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

Abstract

Results of deep seismic reflection survey along a 375 km long transect of the Grenville Province in western Quebec are combined with a review of geological 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 Grenville Front zone thins abruptly to ca. 36 km southeastward, perhaps the result of extension on southeast-dipping surfaces extending to the Moho. Other zones of relatively thin crust, although less pronounced, occur where Proterozoic crust overlies Archean crust, and beneath the Morin anorthosite complex. The thickest crust is found at the extreme southeast of the transect, east of the Morin anorthosite. From northwest to southeast, three main crustal subdivisions are (1) deformed Archean rocks with southeast-dipping reflectors in the Grenville Front zone, (2) an Archean parautochthon with northwest-dipping reflectors extending to the lower crust, and (3) an overlying three-layer crust interpreted as accreted Proterozoic terranes. The boundary between (2) and (3) is a major, southeast-dipping, crustal-scale ramp (Baskatong ramp) interpreted to have accommodated strain during and after accretion. U-Pb and Pb-Pb ages on detrital zircons show that metasedimentary rocks of the allochthons (Mont-Laurier, Réservoir Cabonga, and Lac Dumoine terranes) 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 considered 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 terrane). In the allochthons, U-Pb ages of metamorphic zircon and monazite cluster around 1.17 Ga (Mont-Laurier and Réservoir Cabonga terrane) and 1.07 Ga (Renzy and Lac Dumoine terrane) and are interpreted to record late and post-accretion crustal reworking, a common feature of the Grenville orogen. A final high-grade metamorphic event (ca. 1.0 Ga) documented only in the parautochthon and the Grenville Front zone records large-scale, piggyback-style thrusting of allochthonous slabs onto the parautochthon. The age of transcurrent displacement following peak metamorphism affecting both the allochthons and the parautochthon decreases northwestward from 1.07 to 1.00 Ga. Dating 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 migmatites from the parautochthon cooled from the ca. 1.0 Ga peak of metamorphism through about 450°C (Ar closure in hornblende) at ca. 0.96 Ga with calculated 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 to 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.