The tenuous geological record of Precambrian oceanic spreading centers; revisiting the Paleoproterozoic Purtuniq ophiolite sheeted dike complex, Nunavik, Canada

Sheeted dike complexes (SDC) are primary records of oceanic spreading centers, where dikes feed fissural volcanism above axial magma chambers. Their recognition provides unequivocal constraints on oceanic crust accretion at divergent plate boundaries through Earth's history. However, the identification of Precambrian SDCs has often hinged on field observations while their cogeneticity and coevality with the overlying basalt and underlying gabbro remain unaddressed. Here, we examine the SDC of the Watts Group (2.0 Ga), located in the Ungava Orogen (Canada) and interpreted as the oldest unequivocal remnant of a mid-ocean ridge, the ‘Purtuniq ophiolite’. The dominant calc-alkaline affinity of the mafic dikes contrasts with the tholeiitic host gabbro and basalt, ruling out cogeneticity. SHRIMP UPb dating of zircon from a dike yields an older Archean population from inherited cores (2981 ± 19 Ma to 2663 ± 17 Ma) overgrown by metamorphic rims (1815 ± 11 Ma). The presence of Archean inherited grains in the dikes is consistent with geochemical indicators of contamination by a felsic component. We conclude that the dikes do not represent a SDC but rather an isolated, petrogenetically unrelated dike array intruding the Watts Group. The ‘Purtuniq Ophiolite’ is thus only comprised of basalt and gabbro that are likely part of the Minto-Povungnituk Large Igneous Province (LIP) and does not represent a remnant of oceanic crust formed at a spreading center. A review of 19 possible cases of pre-850 Ma SDC-bearing ophiolites shows there is no definitive evidence for old SDCs formed in a modern-style oceanic spreading center.