Localized Backarc Extension in an Overall Compressional Setting During the Assembly of Nuna: Geochemical and Isotopic Evidence From Orosirian (1883–1848 Ma) Mafic Magmatism of the Aillik Group, Labrador, Canada

Abstract

AbstractBimodal volcanism occurs in a range of extensional environments that are characterized by distinctive but overlapping, lithogeochemical and isotopic signatures and lithological assemblages. Where basaltic magmatism is associated with ferroan (A‐type) rhyolites, it is typically the lithogeochemistry and isotopic systematics of the basaltic rocks that are the most beneficial in constraining the tectonic evolution. This study presents geochemical and isotopic data from mafic volcanic rocks of the bimodal Aillik Group, that formed during the assembly of the Paleoproterozoic supercontinent Nuna. Lithogeochemical signatures define two suites. Suite 1 samples have N‐MORB (Normal‐Mid‐Ocean Ridge Basalt) chemical affinities such as flat HREE, and smooth but flat LREE and have εNd(t = 1860 Ma) values between +2.8 and +4.8. Suite 2 samples typically have compositions between N‐MORB and IAT (Island Arc Tholeiites) basalts, with variable REE patterns ranging from IAT to OIB (Ocean Island Basalt). The basalts display moderately developed subduction zone signatures, such as negative Nb and Ti, and have εNd(t = 1860 Ma) ranging from −3.4 to +2.2. Geochemical variability within the Aillik Group mafic rocks is explained by processes active in arc settings and involves mixing between depleted mantle components with variable contributions from crustal and subducted‐slab sources. Coupled with field evidence, the geochemical and isotopic data support emplacement of the basalt in an extensional, backarc basin setting. This backarc basin formed due to localized extension during Nuna assembly. The Makkovik Province preserves a complex orogen characterized by multiple diachronous, compressional, and extensional events along a long‐lived active margin of the Archean North Atlantic Craton.