The Cornubian batholith is a large mass of granite rock, formed about 280 million years ago, which lies beneath much of Cornwall and Devon in the South West Peninsula of England. The main exposed masses of granite are seen at Dartmoor, Bodmin Moor, St Austell, Carnmenellis, Land's End and the Isles of Scilly. The intrusion is associated with significant quantities of minerals, particularly cassiterite, an ore of tin which has been mined since about 2000 BC. Other minerals include china clay and ores of copper, lead, zinc and tungsten.
It takes its name from Cornubia, the Medieval Latin name for Cornwall.
Extent and geometry
A batholith is a large mass of intrusive rock formed from the crystallization of molten rock beneath the Earth's surface (magma). From gravity and magnetic geophysical data, the batholith is interpreted to extend from about 8°W, more than 100 km southwest of the Isles of Scilly, to the eastern edge of Dartmoor. The negative gravity anomaly, caused by the relatively low density of the granites compared to average continental crust, is linear and trends WSW-ENE, parallel with that associated with the Haig Fras granite.
The shape of the batholith and the relationship between the individual plutons and the main mass of the granite remained entirely speculative until gravity data began to be used to constrain the thickness and shape of the batholith by modelling. The initial work by Martin Bott suggested that the batholith had an overall trapezoidal shape with a base to the batholith at about 10–12 km. However, current understanding of granite pluton shape suggest that most are either laccolithic or lopolithic. Comparisons with other examples would suggest that the thicknesses of the individual plutons would be in the range 3–5 km, based on their outcrop widths. The volume of the batholith was estimated in 1989 to be around 68,000 cubic kilometres.Selwood et al., p. 120
Formation
The Cornubian batholith formed during the Early Permian period, from about 300 to 275 Ma (million years ago) at a late stage in the mountain-building event known as the Variscan orogeny as a suite of late orogenic granites. It has been suggested that stretching of the Earth's crust (crustal extension) allowed the granitic magmas to move to higher levels in the crust. Evidence from neodymium and strontium isotopes suggests that the magmas that formed the batholith were mainly the result of partial melting of the lower crust with a minor component of basaltic magma from a mantle source. This lower crustal source is likely to have consisted of both metasedimentary and metavolcanic rocks, of Proterozoic age (2500 million to 539 million years old).
As the molten rock cooled to around 1000 Celsius it solidified and crystallised and fractures along vertical joints formed. Over time the slate and sandstone rocks covering the granite were eroded exposing the granite in areas such as Dartmoor and Bodmin Moor. The granite also expanded and horizontal joints were formed. These joints are most clearly seen on exposed pieces of rock such as the Tors of Dartmoor and Bodmin Moor. As the granite erodes further, blocks of eroded granite known as clitter are left.
Timing An approximate age of emplacement for the Cornubian batholith was known before radiometric dating methods became commonplace, from the observed relationships with the sedimentary rocks – both those which formed before the intrusion and those which were laid down after it. The youngest rocks into which the granites intrude are the Carboniferous Crackington and Bealsmill formations of Namurian to lower Westphalian age. This provides a lower bound for the timing of granite emplacement of about 310 Ma.
The earliest instance of granitic clasts (fragments of the granite which have weathered away and become part of a new sedimentary rock) in younger sedimentary sequences is from the Late Permian St. Cyres beds. This gives an upper bound of about 250 Ma.
The inferred age of emplacement from this evidence of Late Carboniferous to Early Permian has been confirmed by radiometric dating, although it has shown that the individual intrusions were emplaced over a significant time interval. The earliest dated major intrusion is the Carnmenellis pluton at 293.1±3 Ma. The youngest dated granite is the southern lobe of the Land's End pluton, intruded at 274.5±1.4 Ma. The earliest recorded magmatic activity is the intrusion of the small Hemerdon pluton on the southwestern flank of the Dartmoor pluton at 298.3±2.3 Ma. There is no apparent systematic variation in age of plutons compared to their position within the batholith. This suggests that the batholith grew by the coalescence of a series of separate intrusions over a period of about 25 Ma.
Mechanism of emplacement As with all large intrusive bodies, the method by which the batholith's plutons were emplaced is a matter of debate, due to the space problem of adding such large masses to the upper crust. Four main mechanisms have been proposed; stoping, diapirism, extensional faulting and uplift of the overlying country rock over a laccolithic sill with a relatively small vertical dyke feeder.
Evidence for a stoping mechanism has been described locally from the margin of the Tregonning intrusion, where a series of intrusive sheets extend out from the roof zone of the intrusion into the country-rock. Although the Land's End pluton was once thought to have a diapiric origin, its emplacement is now interpreted to have been accommodated by fault movements during regional extension.
Plutons
The individual plutons that comprise the Cornubian Batholith can be broadly subdivided into five main lithologies: Two mica, Muscovite, Biotite, Tourmaline and Topaz granites, each named for their distinguishing mineral(s). The two-mica and muscovite granites are older, and can be found in the Carnmenellis, Bodmin and Isles of Scilly, whereas the younger biotite and tourmaline granites are found within the Land's End, St. Austell and Dartmoor plutons. Topaz granites outcrop within the Tregonning, Land's End and St Austell plutons.
Dartmoor This is the largest exposed area of granite which also forms the easternmost development of the batholith. The granite consists of two main types, coarse-grained granite with abundant large alkali feldspar megacrysts and coarse-grained granite with few megacrysts. To the southwest there is an area of coarse-grained granite with small megacrysts and several small exposures of fine-grained granite, particularly in the southeastern part of the outcrop. Interpretation of the gravity field over this pluton suggests that it is sheet-like with a thickness of nearly 10 km, and a root at its southern end extending down to about 17 km depth, which may represent the conduit that carried the magma to shallow crustal levels. It appears to have been intruded along the interface between Devonian and Carboniferous rocks. Uranium-lead dating of Monazite from this intrusion gives emplacement ages of 278.2±0.8 Ma and 280.4±1.2 Ma.
Bodmin Moor The Bodmin Moor pluton consists mainly of coarse-grained granite with abundant small megacrysts. Towards the centre and the western margin of the outcrop there are smaller bodies of fine-grained granite. Gravity data suggest that this pluton is in the form of a south-southeastward thickening wedge, reaching a maximum of about 7 km. Monazite from this pluton gives an emplacement age of 291.4±0.8 Ma.
St Austell The St Austell pluton consists of coarse-grained megacrystic granite with large megacrysts at the western and eastern ends of the outcrop. The central part of the pluton is also coarse-grained but lacks the megacrysts. Between the central megacryst poor and the large megacryst rich coarse-grained granites at the western end, a medium-grained granite is developed with lithium-mica. Smaller bodies of fine-grained granite are found in the central part of the outcrop and at the western end. Gravity data indicate that this pluton is wedge-shaped similar to that at Bodmin. Monazite gives an emplacement age of 281.8±0.4 Ma for this pluton.
Carnmenellis The Carnmenellis pluton and the smaller intrusion of Carn Brea appear to be part of a single intrusive body. The central part of the Carnmenellis outcrop is a medium-grained granite with few megacrysts. The bulk of the main outcrop and the Carn Brea and Carn Marth masses consist of coarse-grained megacrystic granite with small megacrysts. Small bodies of fine-grained granite are found towards the west of the Carnemellis outcrop. The shape of this pluton is interpreted to be a sheet about 3 km thick with a near central root extending down to about 7 km depth. Boreholes at Rosemanowes that extend to over 2.5 km deep have shown very little variation in the petrographic composition of the granite with depth in this pluton.Selwood et al., p. 121 Monazite gives an emplacement age of 293.7±0.6 Ma for this pluton.
Tregonning-Godolphin The Tregonning Granite and Godolphin Granite are two distinct granite bodies on the southern coast of Cornwall. The Tregonning Granite is mainly a medium-grained lithium-mica granite with a body of fine-grained granite developed towards the northwest of the outcrop. It has a unique chemistry and differs from the granite found within the Carnmenellis and Land's End granites, likely forming in a different manner. The Godolphin Granite is mineralogically and chemically similar to the nearby Carnmenellis Granite, although is finer grained.
Land's End The Land's End pluton is mainly a coarse-grained granite with abundant large megacrysts. There is an area in the centre which is poor in megacrysts and there are several small and medium-sized masses of fine-grained granite throughout the outcrop. Dating has been carried out on xenotime and monazite samples, from a fine-grained granite and the main coarse-grained granite respectively. These give emplacement ages of 279.3±0.4 Ma for the fine-grained granite and 274.8±0.5 Ma for the main phase granite. This difference is consistent with fine-grained granite being a roof-pendant to the coarse-grained main phase granite intrusion.
Isles of Scilly
The Isles of Scilly all have granite bedrock. The dominant rock type is a megacrystic biotite granite, although the megacrysts are relatively small. In the centre of the pluton a medium-grained granite is developed with few megacrysts, more tourmaline and less biotite than the main variety. Monazite from this pluton suggest an emplacement age of 290.3±0.6 Ma.
Haig Fras
This submarine outcrop, 45 km in length, lies 95 km northwest of the Scilly Isles, rising at one point to 38 m below sea level. In contrast to most granites of the Cornubian batholith the granites here are fine to medium-grained and generally lack megacrysts. It was intruded at 277 Ma and is considered most likely to be a separate but related intrusive body that runs parallel to the Cornubian batholith.
Other intrusions Minor granitic intrusions are present throughout the peninsula. In some cases granitic bodies have been recognised from the mineralization above them, even if the intrusion itself has not been encountered.
A series of minor intrusions are found within the country rock and the granites themselves. Common types are pegmatites, aplites and elvans.
Mineralogy and chemistry of the granite and other rocks
Granite
The main rock forming the batholith is granite, which formed when magma cooled slowly, covered by 2–3,000 metres of slate and sandstone. The slow cooling gave time for crystals to form in the granite which are large enough to see with the naked eye, giving it a granular appearance. These grains are mainly of quartz, feldspar and biotite. The granite is generally coarse grained and in some places very coarse-grained, or pegmatitic (grains larger than 3 cm). Large phenocrysts, several centimetres long, of K-feldspar, are a distinctive feature.