Late Grenvillian deformations in the Parautochthon Belt (Central Grenville Province): Geochronological constraints by coupling U-Pb methods of high spatial resolution and high precision

In the Parautochthonous Belt of the central Grenville Province (Manicouagan Reservoir), age-dating of structures shows that the Grenvillian orogeny obliterated pre-existing fabrics. D1 and D2 structures indicate north-verging reverse shearing with, respectively, SSE-and SSW-trending lineations. A novel approach combining LA-ICP-MS and CA-ID-TIMS zircon U-Pb geochronology constrains the age of these deformation events. Inner and outer overgrowths on zircon grains in a syn-D2 leucosome, which reflect two growth events, are interpreted as being of D1 and D2 age, respectively. Group 1 yields a Pb-206/Pb-207 age of 1008 +/- 18 Ma (LA-ICP-MS) and a concordia age of 1002 +/- 2 Ma (CA-ID-TIMS). Group 2 yields a Pb-206/Pb-207 of 995 +/- 21 Ma (LA-ICP-MS) and a concordia age of 986 +/- 4Ma (CA-ID-TIMS). A late-D2 pegmatite yields a Pb-206/Pb-207 age of 961 +/- 22Ma (LA-ICP-MS). The D1 age is identical the Group 1 age from the leucosome, at 1002 Ma. The zircon could not have grown prior to anatexis, and this group shows features that are consistent with growth during partial melting and S1 is defined by migmatitic banding. The age of D2 is constrained at 986-961 Ma by crystallization of the syn-D2 leucosome coevally with Group 2 zircon, and of the late-D2 pegmatite. Thus, in the central Parautochthonous Belt near the Allocthon Boundary Thrust, where the first deformation event is Grenvillian, no evidence of any pre-Grenvillian tectono-metamorphic event is preserved. [Journal translation]