DIFFERENTIAL EXHUMATION ALONG A TRANSECT THROUGH THE GRENVILLE PROVINCE FROM BAIE COMEAU, QC TO LABRADOR CITY, NL, VIA THE MANICOUAGAN IMPACT STRUCTURE

Apatite fission-track (AFT) ages determined for 13 samples of high-grade metamorphic and igneous rocks of the Grenville province show significant age variations along a transect from Baie Comeau, QC to Labrador City, NL including the nominal rim of the Manicouagan Impact Structure (MIS). This region was tectonically active during the Grenville orogeny (~1200-1000 Ma) and was also the site of a Large Igneous Province around 560 Ma (Higgins and van Breemen, 1998). The 100-km-diameter MIS was formed by the collision with a large bolide at 214 ± 1 Ma ago. Triassic reactivation (~200 Ma) of the St. Lawrence Rift System (SLRS) normal faults, and exhumation of the adjacent region, was associated with the opening of the Atlantic Ocean (Tremblay et al., 2013).

Two samples from the northeast rim of the MIS yielded AFT ages of 205 ± 24 and 210 ± 29 Ma which are comparable to both the U-Pb and (U-Th)/He zircon age of the impact (214 ± 1; 213.2 ± 5.4 Ma) from melt sheet samples (Hodych and Dunning, 1992; van Soest et al., 2011). Three samples from along the southeast rim of the MIS yielded significantly older AFT ages, all of which are within error (256 ± 32 to 265 ±46 Ma). To the northeast of the MIS, five samples yielded AFT ages ranging from 271 ± 31 to 527 ± 75 Ma. Three of these AFT ages (~300 Ma) are within error and are consistent with those determined for the region north of the Sainte Marguerite fault in the Saguenay River Graben ~350 km to the southwest. The other two older AFT ages may be biased by high Cl content of the apatites, which is yet to be evaluated.

The three samples near Baie Comeau, along the St. Lawrence River, yielded significantly younger AFT ages ranging from 159 ± 26 to 209 ± 29 Ma which are consistent with those determined for Grenville hanging wall (~150 Ma) and footwall rocks (~200 Ma) along the SLRS faults from Quebec City to Tadoussac.

Track length distributions for nine samples will be modeled using the HeFTy program (Ketcham, 2005) to help differentiate between possible rapid exhumation following the impact event from slower long-term exhumation of the interior Canadian shield and Mesozoic exhumation along the SLRS associated with late Triassic to Early Jurassic rifting.