Tectonic model (IMAGE) Science China Press Caption The left part shows the >3230 Ma TTGs formed by melting of intracrustal mafic rocks with no involvement of supracrustal sediments or low-temperature altered basalts. Compressional tectonics took place at ca. 3230 Ma and allowed recycling of supracrustal materials in deep crust due to crustal thickening or proto-subduction. The subsequent melting of supracrustal materials and mafic rocks within a partly thickened lithosphere generated the ≤3230 Ma TTGs with systematically elevated oxygen isotopes. Credit ©Science China Press Usage Restrictions Use with credit. License Original content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.