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The volcanogenetic significance of garnet-bearing minor intrusions within the Borrowdale Volcanic Group Eskdale area, Cumbria

Published online by Cambridge University Press:  01 May 2009

B. Beddoe-Stephens  and
I. Mason
B. Beddoe-Stephens
Affiliation:
British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, U.K.
I. Mason
Affiliation:
Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, U.K.
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Abstract

A number of garnetiferous minor intrusions have been mapped within the Borrowdale Volcanic Group. They underlie garnetiferous extrusive volcanic rocks which occur toward the top of a sequence of ignimbrite and lava – the Airy's Bridge Formation – which is the product of a major caldera-forming eruptive episode. Garnet and whole-rock geochemistry indicate that most of the intrusions are indistinguishable from garnetiferous dacite forming the final eruptive unit of the Airy's Bridge Formation: a co-magmatic link is therefore postulated. One of the intrusions, which intrudes the Airy's Bridge Formation, is distinct and may be related to the later Eskdale pluton.

It is suggested that following the emplacement of ignimbrites forming the basal half of the Airy's Bridge Formation, caldera collapse partially sealed a fissure-conduit system and degassed, garnet-bearing magma was intruded as dykes and sills and locally extruded as a post-explosive lava dome. It is also postulated that garnet crystallized in a high-level magma chamber (P < 3 kb) and that reverse chemical zoning was due to growth while sinking through compositionally stratified magma.

Type
Research Article
Information
Geological Magazine , Volume 128 , Issue 5 , September 1991 , pp. 505 - 516
Copyright
Copyright © Cambridge University Press 1991

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Footnotes

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