DEFORMATION AND PETROLOGY OF ORTHOGNEISS BODIES IN THE DULAN AREA OF THE NORTH QAIDAM ULTRA-HIGH PRESSURE TERRANE OF WESTERN CHINA

Orthogneisses surrounding eclogites in the Dulan area of the North Qaidam terrane in western China show differences in deformation textures across the ultra-high pressure terrane. Dulan is one of four metamorphic units in the North Qaidam terrane, and it’s ultra-high pressure metamorphic region is divided into a north belt and a south belt, separated by faults and lower-pressure units. The main unit of study was the Dakendaban Group, which consists of orthogneiss, paraneiss, eclogite, and amphibolite. 30 samples from a 30km² region of the Dulan area were studied and indexed on a scale according to their deformation level based on quartz textures, crystal size and quality, layer thickness, and foliation intensity. The samples with the most deformation intensity were from the north Dulan belt, a 12km² region where 20 samples were obtained. Highest deformation samples tended to show an abundance of epidote and quartz displaying grain boundary migration or chessboard extinction textures indicating deformation temperatures above 500 °C. Less deformed samples showed primarily subgrain rotation, which indicates deformation temperatures of ~400-500 °C. Feldspar augen between 1-3cm in diameter were observed in most less deformed samples. As deformation increased, there was a noticeable change in the augen, resulting in the flattening out and smearing of the felspathic minerals to thin bands less than a millimeter thick that span the whole sample. The higher abundance of epidote in more deformed samples could suggest that stronger deformation promoted growth of epidote at the expense of plagioclase, and/or that fluids required for epidote growth weakened the rock and promoted deformation.