Coesite inclusions and prograde compositional zonation of garnet in eclogite from Zekou in the Su-Lu ultrahigh-pressure terrane, eastern China

Abstract

In the Su-Lu UHP terrane, eastern China, coesite-bearing eclogites have been reported in five areas (Weihai, Yangkou, Rizhao, Donghai and Rongcheng). The presence of a coesite inclusion (70 μm) in garnet in a strongly retrograded eclogite at Zekou (about 40 km southwest of Rongcheng) shows for the first time that this region also underwent coesite-eclogite facies metamorphism. In addition, the garnet of the Zekou eclogite preserves a distinct compositional zonation. The eclogite consists of garnet (20%), symplectite (70%: amphibole+plagioclase) and other accessory minerals (amphibole, rutile, quartz and ilmenite). Garnet porphyroclasts (0.5-1 mm in diameter) show compositional zonation that can be divided into three parts: core (Alm58-59 Sps0.7-0.9 Prp15-17 Adr0.1-0.2 Grs23-24), mantle (Alm55-58 Sps0.5-0.8 Prp14-15 Adr0.1-0.2 Grs25-28), and rim (Alm53-56 Sps0.4-0.6 Prp17-18 Adr0.1-0.2 Grs25-27). Each part has a characteristic inclusion mineralogy: the core part contains rutile, quartz and no coesite, the mantle part contains coesite, quartz-pseudomorphs after coesite, rutile, omphacite and zircon, and the rim part contains rutile. The coesite inclusion (70 μm) is located in the outer mantle part and is associated with radial fractures in the surrounding garnet. The different inclusion assemblages and the chemical zonation from core to rim in garnet porphyroclasts suggest that the Zekou eclogite preserves the information of the prograde stage during UHP metamorphism.