Aphrodite Indares

High-pressure, high-temperature rocks from the base of thick continental crust: Geology and age constraints from the Manicouagan Imbricate Zone, eastern Grenville Province

The Grenville Province of the Canadian Shield is composed of crustal material with Laurentian affinities, involved in continental collision during the Grenvillian Orogeny between 1.19 and 0.98 Ga. In the eastern Grenville Province, the Manicouagan Imbricate Zone (MIZ) is composed of Paleoproterozoic (Labradorian, ∼1.6 Ga) and Mesoproterozoic (Pinwarian, ∼1.4 Ga) crustal segments that were metamorphosed under high-pressure, high-temperature (high-P-T) conditions (P∼1800 MPa and 800°<T<900°C) and intruded by synmetamorphic gabbroic stocks and dykes and postmetamorphic granite, during a ∼1.0 Ga crustal shortening event in the Grenvillian Orogeny. The gabbroic dykes display high-P mineral assemblages and a withinplate tholeiite signature attesting to the intrusion of asthenospheric melts during the high-P metamorphism. The structural configuration of MIZ is that of a thrust stack (Lelukuau terrane) overlain by an extensional assembly of slices (Tshenukutish terrane). To the north, Lelukuau terrane overlies Archean basement and Paleoproterozoic supracrustal rocks of the Gagnon terrane along a thrust contact. Tshenukutish terrane is tectonically overlain to the south by crustal slices composed of pre-Grenvillian and early Grenvillian lithologic units that are correlative with those in MIZ, but which were at midcrustal levels during the Grenvillian Orogeny (low-P segment of the eastern Grenville Province), and were intruded by anorthosite and granitoids farther south. A likely tectonothermal evolution of MIZ at ∼1.0 Ga involves crustal shortening by imbrication in a postsubduction phase of continental collision, followed by (convective?) removal of the underlying thickened lithospheric mantle and the rise of hot asthenospheric material close to the base of the crust. Exhumation of MIZ likely occurred from the basal levels of the crust and was achieved by (1) NW directed thrusting over a crustal-scale ramp (Archean basement of the Gagnon terrane) with coeval extension at the higher levels of the pile (stage 1 extension) and (2) a second extension event (stage 2 extension) coeval with emplacement of postmetamorphic granite at the boundary between MIZ and the low-P segment.

Seismic images of eclogites, crustal-scale extension, and Moho relief in the eastern Grenville province, Quebec

Seismic images from a 250 km Lithoprobe reflection profile in the interior of the eastern Grenville province provide important new constraints on the crustal architecture of this part of the orogen. Prominent upper-crustal reflections can be correlated with exposures of high-pressure metamorphic rocks in the Manicouagan shear belt, providing the first direct evidence for eclogite reflectivity in the Grenville province. The eclogites are cut by major late Grenvillian normal faults that penetrate the deep crust and preserve evidence of extensional collapse of the overthickened orogen. North-to-south crustal thinning, indicated by a change in Moho reflection time from 16 to 13 s, correlates well with regional Bouguer gravity trends and is accompanied by a dramatic increase in the reflectivity of the lower crust. These features underscore the significance of recently recognized along-strike variations in tectonic style within the Grenville province and point to the internal complexity that characterizes the root zones of collisional orogens.

The reaction history of kyanite in high-pressure aluminous granulites

Cathodoluminescence (CL) mapping of kyanite in high-P, aluminous granulites from the central Grenville Province reveals internal structures that are linked to their metamorphic reaction history. In two samples, individual kyanite crystals are shown to be composite porphyroblasts comprising three distinct generations, defined by their CL intensity and Cr (± V, Ti, Fe and Ga) content, and each separated by resorbed interfaces. In contrast, a sub-aluminous sample contains two types of kyanite, one as resorbed inclusions in garnet and another in the groundmass or replacing garnet. These textural variants of kyanite are interpreted within the framework of phase equilibria modelling. In P–T pseudosections, a first generation of kyanite, which is only present in the most aluminous samples, is potentially linked to staurolite breakdown, and its resorption is consistent with a subsequent increase in pressure. This kyanite represents the earliest remnant of prograde metamorphism identifiable in these rocks. The second generation, present in the porphyroblasts in the same samples and as inclusions in garnet in the sub-aluminous sample, is interpreted to be the peritectic product of muscovite dehydration melting. Resorption of this kyanite is consistent with subsequent continuous dehydration melting of biotite, which is also inferred based on microstructural considerations. The final generation of kyanite, present as rims on the prograde kyanite porphyroblasts in aluminous samples and as part of the groundmass or replacing garnet in the sub-aluminous rock, is interpreted to have grown during melt crystallization upon retrogression. The presence of retrograde kyanite implies that the melt crystallized over a wide range of temperatures, and provides an important constraint on the P–T conditions of the metamorphic peak and on the retrograde P–T paths. Cathodoluminescence mapping is crucial for identifying retrograde kyanite in aluminous samples, as it preferentially overgrows existing kyanite rather than replacing other prograde phases. The scarcity of kyanite in sub-aluminous rocks allows retrograde kyanite to grow as discrete crystals that can be identified by optical microscopy. This work attests to the potential of unconventional tools such as CL imaging for deciphering the metamorphic history of rocks. This article is protected by copyright. All rights reserved.

The Ti Record of Quartz in Anatectic Aluminous Granulites

The distribution and concentration of Ti in quartz was assessed in five rutile-bearing anatectic aluminous granulites from the Grenville Province, Canada, each with previously constrained P–T conditions of metamorphism. Characterisation of quartz in these samples with the aid of cathodoluminescence (CL) mapping revealed two distinct types in each sample, resorbed quartz partly consumed by melting reactions during the prograde portion of metamorphism and quartz grown from crystallisation of partial melt during retrogression. In two of the samples, pseudomorphs after former melt in the form of quartz overgrowths on existing quartz were discovered using CL which are not visible by other methods. The P–T conditions recorded by each type based on Ti-in-quartz thermobarometry are in poor agreement with microstructures and P–T results inferred from phase equilibria modelling. The negative correlation between Ti incorporation into quartz and P suggests that prograde quartz, which is expected to record near-peak P–T, should be Ti-poor relative to retrograde quartz, which crystallised at conditions several kbar lower than peak P. The opposite trend was found; retrograde quartz is typically relatively Ti-poor and quartz overgrowths may be nearly Ti-free. Additionally, prograde quartz has variable Ti contents and fails to systematically record near peak P–T. Despite the presence of rutile in the samples, which is generally considered to ensure a Ti-saturated system, these results are strong evidence for Ti-undersaturation of quartz caused by disequilibrium between quartz and rutile. In addition, Zr-in-rutile thermometry generally gives lower temperatures than expected for these rocks, which can be explained by retrograde resetting of Zr in rutile and potentially by disequilibrium between rutile and zircon. Based on these results, the degree of Ti (or Zr) saturation achieved at any stage of metamorphism should be assessed with caution, as co-existence of quartz and rutile (or rutile and zircon) within a thin section or sample may not be a sufficient criterion for equilibrium.