The age and provenance of metasedimentary rocks in the Quetico Subprovince, Ontario, from single zircon analyses: implications for Archean sedimentation and tectonics in the Superior Province

Abstract

U Pb isotopic analyses have been performed on single detrital zircon grains from greywacke samples collected along the northern boundary of the Quetico Subprovince in northwest Ontario. Secondary lead loss was virtually eliminated by choosing crack-free grains and removing the outer surface using air abrasion. Thus, the 207Pb 206Pb age can be considered a good approximation to the age of crystallization of each grain. Zircons in the Quetico metasediments show a range of ages from 2698 ± 3 Ma to 3009 ± 4 Ma. Deposition and metamorphism of the metasediments is bracketed within a 10 ± 5 Ma time span between crystallization of the youngest detrital zircon grain and a late-tectonic intrusion, the Blalock pluton, which cuts the sediments and has a zircon age of 2688 ± 4 Ma. The most easterly metasediment sample contains zircons with ages which cluster around 3007 ± 6 Ma which is within error of the age of a batholith dated in the adjacent Marmion Lake area of the Wabigoon Subprovince. This, along with the fact that most detrital zircons were only slightly abraded by sedimentary transport, suggests that the source of many zircons may have been proximal from the north. There is also a significant cluster of young zircon ages in the range 2698–2709 Ma. Surprisingly, only a few ages correspond to the time of most intense arc-type felsic volcanism (2710–2745 Ma) in the main greenstone belt of the western Wabigoon Subprovince. Sedimentary deposition occurred at a time of extensive deformation in the adjacent Wabigoon Subprovince. The pattern of detrital grain ages indicates that this involved uplift of a complex older sialic landmass. Petrographic evidence for the presence of substantial felsic volcanic detritus as well as the fact that many of the zircons only slightly pre-date deposition indicates that much of the sediment was probably derived from erosion of active felsic volcanic centers. These data combined with regional geochronological and field studies favour deposition in a basin formed by late orogenic collisional events.