Assembly and dispersal of supercontinents: The view from Avalon

Abstract

Recent models for the post-750 Ma Rodinian supercontinent dispersal (e.g. Hoffman, 1991) envision that cratons margined by Grenvillian belts, were reorganized before ca 540 Ma to form the Gondwanan supercontinent. Laurentia and Baltica distanced themselves from Gondwana by moving out of the Rodinian cratonal cluster. West Gondwana, of which Avalon was a part during the late Proterozoic to Cambrian cratonal assembly, consisted mainly of Africa and South America. The main geological evidence is presented for: (1) a transition from continental platform conditions to those of a subduction-related volcanic arc regime in Late Proterozoic time during the dispersal of the Rodinian supercontinent, and the resulting assembly of the Gondwanan supercontinent; and (2) a second transition that marked a reversal from the volcanic arc regime to marine platformal environments by early Cambrian time. Evidence for progressive instability of the continental shelf margining the Rodinian supercontinent is contained in late Proterozoic olistostromes, mylonite zones, calc-alkaline magmatism, and arc-derived clastic rocks, some being glacigenic, during three phases of the Avalonian orogeny. By early Cambrian time the reversal from a tectonically unstable volcanic arc regime to more stable platformal conditions took place as Avalon, Armorica and related microcontinental blocks rifted from Gondwana. These Gondwanan fragments sequentially come into collision, first with each other and Baltica, and then with Laurentia in Mid to Late Paleozoic time as Pangaea was being assembled.