Magnetic Fabric and Inferred Flow Direction of Dikes, Conesheets And

Journal of Volcanology and Geothermal Research 106 92001) 195±210 www.elsevier.nl/locate/jvolgeores

Magnetic fabric and inferred ¯ow direction of dikes, conesheets and sill swarms, Isle of Skye, Scotland

E. Herrero-Berveraa,*, G.P.L. Walkerb, E. CanÄon-Tapiac, M.O. Garciad aPaleomagnetic Laboratory, Hawaii Institute of Geophysics and Planetology, University of Hawaii, Manoa, 2525 Correa Road, Honolulu, HI 96822, USA bGeology Department, University of Bristol, Bristol BS8 1JR, UK cDivision de Ciencias de la Tierra, CICESE, P.O. Box 434843, San Diego, CA 92143, USA dDepartment of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822, USA Received 18 May 2000; accepted 2 November 2000

Abstract We have measured the AMS of 16 dikes, 35 conesheets and three sills associated with the Cuillin Hills magmatic center of the Isle of Skye in the Inner Hebrides of Scotland and of nine dikes located in the regional dike swarm of Skye. Sixty-three intrusives, totalling 734 samples, were studied to determine the plumbing of the Skye volcanic system. Low-®eld susceptibility versus temperature 9k±T ) identi®ed three different mineral phases in the area, namely Ti-rich magnetite, pyrrhotite and titanomaghemite. The petrofabrics of the 63 intrusives yielded coherent ¯ow azimuths regardless of their time of emplacement.

Three main types of magnetic fabric, 9A±C) were found. Fabric Type A 9plane Kmax±Kint parallel to the dike plane) represents the magma-¯ow direction within the intrusives and is the dominant fabric 955% of all the intrusives) within the Cuillin Hills magmatic center and its regional dike swarm. The Kmax inclinations show that 55% of the intrusives were fed by horizontal to sub-horizontal 9AMS inclination of the Kmax axis is equal or less than 308) magma ¯uxes and the rest of them were fed by inclined to vertical ¯uxes. Horizontal magma ¯ow means lateral magma injection inside fractures and becomes more probable as the source is located further away. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: anisotropy of magnetic susceptibility; ¯ow azimuth; Isle of Skye

1. Introduction Trinidade et al., 1999; Raposo and D'Agrella-Filho, 2000). Rocks possess a fabric consisting of an aniso- Paleomagnetism and rock magnetism have contrib- tropy of their magnetic susceptibility 9AMS) that is uted greatly to volcanology because they provide: 91) a capable of yielding important insights into their origin. correlation or dating tool; 92) a means of documenting In particular, AMS study of igneous rocks may yield the the tilting and rotation of rocks; 93) a way of assessing direction of ¯ow not only in lava ¯ows but also in dikes the thermal history of rocks; and 94) a magma-¯ow and other sheet-like intrusions. direction indicator 9e.g. Ernst, 1990; McKenzie et al., The nature of magma transport in the crust is a ®rst- 1992; Tarling and Hrouda, 1993 and references therein; order problem in volcanology. It controls the growth of volcanoes and plays an important role in the modi- ®cation of silicate melts by fractionation, assimilation * Corresponding author. Fax: 11-808-956-3188. E-mail address: [email protected] 9E. Herrero- and mixing. Establishing a technique to determine Bervera). magma-transport directions in dikes and/or sills is

0377-0273/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0377-0273900)00293-6 196 E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210 one of the most important steps forward in our under- depend on the understanding of the mechanisms at the standing of magmatic plumbing systems. Knowing time when fabrics were acquired, which is a problem the emplacement behavior of dike swarms of the common to all petrofabric techniques. Isle of Skye would be a relevant measure forward in Hydrodynamic alignment of crystals such as this direction. The roots of a Tertiary 960 Ma old) olivines, pyroxenes, plagioclases and perhaps titano- volcano are superbly exposed: lavas, feeder dikes magnetites and ilmenohematites within a magma and intrusive core. during ¯ow is the main factor responsible for the This study of the magmatic plumbing of the basaltic generation of primary fabric within igneous rocks. It volcanic system in Skye provides fundamental new is also possible that another mechanism in which late insights into the origin of dikes and the ¯ow direction crystallizing magnetite 9and titanomagnetites) ®ll in of magmas within volcanic rift zones. Intrusion the gaps left between earlier formed and aligned swarms occur in all basaltic volcanic systems plagioclase crystals takes place. Thus, the ferromag- 9Walker, 2000). Much of what is known about rift netic minerals can acquire a ¯ow fabric even though zone processes comes from the monitoring of ground their growth postdates active magma ¯ow 9Hargraves deformation and seismicity in active rift zones. Clas- et al., 1991). Differences in the character of the sic petrologic studies on dike swarms and associated magnetic fabric 9e.g. lineated versus foliated) prob- sills and inclined sheet-like intrusions were made in ably re¯ect differences in the viscosity and rate of the Hebrides three to nine decades ago 9e.g. Harker, ¯ow at the time of emplacement, although insuf®cient 1904) but few quantitative studies on these intrusions studies have been made to quantify such relationships have been attempted. Accordingly, many basic ques- 9Tarling and Hrouda, 1973). Our approach has been tions about the emplacement of these intrusions have mainly to study the plumbing of volcanoes using not been resolved. For example, the geometry and petrofabrics based on the AMS techniques. The distribution of intrusions in dike swarms, variations commonly accepted principle is that the preferred in the dike populations and swarms intensities at vary- orientation of magnetic grains as controlled by the ing distances from the volcanic focus, and how ¯ow of magma determines the AMS of igneous magma is partitioned between dikes and sills are rocks 9for example see Hrouda, 1982; Tarling and some of the unsolved problems. The AMS technique Hrouda, 1993). Experimental results have shown has been employed in the Skye intrusive swarms to that the AMS measurements provide a direct record investigate the signi®cance of the magnetic fabric in of ¯uid-dynamic histories of igneous rocks because these intrusives and to provide information on their they are a direct or indirect re¯ection of the preexist- mode of emplacement, since their petrofabric is ing silicate fabric, thereby supporting the use of this poorly de®ned or almost absent, so no lineations can technique as an indicator of the ¯ow direction in be measured in the ®eld. igneous rocks in addition to the previous empirical observations made in dikes 9e.g. Khan, 1962; Knight 1.1. Anisotropy of magnetic susceptibility in igneous and Walker, 1988; Ernst and Baragar, 1992) and pyro- rocks clastic ¯ows 9e.g. Ellwood, 1982; Knight et al., 1986). The interior of a basaltic volcano is superbly During the past 25 years, the measurement of the exposed on the Isle of Skye. The lavas, feeder dikes AMS has been increasingly utilized as a rapid and and intrusive core of this Tertiary Volcano allow us to sensitive method of determining the petrofabrics of examine magmatic ¯ow directions using the AMS rocks 9e.g. Tarling and Hrouda, 1993 and references determinations. therein). The origin of AMS is intimately related to the crystal shape, degree of alignment and distribution within a rock. The possibility of measuring AMS in 2. The Isle of Skye dike, conesheets and sill swarms the absence of strain markers makes this method unique, although some problems with the interpreta- Flood-basalt volcanic ®elds exempli®ed in Iceland tion of results may arise due to the many factors that and alongside the Dead Sea/Red Sea rift system 9Gass control the measured AMS. Most of these problems and Mallick, 1968; Cox et al., 1970) are issued from E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210 197

Fig. 1. Index map of the Isle of Skye, Scotland showing stippled the dike swarm, the central intrusive and gabbroic core 9rectangular box), and the three sampling sites in the regional dike swarm 9black bar, with numbered dikes). rift zones wherein rifting leads to the eruption of larly numerous and silicic magmas were erupted as voluminous basaltic lava ¯ows. Extinct ¯ood-basalt well as basaltic. ®elds such as those of ca. 60 Ma age that occur in Here we direct attention to one particular linear Skye are eroded to expose abundant rifts. They are dike swarm, that which is focused on the large gabbro linear features and are inferred to be feeders of the intrusions of the Black Cuillin Hills in Skye 9see ¯ood-basalts. In general each dike swarm has a focus Fig. 1). The intrusions include plutons of granite in where magmatic activity was concentrated 9see addition to regional ma®c dikes. They also include a Fig. 1). Generally the focus is identi®ed as the site swarm of ma®c conesheets. These are narrow sheet- of a central volcano wherein intrusions are particu- like intrusions in the immediate vicinity of the Cuillin 198 E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210 e axes of principal principal susceptibilities; aphs of Isle of Skye intrusions: . The upper diagrams show the principal susceptibilities ofsusceptibility the de®ning individual an opposed specimens, imbrication are mean encircled, omitting values the and other two9a) regions axes a of of dike each specimen con®dence 0.5 and m obtained the wide; regions at 9b) of a con®dence any about dike the one 0.5 mean m intrusive. wide; On 9c) a the conesheet lower 0.96 m diagrams wide. th Fig. 2. 9A) Equal area projections of representative dikes illustrating the different types of AMS axial distributions for Skye dikes and conesheets the trend of the intrusive and inferred ¯ow direction is also shown in the lower diagrams. See text for a more detailed discussion. 9B) Selected photogr E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210 199

Fig. 2. 9continued) 200 E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210

Fig. 3. Representative susceptibility versus temperature 9k±T) analysis plots for the Cuillin Hills intrusives and the regional dike swarms intrusives 9Waternish, Drynoch, Armadale).

Hills gabbro pluton which dip toward a focus in the feeder dikes in Iceland and Hawaii ascending in the Hills 9Harker, 1904; Speight et al., 1982; Bell and volcanic center and then propagating laterally as bladed Harris, 1986; Bell et al., 1994). dikes farther away. There are, thus, differences of Opinions differ regarding details of the subsurface opinion that an AMS study might resolve. plumbing system by which magma coming from its More than half of the sampled intrusions are source 9a melting anomaly in the upper Mantle) ascends inclined sheets better known as conesheets. These to the upper crust or the surface. Gudmundsson are intrusions that swarm in the Black Cuillin Hills 91995a,b) inferred that magma in Iceland rose vertically and dip toward a common focus. Usually they are from a long and deep rift, whereas Sigurdsson and interpreted to occupy shear fractures caused by Sparks 91978a,b), Rubin 91990) and others envisage upwardly directed magmatic pressure. Impressed by

Fig. 4. 9a) Dike inferred ¯ow directions on dikes from the Cuillin Hills center. The arrows represent the inferred absolute magma-¯ow

directions based on either paired grouping of the Kmax axes for dikes. The inset represents also the inferred absolute magma-¯ow directions of the sites sampled in three different locations of the regional dike swarms. 9b) Conesheet and sill complex inferred ¯ow directions from the Cuillin Hills and Blaven conesheet and sill complex. The arrows represent the inferred magma-¯ow directions based on either paired grouping

of Kmax axes for the intrusive conesheets and sills. Dashed arrows correspond to sills. E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210 201 202 E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210 the narrow character of basaltic sheets in the Cuillins 982 cm median width of sampled conesheets, see Fig. 2A and B) and the close similarity with dikes 967 cm median width, Fig. 2A and B) in the same outcrops a different mechanism was proposed 9Walker, 1975, 1993b, 1999) closely akin to dike injection. The present paper contributes signi®cantly to the problem.

3. Procedure

We drilled about 734, 5±10 cm long, 2.5 cm in diameter cores from some 63 separate Skye intrusions with an adapted gasoline-driven portable chainsaw. We oriented about half of the samples by sun compass and the remainder 9collected in the prevailing cloudy weather) by magnetic compass. We measured these cores with a Czech-made apparatus 9KAPPABRIDGE KLY-2) at the University of Hawaii Paleomagnetic

Laboratory to determine their K1, K2 and K3,parameters which describe the orientation and relative lengths of the maximum, intermediate and minimum axes of the triaxial AMS representational ellipsoid. The orientation of axes of each sample was plotted on equal-area plots 9Fig. 2A). Most of the samples showed tight clustering of axes. The statistics proposed by Jelinek 91978) were used through out this study. Our sampling of the Isle of Skye intrusives was done in or around the margin of the Cuillin Hills gabbroic complex 9see Figs. 1, 4a and 5a), but nine dikes were collected from three locations outside the Cuillin Hills 9Drynoch, 6 km NE, Waternish, 38 km NE, and Armadale, 18 km SE of the Cuillin Hills) and beyond its epidotic hydrothermal aureole 9Walker, 1993a,b). These nine dikes are in the regional dike swarm and are typically wider than dikes in the Cuil- lin Center 9up to 1.1 m, average width 0.5 m) and one of our objectives was to determine if the magnetic fabric of these wider dikes is similar to that of the narrow dikes. In interpreting the magnetic fabric, we looked for symmetrical arrangements of AMS axes about the plane of the dike, and in particular the symmetrical clustering of AMS axes into paired ®elds. Such segre- gation was postulated by Knight and Walker 91988) for Ko'olau dikes and it has been used extensively by Fig. 5. Histograms of the maximum susceptibility inclination a wide variety of workers 9e.g. Ernst, 1990; Ernst and angles of the mean eigenvectors for the Cuillin Hills: 9a) dikes; Baragar, 1992; Rochette et al., 1991; Staudigel et al., 9b) regional dikes; 9c) conesheets and sills. E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210 203 1992; Raposo and Ernesto, 1995; Raposo, 1997; sponds with the Type Iia of the Ponta Grossa dikes Raposo and D'Agrella-Filho, 2000) to mark a double reported from Brazil by Raposo and Ernesto 91995). imbrication against the two dike margins, and to infer Type-Cintrusions. These are illustrated by Dike 31 the azimuth of ¯ow. Following this idea we have inter- which is one of the large Waternish dikes 97.7 m) and preted our Skye data as detailed below. is the widest dike sampled in the present study 9see Fig. 2A). It is subdivided by partings into three portions of roughly equal thickness, an inner portion 3.1. Three AMS types of intrusions from our Skye and two outer portions 9see Tables 1±4). Although the study partings are not chilled margins, they likely represent boundaries between an earlier and a slightly later Fifty-six intrusions have been studied within the magma pulse into the same ®ssure. This dike gives gabbroic central complex of the Cuillin Hills in the tightest clustering of Kmin of any dikes, but it is the Skye and nine outside the complex. Three distinct most dif®cult to interpret. The reason is that Kmax lies types of fabric were found: mostly near the normal to the intrusion plane, while

Type A intrusions. Most of the intrusions referred to Kint and Kmin lie in the plane. Only two dikes out of 25 here as Type-A have a good clustering of Kmin axes are Type-C dikes. This type also corresponds with approximately normal to the intrusion plane of the Type III reported by Raposo and Ernesto 91995), dikes, and Kmax and Kint axes lying approximately from the Ponta Grossa dikes in Brazil. parallel with that plane. This is the type that predomi- nates also in the Ko'olau swarm 9Knight and Walker, 3.2. Shallowly dipping intrusions 1998) and most of the other dike swarms studied so In addition to dikes there are other types of intru- far, such as the dikes from the Oman ophiolite. Type sive in Skye, namely intrusive-sheets or conesheets A intrusions have ªnormal magnetic fabricº, inter- and a few sills. Formation of a dike complex requires preted as re¯ecting magma ¯ow by Rochette et al. that a volcano be capable of widening suf®ciently to 91991). Other studies report the same type of common accommodate the dikes; otherwise a conesheet and fabric 9e.g. Ernst and Baragar, 1992; Staudigel et al., sill complex that is accommodated by thickening of 1992; Rochette et al., 1992; Raposo and Ernesto, the volcano develops instead. Recent studies 9Gaut- 1995; Raposo, 1997). neb and Gudmundsson, 1992; Gudmundsson, Type-B intrusions. Most of the remaining dikes 1995a,b, 1998) show that conesheets are closely simi- 9Type-B) have a magnetic fabric pattern that deviates lar to dikes in width and in having their dilation vector from the simple Type-A pattern. Thus in Dike 3, K min normal to the intrusion plane 9Walker, 1993a). lies in the plane of intrusion, and is evidently inter- Among the conesheets in the Cuillins, the common- changed with K . K appears to be normal 9see int max est type of petrofabric is the A Type which has the Tables 1±4). Type B is illustrated by dike number Kmin axis in the pole position and Kmax and Kint 48, where there is a very wide scatter of AMS lying in the intrusion plane. Most have a very clear directions, and in many samples K and K both min int clustering of Kmax axes in two areas on either side of lie nearly in the plane of intrusion. This particular the intrusion plane interpreted to be an imbrication dike is very instructive when samples from the rim and hence yielding the ¯ow azimuth. Fig. 2A shows are compared with samples from the middle. Rim a typical conesheet 9number 4) petrofabric, and we samples give a Type-A pattern. can observe that the distance apart from the K Only two dikes out of 25 are Type-B. This type is max mean is 208 or less. interpreted as resulting from magma under stress. One can interpret this type of fabric to imply a vertical compaction of a static magma column with the mini- 4. Results and interpretation mum stress gradient along the dike direction. Type-B may alternatively be explained by dikes having acted 4.1. Magnetic mineralogy as stress conduits, with continuing compression tend- ing to force material along strike. This type also corre- The magnetic mineralogy and magnetic properties 204 E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210

Table 1 Anisotropy of magnetic susceptibility for the Isle of Skye dikes 9D, I, E2:E1 are the declination and inclination of the mean susceptibility directions and semi-angle of the minor and major axes of the 95% con®dence ellipse, respectively 9Jelinek, 1978); azimuth represents the inferred absolute magma ¯ow direction)

Dike no. Dmax Imax E2:E1 Dint Iint E2:E1 Dmin Imin E2:E1 Azimuth Type

12 191 7 3.5/6.1 294 59 3.4/12.5 97 30 5.6/12.7 114 A 14 144 2 8.0/23.9 304 82 12.7/25.9 54 1 6.1/17.6 144 A 16 153 65 5.4/30.9 325 25 11.9/30.8 57 3 3.9/12.8 Undetermined A 16A 33 50 12.8/15.3 124 1 7.4/23.5 215 40 14.8/21.3 Undetermined A 19 196 53 8.7/29.8 306 14 12.3/29.0 46 33 9.1/18.5 126 A 20 281 35 5.1/8.4 50 42 5.5/13.2 168 29 7.9/13.1 101 A 28 109 49 9.5/53.3 345 26 7.7/53.3 239 39 5.8/11.6 109 A 34 330 28 11.2/41.3 112 56 7.3/41.3 230 18 5.7/12.4 146 A 35 2 67 8.0/10.9 157 21 9.2/13.5 251 9 4.0/14.1 2 A 36 22 72 11.3/37.5 219 18 30.6/63.9 128 5 9.7/63.2 22 B 37 115 77 2.3/19.0 335 10 2.5/19.0 244 8 2.1/2.7 115 A 38 133 27 3.2/61.6 18 40 2.2/61.5 247 38 2.2/10.2 247 A 39 338 4 4.3/8.9 74 50 6.0/48.5 245 40 3.7/48.5 158 A 43 206 22 3.5/18.1 114 5 3.5/18.1 11 67 3.7/18.1 294 C 51 259 38 12.0/17.9 87 51 8.2/17.9 352 4 5.5/13.5 79 A 3 301 16 7.7/18.1 202 28 12.8/20.4 57 58 12.2/20.7 121 B List of dikes Ð regional swarm Waternish 29 131 5 11.7/19 36 44 16.1/19.7 226 45 12.3/19.3 131 A 30 191 70 9.7/23.5 326 15 18.4/23.3 60 14 7.6/19.8 191 A 31 76 7 4.3/11.2 345 4 5.2/11.2 225 81 4.3/5.3 76 C 32 279 49 5.2/11.2 102 41 5.2/28.7 11 2 6.0/28.5 279 A 32A 309 8 20.5/31.6 107 81 25.5/40.8 218.3 3 23.9/41.4 309 A 31A 87 4 2.4/14.6 356 11 1.6/15.1 198 79 2.4/4.3 87 C 31B 71 3 2.4/4.4 340 16 2.9/4.3 172 74 2.5/3.1 71 C Armadale 48R 151 59 11.0/12.4 354 29 8.1/12.6 259 10 7.5/11.5 151 C 48C 262 21 6.6/10.2 172 2 9.8/16.7 77 69 6.1/16.7 262 C 49 42 42 12.6/32.4 189 43 20.5/45.2 295 17 12.7/41.1 222 A 50 110 15 3.7/12.3 360 52 7.4/15.2 210 34 4.0/11.8 290 A Drynoch 53 242 26 7.6/11.1 135 30 10.7/14.4 5 48 7.6/14.0 62 C 53A 246 5 1.6/6.4 154 21 3.8/6.4 348 69 1.7/3.9 66 C 54 344 44 16.3/36.9 140 44 15.2/36.6 242 12 5.5/24.2 164 A of the Cuillin Hills gabbroic complex and of the three given in Fig. 3. Some samples exhibit irreversible regional dike swarms were analyzed to identify the thermomagnetic behavior 9e.g. SK-139 and SK- magnetic carriers of the NRM and to determine their 684) where there is production of magnetite during magnetic properties as a check on the reliability of heating. In other cases, a simple thermomagnetic the petrographic record. The temperature depen- behavior characterized by nearly reversible k±T dence of initial magnetic susceptibility was curves with a single Hopkinson peak and a sharp measured using low-®eld susceptibility measure- decrease in susceptibility. These results indicate the ments 9k±T ) on at least one sample per site. Samples attainment of a Curie point at 5808C, which is were progressively heated generally up to 6508Cand characteristic of nearly pure magnetite 9see for subsequently cooled using a KLY2-CS3 apparatus instance SK-624, Fig. 3). 9e.g. Hrouda, 1994; Hrouda et al., 1997). Several The main characteristics of the magnetic miner- typical samples of susceptibility versus 9k±T )are alogy consisting of three magnetic phases seem to E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210 205 b 9I) 9II) 9II) 9I) 9III) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9I) 9II) 2 1 2 1 1 1 1 1 1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Core-sample # Strike Dip direction Dip angle Intrusion width AMS incl. Category a Drynoch Armadale Waternish Intrusion types: C, conesheet; D,Category dike; represents S, the sill. degree of resolution of imbrications: 9I) good; 9II) fair; 9III) poor. a b 3 26 D 18±26 115 15 75 112 16 K Table 2 The sampling locations, types, widths, strikes and dipIntrusion directions # for the Isle of Skye dikes 12 Outcrop # Intrusion type 34 D 109±119 140 50 85 52 59 K 53 91 D 666±678 120 30 70 82 26 K 141616A1920 5628 5634 5635 12336 123 D37 124 D38 D 6639 D 6743 D 6751 D 67 D 67 149±158List of D 67 dikes 182±192 Ð 171±180 125 regional swarm D29 224±232 D 930 233±240 135 D31 340±348 140 D 135 D32 440±456 14032A 45 457±469 120 D 23031A 70 470±480 120 16031B 481±491 120 50 115 492±502 120 34048R 120 135 503±513 D 7048C 120 543±550 65 105 D 8549 120 25 105 D 639±65250 45 155 90 D 75 D 138 15 170 75 150 D 138 15 40 7353A D 65 57 138 360-36854 90 85 80 138 370±378 103 60 D 90 36 370±379 D 50 26 180 397±411 412±429 80 91 40 D 120 380±386 50 140 D 2 82 91 Undetermined 387±396 80 140 70 80 Undetermined 120 130 42 Undetermined 14 581±602 D 50 80 140 49 35 585±598 Undetermined 50 140 50 D 49 140 603±625 42 28 46 K 626±634 50 155 67 50 155 47 90 85 72 K 85 150 679±694 77 K 65 115 85 38 K 65 695±705 85 4 K 60 5 282 85 63 K 130 130 38 K 204 145 70 K 70 122 K 40 96 152 70 K 55 K 70 5 120 K 7 145 8 80 360 4 49 70 3 K 29 K 59 K 212 21 K 126 42 K K K 15 K 5 K 44 K K K K 206 E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210

Table 3 AMS parameters of conesheets and sills from the Isle of Skye 9symbols are the same as Table 1)

Conesheet Dmax Imax E2:E1 Dint Iint E2:E1 Dmin Imin E2:E1 Azimuth Type

1 134 26 14.5/33.4 41 5 24.6/33.4 300 63 5.4/28.4 220 C 2 281 43 6.0/28.7 135 42 23.5/32.2 28 18 7.5/28.3 101 A 4 301 17 10.0/15.3 209 7 12.8/16.5 97 71 9.1/14.9 121 A 6 329 36 4.9/7.9 214 30 6.3/39.6 96 40 6.7/39.6 149 C 7 126 30 16.3/40.1 1 45 29.7/40.3 236 30 18.9/30.2 126 A 8 241 27 6.7/25.6 106 54 19.8/24.3 343 22 10.1/20.5 61 A 9 345 56 6.5/10.1 196 30 5.0/11.5 97 15 6.7/7.9 196 B 10 238 38 16.5/43.1 136 16 16.7/46.1 28 48 16.9/28.8 58 A 11 323 44 8.9/30.2 232 1 17.9/62.3 140 46 26.4/62.3 232 C 13 35 25 14.9/55.0 301 9 26.4/56.0 192 63 11.2/34.4 215 B 15 179 66 3.3/32.1 269 0 3.3/32.1 360 24 3.2/6.3 179 B 17 232 18 6.1/32.6 135 22 6.1/32.6 358 61 6.4/11.7 52 B 17A 297 2 9.1/25.3 206 20 7.5/24.1 33 70 7.2/14.9 26 B 18 121 38 2.9/10.6 348 41 3.2/10.7 233 26 2.6/4.6 301 A 21 44 40 7.0/14.7 175 38 5.4/58.1 288 27 6.6/58.2 355 C 21A 49 10 10.1/22.9 139 3 11.1/23.2 247 79 7.0/14.3 49 C 22 96 40 4.1/21.1 336 31 14.6/31.1 221 34 8.5/29.2 276 B 23 22 44 11.5/28.0 260 28 13.3/28.9 150 32 7.5/20.5 202 B 24 36 38 7.7/10.4 163 37 8.6/38.5 279 30 9.7/38.5 216 A 25 110 29 28.4/61.9 16 7 28.1/61.9 274 60 20.5/35.0 290 A 25A 256 4 14.7/38.5 346 3 22.7/38.3 113 85 17.6/31.2 76 B 26 145 44 12.2/37.9 40 15 18.9/35.9 296 42 14.3/23.2 325 A 27 338 13 4.9/9.4 77 34 7.4/11.6 230 53 4.9/10.1 338 A 40 165 42 3.3/4.7 50 24 3.6/11.2 300 38 3.8/11.0 230 C 41 163 24 5.4/6.2 61 26 5.5/15.1 289 54 5.6/15.0 241 C 42 172 30 1.7/3.2 81 0 2.6/6.6 351 60 2.5/6.6 172 C 44 358 32 3.2/19.6 254 21 4.4/19.2 137 50 3.3/6.1 137 A 45 34 25 2.1/7.8 127 6 1.9/7.2 230 64 2.1/5.4 120 A 47 89 33 56.8/73.6 358 2 33.1/71.2 266 57 16.2/67.6 269 A 52 126 14 5.8/11.2 23 41 7.5/13.5 231 42 4.9/11.9 306 A 56 199 17 8.0/53.5 63 67 14.6/53.6 293 15 6.3/16.9 199 A 57 187 80 9.1/12.7 43 8 9.1/21.3 312 6 7.1/20.6 187 A Sill no. Cuillin Hills sills, AMS data 5 290 17 18.9/2.6 20 1 14.9/25.7 112 73 14.5/21.3 110 A 12A 19 33 9.1/43.5 241 48 8.6/43.4 124 22 9.1/10.4 199 A 33 326 18 8.5/24.4 59 12 10.5/23.4 181 68 8.7/13.0 326 A

be present in the Cuillin Hills gabbros and the 4.2. AMS axes three regional dike swarms. One magnetic phase is observed at a temperature range between 120 In narrow sheet-like intrusions and also in thin and 1408C and corresponds to titanium-rich tabular lava ¯ow units of broadly similar overall magnetite. The second phase is observed at 320± form, two of the three AMS axes would be expected 4008C which corresponds to an iron sul®de such to lie in or close to the intrusion plane while the third as pyrrhotite 9FeS). The third phase is observed at axis would be normal to that plane. These expecta- 580±6108C and might correspond to a partially tions were derived from previous experience of the oxidized titanomagnetite that is commonly fabric of igneous rocks 9e.g. Knight and Walker, referred to as titanomaghemite 9Curie temperature 1988; Rochette et al., 1991; Raposo and Ernesto, 590±6758C). 1995; Raposo, 1997). They were met by 90% of the E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210 207

Table 4 The sampling locations, types, widths, strike and dip directions of conesheets and sills from the Isle of Skye

Intrusion # Outcrop # Intrusion typea Core-sample # Strike Dip direction Dip angle Intrusion width AMS incl. Categoryb

Cones 1 122 C 210±213 60 330 35 24 10 K2 9I) 2 26 C 001±17 150 240 35 105 43 K1 9I) 4 26 C 27±32 10 280 35 57 17 K1 9II) 6 27 C 43±55 10 100 10 84 36 K1 9II) 7 27 C 56±63 10 280 70 45 30 K1 9II) 8 33 C 64±75 20 120 75 98 27 K1 9II) 933C 76±86 359630K2 9III) 10 33 C 87±98 75 165 40 100 38 K1 9I) 11 33 C 99±108 65 155 55 57 1 K2 9I) 13 3 C 134±148 230 140 50 76 25 K1 9I) 15 56 C 159±170 105 15 50 105 66 K1 9III) 17 122 C 193±200 35 305 45 24 18 K1 9I) 17A 122 C 201±209 45 315 40 33 20 K2 9I) 18 123 C 214±223 160 70 70 111 38 K1 9I) 21 62A C 246±254 135 225 35 61 38 K2 9I) 21A 62A C 260±269 55 145 25 95 10 K1 9I) 22 62A C 270±277 85 175 75 44 40 K1 9I) 23 62A C 280±289 55 145 45 75 44 K1 9II) 24 62A C 290±297 50 140 45 125 38 K1 9I) 25 121 C 303±313 10 100 45 96 29 K1 9I) 25A 62A C 298±302 85 175 75 400 41 K1 9I) 26 124 C 317±327 15 105 57 44 K1 9I) 27 124 C 329±339 145 55 60 57 13 K1 9I) 40 125 C 514±524 85 355 25 25 24 K2 9I) 41 125 C 525±533 60 330 30 47 26 K2 9I) 42 125 C 524±542 90 0 50 42 30 K1 9I) 44 126 C 571±576 50 K2 9I) 45 126 C 562±570 30 300 30 56 25 K1 9III) 47 126 C 577±580 105 15 70 19 33 K1 9II) 52 9 C 653±665 155 65 60 127 14 K1 9I) 56 9 C 718±727 130 40 80 212 17 K1 9III) 57 9 C 728±736 15 105 70 33 80 K1 9III) Cuillin Hills sills

5 26 S 33±42 0 73 17 K1 9II) 12A 3 S 120±133 0 62 33 K1 9I) 33 86 S 430±439 0 42 18 K1 9I)

a Intrusion types: C, conesheet; D, dike; S, sill. b Category represents the degree of resolution of imbrications: 9I) good; 9II) fair; 9III) poor. sampled intrusions and are interpreted to mean that, in Ernst and Baragar, 1992). It is based secondly on general, the fabric was generated by a magmatic ¯ow the analogy with many basaltic lava ¯ow units 9e.g. con®ned between top and base of a lava ¯ow or Canon-Tapia et al., 1994, 1995, 1996, 1997), in which between the walls of an intrusion. Another expecta- most of the Kmax is aligned in the downslope 9i.e. ¯ow) 0 tion was that the Kmax AMS axis should generally lie direction. Furthermore, in a a ¯ows containing vesi- in the intrusion plane. This is satis®ed by about 75% cles, the vesicles in general tend to have the form of a of the sampled intrusions. We infer that, in general, K1 triaxial ellipsoid, the orientation of which mimics the delineates the magmatic-¯ow direction. This is based AMS fabric 9Canon-Tapia et al., 1997). ®rst on previous research done on dikes elsewhere In a minority 9about 25%) of lavas and intrusions, the

9Knight and Walker, 1988; Rochette et al., 1991; Kmax axis is found to be normal to the intrusion plane, 208 E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210 while Kint usurps its position in the intrusion plane. It is which the AMS fabric demonstrably coincides with expected that Kint is then the ¯ow direction. In lava ¯ows the macroscopic vesicle fabric. this was attributed by Canon-Tapia et al. 91995) and Herrero-Bervera et al. 92001a) to the lava locally widening more rapidly than lengthening 9in response to chan- 5. Discussion ging topography). In the case of intrusions, we envisage Models for the mechanism of emplacement of Brit- that irregularities such as sidestep structures, which are ish Tertiary province intrusions have been long domi- particularly prevalent in the Cuillin Hills, locally nated by the belief that the prevalent control is the obstruct the free passage of magma and locally cause local or regional stress ®eld. The swarms of predomi- widening to exceed lengthening. nantly NW-trending dikes found throughout the province are thought to be an expression of this control. Alternatively we postulate that intruding 4.3. Double imbrication magma tends to move in the direction of maximum excess hydrostatic pressure. A feature of practically all intrusions is the scat- One results demonstrate that the inclined sheets and tered distribution ®eld, commonly over 208 of poles. dikes are very similar to one another in many charac- The scatter in Kmax is generally not random. Instead, teristics, have a similar distribution pattern 9preferen- the ®eld is bisected by the intrusion plane 9see Fig. tially injected into the main rift zone), and may be 2A). Examination then reveals that the samples from explained as alternative intrusion types. Our AMS one of the sub-®elds were collected from the upper results indicate that the injection of both the dikes side or top of the intrusion, while samples from the and conesheets took place by lateral ¯ow, i.e. horizon- other sub-®eld came from the left-hand side or tal to sub-horizontal ¯ow 9AMS inclination less than bottom. The subdivision into two ®elds was inter- or equal to 308; Figs. 4b and 5b). Another important preted to be caused by an imbrication 9Knight and ®nding is that the inferred ¯ow directions of the dikes, Walker, 1988; CanÄon-Tapia et al., 1997; Walker et conesheets and sills in the entire area studied do not al., 1999). Furthermore, the imbrication enables the show a de®nite ¯ow path as one might have expected ¯ow azimuth to be inferred according to which ®eld but rather a radiating trend outwards from the focus of corresponds with which side of the intrusion. the Cuillin Hills 9Fig. 4a and b). Flow paths are less Imbrication is not, in general, macroscopically visi- well de®ned in the regional swarms 9see Fig. 4a). ble on the Skye intrusions. We have, however, acquired con®dence in our interpretation from the close analogy that exists between the AMS foliation 6. Conclusions and the macroscopic fabric shown by many thin a0a lava ¯ows 9CanÄon-Tapia et al., 1995, 1996). These Using AMS we were able to determine the ¯ow lavas very commonly exhibit a foliation that curves azimuth for dikes and conesheets in the Cuillin Hills around the ¯ow front and sides 9and also commonly gabbroic complex, on the Isle of Skye. As observed in the ¯ow top) and asymptotes both inward and up-¯ow other dikes and sills the fabrics of Skye intrusives are against the ¯ow margins. The foliation is marked by more foliated than lineated. This complex formed planes along which the rock splits preferentially when approximately 60 million years ago during a period hammered, and appears to coincide with a parallelism of 1±3 million years, based on paleomagnetic polarity of microscopic feldspar crystals in the groundmass of 9Herrero-Bervera et al., 2001b). Therefore the the basalt. A foliation is also marked by a plane of magmatic activity must have been prolonged and ¯attening of macroscopic vesicles that is parallel to it. may well have occurred in several episodes. From A lineation also occurs in many lava ¯ow lobes and is the results obtained during our AMS investigations marked by the elongation of vesicles in the foliation we have found that it is possible to determine the plane parallel with the length of the lobe. CanÄon- direction of ¯ow as mentioned above, but so far we Tapia et al. 91995) documented two narrow lava have not obtained a fabric that indicates that the stress lobes on Xitle 9Mexico) and Mauna Kea 9Hawaii) in regime and magmatic supply varied strongly. E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 106 92001) 195±210 209 From the AMS data for the Isle of Skye, the follow- References ing conclusions can be drawn: 91) The anisotropy of low-®eld magnetic suscept- Bell, B.R., Harris, J.W., 1986. An Excursion Guide to the Geology ibility 9AMS) is carried by three different mineral of the Isle of Skye. The Geological Society of Glasgow, McCor- phases, namely, Ti-rich magnetite 9Curie tempera- quodale, Glasgow, 317 pp. Bell, B.R., Claydon, R.V., Rogers, G., 1994. The petrology and tures from 120 to 1408C), and iron sul®des such as geochemistry of cone-sheets from the Cuillin igneous complex, pyrrhotite 9FeS) with a characteristic Curie tempera- Isle of Skye: evidence for combined assimilation and fractional ture range of 320±4008C and a third mineral phase crystallization during lithospheric extension. J. Petrol. 35, with a Curie temperature of 580±6108C correspond- 1055±1094. ing to titanomaghemite. CanÄon-Tapia, E., Herrero-Bervera, E., Walker, G.P.L., 1994. Flow directions and paleomagnetic study of rocks from the Azufre 92) Three primary magnetic fabrics were recog- Volcano, Argentina. J. Geomag. Geoelectr. 46, 143±159. nized. Type A is the most common 960%) and it corre- CanÄon-Tapia, E., Walker, G.P.L., Herrero-Bervera, E., 1995. sponds to the ªnormal typeº of Rochette et al., 1991. It Magnetic fabric and ¯ow direction in basaltic pahoehoe lava is found in other dike swarms worldwide 9e.g. Knight of Xitle Volcano, Mexico. J. Volcanol. Geotherm. Res. 65, and Walker, 1988; Raposo and Ernesto, 1995; 249±264. CanÄon-Tapia, E., Walker, G.P.L., Herrero-Bervera, E., 1996. The Raposo, 1997). internal structure of lavas-insights from AMS measurements I: 93) Fabric Type A is associated with magma-¯ow near vent aa. J. Volcanol. Geotherm. Res. 70, 21±36. direction. It is characterized by a magnetic lineation CanÄon-Tapia, E., Walker, G.P.L., Herrero-Bervera, E., 1997. The internal structure of lavas-insights from AMS measurements II: 9Kmax) parallel to the foliation of the dike 9i.e. dike plane), whereas the K axis is approximately coin- Hawaiian pahoehoe, toothpaste and aa. J. Volcanol. Geotherm. min Res. 76, 19±46. cident with the pole of the plane foliation 9i.e. perpen- Cox, K.G., Gass, I.G., Mallick, D.I.J., 1970. The structural evolu- dicular to the dike plane). tion and volcanic history of the Aden and Little Aden volcanoes. 94) In the central area of the Cuilllin Hills, 50% of Q. J. Geol. Soc. London 124, 283±308. the dikes were fed by horizontal or sub-horizontal Ellwood, B.B., 1982. Estimates of ¯ow direction for calc-alkaline ¯ow. The same conclusion is true of the conesheets welded tuffs and paleomagnetic data reliability from anisotropy of magnetic susceptibility measurements; Central San Juan and sills within the gabbroic complex. Mountains, southwest Colorado. Earth Planet. Sci. Lett. 59, 95) In the regional dike swarm 70% of the dikes 303±314. were fed by horizontal or sub-horizontal ¯ow. Ernst, R.E., 1990. Magma ¯ow directions in two ma®c Proterozoic 96) In the Cuillin Hills and Blaven dike complex the dyke swarms of the Canadian shield, as estimated using aniso- magma-¯ow planes are on average steeper, suggesting tropy of magnetic susceptibility data. In: Parker, A.J., Rick- the proximity of a magma source in the gabbroic wood, P.C., Tucker, D.H. 9Eds.), Ma®c Dykes and Emplacement Mechanisms. Balkema, Rotterdam, pp. 231±235. complex of the Isle of Skye. 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