View source for Borrowdale Volcanic Group, contemporaneous intrusive rocks and petrography of the volcanic rocks, Caradoc magmatism, Ordovician, Northern England

'''From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. [[British regional geology: Northern England|British regional geology: Northern England]]. Fifth edition. Keyworth, Nottingham: British Geological Survey.'''


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== Contemporaneous intrusive rocks ==

[[File:P005180.jpg|thumbnail|Complex relationships along the upper contact of an andesite sill intruded into laminated volcaniclastic sandstone, Middle Fell, Wasdale, Cumbria. The white weathered zone of siltstone between the laminated sandstone and andesite suggests that water-saturated sediment was fluidised during emplacement of the andesite. The hammer head is 17 cm long. (P005180).]]

Basaltic, andesitic, dacitic and rhyolitic sills, individually up to 250 m thick, contributed significantly to the development of the volcanic succession. A very large number are hosted in bedded volcaniclastic units within the Duddon, Kentmere, Ambleside and Helvellyn basin successions. Fewer sills are recognised in the Birker Fell Formation, except in the Eagle Crag Member.

Many of the sills are vesicular and have peperitic margins, the textures indicating the importance of fluidisation during emplacement at shallow levels into water-saturated sediment [[Media:P005180.jpg|(P005180)]]. In some cases, the carapace of sediment was so thin that parts of the overburden slumped off and became mixed with peperitic clasts in associated mass-flow deposits. By contrast, podiform andesite sills up to about 30 m thick, and within the Helvellyn Basin succession south of Ullswater, have thin chilled margins indicating intrusion into consolidated rock, possibly at a very late stage in the volcanism.

Whilst some of the sills were emplaced during pyroclastic eruptions, the timing of intrusion of most others is uncertain. Their occurrence at many levels in the volcanic pile implies that there were multiple episodes of injection. Moreover, localised concentrations of andesite intrusions, such as that in the eastern Lake District associated with the Kentmere Basin succession, may represent the sites of volcanoes or, perhaps, the early phase of caldera cycles.

== Petrography of the volcanic rocks ==
The Borrowdale volcanic rocks contain up to 30 per cent phenocrysts, comprising mainly plagioclase feldspar. Some clinopyroxene is typically present in the mafic rocks, with pseudomorphs after olivine occurring in a few. By contrast, orthopyroxene becomes the dominant mafic mineral in the felsic rocks, with orthoclase and biotite in a few of the most silicic. Almandine garnet phenocrysts are locally abundant in some andesite and dacite, but they are most abundant in the silicic ignimbrites. The garnetiferous rocks are peraluminous and the mineral and whole-rock compositions are related. The tendency for some calc-alkaline magmas to become peraluminous may be due to the assimilation of pelitic sediment. In the Lake District the source of this could be Skiddaw Group mudstone, but this was available only at relatively shallow crustal levels.



== Bibliography ==
Beddoe-Stephens, B, Petterson, M G, Millward, D, and Marriner, G F. 1995. Geochemical variation and magmatic cyclicity within an Ordovician continental-arc volcanic field: the lower Borrowdale Volcanic Group, English Lake District. ''Journal of Volcanology and Geothermal Research'', Vol. 65, 81–110.

Branney, M J, and Kokelaar, B P. 1994. Volcanotectonic faulting, soft-state deformation and rheomorphism of tuffs during development of a piecemeal caldera, English Lake District. ''Geological Society of America Bulletin'', Vol. 106, 507–530.

Branney, M J, and Soper, N J. 1988. Ordovician volcano-tectonics in the English Lake District. ''Journal of the Geological Society of London'', Vol. 145, 367–376.

Branney, M J, and Suthren, R J. 1988. High-level peperitic sills in the English Lake District: distinction from block lavas and implications for Borrowdale Volcanic Group stratigraphy. ''Geological Journal'', Vol. 23, 171–187.

Fitton, J G. 1972. The genetic significance of almandine-pyrope phenocrysts in the calcalkaline Borrowdale Volcanic Group, northern England. ''Contributions to Mineralogy and Petrology'', Vol. 36, 231–248.

Hughes, R A, Evans, J A, Noble, S R, and Rundle, C C. 1996. U-Pb chronology of the Ennerdale and Eskdale intrusions supports subvolcanic relationships with the Borrowdale Volcanic Group (Ordovician, English Lake District). ''Journal of the Geological Society of London'', Vol. 153, 33–38.

Johnson, E W, Briggs, D E G, Suthren, R J, Wright, J L, and Tunnicliff, S P. 1994. Non-marine arthropod traces from the subaerial Ordovician Borrowdale Volcanic Group, English Lake District. ''Geological Magazine'', Vol. 131, 395–406.

Millward, D. 2002. Early Palaeozoic magmatism in the English Lake District. ''Proceedings of the Yorkshire Geological Society'', Vol. 54, 65–93.

Millward, D. 2004. The Caradoc volcanoes of the English Lake District. ''Proceedings of the Yorkshire Geological Society'', Vol. 55, 73–105.

Millward, D, and Evans, J A. 2003. U-Pb chronology and duration of upper Ordovician magmatism in the English Lake District. ''Journal of the Geological Society of London'', Vol. 160, 773–781.

Millward, D, Beddoe-Stephens, B, Williamson, I T, Young, S R, and Petterson, M G. 1994. Lithostratigraphy of a concealed caldera-related ignimbrite sequence within the Borrowdale Volcanic Group of west Cumbria. ''Proceedings of the Yorkshire Geological Society'', Vol. 50, 25–36.

Millward, D, Marriner, G F, and Beddoe-Stephens, B. 2000. The Eycott Volcanic Group, an Ordovician continental-margin andesitic suite in the English Lake District. ''Proceedings of the Yorkshire Geological Society'', Vol. 53, 81–96.

Petterson, M G, Beddoe-Stephens, B, Millward, D, and Johnson, E W. 1992. A pre-caldera plateau-andesite field in the Borrowdale Volcanic Group of the English Lake District. ''Journal of the Geological Society of London'', Vol. 149, 889–906.

Piper, J D A, Stephen, J C, and Branney, M J. 1997. Palaeomagnetism of the Borrowdale and Eycott volcanic groups, English Lake District: primary and secondary magnetisation during a single late Ordovician polarity chron. ''Geological Magazine'', Vol. 134, 481–506. 
[[Category: Northern England]]