Funding Agency: NSF EAR, Award #0087650
Funding dates: April 1, 2001 - March 31, 2004 (Estimated).

Abstract
Knowledge of the distribution of land masses on Earth during the Late Precambrian has wide ranging implications for plate tectonics, paleoclimate and biological evolution. The Precambrian Hebridean Shield is a small fragment of ancestral North America (Laurentia) that detached during break up of Pangaea to become part of Europe. Prior to assembly of Pangaea, it formed the tip of a major promontory of Laurentia (between the Labrador and Greenland margins) and may thus serve as a sensitive probe of interactions between NE Laurentia and other continents. Although the history of this "Scottish promontory" is important to understanding global paleogeography in Neoproterozoic and Early Paleozoic times, its tectonic history remains controversial. Isotopic ages and structural complexities in the metasedimentary Moine and Dalradian supergroups of Scotland have long been interpreted to reflect Neoproterozoic orogenic (Knoydartian) episodes overprinted by Early Paleozoic deformation and metamorphism. Yet Neoproterozoic orogeny is unknown elsewhere in NE Laurentia and is instead a characteristic of Gondwanaland. The existence of this anomalous Neoproterozoic event is based primarily on two independent methods of geochronology, but neither appear water tight. Furthermore, we recognize a regional suite of mafic dikes that may serve as a reliable temporal marker that requires metamorphism to have occurred later than is currently presumed. If correct, Neoproterozoic orogeny in Scotland seems doubtful and deformation in these rocks is more likely related to the assembly of Laurentia and northwestern Europe during the Early Paleozoic (consistent with other regions along the Laurentian margin).

Field mapping of key lithologies form a framework for detailed geochronology and geothermobarometry that will constrain the timing of magmatism, deformation and metamorphism in this region. Absolute ages of events will be used to test correlations of tectonic events along the Laurentian margin. Discounting Neoproterozoic orogeny in Scotland would simplify the pre-Caledonian history of the Scottish promontory of Laurentia in which Neoproterozoic magmas would relate to two separate phases of protracted rifting during the Riphean (~900-800 Ma) and Vendian (~600 Ma) periods. In turn, this would permit correlation of common events along this entire margin, and thus support a proposed two-stage 'breakout' of Laurentia as a discrete continent during the Neoproterozoic, hypothetically from the Rodinian and Pannotian supercontinents, respectively. Alternatively, confirming the existence of Neoproterozoic orogeny in Scotland may dissociate Scotland from the rest of the NE Laurentian margin, thereby inferring crustal exchange between a Gondwanide continent and Laurentia during the latest Precambrian. Either outcome will resolve the timing and nature of tectonic events along the margin of NE Laurentia, thereby contributing to our understanding of global paleogeography during an important segment of Earth history that includes remarkable biodiversity and anomalous climate events.