AV俱乐部

A detailed field, structural, metamorphic, and U-Pb geochronologic study of para- and orthogneisses in the Central Gneiss Belt near Pointe-au-Baril, Ontario, provides important constraints on pre-Grenvillian metamorphism and late Grenvillian extensional deformation in the southwest Grenville orogen. The parautochthonous, polyorogenic Britt domain is overlain by the allochthonous Shawanaga domain (affected only by Grenvillian orogenesis) along a Grenvillian thrust detachment that was reactivated as a southeast-directed, ~3 km-wide, ductile extensional shear zone (the Shawanaga shear zone) late in the Grenvillian orogeny. The position of the overprinted thrust detachment is marked by an abrupt change in histories of structural and metamorphic development, felsic plutonism, and mafic dyke emplacement. Extensional displacement on the Shawanaga shear zone and on the smaller Nares Inlet shear zone (within the Britt domain) was contemporaneous with regional folding about axes parallel to the extensional transport direction. A segment of the Britt domain lying between the shear zones was decoupled from this folding event and preserves pre-Grenvillian granulite-facies assemblages and early Grenvillian tectonic fabrics.

Metamorphic P-T estimates for pre-Grenvillian granulites are distinct from those obtained from peak- and post-peak metamorphic Grenvillian assemblages in the Britt and Shawanaga domains. Grenvillian P-T paths for the domains are also distinct, but both have steep-trajectory segments indicating ~5 kbar of decompression, attributed to tectonic exhumation by extensional shear zones. Pelitic assemblages within the Shawanaga shear zone contain garnet porphyroblasts with strong compositional zoning and record an "inverted" metamorphic isograd marked by the first appearance of sillimanite in kyanite + K-feldspar assemblages. Preservation of both features appears to be due in part to rapid cooling during extensional exhumation.

U-Pb zircon data from fresh granulite-facies assemblages indicate pre-Grenvillian metamorphism at ca. 1450-1430 Ma. Igneous crystallization ages of granitic pegmatite dykes in the Shawanaga shear zone suggest that extensional shearing was waning by 1020 Ma and had ceased by 990 Ma. The time of earlier thrusting is largely unconstrained but it may have occurred at ca. 1080 Ma. Concordant to weakly discordant U-Pb titanite analyses from the study area fall into three 207Pb/206Pb age groups (1028-1018 Ma, 1008-1000 Ma, and 967-956 Ma) that record initial cooling through the titanite closure temperature (~600 ^oC) followed by at least two localized recrystallization and/or titanite growth events.

The contrasting tectonostratigraphic character and orogenic history of allochthonous and parautochthonous domains documented in this study has implications for the position of the Allochthon Boundary Thrust in the southwest Grenville orogen. The late Grenvillian history of the Pointe-au-Baril area resembles those documented in younger collisional orogens that have undergone important late orogenic extension, suggesting that this region may contain analogous, deep crustal structures that have not yet been exhumed in the younger belts.

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