U-Pb zircon ages have been compiled for magmatic and sedimentary rocks from the low metamorphic grade portion of the Flin Flon greenstone belt, now recognized as a Paleoproterozoic tectonic collage. The "Amisk collage" formed in two major magmatic periods that were separated by an interval of intraoceanic accretionary tectonics. Pre-accretionary volcanic and plutonic rocks of arc and ocean-floor tectonic affinities have crystallization ages of 1.906-1.901 and 1.888-1.881 Ga; the earlier period was dominated by juvenile tholeiitic arc basalts and related back-arc-basin basalts, and the younger period by juvenile calc-alkaline volcanic rocks and turbidites. Intraoceanic accretion of the diverse tectono-stratigraphic assemblages may have commenced between 1.90 and 1.89 Ga, but the main period was 1.88-1.87 Ga. The post-accretionary period (1.876-1.838 Ga) was characterized by intrusion of juvenile calk-alkaline plutons generated by a successor arc that stitched the diverse pre-accretionary assemblages. Marine to subaerial volcaniclastic and epiclastic units were deposited in successor basins <=1.87 Ga (Schist-Wekusko suite), succeeded by alluvial-fluvial (Missi Group) to marine (Burntwood Group) sediments after 1.85 Ga. Despite the dominance of juvenile magmatic rocks within the collage, U-Pb zircon age and Nd-isotopic data show that older (>2.2-3.0 Ga) basement fragments were present throughout the development of the Amisk collage. An arc-back-arc system close to an Archean craton is proposed as the most likely tectonic setting during formation and accretion of the Amisk collage from 1.90 to 1.84 Ga. Subsequent continental collision during peak orogeny (1.84-1.81 Ga) is interpreted to have delaminated the lower crust and upper mantle of the collage, preferentially preserving crust of intermediate bulk composition.
Impact of volatiles and variations in local bulk composition on deformation and magma emplacement processes in the deep crust and upper mantle parts of continental rift systems
