of about 3 cm3) using a laboratory-made translation
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inductometer within an electromagnet capable of reaching 1.6 T.
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5. Magnetic Mineralogy
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5.1. Thin sections study
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As the magnetic characteristics of the rocks are tightly controlled by their
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mineralogical content, the various assemblages recorded in the different rock types need to be
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specified. This is particularly important in the case of migmatitic gneisses where partial
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melting induces a redistribution of refractory minerals from the mesosomes to both the
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melanosomes and leucosomes.
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Magnetite and ilmenite are the most abundant oxide minerals. They form discrete grains or
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clusters with direct contact between magnetite and ilmenite. The larger grains (millimetric in
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size) are generally found in the melanosomes and therefore in the foliation and they appear as
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elongated parallel to the lineation defined by the sillimanite when the latter is present (Figure
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III). For ilmenite crystal, deformation also induces a structural axes related lattice preferred
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orientation (LPO), which is characterized by the c-axes strongly concentrated close to the
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normal to the foliation, and by the poles of (2-1-10) and (10-10) prism planes concentrated
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close to the lineation (Figure IV). Oxides are also present in the leucosomes but the fabric
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(grains alignment) appears to be more scattered than in the melanosomes. At the melanosome-
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leucosome boundary, elongated shape grains, maybe rotated, have been observed
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perpendicular to the foliation. In the leucosomes these oxide grains become more discrete and
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more rounded (Figure III).
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Cordierite is also present in migmatitic gneisses and granitic dykes of Pointe Géologie.
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This mineral that occurs mainly as grains of globular shape (up to a few millimetres in size) is
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carefully watched because cordierite presents specific magnetic properties. This mineral
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carries an inverse magnetic fabric (i.e. characterized by a switch between the principal
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maximum K1 and minimum K3 axes of the AMS ellipsoid with respect to the grain shape
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(Potter and Stephenson, 1988; Rochette et al., 1992) that could lead to misinterpretation of the
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measured magnetic fabrics. However the mean magnetic susceptibility (Km) for magnetite,
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which is ubiquitous in studied thin sections, is significantly higher than for cordierite (Km
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ratio > 1000). Therefore magnetite should be the main carrier of measured AMS. For
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relatively large grains of magnetite, the K1 and K3 axes indicate the magnetic lineation and
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magnetic foliation pole, respectively.
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5.2. Rock-magnetism
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The mean magnetic susceptibility Km (average 5.2 10-2) of the studied samples is
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different according to the studied fraction of the migmatites (Figure V). The gneisses with
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subordinate melted component and dominant melanosome fraction have very high
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susceptibility (average 7.4 10-2). Km values for dominant granitic fraction are lower (average
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1.5 10-2). Migmatite samples including both leucosome and melanosome fractions present
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intermediate Km values. This variability in leucosome versus melanosome content of gneissic
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specimens largely explains the relatively large distribution of susceptibilities (Figure V). The
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anatexites samples show the highest variation in Km values.
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All thermomagnetic KT curves (susceptibility K as a function of temperature T) show
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a sharp susceptibility decrease around 580°C during heating (Figure VI), clearly indicating
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the presence of magnetite. In addition low temperature experiments confirm the presence of
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this mineral by a large susceptibility increase around -160°C (Figure VI). This variation
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corresponds to the Verwey transition that is characteristic of pure magnetite. Few
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thermomagnetic curves are perfectly reversible. Susceptibility values from the cooling curve
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are mostly a little bit higher than the initial values, in air as in argon atmosphere (but
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sometimes much higher as for the sample AP35). Partial cooling loops during heating
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evidenced a weak mineral alteration due to heating around 300 to 400°C that probably reflects
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transformation of a weak amount of maghemite in hematite. New magnetite is also often
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formed at the highest temperatures.
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Hysteresis loops show generally a full saturation of the magnetization for moderate
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field (Figure VII). This saturation was partial only for the sample AP35, evidencing the
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presence of another component of high coercivity. This component is probably hematite. In
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most cases, ferrimagnetic minerals have a largely dominant effect for the susceptibility, but
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two of the studied granite samples highlight also a strong contribution of the paramagnetic or
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even diamagnetic minerals. Day plot (Day et al., 1977) for samples with mainly magnetite
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clearly evidences a large size of the magnetite particles (Figure VIII).
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Main magnetic mineral is therefore magnetite with a large size. Obtained AMS data
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are then directly usable for a structural analysis (no inverse magnetic fabric due to small
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Single Domain particles (Potter and Stephenson, 1988).
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6 - Magnetic fabric
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The AMS ellipsoid yields the three principal susceptibility axes defined as
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K1  K2  K3. K1 is the magnetic lineation and K3 is perpendicular to the magnetic foliation.
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The Jelinek (1981) parameters P' and T were used to describe the intensity and the shape of
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the magnetic fabric, respectively. Data for a group of samples were analysed using normalized
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tensor variability (Hext, 1963; Jelinek, 1978) statistics.
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6.1 – Structures from AMS
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Data for migmatitic gneiss and for granites are presented separately (migmatitic
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gneisses included minor melted parts and "granitic" samples small amounts of melanosome
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assemblage and this separation has been made then considering the dominant presence of one
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of these component). Moreover, from the regional structural field pattern, two distinct zones
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have been distinguished for the AMS analyse: the sub-vertical zones and the dome shaped-
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zones (Figure IX and X).
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6.1.1. In migmatitic gneisses of sub-vertical shear zones (sites T, R, D, O, XIII, A of
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Pétrels Island, Figure X, melanosome layers show generally both well clustered magnetic
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foliation pole (K3) and magnetic lineation (K1). The magnetic foliation is dominantly
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subvertical N-S in accord with the observed field foliation and the magnetic lineation presents
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a weak plunge and N-S trending. The granitic rich layers (site XIII) display more diffuse
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fabric, in particular the K1 axes that tend to form a girdle perpendicular to K3. Well-
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individualized leucosomes (e.g. Figure IIC) show a different magnetic fabric signature than
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melanosome layers characterized by strongly inclined to sub-vertical magnetic lineation (site
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XIII
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horizontal plane with two dominant directions: N-S and E-W.
. The K3 axes are more scattered than the K1 axes. They are in the
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Xthe K3
In large coarse-grained pink granite dykes (sites A, E, D
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axes are relatively well clustered defining a subvertical magnetic of strike close to that of
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neighbouring gneisses (sites E, D
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magnetic lineation pattern is characterized by the verticalization of K1-axes in comparison to
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magnetic lineation measured in the melanosome layers of neighbouring gneisses. They form
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girdle perpendicular to K3 (site E or strongly inclined to sub-vertical local maximum
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(sitesD A
a divergent strike (sites A). The

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, Taureau, la Vierge, Balance Islands (sites
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VIIXV, XIV XVIII, XIXFigure IX) and South of Pétrels Island (site
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XX
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Gouverneur anatexite (sites VII, XV
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lies in the horizontal plane, relatively scatted or forms clusters at 90 degrees from each other
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with the N-S and E-W directions that prevails for south Gouverneur Islands (site XV).
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Sampling sites XIV, XVIII (Figure IX) show heterogeneities in the partial melting degree
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that is expressed in the field by more or less regularly presence of migmatitic gneiss rafts in
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the anatexites. For site XVIII
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gneisses show distinct magnetic axes orientations characterized by K1 preferentially sub-
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vertical and K3 horizontal N-S (i.e., magnetic foliation vertical E-W) in anatexites while K1
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is horizontal to strongly plunging N-S and K3 is horizontal E-W (i.e., magnetic foliation
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vertical N-S) in neighbouring gneisses. The N-S orientation of magnetic foliation in gneisses
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fit quite well with the regional vertical shear zone structures. In opposite, the E-W vertical
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magnetic foliation that appears in the larger dykes and anatexites is less well-constrained by
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large-scale
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XIV T
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and the migmatitic gneisses was difficult to perform. That explains the relatively wide
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confidence zones at 95% for K1 and K3. However the magnetic fabric is mainly oriented
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vertical E-W for the foliation and strongly-inclined (westward) for the lineation.
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K1max tends to have strong inclination or to be sub-vertical. For
field
 anatexites and migmatitic
structures
(Figure
IX).
For
site
tinction on the field between the anatexites
In mylonitic gneisses of Rocher Jakobsen (sites II, V,
) the magnetic
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foliation (K1-K2) is dominantly subvertical N-S in accord with the observed field structures
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(Figure IIf). On the contrary, the AMS measurement reveals uncommon highly dipping or
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sub-vertical lineation.
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6.1.2. In the migmatitic gneisses of the dome shaped-zones (sites J, VI, G, K, H, S,
X) magnetic fabric
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XVI, XVII, Figure IX; sites C, IV, Q, III, VIII, I, B, i, XII, F,
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display a stronger variability of K1 and K3-axes orientations from one site to another one in
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comparison to structured vertical shear zones. A relatively strong dip (about 45°) for the
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magnetic foliation is frequently observed, in particular for the sites located in the flanks of
356
domes (sites H, K, Figure IX; sites IV, Q, III, Figure X) for the eastern flanks of the Pétrels-
357
Curies Islands dome). The magnetic foliations are generally in agreement with the field
358
measured strikes and dip of melanosome layers. The magnetic lineations show gently dip and
359
dominant NNW strike. The innermost zones of domes (sites XVI, XVII, Figure IX; sites VIII,
360
I, B, Figure X) show also a strong variability of AMS structure patterns correlated to the
361
complexity of migmatitic structures observed at the domes apex. In these areas are observed
362
the less well-defined magnetic directions (with the widest confidence zones at 95%: site XVI,
363
Figure IX; site I, Figure X). Sampling in a granitic dykelet of centimetric width crosscutting
364
flat-lying gneissic foliation was performed in site XVII (Figure IX). The related AMS pattern
365
is characterized by vertical N-S magnetic foliation and sub-vertical magnetic lineation and
366
then differs from that of gneiss.
367
Large dykes of coarse grained pink granites sampled in site F an XII (Figure X) are
368
characterized by vertical magnetic lineation when compared with the magnetic lineation from
369
the melanosomes of gneisses. This feature is similar to that observed in the granitic bodies
370
from the sub-vertical shear zones. The magnetic foliation in the granitic dykes is either
371
concordant with that of host gneisses (site F) or discordant (site XII).
372
373
6.2 – AMS parameters (P’ and T)
374
The Figure XI presents the characteristics of the susceptibility ellipsoid. The degree of
375
anisotropy P’ is very high in the migmatitic gneisses (mean 1.479; minimum 1.041; maximum
376
1.903). It is lower in granites (mean 1.222; minimum 1.040; maximum 1.413). The shape of
377
the susceptibility ellipsoid is dominantly oblate, in gneisses (for T value: mean 0.30;
378
minimum -0.08; maximum 0.66) as in granites (mean 0.25; minimum -0.10; maximum 0.62).
379
In granites, a clear relationship (Figure V, R2=0.70) between P’ (intensity of the fabric) and
380
Km (related to the concentration in ferrimagnetic minerals) highlights the dominant effect of
381
the magnetite in the AMS (Henry et al., 2004). Another relationship (R2=0.54) between T and
382
P’ in granites suggests that the most prolate fabrics correspond to the weakest anisotropy
12
383
Figure XI. In addition, for granite, the anatexites show the highest variability in P’ and T
384
values, including the extreme values Figure XI.
385
386
7 – Discussion
387
7.1 Mean susceptibility (Km) and anisotropy (P’) signature
388
The amount of magnetite is clearly different in migmatitic gneisses and in granites
389
(leucosomes and dyke), as shown by the susceptibility values (Figure V). This variation of
390
magnetic susceptibility between melanocratic and leucocratic layers is mainly due to the
391
variations in magnetite content. Magnetic studies suggest also mineralogical changes
392
affecting magnetite with formation of ferromagnetic oxides of lower susceptibility like
393
maghemite or hematite. Concerning this mineralogical change, gneiss sample AP35 (close to
394
site R, Figure X) is the only one with hematite; it also presents the strongest formation of a lot
395
of new magnetite at high temperature (susceptibility becomes 14 times higher on the cooling
396
KT curve). It is therefore probably due to a strong local mineralogical alteration (interaction
397
with deep fluids, weathering?) and remains an exception.
398
The intensity of magnetic anisotropy, illustrated by the P’ values, is lower in granites
399
than in gneisses (Figure XI). In granites, it presents a relationship with the Km values,
400
suggesting that this difference could be explained by the lower amount of magnetite in
401
granites, AMS then resulting mainly from strong magnetite anisotropy and weak anisotropy
402
of a low susceptibility component. Owing to its Km values, this last component is the
403
paramagnetic minerals and not the maghemite. The observed inverse correlation between T
404
and P’ in granites (Figure XI) shows that the AMS of this second component is less oblate
405
than the magnetite AMS.
406
Inversely to granitic magmas constituting leucosomes and dykes, a wide range in Km
407
values, according to samples within a single site, is observed in anatexites. It is due to the
408
presence of more refractory gneissic rafts with high concentration in oxides grains.
409
410
7.2 AMS in migmatitic gneisses and granites
411
In addition to the Km and P’ values, the main differences in magnetic fabric between
412
melanosome layers of gneisses and granitic magmas concern the orientation of the K1 axis
413
(Figures IX and X). K1 axis is dominantly close to the horizontal in melanosomes and close to
414
vertical in granites. In some granitic sites (e. g., sites E, XII, Figure IX), its orientation is
415
relatively scattered within a vertical plane, probably because of the presence of melanosome
416
fragments within the granite. Other difference in orientation of susceptibility axes concerns
13
417
K3. Whereas in melanosome the K3 orientation fits to the field structures, it appears in granite
418
a more complex magnetic orientation pattern with two dominant orientations. The first one
419
matches to the gneissic structures (for example, the magnetic foliation is vertical N-S in the
420
shear zone area) and the second is at 90 degrees of the first one. In some sites the two
421
clustering are present (e.g., site XV Figure IX and site XIII, Figure X). The first orientation
422
is associated to the strike-slip deformation, which is at the origin of the large-scale sub-
423
vertical corridors. The cause of the second orientation is less straightforward. In Pétrels
424
Island, the migmatitic layering is locally cut by vertical large granitic dykes of NE-SW to E-
425
W direction (Monnier, 1995). Then emplacement of such E-W trending dykes tectonically
426
controlled and consistent with a component of NNW-SSE to N-S dextral strike-slip (Gapais et
427
al., 2008). In the western area (Gouverneur Island area, Figure IX) where rafts of
428
melanocratic restites are still packed in anatexites, E-W magnetite could correspond to
429
inheritance of older tectonic foliations.
430
The gneisses in sites II and V (Rocher Jakobsen), on the southwestern border of the
431
Pétrels Island (Figure X) also present mainly subvertical K1 axes. However, they are intensely
432
deformed, unlike the granitic leucosomes, dykes and anatexites that have not recorded
433
pervasive solid-state deformation. Thus, AMS fabric is interpreted as resulting of high-
434
strained, late- to post-migmatisation solid-state deformation. The strong K3 clustering
435
indicating a N-S vertical magnetic foliation and the highly tilted to sub-vertical magnetic
436
lineation could be related to deformation in transpressionnal regime mainly restricted to the
437
N-S vertical shear zones in restricted areas.
438
Thin section analyses show a change of orientation of magnetite grains from
439
melanosome to leucosome. In melanosome, the oxide grains are within the foliation and
440
elongated parallel to the lineation also marked by the sillimanite alignment. They are
441
interpreted as resulting from solid-state deformation that occurred during the Paleoproterozoic
442
metamorphic event (1.69 Ga ago). In addition, LPO of ilmenite, which strongly correlated
443
with the tectonic fabric, suggest dislocation glide on (0001) as main deformation mechanism.
444
The basal slip for hematite – ilmenite mineral could reflect high-temperature solid-state
445
deformation (e.g., Bascou et al., 2002; Siemes et al., 2003). Migmatitisation processes are
446
also associated with segregation and transfer of melts that lead to leucosomes, pink granitic
447
dykes and then anatexites when partial melting degree is higher. As discussed in Vigneresse
448
et al. (1996) and recently in Holtzman et al. (2012), the increase of melt fraction produce
449
changes in rheological behaviour from plastic deformation evidenced by elongation of oxide
14
450
minerals that are abundant in the melanosome, to magma flow with transport of isolated solid
451
particles in leucosomes, dykes and anatexites.
452
Such different and successive regimes of deformation affecting magnetite grains first
453
deformed under solid-state conditions and then involved as passive rigid particles during
454
melting, have been already described in migmatitic terrains (e.g., Ferré et al., 2003). In Pointe
455
Géologie migmatites, these successive regimes generate distinctive AMS signatures. The
456
clearest difference concerns the K1 orientation, which tends to a vertical plunge during
457
segregation and circulation of felsic melt in agreement with the gravity-induced upwelling of
458
magmas. The K3 orientation in granitic layer appears more sensitive to the degree of partial
459
melting and the structuration of country rock. At lower degrees of partial melting, liquid
460
segregation remained strongly constrained by the framework of the unmelted gneissic
461
material. Then the felsic melt, forming dykelets and connected lenses is concordant with the
462
foliation of gneisses. For higher degree of melting, the melt was injected as concordant or
463
cross-cutting large veins (Figure IIc,d) resulting in the various orientations of the magnetic
464
foliation. Then the dykes and veins intrude areas of lower stress, generally close to the 3
465
direction (e.g., Cooper, 1990; Hand and Dirks, 1992).
466
467
468
7.3 Methodological and regional-scale implications
469
All these observations evidence that the AMS study of migmatites cannot be
470
considered as a routine method. To be as reliable as possible, the AMS approach requires a
471
preliminary detailed observation of the various leucosomes habitus and then a large number
472
of magnetic measurements of rocks characterized by various degrees of melting. For low
473
degrees of partial melting, the leucocratic layers remain strongly constrained by the earlier
474
structure defined by the unmelted material. In that case, the AMS principal axes are
475
representative of the local and regional finite strain axes. This fits with the conclusions of
476
Ferré et al. (2003) for migmatites of the Superior Province in Minnesota. For more
477
pronounced partial melting degree, well-individualized leucocratic layers can developed
478
specific AMS patterns associated to flow regime. In the Pointe Géologie migmatites, the
479
difference with the local and regional finite strain axes is highlighted by the direction of K1-
480
axes. Large variations of the partial melting rates increase the probability to measure
481
composite AMS and the difficulty to isolate pre-melting and synmelting magnetic fabrics.
482
The strongest variations of AMS principal directions and parameters recorded in anatexites
15
483
are related to the heterogeneity of these rocks, which enclose number of schlierens and
484
refractory material.
485
The structural pattern of Pointe Géologie consists of two domains with respectively
486
flat-lying and sub-vertical foliations related to a long-live tectonic, metamorphic and anatectic
487
event, both associated with partial melting (Monnier, 1995; Pelletier et al., 2002; Pelletier et
488
al., 2005). Commonly, stretching mineral lineations are not easy to observe within the
489
foliation, more especially within the sub-vertical shear zones. Combining structural field
490
evidences, determination of P-T-t metamorphic evolution and analogic simulations of the
491
mantle-crust ductile deformation Gapais et al. (2008) proposed a model in which the
492
Paleaoproterozoic hot and weak lithosphere converges toward a cooler and more rigid craton
493
(Neoarchean in age). In this model, deformation of the Paleoproterozoic crust combines both
494
transpression and horizontal flow and/or lateral escape parallel to the Archean basement and
495
perpendicular to the shortening direction.
496
AMS structural pattern determination provides complementary structural information
497
such as magnetic lineations. They are mainly sub-horizontal in sub-vertical foliated gneisses.
498
It underlines the importance of the strike-slip component during the deformation. In mylonitic
499
gneisses that are considered as late- to post-migmatisation tectonites, nearly vertical lineations
500
are evidenced. This is considered as the effect of transpressionnal regime, which combines
501
strike-slip and across-strike shortening. In migmatites with flat-lying foliation, the magnetic
502
lineation displays a moderate plunge and is commonly parallel to the mineral stretching
503
lineation. Such features are in good agreement with horizontal crustal flow mechanism.
504
Increasing partial melting degree leads to a loss of the rock cohesion related to the
505
migration of the melt trough a solid-state made of previously deformed minerals. Thus the
506
former tectonic structures will be progressively erased. Melt segregation and transfer occur
507
under a viscous flow regime, mainly driven by gravity forces. In such a regime, the well-
508
defined leucosomes, dykes and anatexites will develop a magnetic fabric corresponding to the
509
upward transport of magmas. Then AMS measurements would allow to better map the mass
510
transfers associated with intensive melting at peak temperatures and to characterize the
511
general framework of a warm and buoyant lithosphere.
512
513
8 – Conclusion
514
Pointe Géologie archipelago in Terre Adélie represents a deep-seated crustal section
515
affected by intensive anatectic processes during Paleoproterozoic times, 1.69 Ga ago. This
516
generated different HT rocks types such as: migmatites including leucosomes and
16
517
melanosomes, coarse-grained pink granites dyke and anatexites. Partial melting affects
518
indifferently the regional structures characterized by the juxtaposition of flat-lying and sub-
519
vertical deformation zones.
520
Magnetic mineralogy study shows that oxide minerals are mainly large grains of
521
magnetite. Ilmenite is also observed constituting clusters with magnetite, which are
522
particularly abundant in melanosome layers. The grains are intensely solid-state deformed and
523
aligned parallel to the lineation marked by sillimanite. In the leucosomes, oxides appear more
524
scattered than in the melanosomes and their rounded isometric shape suggests rotations of the
525
former elongated grain during melt segregation.
526
Melanocratic layers on the one hand and felsic leucosomes, dykes and anatexites on
527
the other hand develop contrasted AMS signatures. The mean magnetic susceptibility (Km)
528
and the anisotropy degree (P’) are higher in the gneisses and much lower in granitic samples.
529
Orientation of principal susceptibility axes also differs. In migmatitic gneisses, magnetic
530
foliations are coherent with the local and regional structural pattern. Nevertheless AMS
531
technique provides complementary information on the orientation of lineations when they are
532
difficult to observe in the field. They are subhorizontal in the shear zone, except in narrow
533
vertical late mylonitic zones where high dips evidences the transpressionnal regime of
534
deformation. In well-defined leucosomes, granitic dykes and anatexites, the magnetic
535
ellipsoid is characterized by the vertical plunge of the magnetic lineation. Such a changing of
536
orientation is related to a rheological contrast between the solid-state deformation suffered by
537
the oxide grains and their reorientation in a viscous flow during the aggregation of felsic melt.
538
Magnetic structure pattern do not disagree with the model of deformation of a hot and
539
weak lithosphere converging toward a cooler Archean craton proposed by Gapais et al. (2008)
540
from metamorphic and field structures analysis and analogic simulations. They explained the
541
structural pattern of the pointe Géologie and surrounding areas by horizontal, orogen-parallel,
542
subconstrictional flow that could result from oblique convergence and/or from particular
543
boundary conditions facilitating lateral escape. A comparable model was also assumed from
544
numerical modelling to image the deformation of the hot Neoarchean crust of the Eastern part
545
of the Terre Adélie Craton (Duclaux et al., 2007). The magnetic structures map allows
546
highlighting the role of the gravity-induced upwelling of a crust undergoing partial melting.
547
From methodological point of view, this work emphasizes on the importance of an
548
acute petrological characterization of sites to be selected for AMS measurements in order to
549
avoid pre- and syn-melting composite magnetic fabrics. Finally, it underlines the difficulty of
17
550
AMS analyses in anatectic rocks, which lead to a very large variability for AMS signature;
551
this mainly due by the preservation of refractory rafts and by the heterogeneity of such rocks.
552
553
Acknowledgements
554
We are very grateful to the "Institut polaire français Paul Emile Victor" for support of
555
the program ArLiTA (Architecture de la Lithosphere de Terre Adélie) and to the Japanese
556
"National Institute of Polar Research" for participation to the international exchange program
557
under the Antarctica Treaty and for invitation of BH in Tokyo. LPO were carried from EBSD
558
measurements using a SEM-JSM 5600 at the Montpellier University (France) and ilmenite
559
pole figures were generated from software developed by D. Mainprice (Geosciences
560
Montpellier).
561
562
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Wimmenauer, W., Bryhni, I., 2007. Migmatites and related rocks, in: IUGS (Ed.),
662
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663
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664
665
666
21
667
Figures captions
668
669
Figure I: Simplified geological map of the Terre Adélie Craton (after Ménot et al. 2007).
670
Studied zone (indicated by a star) is located in the 1.7 Ga domain, close to the French
671
Polar Dumont d’Urville station. Inset: location and limits of the Terre Adélie Craton
672
(TAC) in East Antarctica and of the Gawler Craton (GC) in South Australia.
673
674
Figure II : Example of high-temperature rock types outcropping in Pointe Géologie
675
archipelago. (a) Migmatitic gneiss structures illustrating the HT polyphased tectonic in
676
the Paleoproterozoic formation of the TAC: septas of mesosomes (1) and more
677
continuous layers of melanosomes (2) bearing foliation; successive generations of melt
678
with earliest elongated lenses (3) locally affected by intrafolial folds within the
679
foliation, undeformed leucosomes (4) concordant with the foliation and later thin
680
dykes (5) cross-cutting foliation. (b) Folded leucosomes with axial plane parallel to the
681
foliation. (c) Melt segregation and upwards transfer processes generating concordant
682
leucosomes feeding cross-cutting dykelets (Pétrel Island). (d) Coarse-grained pink
683
granites forming metric to decametric thickness dykes cross-cutting the regional
684
foliation (Pétrel Island). (e). Anatexite showing melanocratic schlierens and rafts
685
bearing the foliation (Balance Island). (f) Mylonitic gneisses characterized by vertical
686
foliation (Rocher Jackobsen, Pétrels Island). The solar compass used to determine the
687
geographical orientations of tectonic structures gives the photograph scale.
688
689
Figure III : Photomicrograph of migmatitic gneiss in the vertical shear zone (Lamarck Island).
690
Sillimanite (sil) and elongated oxide grains (mainly magnetite (mag) in this sample)
691
are parallel to foliation in the melanocratic layers. The shape of oxide grains tends to
692
be more rounded and perpendicular to the foliation in more leucocratic areas (see
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arrows).
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Figure IV:
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Ilmenite LPO patterns of migmatitic gneiss in the vertical shear zone, Pétrel Island
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(sample AP85). LPO are represented on equal area, lower hemisphere projection.
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Foliation is marked by a black line and lineation is marked by the structural axis X.
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The density contours are in Multiple Uniform Distribution (MUD); N is the number of
22
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measured grains.
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Figure V : P' parameter as a function of Km values: granitic dyke (open squares), mylonitic
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gneisses (full triangles), gneisses (full squares) and anatexites (open diamonds). Black
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diamond symbol correspond to gneisses of La Vierge Island (Site XVIII).
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Figure VI : Normalized susceptibility K/Ko as a function of the temperature T in gneisses
(samples AP35 and AP85) and in granites (09cjb75, 11cjb41 and 11cjb71).
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709
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Figure VII : Hysteresis loops for gneisses (AP35 and AP85) and granite (11cjb108).
Remanent coercive force (Hcr).
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Figure VIII : Ratios of hysteresis parameters (Day et al., 1977): Jrs/Js (Jrs: remanence at
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saturation; Js: magnetization at saturation) as a function of Hcr/Hc (Hcr: remanent
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coercive force; Hc: coercive force). Areas for Multi-Domain (MD) and Pseudo Single
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Domain (PSD) grain size for pure magnetite.
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Figure IX: Projection of AMS data of each site of Pointe Géologie Archipelago. AMS
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projections on Petrel Island (black frame) are illustrated in Figure X. Distribution of
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maximum axes (K1; blue squares) and minimum axes (K3; violet circle) is in lower
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hemisphere equal area projection along with confidence ellipses. White, yellow and
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green colour of stereonets background represents the AMS projections of gneisses,
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granitic dykes and anatexites, respectively.
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Figure X : Projection of AMS data of each site of Petrel Island in Pointe Géologie
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Archipelago. Distribution of maximum axes (K1; blue squares) and minimum axes
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(K3; violet circle) is in lower hemisphere equal area projection along with confidence
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ellipses. White, yellow and green colour of stereonets background represents the AMS
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projections of gneisses, granitic dykes and anatexites, respectively.
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Figure XI : T parameter as a function of P' values (Jelinek, 1981): granitic dyke (open
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squares), mylonitic gneisses (full triangles), gneisses (full squares) and anatexites
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(open diamonds). Black diamond symbol correspond to gneisses of La Vierge Island
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(Site XVIII).
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24