DETRITAL MINERAL PROVENANCE AND AGE OF THE OAK HILL GROUP IN THE SOUTHERN QUÉBEC APPALACHIANS; PALEOGEOGRAPHIC IMPLICATIONS FOR THE LAURENTIAN MARGIN FROM NEOPROTEROZOIC DETRITAL ZIRCONS

The early development of the Iapetus rift and Laurentian passive margin is recorded in late Neoproterozoic to early Cambrian volcano-sedimentary units of the Oak Hill Group in the Québec and Vermont Appalachians. These rocks may contain archives of the local paleogeographic setting, the identity of adjacent continental blocks and vestiges of rifting episodes from Rodinia’s breakup history. We investigated this geological record using U-Pb zircon geochronology from igneous and detrital samples from the Waterloo, Sutton and Saint-Sylvestre areas in southern Québec.

We generated 834 concordant ages ranging from 2485 ± 17 Ma to 515 ± 11 Ma. The base of the Oak Hill Group is constrained by an age of 549 ± 3 Ma on a felsic volcanic rock of the Tibbit Hill Formation. The detrital age spectra for samples from the overlying Pinnacle Formation, the basal sandstone unit of the Oak Hill Group, all show peaks around 1425 Ma and 1175 Ma. Samples from the Sutton area are rich in heavy minerals (Fe-Ti-Zr) and show a large Cryogenian peak at ~650 Ma, which is unusual for Laurentian sedimentary units. Younger sedimentary rocks show a greater variety of Mesoproterozoic ages and an attenuation of the 650 Ma signature.

Most of the detrital zircon populations documented in the Oak Hill Group can be explained by a provenance from the Grenville Province. However, the 650 Ma signature of the Pinnacle Formation is neither explained by the erosion of the Grenville Province (~1800-955 Ma), nor by Iapetan rift magmatism as documented in the Northern Appalachians (620-550 Ma). This signature could be attributed to magmatism associated with the fragmentation of Rodinia and the opening of the Puncoviscana ocean, which preceded that of the Iapetus Ocean, but whose remains are not preserved or recognized yet along the Laurentian margin in the Northern Appalachians. These Cryogenian zircons may be derived from the erosion of magmatic rocks emplaced within Mesoproterozoic basement, as documented in the Arequipa-Antofalla block of the Central Andes, which is inferred to have drifted away from Laurentia during the opening of Iapetus. Further work on the origin of the Cryogenian detrital zircons is underway in order to establish the nature of potential sources.