CARBONATE AND SILICATE MELTS IN DEEP SUBDUCTION ZONE: PETROLOGICAL CONSTRAINTS FROM UHP ECLOGITE FROM ERZGEBIRGE IN CENTRAL EUROPE

A number of studies have indicated the presence of carbonate melts in subduction zones. However, knowledge about the behavior of carbonate minerals and melts at deep subduction zones is still very limited. We performed a combined study of whole-rock geochemistry, rock-forming mineral chemistry and inclusion analyses as well as zircon U-Pb ages, trace elements and inclusion analyses for UHP eclogite from Erzgebirge in central Europe. The results not only provide evidence for the presence of carbonate and silicate melts, but also place constraints on the timing and mechanism of anatexis in the subduction zone. The occurrence of coesite in garnet and zircon indicate the UHP metamorphism of the target samples. There are two groups of multiphase solid (MS) inclusions in garnet and omphacite. The first group is composed of different proportions of silicate minerals such as Qtz, Mus, Pl, Bt, Amp and Chl, and the second group is composed of carbonate minerals and different proportions of silicate minerals. These MS inclusions are generally euhedral to subhedral, and some show relatively regular shapes approaching the negative crystal shape of the host garnet. This indicates the presence of carbonate and silicate melts in the coesite stability field during the UHP eclogites-facies metamorphism. Two episodes of zircon growth with similar U-Pb ages of ca. 340 Ma are recognized during the subduciton-zone metamorphism. Group I zircons exhibit high Th/U ratios of 0.3 to 1.7, steep HREE patterns and negative Eu anomalies with Eu/Eu* ratios of 0.2 to 0.8, and contains inclusions of Coe/Qtz, Grt, Omp, Cal, Mgs, Ap, Rt, Ky and Phg as well as MS inclusions of Qtz+Mus. In contrast, Group II zircons exhibit high Th/U ratios of 0.1 to 0.8, flat HREE patterns without Eu anomalies and contains inclusions of Coe/Qtz, Omp, Pl, Kfs, Cal and Ap. Combined with the P-T estimation, they are interpreted to grow from hydrous melts at 4.5 GPa and 1000℃, under UHP metamorphic conditions during the peak and/or early exhumation stages. The silicate melts were derived from dehydration melting of hydrous minerals, whereas the occurrence of carbonate melts indicates involvement of carbonate minerals in the anatexis and thus has great bearing on recycling of supracrustal carbon into the deep mantle.