FORMATION OF NANOMETER TWINS OF RUTILE AND THEIR TEXTURAL CHARACTERISTICS IN UHP ECLOGITE

Detailed TEM investigations present the ultra-fine microscopic textures of rutile in eclogites of Maobei, Su-Lu UHP terrane. The samples of eclogite are very fresh with less retrograde metamorphism. The eclogite contains garnet, omphacite, rutile, quartz/coesite and minor apatite. There are two types of rutile in investigated samples. One type rutile associates with garnet and omphacite with the grain-size of 0.2-2.0 mm, which includes Ti-Fe spinel needles and laths inside. The other type is included in the garnets and omphacites with the grain size of 1-5 µm. This paper reports TEM observations and petrological studies on the first type rutile, which is associated with garnet and omphacite. The dense defects and regular oriented nanometer-size deformed twins are discovered in the crystal structure of rutile, which implicate a strong stress effect on the rutile crystals. The occurrence of the deformed rutile twins shows a regular, oriented pattern. Two group twins in the rutile are observed with the sliding planes parallel to (100) and (010) respectively (Fig 1). Coesites also have been found in the internal or inter-grained rutiles that suggest at least 2.8 GPa UHP metamorphism has been reached. In addition, the omphacites, associated with rutiles, contain a lots of oriented exsolution rods of the quartzs, which are parallel to planes of omphacite (010). The exsolution of quartz from omphacite implies the presence of supersilicic primary clinopyroxene in the eclogite. A great deal of studies has revealed that the exsolution of quartz from eclogite experiences 3 GPa, even to 8 GPa UHP metamorphic processes (Mao H K., 1971, Zhu Y F et al., 2002). Former studies indicated that high-pressure phase of titanium dioxide (TiO2) with an PbO2-type structure formed from 4 to 5 GPa at 900 to 1000°ãC£¨Hwang, et al 2000£©£¬and nanometer PbO2-type TiO2 phase produced under 4 GPa (Olsen et al (1999). In short, the investigated rutile eclogites probably have experienced 6-8 GPa UHP metamorphism. The presence of internal defects and nanometer twins in the rutile are likely a structural adjusting process when the primary high-pressure PbO2-tpye rutile transforms into low-pressure rutiles.