EVIDENCE FOR ~ 40 MA TRANSITION FROM A FOREARC TO A FORELAND BASIN IN EASTERN PART OF LABRADOR TROUGH

The New Quebec Orogen (NQO) is a Paleo-Proterozoic supra-crustal belt that underwent reworking when the Superior Craton collided with the Core Zone, during the Trans-Hudson Orogeny. Within the NQO, the Labrador Trough regroups greenschist-facies sedimentary and volcanic sequences (Kaniapiskau Supergroup) inferred to represent the rifted margin of the Superior and a potential oceanic domain, whereas the Laporte Group, composed of similar successions but metamorphosed to higher grades, remains of unclear origin. Competing models purport that the Laporte Group is either a deeper, higher grade exposure of the Labrador Trough, or an exotic terrane. Large scale sampling, petrography and U-Pb detrital zircon geochronology provide insights to elucidate the sedimentary provenance and depositional environments for both zones.

New results reveal a different sedimentary architecture for both units. In particular, the majority of metarenites from Kaniapiskau Supergroup have medium to high textural and compositional maturity, such as meta-quartzarenites and meta-subarkoses; whereas the metarenites from Laporte Group suggest photoliths with low to medium textural/compositional maturity, like feldspathic and some lithic meta-wackes.

A total of 343 zircon analyses from the Kaniapiskau Supergroup yield ages older than 2.5 Ga (2565 ±12 Ma, youngest zircon), with typical Archean Superior-type signature as the main source area and a main statistical age peak at 2722 Ma. There is more than a 150 Ma gap between detrital zircon ages and the interpreted depositional age, suggesting a divergent environment like a rifted and/or passive margin.

In contrast, detrital zircons from the Laporte Group (a total of 234 analyses) yield ages older than 1.8 Ga, with the youngest zircon dated at 1827 ± 25 Ma and a dominant statistical age peak at 1843 Ma. Metamorphic monazite ages from Laporte Group previously dated at 1793 ± 2 Ma suggest a relative short depositional time period (around 40 Ma). The presence of detrital zircon age peaks at 2.2- 2.4 Ga suggests an exotic terrain compatible with the Core Zone as the main source area. The presence of the orogenic-type De Pas Batholith suggests eastward subduction at 1840-1830 Ma and the time period between detrital ages and deposition may represent a transition between forearc to foreland basin.