Melt segregation, pervasive melt migration and AMS fabric development from migmatites to crustally-derived granite: Central Vosges
Jean Bernard Edel1 and Karel Schulmann1,2, Josef Ježek3
1Ecole et Observatoire des Sciences de la Terre, 5 Rue René Descartes, Strasbourg, France
2
Institute of Petrology and Structural Geology, Charles University, Albertov 6, Praha 2, Czech Republic3
Institute of Applied Mathematics, Charles University, Albertov 6, Praha 2, Czech RepublicComplex behaviour of solid state rocks including granitic melts in region of fertile magma segregation was studied in the metasedimentary and leptynitic migmatites of the Central Vosges. The metasedimentary metatexites and diatexites were deformed by homogeneous viscous flow, but show a complete continuity between pre-rheological critical melt percentage AMS fabrics and AMS fabrics associated with viscous magma flows. Metatexites with low proportion of melts (<50%) and metagraywacke mesosomes exhibit rather high degree of anisotropy associated with plane strain to oblate ellipsoid shapes. The leucosomes show plane strain ellipsoid shapes and weaker degree of anisotropy reflecting strain partitioning where the bulk vorticity is split between coaxial high viscosity restites and non-coaxial lower viscosity melts. The diatexites and granites (>50% of melts) show very weak degree of anisotropy and highly variable ellipsoid shapes which may reflect undisturbed rotation of careers of magnetic anisotropy (biotite) in freely moving melts.
Mechanical behaviour of rocks with low ability to melt also depends on the volume of granitic magma. In regions with low volume of melts, the magma is ascending along the main anisotropy in the form of sills. In domains where the magma proportion is high, these lithologies behave as rigid sheet-like bodies. This behaviour was quantified using angular relationships between leptynites and homogeneously flowing magma by calculating the angle between main susceptibility directions of each subsite in leptynitic domain and mean vectors of average tensor of magnetic susceptibility from areas dominated by undisturbed magmatic flow. Low proportion of melt is characterized by preservation of homogeneous AMS fabric of solid state network at high angle to fabric pattern of pervasive homogeneous viscous flow. High proportion of melt is identified by disintegration of pre-melting fabric elements and their progressive reorientation towards directions of main magma flow.
We present a model of distintegration of rigid framework fabric formed by leptynitic gneisses in course of viscous magmatic flow. The model approximates rotation of folded rigid sheets with previously developed anisotropy in simple shear and combined simple-pure shear flows.