PETROLOGY AND U-PB GEOCHRONOLOGY OF MAFIC, HIGH-PRESSURE, METAMORPHICCORONITES FROM THE TSHENUKUTISH DOMAIN, EASTERN GRENVILLE PROVINCE

A suite of mafic and ultramafic rocks from the Tshenukutish domain, ea stern Grenville Province, have been the subject of a detailed petrogra phic and geochemical study combined with U-Pb geochronology. The Tshen ukutish domain is the highest tectonic level in the recently discovere d, eclogite-bearing, Manicouagan Imbricate Zone. The mafic and ultrama fic rocks are characteristic of two of the lithotectonic segments in t he domain, the Lac Espadon segment and the Bale du Nord segment. In th e Lac Espadon segment, cumulate textures and geochemical characteristi cs suggest that the units represent a layered, mafic and ultramafic co mplex and an unrelated apatite-rich, Fe-Ti mafic pegmatite (nelsonite) body. In contrast, the samples from the Pale du Nord segment represen t fairly homogeneous Fe-Ti gabbros and anorthosites. The samples studi ed show abundant petrographic evidence for high-pressure metamorphism, for instance the widespread development of garnet, clinopyroxene and kyanite bearing metamorphic assemblages. Estimates of the conditions o f high-pressure metamorphism in previous studies and melting textures in deformed and hydrated gabbro margins are in the range 750-900 degre es C and 1600-1800 MPa. The gabbroic end-members from both segments su ggest within-plate tholeiite petrotectonic signatures. REE-patterns of gabbros and cumulates in both segments suggest fractionation from a t holeiitic source. Cumulate textures, fractionation and inherited zirco ns suggest high-crustal level emplacement of the magmatic protoliths i n the Tshenukutish domain. In the Lac Espadon segment U-Pb ages of ign eous zircon and baddeleyite give an age range of 1643-1629 Ma for the emplacement of the layered complex. The nelsonite in the LES gives a p oorly constrained igneous zircon age of 1596 + 126/-99 Ma. In the Bale du Nord segment baddeleyite gives an age of 1170+/-5 Ma for emplaceme nt of the Fe-Ti gabbros. This is the youngest intrusion in the TD and thus, gives a maximum limit for the onset of Grenvillian metamorphism. Ages of high-P metamorphism are further constrained by zircon formed during partial melting of deformed and hydrated Fe-Ti gabbro margins ( 1046 +/- 3 Ma), overgrowth of igneous zircon (1042 +/- 22/-28 Ma) in the nelsonite, saccharoidal zircon replacing baddeleyite (1030 + 10/-7 Ma and 1030 +/- 12 Ma) and new growth of equant zircon (1012 +/- 12 M a) from the undeformed interior of the gabbros. These ages indicate th at the high-P metamorphism of the Tshenukutish domain was late-Grenvil lian (Ottawan). Retrogression to amphibolite facies is constrained by the ages of titanite (1006-997 Ma) in deformed gabbro margins. Rutile ages range between 960 and 929 Ma and are interpreted as cooling ages. The difference between the ages of titanite and rutile suggest slow c ooling of the Tshenukutish domain after initial high-pressure and temp erature metamorphism and exhumation. (C) 1998 Elsevier Science B.V.