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Clay minerals as indicators of thermal history of Palaeozoic rocks from Marija Hills, NW Croatia

In Marija Gorica Hills (NW Croatia) few outcrops of Upper Palaeozoic rocks can be found. Having intention to determine thermal history of the rocks and to reveal possible metamorphic event the rocks have been studied by several complementary methods (polarizing microscopy, X-ray powder diffraction, and IR-spectroscopy). Syn- deformationally grown quartz in a strain shadow around an elongate framboidal pyrite is a common microtextural feature for Marija Gorica sandstone. Strain fringes record deformation history in both their internal and external shape indicating a low temperature deformation (metamorphism). Distinct stages of deformation history are also preserved in slightly curved quartz fibres and the crenulation cleavage. At contact points in the quartz grains, effects of pressure solution are visible. These features inspired further thermal history study. The illite crystallinity method, as one of the basic methods for evaluation of very- low grade metamorphism conditions in phyllosilicate bearing rocks was used. The obtained results expressed by Kübler index (KI) indicate anchizonal, i.e. very low-grade metamorphism. According to the results of XRD method minerals classified as kaolinite sub-group clay minerals were present in the investigated rocks. The transformation of kaolinite to dickite, two polytypes belonging to the sub-group, is temperature dependant therefore, in order to obtain information on thermal history of the rocks, polytype determination was performed. It was not possible to give unequivocal answer about polytype present by XRD method, therefore the <4 and <2 μ m rock fractions were investigated by IR spectroscopy. Distinction between polytypes was based on spectrum appearance in OH-stretching region. The presence of illite made the determination of polytypes rather difficult. Nevertheless it was observed that kaolinite to dickite transformation was not complete, especially in fine grained rocks, i.e. rocks with lower permeability, in which kaolinite was the dominant polytype present. Further research is planned in order to clear the eventual effects of various factors (bulk rock and fluid chemistries, etc.) having been responsible for the unusual persistence of kaolinite at relatively high (anchizonal) temperatures.

illite crystallinity ; kaolinite group minerals ; Marija Gorica Hills ; very low grade metamorphism

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