British Tertiary Igneous Province Research Articles

Composite minor intrusions as windows into subvolcanic magma reservoir processes: mineralogical and geochemical evidence for complex magmatic plumbing systems in the British Tertiary Igneous Province

The Rudh’ a Chromain sill is a composite minor intrusion emplaced into Jurassic sandstones on the south coast of the Ross of Mull, NW Scotland, and is part of a suite of basic–silicic sheets associated with the early development of the 58–56 Ma Mull Central Igneous Complex. New whole-rock major and trace element data combined with Sr–Nd–Pb isotope data indicate that two distinct magma end members were involved in the formation of the sill: a tholeiitic basaltic andesite magma generated by contamination and fractional crystallization of regionally available olivine tholeiite basaltic magma, and a rhyolitic magma produced predominantly through crustal melting of basement metasediments. Intermediate compositions at the gradational boundaries between the basic margins and silicic interior of the sill formed by high temperature diffusive hybridization within a compositionally-zoned magma reservoir prior to sheet emplacement. The basic portions of the sheet are replete with a large variety of crustal xenoliths, as well as numerous gabbroic and noritic cumulate fragments. The Rudh’ a’ Chromain sill therefore preserves evidence for the complex interplay between a number of magmatic processes which, when combined with data from the remainder of the suite, suggests that the magmatic plumbing system which fed the sill complex was not simply one large, long-lived magma chamber, but rather a plexus of variably connected sheet-like magma reservoirs.

Patterns of Nd and Sr isotopic ratios produced by magmatic and post-magmatic processes in the Shiant Isles Main Sill, Scotland

The Shiant Isles Main Sill of the British Tertiary Igneous Province is a classic example of a differentiated, alkaline basic sill. Four separate intrusions, each emplaced internally in rapid succession, form a 165-m-thick sill hosted by Lower Jurassic sedimentary rocks. Extensive Nd and Sr isotopic studies were conducted on samples from a vertical section through the sill where the relationships of samples to one another are well defined. The results illuminate patterns of modification of isotopic ratios and clarify the petrogenesis (magma sources, crustal contamination), magmatic processes (bulk mixing, interstitial liquid mixing), and post-magmatic alteration (hydrothermal effects on Sr and Nd). Overall, the whole-rock initial 87Sr/86Sr ratios range from ∼0.7037 to 0.7061 while initial 143Nd/144Nd ratios vary from ∼0.51243 to 0.51286 (ɛNd∼−0.7 to +5.7) – values that contrast markedly with those of the country rock. Acid leaching (HCl) of the whole-rock samples that removes analcime indicates that most of the scatter in the 87Sr/86Sr is caused by the ubiquitous sub-solidus, aqueous alteration during which more-radiogenic Sr was introduced into the sill, especially along the margins, and also reveals magmatic isotopic ratios. In contrast, Nd was immobile during fluid interaction so that the sill 143Nd/144Nd ratios were not affected, even <1 m from the country-rock contact. Using leached rock values, 87Sr/86Sr and 143Nd/144Nd ratios are inversely correlated from magmatic processes. Magmas with two distinct isotopic compositions were involved: a more primitive one with 143Nd/144Nd ∼0.51285 and 87Sr/86Sr ∼0.7035 that produced the first two intrusions and a more evolved one (with 0.51252 and 0.7048) that produced the third intrusion. Mixing of the two magmas was very limited, restricted to near contacts between units, and apparently occurred by interstitial melt migration. The more evolved crinanitic magma was probably produced from a batch of the more primitive picritic melt by a small degree of crustal contamination and crystal fractionation during a short crustal residence prior to ascent and emplacement.

The Sub-lithospheric Source of North Atlantic Basalts: Evidence for, and Significance of, a Common End-member

Palaeogene basalts from the margins of the North Atlantic often show geochemical variations that are consistent with their parental magmas having interacted with the lithosphere en route to the Earth's surface. These geochemical trends vary depending on the nature of the local lithospheric contaminants. Using examples from the British Tertiary Igneous Province and SE Greenland, we construct coherent contamination trends, which converge on a restricted Pb isotope composition, apparently indicating a common uncontaminated asthenospheric mantle component. Significantly, this composition is also suitable as one end-member of the Pb isotope arrays recorded in Recent Icelandic basalts. We conclude that this composition has been a persistent component of the Iceland plume over 60 my, dominating the mantle contribution to the Palaeocene phase of flood basalt magmatism but constituting only one end-member on Iceland. The Pb isotope composition of this 'North Atlantic end-member' is consistent with, but not necessarily demanding of, a primordial source. Recent evidence suggesting a lower- mantle origin for mantle plumes encourages investigation of whether the geochemical evidence supports that hypothesis. Helium isotope data from Palaeogene North Atlantic basalts support a lower-mantle contribution. However, mixing models suggest that it is untimely that the lower-mantle contribution is large enough to dominate the Sr-Nd-Pb isotope compositions and lithophile trace element signatures of any plume-derived basalts.

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.

Palaeomagnetic constraints on the age of the Red Arch Formation and associated sandstone dykes (Northern Ireland)

The Red Arch Formation is a continental red bed succession of uncertain age exposed on the northeast coast of Ireland. There are lithological similarities with the Old Red Sandstone of the Highland Border region, although unconformities within the succession, the presence of reworked aeolian sand grains and variations in the induration of the rocks have prompted earlier workers to suggest that the upper part of the succession may be Triassic in age. This paper reports palaeomagnetic results from the Red Arch Formation. The characteristic magnetizations (ChRMs) are exclusively of reversed polarity and the palaeomagnetic pole position for the sequence bears close comparison with other results from the Old Red Sandstone in Scotland and southern Britain. These poles are regarded as magnetic overprints formed during the Kiaman Superchron because they fall in the Permo‐Carboniferous segment of the apparent polar wander path. Sandstone dykes that cut the Red Arch Formation on a NNE–SSW and NNW–SSE trend have also been studied palaeomagnetically. The dykes show more steeply inclined ChRMs predominantly of normal polarity. The dykes are therefore interpreted to be post‐Kiaman in age, a conclusion consistent with our structural observations. The age of the dykes cannot be determined precisely because they have also been, at least partially, remagnetized. Lithological and regional geological considerations suggest the dykes are Triassic in age and formed in an extensional stress regime prior to the opening of the North Atlantic. They were remagnetized in Tertiary times, possibly associated with uplift and emplacement of the British Tertiary Igneous Province.

The occurrence of zirconian aegirine and calcic catapleiite (CaZrSi 3 O 9 .2H 2 O) within a nepheline syenite, British Tertiary Igneous Province

The occurrence of zirconian aegirine and calcic catapleiite (CaZrSi ₃ O ₉ .2H ₂ O) within a nepheline syenite, British Tertiary Igneous Province

Rare-earth-bearing minerals fergusonite and gadolinite from the Arran granite

Synopsis A suite of rare earth element bearing minerals has been identified from miarolitic cavities in the northern Arran granite. This is the first occurrence in the British Isles of in situ fergusonite and gadolinite to be confirmed by X-ray analysis. The presence of these minerals reflects the enrichment of rare earth elements in the highly fractionated granites in the southern sector of the British Tertiary Igneous Province.

Exceptional REE-enrichment in apatite during the low pressure fractional crystallisation of alkali olivine basalt; an example from the British Tertiary Igneous Province

AbstractThe Cnoc Rhaonastil dolerite boss on Islay, NW Scotland represents a body of alkali-olivine basalt magma which differentiated at low pressure and in situ, from dolerite through teschenite to minor nepheline-syenite. The syenites occur as isolated pods and pegmatitic schlieren within the leucodolerite, and have an exotic mineralogy including Zr-aegirine, Zr-arfvedsonite, Ca-catapleiite, zirconolite and aenigmatite. Fluor-apatite occurs as an accessory phase in the dolerite, but becomes more abundant within the teschenite and syenites. Total REE contents within apatites in the dolerites are typically low (σREE = 0·57–3·21 wt.% oxide), the highest REE contents occurring in irregular, deuterically altered rims and internal patches. The REE-enriched rims also have slightly elevated SiO2 contents at 0·81–0·95 wt.%, suggesting that the substitution scheme Ca2++P5+ ⇔ REE3++Si4+ was operating. These apatites have up to 0·08 wt.% Cl and 3·7 wt.% F, with most being almost pure end-member fluor-apatite. The majority of the teschenite apatites show the least REE-enrichment (σREE = 0·27–0·45 wt.%), coupled with low Na (&lt;0·12 wt.%) and low SiO2 (&lt;0·39 wt.%) contents. However, within the syenites two distinct populations of apatite exist. The first, most common, variety consists of unzoned, low-REE apatites (max. 3·1 wt.% σREE, again in irregular rims and patches), whereas the second variety is often complexly zoned, and has variably enriched zones up to a maximum σREE content of 42 wt.%; this is by far the most REE-enriched natural fluor-apatite so far reported from the British Isles. The REE-enriched zones are often less than 3 μm wide, and have Na content up to 5·4 wt.% Na2O, implying that the substitution scheme Na+ + REE3+⇔2Ca2+ dominated over the more typical scheme involving Si4+ which operated in the dolerites and teschenite. Other zones are either variably enriched in Y (up 2·1 wt.% Y2O3) or Th (up to 0·85 wt.% ThO2). However, there is no correlation between Y and REE contents, suggesting that crystallographic factors were involved in apatite Y and REE partitioning. The REE-rich apatites have very low Cl content (&lt;0·04 wt.%), but high F concentrations (&lt;2·8 wt.%). It is believed that these strongly enriched apatites crystallised under disequilibrium conditions from isolated, variably REE-enriched domains, within the fluid-rich residual syenitic magma. These domains may have been generated by the prior crystallisation of monazite, Ca-catapleiite or zirconolite, which can be found as small inclusions within albite and interstitial analcime. The dynamic process of slumping of the denser teschenite back into the leucodolerite crystal mush is believed to have played an important role in the release of deuteric fluids and the concentration of residual magmas.

The mineral chemistry and petrology of Tertiary pitchstones from Scotland

AbstractSub-silicic to silicic pitchstones are widespread throughout the British Tertiary Igneous Province (BTIP), with examples being found at all the major igneous centres. Both highly porphyritic and almost completely aphyric varieties occur, and take the form of sills, dykes and lava flows. Here we present previously unreported mineral chemistry data on phenocryst and microcrystallite populations from a number of pitchstones from throughout the BTIP. Phenocryst assemblages are completely anhydrous, comprising mixtures of plagioclase, sanidine, fayalite, orthopyroxene, pigeonite, ferroaugite, ferrohedenbergite and quartz. Microcrystallite assemblages are also diverse, consisting of sanidine, ferrohedenbergite, fayalite and, occasionally, almost pure end-member ferrosilite, as well as hydrous phases such as ferrohornblende and biotite. Textural and mineral chemistry observations support interpretations derived from whole-rock and residual glass major element analyses, together with whole-rock trace element and the available Sr-Nd-Pb isotope data, that the Tertiary pitchstones of Scotland are either the products of intimate mixing between a range of basaltic magmas with hydrous crustal melts, or were formed by the crustal contamination of basaltic magmas.

On the nature of the parental magma of the Palaeogene Staffa Magma sub-type, Isle of Mull, Scotland

AbstractThe tholeiitic, columnar (Staffa type) lavas found at, and near, the base of the Tertiary lava succession on the Isle of Mull contrast markedly with overlying transitional tholeiiticalkalic Mull Plateau Group lavas, and as a result previous workers have classed the Staffa type lavas as a distinct group. The Staffa type lavas are among the most contaminated in the British Tertiary Igneous Province and have equilibrated and fractionated at shallow depths within the crust (relative to the overlying Plateau Group lavas), so obscuring the chemical identity of their parental magma. However, Staffa type lavas which are more basic than examples hitherto reported help in the elucidation of the parental magma. It is demonstrated that the Staffa type lavas were derived from a parental magma which was compositionally similar to that of Plateau Group lavas. The primary reasons for the distinctive features of the Staffa type are related to post-mantle melt generation processes, i.e. to extensive crystal fractionation and to crustal contamination at shallow crustal depths. The lavas are compositionally and temporally unrelated to the Hebridean tholeiitic magma types, the Central Mull Tholeiite type and the Skye Preshal More type.

Early Tertiary plume uplift of the North Sea and Faeroe-Shetland Basins

The Cretaceous and Tertiary post-rift subsidence history of the northern North Sea and the Faeroe-Shetland Basins, formed by Jurassic and Cretaceous rifting, show substantial departures from McKenzie post-rift subsidence in the Early Tertiary. These departures can be explained by regional Palaeocene uplift, followed by rapid decay of this uplift in the Eocene through to the present day. Forward and reverse 2-D modelling of syn- and post-rift stratigraphy, using flexural isostasy, has been used to determine the magnitude and timing of this uplift event. In the north Sea Basin (NNSB), modelling predicts Palaeocene uplift to have been of the order of 375–525 m, increasing northwards. In the Faeroe-Shetland Basin, Palaeocene uplift is estimated to have been 900 m. The timing of Palaeocene uplift in the NNSB and northern Britain is consistent with the development of the Early Tertiary Icelandic plume (c. 64 Ma) or a related plume. Palaeocene uplift in the NNSB is attributed to dynamic uplift generated by mantle fluid flow driven by ascending hot plume material. Magmatic underplating or buoyancy from heated lithosphere are not believed to be significant mechanisms for regional Palaeocene uplift in the NNSB. The decay of Palaeocene uplift in the NNSB occurs more rapidly than predicted by simple conductive cooling of heated lithosphere. Rapid decay of uplift in the NNSB is attributed to a temperature decrease of the plume in the Early Eocene (c. 55 Ma), resulting in a decrease in dynamic uplift, and coincides with a decrease in volcanic activity within the British Tertiary and Greenland Igneous Provinces, and the initiation of sea-floor spreading between Greenland and northwest Europe. The plume responsible for Palaeocene uplift in the NNSB and northern Britain may have been located in the Faeroe-East Greenland area, although a smaller transient plume within the British Tertiary Igneous Province may also have contributed to uplift.

Geochemistry of the Tertiary volcanism of Northern Ireland

The Antrim Plateau (Northern Ireland) forms the southwestern part of the British Tertiary Igneous Province and is dominated by basaltic lavas. Previous work divided the province into three stratigraphic formations, viz. the Lower, Middle and Upper Formations, with the Middle Formation having a basaltic member known as the Causeway Member. New chemical and SrNd isotopic data covering the spectrum of lava types are presented. Three distinct geochemical types are recognised: (1) basalts of the Lower Formation and most of those from the Upper Formation are LREE-enriched but have characteristic convex-up REE patterns with a maximum around Nd; (2) some of the Upper Formation basalts and the majority of the Causeway Member basalts are LREE-enriched with no convex shape in their REE pattern; and (3) some of the Causeway Member basalts are LREE-depleted and similar to N-MORB. Isotopic compositions of Sr and Nd show considerable variation throughout the three formations with ε Sr( t) = − 22 to + 107 and ε Nd( t) = − 11 to + 8.5. The exceptionally wide ranges of isotopic compositions are believed to have been produced by assimilation of a crustal (mainly Dalradian derived) component. The REE patterns indicate that most of the basalts were derived from a LREE-depleted mantle and with the exception of the Causeway Member, most have convex-up REE patterns. These are interpreted as being due to residual garnet in the mantle source, which indicates melting at depths in excess of 80 km. This suggests that at the start of Tertiary volcanism, the melt regime was controlled by a thick lithosphere which thinned with time such that the Causeway Tholeiites were produced at shallower levels. The return to the convex-up patterns of the Upper Formation means that simple models of lithospheric stretching and rifting are not able to explain the Antrim situation.

Tectonic controls on sequence development in the Palaeocene and earliest Eocene of southeast England: implications for North Sea stratigraphy

Abstract Depositional sequences in the Palaeocene and early Eocene of southern England are grouped into three unconformity-bounded composite sequences, corresponding to the Ormesby Clay and Thanet Sand formations, the Lambeth Group (including the Woolwich and Reading formations) and the Thames Group (including the Harwich and London Clay formations). A refined chronostratigraphical framework, coupled with a revised Palaeogene chronometry, reveals a clear relationship between long-term Palaeocene to early Eocene sea-level trends and radiometrically dated volcanic events in the North Atlantic Province. In gross terms, it is possible to relate the progressive Palaeocene uplift of NW Europe to the rise of the proto-Icelandic mantle plume beneath the continental crust of East Greenland, and the early Eocene subsidence of NW Europe to the transfer of the plume to the developing NE Atlantic mid-ocean ridge. However, both the sedimentary and volcanic record indicate that the uplift of NW Europe took place in three stages, expressed in the North Sea region as long-term regressive-transgressive facies cycles. The first uplift phase was associated with the onset of Hebridean volcanic activity and with a short-lived influx of coarse clastic sediments into the North Sea Basin (basal Ekofisk Formation). No sediment associated with this cycle is preserved in southern England. The second uplift phase was associated with the main period of Hebridean activity and led to substantial uplift of Scottish source areas. It was followed by regional subsidence and transgression of the basin margins (Ormesby-Thanet composite sequence). The third uplift phase was associated with the onset of rifting and volcanism along the North Atlantic rift zone. Eastward tilting initially led to sedimentation in eastern and southern England (Lambeth composite sequence), but continued uplift of southern and western Britain eventually restricted sedimentation to the central and northern parts of the North Sea basin, with major progradation of the Dornoch delta complex taking place along the eastern margin of the Scottish land mass. This period marked the maximum isolation of the North Sea Basin, with reduced salinities being reflected in a near-absence of marine fauna and a highly restricted microflora. The third uplift phase is believed to mark the culmination of plume-related tectonism in NW Europe and was associated with a resurgence of volcanic activity in the British Tertiary Igneous Province, including intrusion of the Cleveland-Blyth-Acklington dyke system along the uplifted Sole Pit inversion zone. A sharp reduction in intrabasinal activity, coupled with the initiation of a long period of regional subsidence (Thames composite sequence), is interpreted as representing thermal subsidence following the onset of seafloor spreading between Greenland and Scotland. Oceanic crust generation was initially subaerial and was associated with intense, largely basaltic, pyroclastic activity that affected the whole of NW Europe (Balder Formation and equivalent tuffs). The cause of the episodic nature of the plume-related uplift is uncertain. It appears that the mechanics and configuration of plume emplacement were either intrinsically more complex than previously recognized, or were influenced by changes in the regional stress pattern within the NW European crust.

Variation in the form and distribution of dykes in the Mull swarm, Scotland

The dykes of the Mull swarm, part of the British Tertiary Igneous Province, have been examined in a series of well exposed coastal sections. Traverses through the dyke swarm show systematic changes in the thickness and spacing of dykes, both of which are typically log-normally distributed. Both the geometric mean thickness and spacing of dykes increase with distance from the Mull centre, concordant with a decrease in crustal extension. Variations in the thickness and spacing result from changes in character of the magma flow. It is inferred that the magma flux was fairly evenly distributed near the Mull centre, but farther away from the source it becomes focused into larger dykes. The form, orientation and opening direction are also observed to vary with distance away from the volcanic centre on Mull. The dykes intrude a range of pre-existing fractures, the range of fractures used decreases with distance from the centre. This, together with other features of the form of the dykes have been interpreted in terms of the varying magma pressure and regional stress, which is expressed as the R-ratio. Analysis of the variable strikes of fracture-filled dykes is consistent with decreasing magma pressure with distance from the Mull centre, assuming that regional stresses were relatively invariant across the same distance.

Transient regional uplift in the Early Tertiary of the northern North Sea and the development of the Iceland Plume

Abstract Forward and reverse 2D flexural modelling of syn-rift and post-rift stratigraphy, has been used to investigate departures from McKenzie post-rift subsidence for the Cretaceous-Tertiary following Late Jurassic rifting in the northern North Sea Basin. The analysis shows uplift in the Palaeocene of 375–525 m relative to calibrated McKenzie subsidence curves, followed by a rapid decrease in uplift in the Early Eocene of 160–310 m. Both Palaeocene uplift and Eocene subsidence increase northwards. The timing, distribution and magnitude of regional Palaeocene uplift is consistent with an origin relating to the development of a mantle plume between 65 and 55 Ma. Transient regional Palaeocene uplift in the northern North Sea Basin was produced by long wavelength dynamic uplift generated by the flow field of a mantle plume. Heating of the asthenosphere or lithosphere beneath the North Sea by hot plume material, or magmatic underplating are not believed to have contributed to Palaeocene uplift in the northern North Sea Basin. Rapid Early Eocene subsidence was generated by a decrease in dynamic uplift caused by a reduction in plume activity. The plume responsible for North Sea Palaeocene uplift may have been the early Iceland plume or a distinct British Tertiary Igneous Province plume related to the same mantle convection event.

Current research in the British Tertiary Igneous Province

A one-day meeting on current and recent research in the British Tertiary Igneous Province, convened by Andrew Kerr, was held at the Department of Geological Sciences, University of Durham, on 1 May 1993. The meeting attracted over 50 participants, from a wide range of disciplines, including geochemistry, volcanology, petrology and structural geology. There were nine speakers and six poster presentations. After a brief welcome, by R. N. Thompson on behalf of the Durham Department, S. A. Gibson (Durham University) reviewed her work on the Little Minch Sill Complex, Skye. Field and petrographic evidence were used to show that the internal stratigraphy of the sills could be explained by multiple intrusion. Previous workers had proposed that the sill complex were fed by the same magmatic conduits as the Skye lavas. Geochemical data, however, prove that this can not be the case since the sills had been contaminated with amphibolite facies upper crust, in contrast to the predominantly lower crustal contamination of the lavas. G. R. Kitchener (Glasgow University) spoke on oxygen isotope exchange during hydrothermal alteration around the Cullin intrusion, Skye. Focusing on one lava flow, close to the margin of the intrusion, he showed how δ 18 O varied with height throughout the flow, and cast some doubt on the interpretations of δ 18 O values by previous workers on the Skye lavas. Analysis of secondary mineral separates revealed that the chlorite and the zeolites have different δ 18 O, which suggests that the hydrothermal alteration was caused by many different pulses of fluid with variable

C. H. Emeleus and M. C. Gyopari British Tertiary Igneous Province, London (Chapman and Hall), 1992, xii + 259 pp, Price £65.00 hardback.

C. H. Emeleus and M. C. Gyopari British Tertiary Igneous Province, London (Chapman and Hall), 1992, xii + 259 pp, Price £65.00 hardback.

Elemental evidence for an enriched small-fraction-melt input into Tertiary Mull basalts, Western Scotland

Continental basalts enriched in the incompatible trace elements have recently been interpreted as mixtures of depleted asthenospheric melt and an enriched small-fraction-melt lithospheric component. Hebridean Tertiary basalts are relatively depleted in incompatible trace elements, and it has been suggested that this is because the enriched lithospheric component had already been extracted beneath that region, during the Permo-Carboniferous. It was therefore not widely available to contaminate Tertiary magmas. A flow-by-flow geochemical study of the Mull lava succession has nevertheless revealed the presence of lava flows relatively enriched in the incompatible trace elements, at the base of the succession, in some parts of the island. Fractional crystallization, contamination by Archean and Moinian crust, variation in the degree of mantle melting and an asthenospheric, ocean island basalt-like source, have all been ruled out as possible mechanisms of enrichment in favour of 5–10% contamination by an enriched, fusible, small-fraction-melt from the lithospheric mantle. The overall lack of basalts enriched in trace elements in the British Tertiary Igneous Province, argues strongly against the widespread presence of an enriched Tertiary lithospheric mantle below the Hebrides. The origin of the a sodic (rather than potassic) contaminant within the Hebridean lithosphere is also discussed.

New K-Ar ages, and a provisional chronology, for the offshore part of the British Tertiary Igneous Province

Synopsis New K-Ar whole-rock ages are presented from 13 offshore locations to the north and west of Scotland. Ten of these locations have yielded ages which are considered to be geologically significant. Together with previously published ages, these suggest that the earliest sills of the Faeroe-Shetland Intrusive Complex were intruded at c. 80 Ma (Campanian), although the bulk of the complex was probably emplaced at 55–53 Ma (early Eocene). Lavas on the Hebrides Shelf exhibit a range of ages from c. 63–46 Ma (Danian-Lutetian). New K-Ar ages from Rosemary Bank are inconclusive but, combined with calcareous nannofossil analysis and the known magnetic polarity of the bank, suggest that it may have formed during the Late Cretaceous (70–68 Ma or earlier). The new data are incorporated into the chronology of all the major onshore and offshore components of the British Tertiary Igneous Province. Sources for the offshore lavas include the Faeroe Islands, central igneous complexes and large fissures such as those beneath the Wyville–Thomson Ridge and the Hebridean Escarpment.

Magnetic polarity timescales: a new test

Abstract The magnetic polarity timescale, for times older than a few million years, is based upon sea-floor spreading magnetic anomalies, but the relative ages deduced from the anomalies must be calibrated to allow for non-uniform spreading rates. Calibration is carried out by using well-dated samples of known position in the magnetic timescale, but such samples are rare. This paper presents a method for testing the accuracy of a (portion of a) proposed magnetic timescale, using a body of polarity-plus-age data unconnected with sea-floor spreading anomalies. The method also suggests how a timescale could be improved by displacing or offsetting parts of it by a constant amount. When this method is used with data for the British Tertiary Igneous Province, the timescale of Harland et al. (1989) is found to be superior to those of Harland et al. (1982) and Berggren et al. (1985) for part of the early Tertiary period ( c. 62–52 Ma). Over this interval the Harland et al. (1989) timescale is close to optimum for this data set, but it might be improved by subtracting about 0.25 Ma from the ages of all polarity boundaries.