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 degreesC) suggesting high heat flow during eclogite facies metamorphism. Evidence of high-temperatures (700-800 degreesC) 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 degreesC/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 degreesC/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. (C) 2002 Elsevier Science B.V. All rights reserved.