Chlorite-mica aggregates: morphology, orientation, development and bearing on cleavage formation in very-low-grade rocks

-Chlor i te-mica aggregates in slates from northern Spain have been investigated in very-thin thin sections. Specimens from various parts of a fold show different morphologies that are dependent on the operating deformation mechanism. From the limb to the hinge rigid-body rotation becomes less important and folding and intragranular kinking are more common. This is associated with an increasing aspect ratio (width/length) for the aggregates. Chlorite-mica aggregates were, in part, present before cleavage development and are statistically parallel to the bedding enveloping surface. Variation in degree of distortion of chlorite suggests continuous solution and growth during deformation. Muscovite, on the other hand, is generally more highly deformed, suggesting that it was all early. Splitting of micas along (001)-planes occurs, creating extension sites where chlorite growth can take place. Thus, an early diagenetic/metamorphic formation with mimetic growth of chlorite on detrital micas is favoured for the origin of the aggregates. The behaviour of the aggregates during deformation provides information on a mechanism for cleavage development. As a result of intragranular kinking, high angle boundaries are created, with parts of the deformed grains at a low angle to the cleavage plane. Mechanical rotation appears to be the important mechanism for cleavage development in these rocks. Modification by recrystallization, solution and growth processes can account for the observed microstructure.