Chaotic Zone migmatites, 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.

Chaotic Zone migmatites on west side of the igneous complex

Figure 11: (a) Geological map showing location of field stops for Day 3 Part 2, (see Fig. 7 for key to geological map). Also shows Day 1, Stops 1-7 1-8 & 1-9.
Figure 11: (b) Corresponding topographic map showing location of field stops for Day 3 Part 2: (reproduced with permission of the Ordnance Survey).
Photo 13 Stops 3-11 to 3-13. View of a typical exposure of the Chaotic Zone migmatites. Note the isolated fragments of metapelitic hornfels randomly orientated within a granular matrix containing veins. See the description of Stop 3-11 for a fuller description and explanation of these features.
Photo 14 Stops 3-11 to 3-13. Detail of the Chaotic Zone migmatites. An isolated metasedimentary schollen, with relic bedding still preserved, occurs in lower part and is surrounded hr a granular semipelitie matrix. An andalusite-rich domain occurs just below the scale bar in upper part.
Photo 15. View of the contact between granite of the igneous complex (light coloured upper crags) and the Chaotic Zone migmatites (dark lower exposures)

The purpose of Day 3 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 west 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 250m elevation gain for Part 1 (Fraochaidh prograde sequence); and ca. 2 km walking and 200m 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 (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.

Chaotic Zone migmatites

The aim of this traverse is to examine the extensively disrupted and migmatised pelitic and semipelitic Leven Schists occurring at the western margin of the igneous complex in the crags above Lagnaha (Fig. 7 and Coloured Map). In addition, some calsilicate and marble exposures are present in the same vicinity. This excursion, or parts of it, may be clone in 2-4 hours and combined with the Fraochaidh prograde traverse (Day 3, Part 1) to make a complete day. Alternatively, if it is desired to spend plenty of time exploring around the localities suggested, the traverse can occupy a full day. See Fig. 11 for the relevant location maps.

Itinerary of stops: Chaotic Zone migmatites and nearby carbonates and calsilicates

Stop Grid ref. (sheet NN) Features
Stop 3-10 Above Auchindarroch NN 0165 5525 Hybrid granite of the igneous complex
Stop 3-11 NN 0160 5530 Disrupted pelitic and semipelitie rocks of the 'Chaotic Zone'
Stop 3-12 NN 0160 5535 Intrusive veins and dyke cross-cutting disrupted migmatites
Stop 3-13 NN 0160 5565 to 0160 5575 Many good exposures of 'Chaotic Zone' migmatites
Stop 3-14 Above Lagnaha NN 0145 5590 Striped calsilicates of the Ballachulish Limestone lithology
Stop 3-15 NN 0135 5580 to 0125 5560 Marbles, calsilicates and semipelites of the Appin Limestone/Appin Phvllite lithology

Directions to start of Chaotic Zone traverse: The following directions take you by vehicle from the A828, by Duror, to the nearest point to the rock exposures that may be reached by car. The route involves entering the Glen Duror forest at NN 0050 5515 to the east of Auchindarroch – as in the itinerary for Day 2. Note that to drive into the forest, you will need a key to the forestry gate (see section on Logistics for Field Excursions). If you do not have a key it is an extra walk to the exposures of approximately 2.5 km.

Starting from the A828 road, take the Auchindarroch turnoff, about 300 m north of the Duror Hotel. Set odometer to zero at this turnoff. Follow the main paved road 0.75 miles (1.2 km), to where a paved road forks oil to the right, whilst straight ahead the paving gives way to a dirt track which soon forks to give two tracks both barred by gates. You should proceed to the left-hand gate (NN 0050 5515), which is usually locked, but may be opened with the Forest Enterprise key (see above). From the gate follow the 'main' (most 'straight-on' ) forestry road/track, avoiding the road to the right at 0.9 miles (1.5 km). At 1.65 miles (2.6 km as measured from the A828) the road forks (the exposures on the N (left) side of the road comprise Stop 2-1). Take the left hand fork (not the right hand fork as in Day 2). After about 150 m the road bends around to the left and heads obliquely uphill. Follow the main ('straight-on') road up to the NE, as high as it goes, avoiding all small roads coining in from the sides, to an intersection with a prominent upper forestry road at NN 0150 5490 (ca. 2.3 miles or 3.7 km from the A828). At this intersection the main road is tarred around the bend as it swings right, but you proceed left (close to straight-on), and continue for about 100 m to a lay-by, just beyond where the road crosses over a burn. Park in this lay-by (NN 0140 5500). Set altimeter to 275 m.

Stop 3-10. Hybrid granite of the igneous complex. NN 0165 5525, 400m.

Directions: From the lay-by where the vehicle is parked, ascend the hillside (a tree-felled area in 1999) beside the burn. There are extensive exposures in the burn of biotite- and hornblende-bearing granodiorite, with relic clinopyroxene. Continue upstream until a fence marking the boundary of the forested area is reached at about 350m elevation. Cross the fence onto open grassy slopes. Keeping on the left (west) side of the burn, climb uphill for ca. 150 m in a north-easterly direction to light-coloured exposures just to the west of the burn which cuts a deep gullet at this elevation.

Description: The light-coloured rock faces seen here are of hornblende- and biotite-bearing. pinkish-greyish, quartz-bearing monzogranite with mafic clots. They are in the marginal zone of the igneous complex.

Stop 3-11. Disrupted pelitic and semipelitic rocks of the 'Chaotic Zone'. NN 0160 5530, 410m.

Directions: From Stop 3-10), head about 75 in to the NW. away from the burn, to some relatively dark and craggy exposures. Examine several exposures in this vicinity.

Description: Good, moderately clean exposures show many of the features characteristic of the Chaotic Zone migmatites (see Photos 13 and 14). The rocks belong to the Leven schist lithology. Two main rock types are seen: generally granular, relatively homogeneous, quartzofeldspathic rock with a relatively weak, diffuse foliation, which forms most of the volume of the exposures; and layered cordierite- and/or andalusite-rich hornfels lavers and fragments (schollen) which occur in various orientations in the granular matrix. The way that the rigid fragments are suspended at all angles in the granular material, gives the exposures a xenolithic igneous appearance. The mineral assemblage in the granular matrix is rich in Qtz+Kfs+Pl+Bt. with variable amounts of Crd+And, in thin section it shows a number of igneous-like textures (Harte et al., 1991a). The hornfels schollen are rich in Crd±And+Kfs with rare sillimanite. These mineral assemblages place the rocks in metamorphic Zone V.

Significant disruption and loss of cohesion of original sedimentary layering is evident everywhere, but varies in extent from exposure to exposure. The most striking extents of disruption are those showing disorientated fragments of hornfels 'floating' at all angles in the matrix. These features indicate that the granular semipelitic matrix responded to external stresses in a ductile fashion, whereas the hornfels schollen responded in a brittle fashion. In some localities, diffuse planar textures in the matrix that bend around hornfels fragments may represent a crude flow foliation (see Photos 13 and 14).

Locally, 2-10 mm wide veins of a dominantly subvertical, N- to NE-trending orientation, anastomose through the semipelitic matrix, forming upward-weathering ribs on the exposure surfaces (Photo 13). These may branch and interconnect but typically maintain a limited range of trends. The veins are granular and rich in K-feldspar and quartz with variable amounts of plagioclase, biotite and cordierite. The margins of the veins with the enclosing rock are generally well defined but not sharp.

The extensive disruption and range of migmatitic features in the Chaotic Zone is unique in the aureole. The disruption is attributed to widespread melting of the abundant quartzofeldspathic Leven Schist semipelite. The volume of melt produced in the semipelitic matrix appears to have exceeded the critical melt fraction for suspension-like behaviour of crystals in a fluid (melt), accounting for the semipelite's ductile characteristics and ability to flow. Melt-poor pelitic hornfels layers responded to the stress in a brittle way, leading to disaggregation. The most likely cause for the extensive inciting was the release of both heat and fluids from crystallization of quartz diorite which underlies these rocks (Pattison & Harte, 1988; Linklater et al.. 1994). Later intrusion of pink granite, which forms the igneous contact in this vicinity, may have been a source of stress contributing to the local disaggregation of hornfels layers and incipient bulk How of the partially molten semipelitic matrix. The veins are interpreted as relatively late-stage features in melting, segregation and crystallisation history of the migmatites, possibly representing pathways of late, volatile-enriched melts.

It is at first sight paradoxical that the Chaotic Zone migmatites, which are by far the most extensively melted country rocks in the aureole, achieved only modest temperatures within Zone V. The absence of high-grade minerals like spinel, garnet and hypersthene, the rarity of sillimanite, and the stability of chlorite+calcite in adjacent calsilicates (see below and Pattison & Harte, 1997) indicate that these rocks were heated to 670-700 °C, only a little higher than the water-saturated granite solidus. This temperature range is significantly below the 750-800 oC temperatures in other parts of the aureole (e.g. Fraochaidh. Sgorr Dhearg) where the pelitic and semi-pelitic rocks contain higher-grade mineral assemblages, yet generally show evidence for only small degrees of partial melting. The extent of melt formation in the Chaotic Zone migmatites is probably linked with fluid infiltration from the underlying, quartz diorite (Paulson & 1988: Linklater et al., 1994); and thus provides eloquent testimony to the effectiveness of fluid infiltration as a means of generating extensive melting, at relatively modest temperatures.

Stop 3-12. Intrusive veins and and dyke cross-cutting disrupted migmatites. NN 0160 5535, 420m.

Directions: From the first exposure of Stop 3-11, proceed about 75 m in NW and obliquely uphill, examining exposures along the way.

Description: A few cm-wide veins and a 2-3 m wide dyke of granitoid material from the igneous complex cut the disrupted metasediments in this vicinity. The dyke tapers to the SSE and widens upslope to the NNW to join with the main mass of granite: thus giving the impression of an apophysis.

The contrast between the light-coloured, medium-grained, granite-textured dyke material and the darker, finer-grained, granular semipelitic material with hornfels fragments is obvious. Although the veins and dykes make well-defined contacts with the metasediments, the contacts nonetheless vary from diffuse where in contact with the granular semipelitic material, to sharp where in contact with the hornfels schollen. In places, a few metasedimentary fragments have been entrained at the margins of the dykes.

These features demonstrate that there was negligible mixing and interaction of magma from the igneous complex with the nubile metasedimentary material, further indicating that the mobility of the metasedimentary material was due to internal processes, most likely internal melt generation fluxed by magmatic volatiles (see above). These features also suggest that although the granitoid material and granular semipelitic material were both ductile and able to flow, the two magmas were sufficiently viscous that they did not mix very much.

Stops 3-13. Many good exposures of Chaotic Zone migmatites enroute to and in the vicinity of NN 0160 5565 - NN 0160 5575, 470-500m.

Directions: From Stop 3-12, head northwards (azimuth (005°) obliquely uphill towards the shoulder of dark coloured exposures and grass. The route follows close to the contact of the metasediments with granite of the igneous complex, the latter revealed by light coloured exposures and boulders just to the west, which contrast with the dark outcrops of the migmatites (see )Photo 15. At an elevation of about 470-480 m, near the top of the shoulder ( NN 0155 5555, marked on Fig. 11), are numerous large, well-exposed outcrop surfaces that show the full range of 'Chaotic Zone' features described under Stop 3-11. Cm to m-scale pockets of white vein-quartz additionally appear in some exposures, sometimes in vaguely defined trains suggesting that they were mechanically broken apart and separated during disruption of the metasediments. Some very andalusite-rich hornfels occurs in the vicinity of, sometimes on the margins of, these quartz-rich segregations.

Continue heading north to some dark, south-facing exposures adjacent to a burn: the exposures along the burn and about 100m north of the burn at about the same elevation constitute Stop 3-13.

Description: These exposures allow examination in three dimensions of some of the most spectacular features of the Chaotic Zone migmatites described under Stop 3-11. The variable extents of disruption of primary layering are well displayed in this vicinity. Although there is an overall NS-trending, steeply E-dipping planar foliation/layering in the exposures, defined by trains of schollen and by the weak foliation in the granular semipelitic matrix, in detail the coherency of the primary layering is largely lost. There are a variety of vein types and schollen types. Some of the schollen are very rich in andalusite­ and/or cordierite.

Stop 3-14. Striped calsilicates of the Ballachulish Limestone lithology. NN 0145 5590, 430m.

Directions: From the well-exposed disrupted migmatite exposures about 100 m north of the burn of Stop 3-13, bear about 320° downhill for 200 in to a prominent grassy, flat-topped ridge. This ridge has platy, slab-like, exposure along its flanks and lies immediately beyond a steeply incised burn. Descend the slope. crossing a boggy area en route to the ridge. Downslope to the WNW, beyond the boggy area and where a burn goes over a small waterfall (NN 0145 5580, 430 m), are some more light-coloured platy exposures of the same rock type as that on the flanks of the ridge.

Description: The rocks are NS-trending, steeply cast-dipping (000-020/65-­90E), fine-grained, white- and green-striped calcsilicate rocks, with many tight folds and some quartz pods. These calcsilicates belong to the lowermost part of the Ballachulish Limestone unit (see Coloured Map and Fig. 11 ). A ca. 1.5 m-thick, shallowly dipping granitoid sill cuts the calcsilicates near the bottom of the outcrop.

The layering in the calcsilicates is on a mm to cm scale and is quite regular. The white layers are rich in diopside, with variable amounts of tremolitic amphibole, plagioclase and quartz, whereas the darker, green layers are rich in biotite and sometimes chlorite, with variable amounts of the above minerals. K-feldspar, calcite and wollastonite have been observed in some layers. Rare samples containing coexisting chlorite+calcite without spinel restrict the maximum temperature of these rocks to about 670 °C (reaction CI7 of Fig. 6).

Looking up slope from here, the location of the igneous contact can be seen where dark grey outcrops of the migmatitic metasedimentary host rocks pass uphill into lighter-coloured outcrops of granite and granodiorite (see Photo 15).

Stop 3-15. Marbles and calcsilicates of the Appin Limestone. NN 0135 5580 – NN 0125 5560, 410 - 340m.

Figure 2. Outline of lithostratigraphic units and major regional structures around the Ballachulish Igneous Complex. The position of the garnet isograd of regional metamorphism. Which formed prior to intrusion of the complex, is extrapolated across the area of the complex. From Pattison & Harte (1997).
Directions: From Stop 3-14, follow the burn down into an obvious gulley, and descend the NNE-SSW trending gulley for about 200 m until a fence is reached, examining the abundant carbonate and layered calcsilicate rocks along the way.

Description: Many varieties of carbonate-rich and calcsilicate rock, locally interlayered with psammitic and rusty-weathering semipelitic rock, are found in the steep gulley. The first, dark-weathering exposures are of yellowish-brown, dolomite-rich carbonate rock showing dissolution along fractures. Continuing down the gulley, various other carbonate, calcsilicate and semipelitic rocks are encountered, including dolomite-rich carbonate rocks with a mm to cm scale, downward-weathering calcite nodules. Strongly ribbed calcsilicate rocks, sometimes with dark micaceous or psammitic interbeds, are also common: these are distinguished from the striped calcsilicate rocks of the Ballachulish Limestone (Stop 3-14) by their less regular (mm-cm scale) thickness variations and the greater variability of the compositions of individual layers. Many rocks contain mm-cm scale veins, which are often quartz-rich, indicative of fracture-channelled fluid infiltration. In places, especially going up the slope to the west, the carbonate and calcsilicate rocks are interlayered with metapelitic and quartzitic rocks.

The marbles and calcsilicates belong to the Appin Limestone unit. Layering in the rocks overall has a steeply east-dipping, NNE-trending orientation (e.g., 025/60E), although there are many folds visible. Above the gulley to the immediate west is a thick expanse of Appin Quartzite, comprising the folded core of the Beinn Sgluich Anticline. The rocks above the gulley to the east, but downslope of the green- and white-striped calcsilicate unit of the Ballachulish Limestone unit, form an interlayered interval of carbonate rock, cordicrite-rich pelitic hornfels and quartzite, with carbonate increasing towards the gulley. This interlayered sequence comprises the Appin Phyllite and Appin Limestone units, and is equivalent to the mixed, calcsilicate/marble and pelite/semipelite sequence seen at Stop 1-7, which occurs on the other (west) side of the Beinn Sgluich Anticline (Fig. 2). The mixed Appin unit here is separated from the green- and white-striped calcsilicates of the Ballachulish Limestone unit by a major fault, the Glen Stockdale Slide. This slide cuts out the Transition Series, Ballachulish Slate and most of the Ballachulish Limestone units (Fig. 2).

Thin-section analysis reveals that the carbonate-rich rocks typically contain calcite, dolomite, mid various combinations of forsterite (variably altered to serpentine and more rarely talc), diopside, tremolitic amphibole, epidote, chlorite. phlogopite, plagioclase and rarely spinel, wollastonite and quartz. Calcsilicate rocks contain variable combinations of calcite, dolomite, plagioclase, quartz, tremolitic amphibole, diopside, phlogopite, muscovite, K-feldspar and rarely wollastonite. The occurrence in close proximity of Spl+Fo+Cal+Dol-bearing and Chl+Cal-bearing marbles, and of Cal+Qtz-­bearing and Cal+Qtz+Wo-bearing marbles, indicates local variations in fluid composition (Paulson & Harte, 1997). These variations were most likely caused by localised, channelled, water-rich fluid infiltration, consistent with the veining visible in many of the rocks.

Return to the vehicle by climbing cast out of the gulley and following the SE-trending fence back to the burn that was ascended to reach Stop 3-10.

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