Rotated garnets: a mechanism to explain the high frequency of inclusion trail curvature angles around 90° and 180°

摘要

This paper presents numerical data from garnets with inclusion trail curvature angles of up to 260°. Three hundred and twenty-five garnets were studied from an outcrop of greenschist facies phyllite in southern Minas Gerais State, Brazil. Apart from the inclusion trail curvature angle α, also the aspect ratio R and the angle between the long axis of the garnets and the foliation, β, were measured. The results show a remarkable concentration of α at 180° and a minor one at 90°. R varies between 1 and 2 showing that the garnets deviate from sphericity and β shows that all garnets have their long axis in the "forward rotated" quadrant, supporting the rotational interpretation. A model is proposed to explain the concentrations of α, based on preferential growth of the garnets into the mica rich strain caps, orthogonal to the foliation, causing elongated crystals that, because of their shape and position would experience accelerated rotation until relatively stable positions with their long axes parallel to the foliation would be attained. Renewed growth, again into the mica-rich strain caps, orthogonal to the foliation would first restore the spherical shape and then produce an elongated shape, again perpendicular to the foliation, forcing a repetition of the process. It is concluded that this model is capable of explaining the concentration of α in multiples of 90°, in a rotational model, where this concentration was considered earlier as an argument in favor of the non-rotational model.