Cornubian batholith
Encyclopedia
The Cornubian batholith refers to the group of associated granite
intrusion
s which underlie the south-western peninsula of Great Britain
. The main exposed masses of the batholith are seen at Dartmoor
, Bodmin Moor
, St Austell
, Carnmenellis
, Land's End
and the Isles of Scilly
. It formed during the early Permian
period, from about 300 to 275 Ma, at a late stage in the Variscan orogeny
. The granites are classified as S-type, interpreted to be derived from the partial melting
of a sedimentary
protolith
. The intrusions are associated with significant mineralization, particularly tin
, for which the area has been famous since about 2000 BC. It takes its name from Cornubia, the Medieval Latin
name for Cornwall
.
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 pluton
s 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 laccolith
ic or lopolith
ic. 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.
granites, containing both muscovite
and biotite
. Li-mica granite forms a less common type found only in the St. Austell pluton and some smaller intrusions. Many of the granites contain large alkali feldspar megacryst
s. Locally the tourmaline
-bearing granite luxullianite
is developed by the tourmalinization of the original granite.
, caesium
and uranium
and moderately so in fluorine
, gallium
, germanium
, rubidium
, tin, tantalum
, tungsten
and thallium
. Given the overall chemistry, the levels of phosphorus
are also high. Strontium, barium
and the elements from scandium
to zinc
are relatively depleted. This chemistry is consistent with partial melting of a source consisting of greywacke
s. The conditions under which the melts formed is modelled to be temperature of 770°C and a confining pressure
of 50 MPa
.
Differences in chemistry have been identified between an earlier group of plutons (Isles of Scilly, Carnmenellis & Bodmin Moor) and a later group (Land's End, St Austell & Dartmoor). The early group of granites are more aluminous than the later set and have steeper slopes on plots of cerium
against yttrium
. The later group contain more common basic microgranite enclaves
.
for Dartmoor compared to 94 ppm for Bodmin Moor. The concentration of ammonium in these granites correlates well with both their initial / ratios and their peraluminosity
. The relatively high ammonium content is interpreted to indicate that the granites were derived from a sedimentary protolith, or have been contaminated from such a source after emplacement to high crustal levels.
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.
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.
methods became commonplace, from the observed relationships with sedimentary sequences. The youngest rocks intruded by the granites, the Carboniferous
Crackington and Bealsmill formations of Namurian to lower Westphalian in age, provide a lower bound for the timing of granite emplacement of about 310 Ma. The earliest instance of granitic clasts in younger sedimentary sequences, the Late Permian St. Cyres beds, 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 suggest that the batholith has grown by the coalescence of a series of separate intrusions over a period of about 25 Ma.
Several major plutons are recognised, which are all compositionally distinct apart from the Bodmin Moor and Carnmenellis intrusions, which cannot be discriminated on this basis.
megacryst
s 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.
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.
A series of minor intrusions are found within the country rock and the granites themselves. Common types are pegmatite
s, aplite
s and elvan
s.
A feature of the granites of the batholith is the high concentrations of volatile
components. Fluids rich in these have strongly affected the country rock and locally the granites themselves. The first phase recognised is potassium
–metasomatism
followed by sodium
–metasomatism and finally acid metasomatism.
caused by the metasomatism of shales and metabasalts. Typical minerals formed at this time include garnet
, pyroxene
, epidote
, Cl-rich amphiboles, malayaite
, vesuvianite
, siderite
and axinite
. The skarns may contain economic amounts of tin, copper, iron and arsenic. Minerals formed at this stage give ages close to the crystallization age for the particular pluton.
ization and tourmalinization
of the granite by high-temperature late magmatic fluids rich in volatiles. Veins of cassiterite
and wolframite
are found associated with the greisens, the former deposited by high salinity, low , fluids and the latter by low salinity high fluids. Muscovites within the greisens give cooling ages similar to magmatic muscovites in the relevant granite.
The third and main phase of mineralization occurs at a later stage, at lower temperatures and involves deposition in veins by fluids circulating within the country rock, leaching out tin, copper and arsenic. The typical fill of these veins is quartz
-tourmaline-chlorite
-sulfide-fluorite
, with tin, copper, lead, zinc, iron and arsenic sulphides. The dominant trend for mineral veins of this stage is W-E. This mineralization is 25–40 Ma later than the intrusion age for the Carnmenellis pluton.
fluid inclusions
suggest that this mineralization was caused by fluid derived from Permo-Triassic sedimentary sequence that once covered the entire area and is still preserved in the Southwest Approaches
to the English Channel
. Evidence of seawater influence suggest that the mineralization did not start until the Late Triassic
marine incursion at the earliest. The high heatflow from the granites helped drive fluid circulation.
, such as Lee Moor on the western edge of Dartmoor and the St Austell district. The kaolinization is thought to have resulted from intense supergene
weathering
in a tropical to warm climate during the Cretaceous
to Cenozoic
, based on studies of D
/H
and / ratios.
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
intrusion
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
s which underlie the south-western peninsula of Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
. The main exposed masses of the batholith are seen at Dartmoor
Dartmoor
Dartmoor is an area of moorland in south Devon, England. Protected by National Park status, it covers .The granite upland dates from the Carboniferous period of geological history. The moorland is capped with many exposed granite hilltops known as tors, providing habitats for Dartmoor wildlife. The...
, Bodmin Moor
Bodmin Moor
Bodmin Moor is a granite moorland in northeastern Cornwall, England, United Kingdom. It is in size, and originally dates from the Carboniferous period of geological history....
, St Austell
St Austell
St Austell is a civil parish and a major town in Cornwall, England, United Kingdom. It is situated on the south coast approximately ten miles south of Bodmin and 30 miles west of the border with Devon at Saltash...
, Carnmenellis
Carnmenellis
Carnmenellis Hill gives its name to the area of west Cornwall between Redruth, Helston and Penryn. The hill itself is situated approximately three miles south of Redruth....
, Land's End
Land's End
Land's End is a headland and small settlement in west Cornwall, England, within the United Kingdom. It is located on the Penwith peninsula approximately eight miles west-southwest of Penzance....
and the Isles of Scilly
Isles of Scilly
The Isles of Scilly form an archipelago off the southwestern tip of the Cornish peninsula of Great Britain. The islands have had a unitary authority council since 1890, and are separate from the Cornwall unitary authority, but some services are combined with Cornwall and the islands are still part...
. It formed during the early Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
period, from about 300 to 275 Ma, at a late stage in the Variscan orogeny
Variscan orogeny
The Variscan orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica and Gondwana to form the supercontinent of Pangaea.-Naming:...
. The granites are classified as S-type, interpreted to be derived from the partial melting
Partial melting
Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the solid....
of a sedimentary
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
protolith
Protolith
Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
. The intrusions are associated with significant mineralization, particularly tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
, for which the area has been famous since about 2000 BC. It takes its name from Cornubia, the Medieval Latin
Medieval Latin
Medieval Latin was the form of Latin used in the Middle Ages, primarily as a medium of scholarly exchange and as the liturgical language of the medieval Roman Catholic Church, but also as a language of science, literature, law, and administration. Despite the clerical origin of many of its authors,...
name for Cornwall
Cornwall
Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of , and covers an area of...
.
Extent and geometry
From gravity and magneticEarth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
geophysical data
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
, 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
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
, is linear and trends WSW-ENE, parallel with that associated with the Haig Fras
Haig Fras
Haig Fras is a 45 km long submarine granitic rocky outcrop in the southern part of the Celtic Sea, lying about 95 km northwest of the Isles of Scilly. It covers an area of . At one point it reaches within 38 m of the sea surface. It is protected as a Special Area of Conservation because of the...
granite.
The shape of the batholith
Batholith
A batholith is a large emplacement of igneous intrusive rock that forms from cooled magma deep in the Earth's crust...
and the relationship between the individual pluton
Pluton
A pluton in geology is a body of intrusive igneous rock that crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies...
s 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
Martin Bott
Martin Harold Phillips Bott FRS is an English geologist and now Emeritus Professor in the Department of Earth Sciences at Durham University, England.He is a Vice-President of Christians in Science...
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 laccolith
Laccolith
A laccolith is a sheet intrusion that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.Laccoliths tend to form at relatively...
ic or lopolith
Lopolith
A lopolith is a large igneous intrusion which is lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body...
ic. 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.
Mineralogy and chemistry
Mineralogy
The Cornubian intrusions are two-micaMica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
granites, containing both muscovite
Muscovite
Muscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic...
and biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
. Li-mica granite forms a less common type found only in the St. Austell pluton and some smaller intrusions. Many of the granites contain large alkali feldspar megacryst
Megacryst
In geology, a megacryst is a crystal or grain that is considerably larger than the encircling matrix. They are found in igneous and metamorphic rocks.- Notes :*...
s. Locally the tourmaline
Tourmaline
Tourmaline is a crystal boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gem comes in a wide variety of colors...
-bearing granite luxullianite
Luxullianite
Luxullianite is a rare type of granite, notable for the presence of clusters of radially arranged acicular tourmaline crystals enclosed by phenocrysts of orthoclase and quartz in a matrix of quartz, tourmaline, alkali feldspar, brown mica, and cassiterite.The name originates from the village of...
is developed by the tourmalinization of the original granite.
Chemistry
The two-mica granites are strongly peraluminous, with a low ratio of sodium to potassium and overall high level of alkalis. The granites are highly enriched in lithium, boronBoron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
, caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
and uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
and moderately so in fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
, gallium
Gallium
Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...
, germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
, rubidium
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
, tin, tantalum
Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
, tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
and thallium
Thallium
Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin but discolors when exposed to air. The two chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861 by the newly developed method of flame spectroscopy...
. Given the overall chemistry, the levels of phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
are also high. Strontium, barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
and the elements from scandium
Scandium
Scandium is a chemical element with symbol Sc and atomic number 21. A silvery-white metallic transition metal, it has historically been sometimes classified as a rare earth element, together with yttrium and the lanthanoids...
to zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
are relatively depleted. This chemistry is consistent with partial melting of a source consisting of greywacke
Greywacke
Greywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...
s. The conditions under which the melts formed is modelled to be temperature of 770°C and a confining pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
of 50 MPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
.
Differences in chemistry have been identified between an earlier group of plutons (Isles of Scilly, Carnmenellis & Bodmin Moor) and a later group (Land's End, St Austell & Dartmoor). The early group of granites are more aluminous than the later set and have steeper slopes on plots of cerium
Cerium
Cerium is a chemical element with the symbol Ce and atomic number 58. It is a soft, silvery, ductile metal which easily oxidizes in air. Cerium was named after the dwarf planet . Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight...
against yttrium
Yttrium
Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is...
. The later group contain more common basic microgranite enclaves
Xenolith
A xenolith is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption...
.
Ammonium content
The granites are generally rich in ammonium compared to average granites world-wide. There is also considerable variation between the individual plutons, with an average of 11 ppmParts-per notation
In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement...
for Dartmoor compared to 94 ppm for Bodmin Moor. The concentration of ammonium in these granites correlates well with both their initial / ratios and their peraluminosity
Peraluminous
Peraluminous rocks are igneous rocks that have a molar proportion of aluminium oxide greater than that of sodium oxide, potassium oxide and calcium oxide combined. Compare with peralkaline, metaluminous, and subaluminous. Examples of peraluminous minerals include biotite, muscovite, cordierite,...
. The relatively high ammonium content is interpreted to indicate that the granites were derived from a sedimentary protolith, or have been contaminated from such a source after emplacement to high crustal levels.
Tectonic setting and origin
The granitic intrusions that form the batholith were intruded towards the end of the Variscan orogeny, as a suite of late orogenic granites. It has been suggested that crustal extension allowed the granitic magmas to move to higher crustal levels. Evidence from neodymiumNeodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
and strontium
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
isotopes suggests that the magmas that formed the batholith were mainly the result of partial melting
Partial melting
Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the solid....
of the lower crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
with a minor component of basaltic magma from a mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
source. This lower crustal source is likely to have consisted of both metasedimentary and metavolcanic rocks, of Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
age.
Mechanism
As with all large intrusive bodies, the mechanism of emplacement of the Cornubian batholith 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 rockCountry rock (geology)
Country rock is a geological term meaning the rock native to an area. It is similar and in many cases interchangeable with the terms basement and wall rocks....
over a laccolithic sill
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
with a relatively small vertical dyke
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
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.
Timing
An approximate age of emplacement for the Cornubian batholith was known before radiometric datingRadiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
methods became commonplace, from the observed relationships with sedimentary sequences. The youngest rocks intruded by the granites, the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
Crackington and Bealsmill formations of Namurian to lower Westphalian in age, provide a lower bound for the timing of granite emplacement of about 310 Ma. The earliest instance of granitic clasts in younger sedimentary sequences, the Late Permian St. Cyres beds, 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 suggest that the batholith has grown by the coalescence of a series of separate intrusions over a period of about 25 Ma.
Plutons
Dartmoor
The largest exposed area of granite, it also forms the easternmost development of the batholith. The granite consists of two main types, coarse-grained granite with abundant large alkali feldsparAlkali feldspar
The alkali feldspar group are those feldspar minerals rich in the alkali elements like potassium. The alkali feldspars include: anorthoclase, microcline, orthoclase and sanidine....
megacryst
Megacryst
In geology, a megacryst is a crystal or grain that is considerably larger than the encircling matrix. They are found in igneous and metamorphic rocks.- Notes :*...
s 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
Uranium-lead dating
Uranium-lead is one of the oldest and most refined of the radiometric dating schemes, with a routine age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1-1 percent range...
of Monazite
Monazite
Monazite is a reddish-brown phosphate mineral containing rare earth metals. It occurs usually in small isolated crystals. There are actually at least four different kinds of monazite, depending on relative elemental composition of the mineral:...
from this intrusion gives emplacement ages of 278.2±0.8 Ma and 280.4±1.2 Ma.
Bodmin Moor
The Bodmin Moor pluton consist 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 intrusions of Tregonning-Godolphin, Carn Brea and Carn Marth 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 Tregonning-Godolphin outcrop is mainly a medium-grained lithium-mica granite with a body of fine-grained granite developed towards the northwest of the 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. Monazite gives an emplacement age of 293.7±0.6 Ma for this pluton.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 of xenotimeXenotime
Xenotime is a rare earth phosphate mineral, whose major component is yttrium orthophosphate . It forms a solid solution series with chernovite- and therefore may contain trace impurities of arsenic, as well as silicon dioxide and calcium...
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 to a minimum of 38 m below the sea surface. 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. Many of these are connected to one of the larger plutons, such as the Tregonning granite, which appears to be part of the Carnmenellis pluton from its similar composition and indications from gravity data. In some cases granitic bodies have been recognised from the mineralisation 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 pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
s, aplite
Aplite
Aplite in petrology, the name given to intrusive rock in which quartz and feldspar are the dominant minerals. Aplites are usually very fine-grained, white, grey or pinkish, and their constituents are visible only with the help of a magnifying lens...
s and elvan
Elvan
Elvan is a name used in Cornwall and Devon for the native varieties of quartz-porphyry. They are dispersed irregularly in the Upper Devonian series of rocks and some of them make very fine building stones...
s.
Associated metamorphism and metasomatism
Contact metamorphic effects are found around many of the plutons.A feature of the granites of the batholith is the high concentrations of volatile
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
components. Fluids rich in these have strongly affected the country rock and locally the granites themselves. The first phase recognised is potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
–metasomatism
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
followed by sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
–metasomatism and finally acid metasomatism.
Associated mineralisation
Skarns
The first phase of mineralization is the formation of exoskarnsSkarn
Skarn is an old Swedish mining term originally used to describe a type of silicate gangue, or waste rock, associated with iron-ore bearing sulfide deposits apparently replacing Archean age limestones in Sweden's Persberg mining district. In modern usage the term "skarn" has been expanded to refer...
caused by the metasomatism of shales and metabasalts. Typical minerals formed at this time include garnet
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
, pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, epidote
Epidote
Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often...
, Cl-rich amphiboles, malayaite
Malayaite
Malayaite is a calcium tin silicate mineral with formula CaSnO[SiO4]. It is a member of the titanite group.-Discovery:Malayaite was originally found in Perak and was first described in literature in 1961, though it was not yet given a name. In 1965, the mineral was named and recognized by the...
, vesuvianite
Vesuvianite
Vesuvianite, also known as idocrase, is a green, brown, yellow, or blue silicate mineral. Vesuvianite occurs as tetragonal crystals in skarn deposits and limestones that have been subjected to contact metamorphism...
, siderite
Siderite
Siderite is a mineral composed of iron carbonate FeCO3. It takes its name from the Greek word σίδηρος sideros, “iron”. It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus...
and axinite
Axinite
Axinite is a brown to violet-brown, or reddish-brown bladed group of minerals composed of calcium aluminium boro-silicate, 3Al2BO3Si4O12OH...
. The skarns may contain economic amounts of tin, copper, iron and arsenic. Minerals formed at this stage give ages close to the crystallization age for the particular pluton.
Tin and tungsten
The second phase of mineralization recognised is associated with greisenGreisen
Greisen is a highly altered granitic rock or pegmatite. Greisen is formed by autogenic alteration of a granite and is a class of endoskarn.Greisens appear as highly altered rocks, partly coarse, crystalline granite, partly vuggy with miarolitic cavities, disseminated halide minerals such as...
ization and tourmalinization
Tourmaline
Tourmaline is a crystal boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gem comes in a wide variety of colors...
of the granite by high-temperature late magmatic fluids rich in volatiles. Veins of cassiterite
Cassiterite
Cassiterite is a tin oxide mineral, SnO2. It is generally opaque, but it is translucent in thin crystals. Its luster and multiple crystal faces produce a desirable gem...
and wolframite
Wolframite
Wolframite WO4, is an iron manganese tungstate mineral that is the intermediate between ferberite and huebernite . Along with scheelite, the wolframite series are the most important tungsten ore minerals. Wolframite is found in quartz veins and pegmatites associated with granitic intrusives...
are found associated with the greisens, the former deposited by high salinity, low , fluids and the latter by low salinity high fluids. Muscovites within the greisens give cooling ages similar to magmatic muscovites in the relevant granite.
The third and main phase of mineralization occurs at a later stage, at lower temperatures and involves deposition in veins by fluids circulating within the country rock, leaching out tin, copper and arsenic. The typical fill of these veins is quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
-tourmaline-chlorite
Chlorite
The chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.-Oxidation states:...
-sulfide-fluorite
Fluorite
Fluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...
, with tin, copper, lead, zinc, iron and arsenic sulphides. The dominant trend for mineral veins of this stage is W-E. This mineralization is 25–40 Ma later than the intrusion age for the Carnmenellis pluton.
Lead and zinc
Lead and zinc mineralization is associated with north-south to northwest-southeast trending veins, known as the 'crosscourses' as they cross-cut the earlier W-E trending veins. Studies on quartzQuartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
fluid inclusions
Fluid inclusions
thumb|250px|Trapped in a time capsule the same size as the diameter of a human hair, the ore-forming liquid in this inclusion was so hot and contained so much dissolved solids that when it cooled, crystals of halite, sylvite, gypsum, and hematite formed. As the samples cooled, the fluid shrank more...
suggest that this mineralization was caused by fluid derived from Permo-Triassic sedimentary sequence that once covered the entire area and is still preserved in the Southwest Approaches
Southwest Approaches
The Southwest Approaches is the name given to the offshore waters to the southwest of Great Britain. The area includes the Celtic Sea, the Bristol Channel and sea areas off southwest Ireland...
to the English Channel
English Channel
The English Channel , often referred to simply as the Channel, is an arm of the Atlantic Ocean that separates southern England from northern France, and joins the North Sea to the Atlantic. It is about long and varies in width from at its widest to in the Strait of Dover...
. Evidence of seawater influence suggest that the mineralization did not start until the Late Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
marine incursion at the earliest. The high heatflow from the granites helped drive fluid circulation.
China clay
Alteration of feldspars by kaolinization is common throughout the batholith and has locally formed large economic deposits of china clayKaolinite
Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O54. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra...
, such as Lee Moor on the western edge of Dartmoor and the St Austell district. The kaolinization is thought to have resulted from intense supergene
Supergene (geology)
In ore deposit geology, supergene processes or enrichment occur relatively near the surface. Supergene processes include the predominance of meteoric water circulation with concomitant oxidation and chemical weathering. The descending meteoric waters oxidize the primary sulfide ore minerals and...
weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
in a tropical to warm climate during the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
to Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
, based on studies of D
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
/H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and / ratios.