Fraochaidh prograde sequence, Ballachulish Igneous Complex - an excursion

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From: Pattison, D R M and Harte, B. Ballachulish Igneous Complex and aureole: a field guide Edinburgh : Edinburgh Geological Society, 2001.

Fraochaidh prograde sequence - Metamorphic zones in pelitic hornfelses on the SSW side of the igneous complex

The purpose of this excursion is to examine the contact metamorphism of the abundant pelitic country rocks in the aureole, and, as outlined in Fig. 7, the day is divided into two separate excursions:

Day 3, Part 1: Fraochaidh prograde sequence - Metamorphic zones in pelitic hornfelses on the SSW side of the igneous complex.

This excursion provides a continuous section from low to high grade in metapelitic rocks on the NW flank of Fraochaidh hill, and illustrates in Creran Succession rocks the most common sequence of metamorphic zones in the aureole (see introduction). Fig. 10 gives the relevant geological and topographic maps.

Day 3, Part 2:Chaotic Zone migmatites on W side of the igneous complex.

This excursion examines the area where the most extensive migmatisation in seen in the aureole - the Chaotic Zone on the west flank of Beinn a' Bheither above Lagnaha Farm. The localities are shown on the geological and topographic maps of Fig. 11.

Both parts involve moderately short hill traverses, with: ca. 4 km walking and 250 m elevation gain for Part 1 (Fraochaidh prograde sequence); and ca. 2 km walking and 200 m elevation gain for Part 2 (Chaotic Zone). A compass and altimeter are recommended in both cases. Part 1 typically requires about 4 hours of time, but if you wish to examine and collect extensively, or follow the optional extensions to the excursion, it can occupy a whole day.

Note for both excursions: To drive all the way to the starting point of each of the traverses, it will be necessary to drive along the Forestry roads, for which a key to open the Forestry gates is necessary and must be obtained from the Forest Enterprise office in (see section on Logistics for Field Excursions). However, you can walk all the way from the Forestry gates, which will add an extra 2-3 km to the length of each excursion.

Itinerary of stops for Day 3, Part 1: Fraochaidh prograde sequence

Grid ref.

(sheet NN)

Features
Stop 3-1 Below (S of) A’Chruach NN 0140 5340 Fissile cordierite-spotted phyllites of Zone II
Stop 3-2 NN 0170 5290 Hardened cordierite-spotted phyllites of Zone III
Stop 3-3 Coire na Capuill NN 0195 5275 Massive cordierite-rich hornfelses of Zone III
Stop 3-4 NN 0210 5280 Cordierite+K-feldspar hornfelses of Zone IVb with honeycomb structure
Stop 3-5 NN 0215 5275 Andalusite+K-feldspar hornfelses of Zone Va
Stop 3-6 NN 0225 5280 Corundum-bearing hornfelses of Zone Vb)
Stop 3-7 NN 0235 5270 Different types of veins in hornfelses: onset of partial melting
Stop 3-8 Coire Dubh NN 0245 5265 Garnet+cordierite-bearing and spinel‑bearing hornfelses
Stop 3-9 Coire Dubh NN 0250 5265 Biotite+hornblende quartz monzodiorite of the igneous complex

Directions to start of traverse: From the A828 road. take the Auchindarroch turnoff, about 300 m north of the Duror Hotel (see Coloured Map). Set odometer to zero at turnoff. Proceed along the paved road for about 0.25 miles (0.4 km), at which point you will reach a cross-roads where the roads/tracks to left and right are on the bed of the old railway line. The right-hand track is untarred, but is signposted "Home of James of Glen in 1752”. Follow this track, crossing over two small bridges, and continuing on to a junction at 0.55 miles (0.9 km) from the A828. There the line of the old railbed is taken by a track carrying straight on under a bridge, whilst the main track to Acharn Farm and the home of James of the Glen swings strongly around to the left. You take a middle, more-elevated track, which leads to a gate after 50 m. The gate is usually locked, but may be opened if you have obtained a Forestry key (see above). Proceed beyond the gate.

Note: The track that you are now following was built in 1993, and so does not appear on the 1978 Ordnance Survey map, although it has been added to the Coloured Map and Figs. 7 and 10.

Keep to the main track/road, passing numerous exposures of Appin Quartzite. Starting at about 2.0 miles (3.2 km) front the A828, there arc several exposures of the Leven Schist from Zone III of the aureole showing good cordierite-spotted phyllites (see Photo 7). At about 2.2 miles (3.7 km), cross a small bridge over a burn and park in the lay-by (approximately NN 0130 5375).

Photo 7 Stops 1-3 and 3-1. Cordierite porphyroblasts visible as dark patches on cleavage planes of regional phyllites and schists. Note that the size of the patches varies in different parts of the rock. This sample comes from roadside exposures near the parking place for Day 3, Part 1, and shows a coarser development of cordierite patches than at Stop 1-3; but the style of development is similar at both these and other localities in the aureole.

Walk along a bike path leading south-eastwards for about 140 m, at which point the bike path begins to swing downhill to the north-east. Leave the bike path at this point and cross an overgrown area (in 1999) to the south-east for about 30 m, to join an obvious but rough brashed path by a burn. Climb up this steep path through the forest (ca. 150 m of elevation gain and general direction SSW) until you reach a low fence near the edge of the forest. This fence trends roughly on contour in a south-easterly direction. Set altimeter to 330 m The route followed since the parking place is indicated on Fig. 13. The traverse begins on the open grassy slopes above the fence.


Geological map showing location of field stops for Day 3 Part 1, Fraochaidh traverse, (see Fig. 7 for key to geological map).
Figure 10 B Corresponding topographic map showing location of field stops for Day 3 Part 1


Day 3 Part 1

Stop 3-1. Fissile cordierite-spotted phyllites of Zone II. NN 0140 5340, 360m.

Directions: After having climbed over the fence, proceed southwards uphill for 100-150 m, on a grassy slope with boulders, to the lowermost of many exposures.

Description: These exposures arc of green-grey crenulated phyllites and locally show well-developed spots, best seen on clean surfaces parallel to the main schistosity or cleavage. The spots are (pale) brown on clean weathered schistosity surfaces, and darkish grey on fresh schistosity surfaces, with ovoid shapes and diameters of 2-7 mm (see Photo 7).

The spotting is more easily seen in hand specimen than in thin section. In thin section, the spots vary in appearance from irregularly-shaped, brown, scaly film-like patches to fine-grained mixtures of chlorite and muscovite that overprint the foliated matrix. Based on the similarity in size, shape and mode of occurrence of these spots with cordierite spots found in Zone 11 elsewhere in the aureole, they are interpreted to be pseudomorphous after cordierite. Chlorite of regional origin is present in the matrix, resulting in the diagnostic Zone II assemblage Ms+Chl+Crd+Bt+Qtz (ca. 560 °C). The cordierite is thought to be introduced by reaction P1 (Ms + Chl + Qtr = Crd + Bt H2O ; see Introduction).

Location of the cordierite (spotting) isograd. The spotted phyllites of Stop 1 are in Zone II of the aureole. Owing, to poor exposure in the direction of lower grade (WSW), it is difficult to pinpoint exactly where cordierite spotting first appears (i.e., pinpoint the cordierite isograd) in this vicinity. If one walks westwards and uphill from Stop 3-1 towards where a fence heads up a small valley to the south-west, the frequency of spots decreases and they may become absent just before the fence (NN 0125 5335; 390 m), which is also where exposure dies out. Climbing straight uphill southwards from Stop 3-1, the spots decrease in abundance and disappear within 50 to 100 m.

Stop 3-2. Hardened cordierite-spotted phyllites of Zone III. NN 0170 5290, 420m.

Directions: From Stop 3-1, proceed south-eastwards parallel to the south-east-trending, fence above the trees, examining the lowermost exposures of the grassy hillside. Throughout much of the interval between Stops 3-1 and 3-2, the traverse runs parallel to metamorphic grade within Zones II and III. Spots continue to be readily seen in crenelated green phyllites, which form the dominant rock type with interbeds of semipelite and psammite. At NN 0150 5320 (shown on Fig. 10), there is a prominent exposure beyond which the hillside above the forest becomes less steep. At about NN 0155 5315, the edge of the forest swings sharply away to trend ENE, but the fence carries on south-eastwards but now runs obliquely uphill. Continue on, heading gently uphill south-east and keeping just above the fence, for about 300 m until some low, knobbly exposures surrounded by grass, on the other side of a small burn, are encountered in a relatively level area down slope of the steeper, craggier part of the hillside. This is Stop 3-2. On the way at NN 0160 5300 (indicated on Fig. 10), a short distance up the slope to the south, some prominent platy exposures may be examined that contain psammitic beds.

Description: The rocks are generally more spotted (cordierite-richer), and tougher and greyer (less green) in colour than at Stop 3-1, but they still preserve clear evidence of schistosity, and some layers are still readily fissile. Many layers contain fresh cordierite. The most common assemblage is Ms+Crd+Bt+Qtz, diagnostic of Zone III of the aureole. In these samples, there is no evidence of the primary chlorite present at lower grade (Zone II); the chlorite is interpreted to have reacted out by reaction P1 (Ms + Chl + Qtz = Crd + Bt + H2O).

Location of the chlorite-out (Zone II/III) isograd and the width of Zone II. Pinpointing the Chl-out isograd between Stops 3-1 and 3-2 is difficult to do in the field. Based on thin section analysis (unpubl. data), the last appearance of primary chlorite occurs around the exposure at NN 0160 5300. Combined with thin section analysis of other samples in the vicinity, the overall width of Crd+Chl-hearing Zone II is 50-100 m.

Stop 3-3. Massive cordierite-rich hornfelses of Zone 111. NN 11195 5280, 430 m

Photo 11 Stop 3-3. Massive, pitted pelitic hornfels from the high grade end of Zone III. Fraochaidh transect. The pits represent the sites of weathered out cordierite crystals. The matrix surrounding the pits is rich in muscovite, biotite and quartz. Scale bar in cms.

Directions: From Stop 3-2, the fence keeps a roughly SE trend, with some wiggles, until NN 0180 5270 where it bends sharply to trend ENE and crosses a burn. In the interval before the sharp bend in the fence, you may wish to examine the lowermost exposures on the hillside above the fence; it is still obvious that the rocks were once crenelated phyllites, although they arc now quite tough cordierite-spotted rocks from Zone III. Follow along below the fence trending to the ENE, crossing two deeply incised burns. Semipelitic hornfelses in the burns appear to be rather fractured with abundant oxide staining, possibly indicating a fault zone (Pass of Brander Fault? - Pattison & Volt. 1991). Exposures in between the burns are mainly quartzose with some spotting visible. About 100 m further on, the fence swings uphill to the south; do not follow the fence. Instead, bear NE and cross a small burn to a nearby low ridge with a series of good exposures forming Stop 3-3.


Description: The rocks here show a contrast with previous exposures in that they have a marked pitted structure on weathered surfaces (see Photo 11). The roughly equant pits are 2-7 mm across and mark the sites of weathered out cordierite crystals. Around the pits the rock matrix forms a network of paler-coloured material containing tiny dark pits. The pale matrix is rich in muscovite and the tiny dark pits are the sites of biotite crystals. Thin (ca. 1 mm) dark seams rich in biotite mark the presence of former crenulation cleavages. The rocks will still break along these surfaces occasionally. Although these rocks show a somewhat coarser grain size and more massive, pitted appearance than rocks at Stop 3-2, they still contain the same Zone Ill mineral assemblage (Ms+Bt+Crd+Qtz). Craggy exposures uphill to the south show extensive exposures of these pitted hornfelses.

Stop 3-4. Cordierite+K-feldspar hornfelses of Zone IVb with honeycomb structure. NN 0210 5280, 430m

Directions: From Stop 3-3, bear east for about 150 m to very extensive exposures with large expanses of flat rock surfaces parallel to the ground surface, situated beneath prominent north-facing crags.

Photo 12 Stops 3-4, 1-8, 5-2, 5-4 and 5-5. Massive Crd+Kfs-bearing pelitic hornfels characteristic of Zone IVb. The photograph was taken near Stop 5-5 in Coire Giubhsachain, but is representative of Zone IVb in many parts of the aureole. Note the mesh or honeycomb texture, defined by randomly orientated, weathered-out cordierite crystals (represented by pits) within a resistant, light coloured matrix rich in K-feldspar Note also that the matrix surrounding the pits is lighter colorant and more sharply defined than the matrix in hornfelses from Zone III (compare with Photo 1 ).

Description: Clean weathered surfaces commonly show a small-scale network structure of light-coloured ridges surrounding 0.2-0.3 mm pits, the latter representing weathered-out cordierite crystals (see Photo 12). The main distinction from the last locality is that in many rocks the network appears whiter and sharper here, reflecting the change in the matrix mineralogy from K-feldspar-absent (Stop 3-3) to frequently K-feldspar-rich. The presence of K-feldspar (Kfs) places these rocks in Zone IVb (ca. 620 °C). The full assemblage in the layers with the white network is usually Ms+Bt+Crd+Kfs+Qtz. However, not all rocks contain Kfs; most likely reflecting bulk compositional variations, especially Fe-Mg ratio (see Fig. 8 of Pattison & Harte, 1997). In rocks with Kfs, the modal amount of muscovite has decreased markedly from Zone III, consistent with production of Kfs by reaction P2b (Ms+Bt+Qtz = Crd+ Kfs+ H2O).

An additional feature of these exposures, not seen previously, is the presence of some thin (< 2 mm wide) white, generally subvertical seams, several cms in length, which cut across foliation and other structures; these are dominantly composed of K-feldspar and trend generally in a NW-SE direction. A possible interpretation of these seams is that they represent the conduits for the upward escape of fluids generated during production of the Crd+Kfs-bearing Zone IVb assemblage at the expense of the lower grade Zone III Ms+Bt-rich assemblage (i.e., reaction P2b), with the conduits having formed due to fluid (hydro-) fracturing of the strongly recrystallized hornfels. The absence of comparable features at lower grade may have been due to the more fissile nature of the rocks, possibly allowing escape of fluids parallel to the pre­existing schistosity. Although it is not an obvious characteristic of the rocks, one can still see evidence of relict dark cumulation cleavages (rich in biotite) along which the rocks may occasionally break.

Stop 3-5. Andalusite+K-feldspar hornfelses of Zone Va. NN 0215 5275, 450 m

Directions: From Stop 3-4, proceed about 75 m upwards to the east to the craggy exposures at the top of the expanse of flat exposures.

Photo 18 Stops 5-3 to 5-6. 1-8 and 3-5. Randomly orientated andalusite prisms in a massive cordierite+K-feldspar-rich pelitic hornfels (Zone V). The photograph was taken at Stop 5-5.

Description: Layers containing andalusitc crystals are locally interbedded with hornfelses showing the white, Kfs-rich ribbed texture first seen at Stop 3-4. On weathered surfaces, the andalusite protrudes above the hornfels exposure surfaces, occurring as small sharpish points and sometimes in well-defined, 2-20 mm long, slender prisms (see Photo 18). Most rocks have lost all sign of primary, fine-grained muscovite. The loss of muscovite in conjunction with the development of And+Kfs represents the Zone IVb/V isograd, and is ascribed to reaction P3 (Ms+Qtz = And+Kfs+ H2O; ca. 640 °C). Although not visible in hand specimen, volumetrically minor sillimanite also makes its first appearance at this grade in this vicinity.

Stop 3-6. Corundum-bearing hornfelses of Zone Vb. NN 0225 5275, 440 m

Directions: From Stop 3-5, continue bearing about 100 m east at about the same height, examining prominent craggy hornfels exposures locally containing andalusite.

Description: Thin section examination reveals that a few andalusite-rich hornfelses also contain corundum. Identifying corundum in the field is not easy: it is relatively rare and typically occurs in andalusite-rich layers in small upstanding 'pimples' on the clean rock surfaces, typically less than 2 mm across, which are distinct from the larger, more prismatic andalusite crystals (see Photo 19). In thin section corundum occurs in irregularly shaped crystals typically surrounding ilmenite crystals. Corundum-bearing layers are quartz-absent, rarely have muscovite, and contain the assemblage Crn+And±Sil+Crd+Bt+Kfs. The first appearance of corundum corresponds to the Zone Va/Vb isograd, and is inferred to be due to reaction P5 (Ms = Crn+Kfs+ H2O: ca. 670 °C). Interlayered quartz-bearing layers contain the assemblage Crd+Qtz+Bt+Kfs+Bt±And-±Sil.

Rocks in this vicinity show sub-vertical, NS- trending discrete white veins of 1-2 mm thickness which cut across layering and cleavage, forming a parallel set which do not interconnect very much with one another or with other layer-parallel structures. Although of a similar nature to the veins first noted at Stop 3-4, the length and continuity of these veins is generally greater than at Stop 3-4.

Note: More abundant corundum-bearing hornfelses are found in Coire Giubhsachain, as described in the Day 5 excursion.

Stop 3-7. Different types of veins in hornfels: onset of partial melting. NN 0235 5270, 440-460 m

Directions: From Stop 3-6, continue ESE and uphill in the direction of a large light-coloured boulder. After 150-200 m, you will find yourself on the edge of a ridge below which Coire Dubh opens out immediately to the east, and where the major crags swing round to take a more NNW-SSE, rather than WNW-ESE, trend. The exposures of Stop 3-7 occur for about 100 m leading up to this point.

Description: The hornfelses have a generally similar appearance and mineralogy to those of Stops 3-5 and 3-6, with local andalusite- and corundum-bearing layers, but show a greater variety of mm-scale vein types. The veins appear to be a little wider and stand up as ribs on exposure surfaces. Some are sub-vertical, discrete and relatively sharp-margined like those seen at lower grade, whereas a larger proportion show a variety of orientations, are less continuous, and have less sharp contacts with the enclosing hornfels, in places merging with the matrix. Some sub-vertical veins interconnect with more shallowly-dipping veins. Many of the veins are of a fine-grained, granular Kfs+Qtz-rich composition and resemble migmatitic leucosomes, contrasting with the thin Kfs-dominated mineralogy of the veins noted in Stops 3-4 to 3­6. In thin section, the more diffuse veins show a range of igneous-like textures similar to those observed in migmatites elsewhere in the aureole (Haile et al., 1991a). Taken together, these features are tentatively interpreted to be indicative of localized partial melting in the hornfelses. The temperature of these rocks was probably ca. 700 °C (see Introduction).

Stop 3-8. Garnet+cordierite-bearing and spinel-bearing hornfelses. NN 0245 5265, 430-450 m

Directions: Standing on the edge of the corrie, one sees just below and to the east on the lip of the corrie floor, a low smooth rocky whaleback ridge lying just beyond a small gravel/boulder landslip area of similar size. This low rocky whaleback is Stop 3-8.

Description: Granular, medium-grained, biotite-rich hornielses show variably abundant red garnet porphyroblasts around 4 to 7 mm across. The full assemblage in these rocks is Bt+Crd+Grt+Kfs+Qtz+Pl+Ilm. Interlayered light-coloured quartz-absent hornfelses may contain corundum and green hercynitic spinel (not visible in exposure). Although these exposures show few obvious leucosomes or flow structures, thin sections show several of the igneous-like textures found elsewhere in the migmatite zone of the aureole (Harte et al.. 1991). These rocks are among the highest grade hornfelses in the aureole, having formed at temperatures of 750-800 °C (Pattison. 1989).

Stop 3-9. Biotite+hornblende quartz monzodiorite of the igneous complex. NN 0250 5265, 450 m

Directions: Proceed about 50 m SE of Stop 3-8, examining low exposures.

Description: The rock is clinopyroxene+biotite±hornblende-bearing quartz monzodiorite of the igneous complex, locally showing fine-grained, schlieren textures.

Options from Stop 3-9.

(a) Retrace your route back to the vehicle.

(b) Explore further the high-grade cordierite-rich hornfelses of Coire Dubh, many of them contain hercynitic spinel (refer to Fig. 10 for the details of the geology in Coire Duhh).

(c) A more scenic and correspondingly more strenuous option, and one which allows examination of another transect into the aureole, is to hike to the summit of Fraochaidh which forms the head to Coire Duhh.

The best route for option (c) is to ascend the broad shoulder of Fraochaidh to the east of Coire Duhh. Going up the shoulder, one passes from a xenolith-hearing diorite facies of the igneous complex into striped psammitic hornielses that have few distinctive contact metamorphic features. However, the rocks on this eastern shoulder of Fraochaidh fall just within the garnet zone of the earlier regional metamorphism (Fig. 10 and Coloured Map). Therefore, higher up the hill in the lower grade parts of the aureole, relict regional garnet may be seen, whilst cordierite-biotite pseudomorphs after such garnet are found in the higher-grade contact metamorphic rocks. The regional garnets are quite distinct to the contact metamorphic garnets found at the highest grade of contact metamorphism (e.g. Stop 3-9). The summit of Fraochaidh is underlain by cordierite-spotted pelites and semi pelites of Zone III. Regional grade Ms+Chl-bearing rocks of Zone I are found a short distance down the slope south of the summit.

For an easy, rambling descent from the summit of Fraochaidh, walk WNW and then NW down the ridge top to the small loch by A’Chruach, and from there descend to the brashed track at the start of the traverse. For a more geologically orientated descent, one can go clown down the NW-trending salient between Coire na Capuill and Coire Duhh. Going down the salient takes you more or less along strike in the Creran Succession, and upgrade past excellent exposures of Zone III Crd-spotted pelites and IVb massive Crd+Kfs hornfelses, rejoining the morning route around Stop 3-4 or 3-5.


At all times follow: The Scottish Access Codeand Code of Conduct for Fieldwork .