The Donegal batholith is a large granitic

igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

of early

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

age that

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial ...

s in

County Donegal County Donegal ( ; ga, Contae Dhún na nGall) is a county of Ireland in the province of Ulster and in the Northern and Western Region. It is named after the town of Donegal in the south of the county. It has also been known as County Tyrconne ...

Ireland Ireland ( ; ga, Éire ; Ulster Scots dialect, Ulster-Scots: ) is an island in the Atlantic Ocean, North Atlantic Ocean, in Northwestern Europe, north-western Europe. It is separated from Great Britain to its east by the North Channel (Grea ...

. It consists of at least eight separate

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

s, the largest of which is the Main Donegal Granite. It was intruded at a late stage in the

Caledonian orogeny The Caledonian orogeny was a mountain-building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occ ...

about 400 million years ago (Ma).

Extent and geometry

The Donegal batholith extends for about 75 km in a SW-NE direction from just west of the small town of Ardara to

Fanad Fanad (official name: Fánaid) is a peninsula that lies between Lough Swilly and Mulroy Bay on the north coast of County Donegal, Ireland. The origins of the name Fanad are lost in time thought there is some speculation that the name derives from ...

Head. The NW-SE extent is more difficult to determine as much of the Thorr and Fanad plutons lie offshore.

Formation

The Donegal batholith was formed during the Early Devonian, towards the end of the Caledonian orogeny between about 418 and 397 Ma. It is interpreted to have been intruded along a major SW-NE trending sinistral

shear zone In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear ...

. The space to allow the intrusion of such large volumes of granitic magma into the crust is thought to be a result of movement along the shear zone combined with the sinistral strike-slip reactivation of a major SSW-NNE trending fault, running approximately through the centres of the Ardara, Trawenagh Bay, Rosses and Thorr plutons. The chemistry and timing of the granites does not suggest that they are

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

-related. Subduction is thought to have ceased by the end of the Silurian (~419 Ma) and there is no evidence of significant involvement of mid-ocean ridge basalt or

asthenosphere The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is not ...

sources for the granitic melts. Break-off of the subducting slab following the end of subduction would lead to the upwelling of hot asthenosphere, causing melting of the overlying lithosphere producing lamprophyric magmas, underplating and injecting into the crust. The granitic magmas may be a result of either partially melting the lamprophyric underplate or by differentiation from the lamprophyric magmas. Further upwelling would lead to an increase degree of melting within the crust, contributing to a decrease in the amount of mantle component in the granitic melts. This is consistent with an overall change from more basic to more acidic with time observed in the plutons and a reduction of Barium and Strontium.

Plutons

The batholith is made of at least eight separate granitic plutons.

Main Donegal Granite

This is the largest of the plutons, extending for 44 km and about 10 km across. Three main phases of intrusion are evident. The first phase is a fine-grained

granodiorite Granodiorite () is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gra ...

, which was emplaced into the

Dalradian The Dalradian Supergroup (informally and traditionally the Dalradian) is a stratigraphic unit (a sequence of rock strata) in the lithostratigraphy of the Grampian Highlands of Scotland and in the north and west of Ireland. The diverse assemblag ...

country rock Country rock is a genre of music which fuses rock and country. It was developed by rock musicians who began to record country-flavored records in the late 1960s and early 1970s. These musicians recorded rock records using country themes, vocal s ...

, followed by sheeted

tonalite Tonalite is an igneous, plutonic ( intrusive) rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the total ...

s intruded into both the Dalradian and the granodiorite. These early phases were followed by the emplacement of a large volume of

porphyritic Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning all ...

monzogranite Monzogranites are biotite granite rocks that are considered to be the final fractionation product of magma. Monzogranites are characteristically felsic (SiO2 > 73%, and FeO + MgO + TiO2 < 2.4), weakly

Trawenagh Bay

This pluton lies at the southwestern end of the Main Donegal granite. Three distinct types of granite are recognised, an outer biotite monzogranite inside which there is a body of biotite-muscovite monzogranite and in the centre is an aplitic biotite-muscovite microgranite. The intrusion is interpreted to have the form of a

laccolith A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying ap ...

. The main part of the pluton shows only weak evidence of deformation. Studies of the anisotropy of the

magnetic susceptibility In electromagnetism, the magnetic susceptibility (Latin: , "receptive"; denoted ) is a measure of how much a material will become magnetized in an applied magnetic field. It is the ratio of magnetization (magnetic moment per unit volume) to the ap ...

(AMS) have found evidence of consistent west-east linear fabrics, interpreted to result from westward flow of the magma into the Trawenagh Bay pluton from the Main Donegal granite.

Thorr

The Thorr pluton is one of the largest bodies in the batholith, covering an area of about 800 km², although its precise extent is unknown as a major part of it lies offshore.

Ardara

The Ardara pluton has an almost circular outcrop apart from being elongated to towards the east. Most of the outer parts of the intrusion are made of a coarse-grained monzodiorite. Going into the centre of the pluton the next unit is a tonalite-granodiorite, with megacrysts. The central part is a coarse-grained granodiorite.

Rosses

The roughly circular Rosses pluton also has a concentric plan with an outer unit of medium-grained granite. Inwards this is followed over most of the outcrop by a coarser-grained granite, although it is not present to the south. Occupying most of the centre of the intrusion is a finer-grained granite, locally with the development of a small body of muscovite-rich granite.

Barnesmore

This pluton lies well to the south of other parts of the batholith and is not that well studied, partly due to a lack of exposure. Three main units have been identified, one of granodiorite and two of granite. The Barnesmore pluton is relatively enriched in Uranium and Thorium compared to other parts of the batholith. Locally the original biotite

syenite Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger proport ...

mineralogy, lacking in quartz, due to

metasomatism Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical com ...

. The intrusion has been dated as 397±7 Ma using Rb-Sr dating.

Fanad

The Fanad pluton is made up of three separate bodies, with a significant part of its extent being offshore. The main rock types are quartz

monzodiorite Monzodiorite is an intrusive rock with a composition intermediate between diorite and monzonite. It is defined in the QAPF classification as coarse-grained igneous rock in which quartz makes up 0% to 5% of the QAPF mineral fraction, plagioclase ...

and granodiorite. The intrusion has been dated as 402±10 Ma using Rb-Sr dating.

Toories

This pluton is only known from a few exposures of quartz monzodiorite and monzotonalite on islands off the Donegal coast and has been little studied. No radiometric dating has been carried out on this granite, although field relationships suggest that it postdates the Thorr pluton and was intruded before the Rosses pluton. It appears to have a similar internal structure to the Ardara pluton, suggesting it is another example of forceful intrusion.

References

{{Reflist Batholiths of Europe Geography of County Donegal Geology of Ireland Devonian magmatism