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Correlation of organic and inorganic temperature indicators – a case study on low-temperature metamorphic rock series from Mt. Medvednica, Croatia

In the last ca. 40 years numerous papers dealt with the physico-chemical controls of the organic maturation and the phyllosilicate reaction progresses acting during diagenesis and low-T metamorphism. The aim of this work was to correlate inorganic and organic metamorphic grade indicators obtained on the Eoalpine (Paleozoic– Mesozoic) complex, the Jurassic ophiolitic mélange and Late Cretaceous– Paleocene sequence of Mt. Medvednica, Croatia providing additional data on the diagenetic– low-T metamorphic evolution of these rock series, the microstructural, mineral paragenetic and phyllosilicate “ crystallinity” parameters of which were presented earlier. For this purpose vitrinite reflectance, X-ray powder diffraction (XRPD)-based graphite “ ordering” and Raman spectroscopic parameters of the fine-dispersed carbonaceous material (CM) were used as thermal maturity indicators. Paleo-temperatures were calculated on random vitrinite reflectance (Ro) values and on Raman spectral parameters. Organic maturity data were correlated with the phyllosilicate “ crystallinity” indices (illite Kübler and chlorite Árkai indices), apparent mean crystallite thickness and lattice strain values obtained by the Voigt method and results of the chlorite empirical Al(IV) thermometers and geothermobarometer. Fairly good correlation was proven between the temperatures calculated by empirical inorganic and organic “ thermometers” . For anchi-epizonal metapelites from the Eoalpine (Paleozoic– Mesozoic) complex temperatures of ca. 300-420°C were estimated on the basis of mineralogical T indicators and Ro data. However, the Raman thermometers gave temperatures of ca. 390-460°C. In the Jurassic ophiolitic mélange Ro values suggested temperatures between 200-250°C. The Late Cretaceous– Paleocene sequence was characterized by peak temperatures of 100-230°C. T values increased systematically from youngest, Paleocene towards the oldest, Cretaceous rocks. These data corresponded fairly well to the high temperature part of the diagenetic zone. The diagenetically altered rocks were characterized by superimposition of the Raman bands of the different CM generations. Therefore, the Raman thermometer can only be applied with caution in these rock series.

organic and inorganic temperature indicators; low-temaperature metamorphism; Medvednica; Croatia

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