Discovery of Blueschists in the Medvednica Mountain (Northern Croatia) and Their Significance for the Interpretation of the Geotectonic Evolution of the Area

Abstract

Blueschists were discovered in the metamorphic rocks of the Medvednica mountain. Two varieties of blueschists are recognised, one with garnets and another with omphacite. Determined mineral associations are composed of sodic amphibole (glaucophane and crossite), garnet, omphacite and white mica. These mineral associations indicate metamorphic conditions which are transitional between blueschist and eclogite facies, i.e. high pressures at relatively moderate temperatures (LG, HP/LT). Such conditions are often characteristic of subduction zones.

The occurrence of high-pressure metamorphic rocks in the Medvednica mountain, together with local outcrops of ultramafic rocks (serpentinised harzburgite, dunite and serpentinite) indicate subduction related rocks of the Dinaric part of the Tethyan oceanic crust. In the northern part of the Dinarides, ultramafic rocks and glaucophane schists have also been found in the Motajica and Fruska gora mountains. The Medvednica-Motajica-Fruska gora zone probably represents a relict subduction zone in the Dinaridic part of Tethys, along which regional high-pressure metamorphism took place.