A Transition from Diagenesis to Greenschist Facies within a Major Variscan Fold/Thrust Complex in South-West England

T. J. Primmer*
Department of Geology, University of Bristol, Queen's Building, University Walk, Bristol BS8 1TR
*Present address: Department of Geology, Imperial College of Science and Technology, Prince Consort Road, London SW7 2BP.

Abstract: The north coast of Cornwall, from Bude to Newquay, provides a continuous section through a major Variscan fold/thrust complex. Illite crystallinity studies have revealed a transition from diagenesis in the north to greenschist facies metamorphism in the south in the Upper Palaeozoic succession. More detailed studies of mineral assemblages in both metabasites and pelitic rocks support the regional pattern of metamorphism indicated by illite crystallinity, and show that locally in the Tintagel district, the grade of metamorphism may have reached middle to upper greenschist facies. An attempt to correlate the above data with temperatures (108–985°C) derived from O-isotope geothermometers is made. Interpretation of the metamorphic data presented helps to emphasize the tectonic importance of the major structures seen in the fold/thrust complex.

Keywords: diagenesis • greenschist facies • illite • Variscan • Cornwall • England

Mineralogical Magazine; June 1985 v. 49; no. 352; p. 365-374; DOI: 10.1180/minmag.1985.049.352.07
© 1985, The Mineralogical Society
Mineralogical Society (www.minersoc.org)