Geochemical variation and magmatic cyclicity within an Ordovician continental-arc volcanic field: the lower Borrowdale Volcanic Group, English Lake District

Beddoe-Stephens, B.; Petterson, Michael G.; Millward, D.; Marriner, Giz F.

doi:10.1016/0377-0273(94)00092-u

Cited by 11 publications

(23 citation statements)

References 49 publications

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“…The Burtness Comb Fault was therefore one of the flllldamental faults that controlled the accu mulation and preservation of the Borrowdale Volcanic Group (Millward 2002). Within the Borrowdale Volcanic Group, volcan ism was mainly subaerial and corresponded to medium-to high-K calc-alkaline rocks related to subduction along a continental margin (Fitton et al 1982;Beddoe-Stephens et al 1995). The Borrowdale Volcanic Group is llllderlain by anchizonal to epizonal metapelites and sandstones of the upper Cambrian to middle Ordovician Skiddaw Group.…”

Section: Borrowdale Depositmentioning

confidence: 99%

Key factors controlling massive graphite deposition in volcanic settings: an example of a self-organized critical system

Luque

Menor

Barrenechea

et al. 2012

JGS

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Massive graphite deposition resulting in volumetrically large occurrences in volcanic environ ments is usually hindered by the low carbon contents of magmas and by the degassillg processes occurring during and after magma emplacement. In spite of this, two graphite deposits are known worldwide associated with volcanic settings, at Borrowdale, UK, and Huelma, Spain. As inferred from the Borrowdale deposit, graphite mineralization resulted from the complex interaction of several factors, so it can be considered as an example of self-organized critical systems. These factors, in turn, could be used as potential guides for exploration. The key factors influencing graphite mineralization in volcanic settings are as follows: (1) an unusually high carbon content of the magmas, as a result of the assimilation of carbonaceous metasedimen tary rocks; (2) the absence of significant degassing, related to the presence of sub-volcanic rocks or hypabys sal intrusions, acting as barriers to flow; (3) the exsolution of a carbon-bearing aqueous fluid phase; (4) the local structural heterogeneity (represented at Borrowdale by the deep-seated Burtness Comb Fault); (5) the structural control on the deposits, implying an overpressured, fluid-rich regime favouring a focused fluid flow; (6) the temperature changes associated with fluid flow and hydration reactions, resulting in carbon supersaturation in the fluid, and leading to disequilibrium in the system. This disequilibrium is regarded as the driving force for massive graphite precipitation through irreversible mass-transfer reactions. Therefore, the formation of volcanic-hosted graphite deposits can be explained in terms of a self-organized critical system.

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Section: Borrowdale Depositmentioning

confidence: 99%

Key factors controlling massive graphite deposition in volcanic settings: an example of a self-organized critical system

Luque

Menor

Barrenechea

et al. 2012

JGS

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“…Similar cyclic mafic volcanism and a change to more primitive compositions upward in the sequence has been identified in a number of volcanic provinces, for example for the Tertiary volcanic province of Greenland (Hogg et al 1989), Ordovician mafic rocks in the English Lake District (Beddoe-Stephens et al 1995) and the Silurian Passamaquoddy Bay sequence in eastern Canada (Van Wagoner et al 2002). These authors suggest that distinct magma batches underwent similar but decreasing amounts of fractionation over time likely related to maturing of the sub-volcanic plumbing system.…”

Section: Chemical Stratigraphymentioning

confidence: 90%

Extension-related volcanism in the Middle to Late Devonian of the Lachlan Orogen: geochemistry of mafic rocks in the Comerong Volcanics

Dadd

2011

Australian Journal of Earth Sciences

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The Comerong Volcanics are a Middle to Late Devonian bimodal sequence located in the southeastern Lachlan Orogen of NSW, Australia. Magmatism related to subduction was ongoing and located to the east of the continent during this period placing the Comerong Volcanics in a continental back arc setting. Mafic rocks in the Comerong Volcanics occur in three stratigraphically distinct units and range in composition from tholeiitic andesite to basalt. An inverse relationship between volume of erupted lava and degree of fractionation of the magma is evident from the base of the volcanic complex to its top. The lowermost mafic unit is the least voluminous and most fractionated and consists of flows with low Mg#, Ni and Cr and high incompatible trace element abundances. The overlying unit comprises both moderately and extremely fractionated basalt and is characterised by flows with a relatively high TiO 2 content. Lavas in this unit can be divided into higher and lower-Ti types. The uppermost unit is the most voluminous and the least fractionated with relatively high MgO, Ni and Cr and low incompatible trace element abundances. Compositions are similar to the low-Ti lavas in the unit below. Trace element characteristics of all lavas suggest they were derived from a heterogeneously enriched source in the subcontinental lithospheric mantle. Enrichment of the source in LILE and depletion in Ti and Nb likely reflect earlier subduction in the orogen. The mafic rocks were erupted in a continental within-plate setting; a setting consistent with the field relationships.

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“…On the contrary, average Al 2 O 3 content is around 16 %, in fact slightly lower than the average for Borrowdale Volcanic Group crystalline rocks of similar silica content (cf. Beddoe-Stephens et al 1995). Uniform REE abundances and patterns, particularly the consistent Eu anomaly (Fig.…”

Section: Bulk-rock Composition and Magmatic Parametersmentioning

confidence: 95%

“…9) reveal that the succession as a whole defines a coherent fractionation trend, which falls within the field defined for Borrowdale Volcanic Group crystalline rocks (e.g. Beddoe-Stephens et al 1995), and displays relatively minor cross-trend scatter. The exception to this is the more mobile element Sr, which appears variably depleted due to plagioclase alteration.…”

Section: Bulk-rock Composition and Magmatic Parametersmentioning

confidence: 97%

Very densely welded, rheomorphic ignimbrites of homogeneous intermediate calc-alkaline composition from the English Lake District

Beddoe-Stephens

Millward

2000

Geol. Mag.

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Within a largely concealed, caldera-related volcaniclastic succession of the Ordovician Borrowdale Volcanic Group in the western Lake District, two thick (100–350 m) ignimbrites within the Fleming Hall Formation exhibit a number of features that in combination make them unusual deposits. They are both homogeneous with comparatively low-SiO2 (63%) bulk composition, contain only a moderate crystal content, are generally poor in lithic clasts, show uniformly very dense welding (yielding parataxitic to massive vitrophyric texture) throughout and lack associated fall-out or surge deposits. Ignimbrites of comparable bulk composition in this geological setting are usually part of zoned sheets and/or frequently very crystal-rich. Large-scale, unzoned densely welded ignimbrites are usually rhyodacitic to rhyolitic. By contrast, ignimbrites of intermediate composition that display dense welding are relatively small deposits that form by agglutination of hot, plastic spatter.It is postulated that the Fleming Hall ignimbrites were derived from low column height, low explosivity eruptions that conserved heat and minimized entrainment of accidental lithic clasts and the formation of fine ash. The very dense welding and lack of bubble-wall shard vitroclastic textures indicate that pyroclasts were hot and relatively dry, probably occurring as mildly vesicular (scoriaceous) fragments which welded or fused together during aggradational deposition rather than by post-depositional compactional loading. There is little variation in the degree of matrix or melt crystallization throughout the two ignimbrites, despite the fact that high temperatures must have been maintained for many years following deposition. Both display virtually ubiquitous development of micropoikilitic glass devitrification texture, which suggests that the viscosity of the supercooled dacitic melt was sufficiently high, probably due to initial degassing, to inhibit significant melt crystallization after deposition.The eruption of the Fleming Hall magmas was probably initiated by the rise or injection of hotter, more basic, magma, and not by overpressurization due to volatile exsolution resulting from cooling and crystallization. Foundering of the chamber roof caused forcible and rapid eruption of the magma, probably along a series of volcanotectonic faults rather than a central vent, and probably flooded the resultant caldera depression. It is predicted that this type of eruption will not have produced a widely dispersed deposit, the bulk of which may have been largely contained within its own caldera.

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Geochemical variation and magmatic cyclicity within an Ordovician continental-arc volcanic field: the lower Borrowdale Volcanic Group, English Lake District

Cited by 11 publications

References 49 publications

Key factors controlling massive graphite deposition in volcanic settings: an example of a self-organized critical system

Key factors controlling massive graphite deposition in volcanic settings: an example of a self-organized critical system

Extension-related volcanism in the Middle to Late Devonian of the Lachlan Orogen: geochemistry of mafic rocks in the Comerong Volcanics

Very densely welded, rheomorphic ignimbrites of homogeneous intermediate calc-alkaline composition from the English Lake District

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