THE LONG-HOT RIGOLET PHASE OF THE GRENVILLIAN OROGENY

The last phase of the Grenvilllian Orogeny, called Rigolet (ca. 1010-950 Ma), is commonly considered as a short and cold event that developed by foreland-ward propagation of the orogen following convergence renewal. During the last 12 years of field-based studies in the Manicouagan area and along highway 117 of the central-eastern and western Grenville Province of Quebec (Canada), respectively, our research group has systematically conducted U-Pb geochronology in order to constrain the timing of deformation in the Parautochtonous Belt. In the Manicouagan area, nine leucosome and two leucogranite dikes present evidence for melt-present deformation and yield ages, obtained by La-ICP-MS, centred at ca. 990 Ma that are indistinguishable, within errors, from two a syn-deformation titanite in amphibolite and two others late leucogranite dikes. U-Pb geochronology using a higher precision technique (i.e., Ca-ID-TIMS) implies at least 13 Myrs of melt-present deformation. Similarly, in the western Grenville Province, one leucosome and seven leucogranitic dikes present evidence for melt-present deformation for at least 29 Myrs between ca. 970 and 1010 Ma. At both localities, Archean gneisses of the Parautochtonous Belt were pervasively melted and flowed towards the foreland. Leucogranite migrated structurally upward and intruded into the previously metamorphosed rocks of the Allochthonous Belt. This lead to melt-enhanced deformation of these gneisses that were locally detached and incorporated into the underlying flowing channel as metric pods and as km-size slices. Not only that these data imply that the Rigolet phase was long and hot, but it constitutes a spectacular example of heterogeneous channel flow in a Mesoproterozoic orogen.