When did plate tectonics begin on the North China Craton? Insights from metamorphism

Abstract

Understanding when and why plate tectonics began on Earth is one of the most important unresolved problems in earth sciences. As a final result of plate tectonics, continental collisional orogens are formed, and the identification of such orogens is regarded as a milestone in recognizing the operation of plate tectonics in Earth's history. Metamorphism of continental collisional belts is generally characterized by clockwise p-T paths, especially for those involving isothermal decompression following the peak metamorphism. Therefore, clockwise p-T paths involving isothermal decompression are considered one of the important indicators for recognition of plate tectonics, especially in the early history of the earth. As one of the oldest continental blocks in the world, the North China Craton is considered a promising area for applying the large-scale synthesis approach of metamorphic p-T paths in understanding tectonic settings or processes since extensive investigations have been undertaken on the p-T-t evolution of the basement rocks over the craton. Tectonically, the North China Craton is composed of three small continental blocks (Eastern Block, Yinshan Block, and Ordos Block) and three Paleoproterozoic mobile belts (Trans-North China Orogen, Khondalite Belt, and Jiao-Liao-Ji Belt). Metamorphism of late Archean basement rocks in the Eastern and Yinshan Blocks is characterized by anticlockwise p-T paths, mostly involving isobaric cooling, suggesting that the heating source of the metamorphism was related to the underplating or intrusion of large volumes of mantle-derived magmas. Although theoretically such mantle-derived magmas form under continental magmatic arc, mantle plume or continental rift environments, only a mantle plume model can reasonably explain spatial and temporal distribution, lithological association, and structural features of the late Archean basement rocks in the Eastern and Yinshan Blocks. Therefore, mantle plumes may be the major tectonic mechanism that governed the formation and evolution of the late Archean crust in the Eastern and Yinshan Blocks, and plate tectonics may not have operated in the two blocks during late Archean period. Metamorphism of the basement rocks in the Paleoproterozoic Khondalite Belt and the Trans-North China Orogen is both characterized by clockwise p-T paths involving isothermal decompression following peak metamorphism, which suggests that the two orogens underwent initial crustal thickening followed by rapid exhumation/uplift tectonic processes. Such tectonic processes are typical indicators of collisional orogens that accommodate plate tectonics. The Paleoproterozoic Jiao-Liao-Ji Belt can be divided into the southern and northern zones, of which the former is characterized by anticlockwise p-T paths, whereas the latter is characterized by clockwise p-T paths, also indicating a regime of plate tectonics. The time of the initiation of plate tectonics in the North China Craton can be approximated by the age of the earliest juvenile crustal components in the three Paleoproterozoic tectonic belts. So far, the convincing oldest, subduction-related, juvenile crustal component in the North China Craton is the 2.56 Ga Wutai granitoids in the Trans-North China Orogen, which can be used to approximate the timing of the onset of plate tectonics in the North China Craton.