Two contrasting P-T paths for metamorphic sole amphibolites of the Dinaride Ophiolite Zone (Krivaja-Konjuh ultramafic massif, Central Bosnia and Herzegovina) and their geodynamic implications

Abstract

The Inner Dinarides, a part of the Alpine-Dinaride mountain chain in south-eastern Europe, are known for extended ophiolite-derived ultramafic massifs. The sampled rocks from the metamorphic sole of such a massif, the Jurassic Krivaja-Konjuh ultramafic massif in central Bosnia and Herzegovina, are mainly garnet amphibolites, with large grains of pyrope-almandine garnet commonly surrounded by kelyphite coronae, and pargasite amphibolites. The bulk-rock geochemistry points to mantle peridotite as the protolith for pargasite amphibolite and N-MORB for garnet amphibolite. Pressure-temperature (P-T) pseudosections were constructed in the MnNCKFMASHTO system and contoured by isopleths for the modal and chemical composition of minerals. On this basis, a counterclockwise P-T path with maximum P-T conditions of 1.2 GPa and 960 °C was deduced for pargasite amphibolite as a bottom part of an overriding plate. This part was cooled by subducting oceanic crust. In addition, the Jurassic intra-oceanic subduction enabled infiltration of hydrous fluid necessary to form the pargasite amphibolites. A clockwise P-T path with maximum pressure conditions at ca. 2.1 GPa (ca. 65–70 km depth) and temperatures of 810 °C was reconstructed for garnet amphibolite from an upper part of the subducting plate which was exhumed in a subduction channel and came in contact with the pargasite amphibolite. Thus, these amphibolites formed before they were involved in a metamorphic sole at the interface between continental crust and obducting ophiolite.