Alpine metamorphism of organic matter in metasedimentary rocks from Mt. Medvednica (Croatia)

Abstract

The diagenetic to low-temperature metamorphic alteration of Mt. Medvednica (Internal Dinarides, Croatia) has been studied by vitrinite reflectance, ordering data of organic matter determined by X-ray powder diffraction and Raman spectroscopy of extracted carbonaceous material.

In metapelites of the Medvednica Metamorphic Complex (MMC), maximum vitrinite reflectance varies between 6.9 and 9.8%. By X-ray powder diffraction, the finely dispersed carbonaceous material is classified as d₁-graphite. Based on the “Raman Spectroscopy of Carbonaceous Material Thermometer”, peak metamorphic temperatures of ca. 410 °C are estimated. The degree of organic maturation and the estimated peak temperature correlate fairly well with the Cretaceous (120–80 Ma) high-temperature anchizonal to epizonal metamorphism of the unit, determined by illite Kübler index, chlorite “crystallinity” and K-white mica – chlorite thermobarometry.

In the tectonically higher Jurassic Ophiolitic Mélange and in the unconformably overlying Cretaceous–Paleocene Sequence, random vitrinite reflectance between ca. 0.7 and 2.2% suggest a stratigraphical trend of the data and therefore a common burial history. In the Jurassic Ophiolitic Mélange and the Cretaceous–Paleocene Sequence, the synmetamorphic carbonaceous material is classified as d₃-graphite. In both units, organic thermometers provide peak temperatures of 100–240 °C. These estimates are in accordance with phyllosilicate reaction progress indicators, showing no systematic variation in the stratigraphic succession.