LOCALIZED BACKARC RIFTING DURING THE PALEOPROTEROZOIC ASSEMBLY OF LAURENTIA: GEOCHEMICAL AND ISOTOPIC EVIDENCE FROM MAFIC MAGMATISM, LABRADOR, CANADA

The Paleoproterozoic convergence of southeastern Laurentia documents the assembly of the supercontinent Nuna, featuring a long-lived internal ocean closure as various proto-continents collided. This created an overall compressive regime and localized extension, due to the multiple subducting plates. Evidence for localized extension is preserved in mafic volcanic rocks of a bimodal suite (Aillik Group) of the 1.9-1.7 Ga Makkovik Orogen, developed along the margin of Laurentia. Geochemical and isotopic data, from mafic volcanic rocks, constrain the tectonic setting of the early basin, refining the pre-deformation history of the Aillik Group and defining constraints on the accretion of the Laurentian margin.

Lithogeochemical signatures of the basaltic magmatism define two suites. Suite one basaltic rocks have N-MORB chemical affinities, illustrated by low TiO₂ concentrations, and flat REE. These rocks have high εNd₍₀₎ values ranging from +2.8 to +4.8. Suite two basaltic rocks plot as a mixture between N-MORB and IAT basalts, with variable LREE enrichment and depletion in HREE. Suite two basalts display moderately developed subduction zone signatures, such as negative Nb and Ti. The suite two rocks have εNd₍₀₎ that range from -3.4 to +2.2.

The geochemical variability of the Aillik Group mafic volcanic rocks results from the mixing of depleted mantle components with variable contributions from crustal and subducted-slab sources. Coupled with field-mapping evidence, the geochemical and isotopic signatures of the mafic volcanic rocks support derivation in an extensional, backarc basin setting. This backarc basin formed because of localized extension during Nuna assembly, and was initiated prior to 1883 Ma. There was a switch to a compressional tectonic setting by ca. 1836 Ma, marked by the accretion of part of Cape Harrison Arc. This resulted in the inversion of the backarc basin and subsequent northwestward transport of the Aillik Group, concomitant with regional-scale shearing and folding with associated low-grade metamorphism. The Makkovik Province preserves a complex orogen characterized by multiple diachronous, compressional and extensional tectonic events along a long-lived active margin of Laurentia.