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Low-temperature metamorphism of Mt. Medvednica (Croatia) based on organic maturity parameters

The Zagorje– Mid-Transdanubian Zone (ZMTZ) between the Periadriatic-Balaton Lineament and the Zagreb-Zemplin Lineament (or Mid-Hungarian Line) forms the hinge between the Alps and the Dinarides at the western margin of the Pannonian Basin. Only few “ inselbergs” are exposed in the ZMTZ. One of the largest ones is Mt. Medvednica in northern Croatia. This contribution presents the spatial pattern of organic metamorphism within Mt. Medvednica. Vitrinite reflectance data as well as X-ray powder diffraction and Raman spectroscopy of extracted carbonaceous material (CM) characterize the low-temperature metamorphic overprint of the mountain. In the Paleozoic– Mesozoic complex Rmax and Rmin values vary between 6.9 and 9.8% and 1.3-2.3%, respectively. According to Landis (1971), CM finely dispersed in metapelites is classified as graphite-d1. Based on the Raman spectroscopy of CM thermometer (Beyssac et al. 2002 ; Rantitsch et al. 2004), a peak metamorphic temperature of ca. 410 °C is estimated for this unit. Random vitrinite reflectance value (Rr) of ca. 2.2% are measured on samples from the Jurassic ophiolitic mélange. In the Cretaceous– Paleocene sequence Rr data range between 0.7-1.8%. The high-T metamorphic overprint of the Paleozoic– Mesozoic complex might be explained by the tectonic burial beneath a later eroded tectonic cover. Taking into account that the Cretaceous– Paleocene sequence and the Jurassic ophiolitic mélange are characterized by numerous inversions after the Paleocene, a model which involves increasing Rr data towards the thrust plane between the units and underlying Paleozoic– Mesozoic complex can provide an acceptable explanation for the Rr pattern revealed in this study.

low-temperature metamorphism; organic maturity; Internal Dinarides; Medvednica; Croatia

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