The Northern Highland and Grampian terranes
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