Petrologic comparison of the Gyod and Helesfa serpentinite bodies (Tisia Mega Unit, SW Hungary)

Magnetic anomalies in the south-western part of the Tisia Mega Unit were drilled in different geological situations: the Helesfa serpentinite body is surrounded by a Variscan granite, while the host rocks of the Gyod serpentinite body are paragneiss and amphibolite. Nevertheless, these anomalies are on opposite sides of the Mecsekalja Shear Zone, which developed during the Variscan Orogeny. Olivine, enstatite and spinel occur as relicts in the Gyod serpentinite, while the primary ultramafic rock of the Helesfa serpentinite is totally serpentinized. Based on the major-element whole-rock geochemical data, the protolith of the two serpentinites was mainly harzburgite. Based on the similar petrography, textures, mineral composition, major-element geochemistry, as well as metamorphic and structural evolution, a close genetic relationship between the Gyod and Helesfa serpentinite bodies can be supposed. Serpentinite with pseudomorphous lizardite and chrysotile textures possibly formed during the ocean-floor metamorphism. Afterwards, an antigorite +/- talc assemblage developed from the pseudomorphous serpentine texture, which is sheared and mylonitized, supposedly due to subduction. Serpentinites with similar composition and evolution are widespread in the Bohemian Massif (especially in the Polish Central-Sudetic ophiolites).