Textural Characterization and its Formation of Nanometer Ultra-fine Rutile from Eclogite of Sulu UHP terrane, China

Detailed TEM investigations present the ultra-fine microscopic textures of rutile in rutile eclogites of Maobei, Su-Lu UHP terrane. The mineral assemblage contains garnet, omphacite, rutile, quartz/coesite and minor apatite. Two types of rutiles occur in investigated samples. Rutile (A) associates with garnet and omphacite£¬with the grain-size of 0.2-2 mm. The dense defects and modulated structures regularly oriented nanometer-size kinks are discovered in the rutile crystal, which implicate a strong stress and strain effect on the rutile crystals. The kinks in the rutile are observed with the sliding planes {110} and sliding direction [100]. That is intimately related to continental collision. Rutile (B) is included in the garnets and omphacites with the grain size of 1-40 ¦Im. The crystal of later type is well preserved in the host minerals, and has not undergone crystal distortion during exhumation of subducted slab. The rutile eclogite under study occurs in the Daibei-sulu UHPM belt, consisting of garnet (~43 vol. %), omphacite (~45 vol. %), rutile (~7 vol. %), Quartz/Coesite (~3 vol. %) and minor amount of apatite and spinel. There are two different types of rutiles in the samples, in which rutile (A) associates with garnet and omphacite with the grain-size of 0.2-2 mm.; and rutile B included in the garnets and omphacites has grain size of 1-40 µm. The ultra-fine microscopic textures of rutile are discovered in type A rutile, having been investigated in detail by means of TEM technique. Two type rutiles in the eclogites of Maobei were investigated in detail by TEM. They exhibit completely different micro textures, indicating these two types of rutiles form from different tectonic environment and have experienced different geological events. Rutile (A) associated with garnet and omphacite contains a lot of dense defects, modulated texture and nanometer-size kinks in the crystal. The deformed kink twins in the rutile are observed regularly oriented with the sliding planes {110} and sliding direction [100], indicating a strong strain effect on the rutile crystals, and may intimately relate to the subduction, exhumation and subsequent continental collision. The rutile (B) is included in the garnets and omphacites with the grain size of 1-40 ¦Im . Rutile (B) shows a perfect configuration due to the well preservation from host minerals (garnets or omphacites), and no defects has been found in the crystal, which suggests rutile (B) has not undergone crystal distortion during exhumation of subducted slab. Discovery of many oriented exsolution rods of quartz indicates the host primary clinopyroxenes could be supersilicic. Former researches conclude that pyroxenes from deep mantle or deeply subducted UHP terrane could be of supersilicic characteristics. In addition, several coesites have been found inside the rutile and among the grain boundaries of rutiles under TEM in this study, also suggesting the rock has experienced a UHP metamorphism. The formation of rutile (A), contained a lot of dense defects, modulated texture and nanometer-size kinks in the crystal, may intimately relate to the exhumation process of subducted terrane. Rutile (B) crystal is well preserved in the host minerals, and has not undergone crystal distortion during exhumation of subducted slab.