Unravelling the spirals: a serial thin‐section study and three‐dimensional computer‐aided reconstruction of spiral‐shaped inclusion trails in garnet porphyroblasts

Abstract

Abstract Seventy‐seven spatially orientated, serial thin sections cut from a single rock reveal changes in the geometry of spiral‐shaped inclusion trails (SSITs) in garnet porphyroblasts. The observed SSITs are doubly curved, non‐cylindrical surfaces, with total inclusion‐trail curvature decreasing systematically from the cores to the rims of porphyroblasts. The three‐dimensional geometry of the SSITs, reconstructed with the aid of computer graphics, shows that the orientations of spiral axes defined by the SSITs are not related in any expected nor predictable way to the main foliation in the matrix. This suggests continued deformation after or during the latest stages of porphyroblast growth, which has important implications for the use of SSITs as shear‐sense indicators. Whether the formation of SSITs involves significant porphyroblast rotation with respect to a geographically fixed reference frame cannot be determined from the available data.