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Metamorphism in the Upper Devonian of N. Cornwall

  • F. Coles Phillips (a1)
Extract

The geological succession and structure of the district are briefly summarized. Evidence of stress conditions becomes increasingly abundant as one passes from south to north towards the boundary of the Carboniferous.

The thermal alteration due to the Bodmin Moor granite is definitely delimited by tracing the outer limit of spotting in normal pelitic sediments.

Petrographical study of phyllites from outside this limit shows considerable mineralogical reconstruction in the sediments, with crystallization of white mica, chlorite and chloritoid; in some instances garnet is developed. The distribution of these types shows no relationship to the granite outcrop, but accords with the main structural features of the area.

Analogous changes are prominent in the igneous horizons, with the formation of schists of the general type of prasinite.

Within the aureole thermal effects can be traced superposed on the earlier dynamic changes.

From these lines of inquiry it is concluded that many of the changes in the phyllites, notably the formation of porphyroblastic white mica, chlorite and chloritoid, hitherto ascribed with greater or less finality to the action of the granite, are due to an earlier dynamic metamorphism.

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References
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1847. Pattison, S. R.On the Geology of the Tintagel District”: 34th Ann. Rep. Roy. Geol. Soc. Cornwall, pp. 312.
1865. Pattison, S. R.Geology of the Tintagel District”: Trans. Roy. Geol. Soc. Cornwall, vii, p. 3.
1889. Hutchings, W. M. I: “Notes on Altered Igneous Rocks of Tintagel, North Cornwall”: Geol. Mag., pp. 5359, 101–9.
Hutchings, W. M. II: “On the occurrence of Ottrelite in the Phyllites of North Cornwall”: Idem., pp. 214–20.
1903. Parkinson, J.: “Geology of the Tintagel and Davidstow District”: Q.J.G.S., lix, pp. 408–28.
1909. Dewey, H.On Overthrusts at Tintagel (North Cornwall)”: Q.J.G.S., lxv, pp. 265–80.
1910. Reid, C., et. al. “Geology of the Country around Padstow and Camelford”: Mem. Geol. Surv. Eng. and Wales, Sheets 335 and 336.
1911. Reid, C. “Geology of the country around Tavistock and Launceston”: Idem, sheet 337.
1911. Dewey, H., and Flett, J. S.British Pillow-lavas and the Rocks associated with them”: Geol. Mag., pp. 202–9 and 241–8.
1914. Dewey, H.The Geology of North Cornwall”: Proc. Geol. Ass., xxv, pp. 154–79.
1920. Goldschmidt, V. M. “Die Injektionsmetamorphose im Stavanger-Gebiete”: Vidensk. Selsk. Skrift. M. N. Kl., No. 10.
1923. Tilley, C. E.Petrology of the metamorphosed rocks of the Start Area”: Q.J.G.S., lxxix, pp. 172204.
1924. Grubenmann, U. and Niggli, P. Die Gesteinsmetamorphose. Berlin, 1924.
1925. Tilley, C. E.Petrographical Notes on some Chloritoid Rocks”: Geol. Mag., lxii, pp. 309–19.
1927. Ghosh, P. K.Petrology of the Bodmin Moor Granite, Cornwall”: Mem. Mag., xxi, pp. 301–6.
1928. Phillips, F. C.Petrological Notes on three Rock-types from the Shetland Islands”: Geol. Mag., LXV, pp. 500–7.
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Geological Magazine
  • ISSN: 0016-7568
  • EISSN: 1469-5081
  • URL: /core/journals/geological-magazine
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