Newly‐discovered Eoarchean TTG gneisses in the Tarim Craton imply plate tectonics at ∼3.7 Ga

Abstract

Eoarchean (3.6 – 4.0 Ga) TTG (tonalite – trondhjemite – granodiorite) gneisses are the major component of Earth’s oldest persevered continental crust, thereby holding the key to understanding how continental crust originated and when plate tectonics may have started in the early Earth. These rocks are extremely rare in the geological record and so far have been identified only in ~10 areas around the world. Using large-scale mapping and detailed zircon U-Pb dating, we identified a suite of Eoarchean (~3.7 Ga) tonalitic gneisses in the Aktash Tagh area, southeastern Tarim Craton (Ge et al., 2018). These are the oldest rocks in China except for the 3.8 Ga Anshan gneisses from the North China Craton, making Tarim one of the oldest continental blocks in the world. Filed observations indicate that these rocks occur as tectonic enclaves in ~2.0 Ga gneisses and were intruded by ~1.9 Ga carbonatites and ~1.8 Ga mafic and granitic dikes. SHRIMP zircon U-Pb dating indicates that the protolith of these tonalitic gneisses crystalized at ~3.71 Ga and were overprinted by two metamorphic events at ~3.56 Ga and ~2.0 Ga, respectively. Zircon Lu-Hf and O isotopic data suggest that these rocks were largely derived from juvenile material extracted from a near-chondritic undifferentiated mantle, thus representing initial continental growth in the early Earth.