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  • Author or Editor: Gábor Dobosi x
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The newly discovered crustal xenoliths from Miocene andesites in the Mátra Mts (Northeast Hungary) were classified into three petrographic types. Type 1 is a garnetiferous, plagioclase-rich cumulate rock, presumed to originate from the felsic portion of the nearby Szarvaskõ-Darnó mafic igneous complex. The almandine-rich garnet has been inferred to be of granulite facies origin. Garnet - orthopyroxene geothermobarometry on the orthopyroxene - plagioclase corona structure around it indicates a later high temperature (800-950 °C) low pressure (3-4 kbar) retrogression event. Type 2 is also plagioclase-rich cumulate rock with contact metamorphic assemblage containing spinel, corundum and andalusite, which crystallized in the course of dehydration reactions of biotite and/or smectite in hornblende hornfels to sanidinite facies conditions. Type 3 is microsyenite, which contains abundant anorthoclase and minor amount of Na- and K-rich kaersutitic-barkevikitic amphibole.

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New mineral paragenetic, illite Kübler index, chlorite “crystallinity”, apparent crystallite thickness, lattice strain, and K-white mica geobarometric data proved that the Eoalpine (Paleozoic-Mesozoic) metamorphic complex was affected by medium-pressure, high-temperature anchizonal regional metamorphism, whereas the Jurassic ophiolitic mélange and the Late Cretaceous-Paleocene sedimentary sequence of Mt. Medvednica were diagenetically altered. Mineral chemical investigations carried out on phyllosilicate flakes found in various microstructural positions revealed complete mineral chemical homogenization of chlorite and K-white mica of selected slate samples from the Eoalpine (Paleozoic-Mesozoic) metamorphic complex. One possible explanation of this feature is an Alpine (Cretaceous) regional metamorphic event with polyphase deformational history. Variscan low-temperature metamorphism overprinted by an Alpine (Cretaceous) event, with temperatures at least as high as those of the Variscan one, may be an alternative, although more complicated explanation. However, no isotope geochronological evidence supports this assumption. At present only one metamorphic event can be detected. Its physical conditions were ca. 350-400 °C on the basis of illite Kübler index and chlorite Al(IV) empirical thermometers and 3-4 kbar using K-white mica b cell dimension measurements.

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