conference report - lyell collection

Jl geol. Soc. Lond. Vol. 135, 1978, pp. 459-464. Printed in Northern Ireland.

Conference Report Emplacement and crystallisation of minor intrusives

B. G. J. Upton

A joint Tectonic Studies Group and Volcanic Studies Group meeting was held at the Depar tment of Geol- ogy, University of Edinburgh on 20 May 1977, for the purpose of exploring the common ground between structural and petrological/volcanological interests in the subject of minor intrusives.

In the course of the eleven papers presented a wide range of ground was covered although, not surpris- ingly, the problems associated with the intrusion and congelation of basic magmatic sheets occupied the greater part of the meeting. By way of contrast, however, two of the papers presented were directed towards the deformation phenomena within and around granitic intrusions.

Basic sheet swarms were considered from a diversity of viewpoints ranging from petrochemical to structural. G. P. Durant highlighted the wide compositional range from transitional basalt to trachyte within the Islay and Jura dyke swarms and concentrated on their petrological relationships. T. J. Halsall demonstrated that, from a detailed study of the south Arran dyke swarm, it is possible to recognise a sequence of emplacement episodes within which the change of magma type is a reflection of changing stress patterns. M. K. Wells, speaking of the Centre 2 intrusions of Ard- namurchan, indicated how the processes of cone-sheet intrusion, gabbro emplacement and updoming were intimately inter-related processes associated with the lateral spread of magma from a central feeder system. In a detailed treatment of the emplacement and crystallisation of a single sheet intrusion (the Dippin sill, Arran), F. G. F. Gibb and C. M. B. Henderson discussed the role of crystal fractionation and flow differentiation operating on batches of alkali olivine basalt magma. Turning to the Tertiary dyke swarms of the east Greenland coast F. B. Davies considered the geometrical forms of termination and apophyses to the dykes and presented a revised model for their emplacement along the rifted east Greenland coastal zone.

In a paper concerned with tectono-magmatic phenomena along fractured continental margins, C. J. StiUman and A. H. F. Robertson reviewed the late Mesozoic and early Tertiary history of Fuerteventura, paying particular attention to the remarkable and in- tense swarm of alkali basic dykes and associated plutons exposed on the island.

Caledonian dyke swarms formed the subjects of papers presented by D. I. Smith and S. Lippard. Whereas D. I. Smith outlined the intrusive and tec-

tonic history of syn- and post-tectonic dykes of northern Scotland, S. Lippard discussed the late-tectonic tholeiitic sheets pre-dating the latest Caledonian phase of nappe emplacement in the west of Norway.

The oldest rocks discussed at the meeting were those of the west Greenland Archaean, studied by W. R. Fitches who called attention to the unusual potassic characteristics of some of the Archaean basic sheets, whose geochemistry suggested affinity to the shoshonites.

On the subject of granitic intrusives, M. T. Holder described the deformation of wall-rocks and xenoliths associated with the Flamanville and Ardara intrusions and concluded that the style of granitic intrusion was largely controlled by the competence of the country rocks. Wall-rock deformation in the vicinity of a granite intrusive also formed the topic of the paper by M. P. Coward and C. Longman. These authors demonstrated that the Beinn an Dubhaich granite (Skye) had a polyphase intrusive history and that there was over- lap between the period of granite emplacement and that of the Tertiary basic dykes in the vicinity.

Following the one day meeting in Edinburgh, some twenty participants took part in a weekend field- meeting on Arran, led by T. J. HalsaU. This excursion, which was concerned with the Tertiary minor intrusives, was blessed by excellent weather.

B. G. J. Urn-oN, Grant Institute of Geology, West Mains Road, Edinburgh EH9 3JW.

Granite emplacement models M.T. Holder

Field work on the meso-zonal, post-tectonic plutons of Flamanville in Normandy and Ardara in Donegal, has shown that xenoliths within them are commonly ellipsoidal and orientated subparallel to a foliation in the granite concentric to its margins. The foliation decreases in intensity away from the pluton contacts. Xenolith shape also reflects this decrease in deformation and thus appears to be closely linked to the process forming the foliations. Using the xenolith shapes as strain markers, finite strain results derived from them have been corrected for non-statistically viable sampling, forced on the study by lack of xenoliths in some areas of the plutons. The strain pattern exhibited within the plutons and the geometric model formulated by Ramsay (1975) for the Chinamora Batholith have been used; the intrusion of the plutons is found to be related to a mechanism of spheroidal distension. Moreover, the entire history of the growth of the Ardara pluton shows that the 'ballooning' of the pluton has been caused by successive discrete pulses of magma, of

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460 B. G . J . Upton

differing composition, intruding into each other and creating a composite pluton.

Examination of the structures in the country rocks around the edges of both Flamanville and Ardara has confirmed that the bulk of the volume now occupied by the granites has been created by shouldering the country rocks aside and tightening pre-existing regional structures in the immediate vicinity of the plutons.

An investigation of the geographic distribution of the composition of xenoliths at Ardara has shown that each successive pulse of magma contains its own distinctive compositions of xenoliths. The origin of these xenoliths has been determined by an examination of the contacts of the pluton and shown to be migmatitic, in the case of semi-pelitic xenoliths, and due to stoping in the case of quartzitic and basic xenoliths. The discovery of stoping in an otherwise ductile intrusion led to the conclusion that the intrusion mechanism has been controlled by the competence of the country rocks in association with a magma pressure in excess of the lithostatic pressure. This intrusion process could thus explain why the Rosses pluton in Donegal intrudes through the older Thorr granite by stoping, whereas the neighbouring Ardara pluton, of similar age, intrudes the Dalradian pelites and semi-pelites predominantly by 'ballooning'.

Some Caledonian minor intrusions of the Northern D. I. Smith

The Moine rocks north of the Great Glen Fault are cut by several swarms of minor intrusions of 'Caledonian' age. One suite, ranging in composition from acid porphyrite through microdiorite to appinitic types is particularly well represented in the southern part of the region where relationships with other intrusions have been firmly established. They are younger than the garnetiferous amphibolites (meta-dolerites), and are cut by the post-tectonic felsites, porphyrites and lamprophyres (mainly minettes) of the Ratagain Complex dyke swarm. Cross-cutting examples in relation to recently isotopically dated pegmatites suggested a time of intrusion at around 445 Ma. An age-dating programme on selected members of the suite is currently in progress.

Microdiorites (previously classed as 'lamprophyres' and 'lamproschists') ranlzing in thickness from about 2 m down to a few millimeters, are by far the most abundant members of the swarm. A particular study has been made of this group in the district between Loch Sunart and Loch Cluanie. The intrusions are widely distributed, over 3000 having been mapped in an area of 3500 km 2. Local concentrations in discrete domains have been found where densities of up to 15 intrusions per km 2 are attained. Although variable in orientation their most common occurrence is as flat-lying sheets with an inclination of 25-30 ° towards ESE. Interest in this microdiorite suite is centred upon the fact that a majority of them exhibit a variety of fabrics and mineral assemblages which indicate that they have been affected by a late Caledo- nian phase of deformation and metamorphism. Undeformed intrusions with original igneous textures composed of hornblende, biotite and zoned oligoclase, are almost entirely restricted to a coastal belt west of a line through Salen, Lochailort and Kinlochourn. Recrystallised mineral assemblages in deformed sheets to the west of this boundary are typically of greenschist facies, i.e. albite-actinolite-chlorite- epidote, while to the east amphibolite-facies assemblages of hornblende-biotite-oligoclase predominate.

Fabrics within intrusions are of two main types. The most common is an oblique sigmoidal schistosity containing a

down-dip extension lineation signifying the maximum principal strain. The geometry of the intrusive sheets together with their internal fabrics and structures are compatible with an E-W directed horizontal maximum stress. Folded inclusions, and local crenulations of the sclaistosity which appear to be related to marginal irregularities are consistent with this interpretation. Displacements of markers in the country rocks adjacent to schistose sheets are generally greater than can be accounted for by the angular difference between the fabric and the sheet margin suggesting that brittle thrusting took place along the upper and lower surfaces of some of the intrusions. A second type of fabric consists of a subvertical axial plane schistosity striking approximately N-S, usually composed of aligned mica flakes. This is developed in slightly folded subhorizontal sheets in the western part of the region. Neither the sigmoidal nor the axial plane fabrics have any visible counterparts in the country rocks into which the sheets are intruded.

In one restricted area, between Loch Arkaig and Loch Garry minor intrusions of the microdiorite suite with variable orientations show crenulated margins and a consistent steep NW-SE cleavage which is parallel to the axial planes of late minor folds in the Moinian metasediments of that region. Emplacement of these intrusions may either have occurred before those farther west or synchronously under more ductile conditions.

Archaean h ' ~ - K b a ~ sheetB in W G m e n l a ~ W. R. Fitches

The intrusions are chemically similar to more recent 'within-plate' basalts, shoshonitic magmas and perhaps lamprophyres. Such evolved basic magmas contrast strongly with the low-K basaltic and tonalitic rocks typical of Archaean terrains. They are, so far, unique to one part of the W Greenland craton and appear to represent the oldest known high-K basic rocks.

The intrusions crop out in the northern Fiskenaesset region which lies SE of the Godthabsfjord region described by Bridgwater et al. (1974) among others. The sheets cut supra- crustal rock which closely resemble the Malene Supracrustals of Godthabsfjord and tonalitic-granodioritic gneisses whose relations to the Amitsoq (3.7 b.y. +) and Nuk (2.8-2.9b.y.) gneisses of Godthabsfjord are unknown. They were metamorphosed in the amphibolite facies and deformed probably during the widespread 2.8 b.y. event. The intrusions are therefore considered older than 2.8 b.y. but their maximum age is not established.

Two major sheets are known, the larger of which is 6 km in length and 300m across, and each is fringed by parallel swarms of thin sheets. They have the composition of biotite diorite/gabbro or pyroxene kersantite and compz;ise biotite, clinopyroxene, plagioclase, ilmenite and apatite, locally with accessory microcline. Metamorphism has caused extensive replacement of pyroxene by amphibole and quartz, and ilmenite by sphene.

They have been analysed for major, minor, trace and RE elements. For interpretation, emphasis has been placed on elements usually regarded as stable during alteration, although there are reasons to suspect that metasomatism has not been significant. The closest chemical analogues are the 'within-plate' basalts according to Nb/SiO2, Ti/Y vs Zr/Y and Ti-Zr-Y ratios. The analogy with shoshonites is also strong; major element chemistries are very similar, for example, as are their Sr/Zr and KzO/SiO z ratios. REE data show a very strong light element enrichment, with Ce/Yb ratios exceeding



Emplacement and crystallisation o[ minor intrusives 461

20. The REE sample/chondrite pattern is concave and there is a very small positive Eu anomaly.

The more modem, chemically similar basic rocks can be placed with some confidence into plate tectonic models. Not enough is known about the W Greenland craton, however, to interpret the Archaean intrusions in these terms. The more m o d e m analogues are usually explained as having originated below a very thick and stable continental crust. On ascent they acquire their high alkali and other characteristics by repeated or continuous fractionation, mixing and by scaveng- ing crustal material. It is considered that the Archaean intrusions underwent a similar history. They provide magmatic evidence of continental crustal thicknesses of mo d em dimen- sions in W Greenland by 2.8 b.y. at the latest.

The dyke swarm of the Fuerteventura Basal Complex, Ca- nary Islands C . J . Stillman & A. H. F. Robertson

The Fuerteventura Basal Complex dyke swarm is emplaced in a thick sequence of terrigenous, calcareous and volcanic sediments ranging in age from Lower Cretaceous to mid- Tertiary. Early deep-water flysch sedimentation at the base of the West African continental slope was succeeded by Albian uplift and a transition to calcareous pelagic sedimentation, then the onset of basaltic submarine volcanism. After a phase of compression resulting in substantial overturning of the sediments, re-establishment of crustal tension in mid- Tertiary times was linked with the emplacement of a NNE- SSW trending swarm of ankaramitic, basaltic and trachytic dykes.

The dykes occur in families of up to 100 members, often sheeted with one-sided chills, containing no screens of country rock. Each of these families presumably was injected through a single primary fissure and demonstrate a crustal extension of several hundred metres. The observed aggregate extension exceeds 12 km and from geophysical evidence of the lateral extent of the dyke swarm, at least 30 km of extension seems probable. In zones between fissures where dyke intensity is less than 100 per cent, individual dykes show dilational emplacement with little or no buckling of the host rock.

Within the dyke complex are emplaced large high-level basic and ultrabasic plutons; the dyke intensity shows a marked increase approaching each pluton and an abrupt decrease within the pluton. Where later plutons have intruded earlier ones, the pattern of increase in intensity towards the contact is repeated. Contact metamorphic aureoles around the plutons demonstrate the age relations of successive phases of plutons and dykes. Chemistry and mineralogy indicate a consanguinity of plutons and dykes, both being derived from a mildly alkaline basalt magma, and a causative mechanism relating the intrusion of both dykes and plutons within a dominantly extensional stress regime can be deduced. The emplacement involved metamorphism to albite- epid'ote greenschist facies in a water-rich, presumably sub- ocean floor, environment.

Towards middle Miocene times relaxation of this regional stress regime reduced the number of dykes and permitted the emplacement of alkaline, salic ring complexes, followed by uplift, subaerial volcanism and erosion. The erosional period lasted for some 8-10 million years and planed down the tilted basal complex to truncate the dyke swarm, expose the plutons and permit the overstep of Plio-Pleistocene island- capping volcanics onto the deeper levels.

Geophysical and geological data on the regional setting suggest that both the magma generation at depth and its rise

and emplacement at high levels in the Atlantic crust may be related to processes controlled by deep rift structures, in- itiated on the margin of the African continental plate, then possibly reactivated related to successive tectonic pulses in the High Atlas.

Tectonics of Tertiary dykes of east Greenland F .B. Davies The east Greenland dyke swarm has long been interpreted

as intruded radially under tension during the break-up of the north Atlantic (Wager & Deer 1938). The present work records observations on Tertiary dykes of the main dolerite swarm and excludes syenitic and lamprophyric dykes occurring irregularly along the coast.

Two contrasting zonal distributions occur along the coast from Angmagssalik to Kangerdlugssuaq. North from Angmagssalik to Kangerdlugssuatsiaq, the Tertiary dykes are nearly vertical and occur in two directional sets; the com- monest s t r ik ing- 040 ° and the other striking - 090 °, although dykes of both sets often abruptly change orientation to that of the opposite set. Dykes of both directional sets do not obviously vary in distribution between inner fjords and outer coast. Marginal irregularities and offsets indicate that the dykes have dilated with oblique shear movements in the horizontal plane. Dykes of the 040 ° set consistently indicate dextral shearing, and those of the 090 ° set consistently indicate sinistral shearing, which demonstrates that dykes within this overall regional distribution were emplaced into a conjugate system of shear fractures with 0-2 vertical, 0" 1 horizontal and striking 065 ° and 0-3 horizontal and striking 155 ° .

The other zone lies eastward of the former and is confined along the outer coast north of Kangerdlugssuatsiaq and is well known from the work of Wager & Deer (1938). Several zones of dykes striking 050o-060 ° occur north along the coast to Kangerdlugssuaq, where their distribution is confused by, among other features, the Skaergaard intrusion. These dykes dip from 300-60 ° NW and are often cut by dykes dipping 600-900SE. This variation is systematic in the sense that dykes dipping around 30°NW are cut by near-vertical dykes whereas dykes dipping around 600NW are cut by dykes dipping about 600SE. Marginal irregularities of the dykes dipping towards the continent indicate a consistent normal sense of movement with the NW side down, whilst those of dykes dipping toward the ocean indicate a consistent normal shear with the SE side down. These dykes indicate a conjugate shear fracture system during dyke emplacement of the typical rift type (0" 2 horizontal, 0"3 vertical, 0"~ horizontal) particularly where both continent-ward and oceanward dipping dykes have dips of 600NW and SE respectively. Devia- tions of the dyke orientation away from this ideal are not accompanied by variations of the dihedral angle (about 60 ° ) between the continent-ward and oceanward dipping dykes, and since the changes in bulk orientation occur along strike of the swarms, most reasonably suggest that tilting of the entire dyke distribution has occurred towards the ocean. These two contrasting bulk distributions of contemporaneous shear fracture systems infilled by Tertiary dykes, where an inner zone of zig-zag transcurrent shear fractures is bounded by an outer zone of normal fault fractures, show a pattern well known from classic rift systems (e.g. Afar) and demonstrate that the present exposure of the east Greenland coast is a partial section across a coast formed by rift tectonics.

Petrology of tim I)i l~in Sin, Isle of Arran F . G . F . Gibb & C. M. B. Henderson

The Dippin sill of southeast Arran is a basic alkaline intrusion of Tertiary age intruding Triassic sediments. The



462 B. G.J . Upton

contacts and margins of the sill are poorly exposed and two holes were drilled through the sill to augment the vertical sections collected from cliff faces. The sill consists of a thick central unit of crinanite bounded by thinner marginal units of teschenite with minor units of augite teschenite and pegma- tite occurring at various levels within the intrusion. The crinanite unit forms the bulk of the sill and contains a central zone approximately 2 0 m thick in which phenocrysts of plagioclase and dusters of olivine crystals are concentrated.

Three distinct chemical trends are defined respectively by the crinanites, the marginal teschenites and the augite teschenites plus pegmatites. The crinanite trend can be subdivided into three:

(i) a relatively basic part in which the chemical variation is controlled by the accumulation of olivine and plagioclase crystals;

(ii) a part in which the variation is due to the fractionation of olivine, plagioclase and clinopyroxene; and

(iii) a part in which iron oxide appears to have fractionated in addition to olivine, plagioclase and clinopyroxene.

The trend of chemical variation exhibited by the marginal teschenites indicates that they are depleted in clinopyroxene relative to the crinanites: this trend can be ascribed to the fractional crystallisation of clinopyroxene together with a systematic degree of contamination from the country rocks. The third trend, that defined by the augite teschenites and pegmatites, is essentially due to the fractionation of clinopyroxene and iron oxide.

It is proposed that the distinctive chemical trends were formed during a complex continuous differentiation and emplacement process which began with the intrusion of a mantle- derived alkaline basic magma into a deep crustal reservoir. Crystallisation and gravitative accumulation of olivine and calcic plagioclase resulted in the magma within this reservoir becoming compositionally stratified. Subsequently, relatively fractionated magma was expelled from the upper parts of the reservoir, and was intruded less than 3 km below the contem- porary surface to form the marginal facies of the sill. There then followed a much larger injection of less-fractionated olivine-bearing magma which gave rise to the thick central part of the sill. Flow differentiation during the uprise and emplacement of these magmas followed by substantial post- emplacement fractional crystallisation resulted in the evolution of the three distinct chemical trends.

One of the last minerals to form in the sill was the interstitial analcime which has long been regarded by petrologists as primary (i.e. magmatic). However, recent experimental evidence suggests that it should be unstable above solidus temperatures for basic magmas at such low pressures. Experi- ments on a sample of Dippin crinanite indicate that the analcime was formed by subsolidus replacement of primary nepheline (of which traces have been found in the rocks) between 380 ° and 330°C.

The emplacement of the Tertiary dykes of the KBdonan shore, south Arran T . J . Halsall

The south Arran shore between Bennan Head and Dippin displays a section of the Arran dyke swarm in which minor intrusions are so abundant as to enable the erection of a detailed relative age sequence. Over two hundred dykes have been recorded; many intersect extensive sills at Bennan (quartz porphyry), Kildonan Castle (quartz dolerite- craignurite) and Dippin (crinanite-teschenite) which are used as time markers.

Rock types present as dykes are: olivine dolerite (mildly

alkaline/transitional), alkali olivine dolerite (crinanite and teschenite), tholeiitic olivine dolerite, tholeiitic dolerite and quartz dolerite. Olivine dolerite and tholeiitic olivine dolerite dykes are most numerous.

Analysis of dyke orientation patterns shows that the NW- SE trend is dominant with N-S dykes almost as abundant; there are also subsidiary NE-SW dyke sets. Ten episodes of emplacement have so far been recognised by a combination of dyke orientation, petrography, petrochemistry and age relationships.

The Dippin Sill was injected early in the sequence, probably the first event (episode 1). Alkali olivine dolerite dykes trend NW-SE and constitute a single episode (3) which appears to be somewhat later than the crinanite sills; however, the possibility that they represent feeders for the latter is not ruled out.

Olivine dolerite dykes occur at various stages in the sequence. A particularly prominent set is early (episode 2); unusu- ally marked thermal metamorphism associated with it suggests it may represent the feeder system for the Arran lavas, a view with which the petrochemistry of the rocks is entirely consistent.

Tholeiitic olivine dolerite and tholeiitic dolerite dykes trend NW-SE, N-S and NE-SW, and were emplaced at various stages during the development of the swarm; a prominent NW-SE set represents the final event in the sequence (episode 10). Injection of NW-SE quartz dolerite dykes may have been associated with emplacement of the quartz dolerite sills (episode 8). An earlier N-S set of quartz dolerite dykes (episode 4) is also present.

Dyke orientation on Arran reflects the interplay of the regional stress pattern, local stresses arising from activity within the centre and pre-existing joint i~atterns in the country rocks. It is believed that by relating changes in the stress pattern to changes in magma type through the study of dykes, it will be possible to interpret the development of the Arran centre and its relationship to the development of the Tertiary province as a whole.

A contn'bution to the study o! cone-sheets and related inlru- sions o! Centre 2, Ardnamurcimn, Scotland M . K . Wells

Cone-sheets and related minor intrusions are abundant around Centre 2. They comprise an early outer set separated in time and place from a later inner set by emplacement of the outermost of the Centre 2 major intrusions, the Hypersthene-gabbro.

Outer cone-sheets dip inwards at about 30 ° and intrude Mesozoic strata and Tertiary lavas that have been tilted and dip outwards from Centre 2 at about 30 °, forming a concor- dant roof to the gabbro around its western outer margin. Though the Hypersthene-gabbro is olivine-bearing, it has a marginal facies of acid veined quartz-dolerite whose composition is similar to that of the cone-sheets (Wells 1954). These facts suggest that the doming, intrusion of cone-sheets and emplacement of the gabbro mass were inter-related processes, overlapping in time. Doming was initially ascribed by Wells to forceful intrusion of basic magma. It is more likely, however, that most of the doming and initiation of cone- sheet fractures was the work of volatile-rich acid magma, generated above and moving ahead of more basic material. Dips of the cone-sheets are believed to have been reduced by the doming.

Evidence is available from widely distributed xenoliths of igneous and sedimentary origin, from internal intrusive contacts between varied facies, and from characteristics of layer-



Emplacement and crystallisation of minor intrusives 463

ing, to show that the intrusion of the Hypersthene-gabbro grew over a long period by intermittent injection of crystalliz- ing magma which spread laterally from a centralized feeding system. No evidence is apparent of a ring-dyke structure. The funnel-shaped layered structures are regarded as largely original, developed parallel to the floor of the intrusion, but deformed and steepened by gradual subsidence of inner parts of the mass (cf. Skelhorn & Elwell 1971). Petrographic and palaeomagnetic studies (Wells & McRae 1969) show that deformation occurred while the extensively recrystalltzeo rocks were hot. Further injections of basic magma into the Hypersthene-gabbro mass at this stage produced inclined sheets possessing diffuse margins and somewhat granulitic textures due to baking by the host rock. Mineral growth in the gabbro adjoining these intrusive sheets was completed after their emplacement.

A sparse set of inner cone sheets, possessing sharply defined chilled margins, intruded the gabbro after it cooled. Their dips, like those of the granulitized sheets, are often sensibly parallel to layering, and may well have been deter mined by strictly local factors of anisotropy of the host rocks.

Late Caledonian minor intrnsives, Stord, W Norway S. Lippard

On the island of Stord, within the Caledonian allochthon of W Norway a localised basic sheet and dyke swarm is emplaced into Upper Ordovician bagaltic-andesite and rhyolite volcanics on the inverted limb of a large overfold. The intrusions post-date the development of the main FI foliation and greenschist-facies metamorphism of the country rocks. The minor intrusions comprise more than 100 individual bodies and are of two types: earlier sheets of medium- to coarse-grained dolerite emplaced mainly into the rhyolites along pre-existing structures, and later N-S and NW-SE trending dyke sets. The rocks show extensive alteration effects, but igneous textures and part of the primary mineralogy are preserved.

The nature of the minor intrusions indicates emplacement under high-level, tensional conditions late in the sequence of Caledonian events in W Norway, but probably before the final phase of nappe emplacement, as the intrusions are apparently confined to a single tectonic unit. Preliminary K/Ar whole rock age determinations carried out by J. G. Mitchell give a wide range of ages between 610 Ma and 420 Ma. A negative correlation between K and K/Ar age indicates loss of alkalies during alteration.

Geochemically the intrusions are tholeiitic. The sheets are dolerites and ferrodolerites with high Fe/Mg ratios and high P and Ti contents. The dykes are more basic (MgO, 6-11 per cent) and have lower Fe, Ti and P. On the Ti-Y-Zr plot of Pearce & Cann they plot in the field of within-plate basalts.

The geochemistry of the Tertiary lslay and Jura dyke swarms G . P . Durant

A geochemical study of the dykes from the Islay and Jura swarms reveals that they are alkaline transitional rocks characterised by low concentrations of TiO 2, KxO, P205 and trace elements, the values obtained being more typical of oceanic tholeiites than alkali olivine basalts. Most of the analysed rocks are nepheline normative although others contain both normative olivine and hypersthene.

A differentiated sequence from alkali picrodolerite to hawaiite was observed within the dyke swarm, extreme differentiates in the form of syenites only occurring in the northwest to southeast elongated boss at Cnoc Rhaonastil

which is believed to lie on the axis of one of four component sub-swarms defined by earlier structural studies. The geochemical data confirms the existence of the four sub- swarms, as slight but significant differences in chemistry were observed.

Some of the picrodolerites contain cumulus phenocrysts and the high magnesium content of such rocks may have been caused in part by olivine accumulation. However, other high magnesian (15-17 per cent) rocks appear to represent primitive magma compositions.

The differentiated sequence has been produced by low pressure fractionation initially controlled by olivine extrac- tion. The analyses cluster around the low pressure triple- point junction between the forsterite-anorthite-spinel fields in the experimentally determined system, the use of which is justified by the close correspondence of phenocryst phases with the experimental fields. Some of the analyses plot in the spinel field and the early crystallization of magnetite may have thus been expected leading to a depletion in iron. This is contrary to the observed trend to iron enrichment and could infer that low pO E conditions suppressed the early crystallisation of magnetite.

As the magma composition changed throughout the sequence the normative compositions of olivine and anorthite change from Foao to Fo,6 and from An73 to An32.

Several lines of evidence suggest that unlike the swarms of Skye, Mull and Arran, the Islay and Jura dyke swarms exist independently of a plutonic focal region and are not related to the submarine Blackstones igneous centre. This e:gidence includes field evidence, the number of dykes increasing mar- kedly towards the northwest coast of Islay indicating the proximity of a focal region, geophysical evidence, and K-Ar age data, the dykes yielding Tertiary ages (? c. 40 Ma) whilst a second dated sample from the Blackstones pluton yields an Upper Cretaceous age (c. 83 Ma).

Detormation around the Belnn an Dubhaich granite, Skye M . P . Coward & C. Longrnan

Dlb .

The Beinn an Dubhaich granite is a Tertiary epigranite situated in west central Strath. The intrusion is surrounded by Durness Limestone (Cambro-Ordovician), intruded by basaltic dykes, presumably of Tertiary age.

The following sequence of tectonic events is proposed: Dla. Large scale folding of Dumess Limestone and over-

thrust Torridonian sandstone of the Kishorn nappe to produce the Ben Surdal antiform.

Isoclinal folding of the limestones and development of a pronounced cleavage forming a concentric pattern around a centre situated south of Torrin beneath the western end of the granite. There was syn- to post- tectonic high temperature metamorphism associated with this phase.

D2. Intrusion of Tertiary basaltic sykes associated with a stress field apparently affected by a less rigid zone again situated south of Torrin beneath the west end of the granite.

D3. Deformation of many of the dyl~es during or before consolidation with the formation of folds and boudins. The dykes have chilled margins, even in the necked region, indicating that boudinage took place while the dykes were cooling. From the shape and orientation of the surface of no finite longitudinal strain deduced from stereographic plots of poles to boudinaged and



464 B . G . J . Upton

folded dykes, the D3 principal strains were co-axial with those which produced the earlier D l b isoclinal folds in the limestones.

D4. Passive sheet-like intrusion of the granite, truncating the dykes and the zones of high grade metamorphism.

It seems therefore, that the intrusion of the Beinn an Dubhaich granite went through several phases beginning with a forceful phase associated with localised high temperature metamorphism and deformation of the limestones. This was

followed by stress relaxation and intrusion of basaltic dykes associated with the main igneous centres of Skye. Towards the end of this phase of dyke injection there was a second phase of deformation confined to the centre south of Torrin, deforming the dykes co-axially with the earlier deformation of the limestones. This was followed by passive intrusion of the present granite which appears to have played no part in the structural evolution of Skye but hides the main centre south of Torrin about which most deformation took place.

References

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