Flow lobes in granite: The determination of magma flow direction in the Trawenagh Bay Granite, northwestern Ireland, using anisotropy of magnetic susceptibility

Abstract

Anisotropy of magnetic susceptibility (AMS) has been determined from 152 sites over the ∼64 km2 Trawenagh Bay Granite (part of the 400 Ma Caledonian Donegal Batholith). Microscopic observations, thermomagnetic analyses, high-field torque magnetometry, and heating experiments demonstrate that the AMS fabric is carried by biotite with varying amounts of mimetic magnetite in the biotite cleavage planes. The AMS fabric is interpreted as a flow fabric because microstructures indicate magmatic-state deformation: it defines large-scale macroscopic flow features, flow lobes, in this weakly deformed granite. At pluton scale the data set shows a gentle westward-plunging lineation. At a smaller scale many lobate magnetic foliation patterns can be discerned, in which the magnetic lineation is usually symmetrically disposed about the long axes of the lobes. In three dimensions the lobes are tongue-like, <1500 m wide (subunit scale), and elongated east-west (parallel with the general lineation). They close consistently westward, and younger lobes in the east appear to deflect older lobes in the west. The Trawenagh Bay Granite is west of a major synplutonic shear zone that hosts the Main Donegal Granite pluton. These structures are interpreted as frozen flow lobes and indicate that the magma flowed in a westward direction, from within the active shear zone of the Main Donegal Granite into the weakly deformed wall rock (Trawenagh Bay Granite). Intra-unit pulses or surges have long been suspected in granite plutons. However, this study reports the most detailed description of such structures to date.