Conodonts and corals in kimberlite xenoliths confirm a Devonian seaway in central Ontario and Quebec

Abstract

Eighteen samples containing sedimentary rock xenoliths were obtained from cores drilled into eight Mesozoic kimberlite pipes in the Kirkland Lake area, Ontario, and in Ontario and Quebec near Lake Timiskaming. Nine samples from five pipes contained fossils that were used for age determinations. These fossils are Middle or Late Ordovician graptolites, inarticulate brachiopods, and conodonts; Silurian and (or) Devonian conodonts; Early Devonian colonial corals; a Devonian stromatoporoid; and Early to Middle Devonian conodonts. Regionally, conodonts are unaltered (conodont colour alteration index, CAI 1). Conodont CAI values from the xenoliths are elevated (CAI 2), and a few conodonts have surface colour changes, suggesting hydrothermal alteration. Age determinations allow stratigraphic correlation between xenoliths and Paleozoic outcrops. For the Ordovician and Silurian samples, correlations are made to exposures in the nearby Lake Timiskaming outlier. For the Devonian samples, the closest possible correlative outcrops are about 300 km away. These fossils provide the first physical evidence of a connection between a Lake Timiskaming "basin" and other Ontario basins during at least part of the Devonian. These strata persisted at least until the Mesozoic before they were removed by erosion.