the syntectonic foxdale-archallagan granite and its metamorphic aureole, isle of man

THE SYNTECTONIC FOXDALE-ARCHALLAGAN GRANITE AND XTS METAMORPHIC AUREOLE, ISLE OF MAN

ALEXANDER SIMPSON

The small exposed granite a t Foxdale is merely part of a much larger intrusion, the Foxdale- Archallagan granite, which underlies the country-rock of Manx Slate Series at shallow depth. The granite is post-F, in age. It is a syn-tectonic intrusion and was emplaced during the second (Fa) movement- phase in the Caledonian tectonic chronology of the Isle of Man. Its final consolidation was achieved during the following FrF, static interval. The aureole metasediments consist of porphyroblastic contact phyllites and schists similar to those

produced by regional metamorphism. A textural study has demonstrated that the paracrystalline movement responsible for the directional fabric in these rocks is of tectonic origin and F, in age.

Despite the penetrative, syncrystallization Fr deformation of its envelope, the granite itself is little affected though a mild Fa shearing is visible on its southern fringe. Postcrystalline deformation of the cooled aureole by Ss and F, crumples took place during the third movement-phase. Suggestions are made concerning the form of the Foxdale-Archallagan intrusion in depth. The temporal relationship of the pluton to other syn-tectonic F, acid intrusions in the Isle of Man is considered.

I. INTRODUCTION AND PREVIOUS WORK The Foxdale granite outcrop is situated in the southern half of the Isle of Man (Fig. 1)

and lies 4 ml southeast of Foxdale village. It is emplaced in the Manx Slate Series and forms a low, rounded hill termed ‘Granite Mountain’ on the ordnance maps. The granite outcrop has the shape of an irregular oval (Fig. 2). The longer axis runs east-west for 2 ml; the maximum width from north to south is 4 ml. The intrusion was formerly worked in four quarries situated in the northern half of its outcrop.

Brief reference is made to the intrusion by such early observers as Berger, Henslow and Cumming. During his comprehensive survey of the Island, Lamplugh (a, b 165) mapped and described the outcrop of the granite with accuracy. Short petrographic descriptions of two thin-sections from the granite have been given by Watts (in Lamplugh b 315). A few years before the appearance of the Memoir, Harker (a, b) published some observations which he had made on the igneous body.

The granite is a grey muscovite-granite consisting of quartz, orthoclase, microcline, an acid plagioclase and muscovite. Occasional flakes of biotite occur in some thin-sections as do tiny, sporadic grains of garnet and zircon. The bulk of the intrusion has an even, granitic texture with an average grain-size of 2-3 mm. At the margin a fine-grained facies is adopted in which the grain-size is about 0.5 mm. The outcrop of this marginal microgranite is only a few yards wide at the northwestern contact with the country-rock. On the south side of the mass it forms a broader fringe to the intrusion. Occasionally it possesses a porphyritic texture with larger quartz and felspar crystals of 2-3 mm set in the finer base. The junction of the coarser and finer types.is gradational and they seem to be contemporaneous. The latter appears to be a more rapidly cooled marginal facies which sporadically contains diffuse patches, blebs and veins of the coarser material,

Geol. J. Vol. 4, Pt. 2, 1965 415

416 ALEXANDER SIMPSON

Fig. 1. The location of the Foxdale-Archallagan granite in the Isle of Man. Manx Slate Series, unomamented ; Lower Carboniferous, bricks; glacial drift of northern plain, dots.

D, Dhoon granite; F, Foxdale-Archallagan granite.

The Foxdale granite shelves beneath the slates to the north. The shafts of the now disused Foxdale Mines (Fig. 2), which lie just over & ml north of the intrusion, passed from the overlying sediments into the granite at several hundred feet below the surface (Lamplugh b 507). The same records show that the sloping top of the igneous body is irregular. A surface manifestation of this lies in the small granite cupola which rises to the surface & ml east of the main outcrop and is separated from the latter by a strip of the overlying sediments.

THE FOXDALE-ARCHALLAGAN GRANITE AND ITS AUREOLE 417

The granite outcrop at Foxdale is surrounded by a belt of contact alteration within which the slaty country-rock is recrystallized and rendered silvery in colour. On the west and south sides of the granite this altered tract is narrow (Fig. 2). To the north and northeast, however, it extends as a broad belt for 13-2 ml, almost to the central valley. This extensive thermal aureole is out of all proportion to the granite outcrop at Foxdale which lies near its southwestern margin. The simple explanation of the phenomenon must be that the top of the granite, having shelved below the country-rock at Foxdale; remains at shallow depth under a wide tract of sediments to the northeast. Confirmatory evidence is provided by the shaft of the old Cornelly Mine (Fig. 2) situated at Archallagan, 1& ml north-northeast of the surface Foxdale granite. This mine (‘Old Mines’ on the one-inch ordnance map) passed from the overlying sediments into granite, similar to that at Foxdale, at 300-400 ft below the surface (Lamplugh b 517).

Lamplugh was fully aware of these facts (b fig. 29) but was perplexed by his observation that the strata above the concealed granite at Archallagan were not typical contact alteration hornfelses. Instead, these crystalline rocks were garnetiferous mica-schists of regional metamorphic type. Moreover, the degree of alteration in this northeastern part of the recrystallized tract appeared greater than that displayed close to the exposed granite at Foxdale. In view of these uncertainties Lamplugh suggested that the possible underground continuity of the granite between Foxdale and Archallagan “must be reserved for further investigation”.

Gillott referred briefly to the recrystallized sediments between Foxdale and Archallagan. He also provided petrographic descriptions of a few of these rocks.

2. STRUCTURAL SETTING AND PROBLEMS Simpson (a) has recently elucidated the stratigraphy and tectonics of the Manx Slate

Series. The island forms part of the Caledonian orogenic belt and was subjected to polyphase deformation during its protracted structural evolution. Three distinct phases of tectonic movement (Fl, F,, F3) each formed large-scale folds, congruous minor flexures and an axial-plane cleavage (Sl, S2, SJ. The large-scale, polyphase architecture of this part of the island is conveniently outlined as follows.

The Caledonoid axial trace of the fundamental F, Isle of Man syncline lies 14 ml north of Foxdale (Fig. 2). The northwestern limb of this acute fold is steeply inverted. The Foxdale-Archallagan granite and its thermal aureole lie on the opposite, southeastern synclinal limb (Fig. 3). In the north and west the latter is right way up and also dips to the northwestern quadrant at a gentler angle of circa 30°-500. A mild, chlorite-zone metamorphism (Table 1) was impressed on the Manx Slates throughout the Island during the acute F, folding (Simpson c).

The granite aureole is crossed by the hinge of the large-scale F2 Manx synform. This right-angled flexure refolds the southeastern limb of the Isle of Man syncline. As the right way up strata approach the FP hinge they steepen through the vertical and adopt a southeasterly inversion on the lower synformal limb (Fig. 3). The axial plane of the structure has an overall, gentle northwesterly inclination. It crosses Archallagan (Fig. 2)

41 8 ALEXANDER SIMPSON

228 22 9 23 I 232

---- Gobpirnl Bam&k~

SLATE A Dip of Wdiq

SERIES / Vaticd B d i q

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Injebreck Bmdrd Group

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Fig. 2. Geological map of the Foxdale-Archallagan area.

and skirts the southern margin of the Foxdale granite outcrop. The growth of the Fa folding (Simpson c) was first accomplished by flexural-slip along the bedding or the pre- existing first axial-plane cleavage (SJ. In the latter part of the F2 phase this mechanism was largely replaced by planar-slip along the gentle, second axial-plane cleavage (SJ to provide the requisite accommodation.


F M Foxdole I 2 I‘ Sk John’i

+ + + + + + + + + + + + , + +’ -

Archolloqon Plbntation

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I ’ ,‘ , -. - . --!,-2 I I ;.‘ ’, .- / \

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+ + + + + + + + + + + + + + + + ‘ 4 D

Fig. 3. Geological profiles across the Foxdale-Achallagan area (Fig. 2). The lines A - B and C - D are indicated on the margins of Fig. 2. Scale, ornamentation and symbols as in Fig 2. 1 , Isle of Man syncline (F1): 2, Manx synform (Fa).

During the F, movement the open, dextral (White and Jahns 197) Foxdale fold-pair was superimposed on the plicated strata. The steep axial planes of the antiform and synform comprising this structure strike to the north-northwest (Fig. 2). In the short limb of the fold-pair, which crosses the northern fringe of the aureole, the trend of bedding, S1 and earlier linear structures is deflected from its usual Caledonoid orientation towards the east or east-southeast. The same late deformation is also responsible for a gentle undulation of the major Fa axial plane about northwesterly axes.

The country rock of the Foxdale-Archallagan granite and its aureole is largely formed by the Maughold Banded Group (Fig. 2). This formation (Simpson a 371) consists of thin, rapidly alternating bands of pelite and fine-grained psammite usually less than one inch thick. Only at its northern tip, in the deeply cut valley of Glion Darragh, does the aureole impinge on the uniform pelite of the Barrule Slates (op. cit. 371) and the younger Injebreck Banded Group (op. cit. 371) in the core of the Isle of Man syncline.

In the light of these recent findings, three important problems emerge concerning the Foxdale-Archallagan granite.

(1) The time of granite emplacement relative to the lengthy tectonic chronology recorded

(2) The sequence of crystallization in the granite envelope and the relationship of para-

(3) The shape of the granite intrusion.

in the country-rock.

crystalline deformation.


3. METAMORPHIC EVOLUTION OF THE AUREOLE (a) Introduction

The metamorphism of the Manx Slate Series by the Foxdale-Archallagan granite is post-F, in age. Within the thermal aureole the fine-grained Fl textures of the slates (Simpson c 23) are destroyed by recrystallization and replaced by a coarser, crystalline fabric.

A striking feature of the aureole metasediments is that they are not decussate hornfelses. They consist of silvery, fissile contact phyllites and schists which possess all the characters of metamorphic sediments in areas of regional metamorphism.

The formation of this directional fabric in the aureole rocks can only be attributed to movement in the granite envelope during its recrystallization. The textural study described in the following pages has demonstrated that this para-crystalline movement belongs to the second (FJ movement-phase. The lengthy sequence of crystallization within the granite aureole is summarised in Table I .

PLATE 24

A Contact schist from the lnjebreck Banded Group in Glion Darragh, Archallagan (297804). During F, flexural-slip the fine-grained F1 fabric of the pelite has been reconstituted to a coarse felt of muscovite and chlorite with sporadic quartzose slivers. Two thin sandy laminae have recrystallized to an interlocking quartz mosaic in which the individuals are equidimensional or slightly elongate parallel to the bedding-foliation X.N. x37.

B Para-crystalline F, microfolding in contact phyllite of the MaughoId Banded Group close to the northwestern margin of the Foxdale granite outcrop (282776). Horizontal quartz-rich strain-bands delineate the hinges of the microfolds. Lying astride all the complexities of this fabric are small, random porphyroblasts of chlorite, biotite, garnet and tourmaline which grew immediately after the F1 movement had ceased in this part of the aureole. Besides forming small, parallel-sidrd flakes, the chlorite also occurs as larger, irregular blebs some of which can be seen at the top of the field. P.P.L. x37.

C A sandy rib from the Barrule Slates within the granite aureole at Glion Darragh, Archallagan (297803). During F, flexural-slip the rock was reconstituted to an interlocking quartzchlorite fabric and the original grain outlines were lost. The chlorite flakes were aligned parallel to the bedding which dips steeply from left to right and is delineated by two, more quartzose laminae. The recrystallized quartz (and occasional felspar) grains are either equidimensional or slightly elongate parallel to the bedding-schistosity. After the cessation of flexural-slip, F, planar-slip ensued along flat-lying S, slipplanes. Near the centre of the field one of the quartzose laminae has been noticeably offset by displacement along one of the S, surfaces. The chlorite flakes and quartz grains were dragged along the slip-planes but continued syn-kinematic crystallization enabled the fabric to regenerate itself during the deformation. None of the chlorite laths which outline the S, surfaces, and lie oblique to the earlier foliation, are bent nor does the fabric display any strain resulting from the F, movement. P.P.L. x37.

D-F Well-formed porphyroblasts of garnet (D), staurolite (E) and cordierite (F) in contact phyllit? within 4 ml of the Foxdale granite outcrop. These porphyroblasts grew in the deformed fabric during the static interval which followed the F, movement. They have overgrown all the F, microfolding and horizontal S, slipplanes. The internal trends (Si) are clearly in continuity with the enclosing fabric (Se). Jn E a conspicuous quartwse, and indigestible, strain-band continues horizont- ally from the surrounding contact phyllite through the middle of the larger staurolite porphyroblast. While the garnet and staurolite are fresh, the cordierite has been largely replaced by an aggregate of chlorite and muscovite. Besides the large porphyroblasts, small undefonned blebs and rectangular flakes of post-F, chlorite and biotite can also be observed, scattered at random through the fabric. None of the porphyroblasts displays any postcrystalline rotation or other deformation. P.P.L. X37.

Geol. Jotimnl, Vol. 4 PLATE 24


(b) Early Syn-tectonic Crystallization

The basic phyllitic or schistose texture is outlined by the parallel orientation of stout muscovite and chIorite flakes. In pelite or semi-pelite these form a lepidoblastic felt in which are set small, interlocking quartz grains, either equidimensional or lenticular (Pl. 2 4 ~ ) . Original sedimentary boundaries have been destroyed even in medium-grained psammitic ribs which have recrystallized to an interlocking quartz mosaic, equant or slightly elongate in the plane of the fabric.

The grain size of the aureole metasediments is considerably larger than the pre-existing, slaty F, fabric (0.01-044 mm). Around Foxdale the muscovite flakes range in size up to 0-1-0.2 mm. In the eastern half of the aureole around Archallagan the fabric is somewhat coarser and these laths vary up to 0.4 mm. The dominant flaky minerals are muscovite and green chlorite. Red-brown biotite occurs only sporadically. Ilmenite plates, formed at the same time, lie parallel to the foliation. A proportion of the latter are altered, wholly or partially, to leucoxene. Occasional small granules of epidote may also occur. The recrystallization destroys much of the dusty appearance which characterises the F, pelitic fabric in thin-section. Occasionally, however, sporadic trails of this finely divided graphitic material survive the remaking.

The foliation of the contact phyllites and schists is aligned parallel to either bedding or S , in the metasediments. It pre-dates the formation of the gently inclined S , slip-planes which were generated in the latter part of the F2 phase (see below). It is therefore concluded that the para-crystalline movement controlling the development of the planar fabric was thejlexural-slip along bedding or S , which initiated the growth of the F2 Manx synform (p. 418).

Within the aureole, as elsewhere in the island, further development of the F, folding was largely accommodated by small displacements along the gently inclined, axial-planar S2 slip-planes. This strain-slip intersects and offsets the earlier planar fabric (PI. 24c and F) and its emergence produces an F2 lineation on the surface of the silvery phyllites. These structures are especially pronounced close to the hinge of the Manx synform (Fig. 2) where S, is usually accompanied by strong micro-folding of the contact foliation (Pl. 2 4 ~ and D-F). Only on the northern fringe of the aureole, removed from the major hinge, are they weakly developed or absent (e.g. in Glion Darragh).

Outwith the Foxdale-Archallagan aureole comparable F2 deformation is impressed on the unchanged, fine-grained F, fabric of the Manx Slates. It is unaccompanied by recrystallization apart from a mild plasticity sometimes displayed by quartz particles. The pIanar-slip along S, and the crumpling of the intervening microlithons flex and deform the fine-grained micaceous felt which defines the F, cleavage in the slates (Simpson c 26).

Within the granite aureole, however, crystallization continued activeIy during F2 planar- slip and enabled the fabric to regenerate itself during the deformation. The S , surfaces of slip are marked by flakes of muscovite and chlorite aligned in rows oblique to the earlier foliation(P1.24~). Thestout muscovitelaths which outline the associated microfolding near the hinge of the Manx synform are not bent or broken and the deformed fabric shows little sign of strain (Pl. 2 4 ~ , c and E). As the microcrumples formed much of the quartz, N


dessimated within the phyllite, was frequently squeezed out of the limbs and accumulated in the hinges as interlocking polygonal or triangular grains; small quartz bodies in the apices of the folds often display a U-shaped form. In these quartzose bands the form of the fold hinges is outlined by arcs of unbent mica flakes (PI. 2 4 ~ ) . In the predominantly micaceous limbs of the microfolds, residual quartz grains have been compressed to thin lenses and slivers. On the exposure such rocks show a preferential tendency to part along the micaceous and quartz-rich strain-bands (Pl. 2 4 ~ and E) in flat bevels homologous in orientation to S,. The formation of such strain-banding emphasises the syn-kinematic nature of the crystallization.

(c) Static Growth of Porphyroblasts

Ultimately the F, movement ceased and a static condition prevailed throughout the aureole (Table 1). Nevertheless, the sequence of crystallization outlasted the movement. This is demonstrated by the growth of a varied selection of porphyroblasts oriented at random in the earlier directional fabric (PI. 2 4 ~ and D-F).

The porphyroblastic minerals are almandine garnet, staurolite, cordierite, red-brown biotite, green chlorite and muscovite. They lie astride all the complexities of the groundmass and, varying in size up to 1 or 2 mm, produce a spotting in the rocks. The garnets tend to weather out as small knots. A variable proportion of green and brown tourmaline in unbroken needles is always present together with occasional crystals of apatite. A further generation of ilmenite, partially altered to leucoxene, was formed and spears through S2 and Fa microfolds.

The porphyroblasts may either be idioblastic or occur as irregular blebs. Cordierite is more restricted in its occurrence than the other minerals and has only been observed in the vicinity of the Foxdale granite outcrop. Larger garnets ranging up to 3-4 mm occur in the eastern part of the aureole around Archallagan (Fig. 2).

Over the southwestern half of the granite aureole (i.e. around Foxdale) the reconstitution of the metasediments terminated with the growth of the above porphyroblasts. With the exception of cordierite, sometimes replaced by an aggregate of muscovite and chlorite (PI. 24~), the later minerals are largely unaltered. They have suffered no rotation or other F, disturbance since they overgrew the parent fabric. The margins of the static-growth porphyroblasts abruptly truncate, without deflection, the pre-existing foliation, F2 microfolding and strain-banding of the enclosing contact phyllite (Pl. 2 4 ~ - ~ ) . The porphyroblasts frequently contain helicitic inclusions of small, often elongate quartz grains which show a pattern similar to and in continuity with the fabric of the groundmass minerals. The planar foliation of the contact phyllites can be traced through the porphyroblasts with ease and is undeflected. Where F, microfolding (Se) affects the host fabric it can be traced through the later minerals (Pl. 2 4 ~ - ~ ) by the comparable sinuosity in the inclusion trends (Si). Quartzose strain-bands continue undisturbed and in parallelism through the porphyroblasts (Pl. 24~) . Ilmenite plates may also form included relics (PI. 2 4 ~ F ) and occasional trails of graphitic dust sometimes survive.

THE FOXDALE-ARCHALLAGAN GRANITE AND ITS AUREOLE (d) Renewed Crystallization Around Archallagan

After the static growth of the porphyroblasts, further F2 movement took place in the northeastern half of the granite aureole from Glion Darragh across Archallagan Plantation (Fig. 2). This presumably resulted from a recrudescence of the more or less vertical F2 compression (Simpson Q 391) accompanied by a further, though localised, degree of closure in the hinge of the F2 Manx synf'orm. This renewal of F, movement is demonstrated by the para-crystalline deformation and modification of the preexisting textures and fabric of the aureole metasediments around Archallagan (Pl. 2 5 ~ - ~ ) . These changes were not impressed on the southwestern part of the aureole around Foxdale where the porphyroblastic contact phyllites remain undisturbed (Pl. 2 4 ~ and D-F).

In Glion Darragh, on the upper limb of the synform (Fig. 3), S, had only been weakly developed or is absent. The renewed stress was relieved by further flexural-slip along the northward dipping foliation of the silvery contact phyllites. The preexisting porphyroblasts suffered post-crystalline rotation so that their planar inclusion trains now lie at a high angle or are perpendicular to the enclosing schistosity (P1.25~). The latter is deflected round the earlier knots. Eyes of recrystallized quartz are formed about the porphyroblasts where the foliation diverges to enclose them (Pl. 2 5 ~ ) . This rotation is particularly well displayed by the equidimensional garnets. Platy muscovite and biotite porphyroblasts which had grown athwart the foliation are frequently deformed to acute rhombs by the subsequent slip. The long diagonals of the flakes have been rotated by the shearing towards the plane of the enclosing foliation and in extreme cases the angle of divergence is small. Occasional flakes have been cut through and segmented by the movement along the schistosity.

This renewed movement along the foliation of the meta-sediments in the vicinity of Glion Darragh was accompanied by syn-kinematic crystallization. While the porphyroblasts remained inert during their rotation, there was an increase in the grain-size of the enclosing schistosity. The interlocking quartz grains of the surrounding medium (Se) are noticeably larger than the inclusion trains (Si) of this mineral within the porphyroblasts. Moreover, as a result of this continued crystallization, the size of the mica flakes comprising the foliation is somewhat greater than those in the contact phyllites around Foxdale (p. 421).

Towards the close of the renewed F, movement, the foliation in Glion Darragh was sporadically deformed by small, open crumples, a few millimetres in amplitude. These form a conspicuous F, lineation which plunges gently eastward parallel to the long axes of the micaceous spindles round the garnets. The asymmetry of the crinkles and the sense of porphyroblast rotation c o n h the relative northward down-dip movement of the higher layers towards the gently inclined axial plane of the Manx synform at depth (Fig. 3).

Further south, around the old Cornelly Mine and Archallagan (Fig. 2), the Maughold Banded Group'steepens (with some fluctuation) to an overall vertical disposition in the hinge of the Manx synform. In view of the different structural position of these rocks, the renewed F, para-crystalline deformation followed a different course from the pre- dominant flexural-slip displayed in the northwarddipping strata of Glion Darragh. The Archallagan metasediments had already attained a stage of metamorphic and structural

423


development similar to that depicted in P1. 2% and P1. 2 4 ~ - ~ from Foxdale, with large garnet and biotite porphyroblasts overgrowing the complexities of the F, deformation generated near the hinge of the Manx synform. The renewal of compression in the core of the large-scale F, flexure has tightened the earlier F, microfolds of the vertical foliation and flexed them, along with the syn-genetic S, slip-planes and strain-bands, round the upper and lower margins of the porphyroblasts. A continuation of this process has squeezed the limbs of the crumples into parallelism and their transposition, accompanied by further syn-tectonic crystallization, has produced a new, flat-lying S2 schistosity, homologous in orientation to the usual strain-slip S , and peculiar to this localised part of the Isle of Man.

The lepidoblastic muscovite and chlorite flakes of this new, planar fabric are larger than their predecessors, ranging up to 0.4 mm. Within the coarser felt occasional elongate lenticles of granoblastic quartz with oblique muscovite flakes represent the remnants of earlier F2 quartzose strain-bands. The S, schistosity wraps itself closely round the upper and lower surfaces of the garnet and biotite porphyroblasts (PI. 25c). The spaces at the sides of these pre-existing knots, where the schistosity parts to diverge round the crystals, have been filled by quartz segregations.

PLATE 25 A-B Garnet porphyroblasts (partially replaced by late-stage chlorite) in contact schist from Glion

Darragh. Following the static-growth of the garnets, renewed F, flexural-slip along the foliation has rotated the inert porphyroblasts clockwise so that their planar inclusion trains of elongate quartz, iron ore and ilmenite now make a high angle with the enclosing bedding-schistosity. In A the post- crystalline rotation is through about 60" while in B it is 90". The enclosing schistosity is deflected round the equidimensional knots to form micaceous spindles. In B, eyes of recrystallized quartz have formed where the foliation diverges to enclose the porphyroblast. P.P.L. x37.

C Flat-lying Sz schistosity at Archallagan wrapping itself round the upper and lower surfaces of two preexisting garnet porphyroblasts. The replacement of the older, vertical and finer contact phyllitic fabric by thc coarser Sl schistosity is complete in this field. Quartzose strain-bands, relict from the phyllite, cross the larger porphyroblast. As a result of para-crystalline slippage along the S , schistosity, the garnets have been rotated clockwise through 45". The strain-banding which was originally disposed parallel to the schistosity now lies oblique to it. There is a striking difference in grain-size between the small quartz inclusions of the garnets and the coarser granoblastic quartz of the enclosing schist. P.P.L. x17.

D The northeastern quarter of this field is occupied by part of a large garnet porphyroblast in contact schist from Archallagan. Around the garnet all stages can be observed in the paracrystalline transposition of the essentially vertical, contact phyllitic fabric to the flat-lying and coarser S t schistosity. The latter has been moulded by the renewed vertical compressive stress round the lower margin of the pre-existing garnet porphyroblast. P.P.L. x17.

E A higher power view of the same garnet porphyroblast as in D. The small elongate, arcuate or U-shaped quartz inclusions contained by the garnet outline helicitic F, microfolding which existed in the contact phyllite prior to the static-growth of the porphyroblast. During the renewal of Fa para-crystalline compression, which tightened the earlier microflexures and transposed them to the flat-lying S, schistosity, the garnet was rotated through 90" by lateral slippage on the new $plane. As a result, the axial planes of the acutely sinuous Si in the garnet are now vertical and lie at right- angles to the enclosing Sr schistosity and to the horizontal axial planes of the same F, microfolds undergoing transposition in the surrounding pelite (cf. D). P.P.L. x37.

F Foliated acid sheet from Glion Darragh. The planar fabric consists of stout, parallel muscovite flakes and granulitic quartz showing little or no strain. The schistosity is deflected round a large strained porphyroclast of quartz which shows incipient granulitization. X.N. x37.

Grol. Joiimal, Vol. 4 PLATE 25


The replacement of the older, vertical contact phyllites by the coarser, flat-lying S , schistosity is only partially complete. In most thin-sections relics of the earlier, contorted fabric can be observed in all stages of transposition (PI. 2 5 ~ ) . While this para-crystalline deformation was proceeding in the pelitic and semi-pelitic material, the coarser psammitic ribs underwent an increase in the grain-size of their interlocking, granoblastic texture which shows no sign of strain. The stout interspersed flakes of muscovite and chlorite adopt a parallelism to the S, schistosity in the adjacent pelitic bands.

Over the area between Cornelly and Archallagan the development of the S , schistosity was in places accompanied by some degree of lateral slip along this flat-lying S-plane. In consequence, many of the equidimensional garnets, enveloped by the schistosity, were rotated through 90". In such porphyroblasts quartzose strain-bands (PI. 2%) traversing the crystals and the axial planes of helicitic microfolds in the contorted Si, relict from the finer phyllitic fabric (PI. 25~) , have been rotated out of parallelism with the S , schistosity to a steep inclination at right-angles.

As a result of the renewed F2 compression, the porphyroblasts sometimes show cracking, corroded margins and occasional fragmentation. Around Archallagan there is always a noticeable difference in grain-size between the small quartz inclusions of the porphyroblasts, derived from the earlier phyllites, and the coarser, granoblastic quartz of the surrounding fabric resulting from the continued syn-tectonic crystallization (Pl. 2 5 ~ ~ ) . The para-crystalline disruption resulting from lateral slippage on the Sz schistosity has also rendered the bedding obscure in parts of the thinly striped Maughold Banded Group.

Ultimately the revival of F, compressive stress in the hinge of the Manx synform came to an end. Around Archallagan, however, the lengthy sequence of crystallization once again outlasted the movement and was continued for a second time in a static environment (Table 1). In this closing phase of the metamorphism another generation of smaller, random porphyroblasts was added to the already complex fabric. These include flakes of muscovite, chlorite and biotite together with small garnets. These late crystals have overgrown the S , schistosity and have suffered no post-crystalline rotation or other disturbance. The larger, earlier porphyroblasts of garnet and biotite may also be partially or wholly pseudomorphed by chlorite and occasionally by quartz. Clots of green chlorite also repIace portions of the enclosing fabric. Occasionally the terminal biotite can be seen replacing the green chlorite which pseudomorphs the earlier garnet and biotite porphyroblasts.

Over the schistose tract in the hinge of the Manx, synform around Archallagan, the silvery surfaces of the S , schistosity form the dominant plane of fissility and give rise to broad gently sloping pavements in stream and quarry sections. The shredded and contorted bedding of the Maughold Banded Group, outlined by paler sandy bands, merely forms an F, linear stripe on this S-plane.

While the bedding is essentially vertical it is much plicated by F, minor folding which is contained within the axial-plane schistosity and can only be seen satisfactorily on the edges of freshly broken slabs. The rise of temperature and recrystallization induced in these rocks by the heat of the underlying granite rendered them a more amenable medium for the synchronous F, deformation than elsewhere in the Isle of Man. The small F, plications do not possess their usual open, right-angled form (Simpson a 385) but adopt an acute recumbent profile.

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(e) Metamorphism of Acid Sheets

Traversing the length of the Isle of Man in a Caledonoid direction is a swarm of steeply inclined and relatively thin acid sheets. Their original constitution varied from porphyritic to non-porphyritic microgranite and they are emplaced along bedding or the first axial- plane cleavage (Sd. The minor intrusions are confined to a narrow belt, two or three miles wide, flanking the axial trace of the large-scale F, Manx synform (Fig. 1). Previous research by the writer (Simpson d ) has demonstrated that the swarm was emplaced in the synformal hinge early in the second (FJ movement-phase. Subsequent F2 flexural-slip strongly sheared the sheets parallel to their margins and in many instances reduced them, wholly or marginally, to fine-grained acid phyllonite.

Within the aureole of the Foxdale-Archallagan granite several of these acid sheets, ranging up to a few feet thick, are intercalated inthe silvery contact phyllites along bedding or S1. Good examples can be examined in Glion Darragh and in the Foxdale River which flows northward on the western fringe of the aureole. As elsewhere in the Isle of Man, movement parallel to their margins, and the foliation of the enclosing metasediments, has modified or destroyed the original igneous textures in these pale, fissile bodies.

In thin-section the planar, phyllonitic texture is coarser than outwith the aureole (cf. Simpson d pl. 9 ~ - D ) and can appropriately be described as schistose (PI. 25~). The microfabric consists of stout, parallel muscovite flakes, comparable in size to those of the enclosing pelite, within which occur granulitic quartz and a proportion of felspar showing little or no strain. Surviving porphyroclasts (formerly phenocrysts) of quartz, felspar and muscovite form relatively large knots round which the schistosity is deflected (Pl. 25~) . Relict and highly strained lenses display the coarser microgranitic texture in all stages of reduction by granulitisation and recrystallization to the planar schistose fabric. On the outcrop a megascopic striation or fine crinkling is often present which plunges gently parallel to the Fz lineation in the adjacent phyllites.

Where garnet porphyroblasts are present in the enclosing metasediments they also occur in the reconstituted acid sheets. These may adopt an irregular poikiloblastic form or possess an idioblastic outline and have clearly grown in the schistose fabric after the para- crystalline deformation had ceased.

The acid sheets have suffered the same degree of syntectonic recrystallization, in response to the heat of the underlying granite, as the enclosing sediments. It is concluded that their emplacement, early in the second (FA movement-phase, shortly preceded the start of metamorphism in the aureole (Table 1 and p. 430). During subsequent F, flexural-slip, the rise of temperature in the aureole not only promoted the reconstitution of the sediments to contact phyllites but also encouraged the schistose microfabric of the deformed intrusions. With the cessation of F, movement, the static growth of garnet porphyroblasts took place in both the metasediments and in the foliated acid sheets.

(9 F, Deformation in the Aureole Prior to the onset of the third (F3) movement-phase, the prolonged sequence of crystal-

lization in the Foxdale-Archallagan aureole had come to an end (Table 1). The higher temperature which had prevailed in this localised tract of the Isle of Man had been dissipated.


The northern fringe of the aureole was flexed, along with the surrounding Manx Slates, into the open Foxdale fold-pair (p. 419). The third axial-plane cleavage ( S , ) is sporadically developed in the aureole metasediments as a coarse strain-slip which possesses its usual steep northeasterly dip (Simpson a 391) and strikes parallel to the axial traces of the parent flexures (Fig. 2). These late slip-planes cut discordantly through, offset and flex all the earlier crystalline textures. No further change in the mineralogical constitution of the aureole rocks accompanies these F, movements.

Small-scale, open F, folds occasionally deform the foliation of the contact metasediments and plunge steeply north or northwest. Small flexures of the same age sporadically bend the flat-lying S2 schistosity around Archallagan.

4. THE GRANITE

(a) Time of Emplacement The textural study of the Foxdale-Archallagan aureole has demonstrated that the

recrystallization of the fine-grained F, fabric in the metasediments began during the flexural-slip stage in the formation of the large-scale F2 Manx synform (p. 421). Since the reconstitution of its envelope was inaugurated by the arrival of the granite, the emplacement of the intrusion must have taken place at this time during the second (F2) movement- phase (see Table 1 and p. 430).

(b) Deformation of the Granite

After the granite mass was emplaced its consolidation continued pari passu with the later stages in the formation of the Manx synform and the resultant F, movement in the granite envelope. The crystallizing granite was, however, little affected by the contemporaneous deformation in the country-rock. Over most of the granite outcrop at Foxdale there is little sign of strain apart from slight undulatory extinction in some of the quartz and felspar grains.

The intrusion, however, did not remain entirely unscathed. In the southeastern corner of the exposed Foxdale granite the finer-grained, porphyritic marginal facies displays in places a gently inclined platy fracture. This is controlled by close-set S , slip-planes which traverse the rock and possess the same low inclination as in the country-rock to the south. These surfaces of discontinuity are etched out by weathering on the edges of granite slabs. In thin-section the slip-planes wind their way in broad parallelism through the groundmass, curving round the larger phenocrysts of quartz and felspar. They are outlined and healed by lines of small muscovite flakes and the slippage appears to have taken place before the granite was completely solid.

The same gently inclined, planar deformation is clearly apparent in the small granite cupola to the east (Fig. 2). The rock has a streaky aspect and the quartz phenocrysts have been compressed to thin lenses showing internal granditisation. A noticeable striation pervades the rock and plunges gently southwest parallel to the F, linear structures in the neighbouring country-rock. The granite of this small outcrop is traversed by much quartz- veining of F, age. These veins may either anastomose irregularly through the deformed


granite or form planar lenses along the gently inclined S, surfaces. A proportion of the veining has been implicated in the deformation and shows a degree of fracturing and crushing; the remainder, injected at the close of the movement, is undisturbed.

Also intersecting the granite outcrop at Foxdale are three sets of steep, planar joints. These have the following orientations: (i) dip 70"-vertical, strike 327"-359"; (ii) dip 70"- vertical, strike 74"-99"; (iij) dip 45" at c. 119". Representatives of each of the joint-sets may occasionally be occupied by thin veins of quartz.

The jointing was impressed on the granite after its consolidation and is younger than the more penetrative, marginal deformation already described. It is not clear, however, whether the strain responsible for the formation of the jointing should be referred wholly to the close of the second (F,) movement-phase or whether some of the jointing originated during the subsequent F3 flexing of the area (p. 419).

(c) Shape of the Granite

The top of the granite pluton must underlie the tract of country between Foxdale and Archallagan at shallow depth (Fig. 3), extending at least as far as the boundary of the aureole on the map (Fig. 2). Since the intrusion is certainly not concordant with the bedding there appear to be two alternatives concerning its form in depth.

The granite body may have the orthodox shape of a steep-sided pluton with a flatly undulating top. The latter approaches close to the surface under the overlying aureole but only outcrops at Foxdale. The other possibility is that the intrusion has the form of a thick, gently inclined or flat-lying, lenticular sheet or pod emplaced close to the axial plane of the Manx synform. This style is suggested by the fact that the Foxdale-Archallagan granite was emplaced during the F, movement and lies within the hinge of the synform. This connection in time and space between the emplacement of the syn-tectonic intrusion and the formation of the F, fold, indicates that the granite may possess a shape in harmony with the geometry of the large-scale and synchronous flexure. Small-scale replicas of such a style of emplacement are provided by many of the F, quartz bodies (Simpson 6) com- monly observed in the deformed Manx Slate Series throughout the Isle of Man. These may adopt the form of thick pods or augen which thin out rapidly towards their margins. A favourite site for these bodies, up to several feet thick, is along the axial planes of mesoscopic F, flexures.

Whatever the shape of the Foxdale-Archallagan granite mass, it seems probable that it represents only a part of a still larger intrusion which continues to the southwest at depth and persists a t least as far as the Castletown Waterworks Reservoir, 23 ml from Foxdale. In streams draining down the eastern hillside into the reservoir the sombre sediments of the Manx Slate Series are locally transformed into silvery phyllites with small garnets; recrystallized quartzite bands contain abundant tourmaline. The top of the intrusion evidently approaches the surface again under this restricted tract.

5. SEQUENCE OF SYN-TECTONIC ACID INTRUSION IN THE MANX SLATE SERIES

Previous work by the writer (Simpson d) has shown that the small Dhoon granite in the northeast of the island (Fig. 1) and the associated swarm of acid sheets (p. 427) are also


syn-tectonic intrusions which were emplaced synchronously in the Manx Slate Series at an early stage in F, flexural-slip (Table 1). These bodies consolidated rapidly and during the latter part of F, flexural-slip were strongly sheared parallel to their steep margins (p. 427). In the Dhoon granite and its narrow thermal aureole subsequent deformation by F, planar-slip was essentially post-crystalline. The gently inclined Sz slip-planes penetrate into the sheared granite from the country-rock. In the granite envelope this strain-slip cleavage offsets and deforms the planar fabric of the contact phyllites. The foliae of muscovite and chlorite are bent round the hinges of the associated microfolds (Simpson d, pl. 9 ~ ) .

The Foxdale-Archallagan granite was emplaced by a slightly later pulse of magmatic injection as is shown by the paracrystalline deformation of those thin acid sheets which lie within its aureole (p. 427). The emplacement must, therefore, be referred to thelatterpart of the F, flexural-slip which accompanied the growth of the Manx synform (Table 1 and p. 421).

In contrast to the relatively rapid consolidation of the Dhoon intrusion, the crystallization of the much larger Foxdale-Archallagan granite was more prolonged ; it continued throughout the remainder of the second (FJ movement-phase and into the F,-F, static interval (Table 1). During this period the heat supplied to the aureole promoted the lengthy sequence of syn-tectonic and static crystallization already described (pp. 420-427) with the production, for example, of porphyroblastic contact phyllite, para-crystalline strain- banding and the S2 schistosity around Archallagan. The ultimate consolidation of the granite during the pre-F, interval is reflected by the dissipation of heat in the aureole and the termination of its prolonged metamorphic evolution.

Unlike the Dhoon granite, the Foxdale-Archallagan intrusion, apart from mild penetration by S , along its southern fringe, is largely undeformed (p. 428). The intrusion evidently crystallized quietly while the F, movement was proceeding in the metasediments of the envelope. Complete solidification of the bulk of the mass was not achieved until after the F, movement had ceased (Table 1). The steep, early F, shearing, which is strongly impressed on the Dhoon granite, is not present.

6. SOME GENERAL COMMENTS

The two granite masses in the Isle of Man belong to the varied assemblage of acid intrusions, generally referred to as Caledonian, which have been emplaced in the spreads of deformed Lower Palaeozoic strata surrounding the Irish Sea (Hatch, Wells and Wells 450; Eastwood 30; Read 672). It has been generally considered that th is group of plutons is essentially post-tectonic and ‘younger than the acme of the revolution’ (Hatch, Wells and Wells 451). The mild peripheral shearing sometimes present has usually been attributed either to a slight local renewal of Caledonian stress or to the emplacement of such plutons at the close of the Caledonian orogeny but before the dying stress had entirely ceased.

THE FOXDALE-ARCHALLAGAN GRANITE AND ITS AUREOLE 43 1

Both the Foxdale-Archallagan granite and the Dhoon intrusion, however, have been implicated in the orogenic movement to a much greater and more intimate degree than has previously been envisaged for such Caledonian intrusions in Great Britain. The elucidation of the polyphase structural history of the Manx Caledonides has enabled the time at which these intrusions were emplaced to be established with precision in the lengthy tectonic chronology (Table 1). While the intrusions post-date the acute first (F3 movement-phase which formed the tectonic basis of the Isle of Man, their emplacement was synchronous with the F2 deformation and was completed prior to the F3 phase. Thus the Manx granites attained their present position more or less midway in the sequence of Caledonian tectonic events displayed by the country-rock. Such deformation as the plutons exhibit was not due merely to dying Caledonian stress or to a mild, local renewal of such stress. The granites have been implicated in two strong pulses of Caledonian movement each of which was responsible for large-scale folding and cleavage formation. They are syn-tectonic or syn-orogenic intrusions with reference to the Caledonian orogeny as a whole.

It has recently been demonstrated (Simpson, Helm and Roberts) that Caledonian polyphase folding and cleavage are not peculiar to the Isle of Man but also occur in the Lower Palaeozoic strata of the mainland. A comparable tectonic chronology with F,, F2 and F3 movement-phases has been recognised in North Wales and the Lake District. In the Southern Uplands Shiells and Dearman have recorded two phases of pre-Old Red Sandstone deformation in the eastern coastal section; a third episode of Middle Old Red Sandstone flexuring has also been superimposed. In Kirkcudbrightshire Walton (in Johnson and Stewart 92) has observed at least two generations of small-scale folds accompanied by axial-plane cleavages. It may well prove, when the detailed structural history of their contiguous country-rock becomes better known, that some of the other British Caledonian intrusions are also implicated to a greater extent in the Caledonian orogeny than has previously been believed.

The F2 movement and deformation which took place during the recrystallization of the Foxdale-Archallagan aureole has produced an envelope of phyllites and schists similar to those resulting from regional dynamothermal metamorphism in such areas as the Scottish Highlands. The fabric of the aureole metasediments around Archallagan has revealed a remarkable para-crystalline, and entirely F,, alternation of mobile and static episodes during the metamorphic evolution of the envelope. The complex textures displayed by these porphyroblastic foliates are identical to those described recently by several workers in the polymetamorphic and polyphasal Dalradian schists (Rast; Sturt and Harris ; Johnson).

One of the most exhaustive and penetrative studies carried out on a Caledonian intrusion in recent years has been the dissection of the Donegal granite by Pitcher and Read (a, b, c). This large pluton of 140 sq ml is emplaced in the Dalradian Series. It is surrounded by an aureole of porphyroblastic contact schists comparable to those forming the envelope


of the Foxdale-Archallagan granite and likewise the product of synchronous heating and para-crystalline deformation.

Pitcher and Read have shown that the directed stress, and resultant movement, which operated in the Donegal aureole during its recrystallization was produced by the emplacement of the granite. The same intrusive drag has also caused the deformation of the granite fringe and produced small-scale, steep folding in the aureole. The structural and metamorphic development of the granite envelope thus long post-dates the polyphasal deformation displayed by the Dalradian country-rock. The Donegal intrusion is post- tectonic and the marginal deformation which it displays is self-inflicted.

Brindley (a, b) has also described a broad belt of porphyrpblastic contact schist in the aureole of the large Leinster granite, emplaced in Lower Palaeozoic slates. He likewise attributes the para-crystalline movement controlling the formation of these foliates to the intrusive force of the granite and remarks (b 15) that “the metamorphic aureole has evidently behaved as a mobile envelope around the rising pluton”. A comparable mechanism has been suggested by Gillott (152) to account for the schistose character of the metasediments above the concealed granite at Archallagan.

Unlike the Donegal intrusion, the mild marginal deformation displayed by the Foxdale- Archallagan granite and the foliate character of its extensive aureole are primarily of extraneous tectonic origin. Moreover, these phenomena were produced during the Caledonian deformation of the surrounding sedimentary pile.

Following its syn-F, emplacement, the pluton crystallized quietly and passively while the second movement-phase was proceeding in the surrounding terrain. The F2 folds, accompanying S, and second linear structures which occur in the Foxdale-Archallagan aureole are equally well developed in the Manx Slate Series throughout the Isle of Man (Simpson (I 384). Beyond the granite envelope the F, structures are essentially post- crystalline with reference to the fine-grained F, textures which they flex and deform (Simpson c 26). In the environs of the granite, however, their formation was accompanied by syn-kinematic crystallization consequent upon the heat contributed by the underlying intrusion. The greater plasticity of the envelope metasediments has resulted in the unusual, acute profile displayed by small-scale F, folds (p. 425) and the penetrative remaking of the fabric around Archallagan has converted the strain-slip Se into the unique S2 schistosity @. 424). Thus while the Foxdale-Archallagan granite provided the rise in temperature necessary for recrystallization, the movement responsible for the oriented fabric of the contact phyllites and schists was of regional import.

THE FOXDALE-ARCHALLAGAN GRANITE AND ITS AUREOLE

REFERENCES 43 3

BERGER. J. F. 1814. Mineralogical account of the Isle of Man. Trans. geol. SOC. Land. 2, 29.

BRINDLEY, J. C. LI 1948. Metamorphic aureole in the northern end of the Leinster granite. In Geology of E. Ireland, 18th Session Inter, Geol. Congress, 21.

- b 1957. The aureole rocks of the Leinster granite in south Dublin, Ireland. Proc. Roy. Irish Acad. 59, 1.

CUMMTNG, J. G. 1848. The Isle of Man; its history, physical, ecclesiastical, civil, and legendary. London.

EA~WOOD, T. 1953. British Regional Geology: Northern England. Geol. Sum. and Mus.

G I L L O ~ , J. E. 1955. Metamorphism of the Manx Slates. Geol. Mag. 92, 141.

HARKER, A. a 1894. The Foxdale granite. The Naturalist (Leeds). No. 224, 68.

- b 1895. In Carrock Fell-Part 3. The Grainsgill Greisen. Quart. J. geof. Soc. Lond. 51, 143.

HATCH, F. H., WELLS, A. K. and WELLS, M. K. 1961. Thepetrology of the igneous rocks. London.

HENSLOW, J. S. 1821. Supplementary observations to Dr. Berger's account of the Isle of Man. Trans. geol. SOC. Lond. 5, 482.

JOHNSON, M. R. W. 1962. Relations of movement and metamorphism in the Dalradians of Banffshire. Trans. Edin. geol. Sue. 19, 29.

- and F. H. STEWART, 1963. The British Caledonides. Edin. and London.

LAMPLUGH, G. W. a 1898. One-inch geological map of the Isleof Man: Sheet 100 (solid anddrift). Geol. Surv. U.K.

- b 1903. The geology of the Isle of Man. Mem. Geol. Surv. W.K.

PITCHER, W. S. and READ, H. H. a 1959. The main Donegal granite. Quart. J . geol. SOC. Lond. 114 (for 1958), 259.

- and - b 1960. The aureole of the main Donegal granite. Quart. J. geol. SOC. Land. 116, 1.

- and - c 1963. Contact metamorphism in relation to manner of emplacement of the granites of Donegal, Ireland. J. Geol. 71, 261.

RAST, N. 1958. The metamorphic history of the Schichallion complex. Trans. roy. SOC. Edin. 63, 413.

READ, H. H. 1961. Aspects of the Caledonian magmatism in Britain. Lpool Manchr geol. J . 2, 653.

SHELLS, K. A. G. and DEARMAN, W. R. 1963. Tectonics of the Coldingham Bay area of Berwickshire, in the Southern Uplands of Scotland. Proc. Yorks. geol. SOC. 34, 209.

434 ALEXANDER SIMPSON SIMPSON, A. a 1963. The stratigraphy and tectonics of the Manx Slate Series, Isle of Man. Quart. J . geol.

SOC. Lond. 119, 367.

- b 1963. FI quartz veining in the Manx Slate Series. Nature 199, 900.

- c 1964. The metamorphism of the Manx Slate Series, Isle of Man. Geol. Mag. 101,20.

- d 1964. Deformed acid intrusions in the Manx Slate Series, Isle of Man. Geol. J. 4, 189.

-- , HELM, D. G. and ROBERTS, B. 1963. Polyphase folding in the Caledonides south of the Scottish Highlands. Nature 200, 1060.

STURT, B. A. and HARRIS, A, L. 1961. The metamorphic history of the Loch Tummel area, central Perthshire, Scotland. Lpool Manchr geol. J. 2, 689.

WHITE, W. S. and JAHNS, R. H. 1950. Structure of central and east-central Vermont. J. Grol. 58, 179.

DEPARTMENT OF GEOLOGY, EIRKBECK COLLEGE, UNIVERSITY OF LONDON, MALET STREET, LONDON, W.C. I .

the syntectonic foxdale-archallagan granite and its metamorphic aureole, isle of man

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