Geochronological constraints on the last stages of terrane assembly in the central part of the Grenville Province

Abstract

New U–Pb zircon and monazite ages for syn- and post-kinematic pegmatites that cut major shear zones within the western Quebec portion of the Grenville Province constrain the timing of deformation and final terrane assembly within this region. From the Grenville Front SE along the present section, the Grenville Province consists of a reworked Archean promontory partly covered by Proterozoic allochthons and two Proterozoic metasedimentary and metaigneous terranes, which are probably allochthonous. The age of the Grenville Front is still poorly constrained in this area as pegmatites intersecting high-grade rocks exhumed some 30 km south of the Front have upper intercept U–Pb ages of 2642+6/−5 Ma. Field and geochronological data in this paper, however, lend support to the possibility of late-Archean exhumation of high-grade terranes along the Front. South of the Front, two types of ductile shear zone are responsible for the geometry of terranes in this part of the Grenville Province. The first type corresponds to the boundary of allochthons transported onto the Archean parautochthon. The corresponding structures are not directly dated, but reheating of the parautochthon at about 1020 Ma (U–Pb ages on monazite) sets a minimum age for thrusting in this area. Late movement along an east-dipping shear zone located within the parautochthon, the Cabonga thrust (CT; 998+9/−5 Ma), is rather unique in that it involves rocks of the parautochthon and is characterized by a westerly directed transport. The shear zones of the second type are subvertical, NE-trending, generally sinistral and intersect the parautochthon, the allochthon or both. From the Front towards the SE, these shear zones are: the Cadgecrib shear zone (998+16/−6 Ma) intersecting the parautochthon, the Renzy shear zone (1003+4/−5 Ma) involving allochthonous metasediments, the Baskatong shear zone (1020+2/−1 Ma) which marks the NW limit of Proterozoic rocks of the Mont-Laurier terrane, the Labelle shear zone (1078±6 Ma) separating the Mont-Laurier and the Morin terrane, and the Taureau shear zone (1074±4 Ma) bounding the Morin terrane to the east. All the above timing constraints are from U–Pb ages on syn- to post-kinematic pegmatites emplaced within the respective shear zones. This age distribution suggests a propagation of transcurrent deformation towards the NW, which is responsible for the final configuration of terranes.