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

The breakup of the supercontinent Rodinia in the late Neoproterozoic led to the opening of the Iapetus Ocean on the eastern margin of Laurentia. The early development of this margin was recorded in the Humber Zone of the Southern Quebec Appalachians, and more specifically in the Late Neoproterozoic to Cambrian Oak Hill Group. To uncover the early depositional history and paleogeographic evolution of this margin, we investigated the U-Pb ages of igneous and detrital zircons in Southern Quebec sediments, between the Neoproterozoic and the Cambrian.

The felsic igneous and siliciclastic rock samples were collected from the Waterloo, Sutton and St-Sylvestre areas. Igneous and detrital U-Pb (LA-ICP-MS) ages from these rocks generated 834 concordant ages ranging from 515 ± 11 Ma to 2485 ± 17 Ma. A felsic igneous rock from the base of the Oak Hill Group yielded an emplacement age of 549 ± 3 Ma. All the siliciclastic samples from the Oak Hill Group yielded peaks between 1000 and 1500 Ma. In the Sutton area, a Neoproterozoic age group centered on 650 Ma and composed of 77 grains ranging from 561 ± 14 Ma to 707 ± 18 Ma was found in two heavy mineral-rich sandstones (Fe-Ti-Zr rich paleoplacers) and one sandstone near the base of the Oak Hill Group but becomes faint higher up in the stratigraphy. In two of these three samples, a subtle Paleoproterozoic age group centered on 1750 Ma and composed of 7 grains ranging from 1703 ± 36 Ma to 1784 ± 19 Ma is also present and fades out in the upper units. Other punctual Paleoproterozoic ages are present in two samples higher up in the sequence.

The common occurrence of Mesoproterozoic ages with subordinate Paleoproterozoic grains suggests a dominant Grenvillian provenance for the Oak Hill Group sediments. However, the 650 Ma signature is rare in the Laurentian margin rocks and much more common in the sedimentary sequences derived from Gondwanan sources. To explain this Neoproterozoic signature and its quick attenuation upward, we have investigated various potential sources such as rifted continental blocks that were later accreted on Amazonia’s western margin, and punctual rift-related felsic intrusions along Laurentia’s eastern margin.