Neoproterozoic extension on the Scottish Promontory of Laurentia: Paleogeographic and tectonic implications

The Hebridean shield, the northwest foreland of the Caledonian Orogen of Scotland, is a small fragment of Laurentia detached during the Cenozoic opening of the North Atlantic Ocean and is now part of Europe. The shield was at the tip of a major promontory of the ancestral core of North America, between the Newfoundland (Appalachian) and Greenland (Caledonian) margins. Its history is important to understanding late Precambrian and early Paleozoic global paleogeography and tectonics. Isotopic ages and structural complexities in the Moine and Dalradian Supergroups of the Caledonian Orogen have been interpreted as reflecting Neoproterozoic orogenic episodes overprinted by early Paleozoic deformation and metamorphism. A critical body of rock in the Scottish Highlands, the West Highland Granite Gneiss, has been viewed as a synorogenic intrusion into Moine metasedimentary rocks, and its similar to 870-Ma U-Pb zircon age as dating a Riphean "Knoydartian" orogeny. However, field evidence shows that the granitic protolith of the gneiss was emplaced before a regional suite of tholeiitic dikes was intruded into brittle fractures. The dikes carry all the ductile regional deformation. The zircon age thus reflects the crystallization of an anatectic melt, not its subsequent gneissification. Melting is thought to have resulted from advection of heat by emplacement of basaltic magma deep within the Moine sedimentary pile. In this new scenario, deformation and gneissification took place during the early (Grampian/Taconic) phase of the Caledonian Orogeny, not during the Neoproterozoic. Our interpretation is that all the Knoydartian events were extensional. This leads to a substantial simplification of the pre-Caledonian history of the Scottish Promontory of Laurentia. Protracted rifting in the Neoproterozoic was concentrated in two phases, with episodes of major extension and bimodal magmatism in the Riphean (similar to 900-750 Ma) and Vendian (similar to 600 Ma). These episodes coincide with the two-stage breakout of Laurentia as a discrete continent during the Neoproterozoic, hypothetically from the Rodinian and Pannotian supercontinents, respectively.