TONGUE-LIKE FLOW LOBES IN GRANITE, IDENTIFIED USING MAGNETIC FABRICS, ALLOW DETERMINATION OF FLOW DIRECTION

Unequivocal evidence for the movement of magma into the emplacement site has been a long-held goal of emplacement studies. Hitherto this has been attempted using small scale macroscopic/microscopic features, such as tiling and syn-magmatic shears. The use of such features is however equivocal, since they may also be generated by syn-magmatic tectonics that are not necessarily related to the primary infilling process. Here we report the discovery of large scale macroscopic flow features, revealed by anisotropy of magnetic susceptibility (AMS) measurements, which we feel record the fundamental direction of magma movement into a granite emplacement site.

The pluton studied is the Trawenagh Bay Granite (TBG), part of the Caledonian Donegal Batholith, emplaced at around 400Ma. The body, a largely homogeneous monzogranite, was emplaced in the wall of a major, syn-plutonic shear zone within which lies the closely related Main Donegal Granite (MDG). These two plutons appear to share some magmatic units. This has led to the classical view that the MDG fed magma westward into the TBG, although a more recent model has questioned this flow direction.

AMS has been determined from 153 sites over the ~64km² TBG. At each site an orientated block was collected from which about 10 sub-samples were measured. The data set reveals many lobate magnetic foliation patterns in which the magnetic lineation is usually symmetrically disposed about the long axes of the lobes. In 3D the lobes are tongue-like, flattened in a horizontal plane, between 50-100m wide and elongated east-west. They close consistently towards the west and younger lobes in the east appear to deflect older lobes in the west. These structures are interpreted as frozen flow lobes and indicate that the magma flowed from the east towards the west, from within the active shear zone (MDG) into the weakly deformed wall rock (TBG).