| Computer Code: |
ISIN |
Preferred Map Code: |
notEntered |
| Status Code: |
Full, without ratification |
| Age range: |
Carboniferous Period (C)
— Permian Period (P) |
| Lithological Description: |
The intrusion is comprised of two common types of granite; one coarse-grained with porphyritic crystals of feldspar which is dominant across the islands (Outer Granite); the other finer grained and non-porphyritic, and has a more restricted outcrop in the north and west of St. Mary's and the south part of Tresco and Bryher (Inner granite). Typically the transition between the two types of granite is gradational; only locally is the contact sharp (e.g. north-east of Hugh Town [SV 9095 1155]. Both granite varieties contain muscovite, biotite, othoclase and quartz; tourmaline is locally abundant and elsewhere scarce. In the porphyroclastic granite, crystals of feldspar are subhedral to euhedral and up to 2.5 cm long; the mica minerals tend to form in clusters of subhedral crystals with textures indicating that biotite formed later than muscovite. The average diameter of crystals in the fine-grained granite is approximately 2.5 mm. |
| Definition of Lower Boundary: |
Not defined - presumably intrusive. |
| Definition of Upper Boundary: |
Not defined - presumably intrusive. |
| Thickness: |
Not defined. |
| Geographical Limits: |
The Isles of Scilly Intrusion is part of a chain of intrusions that are interconnected at depth and stretch from the Isles of Scilly to Darmoor; it is commonly referred to as the Cornubian Batholith. |
| Parent Unit: |
Not Applicable (-)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Area |
Isles of Scilly Intrusion - Outer granite; Peninnis Heas, south side of St Mary's. Barrow, 1906. |
| Type Area |
Isles of Scilly Intrusion - Inner granite - landing place at the south end of Tresco. Barrow, 1906. |
| Reference(s): |
| De La Beche, H T, 1839. Report on the geology of Cornwall, Devon and west Somerset. Memoir of the Geological Survey. |
| Darbyshire, D P F and Sheppard, T J. 1985. Chronology of granite magmatism and associated mineralisation, SW England. Journal of the Geological Society, London, 142, 1159-1177. |
| Barrow, G. 1906. The geology of the Isles of Scilly. Memoir of the Geological Survey of England and Wales. Explanation of sheets 357 and 360. |
| Carne, J. 1850. On the geology of the Islands of Scilly. Trans. Royal Geol. Soc. Cornwall, vol. vii pp 140 154. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
