The geology and geochemistry of the metasedimentary rocks of the Loch Laggan - Upper Strathspey area, Inverness-shire

Abstract

An area of 120Km2 extending from Loch Laggan to the upper reaches of the River Spey has been mapped in detail on a scale of 1:10,000. The metasediments cropping out in this area have been assigned to the Grampian Division and consist of varied assemblages of psammltes, semi-psammites and semi-pelites which can be subdivided into two lithostratigraphic successions separated by a tectonic discontinuity, the Gairbeinn Slide. Two lithostratigraphic formations were recognised in the lower Glenshirra Succession, while four formations as well as a transitional lithostratigraphic unit were recognised in the overlying Corrieyairack Succession. Sedimentological analysis suggests that the sediments accumulated in environments varying from alluvial to marine and were mostly transported from the south to the north. The scattered calc-silicate pods and bands were produced by localised precipitation of carbonates during diagenesis. Geochemical studies suggest that the original sandstones of the Glenshirra Succession were dominantly immature lithic arenites whereas those of the Corrieyairack Succession were greywackes. The semi pelitic rocks of the Garva Formation appear to be the most chemically "immature". Provenance analysis suggest that their source rocks were upper and middle level crustal complexes composed of gneisses and granites. The rocks in this area were subjected to three phases of ductile deformation followed by one of brittle movements including faults. D1 resulted in minor, isoclinal, recumbent folds and movement on the Gairbeinn Slide. D2 produced a major tight Laggan Antiform with generally, NE-SW trending axis, while D3 is represented by major crossfolds of the earlier structures on NW-SE axes. Calc-silicate and peiitic phase assemblages indicate that middle amphibolite facies metamorphic conditions lasted from early D1 to post-D2 times, while retrograde conditions followed D3 folding. Localised contact metamorphism accompanied the intrusion of the Corrieyairack Granite Complex. Other igneous rocks in the area include the early two-mica granites and pegmatites which were intruded syn-to late-D3. The remainder of the igneous rocks were emplaced later, and are post-orogenic and postmetamorphic.