Template:P916111

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P916111 Summary of the principal phases in the development of the Borrowdale Volcanic Group.
Development phase Location, lithofacies and thickness Main events
11 Marine transgression Post-BVG
Marginal to Lake District in late Caradoc times; S Lake District in Ashgill
Thermal contraction of granites on cooling allowed progressive marine transgression on to Lake District block
10 Batholith emplacement Beneath Scafell and Haweswater calderas. Granite, microgranite and granodiorite Injection of thick tabular granite sheets resulting in uplift and erosion of volcanic field
9b Cross Fell Cross Fell Inlier. Volcaniclastic sedimentary and pyroclastic rocks, one major sheet of silicic lapilli-tuff; >1100 m Volcaniclastic sedimentation interspersed with sporadic pyroclastic eruptions
9a Gosforth succession West Cumbria: stratiform sequence of andesitic and dacitic lapilli-tuff with subordinate volcaniclastic sedimentary rocks and andesite sills; >2500 m Dominantly explosive intermediate and silicic pyroclastic activity producing many densely welded ignimbrites; ?caldera related
8 Helvellyn Basin succession Central Fells, Helvellyn Basin. Stratiform units of volcaniclastic sandstone and breccia; subordinate pyroclastic units and a silicic lava; >1600 m Fluviolacustrine sedimentation within extensional basin with episodic catastrophic influx of eruption-generated sediment-gravity flows; interrupted by small andesite lava shields and ignimbrite emplacement
7 Lincomb Tarns ignimbrite Throughout BVG.
Eutaxitic and parataxitic dacitic lapilli-tuff; columnar jointed; >800 m
Most widespread and voluminous ignimbrite within BVG. Large magnitude silicic pyroclastic eruptions forming ignimbrite shield
6 Ambleside Basin succession Central south Lake District. Oversteps 5a, b, 4a; bedded volcaniclastic sandstone and breccia; >1100 m Fluviolacustrine sedimentation dominated by catastrophic influx of eruption generated sediment-gravity flows.
?Associated with emplacement of many sills
5b Kentmere Basin succession SE Lake District. Bedded units of volcaniclastic sedimentary rocks, intercalated with sheets of pyroclastic rocks, some lavas; <2800 m Intermediate to silicic ignimbrites; andesitic tuff-cone and lava-shield; fluviolacustrine sedimentation at base; major centre for sill emplacement
5a Duddon Basin succession SW Lake District. Stratified sequence of volcaniclastic sedimentary rocks, andesitic and rhyolitic pyroclastic rocks and andesite lavas Explosive intermediate ?caldera-related pyroclastic eruptions; succeeded by catastrophic eruption-generated sedimentation in extensional basin, episodically interrupted by emplacement of silicic ignimbrites. Many sills emplaced?
4b Haweswater Caldera succession Ullswater, Haweswater. Stratified sheets of dacitic and rhyolitic pyroclastic rocks; garnetiferous; <650 m Large magnitude silicic pyroclastic activity; large-volume ignimbrites associated with caldera collapse
4a Scafell Caldera succession Central Fells. Stratified sheets of andesitic, dacitic and rhyolitic pyroclastic rocks; dacite and rhyolite lavas; garnetiferous. Overlain by bedded volcaniclastic sandstone and breccia; >700 m Andesitic phreatoplinian eruption followed by sequence of large volume silicic ignimbrites with associated development of piecemeal hydrovolcanic caldera; postcaldera silicic lavas and caldera basin sedimentary infill
3 Monogenetic andesite volcanoes Throughout BVG. Andesite lavas and sills, some of basalt, basaltic andesite and dacite; subordinate tuff, lapilli-tuff and sandstone; some units garnetiferous; <2700 m Construction of subaerial, multiple-vent, low-profile andesite volcanoes; some high volume basalt lava-fields; episodic silicic volcanism including possible caldera formation
2 Initial phreato- magmatism Devoke Water, Calder Bridge, Millom Park, Ullswater. Mafic lapilli-tuff and tuff-breccia; contains much mudstone and sandstone from Skiddaw Group; <600 m Hydrovolcanism, construction of tuff-cone field(s)
1 Pre-volcanic Latterbarrow Sandstone and Overwater Siltstone locally preserved; 0–400 m Regional uplift and erosion of Skiddaw Group. Local evidence of fluvial and ?marine deposition