NEOPROTEROZOIC ACCRETIONARY OROGENESIS ALONG THE NORTHEAST LAURENTIAN MARGIN OF RODINIA (Invited Presentation)

The formation of Rodinia culminated in the c. 1.0 Ga Grenville-Sveconorwegian orogeny, recording the collision of Laurentia and Baltica with Amazonia. The closure of internal oceans during continental collision was followed by the relocation of subduction systems to the margin of the supercontinent. River systems carried detritus from this collisional orogen to marginal basins located in eastern Laurentia which faced the peri-Rodinian ocean. Sedimentary and igneous units in Scotland, East Greenland, Svalbard, Ellesmere Island and north Norway record the tectonomagmatic record of the Valhalla accretionary orogen. Calc-alkaline magmatism is recognised in Shetland, Svalbard and Ellesmere Island at c. 970-950 Ma. Renlandian crustal thickening and Barrovian metamorphism at c. 940-920 Ma in East Greenland, SW Svalbard, Shetland and North Scotland is attributed either to terrane accretion or flat-slab subduction. In Scotland, roll-back of the subduction zone is inferred to have resulted in a post-900 Ma successor basin and bimodal gabbro-granite magmatism at c. 870 Ma. Younger, Soroy (northern Norway; 840 Ma) and Knoydartian (Scotland; 820-780 Ma and 740-725 Ma) tectonothermal events are attributed to further periods of accretionary orogenesis. In contrast, eastern North America (e.g. Newfoundland, Blue Ridge) lay within the supercontinent interior and does not record Neoproterozoic orogenesis. Sedimentation here did not commence until c. 760 Ma, accompanying initial stages of supercontinent breakup. In the North Atlantic realm, the post-725 Ma time frame is represented by a further phase of extension and accumulation of thick sedimentary successions in Ellesmere Island, East Greenland, and Scotland. By this time, further subduction roll-back had resulted in wide marginal seas, perhaps similar to the present-day west Pacific, with the locus of subduction located significantly outboard from the continental margin.