Pickle Lake revisited: New structural, geochronological and geochemical constraints on greenstone belt assembly, western Superior Province, Canada

摘要

New field work, U-Pb ages, geochemical data, and Sm-Nd isotopic analyses have established the timing and determined the nature of volcanism, deformation, and tectonic assembly of the Pickle Lake greenstone belt in the Uchi subprovince of the western Superior Province of the Canadian Shield. The >2860 Ma Pickle Crow assemblage has been redefined to include the former Northern Pickle assemblage on the basis of stratigraphic continuity and similar volcanic geochemistry between the two units across a previously inferred fault contact. The Pickle Crow assemblage consists of tholeiitic basalt with thin, but laterally extensive, oxide-facies iron formation overlain by alkalic basalts and minor calc-alkaline andesites to dacites with primitive Nd isotopic compositions (epsilon_(Nd)~(2.89 Ga) = +2.1 to +2.4) suggestive of deposition in a sediment-starved oceanic basin. The approx 2 km thick approx 2836 Ma Kaminiskag assemblage (former Woman assemblage) consists of tholeiitic basalt interbedded with intermediate and rare felsic pyroclastic flows with primitive Nd isotopic compositions (epsilon_(Nd)~(2.836 Ga) = +2.4). Two samples of intermediate volcanic rocks interbedded with southeast-younging pillowed basalt, previously inferred to be part of the Pickle Crow assemblage, yielded U-Pb zircon ages of 2744_(-2)~(+3) Ma and 2729 + 3 Ma. These rocks are thus part of the younger Confederation assemblage, which consists of intercalated basalt and dacite (epsilon_(Nd)~(2.74 Ga) = + 0.1 to +0.8) exhibiting diverse compositions probably reflecting eruption in a continental margin arc to back-arc setting. The contact between the Confederation and Kaminiskag assemblages is assumed to be a fault. The greenstone belt is intruded by late syn- to posttectonic plutons including the composite quartz dioritic to gabbroic July Falls stock with a new U-Pb zircon age of 2749_(-2)~(+4) Ma, and the approx 2741 to 2740 Ma trondhjemitic to granodioritic Ochig Lake pluton and Pickle Lake stock, as well as the approx 2697 to 2716 Ma Hooker-Burkoski stock. The earliest recognized deformation (D_1) is recorded by a local bedding-parallel foliation in the Pickle Crow assemblage. This foliation is truncated by the approx 2735 Ma Albany quartz-feldspar porphyry dyke and is not recognized in the volcanic rocks of the Confederation assemblage. The early deformation event is attributed to overturning of the Pickle Crow assemblage prior to deposition of the approx 2144 to 2729 Ma Confederation assemblage. Subsequent deformation and development of a regionally penetrative planar fabric (S_2) postdates approx 2729 Ma volcanism, pre-dates the intrusion of the ca. < 2716 Ma Hooker-Burkoski stock and is host to gold mineralization.