D. Millward

Deposition of highly crystalline graphite from moderate-temperature fluids

Recognized large occurrences of fluid-deposited graphite displaying high crystallinity were previously restricted to high-temperature environments (mainly granulite facies terranes). However, in the extensively mined Borrowdale deposit (UK), the mineralogical assemblage, notably the graphite-epidote intergrowths, shows that fully ordered graphite precipitated during the propylitic hydrothermal alteration of the volcanic host rocks. Fluids responsible for graphite deposition had an average X CO2/(XCO2 + X CH4) ratio of 0.69, thus indicating temperatures of ~500 °C at the fayalite-magnetite-quartz buffered conditions. Therefore, this is the first reported evidence indicating that huge concentrations of highly crystalline graphite can precipitate from moderate-temperature fluids.

The early Palaeozoic evolution of Northwest England

The Lake District and smaller Craven inliers of northwest England contain a Lower Palaeozoic sequence deposited on the Gondwanan side of the Iapetus Ocean, close to the junction with the Tornquist Sea. The Tremadoc to Llanvirn Skiddaw and Ingleton groups are deep water assemblages of turbidite, olistostrome and slump deposits, formed at a continental margin. They experienced uplift and erosion as a precursor to the eruption of two largely subaerial Llandeilo-Caradoc volcanic sequences: the tholeiitic Eycott Volcanic Group in the north and the calc–alkaline Borrowdale Volcanic Group in the central Lake District. The volcanic episodes are the earliest part of a major episode of magmatism, extending through to the early Devonian and responsible for a major batholith underpinning the Lake District. Subsidence in an intra-arc rift zone preserved the subaerial volcanic sequences. A marine transgression marks the base of the Windermere Group, which comprises a mixed carbonate–clastic shelf sequence of Ashgill age, passing upwards through the Silurian into a thick, prograding foreland basin sequence of Ludlow turbidites. Deformation of the Lower Palaeozoic sequences was possibly diachronous from north to south. It is attributed to the late Caledonian (Acadian) Orogeny and resulted in folding, cleavage and thrust development. Granitic intrusions, including those at Shap, Skiddaw and beneath the hydrothermal Crummock Water Aureole, are partly syntectonic and were therefore penecontemporaneous with this deformation event. Some thrust faulting post-dates the intrusive phase. Post-deformation Devonian conglomerates are also present locally.

Palaeoecological and possible evolutionary effects of early Namurian (Serpukhovian, Carboniferous) glacioeustatic cyclicity

Early Namurian (Serpukhovian, Carboniferous), sedimentary cycles in the Throckley and Rowlands Gill boreholes, near Newcastle upon Tyne, UK, consist of fossiliferous limestones overlain by (usually unfossiliferous) black mudstone, followed by sandstones and often by thin coal seams. Sedimentological and regional geological evidence suggests that the largest are high-amplitude cycles, probably of glacioeustatic origin. δ13C (bulk organic matter) delineates marine and non-marine conditions because of the large difference between terrestrial and marine δ13C, and indicates that full marine salinity was only intermittent and resulted from glacioeustatic marine transgression superimposed on a background of inundation by freshwater from large rivers, which killed off the marine biota. Palynology suggests that plant groups, including ferns and putative pteridosperms, were affected by changing sea level, and that there is a theoretical possibility of connection between cyclicity and the first appearance of walchiacean conifer-like monosaccate pollen such as Potonieisporites. Long-term terrestrial and marine increasing δ13C (organic) may reflect the onset of major glaciation in Gondwana, as there is evidence to suggest that the two are coeval, but no specific mechanism can be suggested to link the trends.

A pre-caldera plateau-andesite field in the Borrowdale Volcanic Group of the English Lake District

The 6 km thick Ordovician Borrowdale Volcanic Group is readily divisible into a lower 2.2–2.7 km thick predominantly pre-caldera succession dominated by basalt, andesite and dacite sheets, and an upper succession of caldera-related ignimbrites and volcaniclastic sedimentary rocks of c. 3 km thickness. The lower Borrowdale Volcanic Group rocks, here included within a single lithostratigraphical unit, the Birker Fell Formation, affords a well-exposed section through a pre-caldera sequence. The Birker Fell Formation is dominated by andesites which comprise 60% of the stratigraphy. Thin sequences of reworked volcanic detritus are commonly interbedded with the andesites; locally there are thicker units of volcaniclastic sedimentary rocks. Interpretation of the stratigraphy and eruptive history of the formation has been aided by the recognition of some distinctive lithological units including: (1) a 200-600 m thick sequence of weakly parallel-bedded basaltic tuffs resting on the pre-volcanic basement, (2) a 200-600 m sequence of single flow unit aa basalts, (3) dacite flows and ignimbrites up to > 1100 m thick in the middle and upper parts of the formation and (4) aphyric to pyroxene-phyric, simple and compound flows of basalt, 50-400 m thick, locally present in the uppermost parts. Analysis of the facies comprising the Birker Fell Formation indicates that it was emplaced as a sub-aerial, plateau-andesite sequence, formed by the coalescence of products erupted from a number of centres or fissures in an extensional, subsiding volcano-tectonic rift zone. Volcano-tectonic faulting and eruption of thick ignimbrites locally influenced development of this field.

Northern England Serpukhovian (Early Namurian) farfield responses to southern hemisphere glaciation

Abstract: During the Serpukhovian (early Namurian) icehouse conditions were initiated in the southern hemisphere. However, nearfield evidence is inconsistent: glaciation appears to have started in limited areas of eastern Australia in the earliest Serpukhovian, followed by a long interglacial, whereas data from South America and Tibet suggest glaciation throughout the Serpukhovian. New farfield data from the Woodland, Throckley and Rowlands Gill boreholes in northern England allow this inconsistency to be addressed. δ18O from well-preserved late Serpukhovian (late Pendleian to early Arnsbergian) Woodland brachiopods vary between −3.4 and −6.3‰, and δ13C varies between −2.0 and +3.2‰, suggesting a δ18O seawater (w) value of around −1.8‰ VSMOW, and therefore an absence of widespread ice-caps. The organic carbon δ13C upward increasing trend in the Throckley borehole (Serpukhovian to Bashkirian; c. −24 to c. −22‰) and the Rowlands Gill borehole (Serpukhovian; c. −24 to c. −23‰) suggests large-scale burial of organic material, probably in burgeoning lycophyte-dominated coal forest, implying a fall in pCO2. pCO2 reduction appears not to have caused large-scale glaciation until the early Bashkirian, but a scenario of coalescing upland ice-caps through the Serpukhovian with a background of decreasing pCO2 appears to be similar to the process that initiated Cenozoic Antarctic glaciation.

Phylogenetic and environmental context of a Tournaisian tetrapod fauna

The end-Devonian to mid-Mississippian time interval has long been known for its depauperate palaeontological record, especially for tetrapods. This interval encapsulates the time of increasing terrestriality among tetrapods, but only two Tournaisian localities previously produced tetrapod fossils. Here we describe five new Tournaisian tetrapods (Perittodus apsconditus, Koilops herma, Ossirarus kierani, Diploradus austiumensis and Aytonerpeton microps) from two localities in their environmental context. A phylogenetic analysis retrieved three taxa as stem tetrapods, interspersed among Devonian and Carboniferous forms, and two as stem amphibians, suggesting a deep split among crown tetrapods. We also illustrate new tetrapod specimens from these and additional localities in the Scottish Borders region. The new taxa and specimens suggest that tetrapod diversification was well established by the Tournaisian. Sedimentary evidence indicates that the tetrapod fossils are usually associated with sandy siltstones overlying wetland palaeosols. Tetrapods were probably living on vegetated surfaces that were subsequently flooded. We show that atmospheric oxygen levels were stable across the Devonian/Carboniferous boundary, and did not inhibit the evolution of terrestriality. This wealth of tetrapods from Tournaisian localities highlights the potential for discoveries elsewhere.

U–Pb chronology and duration of late Ordovician magmatism in the English Lake District

U–Pb zircon ages are presented from igneous rocks in the English Lake District for which a wide range of ages had been obtained previously by other methods. The Little Stand and Crinkle tuffs, from the Borrowdale Volcanic Group, have ages of 451.6 ± 1.4 and 452.8 ± 0.7 Ma (2σ), respectively, the Threlkeld Microgranite 451.0 ± 1.1 Ma (2σ) and the Carrock Microgranite 452.4 ± 4.1 Ma (2σ). These dates are identical to the previously published dates for the Ennerdale and Eskdale granites and together are considered to be a good estimate for both the age and length of Caradoc magmatism in the Lake District. This is the first of three magmatic events recognized in the Lake District, the other two occurring during Ashgill and Early Devonian times. Regional comparisons with the Leinster Terrane in SE Ireland and the largely concealed English Midlands basement emphasize the magnitude and significance of the Caradoc event along the Eastern Avalonia margin.

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

Abstract The origin, differentiation and temporal evolution of magmas have been examined from the Ordovician lower Borrowdale Volcanic Group (BVG) — a calc-alkaline, plateau andesite pile of continental arc affinity. The basalt-dacite compositional range can be modelled by POAM-type crystal fractionation of primitive melts, allied with minor crustal assimilation, derived from enriched mantle modified by fluids from the subduction zone. Detailed sampling of sections through 2–3 km of stratigraphy reveals localised and successive highly systematic magmatic evolutionary trends, or cycles. Cycles in which magma becomes less differentiated with time can be viewed as episodes of rapid eruption of a compositionally zoned chamber, possibly due to magma recharge. By contrast, increasing differentiation with time implies reduced recharge and eruption rates whereby fractionation processes dominate. The lack of evidence for compositional change over time represents a balance between recharge, eruption and differentiation processes. Successive cycles within a local sequence can be related in many cases to discrete batches of magma that ascend into sub-volcanic chambers and undergo fractionation. The differences between contemporaneous sequences confirms the current view of the lower BVG as a multicentred volcanic field. It was deposited in a subsiding, extensional volcano-tectonic rift zone, consistent with episodes of rapid upward magma flux, with the eruption locally of primitive (high Mg#, Ni and Cr) basaltic lavas.

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

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.

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

Massive graphite deposition resulting in volumetrically large occurrences in volcanic environments is usually hindered by the low carbon contents of magmas and by the degassing 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 metasedimentary rocks; (2) the absence of significant degassing, related to the presence of sub-volcanic rocks or hypabyssal 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.