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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.

Geoff Tanner writes: Firstly, I would like to congratulate the authors for presenting a very carefully documented, and splendidly illustrated, account of the origin of the Stralinchy conglomerates. The aim of this contribution is to focus on one important question: is their controversial model for the Donegal Dalradian supported by studies of the stratigraphically equivalent Dalradian rocks of the SW Highlands of Scotland? If an orogenic unconformity is present in the Dalradian Supergroup, evidence for this should be found at the base of, or within, the Easdale Subgroup in this region. The field relationships seen at this stratigraphical level in Scotland are reviewed briefly, and an attempt made to reconcile the results with the Donegal model. Jura Quartzite–Easdale Subgroup relationship in SW Highlands of Scotland. The contact between the Jura Quartzite and the Scarba Conglomerate Formation (which represents the base of the Easdale Subgroup) is best seen in excellent coastal exposures between Jura and Loch Creran. It is exposed on the SE side of the island of Jura; is excised by faulting on the islands of Scarba and Lunga; and re-appears on the SE side of the Rubha Aird Peninsula, 7 km north of Oban. At Lussagiven on Jura [NR 6387 8666], the top of the Jura Quartzite is marked by cross-bedded quartzite followed by 23 m of thinly bedded, commonly gritty, sandstones with conglomerate lenses. These beds, which display cross-lamination, graded bedding and channel structures, pass upwards with perfect conformity into a 36 m thick unit of laminated slate with sandstone ribs, the Jura Slate member of the Scarba Conglomerate Formation. Gritty sandstone and conglomerate beds up to 1 m thick, that characterize the Scarba Conglomerate, appear 280m farther up sequence They contain pebbles to 3 cm; rip-up clasts of mudrock; and less common rafts of sandy sediment up …

A large-scale palaeomagnetic study (125 specimens from 88 sampling localities) has been carried out on the Whin Sill complex and its associated dykes in northern England. The dominant carrier of remanent magnetization of the doleritic rocks is relatively pure magnetite of pseudo-single domain size, and the characteristic remanent magnetizations are typically well defined and unidirectional. Regionally consistent palaeomagnetic directions are obtained from three geographically distinct parts of the Whin Sill complex, here named as the Holy Island Sill (Declination/Inclination ( D / I )=189.5/−2.8, α 95 =6.3), the Alnwick Sill ( D / I =194.6/−25.6, α 95 =8.1) and the Hadrian9s Wall–Pennines Sill ( D / I =189.2/3.3, α 95 =3.5). Although the Holy Island Sill and the Hadrian9s Wall–Pennines Sill are of similar age ( c . 296 Ma) and indistinguishable on palaeomagnetic grounds, the difference in palaeomagnetic directions between them and the Alnwick Sill demonstrates the multi-component nature of the complex; magma-flow indicators suggest that the Holy Island Sill and the Hadrian9s Wall–Pennines Sill may also represent separate intrusion events. Dykes local to the three sills have remanence directions corresponding to their respective sills and can tentatively be identified as their feeder dykes. Virtual geomagnetic poles for the three palaeomagnetic sites are: Holy Island Sill and Holy Island Dyke, 346.8E, 35.4S; Alnwick Sill, High Green Dyke and St. Oswald9s Chapel Dyke, 337.1E, 47.1S; Hadrian9s Wall–Pennines Sill and Hett Dyke, 347.1E, 32.9S.

An intriguing and contradictory scenario has recently developed in the Caledonides of the British Isles concerning the ages of deposition and deformation of the Neoproterozoic Dalradian Supergroup. Isotopic evidence, although limited, suggests that the lower parts of the sequence may have been deposited and undergone Precambrian deformation (i.e. pre-600 Ma) prior to deposition of the upper parts of the sequence (i.e. post-600 Ma). Given these existing constraints, it is clear that a major break (or breaks) would be required in the intervening sequence to maintain a coherent tectonostratigraphy. We present evidence for such an unconformity preserved at the base of the Easdale Subgroup in NW Ireland. Reworked clasts that contain a pre-existing tectonometamophic history are identified within a conglomerate that lies along this regionally recognized boundary. The underlying sequence also exhibits pre-existing deformational fabrics that display erosional truncation at the base of the conglomerates. These relationships, together with significant erosion and excision of the footwall sequence, and extensive thickness and facies variations in the hanging-wall units, imply that a major tectonic unconformity exists within this succession. As the conglomerate lies stratigraphically below Precambrian ( c . 600 Ma) lavas, the structural fabrics contained within the clasts and the underlying sequence must also be of Precambrian age and totally unrelated to the well-established Early Palaeozoic (Caledonian) orogenic deformation observed in the younger parts of the sequence. The Dalradian Supergroup, as defined, may actually comprise (at least) two distinct tectonostratigraphic sequences.

Doleritic sill complexes, which are an important component of volcanic continental margins, can be imaged using 3D seismic reflection data. This allows unprecedented access to the complete 3D geometry of the bodies and an opportunity to test classic sill emplacement models. The doleritic sills associated with basaltic volcanism in the North Rockall Trough occur in two forms. Radially symmetrical sill complexes consist of a saucer-like inner sill at the base with an arcuate inclined sheet connecting it to a gently inclined, commonly ragged, outer rim. Bilaterally symmetrical sill complexes are sourced by magma diverted from a magma conduit feeding an overlying volcano. With an elongate, concave upwards, trough-like geometry bilaterally symmetrical sills climb away from the magma source from which they originate. Both sill complex types can appear as isolated bodies but commonly occur in close proximity and consequently merge, producing hybrid sill complexes. Radial sill complexes consist of a series of radiating primary flow units. With dimensions up to 3 km, each primary flow unit rises from the inner saucer and is fed by primary magma tube. Primary flow units contain secondary flow units with dimensions up to 2 km, each being fed by a secondary magma tube branching from the primary magma tube. Secondary flow units in turn are composed of 100-m scale tertiary flow units. A similar branching hierarchy of flow units can also be seen in bilaterally symmetrical sill complexes, with their internal architecture resembling an enlarged version of a primary flow unit from a radial sill complex. This branching flow pattern, as well as the interaction between flow units of varying orders, provides new insights into the origin of the structures commonly seen within sill complexes and the hybrid sill bodies produced by their merger. The data demonstrate that each radially symmetrical sill complex is independently fed from a source located beneath the centre of the inner saucer, grows by climbing from the centre outwards and that peripheral dyking from the upper surface is a common feature. These features suggest a laccolith emplacement style involving peripheral fracturing and dyking during inner saucer growth and thickening. The branching hierarchy of flow units within bilaterally symmetrical sill complexes is broadly similar to that of primary flow units within a radially symmetrical sill complex, suggesting that the general features of the laccolith emplacement model also apply.

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 Slieve League Peninsula of northwest Ireland lies on the western limb of a major orogenic strike-swing in which regional foliation trends have deviated from the northeast–southwest trends typical of much of Scotland, to west–east orientations. Across-strike coastal exposures on the western tip of the peninsula through Neoproterozoic Dalradian metasediments enable a detailed examination and analysis of the structural evolution of a Caledonian orogenic root zone which has been previously correlated with the Loch Awe Syncline of southwest Scotland. Minor structural development may be evaluated in terms of regional strain profiles and overprinting relationships. Over much of the area, a composite, steep northeast–southwest-trending S2–S3 foliation containing a gently southwest-plunging quartz mineral elongation lineation is the dominant fabric at outcrop, and is associated with MP2 almandine–amphibolite facies metamorphism. F2 folds are isoclinal with curvilinear hinges and similar geometry. They typically plunge steeply towards the southwest and display variable (dextral) or north-directed vergence, whilst minor F3 fold hinges plunge moderately towards the southwest and typically verge (sinistrally) towards the south. Major, composite D1–D3 tectonic slides are developed in the Argyll Group. Structural and stratigraphic relationships indicate that D1 induced large-scale reversals in younging across tectonic slides, resulting in reversals in subsequent F2 and F3 facing patterns. Tectonic sliding is associated with an intensification of strain demonstrated by increasingly intrafolial and curvilinear folding, together with extensional crenulations, sheared quartz pods and metre-scale asymmetric boudinage of metadolerites, all of which indicate dextral (D2) and sinistral (D3) shear. After unfolding subsequent folds (F4), this corresponds to top-to-the-north (D2) and top-to-the-south (D3) translations. D4 results in regionally northwest-verging structures, with minor crenulations and the S4 cleavage transecting fold hinges in an anticlockwise sense, suggesting a dextral component of deformation. The detailed kinematic data indicate that the overall geometry of this western, deep-level arm of the root zone is not a product of the classic mushrooming fountain of nappes model, but rather major interference between consistent northerly directed D2 thrusting and a later phase of southeast-directed (D3) retrocharriage (‘back-folding’) which intensifies towards the south.

The rapakivi granites of South Greenland were emplaced into the Ketilidian orogenic belt at about 1.74 Ga during an episode of extensional tectonics. Petrographically, they can be subdivided into a roughly subequal black facies and white facies, distinguished by the colour (turbidity) of the alkali feldspars and by the associated mineralogy which is more hydrous (i.e., amphibole and biotite compared to olivine and pyroxene) in the white facies. The oxygen isotope composition of these rocks is notably homogeneous; from thirteen outcrops sampled over an area extending some 200 km×100 km all alkali feldspars have an average δ 18 O of 10.2±0.4‰. This lack of variation is consistent with the consensus petrogenetic model which invokes a very well mixed two-component protolith; the high abundance of 18 O (mean whole rock δ 18 O =9.4‰) argues that the major contribution (perhaps 90%), although originally mantle-derived, had undergone 18 O enrichment in a low temperature sedimentary cycle. There is no oxygen isotopic distinction between alkali feldspars in the black and white facies, which is argued to be a consequence of the relatively anhydrous nature of the melt. Hydrogen isotope compositions vary widely—by 30‰—but are coherent on a hand specimen scale, as demonstrated by approximately equal δD of biotite and alkali feldspar. It is not clear at which stage the D/H heterogeneity was introduced.

Orogenesis is increasingly interpreted in terms of strain focusing, localization and partitioning processes. Such heterogeneous deformation is considered a consequence of the tectonic framework, with pre-existing structural and stratigraphic variability providing inherent zones of crustal weakness. Detailed structural investigation of Neoproterozoic Dalradian metasediments in the Glencolumbkille region, northwest Ireland, enables patterns of reworking and strain localization to be assessed in terms of four overprinting ductile deformation episodes. A well-defined and intricate Dalradian stratigraphy provides readily distinguishable markers which not only focus deformation along marked rheological boundaries, but also aid in the definition and identification of resultant geometries. Overall structural and stratigraphic relationships show that whilst D1 was not associated with major structures, D2 is related to north–northeast directed folding and ductile thrusting resulting in a major phase of crustal thickening and almandine-amphibolite facies metamorphism. Structures generated during D2 deformation subsequently became the locus of intense D3 strain and were reactivated in an oblique sense associated with south or southwest directed translations. Local overprinting relationships clearly demonstrate S2 fabrics being transposed by S3 resulting in a composite foliation over large areas. Similarly, the L2 mineral lineation is abruptly transposed by L3 over relatively small distances indicating high D3 strain gradients and the susceptibility of lineations to reworking. The final stage of ductile deformation (D4) which was increasingly localized and focused into earlier (D2−D3) high strain zones, is marked by a pronounced phase of sinistral transpression associated with clockwise cleavage and minor fold transection of northwest verging upright folds. Sinistral shear is strongly partitioned in to the steep limbs of mesoscopic F4 folds. The detailed investigation of structures generated within such multiply deformed and reworked zones provides evidence of both the kinematic and tectonic evolution of regional deformation systems.

Hercynian metamorphism and magmatism in the Cap de Creus peninsula (eastern Pyrenees) occurred contemporaneously with non-coaxial deformation in a transpressive regime. An example of this has been taken from a migmatite complex in the northern coast. The studied area is formed by partially melted sillimanite schists together with two different suites of igneous rocks: a calc-alkaline magmatic sequence, consisting of small granitoid bodies, and later peraluminous leucogranites and pegmatites. All these rocks occur within a NE–SW- to E–W-trending sub-vertical high-strain zone, where a first schistosity is tightly folded and transposed. The relations between folds and crenulation cleavage in the metasediments and deformational fabrics in the granitoids and migmatites provide good evidence for the penecontemporaneity of deformational processes, magmatism and migmatization. Furthermore, the analysis of cross-cutting relationships between different granitoids as well as the observation of their different degrees of deformation verifies that magmatism in this area took place as synkinematic consecutive intrusions from the more basic magmas to the peraluminous acid dykes.

Ropy flow structures that form in dikes and sills are very similar to ropy lava structures (pahoehoe), but are of intrusive origin. They are unusual among magma-flow indicators in that they provide a frozen record of the magma flow. Our observations suggest that they occur in large vesicles, which form as a result of local repeated pressure drops during propagation and emplacement of the magma. Adiabatic gas expansion causes a ductile rim to form around a vesicle; this rim is then sheared by the underlying flow. Anisotropy of magnetic susceptibility data from the contact zones confirm the flow data from the ropy flow structures, but the results from deeper within the body, both locally and regionally, yield different, yet consistent flow orientations. This discrepancy suggests that a distinction can be made between the magma flow close to the contact and the deeper regional flow patterns in sills and dikes.

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.

A suite of microconglomerates is recognized in Silurian rocks which occur on both sides of the proposed line of the Iapetus suture in Ireland. Clast composition and palaeocurrent data show that these conglomerates, which grade into the typical quartz-rich Silurian turbidites, were derived from two compositionally similar magmatic arc terranes which lay on either side of the present Silurian outcrop. In the Llandovery, derivation was from both the south and the north. In the Wenlock, derivation was from the north and sedimentation prograded southwards across the ‘suture’ and onto the southern margin. The source terrain in the south was probably the Ordovician Wexford–Lake District arc. We identify the northern source as another arc (Cockburnland) which has since been cut out by sinistral strike-slip against the Ordovician Northern Belt. These data imply that arc activity ceased synchronously on either side of Iapetus during the late Ordovician and this leads us to speculate that subduction of oceanic crust ended at that time. Closure was associated with deformation and uplift of the bounding Ordovician terrains. These rocks then contributed detritus to the Silurian infill of a successor basin. Regional sinistral transpression finally deformed and reorganized these units between the end Silurian and the early Devonian and led to the complete closure of the remaining Silurian seaway.

We argue that there is little which demands the accretionary prism model for the widespread Silurian rocks of the Southern Uplands of Scotland (and Ireland). With new data from Ireland, we reinterpret these rocks as the turbidite fill of a successor basin that formed after the subduction of Iapetus oceanic crust finally stopped at the end of the Ordovician. The structure of the basin is a sinistral transcurrent imbricate stack, the result of oblique terminal collision of Cadomia with Laurasia at the end of the Silurian.