Ultra-high pressure (UHP) metamorphic rocks from the Yankou region in China have been down a subduction zone to more than 200 km and then rebounded to the surface. Kai Ye, Bolin Cong and Danian Ye of the Chinese Academy of Science in Beijing have worked on barometric indicators from eclogites and garnet peridotites to reach this conclusion (Ye, K. et al. 2000. The possible subduction of continental material to depths greater than 200 km. Nature, 407, 734-736). It is no surprise to learn that basaltic and peridotitic materials have been down a subduction zone, because that is what oceanic lithosphere does continually, though how they return to the surface as intact slabs is problematic.
What is surprising is that such highly compressed rocks are associated with similarly UHP materials that are chemically normal materials of the continental crust. The Yankou rocks now hold the record for deep diving. Sialic subduction is not easy because of its reluctance to reach densities that exceed that of the mantle. That being said, there are growing suspicions that continental materials may contribute to the composition of alkaline magmas formed deep beneath hot spots. If sial does not reach 200 km depth, its density always lies above that of the mantle, and it must be buoyant. Taken deeper, however, the situation reverses because of phase changes that compress silica and feldspar, so that at 300 km depth they become much denser than mantle, and must continue sinking to become potential contributors to later mantle melting.
In this case it seems as if the slab of Chinese sial was dragged from the lower crust by its attachment to enough basic and ultrabasic rocks that the whole lot broke the buoyancy barrier by their density change at high pressures. Getting back to the surface poses the big problem, the authors proposing that they were rafted by rocks beneath them. Somehow, a large mass of UHP basic-ultrabasic material must have become detached from sialic materials before the combined slab passed the 300 km boundary and became doomed to long-term mantle residence. That would give them and any eclogites remaining attached to them sufficient buoyancy to bob up once again.