Orosirian cold eclogite from Baltica marks the onset of modern plate tectonics

When plate tectonics first emerged on Earth is still debated, it may have evolved gradually since the Archean. Evidence from the petrologic record suggested that the modern style plate tectonics, characterized by orogenic processes of low T/P (temperature to pressure) metamorphism may have only begun in the Neoproterozoic. Here we present the oldest known cold subduction eclogites from the Belomorian Province (BP) in Baltica. Alongside other contemporaneous low T/P metamorphic records worldwide, we conclude that modern plate tectonics initiated in the Orosirian and was linked to the Columbia supercontinent event. Two types of eclogites with distinct protolith origins and petrological-mineralogical characteristics from the Kuru-Vaara (KV) complex in the BP were investigated. The prograde metamorphism to eclogite facies is defined by the chemical annuli of garnet and therein mineral inclusions, and peak conditions of 680-710 °C and 2.4 GPa (290 ± 6 °C/GPa) are inferred by pseudosection method. The post-peak evolution is recorded by clinopyroxene-plagioclase symplectite and clinopyroxene breakdown with quartz-amphibole/orthopyroxene lamellae exsolution, and various low-grade hydrous minerals. Zircon isotopic-chemical analyses indicate that the protolith of each type of eclogite was emplaced 2.75-2.74 and 2.5 billion years ago (Ga), respectively, under different tectonic settings, but both were metamorphosed to eclogite facies during a single orogenic cycle before 1.89-1.87 Ga. The Orosirian Belomorian eclogite seems to have undergone tectonometamorphic evolution similar to that of the Miocene Himalayan high-pressure counterpart. We propose that a potential prototype mega-orogen existed in the Orosirian, comparable to the Himalayan Orogen, as the result of modern plate tectonics initiation.