A MODEL FOR IAPETAN RIFTING OF LAURENTIA BASED ON NEOPROTEROZOIC DIKES AND RELATED ROCKS

Geologic evidence of the Neoproterozoic rifting of Laurentia during breakup of Rodinia is recorded in basement massifs of the cratonic margin by dike swarms, volcanic and plutonic rocks, and rift-related clastic sedimentary sequences. The spatial and temporal distribution of these geologic features vary both within and between the massifs but preserves evidence concerning the timing and nature of rifting. The most salient features include: (1) a rift-related magmatic event recorded in the French Broad massif and the southern and central Shenandoah massif distinctly older than that recorded in the northern Shenandoah massif and northward; (2) felsic volcanic centers at the northern ends of both the French Broad and Shenandoah massifs accompanied by dike swarms; (3) differences in volume between massifs of cover-sequence volcanic rocks and rift-related clastic rocks; and (4) WNW orientation of the Grenville dike swarm in contrast to the predominately NE orientation of other Neoproterozoic dikes. Previously proposed rifting mechanisms to explain these features include rift/transform and plume/triple-junction systems. The rift/transform system best explains the first three features above, and we propose that it represents the dominant rifting mechanism for most of the Laurentian margin. To explain feature 4, as well as magmatic ages and geochemical trends in the northern Appalachians, we propose that a plume/triple-junction system evolved into the rift/transform system. A ~600 Ma mantle plume centered east of the Sutton Mountains, Quebec, generated the radial dike swarm of the Adirondack massif and the Grenville dike swarm, and a collocated triple junction generated the northern part of the rift/transform system. An eastern branch of this system produced the Long Range dike swarm in Newfoundland, and a subsequent western branch produced the ca. 554 Ma Tibbit Hill volcanics and the ~550 Ma rift-related magmatism of Newfoundland.