THE LATE INVOLVEMENT OF LAURENTIA IN RODINIA ASSEMBLY

The actual Laurentian-centred paradigm for the construction of the Grenville Province during the Grenvillian Orogeny involves a protracted phase of Andean-style tectonism between ca 1.8 and 1.15 Ga followed by a continental collision with another craton, postulated to have been Amazonia, at ca. 1.1 Ga. In this contribution, we first demonstrate that this model is incompatible with global geochemical datasets that are at the center of the Mesoproterozoic tectonics controversy, but also with multidisciplinary geological datasets from the best and widest Grenvillian exposure: the Grenville Province of NE America. We then present a completely revised model for Grenvillian tectonics. In contrast with the current paradigm, our model involves the construction of a newly revealed continent by amalgamation of oceanic arcs/backarcs far away from Laurentia. A continental arc did develop on this amalgamated continent, but it was during the Ottawan phase of the Grenvillian Orogeny, which was previously considered as the main continental-collision phase. The final collision between this continent (perhaps amalgamated to another craton) and Laurentia occurred at ca. 1.03 Ga and marked the final assembly of Rodinia. This new model explains the largely juvenile signature recorded in igneous rocks of the Grenville Province, in global detrital zircon, and in Sr isotopes of carbonates between 1.8 and 1.2 Ga. A later collision further provides an explanation for the absence of Ottawan igneous or metamorphic ages in the Parautochtonous Belt of the Grenville Province, it resolves the paradox of the Mid Continent Rift developing in the hinterland of a major orogen, and is compatible with paleomagnetic data indicating very fast southward migration of Laurentia between 1.15 and 1.08 Ga.