The Trawenagh Bay Granite and a new model for the emplacement of the Donegal Batholith

Abstract

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 A review is presented of the last twenty-five years of published and unpublished work on the batholith, showing that the MDG shear zone was a long-lived structure almost certainly in existence before the emplacement of that body, and that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 emplacement of the Palaeogene Mourne Granite Centres, Northern Ireland: new results from the Western Mourne Centre

“…Subsidiary lineaments that trend SSW-NNE and SE-NW are also recognized in other areas of the NW Scottish and Irish Caledonides with an acknowledged control on the ascent and emplacement of mainly mid-crustal Caledonian plutons (Watson 1984;Jacques & Reavy 1994;Hutton & Alsop 1996;Stevenson et al 2006). Close to the Mourne area, the SE-NW-trending Newry Fault may be part of this system.…”

“…Hutton 1992Hutton , 1996Mahan et al 2003). Although the latter may also provide a crustal-scale ascent route for magma (Hutton & Alsop 1996;Stevenson et al 2006), plutons that are not obviously associated with a deeply penetrating structure, although they require lateral magma flow at least locally during emplacement, are nearly always assumed to have been fed by subjacent transcrustal dykes (e.g. Petford et al 1993).…”

Refined model of incremental emplacement based on structural evidence from the granodioritic Newry igneous complex, Northern Ireland

“…Several deepseated crustal lineaments are thought to have influenced ascent and emplace ment of magma in the UK (Jacques and Reavy, 1994;Hutton and Alsop, 1996;Stevenson et al, 2006;Cooper et al, 2013). These lineaments are known primarily from geophysics (gravity and magnetic data) and are thought to trend approxi mately northsouth over much of the UK to the north of the Iapetus suture.…”

“…The lineaments are thought to have acted as conduits for magma ascent, with higher crustal structures such as shear zones controlling ac commodation (e.g., Stevenson et al, 2006). For example, Jacques and Reavy (1994) presented a model where ascent of magma in SW Argyll shire was focused where NESW-trending Cale donian strikeslip structures related to Iapetus closure intersect older, presumably deeper, lineaments, which are orientated NWSE.…”

“…Where these faults intersected Gala Group tract boundaries, magma would have been able to ascend further, ultimately supplying the Newry igneous complex ( Fig. 10; e.g., Jacques and Reavy, 1994;Stevenson et al, 2006). The magma supplying the Newry igneous complex is considered to have originated from the up per mantle, before occupying a staging site and attaining intermediate composition (Meighan and Neeson, 1979;Meighan et al, 2003).…”