Tectonic history of the Waugh Lake basin based on field, geochemical, and isotopic (U-Pb, Sm-Nd) study: New insights into the age and origin of a collision-related volcano-sedimentary sequence in the southern Taltson orogen

The Waugh Lake Group (WLG) is a middle greenschist-facies supracrustal succession, previously constrained to 2.10–1.97 Ga and interpreted as the remnant of a back- or intra-arc basin along the SW Rae cratonic margin associated with the 2.0–1.93 Ga Taltson orogeny. Previously interpreted as the WLG’s basal sequence but now recognized to occur below an unconformity, is a turbiditic package overlain by basalt and conglomerate, all with evidence of F1 folding. Above that, the WLG itself comprises two cycles of pebbly arkosic subarenite and intermediate–felsic volcanic rocks separated by intermediate tuff interbedded with siltstone–mudstone; all capped by basalt. All units are folded along NNE-trending F2 axes. U-Pb zircon dates of 1958.1 ± 0.7 and 1956.4 ± 1.2 Ma for dacites, the first dated extrusive rocks in the Taltson orogen, constrain the age of the WLG, whereas the prior 2.10–1.97 Ga constraint still applies to the units below the unconformity. Geochemical analyses of WLG volcanic rocks indicate calc-alkaline affinity with subduction-related trace element patterns. Related Sm-Nd isotopic analyses yield depleted mantle model ages of 2.81–2.72 Ga, and εNd(i) values of -4.9 to -5.7. Differences in rock types, structural style, and age between the WLG and underlying units suggest different tectonic histories. A foreland basin setting is proposed for the lower units, with D1 deformation possibly related to early accretion. By contrast, the arkosic detritus, subduction-related affinity and negative εNd(i) values of WLG volcanic rocks points to a continental arc setting, likely an intra-arc or syn- to post-collisional equivalent basin.