Abstract

The Shebandowan greenstone belt of the western Wawa Subprovince consists of successions of volcanic and sedimentary rocks impinging onto the metasedimentary Quetico Subprovince to the north and cored by a batholithic complex to the south. U-Pb geochronology using mainly zircon and titanite demonstrates a relatively rapid accretion of the greenstone belt in the late Archean. The oldest ages were obtained for 2750 Ma tonalitic gneiss and sporadic 2830–2750 Ma detrital or xenocrystic zircons. A major phase of greenstone belt construction at 2720 Ma formed ultramafic to felsic volcanic rocks and peridotitic, gabbroic, and anorthositic bodies, probably in an extensional arc–backarc setting. These units are laterally correlative with volcanogenic massive sulfide-bearing assemblages in the central Wawa Subprovince and probably with similar successions in northern Minnesota. The second major stage of felsic volcanism and plutonism at about 2695 Ma was associated with D1 compression causing thrusting, imbrication, and sedimentation. This was succeeded by the deposition of an unconformable sequence of calc-alkalic to alkalic volcanic and sedimentary rocks and emplacement of tonalitic to syenitic plutons at about 2690 Ma. Transpressive deformation (D2), constrained between 2685 and 2680 Ma, caused the development of locally penetrative structures and deposition of clastic sedimentary packages. The emplacement of plutons at 2683–2680 Ma concluded the evolution of the greenstone belt. Titanite (and rutile) yield ages in the same range as zircon, showing that the greenstone belt was situated at high crustal levels by 2680 Ma and escaped the younger and intense deformational-metamorphic events recorded in greenstone belts farther east in the central Wawa Subprovince as well as in the immediately adjacent migmatitic Quetico Subprovince to the north.

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