engleski

Clay minerals as thermal history indicators of Inner Dinarides lithologies from the surroundings of Hrvatska Kostajnica (Croatia)

In the Eocene clastic rocks, represented by conglomerates, sandstones and shales, which crop out as a part of Inner Dinarides in the vicinity of Hrvatska Kostajnica (Croatia), zeolite minerals have been described earlier. Zeolites occur mainly as cement but sometimes they are present in veinlets. Exact conditions of their formation are not defined yet. Cretaceous clastic and metasedimentary rocks are also present in the area, and it is difficult to distinguish them from Eocene ones due to absence of fossils and clear geological relations. In order to decipher thermal conditions responsible for zeolite formation clay mineralogy of the rocks was studied. In addition clay mineralogy was also used to distinguish petrographically similar rocks of different ages. Investigations comprised standard X-ray powder diffraction determination of clay minerals including Kubler (KI) and Árkai indices (AI) measurements. Preliminary results obtained so far show that clay minerals in clastic Eocene rocks, some of them containing laumontite and stilbite, are represented mainly by swelling clays (smectite or vermiculite), corrensite, chlorite i.e. assemblages that are characteristic for diagenetic zone. Cretaceous rocks can be divided in two groups. The first of them contain chlorite and illite with KI values typical for the anchizone, while in the other swelling clays are present, indicating lower temperatures. Obtained results of thermal history based on clay mineralogy and Kubler and Árkai indices confirm existence of so far vaguely defined/poorly known very-low grade metamorphic event in the period from Cretaceous to pre-Eocene in the northwesternmost part of the Inner Dinarides realm.

clay minerals, diagenesis, very-low grade metamorphism, Cretaceous, Eocene

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