Metamorphic evolution and tectonic significance of Archean ultrahigh-temperature mafic granulites from the Northwestern Superior Province

Ultrahigh-temperature metamorphism represents the most thermally extreme crustal metamorphism, defined by peak conditions of >900 °C and 0.7-1.3 GPa, and is typically preserved in rare Mg-Al-rich rocks. In this contribution, we report the identification of UHT mafic granulites from the Pikwitonei Granulite Domain, located in the Northwestern Superior Province. Based on petrographic observations and mineral chemical analyses, these mafic granulites can be subdivided into two types according to whether the rock includes garnets or not. Both two types of mafic granulites record three distinct metamorphic stage: (1) The pre-T_max stage is marked by clinopyroxene-plagioclase-biotite inclusions in garnet core, and amphibole-plagioclase-magnetite-ilmenite inclusions in clinopyroxene core. (2) The T_max stage is defined by a coarse-grained matrix assemblage of garnet/amphibole, clinopyroxene, orthopyroxene and magnetite. (3) Late development of garnet-quartz symplectite and amphibole along pyroxene rim represents the post-T_max assemblage of garnet-bearing and garnet-free mafic granulites, respectively. Phase equilibrium modelling of these mafic granulites, carried out in the NCKFMASHTO system using the GeoPS software package, yields anticlockwise P-T paths, with peak P-T conditions of 940-1030 °C / 0.82-0.88 GPa (garnet-bearing type) and 1020-1040 ℃ / 0.68-0.85 GPa (garnet-free type). Metamorphic zircon U-Pb dating gives a mean ²⁰⁷Pb/²⁰⁶Pb age of 2680±11 Ma for garnet-bearing granulites and a continuum of ages from 2673±32 to 2543±21 Ma for garnet-free granulites. These results, combined with previous studies, suggest that this UHT event occurred prior to 2.68 Ga and underwent a prolonged cooling period during 2.67 to 2.54 Ga. One-dimensional thermal modelling results indicate that radiogenic heat production merely heated the rock to 640 °C at a depth of 35 km. Integrating metamorphic P-T-t paths with thermal calculation in the Pikwitonei Granulite Domain, we propose that this UHT event was probably triggered by extra mantle heat input from asthenospheric upwelling.