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Journal of the Geological Society; 1989; v. 146; issue.6; p. 965-970;
DOI: 10.1144/gsjgs.146.6.0965
© 1989 Geological Society of London

Article

Snowdon basalts and the cessation of Caledonian subduction by the Longvillian

P. T. LEAT1 & R. S. THORPE2

1 Department of Geological Sciences, Science Laboratories, South Road, Durham DH1 3LE, UK
2 Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK

Basalts of the Bedded Pyroclastic Formation, Snowdon Volcanic Group, North Wales of Longvillian (Caradoc) age, are heterogeneous in trace element composition, forming a mixing trend between ocean island basalt (OIB)-type and magmatic arc basalt. The two end-member compositions were partial melts of two distinct upper mantle sources. The OIB end-member is similar to Icelandic OIB, and is the only known example of basalt of this type in the Ordovician-Silurian Welsh Basin. Since ocean island basalt magma type is thought never to erupt above well-established subduction zones, the eruption of the Longvillian OIB provides a minimum age for the end of subduction of Iapetus oceanic lithosphere below the Welsh Basin. A model for the evolution of the mantle below the Welsh Basin is proposed which accounts for the geochemistry of magmas erupted from the Tremadoc to the early Silurian.




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