Retrograde Processes of Eclogite from 100-2000m in the Main hole of Chinese Continental Scientific Drilling Project,Eastern China

The Chinese Continental Scientific Drilling (CCSD) Project is located in the Maobei Village, Donghai County, Jiangsu Province, Eastern China. Based on the detailed study on mineralogy, petrology, and geochemistry of eclogite from 100 to 2000m of the main borehole, the major achievements are as following: 1. Four retrograde processes have been identified. That is: weakly retrograde stage, partially retrograde stage, retrograde stage and strongly retrograde stage. During the weakly retrograde stage, only a few symplectitic coronas of amphiboles and plagioclases distributed along the boundry of garnet and omphacite. Rutile is stable. During the partially retrograde stage, the rate of omphacite disappear is rapid, omphacite exist only as a relict. Aegirine rim exist along the boundry of quartz and omphacite. Rutile retrograde into ilmenite. During the retrograde stage, garnets were gradually replaced by pargasite or epidote+biotite. omphacite disappear totally, and It is replaced by symplectitic coronas of amphiboles and plagioclases, but the pseudomorph of omphacites preserved. Aegirine rim become wider and sometimes exist as a lump. During the strongly retrograde stage, Aegirine almost disappear. Amphibolite replaced Aegirine, and It distributes along the boundry of quartz. 2. From fresh eclogites to strongly retrograde eclogites, MgO, CaO and FeO of the whole rock gradually decrease, SiO2, K2O and Na2O gradually increase, and Al2O3 almost keep constant. The Pb value increase obviously from weakly retrograde stage to strong retrograde stage. 3. The peak P-T condition of eclogites: 697-831¡æ and 3.0Gpa¡A; partially retrograde phase: 629-776¡æ and 1.2-1.6Gpa; retrograde phase: 550-650¡æ and 0.5-0.7Gpa (Xu, Z Q et al., 2004); strongly retrograde phase: 300-400¡æ and 0.30-0.35Gpa (Xu, Z Q et al., 2004). 4. The UHP metamorphic rocks had undergone two-phase exhumation: first, nearly isothermal and decompression, suggested that fast exhumation occurred before the completely retrograde of the omphacite (during this phase, eclogites had undergone weakly and partially retrograde); second, decreasing temperature and decreasing pressure by slow uplifting after omphacites being completely replaced by symplectite (during this phase, eclogites have undergone retrograde phase and strongly retrograde phase). Acknowledgement: This work is sponsored by the Major State Key Project to CCSD: 2003CB716501 and major project from NSFC: 40399142