A collision of continents
The serene, forested slopes of the Dabie Mountains hide evidence of one of the most violent processes on Earth: a continental collision. Around 240 million years ago, during the Triassic Period, the South China and North China tectonic plates crashed into one another. This was not a typical subduction event where dense ocean crust slides beneath a continent. Here, a slab of buoyant continental crust from the South China Plate was forced down into the Earth's mantle, a place it normally cannot go.
The rocks you can walk on today were dragged to depths of over 100 kilometers. At this depth, pressures skyrocket to more than 3.5 gigapascals (GPa) and temperatures reach between 700-850°C. These are conditions of the deep mantle, and they transformed the common granite and gneiss of the crust into something extraordinary. Geologists call this process ultra-high pressure (UHP) metamorphism. The Dabie-Sulu orogenic belt, of which these mountains are a part, is the largest exposure of UHP rocks in the world.
The discovery of this UHP belt in the late 1980s was an important moment in geology. It provided the first concrete proof that continental crust could be subducted to such extreme depths and then return to the surface, a process called exhumation. Rapid journey back to the surface was essential for preserving the evidence of their deep history.
Minerals from the mantle
The proof of this deep burial lies in the minerals forged under extreme pressure. The most definitive indicators are microscopic diamonds and a dense polymorph of quartz called coesite. These minerals cannot form in the normal pressure and temperature ranges of the Earth's crust. Finding them inside common rocks like gneiss and marble was revolutionary. Diamonds, typically associated with kimberlite pipes from the deep mantle, were found here as inclusions inside garnet crystals, some only 10 to 60 micrometers across.
Another important rock type found here is eclogite. This rock is famous among geologists for its striking green and red colors, a result of its two main components: green omphacite (a high-pressure pyroxene) and red garnet. Eclogite is much denser than typical crustal rock, which helps explain how the continental slab could sink so deep. The UHP rocks are not found everywhere, but occur as lenses and blocks within the surrounding gneisses. Other UHP indicator minerals found in the Dabie Mountains include jadeite quartzite. The study of these rocks shows the processes that occur deep beneath active mountain ranges during continental collisions.