EARLY PROTEROZOIC TECTONIC OVERVIEW OF THE NORTHERN BRITISH-ISLES ANDNEIGHBORING TERRAINS IN LAURENTIA AND BALTICA

Using a modified Piper (1976) pre-Grenville fit of Laurentia and Balti ca, tectonic information from the Lewisian complex of Britain and from neighbouring Early Proterozoic belts of Laurentia and Baltica is revi ewed. It is shown that a remarkable consistency in convergence directi ons existed across the whole region in the period 1.9-1.8 Ga suggestin g that the reconstruction used may be correct, and that a common movem ent pattern can be inferred for the Early Proterozoic period within th e North Atlantic region. A speculative plate tectonic history for the region between approximately 2.6 and approximately 1.6 Ga is discussed in four stages. (1) 2.6-2.4 Ga: development of conjugate shear-zone s ystems in the North Atlantic craton. (2) 2.4-2.0 Ga: rifting and dyke emplacement in older cratons; creation of oceanic and intracontinental basins. (3) 2.0-1.8 Ga: subduction at active margins of older cratons with creation of magmatic arcs; collision of cratons accompanied by c losure of intra-cratonic basins. (4) 1.8-1.6 Ga: development of new ac tive margin discordant to the previous ones, with significant changes in convergence direction within the amalgamated continental assembly.