Retrograde metamorphism of eclogites from the main hole (0-2000 m) of the Chinese Continental Scientific Drilling, Donghai, Jiangsu Province

Abstract：The main hole of the Chinese Continental Scientific Drilling (CCSD) Project is located in the eastern part of the Dabie-Sulu ultrahigh pressure (UHP) metamorphic belt. In the 0–2000 m long core, all kinds of eclogites account for >50 percent and most of them have experienced different degrees of retrograde metamorphism. According to the retrograde degrees of the dominant minerals garnet and omphacite in the eclogites, the process of the retrograde metamorphism may be divided into two major stages and four substages：the first major stage, which may be further divided into the substages of weak retrograde metamorphism and partial retrograde metamorphism，and the second major stage, which may be further divided into the substages of retrograde metamorphism and strong retrograde metamorphism. The general trend of retrograde process is as follows: garnet was gradually replaced by pargasite or epidote+biotite and omphacite was replaced by symplectitic coronas of amphiboles and plagioclase; the jadeite content of omphacites decreases gradually in the retrogression and part of omphacites are transformed to aegirine-augites. The P-T conditions of eclogites are as follows: the peak stage, 697–831℃ and ~3.0 GPa; the substage of partial retrograde metamorphism, 629–776℃ and 1.2-1.6 GPa; the substage of retrograde metamorphism, 550–650℃ and 0.5-0.7 GPa; the substage of strong retrograde metamorphism, 300–400℃ and 0.30-0.35 GPa. According to an integrated study of the petrological and mineralogical characteristics and P-T conditions, it is deduced that eclogites underwent a process of two-stage exhumation: the first major stage witnessed nearly isothermal decompression, suggesting fast exhumation (eclogites experienced retrograde metamorphism of the first major stage during this stage) and the second major stage saw decrease in temperature and pressure due to slow uplift (during this stage, eclogites underwent retrograde metamorphism of the second major stage). The complete retrograde metamorphism of omphacites is not only the marker of distinguishing the two major stages of retrograde metamorphism but also a marker of distinguishing the two major stages of exhumation.