Geochemistry of the Görcsöny Ridge amphibolites (Tisza Unit, SW Hungary) and its geodynamic consequences

The Görcsöny Ridge is part of the complicated Variscan metamorphic basement of the SW Tisza plate. It was penetrated by several wells, one of which (Baksa-2) explored 1200 m borecore available for petrological examination. Amphibolite samples of this core are of two different sorts: one sort contains biotite, rutile, and garnet, while the other contains ilmenite and is free of garnet. The two rock types occur separately along the borehole, defining a lower and an upper unit (LU, UU). Based on their major and trace element compositions, LU samples are within-plate (WP) tholeiites which assimilated pelagic sediments, while those of the UU represent WP alkali basalts. Thermobarometric calculations suggest that the difference in chemical composition itself does not explain the above differences in mineralogy, so the two units must also differ in their metamorphic histories. Consequently, the crystalline basement of the Görcsöny Ridge possibly consists of amalgamated fragments of different origins and evolutions which became juxtaposed during the Variscan collisional orogeny.