Middle Age Earth: Distinctive geology and metallogeny and their origins

The cooling process of Earth′s deep interior serves as the core engine driving the co-evolution of the lithosphere and surficial environment. During Earth′s Middle Age (1.8–0.8 Ga), the Earth’s lithosphere had a unique ″Hotness–Thinness–Slowness″ state. “Hotness” is manifested by the development of ultrahigh-temperature metamorphism, A-type granites, and massif-type anorthosites in the lower crust of global orogenic belts. “Thinness” is characterized by thinning active continental crust (evidenced by decreased Eu/Eu* values of detrital zircon and lowered La/Yb values of whole-rock compositions). “Slowness” is reflected in reduced plate movement velocity and decreased cooling and erosion rates of the crust. The overall surficial environment is featured by a relatively low level of atmospheric oxygen content and widespread hypoxia in the ocean (but with possible intermittent oxygenation pulses). The metallogenic response features are distinct, manifested as the prosperity of extensional environment-related deposits (e.g. iron oxide copper-gold deposits, carbonatite-type rare earth element deposits) and sulfide-favorable sedimentary exhalative lead–zinc deposits, but the scarcity of compressional environment-related deposits (orogenic gold deposits, volcanic-hosted massive sulfide deposits, and porphyry copper deposits) and anoxia-dependent deposits (i.e. banded iron formations). These unique geological and metallogenic records were probably governed by the deep interior–surface interaction mechanisms, including the multi-stage extensional events, mantle thermal anomalies, slowed subduction rates and the development of retreating subduction zones, and metallogenic transformation driven by the sulfidic ocean stratification following the Great Oxidation Event. Abundant geological and metallogenic records of the Middle Age were preserved in the North China and Yangtze blocks, providing key archives for studying geodynamic processes in the deep interior of the Earth and their shallow-surface responses. Future studies should be focused on the establishment of a more comprehensive framework by integrating multidisciplinary evidences, in order to deeply elucidate the coupling evolution history of the deep processes and surface systems of the Middle Age.