LOCALIZED BACKARC EXTENSION DURING THE ASSEMBLY OF NUNA: EVIDENCE FROM VOLCANO-SEDIMENTARY ROCKS OF THE AILLIK GROUP, LABRADOR, CANADA

Volcano-sedimentary successions are a common feature in a range of geodynamic settings, typifying rift and arc environments. Excellent preservation of the 1.88 to 1.85 Ga Aillik Group, Makkovik Province, Canada, provides an ideal example of volcano-sedimentary conditions in a late Paleoproterozoic backarc setting. The Aillik Group is composed of an upper-greenschist to lower-amphibolite grade supracrustal sequence of volcanic and sedimentary rocks that have been intruded by variably deformed Paleoproterozoic granitoid intrusions, and various ages of post-volcanism mafic and felsic dykes. However, the stratigraphy of the Aillik Group is complicated by folding and structural overprinting that results in the structural repetition of units. Volcanic rocks of the Aillik Group are bimodal, composed predominantly of ferroan (A-type) rhyolite, with lesser preserved transitional tholeiitic basalt (<55% SiO₂). Sedimentary rocks consist of conglomerate, massive sandstone and stratified sandstone-siltstone, whereas volcaniclastic rocks are composed of felsic tuff, tuffaceous sandstone, tuffaceous conglomerate and minor volcanic breccia and mafic tuff. Synvolcanic intrusions include felsic porphyry and mafic dykes and sills.

Geochemical and isotopic variability within the Aillik Group volcanic 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 from the volcanic rocks support emplacement 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.