Kyanite-bearing paragneisses from the Manicouagan Imbricate Zone and its footwall (high-P belt of the central Grenville Province) preserve evidence of partial melting with development of metamorphic textures involving biotite-garnet ± kyanite ± plagioclase ± K-feldspar-quartz. Garnet in these rocks displays a variety of zoning patterns with respect to Ca. Pseudosection modelling in the Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 -O (NCKFMASHTO) system using measured bulk rock compositions accounts for the textural evolution of two aluminous and two sub-aluminous samples from the presumed thermal peak to conditions at which retained melt solidified. The prograde features are best explained by pseudosections calculated with compositions to account for melt loss. The intersection of isopleths of grossular content and Fe ⁄ (Fe + Mg) relating to large porphyroblasts of garnet provide constraints on the P-T conditions of the metamorphic peak. These P-T estimates are considered to be minima because of the potential for diffusional modification of the composition of garnet at high-T and during the early stages of cooling. However, they are consistent with textural observations and pseudosection topology, with peak assemblages best preserved in rocks for which the calculated pseudosections predict only small changes in mineral proportions in the P-T interval, in which retrograde reactions are inferred to have occurred between the thermal peak and the solidus. Maximum P-T conditions (14.5-15.5 kbar and 840-890°C) and steep retrograde P-T paths inferred for rocks from the Manicouagan Imbricate Zone are comparable with those determined for mafic rocks from the same area. In contrast, maximum P-T conditions of 12.5-13 kbar and 815-830°C and flatter P-T paths are inferred for the rocks of the footwall to the Manicouagan Imbricate Zone. The general consistency between textures, mineral compositions and the topologies of the calculated pseudosections suggests that the pseudosection approach is an appropriate tool for inferring the P-T evolution of high-P anatectic quartzo-feldspathic rocks.

No abstract

No abstract

We propose that the Grenvillian allochthonous terranes may be grouped into High Pressure (HP) and Low Pressure (LP) belts and examine the HP belt in detail in the western and central Grenville Province. The HP belt is developed in Paleo- and Mesoproterozoic rocks of the pre-Grenvillian Laurentian margin and characterized by Grenvillian eclogite and co-facial HP granulite in mafic rocks. Pressure–temperature (P–T) estimates for eclogite-facies conditions in well-preserved assemblages are about 1800 MPa and 850°C. In the central Grenville Province, HP rocks formed at ~1060–1040 Ma and underwent a single stage of unroofing with transport into the upper crust by ~1020 Ma, whereas farther west they underwent two stages of unroofing separated by penetrative mid-crustal recrystallization before transport to the upper crust at ~1020 Ma. Unroofing processes were comparable in the two areas, involving both thrusting and extensional faulting in an orogen propagating into its foreland by understacking. In detail, thrusting episodes preceded extension in the western Grenville Province, whereas in the central Grenville Province, they were coeval, resulting in unroofing by tectonic extrusion. In the central Grenville Province, the footwall ramp is well preserved, but any former ramp in the western Grenville Province was obliterated by later lower crustal extensional flow. Continuation of the HP belt into the eastern Grenville Province is not established, but likely on geological grounds. However, the pattern of deep crustal seismic reflection in the Lithoprobe Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT) line contrasts with that father west, suggesting that, if present, the HP rocks were exhumed by a different mechanism.

No abstract