The Lewisian of Lochinver, Sutherland; the Type Area for the Inverian Metamorphism

The Lewisian of Lochinver, Sutherland; the type area for the Inverian metamorphism

CALVIN R. EVANS & RICHARD ST J. LAMBERT

CONTENTS t Petrology and chemistry of the Lewisian of Lochinver ~8 (A) Gneisses . i28 (B) The transition from granulite to amphibolite facies ~29 (c) Pegmatites x3o (D) Mafic and ultramafic dykes x33 2 Structural relationships x33 (A) Badcallian structures . ~34 (S) Inverian structures ~35 (c) Correlation of Inverian folding and metamorphism x38 (D) Intrusion of NW dykes ~38 (~) Laxfordian deformation ~38 3 Geochronology and isotope geology x38 (A) Age of formation of the potash pegmatites ~38 (B) Geochronology of the northern sector of the Lewisian I4I 4 Implications of the Inverian metamorphism and correlations with other shield areas I43 5 References x46

SUMMARY The Lochinver district contains gneisses, and structural features of each sector of the pegmatites and dykes generated in five main complex are described and new whole rock episodes: (I) pre-26oom.y, garnet pyroxene Rb-Sr ages given for the pegmatite suite, granulite facies gneisses with low angle dips, suggesting a maximum age of 254 o m.y. for the formed in their final state by the Badcallian suite (plus intrusion of a further suite at 23io metamorphism; (2) potash-rich pegmatites of m.y. or recrystallisation of the first suite at age range 254o to 23io m.y.; (3) 23 to-22oo 23io m.y.). Other pegmatite ages confirm the m.y. amphibolite facies gneisses occurring in presence ofpre-2ooo m.y. activity NE of Scourie vertical WNW zones, produced by a broadly and at Gruinard. Field and petrographic isochemical metamorphic event; (4) vertical evidence at Lochinver demonstrates the ~w-trending dykes emplaced at 22oo m.y. and existence of amphibolite facies metamorphism perhaps down to 19oo m.y. and (5) epidote- in post pegmatite pre-dyke times, which amphibolite facies gneisses produced in narrow, produced well-defned rock types and w~ew discontinuous Nw shear zones during the suc- vertical structures, and is defined as the ceeding Laxfordian events. The petrographic Inverian metamorphic event.

T H F L ~ wI s I AN has been divided by Peach et al. (I 9 o7) and especially by Sutton & Watson (i95I , i962 ) into a pre-dyke (granulite facies) complex 'Scourian', and a post dyke (amphibolite facies) complex 'Laxfordian'. Detailed work in the Scourie area by Watson produced evidence of pre-dyke pegmatite intrusion and amphibolite facies metamorphism, both of which were ascribed to the closing phase of Scourian metamorphism. Partly on the basis of dyke margins chilled against high- grade gneiss, Sutton and Watson postulated a large time-span between the early

Jl geol. Soc. Lond. vol. x3o , i974, pp. x25-I5o , 7 figs. Printed in Northern Ireland. 3

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Scourian metamorphism and the later Laxfordian metamorphism. This was later confirmed by Giletti et al. (196 I) who concluded that the Scourian was older than 2540 m.y. and that the Laxfordian was about 165o m.y. old. (All Rb-Sr ages are calculated using a half-life for STRb of 4.85 × lO l° y.) Dearnley (1962 , 1963) has since recognized two post dyke metamorphic events in the Outer Hebrides which he termed 'early Laxfordian' and 'late Laxfordian'. Studies of the NW dykes by O'Hara (1961 , 1962) and Tarney (1963) led to the suggestion that they were emplaced into hot country rock. The recognition of the Inverian metamorphism at Lochinver (Evans 1963, 1965) has caused an evolution of thought about the history of the mainland Lewisian. Bowes (1969) has evolved the view that the Lewisian was developed in three orogenic episodes, Scourian, Inverian and Laxfordian, dated at about 2600 + to 2460 m.y. ago, 2200 to ?~ooo m.y. ago and 16oo to 13oo m.y. ago respectively. Park (i97o) argues against this view, defining the term Badcallian for the metamorphism which produced the garnet pyroxene granulites and referring the Inverian metamorphism to pre-22oo m.y. time as a major event. Park emphasises the unity of the pre-Nw dyke Inverian event with the dykes themselves and with the Laxfordian and considers that many of the events attributed to the Laxfordian may be Inverian. Evans (1963) and Holland (1966) have both suggested that the Inverian metamorphism affected large areas of the Lewisian. Beach et al. (I973) have described the structure of the area sw of Laxford Bridge and have shown that a set of folds affected the Foindle and Claisfearn zones of Sutton & Watson (195 I) after the pyroxene granulite facies metamorphism and before the intrusion of dykes. They decline to correlate these folds with the Inverian episode, citing a 2540 m.y. pegmatite (sample 7 this paper; sample I, Giletti et al. I96I ) which they state cuts amphibolized gneiss. However, this pegmatite cuts typical nearly fiat- lying felsic pyroxene granulite, partially retrogressed to hornblende gneiss, with green hornblende pseudomorphing primary pyroxene; it is not in a characteristic vertical shear zone of Inverian type. Consequently we do not regard this pegmatite as evidence of pre-254o m.y. amphibolization and re-affirm the view that the transition zones NF. of Scourie show all the characteristics of an Inverian structure, including structural, petrographic and chemical considerations (for the latter see Holland & Lambert 1973). The Laxfordian metamorphism is now known to have reached its maximum at or before 185o m.y. (Lambert & Holland I972 ) and is polyphase in character (Dash 1969, Beach et al. i973). If the dyke swarm was emplaced in hot country rock closely following on the Inverian metamorphism, then the possibility that the Inverian and Laxfordian events are effectively continuous in the northern Lewisian must be considered seriously. In general, the present authors accord with the views of Park (i97o) on the evolution of the northern sector of the Lewisian. The growing implications of the Inverian event require that the facts concerning the type area be recorded. The area studied (Fig. 1) lies in the central belt of the Lewisian. Previous publications (Evans & Tarney I964; Evans I965; Lambert & Holland i972 ; Pidgeon & Bowes I972) have established the following chronology, which will be assumed throughout:

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i I 1 I~ Southern & Northern ~ Ultramafic gneiss pyroxene zones Central zone of amphibolite xxx Agmatite Ultramafic &intermediate -'--Faults dykes' ---- Shear Belt Mafic dykes -~ Lochr~ter Antiform r~Mafic pyroxene gneiss .J.,-:.,~,"Gentle, Steep l I~"~Mafic hornblende gneiss ~ Vertical Dip

24-

~o. ~ 60 5O

g 30 L MAP 2o

lO N 0 10 20 30 40 50 NW.SCOTLAND I (xx INVER GROUP) 2O

PRECAMBRIAN GEOLOGY 19. OF THE LOCHINVER DISTRICT 18- < NATIONAL GRID (Kin) > O6 07 08 09 10 11 12 13 14 I I I I I I I I I FIo. I. Precambriangeology of the Lochinverdistrict.

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About 275 ° m.y.--granulite facies metamorphic event, 254o-231 o m.y.--potash pegmatites, c. 22oo m.y.--amphibolite facies metamorphism, 2 I9o (-I9OO ?) m.y.--intrusion of marie dykes, c. I85o m.y.hend of major recrystallization at Laxford Bridge, c. t 65o m.y.--end of pegmatite formation at Laxford Bridge, I4OO m.y.hend of Rb-Sr recrystallization of minerals in the north.

I. The Petrology of the Lewisian of Lochinver

A) GNEISSES The petrology of the garnet pyroxene granulites, found only in the southern and extreme northern parts of the Lochinver area, is similar to that of the granulites of Scourie. Descriptions have been given by Peach et al. (I9O7) , Sutton & Watson (I95I), O'Hara (I96I), Bowes et al. (I964) and Muecke (I969) and so only a few comments are made. The ultramafic rocks of this zone are olivine bronzite or hypersthene diopside edenitic hornblende spinel magnetite rocks, characterized by anhedral minerals with an interlocking, bulbous individual shape. Irregular banding and the metamorphic texture (Evans i963, Vernon I97o ) do not lend support to the magmatic crystal accumulation theory of Bowes et al. (1964). Marie gneisses occur with or without garnet: the garnetiferous gneisses are of widespread distribution, but one band occurring immediately below the principal ultramafic mass (o9721o) may be genetically connected with it. These marie gneisses contain augite and hypersthene granules with strained, well-twinned andesine (or, rarely, plagioclase near An60), magnetite, ilmenite and garnet. Primary brown hornblende or quartz are characteristic. Analyses of minerals from these rocks (Evans I963 and Muecke pers. comm.) show that the garnets are GrlsSp2AlmsoPyr33; the brown hornblendes are rich in Ti, A1iv and Na + K; and the augites and hypersthenes are highly aluminous (Al~O3 in cpx up to 6. 7 wt.%, Al~O3 in augite/A1203 in hypersthene '~ 1.5, AllY/ A1va averaging 2). These hornblende and garnet analyses are similar to those from the Adirondacks, summarised by de Waard (I965) , but the pyroxenes are much more aluminous. The dominant type of gneiss is felsic, essentially a quartz diorite or tonalite in broad composition (Holland & Lambert I973). It is usually amphibolized, but fresh felsic gneiss occurs south of the Kirkaig River. Fresh leucocratic varieties contain andesine (near An3s) unzoned, antiperthitic and highly strained. Quartz is opalescent blue and highly strained; the ferromagnesian constituents are hypersthene and augite, usually showing some replacement by hornblende. Magnetite and ilmenite are usually fringed with biotite. In the central zone the gneisses are generally similar to the 'non-charnockitic Scourian gneiss' of Sutton & Watson (I95I). Marie hornblende gneisses are widespread and occur in bands up to tens of metres thick; they may contain small euhedral garnets of a new generation. Badcallian garnets are only seen at the margin of the central zone, where they occur as fractured remnants, sometimes with kelyphitic rims. Plagioclase has recrystallized, losing both strain and the

abundant granulite facies twin lamellae. Hornblende occurs in three principal varieties: large prisms containing abundant dusty oxide inclusions often concen- trated near the centres of the crystals, which are believed to represent former granulite facies hornblendes; prisms with quartz-sieved centres, or aggregates of small grains with identical orientation, the latter two types being of a new generation. These hornblendes coexist with abundant quartz and, in some localities, with the newly formed euhedral garnet. Brown biotite, oxide rimmed with sphene and pyrites mantled by hematite are common minor constituents. Epidote occurs in the vicinity of Laxfordian structures. The felsic gneisses are again the most abundant type. They possess exceptionally irregular banding in many places, often containing balls or lenses of ultramafic or marie material, sometimes agmatitic. Most of the felsic gneisses contain mineral assemblages characteristic of the amphibolite facies, but a few contain porphyro- blastic muscovite together with chlorite, which may be due to further retrogression.

(B) THE TRANSITION FROM GRANULITE TO AMPHIBOLITE FACIES The transition from the granulite facies to the amphJbolite facies, although drawn as a line on Figs. I and 2, is of course gradational. The line is drawn at the base of the most northerly mafic band that contains significant amounts of pyroxene. The corresponding transition for the felsic gneisses, which are more susceptible to amphibolization, occurs about a mile to the south but is irregular in detail. Thus the area referred to as southern granulites contains amphibolized felsic gneiss, mainly along its northern margin, but also scattered about in the southern part of this zone. In addition to control by bulk chemical composition, the transition to amphibolized gneiss is controlled by both pre-existing and apparently penecon- temporancous structures. For example, the series of gently dipping mafic pyroxene gneiss bands at the northern limit of the granulite facies zone are completely amphibolized in the vertical part of a typical Scourian structure near the coast. Retrogressive reactions from granulite facies assemblages to those of amphib- olitc facies can be studied throughout the southern area. These reactions produce rocks with a continuous series of textures ranging from unaltered pyroxenc gneiss, through a stage of partial replacement in which amphibole and biotite replace pyroxene, to the stage at which all pyroxene has disappeared. The intensely folded rocks north of Lochinvcr have lost the replacement textures. The principal agent causing this retrogression is presumably water. The distribution of amphibolized rock within this area strongly suggests that metamorphism was dependent to a large extent on the fugacity of water, since the other intensive variables (lithostatic pressure and temperature) were probably about the same in felsic and mafic gneiss. The progressive amphibolization of mafic bands at their contacts with felsic gneiss indicates that the latter acted as sources for water, while the former acted as sinks. It can be shown that approximately o. 5 gm of water is needed to amphibolize i cc ofmafic gneiss (5o% pyroxene), whereas only about o. I o gm/cc is required to amphibolize felsic gneiss (I o % pyroxene). Thus the outward diffusion of water from a central source region into a succession of anhydrous pyroxene gneiss bands would be expected to proceed five or six times faster in acid gneisses than in the basic bands, ff there is no transfer of water across the contacts. Such transfer will of course take place, and lessen the gap between

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the amphibolite front in the two rock types. If the source region for water in the central amphibolite zone is assumed to be the highly deformed belt of vertical gneiss in its core, then the horizontal component of complete hydration in the felsic gneiss extends 3 to 4 km southwards, whereas that in the marie gneiss is seen to only 2 km to the south. This effect has interesting implications with regard to the meaning of the Inverian metamorphism.

C) PEGMATITES The pegmatites are of two genetic types, (I) intrusive pcgmatites which cut across the foliation of the host gneiss, and (2) replacement pegmatites, or granite gneiss, which preserve the foliation of the pre-existing rock. Intrusive pegmatites of the 'Scourian' type ( Type I ). Intrusive pegmatites, where undeformed, occur as dyke-like bodies varying from a few centimetres to a few metres in width, and from a few metres to a few tens of metres in length. They consist of pink mierocline crystals, in places o. 3 to o. 4 m across, with irregular sheets of quartz intergrown to give a texture similar to graphic granite. In addition to microcline and quartz, lesser amounts ofplagioclase, biotite, orthite, and monazite may be present. Large microcline crystals are rimmed by a narrow (

Pegmatites in the amphibolite zone. The apparent lack of Type z pegmatites within this zone appears to be due to the recrystallization and deformation of these bodies to form pegmatitic gneiss. Within the southwest-dipping part of the amphibolite zone, Type z pegmatites of quartz and feldspar are recrystallized to form aggregates of smaller grains. Biotite books are crumpled and highly deformed; new growth of fine-grained biotite and muscovite occurs throughout the pegmatite. They are, however, unfoliated and retain their dyke-like form, cutting the foliation of the gneiss. With increasing deformation towards the central zone, the pegmatites lose their discordant habit, until they become represented by a few lenses of granite gneiss parallel to the foliation of the surrounding gneiss. In these lenses, irregular sheets and tongues of quartz are aligned parallel to the contact and the surrounding foliation, and are separated by aggregates of microcline and very coarse knots of biotite. Replacement pegmatites, or granite gneiss ( Type 2). A band of pinkish grey granite gneiss is exposed at o82 i9o within the southern pyroxene zone. This band, or a series of similar bands, can be traced from this locality to the southern shore of Loch Kirkaig, where, as elswhere, the rock possesses a well defined planar foliation made obvious by streaks of chlorite, but also shown by flattened lenticles of quartz DISTIRIBUTION OF ~ 27 TYPE 1 PEGMATITES 1

•

23 • • "" ;~=~/

21 . •

xxxx Granite-gneiss Lath K/rk~ /" 0/

, • /~~~ , ]Amphibolitefocie, 18 • each gridsquare = one Kilometre

F z O. 2. Outline map of the Lochinvar district, showing localities of Type I pegmatites (undeformed and deformed) and granite gneiss.

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T A B L E I " Geochronologicaldata from Pegmatites

Nature Locality Rb Sr S~Sr]SaSr of Specimen (see appendix) ppm. ppm. STRb/S6Sr today Sample 2

(a) Laxfordian-Scour~n trans~wn zones ofSut~n& Wagon I. Badnabayzone 4o2 ± 8 407 ± Io 2.859 o'76I 4 ± 0-0030 WR 2. Foindle zone I15 x 23 ol 1"448 0"7453 ± 0-0025 WR

(b) Scourie area 3. Scourie 393 ± 8 83 ± 3 13-7I 1"I235 i 0"0030 WR 4. Scourie 651 2451 0'7671 0"7370 ± 0"0033 WR 5. Badcall, near 15 a 26o 1 o'1671 o'7134 ± o"ooi7 P Scourie 6. Badcall, near -- N 22"9 1.4373 ~ o.oo22 KF Scourie 7- Badcall, near 554 4- 1 o 121 4- 3 i3.25 I.I976 ± o-oo23 WR Scourie

(c) Lochinver area 8. Drumbeg road, 422 ± 8 I57 ± 3 7.777 0"9787 ± °"°°I5 WR near Kylesku 9. Drumbeg road, 45 ° ± 9 x8o ± 4 7"258 o.968I ± o.ooI8 WR near Kylesku 1o. Lochinver 249 ± 5 I5O ± 3 4.822 0-8620 ± o'ooi7 WR I x. Lochinver 367 ± 7 I4O ± 6 7.589 0-9599 4- 0.0020 WR 12. Lochinver 343 ± 7 75 ± 3 13"24 x'I557 4- 0.0060 WR 13- Lochinver 332 ± 7 1oo ± 4 9.611 1-o237 4- o'oo5o KF 14 . Lochinver 28.0 ± 0"5 I48 ± 3 o-544 o-7417 ± o'oo14 P 15. Lochinver 416 ± 8 263 ± 5 4"579 o-871o ± o.oo2o WR 16. Lochinver 65 • 1 I53 ± 3 1.23o o.7498 ± o-oo2o WR 17. Lochinver 351 x541 0.6581 0-7220 4- 0.0020 WR 18. Lochinver 171 2661 o.x85 o.7~7 ° ±°'°o4 ° WR

(d) Other areas 19. Loch Glencoul 364 ± 7 79 ± 2 I3-3 ° 1"oo75 ± o'ooi9 WR 20. Gruinard Bay 2431 i8i 1 3.851 o.8149 ± 0-0007 WR 2 I. Gruinard Bay 711 3oo 1 0.6861 0-7322 ± o'ooi5 WR

(e) Scourie area recalculated from Giletti et al. (I96I) data 22. BadcaU, near 752 x52 14"32 1-2382 + 0"0040 Scourie 23. Scourie 723 93"4 22-4 I 1.4844 ± 0"005 ° 24. Scourie 666 93 "9 2o "53 1-392I ± 0"0o45 25. Badcall 934 i x8 22 "91 1-4388 ~ 0"005 °

1 All XRFSTRb/S6Sr ratios, error + 7 %. 2 WR = Whole rock i.e. hand specimens averaging 15 cm cube P = Hand-picked plagioclase KF = Hand-picked microcline

within the bands of pink feldspar. This foliation, or mineral banding, parallels the upper and lower contacts of the band where these are exposed, and also parallels the mineral banding of the surrounding granulites. Granite gneiss from this locality consist of 3 ° to 4:0 per cent granular microcline, 25 to 30 per cent quartz in thin foliae, with a similar amount of granular plagioclase. The remainder is mainly chlorite, in streaks, with lesser amounts of muscovite, apatite, and magnetite, with a few zircons. Calcite and epidote are found in fractures. The southern boundary of this granite gneiss is obscured by dyke-like pseudo- tachylite up to 3 m thick, with small irregular off-shoots veining both the granite gneiss to the north and the altered pyroxene gneiss to the south. Isotopic analyses show that the gneissic body may be genetically related to the Type I pegmatites (specimens 17 & I8; see also the similar granite gneiss body 4, Table i). Relative age of the pegmatites. Although the detailed relations between the Type x pegmatites and their host rocks is imperfectly understood, their age relative to the amphibolite facies metamorphism described in a previous section is clearly indicated on Fig. 2. The relative lack of recognizable discordant pegmatites within the central zone, together with the presence of a gradational suite of deformed relicts, must be considered as strong evidence for their intrusion in the time inter- val between the close of the granulite facies metamorphism, and the close of the following amphibolite facies metamorphism. A similar conclusion was drawn by Watson in the Scourie area (Sutton & Watson I95I ).

(D) MAFIC AND ULTRAMAFIC DYKES The gneisses are cut by numerous mafic dykes, a few ultramafic dykes and one representative of an intermediate variety (Tarney x963). The petrology of the dykes has been described by Teall (I885, I9O 7, In Peach et al.), Tarney (I963) and Burns (I966). Sutton & Watson (x95i) describe the progressive amphibolization of the dykes with increasing Laxfordian metamorphism in the Loch Torridon and Scourie areas, where the metamorphosed dykes are in part garnet- bearing, but absence of garnet in the amphibolized dykes of the Lochinver area was noted by O'Hara (I96I , i962 ). The corresponding dykes which cut the Lewisian of the Outer Hebrides have been described by Dearnley (I962, I963). The marie dykes of the Lochinver district show the first part of the alteration of pyroxene dolerite to hornblende gneiss described by Sutton & Watson (I95I). They differ in that garnet has not been observed, and the original igneous texture is preserved except in very small, fine grained dykes, and in that part of any dyke which is cut by a late shear. All the margins of the dykes in the Lochinver area are amphibolized, whether sheared or not: similarly all shear zones in both dykes and country rock are amphibolized. The dykes of the central amphibolite zone are much less aItered than are bands of marie gneiss.

2. Structural relationships The gneisses of the Lochinver district have been affected by at least three metamorphic episodes and the structure of the area is understandably complex. Minor structures have a very uniform style and it has proved impossible so far to

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distinguish between Badcallian and Inverian folds on the basis of geometry. On Fig. I, only the locations of the main folds are shown. The critical facts are that Badcallian structures are cut by pegmatites which are involved in Inverian folds, while these structures are similarly cut by marie and ultramafic dykes which are deformed by Laxfordian shearing. Finally, these sheared rocks are intruded by a suite of Palaeozoic alkali dykes which are displaced by faults.

(A) BADCALLIAN STRUCTURES The Badcallian structural style is clearly displayed in the pyroxene bearing rocks of the southern zone: deformation is irregular and flow-like and locally intense. Single major structures die out in less than two kilometres indicating the inability of the gneisses to transmit stress over large distances. The gneissic banding is folded by structures ranging in size from about two kilometres to several centimetres, which for purposes of description have been classed as major and minor

074209 "<%.".. III

. v/- / ///>__\-

//// j .z

MAP OF A TYPICAL BADCALLIAN STRUCTURE

/'//Ger,,e, s~eep o, v~,cal di~

Axis of minor fold 0 100 200 SCALE • JMETERS

FIG. 3. Detail of a Badcallian fold in pyroxene granulite, at grid reference 074209.

structures, both of which affect the two distinct types ofgneissic banding which are present. Primary banding on a scale of one to several hundred metres is represented by the alternation offelsic, mafic and ultramafic rocks which appear to have existed as separate entities before the granulite facies metamorphism. Secondary gneissic banding on a scale of a few centimetres or less is developed within the primary bands. Although major structures cause local variation, within the southern pyroxene zone the regional trend of banding is persistently ESE, with a gentle southerly dip. At several localities within this zone the gneiss swings from its EsE trend to a N~. trend which persists for distances up to one kilometre. This swing is usually accomplished by folding about an eastern synclinal axis, and the return to the regional trend is brough about by folding about a western anticlinal axis, so that the outcrop of banding is displaced sinistrally. Figure 3 illustrates the outcrop pattern of this type of structure. Banding within the northerly trending pyroxene gneiss is attenuated, while the dip is very steep and is locally overturned. These particular folds plunge 3 °0 at 185 ° and are included among the folds shown on Fig. 3; they are typical of a group that plunge at low angles between 17 °o and 25 o°. A large structure exposed east of Strathan Bay (the 'Strathan Bay structure' of Evans 1963) on the southern margin of the central amphibolite zone is more complicated, but still of the same general form.

B) INVERIAN STRUCTURES The most significant structural feature at Lochinver in general, and the central amphibolite zone in particular, is the presence of large-scale structures which are pre-dyke, but post granulite assemblage in age. In addition, a characteristic dif- ference in gneissic banding and in the number of minor folds appear to be associated with this second deformation. Badcallian folds are also present within the least deformed parts of the central amphibolite zone, where they are affected on all scales by Inverian deformation. Differences in banding between amphibolite facies and granulite facies (non- charnockitic and charnockitic) rocks within the Lewisian were first described by Watson (Sutton & Watson 195 i). In the Scourie area, the amphibolite facies rocks have the same strike as the neighbouring pyroxene gneiss, but banding is less regular. Felsic injection of mafic bodies is more acute, producing ultramafic shreds, knots, and lenticles within the felsic gneiss. This phenomenon is clearly displayed in the least intensely deformed, marginal parts of the central amphibolite zone of the Lochinver area. Within the intensely deformed gneiss of the major Inverian structures, both primary and secondary banding have a sE trend and are near vertical; an orientation highly atypical of granulite facies gneiss within this map area. The secondary gneissic banding within these zones, although attenuated, is smooth and regular: deformation is always of a continuous style, characteristic of flowing-folding. The dominant structural feature of the district is the Lochinver antiform, forming the core of the central amphibolite zone, while a smaller Inverian structure, the Strathan Line, located near the southern margin of this zone, deserves description because of the clarity with which it displays the relative ages of Bad- callian folding, Inverian folding and dolerite intrusion.

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The Lochinver antiform, consisting of a gentle sw dipping limb, and a vertical, highly attenuated NE limb, can be traced for about 7 km along its strike (Fig. I). The Six Inch Geological Map for Sutherland sw 7° indicates that this structure extends for at least 3 km further to the SE while the aeromagnetic data suggests that it continues as far as the Moine Thrust (Powell 197o ). The sw limb, except for the differences in banding noted above, is continuous with the trend of the pyroxene zone to the south. The NE limb consists of two segments. A central belt of vertical gneiss about one kilometre wide is followed to the northeast by several kilometres of gently northeast dipping gneiss, which, like its counterpart to the south, preserves earlier large-scale Badcallian structures. This zone is terminated to the northeast by gneiss with a NE strike and gentle NW dip, which grades northwards into granulite facies gneiss. Although in plan the Lochinver antiform appears to be an anticline trending about 125 ° and plunging gently to the southeast, the vertical core is actually composed of many tight vertical folds which affect both the primary and secondary banding. These folds are cut by dykes and both are then cut by Laxfordian shears. The dykes are foliated only where they are traversed by late shears, and even then never assume the coarser mineral banding of the mafic gneiss. The pyroxene bearing Lochinver dyke (Fig. I, 097232 ) cuts obliquely across the axis of the Lochinver antiform, and although locally converted to hornblende schist by late shears, it is in general unsheared and unfoliated, preserving its igneous texture. It is within a few degrees of vertical in both the sw and the NE dipping gneiss, suggesting intrusion after Inverian folding, and thus gives a minimum age for that event. The maximum relative age can be determined most precisely by deformation of pegmatites which cut across earlier folds elsewhere (see above). The Strathan Line (Fig. I), a second belt of nearly vertical gneiss, extends southeastwards from Strathan Bay to the northern tip of Loch Bad na Muirichin (100200). This feature truncates the Strathan Bay structure and is therefore presumably post Badcallian in age. Along most of its length, the Strathan Line consists ofa 15 m belt of gneiss striking 125 ° and dipping from 7° to 9 °0 to the southwest. It displays a pronounced mineral lineation which plunges steeply (about 60 °) to the southeast (similar to that in shreared dykes), suggesting a relatively large horizontal component of movement during crushing. The finely banded gneiss near the margins of the Strathan Line differs from that of the centre in two respects: it is not greatly crushed and it does not display the steeply plunging mineral lineation. The transition from steeply marginal gneiss to gently dipping country rock is accomplished by an anticlinal flexure to the northeast, and a synclinal flexure to the southwest. The axes of these flexures plunge IO to 20 ° southeast (similar to those in the Lochinver antiform). Because the strike of the surrounding gneiss is approximately the same as that of the Strathan line, the gentle plunge of its marginal flexures suggests an early phase of vertical movement. It thus appears likely that the Strathan Line was produced by ductile folding about nearly horizontal axes, and that this pre-existing line of weakness was then the focus of later, subhorizontal movement under brittle conditions, in the rupture field of deformation. Minor folds within the central amphibolite zone are usually upright isoclinal or

sub-isoclinal folds of a ductile or flow-fold character, without axial plane structures, together with monoclinal flexures at the margins of the major Inverian features. In detail their style is similar to those in the pyroxene granulites, but the upright folds are more tightly compressed and regular in form. It is difficult if not impossible, however, to tell whether any individual minor fold in the central zone developed during or before the amphibolite facies metamorphism. Figure 4 shows the orientation of I32 fold axes within the amphibolite gneisses; those from the central belt concentrate in an ESE direction, plunging at 0-20 °, but in the adjacent amphibolized granulites of the southern and northern zones, many axes plunge at shallow angles to the north or south-southwest. The plot of fold axes in the amphibolized gneisses shows symmetry about a plane striking approximately t2o ° and

• • • • • O0 • GO q~O • ~ • • • •

• • • • O • • O O • •

O• • o• OOo °° oo* •

0 Oo ~/ • og x • x • G o • o°O W Xxxx • • O_o• • ~~ , "A ~O ,o+ • "" • .~,

EQUAL AREA PROJECTION OF FOLD AXES -I- Granulite facies gneiss • Amphibolite facies gneiss O Sheared gneiss and dykes FIo. 4. Equal-area projection of fold axes in the Lochinver area.

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dipping IO°S, suggesting that the intrafolial folds of the southern partly amphibolized zone (that is, relict folds of pyroxene granulite age) have been rotated about the Lochinver antiform axis to produce a mirror-image effect.

(C) CORRELATION OF INVERIAN FOLDING AND METAMORPHISM Inverian folding and metamorphism appear to occupy the same position in both time and space: both are post pegmatite, and pre-dyke in age, while isotopic dating of these intrusions shows that they were probably separated by only a small time gap (2300-2200 m.y.). Inverian folding and metamorphism are also intimately associated in space, because the tightly folded vertical part of the Lochinver antiform contains the highest grade of amphibolite facies rocks in the area, and the amphibolized rocks are centred about this structural zone. This association is also seen in minor folds; the typical Inverian minor folds are found only within the central zone, or in amphibolized felsic gneiss marginal to it. Because the folding is ductile, and apparently took place under conditions of high temperature and pressure, it seems logical to correlate these two phenomena. (D) INTRUSION OF DYKES The intrusion of the NW trending basic and ultrabasic dykes with an estimated 8 per cent expansion of the earth's crust in this area must be treated as a structural episode equal in magnitude to the preceding Inverian event. The intrusion of these dykes along fissures approximately perpendicular to the previous maximum principal stress axis suggests that they may represent a relaxation phenomenon associated with the close of the Inverian episode of deformation. This suggestion is further strengthened by the evidence for intrusion into relatively hot country rock (discussed above), and by the short time gap (perhaps less than IOO m.y.) between the recrystallization of the pre-Inverian pegmatites, and the post Inverian dykes.

(E) LAXFORDIAN DEFORMATION The style and form of Laxfordian deformation is displayed in the NW dykes. Un- like earlier diastrophic episodes, Laxfordian movement was almost entirely restricted to the rupture field, producing only small parasitic folds or shears. The Laxfordian shears follow a slightly more east-west trend than the Inverian folds or the dykes, but they are close enough in orientation for the earlier structures to act as planes of weakness. Lineations within sheared dykes generally plunge south- eastward at angles greater than 4 °0 . Often drag folds are not visible, and the lineation is caused by the orientation of hornblende c-axes. The sense of movement on the easterly trending shears is sinistral, with the southern block moving east- wards and upwards relative to the northern block.

3. Geochronology and isotope geology

(A) AGE OF FORMATION OF THE POTASH PEGMATITES The 'Scourian' pegmatites, rich in potash, discordant to the main Badcallian structures and deformed by Inverian events, have been described by Sutton & Watson (I95 I), Khoury (1968) and used for reconnaissance dating purposes (Giletti et al. I96I ). The results are given on Table I, plotted in Figs. 5 & 6 and described in the Appendix.

Lewisian type I potash pegmatites 23+

22,

It/3 "12 +3 ao

e19

Ji Scourie District Lochinver 08 I area of detailed Other Areas I diagram t

(~70 ~'- I? ~ 6 ~I I0I 12I 14I 16I 18I 20I 22I

-Lewisian type 1 potash pegmatites

(detail) 0"84-

e20

.< • o 0'80- k_ t/')

oo 0"78- I=_ L/3 +1 + ScourieDistrict ® Lochinver +2 • Other Areas 14e I ±0-002 Error 44-

o.zo, , , , , , , , , , , , ,o ~.o 30 ~o 5.0 RbSZ/Sr ~s F xG. 5. 87Rb/ssSr against 87Sr/SSSr plot of all analysed Type I pegmatites, except three from Giletti et al. (I96I) with low Rb/Sr. Points 5, 6, I3, z4 and 22 to 25 are from separated feldspars; all the others are whole rocks. The 2540 m.y. isochron was calculated from points 7, 8, 9, I5, x6 and 22: the 23zo m.y. isochron from points xo-I3 and 23-25. Fze. 6. STRb/S6Sr against SVSr/SSSr plot of detail of samples with low Rb/Sr from Fig. 5. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/130/2/125/4884711/gsjgs.130.2.0125.pdf by guest on 25 September 2021 I4 ° C. R. Evans & R. St J. Lambert

The analyses were carried out by the authors using the standard techniques at Oxford (Giletti et al. (I96i) and Long & Lambert (I963)). Subsequent analyses of the Eimer and Amend standard SrCO3 showed that the MS5 used for this work produced isotope ratios in a range close to o.7~oo which are o-ooi5 too high (authors' unpublished data). All samples were analysed as whole rocks, except those listed as plagioclase or potash feldspar, which were single crystals. Figures 5 & 6 show that there is a considerable scatter of the results, apparently indicating either a variety of original ages, or widespread open system conditions. Taking the Scourie-Lochinver pegmatites only, which all (except specimen i o) belong to the readily identifiable class of graphic quartz potash feldspar pegmatites (Type i), it can be seen that a majority lie within the error on one of two isochrons, indicated on the figures. Using the least-squares cubic method of York (I966) the isochrons give 2540 4- 20 and 23io zk 4 ° m.y. respectively, with initial sTSr/SeSr ratios of 0.7040 i o.ooi2 and 0.7027 q-0"0024 (weighted mean 0.7035, to be corrected downwards slightly on standardization). This initial ratio is close to the figure for the host gneisses, which had a ratio of o.7ox -4- o-ooI at I54o m.y. ago (Evans 1965, Evans & Lambert unpublished data). The pegmatites are therefore probably chemically related to their host rocks, rather than to any Rb-enriched source. No Lochinver-Scourie whole rock pegmatites give apparent ages higher than the older group, suggesting that 2540 m.y. may well be near the true age of formation. One of this group (I 5) cuts an ultramafic lens giving a K-Ar hornblende age of 2620 m.y. (Evans i965). The meaning of the younger group, lying on the 23IO m.y. isochron, referred to as 2250 m.y. pegmatites by Evans (I965) , is not so clear. The pegmatites lying on the 231 o m.y. isochron include Scourie and Lochinver examples, none showing any appreciable deformation in the field, although all show the feldspar recrystallization recorded by Giletti et al. (I96I). There is no obvious reason why this set should be apparently younger than the examples at 2540 m.y., which are otherwise indistinguishable. Two principal possibilities seem to exist: either that there really are two groups of pegmatites of different age; or that the younger examples have been down-dated during the Inverian metamorphism from an original 2540 m.y. age. One of us (C. R. E.) prefers the first alternative, arguing that these pegmatites all contain some plagioclase which would absorb sTSr migrating from the potash feldspar and that it would be for- tuitous if irregular down-dating (as evidenced by the pegmatites not lying on the isochrons) produced a second isochron from otherwise unmodified pegmatites. The other (R. St J. L.) considers that it is possible that the Inverian metamorphism was in fact active at 23Io m.y. and under hydrous amphibolite facies conditions, affected 254 ° m.y. pegmatites at some distance from the main deformed zones. The remaining Scourie-Lochinver pegmatites and the granite gneiss samples provide numerous problems. No clear reason exists for the divergence of 3, 8, 23 or 25, all of which cut partly amphibolized pyroxene granulites near Scourie. Our interpretation is of irregular sTSr loss from or migration within these pegmatites, evidence for which lies in the high sTSr/SeSr of the plagioclases. The granite gneiss samples also show irregular behaviour, 17 and 18 being two outwardly identical samples, both lying off each isochron. The 'contact' specimen from the same granite gneiss band, i6, clearly lies on the 2540 m.y. isochron, but 4, from the isoclinally-folded granite gneiss sheet on the Scourie road near Loch an Daimh

Mor, may be older. Our interpretation of the Lochinver granite gneiss age is that the granite gneiss is a post granulite facies metamorphism replacement body, probably derived from the same source at the same time as the potash pegmatites. The two whole rock pegmatites analyzed from the area between Scourie and Laxford (I, cutting the Badnabay zone of Sutton & Watson (1951) and 2, cutting the Foindle zone) give minimum ages for the structures and mineralogy of their host rocks. These are 2ooo -+- 1 lO m.y. for the Foindle structures and 14oo -+- 7 ° m.y. for Badnabay assuming the same initial 87Sr/SeSr as for the Type i pegmatites. The latter figure only adds to the list of so-far unexplained ages of c. 14oo m.y. obtained by Giletti et al. (196i) and Evans (1965), but the Foindle figure supplies some evidence for the existence of pre-Laxfordian structures in the Scourie-Laxford transition zones (see also Lambert & Holland 1972). Further evidence for pre-Laxfordian events in the Lewisian is given by the two Gruinard samples, 2o giving 2ooo ~ 5 ° m.y. and 2I giving 286o + i8o m.y. The former pegmatite cuts typical Gruinard gneiss (Peach et al. 19o7, pp. I72-I9O ), but the latter is weakly foliated parallel to the foliation of the adjacent gneiss. One possible interpretation is that the older discordant pegmatite preserves an original intrusive age (establishing the presence of Badcallian events at Gruinard) while the younger pegmatite gives an approximate or minimum age for the foliation affecting the Gruinard gneisses. In any event the existence of pre-Laxfordian structures at Gruinard is confirmed. The one remaining pegmatite, i9, occurs as a discordant dyke-like body in the chloritized Lewisian above the Glencoul Thrust and below the Moine Thrust in the Assynt district: it confirms that the age of these rocks is at least as great as that of the Laxfordian assemblage.

(B) GEOCHRONOLOGY OF THE NORTHERN SECTOR OF THE LEWISIAN Isotopic data relevant to the present discussion have been published by Giletti et al. (1961), Evans & Tarney (I964) , Evans (1965) , Moorbath et al. (I969), Lambert et al. (I969a), Francis et al. (I97i) and Lambert & Holland (1972). The age data show that the Laxfordian metamorphism was declining in intensity by 185 ° m.y., and was effectively concluded by c. 16oo m.y., (Giletti et al. 196I, Lambert & Holland I972 ). There is no corresponding concentration of apparent mineral ages at Lochinver (Evans I965) in parallel with our suggestion that neither the pre- I8oo m.y. metamorphism at Laxford, or any subsequent event associated with it, was of any major importance at Lochinver. One conclusion might therefore be that the post dyke shears and associated amphibolization at Lochinver were spread over an appreciable period and represent local adjustments of the basement to movements of crustal blocks, rather than the effects of a major reworking or 'orogeny'. That no concentration of apparent ages is found at Lochinver is because there was no single cause. If no definite interpretation can be placed on any individual age of < 18oo m.y. and no definable groups exist, the question arises of whether the five hornblendes from amphibolite facies gneiss (apparent ages 2 lOO, 2o7% I88O, I94O and 198o m.y., Evans 1965) are yielding ages related to any particular event or not. Petrographic distinction has proved impossible amongst the analyzed Lochinver central zone

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hornblendes, while their field occurrence does not relate to the individual ages; on geological grounds, none can be older than 231o m.y., the Rb-Sr age of the pre-Inverian pegmatites or younger than 2200 m.y., the age of the dykes. Figure 7 yields some information relating to the spread of ages: four of these hornblendes were fully analyzed (Evans 1963) and a simple relationship between apparent argon loss and composition (Fe content) is apparent. This pattern has been observed by O'Nions et al. (1969) in more definitive circumstances. Circumstantial evidence supporting this pattern is found in the high retentivity of the very Mg-rich primary granulite facies amphiboles (specimens 290 , 494, 547 & 61o, Evans i965). No inferences may therefore be drawn from any individual hornblende age at Lochinver, but collectively they support the general sequence, for no central zone hornblende has an apparent age >2 ioo m.y. The sequence of events so far defined in the horizontal mainland Lewisian is therefore:

c. 2900 m.y. primary age of pyroxene granulites. > 2600 m.y. termination of pyroxene granulite facies metamorphism, formation of the pyroxene gneisses of the Scourie assemblage. 2540 m.y. intrusion of potash pegmatites in both the mainland Lewisian and the Outer Hebrides. 231 o m.y. further potash pegmatites or recrystallization of first set. 23IO-22oom.y. amphibolite facies metamorphism actively affecting wNw- striking vertical belts up to 2 km wide; development of isoclinaI folds, new gneissose banding and destruction of all pre-existing structures. Formation of the Inver assemblage by hydration of the Scourie assemblage. 2200 m.y. intrusion of ultramafic and mafic dykes into hot country rock, followed closely by their autometamorphism. The last mem- bers of the suite retain pyroxenic mineralogy. c. 185o m.y. onset of decline of amphibolite facies metamorphism of the Laxford assemblage followed by slow cooling and foliated granite and intrusive pegmatite activity. Brittle deformation of pyroxene granulites and amphibolites. c. x6oo m.y. cessation of pegmatite activity, closure of biotite and hornblende to argon loss. Termination of brittle shear activity. c. 14oo m.y. closure of biotite to Rb and, or, Sr migration.

I~ ¸

~- 80 J U)q

/t FIo. 7. / Relationship of chemical composition of hornblendes to 20 argon loss, assuming that all 2'o 3'o ,~ hornblendes are of identical F/CMF % original or true age.

4. Implications of the Inverian metamorphism and correlations with other shield areas The term Inverian, or the evidence for it, has found use since the original definition of the name (Evans 1965) in descriptions of the Lewisian complex (Bowes I968, 1969; Khoury 1968; Sutton 1968; Park 197o ). In most of these publications the Inverian metamorphism is considered as part of a cycle or an orogeny. It was suggested above that the Inverian metamorphism is a phenomenon distinct from the previous granulite facies metamrophism and, in the following, it will be argued that at present there is no evidence to associate it with any other stage of Lewisian history as part of a sequence of events, with the possible exception of dyke intrusion. The Inver assemblage (Holland & Lambert I973) can only be recognized directly by its relationship to dated events, particularly to the Type I pegmatites and the dykes. This restricts recognition to areas of the Lewisian where Nw dykes are well-preserved, that is the Outer Hebrides, and on the mainland, the central area (Ben Stack line to Gruinard and Loch Tollie) and again southwest of Gair- loeb (Park 1964) into the area around Loch Torridon (Sutton & Watson 195I ) including the islands of Rona and Raasay. Pegmatite ages (above) suggest that loci of Inverian events may be found northeast of Scourie and Gruinard; but there are no relevant age determinations from the area south of Gairloch, an area for which discussion must be deferred. In the Outer Hebrides there are no obvious correlatives of the Inverian, but recent age determinations (Lambert et al. 1969; Lambert et al. i969a ) indicate the possibility of events approximately in the Inverian range. The pattern of aeromagnetic anomalies in the Lewisian (Powell 197o ) reveals a very close correspondence between aeromagnetic lows and the major Inverian structures of Lochinver and Scourie-Laxford. An off-shoot of the Lochinver minimum anomaly joins the Clashnessie amphibolite belt (King 1955) and another substantial minimum extends southeast along the coast at Achiltibuie (south of Lochinver, o2oo8o). The minima of the three major Nw-trending anomalies separate the Lewisian into rectangular blocks of NE-SW dimension only 15 and 25 km. The other two areas, northeast of Scourie and at Guinard, where gneisses were formed in much their present condition before 2ooo m.y. ago, have been inten- sively described by Peach et al. (I9O7) , Sutton & Watson (I95I, I962 ), Holland (I966) and Beach et al. (1973). The descriptions of the structures and petrographic features of the Claisfearn and Foindle zones northeast of Scourie, reinforced by the author's own field observations, leave little doubt that these zones are another major Inverian feature: attention may be particularly drawn to the analogies in dyke behaviour between the two areas (see the Survey account 19o 7, pp. 126, 152; also Beach et al. 1973). The biotite K-Ar age of 2o6o m.y. from the Ben Stack line (the margin of the 'central zone' of Peach et al. (I9O7) , approximately equal to the NE limit of the Foindle zone of Sutton & Watson) may be confirmatory of this suggestion of a pre- Laxfordian age of the hornblende gneisses of these zones (Lambert & Holland, I972 ). The Gruinard district, however, contains no major structural features

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diagnostic of Inverian as distinct from either Badcallian or Laxfordian events and the true sequence in this area remains unknown. The Inverian metamorphism therefore stands revealed (so far) as the result of tectonic activity producing two major shear zones in the deeper parts of the crust some I- 3 km wide and 20 km apart. Between these two shear zones there appear to be several lesser shear zones of equivalent age: at Loch Duartbeg (Khoury 1968, pl. 2), at Oldany Island-Drumbeg (Peach et al. I9o7, p. I67) and at Clashnessie (King i955). The latter stucture has some affinities (attitude of foliation, general style of folding, plan shape of fold) with the Strathan Bay structure (Badcallian age) but King states that it contains deformed pegmatites. If these are the Type I potash pegmatites, then the Clashnessie structure is either Inverian or was re- juvenated in Inverian times. The Laxfordian features at Clashnessie resemble those at Lochinver very closely. The larger-scale effect of the Inverian event in the Lewisian is more difficult to identify. An event capable of inducing garnet amphibolite facies metamorphism in at least several hundred km ~ of pyroxene granulite crust ought to have produced widespread re-organization within less refractory materials, such as the biotite gneisses of the Laxford assemblage. We would regard it as likely, therefore, that at least some of the allegedly post dyke events in the biotite gneisses in the northern and southern Lewisian blocks, and in the Outer Hebrides, could be immediately pre-dyke (Inverian) in age. The overall tectonic cause for the Inverian structures and the NW dykes is not easy to discover. Events of similar age are scarce in other areas. The Bug-Podolian orogenic cycle is said to be at 2300-2000 m.y. (Semenenko et al. I968 ) ; the Tazin Lake gneisses of Nw Saskatchewan yield concordant biotite and hornblende K-Ar ages at 237 ° :k 4 ° m.y. (Koster & Baadsgaard I97o ) and the basement gneisses of the Andrew Lake area, N~. Alberta give Rb-Sr ages of 2350 and I98o m.y. (Baadsgaard & Godfrey i966 ). Neither of the Canadian examples can yet be related to any definite orogenic or regional rock-forming event, however. Dyke swarms in the Canadian shield yield Rb-Sr ages of 2030 ~ 80 m.y. (Mackenzie swarms II and IV) and 2080 -4- 7° (Abitibi), a little younger than the Lewisian dykes (Gates, Ph.D. thesis, M.I.T., i97I ). In Gondwanaland, a possible plutonic event at 2200-2000 m.y. is recorded in the Pilbara Volcanics (Compston & Arriens 1968); ages of about 2200 m.y. are recorded from the Dharwar Formation, Mysore (Venkatasubramaniam et al. 1968) and a possible event of 2300-2200 m.y. is recorded in the Nyanza shield (Clifford 1968, p. 382). This scattered information gives the impression of localized events only, without any world-wide synchronous orogeny. Most of these areas have been affected strongly by 18oo-I 600 m.y. events, though. The simplest gross interpretation of the Inverian is of regional block movements due to invasion of the lower crust by water at high temperatures, accom- panied by softening of the main shear zones, with differential movement of the intervening blocks and possible major deformation of the overlying metasedi- mentary assemblages. This was followed, apparently closely, by a major episode of intrusion of comparatively undifferentiated tholeiitic basalt dykes. These phenomena may be related to extensional continental rifting tectonics, but the parallel cannot be pressed far. The hot-spot theory of mantle diapirism and

basaltic magma gnesis appears to relate most closely to the Inverian events, but whether we are dealing with an isolated hot-spot situation, such as Tibesti today, or part of a larger-scale continental rifting phenomenon it is not possible to say. Further work in 18oo m.y. regions of Greenland and in the Gairloch district where the Gairloch metabasites might be the supracrustal representatives of the NW dykes, may help to solve the structural-genetic problem. Appendix

LOCALITIES AND DESCRIPTIONS OF ANALYSED PEGMATITES (TABLE I)

(a) Laxfordian-Scourian transition zones of Sutton & Watson (195I) 1. On knoll Ioom NE of main road, NC 212474, 2"4 km at 28o ° from Laxford Bridge: sharply discordant 2 m K-feldspar pegmatite cutting biotite-gneiss (RL 2352). 2. 7 ° m south of main road close to Loch na Claisfearna, NC 2o3467. Sharply discordant 2o cm zoned pegmatite (quartz-rich in centre) cutting biotite gneiss, strike 13 o°, dip 4o°sw: pegmatite vertical, north-south strike (RL 235o ).

(b) Scourie area 3. 65o m at 33 °o from Scourie Hotel, on sv-facing hillside, NC z53452 graphic magnetite K- feldspar quartz pegmatite cutting amphibolized leucocratic 'pyroxene' gneiss (RL 2133). 4. 4oo m NW of road junction, main road with Badcall road; on the east of the road on escarpment; NC 16o434. A concordant sheet of granite gneiss in pyroxene gneiss, showing mesoscale isoclinal folds within the sheet (RL 2137). 5 & 6. NC I47418 , near Badcall, Scourie. Discordant biotite two feldspar quartz pegmatite in pyroxene gneiss (5-----plagioclase; 6 = K-feldspar. Oxford University Geology Department collection no. 2o747). 7. NC I4742o; see Giletti et al. (I96I); from same pegmatite as Oxf. ~o528, now confirmed as the pegmatite II-i of Sutton & Watson I95 I, Fig. 6 (RL 2126).

(c) Lochinver area 8 & 9. Two separate vertical dyke-like biotite K-feldspar plagioclase quartz pegmatites, strike 5 o°, NC 226318, between Drumbeg-Kylesku road and Loch Unapool, discordant to amphibolized pyroxene gneisses (Oxf. 2249oA and B). xo. x "o km sw of Culag Hotel, Lochinver; NC o862 x5. An undeformed white-coloured K-feldspar quartz pegmatite strike N-S, dip vertical, cutting amphibolized pyroxene gneiss at north edge of Strathan Bay structure. Appears later than nearby deformed pegmatites of the pink, graphic- textured type (Oxf. 22537). z z. 4oo m ~s~ of Kirkaig Point, Lochinver; NC o632zo, large K-feldspar quartz pegmatite striking approximately due N, cutting amphibolized leucocratic 'pyroxene' gneiss of southern pyroxene zone (Oxf. 22363). I2, 13 & 14. z-3 km ssw of bridge over R. Kirkaig; NC o78x82 two I m dyke-like pegmatites cutting amphibolized leucocratic 'pyroxene' gneiss of southern pyroxene zone. Strike due north, vertical: assemblage quartz K-feldspar biotite magnetite (Oxf. 224z 9 and 22425, 13 ---- K-feldspar, 14 -----plagioclase). I5. 4oo m E of Kirkaig Point in small bay NC o642x 3. Irregular K-feldspar quartz biotite pegmatite cutting ultramafic gneiss. Same locality as specimen 494, Evans x965, hornblende age 262o m.y. (Oxf. 22495). x6. xoo m south of z5, NC o74x93, granite gneiss at contact with average leucocratic pyroxene gneiss. No chilling or signs of intrusive contact (Oxf. 22689B).

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17 & iS. Shore x .2 km at 280 ° from road bridge over R. Kirkaig; NC o74194. Weakly foliated concordant sheet of granite gneiss strike 90% dip 20 ° south. 17 and x8 are two different hand specimens (Oxf. 22688).

(d) Other areas x9. South shore of Loch Beag (Inner Loch Glencoul) ; NC 27o296. In chloritized hornblende gneiss above Glencoul Thrust Plane, a NE--SW, vertical discordant pink graphic-textured K-feldspar quartz pegmatite, apparently undeformed except for local cataclasis (RL 2 I o7). 2o. North end of cliff overlooking Gruinard River, o"4 km ssF. of Gruinard House, NG 9619x 7. 6o cm feldspar quartz pegmatite strike 34 o°, dip vertical, concordant with weak foliation of host gneisses. Margins grade into country rock, texture similar to Lochinver Type I pegmatites (RL 2860). 2I. Shore above H.W.M., o- 5 km NE of Gruinard pier, Gruinard Bay: NG 96393 I. K-feldspar quartz pegmatite cutting hornblende gneiss (Oxf. 20366 ). 22-25. Specimens 20528 , 2o75i , 20738 and 2o747 respectively from Giletti el al. (I96I), Table 3: samples 2074o, 2o743 and 2o752 were excluded on account of their low Rb-Sr ratios.

ACKNOWLEDGEMENTS. We acknowledge the assistance and advice of our colleagues at Oxford and thank J. G. Holland, S. Moorbath, G. K. Muecke and R. J. Pankhurst for criticising the text. The work by C. R. Evans was carried out during the tenure of an Imperial Oil Graduate Research Fellowship; the isotope studies formed part of the age and isotope project at Oxford then financed by the Department of Scientific and Industrial Research; and publication at this length was assisted by a grant. 5-References BAADSGAARD, H. & GODFREY, J. D. I966. Geochronology of the Canadian Shield in Northeastern Alberta. i. Andrew Lake area. Can. J. Earth Sci. 4, 54x-563 . BEACH, A., COWARD, M. P. & GRAHAM, R. H. I973. An interpretation of the structural evolution of the Laxford front, Northwest Scotland. Scott. J. Geol. 9,. Bowzs, D. R. 1968. The absolute time scale and the subdivision of Precambrian rocks in Scotland. Geol. F6r. Fiirh. Stockholm 90, I75-I88. I969. The Lewisian of Northwest Highlands of Scotland. Amer. Ass. Petrol. Geol. Mem. I2, 575-594. , WRIGHT, A. E. & PARK, R. G. 1964- Layered intrusive rocks in the Lewisian of the North- West Highlands of Scotland. Quart. d. geol. Soc. Lond. 120, I53-I92. BURNS, D. J. i966. Chemical and mineralogical changes associated with the Laxford metamorphism of dolerite dykes in the Scourie-Loch Laxford area, Sutherland, Scotland. Geol. Mag. 103, I9--35- CLIFFORD, T. N. I968. Radiometric dating and the pre-Silurian geology of Africa pp. 299-416. In Radiometric Dating for Geologists, Ed. Hamilton E. I. and Farquhar, R. I., Interscience. COMPSTON, W. & ARRIENS, P. A. x968. The Precambrian geochronology of Australia. Can. J. Earth Sci. 5, 561-585. DASH, B. x969 . Structure of the Lewisian rocks between Strath Dionard and Rhiconich, Sutherland, Scotland. Scott. J. Geol. 5, 347-374. DZARNLEV, R. x962. An outline of the Lewisian complex of the Outer Hebrides in relation to that of the Scottish mainland. Quart. J. geol. Soc. Lond. 118, x43-x 76. 1963. The Lewisian complex of South Harris; with some observations on the metamorphosed basic intrusions of the Outer Hebrides, Scotland. Quart. J. geol. Soc. Lond. 119, 243-312. EVANS, C. R. I963. The geology of the Lochinver district. D.Phil. thesis, Oxford University. x965. Geochronology of the Lewisian basement near Lochinver, Sutherland. Nature, Lond. 207, 54-56 . & TAmCEV, J. x964. Isotopic ages of Assynt dykes. Nature, Lond. 204, 638-64I. FRANCIS, P. W., MOORBATH, S. & WELrm, H. J. x97I. Isotopic age data from Scourian intrusive rocks on the Isle of Barra, Outer Hebrides, north-west Scotland. Geol. Mag. 108, I3-22.

GI~ ETrI, B. J., MOORBATH, S. & LAMBERT,R. ST J. x96I. A geochronological study of the metamorphic complexes of the Scottish Highlands. Quart. J. geol. Soc. Lond. 117, 233-272. HOLLAND, J. G. I966. Geochemical studies in the Lewisian. D.Phil. thesis, Oxford University. & LAMBERT, R. ST J. I973. Comparative major element geochemistry of the Lewisian of the mainland of Scotland. In The Lewisian of Scotland and related rocks of Greenland, Ed. Park, R. G. & Tarney, J. Univ. of Birmingham Press. KHOURY, S. G. 1968. The structural geometry and geological history of the Lewisian rocks between Kylesku and Geisgeil, Sutherland, Scotland. Krystalinikum 6, 4I-78. KINO, B. C. I955. The tectonic pattern of the Lewisian around Clashnessie Bay near Stoer, Sutherland. Geol. Mag. 112, 69-80. KOS~R, F. & B.~DSO~m~, H. i97o. On the geology and geochronology of northwestern Saskat- chewan. :. Tazin Lake region. Can. J. Earth Sci. 7, 919-93 o. LAMBERT, R. ST J., EVANS, C. R. & DE~a~NrEY, R. I969a. Isotopic ages of dykes and pegmatitic gneiss from the southern islands of the Outer Hebrides, Scotland. Scott. d. Geol. 6, 2o8-~,x 3. I-,AMBERT, R. ST J. & HOLLAND,J. G. 1972. A geochonological study ofthe Lewisian of the Laxford area, N.W. Scotland. d. geol. Soc. Lond. 128, 3-I9. LAMBERT, R. ST J., MvEv.S, J. S. & WATSON,J. : 969. An apparent age for a member of the Scourie dyke suite in Lewis, Outer Hebrides. Scott. d. Geol. 6, 214-23o. LoNo, L. E. & L~,'aBERT, R. ST J. i963. Rb-Sr isotopic ages from the Moine Series. In The British Caledonides Ed. Johnson M. R. W. & Stewart, F. H., Oliver and Boyd, Edinburgh, pp. 2,7-247. MOOI~ATH, S., WEL~, H. & GATE, N. H. I969. The significance oflead isotope studies in ancient, high-grade metamorphic complexes, as exemplified by the Lewisian rocks of Northwest Scotland. Earth Planet. Sci. Lett. 6, 245-%6. MuEcav., G. K., : 969. The petrogenesis of the granulite-facies rocks of the Lewisian of Sutherland, Scotland. D.Phil. thesis, Oxford University. O'HAR_~, M. J. x96:. Petrology of the Scourie dyke. Mineral. Mag. 32, 848-865 . I962. Some intrusions in the Lewisian complex near Badcall, Sutherland. Trans. Edinb. geol. Soc. 19, 2ox-2o7. O'NIoNs, R. K., MORTON, R. D. & BAADSG~, H. I969. Potassium-argon ages from the Bamble sector of the Fennoscandian shield in South Norway. Norsk. Geol. Tidsskr. 49, 17 I-I9o. PARK, R. G., i964. The structural history of the Lewisian rocks of Gairloch, Wester Ross, Scotland. Quart. J. geol. Soc. Lond. x2o, 397-434- I97o. Observations on Lewisian chronology. Scott. J. Geol. 6, 379-399. PEACH, B. N., HORNE, J., GUNN, W., CLOUGH, C. T. & HINX~_AN, L. W. x9o7. The geological structure of the North-West Highlands of Scotland. Mem. Geol. Surv. Gt. Britain. PIDGEON, R. T. & BowEs, D. R. I972. Zircon U-Pb ages of granulites from the Central Region of the Lewisian, northwestern Scotland. Geol. Mag. 109, 247-258. POWELT, D. W. I97o. Magnetised rocks within the Lewisian of Western Scotland and under the Southern Uplands. Scott. J. Geol. 6, 353-37o- SEMEN~NKO, N. P., SCHERBAK, A. P., V:NOG~a~OV, A. P., TOUOAIUNOV, A. I., ELISEEVA, G. D., COTLOVSKY, F. I. & DEMIDENKO, S. G. 1968. Geochronology of the Ukrainian Precambrian. Can. J. Earth Sci. 5, 661-672. SUTTON, J. x968. The extension of the geological record into the Pre-Cambrian. Proc. geol. Assoc. Lond. 78, (for i967) , 493-534. SUTTON, J. & WATSON,J. 195 X. The Pre-Torridonian metamorphic history of the Loch Torridon and Scourie areas in the North-West Highlands and its bearing on the chronological classification of the Lewisian. Quart. J. geol. Soc. Lond. 106, (for I95o ), 241-296. I962. Further observations on the margin of the Laxfordian complex near Loch Laxford, Sutherland. Trans. Roy. Soc. Edinb. 65, 9o-Io6. TARNEY, J. 1963. Assynt dykes and their metamorphism. Nature, Lond. 199, 672--674. TEALI., J. J. H. x885. The metamorphosis of dolerite into hornblende-schist. Quart. J. geol. Soc. Lond. 41, x33-x45. VENKATASUBRA~a~'~M, V. S., GOPALAN, K., IYER, S. S., PAL, S. & KRXSHNAN, R. S. I968. Studies on the Rb-Sr and K-Ar dating of minerals from the Precambrian of India. Can. J. Earth Sci. 5, 6o x-6o 4.

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VERNON,R. H. 1970. Comparative grain-boundary studies of some basic and ultrabasic granulites, nodules and cumulates. Scott. J. Geol. 6, 337-35 x. W.~gD, D. de. 1965. The occurrence of garnet in the granulite facies terrane of the Adirondack Highlands. J. Petrolog~ 6, x65-19 I. YORK, D. I966. Least-squares fitting of a straight line. Can. J. Phys. 44, Io79-Io86. Received x December 1972 ; revised typescript received 16 February 1973; read 31 October x973. CALVaN RALPH Ewuqs, Imperial Oil Limited, Calgary, Alberta, Canada. RICHARD ST JOHN LAMBERT,Department of Geology, University of Alberta, Edmonton, Alberta, Canada.

DISCUSSION MR M. BROWN said the author's use of the term agmatite appeared to be incon- sistent with the original intentions of Sederholm (I923, Bull. Comm. geol. Finlande 58) and also with that given by Mehnert (1968, Migmatites and the origin of granitic works. Elsevier). In the speaker's opinion many so-called agmatites are really examples of schollen structure and he entered a plea for greater care to be exercised in the use and application of nomenclature which refers to 'xenolithic' material in 'igneous' material.

DR TARNEY asked whether, in striving to emphasize the importance of Inverian events at Lochinver, the authors had played down the extent of Laxfordian activity, particularly in the vertical north limb of the Lochinver antiform ? Since the structural and metamorphic characteristics of the Inverian and Laxfordian were very similar in the area, what criteria had the authors used to discriminate between features attributable to each event ? In the speaker's opinion, although the Lochinver antiform itself was a major pre-dyke WNW-ESE structure, the excellent exposures around Achmelvich Bay demonstrated equally well that both dykes and gneisses had been involved in strong penetrative deformation in a zone over 500 m wide in places which followed the steep north limb of the antiform for many kilometres inland. It appeared that this steep limb of the antiform with the associated soft talcose picritic dykes following it for much of its length had been a weak zone exploited during the Laxfordian. The speaker would, however, like to lend support to the authors in the use of the term 'Inverian' to cover the events in Assynt and elsewhere when prominent new pre-dyke NW-SE trending structures were imposed on the Scourian gneisses followed by extensive retrogression of the granulite facies assemblages. While it is demonstrable that a large part of the retrogression was caused by influx of volatiles before the Scourie dykes were intruded, it is important to recall that the Inverian metamorphic activity continued during and shortly after dyke emplacement (Tarney I963). The range of Inverian activity thus straddles the all-important Scourie dyke intrusive phase used by Sutton & Watson in I951 to separate Scourian and Laxfordian. The style of the Inverian is so similar to the Laxfordian and so different from the granulite facies phase of the Scourian that it could as justifiably be considered an early phase of the Laxfordian as a late phase of the Scourian. This question may be resolved in the comparable areas of east Greenland. In the meantime there would seem to be ample grounds for retaining the term

'Inverian' for the major series of events recorded by the Lewisian gneisses and dykes of Assynt for a period of several hundred million years following the granulite facies event. DR J. G. HOLLAND noted that the authors had refrained from using the term 'orogeny' in describing the Inverian episode of amphibolization. He wondered, therefore, whether the authors extended their reservations to the terms 'Scourian orogeny' and 'Laxfordian orogeny' which, although not embodied in the original definition by Sutton & Watson (I95I), were now common currency (e.g. Bowes 1958, 23 Int. geol. Congr. 4, 225). PROFESSOR J. SUTTON said that a published account of Dr Evan's work was welcome; his study of late Scourian deformation is of historical interest for when it was accomplished it was an advance of anything that had gone before. As read at the meeting the paper was misleading in its misuse of well-established nomenclature. The Scourian, like the Laxfordian, covers a long period of time; it is quite wrong to restrict the term Scourian as Professor Lambert, perhaps inadvertently, had done while reading the paper. The whole of the sequence granulite facies metamorphism, pegmatite intrusion, followed by amphibolite facies metamorphism as displayed at Scourie and Loch Inver belongs to the Scourian. The matter is very clearly set out by Sutton & Watson (i95I). It will only cause confusion if the present authors introduce a new nomenclature while describing the same phenomena. PROFESSOR R. ST. J. LAMBERT replied that he thought that the usage ofagmatite is consistent with past definitions. Mehnert (I968) describes agmatite as "Fragments of the paleosome (i.e. the unaltered or slightly modified parent rock).., sur- rounded by relatively narrow 'veins' of the neosome (i.e. the newly formed rock portion)." "These terms [paleosome, neosome] are essentially non-genetic.., for "practical use in the field." The Lochinver agmatites consist of randomly orientated rounded blocks of banded mafic gneiss set in a matrix that seems to us to be partially melted, or at least relatively mobile felsic-dominated material derived locally. The process was not 'igneous' in the usual sense, with the matrix molten and detached entirely from its source and we therefore consider that these rocks are migmatites in the non-genetic sense precisely as advocated by Mehnert. In reply to Dr Tarney, it is agreed that the Laxfordian events had been played down in the presentation of the paper, although not in the paper itself. Laxfordian (= post dyke) events were recognizable by epidote-amphibolite facies mineral assemblages, development of finely banded gneisses in shear belts or zones bounded by structural discontinuities, and by the admittedly negative evidence of minerals whose K-Ar or Rb-Sr ages did not exceed that of the dykes. The Canisp shear belt, referred to by Dr Tarney, was the principal Laxfordian structure in the district. Such structures seemed to be controlled by pre-existing Inverian structures, a relationship particularly well seen along the Strathan line (Evans I963) ESE from Strathan Bay. Because the Inverian and Laxfordian structures of Lochinver had overall properties which were of the same type, unravelling them was very difficult.

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Dr Tarney's extension of the term Inverian to cover post dyke events in the Lochinver area (assuming one episode of dyke intrusion) is in our opinion un- warranted and only liable to cause confusion. The dykes were intruded into hot rocks that had previously been amphibolized and were again metamorphosed at a later date, under rather similar conditions. If dyke intrusion was connected with tensional tectonics then the pre-dyke and post dyke amphibolizations must be named separately, despite their apparent structural parallelism in this area. The definition of the Inverian events includes a time boundary at 219 ° m.y. currently regarded as the age of intrusion of the Lochinver dyke swarm. Dr Tarney's close linking of Inverian and Laxfordian events accorded well with the author's and Dr J. G. Holland's interpretation of the two events, but we regard the Laxfordian as ancillary to the Inverian: indeed, in the type area of the Laxfordian gneisses the principal (early) foliation was continuous with and probably formed at the same time as the (Inverian) foliation in the transitional zones between Scourie and Laxford Bridge. Professor Sutton has raised the vexed question of nomenclature. Regrettably, I cannot agree that the matter was very clearly set out by Sutton & Watson (195 I). The terms Scourian and Laxfordian are used therein as prefixes to gneisses, rocks, metamorphism, metamorphic episodes, times and periods. Dr Evans and I preferred at first to equate the Inverian with Scourian as a term of equal status, but as a separate episode at a later time, identifying Scourian with a granulite facies, pre-26oo m.y. metamorphism following Giletti et al. (I96I). We now use Park's term Badcallian for that granulite facies event, and Dr Holland and I have used the term Scourie assemblage elsewhere to denote the rocks formed by that process. We hope that separation of terms for time, metamorphisms and rocks will clarify matters, but note that 'Laxfordian' still has multiple meaning. Dr Holland's query, which also dealt with nomenclature, was answered in the af- firmative. The writer does not see in the mainland, Lewisian rocks whose formation can be unambiguously assigned to orogenic events, other than perhaps the Gairloch assemblage. However, this assemblage has not yet been sufficiently described to determine whether, for instance, it might be composed of island-arc rocks, or ocean-floor rocks, or igneous-dominated assemblages developed in a local graben. Where such ambiguity exists, it is preferable to refrain from applying the term orogeny to any event affecting the mainland Lewisian. The evolution of the Scourie assemblage can now be related to the Badcallian and Inverian metamorphic events, but the former in its type area could conceivably result from an igneous episode, (e.g. an invasion of already warm crust by basic magma at depth) whereas the Inverian seems to be related to deep crustal fracturing, and cannot be widely identified as yet. The 'Laxfordian orogeny' is more complex, because the Laxford assemblage had a supracrustal history, subsequently being metamorphosed to high grade. When this occurred, and what the overall nature of the process was remains obscure. In my opinion the major, largely isochemical, metamorphism of the Laxford assemblage occurred after the Badcallian event but prior to the intrusion of the dolerite swarm at Laxford Bridge, believed to be of the same age as that at Lochinver. It would certainly be a contradiction of the original terminology to apply the term Laxfordian to such a pre-dyke event.

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