Geology of the Aberfoyle district: Metamorphism
|This topic provides a summary of the geology of the Aberfoyle district – covered by the British Geological Survey. 1:50k geological map sheet 38E (Scotland).
Authors: C W Thomas, A M Aitken, E A Pickett, J R Mendum, E K Hyslop, M G Petterson, D Ball, E Burt, B Chacksfield, N Golledge and G Tanner (BGS).
Rocks belonging to the Dalradian Supergroup and Highland Border Complex were metamorphosed during Caledonian orogenic events (e.g. Strachan et al., 2002). Prograde metamorphism occurred during the Grampian event, following initial thickening of the crust by major folding and nappe formation during D1. Based on radiometric data from elsewhere in the Dalradian, peak metamorphism between D2 and D3 during the Grampian event occurred between about 465–470 Ma (Rogers et al., 1994; Friedrich et al., 1999; Baxter et al., 2002). The Grampian event is likely to have been short lived (Friedrich et al., 1999; Oliver et al., 2000), but work by Baxter et al. (2002) suggested that, at least locally, there may have been two relatively closely spaced thermal maxima. Retrogression accompanied the pre-D4 uplift and extension that resulted in the formation of the Highland Border Downbend. There are no absolute constraints on the age of the retrogression. It may have occurred during the late stages of the Grampian event or, by analogy with the structural history, it may have occurred during the later Scandian event (435–425 Ma).
Like the deformation, the metamorphic grade declines towards the Highland Boundary Fault. Rocks in the Highland Border Complex probably share much of the deformation and metamorphic history of the adjacent Dalradian. However, mineral assemblages in the different units within the Highland Border Complex, as defined in this account, reflect manifestly different grades of metamorphism. The amphibolite facies assemblages of the Corrie Burn Hornblende Schist, adjacent to the Highland Boundary Fault in Loch Achray Forest, reveal metamorphism at significantly higher grade than that which affected the surrounding sedimentary rocks. For example, a quartz + dolomite assemblage in the Dounans Limestone at Limecraig Quarry reflects metamorphic conditions at greenschist facies at most, as do adjacent Dalradian lithologies. Thus, units within the Highland Border Complex were not metamorphosed together, at high regional grade, prior to their conjunction with the Dalradian metasedimentary rocks. The amphibolite schists must have undergone metamorphism prior to their juxtaposition with other Highland Border Complex rocks and Dalradian units. Henderson and Robertson (1982) suggested that dynamothermal metamorphism, at the base of a thrust sole, was responsible for the generation of the amphibolite schists during emplacement of the HBC. High heat flow and accompanying strong shearing deformation within the oceanic crust from which these rocks were derived, was probably responsible for the development of amphibolite schist from the mafic igneous protolith. The deformation possibly occurred during the very earliest stages of collision between the continental margin and oceanic island arc masses.
The metamorphic conditions affecting Dalradian rocks ranged from very low-grade, sub-greenschist facies in the south, to thorough greenschist facies in the north (Tilley, 1925; Mather, 1970). The lowest grade rocks occur adjacent to the Highland Boundary Fault. Rocks in the Loch Ard forest area contain primary detrital minerals, typically quartz, feldspar and mica. These rocks are only weakly metamorphosed and show variable alteration, with feldspars commonly altered to a mixture of sericite, albite and carbonate. Potassium feldspar (K-feldspar), in the form of microcline, is unaltered (Mather 1970), except for very local replacement by stilpnomelane (Mather and Atherton 1965; see below).
Further north, around Loch Ard, the primary mineralogy has recrystallised, notably with detrital plagioclase feldspars being replaced by albite. Here the rocks commonly contain quartz, albite, white mica (phengitic muscovite), chlorite and microcline, an assemblage typical of the chlorite zone of the greenschist facies.
Biotite-forming reactions in pelitic rocks define broad metamorphic zones within the greenschist facies across the area (Mather 1970; Tilley 1925). Tilley originally identified the incoming of biotite in pelitic rocks north-west of a line between Ben Lomond and Stronachlachar (NN 402 103). However, Mather recognised the importance of compositional control on the appearance of biotite in rocks in the Aberfoyle district. He established the limit of the incoming of biotite in metamorphosed wacke sandstone lithologies, placing it some 4–5 km to the south-east of Tilley’s biotite limit in pelitic rocks. This limit extends from Loch Lomond, through the southern end of Loch Chon (NN 435 035) and the eastern end of Loch Katrine (NN 488 078), continuing in a north-east direction towards Glen Finglas.
Mather (1970) defined three zones on the basis of biotite-forming reactions in the metasedimentary rocks. At the lowest grade, chlorite + K-feldspar (microcline) are stable together in rocks of appropriate bulk composition within the chlorite zone. The upper limit of this zone is marked in metasandstones by the incoming of biotite due to the continuous reaction:
chlorite + K-feldspar + quartz = biotite + muscovite + H2O (1)
Biotite appears at lower temperatures in bulk compositions with lower Al2O3/(FeO+MgO) ratios. However, in rocks with the same Al2O3/(FeO+MgO) ratio, biotite appears at lower grades in rocks with higher Fe/Mg ratios (Spear, 1993). This reaction is continuous and, with increasing temperature, the biotite and chlorite will move to more Mg-rich compositions. The resulting assemblages will either contain chlorite or K-feldspar, in addition to biotite and white mica. Bulk compositions of biotite-bearing metasandstones from the Aberfoyle district all lie within the biotite + muscovite + chlorite stability field (Mather 1970, Figure 5, Figure 6 and Figure 7).
Recent quantitative thermodynamic study of the phase relations between chlorite, K-feldspar, white mica, biotite and quartz (Simpson et al., 2000) helps to place constraints on the metamorphic conditions at which reaction (1) occurs. Combination of Mather’s phengite composition data (Mather, 1970) with Figure 2 of Simpson et al. (2000), suggests peak temperatures of about 400°C at pressures of about 1–2 kbars. The indicated pressures are likely to be unrealistically low. To the south-west in Argyll and on Islay, Graham et al. (1983) recorded prograde pressures in the region of 10 kbars, and pressures during retrogression of about 6 kbars. Further combined microanalytical and petrographical work would be required to establish more precisely the peak pressure conditions during metamorphism in the Aberfoyle district.
At higher grades, Mather determined that the appearance of biotite in pelitic rocks resulted from the enlargement of the phengitic muscovite + chlorite + biotite stability field, as celadonite solid-solution in muscovite is reduced with increasing temperature (cf. Simpson et al., 2000). This enlargement of the stability field occurs towards more aluminous compositions, thereby including progressively more pelitic lithologies. Consequently Tilley’s record of biotite appearance in truly pelitic rocks is several kilometres north-west of that for metasandstones.
The metamorphic mineralogy of the mafic ‘green beds’ within the Loch Katrine Volcaniclastic Formation has been described by Kamp (1970). To the south of Loch Katrine ‘green bed’ lithologies are dominated by chlorite, albite, epidote and some calcite, an assemblage typical of mafic rocks in the lowermost grades of greenschist facies metamorphism. In the Loch Katrine area, assemblages in the ‘green beds’ are within the biotite zone, as defined by the metasandstones, and contain muscovite and some biotite in addition to the phases in chlorite zone rocks. The presence of biotite is compositionally controlled, as discussed above, arising from the mixing of siliciclastic sediment with mafic volcanic detritus.
Albite porphyroblasts are common in the volcaniclastic-rich, ‘green bed’ lithologies of the Loch Katrine Volcaniclastic Formation. Trails of fine-grained inclusions in the albites are commonly aligned parallel with the main schistosity, but in some samples, these trails are oblique or sigmoidal, indicating rotation of the fabric during porphyroblast growth. The main phase of albite porphyroblast growth in ‘green bed’ lithologies appears to have occurred after D2, but before or during D3, during peak metamorphic conditions. Subsequent development of inclusion-free rims on many albites indicates later recrystallisation and/or growth, possibly during D4. This indication of porphyroblast growth during peak metamorphic conditions accords with the work of Dymoke (1989) and Mathavan and Bowes (2005), but contrasts with the interpretation of Watkins (1983); Watkins proposed that albite porphyroblast growth resulted from later regional fluid infiltration and hydrolysis of garnet-bearing rocks, with porphyroblast growth concentrated in the hinges of regional F3 folds.
D1 and D2 deformation fabrics are defined dominantly by recrystallised phyllosilicates. Deformation and recrystallisation of feldspar and quartz grains also results in the grain alignment fabric described in Section 9. On the north side of Loch Katrine, at Letter Burn [NN 46 11], thin sections of metasandstones record overgrowth rims on quartz and feldspar grains that are synchronous with the chlorite-defined schistosity. Further deformation results in the migration of silica into pressure shadows.
Post-downbend D4 deformation is manifest typically as a weak to moderate crenulation, best developed in pelitic layers in the hinges of F4 folds. Generally, there appears to be no new mineral growth associated with D4. However, stilpnomelane has been recorded locally in metasandstone lithologies near Kinlochard [NN 455 024] (Mather and Atherton, 1965). Because of its geographical distribution in the Aberfoyle district, Mather (1970) suggested that Stilpnomelane may be associated with the development of the Ben Ledi Antiform, then perceived as a relatively late structure. In the south-west Highlands, stilpnomelane is a mineral characteristic of retrogressively altered metabasite lithologies (e.g. Graham et al., 1983). Within the metasandstones in the Aberfoyle district, stilpnomelane is described as lying across the dominant schistosity, wrapping quartz and feldspar grains and locally replacing microcline (Mather and Atherton 1965) and thus appears to be late. It may, therefore, result from retrogression that occurred during uplift and extension.
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