A nivel nacional:

The structural history of Donegal has been described by Pitcher & Berger (1972) which along with the work of other authors, including that of McCall (1953) and Rickard (1962) provides a comprehensive coverage of the regional structure. The sequence of events was placed in chronological order and up to six different deformation events were attributed to the Caledonian.

The structural relationships of the region under study are best seen in the area north of the Ardara pluton, particularly around Gweebara bay (G7950 9900) and are based on the work of Meneilly (1982,1983). Structural characteristics of cleavage and folding were noted in the field and although these did not form the main thrust of the thesis they were used in the structural interpretation of the country rocks and igneous emplacement.

3.4.1 Regional deformation

Major fold structures in Donegal postdate the DI deformation. The best developed phase is the D2-D3 set of recumbent folding. Perhaps the best developed of these are the Errigal syncline and Aghla anticline. These are gently inclined, facing up to the north west and are responsible for large-scale repetitions of stratigraphy in north west Donegal. No major recumbent folds of this type are seen in W. Donegal, instead, tight D2 and upright, open folds of a later (D4) age are dominant.

Before describing the structure in more detail it is important to note the importance of granitic plutons to the deformation history of Donegal. The two most influential plutons in this respect are the Main Donegal and the Ardara intrusions. Both exert strong deformational effects on their country rock envelopes. Meneilly (1983) described the structure from the two best exposed areas, both north of the Ardara pluton,

namely the Portnoo-Rosbeg and Gweebara areas, and the following is a brief summary of the main features:

(a) Portnoo - Rosbeg

The regional deformation is best seen in the Cleengort Pelite (Upper Falcarragh Pelite). The main cleavage is a penetrative S2 fabric dipping to the south. F2 folds are developed on a relatively large scale west of Portnoo and at Rosbeg, these face upwards to the north. In places S2 cleavage is cut by a gently dipping crenulation cleavage (S3) which verges and faces south on the long limbs of F2 folds. In the Portnoo area the S2 and S3 cleavages become composite and cross-cut folds at a low angle, sub-parallel to bedding, generally dipping to the south.

As previously mentioned the intrusion of the Ardara pluton has had an important effect on the structure of the area. The deformational effects of the diapir on its envelope has developed an upright cleavage - S4 which strongly folds S2 and S3 and tightens pre­ existing folds causing tangential flattening and also faulting out bedding (Fig 3.4 a & b).

Bedding and S2, S3 cleavage are greatly steepened close to the contact changing from 20°-40° up to 80°. Very close to the contact these beds are highly disrupted and boudinaged. In the case of east Clooney (G7450 9900) a strike-slip fault, the Maas- Ardlougher dislocation, is related to intrusion of the central component-G3 (Pitcher & Berger, 1972). This fault may have important implications for the emplacement mechanism of both appinite and granite as it is one of many strike-slip faults of the area which may be relatively deep-reaching fractures that have been frequently reactivated and may provide a pathway for emplacement of magma.

(b) Gweebara Bay

Like the Portnoo-Rosbeg area the structure of this area is dominated by a composite S2 S3 cleavage schistosity, deformed by the regional S4 crenulation cleavage. The major folds of the area, the Mulnamin anticline and Maas syncline strike NW-SE at Gweebara Bay and swing to a ENE strike at Mulnamin Hill, and are upright, open folds. Meneilly (1983) interpreted the Mulnamin anticline as a north-facing asymmetric fold with a moderate to gentle south-dipping axial plane coeval with the F4 Maas syncline. The D4 deformation is the dominant phase of deformation here and it is seen to become more intense as the Ardara granite is approached. The most striking deformation occurs 250m from the contact where there is no separate S4 schistosity, instead bedding and cleavage are parallel, planar and steep. No F4 folds are present as the distension involved in the diapirism has flattened out all structures into a steeply dipping planar fabric as on the west side of the pluton.

3.4.2 Regional metamorphism

The regional metamorphism of the study area reached its peak during the second phase of deformation (D2) according to Pitcher and Berger (1972). This metamorphism

Portnoo limestone (a)

oblate flattening strain on

marginal xenoliths

Fig 3.4 (a) Diagramatic section through the north western part of the Ardara pluton showing the flattening strain on the xenoliths and the country rocks (based on Pitcher & Berger 1972). (b) Map of the occurrence of S4 and S5 including intersection lineations ((34 and (35) in the Ardara area (Meneilly 1982)

reached upper greenschist facies in this area and coincided with the development of a major F2 recumbent fold phase and an associated crenulation cleavage. The effects of the regional metamorphism are best seen in the pelitic country rocks, namely the Clooney pelite and Mulnamin calc silicate flags situated to the north of the Ardara pluton.

The first stage of deformation resulted in the formation of SI, the bedding schistosity, which is poorly defined in the area. The dominant schistosity is that of S2 which is a penetrative crenulation schistosity with associated garnet and ilmenite growth, now largely replaced by chlorite and plagioclase which grew later during D3 and D4. The growth of the garnet is thought to have occurred just after D2 deformation had ceased but before D3, as they were rotated by a D3 flattening event which formed inclusion trails within the garnet.

The next major growth of minerals occurred during D4 where random porphyroblasts of biotite overgrow the S2 and S3 cleavages, locally replacing garnet. A retrogression of garnet and biotite to chlorite is thought to have occurred during D4 but in some places D2 garnet is locally replaced by a late retrogressive D2 chlorite, although the D2 garnet is preserved within the Maas semi pelites which acted as a more competent horizon (Meneilly 1983).

This sequence of events is a broad generalisation and it is complicated by the effects of thermal metamorphism in particular the contact effects associated with the diapiric Ardara pluton.

3.4.3 Thermal metamorphic effects of the Ardara diapiric intrusion

The structural effects of the Ardara pluton were to superimpose a steep cleavage, S4, on the contact schists of the envelope. These contact schists have been shown by Akaad (1956) to have an almost contemporaneous recrystallisation extending in an aureole up to 1.5km from the intrusion. In the outer margins of the aureole, biotite may be seen to replace chlorite of the regionally metamorphosed schists, including the chlorite which replaces garnet that now forms tiny aggregates of biotite. As the contact is approached a belt of andalusite-rich rock up to 500m wide is found which may be accompanied by staurolite and sometimes even kyanite, often enclosed within andalusite. The next zone closest to the intrusion is dominated by a sillimanite-rich area which also contains new garnet. The sillimanite zone consists of felted needle masses associated with ragged elongate biotite parallel to the schistosity, or as inclusions within quartz. The thermal garnet in this zone, like that seen at Portnoo, is highly corroded and forms irregular masses with irregular crystal shape.

Finally, closest to the contact, yellow pinitised cordierite is present in bands up to 15cm thick with biotite and quartz inclusions in a hornfels best seen at Clooney (G9895 7250). This zone is adjacent to the main pluton which also contains sporadic tourmaline pegmatite veins and veinlets along the plane of contact.

3.4.4 Thermal metamorphic effects of the intrusion of the Main Donegal Granite

The mineralogical effects of the intrusion of the Main Donegal Granite (MDG) are not well seen in the study area but according to Pitcher and Berger (1972) as the growth of kyanite, staurolite and garnet developed in 'sweat-outs' aligned parallel to the main schistosity in the rocks east and NE of the area. Another mineralogical feature described by Hall (1966) is the loss of triclinicity in microcline in the Ardara pluton which he attributed to reheating of microcline by the main granite. The deformational effects of the intrusion are seen in the form of development of a steep S5 cleavage which crenulates homfelsed aureole rocks, folds pegmatite sheets and deforms thermal muscovite (Meneilly 1983). Most notable of the deformational effects is the rotation of all structures towards 056° at the SW tail of the MDG and the development of a steeply plunging lineation within all rock types in the aureole of the MDG. In the Mulnamin calc silicates upright F5 folds of S4 trend are found parallel to the shear zone associated with the emplacement of the MDG.