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APATITE FISSION-TRACK AGE DISCONTINUITIES ALONG A TRANSECT THROUGH THE GRENVILLE PROVINCE FROM BAIE COMEAU, QC TO LABRADOR CITY, NL
The transect cuts N-S through a complete section of high-grade metamorphic and igneous rocks of the Grenville province, which was tectonically active until ~1000 Ma ago. During the Ediacaran this region was the site of a Large Igneous province centered on the Sept Iles Mafic Intrusion (565 ± 4 Ma; Higgins and van Breemen, 1998). A large bolide hit the region 214 ± 1 Ma ago (Hodych and Dunning, 1992), producing the 100-km-diameter Manicouagan Impact structure (MIS). Finally, parts of the region may have been uplifted about 200 Ma ago, in response to the opening of the Atlantic Ocean. This project was initiated to uncover the influence and importance of these events.
Four samples from this transect yield apatite fission-track (AFT) ages that are significantly different. A sample which was collected within the nominal outer crater rim of the MIS yielded an AFT age of 265 ± 46 Ma which is comparable to the age of the impact. Two other samples which were collected 40 and 140 km northeast of the outer rim of MIS yielded significantly older AFT ages of 409 ± 62 and 527 ± 75 Ma, respectively. These AFT ages are consistent with those of the pervasive Ediacaran large igneous province and with AFT ages (443 – 378 Ma) determined for the hanging wall region north of the Lac Kénogami fault in the Saguenay River Graben to the south. A sample from just north of Baie Comeau near the St. Lawrence River yielded the youngest AFT age of 209 ± 29 Ma. This AFT age is consistent with those determined for Grenville footwall rocks (201 – 184 Ma; Roden-Tice et al., 2011) along the Saint Lawrence Rift System (SLRS) faults from Quebec City to Tadoussac. The dominance of ~ 200 Ma AFT ages along the SLRS from Quebec City to Baie Comeau show the importance of late Triassic reactivation of the SLRS normal faults and exhumation of the region associated with the opening of the Atlantic Ocean.