Cornubian batholith

The Cornubian batholith is a large mass of granite rock, formed about 280 million years ago, which lies beneath much of the south-western peninsula of Great Britain. 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. The Cornubian batholith is a large mass of granite rock, formed about 280 million years ago, which lies beneath much of the south-western peninsula of Great Britain. 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. 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. 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 542 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. 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.