BELT SUPERGROUP DETRITUS IN THE NANAIMO GROUP: A ROBUST PROVENANCE TIE TO NORTHERN LATITUDES

Detrital zircon in the Mesoproterozoic Belt Supergroup occurs in distinct suites, and defines a robust ‘Belt signature’ that is documented in the Supergroup itself, and in Cretaceous, Eocene, Oligocene and modern basins of the northern Rocky Mountains. A strong 1650-1850 Ma population, probably derived from the Yavapai and Mojave terranes, characterizes most Belt sands. A small 1580-1620 Ma population is unrepresented in North America but is present in the Gawler craton of southern Australia. A 1400-1480 Ma suite represents syn-Belt volcanism. Older Paleoproterozoic and Archean zircons are also present. This ‘Belt signature’ provides a latitudinally restricted provenance indicator that constrains the paleogeographic position of allochthonous terranes.

The Late Cretaceous Nanaimo basin was a peripheral foreland basin deposited on the western edge of the Insular superterrane (IS). Paleomagnetic data suggests IS was at the latitude of central Mexico in Late Cretaceous time. Proponents of large-scale latitudinal terrane translation along the continental margin argue that there are no definitive linkages between the North American craton and the Insular superterrane prior to ~50 Ma, but Belt Supergroup detritus firmly links the Nanaimo basin to the northern North American craton during the Late Cretaceous.

The basin contains a distinctly bimodal detrital zircon signature, with a major population of Cretaceous (68 to 100 Ma) zircon, representing first cycle plutonic detritus eroded during active contractional tectonism within the Coast Plutonic Complex. A smaller population of Meso- and Paleoproterozoic (1300-1800 Ma) zircon is consistently present, and is strikingly similar to that of the Belt Supergroup. The ‘Belt signature’ in the Nanaimo Group also contains a 1300-1400 Ma population that is coeval with post-depositional intrusions in the Belt Supergroup. A minor, but ubiquitous Upper Mesoproterozoic population represents Grenville grains recycled from the Windermere Supergroup. Significantly, quartzite pebbles and cobbles from the upper Nanaimo Group contain the same detrital signature, limiting the probable transport distance. Recognition of Belt Supergroup detritus in the Nanaimo Group establishes a direct link between the Insular superterrane and the northern North American craton.