The Biotite Granite at Strontian, Scotland

Igneous emplacement in a shear-zone termination: The biotite granite at Strontian, Scotland

DONALD H. W. HUTTON Department of Geological Sciences, Durham University, Durham DH1 3LE United Kingdom

ABSTRACT younger porphyritic granodiorite, and an innermost and youngest biotite granite (~90 sq km). Both the tonalite and granodiorite contain numerous Deformed xenolith (strain) data together with other structural microdiorite and microgranodiorite xenoliths, but these are very uncom- information indicate that the biotite granite body (~90 sq km) of the mon in the biotite granite. The granodiorite has been dated at 435 ± 10 Strontian complex, Scotland, was emplaced in the extensional termi- Ma (U-Pb, Pankhurst, 1979; Pidgeon and Aftalion, 1978; Halliday and nation of a dextral transcurrent shear zone. This shear zone is a splay others, 1979) and was thus emplaced either very late in, or just after, the of the major Great Glen fault which lies along the southern boundary Caledonian regional deformation. The biotite granite has been separately of the granite. Siting of the shear zone splay was probably controlled dated at ca. 385 Ma (Halliday and others, 1979), although this is now by (a) a slight releasing bend in the Great Glen fault in this area and considered to be incorrect; the actual age is thought to be only slightly (b) a large, pre-existing, asymétrie synform in Proterozoic metasedi- younger than the tonalite and granodiorite bodies (A. N. Halliday, 1983, mentary country rocks which intersects the Great Glen fault trace at a personal commun.). high angle. A model is proposed in which Moine Thrust (Caledonian) compression at ca. 435 Ma activated the Great Glen fault dextrally. Dextral movements around the releasing bend detached a flat segment from the inside fault wall, and the biotite granite was emplaced sideways at depths of about IS km as a sheet into this extensional, listric-fault-bounded, cavity.

INTRODUCTION

The Caledonian Strontian granite, lying just north of the Great Glen fault in the northwest Scottish Highlands, became well known through the work of Kennedy (1946), who correlated the intrusion with the pluton at Foyers (Fig. 1). This provided an apparently offset marker for the Great Glen fault and indicated a post-Caledonian sinistral movement on the fault of about 100 km. Although subsequent work (Pankhurst, 1979) suggests that these plutons were not originally part of the same body, interest has remained in the possible relationship between the Great Glen fault and the siting and emplacement mechanisms of adjacent granite bodies such as at Strontian (Munro, 1973; Watson, 1984). The present contribution deals specifically with only one of the igneous units at Strontian (the biotite granite) because the emplacement of this body appears to be more ob- viously connected to movements on the Great Glen fault.

REGIONAL SETTING AND THE IGNEOUS UNITS DF VON IAN »YOUNGER AT STRONTIAN DALRADIAN The Strontian complex (-200 sq km) (MacGregor and Kennedy, M0INE + M0INE LIKE 1932; Sabine, 1963; Munro, 1965, 1973) is emplaced in the schists, CALEDONIAN metapsammites and metaigneous bodies of the Proterozoic Moine se- FORELAND quence. These country rocks were deformed and metamorphosed to amphibolite facies during the Grenvillian orogeny ca. 1000 Ma (Brook and others, 1976) and were subsequently redeformed at amphibolite facies Figure 1. Summary lithostratigraphic map of northern Scotland. during the Caledonian orogeny (Barr and others, 1986) between ca. 456 S = Strontian granite complex, F = Foyers granite, GGF = Great Glen Ma and about 440 Ma (van Breemen and others, 1979; A. L. Harris, fault, HBF = Highland Boundary fault, MT = Moine Thrust, NT = 1986, personal commun.). The complex contains three major igneous Naver Thrust, QL = Loch Quoich Line, SBT = Sgurr Beag Thrust, units (Fig. 2). An oldest and outermost tonalité, an inner and probably ST = Swordly Thrust.

Geological Society of America Bulletin, v. 100, p. 1392-1399, 9 figs., September 1988.

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Figure 2. Major pétrographie units in the Strontian granite com- Figure 3. Basic structure in the Strontian complex. Based on the plex. Details of biotite-granite sheets from Institute of Geological following: this study; Institute of Geological Sciences (1977); Munro Sciences (1977). The Great Glen fault runs northeast-southwest along (1965, 1973); Sabine (1963). Solid lines with barbs are generalized Loch Linnhe. foliation trend lines for specified dip ranges in the tonalite and granodiorite. Single double barbed symbols are foliations in the biotite granite. Individual shear zones at LC (Loch a' Choire) and BM (Benin Minor igneous units in the complex include small biotite, hornblende, Meadhoin) are referred to in text. LT = Loch Tearnait. Traverse lines rare pyroxene, orthoclase, quartz bodies referred to in the Caledonian for strain data (Fig. 4) are shown. X, Y, Z locations referred to in text. literature as "appinites" (Wright and Bowes, 1979). These predate and postdate the major igneous units. Also found are porphyritic granodiorite dikes which cut the tonalité and granodiorite and are believed to be of STRUCTURE similar age as, and related to, the biotite granite (Sabine, 1963). Minor lamprophyre, aplite, and pegmatite intrusions are also seen. In the northern part of their outcrop, the tonalite and granodiorite The biotite granite (or biotite adamellite; Sabine, 1963) is a medium- form a synformal-shaped intrusion with inward-dipping Moine contacts. grained oligoclase, orthoclase, quartz, biotite (with rare hornblende) body Farther south, the western contact is long, steep, and relatively straight. which clearly intrudes, by sheeting and stoping, the earlier granodiorite These units contain a well-developed foliation which is broadly concor- and tonalité. This unit is characterized by a northern area dominated by dant with the Moine contact. The synformal shape of the northern contact thick and generally steeply inclined sheets which together with the related is thus expressed as a synformal pattern of the foliation, and this extends granodiorite porphyry dikes change in trend from northeasterly in the southward of Loch Sunart (Figs. 2, 3). Adjacent to the long western west to easterly in the middle and southeasterly in the east (Fig. 2). The contact, the foliation is generally steep and parallel to this. In the vicinity of western contact by contrast is long, steep, fairly straight and cuts cleanly the main northern body of the biotite granite foliation, however, dips in the across the older units. In the southwest part of the complex, the biotite granodiorite are modified and become steeper. This is especially seen in granite intrudes directly into the Moine country rocks. the southwestern area where the foliation dips diminish at first eastward

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from the Moine contact, indicating the original continuation southward of pre-full-crystallization-type fabric. The expression of this later deforma- the synform of foliation, before steepening again toward the biotite granite tion within the biotite granite, as stated above, dies off northward and in a contact. broad way images the steepening of foliation in the adjacent granodiorite. Through much of the northern part of the tonalite and granodiorite, The nature of these implied deformation gradients is explored in the next this foliation is a planar and sometimes linear alignment of plagioclase, section with strain data. hornblende, and biotite; the individual minerals showing little sign of internal deformation by creep or dislocation. Quartz and K-feldspar, how- STRAIN DATA ever, are basically unaligned and fill the interstices between the other aligned minerals or in the case of K-feldspar occur also as large late The variation in the mean shape of populations of small xenoliths, as phenocrysts. These minerals show little sign of internal deformation. This a guide to internal strain variations and emplacement mechanism in plu- type of fabric is considered to have formed by deformation which was tons (Holder, 1979; Hutton, 1982a; Courrioux, 1987), has been imposed on the tonalite and granodiorite before both had fully crystallized. extensively utilized in the tonalite and granodiorite at Strontian. The xeno- They can thus be termed "pre-full-crystallization" fabrics (Hutton, 1988) liths, which are a few centimeters up to (rarely) 2 m in length, are com- and are similar to the hypidiomorphic-granular-textured foliations of posed of microdiorite and microgranodiorite (Sabine, 1963). Such mafic Sierra Nevada (Bateman and others, 1983). In the vicinity of the biotite xenoliths or enclaves are common in other intermediate plutons elsewhere granite, however, and especially in the elongated southwestern outcrop of in the world, and although many have clearly been brought up in the the tonalite and granodiorite, the minerals show evidence of creep in magmas from deeper levels (Holden and others, 1987), there are examples quartz and creep, dislocation, nd bending in plagioclase, hornblende, at Strontian where such xenoliths form by the disaggregation of small biotite, and K-feldspar. These latter fabrics have thus a distinctive "crystal appinite bodies in the granodiorite (P. Holden, 1984, personal commun.). plastic strain" aspect to them, and deformation here was probably imposed Whatever their origin, these xenoliths have regular ellipsoidal and lenticu- in the solid state when all phases in the magma had crystallized. In some lar shapes, and their largest principal planes lie in or close to the mineralog- places in the southwest (see below), the earlier pre-full-crystallization ical foliation planes of the host rock. In three dimensions, they are fabrics are preserved or weakly overprinted by the crystal plastic strain generally disc shaped (K = 0), although plane strain (K = 1), and, more fabrics. Therefore, and because most granite bodies evolve by one-way rarely, prolate (1 < K < shapes are seen. cooling of uncrystallized magma, the crystal plastic strain textures record At any individual locality, the shapes of xenoliths were determined deformation increments which were imposed on the tonalite and grano- on two sets of mutually perpendicular joint surfaces which were approxi- diorite later in the deformation history than the deformation increments mately parallel to two of the principal strain planes (as defined by the preserved farther north; furthermore, these later deformation increments foliation). For each principal plane, the long and short axes of individual appear to be concentrated around the western contact zone of the biotite xenoliths were measured, and a shape ratio was calculated. Between 20 granite. and 50 (usually about 30) xenoliths were thus measured. From these data, The biotite granite itself is generally undeformed (Sabine, 1963; a geometric (logarythmic) mean was calculated. This is a measure of the Munro, 1965) particularly in the northern sheeted area. This work, how- mean shape of the ellipses for that strain plane. A similar exercise is carried ever, has found that deformational fabrics are patchily developed in the out on another (orthogonal) joint surface, and the two mean ratios can main body of this unit and especially in a zone along the western contact then be combined to determine the mean overall shape of the xenoliths. which is approximately triangular in shape and broadens southward (Fig. This is repeated at other localities in the pluton; the strain variations, 3). The fabrics here are normally vertical and aligned almost parallel to the augmented where necessary by visual and qualitative estimates of strain western contact with the tonalite and granodiorite, and farther south, with intensity from fabric development, were determined. In this way 11,800 the Moine contact. The foliation, like that in the northern sector of the xenolith measurements have been made on the Strontian granite at 125 tonalite and granodiorite, is a pre-full-crystallization fabric and as such is localities, of which the data from 51 localities (about 2,500 individual in marked contrast to the crystal plastic strain fabrics in the adjacent measurements) are presented in this paper. Data from the remaining locali- southwestern tonalite and granodiorite. The relationship between these ties are relevant to the emplacement of the tonalite and granodiorite units, different fabric types can be examined in the marginal contact zone of the and this will be described elsewhere (Hutton, 1988). biotite granite. Thus at NM 7470 4490 (2 km due south of Loch Tearnait; Before describing the strain variations, the general validity of these X in Fig. 3), the crystal plastic strain fabric in the granodiorite cuts across data should be discussed. It could be argued that the variations in mean into a biotite granite sheet where it becomes a pre-full-crystallization xenolith shape relate more to large predeformational variations in fabric. In addition at NM 7550 4449 (2.5 km south-southeast of Loch the shapes of xenoliths than to variations in the imposed strain; however, Tearnait; Y in Fig. 3), previously deformed xenoliths of granodiorite and (1) variations in apparent strain measured from the xenoliths always appinite occur within deformed biotite granite. In this case, the fabrics correspond to observed variations in the relative intensity of fabrics in the within the xenoliths are strongly misaligned and oblique to the pre-full- pluton; (2) the size of the standard deviations of the log-normalized popu- crystallization fabric in the enclosing biotite granite. Finally at NM 7390 lations of shape ratios from any locality are found to be remarkably 4049 (at the biotite granite-Moine contact on the shore of Loch Linnhe; constant through large variations in mean shape between localities; (3) the Z in Fig. 3), a microdiorite xenolith within a larger xenolith of granodiorite standard deviations are also very similar even where there are variations in set in biotite granite is obliquely sheared by a new crystal plastic strain the population numbers between localities; and (4) the standard deviations fabric parallel to the pre-full-crystallization foliation in the surrounding are fairly constant where there are variations in the trend of the strain biotite granite. In conclusion, these field data indicate that, following an planes (foliation planes) within the pluton. These facts indicate that the early pre-full-crystallization deformation in the tonalite and granodiorite, microdiorite and microgranodiorite xenoliths had originally well-con- both units were intruded by biotite granite. All three units were then strained shapes that did not deviate significantly from spheres. Thus, unlike deformed. This gave rise to (1) a solid state foliation in the tonalite and deformed country-rock xenoliths which do usually show significant varia- granodiorite which by and large reworked and overprinted the earlier tions in initial shape (Hutton, 1982b), there is probably no real need to pre-full-crystallization fabric and (2) a concordant foliation in the biotite include an initial shape calculation in strain determinations from the mi- granite, which, because it had incompletely crystallized at this time, was a crodiorite and microgranodiorite xenoliths.

WEST

G RANODIORITE

WEST EAST WEST EAST Figure 4. X/Z mean strain 20-| 0 (shape ratios) from microdiorite 510 and microgranodiorite xenoliths ^ 8 in the southern part of the " 6H Strontian complex. Locations of £ 4 < 3H traverses shown in Figure 3. 1 2- Note vertical (strain) scale is log- NO OBSERVEO- STRAIN arithmic (to base 10). GD GD BG 1 WEST EAST WEST EAST 30 o INTENSE FABRIC^ -20H SMALL DEXTRAL SHEAR ZONES

8 £ 6 v < 4 STRONGER FABRIC \ x 3 IN BIOTITE GRANITE \ WEAK FABRIC IN uo .X'BIOTITE GRANITE 2 \? GD BG GD BG

2 Km

The viscosity contrast between xenoliths and host (Hutton, 1982a) across the tonalite and granodiorite to the south of Loch Sunart and is can also affect apparent strain variations. Very little quantitative data are included to show a typical X/Z strain profile through the original synfor- available for this at Strontian. Xenoliths, however, often show a slightly mal shape of the older units of the pluton. These show high values at the more intense development of foliation within them than in the surrounding Moine-tonalite contact, a slight "kick" near the tonalite-granodiorite host. This suggests that at the time of deformation they were mechanically boundary, and a general decrease (to values of between 2 and 3) in the weaker than the tonalite and granodiorite, and therefore strain values main body of the granodiorite. measured from the xenoliths are somewhat larger than that in the host. Traverse B lies farther south and runs across from the western Moine The viscosity contrast, however, should remain fairly constant throughout contact to the western limit of the dense biotite granite sheets. Strains at the the pluton, and therefore the variations in strain (strain gradients) are still western margin of the tonalite are (qualitatively) as high as they are farther accurately reflected in the xenolith data. north, and the pattern east of this is similar, but the lowest strains are now Data are presented from five traverses through the southern and about 3; this rises onto a somewhat higher plateau farther east. southwestern part of the tonalite and granodiorite (Figs. 3, 4). The two Traverse C shows the same basic pattern, but the lowest values are joint surfaces on which measurements were made are (1) the surface constant between 3 and 4. The eastern end of this traverse runs into the which is at right angles to the generally steep and north-south-trending main body of the biotite granite. No increase in the strain of the granodio- foliation planes and which is often horizontal or has a north-south strike rite is seen, and no fabrics are developed within the biotite granite. Thus and low dips and (2) the surface which is at right angles to this and to the strain in the latter body is effectively zero. foliation plane. This has generally steep dips and east-west strikes. Al- Traverse D shows a similar type of X/Z strain gradient to C. The though data were collected from both surfaces, they are more complete Moine-tonalite contact strains, however, are much higher than before, because of the nature of the topography for the (generally) horizontal and the low strain "trough" has risen to between 5 and 6. At the eastern surfaces and these data alone are presented. The mean shapes from the end of this traverse, the granodiorite strains rise toward the biotite granite. (generally) vertical and east-west surfaces, however, show similar gradients The biotite granite in this area contains a foliation, although this represents as do the presented data. The combination of these data indicate that in a much lower level of strain than it does in the adjacent granodiorite. The three dimensions the mean xenolith shapes are disclike (K ~ 0) or elongate strain therefore drops in the biotite granite, although not to such a low discs (0 < K < 1) with the long axis consistently horizontal. Thus in level as before. general, the strain ratios from the horizontal surfaces are X/Z ratios of the Traverse E is across almost the narrowest part of the tonalite and finite strain ellipsoid, and the ratios from the vertical east-west surfaces are granodiorite at Loch Ternait. The marginal tonalite here is extremely Y/Z ratios. deformed. It should be realized that the accurate measurement of very The northernmost traverse A (Fig. 4, location in Fig. 3) runs right large strains from highly elongate xenoliths is hampered (both here and at

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the western end of traverse D) by vegetation cover. Measurements in this situation will often underestimate the length of the longest (X axis) of xenoliths, and hence the mean shape ratios are minima. Farther east along the Lochside, exposure is better, and the strains show a high and generally erratic profile. The high peaks correspond to observed small- and medium- scale shear zones in the granodiorite. The minimum values in this traverse are therefore fairly consistent but are now at values of between 6 and 7. Strains rise again toward the east, and fabric development is intense in the granodiorite and in a long thin appinite body along the biotite-granite contact. Foliation in the biotite granite in this general area is well developed and always obvious. One locality in the biotite granite contains xenoliths, and these show a mean value of about 3. Thus, as before, the strain drops in the biotite granite but to a higher level than that seen farther north. In general, one can see that on moving south along the gradually narrowing outcrop of the tonalite and granodiorite: (a) the Moine-tonalite contact strains become higher; (b) the minimum strains in the medial granodiorite continually increase; (c) the strains eventually come to rise toward the biotite granite; and (d) the strains in the adjacent biotite granite rise from zero in the north to higher, but still very low (when compared to the contact granodiorite strains), in the south. This occurs with a concordant increase in fabric development and a southward broadening of the zone of fabric within the biotite granite. It also occurs with a southward Figure 5. Structural data increase in the development of late-stage crystal plastic strain foliations in from the southwest part of the the tonalite and granodiorite and the steepening of these fabrics toward the pluton. FiHed barb single lines biotite granite contact. As noted before, however, the fabric in the biotite are biotite-granite foliations. granite is essentially of pre-full-crystallization type. Empty barb single lines are to- These data taken together imply that there was a zone of high strain nalite and granodiorite folia- (a shear zone) in the site of the western margin of the biotite granite before tions; T = tonalite, GD = grano- this granite was emplaced. The amount of strain in the shear zone in- diorite, stipple = western margin creased toward the south, and at a late stage in its development, the shear of biotite granite, LT = Loch zone was intruded by the biotite granite which was itself deformed by the Tearnait, LU = Lough Usige. last increments of the strain. The geometry of this shear zone is described in the following section.

SHEAR SENSE Glen fault), the granodiorite is affected by a number of medium-scale As stated above, the mean xenolith shapes throughout the southwest- dextral shear zones exposed along SOO m of coastline. These shear zones ern part of the tonalite and granodiorite have their long (X) axes arrayed are picked out by measured X/Z strain variations combined with gross subhorizontally. This implies that the shear zone is basically transcurrent. fabric reorientations. Thus low strains (average 3-4) are associated with A map of the foliation pattern in the southwest of the pluton (Fig. 5) shows a sigmoidal swing from north or north-northeast trending in the Moine contact zone through north-northwest trends in the medial lower strain part of the granodiorite, back to a north trend in the high strain adjacent to the biotite granite in the east. This pattern is compatible with LOCH ÄCH0IRE transcurrent dextral shear superimposed on an originally north-south- trending early fabric in the tonalite and granodiorite (Soper and Hutton, 1984). Smaller scale evidence for late-stage dextral shear on northerly trends is seen both in the southwest and in other parts of the pluton. At Loch Tearnait, small- and medium-scale shear zones 2-10 m wide show consistent dextral offsets and dextral deflections of tonalite and granodiorite fabrics. Two of these shear zones contain granodiorite-porphyry sheets emplaced along the shear zones. One is foliated parallel to the contacts, J!^ LOCH /(if LINNHE and the other is undeformed. The granodiorite porphyry is considered to y*-3-6 be closely related in intrusion age to the biotite granite (Sabine, 1963). Emplacement is thus syndeformation and postdeformation. In the extreme southwest of the biotite-granite contact at NM 7390 4049, the previously Figure 6. Structural details in granodiorite on the southern shore described microdiorite xenolith within the granodiorite xenolith is re- of Loch a' Choire (location in Fig. 3). Fine lines are extrapolations of worked by dextral shearing with a new crystal plastic strain fabric that is foliation pattern around shear zones. Heavy barbed lines are observed concordant with the adjacent biotite-granite foliation. In the east of the foliations along shore. Numbers are mean X/Z xenolith ratios at dif- pluton at Loch a' Choire (Fig. 6) near Loch Linnhe (the site of the Great ferent localities.

I west-northwest-east-southeast foliation strikes, and as these swing to ap- I LOCH OUOICH proximately northerly trends, the strains increase markedly. In some cases, PS LINE the central parts of the shear zones are exposed. They show intense devel-

I MQINE opment of crystal plastic strain fabrics sometimes expressed as a second GENTLY cleavage, "S2", overprinting earlier pre-full-crystallization fabrics in the INCLINED granodiorite. Small biotite-granite sheets occur in this area. In some cases, Figure 7. Generalized they lie along and parallel to the shear zones and are deformed by them; simple model for the em- whereas in other places, the sheets cut across the shear zones and are BIOTITE GRANITE placement of the biotite gran- undeformed. This, like Loch Tearnait, implies that biotite-granite intrusion SHEET COMPLEX ite in a dextral shear-zone occurred concurrently with and after dextral shearing of the granodiorite. termination related to the Finally in the middle of the main biotite-granite body at NM 7910 5115 Great Glen fault. See text for south of Beinn Meadhoin (Fig. 3), a north-northeast-trending, 300 m long, further details. 50 m wide, shear zone occurs. This focuses around a small appinite body within the biotite granite, and strain determinations together with foliation deflections show that this is also dextral. These smaller scale data are all consistent and indicate, as with the major foliation swing in the southwest of the pluton, that dextral shearing on broadly northerly trends occurred after granodiorite emplacement and concurrently with or after biotite granite emplacement.

Figure 8. More specific model showing mechanism for biotite-granite emplacement in a space created by removal of a flat, but ramp- bounded, segment along the Quoich Line by dextral transtensional movements on the Great Glen fault. Sections A-A' and B-B' further illustrate the geometry (biotite granite shown as crosses). Fold details in Moine west of Quoich Line redrawn from Roberts and Harris (1983).

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Harris, 1983) (Fig. 1). The southerly projection of this line runs more or less along the axis of the foliation synform in the Strontian tonalite and granodiorite and approximately along the western margin of the biotite granite (Fig. 7). The implication, therefore, is that the dextrai shear zone was controlled in orientation and position by the pre-existing steep wall of the Quoich Line. That being the case, one can speculate that the flat pre-existing structure east of the line also controlled emplacement and generated a lower flat detachment and contact to the biotite granite. Be- cause the Quoich Line in this area is a southward plunging synform, the northern termination of the lower detachment must steepen northward, possibly in a listric manner (Fig. 8, section B-B'). This moderately inclined footwall then swings east into the steep transcurrent sidewall along the western side of the Quoich Line. Thus if the flat rocks east of the pre- existing Caledonian structure do control emplacement, as the steep rocks to the west of it appear to do, then the biotite granite may have been emplaced in an extensional listric fault cavity. Figure 8 shows how this model would work. The hanging wall to the intrusion would have to be coupled in some way to the south side of the Great Glen fault. Dextrai and transtensional movements on the fault open a space in the main fault itself and pluck a segment of Moine and earlier Strontian granite units out of the ca. 435 Ma. This shows how Moine Thrust com- Quoich Line synform. Biotite granite magma injects upward from the pression generates dextrai transtension in a con- main fault zone and into the flat space that is being generated (Fig. 8, cave northward-releasing bend of the Great Glen section A-A')- Emplacement occurred at pressures of ~4 kbar (~15 km fault and the detachment of a wall segment from depth) (Ashworth and Tyler, 1983). the Quoich Line. Taken on a larger scale (Fig. 9), this tectonic relationship appears to fit with northwest Highland tectonics. The best dating on the Strontian EMPLACEMENT MODEL AND DISCUSSION complex is 435 ± 10 Ma (Pankhurst, 1979; Halliday and others, 1979). Although this is for the granodiorite, the biotite granite is, by general The simplest model is one in which a dextrai shear zone diminishes in consensus, only slightly younger. During this general period, displacement its displacement northward and terminates in a zone of extensional frac- was occurring on the Moine Thrust system farther west (Halliday and turing and jointing. Into this pull-apart, the biotite granite is emplaced others, 1987). Movement directions were west-northwest to east- (Fig. 7). The arcuate northern sheeted complex of the biotite granite southeast, and the thrust zone is between 10° and 20° oblique to the Great represents the extensional termination, and the western contact is con- Glen fault. Assuming that the latter fracture was already in place as a strained to be long and straight by the deformation in the shear zone. steep, deep-reaching fault, Moine Thrust compression would have created Dextrai shearing began when the tonalite and granodiorite had cooled a dextrai component of shear on the Great Glen and its wallrocks (Fig. 9). sufficiently for crystal plastic strain fabrics to be produced in them. Shear- In published maps, the Great Glen fault is shown as bending slightly ing would have probably been heterogeneous on a small scale (as pre- toward a more westerly trend around the south side of Strontian and served at Loch a' Choire) but with a main locus in the southwest of the farther west (Figs. 1, 9). Dextrai movement on the fault would have pluton. The biotite granite intruded into the zone in an essentially passive created a releasing bend here and a local transtensional environment. Thus manner by sheeting and stoping of the granodiorite and tonalite. Limited if magma were available in the general area, it would have been sited here. deformation continued during and after emplacement, giving rise to the This tectonic setting therefore provides the prerequisite for extensional, marginal zone of foliation along the western contact and smaller, more sideways emplacement of the biotite granite at Strontian. discrete shear zones within the body itself. Because the amount of strain, and hence displacement, in the main dextrai shear zone diminishes north- ACKNOWLEDGMENTS ward, it reasonably follows that it increases southward beyond the present coastline toward the Great Glen fault. It seems highly likely, therefore, that This work was carried out through a Natural Environment Research this fault operated dextrally at this time and that a northerly trending splay Council Grant GR3/4633 and is part of a multidisciplinary study of of the Great Glen controlled emplacement of the biotite granite at Caledonian granites involving Alex Halliday (Scottish Universities Re- Strontian. search and Reactor Centre, Glasgow; now at University of Michigan), Ed The reason for siting the splay at this location can be examined by Stephens (University of St. Andrews), and Bruce Yardley (University of considering the pre-existing Caledonian structure of the Moine rocks in Leeds). Thanks to Ed Stephens and Peter Holden for their companionship this area. Of particular interest is the north-northeast-trending Loch in the field and their discussions of these ideas; Andrew Bell and Rob Quoich line (Roberts and Harris, 1983) (Figs. 1, 7) which separates Butler for discussion of extensional tectonics; the proprietors of the Ben steeply inclined and upright, tightly folded rocks in the west from weakly View Hotel, Strontian, for their hospitality; and Art Ford (U.S. Geological deformed, gently inclined rocks in the east (see Fig. 8, section A-A'). It is Survey), who showed me the easy way to crunch strain data during the essentially, therefore, an asymmetric synform and is considered to be the evenings while we were on field work in Haines, southeast Alaska. I would eastern limit of Caledonian reworking (between about 456 Ma and 440 also like to thank M.R.W. Johnston, A. L. Harris, and an anonymous Ma) of the Moine rocks in the northwest Scottish Highlands (Roberts and reviewer for their helpful comments.

REFERENCES CITED Kennedy, W. Q., 1946, The Great Glen fault: Geological Society of London Quarterly Journal, v. 102, p. 41-76. Munro, M., 1965, Some structural features of the Caledonian granite complex at Strontian, Argyllshire: Scottish Journal Ashworth, J. R., and Tyler, I. M„ 1983, The distribution of metamorphic temperatures around the Strontian Granodiorite: of Geology, v. l,p. 152-175. Geological Magazine, v. 120, p. 281-290. 1973, Structures in the south-eastern portion of the Strontian granitic complex, Argyllshire: Scottish Journal of Barr, D., Holdsworth, R, E., and Roberts, A. M., 1986, Caledonian ductile thrusting in a Precambrian metamorphic Geology, v. 9, p. 99-108. complex: The Moine of northwestern Scotland: Geological Society of America Bulletin, v. 97, p. 754-764. MacGregor, A. G., and Kennedy, W. Q., 1932, The Morvern-Strontian 'Granite': Summary of Progress of the Geological Bateman, P. C , Busacco, A. J., and Sawka, W. H., 1983, Cretaceous deformation in the western foothills of the Sierra Survey, London (for 1931), p. 105-119. Nevada, California: Geological Society of America Bulletin, v. 94, p. 30-42. Pankhurst, R. J., 1979, Isotope and trace element evidence for the origin and evolution of Caledonian granites in the van Breemen, 0„ Aftalion, M„ Pankhuret, R. J., and Richardson, S. W., 1979, Age of the Glen Dessary Syenite, Scottish Highlands, in Atherton, M. P., and Tarney, J., eds., Origin of granite batholiths, geochemical evidence: Inverness-shire: Diachronous Palaeozoic metamorphism across the Great Glen: Scottish Journal of Geology, v. IS, Kent, England, Shiva Publishing, p. 18-33. p. 49-62. Pidgeon, R. T., and Aftalion, M., 1978, Oogenetic and inherited zircon U-Pb systems in granites: Palaeozoic granites of Brook, M., Powell, D., and Brewer, M. S., 1976, Grenville age for rocks in the Moine of northwestern Scotland: Nature, Scotland and England, in Bowes, D. R., and Leake, B. E., eds., Crustal evolution in northwestern Britain and v. 260, p. 515-517. adjacent regions: Geological Journal Special Issue No. 10, p. 183-248. Courrioux, G., 1987, Oblique diapirism: The Criffel granodiorite/granite zoned piuton (southwest Scotland): Journal of Roberts, A. M., and Harris, A. L., 1983, The Loch Quoich Line—A limit of early Palaeozoic crustal reworking in the Structural Geology, v. 9, p. 313-330. Moine of the northern Highlands of Scotland: Journal of the Geological Society, London, v. 140, p. 883-892. Halliday, A. N., Aftalion, M., van Breemen, O., and Jocelyn, J., 1979, Petrogenetic significance of Rb-Sr and U-Pb Sabine, P. A., 1963, The Strontian granite complex, Argyllshire: Geological Survey of Great Britain Bulletin, v. 20, isotope systems in the c. 400 Ms old British Isles granitoids and their hosts, in Harris, A. L., Holland, C. H.. and p. 6-42. Leake, B. E., eds., The Caledonides of the British Isles—Reviewed: Geological Society of London Special Soper, N. J., and Hutton, D.H.W., 1984, Late Caledonian sinistral displacements in Britain: Implications for a three-plate Publication No. 8, p. 653-677. collision model: Tectonics, v. 3, p. 781-794. Halliday, A. N., Aftalion, M., Parsons, I., Dickin, A. P., and Johnston, M.R.W., 1987, Syn-orogenic alkaline magmatism Watson, J., 1984, The ending of the Caledonian orogeny in Scotland: Journal of the Geological Society, London, v. 141, and its relationship to the Moine thrust zone and thermal state of the lithosphere in northwest Scotland: Journal of p, 193-214. the Geological Society, London, v. 144, p. 611-617. Wright, A. E., and Bowes, D. R., 1979, Geochemistry of the appinite suite, in Harris, A. L., Holland, C. H., and Leake, Holden, P., Halliday, A. N., and Stephens, W. E., 1987, Neodymium and strontium isotope content of microdiorite B. E., eds., The Caledonides of the British Isles—Reviewed. Geological Society of London Special Publication enclaves points to mantle input to granitoid production: Nature, v. 330, p. 53-56. No. 8, p. 699-704. Holder, M. T„ 1979, An emplacement mechanism for post-tectonic granites and its implications for their geochemical features, in Atherton, M. P., and Tamey, J., eds., Origin of granite batholiths, geochemical evidence: Kent, England, Shiva Publishing, p. 116-128. Hutton, D.H.W., 1982a, A tectonic model for the emplacement of the Main Donegal Granite, NW Ireland: Journal of the Geological Society, London, v. 139, p. 287-294. 1982b, A method for the determination of the initial shapes of deformed xenoliths in granitoids: Tectonophysics, v. 85, p. 45-50. 1988, Granite emplacement mechanisms and tectonic controls: Inferences from deformation studies: Royal Society MANUSCRIPT RECEIVED BY THE SOCIETY MAY 13,1987 of Edinburgh Transactions: Earth Sciences (in press). REVISED MANUSCRIPT RECEIVED FEBRUARY 18,1988 Institute of Geological Sciences, 1977, Strontian, Scotland sheet 52(E) (solid). 1:50,000 Map Series. MANUSCRIPT ACCEPTED FEBRUARY 23,1988

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