Rb‐Sr isochron study of the Thorr and Main Donegal Granites, Ireland

O’Connor, Patrick J.; Long, C. B.; Kennan, P. S.; Halliday, Alex N.; Max, M. D.; Roddick, J. C.

doi:10.1002/gj.3350170403

Cited by 25 publications

(7 citation statements)

References 28 publications

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“…Emplacement of the late to post-orogenic Donegal Batholith (Fig. 1) occurred c. 428-400 Ma (O'Connor et al 1982(O'Connor et al , 1987Condon et al 2004) and we suggest generation of the Bt 2 biotites as a result of a thermal perturbation related to its emplacement. In that the Bt 2 biotites post-date formation of the shallow-dipping S L foliation and their 40 Ar/ 39 Ar ages reflect crystallization, they provide a minimum age constraint upon the development of S L .…”

Section: Geological Implicationsmentioning

confidence: 80%

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Laser ablation ⁴⁰ Ar/ ³⁹ Ar dating of metamorphic fabrics in the Caledonides of north Ireland

Condon

Hodges

Alsop

et al. 2006

JGS

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In situ 40 Ar/ 39 Ar dating of two texturally distinct biotite generations within a phyllite from the Caledonian orogen of Ireland was performed using a high spatial resolution laser microprobe. In this sample, an older generation of biotite (Bt 1 ) is aligned subparallel to a steeply dipping crenulation foliation, which itself is overprinted by a regionally developed shallow-dipping foliation. The second biotite generation (Bt 2 ) occurs as randomly oriented porphyroblasts that overprint and therefore postdate both foliations. A dataset of 89 ages from a single thick section was acquired for six Bt 1 and eight Bt 2 crystals. The two biotite generations yielded two statistically distinct age distributions with Bt 1 maxima at c. 433 Ma and Bt 2 maxima at c. 417 Ma. A small number of analyses (11% Bt 1 and 26% Bt 2 ) from within crystals yielded 'main population' ages that conflict with the dominant age populations and relative chronology established by field relationships. Discordant analyses from the earlier Bt 1 crystals are younger (than the Bt 2 population) and probably reflect intracrystalline partial resetting during a later thermal perturbation. The discordant Bt 2 analyses are predominantly older (than the Bt 2 population) and are interpreted as indicating localized excess 40 Ar. Such dis cordant analyses indicate intracrystalline variation reflecting both excess 40 Ar (Bt 2 population) and 40 Ar loss (Bt 1 population), and were therefore discarded when determining geologically meaningful ages.

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Section: Geological Implicationsmentioning

confidence: 80%

“…1) that form part of the late to postorogenic 'Newer Granite' suite. Geochronological constraints indicate that construction of the Donegal Batholith was protracted and occurred from c. 428 until c. 400 Ma (O'Connor et al 1982(O'Connor et al , 1987Condon et al 2004).…”

Section: Geological Setting and Sample Descriptionmentioning

confidence: 93%

Laser ablation ⁴⁰ Ar/ ³⁹ Ar dating of metamorphic fabrics in the Caledonides of north Ireland

Condon

Hodges

Alsop

et al. 2006

JGS

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“…The Donegal Batholith of NW Ireland (Fig. 1), made famous by the classical work of Pitcher and Berger (1972), is made up of a suite of Caledonian granitic plutons emplaced into Laurentian crust in the mid-Silurian-early Devonian (428-400 Ma, U/Pb: Condon et al 2004;418-397 Ma, Rb/Sr: Halliday et al 1980;O'Connor et al 1982O'Connor et al , 1987, during the final closure of the Iapetus Ocean (Ghani and Atherton 2008). During the development of the Caledonian orogen, the Laurentian margin http://www.dx.doi.org/10.12789/geocanj.2019.46.144 was affected by two distinct orogenic episodes (e.g.…”

Section: Geological Settingmentioning

confidence: 99%

Age, Geochemistry and Origin of the Ardara Appinite Plutons, Northwest Donegal, Ireland

Murphy

Nance

Gabler

et al. 2019

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In northwest Donegal, Ireland, a large number of coeval appinitic (hornblende-plagioclase-rich) plutons and lamprophyre dykes occur around the Ardara pluton, a granitic satellite body and one of the oldest phases of the ca. 428–400 Ma composite Donegal Batholith. The appinite units form a bimodal (mafic–felsic) suite in which hornblende is the dominant mafic mineral and typically occurs as large prismatic phenocrysts within a finer grained matrix. Lamprophyre dykes are mafic in composition with a geochemistry that is very similar to that of the mafic appinite bodies. Both mafic rocks are subalkalic, with calc-alkalic and tholeiitic tendencies, and show trace element abundances indicating that the mantle source was contaminated by subduction zone fluids. 40Ar/39Ar analysis of hornblende separated from two samples of appinite yield mid-Silurian (434.2 ± 2.1 Ma and 433.7 ± 5.5 Ma) cooling ages that are interpreted to closely date the time of intrusion. Hence, according to the available age data, the appinite bodies slightly predate, or were coeval with, the earliest phases of the Donegal Batholith. Sm–Nd isotopic analyses yield a range of initial εNd values (+3.1 to –4.8 at t = 435 Ma) that, together with trace element data, indicate that the appinitic magmas were likely derived from melting of metasomatized sub-continental lithospheric mantle and/or underplated mafic crust, with only limited crustal contamination during magma ascent. The appinitic intrusions are interpreted to have been emplaced along deep-seated crustal fractures that allowed for mafic and felsic magma to mingle. The magmas are thought to be the products of collisional asthenospheric upwelling associated with the closure of Iapetus and the ensuing Caledonian orogeny, either as a result of an orogen-wide delamination event or as a consequence of more localized slab break-off.RÉSUMÉDans le nord-ouest du Donegal, en Irlande, un grand nombre de plutons appinitiques (riches en hornblendes ou en plagioclases) et de dykes de lamprophyres contemporains se retrouvent autour du pluton d’Ardara, un corps satellite granitique et l’une des phases les plus anciennes du batholite composite de Donegal, âgé d’environ 428–400 Ma. Les unités de l’appinite forment une suite bimodale (mafique–felsique) dans laquelle la hornblende est le minéral mafique dominant et se présente généralement sous forme de grands phénocristaux prismatiques au sein d’une matrice à grains plus fins. Les dykes de lamprophyres ont une composition mafique dont la géochimie est très similaire à celle des corps d’appinite mafique. Les deux roches mafiques sont subalcaliques, avec des tendances calcoalcalines et tholéiitiques, et elles montrent des teneurs en éléments traces indiquant que la source du manteau a été contaminée par des fluides de zone de subduction. L'analyse 40Ar/39Ar des hornblendes provenant de deux échantillons d'appinite donne des âges de refroidissement du Silurien moyen (434,2 ± 2,1 Ma et 433,7 ± 5,5 Ma) qui sont interprétés comme étant proches de la date de l’intrusion. Par conséquent, selon les données d’âge disponibles, les corps d’appinite sont légèrement antérieurs ou contemporains des toutes premières phases du batholite de Donegal. Les analyses isotopiques Sm–Nd aboutissent à une gamme de valeurs εNd initiales (+3,1 à -4,8 à t = 435 Ma) qui, associées aux données des éléments traces, indiquent que les magmas appinitiques sont probablement dérivés de la fusion d'un manteau lithosphérique souscontinental métasomatisé et / ou d’une croûte mafique sousplaquée, avec une contamination crustale limitée lors de l'ascension du magma. Les intrusions appinitiques sont interprétées comme s'étant mises en place le long de fractures profondes de la croûte qui ont permis au magma mafique et au magma felsique de se mélanger. On pense que les magmas sont les produits de la remontée (upwelling) asthénosphérique collisionnelle associée à la fermeture de l’océan Iapetus et à l'orogenèse calédonienne qui s'ensuit, soit à la suite d'un délaminage à l'échelle de l'orogène, soit à la suite d'une rupture plus localisée de la plaque.

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“…This has remained enigmatic, as not all of the plutons have direct crosscutting relationships. Furthermore, radiometric dates have so far been unable to distinguish the absolute emplacement ages of individual plutons and the units within them (Halliday et al 1980;Siegesmund & Becker 2000;O'Connor et al 1982). The present model makes the reasonable assumption that the timing of emplacement of the constituent plutons is very close and individual plutons are likely to overlap significantly.…”

Section: Some Concluding Remarksmentioning

confidence: 99%

“…The Donegal Batholith is a well known and well studied late Caledonian batholith (Pitcher & Berger 1972), the constituent plutons of which were intruded penecontemporaneously at around 400 Ma (Pitcher & Berger 1972;O'Connor et al 1982) (Fig. 1).…”

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confidence: 99%

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

Stevenson

Hutton

Price³

2006

Transactions of the Royal Society of Edinburgh: Earth Sciences

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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.

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Rb‐Sr isochron study of the Thorr and Main Donegal Granites, Ireland

Cited by 25 publications

References 28 publications

Laser ablation ⁴⁰ Ar/ ³⁹ Ar dating of metamorphic fabrics in the Caledonides of north Ireland

Laser ablation ⁴⁰ Ar/ ³⁹ Ar dating of metamorphic fabrics in the Caledonides of north Ireland

Age, Geochemistry and Origin of the Ardara Appinite Plutons, Northwest Donegal, Ireland

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

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Rb‐Sr isochron study of the Thorr and Main Donegal Granites, Ireland

Cited by 25 publications

References 28 publications

Laser ablation 40 Ar/ 39 Ar dating of metamorphic fabrics in the Caledonides of north Ireland

Laser ablation 40 Ar/ 39 Ar dating of metamorphic fabrics in the Caledonides of north Ireland

Age, Geochemistry and Origin of the Ardara Appinite Plutons, Northwest Donegal, Ireland

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

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Laser ablation ⁴⁰ Ar/ ³⁹ Ar dating of metamorphic fabrics in the Caledonides of north Ireland

Laser ablation ⁴⁰ Ar/ ³⁹ Ar dating of metamorphic fabrics in the Caledonides of north Ireland