How Earth’s continents formed and evolved remains a fundamental question in geology. Subduction zones are key places where continents are both built and destroyed. One important process of destruction is subduction erosion, where pieces of the overriding continental crust can be removed into the deep Earth during subduction. This process plays a major role in balancing crust formation and loss, but it is very hard to detect in ancient, inactive plate boundaries. In this study, we investigate 800-million-year-old plutonic rocks from the Yangtze Block in South China, once part of an active subduction system. By combining mercury (Hg) stable isotopes with neodymium (Nd) and hafnium (Hf) radiogenic isotopes, we track the origin and recycling of the materials that formed these magmas. Hg isotopes are especially useful because they preserve chemical signals from Earth's surface. Crust removed by tectonic erosion carries distinct Hg signatures that help identify recycled materials in arc magmas—signatures that are hardly detectable by traditional methods. Our results reveal that the crust formed in the arc region was rapidly eroded and recycled into the mantle. These findings provide evidence of ancient erosive margins and help us better understand how subduction zones shape the continents over time.