Contribution of AMS studies to understanding the deformation in the migmatitic terrains of
Pointe Geologie, Terre Adélie (East Antarctica)
1/ Introduction (Jerome-Bernard)
-
Complexité des caractéristations structurales dans les terrains migmatitiques
Apport de l’ASM dans ce type de terrain (travaux de Férré et al, 2003;
2/ Geological setting (René Pierre-Jerome)
3/ Regional structures (René Pierre-Jerome)
4/ Magnetic Mineralogy
Image microsopie optique, Courbes K(T), hysteresis
5/ Structures from AMS (Jerome –Bernard)
Mise en evidences des differences de pattern entre granite et gneiss
5. AMS parameters
5.1. Mean susceptibility
(Gneiss/Granite)
5.2. Anisotropy (P’) and shape (T)
(Gneiss/Granite)
6/Discussion
7/ Conclusion
Introduction
………..
AMS study has been applied to the migmatitic and anatectic formations of
the Pointe Géologie archipelago in Terre Adélie, East Antarctica. Previous
structural studies have been performed based on foliation and lineation
measurements on migmatites (Pelletier et al., 2002; Gapais et al., 2008). But we
believe they may be supplemented by taking into account the magnetic fabric of
the leucosomes and anatexites, which correspond to more than 50% of the
outcrops and which are devoid of clear markers of deformation. This would
greatly improve the structural pattern of this region.
Geological setting
The Terre Adélie Craton (TAC) is located on the easternmost part of the
East Antarctic Shield. It crops out as islands and capes scattered along the coast
between 135° and 145°E but most of the outcrops are located in three main
archipelagos, from West to East: around Pointe Géologie (66°50’S, 140°E), East
of Port Martin (66°80’S, 142°25’E) and East of Cape Denison (67°S, 142°25’E)
up to the Mertz glacier. This crystalline basement consists of Late Archean and
Paleoproterozoic formations devoid of any significant reworking since 1,5 Ga
and then it is considered as part of the Mawson block or Mawson continent
(Fanning et al., 1995, 2002). At the scale of the TAC, the Late Archean and
Paleoproterozoic terrains are distributed respectively westwards and eastwards
from Port Martin (Monnier et al., 1996; Ménot et al, 2007).
- Fig. 1 -
This paper deals only with the Paleoproterozoic formations of the Pointe
Géologie area, i.e. close to the permanent French Polar Station Dumont
d’Urville. As described by Bellair (1961 a & b), the main rock types are
migmatitic to anatectic gneisses that are cross cut by dykes of isotropic coarsegrained pink granites. The detailed mapping, carried on during the GEOLETA
project, improves this previous description and points out some new and
significant features (Monnier et al., 1996; Peucat et al., 1999; Gapais et al.,
2002). Migmatitic gneisses and anatexites derived from a sedimentary sequence
dominated by aluminous pelites together with subordinate greywackes,
quartzites and calc-silicates. Metagreywakes appear as intercalations ranging
from tens of centimeters to hundreds of meters in thickness. Quartzites and calcsilicates form dismembered layers of mesosomes within the migmatites and they
mark out a poorly preserved sedimentary layering. Amphibolitized dolerites and
gabbros occur as dikes and sills and they are coeval with the anatectic event as
shown by magma mingling features between the mafic intrusions and the
granitic melts. Isotopic data indicate Paleoproterozoic sources for both
sediments and magmas (Peucat et al., 1999 and ref. therein). According to
Monnier et al. (1996), mesosomes, neosomes and anatexites represent
respectively 10%, 60% and 30% of the rock types in the Pointe Géologie
archipelago.
These mesocrustal formations underwent a polyphased tectonic and
metamorphic evolution with three phases of deformation leading to a composite
subvertical regional S0-S1-S2 foliation (Monnier et al., 1996; Pelletier et al.,
2002). Neosomes display two successive parageneses related to both the S 1 and
S2 metamorphic foliations and bearing comparable Sil+/-Kfs+/-Pl+/-Bt+/- Cd+/Grt+/-Ilm+/-Qtz mineral assemblages. Moreover, mineral chemistry of the S1
and S2 parageneses is similar and argues for a single continuous LP-HT
metamorphic event, the peak conditions of which reaching 750-700°C at 0.6-0.4
Gpa. A weak retrograde imprint with Bt+/-Ms+/-And assemblages marks the
later evolution to about 550-450°C and 0.5-0.4 Gpa (Monnier et al., 1996;
Pelletier et al., 2002, 2008).
Evolution tectono-metamorphique
 fusion partielle continue pendant deformation polyphasée
 single continuous metamorphic event under constant P-T
conditions P= T= ; puis retrogression
Geochronology (Bellair et Delbos, 1962; Peucat et al., 1999)
Regional structures
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