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Early Palaeozoic magmatism in the English Lake District

D. Millward
Early Palaeozoic magmatism in the English Lake District
Proceedings of the Yorkshire Geological Society (November 2002) 54, Part 2: 65-93


The Lower Palaeozoic rocks of the Lake District record the rifting of the microcontinental terrane of Avalonia from Gondwana early in the Ordovician, its drift northwards across the Iapetus Ocean and collision with Laurentia and Baltica in the Silurian, and its Acadian deformation in Early Devonian times. Three discrete episodes of magmatism occurred during these events. In late Ordovician (Caradoc) times large volumes of magma were generated in response to subduction of Iapetus oceanic crust beneath Avalonia. Large-scale eruptions formed the basalt-andesite-rhyolite volcanic fields that are now preserved as the Eycott and Borrowdale volcanic groups. Large bodies of mainly granitic magma were emplaced beneath the Borrowdale Volcanic Group as part of the largely Ordovician Lake District batholith; these include, for example, the Eskdale and Ennerdale intrusions. Less than 10 Ma later, a brief episode of silicic volcanism is recorded within the marine sedimentary strata of the lowest part of the overlying Windermere Supergroup. A period of about 40 million years of apparent quiescence then ensued before further granitic masses, the Shap and Skiddaw plutons were emplaced around the margins of the batholith in Early Devonian times. All of these magmatic episodes were accompanied by minor intrusions. The Eycott and Borrowdale volcanic groups are rare examples in the geological record of the products of subaerial volcanism. Preservation is entirely due to subsidence keeping pace with the emplacement of new material. Subsidence resulted both from extensional tectonic processes and, locally, during the movement of large volumes of magma to the surface. The Eycott and the lower part of the Borrowdale volcanic groups accumulated in two adjacent volcanotectonic depressions. At least five depositional centres located within the central Lake District, along ENE structural trends, were active during aggradation of the upper part of the Borrowdale Volcanic Group. The major caldera-related silicic volcanic centres were underpinned by components of the Lake District batholith. It seems likely that these two very thick volcanic sequences accumulated within a period of less than 5 Ma. The superb exposure of these rocks and depth of erosion in the Lake District allow a view into the depths of the magmatic system rarely seen in modern examples.

ISSN: 0044-0604
EISSN: 2041-4811
Serial Title: Proceedings of the Yorkshire Geological Society
Serial Volume: 54, Part 2
Title: Early Palaeozoic magmatism in the English Lake District
Author(s): Millward, D.
Affiliation: British Geological Survey, Edinburgh, United Kingdom
Pages: 65-93
Published: 200211
Text Language: English
Publisher: Yorkshire Geological Society, Leeds, United Kingdom
References: 63
Accession Number: 2004-053684
Categories: Igneous and metamorphic petrologyStructural geology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Presidential address, Yorkshire Geol. Soc. meeting, York, Dec. 8, 2001
Illustration Description: illus. incl. sects., 5 tables, geol. sketch maps
N54°04'60" - N55°15'00", W03°40'00" - W02°10'00"
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200416
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