Development of porphyroclast geometries during non-coaxial flow : an experimental and analytical investigation

Abstract

A porphyroclast is a remnant of a resistant mineral grain, preserved in a deformed rock, which is of a size larger than the grains in the surrounding matrix. Porphyroclasts develop because of a difference in rheology between constituent minerals during deformation; relatively hard minerals tend to form porphyroclasts while relatively soft minerals form the matrix, the finer grain size generally reflecting extensive dynamic recrystallisation. This thesis is concerned with establishing a systematic basis for the kinematic interpretation of porphyroclast microstructures, and in particular mantled porphyroclasts, developed in deformed metamorphic rocks. Provided that a deformable mantle exists at the periphery of a relatively rigid porphyroclast, the final geometry of a core-mantle-matrix system is a direct result of the flow pattern, or velocity distribution, in the immediate vicinity of the core. Porphyroclast geometries therefore in some way reflect the kinematics or pattern of flow. These relatively rigid objects and associated features are widely used as shear sense indicators, particularly in mylonites, and may also hold information on the rheology of the clast and surrounding material during the time of their development. When the kinematic conditions or flow geometry that lead to the development of a particular microstructure are known, the microstructural feature in question can potentially be used to define the sense of shear and the distribution of strain and strain rate. Accordingly, the most reliable information is obtained from microstructures that develop under unique conditions.