APATITE FISSION-TRACK EVIDENCE FOR DIFFERENTIAL EXHUMATION ON A TRANSECT THROUGH THE GRENVILLE PROVINCE (QUEBEC TO NEWFOUNDLAND-LABRADOR) CROSSING THE MANICOUAGAN IMPACT STRUCTURE

Apatite fission-track (AFT) ages determined for 17 samples of high-grade metamorphic and igneous rocks of the Grenville province show significant age variations along a transect from Baie Comeau, QC on the St. Lawrence River to Labrador City, NL crossing 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).

Five new AFT ages were determined in this study. A sample from Baie Comeau on the St. Lawrence River yielded an AFT age of 198 ± 30 Ma which is consistent with previously published AFT ages from this investigation (159 ± 26 to 223 ± 17 Ma) and those determined for Grenville hanging wall (~150 Ma) and footwall rocks (~200 Ma) along the SLRS faults from Quebec City to Tadoussac (Tremblay et al., 2013). Two new samples from 75 -100 km north of the St. Lawrence River gave comparable AFT ages of 222 ± 28 and 213 ± 32 Ma.

A sample from the northeastern rim of the MIS yielded an AFT age of 225 ± 37 Ma which is 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). This AFT age is consistent with a previously published AFT age of 206 ± 24 Ma from the same part of the MIS. To the northeast of the MIS across the Newfoundland-Labrador border, a new AFT age of 478 ± 75 Ma was determined and is consistent with four previously published ages from that area (271 ± 31 to 433 ± 43 Ma).

Track length distributions for five new samples will be modeled using the HeFTy program (Ketcham, 2005) to help differentiate between different exhumation processes in the region. Initial models for samples from the St. Lawrence River area indicate phases of rapid cooling of 1°C/myr from 250 to 180 Ma and 0.5°C/myr at 75 Ma. Additional HeFTy models for samples within the MIS impact zone will help differentiate between impact-related and regional cooling and exhumation.