Paleoproterozoic volcanism and rift basin evolution during Nuna assembly: Evidence from the Ingrid group, Labrador, Canada

• The Ingrid group records Paleoproterozoic volcanic activity in the Torngat Orogen. • Transtensional strike-slip faulting leads to rift basin formation during the final stages of supercontinent assembly. • Lithosphere-scale weaknesses serve as conduits for magma during transtensional tectonics. • Nd isotopic data suggest contamination of varying ages of lithosphere. Orogenic belts preserve evidence of supercontinent assembly, recording multiple collisions of terranes of various ages, tectonic affinities and magnitudes. In Labrador, the amalgamation of the supercontinent Nuna (Columbia) is preserved within the Southeastern Churchill Province, the easternmost segment of the Paleoproterozoic Trans-Hudson Orogen. The eastern accretionary zone, the 1.9–1.7 Ga Torngat Orogen, represents the deeply exhumed structural core of the orogen, where the Ingrid group was deposited during transtensional strike-slip faulting and rift-basin formation. The Ingrid group comprises siliciclastic (sandstone, conglomerate), pyroclastic (felsic tuff) and volcanic rocks (basalt, andesite, rhyolite). Geochronological data constrain the timing of felsic volcanism to 1807.5 ± 3.2 Ma; representing a previously unrecognized magmatic event. Detrital zircon data record peaks at ca. 1810 and 1890 Ma reflecting active erosion of the Ingrid group and Torngat Orogen rocks during deposition. Lithogeochemistry and Nd isotopic signatures of the volcanic rocks indicate a role for subduction zone-modified subcontinental lithospheric mantle and/or crustal contamination. Basalt samples are interpreted to originate from shallow mantle depths based partly on high Tb/Yb (cn) and La/Yb (cn) ratios indicating the progressive diminishing of the garnet signature within the spinel peridotite stability field. The degree of contamination in basalt and andesite samples correlates with negative εNd (t=1810 Ma) = −6.3 to −1.3 values, compared to the felsic volcanic rocks with εNd (t=1810 Ma) = 0.6 to 1.9 values, indicating higher contamination from Th- and La-enriched lithospheric material in the mafic to intermediate rocks than the felsic rocks. This suggests that mafic to intermediate rocks were derived from older (Meso- to Neoarchean) lithosphere, while felsic magmas arose from younger (Paleoproterozoic) lithosphere. Magmatism utilized long-lived zones of weakness in the Torngat Orogen’s transtensional strike-slip shear/fault system, highlighting the role of lithosphere-scale discontinuities, often remnants of earlier plate boundary sutures and faults, in supercontinent assembly.