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Contribution of AMS characterization 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, 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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Keywords: Migmatite, anatexite, AMS, Pointe Géologie, Antarctique
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1. Introduction
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1er paragraphe, il me semble important de definer la terminologie utilise pour la
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description des migmatites et anatexites, soit par une ou deux phrases préliminaires soit
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comme ci dessous dans le fil du texte. A voir. (ref Wimmenauer and Bryhni, 2007
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Migmatite and Anatexites result from progressive melting of rocks under high-grade
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metamorphism conditions ref Wimmenauer and Bryhni, 2007. These rocks are often the only
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key to determine the structural patterns in the deep crust. However, the structural analysis of
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migmatites may be not very easy because of their complex and multiphase evolution.
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Moreover, the products, leucosomes and melanosomes, from the melting of an initial highly
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deformed rock acquire a fabric that mostly cannot be determined by classical structural
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approaches. In partially melted gneisses, i.e. migmatites, this fabric can differ from that of the
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initial parent rock. IIn dominantly melted rocks, the only visible fabric is mostly related to the
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orientation of rafts of melanosomes or mesosomes, i.e. respectively restites or protoliths, and
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therefore this fabric only offers an incomplete signature of the tectonic events suffered by the
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rocks during melting. Melanosomes and mesosomes usually exhibit features of metamorphic
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rocks whereas the leucosomes display an igneous-like appearance. Therefore, one of the
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characteristics of the migmatite structures is their local variability and then their analysis
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passes by a statistical approach. To this aim, the field observations are limited to part of the
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elements (markers ?) of the fabric and generally not 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 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).
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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, 1996 #3766;Pelletier, 2002 #3923;Pelletier, 2005
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#867}. Neosomes display two successive parageneses related to both the S 1 and S2
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metamorphic foliations and bearing comparable Sil+/-Kfs+/-Pl+/-Bt+/- Cd+/-Grt+/-Ilm+/-Qtz
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mineral assemblages. Moreover, mineral chemistry of the S1 and S2 parageneses is similar and
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argues for a single continuous LP-HT metamorphic event, the peak conditions of which
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reaching 750-700°C at 0.6-0.4 GPa. Large amounts of anatectic melts have been produced all
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along the tectonic and metamorphic evolution as suggested by the various habitus of the
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leucosomes. 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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{Pelletier et al 2005 #867}{Monnier et al., 1996 #764} and they are the dominant facies of
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the area, especially on the Pétrels Island (Dumont d’Urville station, Figure I). We will discuss
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more specifically the space and time relationships between leucosomes and melanosomes in
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the 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 VIII). 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 IX). 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 VIII). Most sampled sites correspond to a surface up to 50m2.
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As a test, one site (site i, Figure IX) corresponds to a sampling made on a reduced surface
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(about 0.5 m2) in gneisses showing visible foliation and lineation.
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X large samples have been taken for petrographic and rock-magnetism studies. For
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magnetic fabric study, cores were sampled with a gasoline-powered portable drill. All the
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cores were oriented only with a sun compass (Figure IIf) because of the immediate proximity
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of the southern magnetic pole. That introduced a strong limitation for sampling because of
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climatic conditions. The sampling was made during three different summer fieldworks, in
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1992, 2009 and 2011. As a whole, 819 rock cores were obtained in 33 sites, including 20 on
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the Pétrels Island (Figure IX). When possible, different facies (from gneiss to anatexites) were
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sampled on the same site. The cores were cut in the laboratory in specimen of standard
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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, 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
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concentrated close to the lineation (Figure IIIbis). Oxides are also present in the leucosomes
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but the fabric (grains alignment) appears more scattered (equant ?) than in the melanosomes.
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At the melanosome-leucosome boundary, elongated shape grains, may-be rotated, have been
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observed perpendicular to the foliation. In the leucosomes these oxide grains become more
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discrete and 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
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the measured magnetic fabrics. However the mean magnetic susceptibility (Km) for
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magnetite, which is ubiquitous in studied thin sections, is significantly higher than for
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cordierite (Km ratio > 1000). Therefore magnetite should be the main carrier of measured
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AMS. For relatively large grains of magnetite, the K1 and K3 axes indicate the magnetic
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lineation and 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 IV). The gneisses with
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subordinate melted parts and dominant melanosome fraction have very high susceptibility
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(average?). Km values for dominant granitic fraction are lower (average?). Migmatite samples
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including both leucosome and melanosome fractions present intermediate Km values. This
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variability in leucosome versus melanosome content of gneissic specimens largely explains
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the relatively large distribution of susceptibilities (Figure IV). The anatexites samples show
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the highest variation in Km values (from X to Y).
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All thermomagnetic KT curves (susceptibility K as a function of temperature T) show
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during heating a sharp susceptibility decrease around 580°C (Figure V), clearly indicating the
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presence of magnetite. In addition low temperature experiments confirm the presence of this
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mineral by a large susceptibility increase around -160°C (Figure V). 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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inversiontransformation of a weak amount of maghemite in hematite. New magnetite is also
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often 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 VI). 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 VII).
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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 components). 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 VIII and IX).
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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 IX, 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 in agreement with the orientation of mineral lineation
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outlined in few places by a preferred alignment of sillimanite crystals (je n’ai pas de souvenir
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de lineation à sillimanite dans les SZ verticals, de plus le site I est assez marginal, voir
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extérieur à la SZ majeure). The granitic rich layers (site XIII) display more diffuse fabric, in
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particular the K1 axes that tend to form a girdle perpendicular to K3. Well-individualized
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leucosomes (e.g. Figure IIC) show a different magnetic fabric signature than melanosome
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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.
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. The K3 axes are more scattered than the K1 axes. They are in the
IX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 VIII) 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 VIII) 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
structures
 anatexites and migmatitic
(Figure
VIII).
For
site
tinction on the field between the anatexites
In mylonitic gneisses of Jakobsen (sites II, V,
) the magnetic foliation
(K1-K2) is dominantly subvertical N-S in accord with the observed field structures (Figure
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IIf). On the contrary, the AMS measurement reveals uncommon highly dipping or sub-
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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,
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XVI, XVII, Figure VIII; sites C, IV, Q, III, VIII, I, B, i, XII, F,
) magnetic
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fabric display a stronger variability of K1 and K3-axes orientations from one site to another
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one in comparison to structured vertical shear zones. A relatively strong dip (about 45°) for
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the magnetic foliation is frequently observed, in particular for the sites located in the flanks of
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domes (sites H, K, Figure VIII; sites IV, Q, III, Figure IX) for the eastern flanks of the
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Pétrels-Curies Islands dome). The magnetic foliations are generally in agreement with the
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field measured strike and dip of melanosome layers. The magnetic lineations show gently dip
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and dominant NNW strike. The innermost zones of domes (sites XVI, XVII, Figure VIII; sites
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VIII, I, B, Figure IX) show also a strong variability of AMS structure patterns correlated to
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the complexity of migmatitic structures observed at the domes apex. In these areas are
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observed the less well-defined magnetic directions (with the widest confidence zones at 95%:
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site XVI, Figure VIII; site I, Figure IX). Sampling in a granitic dykelet of centimetric width
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crosscutting flat-lying gneissic foliation was performed in site XVII (Figure VIII). The
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related AMS pattern is characterized by vertical N-S magnetic foliation and sub-vertical
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magnetic lineation and then differs from that of gneiss.
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Large homogénéiser dykes of coarse grained pink granites sampled in site F an XII
349
(Figure IX) are characterized by vertical magnetic lineation when compared with the
350
magnetic lineation from the melanosomes of gneisses. This feature is similar to that observed
351
in the granitic bodies from the sub-vertical shear zones. The magnetic foliation in the granitic
352
dykes is either concordant with that of host gneisses (site F) or discordant (site XII). Relire
353
les phrases avec attentiuon
354
355
6.2 – AMS parameters (P’ and T)
356
The Figure X presents the characteristics of the susceptibility ellipsoid. The degree of
357
anisotropy P’ is very high in the migmatitic gneisses (mean 1.479; minimum 1.041; maximum
358
1.903). It is lower in granites (mean 1.222; minimum 1.040; maximum 1.413). The shape of
359
the susceptibility ellipsoid is dominantly oblate, in gneisses (for T value: mean 0.30;
360
minimum -0.08; maximum 0.66) as in granites (mean 0.25; minimum -0.10; maximum 0.62).
361
In granites, a clear relationship (Figure IV, R2=0.70) between P’ (intensity of the fabric) and
11
362
Km (related to the concentration in ferrimagnetic minerals) highlights the dominant effect of
363
the magnetite in the AMS (Henry et al., 2004). Another relationship (R2=0.54) between T and
364
P’ in granites suggests that the most prolate fabrics correspond to the weakest anisotropy
365
Figure X. In addition, for granite, the anatexites show the highest variability in P’ and T
366
values, including the extreme values Figure X.
367
368
7 – Discussion
369
7.1 Mean susceptibility (Km) and anisotropy (P’) signature
370
The amount of magnetite is clearly different in migmatitic gneisses and in granites
371
(leucosomes and dyke), as shown by the susceptibility values (Figure IV). This variation of
372
magnetic susceptibility between melanocratic and leucocratic layers is mainly due to the
373
variations in magnetite content. Magnetic studies suggest also mineralogical changes
374
affecting magnetite with formation of ferromagnetic oxides of lower susceptibility like
375
maghemite or hematite. Concerning this mineralogical change, gneiss sample AP35 is the
376
only one with hematite; it also presents the strongest formation of a lot of new magnetite at
377
high temperature (susceptibility becomes 14 times higher on the cooling curve). It is therefore
378
probably due to a strong local mineralogical alteration (interaction with deep fluids,
379
weathering?) and remains an exception.
380
The intensity of magnetic anisotropy, illustrated by the P’ values, is lower in granites
381
than in gneisses (Figure X). In granites, it presents a relationship with the Km values,
382
suggesting that this difference could be explained by the lower amount of magnetite in
383
granites, AMS then resulting mainly from strong magnetite anisotropy and weak anisotropy
384
of a low susceptibility component. Owing to its Km values, this last component is the
385
paramagnetic minerals and not the maghemite. The observed inverse correlation between T
386
and P’ in granites (Figure X) shows that the AMS of this second component is less oblate than
387
the magnetite AMS.
388
Inversely to granitic magmas constituting leucosomes and dykes, a wide range in Km
389
values, according to samples within a single site, is observed in anatexites. It is due to the
390
presence of more refractory rafts of gneisses with high concentration in oxides grains.
391
392
7.2 AMS in migmatitic gneisses and granites
393
In addition to the Km and P’ values, the main differences in magnetic fabric between
394
melanosome layers of gneisses and granitic magmas concern the orientation of the K1 axis
395
(Figures VIII and IX). K1 axis is dominantly close to the horizontal in melanosomes and close
12
396
to vertical in granites. In some granitic sites (e. g., sites E, XII, Figure IX), its orientation is
397
relatively scattered within a vertical plane, probably because of the presence of melanosome
398
fragments within the granite. Other difference in orientation of susceptibility axes concerns
399
K3. Whereas in melanosome the K3 orientation fits to the field structures, it appears in granite
400
a more complex magnetic orientation pattern with two dominant orientations. The first one
401
matches to the gneissic structures (for example, the magnetic foliation is vertical N-S in the
402
shear zone area) and the second is at 90 degrees of the first one. In some sites the two
403
clustering are present (e.g., site XV Figure VIII and site XIII, Figure IX). The first
404
orientation is associated to the strike-slip deformation, which is at the origin of the large-scale
405
sub-vertical corridors. The cause of the second orientation is less straightforward. In Pétrels
406
Island, the migmatitic layering is locally cut by vertical large granitic dykes of NE—SW to E-
407
W direction (Monnier, 1995). Then emplacement of such E-W trending dykes is tectonically
408
controlled and consistent with a component of NNW-SSE to N-S dextral strike-slip (Gapais
409
et al., 2008). In the western area (Gouverneur island area, Figure VIII) where rafts of
410
melanocratic restites are still packed in anatexites, E-W magnetite could correspond to
411
inheritance of older tectonic foliations.
412
The gneiss sites II and V, on the southwestern border of the Pétrels Island (Figure IX)
413
also present mainly subvertical K1 axes. However, the gneisses of sites II and V (rocher
414
Jakobsen, voir aussi plus haut+carte pour nom) are intensely deformed, unlike the granitic
415
leucosomes, dykes and anatexites that have not recorded pervasive solid-state deformation,.
416
Thus, AMS fabric is interpreted as resulting of high-strained, late- to post-migmatisation
417
solid-state deformation. The strong K3 clustering indicating a N-S vertical magnetic foliation
418
and the highly tilted to sub-vertical magnetic lineation could be related to deformation in
419
transpressionnal regime mainly restricted to the N-S vertical shear zones.
420
Thin section analyses show a change of orientation of magnetite grains from
421
melanosome to leucosome. In melanosome, the oxide grains are within the foliation and
422
elongated parallel to the lineation also marked by the sillimanite alignment. They are
423
interpreted as resulting from solid-state deformation that occurred during the Paleoproterozoic
424
metamorphic event (1.69 Ga ago). In addition, LPO of ilmenite, which strongly correlated
425
with the tectonic fabric, suggest dislocation glide on (0001) as main deformation mechanism.
426
The basal slip for hematite – ilmenite mineral could reflect high-temperature solid-state
427
deformation (e.g., Bascou et al., 2002; Siemes et al., 2003). Migmatitisation processes are
428
also associated with segregation and transfer of melts that lead to leucosomes, pink granitic
429
dykes and then anatexites when partial melting degree is higher. As discussed in Vigneresse
13
430
et al. (1996), the increase of melt fraction produce changes in rheological behaviour. From
431
plastic deformation evidenced by elongation of oxide minerals, which are abundant in the
432
melanosome to magma flow with transport of isolated solid particles in leucosomes, dykes
433
and anatexites.
434
Such different and successive regimes of deformation affecting magnetite grains first
435
deformed under solid-state conditions and then involved as passive rigid particles during
436
melting, have been commonly observed in various migmatitic terrains voir labrousse
437
(scandinavie)Ferré et al., 2003). In Pointe Géologie migmatites, these successive regimes
438
generate distinctive AMS signatures. The clearest difference concerns the K1 orientation,
439
which tends (reaches) to a vertical orientation during segregation and circulation of felsic melt
440
in agreement with the gravity-induced upwelling of magmas. K3 orientation in granitic layer
441
appears more sensitive to the degree of partial melting and the structuration of country rock.
442
At lower degrees of partial melting, liquid segregation remained strongly constrained by the
443
framework of the unmelted gneissic material. Then the felsic melt formed dykelets and
444
connected lenses concordant with the foliation of gneisses. For higher degree of melting, the
445
melt was injected as concordant or cross-cutting large veins (Figure IIc,d) resulting in the
446
various orientations of the magnetic foliation. Then the dykes and veins intrude areas of lower
447
stress, generally close to the 3 direction (e.g., Cooper, 1990; Hand and Dirks, 1992).
448
449
450
7.3 Methodological and regional-scale implications
451
All these observations evidence that the AMS study of migmatites cannot be
452
considered as a routine method. To be as reliable as possible, the AMS approach requires a
453
preliminary detailed observation of the various leucosomes habitus and then a large number
454
of magnetic measurements of rocks characterized by various degrees of melting. For low
455
degrees of partial melting the leucocratic layers remain strongly constrained by the earlier
456
structure defined by the unmelted material. In that case, the AMS principal axes are
457
representative of the local and regional finite strain axes. This fits with the conclusions of
458
Ferré et al. (2003) for migmatites of the Superior Province in Minnesota. For more
459
pronounced partial melting degree, well-individualized leucocratic layers can developed
460
specific AMS patterns associated to flow regime. In the Pointe Géologie migmatites, the flow
461
regime differs from the local and regional finite strain axes by the direction of K1-axes. Large
462
variations of the partial melting rates increase the probability to measure composite AMS and
463
the difficulty to isolate pre-melting and synmelting magnetic fabrics.. The strongest variations
14
464
of AMS principal directions and parameters recorded in anatexites are related to the
465
heterogeneity of these rocks, that enclose number of schlierens and refractory material.
466
The structural pattern of Pointe Géologie consists of two structural domains with respectively
467
flat-lying and sub-vertical foliations related to a long-live tectonic, metamorphic and anatectic
468
event (Monnier, 1995; (Pelletier et al., 2002, 2005). Commonly, stretching mineral lineations
469
are not easy to observe within the foliation, more specially within the sub-vertical shear
470
zones. Combining structural field evidences, determination of P-T-t metamorphic evolution
471
and analogic simulations of the mantle-crust ductile deformation ,{Gapais, 2008 #437 and ref.
472
therein} propose a model in which the Paleoproterozoic hot and weak lithosphere converges
473
toward a cooler and more rigid craton (Neoarchean in age). In this model, deformation of the
474
Paleoproterozoic crust combines both transpression and horizontal flow and/or lateral escape
475
parallel to the Archean basement and perpendicular to the shortening direction.
476
AMS structural pattern determination provides complementary structural information
477
such as magnetic lineations. They are mainly sub-horizontal in sub-vertical foliated gneisses.
478
It underlines the importance of the strike-slip component during the deformation. In mylonitic
479
gneisses, which are considered as late- to post-migmatisation tectonites, nearly vertical
480
lineations are evidenced. This is considered as the effect of transpressionnal regime, which
481
combinates strike-slip and across-strike shortening. In migmatites with flat-lying foliation, the
482
magnetic lineation displays a moderate plunge and is commonly parallel to the mineral
483
stretching lineation. Such features are in good agreement with an horizontal crustal flow
484
mechanism.
485
Increasing partial melting degree leads to a loss of the rock cohesion related to the
486
migration of the melt trough a solid-state frame made of previously deformed minerals .Thus
487
the former tectonic structures will be progressively erased. Melt segregation and transfer
488
occur under a viscous flow regime, mainly driven by gravity forces. In such a regime, the
489
well-defined leucosomes, dykes and anatexites will develop a magnetic fabric corresponding
490
to the upwards transport of magmas. Then AMS measurements would allow to better map the
491
mass transfers associated to intensive melting at peak temperatures and to characterize the
492
general framework of a warm and buoyant lithosphere.
493
494
8 – Conclusion
495
Pointe Géologie archipelago in Terre Adélie represents a deep and hot crust affected
496
by intensive anatectic event during Paleoproterozoic times, 1.69 Ga ago. This generated
497
different HT rocks types such as: migmatites including leucosomes and melanosomes, coarse15
498
grained pink granites dyke and anatexites. Partial melting affects indifferently the regional
499
structures characterized by the juxtaposition of flat-lying and sub-vertical deformation zones.
500
Magnetic mineralogy study shows that oxide minerals are mainly large grains of
501
magnetite. Ilmenite is also observed constituting cluster with magnetite, which is particularly
502
abundant in melanosome layers. The grains are intensely solid-state deformed and aligned
503
parallel to the lineation marked by sillimanite. In the leucosomes, oxides appear more
504
scattered than in the melanosomes and their rounded shape suggests rotation of the former
505
elongated grain during melt segregation.
506
Melanocratic layers on the one hand and felsic leucosomes, dykes and anatexites on
507
the other hand develop contrasted AMS signatures. The mean magnetic susceptibility (Km)
508
and the anisotropy degree (P’) are high in the gneisses and lower in granitic samples.
509
Orientation of principal susceptibility axes also differs. In migmatitic gneisses, magnetic
510
foliations are coherent with the local and regional structural pattern. Nevertheless AMS
511
technique provides complementary information on the orientation of lineations, when they are
512
difficult to observe in the field. They are subhorizontal in the shear zones, (dans les domes
513
non ?) except in vertical mylonitic zone where high plunge evidences the transpressionnal
514
regime of deformation. In well-defined leucosomes, granitic dykes and anatexites, the
515
magnetic ellipsoid is characterized by the vertical orientation of the magnetic lineation. This
516
change of orientation is related to a rheological contrast between the solid-state deformation
517
suffered by the oxide grains and their reorientation in a viscous flow during the aggregation of
518
felsic melt.
519
Magnetic structure pattern do not disagree with the model of deformation of a hot and
520
weak lithosphere converging toward a cooler Archean craton proposed by Gapais et al. (2008)
521
from metamorphic and field structures analysis and analogic simulations. They explained the
522
structural pattern of the Pointe Géologie and surrounding areas by an horizontal, orogen-
523
parallel, subconstrictional flow that could result from oblique convergence and/or from
524
particular boundary conditions facilitating lateral escape. A comparable model was also
525
assumed from numerical modelling to image the deformation of the hot Neoarchean crust of
526
the Eastern part of the Terre Adélie Craton (Duclaux et al., 2007). The magnetic structures
527
map allows to highlight the role of the gravity-induced upwelling of a crust undergoing partial
528
melting.
529
From methodological point of view, this work emphasizes on the importance of an
530
acute petrological characterization of sites to be selected for AMS measurements in order to
531
avoid pre- and syn-melting composite magnetic fabrics. Finally, it underlines the difficulty of
16
532
AMS analyses in anatectic rocks, which lead to a very large variability for AMS signature;
533
this mainly cause by the preservation of refractory rafts and by the heterogeneity of such
534
rocks . .
535
536
Acknowledgements
537
We are very grateful to the "Institut polaire français Paul Emile Victor" for support of
538
the program ArLiTA (Architecture de la Lithosphere de Terre Adélie) and to the Japanese
539
"National Institute of Polar Research" for participation to the international exchange program
540
under the Antarctica Treaty and for invitation of BH in Tokyo. LPO were carried from EBSD
541
measurements using a SEM-JSM 5600 at the university of Montpellier (France) and ilmenite
542
pole figures were generate from software developed by D. Mainprice (Geosciences
543
Montpellier).
544
545
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546
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19
643
Figures captions
644
645
Figure I : Simplified geological map of the Terre Adélie Craton (after Ménot et al. 2007).
646
Studied zone (red star) is located in the 1.7 Ga domain, close to the French Polar
647
Dumont d’Urville station. Inset: location and limits of the Terre Adélie Craton (TAC)
648
in East Antarctica and of the Gawler Craton (GC) in South Australia.
649
650
Figure II : Example of high-temperature rock types outcropping in Pointe Géologie
651
archipelago. (a) Migmatitic gneiss structures illustrating the HT polyphased tectonic in
652
the paleproterozoic formation of the TAC: septas of mesosomes and more continuous layers of
653
melanosomes bearing foliation (e.g., hammer area); successive generation of melt segregation
654
with earliest elongated lenses locally affected by intrafolial folds within the foliation,
655
undeformed and concordant leucosomes with the foliation and later thin dykes cross-cutting
656
foliation (arrowed). (b) Folded leucosomes with axial plane parallel to the foliation. (c)
657
Melt segregation process generating concordant leucosome that supplies “alimente”
658
cross-cutting dykelet transferring melts upward (Petrel Island). (d) Coarse-grained pink
659
granites forming metric to decametric thickness dykes cross-cutting the regional foliation
660
(Pétrel Island). (e). Anatexite showing melanocratic layers bearing foliation (Balance Island).
661
(f) Mylonitic gneisses characterized by vertical foliation (Rocher Jackobsen, Pétrels Island).
662
The solar compass used to determine the geographical orientations of tectonic structures gives
663
the photograph scale.
664
665
Figure III : Photomicrograph of migmatitic gneiss in the vertical shear zone (Petrels Island).
666
Sillimanite (sill) and elongated oxide grains (mainly magnetite (mt) and ilmenite) are
667
parallel to foliation in the melanocratic layers. The shape of oxide grains tends to be
668
more rounded and perpendicular to the foliation in more leucocratic areas (arrowed).
669
670
Figure IIIbis:
671
672
673
674
675
676
677
Ilmenite LPO patterns of migmatitic gneisses in the vertical shear zone, Pétrel Island
(sample AP85). LPO are represented on equal area, lower hemisphere projection.
Foliation is marked by a black line and lineation is marked by the structural axis X.
The density contours are in Multiple Uniform Distribution (MUD); N is the number of
measured grains.
Figure IV : P' parameter as a function of Km values: granitic dyke (open squares), mylonitic
678
gneisses (full triangles), gneisses (full squares) and anatexites (open diamonds).
20
679
680
681
Figure V : Normalized susceptibility K/Ko as a function of the temperature T in gneisses
(samples AP35 and AP85) and in granites (09cjb75, 11cjb41 and 11cjb71).
682
683
684
Figure VI : Hysteresis loops for gneisses (AP35 and AP85) and granite (11cjb108). Remanent
coercive force (Hcr).
685
686
Figure VII : Ratios of hysteresis parameters (Day et al., 1977): Jrs/Js (Jrs: remanence at
687
saturation; Js: magnetization at saturation) as a function of Hcr/Hc (Hcr: remanent
688
coercive force; Hc: coercive force). Areas for Multi-Domain (MD) and Pseudo Single
689
Domain (PSD) grain size for pure magnetite.
690
691
Figure VIII : Projection of AMS data of each site of Pointe Géologie Archipelago. AMS
692
projections on Petrel Island (black square) are illustrated in Figure IX. Distribution of
693
maximum axes (K1; blue squares) and minimum axes (K3; violet circle) is in lower
694
hemisphere equal area projection along with confidence ellipses. White, yellow and
695
white colour of stereonets background represents the AMS projections of gneisses,
696
granitic dykes and anatexites, respectively.
697
698
699
Figure IX : 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 white colour of stereonets background represents the AMS
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projections of gneisses, granitic dykes and anatexites, respectively.
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Figure X : 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).
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