Hostname: page-component-cd9895bd7-8ctnn Total loading time: 0 Render date: 2024-12-24T08:08:43.705Z Has data issue: false hasContentIssue false
  • English
  • Français

Nature and origin of meladiorite layers in northern Guernsey, Channel Islands

Published online by Cambridge University Press:  05 July 2018

A. C. Bishop  and
W. J. French
A. C. Bishop
Affiliation:
Department of Mineralogy, British Museum (Natural History), London SW7 5BD
W. J. French
Affiliation:
Department of Geological Sciences, Queen Mary College, London E1 4NS
Get access Rights & Permissions [Opens in a new window]

Abstract

The Cadomian igneous complex of N. Guernsey includes gabbro, diorite, granodiorite, and adamellite. The gabbro is layered, shows variations in colour index and texture, contains abundant amphibole, and is cut by the diorites. The diorites show little variation in mineral compositions despite considerable variation in modal proportions. They contain numerous thin and roughly parallel veins of leucodiorite and granodiorite, and layers of meladiorite occur in several places within the veined diorites. These meladiorites, previously interpreted as separate intrusions, form individual layers from a few centimetres to several metres thick which can be traced for some tens of metres along strike. The upper surfaces of the meladiorites are planar but the lower tend to be indented and irregular. The layers thin out laterally and usually pass into the host diorite by way of a zone containing dark spots and both the spots and the meladiorite layers are composed of actinolitic hornblende and Mg-biotite.

Chemical and petrographic data are presented for most of the rocks and minerals especially for the meladiorites and their host, the veined diorites. The rock chemistry and particularly the compositions of the amphiboles and micas of the meladiorites and the mafic spots, taken together with the presence of unmixing lamellae in the amphibole and the details of the field relationships, indicate that the rocks tended to equilibrium under subsolidus conditions and are most probably of metasomatic origin.

Acid pipes and pegmatites occur within and are restricted to the meladiorites. The forms of the pipes were previously taken to indicate that they were magmatic at the same time as the meladiorites. The metasomatic origin for the meladiorites proposed here calls into question this magmatic interpretation and it is suggested that the pipe formation is connected with the metasomatic process. It is envisaged that the alteration of the veined diorites took place under conditions in which high gas pressures allowed acidic melt to be produced at temperatures only a little above the solidus of the diorites. The thermal impulse for this recrystallization may have resulted from the polyphase intrusive history of the complex as a whole.

Type
Research Article
Information
Mineralogical Magazine , Volume 46 , Issue 340 , September 1982 , pp. 301 - 321
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1982

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Adams, C. J. D. (1976) J. Geol. Soc. Lond. 132, 233–50.10.1144/gsjgs.132.3.0233CrossRefGoogle Scholar
Bishop, A. C. (1963) Proc. Geol. Assoc. 74, 289–300.CrossRefGoogle Scholar
Cameron, E. P., and French, W. J. (1977) Mineral. Mao. 41, 239–51.10.1180/minmag.1977.041.318.12CrossRefGoogle Scholar
Cameron, K. L. (1971) Carnegie Inst. Washington Yearb. 70, 145–53.Google Scholar
Cameron, M., and papike, J. J. (1979) fortschr. mineral. 57, 28–67.Google Scholar
de Albuquerque, C. A. R. (1973) Geochim. Cosmochim. Acta, 37, 1779–802.CrossRefGoogle Scholar
Drysdall, A. R. (1957) The petrology of the plutonic complex of north Guernsey. Unpubl. Ph.D. thesis, Univ. of Southampton.Google Scholar
Elwell, R. W. D., Skelhorn, R. R., and Drysdall, A. R. (1960) Geol. Mag. 97, 89–105.CrossRefGoogle Scholar
Elwell, R. W. D., Skelhorn, R. R., and Drysdall, A. R. (1962) J. Geol. 70, 215–26.10.1086/626810CrossRefGoogle Scholar
Engel, A. E. J., and Engel, C. G. (1960) Geol. Soc. Am. Bull. 71, 1–57.CrossRefGoogle Scholar
Ernst, W. G. (1968) Amphiboles, crystal chemistry, phase relations and occurrence. Heidelberg, Berlin, New York (Springer-Verlag), 125 pp.Google Scholar
Eugster, H. P. (1956) Carnegie Inst. Washington Yearb. 55, 158–61.Google Scholar
Foster, M. D. (1960) U.S. Geol Surv. Prof. Pap. 354B.Google Scholar
French, W. J. (1966) Proc. R. It. Acad. 64B, 303-22.Google Scholar
Harry, W. T. (1950) Mineral. Ma#. 29, 142–9.Google Scholar
Heinrich, E. W. (1946) Am. J. Sci. 244, 836–48.10.2475/ajs.244.12.836CrossRefGoogle Scholar
Hietanen, A. (1963) U.S. Geol. Surv. Prof. Pap. 344D.Google Scholar
Hofmann, A. (1972) Am. J. Sci. 272, 69–90.CrossRefGoogle Scholar
Leake, B. E. (1968) Geol. Soc. Am. 98 Spec. Pap. Google Scholar
Leake, B. E. (1971) Mineral Mag. 38, 389–407.10.1180/minmag.1971.038.296.01CrossRefGoogle Scholar
Le Maitre, R. W. (1976) J. Petrol. 17, 589–637.CrossRefGoogle Scholar
Nockolds, S. R., and Mitchell, R. L. (1948) Trans. R. Soc. Edinb. 61, 533–75.CrossRefGoogle Scholar
Phillips, E. R., and Rickwood, P. C. (1975) Lithos, 8, 275–81.CrossRefGoogle Scholar
Roach, R. A. (1967) Rep. Trans. Soc. Guernes. 17, 751–75.Google Scholar
Ross, M., Papike, J. J., and Shaw, K. W. (1969) Mineral. Soc. Am. Spec. Pap., 2, 275–99.Google Scholar
Ross, M., Papike, J. J., and Shaw, K. W. and Weiblen, P. W. (1968) Science 159, 1099–102.CrossRefGoogle Scholar
Spear, F. S. (1976) Carnegie Inst. Washington Yearb. 75, 775–9.Google Scholar
Streckeisen, A. L. (1967) Neues Jahrb. Mineral., Abh. 107, 144–240.Google Scholar
Thornton, C. P., and Turtle, O. F. (1960) Am. J. Sci. 258, 664–84.CrossRefGoogle Scholar