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The Heavy Minerals of the Plutonic Rocks of the Channel Islands: 2. Guernsey, Sark, and Alderney

Published online by Cambridge University Press:  01 May 2009

A. W. Groves
A. W. Groves
Affiliation:
Research Student, Royal College of Science, London.
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Summary of results

The method attempted of correlation of plutonic masses by their accessory minerals is successful, and particularly so with the acid rocks. The results effected in the Channel Islands are as follows:—

1. Confirmation of the existence of granites of two ages in Jersey, and correlation of the Fort Regent and Gorey masses for the first time.

2. Correlation of the Cobo Granite of Guernsey with the “Older” Granite of Jersey (Gorey and Fort Regent).

3. Conclusive evidence to show that the Cobo Granite is the last stage of the Differentiation Series of North Guernsey, previously thought improbable.

4. The probable occurrence of both “Newer” and “Older” Granites in Alderney, making a Pre-Cambrian age for the “Newer” Granite highly probable.

5. The dissimilarity of the heavy residue of the “Newer” Granite to that of the Dartmoor Granite.

6. The similarity of the residue of the “Older” Granite with that of the Granite Gneiss of the Lizard and their suggested correlation.

7. The individuality of the gneisses of South Guernsey.

8. The occurrence in hybrid rocks of the mineral suites (particularly the zircons) of both rocks involved, consequently we have here a new method of demonstrating the hybrid nature of a rock.

Many rocks previously termed diorites in Jersey, Guernsey, and Alderney are shown petrographically not to be true diorites, some being gabbros, others hybrids.

The occurrence of acid-soused basic rocks in Jersey, Guernsey, and Alderney is pointed out, and their petrography described.

The occurrence of amphibolites in Roselle Bay, Alderney, is made known for the first time, and their petrography described.

In the course of the whole work sixty-three rock specimens have been crushed and passed through bromoform according to the method of procedure previously outlined, making an aggregate of about 15 lb. of rock crushed. From this crushed rock nearly four hundred slides of grains have been prepared, and of these about 90 per cent are nonmagnetic. In addition eighteen thin sections have been prepared.

I wish to thank Mr. A. E. Mourant, B.A., for some of the specimens of gneisses from southern Guernsey, and also Dr. G. H. Plymen for the specimens from Alderney. The rest of the material. was collected by myself.

Type
Original Articles
Information
Geological Magazine , Volume 64 , Issue 10 , October 1927 , pp. 457 - 473
Copyright
Copyright © Cambridge University Press 1927

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References

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