Banded Eclogitization on Serpentinites – effect of Shock Wave Action

A rare case of rhythmic banded eclogitization of serpentinites in crustal conditions is interpreted as a possible effect of paleoseismic waves impact on serpentine structure. The serpentinites are a part of the Ophiolite Association in the Rhodope Massif, South Bulgaria. The Association is included in Lukovitsa Variegated Formation, which in the area of Avren village, consists of amphibolites, marbles and schists. Avren serpentinite body is in contact with leptite gneisses. The peripheral parts (30-40m) are affected by eclogitization. Bands of garnet-lherzolites (1- 20mm), which are parallel to the contact, alternate with strips of unchanged serpentinite. The bands, close to the contact are more frequent and consist of pyrope-garnet, diopside, enstatite, olivine and spinel, crystallized under conditions of T = 560-820oC /P = 8-15 kbar. Moving away from the contact, the stripes become rarer and gradually disappear. Obviously eclogitization is associated with the contact between serpentinites and gneisses, which has been most probably a paleoseismic zone of friction. Every seismic event provokes emergence of shock body waves, whereas the longitudinal P-waves create spaces of delamination with rarefaction of the particles and spaces of compression. We assume that the weak Van der Waals bonds of the serpentine structure could break in the dilation zones during expansion and then layers of mobilized particles in chaotic motion occurred. The following S-waves enhanced the motion and increased the temperature and pressure in the layers. After crystallization of the mobilized particles, a striped texture of alternating bands of garnet-lherzolite and unchanged serpentinite was formed. The described case demonstrates the influence of seismic waves on crystallization processes and at the same time the appearance of HP metamorphism in situ in the middle levels of the crust.