High-Resolution TEM Study of Inclusions / Precipitates in Omphacite from Sulu and Dabie UHP Eclogites, Eastern China

The mineralogy of micro-phases / nano-phases and their textural relationship in host minerals from ultra-high pressure (UHP) metamorphic rocks can provide information about the history of subduction and exhumation of their host rocks. Using high-resolution transmission electron microscopy (HRTEM), we examined the omphacite in UHP eclogite from Qinglongshan area of the Sulu terrane. Selected-area electron diffraction patterns confirm that the needle-like precipitates in the central part of the omphacite are quartz micro-crystals. This kind of texture indicates UHP origin of the pyroxene. The omphacite is composed of a mixture of C- and P-lattice domains with several tens of nano-meters in size. Anti-phase boundaries did not develop in the omphacite. The domain structure in the omphacite indicates early stage ordering between Na-Al and Ca-Mg that will result in phase transition from disordered omphacite (C-lattice) to ordered omphacite (P-lattice). The domain structure of omphacite also indicate fast cooling (or exhumation) of the host rock. The omphacite contains Ca-poor pyroxene lamellae with P-lattice and no anti-phase boundaries, which suggests that the pyroxene lamellae formed as P-lattice pyroxene, instead of high-clinoenstatite. The omphacite from Northern Dabie UHP terrane is disordered and with C-lattice. It can be inferred that the cooling rate for the Northen Dabie omphacite is higher than that of Qinglongshan omphacite.