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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

The structural evolution and emplacement of the Eastern Mourne pluton was investigated using anisotropy of magnetic susceptibility (AMS) measurements (carried out on 112 oriented block samples) and structural data from the host rocks. From these new data cauldron subsidence, as the emplacement mechanism, is disputed and evidence for an alternative, laccolithic style model involving inflation is presented. This includes deflection and uplift of host-rock bedding close to contacts and the magnetic fabric pattern, which has a gentle dome geometry, even close to contacts. The magnetic lineations usually plunge down-dip near the external margins but otherwise have a general SSW–NNE trend that diverges northward. This suggests a northward-directed inflow direction. The model for the emplacement of the Eastern Mourne pluton is a laterally fed laccolith, emplaced south to north. The eastern margin is interpreted as a faulted contact facilitating the inflation of an asymmetrical ‘breached’ laccolith.

The Trawenagh Bay Granite (TBG) is shown to be a tabular pluton with gently inclined contacts that, from anisotropy of magnetic susceptibility (AMS) studies, was emplaced as a series of flow lobes whose geometries indicate that it flowed horizontally towards the W out of late stage adjacent steeply inclined monzogranite sheets of the Main Donegal Granite (MDG). We thus confirm in detail the central broad idea of the Pitcher & Read (1959) model that the Main Donegal Granite fed the Trawenagh Bay Granite. Early TBG flow lobes cut and are cut by deformation associated with the sinistral shear zone in which the MDG lies, thus demonstrating synchronicity of shearing and magmatism. The TBG magma leaked out of the shear zone and emplaced into undeformed country rocks and was probably guided by shear zone splays that die out along its northern and southern margins. At a late stage in the development of MDG, the splays developed from the NNE-trending SW boundary of the shear zone and caused a gap in this structure through which TBG magma was channelled out of the MDG. A review is presented of the last twenty-five years of published and unpublished work on the batholith, showing that the MDG shear zone was a long-lived structure almost certainly in existence before the emplacement of that body, and that four of the contiguous granitiods (Thorr, Ardara, and Rosses, as well as Trawenagh Bay) were all sourced within the shear zone. A new model is presented for the development of the batholith. The pre-existing crustal structure was a deep-seated N12°E fault in the basement to the Dalradian wall rocks of the granites, that was coupled to up to six other more minor WNW–ESE basement faults in the W. A NE–SW-trending sinistral shear zone was initiated at the end of the Caledonian orogeny, as calc-alkaline and deep-seated appinites were generated in the area. This shearing activated the pre-existing structures at the current crustal level, and the N12°E structure acted as a continental transform fault which allowed the dilation needed to facilitate the wedging space requirements of the MDG and the other units in the shear zone, as well as transferring regional sinistral shear through the system. The Thorr and Ardara plutons were emplaced first into the shear zone and then those magmas leaked out into the adjacent wall rocks: one to form a large laccolith, the other to form a balloon. Steep early MDG complex sheets (granodiorites and tonalities) were emplaced in the shear zone between the Thorr and Ardara emplacement sites. Dilation continued until late stage extensive monzogranite sheets were intruded in the NW and SE of the pluton. One of these probably leaked material westward to form the Rosses laccolith and southwestwards to form the TBG in the final stages of shear zone movement.

The early Proterozoic rapakivi suite in the Ketilidian mobile belt of South Greenland is bimodal with an abundance of mafic plutons which mingled and hybridised with the rapakivi granites. Basic rocks lacking signs of hybridisation and with mineralogy and bulk compositions apparently unaffected by proximity to the granites are enriched in large ion lithophile element (LILE) relative to high field strength element (HFSE) and have moderate Nb depletion. These are characteristics common to basic rocks found in back-arc basins. This tectono-magmatic environment supports a model for the late Ketilidian of crustal extension, probably caused by slab roll-back and is in keeping with the low-pressure, high-temperature metamorphic conditions which prevailed when the rapakivi suite was emplaced. Nd and Sr isotopes confirm the juvenile character of the Ketilidian crust and rapakivi granites. Hybrid mafic rocks have low eNd compared to depleted mantle, while apparently unhybridised examples have also undergone some isotopic modification.

The emplacement mechanism of the Ardara granite (a Caledonian pluton located on the northwestern seaboard of Ireland) has been a widely cited and a key point of reference in the debate about the granite space problem for nearly half a century. Early models related its subcircular tailed shape, concentric foliation pattern, and concentrically deformed envelope to diapirism. It was subsequently reinterpreted, using strain data, as a ballooning pluton that was created by ∼70% in situ expansion at the emplacement site and was later cut by a regional shear zone. Recently it was the leading example in a wide-ranging review of forcefully emplaced plutons of this type and was reinterpreted as a series of nested diapirs associated with only modest amounts (30%) of forceful space creation, but a large component of inferred early block stoping. We report the results of the complete remapping and extensive collection of new structural data from this 8-km-diameter, normally zoned, three-phase, calc-alkaline pluton. We confirm the existence of the steeply inclined internal foliation pattern and that it was formed in the magmatic state, although this and the outline of the pluton are modified by late-stage (high temperature solid state) regional transcurrent shear zones along two of the pluton contacts. Structural mapping has failed to identify a macroscopic lineation in the magmatic foliation planes, and this is confirmed by strain determinations (using mafic enclave populations and magmatic crystal separations) for the magmatic state deformation that show nearly pure flattening type strains (K = 0). Strains measured by these techniques show an apparently smooth gradient increasing outward toward the pluton contacts from an inner low-strain zone. We argue that these features, together with the simple normal petrographic zonation, a lack of small-scale structures that would indicate the pluton had moved up relative to the wall rocks, and an exposure level that is not in the roof zone, are inconsistent with diapirism (either singly, or multiply) but are most easily explained by a ballooning emplacement mechanism. Synmagmatic shear-sense data from both the pluton and the adjacent country rocks indicate, however, that within the overall context of radial ballooning there was a limited amount of late-stage nonradial (northward) injection of magma. An inversion of the deformed enclave and crystal spacing strain determinations and strain determinations within the country rocks suggest that more than 80% of the volume of the pluton was accommodated by the ballooning mechanism.