St Michael's Mount Granite

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The St Michael's Mount Granite is a small part of intrusion of Cornu Bischen batholith on the south coast of Cornwall . The granite penetrated weakly metamorphic sediments of the Mylor Slate Formation in the Unterperm .

geography

St Michael's Mount from the south. In the core of the tourmaline muscovite granite. To the south of the wall the old age zone passes by.

The St Michael's Mount Granite underlies the southern and southwestern part of the Mount's Bay situated St Michael's Mount . The tidal island is about 450 meters south of Marazion and can be reached on foot at low tide. It is 4 kilometers to Penzance in the west. The conical castle hill consists mainly of granite, which is exposed in an area about 310 meters long (in east-west direction) and 230 meters wide - about a third of which is under water at high tide.

geology

St Michael's Mount Granite is a small stock (with an estimated 0.1 square kilometer base) made of porphyry granite that penetrated the metapelite of the Upper Devonian ( Famenne ) and transformed it into Hornfelse with contact metamorphosis . In the very close contact aureole, the former slate clays have completely lost their ability to be split, and biotite , cordierite and andalusite were also formed . It is believed that the isolated tubular stick is connected at depth to the actual Cornubian batholith. The Hornfelse or Metapelite underlay the flatter northern section of the island.

Petrology

Subtropical gardens in tourmaline-muscovite granite

In the peraluminous St Michael's Mount Granite - an S-type granite - two main petrological facies can be distinguished. A biotite granite stands at the contact area to the Hornfelsen. The inside of the stick is formed by a partly porphyry tourmaline-muscovite granite, which clearly predominates in terms of volume. Between these two types of granite lies a pegmatite with a comb texture known as a stick separator . Banded layers often appear in this transition area, which also develop comb textures. These ligaments are viewed as advancing crystallization fronts frozen in place in a solidifying magma , with the branching alkali feldspar crystals opening towards the magma. Their positions within the intrusion depend on the prevailing temperature gradient and the associated degree of hypothermia.

The biotite granite is a medium to coarse-grained leucocratic rock that weathered light orange (grain sizes 1 to 5 millimeters and> 5 millimeters) and occurs only at the edges of the intrusion. It does not contain any conspicuous megacrystals, but instead clumps of biotite up to 3 millimeters in size. The biotite granite does not form a continuous ring and its transitions to the tourmaline-muscovite granite in the interior are irregular.

The tourmaline muscovite granite is also a leucocratic, but gray-white weathered rock. Its grain sizes are variable - as are the proportions of its feldspar megacrystals, which reach 1 to 2 centimeters. In the digestion can be tourmaline , muscovite , alkali feldspar , plagioclase (albitisch) and quartz recognize. Biotite, topaz and apatite appear as accessories . In its porphyry formation, in addition to alkali feldspar, idiomorphic bipyramidal quartz occurs as a phenocrystal. In addition to quartz and feldspar, muscovite and light brown zinnwaldite are also found in the matrix .

tectonics

The castle stands on tourmaline-muscovite granite, view from the southeast.

Host rocks

The Mylor Slate formation in the North tidal island has a coating parallel cleavage (S 1), which sweeps between 158 N and 170 N and incident at 24 to 30 degrees to the east-northeast. Numerous folds (F 3) can also be seen, the planes of which, however, dip to the south or south-south-west. Their fold axes descend at 8 to 25 degrees to the east or east-southeast. This difference in the direction of vergence between foliation and folding by up to 95 degrees can best be explained by a transpressive block rotation in the stiff deck sediments, which was triggered by the encroaching granite dome and / or by local, north-northwest-south-south-east trending lateral shifts. The metapelites are also clearly steeply fissured , the two predominant groups strike N 075 and N 115. The granite had penetrated into the neighboring rock in places along these two groups. Before the granitic veins penetrated, quartz veins up to 2 centimeters thick had formed in the metapelites - with strike directions N 004 and N 146.

granite

The granite has also developed a very clear, steepest and also horizontal fracture when it reaches the brittle area. Two directions are particularly pronounced - N 155 and N 115, the latter direction already being mapped out in the metasediments. N 090 also appears subordinate.

Mineralization

The old age zone appears under rubble blocks to the right of the pillbox

Another special feature of St Michael's Mount Granite is its tin - tungsten mineralization, which is formed in the south of the granite stock as an east-west striking, steep, endogranitic old zone around 50 to 75 meters wide . Cassiterite , wolframite , lollingite and arsenopyrite appear as minerals . In addition to quartz, the matrix contains the minerals tourmaline (Schörl), topaz and beryl . Accessory is also apatite, can lepidolite (lithium mica) and occasionally sulfides such as chalcopyrite and Varlamoffite verwitternder Stannit should join. The deposit was never mined and is therefore in the fresh state.

The core of the greisen is formed by quartz veins that are centimeter thick (up to a maximum of 5 centimeters) in the direction of strike, on which up to 15 millimeter thick muscle margins have grown. The predominant strike direction is N 070 and thus follows one of the main fracture directions in the Metapelites. The subordinate direction N 050 runs diagonally to this, which is offset orthogonally from the main direction with expansion bridges of 1 centimeter. The directions N 010 and N 150 are also present. Around the quartz core on both sides there is an up to 25 centimeter wide eroded zone, which makes up around 25 percent of the total rock volume. The veins are fissures, which can be recognized by the structure that fills the cavity - with a right-angled, bridging crystal growth and occasional interstitial openings ( English vugs ). Many veins were filled in one go, but several growth stages can be observed in some. Since fissures open hydraulically through excess fluid pressure, multiple backfills indicate pulsating pressure conditions.

Investigations on liquid inclusions in quartz showed a hydrothermal temperature range of 100 to 400 ° C, the peaks of the inclusion temperatures should have been between 250 and 350 ° C. The depth of mineralization is assumed to be 1 to 3 kilometers. The salinity of the solutions was 5 to 12 equivalent percent NaCl.

The swarm of old people is generally viewed as the result of excess pressure from fluids that have accumulated under the roof vault of the granite cane. Their overpressure was able to exceed the critical tensile stress σ 3 , as a result of which the rock cohesion was lost and the set of tensile crevices parallel to N 070 opened. This was essentially a closed, static liquid system. The stretching thus took place to the south-southeast - in accordance with the general late variscan stretching direction.

Age

The St Michael's Mount Granite was dated 281.4 million years ago by Clark and colleagues (1993) - this corresponds to the early Kungurian .

See also

literature

  • EH Davison: On the Geology of St Michael's Mount . In: Transactions of the Royal Geological Society of Cornwall . tape 15 , 1920, p. 313-321 .
  • Dominy, SC, Camm, GS, Bussell, MA, Scrivener, RC and Halls, C .: A review of tin stockwork mineralization in the south-west England orefield . In: Proceedings of the Ussher Society . tape 8 , 1995, p. 368-373 .
  • C. Halls, JW Cosgrove and GS Camm: The influence of fluid pressure in governing fracture geometry and mineral textures in the pneumatolytic lode systems of south west England . In: Geoscience in south-west England . tape 10 , 2000, pp. 58-63 .
  • KFG Hosking: The Vein System of St. Michael's Mount, Cornwall . In: Transactions of the Royal Geological Society of Cornwall . tape 18 , 1953, p. 493-509 .
  • EB Selwood, EM Durrance and CM Bristow: The Geology of Cornwall . University of Exeter Press, 1998, ISBN 0-85989-432-0 .

Individual evidence

  1. ^ GEA Jackson and GM Power: Columnar, branching and curved feldspar growth in the St Michael's Mount Granite, Cornwall . In: Proceedings of the Ussher Society . tape 8 , 1995, p. 363-367 .
  2. ^ F. Moore: The Occurrence of Topaz-rich Greisens in St. Michael's Mount, Cornwall . In: Proceedings of the Ussher Society . tape 4 , 1977, pp. 49-56 .
  3. ^ Nicholas Le Boutillier: The tectonics of Variscan Magmatism and Mineralization in Southwest England (doctoral thesis) . University of Exeter, 2002.
  4. ^ SC Dominy, GS Camm, RC Scrivener, C. Halls and MA Bussell: Stockwork mineralization in south-west England: structural and paragenetic aspects . In: Proceedings of the Ussher Society . tape 8 , 1995, p. 370-375 .
  5. ↑ PD Wheeler, G. Edwards, GS Camm, C. Halls and JW Cosgrove: An investigation into the controls on the formation of greisen-bordered veins, St Michael's Mount, Cornwall . In: Geoscience in south-west England . tape 10 , 2001, p. 249 .
  6. ^ RG Taylor: Ore textures recognition and interpretation. Volume 1, Infill textures . In: EGRU . James Cook University of North Queensland, 1992, p. 24 .
  7. ^ C. Halls: Energy and mechanism in the magmato-hydrothermal evolution of the Cornubian Batholith - a review . In: Seltmann, Kampf and Moller (eds.): Metallogeny of collisional orogens . Czech Geological Survey, Prague 1994, p. 274-294 .
  8. ^ AR Campbell and KS Panter: Comparison of fluid inclusions in coexisting (cogenetic?) Wolframite, cassiterite and quartz from St. Michaels Mount and Cligga Head, Cornwall, England . In: Geochimica et Cosmochimica Acta . tape 54 , 1990, pp. 673-681 .
  9. ↑ AH Clark, Y. Chen, HA Farrar, JA Stimac, MJ Hodgson, J. Willis-Richards and AV Bromley: The Cornubian Sn-Cu (As, W) metallogenic province: product of a 30 MY history of discrete and concomitant anatectic , intrusive and hydrothermal events . In: Proceedings of the Ussher Society . tape 8 , 1993, pp. 112-116 .