British Tertiary Igneous Province Research Articles

Anorogenic granite magma genesis: new isotopic data for the southern sector of the British Tertiary Igneous Province

ABSTRACTIt is now generally accepted that British Tertiary granites contain crustal and mantle components. Genesis principally by differentiation of crustally contaminated basaltic magmas is widely held and silicic melts with some remarkable trace element similarities were generated within different upper crust along the St Kilda/Skye - Carlingford zone.New whole-rock (and mineral) O isotope data for the southern sector of the province (N Arran, Ailsa Craig, Mourne Mountains, Slieve Gullion, etc) reveal that δ18O lies in the range +5·1 to +9·7‰ for most of the analysed granites, meteoric water-rock interaction having been in general less intensive than at Skye and Mull. Nevertheless, highly 18O-depleted country rocks (with δ18O<0) exist adjacent to the N Arran and Mourne Mountains granite plutons. There is as yet no evidence for the existence of low -18O granitic melts in this southern sector where magmatic δ18O compositions (up to c. + 9·5‰) can be inferred for some of the intrusions.New Nd (and Sr) isotope data indicate that although there is some similarity in initial 87Sr/86Sr ratios between the northern and southern sector granites, in northeastern Ireland initial εNd values for the analysed Tertiary acid major intrusions range from −3·9 to −4·5. This is in marked contrast to the Skye granites, some of which have values below −20, reflecting the involvement of different lithosphere.

Subcontinental mantle plumes, hotspots and pre-existing thinspots

The magmatism occurring when a hot convective mantle plume is sited beneath a lithospheric plate may be more complex if the latter is continental, rather than oceanic, because of the characteristic local physical inhomogeneity of continents. Thus, the surface volcanic expression of the plume (hotspot) may be displaced from immediately above its rising stem, if the continent has been previously locally thinned nearby. The magmatism of the British Tertiary Igneous Province and Parana basin, South America, appears to fit this model, and it may also explain why the Miocene Columbia River basalts of the NW United States overlie an Early Tertiary sedimentary basin.

40Ar/ 39Ar dating of alkaline and tholeiitic magmatism of Saudi Arabia related to the early Red Sea Rifting

40Ar/ 39Ar plateau-ages at 27–28 Ma obtained on feeder dykes and one lava flow of the alkaline massif of Harrat Hadan are in agreement with the assumed age partly deduced from the conventional K/Ar data of the early activity of other alkaline volcanic massifs from Saudi Arabia. This magmatic activity is spatially distributed over a large area, along the western edge of the Arabian plate, and their N-S to NW-SE volcano-tectonic directions are similar to those of the future Red Sea Rift. Preliminary results obtained on tholeiitic lava flows, dykes and plutons gave 17 plateau-ages which, combined with 6 ages deduced from more disturbed age-spectra, display a main histogram peak from 24 to 21 Ma, much narrower than that obtained with conventional K/Ar ages on the same formations. Therefore, a strong tholeiitic activity affected a narrow linear area following the actual eastern Red Sea coast, over nearly 1700 km, during a brief period of time, and without showing any apparent migration. The main volcano-tectonic features of the future Red Sea are strongly outlined during this event. Such brief magmatic episodes related to continental rifts have also been documented by precise 40Ar/ 39Ar analyses on the British Tertiary Igneous Province, the Deccan traps and the eastern Central Atlantic.

Magnetostratigraphy of the Tertiary igneous rocks of Arran

Synopsis The Tertiary igneous succession of Arran is poorly known, partly because of a paucity of informative field relationships, so palaeomagnetic polarities and Ar–Ar dates have been measured to provide additional constraints. New palaeomagnetic results for over 200 sites have been combined with published data. The Northern Granite has normal polarity (N), while the later Central Ring Complex has reversed polarity (R). All sampled quartz-porphyries are N and all felsites (apart from some intermediate direction (I) sites) are R, the only known correlations between polarity and petrography in the British Tertiary Igneous Province. The remaining petrographic types may be found with either polarity. All dykes sampled on the NW coast are N, as are 70% of those on the NE coast, but elsewhere R polarity predominates. Radiometric dates show that the activity occurred approximately 61 to 58 Ma ago. It spanned at least the magnetic polarity sequence R–N–R, and although additional polarity intervals cannot be ruled out, a tentative succession consistent with R–N–R and with as few as possible recurrences of petrographic types has been constructed. The mean direction of magnetisation of the sites is not consistent with that of an axial, centred dipole for the present latitude: several explanations are offered.

Further evidence for a single polarity and a common source for the quartz-porphyry intrusions of the Arran area

Synopsis New evidence supports the proposition that all the Tertiary quartz-porphyry intrusions of Arran and Bute, which have a geographic span of 35 km, are normally magnetised. This is the only known correlation between magnetic polarity and petrography in the British Tertiary Igneous Province. It is most easily explained by a single magma source supplying all the quartz-porphyry bodies. The bodies were probably intruded during a single normal magnetic interval lasting about 1/2 Ma, about 58 Ma ago. This significant correlation between petrography and polarity could help to erect the Tertiary igneous succession of Arran by use of the quartz-porphyry intrusions as a ‘marker horizon’.

Asthenospheric and lower-lithospheric mantle contributions to continental extensional magmatism: An example from the British Tertiary Province

The compositions of Palaeocene basic magmas in Skye and Mull, British Tertiary Igneous Province (BTIP), shed light on the general problem of the relative importance of lithospheric and asthenospheric mantle as sources of continental magmatism during extension. After the effects of crustal contamination in some magmas have been detected, and the samples showing it eliminated from consideration, the remainder show a significant diversity of trace-element compositions. Basic lavas from Skye and Mull which lack crustal contamination have contents of compatible and moderately-compatible elements (e.g., Ni, Cr, Y, Yb) and also of moderately-incompatible elements (e.g., Ti, P, Zr, Hf, Sr, MREE) which are within the range shown by the tholeiitic and mildly-alkalic ocean-island basalts (OIB), as from Iceland. In contrast, their contents of strongly- and very-strongly-incompatible elements (e.g., K, Rb, Ba, U, Th, Nb, Ta, LREE) are within the range shown by normal mid-ocean-ridge basalts. These BTIP magmas are interpreted as second-stage melts from mantle which had already lost a very small melt fraction. The latter is thought to be represented by the strongly-alkalic Permo-Carboniferous dyke swarms associated with rifting at that time in NW Scotland. Residual uppermost asthenosphere from the late Palaeozoic magma genesis event is postulated to have become attached to the thickening lithosphere beneath NW Scotland and thus to have remained coupled to it throughout the Mesozoic, until reactivated by Palaeocene stretching. Occasional hawaiites in the top third of the Skye lava pile remnant, and also basalt dykes of the swarm cutting the granites of the central complex (the last significant magmatic event at this centre) have the elemental characteristics of typical North Atlantic OIB - notably La Ta =10–12 . These compositions are thought to demonstrate input from asthenospheric-source basic magmas throughout the lithospheric-source-dominated Skye and Mull magmatism. It is suggested that all the basic magmas in this province are ultimately derived from asthenospheric sources, but that the larger magma batches became contaminated during uprise by melting and assimilating the most-fusible parts of overlying lithospheric mantle (followed by lower crust in many cases). Smaller magma batches traverse the overlying lithosphere with little or no contamination. Perhaps the difference between the two lies in their flow behaviour - laminar or turbulent - during uprise.

Time and duration of activity in the British Tertiary Igneous Province

Summary The time and duration of igneous activity in the separate component areas of the British Tertiary Igneous Province (BTIP) are investigated using radiometric dates that pass internal consistency tests, used in conjunction with palaeomagnetic polarities and the available stratigraphic information. Reliable results are available only for the currently subaerial parts of the Province. It is found that activity occurred within the approximate interval 63–52 Ma, with most activity at about 59 Ma. Later activity was predominantly of acid magmas though basic rocks preponderate in the province as a whole. The types and span of igneous activity in the separate areas followed no common pattern but areas of more complex geology tend to have had a longer span of activity. Magnetic polarities are predominantly reversed, with all the lavas having this polarity. This predominance is probably largely, but not entirely, due to reversed polarity intervals being longer than normal ones at this time, and with much activity occurring in a single reversed interval. The sequence of polarities found within the BTIP cannot be fully reconciled with the polarity timescales of either Harland et al. (1982) or Berggren et al. (1985).

Volcanic ash in a cored borehole W of the Shetland Islands: evidence for Selandian (late Palaeocene) volcanism in the Faeroes region

Summary A volcanic ash layer, containing basic glass shards compositionally comparable to the basalts of the Faeroe-East Greenland Province, has been discovered in early Selandin sediments from a borehole W of the Shetland Isles. The available magnetostratigraphical, biostratigraphical and tephrachronological information indicates that basaltic volcanism in the province began during magnetic anomaly chron C26R, earlier than the currently accepted C24R interval, but correlating with the main period of basaltic volcanism in the British Tertiary Igneous Province.

A palaeomagnetic study of lower Tertiary lavas in E Greenland and comparison with other lower Tertiary observations in the northern Atlantic

Summary Rock samples from five separate profiles, totalling 2.3 km, in the E Greenland lower Tertiary plateau basalts exhibit well-defined magnetic vectors and stability to both alternating magnetic fields and thermal demagnetization. Stability is confirmed by baked-contact evidence and the preservation of a secular variation record. The remanences are all of reversed polarity and the age relationships suggest that these were acquired during chron C24R, immediately prior to the formation of oceanic magnetic anomaly 24. Radiometric age determinations from the N Atlantic Igneous Province suggest that the plateau basalts of E Greenland may be of similar age to those of the Faeroes, but differ in age from those of the British Tertiary Igneous Province. Palaeomagnetic pole positions for each province suggest an age similarity between the Faeroes and British plateau basalts, while their age relationship to the E Greenland basalts is not clear. The pole positions generally confirm the published continental reconstructions for this period. The simplest polarity solution is for the magnetization of the British and Faeroes basalts to be predominantly associated with chrons C24R and C26R, with only chron C24R being represented in E Greenland. However, this model is dependent on the radiometric constraints on the Palaeocene/Eocene boundary.

Revised 40 AR– 39 AR age for granites of the Mourne Mountains, Ireland

The British Tertiary Igneous Province (BTIP) consists of many geographically separate areas throughout the British Isles and the question arises whether activity occurred contemporaneously throughout the province. In the absence of palaeontological evidence, this is best answered using radiometric dating, and many potassium-argon analyses have been carried out. However, experience has shown that conventional K–Ar dating in the BTIP is prone to error, often giving spuriously high or low dates. For this reason, Rb–Sr isochron and 40Ar–39Ar step–heating plateau dates are preferred, for internal acceptance criteria can be applied to them. The Mourne Mountain granites consist of five successive granite intrusions, G1 to G5, and lie to the NE of the Carlingford Complex and Slieve Gullion central intrusive complexes; together these form the only major masses of Tertiary intrusive rocks in Ireland. They are described in Meighan et al. (1984). Table 1 lists available dates. The most reliable are the 40Ar–39Ar plateau dates of 59.6 ± 1.6 Ma for G2 and 59.5 ± 1.6 Ma for G5; though acceptance criteria were not applied to establish the validity of the plateaus chosen, the steps selected for each plateau, although increasing in apparent age, are concordant at the 1σ level and contain the bulk of released 39Ar. Thus they appear to establish the age of the Mournes as about 59.5 Ma. However, three new 40Ar–39Ar step-heating analyses are reported here that indicate that the age is significantly younger. An analysis has been performed for a biotite from a mafic-rich facies within G2 (Meighan . . .

The British Tertiary Igneous Province: Young Rb-Sr Ages for the Mourne Mountains Granites

Radiometric, palaeomagnetic and palaeontological studies continue to refine information on the precise sequences of events in the various parts of the North Atlantic Tertiary Igneous Province before and after the initiation of seafloor spreading. On timing models of approximately 53 or 54 Ma for the main opening of the NE Atlantic Ocean, major intrusive activity in the British Isles mostly preceded the latter at around 59 Ma (Macintyre, McMenamin and Preston 1975, 1979; Mussett 1980). By contrast in East Greenland magmatism was closely associated in time with this major event (Soper, Higgins et al. 1976; Soper, Downie et al. 1976; Myers 1980) and continued intermittently for more than 15 Ma after it adjacent to the rifted continental margin (Brown et al. 1977; Gleadow and Brooks 1979). This contribution complements the data available for Scottish Tertiary igneous rocks, presenting the results of Rb–Sr age determinations on each of the high level Mourne Mountains granite intrusions (G1–G5), the first by this method on Irish Tertiary igneous rocks. These imply that there were major intrusive events at approximately 56 Ma and 52 Ma in NE Ireland, with G5, the latest ring dyke exposed in the complex, appearing to be significantly younger than its predecessors G1–G4 and contemporaneous with some of the earlier intrusions in East Greenland. The Mourne Mountains central intrusive complex comprises five metaluminous-peraluminous (biotite ± calcic amphibole) granite intrusions, three (G1, G2 and G3) in the eastern centre (Richey 1928) and two (G4 and G5) in the younger, contiguous western centre . . .

Palaeomagnetism and age of the quartz-porphyry intrusions, Isle of Arran

Synopsis Most of the quartz-porphyry intrusions of Arran, many of which form the acid member of composite intrusions, have been sampled palaeomagnetically. Though both normal and reversed polarity may be found in a single intrusion it is concluded that the reversed polarities are due to overprinting by later reversely magnetised intrusions and that all the quartz-porphyries were intruded while the magnetic field had normal polarity. The age of the Drumadoon quartz-porphyry dyke and, by inference, that of all the other quartz porphyry intrusions, is 58.5 ± 0.8 (lσ) Ma. It is suggested that normal polarity may provide a stratigraphic ‘horizon’ useful for elucidating the poorly-known succession of Arran Tertiary igenous rocks, most of which are reversely magnetised. This work forms part of a comprehensive study, using palaeomagnetic polarity and radiometric dating, of the timing and duration of activity in the British Tertiary Igneous Province, and the age quoted above lies in the period of peak activity. This is the first time that a correlation of polarity with a distinctive petrographic rock type has been found.

40 Ar– 39 Ar step-heating ages of the Tertiary igneous rocks of Mull, Scotland

Twenty-one samples from throughout the Mull Tertiary igneous succession were dated. Of these, eight gave a satisfactory age plateau and the dates show a systematic decrease throughout the succession. Extrusion of the lavas began c . 60 Ma ago, contemporaneous with the bulk of activity in the British Tertiary igneous province. Igneous activity continued until c . 57 Ma. Probably the lavas were extruded during the reversely magnetized chron 26r; development of the central igneous complex continued into the normal chron 26, while the late dykes were intruded during 25r. If this is so then, according to the polarity timescale of Harland et al. 1982, the duration of igneous activity lies between 0.7 and 4.7 Ma, consistent with the dates. The usefulness of the age plateau selection criteria of Lanphere & Dalrymple (1978) is supported by the results.

Palaeomagnetism and radiometric dating of the British Tertiary Igneous Province: Muck and Eigg

The lavas, dykes and other intrusive bodies of Muck and Eigg have been sampled for palaeomagnetic purposes. All the bodies are reversely magnetized except for four dykes of Muck which are normally magnetized. The lavas show variations of direction of magnetism believed to be a record of secular variation. The radiometric dates support the geological evidence that the lavas of Eigg and Muck predate parts of the Rhum central igneous complex, the lavas of Rhum and Canna and some of those of Skye. The combined evidence shows that most of the activity took place during an R-N-R-N sequence between 63 and 58 Ma. The younger R-N sequence is the same as the first part of the R-N-R sequence found in Mull but the Sgurr pitchstone of Eigg and parts of the Eastern Redhills of Skye belong to later reversed intervals and correspond to Chron 26r and Chron 26. The igneous activity occurred over a 10 Myr period from about 63 Ma and can probably be associated with Chrons 27r to 23r of the Harland et al. time-scale.

Some aspects of Tertiary acid magmatism in NE Ireland

AbstractGeochemical data (including REE determinations) are presented for all five Mourne Mountains granites and three Northern Ireland rhyolites. These confirm (1) the extremely fractionated nature of some of the rocks (Sr and Ba < 10 ppm, Rb > 400 ppm, Eu/Eu* < 0.1, and K/Rb < 100), and (2) a major revision to the outcrops of the E. Mourne granites G1 and G2 in which much of the former is reclassified as G2. Combined petrographic and geochemical studies have also indicated that magmatic pulses were involved in the emplacement of Mourne intrusions G2 (Revised)-G5 inclusive.The N. Ireland Tertiary acid rocks exhibit general geochemical similarities to their analogues elsewhere in the British Tertiary Igneous Province (in which Sr is generally < 100 ppm and CeN/YbN generally < 8 with Eu/Eu* often < 0.6), but as a suite the Mourne granites are enriched in Rb and some other LIL elements relative to their N. Arran counterparts.The more fractionated acid magmas of NE Ireland are believed to have evolved from primitive granitic parent liquids by crystal fractionation at depth which involved major and accessory phases (including zircon and allanite). In the Mourne (and County Antrim) areas the primitive acid compositions lie at the ends of basaltic (tholeiitic) differentiation series, and in the Mourne central complex there is a complete geochemical sequence from basic rocks through intermediate members to primitive and ultimately highly evolved, subalkaline, granitic intrusions. It is concluded that the data are consistent with the Mourne granites and Northern Ireland rhyolites being essentially basaltic differentiates, although Sr isotope evidence indicates some (probably minor) crustal involvement.

Evidence for early Tertiary remagnetization of Devonian rocks from the Orcadian Basin, northern Scotland, and associated transcurrent fault motion

The moderately inclined, reversed field vectors exhibited by many of the Devonian rocks of the Orcadian Basin are reinterpreted as resulting from a regional, early Tertiary remagnetization event in northern Scotland, relating to the emplacement of the British Tertiary Igneous Province. The nature of the remagnetization appears to be primarily chemical and may relate to the circulation of magma-derived hydrothermal fluids in the porous Old Red Sandstone formations. Fine-grained matrix hematite appears to be an important carrier of this remagnetization component, but other secondary opaque minerals also contribute to the magnetizations of certain sites. Although Orcadian Basin rocks are currently at some distance from the nearest Tertiary igneous center (Skye), geologic and tectonic evidence may allow for southward translation of the Skye igneous center in earliest Tertiary time.

Origin of some mixed-magma and net-veined ring intrusions

Most of the ring complexes in the British Tertiary Igneous Province include at least one net-veined or mixed-magma ring-dyke. Net-veined ring-dykes consist of a granophyre host rock containing mafic pillows, with areas of granophyric net-veined mafic rock. Mixed-magma ring-dykes contain mafic micropillows dispersed in a silicic host rock. The ‘mafic’ component covers a wide compositional range with basaltic-andesite to andesite abundant. These magmas represent both fractionates of basaltic magma and hybrids of silicic, intermediate and mafic magma. Analogous compositional diversity in extrusive rocks of Hekla and Askja, Iceland, indicate that the Hebridean composite ring-dykes originate from compositionally zoned magma chambers. Density relationships suggest that zoning in chambers of tholeiitic systems can only result for magmas more evolved than ferrobasalt. Zonation can result from crystallization on the sidewalls and roof of the chamber (producing a boundary layer flow of less dense fluid), wall melting and magma replenishment. Hybrid mafic pillows originate by mixing within the chamber, especially when lower-density olivine tholeiite is emplaced beneath ferrobasalt magma. Subsidence of a crustal block into an underlying compositionally and thermally zoned chamber results in commingling of silicic, intermediate and mafic magmas within both the reservoir and the ring-dyke, producing pillows and net-veining. Mixing is incomplete due to the contrasted viscosities and solidus temperatures of the magmas. Mixing and net-veining is also enhanced by volatile exsolution and explosive emplacement of magma at sub-volcanic levels, with mixed magma eruptions sometimes occurring at the surface.

The British Tertiary igneous province: palaeomagnetism of the dyke swarm along the Sleat coast of Skye

Summary. A palaeomagnetic investigation has been made of a swarm of more than 400 dykes along the south coast of Skye, Scotland, by the Sound of Sleat. Seven red lamprophyre dykes have palaeomagnetic directions inconsistent with Tertiary age, and not inconsistent with their previously held Caledonian age. The remaining 409 dykes have palaeomagnetic directions that are consistent with a Lower Tertiary age. We present evidence suggesting that the Tertiary dykes might have been emplaced during a short time, over which the geomagnetic polarity occupied as few as three polarity intervals (NRN or RNR). Certain ‘intermediate’ directions of magnetization have also been found, and are presented here.

Thermal aspects of the origin of Hebridean Tertiary acid magmas. I. An experimental study of partial fusion of Lewisian gneisses and Torridonian sediments

AbstractThermal aspects of the conflicting hypotheses that the acid magmas of the British Tertiary Igneous Province were generated by either fractional crystallization of basic magmas or partial fusion of country rocks around basic intrusions are investigated by combining petrological and experimental data. Sparse large xenoliths (up to 12 m2 outcrops) of Lewisian gneiss occur in the ferrodiorite member of the Marscoite Suite in the Western Redhills Eocene intrusive complex of Skye. Most of the blocks are of granodioritic-tonalitic gneiss which is very similar to the grey gneisses of the northern and southern regions of the mainland Lewisian. One is a two-pyroxene basic granulite, closely resembling in petrography and composition the granulite-facies basic gneisses of the central region of the Scottish mainland Lewisian. During Tertiary thermal metamorphism the xenoliths recrystallized patchily to pyroxene hornfelses and up to 50% of melting occurred in any parts of blocks that were rich in alkali feldspar and quartz. Microprobe analyses of intergrown Ca-rich and Ca-poor pyroxenes in a hornfelsed ultramafic mica pyroxenite lens within a leucogneiss block give a calculated temperature of 950 °C for the Tertiary metamorphism. Melting experiments at 1 kb PH2O were carried out on leucocratic and mesocratic Lewisian gneisses (from the xenoliths), Torridonian arkose and shale. Extensive melting of the leucogneiss and arkose, to peraluminous liquids, occurs within a few degrees above their solidus (715 °C). The silicate liquidus of the leucogneiss is at 930 °C. Microprobe analyses of the glass products of melting experiments (mostly of leucogneiss) show that liquid composition trends during progressive melting are consistent with theoretical equilibria in the synthetic system Ab-Or-An-Qz-H2O. Partial fusion in the Skye Lewisian gneiss xenoliths took place at a water vapour pressure of a few hundred bars, probably under water-undersaturated conditions.

Palaeomagnetism of the British Tertiary igneous province: Rhum and Canna

Summary Palaeomagnetic measurements have been made on samples of lavas, dykes and plutonic rocks from Rhum, and of lavas and dykes from Canna. All the samples are reversely magnetized, except for three dykes cutting the Rhum lavas and two dykes cutting the Canna lavas. The magnetizations are believed to be primary, or to have been acquired only a short time after the units were formed. The simplest interpretation of the polarities is that the majority of the rocks were formed in a single reversed interval, with the normally magnetized dykes being intruded in the following normal interval. However, if the time during which the Rhum centre was eroded, prior to the extrusion of the lavas, was long, additional inversions of the Earth's magnetic field could have escaped detection. In this case the polarity sequence is extended from R—N to R… N… R—N, with the final R—N corresponding to the lavas and the normal dykes cutting them. Igneous activity began about 59 Ma ago, contemporary with activity elsewhere in the British Tertiary igneous province.