DETERMINING DEFORMATION CONDITIONS USING QUARTZ TAILS ON FELDSPAR CLASTS: AN EXAMPLE FROM THE KOREAN COLLISION BELT, SOUTH KOREA

Asymmetric tails on porphyroclasts are often used as shear sense indicators in ductile deformation zones. There are three main types of porphyroclast systems, and they form by different processes. These include mantled porphyroclasts formed by dynamic recrystallization, porphyroclasts with strain shadows formed by dissolution and precipitation, and porphyroclasts with tails formed from reaction rims. Questions still remain about the processes through which porphyroclasts with reaction rims form; to determine the main processes involved we looked at a common example, quartz-mica tails on feldspar porphyroclasts. Samples from basement gneisses in the Korean Collision Belt with good examples of feldspar clasts with quartz-mica tails were selected for a series of analyses. Microstructural studies of the quartz in the tails included analyses such as grain size, grain area, and grain shape. These data and microstructural grain relations were used to determine the processes by which the tails developed. Ti concentrations within the quartz tails were measured using LA-ICP-MS and used to determine temperature using Ti in quartz geothermometry. This analysis gave estimates of the temperature of formation of the quartz tails (i.e., deformation temperature) and was used to see if there was any temperature variation within the tails. Having collected microstructural and temperature data, a deformation mechanism map plotting grain size (d) against temperature (T) was constructed using flow laws for quartz. This allowed us to directly plot our d-T data on the map, assuming a constant stress, to be able to identify the deformation mechanisms within the quartz grains in the tails. Combining the deformation mechanism map, microstructural evidence, and temperature data, a model of how quartz tails grow on feldspar porphyroclasts was developed that provides insight into how reaction rim tails on porphyroclasts form, as well as the deformation conditions at the time of tail formation.