Early Palaeozoic magmatism in the English Lake District

Millward, D.

doi:10.1144/pygs.54.2.65

Cited by 18 publications

(16 citation statements)

References 102 publications

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“…I). This E-to ENE-striking fault with hade to the south, is inferred to lie above a repeatedly re activated, deep-seated basement structure, and hence was one of the fundamental faults that controlled accumulation and preservation of the BVG (Millward, 2002). The fault may in part also mark the northern margin of the Scafell Caldera, a major piecemeal, hydrovolcanic system within the BVG (Branney and Kokelaar, 1994).…”

Section: Controls On the Emplacement Of The Graphite Depositmentioning

confidence: 94%

The graphite deposit at Borrowdale (UK): A catastrophic mineralizing event associated with Ordovician magmatism

Menor

Millward

Luque

et al. 2010

Geochimica et Cosmochimica Acta

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The volcanic-hosted graphite deposit at Borrowdale in Cumbria, UK, was formed through precipitation from C-O-H flu ids. The 0 13 C data indicate that carbon was incorporated into the mineralizing fluids by assimilation of carbonaceous met a pelites of the Skiddaw Group by andesite magmas of the Borrowdale Volcanic Group. The graphite mineralization occurred as the fluids migrated upwards through normal conjugate fractures forming the main subvertical pipe-like bodies. The min eralizing fluids evolved from COrCH4-H20 mixtures (XC02 = 0.6-0.8) to CHCH20 mixtures. Coevally with graphite depo sition, the andesite and dioritic wall rocks adjacent to the veins were intensely hydrothermally altered to a propylitic assemblage. The initial graphite precipitation was probably triggered by the earliest hydration reactions in the volcanic host rocks. During the main mineralization stage, graphite precipitated along the pipe-like bodies due to CO2 --> C + O2. This agrees with the isotopic data which indicate that the first graphite morphologies crystallizing from the fluid (cryptocrystalline aggregates) are isotopically lighter than those crystallizing later (flakes). Late chlorite-graphite veins were formed from CH4-enriched fluids following the reaction CH4 + O2 --> C + 2H20, producing the successive precipitation of isotopically lighter graphite morphologies. Thus, as mineralization proceeded, water-generating reactions were involved in graphite precipitation, further favouring the propylitic alteration. The structural features of the pipe-like mineralized bodies as well as the isotopic homogeneity of graphite suggest that the mineralization occurred in a very short period of time.

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Section: Controls On the Emplacement Of The Graphite Depositmentioning

confidence: 94%

The graphite deposit at Borrowdale (UK): A catastrophic mineralizing event associated with Ordovician magmatism

Menor

Millward

Luque

et al. 2010

Geochimica et Cosmochimica Acta

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“…1). This east-to ENE-striking fault is inferred to lie above a repeatedly reac tivated, deep-seated basement structure, and hence it was one of the flllldamental faults that controlled the accumulation and preserva tion of the Borrowdale Volcanic Group (Millward 2002). The fault may in part also mark the northern margin of the Scafell Caldera, a major piecemeal, hydrovolcanic system within the Borrowdale Volcanic Group (Branney & Kokelaar 1994).…”

Section: Borrowdale Depositmentioning

confidence: 96%

“…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).…”

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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“…1 and 2; Branney and Kokelaar 1994) lies within a 6 km-thick succession of predominantly subaerial calc-alkaline lavas, pyroclastic rocks, volcanogenic sedimentary rocks and high-level intrusions, which together comprise the Ordovician (Caradoc) Borrowdale Volcanic Group (Millward 2002). The volcano is one of several nested centres preserved by caldera collapse and extension-related subsidence within a continental arc (Branney 1988;Branney and Soper 1988).…”

Section: Scafell Caldera Volcanomentioning

confidence: 99%

Incursion of a large-volume, spatter-bearing pyroclastic density current into a caldera lake: Pavey Ark ignimbrite, Scafell caldera, England

2007

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The Scafell caldera-lake volcaniclastic succession is exceptionally well exposed. At the eastern margin of the caldera, a large andesitic explosive eruption (>5 km 3 ) generated a high-mass-flux pyroclastic density current that flowed into the caldera lake for several hours and deposited the extensive Pavey Ark ignimbrite. The ignimbrite comprises a thick (≤125 m), proximal, spatter-and scoria-rich breccia that grades laterally and upwards into massive lapillituff, which, in turn, is gradationally overlain by massive and normal-graded tuff showing evidence of soft-state disruption. The subaqueous pyroclastic current carried juvenile clasts ranging from fine ash to metre-scale blocks and from dense andesite through variably vesicular scoria to pumice (<10 3 kg m −3 ). Extreme ignimbrite lithofacies diversity resulted via particle segregation and selective deposition from the current. The lacustrine proximal ignimbrite breccia mainly comprises clast-to matrix-supported blocks and lapilli of vesicular andesite, but includes several layers rich in spatter (≤1.7 m diameter) that was emplaced in a ductile, hot state. In proximal locations, rapid deposition of the large and dense clasts caused displacement of interstitial fluid with elutriation of low-density lapilli and ash upwards, so that these particles were retained in the current and thus overpassed to medial and distal reaches. Medially, the lithofacies architecture records partial blocking, channelling and reflection of the depletive current by substantial basinfloor topography that included a lava dome and developing fault scarps. Diffuse layers reflect surging of the sustained current, and the overall normal grading reflects gradually waning flow with, finally, a transition to suspension sedimentation from an ash-choked water column. Fine to extremely fine tuff overlying the ignimbrite forms ∼25% of the whole and is the water-settled equivalent of co-ignimbrite ash; its great thickness (≤55 m) formed because the suspended ash was trapped within an enclosed basin and could not drift away. The ignimbrite architecture records widespread caldera subsidence during the eruption, involving volcanotectonic faulting of the lake floor. The eruption was partly driven by explosive disruption of a groundwaterhydrothermal system adjacent to the magma reservoir.

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Early Palaeozoic magmatism in the English Lake District

Cited by 18 publications

References 102 publications

The graphite deposit at Borrowdale (UK): A catastrophic mineralizing event associated with Ordovician magmatism

The graphite deposit at Borrowdale (UK): A catastrophic mineralizing event associated with Ordovician magmatism

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

Incursion of a large-volume, spatter-bearing pyroclastic density current into a caldera lake: Pavey Ark ignimbrite, Scafell caldera, England

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