Brian R. Bell

Constraints on mantle plumes from the helium isotopic composition of basalts from the British Tertiary Igneous Province

Helium isotopes have been measured in olivines from igneous rocks spanning the history of the Palaeocene volcanic centre at the Isle of Skye, northwest of Scotland. 3He/4He in olivine phenocrysts from basalts from the Skye Lava Field are up to 22 times the atmospheric ratio which is consistent with their origin from the proto-Iceland mantle plume despite being erupted 1300 km from the plume axis. The 3He-anomaly of the Palaeocene starting plume in the North Atlantic appears to have been up to three times greater, for the same distance from the plume axis, than the modern Iceland plume. The Pb isotope and trace element (ΔNb) composition of the least crustally contaminated Skye magmas resemble North Atlantic MORB rather than Iceland basalts. The proto-Iceland plume head appears to have been dominated by upper mantle material with a small deep mantle He contribution rather than undiluted deep mantle material as predicted in the classic plume model.

Three-dimensional seismic imaging of Paleogene dike-fed submarine volcanoes from the northeast Atlantic margin.

We use three-dimensional seismic data to describe the architecture of shallow intrusive igneous sill and dike complexes intruded into the upper crust as well as the internal and external geometry of extrusive conical igneous mounds that were fed by the magma conduits. The conical mounds accreted on the seabed, directly above the tips of the basaltic dikes, between 54.9 and 54.6 Ma. In plan view the mounds are broadly circular, 1–1.7 km in diameter, and have a relief of 50–300 m. They occur above a complex network of linked cuspate-shaped sills and dikes intruded into Paleocene and Cretaceous sediments of the Faeroe-Shetland Basin, northeast Atlantic margin. The highly organized internal structure, consisting of bulbous layers arranged concentrically around a central axis, along with the clear vertical link to the underlying sills and dikes, indicates that they most likely represent pillowed basaltic lava and hyaloclastite mounds that were fed by the dikes and extruded onto the seafloor. Mounds of similar external geometry have been described from sidescan sonar images above active spreading centers in oceanic settings. However, such structures are rarely recognized in passive margin settings, and prior to this investigation our knowledge of their three-dimensional internal geometry has been largely speculative.

Crystallization of the Shap Granite: evidence from zoned K-feldspar megacrysts

Textural, chemical and Sr isotopic studies of feldspars from the subsolvus Shap granite, northern England, demonstrate that a number of magma mixing events have dominated the evolution of this pluton. K-feldspar megacrysts are phenocrysts formed in the magma chamber. They contain a number of Ba-rich zones that developed during periods of slight dissolution and regrowth linked to the hybridization of the granite by the intrusion of basic magmas. Diorite enclaves represent the relicts of these magmas and these also contain K-feldspar megacrysts, which show evidence of major dissolution. They are xenocrysts picked up from the host granite and incorporated in the basic magma. Increasing H2O contents during fractional crystallization caused a late switch from growth of megacrysts to finer-grained K-feldspars in the matrix. The chemically and isotopically zoned K-feldspar megacrysts preserve an exceptional record of the evolution of the magma, and the zones also had a significant influence on the development of exsolution microtextures during cooling.

The stratigraphy, environment of eruption and age of the Faroes Lava Group, NE Atlantic Ocean

Abstract Palaeogene volcanism on the NW margin of the Faroe-Shetland Basin is represented by the Faroes Lava Group, within an age range of 57.5–60.56 Ma. The volcanic sequence comprises >1000 m of basaltic volcaniclastic rocks deposited in estuarine or marginal lagoons, overlain by three laterally-extensive formations of subaerial facies basaltic lavas: Lower, c. 3250 m; Middle, c. 1400 m; Upper, at least 900 m (top not preserved). The Lower and Upper formations comprise high-volume sheet flows, commonly with ferrallitized tops, interbedded with reddened, thin, fluvial claystone and basaltic siltstone deposits. Laterally-impersistent coals occur within the Lower Lava Formation. The Coal-bearing Formation (c. 20 m) was deposited in an overbank floodplain environment during an hiatus in the volcanism between the Lower and Middle formations. The Volcaniclastic Sandstone Sequence comprises hydroclastic and pyroclastic deposits which post-date the Coal-bearing Formation and represent a return to volcanism, prior to the eruption of the Middle Lava Formation which is mainly characterized by inflated pahoehoe flows. The onshore sequence of the Faroes Lava Group can be correlated with basaltic flows within the Faroe-Shetland Basin, where lavas in Well 205/9-1 are interpreted to be of Lower Lava Formation affinity, possibly erupted from a local vent system. Seismic and gravity mapping and modelling suggest that the offshore extension of the Lower Lava Formation, together with the oldest part of the Middle Lava Formation, comprise subaqueous hyaloclastites deposited in a prograding Gilbert-type lava delta system. The youngest part of the Middle Lava Formation and all of the Upper Lava Formation occur as subaerial facies lavas within the basin.

On the emplacement of sill complexes: evidence from the Faroe-Shetland Basin

Abstract The North Faroe-Shetland Basin (NFSB) Sill Complex is of late Paleocene/earliest Eocene age and was emplaced within Cretaceous and Paleocene sedimentary rocks, in places to depths as shallow as a few hundred metres below the contemporaneous basin floor. Intersections of the Complex occur in exploration wells drilled by the oil industry and indicate tholeiitic basaltic compositions. High quality 3D seismic data, obtained during hydrocarbon exploration along the NE Atlantic Margin, provide a unique view of an uneroded suite of these sheet-like intrusions in UK Quadrants 218 and 219 and indicate the multi-centred nature of the NFSB Sill Complex, with upward-fingering terminations from broad bowlshaped foci of intrusion. Where the intrusion depth is very shallow, depending upon the host lithology, sill emplacement has lead to the development of structures on the contemporaneous basin floor interpreted as submarine hyaloclastite-dominated vents, up to c. 2 km across and with heights of up to c. 100 m. Where intrusion depth is greater, ‘seismic chimney’ structures are interpreted as the fluidescape feeders of sedimentary-hydrothermal mounds. Subsequent differential compaction of sedimentary sections, with and without shallow-emplaced sills, has given rise to distinctive ‘eye’ structures, as seen in seismic sections.

The evolution of the North Atlantic Igneous Province and the opening of the NE Atlantic rift

The papers in this volume represent a step forward in our knowledge of the geological evolution of the North Atlantic from the Cretaceous-Palaeogene boundary through to the early Eocene. With the increase in hydrocarbon exploration activities in the Faroe-Shetland Basin (Fig. 1), new interpretations of the regional geology have become increasingly important, and the accuracy of the time frame for this work is vital to our understanding of the sequence of volcanic and sedimentary events. The synthesis of data relating to Palaeogene volcanism and sedimentation along the Norwegian Margin by Eldholm et al. emphasizes the importance of transfer zones, possibly inherited from the Proterozoic basement, in the distribution of sediments and volcanic products during rifting (Fig. 2). Furthermore, subsequent uplift and the development of marginal highs are invoked as factors which affected water circulation within the basins, leading to a deterioration in the Eocene climate. This work identifies the relevance of the North Atlantic Igneous Province (NAIP) as an influence on environmental systems on a global scale. Ar-Ar and Pb-U isotopic age data show that the main period of continental flood basalt volcanism in the NAIP extended from c. 60.5 Ma through to c. 54.5 Ma (Table 1). Biostratigraphical analysis of these volcanic-sedimentary sections (Jolley et al. 2002) shows that the onset of this interval equates to the Late Paleocene Thermal Maximum (LPTM). New isotopic dating of the oldest part of the volcanic sequence on the Faroe Islands, the Lower Formation, by Waagstein et al. has further confirmed the age

The cumulus and post-cumulus evolution of chrome-spinels in ultrabasic layered intrusions: evidence from the Cuillin Igneous Complex, Isle of Skye, Scotland

Chrome-spinels from the layered Peridotilte Series of the unmetamorphosed, anorogenic 60 Ma Cuillin Igneous Complex, Isle of Skye, display a wide variety of compositions. Cumulus (within seams) chrome-spinels from the lowest exposed portion of the Peridotite Series exhibit features indicative of textural equilibrium, are rich in Al and Mg, and have low values of the ratio Cr/(Cr+Al). Cumulus chrome-spinels from higher up in the series are different from these: particularly, textural disequilibrium is evident, intercumulus plagioclase and olivine are present, and the chrome-spinels are rich in Cr, Fe and Ti, with high values of the ratio Cr/(Cr+Al). Intercumulus (dispersed) chrome-spinels tend towards anhedral forms and define enrichment trends towards Fe (both Fe2+ and Fe3+) with decreasing Mg, Cr and Al, and towards Al, with decreasing Fe2+ and Cr (and increasing Mg). Individual crystals are completely homogeneous and are devoid of reaction rims. The observed textural characteristics and compositional data of the chrome-spinels documented here suggest that the semi-quantitative peritectic reaction: aluminous chrome-spinel + melt→plagioclase + olivine + chromian chrome-spinel, is responsible for the observed parageneses, and that both the environment of crystallization (eumulus or intercumulus) and the role of plagioclase ±olivine crystallization are critical parameters for this geochemical trend in spinels within upper crustal magmatic systems. The effects of pyroxene crystallization on the development of this geochemical trend are also considered. This investigation highlights the need to consider the role of post-cumulus mineral-melt reactions and their influences upon the final compositions of major oxide and silicate phases within layered intrusions.

Sedimentary and volcano-tectonic processes in the British Paleocene Igneous Province: a review

Research on the British Paleocene Igneous Province (BPIP) has historically focused on the emplacement, chemistry and chronology of its elaborate central intrusive complexes and lava fields. However, the BPIP has also been dramatically shaped by numerous erosion, sedimentation and volcano-tectonic events, the significance of which becomes ever clearer as localities in the BPIP are re-investigated and our understanding of volcano-sedimentary processes advances. The resultant deposits provide important palaeo-environmental, palaeo-geographical and stratigraphical information, and highlight the wide range of processes and events that occur in ancient volcanic settings such as the BPIP. In this paper we review the sedimentary and volcano-tectonic processes that can be distinguished in the BPIP, and conceptualize them within a generalized framework model. We identify, and describe, the sedimentary responses to four broadly chronological stages in the history of the BPIP volcanoes: (1) the development of the lava fields, (2) early intrusion-induced uplift, (3) caldera collapse and (4) post-volcano denudation and exhumation of central complexes. We highlight and illustrate the range of sedimentary processes that were active in the BPIP. These operated on and helped shape a dynamic landscape of uplands and lowlands, of alluvial fans, braided rivers, lakes and swamps, and of volcanoes torn apart by catastrophic mass wasting events and/or caldera collapse.

Application of palynological data to the chronology of the Palaeogene lava fields of the British Province: implications for magmatic stratigraphy

New high-precision ages, determined from palynomorph assemblages within intercalated sedimentary deposits, are presented for the Palaeogene lava fields (Skye, Mull and Antrim) of the British Province. These data reveal very rapid averaged eruption rates (1 m/200 years) and the non-synchronous formation of the lava fields. Eruption of the volumetrically dominant transitional to mildly alkaline lavas of the Skye (58.25-58.0 Ma) and Mull (post 55 Ma) lava fields (the Skye Main Lava Series, and the Plateau Group of Mull, respectively) is separated by the eruption of the MORB-like Preshal More flows of olivine tholeiite found at the top of the preserved sequence on Skye and at the base of the Mull Lava Field. The Lower Formation of the Antrim Lava Field correlates with the Skye Lava Field and the Upper Formation correlates with the Preshal More flows. The new ages indicate that the eruption of the Preshal More flows was synchronous with the main ocean floor spreading event which occurred 500 km to the NW, at c. 55 Ma, during Chron 24r. A combined thinspot and channelized plume model may best explain the temporal and spatial distributions of the lava fields and associated subvolcanic complexes of the British Province.

Intrusion-induced uplift and mass wasting of the Palaeogene volcanic landscape of Ardnamurchan, NW Scotland

Fragmental rocks associated with the exhumed hearths of the Palaeogene volcanoes of NW Scotland have previously been interpreted as classic examples of vent-filling agglomerates of pyroclastic origin. Mapping and logging of these lithologies at the Ardnamurchan Central Complex indicate that they comprise conglomerates of debris flow origin, interbedded with fluvial–lacustrine units, which developed on a dissected landscape during the volcanic and intrusive activity. Clast-matrix analysis allows the palaeo-topography and drainage systems to be inferred, and palynological data are used to develop a palaeogeographical model for the volcanic landscape. A source-region uplift model links the catastrophic mass wasting events to the shallow emplacement of the central complex intrusions.