CHAPTER THREE GEOLOGICAL SETTING
3.5. THE DONEGAL GRANITES
The dominant form of magmatism in Donegal is granitic in the form of the eight Caledonian granitic plutons of Thorr, Ardara, Rosses, Trawenagh Bay, Fanad, Main Donegal, Barnesmore and Toories. Several methods of intrusion are involved (Pitcher & Berger 1972) ranging from the different forms of stoping exhibited by Thorr, Fanad and Trawenagh Bay to the emplacement by magmatic wedging in a sinistral shear zone of the Main Donegal granite, while the Rosses and Barnesmore plutons were emplaced by a process of cauldron subsidence. The Toories and Ardara plutons however were emplaced by a process of diapiric distension of the wall rocks. The various plutons are all related to one another temporally and spatially and are emplaced within the so-called Donegal batholith. Evidence from field observation and radiogenic methods of Rb-Sr dating by Halliday et al. (1980) indicates that much of the granite magmatism occurred around 400 Ma and developed in the following sequence: MDG, Rosses, Ardara, Trawenagh Bay, Fanad and Barnesmore.
Appinitic intrusions have been documented from both the Fanad and Barnesmore intrusions (Walker & Leedal 1954, Pitcher & Berger 1972) but the pluton with the most numerous appinite intrusions by far is Ardara.
3.5.1 The Ardara Pluton
Various aspects of the diapiric pluton of Ardara have been described in detail by Akaad (1956), French (1966), Hall (1966), Pitcher & Berger (1972) and Holder (1979). Thus the description here is based on their observations coupled with new field and petrographic observations made during this present work. The Ardara pluton (Fig 3.4 b) has a broadly rounded outline which narrows in the east to form a 'tail' region where it comes into direct contact with the deformation associated with the MDG. It measures
8.5km from the northern margin to the southern and 12.35km from the western margin to the end of the 'tail'. The pluton is estimated to have a total area of 240km2 (Holder 1979).
(i) Pluton zoning
The pluton is zoned from margin to core, with a quartz monzodioritic outer facies passing inwards to a quartz dioritic zone which grades into a central granodiorite (Plate 3.1 a-c).
The margin (Gl) consists a coarse grained megacry Stic quartz monzodioritic facies (Gl). The mineralogy of Gl comprises megacrystic plagioclase with biotite and hornblende as the ferromagnesian phases, biotite is dominant over hornblende and together with the enclaves define the foliation which is concentric with the marginal contacts. Quartz and orthoclase are minor phases in comparison to plagioclase. Accessories include titanite, zircon and epidote. This zone is largely continuous around the margin of the whole body but is broken in the NE by the intrusion of an inner facies of quartz monzodiorite granodiorite (G2). The contact between the two is sharp in the area of L. Nagurragh but north of L. Fad this contact is gradational and interdigitating.
The G2 facies is finer grained than Gl with a more equigranular texture and coarse plagioclase laths decreasing in grain size towards the core and whilst biotite is dominant over hornblende it becomes texturally less distinctive. The percentage of quartz increases slightly while that of orthoclase stays the same. The foliation is defined by biotite and enclaves which, like that of Gl lie parallel to the margin at a relatively steep angle. North of L. Derryduff G2 passes gradationally over a distance of 10m into the central granodiorite (G3).
G3 forms a circular outcrop which takes in the low-lying terrain in the core of the intrusion. It has distinctive field characteristics of decreased biotite with increased quartz content and a more equigranular isotropic nature, these quartz crystals are often as big as the plagioclase and orthoclase and thus when they weather they give the granodiorite a rubbly, pellet-like texture.
(ii) Contact Relationships (a) Pelite-Gl Contact:
The contact between the Falcarragh pelite and Gl is a relatively sharp, planar and steep junction. The pelite is steepened up in the contact zone and has developed a strong flattening cleavage (S4) (see section 3.4.3). In the region of Narin crossroads (G7205 9865) and Clooney (G7330 9907) sinsitral shear bands can be discerned both in the pelite and the quartz monzodiorite of Gl and the pelite/Gl contact is defined by a 10cm thick tourmaline pegmatite that lies in the plane of the contact and strikes into the pelite west of Clooney (G 12T1 9900). It is apparent that sinistral shearing has opened up gaps for pegmatites and caused simple shear within the pelite, as well as deforming S4 folds within the shear zones (Fig 3.5). The intense strains described by Holder (1979) within the pluton
Plate 3.1 (a). Quartz monzodiorite (861) from the outer unit (Gl) of the Ardara pluton showing clot-like form of biotite and hornblende defining a crude foliation. Plagioclase phenocrysts form subhedral laths with smeared biotite grains. K feldspar and quartz occupy the mesostasis.
Plate 3.1 (b). Quartz monzodiorite/granodiorite (Y32) from the inner unit (G2) of the Ardara pluton with fewer plagioclase megacrysts than Gl. Texture appears almost equigranular with interlocking plagioclase, K feldspar and quartz. Biotite occurs as small euhedral flakes.
Plate 3.1 (c). Central granodiorite (Y37) from the central unit (G3) of the Ardara pluton. Texture is typically equigranular with distinctive rounded quartz crystals along with plagioclase and K feldspar and ragged, squat biotite crystals.
ww-w.
■ 1 5 ' ’ h J’, jifrr ‘*i
quartz developed in
pressure shadow
fabric
muscovite \
Fig. 3.5 The shear fabric associated with the intrusion of the Ardara pluton. From the
northern contact aureole, Clooney.
are most apparent at the margins of the outer unit (Gl) with a relatively rapid decrease in strain fabrics within Gl. The presence of strain fabric within G2 in the form of deformed enclaves may correlate with discontinuities in the strain regime at a local level.
Within the quartz monzodiorite there is evidence of sinistral shear shown by the smearing of biotite around the megacrystic plagioclase in an S/C style fabric (Plate 3.2) and it also apparent from the average X/Z ratio of the basic enclaves at the contact (about 6:1) that there has been intense flattening deformation. Above Maas (G7650 9762) the pelite is steeply folded and contorted parallel to the contact and in places it appears to be almost partially mobilised by the intrusion, whilst at Summy Lough the outer Gl facies is deformed by crude dextral shear bands striking along 020°. The relationship between the sinistral and dextral shear bands may relate to the emplacement mechanism of the Ardara pluton, the shear bands are localised and affect the grain size of Gl over a distance of 2m
where the plagioclase is granulated and broken down and the biotite defines a strong shear foliation (Fig 3.5). At that time Gl must have been in a highly ductile state as the foliation is rotated away from 020° by miniature dextral shears in small zones (30 x 5cm). This deformation may be related to the large shear zone which passes north of the Gl- Summy Lough diorite contact but which shears the Summy Lough diorite.
(b) G1-G2 Contact:
The contact relationships between the coarse grained and equigranular G2 are difficult to find in the field. A sharp boundary between Gl and G2 has been observed only in a few places. At SE L. Namanlagh (G7615 9745) it is a steep sharp but sinuous contact, while at Letterilly House Gl also has a sharp, steep contact with G2 and in places G2 can be seen to intrude Gl in the form of metre-wide dykes. Elsewhere in the north of the pluton no such contact was seen, instead a traverse across the two facies reveals a possible intermixing contact up to 10m wide with subtle changes over a few metres then a reversion to the Gl until G2 proper becomes dominant (Fig 3.6).
(c) G2-G3 Contact:
This contact is wholly gradational over a distance of ~30m. The main difference is the increased quartz content of G3 compared with G2, the relative lack of enclaves and the smaller degree of strain. This is not always the case as the composition of G3 and the numbers of enclaves are variable. Plagioclase is no longer megacrystic but more equigranular and it has mutually interfering crystals with igneous contacts.
(d) G2-G1 and Main Donegal Granite contact
The Main Donegal granite comes into contact in the area of west Tievebrack Hill (G825 965) along a SW strike to Woodtown (G8220 9535). West of Tievebrack Hill the G2 Main Donegal granite contact is poorly exposed but from the evidence of a few rock knolls it appears to be relatively sharp and maps out as interdigitating with fingers of Main Donegal Granite up to 30m long and 10m wide veining G2. These fingers generally strike parallel to the main foliation of the Main Donegal granite. Numerous Main Donegal pegmatites vein G2 and some of these are folded by later shear movements within the Main Donegal granite.
Contact of the Main Donegal granite with Gl occurs in the area of Woodtown (G8220 9535). Again parallel to the Main Donegal granite foliation of 056°, Gl is rotated and has a steep, sharp contact, marked by a break in slope in a stream valley. Gl, like G2 has a strong foliation, imposed by the Main Donegal granite and is highly veined by aplo- pegmatite veins related to the Main Donegal granite.
Plate 3.2. Sinistral shear zone within outer unit (Gl) of the main Ardara pluton. Note the heterogeneous texture of Gl within the S plane. Mafic banding of amphibole and biotite is developed along the C plane. Quarry in Gl, Clooney. Lens cap is 50 mm in diameter.
/
Clooney
\ / I \ / / V/; i ' - \ " _ I \ i ~ ' f'' intersheeted contact diffuse contact /\ ' fXy(y <' \""- /C/C' z'-/ "xT7 j/ e,'f~ / diffuse contact / ’ -•' sintersheeted contact - / - /,s< rr/
Az y-'zv A'' ■ VG1
G2
" t — / , A,'' ■" ' 'Z /
'r-''/- z" - diffuse contact 1^ I f I \\ "-v- I ( 1 intersheeted contact m'/'I
i y/y > '-/' z. , s I x '1 - - - i . " l - .-t , .-'"J-"’ *' "i'_ ' - v/' y . < ,> j' J
u, - I - L» i .1 , Z , x I I' l i i \ ‘ Z, -Z - /? ( - zLough Fad
\
Fig. 3.6 Nature of the contacts between the outer unit (Gl) and the inner unit (G2) of the Ardara pluton in the northern part of the pluton.
(iii) Lithological Variation Facies Gl
The outer facies is a coarse grained quartz monzodiorite forming an almost continuous ring 250 - 1000m wide around almost the whole intrusion except for the stalk where the inner G2 intrusion is in direct contact with the MDG. Gl is strongly foliated and coarsely porphyroblastic with numerous enclaves (see section 3.5.2). In the field Gl appears as a grey white speckled rock with abundant porphoryblastic plagioclase which is
sporadic in occurrence and non-uniform in terms of texture and grain size. At its margins biotite is abundant and can define a strong foliation fabric which, when associated with plagioclase, defines a weak sinistral S/C fabric along the northern margin and dextral shear along the western margin.
Facies G2
This rock type varies from a quartz monzodiorite to a granodiorite depending on the amount of plagioclase and quartz which are highly variable. In the field it appears to be more equigranular than Gl with a pink grey colour and dominant plagioclase with lesser amounts of quartz, orthoclase and abundant biotite. As the intrusion is traced inwards towards the south it passes transitionally into a more quartz rich-facies with equigranular texture.
Facies G3
The central member of the Ardara pluton forms a rounded body 3-4km wide in the low-lying boggy ground around L. Nagurragh (G7310 9710), L. Derryduff (G7450 9700) and L. Machugh (G7650 9550). It is compositionally and texturally variable and spatially has a variable number of enclaves. In the area around L. Nagurragh it appears as a quartz rich rock, pale pink in colour with pebbly texture due to the erosion of quartz fragments. The rock contains more quartz than G2 and less biotite.
3.5.2. Enclaves in the Ardara pluton
Enclaves within the different facies of the Ardara pluton are variable in terms of composition, number, occurrence, size and strain:
(a) Enclave composition
The following types are abundant within the pluton:
(i) Pelite and calc pelite: these probably represent fragments of Falcarragh pelite, and are most commonly found in the outer facies of the pluton, Gl. These are true xenoliths accidentally included in the magma. They tend to have straight edged contacts with the enclosing medium and show little or no reaction. However in some cases the pelites are resorbed into the granitoid as schlieren broken up along bedding laminae with increased growth of biotite.
(ii) Basic clots: these include basaltic, homblenditic and dioritic xenoliths all of which show similar textures but vary according to grain size, composition and texture. The basic clots are mostly biotite-rich as opposed to hornblende-rich and only a very few have a textural similarity to "appinite". Commonly the basaltic and metadoleritic xenoliths show the least alteration and may have sharp uncomplicated contacts with the granitoid. In some cases, as with the dioritic and hornblendite xenoliths, reaction occurs in the form of varying degrees of assimilation where edge crystals grow across the contacts which are often diffuse and sinuous. Porphyroblasts of plagioclase and orthoclase often grow within the xenoliths and in extreme cases the xenoliths become schlieric and are wholly or
incompletely resorbed. Some of these enclaves may be xenoliths of host basic rocks (e.g. metadolerite), but many (possibly the majority) are enclaves of igneous material posssibly included at depth derived from a pre-existing basic magma.
(b) Occurrence
In general the number of metasedimentary xenoliths is greatest at the margin of the contact between Gl and the pelite. Basic enclaves are present within Gl as part of the overall suite but this number increases near the contact with G2 whilst the number of metasedimantary xenoliths falls. As G3 is approached the number of enclaves decreases in the north of the area but in the south G3 contains more enclaves.
(c) Size of the enclaves
The enclaves range in size from a few centimetres to several tens of metres, as in the area NE of Summy Lough (G7080 9740). Most commonly they average ~9cm in length and 4.5 cm in width but dimensions depend on location within the pluton. Marginal enclaves are more elongate and narrower than those within the centre of the pluton, which has been attributed to complex strain histories within the pluton.
3.5.3 Emplacement of the Ardara pluton and its implication for appinite emplacement The Ardara pluton has long been considered to be a diapir, and Akaad (1956), Pitcher & Berger (1972) and Holder (1979) have all described its form. Its markedly rounded shape and intensely deformed aureole support the diapir model. Pitcher & Berger (1972) interpret the intrusion as being initially diapiric but later becoming distensional as the diapir "ballooned". The age of the pluton in relation to the regional structure of the envelope is relatively determined by its thermal metamorphism. This metamorphism is synchronous with D4 ; andalusite and staurolite porphyroblasts overprint the S4 cleavage and are later tightened around them. The S4 cleavage is also later overprinted by biotite formed during the thermal reaction, which strongly suggests that the intrusion was synchronous with D4.
Holder (1979) showed, using enclave shapes that the strain increases along with the foliation away from the core (which shows negligible strain values) to an X/Z ratio of ~5 at the margin. This strain is oblate with the X/Y plane lying parallel to the contact (see Fig 3.7).
Sanderson & Meneilly (1981) analysed the 3D strain of andalusite fabrics from the northern aureole of the pluton. They found that rotation of andalusite porphyroblasts was due to deformation of the envelope by the expanding pluton. They calculated that the strain was a flattening strain (K<1) of the order of X/Z-l 1. They reconcile this large strain ratio by arguing that the initial diapiric strain may not be recorded by enclaves in the early magma. The higher strains found on the aureole side of the contact may thus be a record of the combined strains during the development of the initial diapir and its later expansion. Meneilly (1982) noted various things may occur because of the intrusion of a diapir:
(i) A change in rheology: rocks which deform by dislocation creep or diffusion
creep have apparent viscosities which are strongly dependent on temperature (White 1976) -I
and thus the hotter rocks around the granite will have lower viscosities. ?
(ii) Stress redistribution: pressure exerted from within the expanding pluton will produce a radial distribution of maximum principal stress, ap This could be superimposed on a regional stress system but close to the granite ctj will be at a high angle to the contact.
(iii) Low angle stress: if expansion is not always perpendicular to the contact but is | oblique then depending on the competence of the contact rocks, the influence of the shear ; systems at this time may be variable and may also depend on any inhomogeneities in the
magma. Oblique expansion may develop tension gashes on a large scale along pre-existing 4 weaknesses such as bedding planes, and this may aid the emplacement of magma outside
the main pluton area.
(iv) Enclave strain: Holder (1979) measured the dimensions of a number of 4 enclaves from within the Ardara pluton and using the balloon model of Ramsay (1980)
determined changes in the shape of enclaves within the pluton and related these to changes in the volume of the pluton as due to influxes of magma in different pulses. He noted that the position of magma influxes gained from freezing surfaces (ratio of xenolith radius and total radius) coincided with the position of internal contacts between Gl, G2, and G3. From the evidence of the enclaves he concluded that 72% of the volume of the pluton was achieved by magma intruding and deforming country rocks in three distinct plutonic and thermal events as three pulses of magma. He also concluded that a degree of current activity
was the cause of distribution of basic enclaves from the Meenalargan appinitic intrusion. 4
The emplacement of appinites requires a mechanism that takes into account the deep
structure of the crust, probably related to a wrench fault shear zone mechanism. The j observation of sinistral S/C fabric on the northern side of the pluton (Plate 3.2) during this 4 study may have important implications for the emplacement mechanism of the Ardara
pluton and the satellitic appinitic intrusions. Sinistral shear at Clooney is apparent from the 4 evidence of biotite and pelitic schlieren. This shear is most obvious at the margin close to
Fig 3.7 Cartoon showing the principal strain orientations at the margin of the Ardara pluton. The deformed enclaves exhibit most intense strain at the contact of the pluton with the country rocks. The foliation planes marked on the diagram define the orientation of the X-axis corresponding to the X-axis shown on the strain elipse.
the contact and is less obvious further within Gl. Shear bands can initiate very early during