High-pressure granulites at the dawn of the Proterozoic

Abstract

High-pressure metamorphism is uncommon in the ancient geological record. Kanja Malai, in the Salem crustal block (southern India), contains high-pressure kyanite-garnet−bearing felsic granulites that equilibrated at 14–16 kbar and ∼820–860 °C. Laser ablation−inductively coupled plasma−mass spectrometry U-Pb zircon and in situ monazite geochronology indicate that these assemblages grew ca. 2490 Ma. These pressure-temperature-time constraints provide a rare record indicating that thickened crust and low apparent thermal gradient conditions existed during the Archean-Proterozoic transition, a period of Earth history for which the rock record commonly preserves evidence for comparatively high apparent thermal gradients. The thermal regimes required to generate these metamorphic conditions are typical of collisional orogenesis, and suggest that the continental lithosphere was capable of supporting crustal thickening to ≥45–50 km. Such crustal thickening provides supporting evidence that tectonic regimes similar to modern Earth–style tectonics were in operation at the Archean-Proterozoic transition.