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

The north-east Grampian Highlands, as described here, are bounded to the north-west by the Grampian Group outcrop of the northern Grampian Highlands and to the south by the Southern Highland Group outcrop in the Highland Border region. The Dalradian succession therefore encompasses the whole of the Appin and Argyll groups, but also includes an extensive outlier of Southern Highland Group strata in the north of the region. The succession includes shallow-marine sequences, glacigenic deposits at two stratigraphical levels, the earliest evidence for volcanism in the Dalradian, a later major development of basaltic and picritic sub-marine lavas, and thick turbiditic sequences. In the south, the Grampian–Appin group boundary is a high-strain zone, with no obvious dislocation or stratigraphical excision, which was formerly termed the Boundary Slide. Shear-zones at higher structural levels are associated with pre-tectonic granites, such as the Ben Vuirich Granite, which have been dated at c. 600 Ma and hence place limits on the timing of sedimentation, deformation and metamorphism. The region is divided from north to south by a major zone of shearing and dislocation with associated igneous intrusions, termed the Portsoy Lineament. To the west of the lineament, the stratigraphy is more-or-less continuous along strike with that of the central Grampian Highlands. D1, D2 and D3 structures extend from the Tummel Steep Belt north-eastwards throughout this area. The stratigraphical succession is broadly continuous across the Portsoy Lineament but to the east, in the Buchan Block, correlations are more tenuous and do not extend below subgroup level. High-grade migmatitic paragneisses were once interpreted as pre-Dalradian basement but they are now assigned to the Crinan Subgroup, within the Dalradian succession. Within the Buchan Block the outcrop pattern is controlled by two broad, open, post-metamorphic folds, the Turriff Syncline and the Buchan Anticline. The Buchan Block is the international type area for the high-temperature/low-pressure Buchan-type regional metamorphism. To the south and west, this passes into higher pressure Barrovian-type metamorphism. South of Deeside, metamorphic conditions reached 820 8C and over 8 kbar, well into granulite facies and the highest recorded in the Grampian Terrane. The detailed relationship between the high heat-flow and the emplacement of large bodies of basic and silicic magma is a matter of ongoing research. Plutons of the north-east Grampian Basic Suite, emplaced at c. 474–470 Ma, during or shortly after the peak of metamorphism and the D3 deformation, provide key evidence for the timing of the Grampian orogenic event.