Temperature–time paths in the high-P Manicouagan Imbricate zone, eastern Grenville Province:: Evidence for two metamorphic events

The Manicouagan Imbricate zone (MIZ), eastern Grenville Province is a series of lithotectonic units assembled from different crustal levels during the Grenvillian orogeny. Structural evidence along with U–Pb dating suggests that two separate stages of metamorphism occurred at ca. 1050 and 1020 Ma, respectively. The first stage is characterised by high-pressure and temperature (high-PT) metamorphism, up to eclogite facies, with a range of maximum recorded pressures, between 14 and 20 kbar, consistent with variations in burial depth. Maximum T-conditions, recorded from apparently different structural levels, are rather similar (800–900 °C) suggesting high heat flow during eclogite facies metamorphism. Evidence of high-temperatures (700–800 °C) recorded during the early stage of tectonic exhumation indicate near-isothermal decompression. The second stage is a high-T event coeval with the emplacement of syn-metamorphic granite and is recorded by rocks in the upper-most tectonic slice (Boundary zone) in the south-east MIZ. This event may also be responsible for amphibolite facies overprinting across the rest of the MIZ. Using the U–Pb and PT-data sets, along with published diffusion rates for Pb in monazite, titanite and rutile, Tt-paths have been modelled for several of the lithotectonic units. In most of the units, suggested cooling rates appear to have been relatively fast during initial exhuamtion, but show a prolonged period of slower cooling (5–10 °C/Ma for 30–40 Ma) apparently at odds with the steep (near isothermal) PT-paths, preservation of eclogitic textures and preservation of garnet zoning in the highest grade rocks. On the other hand, in the Boundary Zone, the recorded cooling rates are much faster (>20 °C/Ma) indicative of tectonic exhumation. This phase of rapid cooling in the Boundary zone corresponds in time with the slow cooling rates (at 1030–1000 Ma) in the rest of the MIZ. Thus, thrusting of the Boundary Zone over the rest of the MIZ appears to have caused a thermal overprint which was superimposed on the Tt-paths. Taken as a whole, the Tt-paths for the MIZ are interpreted as representing two separate metamorphic events which were both relatively short in duration.