Petrology of eclogites from the main hole of the Chinese Continental Scientific Drill Project

The drill site of the 5000 m deep main hole of the Chinese Continental Scientific Drilling Project (CCSD) is located in the southern Sulu ultrahigh-pressure (UHP) metamorphic belt. The 0 to 2000 m interval of the CCSD main hole consists of eclogite, gneiss, garnet peridotite and minor schist and quartzite. The eclogites with a cumulative thickness of about 1000 m have different mineral assemblages and model contents, as well as variable petrochemical compositions. They can be divided into five types:Si-rich quartz eclogite, Ti-rich rutile eclogite, Al-rich phengite and kyanite eclogite, Mg-rich two-mineral eclogite and normal eclogite. Protoliths of eclogites include layered mafic to ultramafic cumulates and supracrustal rocks. The whole-rock composition of eclogites has an apparent controlling effect on the contents of some components in garnet, omphacite and phengite and directly influences the P-T estimates of UHP metamorphism. The extensive development of diffusional compositional zoning in the eclogitic garnet, omphacite and phengite indicates that these UHP minerals were re-equilibrated during the early-stage retrograde metamorphism. This fact and the existence of garnet porphyroblasts with growth zoning suggest that the eclogites formed at higher temperature (940℃) and pressure (4.5GPa) than those estimated by previous workers.