Contribution of AMS measurements in understanding the migmatitic terrains of Pointe Géologie, Terre Adélie (East-Antarctica)

摘要

A detailed magnetic mapping using Anisotropy of Magnetic Susceptibility (AMS) technique was carried out in Pointe Géologie archipelago (Terre Adélie, East Antarctica) that represents a hot crust having experienced a long-lived anatectic event during Paleoproterozoic times, 1.69. Ga ago. AMS measurements allowed to better analyse the tectonic structure of the crystalline basement that is built up by rocks affected by various degrees of partial melting and then, devoid of clear strain markers. AMS sampling was performed from main rocks types of Pointe Géologie: migmatites including leucosomes and melanosomes, coarse-grained pink granites, anatexites and mylonitic gneisses. For melanosomes, the magnetic foliation is dominantly in agreement with the observed field foliation, i.e. dominantly N-S sub-vertical in shear zones and gently inclined in dome structures. AMS technique reveals a sub-horizontal magnetic lineation in migmatites from shear zones and a gently plunging one in dome structures. Magnetic properties of leucosomes and of coarse-pink granitic dykes contrast with melanosomes. The bulk susceptibility and anisotropy degree are significantly lower in granitic magmas that in melanosomes. In addition, in well-defined leucosomes, granitic dykes and anatexites, the magnetic ellipsoid is characterized by a higher plunge of the magnetic lineation, which tends to be vertical. This is associated to a rheological contrast between the solid-state deformation suffered by oxide grains in the melanosomes and their reorientation in a viscous flow during the transfer of felsic melt to the granitic dykes. Magnetic structure of leucosomes, granitic dykes and anatexites highlights the role of the gravity-induced upwelling of a crust undergoing high degree of partial melting in a transpressional regime.