Isles of Scilly Pluton

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The Isles of Scilly Pluton is the Isles of Scilly with overlapping Permian granite intrusion that for Cornu Bischen batholith is expected.

geography

Granite at Trenemene, Western Rocks, Isles of Scilly

The Isles of Scilly consist of 5 larger and more than 50 smaller islands , which are about 40 kilometers in a west-south-west direction from Land's End , the south-western tip of Great Britain . The islands of 120 square kilometers cover the northwest two-thirds of the oval pluton .

geology

Remapping has shown that the Isles of Scilly Pluton is composed of several smaller, separate, individual granite intrusions. This is in contrast to previous interpretations, which were based on only two or three different facies. The host rocks of the Pluton are nowhere exposed.

Petrology

The Daymark at the northeast end of St. Martin’s is underlain by the coarse-grained porphyry facies.

Mullis and colleagues (2001) now distinguish six petrological facies in Pluton, which differ in their grain size and their respective proportion of phenocrystals ( porphyry ):

  • coarse-grained granites> 2 millimeters
  • medium-grain granites
  • fine-grain granite <1 millimeter.

The coarse-grained granites can be correlated with the facies G 1a in the Cornubian batholith. They are further subdivided into a richly porphyry facies with more than 10 percent phenocrystals, a moderately porphyry facies, and a weakly porphyry facies with less than 5 percent phenocrystals. The medium-grain granite ( G 1b ) consists of a weakly porphyry facies and a non-porphyry facies. The fine-grain granite ( G 1c ) is not further subdivided. The distribution of the individual facies is irregular.

mineralogy

Mineralogically, it is two- mica granite with biotite (siderophyllite) and muscovite , roughly equivalent. In addition to orthoclase , plagioclase and quartz , cordierite (pinitized), tourmaline , andalusite , monazite , apatite , ilmenite , zircon , uraninite and anatase appear as accessories . The tabular, subidiomorphic orthoclase phenocrystals can be up to 40 millimeters in size, but their grain size is usually between 15 and 25 millimeters. They can have magmatic foliation . Cordierite forms phenocrystals up to 8 millimeters in size. Quartz usually occurs as agglomerations of individual grains.

Geochemical composition

The following geochemical analyzes were carried out on the pluton - to compare the average value on the Cornubian batholith:

Oxide
wt.%		 Average
Cornubian batholith		 Coarse-grained
G 1a 		Medium grain
G 1b 		G 1c
SiO 2 72.35 71.52 72.96 72.34
TiO 2 0.26 0.24 0.08 0.20
Al 2 O 3 14.52 14.84 14.42 14.77
Fe 2 O 3 0.30 0.53 0.57
FeO 1.56 1.02 0.36 1.58 dead
MnO 0.06 0.02 0.02 0.02
MgO 0.41 0.36 0.17 0.33
CaO 0.79 0.81 0.54 0.60
Na 2 O 2.96 2.94 3.28 2.81
K 2 O 5.12 5.42 5.04 5.29
P 2 O 5 0.25 0.23 0.24 0.19
H 2 O -
H 2 O + 1.44

Age

With the help of the uranium-lead method, a high-precision age of 292 million years was determined for the pluton by Chen and colleagues (1993) at Monazite . The sample came from Penninis Head on St Mary’s . The same sample provided a cooling age of 287 million years using the argon method. Another age was 290.3 ± 0.6 million years - which corresponds to the sacmarium . The Isles of Scilly Pluton therefore belongs with the Carnmenellis Granite and the Bodmin Moor Granite to the early stage in the Cornubian batholith.

literature

  • M. Stone and CS Exley: Geochemistry of the Isles of Scilly pluton . In: Proceedings of the Ussher Society . tape 7 , 1989, pp. 152-157 .

Individual evidence

  1. ^ G. Barrow: The geology of the Isles of Scilly . In: Memoir of the Geological Survey of England and Wales . 1906.
  2. ^ 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.
  3. ^ Y. Chen, AH Clark, E. Farrar, HAHP Wasteneys, MJ Hodgson and AV Bromley: Diachronous and independent histories of plutonism and mineralization in the Cornubian batholith, south-west England . In: Journal of the Geological Society . tape 150 . London 1993, p. 1183-1191 .