The 5158-m-deep main hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) at Maobei, SW Sulu Belt, penetrated five major lithologic units: eclogite and/or retrograded eclogite, ultramafic rock, paragneiss, orthogneiss and amphibolite. All analyzed samples are overprinted to various extents by amphibolite facies retrograde metamorphism. Ultrahigh-pressure (UHP) index minerals are preserved as inclusions in zircons separated from 137 core samples.
These index assemblages include Coe + Phe + Grt and Coe + Jd + Grt + Phe + Ap from paragneiss, Coe + Phe + Ky + Ttn + Ap from orthogneiss, and Coe + Grt + Omp + Phe from amphibolite and retrograde eclogites. The common occurrences of these UHP assemblages throughout the section indicate that the drill hole did not penetrated through the UHP tectonic slice. Zircons separated from surface samples distributing more than 11000 km2 and cores from other shallow holes with depths of 432 m to 1028 m in the Sulu UHP terrane also contain similar coesite-bearing UHP mineral inclusions. All of the UHP mineral assemblages, whether from retrograde ecologites or from the surrounding gneisses, record similar metamorphic conditions of 732-843°C and 3.2-4.0 GPa.
This widespread occurrence of UHP assemblages in the Sulu area suggests that voluminous continental materials, consisting mainly of sediments and granitic crustal materials of passive continental margins including various sandstone, shale, limestone and minor mafic-ultramafic intrusive and extrusive rocks, were subducted simultaneously to mantle depths in excess of 100 km and metamorphosed, and then rapidly returned to the crustal level.