USING TEMPERATURE AND MICROSTRUCTURAL ANALYSES OF REACTION RIM TAILS ON PORPHYROCLASTS TO DETERMINE DEFORMATION CONDITIONS IN GRANITIC BASEMENT ROCKS IN THE KOREAN COLLISION BELT, SOUTH KOREA

Within shear zones, a variety of different shear sense indicators have been identified and examined, one of the most common being porphyroclast systems. These include mantled clasts and clasts with strain shadows, which have both been well studied. The processes that occur during formation of the tails of these two porphyroclast systems are relatively well established, with mantled clasts often forming by dynamic recrystallization and clasts with strain shadows forming by dissolution and precipitation. These systems may represent two end members of tail growth. The third member of porphyroclast systems are clasts with reaction rims. The processes that form the tails in these systems are not as straightforward. Feldspar porphyroclasts with quartz tails are an interesting case of clasts with reaction rims. To gain insight into tail formation of these reaction rims, several samples with good examples of feldspar clasts with quartz tails were collected from basement gneisses in the Korean Collision Belt. A series of microstructural analyses were conducted including grain size, grain area, and grain shape. Temperatures were determined within the tails by using LA-ICP-MS to measure Ti concentrations in quartz grains along transects from clasts to tail tips. A deformation mechanism map was constructed using flow laws for quartz and data from grains within the tails were plotted using grain size and temperature. Microstructural analyses, temperature data, and the deformation mechanism map were used to determine progressive change in processes and deformation conditions during the formation of the quartz tails. These studies can also be used to determine regional variations in deformation conditions.