Borrowdale Volcanic Group, lower andesitic eruptive phase, 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.
Fifth edition. Keyworth, Nottingham: British Geological Survey.
Initial phreato-magmatic eruptions (P916111), phase 2
Initially, phreatomagmatic eruptions occurred widely and their products are preserved in units varying from a few metres to more than 600 m thick. The Devoke Water Tuff is the most extensive of these units, covering an area of about 30 km² in the western Lake District. Very similar rocks also occur about 5 km north of Calder Bridge (NY 065 105) and around Ullswater. In the southwest of the Lake District, phreatomagmatic deposits comprise much of the Whinny Bank, Po House and Greenscoe formations. In addition to a juvenile component of non-vesicular basaltic andesite, these rocks contain abundant mudstone and sandstone fragments along with accretionary mud pellets derived by pyroclastic fragmentation of the underlying Skiddaw Group and, north of Calder Bridge, the Latterbarrow Sandstone. They have very little ash matrix and were explosively erupted when non-vesiculated magma came into contact with either surface or ground water, resulting in the construction of tuff rings or cones. Clasts that might have an origin deeper in the crust, or from the mantle, have not been recorded, indicating that the explosions occurred at a shallow depth.
The Whinny Bank Tuff Formation comprises planar-bedded mudstone intercalated with an upwards-increasing proportion of andesite lapilli and lapilli-tuff beds. Much of the mud occurs as accretionary lapilli deposited from fall-out ash and/or from pyroclastic surges. In the Furness Inlier, the mudstone-rich lapilli-tuff and breccia of the Greenscoe Formation were deposited within a valley cut into the Skiddaw Group. There is no unambiguous evidence in the basal parts of these units that establishes whether the initial eruptions occurred either in a subaerial, a lacustrine or a marine environment. If subaqueous conditions did prevail, then water depths were less than about 1 km. However, higher parts of the Devoke Water Tuffs, for example, contain discordances and drapes that are typically subaerial in form. chapter three: caradoc magmatism
Low-profile andesite volcanoes (P916111), phase 3
The Birker Fell Formation is interpreted as the remnants of a plateau-andesite lava field erupted from numerous low-profile volcanoes. This voluminous unit crops out over an area of 315 km2 and probably underlies much of the remainder of the volcanic succession (P916047). Andesite sheets with blocky autobrecciated margins dominate the succession, locally comprising 30 to 90 per cent of the observed thickness. Individual sheets are 10–200 m thick and may be mapped laterally for up to 3 km. The sheets are interpreted as mainly block lavas, though sills may comprise up to about 30 per cent of the formation, for example, on High Rigg (NY 307 214). Locally, erupting lavas fed block-and-ash flows to form substantial accumulations of breccia.
The andesite sheets are remarkably parallel over considerable distances, as illustrated on High Rigg, east of Derwent Water (P704123). Cavities in autobreccia in the upper part of many sheets are filled with laminated sandstone. The lamination is parallel with bedding in overlying clastic rocks, showing that the sequence was near-horizontal when the cavities were filled. There is no evidence for steep slopes, as are associated with classical stratovolcanoes, and instead, an overlapping cluster of shield-like edifices with relatively small diameters and gently sloping flanks is more likely. The facies model is shown in (P916048).
Systematic geochemical cycles characterise the andesite sequence and may be related to fractionation of discrete magma batches within subvolcanic chambers. Cycles in which the rocks became less differentiated with time represent rapid eruption from a compositionally zoned magma chamber, whereas those in which the rocks became more differentiated imply much reduced eruption and recharge rates such that crystal fractionation processes dominated the composition of each successive unit. In addition, some parts of the succession show almost no geochemical evolution with time implying a balance between eruption, magma recharge, mixing and crystal fractionation processes. The occurrence of different sequences of cycle types at various locations supports the view that the lava pile was constructed from a number of volcanic centres.
Basaltic andesite aa-lavas, typically 5 to 30 m thick, occur in the lower part of the Birker Fell Formation in Eskdale and Wasdale, on High Rigg, Hallin Fell (NY 420 190) and around Haweswater (NY 500 140). Basaltic compound lava fields are developed near the top of the formation in the western part of the outcrop; one such unit comprises basaltic compound aa-flows up to 310 m thick, ponded within a depression, and at least 3 km3 in volume.
The effusive sheets are separated by lenticular units of volcaniclastic rocks, deposited from mass-flows, sheet floods and ephemeral streams, and possibly by wind. Tephra is readily recycled from the flanks of subaerial volcanoes, and thin volcaniclastic units within the Birker Fell Formation may represent considerable time spans during which the landscape may have been buried and stripped more than once. Where intercalations of volcaniclastic rocks are generally thin or absent, for example around Ullswater, the andesite succession was probably emplaced rapidly. By contrast, the Eagle Crag Member, between Haycock and Brown Knotts, is the most extensive and thickest volcaniclastic intercalation within the andesite pile, and it also hosts abundant peperitic sills. The geometry of the unit and the predominantly subaqueous environment of deposition imply the existence of a basin, perhaps controlled by extensional faults.
The lava plateau had largely a constructional topography, but there are also valleys filled with lavas and pyroclastic rocks, or choked with debris-flow and sedimentary deposits. For example, between Yoadcastle (SD 155 949) and Pike of Blisco (NY 270 044), two silicic ignimbrites, the nodular Little Stand Tuff and the lithic-rich Grey Friar Tuff, and the Great Whinscale Dacite lava were confined within an east-north-east-trending valley, at least 16 km long. The amount of topographical relief at any level within the sequence is difficult to estimate, though prior to the succeeding pyroclastic eruptions of the Scafell Caldera succession, the upper surface of the lava field had a relief of no more than about 110 m.
Silicic lavas and pyroclastic rocks were erupted episodically, particularly in the western Lake District. The main units are welded ignimbrites of the Cockley Beck (P006920) and Crag-house tuffs, and the succession of silicic rocks associated with the Great Whinscale Dacite. The aphyric Great Whinscale Dacite is an unusually extensive lava flow of this composition, having a length of at least 13 km and a minimum volume of 4 km3. Porphyritic dacite lavas and domes, many of them up to 250 m thick, also occur at other levels within the formation.
In the western Lake District, the andesitic to dacitic Craghouse Tuff represents the earliest large-volume, densely welded ignimbrite in the Borrowdale Volcanic Group, and possibly provides the first evidence therein of caldera formation. The ignimbrite sequence crops out south-east of the Ennerdale Intrusion, between the Burtness Comb and Thistleton faults. Thickness variation in the ignimbrite is systematic and fault controlled (P916048). Compositional zoning, from dacite at the base to acid andesite towards the top is typical of many ignimbrites erupted from zoned magma chambers.
|Development phase||Location, lithofacies and thickness||Main events|
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|
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