Crustal evolution along a seismic section across the Grenville Province (western Quebec)

Results of deep seismic reflection survey along a 375 km long transect of t he Grenville Province in western Quebec are combined with a review of geolo gical observations and published isotopic ages. The seismic profile offers a remarkably clear image of the crust-mantle boundary and a good definition of the various crustal blocks. Crust about 44 km thick beneath the Grenvil le Front zone thins abruptly to ca. 36 km southeastward, perhaps the result of extension on southeast-dipping surfaces extending to the Moho. Other zo nes of relatively thin crust, although less pronounced, occur where Protero zoic crust overlies Archean crust, and beneath the Morin anorthosite comple x. The thickest crust is found at the extreme southeast of the transect, ea st of the Morin anorthosite. From northwest to southeast, three main crusta l subdivisions are (1) deformed Archean rocks with southeast-dipping reflec tors in the Grenville Front zone, (2) an Archean parautochthon with northwe st-dipping reflectors extending to the lower crust, and (3) an overlying th ree-layer crust interpreted as accreted Proterozoic terranes. The boundary between (2) and (3) is a major, southeast-dipping, crustal-scale ramp (Bask atong ramp) interpreted to have accommodated strain during and after accret ion. U-Pb and Pb-Pb ages on detrital zircons show that metasedimentary rock s of the allochthons (Mont-Laurier, Reservoir Cabonga, and Lac Dumoine terr anes) range from Archean to as young as 1.21 Ga. A single zone with 1.4 Ga old Sm-Nd model ages appears to lack Archean components and may be consider ed as a fragment of juvenile Mesoproterozoic crust pinched in a shear zone (Renzy shear zone) that could be raised to the status of terrane (Renzy ter rane). In the allochthons, U-Pb ages of metamorphic zircon and monazite clu ster around 1.17 Ga (Mont-Laurier and Reservoir Cabonga terrane) and 1.07 G a (Renzy and Lac Dumoine terrane) and are interpreted to record late and po st-accretion crustal reworking, a common feature of the Grenville orogen. A final high-grade metamorphic event (ca. 1.0 Ga) documented only in the par autochthon and the Grenville Front zone records large-scale, piggyback-styl e thrusting of allochthonous slabs onto the parautochthon. The age of trans current displacement following peak metamorphism affecting both the allocht hons and the parautochthon decreases northwestward from 1.07 to 1.00 Ga. Da ting thus shows that Grenvillian deformation in western Quebec occurred in pulses over an interval of 180 million years, with a tendency to propagate from the inner part of the orogen toward the Grenville Front. Reworked migm atites from the parautochthon cooled from the ca. 1.0 Ga peak of metamorphi sm through about 450%C (Ar closure in hornblende) at ca. 0.96 Ga with calcu lated cooling rates of about 6%C per million years, and unroofing rates of 0.33 km per million years. The cooling-unroofing history of the allochthons is not so straightforward, probably due to tectonic disturbances related t o allochthon emplacement. Cooling through 450%C occurred between 1.04 and 1 .01 Ga, at least 50 million years earlier than cooling in the parautochthon ; this contrast agrees with the northwestward propagation of the orogen.