Magnetic Fabric of the Trawenagh Bay Pluton, NW Ireland. Implications for granite emplacement in the wall of a major shear zone.
C. Stevenson*, D. W. H. Hutton & W. H. Owens
University of Birmingham, Department of Earth Science
cts159@bham.ac.uk
The Trawenagh Bay Pluton (TBP) is one of six adjoining plutons comprising the late Caledonian Donegal Batholith in the NW of Ireland, and forms a distinctive lobe of the Main Donegal Granite (MDG) within this batholith. Emplacement of the Donegal Batholith has been attributed to the synplutonic Donegal Shear Zone (DSZ) occupied by the MDG. The TBP differs from the MDG in its paucity of visible structure and cross cutting nature implying that a different mechanism operated in its emplacement into the wall of this shear zone than into the shear zone itself; a scenario that has not been investigated before.
The lack of visible structure and apparent petrographic and textural homogeneity has allowed the intrusion relationship between the TBP and the MDG to also remain enigmatic. Measurement of the anisotropy of the magnetic susceptibility (AMS) provides a tool by which the magnetic fabric may be determined and related to the petrofabric. This allows a more confident and, in some respects, more complete structural appraisal.
Based on previous work, initially undertaken by Gindy (1952) and more recently by Pitcher and Price (1999), a study of the AMS of the TBP is being undertaken. These data combined with new field observations of the TBP and some of its host rocks are part of a larger study. This is aiming to gain a better understanding of this intrusion scenario and the interplay between magma emplacement in the wall of an active shear zone and development of that shear zone.
A suite of orientated blocks was collected from the TBP. These blocks were then drilled, reoriented, and the AMS measured in the laboratory to produce a magnetic fabric map of the TBP that closely agrees with field observations of mineral alignments and faint fabrics, considerably augmenting this data.
Initial AMS findings are showing a swing in magnetic fabric trace (principally the magnetic lineation) from NE-SW on the transitional contact with the MDG and parallel to macroscopic fabrics of the MDG, to WNW-ESE towards the west of the TBP. Close to the western contact of the TBP contact parallel magnetic foliation dipping steeply towards the W becomes more evident. The magnetic fabric properties also reflect petrographic variations (internal zonation) of Price and Pitcher (1999).
An elaboration of Price and Pitchers’ (1999) multipulse inclined sheet protruding from the MDG in a tensional environment is put forward. These tensional stresses are created around the bowed flank of the shear zone caused by differential displacement. As a result magma that is present is able to exploit incompatibilities in the strain field that are created. Detailed field structural observation has provided evidence for N directed extension allowing a tongue like ‘space’ to open up in the flank of the DSZ and be passively filled with MDG related magma to form the TBP.
References:
A. R. Price and W. S. Pitcher, (1999). The Trawenagh Bay Granite: A multipulse, inclined sheet intruded in the flank of a synplutonic shear zone.