Age and duration of activity at the Isle of MullTertiary igneous centre, Scotland, and con¢rmation of the

existence of subchrons during Anomaly 26r

Lynne M. Chambers a;*, Malcolm S. Pringle b

a Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UKb Scottish Universities Environmental Research Centre, Rankine Avenue, Scottish Enterprise Technology Park, East Kilbride,

Glasgow G75 0QF, UK

Received 1 September 2000; received in revised form 20 August 2001; accepted 4 September 2001

Abstract

The duration of activity that produced the Isle of Mull Tertiary igneous centre has been constrained to 2.52 þ 0.36million years (1c) using new high-resolution 40Ar/39Ar radiometric dating techniques. These new 40Ar/39Ar ages forMull and also one from Skye, presented here, confine the duration of igneous activity to within one magnetic chron(3 million years within Chron C26r). However, the Mull and Skye Tertiary centres record a sequence of reversed andnormal polarity intervals. Recalibration of tie points used to determine the magnetic polarity time scale cannot accountfor all of the observed changes in polarity from Mull and Skye. We propose that the new ages for the Mull and Skyecentres indicate that these parts of the British Tertiary volcanic province developed fast enough to record short-periodpolarity intervals, which correspond to tiny wiggles in the marine magnetic anomaly record. ß 2001 Elsevier ScienceB.V. All rights reserved.

Keywords: Ar-40/Ar-39; Mull Island; Isle of Skye; reversals; magnetization; secular variations; magnetic anomalies

1. Introduction

The majority of the Isle of Mull comprises oneof several Tertiary igneous centres that collec-tively are known as the British Tertiary IgneousProvince (BTIP) (Fig. 1), which is the easternmostmanifestation of igneous activity within the widerNorth Atlantic Large Igneous Province. With an

area of 941 km2, Mull is one of the largest of theindividual centres within the BTIP. As with mostof these igneous centres, development of the Mullcentre began with ¢ssure eruptions that produceda V2 km thick lava £ow section [1], 1000 m ofwhich is still present today. Fissure eruptionseventually gave way to the development of centralvolcanoes and ¢nally a NW^SE-trending regionaldyke swarm cut the whole area. Cross-cutting arc-uate intrusions within the sub-volcanic root indi-cate that the igneous activity was concentratedaround three centres [2], C1, C2 and C3, whichmoved progressively north-westward through time(Fig. 1). The last intrusion within the central com-

0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 4 9 9 - X

* Corresponding author. Tel. : +44-131-650-8524;Fax: +44-131-668-3184.

E-mail addresses: lynne.chambers@glg.ed.ac.uk(L.M. Chambers), m.pringle@suerc.gla.ac.uk (M.S. Pringle).

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plex, the Loch Ba ring dyke, is cut by the NW^SE-trending regional dyke swarm.

Previous 40Ar/39Ar ages placed the base of theMull lava pile at 60 þ 0.5 (1c) Ma and the LochBa ring dyke at 56.5 þ 1 (1c) Ma [3]. In this study,we provide new, higher resolution, 40Ar/39Ar agesfor the stratigraphic development of the Mull lavapile and central volcano. Developments of the40Ar/39Ar dating technique, including calibrationwith better known standards, have allowed thetiming of igneous activity in Mull to be moretightly constrained. We also use these dating con-straints, along with previously published palaeo-magnetic data [4], to reinterpret the meaning ofthe polarity record preserved by the Mull lavapile.

2. Analytical technique

Radiometric dates were obtained from 40Ar/39Ar incremental-heating analysis of whole rock(250^500 Wm) and felsic concentrate (106^250Wm) sieved fractions that were prepared at theUniversity of Edinburgh. Following petrographicexamination of approximately 100 thin sectionsfrom throughout the stratigraphy in Mull, 10were deemed suitable for 40Ar/39Ar analysis andtheir locations are shown on Fig. 1. The irradia-tion £ux factor (J) was monitored approximatelyevery 20 mm using the USGS standard TaylorCreek rhyolite (TCR) sanidine 85G003 at 27.92Ma. All errors are reported as 1 standard devia-tion of analytical precision unless otherwisestated. A more detailed description of the 40Ar/39Ar analytical procedure can be found in [5,6]and details of the irradiation conditions can befound in [7].

3. Results

The reliability of each 40Ar/39Ar date is assessedusing the criteria of [8,9]. Data from the incre-mental-heating experiments, including J values,are presented in Table 1 and the complete analysisof each sample is available in the BackgroundData Set1. Age spectrum diagrams and K/Ca for

all of the incremental-heating experiments, whichful¢lled the aforementioned criteria, are presentedin Fig. 2.

Of the 10 Mull samples analysed, eight met allof the reliability criteria. Of those eight, sampleB1 preserves McCulloch's tree at the base of theplateau lava succession in the west of the islandand is stratigraphically the earliest in the succes-sion. Sample B1 reached an incremental-heatingcrystallisation age of 60.6 þ 0.3 Ma, which coin-cides with the weighted mean age of 60.5 þ 0.5 Ma[3] for the base of the lava succession. BM64 andBM67 (three £ows above BM64) are lavassampled by Kerr [10] from the Ben More se-quence. These lavas are the highest (at approxi-mately 700 m above sea level) in the lava succes-sions that were analysed. Both yield concordantapparent plateau ages, which are within error ofeach other. The best age estimate of these BMsamples is a weighted mean age of both experi-ments at 58.38 þ 0.19 Ma.

Plateau lava eruptions were eventually super-seded by the development of a central volcano.Samples SO33 and SO17 are lavas from withinthe ¢rst of the three central volcanoes (C1) andare also known as the Central Group lavas (CG).These samples have normal magnetic polarity [4].Using samples with known magnetic polarity al-lows direct correlation between the geomagneticpolarity time scale and the radiometric ages.SO33 reached an apparent age plateau at58.94 þ 0.3 Ma, whereas SO17 contained slight ex-cess argon, so the isochron age of 59.66 þ 0.64 Mais the best estimate of the crystallisation age ofthis sample. The combined weighted mean ageof 59.05 þ 0.27 Ma is used as the age of the SOor C1 samples. Sample LB1 is a sanidine concen-trate from the Loch Ba ring dyke, which was thelast magmatic event within the central volcano(C3). It gave an apparent age plateau of58.48 þ 0.18 Ma. The cross-cutting dykes ofMD1^MD3 give similar apparent age plateausof 58.0 Ma. MD1 and MD3 are two splits ofthe same sample; MD1 was the whole rock sam-ple and MD3 a felsic concentrate. A weighted

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Scotland

Tertiary Lava Succession

Tertiary Central Complex

0 50Km

LochNa Keal

Loch BaLB1 + MD1-3

C1

C2

C3

Ross ofMullGranite

0 6Km

Tertiary Lava Succession

Tertiary Central Complex

Mesozoic SedimentsPre-Mesozoic Rocks

Tertiary Igneous Centre

Staffa

BeinnTalaidh

Beinn

Beinna Ghraig

Bein Buie Gabbro

Corra -Bheinn

Loch NaKeal

LochBa

Glen

Cannel

Chaisgidle

0 2Km

N

Loch BaRing Dyke

FelsiteGranophyre

DoleriteGabbro Faults

Ring-dykes

Glen Cannel FelsitesGlen Cannel Granophyre

Central Intrusions

(a) (b)

(c)

Mull

Skye

Sample SiteMull

Ulva

B1

BM64 + 67

Ben More

SO17 + 33

Fig. 1. (a) Geographical location of Mull and Skye within the BTIP. (b) A simpli¢ed geological map of the Isle of Mull showingthe location of the sample sites and the position of the three centres of igneous activity (C1^C3). (c) A detailed map of the Mullcentral complex after Bailey et al. [2].

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mean age using all three dyke samples gives a bestage estimate of 58.12 þ 0.13 Ma for the cross-cut-ting dykes.

It should be noted that the weighted mean age

of 59.05 þ 0.27 Ma for the SO samples is olderthan the BM samples at 58.38 þ 0.19 Ma, whichoccur earlier in the magmatic succession. The SOweighted mean age, rather than the younger BM

Table 1A summary of the results from all of the rocks analysed in this study from the Isle of Mull and the Isle of Skye

Sample Name Material Age Spectrum Analysis Isochron Analysis39Ar(%)

stepsused

Age(Ma)

1c MSWD Age(Ma)

1c Sums/N-2

40Ar/36Arintercept

1c

B1 Lower lavas wr lch 60.09 6 of 16 60.56 0.29 14.88 60.30 0.43 11.37 284.60 132.95BM64 Ben More wr lch 60.47 11 of 17 58.19 0.26 17.60 58.08 0.36 14.31 307.80 18.04BM67 Ben More wr lch 80.90 13 of 17 58.66 0.25 25.85 58.18 0.34 20.77 309.95 9.14

BM64and 67

weightedmean age

58.38 0.19 1.45

SO33 Centre 1 lavas wr lch 51.69 7 of 12 58.94 0.30 18.88 57.95 1.18 26.27 321.96 33.81SO17 Centre 1 lavas wr lch 55.93 9 of 16 61.54 0.24 2.02 59.66 0.64 1.28 315.50 6.22

SO17and 33

weightedmean age

59.05 0.27 1.13

LB1 Loch Baring dyke

sanidine 55.54 12 of 20 58.48 0.18 3.05 58.45 0.29 3.90 298.49 30.12

LB2 Loch Baring dyke

wr lch 66.90 10 of 17 56.60 0.21 5.72 56.57 0.23 6.92 298.77 3.65

MD1 Dyke felsicgmass

66.77 7 of 18 58.07 0.23 9.13 58.53 0.40 10.24 285.58 7.20

MD2 Dyke felsicgmass

58.11 10 of 18 58.33 0.27 14.89 58.40 0.32 19.23 293.48 2.89

MD3 Dyke felsicgmass

73.46 10 of 18 58.04 0.20 1.68 58.09 0.33 2.12 294.20 4.84

MD1-3 weightedmean age

58.12 0.13 0.42

SG16 Loch Ainortgranite

biotite 53.52 2.8-4 58.40 0.21 0.86 58.19 0.51 0.79 311.67 28.87

SG16 Loch Ainortgranite

biotite 56.71 2.1-fuse 59.22 0.29 0.63 59.95 0.72 0.56 278.72 11.13

SG16 Loch Ainortgranite

biotite 84.90 1.8-fuse 64.42 0.53 3.37 64.23 1.26 5.57 297.06 15.56

SG16 Loch Ainortgranite

biotite 50.94 1.6-2.2 58.43 0.23 1.49 58.07 0.44 1.42 313.35 18.44

SG16 Loch Ainortgranite

biotite 87.76 2.1-fuse 74.34 1.63 73.96 77.95 3.67 112.72 251.86 34.82

SG16 Loch Ainortgranite

biotite 68.56 1.8-fuse 52.88 1.11 97.92 60.40 0.73 4.00 78.00 19.94

SG16 Loch Ainortgranite

biotite 75.99 2.0-fuse 58.41 0.56 11.90 59.73 0.67 6.68 235.21 22.81

SG16 Loch Ainortgranite

biotite 84.24 1.5-fuse 65.14 0.85 6.26 66.12 1.89 11.17 281.77 18.69

SG16 weightedmean age

58.58 0.13 2.04

Complete analyses are available in the Background Data Set1 (Appendix 1). Experiments shown in italics failed the rigorous sta-tistical acceptance criteria of [8,9]. All ages are calculated relative to the USGS TCR sanidine 85G003 at 27.92 Ma (isochronage). Plateau steps are in % CO2 laser power for the Loch Ainort biotite (SG16) experiments.

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age, is re-enforced by ages for the younger LochBa ring dyke and the cross-cutting dykes. Analy-sis of the BM samples by N18O indicates that theyare altered by hydrothermal £uids with low N18Ovalues [7]. Detailed oxygen isotope studies [11^13]have established that large-scale interaction be-tween heated meteoric ground waters and shal-low-level plutons is a common phenomenon inTertiary volcanic centres. They found that theoldest rocks and those adjacent to the calderaring fractures have the lowest N18O values (32.9to 35.7). Although the BM samples show no pet-rographic signs of alteration, they have N18O val-ues of +4.7 and +3 [7], whereas normal values forbasic igneous rocks are +5.5 to +7.4 [14]. In ad-dition, the BM weighted mean age of 58.38 þ 0.19Ma is comparable to that for the Loch Ba ringdyke at 58.48 þ 0.18 Ma. In light of these results,the BM lava age should be treated with caution asit most likely represents the hydrothermal altera-

tion of the lavas during the development of theyounger central volcanoes (C2 and C3).

4. Correlation to the magnetic polarity time scale

4.1. Magnetostratigraphy of Mull

Palaeomagnetic studies of the Mull volcanowere carried out by [4,15,16], who concludedthat Mull had a palaeomagnetic sequence of re-versed^normal^reversed (R-N-R) polarity. Thisinterpretation assumed that the Central Grouplavas (C1) were normally magnetised by over-printing due to a later thermal (volcanic) eventor by slow cooling which produced an apparentsequence of R-N-R-N-R polarity rather than R-N-R polarity [16]. Later palaeomagnetic work [4],discussed further in the next paragraph, whichsampled the central complex in more detail,

Fig. 2. Age spectra and K/Ca plots for the eight samples that met all of the statistical acceptance criteria of reliability [8,9]. Ar-rows mark the steps that were used in the plateau and isochron age calculations. The error bars shown for each plateau markthe age ( þ 2c).

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showed that there was a minimum of seven polar-ity intervals (Fig. 3a). However, available 40Ar/39Ar ages indicated that this polarity zonationmust fall within the R-N-R sequence correspond-ing to Chrons C26r, C26n and C25r. Remagneti-sation was therefore invoked to explain the addi-tional apparent reversed and normal polarityintervals.

Palaeomagnetic data for Mull [4,15,16] areavailable in full as Supplementary PublicationNo. SUP 18048 (7 pages) from the British LibraryDocument Centre or from the Geological Societyof London Library. Detailed examination of thesedata, summarised in Fig. 3, and the cross-cuttingrelationships in Mull reveal an apparently compli-cated magnetic history. First, there are 1000 m ofplateau lava £ows that can be traced from thewest coast to the top of Ben More, all of whichhave reversed polarity. These plateau lavas are cutby Centre 1 (C1) of the central complex. WithinC1 are lava £ows and pillow lavas that formedduring an eruption into a caldera lake. Insidethe main ring fault of the caldera, these lavashave normal magnetic polarity and outside thering fault they have reversed polarity. The lavaswithin the central complex that possess N polarityshow no evidence for a secondary remanencecomponent during thermal demagnetisation [4].Correlation across the ring fault is di¤cult, sothese normal polarity lavas could be of a slightlydi¡erent age to the reverse polarity lavas outsidethe ring fault. Samples SO33 and SO17 are fromlava £ows inside the ring fault and have normalpolarity, whereas the remaining intrusions of C1have reverse polarity, including the early acid andbasic cone sheets. These early acid and basic conesheets (R polarity) cut the lavas inside the caldera(N polarity) [2,17]. This places the timing of thenormal polarity pillow lavas between the plateaulava succession (R polarity) and the remainingintrusions of C1 (R polarity). The intrusion ofacid cone sheets (N polarity) and later cone sheetscutting C1 (R, N and intermediate polarity) markthe beginning of C2. The remainder of C2 has Npolarity, as does the whole of Centre 3 (C3), in-cluding the Loch Ba ring dyke. The ¢nal stage,the cross-cutting dykes, have R polarity. Thisgives a total of seven polarity intervals recorded

in the Mull succession [4], which is shown sche-matically in Fig. 3.

Dagley et al. [4] identi¢ed additional apparentpolarity changes not highlighted in grey (Fig. 3a),that are the result of remagnetisation. The BenBuie layered gabbro (stratigraphic level 7 in Fig.3a) appears to have mixed polarity. North^southtransects through the gabbro reveal that near tothe Glen More ring dyke and the Loch Airdeglaisgranophyre, the gabbro has normal magnetic po-larity, while to the south, away from the contact,the gabbro has reverse polarity. The results dem-onstrate that the true magnetic polarity is re-versed with some samples in the north having aremagnetised normal polarity [4]. These samplesexhibit evidence for low grade metamorphism,such as turbid feldspar and the replacement ofpyroxene by hornblende and chlorite [4].

The samples at Creag na h-Iolaire (stratigraphiclevel 5 in Fig. 3a) with apparent normal polarityyielded internally inconsistent results and a ther-mally induced N overprint is apparent in thesesamples. The samples were taken extremely closeto the large intrusions of C2 and appear to haveundergone low grade metamorphism. The Derry-naculen granophyre (event 2, Centre 1) appears tohave mixed polarity (only samples with normal orreversed are shown in Fig. 3a). During alternating¢eld demagnetisation, the one sample with normalpolarity (S151) has positive inclinations that be-come shallower at higher applied ¢elds [4]. Dagleyet al. [4] used this evidence to conclude that theDerrynaculen granophyre was partially remagne-tised by later intrusions of C2, but that it wasoriginally emplaced at the same time as the Tor-ness felsite and the Glas Bheinn granophyre (dur-ing a reversed polarity interval).

Using the new 40Ar/39Ar ages presented here(Table 1) to constrain the timing of the Mull vol-cano and the palaeomagnetic results of Dagley etal. [4], it is apparent that all seven short polarityintervals fall within Chron C26r (using the timescales of [19,20]). The samples SO33 and LB1,which both have normal magnetic polarity, bothhave ages that plot well within Chron C26r. Re-gardless of whether the rocks of Mull have beenmagnetically overprinted by thermal events subse-quent to emplacement, the new age determina-

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tions indicate that the whole of the sequence wasemplaced within Chron C26r (60.92^57.91 Ma[19,20]). Remagnetisation may no longer be re-quired to explain the extra polarity intervalsfound by Dagley et al. [4].

In addition to Mull, the Tertiary centre of Skyealso records a series of polarity intervals [21]. Pa-laeomagnetic data for Skye [21] are available infull as Table A1 deposited in the Royal Astro-

nomical Society Library as m/s 191. These datasuggest that a total of nine polarity intervals wererecorded in the Skye succession (Fig. 3). One ad-ditional apparent normal polarity interval is visi-ble in Fig. 3b, but this is represented by only onegabbro sample (stratigraphic level 2). This gabbrois part of the Outer Gabbro from the CuillinCentre, in Glen Sligachan, and is present as athin slice between the Marsco and Glamaig epi-

Fig. 3. Plots of inclination (degrees, positive downward = normal polarity), declination (degrees east of true north) versus relativestratigraphic order for (a) Mull and (b) Skye. The relative stratigraphic order is based on a numbering scheme given to individualvolcanic units devised by Skelhorn et al. [18] for Mull, and Bell [40] for Skye. This means that the true thickness of the units isnot represented. Normal polarity intervals are marked in grey and reversed polarity intervals in white. Palaeomagnetic data arefrom Dagley et al. [4,21]. Further details are available as Appendices 2 and 3 in the Background Data Set1. Interpreted changesin magnetic polarity (used in Fig. 5) recorded in the Mull and Skye successions are shown with normal polarity intervals in blackand reversed polarity intervals in white. CG = Central Group lavas; C1^3 = centres 1^3; SGG = Strath na Creitheach centre. Theinferred reversed polarity interval between Centres 2 and 3 in Mull (above stratigraphic level 15 in panel a) is shown. Samplesfrom the Ben Buie gabbro and Creag na h-Iolaire (stratigraphic levels 5 and 7 in panel a) have apparent normal polarity, butyield internally inconsistent results and a thermally induced N overprint is apparent in these samples. The additional apparentnormal polarity interval is visible in panel b) (stratigraphic level 2) and is also inferred to be remagnetised, as the gabbro ispresent as a thin slice between two later large granite intrusions that have normal polarity [4].

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granites. Both of these granites have normal po-larity, so remagnetisation was inferred by Dagleyet al. [21] for this gabbro sample.

Because the existing 40Ar/39Ar ages from Skye[21] had large associated errors, the nine magneticpolarity intervals were assumed to be real and notdue to remagnetisation. The Loch Ainort granite(reversed polarity) is key to the interpretation ofnine polarity intervals, therefore individual biotitecrystals from the Loch Ainort granite were ana-lysed by laser incremental heating using a CO2

laser as part of this study. A weighted mean ageof 58.58 þ 0.13 Ma is based on a total of fourincremental-heating experiments, which are alsoincluded in Table 1. When used in conjunctionwith a Cuillins U/Pb age [22], this additional40Ar/39Ar age indicates that the majority of theSkye Tertiary centre also formed during ChronC26r.

4.2. Models that account for normal polarityintervals within Chron C26r

Three models are proposed to account for theoccurrence of normal polarity intervals withinChron C26r and each one is discussed in turn.

In the ¢rst model, it is assumed that the normalpolarity intervals result from later remagnetisa-tion associated with contact metamorphism re-sulting from emplacement of subsequent volcanicunits. However, preservation of cross-cutting rela-tionships with rocks of di¡erent magnetic polarity[4,21] implies that no major widespread remagne-tisation has occurred. Most samples show no sig-ni¢cant secondary remanence component, andwhen a secondary component is present, it canbe removed (e.g. the Glen More ring dyke andother examples previously discussed) to give theprimary magnetisation direction [4]. This also sug-gests that complete remagnetisation has not oc-curred within the central complex. We thereforeconsider this hypothesis to be the least likely ofthe three proposed.

The second and third models involve the exis-tence of short polarity intervals within a longerchron. Distinctive and globally coherent patternsof tiny wiggles have been found in sea£oor mag-netic anomaly patterns during Chron C26r [23],which are superimposed onto the larger-scaleanomaly pattern due to longer polarity intervals(chrons) (Fig. 4). These patterns of tiny wiggles,which have uncertain origin and are therefore re-ferred to as cryptochrons, can be attributed to£uctuations in the intensity of the dipole geomag-netic ¢eld [23,24], or short polarity intervals [25].Cryptochrons, as de¢ned by Cande and Kent [23],are magnetic anomalies that are less than 30 kyrin duration, in a calibrated time scale, with awavelength of 8^25 km and an amplitude of 25^100 nT. In the revised time scale, the existence ofseven cryptochrons is proposed in Chron C26r(see Fig. 4) [20,26]. Cryptochrons are only inter-preted to represent short polarity intervals in thepolarity time scale when they are clearly recog-nised in a magnetostratigraphic study. In suchcases, cryptochrons would then be recognised assubchrons. A modelling study has suggested thatthe tiny wiggles seen in Chron C26r are due tovariations in palaeointensity rather than true po-larity intervals [23].

The normal polarity intervals seen in Mull andSkye (Fig. 3) could provide evidence that trueshort polarity intervals can be seen within ChronC26r (Fig. 5). It is uncertain whether there is a

Fig. 4. Sea£oor magnetic records redrawn from Cande andKent [23]. Records from three spreading centres have beenstretched to the same length. The faster spreading ridgeshows better resolution of the tiny wiggles within ChronC26r. The locations of the seven cryptochrons are markedwith dots and labelled using the scheme of Cande and Kent[20]. A tiny wiggle is evident within Chron C26n, which maycorrespond to the Loch Ainort granite, Skye.

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reversed polarity period between C2 and C3 inMull (Fig. 3). This is possible, especially sincecryptochrons have durations of 6 30 kyr. If so,this suggests that the Mull and Skye volcanicunits both record nine polarity intervals. A de-tailed understanding of the magmatic historyand cross-cutting relationships provides a tempo-ral control for Mull and Skye, which is consistentwith the interpretation that tiny wiggles in ChronC26r can be represented by the observed shortpolarity intervals from these areas.

In model 2, it is assumed that all of the normalpolarity intervals recorded in the Mull successioncorrelate with tiny wiggles from marine magneticanomaly records (Fig. 5). However, in model 3(Fig. 5) we use an alternative, in addition totiny wiggles, to account for the lack of correlation

between the conventional polarity time scale andthe 40Ar/39Ar ages. In this model, we also considerthe way that the geomagnetic polarity time scaleis generated. Tie points in the time scale are ¢xedusing 40Ar/39Ar ages or biostratigraphic evidence.The widths of the sea£oor polarity intervals aremeasured between two tie points and the dura-tions of the intervals are calculated assuming con-stant sea£oor spreading rates. The time scales of[19,20] use nine tie points to calibrate the sea£oorspreading curve during the Cenozoic. The tiepoints relevant to the position of Chron C26rare the Cretaceous^Tertiary (K^T) boundary (65Ma) and the Palaeocene^Eocene (P^E) boundary(55 Ma) [19,20]. The K^T boundary is tightly con-strained by a weighted mean 40Ar/39Ar age of 64.6Ma [27] and 65.44 þ 0.07 Ma [28]. The P^E

Fig. 5. Two schematic models showing the possible correlation between the magnetic histories of Skye and Mull and the geomag-netic polarity time scale. Normal polarity intervals are shown in black, reversed polarity is white and cryptochrons from Candeand Kent [20] are shown as dashed lines. The stratigraphy in Skye and Mull is taken from Emeleus et al. [17] and the palaeo-magnetic sequences are from Dagley et al. [4,21]. In model 2, the time scale and individual cryptochrons within Chron C26r arecorrelated with the normal polarity intervals seen in Mull and Skye. In model 3, the later events of Centres 2 and 3 in Mull andthe corresponding sequence in the Redhills of Skye are correlated to Chron C26n. In this model, the reversed polarity Loch Ai-nort granite becomes a reversed polarity interval within Chron C26n. In model 3, the short normal polarity intervals below C26nwould correlate with tiny wiggles in C26r. All of the ages used are in Ma and are taken from this study except the Cuillins U/Pbage [22].

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boundary is less well constrained and is based onan age of the 317 ash layer within the Mo claysequence, Denmark. The weighted mean age of55.07 þ 0.16 [29] and unpublished ages [30] forthe British equivalents of the 317 ash at54.51 þ 0.05 Ma and 54.56 þ 0.14 Ma are used.The 317 ash is also present in DSDP Hole 550where it lies 7 m above the NP9/NP10 calcareousnannofossil zonal boundary [19]. An age of 55 Mafor this zonal boundary was estimated [30] andwas ¢rst used by Cande and Kent [26] for theP^E boundary. Recent palaeomagnetic work onDSDP Hole 550 [31] suggests that the originalmagnetic stratigraphy for this hole was in errorand that the position of the base of Chron C24nin the geomagnetic polarity time scale should beolder than its present position. If the position ofthe P^E tie point is adjusted, when more reliableages are available, then the positions of the Chronboundaries will move accordingly. If this is thecase, then the P^E boundary would need to beat least 0.5 Myr older than its present positionfor the position of Chron C26n to correspond tothe youngest age for an intrusion with normalpolarity in Mull, as shown in model 3 (Fig. 5).However, all of the intrusions in Centre 3 havenormal polarity, and the LB1 age (58.48 þ 0.18Ma) is for the last of these intrusions. The se-quence of seven or possibly nine stratigraphicallycontrolled polarity intervals recorded at Mull can-not be explained only by making Chron C26nolder. This does not mean that the last normalpolarity intervals recorded in Centres 2 and 3,or even just Centre 3, could not correlate withChron C26n. However, cross-cutting relationshipsindicate that calibration of the polarity time scaleand the resulting adjustment in the age of thepolarity intervals cannot account for all of thechanges in magnetic polarity, and that the exis-tence of at least two cryptochrons is required toexplain all of the observed short polarity intervals.

We consider model 3 to be the most likely ex-planation for the recorded polarity successionsbecause the intrusions comprising Centres 2 and3 on Mull and the Western Red Hills on Skyerepresent the sub-volcanic roots of large centralvolcanoes. For comparison, the duration of theSnaefellsjo«kull volcano in Iceland, 150 km from

the spreading centre, is also well constrained by40Ar/39Ar ages. The Pleistocene volcanics of theSnaefellsjo«kull system rest unconformably on theeroded Tertiary base. The ¢rst lava £ow abovethis unconformity gives an 40Ar/39Ar age of842 þ 10 kyr [32] and its last major explosive erup-tion was 1750 þ 150 years BP [33]. 1 million yearsis the proposed average life span of a ¢ssureswarm from the initial ¢ssure eruptions to its ex-tinction as a central volcano [34], but some sys-tems may persist for up to 2.5 million years in thecase of the large non-rifting central volcanoes [35^37]. It is therefore unlikely that the large centralvolcanoes in Mull and Skye, which are made ofmultiple intrusions with normal polarity, formedwithin the duration of one or even two crypto-chrons (6 30 000 years). It is more likely thatthese short polarity intervals are represented bythe suites of cone sheets and Central Group lavas,for example, while Centres 2 and 3 of Mull andthe Western Red Hills of Skye represent ChronC26n (Fig. 5).

4.3. Correlating 40Ar^39Ar ages and thegeomagnetic polarity time scale

The 40Ar/39Ar ages presented here were calcu-lated relative to the USGS standard TCR sani-dine 85G003 at 27.92 Ma, which is equivalent toFish Canyon tu¡ sanidine at 27.62 þ 0.02 Ma.These standard values give an age of64.81 þ 0.03 Ma for K^T tektite analysed at SU-ERC during the course of this study. This age isconsistent with the 65 Ma used for the K^T tiepoint [19,20] for the polarity time scale. Unlikethe position of the P^E boundary, if the K^T tiepoint is moved then all of our ages would have tobe adjusted accordingly. The 40Ar/39Ar techniquerelies on the ages of standards used, therefore alist of suggested standard values has been pub-lished [38]. The results presented here can be cal-culated to other widely used values for these mon-itor minerals. For example, using published agesfor TCR sanidine at 28.34 Ma [38], all of the agesreported here would have to be multiplied by1.0149. However, this would mean that the SU-ERC K^T tektite age would change from 64.81 to65.78 Ma, which does then not correspond to the

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65 Ma used in the standard time scale. Changingthe ages of the standards used in this study in linewith recommended values of Renne et al. [38]would make all of the ages older, including theK^T tektite, but would still not account for thepresence of normal polarity intervals withinChron C26r.

The 40Ar/39Ar dating technique has been usedrecently to separate magnetic reversals in morerecent parts of the time scale [5]. By directly ana-lysing rocks that bracket the transitions betweenpolarity intervals, many previously joined eventshave been separated into di¡erent polarity inter-vals. For example, three new ages from the Pu-naruu Valley, Tahiti, previously correlated withthe Cobb Mountain normal polarity subchron in-dicate that the eruption occurred about 76 kyrafter the Cobb Mountain polarity interval andthat these rocks therefore represent a younger po-larity interval. Singer et al. [5] concluded thatthere were no fewer than seven, and maybemore than 11, attempts of the geodynamo to re-verse polarity between 1.18 and 0.78 Ma. Shortpolarity intervals are also found in sedimentaryrecords where sedimentation rates are V2 cm/kyr or above. Recent work in the North Paci¢cOcean [39] found evidence for two short polarityintervals and a geomagnetic excursion withinChron 5n.2n, which correspond to three tiny wig-gles visible on marine magnetic anomaly records.This evidence suggests that there is no reason tosuppose that frequent polarity reversals are lim-ited to recent parts of the time scale.

5. Conclusions

Using 1c errors on the 40Ar/39Ar ages reportedin this study, the total time taken for the plateaulavas and the Mull Tertiary centre to form was2.52 þ 0.36 million years. The measured palaeo-magnetic sequence (Fig. 3), in conjunction withthe new ages, indicates that the whole igneousand magnetic sequence in Mull formed withinChron C26r [19,20].

The new age of 58.58 þ 0.13 Ma for the LochAinort granite, Skye, also indicates that the bulkof igneous activity in this Tertiary centre occurred

during Chron C26r. The sequence of nine polarityintervals on Skye (Fig. 3) can be directly corre-lated to most of those on Mull (Fig. 5).

Three models, presented here, can be used toexplain the palaeomagnetic sequences observedin Mull and Skye (Fig. 3) and their lack of corre-lation to the conventional geomagnetic polaritytime scale. In the ¢rst model, it is assumed thatthe normal polarity intervals recorded in the rocksof Mull and Skye are remagnetised. We considerthis to be unlikely for the reasons discussed pre-viously. In model 2 (Fig. 5), all of the normalpolarity intervals are interpreted to correlatewith tiny wiggles observed on marine magneticanomaly records (Fig. 4). However, in model 3,the normal polarity intervals of Centres 2 and 3might correlate to older ages on Chron C26n ifthe time scale is miscalibrated (Fig. 5). We con-sider this last hypothesis to be the most likely forreasons stated earlier. In model 3, the Loch Ai-nort granite would correspond to a reversed po-larity interval within Chron C26n, which is visiblein some of the sea£oor spreading records [23](Fig. 4).

We propose that eruption and formation of theMull and Skye volcanic centres was rapid enoughto preserve a detailed record of short polarity in-tervals during the 3 million years of anomaly 26r.If the conventional geomagnetic polarity timescale is miscalibrated, and after better calibrationof the P^E boundary the position of Chron 26n isadjusted, the existence of at least two short polar-ity intervals is still required to account for all ofthe normal polarity intervals recorded in the rocksof Mull and Skye. These short polarity intervalswould be most likely correlated with tiny wiggleswithin C26r.

Acknowledgements

We thank Robert Duncan and Andrew Robertsfor their detailed and helpful reviews. This workwas carried out as part of a NERC funded PhDproject and was supported by NERC Grant GT4/96/87/E. Samples were kindly provided by PeterDagley (SO33 and SO17) and Andrew Kerr(BM67 and BM64). Godfrey Fitton is thanked

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for helpful discussions and Lee Newcombe forassistance in the ¢eld.[RV]

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