Acta Musei Moraviae

Metamorphic evolution metadiorite subzone in the Brno Batholith

Metadiorite subzone forms the western part of the Central basic belt of the Brno Batholith. It can be interpreted as relic of Neoproterozoic ophiolite complex. Cumulate gabbros, diorites and ultramafic rocks were obducted on to the Thaya terrane and subsequently deformed and/or metamorphosed. The mineral assemblage of rocks forming the metadiorite subzone and the mineral textures (zonation in amphibole, chromite) indicate polyphase metamorphic evolution. Three major metamorphic stages can be distinguished. The first stage comprises a regional metamorphism M1, which reached a greenschist facies grade. The second stage is represented by a contact metamorphism (M2) related to the intrusion of the Cadomian granitoids, with intensity reaching greenschist or up to amphibolite facies. The last stage is characterized by a regional Variscan metamorphism (M3) under prehnite–pumpellyite to greenschist facies conditions.

Spinels from ultramafic rocks feature heterogeneous chemical zonation. The degree of chromite grains alteration depends on the chemical composition of coexisting silicates and on the P-T conditions of metamorphism. The individual alteration zones represent different steps of the prograde metamorphism (M2) that affected primary chromitites.

The estimated thermal metamorphic conditions for the metadiorites and metagabbros are in agreement with the data obtained from metamorphic assemblage of the ultramafic rocks.