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Journal of the Geological Society; 1992; v. 149; issue.1; p. 139-147;
DOI: 10.1144/gsjgs.149.1.0139
© 1992 Geological Society of London

Article

Timing of post-tectonic Cadomian magmatism on Guernsey, Channel Islands: evidence from 40Ar/39Ar mineral ages

R. D. DALLMEYER1, R. S. D’LEMOS2 & R. A. STRACHAN2

1 Department of Geology, University of Georgia, Athens, GA 30602, USA
2 Department of Geology, Oxford Polytechnic, Oxford OX3 0BP, UK

The Channel Island of Guernsey exposes pre- and post-tectonic calc-alkaline plutonic rocks attributed to late Precambrian (Cadomian) subduction activity. 40Ar/39Ar mineral cooling ages presented here clarify the age and time-span of post-tectonic intrusions. Amphibole from the Bon Repos meladiorite yields an isotope correlation age of 605.3 ± 4.6 Ma, indicating emplacement during late stages of the tectonothermal event recorded within its host rocks. Igneous hornblendes from the Northern Igneous Complex record consistent isotope correlation ages (St Peter Port gabbro 570.2 ±1.1 Ma, Bordeaux diorite complex 569.0 ± 1.1 and 565.3 ± 2.2 Ma, and a comagmatic enclave within Cobo granite 570.2 ± 1.5 Ma). Hornblende and biotite from contact-metamorphosed host rocks adjacent to the Cobo granite yield isotope correlation ages of 569.6 ±1.1 and 563.8 ± 3.3 Ma respectively. These ages, together with field data, demonstrate that the various members of the Northern Igneous Complex were emplaced contemporaneously at c. 570 Ma, significantly earlier than a c. 500 Ma Rb-Sr whole-rock isochron age previously reported for the Cobo granite. The 40Ar/39Ar ages indicate that post-tectonic magmatism on Guernsey was broadly coeval with post-metamorphic cooling around the Baie de St Brieuc on the adjacent French mainland, and occurred c. 30 Ma prior to peak Cadomian deformation and metamorphism in the St Malo area.





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