The emplacement of the Rieserferner Pluton (Eastern Alps, Tyrol): constraints from field observations, magnetic fabrics and microstructures

Within the alpine chain several Tertiary intrusions (e.g. Bergell, Adamello) intruded along the Periadriatic Lineament. This study attempts to show how the Rieserferner Pluton (Vedrette di Ries) is related to the Defereggen-Antholz-Vals (DAV)-Line. For a survey of the fabric pattern over the pluton the anisotropy of the magnetic susceptibility (AMS) was determined on nearly 170 samples. Low bulk susceptibilities of Kvol<450×10–6 SI and a total degree of anisotropy P′<1.2 point to a paramagnetic control of the magnetic properties. The coincidence between the magnetic fabric and the macroscopic fabric was proven via texture measurements and the HFA technique. Within the western Rieserferner Pluton microstructures gave no evidence for a solid-state deformation of the plutonic rocks while the central domain presents a continuum from magmatic structures in the south to a penetrative solid-state deformation along the northern margin. Structural observations indicate that the magma was locally emplaced along an active strike-slip fault. The magnetic foliation of the pluton forms three interconnected asymmetric dome structures corresponding to the Western, Central and Eastern Rieserferner. Generally, E–W-trending lineations are sigmoidally deflected along the northern and southern contact of the Central Rieserferner. Weakly defined fabrics in the Western Rieserferner suggest magma accommodation in some kind of pull apart structure related to the left lateral shear on the DAV-Line. In the Central Rieserferner preferred orientation of contact metamorphic sillimanite as well as syn-intrusive isoclinal folds indicate the thinning of the partly preserved roof envelope of the pluton, implying some ballooning of the topmost intrusion during the final stage of emplacement.