Cornubian batholith

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The Cornu Bische Batholith is a batholith of granite standing in front of some 280 million years ago in the Permian was born. It underlies much of the south-western peninsula of Great Britain and occurs in Dartmoor , Bodmin Moor , around St Austell , Carnmenellis , at Land's End and on the Isles of Scilly . Very important mineral deposits are closely related to the intrusion , especially the tin ore cassiterite , which has been in existence since 2000 BC. Is mined here. Ores of copper , lead , zinc and tungsten are also present . The mining of kaolin is also important for the porcelain factory .

etymology

Location map of the Cornubian Batholite in South West England

The name Cornubian batholith is derived from Cornubia - the name for Cornwall in Middle Latin .

geography

Bouguer gravity anomaly map of south-west Great Britain - clearly recognizable the negative gravity troughs of the Cornubian batholith and the granite of Haig Fras

The Cornubian batholith emerged as the crystallization product of a melt ( magma ) below the earth's surface. Gravity measurements and geomagnetic investigations show that the 250 kilometers long and 40 to 60 kilometers wide intrusion extends from around 8 ° W (around 100 kilometers southwest of the Isles of Scilly) to the eastern edge of Dartmoor. Since granite is lighter than the average continental crust, the intrusion can be identified by a negative gravity anomaly. This runs roughly in a straight line from west-southwest to east-northeast. Haig Fras Granite , a little further north in the Celtic Sea , follows the same direction.

Until the use of gravity measurements, which only allowed models for the thickness and spatial extent of the batholith, the outlines of the batholite as well as the arrangement of its granitic individual plutons were completely speculative in nature. Martin Bott's original work suggested that the batholith was roughly shaped like a trapezoid with a base 10 to 12 kilometers wide. In the meantime, however, the shape of a Lakkolite or a Lopolite is considered more likely. On the basis of comparisons with other intrusion examples, with the aid of the smear width, it can be assumed that the individual plutons are between 3 and 5 kilometers thick. The total volume of the batholith was estimated at around 68,000 cubic kilometers in 1989.

Emergence

Hound Tor in Dartmoor with beautiful horizontal and vertical clefts

The for Rhenoherzynikum belonging Cornu Bische Batholith is a composite batholith, which had formed over a period of 25 million years from 300 to 275 million years ago in the Permian. In this respect, its granites represent a late phase within the Variscan orogenesis . It is assumed that after the collision, crustal expansion in the final orogenic stage enabled the magmas to rise to higher crustal areas. Neodymium - and strontium isotopes suggest that the constituent magmas are due to a partial melting of the lower crust, in which additionally quantitatively minor, from the Earth's mantle -derived basaltic melts and lamprophyre had invaded. The lower crust may have consisted of Proterozoic metasediments and metavolcanites.

The granitic melts were mainly enriched in volatile elements, but also in rare earths . Their rise is likely to have taken place as a tough crystal slurry. After the anatectic melts , which were around 800 ° C and under a pressure of 0.5 GPa, had finally cooled to around 700 ° C, the crystal slurry gradually solidified and solidified. Due to a high content of radioactive elements and the resulting internally generated heat, the pulp was again punctually melted and thus further igneous pulses - which is why the batholith is now composed of several individual intrusions, the contacts of which are often separated from each other by pegmatitic stick separators .

In the lower tempered area, brittle deformation then occurred later with the formation of, in particular, vertical and horizontal fissures . The horizontal fissures are traced back to the expansion of the batholith through pressure relief. They can be seen very beautifully at the so-called Tors in Dartmoor and Bodmin Moor.

The overburden of only weakly metamorphic schists and sandstones of the Devonian and Carboniferous (metasediments and metavolcanites of a passive continental margin) was then slowly removed, so that the granites in areas such as Dartmoor and Bodmin Moor finally emerged on the surface. Continued erosion severely affected the granites, so that these are now often only preserved as loose individual blocks - so-called clitters .

Seated

The seat-taking mechanism of the batholith with its single plutons is still a controversial subject - not least because of the spatial problem of such massive intrusions within the upper crust. Four mechanisms have so far been considered: active scour out the host rock ( English stoping ), diapirs , Ausdehnungstektonik (grave education) and increase in the overlying rock package by a lakkolithischen storage aisle with a relatively short, steeply inclined conveyor area.

There are signs of stoping at the edge of the tregonning intrusion , where several intrusive layers penetrate from the roof of the intrusion into the neighboring rock. The Land's End Granite was formerly regarded as still Diapir be forced upon but more recently on distortions tied, which had been formed in a regional strain field. The main direction of faults in the Cornubian batholith is generally northwest-southeast.

Regional petrology

According to Simons and colleagues (2016), the individual plutons of the Cornubian batholith reveal five different lithologies , named after the predominant minerals: two mica ( G 1 ), muscovite ( G 2 ), biotite ( G 3 ), tourmaline ( G 4 ) and topaz granite ( G 5 ). The two-mica and muskowite granites are older, they appear in Carnmenellis, Bodmin Moor and the Isles of Silly. The younger biotite and tourmaline granites appear at Land's End, St Austell and Dartmoor. Topaz granites are found in the Tregonning , Land's End, and St Austell plutons .

The medium to coarse-grained two-mica granite G 1, which is by far predominant in terms of volume at more than 90 percent, consists of 34% quartz, 32% alkali feldspar, 22% plagioclase, 6% biotite, 4% muscovite, 1% tourmaline and 1% others Minerals - with some slight variations in these ratios. It thus represents a developed S-type. In addition to the main minerals mentioned, allanite , almandine , andalusite , apatite , cordierite , fluorite , ilmenite , topaz and zircon , the radioactive minerals monazite , xenotime and uraninite and the subsolidus formations chlorite , rutile and titanite appear as accessories .

The G 2 granite is quite similar to the G 1 granite, but has more muscovite. It only occurs in small numbers.

It is assumed that the G 1 granite was formed from a metagrauwacke by melting 20 percent muscovite and only a little biotite, whereby the physical conditions were 731 to 806 ° C and more than 0.5 GPa. In the case of the younger G 3 granites, the melting of biotite was decisive with the same source, the temperature of the process was higher at 768 to 847 ° C, but the pressures were somewhat lower at around 0.4 GPa. Based on the geochemical data, two series of fractionation can be identified - G 1 → G 2 and G 3 → G 4 . The more highly developed G 2 and G 4 granites were created through 10 to 30 percent fractionation of a mineral mixture of plagioclase, alkali feldspar and biotite - which can be derived from the main elements such as barium , strontium and rubidium . Metasomatic transformations in the G 2 and G 4 granites can be recognized by secondary formations of muscovite and tourmaline, but also by a trend change in the main and trace elements, especially in P 2 O 5 and rubidium. The G 5 granite represents a completely unique type of magma. The partial melting of a biotite- rich restite that was left in the magmas of G 1 and / or G3 is now used as an explanation . Its melting was facilitated by granulite facial fluids of the lower crust.

Dartmoor

Granite deposits at Haytor in Dartmoor

At 650 square kilometers, the Dartmoor Granite is the largest single intrusion that extends furthest to the east. It consists of two main facies, a coarse-grained granite with many alkali feldspar megacrystals (facies G 3a ) and a coarse-grained granite that is quite poor in megacrystals ( G 3b ). In the southwest, the coarse-grained granite has somewhat smaller megacrystals and in some places medium-grain and small-grain granites also occur ( G 3c and G 3d ). Tourmaline granites with globular quartz ( G 4b ) are also found selectively. Gravity measurements on the Dartmoor Pluton suggest a stratified occurrence that is almost 10 kilometers thick. The root in the south is 17 kilometers deep. Possibly this is a conveyor dike in which the magma penetrated into shallower crustal areas. It appears that the laminate has intruded on the Devonian-Carboniferous boundary. Age determinations on Monazite using the uranium-lead method yielded 278.2 ± 0.8 and 280.4 ± 1.2 million years. Geochemically, the Dartmoor Granite differs significantly from the other granites in the Batholith and therefore represents its own, separate, igneous pulse.

Bodmin Moor

The Bodmin Moor Granite at Rough Tor

The Bodmin Moor Granite (220 square kilometers) consists mainly of a coarse-grained granite which is very rich in smaller megacrystals ( G 1a ). A medium-grain granite with phenocrystals of variable sizes appears in the central part ( G 1b ). The pluton has even smaller bodies of fine-grain granite on its western edge and in its center ( G 1c ). Gravity measurements show that the pluton has a wedge-shaped shape, which thickens to the south-southeast and there reaches a maximum thickness of around 7 kilometers. Age measurements on Monazite showed 291.4 ± 0.8 million years.

St Austell

The 85 square kilometer St Austell Granite consists on its eastern half of a coarse-grained, megacrystal-bearing granite (facies G 3a ). The western and eastern edges of the intrusion are characterized by very large megacrystals, whereas these are absent in the center. A medium-grain granite has formed between the west end and the center, the mica of which contains lithium ( G 5a ). Small bodies made of fine-grain granite appear in the center and on the western edge ( G 3d ). Topaz granites ( G 4 ) form the western central section. The pluton of St Austell is also wedge-shaped. Month ages result in 281.8 ± 0.4 million years.

Carnmenellis

The intrusive body with Carnmenellis granite (135 square kilometers) consists of the actual Carnmenellis pluton and the much smaller intrusion of Carn Brea. The center and the eastern edge of the intrusion are built up from a medium-grain granite, which contains only a few megacrystals (facies G 1b ). The lion's share of the intrusion and the masses of Carn Brea and Carn Marth are a coarse-grained, megacrystal-bearing granite with small megacrystals ( G 1a ). Fine-grain granite ( G 1c ) is only found in two small deposits in the west. The pluton is interpreted as a 3-kilometer-thick layer body, the central root of which extends down to a depth of 7 kilometers. Wells at Rosemanowes reached a depth of 2.5 kilometers and found only minor changes in the petrographic composition of the granite as a function of depth. Month ages resulted in 293.7 ± 0.6 million years.

Tregonning and Godolphin

The Tregonning Granite and the Godolphin Granite are two separate intrusion bodies on the south coast of Cornwall. The Tregonning Granite is uniformly grained and mainly designed as a medium-grain, lithium mica-bearing topaz granite (facies G 5 ), which becomes fine-grained towards the northwest. Its chemical composition is unique and clearly different from the granites of Carnmenellis and Land's End. This is likely to be due to a different origin.

The Godolphin Granite is mineralogically and chemically related to the nearby Carnmenellis Granite, but more fine-grained than this.

Land's End

Granite rocks at Land's End

The Land's End Granite (190 square kilometers) is a predominantly coarse-grained granite that contains abundant megacrystals (facies G 3a ). Only a few megacrystals appear in the central area. Fine-grained granite (facies G 4b ) is found in several small and medium-sized clusters all over the pluton. Age determinations were made on Xenotim and Monazite. The xenotime sample from the fine-grained facies gave 279.3 ± 0.4 million years and the monazite sample from the coarse-grained main facies 274.8 ± 0.5 million years. The age difference is explained by the assumption that the fine-grained facies represent the roof of the intrusion and therefore crystallized far earlier (and faster) than the bulk of the intrusion.

Isles of Scilly

Granite at Trenemene, Isles of Scilly

The 120 square kilometers of the Isles of Scilly are all underlain by granite (facies G 1a , G 1b and G 1c ). The type of rock is dominated by biotite granite containing megacrystals, although the megacrystals are not too large. The center of the pluton is medium-grained and, compared to the main facies, has only a few megacrystals, less biotite, but all the more tourmaline . The monthly ages are 290.3 ± 0.6 million years.

Haig Fras

The Haig Fras Granite is a 45 kilometer long submarine granite outcrop. It is 95 kilometers northwest of the Isles of Scilly and is at its highest point 38 meters below the surface of the sea. In contrast to most granites of the Cornubian batholith, the Haig Fras Granite is a fine-grained to medium-grained granite that does not contain any megacrystals. Its intrusion age is 277 million years. It is probably an independent intrusion that runs parallel to the Cornubian batholith.

More intrusions

On the south-west English peninsula there are other granite intrusions such as St Michael's Mount , St Agnes ( G 1a ), Cligga Head ( G 2 ), Carn Marth ( G 2 ), Castle an Dinas , Kit Hill ( G 2 ), Hingston Down ( G 2 ) and Hemerdon Ball ( G 2 ). In some cases, granite bodies have been identified by the mineralization they induced in the superstructure, although the intrusions themselves remained hidden. Smaller intrusions can be found in the adjacent host rock or in the granite itself. These usually include pegmatites , aplites and so-called elvans , a rhyolite gangrene .

Mineralogy and Geochemistry

granite

Coarse-grained granite G 3a with large alkali feldspar megacrystals from Dartmoor

The Cornubian batholith is made up of granite, the magma of which was initially only slowly cooled to around 320 ° C over a period of 4 to 5 million years. The intrusion was covered by 2000 to 3000 meter thick slates and metasandstones. The slow cooling process allowed the crystals that formed to reach grain sizes that can be seen with the eye. Their structure is granular. Essentially, the newly formed crystals are quartz , feldspar and biotite . The granite is usually coarse-grained, sometimes very coarse-grained or even pegmatitic (with grain sizes up to 3 centimeters). The salient feature, however, are very large phenocrystals of alkali feldspar ( orthoclase ) that can grow to be several centimeters long.

The geochemistry and mineralogy of the granites are variable and change depending on the occurrence. In the classification scheme of Chappell and White, however, they all fall under the S-type (sedimentary type), ie their parent rock (protolith) was originally of a sedimentary nature.

mineralogy

Luxullianite

The intrusions of the Cornubian batholith are predominantly based on monzogranitic two-mica granite, which contains both muscovite and biotite. Lithium mica granite is quite rare and only appears in the St Austell pluton and a few smaller intrusions. Many granites carry large alkali feldspar phenocrystals. In some places the original granite was converted to the tourmaline- leading variety Luxullianit . Tourmalinization occurred in the late cooling stage of the batholith, with feldspar and mica being partially replaced by tourmaline.

geochemistry

Landscapes of the different granite facies: A) Carnmenellis granite G 1a B) Isles of Scilly, G 1c inclusion within G 1a C) G 2 granite from Cligga D) G 3a granite from Dartmoor E) G 4b granite from Land's End F) G 3a Granite from Land's End G) Topaz Granite G 5 from Tregonning
Oxide
wt.%		 average Dartmoor Bodmin St Austell Land's End
G 3a 		Carnmenellis
G 1a 		Isles of Scilly
G 1a 		Tregonning
G 5a
SiO 2 72.35 73.69 71.41 72.32 70.46 71.74 71.52 71.13
TiO 2 0.26 0.22 0.22 0.22 0.38 0.24 0.24 0.07
Al 2 O 3 14.52 13.63 15.22 14.43 14.81 15.20 14.84 15.91
Fe 2 O 3 0.30 0.16 0.21 0.28 0.32
FeO 1.56 1.62 1.22 1.33 2.19 1.82 dead 1.66 dead 1.39 dead
MnO 0.06 0.08 0.05 0.04 0.07 0.04 0.02 0.06
MgO 0.41 0.40 0.41 0.36 0.65 0.43 0.36 0.10
CaO 0.79 0.54 0.89 0.68 0.85 0.93 0.81 0.60
Na 2 O 2.96 2.98 3.10 2.95 2.52 3.00 2.94 3.79
K 2 O 5.12 5.01 4.96 5.21 5.57 5.19 5.42 4.71
P 2 O 5 0.25 0.18 0.29 0.28 0.29 0.23 0.23 0.50
H 2 O - 0.22 0.17 0.18 0.22
H 2 O + 1.00 1.01 0.78 1.00

The two-mica granites have a high molecular ratio A / CNK, ie Al 2 O 3 / CaO + Na 2 O + K 2 O> 1.2 and are therefore clearly peraluminous . Their Na / K ratio is low, while the total concentration of alkalis is quite high. The granites show a high concentration of potassium , lithium, boron , cesium and uranium and a moderate concentration of fluorine , gallium , germanium , rubidium , tin , lead , tantalum , tungsten and thallium . Relatively high concentrations also apply to phosphorus . Strontium, barium and the elements from scandium to zinc are depleted . These geochemical facts point to partial melting of an original rock consisting of Grauwacken . Model calculations show a temperature of 770 ° C for melting at a load pressure of 50 MPa.

Because of the very high content of radioactive elements - uranium 12 to 23 ppm and thorium 11 to 17 ppm - the Cornubian batholith is characterized by a not insignificant heat production with a lower geothermal depth .

Geochemical differences can also be seen between an early group of plutons (Isles of Scilly, Carnmenellis and Bodmin Moor) and a later group (Land's End, St Austell and Dartmoor). The first group is richer in aluminum and shows a steeper curve slope in the Ce / Y diagram. In the latter group, xenolites made from mafic micro-granites are more common.

In the case of the ammonium concentration, an enrichment compared to average granites can be observed. There are large differences in the ammonium concentration between the individual plutons - only 11 ppm were measured for the Dartmoor Granite, whereas the Bodmin Moor Granite had a value of 94 ppm. The ammonium concentration in these granites clearly correlates with their initial 87 Sr / 86 Sr ratio and their pereraluminosity. Very high ammonium values ​​are interpreted either as a sign of sedimentary parent rock or as crustal contamination after penetration into the upper crust.

Contact metamorphosis and metasomatosis

The granite contacts with the host rock are sharp. Along the edges of many individual intrusions, the host rocks were metamorphically changed by the heat given off by the batholith . The contact aureole can be tracked up to a distance of 3 to 4, sometimes up to 6 kilometers into the adjacent rock. The efficiency of this process depends on the type of rock in question and on its distance from the edge of the intrusion. Fine-grained sediments were transformed into horn rock , with new minerals such as amphiboles and pyroxenes growing. At a greater distance, the contact metamorphosis can only be recognized by the patchy appearance of the affected rocks (as blotch slate). In addition to amphiboles and pyroxenes, grenades also formed in calcareous sediments . Green stones were generally transformed into hornblende plagioclase stones.

The granites of the Cornubian batholith were characterized by very high contents of volatile components. These - dissolved in liquids - metasomatically very heavily on the host rocks and also on the granite itself . In the first phase of the metasomatic changes there was an accumulation of alkalis, which affected the interior and the edge areas of the granite. An initial potassium metasomatosis was followed by a sodium metasomatosis. In the final phase, acids accumulated and alkalis were removed again (acid metasomatosis). The end result was the formation of old men and a general tourmalinization .

Age

An approximate relative age of the Cornubian batholite could be determined from the stratigraphic conditions even before the use of radiometric dating methods . The most recent Variscan metamorphic rocks to be penetrated by the batholith are the Carboniferous Crackington and Bealsmill Formations . They have ages from the Namurian and the Westphalian and thus result in a maximum age of the intrusion of 310 million years.

The earliest granite clasts come from the Upper Permian St. Cyres Beds of the Exeter Group , the minimum age of the Cornubian batholith is thus around 250 million years.

These relationships could be confirmed by the absolute age dating. It also became apparent that the individual intrusions were spread over a fairly long period of time. The oldest, radiometrically dated, significant single intrusion is the Carnmenellis pluton with 293.1 ± 3 million years. The most recent granite intrusion is found in the southern lobe of Pluton's Land's End with an age of 274.5 ± 1.4 million years. The small Hemerdon pluton on the southwest flank of the Dartmoor pluton is even 298.3 ± 2.3 million years old. The intrusion history of the Cornubian batholith thus extends over around 25 million years. A systematic spatial variation in the age distribution could not be determined, but there is certainly an age dependency on the granite type. Facies G 1 and G 2 are the oldest and extend over the period 293.7 to 281.7 million years. The facies G 3 , the associated facies G 4 and also G 5 are younger and cover the period 281.8 to 272.3 million years. Overall, it can be concluded that the Cornubian batholith was formed by a merging of individual intrusions during the Cisuralium .

Mineralization

Cassiterite from the Botallack mine

In the vicinity of the batholites there are large postmagmatic, hydrothermal mineral deposits that have been mined for thousands of years. Tin mining, for example, goes back to 2000 BC. BC back. The minerals formed over a period of 286 to 265 million years when fluids in the gradually cooling hot granite escaped through fracture systems. The ores accumulated in veins or were later deposited and enriched by rivers as alluvium. The ore veins mostly follow steep crevices or fractures that run through both the granite and its host rock. They can withstand several kilometers and are on average between 0.5 and 3 meters wide.

So far, 2.77 million tons of tin, 2.0 million tons of copper, 2.0 million tons of iron ore, 250,000 tons of lead and 250,000 tons of arsenate have been mined in the Cornubian batholith.

Four stages can be distinguished in the mineralization process, which reflect different conditions during the cooling. Each individual stage corresponds to a temperature range that has specific, economically significant metal concentrations and their gaits. The first three stages are related to the penetration and slow cooling of the batholith, while the fourth stage is likely due to the heat generated by radioactive decay.

Level 1 - Exoscarne

The earliest mineralization had started while the granite penetrated. Hot solutions secreted from the Si, Al, Fe and Mg-rich magma penetrated into shale and metabasalts, dissolved them and metasomatically transformed them into exoscarins. The new mineral formations typical for this stage include garnets, pyroxenes, epidotes , chlorine-rich amphiboles, malayaite , vesuvianite , siderite and axinite . The skarns can contain buildable concentrations of tin, copper, iron and arsenic . The temperatures for the first stage were 450 to 375 ° C.

Stage 2 - Greisen and Tourmalinization

Wolframite from the Camborne - Redruth - St Day district

The second mineralization phase saw the transformation of granite into greisen and tourmaline (temperature range 450 to 350 ° C). This was achieved through the action of late magmatic fluids that were enriched in volatile components. Associated with the greisen are veins of cassiterite (tin oxide SnO 2 ) and wolframite - an iron, manganese and tungsten mineral, (Fe, Mn) WO 4 . Cassiterite was deposited from liquids that had a fairly high salinity (5 to 10 percent by weight), but only a low concentration of carbon dioxide. Wolframite, on the other hand, comes from liquids with a low salinity and a high CO 2 concentration. The muskowites in the old men give the same cooling ages as muskowites in the corresponding granites.

Stage 3 - main mineralization phase tin / copper

The third stage occurred later in the cooling process and was heated to 400 to 200 ° C lower. Liquids circulating in the host rocks with a salinity of 3 to 15 percent by weight dissolved tin, copper and arsenic and deposited them in veins with a general east-west strike. The typical mineral filling of these veins consists of quartz-tourmaline-chlorite-sulfide-fluorite with additions of tin, copper, lead, zinc, iron and arsenic sulfides. Mineralization had occurred 25 to 40 million years after the Carnmenellis Pluton intrusion. These veins are the economically most important mineral deposits in the Cornubian batholith.

Level 4 - ricochet

The fourth and final mineralization stage was the lowest temperature - temperatures ranged between 170 and 100 ° C. Associated are polymetallic veins that contain lead, zinc, silver and uranium . The veins are known as so-called ricochets ( English crosscourses ), as their north-south or north-west-south-east strikes cross the older, east-west running veins. Quartz, barite and fluorite serve as the passage filling . Investigations on liquid inclusions have shown that the mineralizing solutions of high salinity (up to 26 percent by weight) have compositions corresponding to deep sedimentary solutions, which are generally very rich in sodium, calcium and chlorine. In this case, they come from the Permo-Triassic sediments that once covered the entire region. These rocks are still preserved in the Southwest Approaches at the entrance to the English Channel. The seawater impact in the solutions suggests that the earliest mineralization began with transgression in the Upper Triassic. The high heat flow of the granitic batholith was the engine of the liquid circulation.

Kaolin mining

Kaolin mining at Lee Moor

Satellite images of southwest England reveal bright spots that go back to the mining of kaolin. Kaolin deposits of economic importance appear in several places in the southwestern peninsula, particularly at Lee Moor on the western edge of Dartmoor and in the district of St Austell. The kaolin was created from the decomposition of feldspars in a process known as kaolinitization . The origin and age of these deposits are still a matter of debate. It is generally believed that they were caused by circulating meteoric waters (from rain or snow) during the batholith's late cooling phase. Investigations based on isotope ratios of D / H or 18 O / 16 O see the causes of kaolinization in the very warm surface conditions (ie intensive weathering) of a tropical to subtropical warm climate during the Cretaceous and the Cenozoic Era.

So far 150 million tons of kaolin have been mined.

Further use

The granites are still quarried in numerous quarries . They are generally used as a building block, also for sacred buildings and tombs. In road construction, they are used as delimitation and border stones and processed into gravel and gravel.

Conclusion

The intrusion of the Cornubian batholith into the Rheno-Hercynian Zone of the Variscan Orogen in south-west England was followed by significant hydrothermal mineralization of lithophilic non-ferrous metals . Both processes took place in the final phase of the orogenic collision or immediately afterwards. They were made possible by the late orogenic collapse caused by stretching / straining, which had occurred in a large number of flat and steeper faults. The large anatectic enamel volume, the penetration of mafic mantle magmas at the base, the exhumation of the metamorphic rocks and the simultaneous volcanism, as manifested in the lamprophyric and basaltic Exeter Volcanic Series , all point to a substantial subcrustal heat influx into a section of the crust that is active Crust thinning was affected.

See also

literature

  • EB Selwood, EM Durrance and CM Bristow: The Geology of Cornwall . University of Exeter Press, 1998, ISBN 0-85989-432-0 (English).
  • B. Simons et al: The petrogenesis of the Early Permian Variscan granites of the Cornubian Batholith: Lower plate post-collisional peraluminous magmatism in the Rhenohercynian Zone of SW England . In: Lithos . tape 260 , 2016, p. 76–94 , doi : 10.1016 / j.lithos.2016.05.010 (English).

Individual evidence

  1. ↑ RC Scrivener: Cornubian Granites and Mineralization of SW England . In: PJ Brenchley and PF Rawson (Eds.): The Geology of England and Wales . The Geological Society, 2006, p. 257-267 .
  2. ↑ JWF Edwards: Interpretations of seismic and gravity surveys over the eastern part of the Cornu Bian platform . In: DHW Hutton and DJ Sanderson, Variscan Tectonics of the North Atlantic Region (Eds.): Special Publications . tape 14 . Geological Society, London 1984, p. 119-124 .
  3. ^ GK Taylor: Pluton shapes in the Cornubian Batholith: new perspectives from gravity modeling . In: Journal of the Geological Society . tape 164 (3) , 2007, pp. 525-528 , doi : 10.1144 / 0016-76492006-104 .
  4. ^ EB Selwood, EM Durrance and CM Bristow: The Geology of Cornwall . University of Exeter Press, 1998, ISBN 0-85989-432-0 .
  5. Jump up ↑ Nance, RD, Gutiérrez-Alonso, G., Keppie, JD, Linnemann, U., Murphy, JB, Quesada, C., Strachan, RA and Woodcock, NH: Evolution of the Rheic Ocean . In: Gondwana Research . tape 17 , 2010, p. 194-222 .
  6. ^ DPF Darbyshire and TJ Shepherd: Nd and Sr isotope constraints on the origin of the Cornubian batholith, SW England . In: Journal of the Geological Society . tape 151 (5) , 1994, pp. 715 , doi : 10.1144 / gsjgs.151.5.0795 .
  7. ↑ RK Shail and BE Leveridge: The Rhenohercynian passive margin of SW England: development, inversion and extensional reactivation . In: CR Geoscience . tape 341 , 2009, p. 140-155 .
  8. ^ Robert Hesketh: Devon's Geology, an introduction. Bossiney Books, 2006, ISBN 1-899383-89-1 , pp. 10-12 .
  9. ^ RG Park: Foundation of Structural Geology (3 ed.) . Routledge, 2004, ISBN 0-7487-5802-X , pp. 128-129 .
  10. ↑ Nick Leboutillier: "Megiliggar Rocks" . In: Cornish Geology website . 2003.
  11. ^ T. Powell, S. Salmon, AH Clark and RK Shail: Emplacement styles within the Land's End Granite, west Cornwall . In: Geoscience in South-West England . tape 9 , 1999, p. 333-339 .
  12. ↑ a b B. Simons et al.: The petrogenesis of the Early Permian Variscan granites of the Cornubian Batholith: Lower plate post-collisional peraluminous magmatism in the Rhenohercynian Zone of SW England . In: Lithos . tape 260 , 2016, p. 76-94 , doi : 10.1016 / j.lithos.2016.05.010 .
  13. ^ CS Exley and M. Stone: Hercynian intrusive rocks . In: DS Sutherland (Ed.): Igneous Rocks of the British Isles . Wiley, Chichester 1982, pp. 287-320 .
  14. ^ A b J. T. Chesley et al.: Thermochronology of the Cornubian batholith in southwest England: Implications for pluton emplacement and protracted hydrothermal mineralization . In: Geochimica et Cosmochimica Acta . tape 57 (8) , 1993, pp. 1817-1835 , doi : 10.1016 / 0016-7037 (93) 90115-D .
  15. ↑ Bruce W. Chappell and Rick Hine: The Cornubian Batholith: an Example of Magmatic Fractionation on a Crustal Scale . In: Resource Geology . Vol. 56, No. 3 , 2006, p. 203-244 .
  16. ^ J. Dangerfield and JR Hawkes: The Variscan Granites of south-west England: additional information . In: Proceedings of the Ussher Society . tape 5 , 1981, pp. 116-120 .
  17. ↑ M. Stone and CS Exley: Geochemistry of the Isles of Scilly pluton . In: Proceedings of the Ussher Society . tape 7 , 1989, pp. 152-157 .
  18. ↑ JWF Edwards, M. Briant and MJ Arthur: Proposed Mesozoic dykes in the Celtic Sea . In: Proceedings of the Ussher Society . tape 7 , 1991, pp. 344-349 .
  19. ^ SJL Mullis, S. Salmon and T. Powell: Insights into the formation of the Isles of Scilly pluton . In: Geoscience in South-West England . tape 10 , 2001.
  20. ^ A. Hall: The distribution of ammonium in granites from South-West England . In: Journal of the Geological Society . tape 145 (1) , 1988, pp. 37-41 , doi : 10.1144 / gsjgs.145.1.0037 .
  21. ^ EA Edmonds, MC McKeown and M. Williams: South West England . In: British Regional Geology (3rd ed.) . Her Majesty's Stationery Office, 1969, ISBN 0-11-880074-4 , pp. 49-50 .
  22. ^ F. Pirajno: Intrusion-related hydrothermal mineral systems . In: Hydrothermal processes and mineral systems . Springer Science & Business Media, 2007, ISBN 978-1-4020-8613-7 , p. 241 .
  23. ↑ J. Dangerfield and JR Hawkes: Unroofing of the Dartmoor granite and possible consequences with regard to mineralization . In: Proceedings of the Ussher Society . tape 2 (2) , 1969, p. 122-131 .
  24. ^ SA Gleeson, JJ Wilkinson, FM Stuart and DA Banks: The origin and evolution of base metal mineralizing brines and hydrothermal fluids, South Cornwall, UK . In: Geochimica et Cosmochimica Acta . tape 65 (13) , 2001, pp. 2067-2079 , doi : 10.1016 / S0016-7037 (01) 00579-8 .
  25. ↑ SMF Sheppard: The Cornubian Batholith, SW England: D / H and 18 O / 16 O studies of kaolinite and other alteration minerals . In: Journal of the Geological Society . tape 133 (6) , 1977, pp. 573-591 , doi : 10.1144 / gsjgs.133.6.0573 .
  26. ^ Nigel Woodcock and Bob Strachan: Geological history of Britain and Ireland . Blackwell Science Ltd, Oxford 2000, ISBN 0-632-03656-7 .