An ocean floor on the roof of the world
Stretched across southern Tibet for over 1,500 kilometers is a geologic seam known as the Yarlung-Zangbo Suture Zone. This zone marks the line where the ancient Neo-Tethys Ocean was consumed as the Indian subcontinent collided with Asia, a process that began around 65 to 55 million years ago. The rocks in this suture are not continental; they are ophiolites—complete sections of oceanic crust and the underlying upper mantle that were scraped off the down-going Indian plate and thrust onto the Tibetan plateau.
A textbook ophiolite sequence consists of mantle peridotite at the base, layered gabbro, a complex of sheeted dikes, and pillow lavas on top. The Tibetan ophiolites exhibit these features, though often tectonically scrambled. The pillow lavas, with their distinct bulbous shapes, formed from underwater volcanic eruptions on the seafloor more than 120 million years ago. Radiometric dating of zircon crystals from mafic rocks within the belt confirms a period of formation between 130 and 120 million years ago, long before the collision began. These rocks are the physical remnants of the ocean floor that once separated India from the rest of Asia.
Messengers from the deep mantle
Within the eastern part of the belt, the Luobusa ophiolite contains some of the world's largest deposits of chromitite, a rock composed primarily of the mineral chromite (FeCr₂O₄). While chromite itself is not rare, the Luobusa chromitites contain a collection of unexpected minerals. Geologists have separated thousands of microscopic grains of diamond, moissanite (silicon carbide), and native elements from these rocks.
These minerals are considered ultra-high-pressure (UHP) phases, meaning they can only form under the immense pressures found deep within the Earth. Their presence suggests they originated in the mantle transition zone, a layer between 410 and 660 kilometers deep. The prevailing theory is that these minerals were captured by rising mantle material, possibly a mantle plume, and transported to a shallow depth beneath a mid-ocean ridge. There, they were incorporated into the oceanic lithosphere and trapped within the crystallizing chromite. The chromite acted as a capsule, preserving the UHP minerals during their rapid journey to the surface and preventing them from converting to lower-pressure forms. These tiny mineral inclusions are a physical sample of Earth's deep mantle, a region otherwise studied only indirectly through seismic waves.