Learning from mafic dykes in polymetamorphosed granulite terranes: an example from the Grenville Front Tectonic Zone

Mafic dykes can preserve near-peak metamorphic assemblages in granulitic terranes and often are key to deciphering their pressure-temperature-time-deformation (P-T-t-D) history. They are particularly important in high-grade polymetamorphic terranes, where younger metamorphic events may be poorly recorded in other units. The middle to lower crustal units of the Grenville Front Tectonic Zone (GFTZ) in western Québec, Canada, present a unique opportunity to address this issue, having experienced two metamorphic events separated by more than 1.6 billion years. The GFTZ exposes parautochthonous restitic orthopyroxene + garnet-bearing paragneisses and associated two-micas pegmatite dykes. These were formed in the Superior Craton during c. 2.6 Ga granulite-facies metamorphism (M1) and affected by a loosely constrained c. 1.0 Ga overprint during the Grenvillian orogeny (M2). Following M1 and prior to M2, the GFTZ have been intruded by Proterozoic gabbro dykes that can be used to monitor the conditions and timing of M2. In this contribution, we present new field relationships, whole rock geochemistry, in situ laser ablation U-Pb titanite and Lu-Hf garnet geochronology, along with isochemical phase equilibria modeling to provide quantitative P-T-t-D estimates of the metamorphism preserved in the mafic dykes.

Field evidence shows that ~10 m thick metagabbro dykes cross-cut foliated paragneiss and pegmatite dykes. Their immobile element geochemical signatures are consistent with those from regionally recognized Proterozoic mafic dyke swarms intruding the Superior Craton. Metagabbros are characterized by an assemblage of plagioclase + hornblende + clinopyroxene + garnet + orthopyroxene + quartz + titanite. Garnet coronas surrounding relict magmatic clinopyroxene in contact with plagioclase are common in dyke cores, contrasting with granoblastic and migmatitic assemblages in dyke margins. Five metagabbro samples returned in situ Lu-Hf garnet dates in the range of 1124 to 920 Ma and U-Pb titanite dates from 1022 to 985 Ma, interpreted as the timing of M2 metamorphism and consistent with documented Grenvillian metamorphism. Isochemical phase equilibria modeling of a granoblastic hornblende + plagioclase + clinopyroxene + quartz + garnet migmatitic assemblage in a dyke margin indicate equilibrium conditions of 833 ± 12 °C and 9.9 ± 0.4 kbar, corresponding to mid- to high-pressure granulite conditions.

Our results confirm a c. 1.0 Ga granulite-facies overprint attributed to the Grenvillian Orogen (M2). It is noteworthy that the high-grade assemblage is only expressed in the Proterozoic mafic dykes and that the host migmatitic paragneiss did not pervasively recrystallize during M2, potentially due to its restitic nature. This contrast in lithological reactivity resulted in a differential metamorphic record, emphasizing that the Grenvillian granulitic overprint could easily be overlooked if metagabbros are neglected. In conclusion, the results presented herein underscore the critical role of metamorphosed mafic dykes in disentangling the complex evolution of polymetamorphic granulitic terranes.