Cornubian Batholith

Geological map of the region showing the Permian Cornubian Batholith hosted by Devonian and Carboniferous rocks. Contains British Geological Survey materials © UKRI 2013

The Cornubian Batholith is the large mass of granite that forms much of the peninsula of SW England. It is exposed in a series of six major plutons (large igneous bodies exposed as irregular outcrops) and several minor stocks, intruded into Devonian and Carboniferous aged metasedimentary and igenous rocks. Gravity survey data indicate that the granite is continuous at depth (i.e. all the plutons are joined below the surface) and as a whole it is over 250 km long, 20-40 km wide and up to 10 km thick. There’s also an extra little pluton, the Haig Fras Batholith, to the northwest, hidden beneath the ocean. So that’s a whole lot of granite!

The batholith owes its origin to the Variscan Orogeny, a mountain building event that occurred in the late Palaeozoic. You can read more about the Variscan here.

But isn’t granite all just granite? Errrrr…no. Granites are super fascinating, and show considerable variation in their textures, mineralogy and chemistry – even down to the metre scale. They vary so much you could spend a lifetime trying to figure out even a single location, let alone what is going on in an entire batholith.

Perhaps rather ambitious, but my PhD looked at the regional variations within the Cornubian Batholith and what that meant for the behaviour of low carbon technology metals across the region (e.g. the lithium in your electric car and the indium in your touch screen). All the conclusions are based on the current exposure level of the batholith, alongside observations made from the Rosemanowes and Bosworgy boreholes drilled within the granite. We are literally only scratching the surface of the batholith, and can only hazard a guess at what is happening at depth.

You can find the complete PhD here, but the sections below very briefly summarise the main findings. You can also read the papers here (£) and here (open access) for more information.

There’s not a lot about the regional tectonic evolution, lamprophyres and basalts or mineralisation in this section. They’re stories that deserve their own pages: Tectonics, Lamprophyres, Timeline, Mineralisation. There’s also a separate page about elvans and rhyolites, which are associated with the batholith, but would make this page far too long! Have also left out what is happening to the rare metals – get in touch if you want this information.

Granite Map
This is map is formed from previous studies, including observations in mines and quarries across the region. Granite also varies down to the metre scale, so you could spend a lifetime re-analysing outcrops and this map should be regarded as a general overview. You can visit locations and find multiple types of granite in a single place.

Geological map showing the principal mineralogical and textural variations in the Cornubian Batholith. Map from Simons et al. 2016 and compiled from Ghosh (1927), Exley and Stone (1982), Dangerfield and Hawkes (1981), Manning et al. (1996), (Selwood et al., 1998); Müller et al. (2006). Contains British Geological Survey materials © UKRI 2016.

Batholith Age
Age dating (U-Pb monazite / xenotime) of different plutons indicate that the batholith was constructed over a period of 20 million years, in two main phases. The granites that comprise the Isles of Scilly, Carnmenellis and Bodmin plutons are typically aged over 290 million years old, whereas the plutons that form the Land’s End, St Austell and Dartmoor plutons are largely younger than 282 million years old. The two groups of granites also form different granite variants which can be distinguished by mineralogy and geochemistry (see further below).

Within individual plutons there is age variation, indicated a gradual construction of individual plutons within the batholith. For example, in the Land’s End Granite, central to northern locations at Zennor, Crippletease and Castle-an-Dinas are all aged from 276-277 million years. Lamorna, in the south, is aged 274.7 million years (Chesley et al 1993).

Radiometric dates for the Cornubian Batholith ± 2σ. Blue – Pb-Pb whole rock; Red – U-Pb monazite; Green – U-Pb xenotime; Orange – Ar-Ar muscovite. CGG- coarse-grained granite; FGG- fine-grained granite. 1- Chesley et al. (1993); 2- Chen et al. (1993).

Mineralogy & Textures
Granites across SW England are dominantly composed of the minerals: quartz, plagioclase feldspar, alkali feldspar, biotite mica, muscovite mica, tourmaline and topaz in variable abundances. These seven minerals – both their abundance and size – can be used to determine which granite type you are looking at in the field.

The Cornubian Batholith can be broadly subdivided into 5 main granite types, which can be further subdivided into different textures. In the field, the following guide can be used to determine the main granite type:

Granite Two-mica (G1) Muscovite (G2) Biotite (G3) Tourmaline (G4)  Topaz (G5)
Where? Major composite plutons (Bodmin, Carnmenellis, Isles of Scilly) Small stocks associated with NW-SE trending faults Major composite plutons (Land’s End, Dartmoor) Small stocks, spatially associated with G3 Small stocks associated with NW-SE trending faults
Main features Fine- to coarse- grained, variably common (<5-25%) alkali fedlspar phenocrysts (<25 mm), Biotite approximately equal to muscovite, whitish grey Fine- to coarse-grained, variably common (<5-25%) alkali feldspar phenocrysts (<25 mm), muscovite more common than biotite, whitish grey Medium- to coarse-grained, common (15-25%) alkali feldspar phenocrysts (>25 mm), biotite, minor tourmaline, grey Fine- to medium-grained, variable alkali feldspar and quartz phenocrysts (>25 mm), biotite, tourmaline often the dominant dark mineral, grey to pinkish grey Medium-grained, equigranular, rare alkali feldspar phenocrysts, topaz as major mineral, pale micas, white or greenish white
Major minerals Quartz, microperthitic alkali feldspar, plagioclase (An15-30), Mg siderophyllite (biotite), muscovite.
Bt ≈ msc, Kfs > pl
Quartz, microperthitic alkali feldspar, plagioclase (An<10), muscovite, Li siderophyllite (biotite). Msc > kt, kfs > pl Qtuartz, microperthitic alkali feldspar, plagioclase (An15-30), magnesium siderophyllite (biotite), tourmaline, cordierite. Kfs ≈ pl, bt > tur Quartz, microperthitic alkali feldspar, plagioclase (An<8), tourmaline, ferroan polylithionite (biotite). Tur ≠ or > bt, kfs ≈ pl Quartz, microperthitic alkali feldspar, plagioclase (An<5), polylithionite (mica), topaz, tourmaline. Pl > kfs
Accessory minerals Tourmaline, andalusite, apatite, zircon, monazite, rutile. Ilmenite, xenotime, pyrite. Tourmaline, topaz, apatite, zircon, rutile, ilmenite, fluorite. Muscovite, zircon, apatite, ruitle, ilmenite, monazite, xenotime, pyrite. Muscovite, topaz, apatite, zircon, fluorite, Nb-Ta-rutile, ilmenite, thorite, phosphates. Apatite, Nb-Ta-rutile, ilmenite, muscovite, zircon, fluorite, phosphates.
Enclaves Metasedimentary None seen Metasedimentary, mafic microgranular Mafic microgranular Metasedimentary
Other features Strained quartz widespread Some stocks associated with greisen deposits Qtz-Tur orbicules common Variable textures see Manning et al. (1996) Associated with aplites (e.g. Megiliggar Rocks)

Abbreviations: Bt = biotite, Kfs – alkali feldspar; Msc – muscovite; Pl – plagioclase; Tur – tourmaline.

Examples of granite variation. Clockwise from top left: G1a Carnmenellis, G2 Hemerdon, G3a Land’s End, G5 Tregonning, G4b Land’s End, G4c St Austell.
Enclaves & Fabrics


Inclusions within the granites include country rock xenoliths and microgranular mafic enclaves (MME). The xenoliths tend to have sharp contacts with their enclosing granite and often retain their metasedimentary rock structures. In some locations there is partial dissolution of the edges of xenoliths. All MME have lobate (“blobby”) contacts with the enclosing granite, typical of magma mingling. MME are common within the G3 and G4 granites; they are not reported from other granite types.

Non-igneous enclaves (NIE) are found in the G1, G3 and G4 granites. They typically comprise biotite, sillimanite, cordierite, quartz and feldspars. Stimac at el (1995) interpreted these NIE as either representing material left from the source zone after melting (restite) or metamorphosed rocks from a greater depth than those exposed at the current level.

Magmatic fabrics

Within the batholith, particularly described in the Land’s End and Carnmenellis Granites, there are numerous locations where the larger alkali feldspar phenocrysts are aligned demonstrating the direction of magmatic flow. Mineral chemistry shows that the alkali feldspars are phenocrysts (i.e. have grown in an evolving melt) rather than forming through metasomatism (fluids) and therefore these textures can be used to determine directions of magmatic transport. Ghosh (1934) and Kratinova et al (2003) have described these textures in more detail.

Solid state fabrics

Deformation of the granites is confined to the northern edges of the older G1 (two-mica) Bodmin, Carnmenellis and Isles of Scilly granites. Mylonites are developed – the granite is sheared and recrystallised. The alkali feldspar phenocrysts may be mantled or there may be total recrystallisation of the granite into fine-grained, clay-rich zones.

Left to right: Mafic microgranular enclave (MME) in the St Austell Granite; Killas block 1.5 across in the Tregonning Granite; Aligned alkali feldspars in the Land’s End Granite.

Studies in the early 20th Century demonstrated that the batholith had been constructed through a series of different melt batches. You can see this in many locations – there are internal contacts within individual plutons. Within the Land’s End Granite there are commonly sharp, near-horizontal contacts between different granite types. Contacts may also be undulose, or there may be “blobs” of one granite in another, indicating different granite melts mingling whilst cooling. G3 biotite and G4 tourmaline granite contacts may be gradational, particularly described by Dave Manning in St Austell and Axel Müller in Land’s End. Comb-shaped pegmatitic contacts (“stockscheider”) are also observed, such as the one in the St Austell Granite at Goonbarrow.

Granite contacts, left to right: Adjacent sheets of biotite granite with differing textures in the Land’s End Granite; Fine-grained grained with medium-grained granite on Dartmoor; Pegmatite contact within the St Austell Granite.

Mineral Chemistry
The mineral chemistry across the granite types can also be utilised to determine granite types, and helps to tell the story of how the granites formed. Biotite group minerals and plagioclase in particular can effectively track granite evolution across the entire batholith.

Within G1 and G3 granites, magnesian siderophyllite micas are the most common composition. G2 and some G4 granites have pale brown lithium siderophyllite micas as the dominant composition. Pale pink zinnwaldite mica is common in the remaining G4 granites, and G5 granites. Lepidolite is present in fine-grained topaz aplites. These mica compositions (G1+G3 to G2 + G4 to G4 and G5) represent a chemical continuum in which the lithium content of the mica increases and iron and magnesium decrease. More chemically evolved granites have iron-poor micas.

Biotite group mica compositions in different granites types. Data from Henderson (1989) and Stone (1988) are shown for comparison.

Plagioclase also shows a changing composition. Plagioclase cores are more calcic than crystal rims and more evolved G2, G4 and G5 granites have plagioclase compositions of almost pure albite. Tourmaline is dominantly schorl, but in G5 granites and topaz aplites may contain sufficient lithium to reach elbaite compositions.

All of the granites are silica rich (>70% SiO2) and peraluminous – they have a higher abundance of aluminium than potassium and sodium together. Their chemical characteristics are broadly similar to granites termed “S-type” around the world (do not make me rant about alphabet characterisation of granites). However, Cornubian granites have higher potassium, rubidium, tin and tungsten relative to many other peraluminous granites around the world. Also notable in the Cornubian Batholith, are the high levels of boron, reflected in the tourmaline which is pervasive across the region.

Along with the mineralogy, textures and mineral chemistry, geochemistry can also be used to classify the granites. In particular, plotting zirconium against niobium, as suggested by Manning et al. in 1996, gives an indication from geochemistry which granite is present. G1, G2 and G3 granites have a wide zirconium range, but relatively consistent niobium, whereas G4 granites have moderate abundances of both elements. G5 granites have high niobium and low zirconium.

There is a lot more that could be said about geochemistry, this is really just a snapshot. If you would like more information on which granites are enriched in particular metals, especially rare metals, and the processes that control this, do get in touch.

Granite Source
This section will seemingly present some ideas without much discussion – if you check the papers (2016, 2017) there is more information. Otherwise, this entire section would be getting rather long!

The mineralogy and chemistry of the granites indicates that they formed from sedimentary rocks. This is consistent with what we know about the tectonic evolution of southwest England. Comparing the natural data from the Cornubian Batholith with experimental melting studies of sedimentary rocks also confirms that the granites have this source. Due to the tectonic setting and coeval mantle-derived melts (lamprophyres & basalts), a mantle component has to be considered.

The G1 two mica granites formed from melting of a greywacke-like source at >4 kbar and temperatures of 750-800 degrees. Melting was muscovite dominated, and there is no mantle component. G3 biotite granites, typically younger than 282 million years old, also formed from melting of a greywacke-like source, but at lower pressures (<3 kbar) and higher temperatures (780-820 degrees). Melting at higher temperatures is biotite-dominated, and there is a very minor mantle component within the granites.

Geochemical plots and trends indicate processes that helped to form the range of granite compositions seen. There are geochemical continuums between G1-G2 and G3-G4 granites, indicating they are linked by common processes; this is supported by their ages. The two sets of granites appear to be linked by fractionation, although fluids have likely also impacted the composition of evolved granite melts. The G2 granites and G4 granites represent geochemically evolved G1 and G3 granites respectively.

The G5 topaz granites are the outliers, with a unique chemistry and unusual mineralogy. It is a little tricky to ascertain their origin, which could be through a number of mechanisms, as noted by Maurice Stone (1992). In my thesis, I suggested that the G5 granites formed through partial melting of biotite-rich residue left after melting that formed the G1 granites. Melting was induced by fluids derived from granulite facies dehydration melting of the lower crust. This is the model invoked for the Beauvoir Granite in the Massif Central, France (Cuney, 2014) and invoked here because it was not possible to resolve fractionation models or melting models of unusual source rocks to attain the compositions show by the topaz granites. Science is largely guesswork tough – so fluid involvement or unusual source rock melting should still be considered possible mechanisms.

Granite Emplacement
Granites do not rise up through the crust like a lava lamp. The internal contacts within the granite plutons indicate that batholith has been constructed through accumulation of multiple granitic melt batches added as sheet-like intrusions. The emplacement of the granites has been facilitated by ENE-WSW and NW-SE trending faults that extend to depth and crosscut the region. As shown by the strained quartz and mylonites, melt was emplaced both after fault movement and whilst the faults were still active and the melt had not yet cooled.

Why does this even matter?

You might ask – why do we need to know all this about the granite in SW England? The Cornubian Batholith, and the fluids derived from it, are responsible for a large proportion of the mineralisation in southwest England. Our mining heritage is an important part of our history, and discoveries and inventions in Cornwall and Devon have been, and still are, used the world over. Understanding the granites also helps to understand the tectonic evolution of this part of the world and understand why our landscape is the way it is today.


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