Petrology and U–Pb geochronology of mafic, high-pressure, metamorphic coronites from the Tshenukutish domain, eastern Grenville Province

Abstract

A suite of mafic and ultramafic rocks from the Tshenukutish domain, eastern Grenville Province, have been the subject of a detailed petrographic and geochemical study combined with U–Pb geochronology. The Tshenukutish domain is the highest tectonic level in the recently discovered, eclogite-bearing, Manicouagan Imbricate Zone. The mafic and ultramafic rocks are characteristic of two of the lithotectonic segments in the domain, the Lac Espadon segment and the Baie du Nord segment. In the Lac Espadon segment, cumulate textures and geochemical characteristics suggest that the units represent a layered, mafic and ultramafic complex and an unrelated apatite-rich, Fe–Ti mafic pegmatite (nelsonite) body. In contrast, the samples from the Baie du Nord segment represent fairly homogeneous Fe–Ti gabbros and anorthosites. The samples studied 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 of high-pressure metamorphism in previous studies and melting textures in deformed and hydrated gabbro margins are in the range 750–900°C and 1600–1800 MPa. The gabbroic end-members from both segments suggest within-plate tholeiite petrotectonic signatures. REE-patterns of gabbros and cumulates in both segments suggest fractionation from a tholeiitic source. Cumulate textures, fractionation and inherited zircons suggest high-crustal level emplacement of the magmatic protoliths in the Tshenukutish domain. In the Lac Espadon segment U–Pb ages of igneous 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 poorly constrained igneous zircon age of 1596+126/−99 Ma. In the Baie du Nord segment baddeleyite gives an age of 1170±5 Ma for emplacement 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 Ma) from the undeformed interior of the gabbros. These ages indicate that the high-P metamorphism of the Tshenukutish domain was late-Grenvillian (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 cooling of the Tshenukutish domain after initial high-pressure and temperature metamorphism and exhumation.