Abstract This contribution presents a new model for the Grampian-age tectonothermal development of the Buchan Block and Barrovian-type regions to its west, in the Grampian Terrane, Scotland. The model has drawn on evidence gathered from field mapping, microstructural analysis, metamorphic petrology and mafic magma geochemistry to propose that emplacement of the Grampian gabbros and regional metamorphic heating associated with production of Barrovian- and Buchan-type units occurred during syn-orogenic (Grampian-age), lithospheric-scale extension. Extension followed lithospheric thickening associated with the initiation of Grampian orogenesis and was followed by renewed lithospheric thickening and termination of the extensional heating. Mantle melting to produce the Grampian gabbros of the Grampian Terrane was achieved by extensional thinning of the lithosphere and decompression melting of the asthenosphere at depths of less than 70 km. Advection of heat from the mantle with emplacement of the Grampian gabbros augmented elevated heat budgets associated with attenuation of isotherms during extension. Deposition of the uppermost Dalradian (the Whitehills and Boyndie Bay Groups and the Macduff Slates) occurred during Grampian-age lithospheric extension. A gently-dipping, mid-crustal detachment focused metamorphic heat sources and accommodated significant lithospheric-scale strain, allowing independent thermal evolution of units in its hanging wall (the Buchan Block) and footwall (Barrovian-type units).

Abstract The Dalradian Supergroup, comprising the Grampian (oldest), Appin, Argyll and Southern Highland (youngest) groups (Fig. 13) consists of lithologically diverse metasediments and typically mafic metavolcanic rocks. The supergroup is probably entirely late Precambrian in age and underlies an area of about 48000 km 2 in north and west Ireland, Scotland and Shetland. Neither the base nor the top of the Dalradian is seen but, taking the maximum thickness preserved for every group and making no allowance for tectonic thickening, it is calculated to have a total cumulative thickness of c. 25.5 km.

Abstract The Northern Highland and Grampian terranes together comprise an extensive tract of structurally complex and generally high-grade metamorphic rocks within the Caledonian orogenic belt of Scotland ( Fig. 4.1 ). This part of the orogen is dominated by two thick sequences of mainly Neoproterozoic metasedimentary rocks. The older sequence comprises the Moine Supergroup of the Northern Highland terrane, and possibly also the Dava Succession of the Grampian terrane. Both were deposited between c. 1000 Ma and c. 870 Ma, and subsequently affected by a controversial Knoydartian tectonothermal event at c. 800 Ma. The younger Dalradian Supergroup of the Grampian terrane accumulated between c. 800 Ma and the Early Cambrian during the break-up of the late Precambrian supercontinent Rodinia and the formation of the Iapetus Ocean. Inliers of Archaean to Palaeoproterozoic orthogneisses ( Fig. 4.1 ) probably represent fragments of the Laurentian continental basement on which the Moine and Dalradian successions accumulated. Caledonian orogenesis in the North Atlantic region resulted from the closure of the Iapetus Ocean and the convergence of three crustal blocks: Laurentia, Baltica and Avalonia (Soper & Hutton 1984; Pickering et al. 1988; Soper et al. 1992b). Early orogenic activity along the Iapetan margin of Laurentia resulted from an arc-continent collision that occurred during initial ocean closure in the Early to Mid-Ordovician. This phase of the Caledonian orogenic cycle is known as the Grampian event and it affected both the Northern Highland and Grampian terranes. Ocean closure and final amalgamation of crustal blocks occurred in the Late Silurian