Strain partitioning and crack-seal growth of chlorite-muscovite aggregates during progressive noncoaxial strain: an example from the slate belt of Taiwan

Abstract

Chlorite—muscovite aggregates from the slate belt of southeastern Taiwan have an ellipsoidal geometry (~ k = 1 on a Flinn diagram) with an aspect ratio a:c ranging from about 7:1 to 12:1 and a crystallographic preferred orientation with the basal planes of the chlorite and muscovite at a high angle to the slaty fabric in the rock. The principal axes of the aggregates are approximately coaxial with the principal finite strain directions determined from fibrous pressure shadows. In cleavage parallel sections, straight fibers parallel to the downdip mineral lineation record plane strain. In cleavage perpendicular sections, however, pressure shadows have curved fibers that are consistent with W-directed thrusting. We propose a model in which the chlorite-mica aggregates grow syntectonically by cracking of detrital chlorite grains parallel to the basal planes and precipitation of secondary chlorite and mica in the open cracks. This model involves (1) rotation of preexiting detrital chlorite grains during progressive noncoaxial deformation, (2) cracking of these grains parallel to the basal plane when the (001) planes rotate into the incremental shortening field, with nucleation and growth of secondary chlorite and muscovite in the open microcracks. As the aspect ratio increases, the grains stabilize in orientation, with the long axes (normal to the basal planes) at a small angle to the shear direction. Extension normal to (001) is also accompanied by some shortening parallel to (001), but with an overall increase in size of the aggregates. The final grain size is controlled by the cumulative displacement associated with crack-seal events. Microprobe analyses of both detrital and secondary chlorite grains show distinct compositional differences within individual grains perpendicular to (001), particularly in the detrital grains, which we interpret as primary and secondary chlorite, consistent with a primary-and-secondary origin for the aggregates.