Structure of remanent magnetization in some skye lavas, NW Scotland

Physics of the Earth and Planetary Interiors, 16 (1978) 45-58

45

© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

STRUCTURE OF REMANENT MAGNETIZATION IN SOME SKYE LAVAS, NW SCOTLAND K.M. STORETVEDT

Department of Geophysics, University of Liverpool, Liverpool (Great Britain) and Department of Geophysics, University of Bergen, N-5014 Bergen (Norway) (Accepted for publication June 14, 1977)

Storetvedt, K.M., 1978. Structure of remanent magnetization in some Skye lavas, NW Scotland. Phys. Earth Planet. Inter., 16: 45-58. In the British Tertiary igneous province one commonly observes reversed magnetizations with an abnormally large range of inclinations. Two of the Skye lava sequences which are of.Early Eocene age have been chosen to investigate why this range of inclinations exists. Various laboratory studies of the natural remanenee reveal a composite palaeomagnetic record. There are two axes of magnetization present: one steeply inclined (~75 °) and one with an intermediate inclination (~35°). There is an excess of reversed polarity components in the bulk fossil remanence of most lavas and the inclination spread seems basically caused by superposition of these components. The experimental problem of splitting the polyphase magnetization into its separate sub-components is demonstrated by many examples. It is concluded that processes of low-temperature mineral alteration (which strongly overprints the high-temperature exsolution structures) and remagnetization must have been active for a minimum time span of 20 m.y. after the original cooling of the lavas, involving both polarity inversions and a major geomagnetic axis shift in mid-Tertiary times. As a consequence, the original TRM has probably been erased to a major extent and replaced by CRM's in subsequent times. The polar estimate based on the shallow magnetization group agree well with suggested Lower Tertiary palaeopoles from Northern Ireland and from the Faeroe Islands. The multivectorial nature of the remanent magnetization in the Skye lavas signifies that even for geologically very young rocks it is necessary to employ much more rigorous analysis techniques than are currently being used in many palaeomagnetic laboratories.

1. Introduction Over the past fifteen years or so the British Tertiary igneous province has been subjected to an intensive palaeomagnetic and opaque petrological study. References to the quite extensive literature can be found in papers b y R.L. Wilson, J.M. Ade-HaU, and coworkers. However, as the amount of data has increased it has gradually become clear that on the whole the observations are difficult to interpret - though dominantly reversely magnetized the various rock formations generally exhibit pronounced elongated distributions (smeared in the direction o f inclination) so that in many cases one has refrained from calculating overall mean directions. It is o f great geophysical importance to know whether the observed distributions of remanent magnetization in the British Tertiary igne-

ous province are caused either b y a combination of complex geomagnetic field behaviour and irregular extrusive/intrusive activity or whether they simply are the result of lengthy magnetization processes with b o t h axial shift(s) and/or polarity changes involved. In order to initiate a consideration o f this question it was decided to take a further look into the remanence structure of some o f the Skye lavas. Earlier work on the Skye igneous complex which is thought to be of Early Eocene age (Evans et al., 1973) includes studies o f 53 igneous units (lavas, dykes, etc.) b y Kahn (1960), a detailed petrological and magnetic investigation of a single lava (Ade-HaU et al., 1968a, b) and general palaeomagnetic studies o f the entire lava sequence (Wilson et al., 1972) as well as o f the Vaternish dyke swarm on North Skye (Wilson et al., 1974).

46

2. Rock collection and experimental procedure The experiments were carried out on the Liverpool University rock collection from the Skye lavas. Based on previous palaeomagnetic results from these rocks (Wilson et al., 1972) material of all available lavas from two of the five possible lava sets (Anderson and Dunham, 1966) was chosen for further experimental work; i.e. the top sequence or Osdale set, referred to as section M by Wilson et al., and the second lowest or Ramascaig set, called section J by Wilson et al. In the following the M and J notations will be used. Both sections were characterized by an abnormal variation in inclination only. The available rock material from each flow consisted of a few 2.5-cm specimens cut from independently oriented field cores (in general four) taken from the same site. As a general rule, two specimens A (top) and B (bottom), had been cut from each core. In the previous study the B-specimens had been given AF treatment (in steps of 50 Oe) up to 200 Oe and occasionally to 400 Oe. In the present analysis some specimens which proved to be particularly resistant to AF treatment were demagnetized up to a maximum of 1,500 Oe, but in general the progressive demagnetization was discontinued at a lower AF (ranging upwards from 250 Oe). The reason for not taking all specimens up to the maximum available field was two-fold: (1) experiments showed that if systematic directional changes did not take place at fields of a few hundred oersteds there appeared to be a very small chance that such movements should occur within a field range of 1,500 Oe; and (2) several specimens carrying a relatively "soft" NRM acquired viscous or stray moments during the demagnetization process which gradually became more and more troublesome (as the intensity decreased) for a sufficiently accurate determination of the NRM parameters. When one suspected that problems of this kind were affecting the results (abnormal intensity variation was the best guide in this respect) the demagnetization steps were tested by repeated demagnetization (in the same field) followed by subsequent remeasurements. In order to avoid a gradual buildup of either rotational magnetization (Wilson and Lomax, 1972), or anhysteretic remanence, causing a systematic directional trend (which easily might be interpreted as a true NRM property) the specimens, on repeated demagnetization, were

mounted differently each time in the specimen holder of the AF apparatus. On the average about three specimens from each lava were subjected to the more detailed AF treatment. In part these included previously studied specimens though in the intervening time many of them had been somewhat reduced in size due to manufacturing of thin sections. A total of 21 specimens from the two investigated lava sections (nine from the M section and twelve from the J section) - a few previously untreated ones, along with some which had a fair proportion of their remanence intensity left after the AF analysis - were subjected to thermal demagnetization. The thermal work was carried out at the University of Bergen. In order to fit the specimen holder of the spinner magnetometer the cylinders had to be cut to a height of about 1.6 cm. In order to avoid the possible biassing effect of small residual fields within the furnace the specimens were given a new orientation before each succeeding demagnetization step. Only magnetization directions associated with the temperature range having systematically decaying intensities vs. progressive demagnatization have been accepted for further consideration. For specimens showing systematic directional trends (AF or thermal) the general rule adopted here for acceptance of "stable end-points" is that the terminal direction of a specimen must be experimentally confirmed by at least three successive demagnetization steps (AF or thermal). However, owing to the problem of scattered results, at some stages of demagnetization (not included in the diagrams) this requirement could only be fulfilled for relatively few specimens. Lava mean directions were only estimated for flows which exhibited very good internal directional consistency, and where smeared distribution of the bulk remanence and/or systematic directional trends (without stable end-points) have not been observed. Other studies were performed on M-section lavas only. These included general microscopy of the opaque mineralogy [following procedures and descriptions as outlined by Ade-Hall et al. (1968a)] and measurements of saturation magnetization vs. temperature, Js(T). The magnetic colloid technique (Soffel, 1968) was used in order to provide a deeper look into the structure of apparently homogeneous oxide grains. The latter analysis was applied on specimens from about half of the M-section lavas.

47

3. Opaque petrology; lava section M 3.1. Microscopic examination Two polished sections from each of 26 flows have been examined. The dominant oxide mineralogy consists of "titanomagnetite" (TM) and discrete ilmenite, the first in general accounting for more than 99% by area of the opaques present. The degree of hightemperature oxidation states is generally low, the average magnetite oxidation number (Ade-Hall et al., 1968a) being less than 2 for the great majority of analysed specimens. Likewise, the titanomagnetite granulation (Ade-Hall et al., 1968a) is only slightly developed. Fig. 1 shows the frequency of average TM oxidation number (M) and of average TM granulation number (G). Class-1 TM grains frequently show patchy lightening in colour while still remaining optically isotropic. The occurrence of white bands (subdividing larger grains) is another characteristic feature. These latter observations provide an indication that mineralogical changes of palaeomagnetic importance may have taken place below the level of optical detection. In order to look into this problem, apparently homogeneous TM grains were covered by a magnetic colloidal suspension for observations of grain-colloid in. teraction (Soffel, 1968). These experiments show that the initially precipitated Fe-Ti oxides have suffered a general alteration in these rocks. Thus, class-1 grains in general exhibit a strong magnetic inhomogeneity

-30

~20

with sub-division of larger gains into an irregular pattern of smaller regions, some of which are strongly magnetic, while others have no visible magnetic effect. These features which are probably late- or post-deuteric in origin also overprint the oxidation-exsolution structures of deuterically more altered grains. In some cases the rims of larger grains (where also an embryonic granulation often can be seen) show the strongest magnetism. This latter observation is certainlyrelated to mineral alteration and not to domain structure in that the "TM" grains may show little magnetic effect in the interior of the grains even when subdivided by bands (or lamellae) of various non-magnetic (or weakly magnetic) oxide minerals. Also, the grain sizes concerned (~50 tan) are far above that of TM single-domain structure. Faint traces of sulphide are commonly observed. Lavas M017-M022 differ from the rest of the collection by having more numerous patches of reddish• brown silicate groundmass interspersed with haematite bands. Of these latter lavas M018 and M020-M022 have magnetite oxidation number ~3 while M017 and M019 are class-1 rocks.

3.2. Thermomagnetic analysis Curie-point estimates associated with various thermal cycling experiments were obtained by heating in air in a field of approximately 1 kG as described by Ade-HaU et al. (1965). A total of some 45 specimens from 20 different lavas were subjected to such studies. As shown in Fig. 2 the Tc distribution on heating is concentrated in two temperature intervals, 200-400 and 500-600°C respectively. The "300°C"-phase is

10

"3 10

8 4

2

3

4

g

6

M

2

3 G

Fig. 1. Frequency of "magnetite" oxidation number, M, and granulation number, G, for the Skye M section lavas.

2

1-

,qn 4 5

6

T°C x l O 0

Fig. 2. Distribution of measured Curie points in Skye M lavas.

48

the dominating one, contributing 80-100% of total induced magnetization in the majority of specimens. Exceptions are twelve specimens having single hightemperature Curie points. Of the latter group eight come from lavas having average magnetite oxidation numbers :->2.8 while four specimens come from two lavas (M007 and MOO8) classified by Mvalues close to unity (1.00-1.21). In most of the two-phase samples the high Curie points are weak and poorly defined the Js(T) curves having a monotonic decay above 400°C as seen for example in many red sediments. Therefore, it has not been possible to def'me precisely the high-temperature Curie point in many of the twophase specimens. In order to study the thermal stability of the "300°C"-phase a number of isothermal heating experiments and subsequent thermal cyclings were carried out at intermediate temperatures. For experimental times of the order of 1 h the thermomagnetic curves are reversible for temperatures not exceeding about 300°C. On the other hand, the 300°C-phase alters rapidly at temperatures of 350°C. Isothermal heat treatment at about 350°C leads to a gradual Js increase (Fig. 3, MOOS-B): a 45 rain heating causes an alteration of induced magnetization to the same extent as a direct heating to 600°C. The break-down of the "300°C"-phase results in general in at least two magnetic phases, having Curie points above 500 and below 200°C respectively, but the high-temperature inflextion may be ill-defined. In connection with the disappearance of the 300°C-phase there are some cases of a drastic decrease in magnetic moment below about 300°C (on heating) associated with a monotonic and low-intensity cooling curve (of. specimen M019-2B of Fig. 3). The latter observations may reflect breakdown processes in maghaemite or titanomaghaemite. Various types of Js(T) behaviour are shown in Fig. 3. 3.3. Conclusion on magnetic mineralogy The many microscopic indications of alterations of the primary opaques and the observations of a general magnetic inhomogeneity within apparently optically homogeneous titanomagnetite grains suggest that the oxide mineralogy of the Skye M-section lavas is more complex than that resulting from a single high-temperature oxidation-exsolution process, i.e. ~ere is an important secondary mineralogical alteration (late

1.0 Js

L

0.5

t:oo,,. ,,\

1

,.iI% r

30C~C

/

i 600eC

' 300°C

600"(::

1.0 JS

0.5

M 0 1 9 - 2B ~ ~ - 1.50

,

,

i

300°C

i

i

600'

300°C

600°C

Fig. 3. Examples of temperature dependence of induced magnetization in a field of about 1 kG. Specimen M005-1B illustrates the commonly observed rise in "spontaneous" magnetization during isothermal heat treatment for 45 rain around 350°(:. Note that single high-temperature Curie points can be found both in specimenshaving highly exsolved titanomagnetite grains as well as in specimens where high-temperatureexsolution structures are practically absent (21'/~ 1.0).

deuteric or younger) overprinting the high-temperature exsolution structure. Carmichael and Nicholis (1967) have concluded that titanomagnetites that crystallize from a basaltic melt will contain 5 0 - 8 0 reel% ulvospinel in solid solution with magnetite provided that no oxidationexsolution has occurred. This would imply Curie points between 0 and 300°C with a probable peak in the 100-200°C range (Creer and Petersen, 1969; Ade-Hali et al., 1971) which is lower than most of the Curie points encountered in the present study. According to Ade-Hall et al. (1971) a steady rise in Curie point is the fLrStresponse of a non-deuterically oxidized basalt to hydrothermal alteration. When comparing the presently obtained Js(T) results with some relevant laboratory experiments by Creer and Petersen

49 (1969) it seems that the fairly rapid change of magnetic properties at about 350°C is unlikely to be caused by laboratory oxidation of a virgin-state titanomagnetite. These authors have reported on extensive thermal treatment of assumed stoichiometric (tested by electron microscopy) titanomagnetite at 400°C in order to produce either a single high-temperature Curie point or double Curie point curve, which readily forms on laboratory heating of the considered lavas. Therefore, it seems more plausible in the present case to ascribe the observed irreversible thermomagnetic behaviour to an unmixing of cation-deficient phases, forming an intergrowth of a variety of magnetic and non-magnetic oxides depending on the chemical composition of the spinel phase before inversion (Readman and O'Reilly, 1970; O'Reilly and Readman, 1971) rather than to laboratory oxidation processes. Unmixing processes may also have taken place in nature: relatively well-defined high-temperature (500-550°C) Curie points in some lavas having averaged M values close to unity may be explained in this way. Five of the investigated flows, Nos. 10, 18 and 20-22, differ from the rest of the sequence by having a higher average deuteric oxidation state (M > 2.8) coupled with reversible single high-temperature Js(T) curves. One might think that these lavas which suffered a relatively pronounced high-temperature alteration may have been less susceptible to further mineralogical changes in post-cooling time than the rest of the M section. However, the general overprinting of probably low-temperature alterations even for these lavas poses a quite complex situation from a magneto-mineralogical point of view. The fairly pronounced alteration of the original F e - T i oxides in the M-section lavas have most likely lead to important modifications of the initially impressed magnetization (TRM). In fact, one may expect to find a simple palaeomagnetic record (one direction of stable remanence in a single lava) only if: (a) the secondary mineral changes took place while the ambient field retained the axis and polarity as during original cooling of the lavas, or (b) the original remanence has been totally replaced during subsequent thermochemical processes during which both the geomagnetic axis and the polarity remained constant. However, the question of whether chemical magnetization (CRM) is of importance in these lavas and, if so, the length of time involved in these magnetization processes, can

only be answered by a detailed analysis of the natural remanence itself.

4. Demagnetization results 4.1. Lava section M

The directional data after AF treatment in 200 Oe confirmed the meridional distribution of lava mean directions as obtained in the early study (Wilson et al., 1972) though in two or three cases the new site (lava) mean directions were significantly displaced either steeper or shallower than those previously estimated. This non-repeatability of directions of magnetization is, however, more frequently noted for separate specimens. It appears that these discrepancies arise from a slight reduction of specimen size which had taken place between the two studies due to thin-section preparation. These observations indicated a rather complex remanence buildup (there must be more than one stable magnetization component in these rocks and sometimes the different components are not evenly distributed throughout the volume of a specimen) and this has been confirmed by the more extensive demagnetization experiments. The resistance of the NRM to AF or thermal demagnetization may vary greatly demanding individual treatment of specimens. Directional trends on progressive demagnetization are pronounced features of the M lavas but unfortunately one very rarely succeeded in following these vectorial movements into final terminal directions. Consequently, neither of the two demagnetization techniques employed proved satisfactory for retrieving the full palaeomagnetic information contained in these rocks. It appears that in general neither coercivity nor blocking temperature spectra of the sub-components present are sufficiently dissimilar to allow one of the remanences to be clearly separated prior to the more "erratic" stage of stepwise demagnetization. Before considering the overall directional information it seems necessary to have a closer look at the remanence structure within a representative selection of lava flows. Such results have been illustrated in Figs. 4 and 5. Intensity decay of NRM for specimens having the most marked direction change on thermal demagnetization are illustrated in Fig. 6. Lava MOO2 shows a quite stable bulk remanence,

SO

4A(NRM-490°C) ~ T .52O'Co ~ I~ I " 4 B ( N R M -1500 0e)

W

T

t

.~~%~og;.

~vo M 002

490 >i_ # 460 ° NRM

/

I

Lava

-

I

M

I

I

I

I

003

I

E

5000e-- °1550 0e

45o°%- %4A

~:r~:::'I00-300

350-°~

~oo-~,,

o\ _'4_oo

Oe

:26oo

"5oo

\

'~

/ ~ ~ I B ~,~4o-" ~oo ~oo.F~o. /

L _ T _.J S

i

I

I

I

I

18 (50- 500 Oe)

Lava

wC

T 1

/ a 1A(50-

S

I

500 0e) --

T

I

~

2B (NRM- 1200 . ~

3B ( N R M - 4 0 0

Lava

300

I

41:&(NRM- 1OOOO e ) ~ /

--

M 006

/

I

_350 Oe

M

Oe)--

007

/

-/

38 (1OO- 30

/

//

0

~'~

150 O e L ~

l

S

+,,, Lava

/

'~B

S

I

M 010

900 Oe

2 Bt.B.~ BOO 70 0

t

5SOoi~_.a i S (I00-6o00e) ~500e _I~00 --I ~

38(NRM-1500

\ 0e)

\ \

L_TL~ $

I

I

I

I

I

7 |

350 Oeo,,,,,,~ l ~'~ 300 2A (100-350 O e ) " ~ . u-4 B 3A(100- 350 O e ) ~ ~ 25O x~ ~ 5 o Lava M 011 ~I00 1B ( 1 0 0 - 8 0 0 O e )

6 5 0 Oe --I~

I

/

/ / /

~T~ S

Fig. 4. Examples of within-lava directional variation for some M sequence flows. Projection is equal area. Open symbols are upward-pointing magnetizations while closed symbols are downwazd-pointing vectors. Squares represent bulk directions of magnetization (the average direction of magnetization after removal of possible low-stability remanence but disregarding any systematic directional change at high-temperatures/fields). C>osses are previously published lava mean directions. All directions are without correction for tectonic tilt (small).

51

N 3

o

W

l

I

|

\

I

I

I

I

4B (100- 450 Oe)

\

. . L

Lava M 013 ..-

%,

/

L a v a M 012

~-7

.......-----t..~o 3 B ( 1 0 0 - 4 0 0 Oe)/~"' 3A ( 2 0 0 - 4 0 0

18(100-500 _- -

Oe)

/ /

wI,

-

I

I

I

I

I

I

I I (NRM-550

/ Oe)

/

L_._ T

3A (NRM - 490"C)

wC

S

io~~ C OoW )~__.~ 2B( 50 40 L a v a M 014 ~/-,

,00 50 -

_...J

.L# 200

-~Oo ~.-.'~°~3 B oo

016

-Lava M

--4 B(100-300)

T25ooe

2A ( N R M - 3 O O O e )

/

4B (100- 35Q Oe)

B

/

B

/

/

/

/

/

'

LT

S

S

_.J

N W

I

...

I

,Lava

I

I

I

I

@

M 017

,, x \ \ _

__

4A (1OO-3OO Oe)

%`

_~

I

..

18 (ISO

2oo-c

/

o

~OO '

--V~.//

'

,4o~o-_" .....

~

~orc-

120"

Oe) L . . . . . .

S

2OOpC

Lava M 027

Fig.5.TextasforFig.4.

,,,-..~o;~

~ 1 4 0 ° __

.

1oo c

Oe)

52 o a M O02-3A A, M 012-3A • • M 027-1A

In

J J

1

2

1 3

4

5

O71- 1A 074- 2A

I 6

3t T*C×IOO

Fig. 6. Natural intensity of magnetizationvs. temperature. The specimens concerned are all represented in Figs. 4, 5 and 9 and are basically those giving the mast pronounced directional changes on progressivethermal demagnetization.

having a NNW spread of some 35 degrees of arc. It is interesting to note that thermal demagnetization of two specimens shows directional changes roughly in the plane of this elongated distribution (the directional movements are basically found between 490 and 520°C, where the remanence intensity has the most marked decay, but for specimen 3A one notices small incremental changes in a NW direction also at lower temperatures). These observations are probably caused by a polyphase magnetization consisting of at least two diverging components of magnetization (probably one reversed and one normal). A lava mean direction would in this case be of doubtful palaeomagnetic significance. For lava M003 AF treatment of the two analysed specimens showed direction movements towards steeper inclinations. Specimen 1B was subsequently subjected to thermal demagnetization. The latter measurements were done in the Bergen laboratory where the specimens had to be slightly cut to fit the specimen holder of the new magnetometer. Note the displacement of remanence direction by more than 50 ° after cutting (of. points 500 Oe and 200°C). One may wonder whether the rock saw, which is of the standard type used in palaeomagnetic laboratories (phosphor bronze), may give an additional stable remanence during cutting. However, such cutting had already been done at the Liverpool laboratory (for thin-section preparation between the two studies) and causing similar discrepancies of specimen directions

(see below). Also, the direction changes which may result from cutting are well in harmony with observed trends of within-flow variation of bulk remanence and with direction changes on demagnetization. This suggests that the "cutting effect" is due to inherent palaeomagnetic properties rather than to spurious components impressed by the rock saw (cf. Roy and Lapointe (1978) for further information). The perhaps most convincing evidence in this respect comes from lava J078 (see below) where badly scattered within lava directions after AF treatment were brought into excellent agreement by cutting. Therefore, the most likely explanation of the lava-M003 results is that the underlying more stable remanence consists of two or more components of magnetization and at least one of them must be somewhat irregularly distributed throughout the rock. It is important to notice that progressive thermal demagnetization of specimen 1B also causes a northerly direction trend - as it takes its path through the lower hemisphere this may imply that one of the components involved is of normal polarity. Lava M006 is interesting in that out of four tested specimens, two show steep inclinations and two fairly shallow inclinations, defining the "end-points" in the original smeared distribution of lava means (cf. Wilson et al., 1972). When such a magnetic structure is seen within a single flow it casts severe doubt on the possibility that the smeared distribution is due to a true geomagnetic feature (at least if the remanence is of TRM origin). Lava M007 (Fig. 4) and M017 (Fig. 5) give examples of simple directional results. When lava mean directions are very well defined (and apparently without smeared distribution) there is always a good agreement with the earlier published data. Results of this kind are clearly acceptable for further palaeomagnetic consideration. Lavas such as M010, M011, M013, M014 and M016 are regarded as being doubtful palaeomagnetitally due to features such as within-flow smeared distribution of remanent magnetization or observations of northly direction changes (the directions may become very steep before the trends are being disrupted). Though thermal demagnetization has only been carried out on nine specimens from this lava sequence, it seems that interesting results have been obtained. In flow M027 for example (eL Fig. 5) thermal analysis of two specimens gave directional trends which fit in with

53 the idea of there being a normal component of magnetization present superposed on a dominantly reversed magnetization. This flow has extremely scattered directions of magnetization and AF treatment provides no change in this respect. Since the lava comes from the top of a hill Wilson et al. (1972) considered it as having become struck by lightning and it was consequently disregarded. However, except for a relatively strong low-stability component in some of the specimens of this flow, the intensity of the more stable magnetization (above approximately 100 Oe AF treatment) is on the normal intensity level for the investigated flows. This observation, in addition to the thermal results, makes one suspect that the scattered within-lava magnetism may be caused rather by a complex palaeomagnetic structure in which a normal magnetization component plays an important role. Similarly, the remanence vector at 520°C for specimen M012-3A may form part of a movement which might eventually have ended up somewhere antiparallel to the reversed group(s) if reliable results above 520°C had been obtained. On the whole, systematic direction changes during demagnetization frequently take place in the M-section lavas, defining trends which line up fairly well with observed elongated within-lava distribution of the bulk remanence. The information on vectorial movement, over the range of demagnetization for which acceptable directions were obtained, has been summarized in Fig. 7. From the fact that a series of lavas exhibit certain preferred vectorial characteristics one feels that possibilities of orientation errors, lightning, faulty demagnetization technique, etc. are not appropriate to explain the overall results. The stable lava mean directions for the M section which are given in Fig. 8 define two distinctly different groups (having very steep or fairly shallow inclination respectively) corresponding well to the "endpoints" of the smeared distribution obtained after more limited demagnetization (200 Oe). To a reasonable approximation the two groups of directions obtained may both represent true relative palaeomagnetic field axes, but one should keep in mind that the remanence of these rocks seems quite complex and the acceptable M-sequence data are few, so in principle the estimated lava means may be somewhat displaced away from the direction of underlying palaeomagnetic axes.

N

\

/

\

/

W

i I J I ~~E

\ /

/

\

\ /

L___T .__3

\

S

Fig. 7. Vector diagram showkngdirectional trends in the M lavas obtained during stepwise demagnetization. Circles are bulk directions and the different arrows end in the last point for which the vectorial change could be followed.

4.2. Lava section J

In comparison with the M lavas the magnetization of the J sequence is considerably more stable against AF treatment. There are some tendencies of NS-direction changes but these features are much less pronounced than for the preceding sequence. As a result, the originally elongated distribution of lava mean direction is hardly changed after applying more extensive AF demagnetization in addition to a more strict acceptance criterium (eft Fig. 8). By first approximation, this gives the impression of there being a domi-

Lava section J /

Lava section

M

-

"

o-~75 --0 "69

o~'-6a

lt7 19 --

/

57

60

22

\

/

\

S

S

Fig. 8. Distributions of acceptable lava mean directions for the Skye M and J sequences (without tilt correction).

~550oc

I

I

I

I

I

I

J

. .'~LO,;sO-

E

I

I

I

I

I

I 4 3 (~--460"

20 °- 400 °

- Lava J 067 4 B ( N R M - 550 Oe) \ \

L._ ]- _.J $

\

1A

/ /

'

"+. 3A

1OO~:),,~O3OO-12OO v

Lava J 071

-//'ix

( 4 O O - 13OO O e )

/

\

L

~.a

~ T ~ S

oe)W

I

I

I

I

°°

I loo" T

I

200~

p

h~32~r

43d

3oo--"°-"-"-430.

01~4~0°

460 o-

L

1A

L a v a J 071

X

I

~.

--/ D (NRM-12OO Oe)

f

/

180°C

\

/

\

/

s

S

"

I200°C

Z

-C' 4~(4oo~oo&,4o. ,B("\'2°''°°-5°° Ja~°e'" 2o /

I

I

I

•-"

-o,;~"

4A (400 - 500 Oe)

_-

3B ( 2 0 0 - 5 0 0 0 e ) /

I

2A (400.

3A ( 400 - 500 Oe)

~\o/ %/

5000e)

Lava J 078

(2o_46ooc)

/

--

° 1B (2oo- 500 Oe) s

200-5000e/----~

/

"-,~d" 1B

\

' abrupt, direction c h a n g e a f t e r cutting,

Fig. 9. Examples of directional variation within some J sequence lavas. For further explanations refer to text of Fig. 4.

55 nantly single-component remanence which in turn may imply that the meridional distributiOn of lava mean directions reflect a true geomagnetic field behaviour (impressed at the time at which the lavas originated). However, such conclusions seem not to be supported by the available thermal demagnetization data, some of which are illustrated in Fig. 9 along with corresponding bulk directions based on AF analysis. Examples of intensity decay patterns are shown in Fig. 6. Flow J063 has a very good internal consistency of three AF-based directions but these are systematically displaced (~15 °) north of the previously obtained lava mean direction (Wilson et al., 1972). Also, thermal analysis of a fourth specimen shows a gradual movement in a NW direction, the trend of which corresponds to the NW lineament seen in a fair number of steeply-magnetized M lavas and which also is a pronounced feature in partially remagnetized rocks of similar age from the Faeroes (LCvlie, 1976). Despite the well-defined characteristic magnetization of lava J063 after AF analysis one may therefore begin to wonder whether this technique really has given us the necessary information for a reasonable palaeomagnetic evaluation of the results. Similarly, examples from lavas J067 and J071 show directional trends which are well in line with observations in the M sequence. It is worth remembering that for the latter two lavas thermal demagnetization is carried out "on top of" previous AF treatment and corresponding therefore to a more advanced stage of demagnetization. Specimen J071-1A demonstrates the same interesting feature as has been seen in some M lavas: a certain reduction of the specimen size causes an abrupt displacement of the bulk remanence direction and again it is in the NW direction. Lava J074 shows a marked linear spread in declination of the bulk remanence in four AF-tested specimens. Further, thermal demagnetization of a fifth specimen shows a directional variation which is such that one might suggest that the magnetization structure seen in this flow is caused by the involvement of at least two components of magnetization with opposite polarity. In flow J078, specimen 1B is an example where AF demagnetization (at least up to 500 Oe) makes extremely limited "improvement" to a deviating direction of magnetization while slicing-off a few millimetres of the specimen is sufficient to bring it into a very good agreement with the other specimens of this flow (subsequent thermal demagnetization confirms the stability of the latter

"point"). In the first study, flow J078 directions were so badly scattered that one had desisted from calculating a lava mean direction while in the present study the specimen directions are faily well grouped even without further demagnetization. It should be noted that, in addition to specimen 1B already dealt with, the three A specimens concerned had been cut between the two studies. Of the remaining six thermally examined specimens (not shown in diagram) J064-1A seems to have a stable "end-point" around 180 °, - 7 4 ° (in very good agreement with AF-based data of the same flow), J057-2A becomes stable at about 177 °, - 5 1 o (again in agreement with AF-based data of the same flow) while J060-5A, J065-3A, J066-1A and J067-3A tend to follow NW-SE and NNE directional paths at higher temperatures. Though the thermal data are few the available results suggest therefore that one probably is facing the same magnetization intricacy as found in the M section. When comparing the experimental resuits of the two lava sequences one gets the impression that for some unknown reason the AF technique is not sufficiently powerful for discriminating between the various sub-components. One dearly needs many more thermal results from the J lavas in order to present a firm conclusion regarding their magnetization buildup, but it seems a reasonable guess that many of the lava mean directions acceptable after AF analysis (Fig. 8) would no longer be adequate after thermal tests.

5. Conclusion and discussion It seems to be experimentally well established that the palaeomagnetic record of the investigated Skye lavas is complex. For the M section there are good reasons to believe that the anomalous large inclination spread, which characterizes the bulk remanence, is not reflecting a true property of the geomagnetic field, but is rather the result of a superposition of basically two reversed components of magnetization having inclinations of around - 3 5 ° and - 7 5 °, respectively (cf. Fig. 8). The situation is much less clear in the J section. Here, the AF method does not seem to possess the necessary "resolution power" for splitting the polyphase magnetization suggested from thermal demagnetization studies. However, it is interesting to

56 note, that, in the three specimens (of a total of ten specimens analysed) where thermally stable endpoints were obtained, two were in reasonably good agreement with the intermediate/shallow inclination group of the M lavas while the third agreed with the steeply magnetizated M lavas. One is tempted to believe therefore that also in the J section the inclination spread is basically caused by a two-axis remanence though this idea dearly needs to be underpinned by many more thermal experiments. The evidence for a two-axis magnetization (even in the same flow) lines up with the fairly pronounced low-temperature alteration of the opaque mineralogy and suggests strongly that a substantial amount of chemical remanent magnetization (CRM) must be involved. An axis variation relative to Europe between Lower and Upper Tertiary was reported more than ten years ago (Irving, 1964). More recently, this palaeomagnetic discordance has been found within some separate European Tertiary volcanics (Storetvedt, 1972, 1973; I_~vlie, 1975; I4hvlie and Kvingedal, 1975). Based on geological evidence (Storetvedt, 1972), it has been inferred that this axial change must have occurred at around the Eocene-Oligocene boundary. The evidence of a two-axis structure in the Skye lavas is therefore supported by other observations; the shallow inclined remanence is likely to correspond to the Lower Tertiary field while the steep directions represent the Upper Tertiary (post-Eocene) field. One would suggest that lavas which suffered a relatively strong high-temperature oxidation during the initial cooling (.~ ~ 3) would be less susceptibleto post-cooling alteration than lavas with low M values. This is in part supported by observations in that none of the high inclination lavas (the remanence probably impressed in post-Eocene time) seem to be in a high deuteric oxidation state. On the other hand, the situation is more complex in that there are also low inclinations associated with lavas having low M values. Based on the observed low-temperature alteration one may suggest that the remanence of the latter rocks was imposed either at a late stage of cooling or at some subsequent time prior to the axis shift. On the average, the low-inclination lavas (irrespective of the extent of high-temperature oxidation) differ from the rest by having a more reddish silicate ground mass, but at present it seems difficult to assess the palaeomag-

netic significance of this observation (it can be both of primary and of secondary origin). Magnetization directions and polar estimates (before and after tilt correction) of the shallowly magnetized M lavas are shown in Table I. Comparison with other palaeopole determinations from rocks of similar age, i.e. the Antrim laterites (I~vlie et al., 1972) and the basalt sequences of the Faeroes (L~vlie, 1975; L~vlie and Kvingedal, 1975) and of east Greenland (Hailwood et al., 1978) is shown in Fig. 10. The Sloye palaeopole is in closer agreement with the other poles before tilt correction which supports geological evidence (Anderson and Dunham, 1966) that the small stratal dip of this formation is of primary origin. Also, by allowing for a certain continental separation between Europe and Greenland in Middle-Late Tertiary times the east Greenland pole would move closer to the European ones. The agreement of the Skye results with other data, despite few results; the general observation that suggested low-temperature mineralogical structures overprint the high-temperature features, and the very good agreement between lava mean directions may indicate that the magnetization of individual lavas within the shallow group probably represents average geomagnetic fields (secular variation may to a large extent be averaged out even in a single specimen) rather than to spot readings. In other words, it appears most reasonable to suggest that the original TRIVlonly plays an insignificant role in the present magnetization of the investigated Skye lavas. The directional variation vs. progressive demagnetization and the occurrence of elongated distributions of bulk remanence (corresponding to the directional trends observed in separate specimens) seem to imply that the total magnetization contains palaeomagnetic components acquired over a sufficient length of time to involve both an axial shift as well as polarity inversion(s). The major experimental problem in deterrnin, ing separate sub-components is that they have closely overlapping stability spectra against AF demagnetizaCon whereby only the resultant (which may not correspond to any true palaeomagnetic field) may be measured. This probably explains why the elongated distribution of lava mean directions (filling in the "space" between the two magnetization groups revealed by the M section) for the J lavas still remains after AF treatment. On the other hand, thermal demagnetization ap-

57 TABLE I Palaeomagnetic parameters of the shallowly magnetized M lavas (for acceptance criteria see text), before (A) and after (B) tectonic correction N

A B

4 4

Mean direction (°) decl.

incl.

174.9 174.0

-36.0 -30.2

c~gs (°)

4.0 4.0

K

540 540

pears to have a greater "splitting" effect on the bulk remanence. The general argument against the thermal method is that it commonly introduces mineral changes in the rock (which probably the AF method does not). However, mineral alteration may in itself be a very useful element in demagnetization processes. For example, if cation-deficient phases are present (as most likely in the present case) heat treatment may lead to mineral disintegration whereby a certain fraction of the natural remanence may be eliminated simply by reconstitution of Fe oxides (provided the ambient field has been reduced to a sufficiently low level to prevent possible new magnetic phases acquiring a TRM). The presence of a normal magnetization is suggested by directional variation, but no stable end-points have been reached. However, to judge from extended remagnetization circles both the steep and the shallow normal component are present though the great-circle paths have a clear preference for the steeply inclined intersection "point". Steeply-inclined reversed magnetizations often seem to have a declination about due west. Such odd

70*

60.

Fig. 10. Comparison of Skye palaeopoles [before (A) and after (B) tectonic correction] with other poles from the Thulean basalt province. Aa = Antrim; EG = east Greenland; Fa = Faeroes.

Pole lat. (°N)

long. (°E)

52.3 48.5

181.2 182.2

directions (with reference to experimental data they cannot be explained in terms of secular variation) coupled with a frequently NW directional spread suggest that a certain normal magnetization adds to the reversed component(s) producing deviating resultants. The normal magnetization appears to cause particular problems for an accurate determination of the steeply reversed component which is poorly defined in the present study. The inferred magnetization complexity as here described is by no means a unique case within the British Tertiary igneous province in that data from the Mull lavas (Wilson, 1964) aswell as from the Antrim basalts of Northern Ireland (L¢vlie et al., 1972) indicate an equally complex palaeomagnetic structure; further support in the same direction comes from the basalt province of the Faeroes (l.~vlie, 1976, 1978). Though remagnetization coupled with a two-axis field seems to provide the simplest explanation of the present data one clearly needs first of all a greater additional amount of thermal demagnetization results before the magnetization problem of the Skye sequence can be satisfactorily settled. However, if the two-axis model is the correct one it implies that certain thermochemical processes (causing mineral alteration and remagnetization) were operating in the Skye complex for a period of at least 20 m.y. after the original coolhag of the lavas. In a remagnetization system rapid polarity fluctuations are likely to give only a very indistinct palaeomagnetic record. Thus the bulk remanence will be dominated by that polarity which, on a whole, was the most important one for the period concerned or, alternatively, had a sufficient persistence during the most active time of remagnetization. As the various sub-components, in general, have closely overlapping stability spectra the opposite polarity remanence will show up at best in directional trends at low inten-

58 sity levels of demagnetization. However, from such observations one cannot conclude that the opposite polarity component is of minor importance in the palaeomagnetic buildup.

Acknowledgements I am very grateful to the Department of Geophysics, Liverpool University, for giving me free access to their rock collection from the Skye lavas and to Prof. R.L. Wilson for numerous discussions during the course of this study in addition to the many constructive comments on the manuscript. I should like to acknowledge the extensive help, during laboratory analysis, of Mrs. B. Bridges, Mrs. M. Catterall and Mr. K. Breyholtz. Dr. R. L~vlie kindly helped with parts of the rnieroscopy analyses. A major part of this work was carried out during a six-month leave of absence at the Department of Geophysics, Liverpool University, from the Department of Geophysics, University of Bergen. This research has been financially supported by the Norwegian Research Council for Science and the Humanities.

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