British Tertiary Igneous Province Research Articles

Optimal resolution tomography with error tracking and the structure of the crust and upper mantle beneath Ireland and Britain

SUMMARYThe classical Backus–Gilbert method seeks localized Earth-structure averages at the shortest length scales possible, given a data set, data errors, and a threshold for acceptable model errors. The resolving length at a point is the width of the local averaging kernel, and the optimal averaging kernel is the narrowest one such that the model error is below a specified level. This approach is well suited for seismic tomography, which maps 3-D Earth structure using large sets of seismic measurements. The continual measurement-error decreases and data-redundancy increases have reduced the impact of random errors on tomographic models. Systematic errors, however, are resistant to data redundancy and their effect on the model is difficult to predict. Here, we develop a method for finding the optimal resolving length at every point, implementing it for surface-wave tomography. As in the Backus–Gilbert method, every solution at a point results from an entire-system inversion, and the model error is reduced by increasing the model-parameter averaging. The key advantage of our method stems from its direct, empirical evaluation of the posterior model error at a point. We first measure inter-station phase velocities at simultaneously recording station pairs and compute phase-velocity maps at densely, logarithmically spaced periods. Numerous versions of the maps with varying smoothness are then computed, ranging from very rough to very smooth. Phase-velocity curves extracted from the maps at every point can be inverted for shear-velocity (VS) profiles. As we show, errors in these phase-velocity curves increase nearly monotonically with the map roughness. We evaluate the error by isolating the roughness of the phase-velocity curve that cannot be explained by any Earth structure and determine the optimal resolving length at a point such that the error of the local phase-velocity curve is below a threshold. A 3-D VS model is then computed by the inversion of the composite phase-velocity maps with an optimal resolution at every point. The estimated optimal resolution shows smooth lateral variations, confirming the robustness of the procedure. Importantly, the optimal resolving length does not scale with the density of the data coverage: some of the best-sampled locations display relatively low lateral resolution, probably due to systematic errors in the data. We apply the method to image the lithosphere and underlying mantle beneath Ireland and Britain. Our very large data set was created using new data from Ireland Array, the Irish National Seismic Network, the UK Seismograph Network and other deployments. A total of 11 238 inter-station dispersion curves, spanning a very broad total period range (4–500 s), yield unprecedented data coverage of the area and provide fine regional resolution from the crust to the deep asthenosphere. The lateral resolution of the 3-D model is computed explicitly and varies from 39 km in central Ireland to over 800 km at the edges of the area, where the data coverage declines. Our tomography reveals pronounced, previously unknown variations in the lithospheric thickness beneath Ireland and Britain, with implications for their Caledonian assembly and for the mechanisms of the British Tertiary Igneous Province magmatism.

The Hebridean Igneous Province plumbing system: A phase equilibria perspective

The architecture of magmatic plumbing systems and the behaviour of the crystal mush underlying active volcanoes can be assessed by examining the crystal chemistry and microstructure preserved in solidified magma bodies, i.e. dykes, sills, and layered intrusions. Combining field and petrographic observations of the Little Minch Sill Complex in Scotland, with analyses of its mineral chemistry, demonstrates that these shallow tabular intrusions formed by two distinct magma pulses, a cargo-rich magma and a cargo-free magma, in a relatively closed system. The co-genetic nature of the crystal cargo and its carrier liquid permits the application of phase-equilibria modelling to estimate the pressures and temperatures of crystallisation and sill emplacement. Our calculations using thermodynamic modelling software (Perple_X) suggest the presence of a of magma chamber at ∼25 km depth and the formation of sill and dyke network at ∼3–16 km below the surface.Our new estimates of the pressures of magma storage and crystallisation are similar to those found for the Iceland and the rest of the British Tertiary Igneous Province, suggesting the behaviour of magmas in the crust and the architecture of the plumbing system are comparable. This apparent temporal consistency supports the use of preserved intrusive bodies as a means for understanding and constraining processes occurring in the upper crust beneath active volcanoes.

Cenozoic cooling and denudation in the North Pennines (northern England, UK) constrained by apatite fission-track analysis of cuttings from the Eastgate Borehole

The Cenozoic landscape development of Britain remains relatively poorly understood. On the one hand, ‘plumists’ have tried to explain the present-day topography as a consequence of effects of the Iceland mantle plume during the Palaeocene-Eocene British Tertiary Igneous Province (BTIP) magmatism, with little or no subsequent modification. On the other hand, abundant evidence exists from fluvial and marine terraces and superimposed karstic levels for significant vertical crustal motions during the Quaternary, which clearly has nothing to do with any mantle plume. To shed light on this issue, we present the first publication of data that constrain the Cenozoic thermal history of the North Pennine uplands of northern England, from apatite fission-track analysis of drill cuttings from the Eastgate Borehole in Weardale, in the western part of County Durham. Our results indicate ∼650m of regional denudation since the latest Oligocene/Early Miocene, plus the ∼400m of localized entrenchment that has created the modern Weardale valley. Before the latest Oligocene/Early Miocene, but following the BTIP magmatism, the crust in this region experienced significant cooling, mainly due to a decrease in the geothermal gradient from ∼55 to 61°Ckm−1 to the present 38°Ckm−1, along with ∼300±200m of denudation. Although significant BTIP magmatism occurred in northern England, it thus had only a limited net effect; the crust experienced dramatic heating, but cooled back to its original thermal state within, at most, a few tens of millions of years. We suggest that this rapid cooling effect resulted from westward flow of relatively cold material within the mobile lower-crustal layer, driven by the lateral pressure gradient induced by earlier heating effects and effects of surface processes. Whatever topography developed during the Palaeogene, as a direct result of these heating effects, underplating at the base of the crust, and the associated modest denudation, was presumably also short-lived; significant changes to the crustal thickness, and thus to the topography, can be envisaged as a consequence of subsequent lower-crustal flow.

Formation of flint horizons in North Sea chalk through marine sedimentation of nano-quartz

AbstractIn the Upper Cretaceous-Danian North Sea chalk, silica composed of nano-size quartz spheres is dispersed in the chalk matrix, and quartz is present in bands and nodules of flint. In the present investigation of the North Sea Danian chalk the nano-quartz in the chalk matrix is compared with the silica in the flint. Samples of chalk and flint layers from four North Sea wells have been investigated. Atomic Force Microscopy (AFM) has been applied to image the quartz in the chalk and in the flint. X-ray diffraction (XRD), including analysis of the positions and profiles of hkl reflections in powder diffraction patterns, has been applied to characterize the lattice of the quartz in both the chalk matrix and in the flint. The quartz in the chalk matrix and in the flint is composed of nano-quartz spheres having identical cell parameters. Based on the results we propose a new model for formation of flint in North Sea chalk: (1) The nano-quartz in the flint, like the nano-quartz in the chalk matrix, has crystallized in the marine Chalk Sea environment. The colloidal quartz particles flocculated and were deposited on the sea floor mixed with calcitic bioclastic material. (2) Regional variations in the concentration of nano-quartz particles in the sediment reflect different degrees of acidification of the Chalk Sea. (3) This resulted in areas where practically all the calcite bioclasts were dissolved leaving a high concentration of nano-quartz particles to form flint layers; where there was less dissolution, indurated chalk with abundant nano-quartz particles is now preserved. (4) The acidification could have been caused by the effects of enhanced atmospheric CO2 linked to massive short-lived volcanic eruptions in the British Tertiary Igneous Province.

The North Atlantic Igneous Province reconstructed and its relation to the Plume Generation Zone: the Antrim Lava Group revisited

SUMMARY Large igneous provinces (LIPs) have recently been suggested to originate at the edges of low-velocity zones on the core mantle boundary (Plume Generation Zones). If true, LIPs can potentially be used to constrain paleolongitude in plate tectonic reconstructions. To validate the hypothesis, it is essential to study LIPs of which the paleolongitude can be constrained by other methods, such as hotspot reference frames. An ideal candidate to this end is the early Cenozoic North Atlantic Igneous Province (NAIP). Despite being the largest volcanic unit of the British Tertiary Igneous Province (BTIP, part of the NAIP), the age and paleoposition of the Antrim Lava Group (ALG) in Northern Ireland, which is key to the NAIP as a whole, was hitherto poorly constrained. In this paper, we therefore present an integrated high-resolution paleomagnetic and geochronological study. The ALG is divided into three formations: the Lower Basalt Formation (LBF), Interbasaltic Formation (IBF) and the Upper Basalt Formation (UBF). The IBF is mostly lateritic and encloses the Tardree rhyolite. We offer new age constraints from all three formations using the 40 Ar/ 39 Ar method and propose that 62.6 ± 0.3, 61.3 ± 0.3 and 59.6 ± 0.3 Ma (1σ , internal uncertainties) are sound estimates of the age of emplacement of the LBF, Tardree rhyolite (IBF) and UBF, respectively. This constrains the nominal duration of emplacement of the ALG to 3 ± 0.6 Ma (1σ ). This reevaluation of the magnetic signature in the ALG revealed reverse polarity remanence in all three formations and an overall paleomagnetic north pole at latitude 78.9 ◦ N, longitude 167 ◦ E( A95= 6.3; age ∼61 Ma) in the European reference system. This appears consistent with paleomagnetic poles from the rest of the NAIP; both in Europe and Greenland, as well as predictions from modern apparent polar wander paths. The new radiometric ages span magnetochron C26r, C27n and C27r. The normal polarity chron C27n most probably occurred during the IBF hiatus, explaining why no normal polarity remanence was detected in the paleomagnetic investigation. Emplacement of the LBF falls in magnetochron C27r, making this one of the oldest lava sequence in the NAIP; older than the C27n lava pile in Western Greenland. The 60 Ma position of the NAIP in a paleomagnetic reference frame, puts it close to the northern edge of the African large low shear wave velocity anomaly at the core–mantle boundary and therefore in the line with the Plume Generation Zone hypothesis. However, the back-projected Icelandic hotspot, normally considered to have formed the NAIP, is located ∼1500 km north of the latitude at which the NAIP erupted. The northward motion of the north Atlantic lithosphere since the late Cretaceous challenging the existing correlation of the NAIP to the Icelandic hotspot, normally used to explain the observed pre- and syn-breakup North Atlantic magmatism (63–55 Ma), and either an additional plume located further south in the North Atlantic may be invoked to create the NAIP, or the Icelandic hotspot must have

Cenozoic uplift of southwest England

AbstractThe uplift history of southwest England is inferred using a composite dataset including marine and fluvial terraces and karstic data. The estimated post‐Mid‐Pliocene uplift increases eastward from ∼130 m in west Cornwall and ∼135 m in south Devon to ∼150 m in the Hampshire Basin. The post‐Early‐Pleistocene uplift likewise increases eastward, from ∼55 m in west Cornwall to ∼60 m in south Devon and ∼80 m in Hampshire. Landscape and thermochronological evidence also indicates Eocene uplift, caused by the British Tertiary Igneous Province magmatism; this component tapers eastward from ∼300 m in west Cornwall to ∼50 m in south Devon, with subsidence in east Devon. This uplift accompanied magmatic underplating; the mafic layer added to the basal crust thins eastward from ∼6 km in west Cornwall to ∼2 km in south Devon, evidently tapering to zero farther east. The laterally variable crustal properties caused by this variation in underplating have affected the post‐Mid‐Pliocene uplift; the study region is thus intermediate, in terms of crustal strength and landscape evolution, between central‐southern England, with no underplating, and Ireland, where ∼10 km thick underplating has resulted in extreme Late Cenozoic landscape stability. The Eocene mantle‐plume‐related uplift is distinct from the post‐Mid‐Pliocene phase which, the modelling indicates, has been driven by surface processes and, thus, by climate change. Copyright © 2010 John Wiley & Sons, Ltd.

Reassembling the Paleogene–Eocene North Atlantic igneous province: New paleomagnetic constraints from the Isle of Mull, Scotland

The paleomagnetic data sets from the British Tertiary Igneous Province (BTIP) have recently been criticized as being unreliable and discordant with data from elsewhere in the North Atlantic Igneous Province (NAIP) [Riisager et al. Earth Planet. Sci. Lett. 201 (2002) 261–276; Riisager et al. Earth Planet. Sci. Lett. 214 (2003) 409–425]. We offer new paleomagnetic data for the extensive lava flow sequence on the Isle of Mull, Scotland, and can confirm the paleomagnetic pole positions emanating from important earlier studies. Our new north paleomagnetic pole position for Eurasia at 59 ± 0.2 Ma has latitude 73.3°N, longitude 166.2°E (dp/dm = 5.2/7.0). A re-evaluation and an inter-comparison of the paleomagnetic database emanating from the NAIP were carried out to test for sub-province consistency. We find a general agreement between the Eurasian part of NAIP (BTIP and Faeroes) and East Greenland data. However a compilation of West Greenland data displays a large and unexplained dispersion. We speculate on if this is related to different sense of block rotation of the Tertiary West Greenland constituents. Combining all data from the NAIP constituents, give a pole position at 75.0°N, 169.9°E ( N = 25, K = 84.3, A 95 = 3.2) in Eurasian reference frame.

The British Tertiary Igneous Province: Palaeomagnetism and Ages of Dykes, Lundy Island, Bristol Channel

Summary Of 66 dykes collected 52 gave stable directions: 44 reversed, 4 normal and 4 intermediate. The mean direction of magnetization was N3 - 6 E, + 62 7 (down,) equivalent to a pole position of 82.6 ON, 155.0 E. This is significantly different from the present pole and displays right-handed farsidedness. The bulk, at least, of the dykes fall within the apparent span 45-54 My. Yet they display approximately the same small proportion of normal dykes as do the older dyke swarms of the province which were formed during a different polarity period or periods.

EMELEUS, C. H. & BELL, B. R. 2005. British Regional Geology: the Palaeogene Volcanic Districts of Scotland, 4th ed. x + 214 pp. + map in folder. Keyworth: British Geological Survey. Price £18.00 (paperback). ISBN 0 85272 519 1

This, the long-overdue fourth edition of this book, has been completely rewritten by two of the most knowledgeable petrologists currently working on the British Tertiary Igneous Province: Henry Emeleus (Durham) and Brian Bell (Glasgow). Given the undoubted petrological pedigree of the two authors it is no surprise that this publication is first class and is suitable for use by a wide variety of people ranging from undergraduates, school teachers, researchers, lecturers, field trip leaders, civil engineers to enthusiastic amateur geologists. The book is written in clear and lucid style and is lavishly illustrated with numerous colour maps, tables, diagrams and stratigraphic columns, all of which are ideal for teaching purposes. A significant number of well chosen clear photographs and some photomicrographs also illustrate the most salient geological features of the area. A useful A3 foldout colour map of the Scottish Palaeogene igneous province is also …

Crustal velocity structure of the British Isles; a comparison of receiver functions and wide-angle seismic data

Magmatic underplating at the base of the continental crust is thought to be an important consequence of mantle plume activity. Geochemical evidence supports the hypothesis of underplating beneath the British Tertiary Igneous Province during Palaeocene times, a process that has been invoked to explain at least part of the observed pattern of Cenozoic denudation and consequent offshore deposition. This study uses receiver functions to independently test the results of modelling of four wide-angle seismic lines across the British Isles, several of which indicate the presence of a lens of high-velocity material at the base of the crust. This lens has been interpreted as evidence of Palaeogene magmatic underplating. Receiver functions were calculated for three permanent and two temporary three-component broad-band seismometers. In order to detect P-to-S conversions from intracrustal velocity discontinuities, high frequencies were preserved in the calculation of the receiver functions. Receiver functions were modelled using the delay times of the P-to-S converted arrivals only, with constraints on the VP and VP/VS structure imposed from controlled source data. This pragmatic method of interpreting receiver function data agrees well with the results of applying an established inversion technique. Resulting crustal velocity models are broadly consistent with the velocity models from the wide-angle lines, showing evidence for a four-layer crust with laterally varying Moho depths of between 33 ± 2.5 km beneath southwest Ireland, 32 ± 2.5 km beneath northern and eastern Ireland, 31 ± 2 km beneath the Isle of Man and 35 ± 3 km beneath northern England. While these velocity models are consistent with the presence of a lower crustal layer, receiver functions are insensitive to the P-wave velocity structure, and so unfortunately they cannot be used to independently constrain the P-wave velocity of the lower crust, which is an important diagnostic of the presence of magmatic underplating.

Cenozoic vertical motions in the Moray Firth Basin associated with initiation of the Iceland Plume

It is likely that the Iceland mantle plume generated transient uplift across the North Atlantic region when it initiated in earliest Cenozoic time. However, transient uplift recorded in sedimentary basins fringing the region can be overprinted by the effects of permanent uplift. Identifying and quantifying transient uplift can only be achieved in areas which have a well‐constrained stratigraphic record and across which the relative importance of permanent and transient uplift varies (e.g., the Moray Firth Basin, North Sea). By analyzing the subsidence of 50 boreholes from the Moray Firth Basin (MFB), residual vertical motions unrelated to rifting have been isolated. Transient uplift of 180–425 m occurred during Paleocene times. The western MFB has also been affected by permanent Cenozoic uplift, with denudation decreasing from 1.3 ± 0.1 km in the west of the basin to zero denudation east of 1°W. Dynamic support above the Iceland Plume led to transient uplift of the entire MFB in early Paleocene times, peaking in latest Paleocene times. In early Eocene times the effect of the plume waned, and subsidence occurred. Paleocene permanent uplift of the NW British Isles is generally accepted to have been due to magmatic underplating of the crust emplaced during the British Tertiary Igneous Province (61–58.5 Ma). The cause of Neogene uplift events is poorly understood, but it could also be associated with the Iceland Plume.

Palaeogene igneous rocks reveal new insights into the geodynamic evolution and petroleum potential of the Rockall Trough, NE Atlantic Margin

AbstractData acquired from petroleum exploration well 164/7‐1 drilled in the UK sector of the Rockall Trough have yielded fresh insights into the igneous and thermal history of this frontier region. The well targeted a large four‐way dip closed structure of presumed Mesozoic age named ‘The Dome Prospect’. The structure is now known to have a magmatic, rather than a purely structural origin, which was the preferred pre‐well interpretation. The well encountered 1.2 km of Palaeocene age basaltic lavas, overlying Late Cretaceous mudstones which were intruded by over 70 dolerite sills ranging from <1.5‐ to 152‐m thick. 40Ar/39Ar dating of the dolerite intrusions indicates an Early Palaeocene age (63–64±0.5 Ma), which are among the oldest 40Ar/39Ar dates recognised in the North Atlantic Igneous Province. Radiometric dating of the overlying basaltic lavas proved unsuccessful, because of excessive alteration. Biostratigraphic dating of underlying and overlying sedimentary strata was utilised to constrain the age of the lavas to Late Paleocene to Early Eocene age (∼55 Ma). Despite being related to two distinct events separated by ∼8 Ma, the intrusives and extrusives are compositionally similar. The basaltic rocks from well 164/7‐1 possess Sr–Nd isotopic, major and trace‐element geochemical compositions similar to other volcanic and intrusive rocks of the British Tertiary Igneous Province and represent partial melts of both lithospheric and asthenospheric mantle associated with the proto‐Icelandic mantle plume head.Joint consideration of thermal maturity, potential fields and 3D seismic data indicate a deeper igneous body in addition to the sills encountered in well 164/7‐1. Jack‐up and arching mechanisms associated with both scales of intrusive body are believed to have developed the dome structure. The preferred interpretation is of a mafic laccolith, 17 km in diameter, ∼7 km thick, intruded at 64.5 Ma, situated ∼2.5 km below the bottom of the well. 3D thermal modelling suggests that all of Tranche 52 was thermally affected by the intrusion of the magmatic body. The thermal aureole, between 27 and 51 km in diameter, is not thought to play an important role in the hydrocarbon prospectivity of the surrounding Tranches in the NE Rockall Basin.Results show that hydrocarbon exploration prospects that are circular in map view should be interpreted with caution on volcanic continental margins. In sedimentary basins, where salt domes and shale diapirs are absent and igneous rocks prevalent, periclinal structures such as ‘The Dome Prospect’ should undergo a thorough multi‐disciplinary risk assessment.

Rapid formation of the Small Isles Tertiary centre constrained by precise 40Ar/ 39Ar and U–Pb ages

The relative chronology of magmatic and tectonic events is key to an understanding of the influence of the Iceland plume on the North Atlantic. In particular, the location and duration of magmatism is of fundamental importance. Initial widespread flood basalt formation occurred in Baffin Island, Greenland, and Britain before complete plate break up at 56 Ma after which time magmatism became concentrated in the active rift zone. Historically the British Tertiary Igneous Province (BTIP) has been instrumental in advancing many concepts of igneous petrology. However, the absolute age and duration of the province remains unresolved. Here, we present new internally consistent 40Ar/ 39Ar ages that help to constrain the volcanic activity in the Small Isles centre to within 2 my. This short duration has implications for the onset of magmatism in the larger North Atlantic province, the rapid unroofing of the Rum volcano, and more significantly, some of the evidence used to propose pulsing of the Iceland plume.

The 3D facies architecture of flood basalt provinces and their internal heterogeneity: examples from the Palaeogene Skye Lava Field

Quantifying the facies architecture of flood basalt provinces is important as it can be used to understand the physical volcanology and rock property variations throughout the lava sequence. The 3D facies architecture and internal heterogeneity of the Skye Lava Field, for example, provides important insights into the evolution of the British Tertiary Igneous Province, and valuable information to aid in the exploration of potential offshore reservoirs underlying significant flood lavas along the North Atlantic margin. The volcanic stratigraphy of the Talisker Bay area of Skye comprises: (1) lower compound-braided lavas (flow lobes <3 m thick); (2) transitional lavas (flows <8 m thick); (3) upper tabular-type lavas (flows <20 m thick), representing a relative increase in eruptive volume. A 3D model of the lava sequence was reconstructed using detailed digital geological mapping, revealing estimated volumes of: the lower sequence (12.7 km 3 ), the transitional sequence (7.4 km 3 ) and the upper sequence (17.0 km 3 ). The lower sequence lavas formed on the flanks of a shield volcano and were sourced from the NE. Volcanological features such as lava feeder tubes, pahoehoe textures and lobes indicate a scale of volcanism similar to that of present-day Hawaii. The within-flow heterogeneity of the basalts is characterized using an ‘intrafacies scheme’, allowing comparison of variations in lithofacies with characteristic (geophysical) rock properties of compressional-wave velocity and density.

Development of volcanic passive margins: Three‐dimensional laboratory models

Continental breakup above an anomalously hot mantle may lead to the development of volcanic margins. Volcanic margins are characterized by (1) thick seaward dipping lava flow sequences, (2) central intrusive complexes associated with dyke swarms parallel to the coast, and (3) high seismic velocity bodies in the lower crust attributable to magma underplating. A conceptual model for volcanic margins development has recently been proposed based on onshore studies of the Greenland margins and the British Tertiary Igneous Province. It is proposed that the long‐lived central intrusions are genetically linked to underlying persistent zones of mantle fusion. These localized melting domains (or soft spots), equivalent to small mantle diapirs, may locally soften the extending continental lithosphere. The low‐viscosity diapirs would (1) localize tectonic strain and (2) feed the volcanic margin with magma. Thus such soft spots can control the along‐strike magmatic and tectonic segmentation of volcanic margins. Recent geophysical investigations appear to show that the along‐strike structure of volcanic passive margins is compatible with such a segmentation process. Here we present a set of scaled experiments designed to study how such localized rheological heterogeneities in the sub‐Moho mantle may have a mechanical effect on continental breakup. Four‐layer models were constructed using sand and silicone putties to represent the brittle and ductile layers of both crust and mantle. The soft spots are simulated by low‐viscosity silicone putty emplaced within the brittle material. At the scale of the entire breakup zone, the soft spots display an oceanic‐type strength profile defining low‐strength zones where continental breakup is initiated. The rift orientation and segmentation are strongly controlled by the distribution of the low‐viscosity heterogeneities, rather than by the direction of regional extension. The experiments are compared with the geometry and segmentation of the onshore part of the Greenland volcanic margins.

Pb-isotope evidence for contrasting crustal contamination of primitive to evolved magmas from Ardnamurchan and Rum: implications for the structure of the underlying crust

Synopsis Using Pb isotope ratios we compare crustal contamination of primitive to evolved magmas from the Ardnamurchan and nearby Rum Igneous Centres, located on different crustal provinces in the British Tertiary Igneous Province (BTIP). The results confirm that compositional variations of parental cone-sheet magmas in Ardnamurchan can be explained by assimilation of granulite facies Lewisian gneiss at moderate crustal levels and subsequent contamination of evolved magmas with Moine schist metasediments within the uppermost crust during fractional crystallization. In contrast, samples from the Rum Centre have an uncontaminated mantle signature for the basaltic end-member, whereas the evolved rocks show Pb isotope evidence of contamination by Lewisian amphibolite facies rocks. Rum is separated from Ardnamurchan by a major thrust. The absence of a Moine-type isotopic influence in the Rum rocks supports an earlier interpretation, based on field evidence, that these overthrust rocks were eroded from Rum prior to the Palaeocene magmatic activity.

The Blackstones Bank igneous complex: geochemistry and crustal context of a submerged Tertiary igneous centre in the Scottish Hebrides

The Blackstones Bank, located about 60 km WSW of the Isle of Mull in Western Scotland, is a submarine plutonic complex in the British Tertiary Igneous Province. Geochemical and isotopic analysis of gabbros, microgabbros and basic dykes shows that the magmas interacted strongly with crustal rocks during their emplacement. The isotopic signature of the contaminated Tertiary intrusions shows no evidence of any interaction with Archaean basement, despite the location of the Blackstones complex to the west of the Great Glen fault. Instead, the Blackstones rocks have crustal signatures resembling the Proterozoic basement and cover rocks of western Islay. It is therefore inferred that Early Proterozoic crust extends to the west of the Great Glen fault at this point on the Scottish continental shelf. In addition, the occurrence of similar isotopic signatures in Tertiary igneous rocks east of the Loch Gruinard fault confirms that Early Proterozoic basement extends under the Grampian block of mainland Scotland. When combined with published evidence from the Rockall bank, the new data constrain the location of an Archaean–Proterozoic crustal suture with a WNW trajectory which cuts across the continental shelf of northwest Britain.

Evolution of Paleocene sediment dispersal systems in the Foinaven Sub-basin, west of Shetland

Abstract Heavy mineral assemblages in deepwater Paleocene sandstones of the Foinaven Sub-basin (west of Shetland) reflect the influence of three main provenance and dispersal systems. The Schiehallion system has relatively uniform characteristics and persisted through much of the early-mid Paleocene. Its sphere of influence was centred on the Schiehallion Field, but it progressively encroached into the Foinaven area with time. Its main source was the Triassic Foula Formation, with minor supply from Lewisian and Moine basement rocks. The Foinaven system shows marked changes in character, related to evolution of the source area. The earliest sands were derived from a heavily weathered late Cretaceous regolith, and represent the onset of uplift and exhumation of the Shetland Platform. Progressive unroofing led to the incorporation of increasing amounts of Lewisian- and Moine-sourced detritus. Following deposition of the main reservoir sandstones in Foinaven and Schiehallion, there was a progressive change in provenance, with a gradual increase in the amount of sediment shed directly from metamorphic basement. Pulsing of the proto-Icelandic plume has been proposed as the mechanism for repeated influx of sand to the basins around Scotland. However, on the basis of available geochronological data, there does not appear to be a direct link between events in the British Tertiary Igneous Province (BTIP) and changes in provenance in the Foinaven Sub-basin. The initial influx of sediment from a weathered land surface may have been coeval with the onset of magmatism in the BTIP, but the cessation of supply through the Foinaven system at c. 59 Ma does not appear to be related to magmatic events in the BTIP. The waning of magmatism in the BTIP around 58 Ma is broadly coincident with a gradual increase of first-cycle basement detritus.

Felsites and breccias in the Northern Marginal Zone of the Rum Central Complex: changing views, c. 1900–2000

SUMMARY As in several other parts of the British Tertiary Igneous Province, breccias and felsite sheets are closely associated on the Isle of Rum. This association has been described and interpreted by several workers over the last 125 years. Opinion has divided into an intrusive origin for both rock types, as explosion breccias and felsite intrusions, versus a sedimentary origin for the breccias and an extrusive origin for the felsite. Evidence is reviewed for both opinions and it is concluded that the latter is substantially correct, as indicated by the presence of sedimentary structures and interbedded tuffs in the breccias and eutaxitic textures in the felsites. The breccias formed by inwards slumping of rocks from the oversteepened walls of a caldera, whereas the felsites formed by eruption of pyroclastic flows which were thick and hot enough to weld. It is inferred that the caldera formed initially and subsided progressively without any accompanying eruptions, and this is attributed to growth of the underlying magma chamber. The breccias accumulated during this stage. There followed a resurgent stage in which caldera collapse occurred in response to repeated ignimbrite eruptions partially emptying the magma chamber. The chamber is inferred to have been chemically and mineralogically zoned.

Oxygen isotope ratios of zircon: magma genesis of low δ18O granites from the British Tertiary Igneous Province, western Scotland

Igneous zircons from granites of the British Tertiary Igneous Province (BTIP), in western Scotland, preserve magmatic oxygen isotope ratios in spite of hydrothermal alteration and thus provide new evidence for the genesis of granites. CO2 laser and mass spectrometer based bulk (1–3 mg) oxygen isotope analyses from individual plutons show remarkable homogeneity in zircon δ18O, regardless of zircon size and magnetism (average reproducibility of δ18O (Zrc) in a single pluton is ±0.2‰). In addition, ion microprobe analysis shows that individual zircons all have the same δ18O and thus, bulk samples are homogeneous. Fourteen granite bodies from the Isle of Skye show extreme variability in zircon δ18O values from pluton to pluton (δ18O (Zrc)=0.6–5.3‰ VSMOW). All the granites record zircon δ18O values less than would be in high temperature equilibrium with ‘normal’ mantle (δ18O (Zrc) ∼5‰) except for the Beinn an Dubhaich granite. Zircons from one intrusion in the mafic Cuillins center also have low δ18O (2.5‰). Similar variability is observed in δ18O amongst three intrusions on the Isle of Mull (δ18O (Zrc)=3.4–5.9‰). Both Mull and Skye are characterized by intense shallow paleohydrothermal systems, and the low and variable δ18O values of the magmas likely result from melting or interaction with hydrothermally altered country rock. Analysis of granites on the Isle of Arran, where the percentage of low δ18O country rocks is much less, yields higher δ18O (Zrc) of 6.8 and 5.4‰. Published trace element and radiogenic isotope data for BTIP mafic and felsic rocks have supported the hypothesis that the granites are formed primarily by differentiation from the mantle with magmas experiencing minimal crustal input [Dickin et al., Phil. Trans. R. Soc. London A 310 (1984) 755–780]. However, conservative estimates based on oxygen isotope compositions suggest that some of the BTIP granites can only be formed from normal magmas (δ18O (WR) ∼6‰) if they assimilated significant amounts of hydrothermally altered crustal material. Some granites require up to 40% bulk crustal input and are better explained as the result of wall rock melting by an adjacent mafic pluton. Whereas Pb and Sr suggest a general trend towards increasing input of shallow crust for the younger granites [Dickin et al., Phil. Trans. R. Soc. London A 310 (1984) 755–780], the correlation between age and δ18O is less apparent suggesting that Pb and Sr isotopic compositions are decoupled from oxygen isotopes. This would be expected of melted country rock that was hydrothermally altered in oxygen isotope ratios, but not in Pb and Sr isotope ratios.