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Contribution of AMS studies in understanding the deformation of 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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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, PRES Sorbonne Paris Cité
and CNRS, 4 avenue de Neptune, 94107 Saint-Maur cedex, France
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Université de Lyon, Université Jean Monnet and UMR-CNRS 6524, Laboratoire
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:
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1. Introduction
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Anatexites result from progressive melting of rocks under very high-grade
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metamorphism conditions. These rocks are often the key to determine the structural
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conditions in the deep crust. However, the structural analysis of migmatites is often very
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difficult because of their complex and multiphase evolution. Moreover, the products from the
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melting of an initial highly deformed rock have a fabric that mostly cannot be determined by
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the classical structural approaches. In partially melted gneisses, this fabric can modify the
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initial one, and in dominantly melted rocks the single visible fabric is often related to the
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melanosome orientation and therefore offers an incomplete signature of tectonic events
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suffered by the rocks. One of the characteristics of the structures of the migmatites is
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frequently their local variations and for their analysis the key is then a statistical approach. To
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this aim, the field observations are limited to part of the elements of the fabric and generally
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not well-adapted.
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The anisotropy of magnetic susceptibility (AMS) (Graham, 1954; see Tarling and
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Hrouda, 1993; Borradaile and Henry, 1997; Borradaile and Jackson, 2004;and references
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therein) measurements has been widely applied 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) applied this magnetic
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tool for migmatites studies. They showed that despite rheological variations during
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deformation (solid-state deformation followed by viscous flow), AMS axes were consistent
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with observed regional field structure and thus highlighting the interest of the AMS data for
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the strain characterizations in high-temperature metamorphic rocks. In paramagnetic
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migmatites, the biotite fabric can evidenced these two types of deformation (Hasalovà et al.,
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2008). Others authors have shown that anatexite melts could have specific AMS signature,
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associated with changes towards weak solid-state rheology and that AMS characterizations
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could therefore help to better model the different stages of migmatitic terranes (Charles et al.,
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2009; Schulmann et al., 2009; Kruckenberg et al., 2010).
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The Pointe Géologie archipelago (Terre Adélie, East Antarctica) represents a deep and
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hot crust that experimented anatectic event during Paleoproterozoic times, 1.69 Gy ago
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(Ménot et al., 2007 and references therein). This archipelago is partially covered by the ice
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cap and the 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 area based on foliation and lineation measurements on migmatites. But we believe
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these structural data may be supplemented by taking into account the magnetic fabric of the
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leucosomes and anatexites, which correspond to more than 50% of the outcrops and which are
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devoid of clear markers of deformation. This would greatly improve the structural pattern of
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this region.
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The first aims of this paper is thus to precise the deformation framework associated to
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the Paleoproterozoic event in the Terre Adélie Craton from an enlarge sampling in the Pointe
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Géologie rocks. For this, a detailed AMS study has been performed in the migmatites, the
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larger granitic dykes and the anatexites observed on the field. Finally, we discuss on the
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relationships between the various observed migmatitic structures of centimetric to decametric
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scale, which correspond to various degree of segregation and transportation of magma, the
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AMS signature and the 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 respectively westwards and eastwards from Port Martin (Ménot et al., 2007;
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Monnier et al., 1996), Figure 1.
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This paper deals only with the Paleoproterozoic formations of the Pointe Géologie
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area, i.e. close to the permanent French Polar Station Dumont d’Urville (DDU). As described
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by (Bellair, 1961a, b), the main rock types are migmatitic to anatectic gneisses that are cross
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cut by dykes of apparently isotropic coarse-grained pink granites. The detailed mapping,
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carried on during the GEOLETA project, improves this previous description and points out
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some new and significant features ( Monnier et al., 1996; Peucat et al., 1999; Gapais et al.,
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2008). 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 melts. 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 polyphase 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). Neosomes display two
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successive parageneses related to both the S1 and S2 metamorphic foliations and bearing
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comparable Sil+/-Kfs+/-Pl+/-Bt+/-Cd+/-Grt+/-Ilm+/-Qtz mineral assemblages. Moreover,
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mineral chemistry of the S1 and S2 parageneses is similar and argues for a single continuous
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LP-HT metamorphic event, the peak conditions of which reaching 750-700°C at 0.6-0.4 Gpa.
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Large amounts of anatectic melts have been produced all along the tectonic and metamorphic
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evolution as suggested by the various habitus of the leucosomes. The oldest leucosomes
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appear as mylonitic lenses isoclinally folded by the S1-S2 transposition, but most of them are
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undeformed and concordant within the S2 foliation and the latest melts cross cut the S2 planes.
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A retrograde imprint with Bt+/-Ms+/-And assemblages marks the later evolution to about
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550-450°C and 0.5-0.4 Gpa (Monnier et al., 1996; Pelletier et al., 2002) Pelletier 2008?.
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Bellair and Delbos (1962) assumed a 1.5 Ga age (Rb/Sr on micas) for the partial
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melting event. More recently (Peucat et al., 1999), new radiometric data (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 and
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they are the dominant facies of the area, especially on the Pétrels Island (Figure I). We will
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discuss more specifically the space and time relationships between leucosomes and
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melanosomes in the migmatitic gneisses. As partial melting developed all along a polyphased
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tectonic and metamorphic event, leucosomes display various habitus relative to the composite
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foliation, which is marked on the field by melanosomes and septas of mesosomes. The
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melanosomes 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 migmatites
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foliation and they may be later folded together with the foliation. They represent
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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 metric to decametric thickness
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which cross-cut the regional foliation (Figure IId). Their mineralogy is similar to that of
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anatexites and they are devoid of any visible markers of orientation. Such dykes, collecting
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the dykelets, drain the more evolved melts during the late- to post-tectonic stages of the
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partial melting and metamorphic event.
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Anatexites: They outcrop widely on the Gouverneur and the NW area of the Pétrels
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Island (Figure VIII). They derived from a higher degree of partial melting than migmatitic
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gneisses and show a mineral assemblage comparable to that of leucosomes but more enriched
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in Qtz and depleted in Crd. Anatexites display more isotropic textures but a flow fabric or a
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palimpsest structure of the parent gneisses is often marked by some oriented rafts of
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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), i.e. along a major shear zone (sites II and V, Figure IX).
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They consist of Bt + Grt gneisses with only tiny Grt-bearing leucosomes and are characterized
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by a vertical mylonitic foliation. They derive from the K-feldspar migmatitic gneisses and
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then may be considered as late- to post-migmatisation 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 which develop during the metamorphic and anatectic event. Between
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the vertical zones, foliation attitudes point out dome-shaped structures marked by flat lying to
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gently dipping fabrics. Dome displays a mineral lineation outlined by a preferred orientation
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of sillimanite crystals while such a lineation is almost invisible in the vertical zones. The
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transition from flat lying to vertical foliation zones may be sharp, with development of
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vertical foliation planes cutting across the horizontal foliation of domes, or gradual when
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folds, with horizontal axes, induce vertical planes. Such a transition from dome to vertical
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shear zone never exceeds five hundred meters in thickness. In both cases, vertical structures
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seem to be later than flat lying ones. According Gapais et al. (2008), the kinematic markers
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(lineations, boudinage, ...) suggest that shear zones reflect a dextral transpressive movement
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while flat areas are the result of shortening combined with stretching parallel to the shear
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zones. Such patterns might be explained in the frame of a regional convergence of a hot and
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weak juvenile crust leading to horizontal crustal flow and strain partitioning with localized
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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 sites correspond to a surface up to 50m2. As a
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test, a site (site I) corresponds to a sampling made on a reduced surface (about 0.5 m2) in
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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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199Y, 2009 and 2011. As a whole, 479 cores of migmatitic rocks were obtained in 37 sites.
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When possible, different facies (from gneiss to anatexites) were sampled. 20 of the sites have
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been chosen in the Pétrels Island, including 3 sites within granitic dykes crossing the main
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structures (sites A , E , D
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specimen of standard paleomagnetic size.
XX , Figure IX . The cores were cut in the laboratory in
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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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precised. 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
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grains or clusters with direct contact between magnetite and ilmenite. The larger grains
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(millimetric in size) are generally found in the melanosomes. They are then in the foliation
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and elongated parallel to the lineation defined by the sillimanite when the latter is present.
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Oxides are also present in the leucosomes but the fabric (grains alignment) appears more
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scattered than in the melanosomes. At the border melanosome-leucosome, elongated shape
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grains are observed perpendicular to the foliation. In the leucosomes oxide grains tend to be
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more discrete and more rounded (Figure III).
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Cordierite is also present in migmatite 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 millimeters 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 maximum K1
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and minimum K3 axes of the AMS ellipsoid with respect to the grain shape - Potter and
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Stephenson, 1988; Rochette et al., 1992) that could lead to misinterpretation of the measured
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magnetic fabrics. However the mean magnetic susceptibility (Km) for magnetite is
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significantly higher than for cordierite (Km ratio > 1000) that is ubiquitous in studied thin
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sections and it is therefore magnetite that should be the main carrier of measured AMS. For
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magnetite, the K1 and K3 axes indicate the magnetic lineation and magnetic foliation pole,
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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 (Fig. PKM). The gneisses with
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minor melted parts and dominant melanosome fraction have very high susceptibility. Km
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values for dominant granitic fraction are lower. Granitic samples including small amounts of
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melanosome fraction present intermediate Km values. This variability in leucosome versus
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melanosome content of gneissic specimens largely explains the relatively large distribution of
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susceptibilities (Fig. PKM).
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All thermomagnetic curves (susceptibility as a function of temperature) show during
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heating a sharp susceptibility decrease around 580°C (Fig. KT), 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 -170°C (Fig. KT). This variation corresponds
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to the Verwey transition that is characteristic of pure magnetite. Few thermomagnetic curves
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are perfectly reversible. Susceptibility values from the cooling curve are mostly a little bit
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higher than the initial values, in air as in argon atmosphere (but sometimes much higher as for
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the sample AP35). Partial cooling loops during heating evidenced a weak mineral alteration
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due to heating around 300 to 400°C that probably reflects inversion of a weak amount of
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maghemite in hematite. New magnetite is also 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 (Fig. loops). 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 (Fig. Day).
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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 analyzed using
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normalized tensor variability (Hext, 1963; Jelinek, 1978) statistics.
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6.1 – Structures from AMS
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Data for gneiss and for granite are presented separately (often "gneisses" included
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minor melted parts and "granitic" samples small amounts of melanosome minerals and this
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separation has been made then considering the dominant presence of one of these facies).
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Moreover, from the regional structural field pattern, two distinct zones have been
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distinguished for the AMS analysis: the sub-vertical zones (i) and the dome shaped-zones (ii),
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Figure VIII and IX.
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i) 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 by a preferred alignment of sillimanite crystals on few samples. That is the case in
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the site test I where foliation and lineation are clearly visible. The layers richer in melt (site
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XIII) display more diffuse fabric, in particular the K1 axes that tend to form a girdle
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perpendicular to K3. Well-individualized leucosomes (e.g. Figure IIC) show a different
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magnetic fabric signature than melanosome layers characterized by strongly inclined to sub-
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vertical magnetic lineation (site XIII
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than the K1 axes. They are in the horizontal plane with two dominant directions: N-S and E-
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W.
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In
large
coarse
grained
. The K3 axes are more scattered
pink
granite
dykes
(sites
A,
E,
D
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XX
IXthe K3 axes are relatively well clustered defining a subvertical
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magnetic of strike close to that of neighboring gneisses (sites E, D
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divergent strike (sites A XX). The magnetic lineation pattern is characterized by the
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verticalization of K1-axes in comparison to magnetic lineation measured in the melanosome
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layers of neighboring gneisses. They form girdle perpendicular to K3 (sites D XX or
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strongly inclined to sub-vertical local maximum (sites AD
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islands (sites XV, VII,
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XVIII, Figure VIII) K1max is sub-vertical. For Gouverneur anatexite (sites VII,
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XV
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relatively scatted or forms clusters at 90 degrees from each other with the N-S and EW
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directions that prevails for south Gouverneur Islands (site XV).
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lies in the horizontal plane,
In mylonitic gneisses of Jakobsen (sites II, V,
) the magnetic foliation
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(K1-K2) is dominantly subvertical N-S in accord with the observed field structures (Figure
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IIf). In addition, AMS measurement reveals uncommon highly dipping or sub-vertical
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lineation.
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ii) In the migmatitic gneisses of the dome shaped-zones (sites J, VI, G, K, H, S, XVI,
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XVII, Figure VIII; sites C, IV, Q, III, VIII, I, B, i, XII, F,
) magnetic fabric
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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
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domes (sites H, K, Figure VIII; sites IV, Q, III, Figure IX) for the East flanks of the Petrel-
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Curies Islands dome). The magnetic foliations are generally in agreement with the field
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measured strike and dip of melanosome layers. The magnetic lineations show gently dip and
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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. It is in these areas that
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are observed the less well defined magnetic directions (with the widest confidence zones at
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95%) (site XVI, Figure VIII; site I, Figure IX). Sampling in a granitic dykelet of centrimetric
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width crosscutting flat-lying gneissic foliation was performed in site XVII (Figure VII).
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AMS pattern is characterized by vertical N-S magnetic foliation and sub-vertical magnetic
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lineation and then differs from that of gneiss.
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Broad dyke of coarse grained pink granites sampling in site F an XII (Figure IX) are
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characterized as for the granitic sites located in vertical shear zone by verticalization of
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magnetic lineation in comparison to the magnetic lineation measured in the melanosomes
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layers of gneisses The magnetic foliation in the dykes is either the same direction as that
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measured in the gneiss (site F) or discordant (site XII).
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6.2 – AMS parameters
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The figure P'T presents the characteristics of the susceptibility ellipsoid. The degree of
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anisotropy P’ is very high in the gneisses (mean 1.478; minimum 1.041; maximum 1.903)
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(regarder séparément gneiss mylonitique; anisotropie plus forte?). It is lower in granites
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(mean 1.224; minimum 1.038; maximum 1.413). The shape of the susceptibility ellipsoid is
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dominantly oblate, in gneisses (for T value: mean 0.30; minimum -0.08; maximum 0.66) as in
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granites (mean 0.25; minimum -0.15; maximum 0.64). In granites, a clear relationship (Fig.
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PKm, R2=0.70) between P’ (intensity of the fabric) and Km (related to the concentration in
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ferrimagnetic minerals) highlights the dominant effect of the magnetite in the AMS (Henry et
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al., 2004). Another relationship (R2=0.54) between T and P’ in granites suggests that the most
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prolate fabrics correspond to the weakest anisotropy (Fig. P’T).
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Voir la variation en carte
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7 – Discussion
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7.1 Mean susceptibility Km (à reprendre en tenant compte du descriptif pétrologique
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The amount of magnetite is clearly different in gneisses and in granites, as shown by
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the susceptibility values (Fig. PKM). Three possible mechanisms to explain the lower Km
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values in granites can be envisaged.
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(i) The granitic part came from in situ melting mainly of paramagnetic gneiss minerals, giving
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a concentration of the magnetite in the non-melted part.
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(ii) The granitic part resulted from a melt infiltration into banded gneiss from a ferrimagnetic-
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poor external source (Hasalovà et al., 2008).
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(iii) A mineralogical change affecting magnetite occurred during melting, with formation of
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ferrimagnetic oxides of lower susceptibility (like maghemite or hematite) or paramagnetic
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components.
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Concerning this mineralogical change, gneiss sample AP35 is the only one with
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hematite; it also presents the strongest formation of a lot of new magnetite at high temperature
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(susceptibility becomes 14 times higher on the cooling curve). It is therefore probably due to a
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strong local mineralogical alteration (interaction with deep fluids, weathering,?) and remains
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an exception. In all the other gneissic samples, no or few maghemite alteration during
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thermomagnetic experiments has been observed, while such alteration is more important in
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the granitic ones. This mechanism therefore likely had a significant effect for the
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susceptibility decrease in granites related to melting.
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360
The assumption of melt infiltration for the granitic thin layers within the gneiss
361
foliation seems possible only with melt concentration in some layers and very limited
362
migration of the melted components. A far external source should have given large intrusions
363
rather than widespread thin layers. In practice, mechanisms (i) and (ii) should be then not
364
significantly different about the effects on Km values, the “external source” for (ii) being
365
located within the same “initial” gneisses (see Ferré et al., 2003). Melt migration is on the
366
contrary obvious for granitic dykes crossing the gneissic structures, but the external source is
367
probably some local melt concentrations within the gneiss, the granitic facies within the dykes
368
being very similar to that within the melted thin layers into the gneiss. This mechanism is
369
likely the same for the largest granitic layers into the gneiss foliation.
370
Assuming only a local differentiation between “gneiss” layers enriched in magnetite
371
and magnetite-depleted granitic layers, the mean susceptibility has to be unchanged at the site
372
scale. The comparison between Km values in gneisses and granites sites (Fig. PKM) shows
373
that it is not the case. However, the large variation of the Km values according to the samples
374
within a same site, even in granitic sites, strongly argues for a differentiation mechanism.
375
The difference in Km between gneisses and granites therefore probably results from a
376
complicated evolution, combining in situ differentiation, locally melt migration and
377
mineralogical change.
378
7.2 AMS in gneisses and granites
379
In addition to the Km values, the main differences in magnetic fabric between gneisses
380
and granites concern the orientation of the K1 axis and the anisotropy parameters, K3 axis
381
having roughly the same orientation in both cases (Figure stereos). K1 axis is dominantly
382
close to the horizontal in gneisses and close to vertical in granites. In some granitic sites
383
(Figures Pétrels and DDU), its orientation is relatively scattered between within a vertical
384
plane, probably because of the presence of restites within the granite. The gneiss sites II and
385
V, on the south-western border of the Pétrels Island (Figure Pétrels) also present mainly
386
subvertical K1 axes. Km value in the site II has the lowest value for the gneiss sites,
387
suggesting a significant effect of a granitic component.
388
The intensity of the fabric, illustrated by the P’ values, are lower in granites than in
389
gneisses (Fig. PKM). In granites, it presents a relationship with the Km values, suggesting
390
that this difference could be explained by the lower amount of magnetite in granites, AMS
391
then resulting mainly from strong magnetite anisotropy and weak anisotropy of a low
392
susceptibility component. Owing to its Km values, this last component is the paramagnetic
393
minerals and not the maghemite. The observed inverse correlation between T and P’ in
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394
granites (Fig. P’T) shows that the AMS of this second component is less oblate than the
395
magnetite AMS.
396
7.3 Methodological implications
397
All these observations evidence that the obtained magnetic fabrics in gneisses as well
398
as in granites are composite AMS. It is then difficult to isolate pre-melting and synmelting
399
magnetic fabrics (Ferré et al., 2003). For migmatites, the AMS study is clearly not a routine
400
method, the single possible approach having to be based on the obtaining of numerous data
401
from rocks showing various degree of melting.
402
7.4 AMS and regional structures
403
404
7.5 Structural implications
405
406
407
8 – Conclusion
408
409
410
Acknowledgements
411
We are very grateful to the "Institut polaire français Paul Emile Victor" for support of
412
the program ArLiTA and to the Japanese "National Institute of Polar Research" for
413
participation to the international exchange program under the Antarctica Treaty and for
414
invitation of BH in Tokyo.
415
416
References
417
418
419
Figures captions
420
Figure I : Carte de localisation (carte archipel avec structure Gapais + localisation mondiale)
421
Figure II : Planche 6 photos (incluant photo lame-mince)
422
Figure III : Photo lame mince
423
Figure IV : PKm : ensemble des données
424
Figure V : Figure KT: AP85 par exemple + …
425
Figure VI : Figure loops: AP90 par exemple + …
426
Figure VII :Figure Day:
427
Figure VIII : Carte ASM générale
13
428
Figure IX : Carte ASM Pétrel
429
Figure X : Figure P'T: ensemble des données ??? (Possibilité de faire moyennes sur figures
430
431
précédentes)
Figure XI/ Synthèse, bloc diagramme ? Synthèse stéréo ? granite versus gneiss ?
432
433
434
Figure stereos: ensemble des données
435
Figure Petrels: diagrammes par site
436
Figure DDU: diagrammes par site
437
Figure P'T: ensemble des données
438
439
References
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