TESTING THE SUPERIA SUPERCRATON RECONSTRUCTION: U-PB AGE AND GEOCHEMICAL COMPARISON OF NIPISSING (CANADA) AND KARJALITIC (FINLAND) SILLS AND RELATED DYKES

The Superia supercraton reconstruction suggests that the “greater Karelia craton” (Karelia craton plus Kola) and Canada’s Superior craton share a common ancestry, with Karelia representing a fragment from the southeastern margin of the Superior. This configuration is constrained by matching Paleoproterozoic mafic dyke swarms and sill provinces through space and time. Many of the matching contemporaneous magmatic provinces, however, have not been compared in terms of their geochemistry. Here, we present a precise, single zircon, U-Pb CA-ID-TIMS age for the Haaskalehto sill, part of the extensive ca. 2220 Ma Karjalitic sill province, Karelia, and draw on previous studies to compare these sills to the contemporaneous Nipissing sills (ca. 2217 Ma, Canada), and Senneterre-Maguire (ca. 2216 Ma) dykes in terms of their trace element and isotope geochemistry.

The Nipissing and Karjalitic sills are hosted by 2500 - 2200 Ma cover sequences and subjacent basement and are postulated to have been emplaced across a more or less contiguous basin fed via laterally propagating dykes from a common magmatic center termed Ungava. We have determined that the Haaskalehto locality of the Karjalitic sills was emplaced at 2216 Ma, an age that matches the peak of Nipissing sill emplacement across the southern Superior craton.

Geochemically, these sills and dykes are dominantly basaltic tholeiites. The Maguire and Senneterre dykes, and Nipissing sills, have a flat HREE slope, a high La/Sm ratio representative of a steep LREE slope, a negative Nb trough, and an enrichment in Th. The Karjalitic sills show a shallow HREE slope as demonstrated by slightly higher Tb/Yb ratios of up to ~2, lower La/Sm ratios resulting in a shallower negative LREE slope, a negative Nb anomaly, and enrichment in Th. Overall, the Maguire and Senneterre dykes and Nipissing sills show broadly comparable patterns to those of the Karjalitic sills. Isotopic data indicate that the Nipissing sills saw more crustal assimilation (ɛNd values from +0.15 to -4.5) compared to the Karjalitic sills (ɛNd values ranging from +4.7 to -0.5). Therefore, the Karjalitic sills are not simply a “down-stream” continuation of the Nipissing sill system. Karjalitic sills may have been fed by a separate branch of dykes from the Ungava magmatic center or by a second magmatic center.