U–Pb ages of 2713–2711 Ma for zircon and titanite from tonalitic gneiss of the Lapparent Massif in the northeastern Abitibi belt are younger than those obtained for surrounding supracrustal rocks. These data indicate that the Lapparent gneisses are not old basement on which the supracrustal rocks were deposited, as has been postulated by some workers, but rather represent a late synvolcanic intrusion that was emplaced at a relatively deep crustal level. The earliest phase of penetrative deformation recognized in the gneiss must have occurred during or immediately after emplacement of the body and represents a deformation event that is not presently recognized in the higher level rocks in this area. The Lapparent gneisses were apparently juxtaposed against adjacent supracrustal rocks along outward-dipping mylonite zones.

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