XIX.—Metamorphic History of the Schichallion Complex (Perthshire)

Abstract

SynopsisThe Schichallion complex is situated in the Central Highlands of Scotland between the villages Struan and Kinloch Rannoch. The area is of considerable geological interest and has been investigated stratigraphically and structurally by E. M. Anderson and Bailey and McCallien. As a consequence of their researches a complete stratigraphical succession has been established. Thus, the metamorphic rocks of the area are classified into the Moinian and Dalradian systems, which are separated by a plane of tectonic discontinuity known as the Boundary Slide. The Moinian rocks are quartz-felspathic granulites, whereas the Dalradian system includes pelitic schists, quartzites, limestones as well as a variety of meta-igneous rocks.In his previous research the present author has established the tectonic history of the complex. In particular three episodes of folding (F1to F3) and a much later episode of faulting (F4) were recognized. Of these the F3episode is of least significance. Consequently, events after the F2movements can be in many cases regarded as post-folding.The present contribution is concerned essentially with the mapping of the metamorphic zones (garnet and staurolite-kyanite) and with a detailed study of the mineralogical evolution of regionally metamorphosed rocks within these zones.In the field it is possible to prove that the staurolite-kyanite zone is essentially post-folding, since the kyanitepegmatites are found to cut across the minor F2-folds. In this respect textural studies confirm the field observations. The study of the internal inclusions in garnets indicates that the garnet zone has a much longer history, since pre-F2garnets are found in the southern part of the complex and throughout the central part of the area syn-tectonic F2garnets are apparent. The pre-F2garnets contain a very fine-grain F1fabric. Hence the garnets in relation to the F1movements are post-tectonic. Thus, the regional metamorphism can be subdivided into three phases: the F1metamorphism, the F2metamorphism and the post-F2metamorphism. The latter, on structural evidence appears to be at least in part contemporaneous with the F3movements.The meta-igneous rocks of the area are grouped into the hornblende-schists and granular epidiorites. The hornblende-schists appear to have suffered deformation and recrystallization during F1and F2episodes of movement. On the other hand the granular epidiorites are later than the F1and the F2movements. Although in the southern parts of the district the epidiorites have been slightly deformed, elsewhere they preserve the original ophitic texture and cut across the F1and F2folds. The deformation in the south is attributed to the effects of the F3folding. The epidiorites have been evidently emplaced as dolerites after the F2movements and before the F3metamorphism.The localized retrogressive metamorphism is associated with the F4movements, which are responsible for the Loch Tay Fault. The Fault is later than the minor intrusives associated with the Younger Granites of the Scottish Highlands and is suggested to be of a Lower or Middle O.R.S. age.On the basis of the chemical composition of the plagioclase felspars it is proposed to include all the staurolite and kyanite bearing rocks into the epidote-amphibolite facies. In this respect temperature and the hydrostatic pressure are assumed to have been the main factors, since similar minerals came into existence during static and dynamic stages of metamorphism alike.

The Dalradian rocks of the central Grampian Highlands of Scotland

“…This interpretation (Treagus and King, 1978;Treagus, 1987Treagus, , 2000 contradicts previous interpretations (Bailey and McCallien, 1937;Rast, 1958) that this is a folded sequence of formations now assigned to the Blair Atholl Subgroup and the lower Argyll Group, and that the contact with the Grampian Group is a 'slide'.…”

“…This interpretation was supported subsequently by Rast (1958), who interpreted a persistent outcrop of schist immediately above the slide, the Beoil Schist, not as a stratigraphical formation but as a product of localized ductile deformation.…”

“…This late phase is associated with the major N-S-trending Errochty Synform and the Bohespic Antiform, described by Rast (1958) from the boundary of the Grampian Group with the Appin and Argyll groups to the south (see the Meall Dail Chealaich GCR site report). Minor structures associated with this phase become remarkably intense in the mid-part of the Glen Garry section, where the Errochty Synform produces the tight reclined structures at locality 3.…”

“…It was not however noted by subsequent workers in the area, Anderson (1923), Bailey and McCallien (1937) or Rast (1958). Its significance was realized by Thomas (1965Thomas ( , 1980, who interpreted it as the steep limb of one of a number of major post-metamorphic angular kink folds in this area and named it the Trinafour Monoform (Ft on Figure 39a, following the nomenclature of Treagus, 2000).…”

The Dalradian rocks of the north-east Grampian Highlands of Scotland

“…Pantin (1961), who introduced the simpler term Glen Banvie ‗Series' for the ‗Blair Atholl Series of the Banvie Burn Belt', found no evidence for sliding at Gilbert's Bridge. However, Harris (1963) interpreted the junction to be a thrust slide carrying the Dalradian rocks to the north-west, and Thomas (1965Thomas ( , 1980 concluded that the Boundary Slide is associated with a tectonic schist equivalent to the Beoil Schist of the Schiehallion area (Rast, 1958) (see the Strath Fionan GCR site report in Treagus et al, 2013).…”