Main Donegal Granite Research Articles

The relationship between forceful and passive emplacement: The interplay between tectonic strain and magma supply in the Rosses Granitic Complex, NW Ireland

The Rosses Granitic Complex, NW Ireland, part of the late Caledonian (c. 400 Ma) Donegal Batholith, provides the opportunity to study the interplay between relative tectonic strain and magma supply rates in the wall of a major tectonic shear zone. Anisotropy of magnetic susceptibility (AMS) measurements, structural analysis and examination of key intrusive relationships were used to assess the accommodation of magma, associated deformation and magma flow pathways in this complex. In this case the varying emplacement styles, switching from forceful to passive, indicate that relative tectonic strain and magma supply rates were not constant. In the earliest component (a suite of microgranite sheets), AMS reveals subtle fabrics discordant to the sheet margins and is interpreted as post-emplacement deformation fabrics. The concordance of these fabrics to the next component of the complex, the main pluton (G1 and G2 monzogranite), indicates that this deformation was caused by the forceful emplacement of the pluton. AMS fabrics in G1 and G2 reveal a dome shaped foliation with an east-west lineation, indicating an east-west oriented magma transport direction. Outcrops of small stocks or cupolas similar to G2, east of the main pluton, link it to similar lithologies in the Main Donegal Granite further to the east via a partially exposed lateral feeder. This suggests east to west emplacement. The next component, a suite of subvertical porphyritic felsite dykes, is shown (from AMS and visible shearing fabrics) to have been emplaced passively under east-west tension. The final component comprises G3 and G4 of the main pluton, which passively cut all earlier components and contains significant amounts of aplite, pegmatite and greisen. These are interpreted to be cupolas or stocks emanating from an unexposed sheet probably similar in composition and mode of emplacement to G1 and G2. Thus, a general model is put forward where initially forceful subhorizontal sheets extending laterally from the nearby Main Donegal Granite shear zone, gave way to passive emplacement where the roofs of these and subjacent sheets failed. The control on siting of the complex can be related to lower crustal structures, which control upper crustal shear zones. The reason for the switch in emplacement style is suggested to be due to a waning magma supply, in a tectonic strain field where the extensional component was oriented east-west. The waning magma supply then possibly due to switching of the site of emplacement of magma, supplied by the Main Donegal Granite shear zone, to another member of the Donegal Batholith – the Trawenagh Bay Granite.

Flow lobes in granite: The determination of magma flow direction in the Trawenagh Bay Granite, northwestern Ireland, using anisotropy of magnetic susceptibility

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, <1500 m wide (subunit scale), and elongated east-west (parallel with the general lineation). They close consistently westward, and younger lobes in the east appear to deflect older lobes in the west. The Trawenagh Bay Granite is west of a major synplutonic shear zone that hosts the Main Donegal Granite pluton. These structures are interpreted as frozen flow lobes and indicate that the magma flowed in a westward direction, from within the active shear zone of the Main Donegal Granite into the weakly deformed wall rock (Trawenagh Bay Granite). Intra-unit pulses or surges have long been suspected in granite plutons. However, this study reports the most detailed description of such structures to date.

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

ABSTRACTThe 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 &amp; 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.

Discussion on a tectonic model for the emplacement of the Main Donegal Granite, NW Ireland

Mr M. J. Arthur writes: Hutton (1982) suggested that the Main Donegal Granite (MDG) was accommodated in a NE–SW zone of tension ('elongate hole'), contemporaneously with NE–SW sinistral shearing around that zone about 400 Ma ago (end Silurian–early Devonian times). Calculations from strain studies suggested that the sinistral displacement was about 20 km at its maximum, across the NW boundary of the MDG, but only about 5 km where the NW and SE boundaries united and continued to the SW (Hutton 1982). No figure was given for the offset across the SE boundary, though it was considered to be a major sinistral shear zone (Hutton 1982). However, Hutton (1982) proposed an unorthodox ‘crack opening model’ for the emplacement, which is adequately explained by the well established pull-apart model. The MDG has the rhomboidal/elliptical shape (Fig. 1) typical of a pull-apart produced at a releasing bend (cf. Crowell 1974, fig. 3), resulting from the intersection of the two throughgoing parts of a left-stepping sinistral wrench fault (cf. Ramsay 1980, fig. 18). The MDG is about 50 km long and 11 km wide and thus has a length/width ratio of about 4.5, whilst the ratio for pull-aparts is commonly between 2 and 5 with an average of about 3 (Aydin &amp; Nur 1982). Emplacement into an ‘elongate hole’ contemporaneously with shearing around it, is precisely a characteristic expected of a pull-apart. Although pull-aparts are perhaps best known for their occupation by sedimentary deposits they also often contain contemporaneous igneous

Radioelement abundance data for some Dalradian rocks from Co. Donegal, Ireland

AbstractData for U, Th, and K, determined by epithermal neutron activation methods, are reported for Lower and Middle Dalradian lithologies from Co. Donegal, Ireland. Abundances are identical to rocks with stratigraphical correlation in the Scottish Dalradian and appear not to have been affected by regional metamorphism to biotite and possibly even garnet grade during the Grampian orogenic phase, or later contact metamorphism in the aureole of the Main Donegal pluton. Uraniferous Dalradian lithologies have not been encountered in Donegal. Incorporation of Dalradian rocks, either by assimilation or as extensive raft trains within the Main Donegal granite, is therefore unlikely to have contributed to the development of known pegmatite-hosted U mineralization associated with this late Caledonian pluton.

Discussion on a tectonic model for the emplacement of the Main Donegal Granite, NW Ireland

P rofessor W. S. P itcher remarked: Dr Hutton has enthusiastically addressed himself to the main problem highlighted by the previous work: the reason for the wide variety of emplacement mechanisms within coeval and contiguous plutons at the same level in the crust, central to which is the relationship in time between the Main Donegal Granite and the localized but intense deformation. This new structural analysis clearly establishes the presence of an important sinistral shear component, and my reservations concern only the timing and the magnitude of the calculated displacements. Concerning the latter, the evidence is that where the ductile shear zone leaves the Main Donegal Granite pluton it seems not to displace the contact zone, and where it runs off SSW it barely displaces an earlier dislocation known as the Knockateen slide. I am equally in doubt of the extension of the postulated shear zone far into northern Fanad, unless it has been conveniently cut out by movement on the post-shear Cashelmore Fault. Within the granite outcrop itself, where the shear zone runs along the apparent boundary between the Trawenagh Bay lobe and the Main Granite, to the S of Meenderryherk, a train of xenoliths straddles the zone of deformation, by which it is distorted but not dislocated. I conclude that whilst sinistral rotational strain has been proved, the displacements suggested seem excessive. I am much attracted to the proposed model whereby magma was emplaced into space created by a curvature in an active shear zone, particularly so as the early

Deformational history of an area with well-developed tectonic slides: Dalradian rocks of Horn Head, NW Irish Caledonides

ABSTRACTA structural reinterpretation of Appin Group metasediments of the Dalradian Supergroup in part of the northwestern marginal zone of the Irish Caledonides is presented. No major F1 folds have been recorded although bedding occasionally faces upwards NW in S1. The structure is dominated by a series of NW-facing F2 recumbent folds and associated D2 tectonic slides. The slides, which have thrust the overlying rocks to the NW, are generally oblique to the fold axial planes and commonly cut out major F2 fold hinges. Rotation of the hinges towards the NW-SE (Caledonian) stretching direction in areas adjacent to the slides may have contributed to fold excision: an example of a major dislocated sheath fold is described. DM3 produces major SE-verging inclined folds. Although D4, D5 and D6 are of minor importance in the area D6 intensifies towards the SE to become the principal fabric in a major NE–SW-trending sinistrai shear zone that contains the c. 400 Ma Main Donegal granite. The early major structures of this area are compared with those in Dalradian Supergroup of adjacent parts of Scotland. It is suggested that the absence of a major F1 fold (analagous to the Islay anticline) and the occurrence of D2 recumbent folds and low angle thrusts in NW Ireland reflect the close proximity of the foreland, composed of the Lewisian complex, to the NW; such a situation is analagous to the Moine thrust zone in Scotland. The progressive development of low angle structures and thrusts to the W of the Scottish mainland is attributed to a westward shallowing of the Lewisian basement between the SW Highlands of Scotland and NW Ireland.

A tectonic model for the emplacement of the Main Donegal Granite, NW Ireland

The Main Donegal Granite is shown to lie in a major, late stage, NE-SW trending transcurrent sinistral shear zone in the Caledonides of NW Ireland. The zone, which is bent both laterally and upwards around the pluton, has displacements which diminish SW along its length from a maximum of approximately 20 km at the NE end of the pluton. The granite has also been deformed by this sinistral shear but it has a strain component of its own, the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone. Strain variations between granite and wall rock, and an early plagioclase fabric associated with the petrographic banding of the granite, suggest that intrusion and deformation were more or less synchronous (at around 400 Ma). It is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend, split lengthwise, and progressively create an internal low pressure zone into which the Main Granite was emplaced. The wide variety of emplacement styles of the other, radiometrically synchronous, plutons of the batholith can be attributed to the progressive development of the shear zone: Thorr and Fanad in tension between the zone and a complimentary structure to the N; Ardara and Toories in local compression at the shear zone termination; Rosses in tension on the outer arc of the bending zone; and Trawenagh Bay, as displaced Main Granite magma, in a similar tensional setting during the final stages of deformation.

Regional structure and syntectonic granite intrusion in the Dalradian of the Gweebarra Bay area, Donegal

6 phases of deformation are recognized in the Appin Group (Lower Dalradian) metasediments around Gweebarra Bay. The major early folds are F 2 in age (including the Crohy and Maghery Anticlines) facing up to the N or NE. S 2 transposes an earlier fabric (S 1 ) which is only rarely preserved. No major or minor F 1 folds are recognized and the facing of S 1 is unknown. D 3 produces structures verging S on S 2 but does not produce major folds comparable with D 2 . The Maghery Anticline, previously interpreted as changing from an open upright fold in the SE to overturned in the NW, is shown to be overturned throughout. Previous interpretation involving the re-folding of the Maas Syncline by the upright Mulnamin Anticline is not supported and both major folds are now interpreted as D 4 , in age, with the axial plane of the Mulnamin Anticline dipping moderately or gently S. D 4 , is a regional deformation phase synchronous with the intrusion of the Ardara Granite. D 5 , structures are upright or face to the N. The development of a major NE–SW trending sinistral shear zone adjacent to the SW end of the Main Donegal Granite is synchronous with D 5 , deformation, F 5 , folds rotating into sub-parallelism with the shear planes.

The structural setting of the main donegal granite; a reply to comments on a recent interpretation

The structural setting of the main donegal granite; a reply to comments on a recent interpretation

On the age of the Donegal Granite

Synposis Argon–40/argon-39 step heating analysis of a muscovite concentrate from the pegmatitic facies of the main Donegal granite indicates an age of 400 ± 2 Ma (IUGS standard constants) for this important intrusive event in British Caledonian history. This suggests that the previously accepted Rb-Sr regression line age of 498 ± 5 Ma ( λ 87 Rb = 1.39 × 10 −11 y −1 ) may be in error.

Deformation studies in the Irish Caledonides

An interpretation of the major structure of parts of the Irish Caledonides is made, based on the results of mapping of the main early penetrative fabrics. Interpretation of major changes in fabric are attributed to strain variation, and models are developed to explain these changes. The regional nappe structures in the Dalradian rocks have been modified by (1) D 2 slide zones, and (2) a D 4 sinistral shear zone adjacent to the Main Donegal Granite in NW Donegal. In N Mayo the major structure is interpreted in terms of a steep dextral E-W shear zone modifying the early formed nappe pile. The strain history adjacent to the Iapetus suture in central Ireland is described and a wide zone of non-axial planar cleavage discussed. In SE Ireland the regional strain pattern is modified in the tectonic aureole of the Leinster Granite.

The structural setting of the Main Donegal Granite; comments on a recent interpretation

AbstractA recent interpretation of the structural geology and chronology of the northwestern envelope of the Main Donegal Granite by Hutton (1977) is critically examined.Although many aspects of this analysis are sound, there remains serious doubt concerning Hutton's main conclusion that the intense deformation near the granite resulted from its forceful intrusion.

Dalradian structure in the Creeslough area, NW Donegal, Ireland

Summary The principal structure of the Appin Group metasediments in the Creeslough area is a series of D 2 NW-facing recumbent folds and two important slides that are thrusts (to the northwest). D 3 produces large open folds that face to the southeast. D 4 is associated with a major synform and shear zone in the aureole of the Main Donegal granite. No major F 1 fold hinges are present, although D 1 faces northwest. It is concluded that of the early deformations in Donegal, D 2 can be regarded as the Caledonian thrust front.

A structural cross‐section from the Aureole of the Main Donegal Granite

AbstractSeven phases of deformation are recognised in Dalradian metasediments within the NW aureole of the Main Donegal Granite. Major NW facing F2 folds (the Aghla Anticline and Errigal Syncline) refold an originally NW facing slaty cleavage, which is usually parallel to bedding. D3 structures cross‐cut the major F2 folds and verge and face to the SE on their normal limbs. A fourth phase of deformation intensifies towards the granite contact and is shown to be broadly coeval with intrusion and responsible for a major structure in the aureole. Three later phases are variably developed throughout the aureole. The kinematics of F4 folds in relation to the granite intrusion are briefly discussed.

The origin of banding in the Main Donegal Granite, N.W. Ireland

AbstractThis Caledonian pluton exhibits a regular, sub‐vertical banding in which layers of dark, relatively fine grained trondhjemite alternate with layers of light, coarse grained granite. The textures and the presence of cross‐cutting bands of granite indicate that the light bands were derived by potash feldspathization of the trondhjemite. The geometry of the banding indicates that it formed during the “synplutonic” deformation of the pluton, largely in situ. The bands formed by segregation of alkalies into dilatant zones of movement produced during the deformation of partly consolidated magma.

The alkali feldspars of the Ardara pluton, Donegal

SummaryThe alkali feldspars in the Ardara pluton are microcline- and orthoclase-microperthites. Their compositions have been determined by the 20 X-ray method, both before and after homogenization. The bulk compositions are very rich in potassium as a result of postmagmatic recrystallization of the alkali feldspar. There is no significant variation in either the bulk composition or the composition of the potassic phase in different parts of the intrusion. The structural state of the feldspars has been studied by measurement of 2V and obliquity of the potassic phases, and the reciprocal lattice angles α* and γ* of the potassic and sodic phases in selected crystals. Most of the specimens contain near-maximum microcline, but there is a decrease in the obliquity of the potassic phase at the eastern end of the intrusion. This can be related to the subsequent intrusion of the Main Donegal Granite.

On Some Early Dyke-Like Bodies in the Tonigala Granite, Ceylon

Abstract Several dyke-like bodies of uniform width (called "purple dykes" in the field) are found within the Tonigala Granite in north-west Ceylon. These are shown to be true dykes and comparable in some respects to the "relict dykes" of the Cornucopia and the Vancouver regions (Goodspeed, 1955; Roddick and Armstrong, 1959) and to the "early dykes" in the Main Donegal Granite (Pitcher and Read, 1960). The complex relations of the purple dykes to such elements in the Tonigala Granite as pegmatites, granitic veinlets, and biotite-rich foliae indicate that the dykes were intruded early in the history of the granite, after tensional stresses had been set up and during a long period of pegmatisation. Little evidence is however available to show the original composition and the actual mode of emplacement of the dykes. Petrographical and petrochemical similarities between the dyke rock and the surrounding granite suggest that the dykes underwent the same metasomatic changes viz. granitisation, as affected the granite in the later stages of its history.

THE AUREOLE OF THE MAIN DONEGAL GRANITE

The Main Donegal Granite, intruded into Dalradian metasediments a n d metadolerites under conditions that combined high temperature and strong directed pressure, has produced a wide aureole of contact schists that are like many regionally metamorphosed rocks. The aureole rocks were primarily of low metamorphic grade and, towards the granite contact, intense new structures and high-grade mineral assemblages have replaced the regional characters. Metadolerites occur throughout the aureole and can be used for metamorphic correlation of the diversified metasedimentary contact schists. The regional metadolerites are assemblages of albite, zoisite, actinolite and quartz, with relict pyroxene ; ophitic textures are preserved. Towards the granite contact, these rocks pass into coarse plagioclase-amphibolites with strong foliation and lineation. Mineralogical and structural transformations in the aureole are interconnected. Calc-pelites, outside the aureole showing calcite, quartz and phlogopitic mica, develop tremolite within the aureole with diopside at the immediate contact. Pelitic rocks develop kyanite, staurolite, garnet, andalusite, chloritoid, plagioclase, muscovite, biotite, with fibrolite near the contact. Structural effects are recorded farther out than mineralogical reconstruction. Small open folds appear and become tightened and steeper towards the granite; strain-slip cleavage first appearing in the outer aureole becomes the powerful schistosity of the inner aureole. Foliation, lineation, mullioning and boudinage of the rocks of the inner aureole are shared by the marginal parts of the granite. Certain minerals, such as staurolite, kyanite, plagioclase and garnet, formed during the early stages of deformation, were rotated and ruptured during a later stage in which other minerals continued to grow to make the sehistosity. A final episode is marked by great cataclastic ruptural belts that form most of the external edge of the aureole. The conditions in the aureole began with a brittle state, passed to plastic deformation and returned to brittleness. In the rafts enclosed in the granite, stability is reached by the formation of granoblastic hornfels-like rocks of simple character. Pelites give sillimanite- or andalusitegneisses, metadolerites give granoblastic hornblende-andesine-quartz rocks. The question of the controls of aluminium silicate formation is discussed. Fibrolite is considered to be metasomatic. Kyanite and andalusite are not replacements of one another and are coeval with the staurolite, garnet and plagioclase. Andalusite tends to occur in the outer aureole, kyanite in the inner. This suggests that andalusite forms at lower temperature and stress, kyanite at higher temperature and stress. But their development is not that of simple isochemical zoning, since andalusite seems to occur in rocks of special composition––pelites rich in divalent cations, Fe and Mg, the concentration of which may control the species of aluminium silicate formed. The mineral composition, the texture and structure of the aureole rocks agree with the interpretation of the emplacement of the Main Donegal Granite previously put forward by the authors.

Early Transverse Dykes in the Main Donegal Granite

AbstractCertain microtonalite dykes cut across the flow banding of the Main Donegal Granite in the direction of an important cross-joint, yet they and their granite host are shown to have crystallized together and in the same manner as both the darker flow bands in the granite and the pelitic xenoliths within it. It appears that an embryonic joint system existed in the Donegal Granite before consolidation and crystallization were complete. It is suggested that the early dykes represent metasomatized pelitic enclave material and are not differentation products of granitic magma. Comparison is made with the basic analogue provided by the beerbachite enclaves and dykes of the hypersthene-gabbro of Ardnamurchan.